Multidrug resistance (MDR) in cancer: Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

European Journal of Pharmaceutical Sciences 11 (2000) 265–283www.elsevier.nl / locate /ejps

Review

Multidrug resistance (MDR) in cancerMechanisms, reversal using modulators of MDR and the role of MDR

modulators in influencing the pharmacokinetics of anticancer drugs*Rajesh Krishna , Lawrence D. Mayer

Department of Advanced Therapeutics, British Columbia Cancer Agency, Vancouver, BC V5Z 4E6, Canada

Received 18 February 2000; received in revised form 27 April 2000; accepted 27 June 2000

Abstract

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition,its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated invarious studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR),reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokineticinteractions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first andsecond generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible ineliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDRmodulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution,clearance, and interactions) will also be discussed. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Drug–drug interactions; P-glycoprotein; Canalicular multispecific organic anion transporter; Pharmacokinetics; Liposomal doxorubicin; PSC833

1. Biology of multidrug resistance in the following sections in the context of the overall MDRphenomenon.

Of the approximately 1.3 million new cases of cancer A number of mechanisms have been described toeach year in North America (Landis et al., 1998), a fair explain the phenomenon of MDR in mammalian cells.proportion are drug resistant (Gottesman, 1993). This is They have been broadly classified into cellular and non-often due to the fact that these cancers either are inherently cellular mechanisms (Fan et al., 1994), as described below.untreatable or are resistant to a wide variety of anticancerdrugs or their combinations. MDR is a term used to 1.1. Non-cellular resistance mechanismsdescribe the phenomenon characterized by the ability ofdrug resistant tumors to exhibit simultaneous resistance to Non-cellular drug resistance can arise as a consequencea number of structurally and functionally unrelated chemo- of in vivo tumor growth. These phenomena are typicallytherapeutic agents. Although the focus of this review is associated with solid tumors which exhibit unique physio-P-GP-mediated MDR, a general overview of the underly- logical properties compared to circulating tumors such asing mechanisms and types of drug resistance is presented hematological malignancies. Solid tumors are composed of

a vasculature that is characterized by a higher geometricresistance (Sevick and Jain, 1989) and is heterogeneous,where the tumor blood vessels are dilated, tortuous, and

*Corresponding author. Correspondence address: Department of Me- saccular (Jain, 1987). In particular, the branching patternstabolism and Pharmacokinetics, Bristol-Myers Squibb Pharmaceutical

in tumor tissue are much different from normal vessels,Research Institute, P.O. Box 4000, Princeton, NJ 08543-4000, USA. Dr.with vascular shunts and sprouts being present (Jain,Rajesh Krishna. Tel.: 11-609-252-3579; fax 11-609-252-6607.

E-mail address: [email protected] (R. Krishna). 1988). The extracellular environment associated with solid

0928-0987/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0928-0987( 00 )00114-7

266 R. Krishna, L.D. Mayer / European Journal of Pharmaceutical Sciences 11 (2000) 265 –283

tumors is characterized by increased interstitial fluid et al., 1985). Among its various activities, GST plays anpressure compared to normal tissues. This is due to two important role in protecting cells from reactive epoxidescontributing factors, namely, higher vascular permeability (Kuzmich and Tew, 1991). This is believed to occur viaand absence of a functional lymphatic system (Jain, 1987). the catalytic addition of GSH to the epoxide moiety, suchConsequently, poor tumor vascularization can result in as that observed for the metabolism of aflatoxin B1 to anreduced drug access to regions within solid tumors and 8,9-epoxide which is detoxified by GST (Ramsdell andthus protect tumor cells from cytotoxicity. The physiologi- Eaton, 1990). The GST enzyme also protects the cell fromcal properties of solid tumors also result in tumor regions damage due to free radicals (Ketterer et al., 1990).that are deficient in nutrients and oxygen. Such properties The GSTs are extensively involved in the metaboliccan induce additional resistance mechanisms that arise biotransformation of many anticancer drugs. Among thesefrom extracellular influences. An example of this type of are nitrogen mustards, such as BCNU, and cyclophos-resistance is the increased presence of non-cycling tumor phamides. Several resistant cell lines have been shown tocells in poorly vascularized sections of solid tumors. These overexpress GST (Lewis et al., 1989; Hao et al., 1994).cells are often viable, but non-dividing, and consequently The GST-p isoenzyme has been shown to be overex-are resistant to drugs dependent on cell proliferation. The pressed in MCF7/ADR cells (Batist et al., 1986). Theseacidic environment in tumors, due to lactic acid generation MCF7/ADR cells, which also express elevated levels ofby hypoxic tumor cells, has also been suggested to confer a P-GP, exhibit increased peroxidase activity due to theresistance mechanism for weak bases, where cellular enhanced levels of GST-p. Similar increases in GST-puptake is dependent on the pH gradient across membranes levels have also been reported in other MDR cell lines(Demant et al., 1990). (Cole et al., 1989; Chao et al., 1991; Raghu et al., 1993).

However, evidence suggests that the GST-p activity is not1.2. Cellular based resistance mechanisms a resistance mechanism for doxorubicin (DOX). Specifi-

cally, when the gene expressing GST-p was transfectedCellular mechanisms are categorized in terms of altera- into MCF7 cells, it resulted in over 15-fold increase in

tions in the biochemistry of malignant cells. Such mecha- GST-p activity compared to WT cells (Moscow et al.,nisms can be further classified into two major categories: 1989), however, the transfected line was not DOX resistant(1) non-classical MDR phenotypes and (2) transport-based suggesting that GST-p did not contribute to DOX resist-classical MDR phenotypes (see Section 1.2.2. for descrip- ance. This is in contrast to observations with other GSTtion). isoforms where an 8-fold increase in resistance to drugs

such as chlorambucil was observed when GST-a was1.2.1. Non-classical MDR phenotypes transfected into yeast cells (Black et al., 1990), while for

The term non-classical MDR is used to describe non- nitrogen mustards, a 2-fold increase in GST activitytransport based mechanisms that affect multiple drug resulted in drug resistance (Schisselbauer et al., 1990).classes. This type of resistance can be caused by altered Agents such as ethacrynic acid and prostaglandin analogsactivity of specific enzyme systems (such as glutathione block GST activity and can be used to increase theS-transferase, GST and topoisomerase), which can de- sensitivity of chlorambucil (Tew et al., 1988) or mephalancrease the cytotoxic activity of drugs in a manner in- (Clapper et al., 1990).dependent of intracellular drug concentrations, which In addition to GST, the cellular regulation of the thiolremain unaltered. In addition, changes in the balance of tripeptide, glutathione (GSH) also appears to play a keyproteins that control apoptosis can also reduce chemosen- role in detoxification and cellular repair following thesitivity since most anticancer drugs are believed to exert damaging effects of DOX and alkylating agents. Increasestheir cytotoxic effects via apoptotic processes. This section in GSH levels have been observed in many alkylatingoutlines some of these MDR mechanisms and their role in agent-resistant cell lines (Calcutt and Connors, 1963; Ballthe overall MDR phenomenon. et al., 1966; Meister, 1991, 1994). This suggests that

reductions in intracellular GSH will result in chemosensiti-1.2.1.1. Glutathione S-transferease (GST) zation of drug resistant cells. In support of this, use of such

GST is an enzyme system involved in drug and xeno- agents as buthionine sulfoximine (BSO) has led to thebiotic detoxification. Specifically, biotransformation pro- modulation of GSH-mediated drug resistance by reducingcesses catalyzed by GST conjugate organic molecules with GSH levels (Batist et al., 1986).glutathione (GSH), resulting in excretable polar molecules.There are two intracellular pools of GST, one residing in 1.2.1.2. Topoisomerase activitythe cytosol and the other in the microsomal compartment. Two types of topoisomerase have been shown to beCytosolic GSTs are composed of 23–29-kDa subunits present in all eukaryotic cells (Wang, 1985; Osheroff,which may be homo or hetero-dimers (Mannervik and 1989). Type I topoisomerase, a monomeric 100-kDaJensson, 1982). In contrast, microsomal GSTs are trimeric protein (Liu and Miller, 1981), serves to alter DNAand composed of identical 17-kDa subunits (Morgenstern topology via single strand break (Wang, 1985), while

R. Krishna, L.D. Mayer / European Journal of Pharmaceutical Sciences 11 (2000) 265 –283 267

topoisomerase type II alters DNA topology by causing plays a key role in inducing cell cycle arrest in G and1

transient double strand breaks. Both enzyme classes are apoptosis following DNA damage caused by anticancerintrinsically involved in the processes of DNA replication drugs, but also in the regulation of expression of down-(Wang, 1987). There are two subclasses of type II — (1) a stream proteins, bcl-2 and bax (Lowe et al., 1993a;170-kDa a and (2) a 180-kDa b (Drake et al., 1987; Miyashita et al., 1994). The ability of cells to undergoChung et al., 1989). Consequently, these enzymes consti- apoptosis has been linked to the formation of hetero- andtute therapeutic targets in rapidly dividing tumor cells for homo-dimers generated via bcl-2–bax interactions. Foranticancer drugs. For example, DOX and etoposide spe- example, bax–bcl-2 heterodimers as well as bax homo-cifically target topoisomerase II (Tewey et al., 1984a,b; dimers promote apoptosis, whereas apoptosis is inhibitedRoss et al., 1984), while campothecin analogs target when bcl-2 forms homodimers (Reed, 1995). Resistancetopoisomerase I (Kunimoto et al., 1987; Johnson et al., may, therefore, develop with loss of genes required for cell1989). Anthracyclines stabilize the transient covalent death such as p53 or overexpression of genes that blockcomplex between DNA and topoisomerase II a. Rejoining cell death. B-cell lymphoma-2 (bcl-2) is a gene that playsof DNA strands is blocked, resulting in the formation of a key role in the regulation of cell death pathways.stable enzyme–DNA complex with double-strand DNA In lymphomas, particularly follicular lymphoma, evi-breaks (Liu, 1989), resulting in tumor cell death. These dence suggests that chromosomal translocations cause theDNA strand breaks appear to trigger apoptosis (Roy et al., movement of the bcl-2 gene from chromosome 18 to1992; Onishi et al., 1993), which leads to tumor cell death. chromosome 14, due to errors in normal DNA recombina-

Cells become resistant to topoisomerase II inhibitors tion mechanisms (Weiss et al., 1987; Tsujimoto et al.,such as DOX and etoposide (Drake et al., 1987) due either 1988; Zelenetz et al., 1991). Bcl-2 serves to protect the cellto the under-expression of topoisomerase II or topoisomer- from stimuli causing cell death, such as UV or gammaase II gene mutations. Resistance may occur alone or radiation, tumor necrosis factor, or drugs that induce free-concurrent to P-GP overexpression (Kunikane et al., 1990; radical production (Reed, 1995). Bcl-2 has also beenKamath et al., 1992). Reduced activity of topoisomerase II reported to be overexpressed in patients with AML andactivity (Matsuo et al., 1990) as well as reductions in CLL (Campos et al., 1993; Hanada et al., 1993), whereastopoisomerase II mRNA levels have been shown to explain in carcinomas and other undifferentiated cancers, Bcl-2cellular resistance to topoisomerase II inhibitors. expression is altered (Reed et al., 1991; Castle et al., 1993;Topoisomerase II gene mutations can also occur, where Colombel et al., 1993; Bronner et al., 1995). Bcl-2 canreduced enzyme synthesis occurs following topoisomerase confer cellular resistance to the cytotoxic effects of aII gene transcription (Deffie et al., 1989; Tan et al., 1989). number of anticancer agents including DOX, taxol,The exact mechanism of down regulation is poorly under- etoposide, camptothecin, mitoxantrone and cisplatin (Reed,stood, but believed to be due to hypermethylation of the 1995). Unlike P-GP mediated MDR, overexpression oftopoisomerase II gene. bcl-2 does not prevent drug influx into tumor cells (Fisher

A compensatory overexpression of topoisomerase I has et al., 1993; Walton et al., 1993). When bcl-2 is over-been observed in cells resistant to topoisomerase II in- expressed and contributes as a resistance mechanism, it hashibitors, where cells have reduced expression of been shown that the anticancer drugs promote cell cycletopoisomerase II (Tan et al., 1989). One approach to arrest; however, their effects are cytostatic rather thancircumvent topoisomerase II-mediated MDR is to target cytotoxic (Fisher et al., 1993).both enzyme classes at the same time. For instance, when Down regulation of bcl-2 can occur with loss of p53, thetopoisomerase I inhibitor CPT-11 was pretreated in nude tumor suppressor gene. Stimuli such as radiation thatmice bearing human xenografts, enhanced activity of induce DNA strand breaks are associated with increases inDOX, a topoisomerase II inhibitor, was observed (Kim et p53 protein as well as increases in p53 transcriptionalal., 1992), presumably due to overexpression of activity (Zhan et al., 1993). Further, in vitro results havetopoisomerase II activity mediated by CPT-11 pretreat- demonstrated that cell lines lacking p53 exhibit increasedment. resistance to induction of apoptosis by anticancer drugs or

radiation (Lowe et al., 1993b).1.2.1.3. Altered apoptosis regulation

Anticancer drugs typically induce programmed cell 1.2.2. Transport-based classical MDR mechanismsdeath or apoptosis. This form of cell death is characterized The ATP-binding cassette (ABC) family of membraneby nuclear condensation within cells, leading to DNA transport ATPases are of considerable clinical importance.fragmentation caused by endonucleolytic cleavage of The family is phylogenetically ancient and its normalgenomic DNA. The decision, whether a cell continues functions in eukaryotic cells remain to be fully character-through cell cycle or undergoes apoptosis, is dependent ized. The first of these proteins was found in bacteria andupon a complex interplay of a team of genes and proteins yeast (Van Veen and Konings, 1998). These transportthat exert a regulatory role in cellular events. It has been ATPases are generally composed of four structural do-suggested that the tumor suppressor gene, p53, not only mains, two that span the membrane (each containing


several transmembrane segments) and two that remain in ance properties, MRP1 also plays a normal physiologicalthe cytoplasm. The last two units, called nucleotide role in the ATP-dependent unidirectional membrane trans-binding domains, play a role in cleaving ATP (hydrolysis) port of glutathione conjugates, such as leukotriene C and4

to derive energy necessary for transporting cell nutrients, S-(2,4-dinitrophenyl)glutathione (Jedlitschky et al., 1994;such as sugars, amino acids, ions and small peptides across Leier et al., 1994b). Inhibition of this transport has beenmembranes. demonstrated with leukotriene receptor antagonists (Jed-

As an example, one of the proteins responsible for litschky et al., 1994; Leier et al., 1994a,b). Effectiveresistance to the antimalarial agent, chloroquine, is a modulation of MRP-mediated MDR has been observedmember of the ABC family, in which resistant Plasmodium with the leukotriene LTD receptor antagonist MK 5714

falciparum (a malaria causing parasite) exhibit amplified (Gekeler et al., 1995). It has also been shown that MRP1levels of an ABC transporter that pumps chloroquine out transports drugs conjugated to glucuronic acid and gluta-of the cell (Foote et al., 1989; Wilson et al., 1989). The thione in addition to oxyanion complexes, while MRP2cystic fibrosis transmembrane regulator protein (CFTR) is transports agents such as vinblastine (Kool et al., 1998).also a member of the ABC family. Cystic fibrosis is a This would suggest that MRP-mediated MDR requiresdisease caused by a mutation in the gene encoding an ABC drug conjugation or modification prior to efflux across the

2transporter that plays a role as a Cl channel in epithelial plasma membrane.cell plasma membranes. It has been suggested, therefore, MRP is also expressed in normal human tissues, such asthat these ABC transporters may play a dual role as ion muscle, lung, spleen, bladder, adrenal gland and gallchannels as well as carrier pumps for influx/efflux of bladder (Zaman et al., 1993). Reduced glutathione (GSH)nutrients and hydrophobic drugs. has been suggested as an important component of MRP-

Among the ABC transporters involved in MDR are mediated MDR and drug transport (Olsen et al., 1998). AnP-glycoprotein (P-GP) and multidrug resistance associated isoform of MRP was first shown to be expressed in theprotein (MRP), which can be overexpressed in malignant liver which functions in the excretion of glutathione andcells and serve to pump anticancer drugs out of the cell, glucuronate conjugates across the cannalicular membraneresulting in lack of intracellular levels of the drug neces- into bile (Mayer et al., 1995). Excretion of glutathione,sary for effective therapy. These two drug transporter glucuronate and sulfate conjugates across the hepatocytepumps constitute well characterized mechanisms of MDR canalicular membrane was functionally characterizedand are discussed in detail in the following sections. (Ishikawa et al., 1990; Kitamura et al., 1990; Akerboom et

al., 1991; Fernandez-Checa et al., 1992; Keppler et al.,1.2.2.1. Multidrug resistance associated protein (MRP) 1996) as being due to an ATP-dependent mechanism/

MRP, a member of the ABC family, has been described transporter, which has been described under variousas a GS-X pump capable of transporting organic anion names: as a leukotriene export pump (Keppler, 1992), adrug conjugates as well as intact anticancer drugs (Cole et non-bile acid organic anion transporter (Arias et al., 1993),al., 1992; Krishnamachary and Center, 1993; Barrand et MOAT (Oude Elferink and Jansen, 1994; Oude elferink etal., 1994; Grant et al., 1994; Borst et al., 1997). MRP, al., 1995), and as a glutathione-S-conjugate export pumpwhich was isolated as a transmembrane glycoprotein in (Ishikawa, 1992). In addition to being expressed in thenon-P-GP expressing small cell lung cancer DOX resistant cannalicular membrane, MRP2/cMOAT is also expressedcell lines, is an asymmetrical molecule with eight subunits in the human kidney proximal tubule epithelia on theand four membrane-spanning domains (Almquist et al., apical side. Specifically, it is localized to the brush border1995). The clinical cancers exhibiting MRP expression membranes of proximal tubule segments S1–S3 (Schaub etinclude hematological (Burger et al., 1994; Broxterman et al., 1998). MRP2, therefore, is implicated to play a role inal., 1994), lung (Rubio et al., 1994; Savaraj et al., 1994), the renal excretion of endogenous substances and xeno-acute lymphoblastic leukemia relapses and chronic biotics, in normal conditions.myeloid leukemia (Beck et al., 1994).

The classes of anticancer drugs that are substrates of 1.2.2.2. P-Glycoprotein (P-GP)MRP include anthracyclines such as DOX, vinca alkaloids, P-GP, a member of a superfamily of ATP-dependentand etoposide. By pumping these agents out of the tumor membrane transport proteins, is a plasma membranecells, MRP causes reduced intracellular accumulation of protein that was first characterized in multidrug resistantdrugs, leading to resistance. Although MRP is remarkably chinese hamster ovary cells by Ling and co-workers (Lingsimilar to P-GP in terms of substrate specificity and action, and Thompson, 1974; Juliano and Ling, 1976; Riordan andonly a 15% amino acid homology exists between the two. Ling, 1979; Kartner et al., 1983). It has been shown toSeveral isoforms of MRP have recently been identified, pump substrates out of tumor cells through an ATP-namely MRP1, MRP2 (or cannalicular multispecific or- dependent mechanism in a unidirectional fashion. In tumorganic anion transporter or cMOAT), and MRP3-6. MRP 1 cells expressing P-GP, this results in reduced intracellularand 2 have been identified as organic anion transporters drug concentrations which decreases the cytotoxicity of a(Borst et al., 1997). Although associated with drug resist- broad spectrum of antitumor drugs including anthracy-


clines (e.g. DOX), vinca alkaloids (e.g. vincristine), processes involved in normal physiology, overexpressionpodophyllotoxins (e.g. etoposide) and taxanes (e.g. taxol). of this protein in tumor cells results in reduced intracellularP-GP is present in human (two human genes), mouse accumulation of anticancer agents due to increased drug(three rodent genes), and hamster cells (Roninson et al., efflux. Direct evidence of P-GP-mediated drug transport1986; Van der Bliek et al., 1987; Croop et al., 1989; de came from initial studies employing partially purifiedBruijn et al., 1986; Gros et al., 1988). Gene sequence membrane vesicles (Horio et al., 1988). It was shown thatanalysis for mammalian P-GP has revealed the presence of vesicles prepared from resistant cells were more capable oftwo similar halves, each containing 6 putative transmem- binding and transporting radiolabeled vinblastine thanbrane segments, and an ATP-binding consensus motif. The those prepared from sensitive cells (Horio et al., 1988).human protein is comprised of 1280 amino acids with 12 Vesicle transport experiments have also been performed intransmembrane domains and 43% sequence homology biliary canalicular vesicles expressing P-GP, where ATP-between the two halves. Three glycosylation sites on the dependent transport of daunomycin was shown in inside-first extracytoplasmic domain are present (Chen et al., out membrane vesicles obtained from apical membranes of1986). hepatocytes (Kamimoto et al., 1989). Transepithelial trans-

There are three known isoforms of P-GP, namely, class port of vinca alkaloids, and anthracyclines mediated byI, II and III. Rodent cells have all three P-GP genes, P-GP was also demonstrated in epithelia prepared fromencoding classes I, II, and III P-GP, whereas human cells Madin-Darby canine kidney cells (Horio et al., 1989),have two, encoding class I and III P-GP (Lee et al., 1993). where a retrovirus carrying a human mdr1 cDNA wasClasses I and II P-GP genes confer MDR when transfected transfected resulting in epithelia with P-GP polarized oninto sensitive wild type (WT) cells, whereas the class III the apical surface. Further, transport of radiolabeled col-P-GP gene is not shown to be associated with drug chicine was demonstrated by partially purified and recon-resistance. All three classes of P-GP are inherently ex- stituted P-GP (Sharom et al., 1993).pressed in several normal tissues. Specifically, in mam- Several mechanisms have been put forward to explainmalian tissues, class I P-GP is present in intestinal lining this transport function of P-GP. The model of Higgins andepithelium, endothelial cells, bone marrow progenitor Gottesman (1992) postulates that P-GP encounters xeno-cells, peripheral blood lymphocytes and natural killer cells, biotics in the inner leaflet of the plasma membrane andwhereas class II is present in the adrenal cortex. The class flips the agents to the outer leaflet, where they diffuse intoIII P-GP is localized in hepatocytes, cardiac and striated the extracellular region (Fig. 1). P-GP has also beenmuscle (Thiebaut et al., 1987; Croop et al., 1989; postulated to increase intracellular pH (Roepe, 1992),Chaudhary et al., 1992). Human class I P-GP is closelyrelated to rodent class I and II P-GPs, whereas human classIII P-GP corresponds to rodent class III P-GP. In the liver,class III P-GP is localized to the canalicular membrane ofthe hepatocytes. Mice lacking class III P-GP expression areunable to secrete phospholipid into bile and this wouldsuggest that class III P-GP may act as a phospholipidtransporter in hepatocyte membranes.

These localization studies suggest that P-GP has func-tional roles in normal tissues. P-GP activity in normaltissues suggests an important role in transepithelial trans-port. P-GP is expressed in such tissues as transportingepithelia of the liver, kidney, colon, small intestine,pancreas, placenta, uterus and in specialized capillaryendothelial cells in the brain and testis (biliary face ofhepatocytes, brush border of kidney proximal tubule cells,see Fojo et al., 1987; Thiebaut et al., 1987, 1989; Arceci etal., 1988; Sugawara et al., 1988a,b; Cordon-Cardo et al.,1989). That P-GP is expressed in the brain suggests a rolein the blood–brain barrier, thus preventing the permeationand persistence of hydrophobic agents in the centralnervous system. These observations indicate a role ofP-GP in the transepithelial secretion of substrates into bile,

Fig. 1. Function of the P-GP pump. The model illustrates a protein whichurine, or gastrointestinal tract lumen. P-GP may alsouses ATP energy to actively efflux drug substrate across the plasma

confer a protective role to mediate xenobiotic efflux in membrane. The MDR modulator (chemosensitizer) may act as a competi-tissues such as the brain, testis, and placenta. tive inhibitor by occupying drug binding sites or as a non-competitive

Whereas P-GP fulfills critical functions in transport inhibitor at chemosensitizer binding sites (modified from Ford, 1995).


depolarizing plasma membrane electrical potential of the detection also appears to correlate well with poor responsecell by acting as a proton pump, or a chloride channel, thus to chemotherapy (Bradley and Ling, 1994).reducing intracellular accumulation of weak bases orreducing pH-dependent binding of agents to their intracel- 1.4. Modulation of P-GP-mediated MDRlular targets (Roepe et al., 1993).

In the hydrophobic vacuum cleaner model proposed by The process of chemosensitization involves the co-ad-Gottesman and Pastan, 1993, P-GP interacts directly with ministration of a P-GP inhibitor (MDR modulator) with ansubstrates in the plasma membrane and pumps them out of anticancer drug in order to cause enhanced intracellularthe cell. Evidence of competitive binding of drugs on the anticancer drug accumulation via impairing the P-GPP-GP molecule has been demonstrated with radiolabeled function. Numerous compounds have been shown to

125photoaffinity probes, such as N-( p-azido-[3- I]-salicyl)- inhibit the drug efflux function of P-GP and therefore,3N9-b-amino vindesine and [ H]azidopine. This technique reverse cellular resistance.

is based on energy transfer from a chromophoric substrate P-GP directed MDR modulators belong to a number offor P-GP to a photoactive radiolabeled probe. Utilizing the chemical classes including calcium channel blockers,

125photoactive analog of vinblastine, N-( p-azido-[3- I]- calmodulin inhibitors, coronary vasodilators, indole al-salicyl)-N9-b-amino vindesine, as a probe, the presence of kaloids, quinolines, hormones, cyclosporines, surfactants,vinblastine-binding sites on the P-GP molecule was shown and antibodies (Ford and Hait, 1990, 1993). In general,in membrane vesicles prepared from MDR chinese hamster they have been classified as those belonging to the first,lung cells and human carcinoma cells (Cornwell et al., second or third generation, as described in the following1986; Safa et al., 1986). This labeling was inhibited by sections.vincristine and daunorubicin, indicating that the bindingwas competitive (Cornwell et al., 1986). Since colchicine 1.4.1. First generation modulatorsand actinomycin D did not compete for vinblastine-binding Tsuruo and co-workers (Tsuruo et al., 1981, 1982a,b;sites, the presence of multiple drug binding domains was 1983a,b,c) were the first to demonstrate the ability of thesuggested (Cornwell et al., 1986; Akiyama et al., 1988). calcium channel blocker, verapamil (VRP), to reverseThese observations were corroborated by the fact that MDR. VRP enhanced intracellular accumulation of manybinding of the photoactive calcium channel blocker, anticancer drugs, including DOX in numerous cell lines

3[ H]azidopine to P-GP was blocked by colchicine and (Inaba et al., 1979; Harker et al., 1986; Feng et al., 1992;actinomycin D among others (Safa et al., 1987; Yang et al., Roepe, 1992). Subsequent studies revealed that this MDR1988). Based on recent methods of purifying and recon- reversing property is shared by many other calciumstituting class I hamster P-GP into liposomes with greater channel blockers. Clinically available calcium antagonistsretention of ATPase activity (Shapiro and Ling, 1994), were demonstrated to reverse MDR in vitro. These in-transport of the P-GP substrate, Hoechst 33342 was cluded felodipine (Hollt et al., 1992), isradipine (Hollt etdemonstrated (Shapiro and Ling, 1995), lending further al., 1992), nicardipine (Tsuruo et al., 1983b; 1983c; Ramusupport of the hypothesis that P-GP is itself capable of et al., 1984a), nifedipine (Tsuruo et al., 1983b; 1983c;direct substrate transport. Ramu et al., 1984b), bepridil (Schuurhuis et al., 1987), and

diltiazem (Tsuruo et al., 1983b; Klohs et al., 1986). These1.3. Clinical relevance of MDR agents modulated MDR at very high concentrations,

ranging from 5 to 50 mM. At these high concentrations,A number of cancers are treatable either by surgery, there was enhanced cytotoxicity observed in normal cells

radiation therapy or chemotherapy. However, a consider- (Lampidis et al., 1986; 1990). In addition, it was laterable number of cancers are either intrinsically resistant or demonstrated that even calmodulin antagonists such asexhibit treatment induced acquired resistance, which com- trifluorperazine (Tsuruo et al., 1982a,b; Ganapathi andplicates efforts to successfully cause long-term regression Grabowski, 1983; Akiyama et al., 1986; Ganapathi et al.,or cure. As described above, a number of mechanisms 1984, 1986; Klohs et al., 1986; Ford et al., 1989),have been shown to exist under the generalized MDR chlorpromazine (Ganapathi et al., 1984; Akiyama et al.,group. Among these mechanisms, the role of P-GP in 1986; Ford et al., 1989) and prochlorperazine (GanapathiMDR is best characterized. P-GP occurrence in clinical et al., 1984; Ford et al., 1989) all reversed MDR sig-tumors has been extensively characterized and P-GP nificantly at concentrations ranging from 1 to 10 mM.overexpression has been shown to occur both during Other more potent calmodulin antagonists included clopen-diagnosis as well as during relapse. For example, P-GP thixol (Ford and Hait, 1990), trifluopromazine (Ford et al.,was detected at the time of diagnosis in leukemias (Cam- 1989), and flupenthixol (Ford et al., 1989; Ford and Hait,pos et al., 1992; Goasguen et al., 1993); lymphomas 1990), all of which were effective at concentrations of(Niehans et al., 1992); adult and childhood sarcomas approximately 3–5 mM. Indole alkaloids, the anti-malarial(Gerlach et al., 1987; Goldstein et al., 1989; Chan et al., quinine and the anti-arrhythmic quinidine, have also been1990), and neuroblastomas (Chan et al., 1991). P-GP shown to reverse MDR in vitro in experimental cell lines


in conjunction with DOX (Sehested et al., 1989; Eliason et a study using the P388 ascites leukemia, murine C26 colonal., 1990; Solary et al., 1990, 1991). Cyclosporin A, a carcinoma, and HCT-15 human xenografts in nude mice,commonly used immunosuppressant for organ transplanta- the chemosensitizer PSC 833 along with DOX providedtion, remains one of the most effective first generation varying degrees of MDR reversal. While providing signifi-MDR modulators studied. In conjunction with several cant increases in median life span in ascites leukemia andanticancer agents, it effectively reverses MDR in many cell significant tumor regression in C26 carcinoma (whichlines, such as P388 (Kukl et al., 1992), CHO (Neumann et expresses moderate to low levels of P-GP), coadministra-al., 1992), K562 and CEM (Chao et al., 1990), L1210 tion of PSC 833 and DOX, at non-dose reduced conditions,(Dorr and Liddil, 1991), and C26 (Spoelstra et al., 1991). afforded incomplete tumor regression in the human xeno-

A number of these first generation MDR modulators graft model (Watanabe et al., 1996).displaying excellent MDR reversal activities in vitro havebeen tested in murine tumor models. A common model is 1.4.3. Third generation modulatorsthe evaluation of MDR efficacy following i.p. inoculation Several MDR modulators have recently been developedof sensitive and MDR tumor cells, such as P388 or L1210, using structure–activity relationships and combinatorialto generate ascites. Subsequently, the drug and chemosen- chemistry approaches targeted against specific MDR mech-sitizer are administered i.p (Skovsgaard et al., 1984). anisms. These agents exhibit effective reversing concen-Several MDR modulators including VRP, nicardipine, trations in the nanomolar range (20–100 nM), thus requir-quinacrine, and CsA demonstrated significant increases in ing low doses to achieve effective reversing concentrationsmean and median life span in this i.p.–i.p. model (Slater et in vivo. Examples include specific P-GP blockers such asal., 1982; 1986; Tsuruo et al., 1981; 1983c; 1984). the cyclopropyldibenzosuberane LY 335979 (Dantzig et

al., 1996), the acridonecarboxamide GF 120918 (Hyafil et1.4.2. Second generation modulators al., 1993), the diketopiperazine XR9051 (Dale et al.,

A unique property shared by most first generation MDR 1998), the diarylimidazole OC144-093 (Newman et al.,modulators is that they are therapeutic agents and typically 2000), as well as bispecific (both P-GP and MRP, thereverse MDR at concentrations much higher than those multidrug resistance-associated protein) chemosensitizersrequired for their individual therapeutic activity (see such as VX-710 and VX-853 (Germann et al., 1996; PeckSection 1.5). The search for non-toxic second generation et al., 1996). The cyclopropyldibenzosuberane LY 335979,modulators resulted in newer analogs of the first generation is characterized by a 10-fold increased potency comparedagents, which were more potent and considerably less to CsA, latent modulating activity and a blockade mecha-toxic. nism specific for P-GP (Dantzig et al., 1996). The ac-

Structural analogs of VRP, dexverapamil (less cardiotox- ridonecarboxamide GF 120918 (Hyafil et al., 1993) ex-ic R-enantiomer of VRP), emopamil, gallopamil, and hibits similar characteristics to LY 335979, with increasedRo11-2933 (a tiapamil analog) reversed MDR in vitro to a potency and a P-GP-selective blockade mechanism. VX-degree equivalent to VRP, but with marginal toxicity in 170 and VX-853 are examples of bispecific chemosensitiz-many animal models (Nawrath and Raschack, 1987; Pirker ers that block both P-GP and MRP (Germann et al., 1996;et al., 1989, 1990). In addition, some of these compounds Peck et al., 1996). Using a combinatorial chemistrydemonstrated increased potency such as the tiapamil approach, XR9051, a novel diketopiperazine derivative,analog Ro11-2933 which is effective at 1–2 mM con- was shown to be specific in modulating P-GP-mediatedcentrations compared to 5–10 mM required for VRP MDR (Dale et al., 1998). Although these agents appear to(Kessel and Wilberding, 1985; Alaoui-Jamali et al., 1993). be well tolerated in combination with anticancer drugsThe non-immunosuppressive analog of CsA, PSC 833, has such as DOX, it is yet to be determined whether thesedemonstrated superior MDR reversal efficacy in many compounds can achieve full chemosensitization in moreexperimental cell lines in vitro (Boesch et al., 1991a,b; stringent MDR solid tumor models or beyond that, inJonsson et al., 1992; Kreis et al., 1993; Van der Graaf et clinical cancers.al., 1993; Gaveriaux et al., 1989; 1991). PSC 833, hasbeen shown to reverse MDR in conjunction with 1.5. Complications with MDR modulatorsdaunorubicin, DOX, vincristine, vinblastine, taxol, ormitoxantrone in MDR P388 leukemic cell lines at in vitro First generation modulators such as VRP and CsAconcentrations of 0.5–2 mM (Boesch et al., 1991b; Jonsson exhibit inherent pharmacological activity (cardiovascularet al., 1992; Duran et al., 1993; Kreis et al., 1993; Van der for VRP, immunosuppression for CsA) and have beenGraaf et al., 1993; Krishna et al., 1997). shown both preclinically and clinically to require high

Considerable improvements in antitumor activity and of doses to achieve in vivo plasma concentrations sufficient toanticancer agents in conjunction with PSC 833 have been reverse MDR (Ozols et al., 1987; Fisher et al., 1994a). Atdemonstrated in in vivo ascites models as well as solid these elevated doses, both compounds exhibited severe andtumor MDR models (Boesch et al., 1991a; Colombo et al., sometimes life-threatening toxicities. These dose-limiting1996; Watanabe et al., 1996; 1995; Keller et al., 1992). In toxic effects have precluded their application as effective


MDR modulators for in vivo applications, particularly in 1986; Ozols et al., 1987; Bissett et al., 1991; Dalmark etthe clinic. For VRP, this is exemplified by the fact that al., 1991; Hendrick et al., 1991; Rodenburg et al., 1991;plasma concentrations required to obtain desirable car- Verweij et al., 1991; Mross et al., 1993; Miller et al., 1994;diovascular effects average 0.4–1.2 mM (Anderson et al., Murphy et al., 1994; Wishart et al., 1994). Due to the lack1982). Clinical manifestations of toxicity (arterio-ventricu- of selectivity for the tumor tissue P-GP, it is not understoodlar block and hypotension) have been observed at con- whether pharmacokinetic alterations induced by thecentrations higher than these, particularly at plasma con- modulator may adversely affect therapy; however, suchcentrations required for MDR modulation activity (2–6 relationships have been demonstrated (Krishna and Mayer,mM). 1997; Krishna et al., 2000).

These complications of first generation modulatorsstressed the need to develop MDR modulators with lowerinherent toxicities. This initiated the search to identify 2. Role of P-GP in affecting the ADME of anticancercongeners of first generation MDR modulators with re- drugsduced inherent toxicity while retaining MDR modulatingefficacy. These efforts resulted in the development of 2.1. Absorption and distributionsecond generation MDR modulators which were eitherstereoisomers of their first generation racemic counterparts, The role of drug transporters in absorption of drugs hassuch as dexVRP (R-enantiomer of VRP) and dexnigul- been a subject of numerous scientific contributions and isdipine (R-enantiomer of niguldipine) or structural analogs beyond the scope of the review. One potential role ofof first generation agents, such as Ro11-2933 (a tiapamil membrane transporters which has been underrepresented inanalog) and PSC 833 (a CsA analog). All of these the past is its inducibility which may have implications incompounds exhibited decreased inherent toxicity while the oral bioavailability of drugs via increased clearance.retaining MDR reversal efficacy compared to their parent Reduction in the oral bioavailability may occur as acompounds. An added feature was that some of these consequence of induction of intestinal activity of mem-agents exhibited increased potency, and consequently brane transport proteins, while increased bioavailability isrequired low doses to achieve effective in vivo plasma more common consequence of coadministration of aconcentrations required to modulate MDR. transporter blocker with a transport substrate (refer to the

Although these agents circumvented many of the prob- section on drug interactions). Reduced bioavailability oflems experienced with first generation MDR modulators, taxol due to intestinal P-GP is one such example (Spar-when these agents were co-administered with anticancer reboom et al., 1997). Here, P-GP limited the oral uptake ofagents for modulating P-GP-based MDR they influenced taxol by mediating direct excretion of the drug from thepharmacokinetics and biodistribution properties of the central compartment into the intestinal lumen. Decreasedanticancer drugs (reviewed in Lum and Gosland, 1995). biovailability of cyclosporin and tacrolimus via intestinalEarly reports suggested that this was perhaps due to P-GP P-GP activity has also been demonstrated (Kaplan et al.,blockade in normal tissues, because P-GP was present in 1999). P-GP has been shown also to be induced in thesuch tissues as the liver and kidney which exhibit a kidney and other tissues by cyclosporin A treatment (Jettesecretory function (see Section 1.2.2.2.). Examples of such et al., 1996). Increased bioavailability of taxol by PSC 833pharmacokinetic interactions include an 8-fold increase in and cyclosporin A has been shown to result via inhibitionplasma AUC of daunorubicin observed when the anti- of drug transport proteins (Van Asperen et al., 1997, 1998).cancer drug was combined with the P-GP inhibitor VRP in The latter approach has been applied in the development ofrats (Nooter et al., 1987), which resulted in increased suitable oral formulations of anticancer agents.toxicity and a need to reduce the anticancer drug dose. It has been also shown that a single injection of cisplatinThis was corroborated by observations that high dose VRP can induce the expression of drug transporters such asinfusion resulted in 8-fold increases in tissue uptake of P-GP and canicular multispecific organic anion transportervincristine in liver, kidney and intestine, necessitating an (cMOAT) in the liver, kidney, and intestine (Demeule et8-fold reduction in drug dose (Horton et al., 1989). A al., 1999). Interestingly, although the pharmacokinetics ofrecent report demonstrated that VX-853, a potent third cisplatin may not be affected by this induction sincegeneration modulator caused a 3-fold reduction in plasma cisplatin is not a P-GP substrate, it may have implicationsclearance of coadministered paclitaxel, necessitating anti- in modifying the absorption of drugs that are substrates ofcancer drug dose reduction (Wang and Chaturvedi, 1998). these proteins. Several compounds have also been shownThe influence of P-GP-mediated pharmacokinetic interac- to induce the expression of other related transport proteinstions are discussed in greater detail in Section 2. such as cMOAT and these may include phenobarbital

Presumably due to the above complications, results from (Kiuchi et al., 1998), 2-acetylaminofluorene (Kauffmann eta number of Phase II clinical trials in patients with al., 1997), and cycloheximide (Kauffmann et al., 1997).colerectal, lung, ovarian, breast, renal cell, myeloma, and P-GP is also expressed in the blood–brain barrier asleukemic cancers have been disappointing (Presant et al., well as the placenta and therefore may play a critical role


in modulating the distribution of compounds (Cordon- canalicular membranes of the biliary tract and anticancerCardo et al., 1989). It has been speculated that P-GP may drug transport in this tissue has been shown to be inhibitedplay a role in regulating the permeation of xenobiotics into by MDR modulators (Gatmaitan and Arias, 1993; Bohmethe central nervous system (CNS), thereby limiting the et al., 1993; 1994). Blockade of biliary clearance of severalsystemic entry of drugs. This is consistent with in vitro anticancer drugs by MDR modulators have also beenobservations that suggest unidirectional transepithelial demonstrated. This includes reports of colchicine blockadetransport of compounds from the basal side to the apical by CsA (Speeg et al., 1992a), reduced vinblastine elimina-side. Given the localization of P-GP in mouse brain tion caused by CsA (Samuels et al., 1993), as well as DOXcapillary endothelial cells to the apical side of the cells, biliary clearance inhibition by PSC 833 (Speeg andthis would suggest that P-GP in the endothelium plays a Maldonado, 1994; Krishna et al., 1999) and GF120918role in limiting the access of drugs such as vincristine into (Brouwer and Mellon-Kusibab, 1996; Booth et al., 1998).the CNS (Tatsuta et al., 1992). Recent evidence using These observations suggest that MDR modulators blockP-GP knock out models suggests that the drug efflux pump P-GP expressed in the biliary canaliculi and impede drugregulates the transport of morphine across the blood brain transport. In the kidney, P-GP is highly expressed on thebarrier (Xie et al., 1999). Consequently, it may be ex- brush border of proximal renal tubule, and colchicine renalpected that use of P-GP antagonists may alter the per- clearance was inhibited by CsA (Speeg et al., 1992b),meability of substrates across the blood brain barrier. This suggesting that MDR modulators may also alter the renalhas been highlighted in a recent article where P-GP elimination processes of anticancer drugs by blockinginhibitors such as verapamil, cyclosporin A and GF120918 P-GP in the kidneys. These observations of impairedresulted in enhanced bovine brain capillary endothelial biliary and renal excretion mediated by first and secondcellular accumulation of morphine and rhodamine 123 generation MDR modulators are consistent with clinical(Letrent et al., 1999). observations with a variety of MDR modulators and

Work on the toxicological issues involving maternal– anticancer drugs where increases in anticancer drug plasmafetal transport stemmed from studies with avermectins and exposure (i.e. increased elimination half-lives and AUC)related compounds that demonstrated neurotoxicity in and hematological toxicities warranted dose reductionmdr1 knockouts and in CF1 mice which is a naturally (Bartlett et al., 1994; Erlichman et al., 1994; Giaccone etoccurring mdr1a mutant mouse, a subpopulation of the al., 1994; Tolcher et al., 1994; Fisher et al., 1994a,b; BooteCF1 outbred strain (Lankas et al., 1998), where deficiency et al., 1996; Sarris et al., 1996). In contrast, little phar-of P-GP increased the susceptibility to birth defects macokinetic interactions have been observed with thebecause of a lack of P-GP expression in the placenta, MDR modulators LY335979 and OC144-093 (Dantzig etbrain, and the intestine. In humans, the distribution of al., 1996; Newman et al., 2000). The increased specificityP-GP has been shown to be stage-specific in the first and the lack of significant pharmacokinetic interactionstrimester and full-term (MacFarland et al., 1994). Recent- exhibited by these newer agents suggest that the phar-ly, an organic anion transporter has been cloned and macokinetic alterations observed by older first and secondcharacterized in the human placenta called OAT4 which generation MDR modulators may have been due to bloc-has been speculated to play a role in the excretion of toxic kade of transporters other than P-GP.anionic compounds from the fetus (Cha et al., 2000).

The use of P-GP antagonists coupled with a lack ofP-GP expression may therefore dramatically increase fetal 3. Involvement of multiple transporters in clearanceexposure to the substrates. It has been shown that the of anticancer agentsintravenous administration of P-GP substrates such asdigoxin, saquinavir, and taxol, may result in 2.4–16 fold It is possible that first and second generation MDRincreases in drug exposure in knock-out fetuses than wild- modulators may have induced pharmacokinetic interactionstype fetuses, indicating that the absence of P-GP expres- due to non-specific blockade of other ABC transporterssion is a contributing factor (Smit et al., 1999). Additional- other than P-GP. For example, both cyclosporin A (a firstly, the oral administration of P-GP blockers such as PSC generation MDR modulator) and its non-immunosuppres-833 may also elicit a similar result via blockade of sive analog PSC 833 (a second generation MDRplacental P-GP activity (Smit et al., 1999). modulator) are close structural analogs that were initially

developed for P-GP-mediated MDR reversal (Fig. 2).2.2. Clearance Whereas cyclosporin A inhibits both cMOAT and P-GP, it

is a more potent inhibitor of cMOAT than PSC 833. OnWhen MDR modulators are coadministered with anti- the other hand, PSC 833 is 5-times more specific for P-GP

cancer agents for modulating P-GP-based MDR they can than cyclosporin A (Bohme et al., 1993), as shown insignificantly alter the clearance of the anticancer drugs Table 1.(reviewed in Lum and Gosland, 1995). Compared to P-GP, cMOAT is a relatively newly

In the liver, expression of P-GP is primarily on the identified drug transport protein that has also been shown


in biliary transport in mutant EHBRs compared to normalanimals, whereas the glucuronide metabolites of SN-38show a dramatic 34–81 fold reduction in biliary clearancein the mutant animals (Chu et al., 1997). Significantdifferences between parent (3.6 fold reduction in biliaryclearance) and glucuronide conjugate metabolites (172 foldreduction in biliary clearance) have been observed forgrepafloxacin (Sasabe et al., 1998). In addition to thesestructural differences, cMOAT has been shown to havestereospecificity in drug transport. For example, HSR-903is a quinolone antibacterial that possess two opticalisomers, namely the S- and the R-isomer (Murata et al.,1998). The S-isomer apparently exhibits 10-fold greaterantibacterial activity compared to the R-isomer. Significantreduction in biliary excretion of S- and R-isomers for boththe intact and glucuronide forms have been observed inEHBRs compared to normal rats. Specifically, the foldreduction in biliary excretion was 3-, 7-, 44- and 94-foldfor intact S-isomer, intact R-isomer, conjugate S-isomer,and conjugate R-isomer, respectively, in EHBRs comparedto normal rats (Murata et al., 1998).

These observations highlight the interplay of multipleFig. 2. Structures of cyclosporin A and PSC 833. drug transport processes and interactions with MDR

modulators designed to cause blockade may have consider-to be involved in the biliary transport of drugs and their able implications in the elimination of compounds in themetabolites. While P-GP mediates the transport of cationic hepatobiliary system.and neutral species, cMOAT has been implied in thetransport of anionic species. Transporter-dependent inter-actions in the hepatobiliary system may cause alterations in 4. P-GP-mediated MDR reversal via avoidance ofclearance of substrates of these transporters when coad- pharmacokinetic interactionsministered with inhibitors of P-GP/cMOAT. Currently, nocMOAT/MRP2 knockout animal model is available and 4.1. Altering specificity of MDR modulatorsexperimental studies involving this transporter have reliedon the naturally available mutant Wistar (transport de- Certain third generation highly potent and selectiveficient, TR- or Groningen yellow, GY) and Sprague modulators such as LY 335979 (Dantzig et al., 1996),Dawley (Eisai hyperbilirubinemic, EHBR) rat models OC144-093 (Newman et al., 2000), and R101933 (Van(Oude Elferink and Jansen, 1994). These mutant models Zuylen et al., 2000) have been shown to exhibit minimalexhibit MRP2 mutation and hyperbilirubinemia causing pharmacokinetic interactions with anticancer agents thatdefective transport of bilirubin glucuronide, an endogenous are P-GP substrates. This may present a unique approachsubstrate for cMOAT (Oude Elferink and Jansen, 1994). for designing MDR modulators that are less likely to elicitThe role of cMOAT in drug transport in these models have adverse pharmacokinetic interactions. The increasedbeen related to observations of impaired biliary excretion potency and specificity demonstrated by these newerof substrates in mutant rats compared to their normal agents and the lack of significant pharmacokinetic interac-counterparts (Oude Elferink and Jansen, 1994). Com- tions exhibited in combination with anticancer agentspounds such as the lactone and carboxylate forms of suggest that the pharmacokinetic alterations observed byCPT-11 and SN-38 show a remarkable 2–10 fold reduction older first and second generation MDR modulators may

Table 1Differential inhibition of various transporters by two structurally similar MDR modulators

aK (mM)i

Taurocholate Leukotriene C4 Daunorubicin(canalicular bile salt transporter) (cMOAT) (P-GP)

CsA 0.2 3.4 1.5PSC 833 0.6 29 0.3

a Values for transport of taurocholate, leukotriene C4 and daunorubicin (adapted from Bohme et al., 1993).


have been due to blockade of transporters other than P-GP. tumor regression in C26 colon carcinoma cells (Huang etThe examples cited in the above sections highlight this al., 1992; Mayhew et al., 1992) by delivering highercomplex interplay of multiple transporters which may play amounts of the drug supports this argument. However, fora pivotal role in mediating pharmacokinetic interactions tumors exhibiting higher resistance levels, liposomes byand/or alterations in drug absorption, distribution, and themselves may be unable to circumvent MDR signifi-clearance. Although recent data points at cMOAT/related cantly as demonstrated in a rat glioblastoma tumor modelproteins (see Section 3 on the blockade of P-GP and (Hu et al., 1995) and in murine MDR models (Krishna andcMOAT by the first generation MDR modulator cyclos- Mayer, 1997; Krishna et al., 2000).porin A), there may be other transporters involved in the The reversal of MDR using the chemosensitizationoverall pharmacokinetic interaction phenomenon which are approach therefore presents a complex challenge in target-affected by older non-specific MDR modulators. This may ing P-GP in tumor tissue. The lack of specificity for actioninclude but not be limited to high and low affinity forms of on tumor P-GP is an inherent disadvantage for most, if nota single transporter or different class of transporters, e.g. all, MDR modulators. Given the fundamental disadvantageorganic cation transporters. This is consistent with recent of MDR modulation with respect to the inability toobservations that first generation MDR modulators such as differentiate between tumor and non-tumor P-GP, alter-verapamil, quinine, and quinidine significantly inhibited native methods to improve the selectivity of MDR modula-the transport of tetraethylammonium in HeLa cells con- tion may rely on physiological differences between healthytaining the recently cloned polyspecific organic cation and tumor tissue. For example, we have proposed the usetransporter, hOCT1 (Zhang et al., 1998). of liposomes to circumvent the adverse pharmacokinetic

interactions between the anticancer drug and MDR4.2. Altering drug delivery properties modulators (Krishna and Mayer, 1997; Krishna et al.,

1999; Krishna et al., 2000). This is based on the hypothesisBecause small (100-nm vesicles) liposomes can passive- that, by improving anticancer drug delivery to the tumor

ly extravasate in tumor tissues, increased selectivity of site and reducing delivery to healthy susceptible tissues,anticancer drug delivery at the tumor site, while markedly liposomes could increase the selectivity of MDR modula-reducing drug accumulation and toxicity in many suscep- tion at the tumor site. We observed that by coadministeringtible healthy tissues can be achieved (Gabizon and liposomal doxorubicin and PSC 833, there was significantPapahadjopoulos, 1988; Mayer et al., 1989; Gabizon, regression of MDR tumors both in murine (Krishna and1992). This increased delivery of anticancer drugs may Mayer, 1997) and in human xenograft models (Krishna etovercome MDR based only on mass action, provided that al., 2000). In addition, there was increased intracellularthe level of drug resistance is of a magnitude comparable accumulation of doxorubicin as was evidenced usingto the increase in tumor drug levels. Early indications that confocal microscopy and reduced toxicity compared to theliposomes may be beneficial for MDR reversal stemmed free (unencapsulated) doxorubicin. Additionally, whereasfrom observations of enhanced activity of a long circulat- PSC 833 significantly inhibited renal and biliary excretioning liposomal formulation incorporating either the gan- of free doxorubicin, the clearance values remained un-glioside GM1 or PEG-polymerized DSPE for doxorubicin changed in the presence of PSC 833 for the liposomaland epirubicin against the murine C26 colon carcinoma doxorubicin (Krishna et al., 1999). Two factors may have(Papahadjopoulos et al., 1991; Huang et al., 1992; contributed to the increased selectivity and degree of MDRMayhew et al., 1992). These C26 colon carcinoma cells reversal: (1) due to the high doxorubicin localization in theexpress moderate levels of P-GP and doses of free tumor via liposomal delivery, P-GP blockade caused bydoxorubicin up to 10 mg/kg were ineffective (Huang et the coadministered drug efflux blockade component leadsal., 1992) in vivo. However, mice that were treated with to increased cellular DOX uptake in the malignant cells,stealth formulations of DOX or epirubicin exhibited sig- and (2) due to the reduced doxorubicin accumulation innificant tumor regression to nonmeasurable sizes, with 90 other P-GP expressing healthy tissues following liposomaland 100% long-term 120-day survivors in groups that were delivery, doxorubicin is not significantly taken up in thesetreated with sterically stabilized liposomal epirubicin and susceptible tissues even under conditions of P-GP bloc-DOX, respectively. In comparison, free drugs did not have kade.any effect on delaying tumor growth (Huang et al., 1992).The authors attributed this enhanced activity in a relativelyresistant tumor model to be due to increased delivery of 5. Conclusionsthe drug in tumor tissue (Huang et al., 1992).

This ability of the liposomes to inherently overcome a Since the elucidation of P-GP as a membrane transportcertain degree of MDR stems from the fact that liposomes pump in mid-1970s, considerable work has been done incan markedly enhance (between 5–15 fold) the amount of understanding the physiological, biochemical, and thedrug that can be delivered to tumors compared to free pharmacological role of P-GP. In particular, two keydrug. The fact that stealth liposomes were able to provide pharmacological issues have surfaced in the past decade:


Barrand, M.A., Heppell-Parton, A.C., Wright, K.A., Rabbits, P.H.,(1) strategies to block the transport pump as a means ofTwentyman, P.R., 1994. A 190-kilodalton protein overexpressed incircumventing MDR and (2) the role of membrane trans-non-P-glycoprotein-containing multidrug-resistant cells and its rela-

porters in modifying the pharmacokinetics of drugs. This tionship to the MRP gene. J. Natl. Cancer Inst. 86, 110–117.review highlighted several examples where P-GP blockers Bartlett, N.L., Lum, B.L., Fisher, G.A., Brophy, N.A., Ehsan, M.N.,

Halsey, J., Sikic, B.I., 1994. Phase I trial of doxorubicin withnon-specifically inhibited P-GP localized in normal tissuescyclosporine as a modulator of multidrug resistance. J. Clin. Oncol.(biliary canaliculi, kidney proximal tubule, blood–brain12, 835–842.

barrier, and placenta), impeding drug absorption, distribu- Batist, G., Tulpule, A., Sinha, B.K., Katki, A.G., Myers, C.E., Cowan,tion, and clearance. In addition, from the biopharmaceutic K.H., 1986. Overexpression of a novel anionic glutathione transferase

in multidrug-resistant human breast cancer cells. J. Biol. Chem. 33,standpoint, the inducibility of transporters may contribute15544–15549.to the poor bioavailability of drugs while the coadministra-

Beck, J., Gekeler,V., Handgretinger, R., Niethammer, D., 1994. PCR genetion of a P-GP blocker with an oral P-GP substrate mayexpression analysis of multidrug resistance associated genes in pri-

enhance the bioavailability of the substrate. Consequently, mary and relapsed state leukemias: comparison of mdrl, mrp,alterations in P-GP function may impact several physiolog- topoisomerase IIa, topoisomerase IIb and cyclin A mRNA levels.

Proc. Am. Assoc. Cancer Res. 35, 337.ical processes, some of which may have conflicting effectsBissett, D., Kerr, D.J., Cassidy, J., Meredith, P., Traugott, U., Kaye, S.B.,on drug pharmacological properties. Our ability to control

1991. Phase I and pharmacokinetic study of D-verapamil and doxorubi-such interactions will depend not only on the generation of cin. Br. J. Cancer 64, 1168–1171.compounds exhibiting more specific interactions with drug Black, S.M., Beggs, J.D., Hayes, J.D., Bartoszek, A., Muramatsu, M.,

Sakai, M., Wolf, C.R., 1990. Expression of human glutathione S-transporters but also on an increased understanding of thetransferases in Saccharomyces cerevisiae confers resistance to thelarge class of ABC transporters, many of which have notanticancer drugs adriamycin and chlorambucil. Biochem. J. 268, 309–

yet been identified and/or characterized. As more in- 315.formation on these proteins becomes available, we may be Boesch, D., Gaveriaux, C., Jachez, B., Pourtier-Manzanedo, A., Bollinger,

P., Loor, F., 1991a. In vivo circumvention of P-glycoprotein mediatedable to more effectively design drug combinations that willmultidrug resistance of tumor cells with PSC 833. Cancer Res. 51,provide increased selectivity of action at the desired tissue4226–4233.site.

Boesch, D., Muller, K., Pourtier-Manzanedo, A., Loor, F., 1991b.Restoration of daunomycin retention in multidrug-resistant P388 cellsby submicromolar concentrations of SDZ PSC 833, a nonimmunosup-pressive cyclosporine derivative. Exp. Cell Res. 196, 26–32.

References ¨ ¨Bohme, M., Buchler, M., Muller, M., Keppler, D., 1993. Differentialinhibition by cyclosporines of primary-active ATP-dependent transpor-

Akerboom, T.P.M., Narayanaswami, V., Kunst, M., Sies, H., 1991. ATP- ters in the hepatocyte canalicular membrane. FEBS Lett. 333, 193–dependent S-(2,4-dinitrophenyl) glutathione transport in canalicular 196.plasma membrane vesicles from rat liver. J. Biol. Chem. 266, 13147– Bohme, M., Jedlitschky, G., Leier, L., Buchler, M., Keppler, D., 1994.13152. ATP-dependent export pumps and their inhibition by cyclosporines.

Akiyama, S., Shiraishi, N., Kuratomi, Y., Nakagawa, M., Kuwano, M., Adv. Enzyme Reg. 34, 371–380.1986. Circumvention of multiple-drug resistance in human cancer cells Boote, D.J., Dennis, I.F., Twentyman, P.R., Osborne, R.J., Laburte, C.,by thioridazine, trifluoperazine, and chlorpromazine. J. Natl. Cancer Hensel, S., Smyth, J.F., Brampton, M.H., Bleehen, N.M., 1996. PhaseInst. 76, 839–844. I study of etoposide with PSC-833 as a modulator of multidrug

Akiyama, S.-I., Cornwell, M.M., Kuwano, M., Pastan, I., Gottesman, resistance in patients with cancer. J. Clin. Oncol. 14, 610–618.M.M., 1988. Most drugs that reverse multidrug resistance also inhibit Booth, C.L., Brouwer, K.R., Brouwer, K.L.R., 1998. Effect of multidrugphotoaffinity labeling of P-glycoprotein by a vinblastine analog. Mol. resistance modulators on the hepatobiliary disposition of doxorubicinPharm. 33, 144–147. in the isolated perfused rat liver. Cancer Res. 58, 3641–3648.

Alaoui-Jamali, M.A., Schecter, R.L., Rustum, Y.M., Centurioni, M.G., Borst, P., Kool, M., Evers, R., 1997. Do cMOAT (MRP2), other MRPLehnert, S., Batist, G., 1993. In vivo reversal of doxorubicin resistance homologues, and LRP play a role in MDR. Cancer Biol. 8, 205–213.by a new tiapamil analog Ro11-2933. J. Pharmacol. Exp. Ther. 264, Bradley, G., Ling, V., 1994. P-Glycoprotein, multidrug resistance and1299–1304. tumor progression. Cancer Metastasis Rev. 13, 223–233.

Almquist, K.C., Loe, D.W., Hipfner, D.R., Mackie, J.E., Cole, S.P., Bronner, M., Culin, C., Reed, J.C., Furth, E.E., 1995. Bcl-2 proto-Deeley, R.G., 1995. Characterization of the M 190 000 multidrug oncogene and the gastrointestinal epithelial tumor progression model.r

resistance protein (MRP) in drug selected and transfected human Am. J. Pathol. 146, 20–26.tumor cell. Cancer Res. 55, 102–110. Brouwer, K.R., Mellon-Kusibab, K., 1996. Effect of GF120918, a potent

Anderson, P., Bondesson, U., Sylven, C., Astrom, H., 1982. Plasma multidrug resistance modulator, on the pharmacokinetics and pharma-concentration–response relationship of verapamil in the treatment of codynamics of doxorubicin in the dog. Pharm. Res. 13, S455.angina pectoris. J. Cardiovasc. Pharmacol. 4, 609–614. Broxterman, H.J., Kuiper, C.M., Schuurhuis, G.J., Ossenkoppele, G.J.,

Arceci, R.J., Croop, J.M., Horwitz, S.B., Housman, D.E., 1988. The gene Feller, N., Scheper, R.J., Lankelma, J., Pinedo, H.M., 1994. Analysisencoding multidrug resistance is induced and pressed at high levels of P-glycoprotein (P-gp) and non-P-gp multidrug resistance in acuteduring pregnancy in the secreting epithelium of the uterus. Proc. Natl. myeloid leukemia. Proc. Am. Assoc. Cancer Res. 35, 348.Acad. Sci. USA 85, 4350–4354. Burger, H., Nooter, K., van Wingerden, K.E., Oostrum, R.G., Sonneveld,

Arias, I.M., Che, M., Gatmaitan, Z., Leveille, C., Nishida, T., Pierre, P., Zaman, G.J.R., Borst, P., Splinter, T.A.W., Stoter, G., 1994.M.St., 1993. The biology of the bile canaliculus. Hepatology 17, Analysis of the expression of multidrug resistance-associated protein318–329. (MRP) in hematological malignancies. Proc. Am. Assoc. Cancer Res.

Ball, C.R., Connors, T.A., Double, J.A., Ujhazy, V., Whisson, M.E., 1966. 35, 206.Comparison of nitrogen-mustard-sensitive and -resistant Yoshida sar- Calcutt, G., Connors, T.A., 1963. Tumor sulfhydryl levels and sensitivitycomas. Int. J. Cancer 1, 319–327. to the nitrogen mustard merophan. Biochem. Pharmacol. 12, 839.


Campos, L., Guyotat, D., Archimbaud, E., Calmard-Oriol, P., Tsuruo, T., Cornwell, M.M., Safa, A.R., Felsted, R.L., Gottesman, M.M., Pastan, I.,Troncy, J., Treille, D., Fiere, D., 1992. Clinical significance of 1986. Membrane vesicles from multidrug-resistant human cancer cellsmultidrug resistance P-glycoprotein expression on acute nonlymphob- contain a specific 150- to 170-kDa protein detected by photoaffinitylastic leukemia cell at diagnosis. Blood 79, 473–476. labeling. Proc. Natl. Acad. Sci. USA 83, 3847–3850.

Campos, L., Roualult, J-P., Sabido, O., Oriol, P., Roubi, N., Vasselon, C., Croop, J.M., Raymond, M., Haber, D., Devault, A., Arceci, R.J., Gros, P.,Archimbaud, E., Magaud, J-P., Guyotat, D., 1993. High expression of Housman, D.E., 1989. The three mouse multidrug resistance (mdr)bcl-2 protein in acute myeloid leukemia cells is associated with poor genes are expressed in a tissue specific manner in normal mouseresponse to chemotherapy. Blood 81, 3091–3096. tissues. Mol. Cell Biol. 9, 1346–1350.

Castle, V.P., Heidelberger, K.P., Bromberg, J., Ou, X., Dole, M., Nunez, Dale, I.L., Tuffley, W., Callaghan, R., Holmes, J.A., Martin, K., Lus-G., 1993. Expression of the apoptosis-suppressing protein bcl-2, in combe, M., Mistry, P., Ryder, H., Stewart, A.J., Charlton, P.,neuroblastoma is associated with unfavorable histology and N-myc Twentyman, P.R., Bevan, P., 1998. Reversal of P-glycoprotein-me-amplification. Am. J. Pathol. 143, 1543–1550. diated multidrug resistance by XR9051, a novel diketopiperazine

Cha, S.H., Sekine, T., Kusuhara, H., Yu, E., Kim, J.Y., Kim, D.K., derivative. Br. J. Cancer 78, 885–892.Sugiyama, Y., Kanai, Y., Endou, H., 2000. Molecular cloning and Dalmark, M., Pals, H., Johnsen, A.H., 1991. Doxorubicin in combinationcharacterization of multispecific organic anion transporter 4 expressed with verapamil in advanced colorectal cancer: A phase II trial. Actain the placenta. J. Biol. Chem. 275, 4507–4512. Oncol. 30, 23–26.

Chan, H.S.L., Haddad, G., Thorner, P.S., DeBoer, G., Lin, Y.P., Ondrusek, Dantzig, A.H., Shepard, R.L., Cao, J., Law, K.L., Ehlhardt, W.J.,N., Yeger, H., Ling, V., 1991. P-glycoprotein expression as a predictor Baughman, T.M., Bumol, T.F., Starling, J.J., 1996. Reversal of P-of the outcome of therapy for neuroblastoma. New Engl. J. Med. 325, glycoprotein-mediated multidrug resistance by a potent cyclop-1608–1614. ropyldibenzosuberane modulator LY335979. Cancer Res. 56, 4171–

Chan, H.S.L., Thorner, P., Haddad, G., Ling, V., 1990. Immunohistoch- 4179.emical detection of P-glycoprotein prognostic correlation in soft tissue de Bruijn, M.J., Van der Bliek, A.M., Biedler, J.L., Borst, P., 1986.sarcoma of childhood. J. Clin. Oncol. 8, 689–704. Differential amplification and disproportionate expression of five

Chao, C.C., Huang, Y.T., Ma, C.M., Chou, W.Y., Chao, S.L., 1991. genes in three multidrug-resistant Chinese hamster lung cell lines.Overexpression of glutathione S-transferase and elevation of thiol Mol. Cell Biol. 6, 4717–4722.pools in a multidrug resistant human colon cancer cell line. Mol. Deffie, A.M., Batra, J.K., Goldenberg, G.J., 1989. Direct correlationPharmacol. 41, 69. between DNA topoisomerase II activity and cytotoxicity in Ad-

Chao, N.J., Aihara, M., Blume, K.G., Sikic, B.I., 1990. Modulation of riamycin-sensitive and resistant P388 leukemia cell lines. Cancer Res.etoposide (VP-16) cytotoxicity by verapamil or cyclosporine in 49, 58–62.multidrug-resistant human leukemic cell lines and normal bone Demant, E.J., Sehested, M., Jensen, P.B., 1990. A model for computermarrow. Exp. Hematol. 18, 1193–1198. simulation of P-glycoprotein and transmembrane delta pH-mediated

Chaudhary, P.M., Mechetner, E.B., Roninson, I.B., 1992. Expression and anthracycline transport in multidrug-resistant tumor cells. Biochim.activity of the multidrug resistance P-glycoprotein in human peripheral Biophys. Acta 1055, 117–125.blood lymphocytes. Blood 80, 2735–2739. Demeule, M., Brossard, M., Beliveau, R., 1999. Cisplatin induces renal

Chen, C.J., Chin, J.E., Ueda, K., Clark, D.P., Pastan, I., Gottesman, M.M., expression of p-glycoprotein and canalicular multispecific organicRoninson, I.B., 1986. Internal duplication and homology with bacterial anion transporter. Am. J. Physiol. 277, F832–840.transport proteins in the mdr1 (P-glycoprotein) gene from multidrug- Dorr, R.T., Liddil, J.D., 1991. Modulation of mitomycin C-inducedresistant human cells. Cell 47, 381–389. multidrug resistance in vitro. Cancer Chemother. Pharmacol. 27, 290–

Chu, X-Y., Kato, Y., Niinuma, K., Sudo, K.I., Hakusui, H., Sugiyama, Y., 294.1997. Multispecific organic anion transporter is responsible for the Drake, F.H., Zimmerman, J.P., McCabe, F.L., Bartus, H.F., Per, S.R.,biliary excretion of the camptothecin derivative irinotecan and its Sullivan, D.M., Ross, W.E., Mattern, M.R., Johnson, R.K., Crooke,metabolites in rats. J. Pharmacol. Exp. Ther. 281, 304–314. S.T., Mirabelli, C.K., 1987. Purification of topoisomerase II from

Chung, T.D.Y., Drake, F.H., Tan, K.B., Per, S.R., Crooke, S.T., Mirabelli, amsacrine-resistant p388 leukemia cells: Evidence for two forms of theC.K., 1989. Characterization and immunological identification of enzyme. J. Biol. Chem. 262, 16739–16747.cDNA clones encoding two human DNA topoisomerase II isozymes. Duran, G.E., Jaffrezou, J.-P., Gosland, M.P., Ross, K.L., Sikic, B.I., 1993.Proc. Natl. Acd. Sci. USA 86, 9431–9435. Inhibition of the multidrug transporter, P-glycoprotein, by the cyclo-

Clapper, M.L., Hoffman, S.J., Tew, K.D., 1990. Sensitization of human sporine analogue PSC 833, cyclosporine, and verapamil. Proc. Am.colon tumor xenografts to L-phenylalanine mustard using ethacrynic Assoc. Cancer Res. 34, A1922.acid. J. Cell. Pharmacol. 1, 71. Eliason, J.F., Ramuz, H., Kaufmann, F., 1990. Human multi-drug-resis-

Cole, S.P.C., Bhardwaj, G., Gerlach, J.H., Mackie, J.E., Grant, C.E., tant cancer cells exhibit a high degree of selectivity for stereoisomersAlmquist, K.C., Stewart, A.J., Kurz, E.U., Duncan, A.M.V., Deeley, of verapamil and quinidine. Int. J. Cancer 46, 113–117.R.G., 1992. Overexpression of a transporter gene in a multidrug- Erlichman, C., Moore, M., Thiessen, J., De Angelis, C., Goodman, P.,resistant human lung cancer cell line. Science 258, 1650–1654. Manzo, J., 1994. A Phase I trial of doxorubicin (DOX) and PSC 833, a

Cole, S.P.C., Downes, H.F., Slovak, M.L., 1989. Effect of calcium modulator of multidrug resistance (MDR). Anti-Cancer Drugs 5, 42.antagonists on the chemosensitivity of two multidrug-resistant human Fan, D., Beltran, P.J., O’Brien, C.A., 1994. Reversal of multidrugtumour cell lines which do not overexpress P-glycoprotein. Br. J. resistance. In: Kellen, J.A. (Ed.), Reversal of Multidrug Resistance inCancer 59, 42–46. Cancer. CRC Press, Boca Raton, FL, pp. 93–124.

Colombel, M., Symmans, F., Gil, S., O’Toole, K.M., Chopin, D., Benson, Feng, M.R., Liebert, M., Wedemeyer, G., Grossman, H.B., Mancini, W.R.,M., Olsson, C.A., Korsmeyer, S., Buttyan, R., 1993. Detection of the Williams, M., Wagner, J.G., 1992. Effects of verapamil on the uptakeapoptosis-suppressing oncoprotein bcl-2 in hormone-refractory human and efflux of etoposide (VP16) in both sensitive and resistant cancerprostate cancers. Am. J. Pathol. 143, 390–400. cells. Selective Cancer Ther. 7, 75.

Colombo, T., Paz, O.G., D’Incalci, M., 1996. Distribution and activity of Fernandez-Checa, J.C., Takikawa, H., Horie, T., Ookhtens, M., Kap-doxorubicin combined with SDZ PSC 833 in mice with P388 and lowitz, N., 1992. Canalicular transport of reduced glutathione inP388/DOX leukemia. Br. J. Cancer 73, 866–871. normal and mutant Eisai hyperbilirubinemic rats. J. Biol. Chem. 267,

Cordon-Cardo, C., O’Brien, J.P., Casals, D., Rittman-Grauer, L., Biedler, 1667–1673.J.L., Melamed, M.R., Bertino, J.R., 1989. Multidrug-resistance gene Fisher, G.A., Bartlett, N.L., Lum, B.L. et al., 1994a. Phase I trial of taxol(P-glycoprotein) is expressed by endothelial cells at blood–brain (T) with high dose cyclosporine (CsA) as a modulator of multidrugbarrier sites. Proc. Natl. Acad. Sci. USA 86, 695–698. resistance (MDR). Proc. Am. Soc. Clin. Oncol. 13, 144.


Fisher, G.A., Hausdorff, J., Lum, B.L. et al., 1994b. Phase I trial of Goasguen, J.E., Dossot, J-M., Fardel, O., Le Mee, F., Le Gall, E., Leblay,R., LePrise, P.Y., Chaperon, J., Fauchet, R., 1993. Expression of theetoposide with the cyclosporine SDZ PSC 833, a modulator ofmultidrug resistance-associated P-glycoprotein (P-170) in 59 cases ofmultidrug resistance (MDR). Proc. Am. Soc. Clin. Oncol. 13, 143.de novo acute lymphoblastic leukemia: prognostic implications. BloodFisher, T.C., Milner, A.E., Gregory, C.D., Jackman, A.L., Aherne, G.W.,81, 2394–2398.Hartley, J.A., Dive, C., Hickman, J.A., 1993. Bcl-2 modulation of

Goldstein, L.J., Galski, H., Fojo, A., Willingham, M., Lai, S., Gazdar, A.,apoptosis induced by anticancer drugs: resistance to thymidylate stressPirker, R., Green, A., Crist, W., Brodeur, G.M., Lieber, M., Cossman,is independent of classical resistance pathways. Cancer Res. 53,J., Gottesman, M.M., Pastan, I., 1989. Expression of a multidrug3321–3326.resistance gene in human tumors. J. Natl. Canc. Inst. 81, 116–124.Fojo, A.T., Ueda, K., Slamon, D.J., Poplack, D.G., Gottesman, M.M.,

Gottesman, M.M., 1993. How cancer cells evade chemotherapy: SixteenthPastan, I., 1987. Expression of a multidrug-resistance gene in humanRichard and Hinda Rosenthal Foundation award lecture. Cancer Res.

tumors and tissues. Proc. Natl. Acad. Sci. USA 84, 265–269.53, 747–754.

Foote, S.J., Thompson, J.K., Cowman, A.F., Kemp, D.J., 1989. Amplifi-Gottesman, M.M., Pastan, I., 1993. Biochemistry of multidrug resistance

cation of the multidrug resistance gene in some chloroquine-resistantmediated by the multidrug transporter. Annu. Rev. Biochim. 62,

isolates of P. falciparum. Cell 57, 921–930.385–427.

Ford, J.M., 1995. Modulators of multidrug resistance: Preclinical studies. Grant, C.E., Valdimarsson, G., Hipfner, D.R., Almquist, K.C., Cole, S.P.,Hematol. Oncol. Clin. North Am. 9, 337–361. Deeley, R.G., 1994. Overexpression of multidrug resistance-associated

Ford, J.M., Hait, W.N., 1990. Pharmacology of drugs that alter multidrug protein increases resistance to natural product drugs. Cancer Res. 54,resistance in cancer. Pharmacol Rev. 42, 155–199. 357–361.

Ford, J.M., Hait, W.N., 1993. Pharmacologic circumvention of multidrug Gros, P., Raymond, M., Bell, J., Housman, D., 1988. Cloning andresistance. Cytotechnology 1–3, 171–212. characterization of a second member of the mouse mdr gene family.

Ford, J.M., Prozialeck, W.C., Hait, W.N., 1989. Structural features Mol. Cell Biol. 8, 2770–2778.determining activity of phenothiazines and related drugs for inhibition Hanada, M., Krajewski, S., Tanaka, S., Cazals-Hatem, D., Spengler, B.A.,of cell growth and reversal of multidrug resistance. Mol. Pharmacol. Ross, R.A., Biedler, J.L., Reed, J.C., 1993. Regulation of Bcl-235, 105–115. oncoprotein levels with differentiation of human neuroblastoma cells.

Gabizon, A., 1992. Selective tumor localization and improved therapeutic Cancer Res. 53, 4978–4986.index of anthracyclines encapsulated in long-circulating liposomes. Hao, X.Y., Widersten, M., Ridderstrom, M., Hellman, U., Mannervik, B.,Cancer Res. 52, 891–896. 1994. Co-variation of glutathione transferase expression and cytostatic

Gabizon, A., Papahadjopoulos, D., 1988. Liposome formulations with drug resistance in HeLa cells: establishment of class Mu glutathioneprolonged circulation time in blood enhanced uptake by tumors. Proc. transferase M3-3 as the dominating isoenzyme. Biochem. J 297,Natl. Acad. Sci. USA 85, 6949–6953. 59–67.

Ganapathi, R., Grabowski, D., 1983. Enhancement of sensitivity to Harker, W.G., Bauer, D., Etiz, B.B., Newman, R.A., Sikic, B.I., 1986.adriamycin in resistant P388 leukemia by the calmodulin inhibitor Verapamil-mediated sensitization of doxorubicin-selected pleiotrophictrifluoperazine. Cancer Res. 43, 3696–3699. resistance in human sarcoma cells: selectivity for drugs which produce

Ganapathi, R., Grabowski, D., Schmidt, H., 1986. Factors governing the DNA scission. Cancer Res. 46, 2369–2373.modulation of vinca-alkaloid resistance in doxorubicin-resistant cells Hendrick, A.M., Harris, A.L., Cantwell, B.M.J., 1991. Verapamil withby the calmodulin inhibitor trifluoperazine. Biochem. Pharmacol. 35, mitoxantrone for advanced ovarian cancer: A negative phase II trial.673–678. Ann. Oncol. 2, 71–72.

Ganapathi, R., Grabowski, D., Turinic, R., Valenzuela, R., 1984. Correla- Higgins, C.F., Gottesman, M.M., 1992. Is the multidrug transporter ation between potency of calmodulin inhibitors and effects on cellular flippase? Trends Biochem. 17, 18–21.levels and cytotoxicity activity of doxorubicin (adriamycin) in resistant Hollt, V., Kouba, M., Dietel, M., Vogt, G., 1992. Stereoisomers of calciumP388 mouse leukemia cells. Eur. J. Cancer Clin. Oncol. 20, 799–806. antagonists which differ markedly in their potencies as calcium

Gatmaitan, Z., Arias, I., 1993. Structure and function of P-glycoprotein in blockers are equally effective in modulating drug transport by P-normal liver and small intestine. Adv. Pharmacol. 24, 77–97. glycoprotein. Biochem. Pharmacol. 43, 2601–2608.

Gaveriaux, C., Boesch, D., Jachez, B., 1991. SDZ PSC 833, a non- Horio, M., Chin, K.V., Currier, S.J., Goldenberg, S., Williams, C., Pastan,immunosuppressive cyclosporine analog is a very potent multidrug- I., Gottesman, M.M., Handler, J., 1989. Transepithelial transport ofresistant modifier. J. Cell. Pharmacol. 2, 225. drugs by the multidrug transporter in cultured Madin-Darby canine

Gaveriaux, C., Boesch, D., Boilsterli, J.J., Bollinger, P., Eberie, M.K., kidney cell epithelia. J. Biol. Chem. 264, 14880–14884.Hiestand, P., Payne, T., Traber, R., Wenger, R., Loor, F., 1989. Horio, M., Gottesman, M.M., Pastan, I., 1988. ATP-dependent transportOvercoming multidrug resistance in chinese hamster ovary cells in of vinblastine in vesicles from human multidrug-resistant cells. Proc.vitro by cyclosporine A (sandimmune) and non-immunosuppressive Natl. Acad. Sci. USA 85, 3580–3584.derivatives. Br. J. Cancer 60, 867–871. Horton, J.K., Thimmaiah, K.N., Houghton, J.A., Horowitz, M.E., Houg-

Gekeler, V., Ise, W., Sanders, K.H., Ulrich, W.R., Beck, J., 1995. The hton, P.J., 1989. Modulation by verapamil of vincristine phar-leukotriene LTD receptor antagonist MK571 specifically modulates macokinetics and toxicity in mice bearing human tumor xenografts.4

MRP associated multidrug resistance. Biochem. Biophys. Res. Com- Biochem. Pharmacol. 38, 1727–1736.mun. 208, 345–352. Hu, Y.P., Henry-Toulme, N., Robert, J., 1995. Failure of liposomal

Gerlach, J.H., Bell, D.R., Karakousis, C., Slocum, H.K., Kartner, N., encapsulation of doxorubicin to circumvent multidrug resistance in anRustum, Y.M., Ling, V., Baker, R.M., 1987. P-glycoprotein in human in vitro model of rat glioblastoma cells. Eur. J. Cancer 31A, 389–394.sarcoma: evidence for multidrug resistance. J. Clin. Oncol. 5, 1452– Huang, S.K., Mayhew, E., Gilani, S., Lasic, D.D., Martin, F.J., Papahad-1460. jopoulos, D., 1992. Pharmacokinetics and therapeutics of sterically

Germann, U.A., Shlyakhter, D., Mason, V.S., Ford, P.J., Harding, M.W., stabilized liposomes in mice bearing C-26 colon carcinoma. Cancer1996. VX-853: A novel bispecific chemosensitizer which reverses Res. 52, 6774–6781.P-glycoprotein and MRP-mediated multidrug resistance. Proc. Am. Hyafil, F., Vergely, C., Du Vignaud, P., Grand-Perret, T., 1993. In vitroAss. For Cancer Res. 37, 335. and in vivo reversal of multidrug resistance by GF120918, an

Giaccone, G., Linn, S.C., Catimel, G. et al., 1994. Phase I and phar- acridonecarboxamide derivative. Cancer Res. 53, 4595–4602.macokinetic study of SDZ PSC 833 per os in combination with Inaba, M., Kobayashi, H., Sakurai, Y., Johnson, R.R., 1979. Active effluxdoxorubicin in patients with solid tumors. Proc. Am. Soc. Clin. Oncol. of daunorubicin and adriamycin in sensitive and resistant sublines of13, 142. P388 leukemia. Cancer Res. 39, 2200–2203.


Ishikawa, T., 1992. The ATP dependent glutathione S-conjugate export Kitamura, T., Jansen, P., Hardenbrook, C., Kamimoto, Y., Gatmaitan, Z.,pump. Trends Biochem. Sci. 17, 463–466. Arias, I.M., 1990. Defective ATP-dependent bile canalicular transport

¨ ¨Ishikawa, T., Muller, M., Klunemann, C., Schaub, T., Keppler, D., 1990. of organic anions in mutant (TR-) rats with conjugated hyper-ATP-dependent primary active transport of cysteinyl leukotrienes bilirubinemia. Proc. Natl. Acad. Sci. USA 87, 3557–3561.across liver canalicular membrane. Role of the ATP-dependent trans- Kiuchi, Y., Suzuki, H., Hirohashi, T., Tyson, C.A., Sugiyama, Y., 1998.port system for glutathione S-conjugates. J. Biol. Chem. 265, 19279– cDNA cloning and inducible expression of human multidrug resistance19286. associated protein 3 (MRP3). FEBS Lett. 433, 149–152.

Jain, R.K., 1987. Transport of molecules in the tumour interstitium: a Klohs, W.D., Steinkampf, R.W., Havlick, M.J., Jackson, R.C., 1986.review. Cancer Res. 47, 3039–3051. Resistance to anthrapyrazoles and anthracyclines in multidrug-resistant

Jain, R.K., 1988. Determinants of tumor blood flow: a review. Cancer P388 murine leukemia cells: reversal by calcium blockers andRes. 48, 2641–2658.

calmodulin antagonists. Cancer Res. 46, 4352–4356.Jedlitschky, G., Leier, I., Buchholz, U., Center, M., Keppler, D., 1994.

Kool, M., van der Linden, P.A., Mol, C.A.A.M., van Deemter, L.,ATP-dependent transporter of glutathione S-conjugates by the multi-

Scheper, R.J., Baas, F., Borst, P., 1998. The contribution of the MRPdrug resistance-associated protein. Cancer Res. 54, 4833–4836.

(multidrug resistance protein) family to drug resistance. Proc. Am.Jette, L., Beaulieu, E., Leclerc, J.M., Beliveau, R., 1996. Cyclosporin A

Ass. Cancer Res. 39, 167.treatment induces overexpression of P-glycoprotein in the kidney andKreis, W., Budman, D.R., Broome, J., Akerman, S., Calabro, A.,other tissues. Am. J. Physiol. 270, F756–765.

Finamore, P., Cowan, K., Caron, J., Israel, R., Vinciguerra, V., 1993.Johnson, R.K., McCabe, F.L., Faucette, L.F., Hertzberg, R.P., Kingsbury,Evaluation of three established cell lines derived from patients withW.D., Boehm, J.C., Caranfa, M.J., Holden, K.G., 1989. SKF10864, aprostate cancer (DU145, PC-3, LnCap) for phenotype and sensitivitywater-soluble analog of camptothecin with broad-spectrum activity into adriamycin (ADR) and vinblastine (Velb) with or without the newpreclinical tumor models. Proc. Am. Assoc. Cancer Res. 30, 623.cyclosporine A derivative PSC 833 (PSC). Proc. Am. Assoc. CancerJonsson, B., Nilsson, K., Nygren, P., Larsson, R., 1992. SDZ PSC 833 —Res. 34 (PSC), A1894.a novel potent in vitro chemosensitizer in multiple myeloma. Anti-

Krishna, R., Mayer, L.D., 1997. Liposomal doxorubicin circumvents PSCCancer Drugs 3, 641–646.833-free drug interactions, resulting in effective therapy of multidrugJuliano, R.L., Ling, V., 1976. A surface glycoprotein modulating drugresistant solid tumors. Cancer Res. 57, 5246–5253.permeability in Chinese hamster ovary cell mutants. Biochim. Bio-

Krishna, R., de Jong, G., Mayer, L.D., 1997. Pulsed exposure of SDZphys. Acta 455, 152–162.PSC 833 to multidrug resistant P388/ADR and MCF7/ADR cells inKamath, N., Grabowski, D., Ford, J., Kerrigan, D., Pommier, Y.,the absence of anticancer drugs can fully restore sensitivity toGanapathi, R., 1992. Overexpression of P-glycoprotein and alterationsdoxorubicin. Anticancer Res. 17, 3329–3334.in topoisomerase II in p388 mouse leukemia cells selected in vivo for

Krishna, R., McIntosh, N., Riggs, K.W., Mayer, L.D., 1999. Doxorubicinresistance to mitoxantrone. Biochem. Pharmacol. 44, 937–945.encapsulated in sterically stabilized liposomes exhibits renal andKamimoto, Y., Gatmaitan, Z., Hsu, J., Arias, I.M., 1989. The function of

GP170, the multidrug resistance gene product, in rat liver canalicular biliary clearance properties that are independent of valspodar (PSCmembrane vesicles. J. Biol. Chem. 264, 11693–11698. 833) under conditions that significantly inhibit nonencapsulated drug

Kaplan, B., Lown, K., Craig, R., Abecassis, M., Kaufman, D., Leventhal, excretion. Clin Cancer Res. 5, 2939–2947.J., Stuart, F., Meier-Kriesche, H.U., Fryer, J., 1999. Low bioavail- Krishna, R., St.-Louis, M., Mayer, L.D., 2000. Increased intracellularability of cyclosporine microemulsion and tacrolimus in a small bowel drug accumulation and complete chemosensitization achieved intransplant recipient: possible relationship to intestinal p-glycoprotein multidrug-resistant solid tumors by coadministering valspodar (PSCactivity. Transplantation 67, 333–335. 833) with sterically stabilized liposomal doxorubicin. In: Int. J.

Kartner, N., Riordan, J.R., Ling, V., 1983. Cell surface P-glycoprotein Cancer, Vol. 85, pp. 131–141.associated with multidrug resistance in mammalian cell lines. Science Krishnamachary, N., Center, M.S., 1993. The MRP gene associated with221, 1285–1288. a non-P-glycoprotein multidrug resistance encodes a 190-kDa mem-

Kauffmann, H.M., Keppler, D., Kartenbeck, J., Schrenk, D., 1997. brane bound glycoprotein. Cancer Res. 53, 3658–3661.Induction of cMrp/cMoat gene expression by cisplatin, 2-acetylamino- Kukl, J.S., Sikic, B.I., Blume, K.G., Chao, N.J., 1992. Use of etoposide influorene, or cycloheximide in rat hepatocytes. Hepatology 26, 980– combination with cyclosporine for purging multidrug resistant985. leukemic cells from bone marrow in a mouse model. Exp. Hematol.

Keller, R.P., Altermatt, H.J., Nooter, K., Poschmann, G., Laissue, J.A., 20, 1048–1054.Bollinger, P., Hiestand, P.C., 1992. SDZ P.S.C. 833, a non-immuno- Kunikane, H., Ohta, S., Kubota, N., Takeda, Y., Nishino, K., Saijo, N.,suppressive cyclosporine: Its potency in overcoming P-glycoprotein Krishan, A., 1990. Relationship between amount of P-glycoprotein andmediated multidrug resistance of murine leukemia. Int. J. Cancer 50, cellular adriamycin retention in lung cancer cell lines. Proc. Am.593–597. Assoc. Cancer Res. 34, 1836.

Keppler, D., 1992. Leukotrienes: Biosynthesis, transport, inactivation, and Kunimoto, T., Nitta, K., Tanaka, T., Uehara, N., Baba, H., Takeuchi, M.,analysis. Rev. Physiol. Biochem. Pharmacol. 121, 1–30. Yokokura, T., Sawada, S., Miyasaka, T., Mutai, M., 1987. Antitumor

¨Keppler, D., Leier, I., Jedlitschky, G., Mayer, R., Buchler, M., 1996. The activity of 7-Ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxy-cfunction of the multidrug resistance proteins (MRP and cMRP) in drug amptothecin, a novel water soluble derivative of camptothecin, againstconjugate transport and hepatobiliary excretion. Adv. Enz. Regul. 36, murine tumors. Cancer Res. 47, 5944–5947.17–29. Kuzmich, S., Tew, K.D., 1991. Detoxification mechanisms and tumor cell

Kessel, D., Wilberding, C., 1985. Promotion of daunorubicin uptake and resistance to anticancer drugs. Medic. Res. Rev. 11, 185–217.toxicity by the calcium antagonist tiapamil and its analogs. Cancer Lampidis, T.J., Kolonias, D., Tapiero, H., Savaraj, N., Cahn, J., 1990. InTreat. Rep. 69, 673–676. vitro cardiac potencies of multidrug resistance modulators. Proc. Am.

Ketterer, B., Meyer, D.J., Taylor, J.B., Pemble, S., Coles, B., Fraser, G., Assoc. Cancer Res. 31, 373.1990. Glutathione S-transferases and protection against oxidative Lampidis, T.J., Krishan, A., Planas, L., Tapiero, H., 1986. Reversal ofstress. In: Haye, J.D., Pickett, C.B., Mantle, T.J. (Eds.), Glutathione intrinsic resistance to adriamycin in normal cells by verapamil. CancerS-Transferases and Drug Resistance. Taylor and Francis, London. Drug Deliv. 3, 251–259.

Kim, R., Hirabayashi, N., Nishiyama, M., Jinushi, K., Toge, T., Okada, Landis, S.H., Murray, T., Bolden, S., Wingo, P.A., 1998. Cancer statisticsK., 1992. Experimental studies on biochemical modulation targeting 1998. CA-A Cancer J. Clinicians 48, 6–29.topoisomerase I and II in human tumor xenografts in nude mice. Int. J. Lankas, G.R., Wise, L.D., Cartwright, M.E., Pippert, T., Umbenhauer,Cancer 50, 760–766. D.R., 1998. Placental p-glycoprotein deficiency enhances susceptibility


to chemically induced birth defects in mice. Reprod Toxicol. 12, Liebermann, D.A., Hoffman, B., Reed, J.C., 1994. Tumor suppressor457–463. P53 is a regulator of bcl-2 and bax gene expression in vitro and in

Lee, C.H., Bradley, G., Zhang, J-T., Ling, V., 1993. Differential expres- vivo. Oncogene 9, 1799–1805.sion of P-glycoprotein genes in primary rat hepatocyte culture. J. Cell Morgenstern, R., DePierre, J.W., Jornvall, H., 1985. Microsomal gluta-Physiol. 157, 392–402. thione transferase. Primary structure. J. Biol. Chem. 260, 13976–

Leier, I., Jedlitschky, G., Buchholz, U., Keppler, D., 1994a. Characteriza- 13983.tion of the ATP-dependent leukotriene C export carrier in mas- Moscow, J.A., Townsend, A.J., Cowan, K.H., 1989. Elevation of pi class4

tocytoma cells. Eur. J. Biochem. 220, 599–606. glutathione S-transferase activity in human breast cancer cells byLeier, I., Jedlitschky, G., Buchholz, U., Cole, S.P.C., Deeley, R.G., transfection of the GST pi gene and its effect on sensitivity to toxins.

Keppler, D., 1994b. The MRP gene encodes an ATP-dependent export Mol. Pharmacol. 36, 22–28.pump for leukotriene C and structurally related conjugates. J. Biol. Mross, K., Bohn, C., Edler, L., Jonat, W., Queisser, W., Heidemann, E.,4

Chem. 269, 27807–27810. Goebel, M., Hossfeld, D.K., 1993. Randomized phase II study ofLetrent, S.P., Polli, J.W., Humphreys, J.E., Pollack, G.M., Brouwer, K.R., single-agent epirubicin6verapamil in patients with advanced meta-

Brouwer, K.L., 1999. P-glycoprotein mediated transport of morphine static breast cancer. Ann. Oncol. 4, 45–50.in brain capillary endothelial cells. Biochem. Pharmacol. 58, 951–957. Murata, M., Tamai, I., Sai, Y., Nagata, O., Kato, H., Sugiyama, Y., Tsuji,

Lewis, A.D., Forrester, L.M., Hayes, J.D., Wareing, C.J., Carmichael, J., A., 1998. Hepatobiliary transport kinetics of HSR-903, a newHarris, A.L., Mooghen, M., Wolf, C.R., 1989. Glutathione S-transfer- quinolone antibacterial agent. Drug Metab. Disp. 26, 1113–1119.ase isoenzymes in human tumours and tumour derived cell lines. Br. J. Murphy, B.R., Rynard, S.M., Pennington, K.L., Grosh, W., Loehrer, P.J.,Cancer 60, 327–331. 1994. A phase II trial of vinblastine plus dipyridamole in advanced

Ling, V., Thompson, L.H., 1974. Reduced permeability in CHO cells as a renal cell carcinoma. Am. J. Clin. Oncol. (CCT) 17, 10–13.mechanism of resistance to colchicine. J. Cell Physiol. 83, 103–116. Nawrath, H., Raschack, M., 1987. Effects of (2)-desmethoxyverapamil

Liu, L.F., 1989. DNA topoisomerase poisons as antitumor drugs. Annu. on heart and vascular smooth muscle. J. Pharmacol. Exp. Ther. 242,Rev. Biochem. 58, 351–375. 1090–1097.

Liu, L.F., Miller, K.G., 1981. Eukaryotic DNA topoisomerases: Two Neumann, M., Wilisch, A., Diddens, H., Probst, H., Gekeler, V., 1992.forms of type I DNA topoisomerases from HeLa cell nuclei. Proc. MDR hamster cells exhibiting multiple altered gene expression: effectsNatl. Acad. Sci. USA 78, 3487–3491. of dexniguldipine–HCl (B859-35), cyclosporine A and buthionine

Lowe, S.W., Rusley, H.E., Jacks, T., Houseman, D.E., 1993a. p53- sulfoximine. Anticancer Res. 12, 2297–2302.dependent apoptosis modulates the cytotoxicity of anticancer agents. Newman, M.J., Rodarte, J.C., Benbatoul, K.D., Romano, S.J., Zhang, C.,Cell 74, 957–967. Uyeda, R.T., Moran, E.J., Dixon, R., Guns, E.S., Mayer, L.D., 2000.

Lowe, S.W., Schmitt, E.M., Smith, S.W., Osborne, B.A., Jacks, T., 1993b. Discovery and characterization of OC144-093, a novel inhibitor ofp53 is required for radiation-induced apoptosis in mouse thymocytes. P-glycoprotein-mediated multidrug resistance. Cancer Res. 60, 2964–Nature 362, 847–849. 2972.

Lum, B., Gosland, M., 1995. MDR expression in normal tissues: Niehans, G.A., Jaszcz, W., Brunetto, V., Perri, R.T., Gajl-Peczalska, K.,Pharmacologic implications for the clinical use of P-glycoprotein Wick, M.R., Tsuruo, T., Bloomfield, C.D., 1992. Immunohistochemi-inhibitors. Hematol. Oncol. Clin. North Am. 9, 319–336. cal identification of P-glycoprotein in previously untreated diffuse

MacFarland, A., Abramovich, D.R., Ewen, S.W., Pearson, C.K., 1994. large cell and immunoblastic lymphomas. Cancer Res. 52, 3768–3775.Stage-specific distribution of p-glycoprotein in first-trimester and full- Nooter, K., Oostrum, R., Deurloo, J., 1987. Effects of verapamil on theterm human placenta. Histochem. J. 26, 417–423. pharmacokinetics of daunomycin in the rat. Cancer Chemother.

Mannervik, B., Jensson, H., 1982. Binary combinations of four protein Pharmacol. 20, 176–178.subunits with different catalytic specificities explain the relationship Olsen, S.L., Leslie, E.M., Loe, D.W., Ooi, A.L.-J., Deeley, R.J., Cole,between six basic glutathione S-transferases in rat liver cytosol. J. S.P.C., 1998. Site-directed mutagenesis of vicinal cysteine residues inBiol. Chem. 257, 9909–9912. human multidrug resistance protein, MRP. Proc. Am. Ass. Cancer Res.

Matsuo, K., Kohno, K., Takano, H., Sato, S., Kiue, A., Kuwano, M., 39, 167.1990. Reduction of drug accumulation and DNA topoisomerase II Onishi, Y., Azuma, Y., Sato, Y., Mizuno, ak Y., Tadakuma, T., Kizaki, H.,activity in acquired teniposide-resistant human cancer KB cell lines. 1993. Topoisomerase inhibitors induce apoptosis in thymocytes.Cancer Res. 50, 5819–5824. Biochim. Biophys. Acta 1175, 147–154.

Mayer, L.D., Tai, L.C.L., Ko, D.S.C., Masin, D., Ginsberg, R.S., Cullis, Osheroff, N., 1989. Biochemical basis for the interactions of type I and IIP.R., Bally, M.B., 1989. Influence of vesicle size, lipid composition, topoisomerases with DNA. Pharmacol. Ther. 41, 223–241.and drug-to-lipid ratio on the biological activity of liposomal doxoru- Oude Elferink, R.P., Jansen, P.L., 1994. The role of canalicular multis-bicin in mice. Cancer Res. 49, 5922–5930. pecific organic anion transporter in the disposal of endo- and xeno-

¨Mayer, R., Kartenbeck, J., Buchler, M., Jedlitschky, G., Leier, I., Keppler, biotics. Pharmacol. Ther. 64, 77–97.D., 1995. Expression of the MRP gene-encoded conjugate export Oude elferink, R.P.J., Meijer, D.K.F., Kuipers, F., Jansen, P.L.M., Groen,pump in liver and its selective absence from the canalicular membrane A.K., Groothuis, G.M.M., 1995. Hepatobiliary secretion of organicin transport-deficient mutant hepatocytes. J. Cell Biol. 131, 137–150. compounds: molecular mechanisms of membrane transport. Biochim.

Mayhew, E.G., Lasic, D., Babbar, S., Martin, F.J., 1992. Phar- Biophys. Acta 1241, 215–268.macokinetics and antitumor activity of epirubicin encapsulated in Ozols, R.F., Cunnion, R.E., Klecker, R.W., Hamilton, T.C., Ostchega, Y.,long-circulating liposomes incorporating a polyethylene glycol-deriva- Parrillo, J.E., Young, R.C., 1987. Verapamil and adriamycin in thetized phospholipid. Int. J. Cancer 51, 302–309. treatment of drug-resistant ovarian cancer patients. J. Clin. Oncol. 5,

Meister, A., 1991. Glutathione deficiency produced by inhibition of its 641–647.synthesis and its reversal: applications in research and therapy. Papahadjopoulos, D., Allen, T.M., Gabizon, A., Mayhew, E., Matthay, K.,Pharmacol. Ther. 51, 155–194. Huang, S.K., Lee, K.D., Woodle, M.C., Lasic, D.D., Redemann, C. et

Meister, A., 1994. Glutathione, ascorbate, and cellular protection. Cancer al., 1991. Sterically stabilized liposomes: improvements in phar-Res. 54, 1969S–1975S. macokinetics and antitumor therapeutic efficacy. Proc. Natl. Acad. Sci.

Miller, T., Dahlberg, S., Grogan, T. et al., 1994. Southwest Oncology USA 88, 11460–11464.Group clinical trials of chemosensitizers in non-Hodgkin’s lym- Peck, R.A., Marshall, J., Ziessman, H., Hewett, J., Schmalbach, T.,phomas. Anticancer Drugs 5, 73. Chaturvedi, P., Hawkins, M., 1996. A phase I trial of doxorubicin and

Miyashita, T., Krajewski, S., Krajewska, M., Wang, H.G., Lin, H.K., VX-710. Proc. Am. Ass. Cancer Res. 37, 165.


Pirker, R., FitzGerald, D.J., Raschack, M., Frank, Z., Willingham, M.C., A.R., O’Brien, S.M., Ratain, M.J., 1993. Modulation of vinblastinePastan, I., 1989. Enhancement of the activity of immunotoxins by resistance with cyclosporine: A phase I study. Clin. Pharmacol. Ther.analogues of verapamil. Cancer Res. 49, 4791–4795. 54, 421–429.

Pirker, R., Keilhauer, G., Raschack, M., Lechner, C., Ludwig, H., 1990. Sarris, A.H., Younes, A., McLaughlin, P., Moore, D., Hagemeister, F.,Reversal of multidrug resistance in human KB cell lines by structural Swan, F., Rodriguez, M.A., Romaguera, J., North, L., Mansfield, P.,analogs of verapamil. Int. J Cancer. 45, 916–919. Callendar, D., Mesina, O., Cabanillas, F., 1996. Cyclosporin A does

Presant, C.A., Kennedy, P.S., Wiseman, C., Gala, a K., Bouzaglou, A., not reverse clinical resistance to paclitaxel in patients with relapsedWyres, M., Naessig, V., 1986. Verapamil reversal of clinical doxorubi- non-Hodgkin’s lymphoma. J. Clin. Oncol. 14, 233–239.cin resistance in human cancer. Am. J. Clin. Oncol. 9, 355–357. Sasabe, H., Tsuji, A., Sugiyama, Y., 1998. Carrier-mediated mechanism

Raghu, G., Pierre-Jerome, M., Dordal, M.S., Simonian, P., Bauer, K.D., for the biliary excretion of the quinolone antibiotic grepafloxacin and1993. P-Glycoprotein and alterations in the glutathione /glutathione- its glucuronide in rats. J. Pharmacol. Exp. Ther. 284, 1033–1039.peroxidase cycle underlie doxorubicin resistance in HL-60-R, a Savaraj, N., Wu, C.J., Bao, J.J., Solomon, J., Donnelly, E., Feun, L.G.,subclone of the HL-60 human leukemia cell line. Int. J. Cancer 53, Lampidis, T.J., Kuo, M.T., 1994. Multidrug resistance associated804–811. protein gene expression in small cell and non small cell lung cancer.

Ramsdell, H.S., Eaton, D.L., 1990. Mouse liver glutathione s-transferase Proc. Am. Assoc. Cancer Res 35, 242.¨ ¨isoenzyme activity toward aflatoxin B1-8,9-epoxide and Schaub, T.P., Kartenbeck, J., Konig, J., Staehler, G., Storkel, S., Thon,

benzo[a]pyrene-7,8-9,10-epoxide. Toxicol. Appl. Pharmacol. 105, W.F., Keppler, D., 1998. Expression of the MRP2 gene-encoded216–225. conjugate export pump in human kidney proximal tubule epithelia and

Ramu, A., Glaubiger, D., Fuks, Z., 1984a. Reversal of acquired resistance in renal-cell carcinomas. Proc. Am. Ass. Cancer Res. 39, 168.to doxorubicin in P388 murine leukemia cells by tamoxifen and other Schisselbauer, J.C., Silber, R., Papadopoulos, E., Abrams, K., LaCreta,triparanol analogues. Cancer Res. 44, 4392–4395. F.P., Tew, K.D., 1990. Characterization of glutathione S-transferase

Ramu, A., Spanier, R., Rahamimoff, H., Fuks, Z., 1984b. Restoration of expression in lymphocytes from chronic lymphocytic leukemia pa-doxorubicin responsiveness in doxorubicin-resistant P388 murine tients. Cancer Res. 50, 3562–3568.leukemia cells. Br. J. Cancer 50, 501–507. Schuurhuis, G.J., Broxterman, H.J., van der Hoeven, J.J.M., Pinedo,

Reed, J.C., 1995. Regulation of apoptosis by bcl-2 family proteins and its H.M., Lankelma, J., 1987. Potentiation of doxorubicin cytotoxicity byrole in cancer and chemoresistance. Curr. Opin. Oncol. 7, 541–546. the calcium antagonist bepridil in anthracycline-resistant and sensitive

Reed, J., Meister, L., Cuddy, M., Geyer, C., Pleasure, D., 1991. cell lines. Cancer Chemother. Pharmacol. 20, 285–290.Differential expression of the bcl-2 proto-oncogene in neuroblastomas Sehested, M., Jensen, P.B., Skovsgaard, T., Bindslev, N., Demant, E.J.,and other human neural tumors. Cancer Res. 52, 6529–6538. Friche, E., Vindelov, L., 1989. Inhibition of vincristine binding to

Riordan, J.R., Ling, V., 1979. Purification of P-glycoprotein from plasma plasma membrane vesicles from daunorubicin-resistant Ehrlich ascitesmembrane vesicles of Chinese hamster ovary cell mutants with cells by multidrug resistance modulators. Br. J. Cancer 60, 809–814.reduced colchicine permeability. J. Biol. Chem. 254, 12701–12705. Sevick, E.M., Jain, R.K., 1989. Geometric resistance to blood flow in

Rodenburg, C.J., Nooter, K., Herweijer, H., Seynaeve, C., Oosterom, R., solid tumors perfused ex vivo: effects of tumor size and perfusionStoter, G., Verweij, 1991. Phase II study of combining vinblastine and pressure. Cancer Res. 49, 3506–3512.cyclosporine A to circumvent multidrug resistance in renal cell cancer. Shapiro, A.B., Ling, V., 1994. ATPase activity of purified and reconsti-Ann. Oncol. 2, 305–306. tuted P-glycoprotein from Chinese hamster ovary cells. J. Biol. Chem.

Roepe, P.D., 1992. Analysis of the steady-state and initial rate of 269, 3745–3754.doxorubicin efflux from a series of multidrug-resistant cells expressing Shapiro, A.B., Ling, V., 1995. Reconstitution of drug transport by purifieddifferent levels of P-glycoprotein. Biochemistry 31, 12555–12564. P-glycoprotein. J. Biol. Chem. 270, 16167–16175.

Roepe, P.D., Wei, L.Y., Cruz, J., Carlson, D., 1993. Lower electrical Sharom, F.J., Yu, X., Doige, C.A., 1993. Functional reconstitution of drugmembrane potential and altered pHi homeostasis in multidrug-resistant transport and ATPase activity in proteoliposomes containing partially(MDR) cells: further characterization of a series of MDR cell lines purified P-glycoprotein. J. Biol. Chem. 268, 24197–24202.expressing different levels of P-glycoprotein. Biochemistry 32, 11042– Skovsgaard, T., Dano, K., Nissen, N.I., 1984. Chemosensitizers coun-11056. teracting acquired resistance to antharacyclines and vinca alkaloids in

Roninson, I.B., Chin, J.E., Choi, K.G., Gros, P., Housman, D.E., Fojo, A., vivo. A new resistance treatment principle. Cancer Treat. Rev. 11Shen, D.W., Gottesman, M.M., Pastan, I., 1986. Isolation of human (suppl. A), 63–72.mdr DNA sequences amplified in multidrug-resistant KB carcinoma Slater, L.M., Murray, S.L., Wetzel, M.W., Wisdom, R.M., DuVall, E.M.,cells. Proc. Natl. Sci. USA 83, 4538–4542. 1982. Verapamil restoration of daunorubicin responsiveness in

Ross, W., Rowe, T., Glisson, B., Yalowich, J., Liu, L.F., 1984. Role of daunorubicin-resistant Ehrlich ascites carcinoma. J. Clin. Invest. 70,topoisomerase II in mediating epipodophyllotoxin-induced DNA 1131–1134.cleavage. Cancer Res. 44, 5857–5860. Slater, L.M., Sweet, P., Stupecky, M., Wetzel, M.W., Gupta, S., 1986.

Roy, C., Brown, D.L., Little, J.E., Valentine, B.K., Walker, P.R., Sikorska, Cyclosporine A corrects daunorubicin resistance in Ehrlich ascitesM., LeBlanc, J., Chaly, N., 1992. The topoisomerase II inhibitor carcinoma. Br. J. Cancer 54, 235–238.teniposide (VM-26) induces apoptosis in unstimulated mature murine Smit, J.W., Huisman, M.T., van Tellingen, O., Wiltshire, H.R., Schinkel,lymphocytes. Exp. Cell Res. 200, 416–424. A.H., 1999. Absence or pharmacological blocking of placental p-

Rubio, G.J., Pinedo, H.M., Gazdar, A.F., Van Ark-Otte, J., Giaccone, G., glycoprotein profoundly increases fetal drug exposure. J. Clin. Invest.1994. MRP gene assessment in human cancer, normal lung, and lung 104, 1441–1447.cancer cell lines. Proc. Am. Assoc. Cancer Res. 35, 206. Solary, E., Velay, I., Chauffert, B., Bidan, J.M., Caillot, D., Dumas, M.,

Safa, A.R., Glover, C.J., Meyers, M.B., Biedler, J.L., Felsted, R.L., 1986. Guy, H., 1991. Sufficient levels of quinine in the serum circumvent theVinblastine photoaffinity labeling of a high molecular weight surface multidrug resistance of the human leukemic cell line K562/ADM.membrane glycoprotein specific for multidrug-resistant cells. J. Biol. Cancer 68, 1714–1719.Chem. 261, 6137–6140. Solary, E., Velay, I., Chauffert, B., Caillot, D., Bidan, J.M., Dumas, M.,

Safa, A.R., Glover, C.J., Sewell, J.I., Meyers, M.B., Biedler, J.L., Felsted, Casasnovas, O., Guy, H., 1990. Quinine circumvents the doxorubicinR.L., 1987. Identification of the multidrug resistance-related mem- resistance of a multidrug resistant human leukemia cell line, K562/brane glycoprotein as an acceptor for calcium channel blockers. J. DXR. Nouv. Rev. Fr. Hematol. 32, 361–363.Biol. Chem. 262, 7884–7888. Sparreboom, A., van Asperen, J., Mayer, U., Schinkel, A.H., Smit, J.W.,

Samuels, B., Mick, R., Vogelsang, N., Williams S.F Schilsky, R.L., Safa, Meijer, D.K., Borst, P., Nooijen, W.J., Beijnen, J.H., van Tellingen,


1997. Limited oral bioavailability and active epithelial excretion of Tsuruo, T., Iida, H., Kitatani, Y., Yokota, K., Tsukagoshi, S., Sakurai, Y.,1982b. Enhancement of vincristine and adriamycin-induced cytotoxici-paclitaxel (taxol) caused by P-glycoprotein in the intestine. Proc. Natl.ty by verapamil in P388 leukemia and its sublines resistant toAcad. Sci. USA 94, 2031–2035.vincristine and adriamycin. Biochem. Pharmacol. 31, 3138–3140.Speeg, K.V., Maldonado, A.L., 1994. Effect of the non-immunosuppres-

Tsuruo, T., Iida, H., Naganuma, K., Tsukagoshi, S., Sakurai, Y., 1983a.sive cyclosporine analogue SDZ PSC 833 on colchicine and doxorubi-Promotion of verapamil of vincristine responsiveness in tumor cellcin biliary secretion in the rat in vivo. Cancer Chemother. Pharmacol.lines inherently resistant to the drug. Cancer Res. 43, 808–813.34, 133–136.

Tsuruo, T., Iida, H., Nojiri, M., Tsukagoshi, S., Sakurai, Y., 1983b.Speeg, K.V., Maldonado, A.L., Liaci, J., Muirhead, D., 1992a. Effect ofPotentiation of vincristine and adriamycin in human hematopoieticcyclosporin on colchicine secretion by a liver canalicular transportertumor cell lines by calcium antagonists and calmodulin inhibitors.studied in vivo. Hepatology 15, 899–903.Cancer Res. 43, 2267–2272.

Speeg, K., Malonado, A., Liaci, J., Muirhead, D., 1992b. Effect ofTsuruo, T., Iida, H., Nojiri, M., Tsukagoshi, S., Sakurai, Y., 1983c.

cyclosporine on colchicine secretion by the kidney multidrug transpor-Circumvention of vincristine and adriamycin resistance in vitro and in

ter studied in vivo. J. Pharmacol. Exp. Ther. 261, 50–55.vivo by calcium influx blockers. Cancer Res. 43, 2905–2910.

Spoelstra, E.C., Dekker, H., Schuurhuis, G.J., Broxterman, H.J., Lan-Tsuruo, T., Iida, H., Kitani, Y., Yokota, K., Tsukaggoshi, S., Sukurai, Y.,

kelma, J., 1991. P-glycoprotein drug efflux pump involved in the 1984. Effects of quinidine and related compounds on cytotoxicity andmechanisms of intrinsic drug resistance in various colon cancer cell cellular accumulation of vincristine and adriamycin in drug-resistantlines. Evidence for a saturation of active daunorubicin transport. tumor cells. Cancer Res. 44, 4303–4307.Biochem. Pharmacol. 41, 349–359. Van Asperen, J., van Tellingen, O., Sparreboom, A., Schinkel, A.H.,

Sugawara, I., Kataoka, I., Morishita, Y., Hamada, H., Tsuruo, T., Itoyama, Borst, P., Nooijen, W.J., Beijnen, J.H., 1997. Enhanced oral bioavail-S., Mori, S., 1988a. Tissue distribution of p-glycoprotein encoded by a ability of paclitaxel in mice treated with the p-glycoprotein blockermultidrug-resistant gene as revealed by a monoclonal antibody, MRK- SDZ PSC 833. Br. J. Cancer 76, 1181–1183.16. Cancer Res. 48, 1926–1929. Van Asperen, J., van Tellingen, O., van der Valk, M.A., Rozenhart, M.,

Sugawara, I., Nakahama, M., Hamada, H., Tsuruo, T., Mori, S., 1988b. Beijnen, J.H., 1998. Enhanced oral absorption and decreased elimina-Apparent stronger expression in the human adrenal cortex than in the tion of paclitaxel in mice cotreated with cyclosporin A. Clin. Cancerhuman adrenal medulla of Mr 170 000–180 000 P-glycoprotein. Res. 4, 2293–2297.Cancer Res. 48, 4611–4614. Van der Bliek, A.M., Baas, F., Ten Houte de Lange, T., Kooiman, P.M.,

Tan, K.B., Mattern, M.R., Eng, W.-K., McCabe, F.L., Johnson, R.K., Van der Velde-Koerts, T., Borst, P., 1987. The human mdr3 gene1989. Nonproductive rearrangement of DNA topoisomerase I and II encodes a novel P-glycoprotein homologue and gives rise to alter-genes: Correlation with resistance to topoisomerase inhibitors. J. Natl. natively spliced mRNAs in liver. EMBO J. 6, 3325–3331.Cancer Inst. 81, 1732–1735. Van der Graaf, W.T.A., Mulder, N.H., Timmer-Bosscha, H., Meijer, C.,

Tatsuta, T., Naito, M., Oh-hara, T., Sugawara, I., Tsuruo, T., 1992. Meersma, G.J., De Vries, E.G., 1993. Different effects of amiodarone,Functional involvement of p-glycoprotein in blood–brain barrier. J. cyclosporine A and PSC 833 on mitoxantrone sensitivity in humanBiol. Chem. 267, 20383–20391. tumor cell lines with various resistance mechanisms. Proc. Am. Assoc.

Tew, K.D., Bomber, A.M., Hoffman, S.J., 1988. Ethacrynic acid and Cancer Res. 34, A1899.piriprost as enhancers of cytotoxicity in drug resistant and sensitive Van Veen, H.W., Konings, W.N., 1998. The ABC family of multidrugcell lines. Cancer Res. 48, 3622–3625. transporters in microorganisms. Biochim. Biophys. Acta 1365, 31–36.

Tewey, K.M., Chen, G.L., Nelson, E.M., Liu, L.F., 1984a. Intercalative Van Zuylen, L., Sparreboom, A.,Van der Gaast, A.,Van der Burg, M.E.L.,antitumor drugs interfere with the breakage–reunion reaction of Van Beurden, V., Bol, C.J., Woestenborghs, R., Palmer, P.A., Verweij,mammalian DNA topoisomerase II. J. Biol. Chem. 259, 9182–9187. J., 2000. The orally administered P-glycoprotein inhibitor R101933

Tewey, K.M., Rowe, T.C., Yang, L., Halligan, B.D., Liu, L.F., 1984b. does not alter the plasma pharmacokinetics of docetaxel. Clin. CancerAdriamycin-induced DNA damage mediated by mammalian DNA Res. 6, 1365–1371.topoisomerase II. Science 226, 466–468. Verweij, J., Herweijer, H., Oosterom, R., Van der Burg, M.E., Planting,

Thiebaut, F., Tsuruo, T., Hamada, H., Gottesman, M.M., Pastan, I., A.S., Seynaeve, C., Stoter, G., Nooter, K., 1991. A phase II study ofWillingham, M.C., 1987. Cellular localization of the multidrug epidoxorubicin in colorectal cancer and the use of cyclosporine A inresistance gene product P-glycoprotein in normal human tissues. Proc. an attempt to reverse multidrug resistance. Br. J. Cancer 64, 361–364.Natl. Acad. Sci. USA 84, 7735–7738. Walton, M.I., Whysong, D., O’Conner, P.M., Hockenbery, D., Korsmeyer,

Thiebaut, F., Tsuruo, T., Hamada, H., Gottesman, M.M., Pastan, I., S.J., Kohn, K.W., 1993. Constitutive expression of human Bcl-2Willingham, M.C., 1989. Immunohistochemical localization in normal modulates mitrogen mustard and camptothecin induced apoptosis.tissues of different epitopes in the multidrug transport protein, P170: Cancer Res. 53, 1853–1861.evidence for localization in brain capillaries and cross-reactivity of one Wang, J.C., 1985. DNA topoisomerases. Annu. Rev. Biochem. 54, 665–antibody with a muscle protein. J. Histochem. Cytochem. 37, 159– 697.164. Wang, J.C., 1987. Recent studies of DNA topoisomerases. Biochim.

Tolcher, A.W., Cowan, K.H., Solomon, D. et al., 1994. A phase I study of Biophys. Acta 909, 1–9.paclitaxel (T) with R-verapamil (RV) in metastatic breast cancer Wang, Y.-M.C., Chaturvedi, P.R., 1998. Pharmacokinetic interaction(MBC). Proc. Am. Soc. Clin. Oncol. 13, 139. between VX-853, an MDR reversal agent, and paclitaxel in beagle

TMTsujimoto, Y., Louie, E., Bashir, M.M., Croce, C.M., 1988. The reciprocal dogs. AAPS Proceedings (PharmSci ), abstract 4023.partners of both the t(14;18)and t(11;14) translocations involved in Watanabe, T., Naito, M., Oh-hara, T., Itoh, Y., Cohen, D., Tsuruo, T.,B-cell neoplasms are rearranged by the same mechanism. Oncogene 2, 1996. Modulation of multidrug resistance by SDZ PSC 833 in347–351. leukemic and solid tumor bearing mouse models. Jpn. J. Cancer Res.

Tsuruo, T., Iida, H., Tsukagoshi, S., Sakurai, Y., 1981. Overcoming of 87, 184–193.vincristine resistance in P388 leukemia in vivo and in vitro through Watanabe, T., Tsuge, H., Oh-hara, T., Naito, M., Tsuruo, T., 1995.enhanced cytotoxicity of vincristine and vinblastine by verapamil. Comparative study on reversal efficacy of SDZ PSC 833, cyclosporinCancer Res. 41, 1967–1972. A and verapamil on multidrug resistance in vitro and in vivo. Acta

Tsuruo, T., Iida, H., Tsukagoshi, S., Sakurai, Y., 1982a. Increased Oncol. 34, 235–241.accumulation of vincristine and adriamycin in drug resistant P388 Weiss, L.M., Warnke, R.A., Sklar, J., Cleary, M.L., 1987. Moleculartumor cells following incubation with calcium antagonists and cal- analysis of the t(14;18) chromosomal translocation in malignantmodulin inhibitors. Cancer Res. 42, 4730–4733. lymphomas. New Engl. J. Med. 317, 1185–1189.


Wilson, C.M., Serrano, A.E., Wasley, A., Bogenshcutz, M.P., Shanakar, Zaman, G.J.R., Versantvoort, C.H.M., Smit, J.J.M., Eijdems, E.W.H.M.,A.H., Wirth, D.F., 1989. Amplification of a gene related to mammalian De Haas, M., Smith, A.J., Broxterman, H.J., Mulder, N.H., De Vries,mdr genes in drug-resistant Plasmodium falciparum. Science 244, E.G.E., Baas, F., Borst, P., 1993. Analysis of the expression of MRP,1184–1186. the gene for a new putative transmembrane drug transporter, in human

Wishart, G.C., Bissett, D., Paul, J., Jodrell, D., Harnett, A., Habeshaw, T., multidrug resistant lung cancer cell lines. Cancer Res. 53, 1747–1750.Kerr, D.J., Macham, M.A., Soukop, M., Leonard, R.C. et al., 1994. Zelenetz, A.D., Chu, G., Galili, N., Bangs, C.D., Horning, S.J., Donlon,Quinidine as a resistance modulator of epirubicin in advanced breast T.A., Cleary, M.L., Levy, R., 1991. Enhanced detection of the t(14;18)cancer. Mature results of a placebo-controlled trial. J. Clin. Oncol. 12, translocation in malignant lymphoma using pulsed-field gel electro-1771–1777. phoresis. Blood 178, 1552–1560.

Xie, R., Hammarlund-Udenaes, M., de Boer, A.G., de Lange, E.C., 1999. Zhan, Q., Carrier, F., Fornace, Jr. A.J., 1993. Induction of cellular p53The role of p-glycoprotein in blood–brain barrier transport of mor- activity by DNA-damaging agents and growth arrest. Mol. Cell Biol.phine: transcortical microdialysis studies in mdr1a (2 / 2) and mdr1a 13, 4242–4250.(1 / 1) mice. Br. J. Pharmacol 128, 563–568. Zhang, L., Schaner, M.E., Giacomini, K.M., 1998. Functional characteri-

Yang, C.P.H., Mellado, W., Horwitz, S.B., 1988. Azidopine photoaffinity zation of an organic cation transporter (hOCT1) in a transientlylabeling of multidrug resistance-associated glycoproteins. Biochem. transfected human cell line (HeLa). J. Pharmacol. Exp. Ther. 286,Pharmacol. 37, 1417–1421. 354–361.

Documents

Multidrug resistance (MDR) in cancer: Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs