Available online at www.sciencedirect.com
Prodrugs: bridging pharmacodynamic/pharmacokinetic gapsBernard Testa
In this mini review, prodrugs are discussed with a focus on their
pharmaceutical, pharmacokinetic, and pharmacodynamic
objectives, as well as on the resulting therapeutic benefits.
Carrier-linked prodrugs remain the most extensively
investigated and receive due attention here with recent
successes highlighted. A clear trend is apparent in modern
prodrug research, namely the increased attention given to the
knowledge-based design of bioprecursors, namely prodrugs
devoid of a detachable promoiety. In most cases, such
prodrugs are activated by in situ reduction, hence their
designation as bioreductive prodrugs. This is a particularly
active field in the design of more selective, small-molecule
antitumor agents. New antimicrobial agents are also in the
pipeline. In addition, biooxidative bioprecursors offer a
promising strategy in specific cases, as illustrated by the
successful antiaggregating agent clopidogrel.
Address
Pharmacy Department, University Hospital Centre, CHUV/BH-04, 46
Rue du Bugnon, CH-1011 Lausanne, Switzerland
Corresponding author: Testa, Bernard ([email protected])
Current Opinion in Chemical Biology 2009, 13:338–344
This review comes from a themed issue on
Next-generation therapeutics
Edited by Karl–Heinz Altmann and Dario Neri
Available online 25th May 2009
1367-5931/$ – see front matter
# 2009 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.cbpa.2009.04.620
IntroductionWhat makes prodrugs different from other drugs is the
fact that they are devoid of intrinsic pharmacological
activity [1�,2��,3–15]. Thus, the simplest and clearest
definition, in this writer’s view, is that given by Albert
[16], who coined the term. In modified form, the defi-
nition reads: ‘Prodrugs are chemicals with little or no
pharmacological activity, undergoing biotransformation
to a therapeutically active metabolite.’
Drug discovery projects are in danger of failure should
pharmacodynamic (PD) and pharmacokinetic (PK) pre-
requisites prove chemically incompatible that is imposs-
ible to combine in a single molecule. Because prodrugs
and their active metabolite have different chemical struc-
tures, they must differ in their biological behavior, a fact
that opens the door to dissociating PD and PK properties.
Current Opinion in Chemical Biology 2009, 13:338–344
Why prodrugs?The prodrug concept has found a number of useful
applications in drug research and development. These
correspond to a number of objectives, as presented in
Figure 1. However, it should be clear that such a view is
too schematic, these objectives being often intertwined.
Thus, an improved solubility can greatly facilitate oral
absorption, while improving the chemical stability of an
active agent can allow tissue-selective delivery and even
lead to its in situ activation.
Pharmaceutical objectives
Pharmaceutical scientists are often faced with serious
formulation problems resulting from poor solubility,
insufficient chemical stability or poor organoleptic prop-
erties. While pharmaceutical technology can solve such
problems in favorable cases (e.g. by improving the solu-
bility of cyclosporine), success is not guaranteed and may
be time-consuming to achieve.
An example of chemical interest is that of isotaxel (1 in
Figure 2), a prodrug of the potent antitumor agent pacli-
taxel (2 in Figure 2) [17,18]. Paclitaxel has a very poor
water solubility (about 0.25 mg/L) which necessitates
coinjection of a detergent. Taking advantage of the
well-known mechanism of intramolecular nucleophilic
transacylation, an analog of paclitaxel was prepared hav-
ing the benzoyl group located on the a-hydroxy group
rather than on the amido group. What is an amido func-
tion in paclitaxel thus became a basic amino group in
isotaxel, and solubility was greatly improved by protona-
tion (water solubility of 1 HCl is 0.45 g/L). O,N-Migration
of the benzoyl moiety to the active paclitaxel was fast
under physiological conditions of pH and temperature,
but much slower under slightly acidic, pharmaceutically
compatible conditions.
Note that increasing solubility is not only a pharmaceu-
tical but also a PK objective. Indeed, and as made explicit
by the biopharmaceutics classification scheme (BCS) [19],
solubility is one of the main factors influencing oral
absorption.
Pharmacokinetic objectives
PK objectives are currently the most important ones in
prodrug research. Foremost among these is firstly, a need
to improve oral bioavailability, be it by improving the oral
absorption of the drug and/or by decreasing its presyste-
mic metabolism. Other objectives are secondly, to
improve absorption by parenteral (nonenteral, e.g. dermal
and ocular) routes; thirdly, to lengthen the duration of
action of the drug by slow metabolic release; and finally to
www.sciencedirect.com
Bridging pharmacodynamic/pharmacokinetic gaps Testa 339
Figure 1
A schematic classification of objectives in prodrug research. Note that
many of these objectives are in fact intertwined (modified from [1�]).
achieve the organ/tissue-selective delivery of an active
agent. Some of these objectives are exemplified below
with clinically or potentially useful prodrugs.
Achieving improved oral absorption by a prodrug strategy
is a frequent rationale in marketed prodrugs, as aptly
illustrated by neuraminidase inhibitors of therapeutic
value against type A and B influenza in humans [20].
Target-oriented rational design has led to highly hydro-
philic agents which are not absorbed orally, as exempli-
fied by the zwitterion zanamivir which is administered in
aerosol form. Another active agent is Ro-64-0802, which
also shows very high in vitro inhibitory efficacy toward the
enzyme but low oral bioavailability, also being a (too)
hydrophilic zwitterion by virtue of basic amino and acidic
carboxylate groups. To circumvent this problem, the
active agent was derivatized to its ethyl ester known as
oseltamivir (3 in Figure 2). This prodrug is well absorbed
orally and undergoes rapid enzymatic hydrolysis to pro-
duce high and sustained plasma levels of the active agent
(structure marked in red).
Another PK objective is the organ-selective or tissue-
selective delivery of a given drug, also known as the
search for the ‘magic bullet’. Targeting a prodrug to
enzymes highly expressed in the target organ or tissue is
a useful strategy for selective delivery. Given the high
concentration of cytochromes P450 (CYPs) in the liver,
it comes as no surprise that CYP-activated prodrugs
should have been designed with a view of selectively
delivering antihepatitis virus agents to the liver. A good
example is provided by pradefovir (4 in Figure 2), a
prodrug currently in clinical trials as the cis-(2R,4S)-
isomer shown here [21]. When examined in rats, this 1-
aryl-1,3-propanyl prodrug showed a 12-fold improve-
www.sciencedirect.com
ment in the liver/kidney ratio of adefovir (structure
marked in red) over adefovir dipivoxil, the prodrug in
current use.
Targeting prodrugs to enzymes overexpressed in cancer
tissues is a topic of major interest and promise in drug
discovery [22]. A clinically significant example is that of
capecitabine (5 in Figure 2), a multistep, orally active
prodrug of 5-fluorouracil (structure in red) [23]. Capeci-
tabine is well absorbed orally and undergoes three acti-
vation steps resulting in high tumor concentrations of the
active drug. It is first hydrolyzed by liver carboxylesterase,
the resulting metabolite being a carbamic acid which
spontaneously decarboxylates to 50-deoxy-5-fluorocyti-
dine. The enzyme cytidine deaminase present in the
liver and tumors then transforms 50-deoxy-5-fluorocyti-
dine into 50-deoxy-5-fluorouridine. Transformation into
5-FU is catalyzed by thymidine phosphorylase and occurs
selectively in tumor cells.
Capecitabine is of great interest in the context of this
review as it affords an impressive gain in therapeutic
benefit compared to 5-FU due to its oral bioavailability
and a relatively selective activation in and delivery to
tumors. Nevertheless, most of the current attention in
antitumor prodrug design is focused on reductive biopre-
cursors, namely prodrugs reduced in situ to a cytotoxic
metabolite.
Pharmacodynamic objectives
PD objectives can be understood as being synonymous
with decreasing systemic toxicity. Two major cases are
illustrated here, namely the masking of a reactive agent to
improve its therapeutic index, and the in situ activation of
a chemotherapeutic agent.
The masking of a reactive agent to improve its thera-
peutic index is aptly exemplified by the successful anti-
aggregating agent clopidogrel (6 in Figure 3). This
compound, whose molecular mechanism of action was
poorly understood for years, is now known to be a prodrug
[24,25,26�]. It is of interest among prodrugs in that its
major metabolic route in humans (about 80% of a dose) is
one of hydrolysis which leads to the inactive acid. In
contrast, much of the remaining dose is activated in a
three-step sequence. First, CYP3A oxidizes clopidogrel to
2-oxo-clopidogrel (7 with the corresponding substituents
R and R0). This is followed by a CYP-catalyzed sulfox-
idation to the cyclic sulfoxide intermediate. The latter
hydrolyzes spontaneously to a highly reactive sulfenic
acid metabolite (8 with the corresponding substituents R
and R0) which forms a covalent S–S bridge with a thiol
group in platelet ADP receptors, thereby causing their
long-lasting inactivation. The sulfenic intermediate can
also be reduced by glutathione, leading to the correspond-
ing thiol which for some years was thought to be the active
metabolite.
Current Opinion in Chemical Biology 2009, 13:338–344
340 Next-generation therapeutics
Figure 2
Some prodrugs discussed in the text. An illustrative prodrug developed for improved water solubility is namely isotaxel (1), a promising prodrug of
paclitaxel (2). Examples of carrier-linker prodrugs (active drug shown in red) are the anti-influenza virus agent oseltamivir (3), the antihepatitis virus
agents pradefovir (4), and capecitabine (5), a well-known pro-prodrug of 5-fluorouracil.
It is interesting to compare the activation of clopidogrel
with that of prasugrel (9 in Figure 3), a more recent and
smartly designed analog [27–29]. In contrast to the for-
mer, the latter is activated by carboxylesterases 1 and 2 in
humans. Indeed, C(2) in prasugrel carries an acetyl ester
function whose hydrolysis yields the corresponding
hydroxy-thiophene which tautomerizes to the thiolactone
(7 with the corresponding substituents R and R0), a close
analog of 2-oxoclopidogrel. Sulfoxidation followed by
hydrolytic ring opening yields the (yet to be discovered)
active sulfenic acid metabolite (8 with the corresponding
substituents R and R0).
In situ activation to toxic agents is a part of the well-known
mechanism of action of the antibacterial and antiparasitic
nitroarenes such as metronidazol. This strategy is finding
a recent and promising revival with the discovery of
Current Opinion in Chemical Biology 2009, 13:338–344
potent bioreductive prodrugs. These are designed to
be activated by a metabolic reaction of reduction in target
tissues or organisms with a low oxygen concentration.
Such a state of hypoxia may favor reduction reactions, but
another factor accounting for the shift toward reduction is
the hypoxia-increased expression of genes coding for
reductive enzymes. A recent and promising illustration
of bioreductive antimicrobial prodrugs is provided by the
antitubercular nitroimidazooxazine PA-824 (10 in
Figure 3) now in clinical trials [30�]. There is evidence
to indicate that the active species in antimicrobial nitroar-
enes is their nitrenium cation, a strong and highly reactive
electrophile [31].
The case of antitumor bioprecursors is best illustrated
with tirapazamine (11 in Figure 3), since this much
studied agent and its analogs have revealed inspiring
www.sciencedirect.com
Bridging pharmacodynamic/pharmacokinetic gaps Testa 341
Figure 3
A comparison of the activation pathways of the antiaggregant prodrugs clopidogrel (6) and prasugrel (9). The former is activated by two sequential
CYP-catalyzed reactions, first to the thiolactone 7, then to its sulfoxide (not shown), followed by spontaneous hydrolytic ring opening to the reactive
sulfenic acid (8). The latter antagonizes platelet ADP receptors via a covalent S–S bridge. The first activation step of prasugrel (9) is by
carboxylesterase-catalyzed deacetylation, followed by the same subsequent steps as clopidogrel. The figure also shows the structure of the
antitubercular nitroimidazooxazine PA-824 (10) whose activation is by nitroreduction, and the activation of the hypoxia-selective antitumor agent
tirapazamine (11) to the radical 12.
information on their molecular mechanism of action
[32–34]. Tirapazamine is inactivated by two-electron
reduction steps catalyzed by quinone reductase, but
activated to a cytotoxic radical (12 in Figure 3) by a
one-electron reduction catalyzed by NADPH-cyto-
chrome P450 reductase and multiple reductases in the
nucleus. The strong redox potential of this benzotriazi-
nyl radical allows it to abstract a hydrogen radical from
DNA, thereby causing single-strand and double-strand
breaks.
How prodrugs?Given the stated objectives, how can medicinal chemists
design clinically useful prodrugs? As implicit above, two
main classes of prodrugs exist, namely the carrier-linked
prodrugs and the bioprecursors [3].
www.sciencedirect.com
In carrier-linked prodrugs, the active agent (the drug) is
linked to a carrier (also known as a promoiety). In many
cases, carrier-linked prodrugs are esters activated by
enzymatic hydrolysis, although nonenzymatic hydrolysis
may contribute [11]. The variety of ester prodrugs in the
literature is immense and has served to mask carboxylate
groups (e.g. oseltamivir in Figure 2), phosphate groups
(e.g. pradefovir Figure 2), acidic tetrazolate groups, or
alcoholic or phenolic functions. Examples of promoieties
used to derivatize such active agents include simple or
functionalized alkyl groups, aryl groups, simple or com-
plex carboxylic acids, amino acids, and fractionable moi-
eties (e.g. carbonic acid esters).
Amides form another major group of carrier-linked pro-
drugs, the active agent usually being an amine. Here,
Current Opinion in Chemical Biology 2009, 13:338–344
342 Next-generation therapeutics
Figure 4
Two schematic situations in a research project. The two large ovals symbolize the undefined chemical space theoretically accessible in a given
research project. PD space symbolizes the ensemble of all molecules within this chemical space having the required pharmacodynamic profile. PK
space symbolizes the ensemble of all molecules within this chemical space having the required pharmacokinetic behavior. Case A represents the
favorable situation where there is partial overlap of the PD and PK chemical spaces, allowing lead optimization to evolve toward candidates which
combine good PD and PK properties. Case B represents the difficult situation where there is incompatibility between the structural conditions for the
required PD and PK properties, and where a traditional optimization strategy must fail. Here, a prodrug strategy may be the only alternative to bridge
the gap and save the project. The sooner a given research project is found to belong to Case B, the more successful a prodrug strategy may be
(modified from [9]).
useful promoieties include carboxylic acids, amino acids,
and carbonic acids to yield carbamates. Mannich bases
and even imines have also been prepared as potential
prodrugs of amines. Extensive compilations of promoi-
eties have been published [5,6,11].
In contrast to carrier-linked prodrugs, bioprecursors do
not contain a promoiety yet may be activated by hydration
(e.g. lactones such as some statins), oxidation (e.g. prasu-
grel in Figure 3), or reduction (e.g. PA-824 and tirapaza-
mine in Figure 3). This is currently a highly active field in
the search for more selective chemotherapeutic agents
(antibacterial, antiparasitic, or anticancer prodrugs) acti-
vated by reductases [35–41]. Such prodrugs include
nitroarenes, quinones, amidoximes, platinum(IV) com-
plexes, and N-oxides.
ConclusionThe gain in therapeutic benefit provided by prodrugs
relative to the active agent is a question that knows no
general answer. Depending on both the drug and its
prodrug, the therapeutic gain may be negligible, modest,
marked, or even significant. Nevertheless, a trend is
apparent from innumerable data in the literature and
Current Opinion in Chemical Biology 2009, 13:338–344
when comparing marketed drugs and candidates in
R&D. In the case of marketed drugs endowed with useful
qualities but displaying some unwanted property ( post hocdesign), the therapeutic gain is usually modest yet real,
but it will be negligible when the drug defect is tolerable
or barely improved by transformation to a prodrug. In
contrast, it will be marked or significant when much
improved targeting is achieved.
In the case of difficult candidates showing excellent
target properties but suffering from some severe or
intractable physicochemical and/or PK drawbacks (e.g.
high hydrophilicity restricting bioavailability), ad hocdesign may lead to a marked or even significant benefit.
Here indeed, a prodrug form may prove necessary, and its
design should be integrated into the iterative process of
lead optimization. In other words, a prodrug approach is
most fruitful when optimization of a traditional lead
candidate fails because the structural conditions for PD
activity (i.e. the pharmacophore) are incompatible with
the preset pharmaceutical, PK or PD properties. In such
cases, the gap between activity and other drug-like prop-
erties may be of such a nature that only a prodrug strategy
can bridge it (Figure 4).
www.sciencedirect.com
Bridging pharmacodynamic/pharmacokinetic gaps Testa 343
Conflicts of interestNone.
References and recommended readingPapers of particular interest, published within the period of review,have been highlighted as:
� of special interest
�� of outstanding interest
1.�
Testa B, Kramer SD: The biochemistry of drug metabolism —an introduction. Part 5. Metabolism and bioactivity.Chem Biodivers 2009, 6: in press.
An extensive (90 pages), structured and didactic review placing prodrugsin the broader context of metabolic activation and inactivation, toxifica-tion, and detoxification.
2.��
Raito J, Kumpulainen H, Heimbach T, Oliyai R, Oh D, Jarvinen T,Savolainen J: Prodrugs: design and clinical applications. NatRev Drug Discov 2008, 7:255-270.
A well-organized, well-written, and information-rich review on the designof prodrugs and their clinical objectives.
3. Wermuth CG: Designing prodrugs and bioprecursors. In ThePractice of Medicinal Chemistry, edn 3. Edited by Wermuth CG.Amsterdam: Elsevier; 2008:721-746.
4. Testa B: Prodrugs. In Drug Bioavailability — Estimation ofSolubility, Permeability, Absorption and Bioavailability, edn 2.Edited by van de Waterbeemd H, Testa B.Weinheim: Wiley–VCH;2008:559-570.
5. Testa B: Prodrug objectives and design. In ADME-ToxApproaches. Edited by Testa B, van de Waterbeemd H.Volume 5 in Comprehensive Medicinal Chemistry II.Edited by Taylor JB, Triggle DJ. Oxford, UK: Elsevier;2007:1009–1041.
6. Stella VJ, Borchardt RT, Hageman MJ, Oliyai R, Maag H, Tilley JW(Eds): Prodrugs: Challenges and Rewards. Parts 1 and 2. NewYork: Springer; 2007.
7. Smith DA: Do prodrugs deliver? Curr Opin Drug Discov Dev 2007,10:550-559.
8. Testa B: Prodrug research: futile or fertile? Biochem Pharmacol2004, 68:2097-2106.
9. Ettmayer P, Amidon G, Clement B, Testa B: Lessons learnedfrom marketed and investigational prodrugs. J Med Chem2004, 47:2393-2404.
10. Mackman RL, Cihlar T: Prodrug strategies in the design ofnucleoside and nucleotide antiviral therapeutics. Annu RepMed Chem 2004, 39:305-321.
11. Testa B, Mayer JM: Hydrolysis in Drug and Prodrug Metabolism —Chemistry, Biochemistry and Enzymology. Zurich: Wiley–VHCA;2003.
12. Beaumont K, Webster R, Gardner I, Dack K: Design of esterprodrugs to enhance oral absorption of poorly permeablecompounds: challenges to the discovery scientist. Curr DrugMetab 2003, 4:461-485.
13. Wang W, Jiang J, Ballard CE, Wang B: Prodrug approaches inthe improved delivery of peptide drugs. Curr Pharm Des 1999,5:265-287.
14. Lee HJ, You Z, Ko DH, McLean HM: Recent advances inprodrugs and antedrugs. Curr Opin Drug Discov Dev 1998,1:235-244.
15. Bradley DA: Prodrugs for improved CNS delivery. Adv DrugDeliv Rev 1996, 19:171-202.
16. Albert A: Chemical aspects of selective toxicity. Nature 1958,182:421-422.
17. Hayashi Y, Skwarczynski M, Hamada Y, Sohma Y, Kimura T,Kiso Y: A novel approach of water-soluble paclitaxel prodrugwith no auxiliary and no byproduct: Design and synthesis ofisotaxel. J Med Chem 2004, 47:3782-3784.
www.sciencedirect.com
18. Skwarczynski M, Sohma Y, Noguchi M, Kimura M, Hayashi Y,Hamada Y, Kimura T, Kiso Y: No auxiliary, no byproduct strategyfor water-soluble prodrugs of taxoids: scope and limitation ofO–N intramolecular acyl and acyloxy migration reactions.J Med Chem 2005, 48:2655-2666.
19. Amidon GL, Lennernas H, Shah VP, Crison JR: A theoretical basisfor a biopharmaceutic drug classification: the correlation of invitro drug product dissolution and in vivo bioavailability. PharmRes 1995, 12:413-420.
20. Abdel-Magid AF, Maryanoff CA, Mehrman SJ: Synthesis ofinfluenza neuraminidase inhibitors. Curr Opin Drug Disc Dev2001, 4:776-791.
21. Reddy KR, Matelich MC, Ugarkar BG, Gomez-Galeno JE, DaRe J,Ollis K, Sun Z, Craigo W, Colby TJ, Fujitaki JM et al.: Pradefovir: aprodrug that targets adefovir to the liver for the treatment ofhepatitis B. J Med Chem 2008, 51:666-676.
22. Rooseboom M, Commandeur JNM, Vermeulen NPE: Enzyme-catalyzed activation of anticancer prodrugs. Pharmacol Rev2004, 56:53-102.
23. Desmoulin F, Gilard V, Malet-Martino M, Martino R: Metabolismof capecitabine, an oral fluorouracil prodrug: 19F NMR studiesin animal models and human urine. Drug Metab Dispos 2002,30:1221-1229.
24. Pereillo JM, Maftouh M, Andrieu A, Uzabiaga MF, Fedeli O, Savi P,Pascal M, Herbert JM, Maffrand JP, Picard C: Structure andstereochemistry of the active metabolite of clopidogrel. DrugMetab Dispos 2002, 30:1288-1295.
25. Clarke TA, Waskell LA: The metabolism of clopidogrel iscatalyzed by human cytochrome P450 3A and is inhibited byatorvastin. Drug Metab Dispos 2003, 31:53-59.
26.�
Dansette PM, Libraire J, Bertho G, Mansuy D: Metabolicoxidative cleavage of thioesters: evidence for the formation ofsulfenic acid intermediates in the bioactivation of theantithrombotic prodrugs ticlopidine and clopidogrel. ChemRes Toxicol 2009, 22:369-373.
A remarkable research paper revealing new and significant details on theenzymatic mechanism of activation of antiaggregating prodrugs.
27. Farid NA, Smith RL, Gillespie TA, Rash TJ, Blair PE, Kurihara A,Goldberg MJ: The disposition of prasugrel, a novelthienopyridine, in humans. Drug Metab Dispos 2007,35:1096-1104.
28. Williams ET, Jones KO, Ponsler GD, Lowery SM, Perkins EJ,Wrighton SA, Ruterbories KJ, Kazui M, Farid NA: Thebiotransformation of prasugrel, a new thienopyridine prodrug,by the human carboxylesterases 1 and 2. Drug Metab Dispos2008, 36:1227-1232.
29. Riley AB, Tafreshi MJ, Haber SL: Prasugrel: a novel antiplateletagent. Am J Health Syst Pharm 2008, 65:1019-1028.
30.�
Thompson AM, Blaser A, Anderson RF, Shinde SS, Franzblau SG,Ma Z, Denny WA, Palmer BD: Synthesis, reduction potentials,and antitubercular activity of ring A/B analogues of thebioreductive drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824). J Med Chem 2009, 52:637-645.
While nitro aromatic compounds are known since decades as antibac-terial and antiparasitic bioprecursors, this study generates new hopes byapplying the concept to the fight against tuberculosis.
31. Testa B, Kramer SD: The biochemistry of drug metabolism —an introduction. Part 2. Redox reactions and their enzymes.Chem Biodivers 2007, 4:257-405.
32. Anderson RF, Shinde SS, Hay MP, Gamage SA, Denny WA:Activation of 3-amino-1,2,4-benzotriazine 1,4-dioxideantitumor agents to oxidizing species followingtheir one-electron reduction. J Am Chem Soc 2003,125:748-756.
33. Hay MP, Pchalek K, Pruijn FB, Hicks KO, Siim BG, Anderson RF,Shinde SS, Phillips V, Denny WA, Wilson WR: Hypoxia-selective3-alkyl 1,2,4-benzotriazine 1,4-dioxides: the influence ofhydrogen bond donors on extravascular transport andantitumor activity. J Med Chem 2007, 50:6654-6664.
Current Opinion in Chemical Biology 2009, 13:338–344
344 Next-generation therapeutics
34. Solano B, Junnotula V, Marin A, Villar R, Burguete A, Vicente E,Perez-Silanes S, Aldana I, Monge A, Dutta S et al.: Synthesis andbiological evaluation of new 2-arylcarbonyl-3-trifluoromethylquinoxaline 1,4-di-N-oxide derivatives andtheir reduced analogues. J Med Chem 2007, 50:5485-5492.
35. Chen Y, Hu L: Design of anticancer prodrugs for reductiveactivation. Med Res Rev 2009, 29:29-64.
36. Denny WA: Hypoxia-activated anticancer drugs. Expert OpinTher Patents 2005, 15:635-646.
37. Guise CP, Wang AT, Theil A, Bridewell DJ, Wilson WR,Patterson AV: Identification of human reductases that activatethe dinitrobenzamide mustard prodrug PR-104A: a role forNADPH-cytochrome P450 oxidoreductase under hypoxia.Biochem Pharmacol 2007, 74:810-820.
38. Benz CC, Atsriku C, Yau C, Britton D, Schilling B, Gibson BW,Baldwin MA, Scott GK: Novel pathways associated with
Current Opinion in Chemical Biology 2009, 13:338–344
quinone-induced stress in breast cancer cells. Drug Metab Rev2006, 38:601-613.
39. Midgley I, Fitzpatrick K, Taylor LM, Houchen TL, Henderson SJ,Wright SJ, Cybulski ZR, John BA, McBurney A, Boykin DW et al.:Pharmacokinetics and metabolism of the prodrugDB289 (2,5-bis[4-(N-methoxyamidino)phenyl]furanmonomaleate) in rats and monkey and its conversionto the antiprotozoal/antifungal drug DB75(2,5-bis(4-guanylphenyl)furan dihydrochloride).Drug Metab Dispos 2007, 35:955-967.
40. Hall MD, Mellor HR, Callaghan R, Hambley TW: Basis for designand development of platinum(IV) anticancer complexes.J Med Chem 2007, 50:3403-3411.
41. Nemirovski A, Kasherman Y, Tzaraf Y, Gibson D: Reduction ofcis,trans,cis-[PtCl2(OCOCH3)2(NH3)2] by aqueous extracts ofcancer cells. J Med Chem 2007, 50:5554-5556.
www.sciencedirect.com