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Annals of Oncology 18 (Supplement 10): x3x10, 2007

doi:10.1093/annonc/mdm408symposium article

A preclinical review of sunitinib, a multitargeted

receptor tyrosine kinase inhibitor with anti-angiogenic

and antitumour activitiesJ. G. Christensen*Pfizer Global Research and Development, San Diego, CA, USA

Sunitinib malate is an oral, multitargeted tyrosine kinase inhibitor that targets both angiogenic pathways (i.e.,

vascular endothelial growth factor receptor and platelet-derived growth factor receptor) and direct pro-

oncogenic pathways (e.g., stem-cell factor receptor and FMS-like tyrosine kinase-3). Preclinical studies with this

agent have indicated that it exhibits robust inhibitory activity against these targets. Clinical trial results have

demonstrated the therapeutic potential of this agent and have implicated sunitinib targets in the

pathophysiology of malignancies such as renal cell carcinoma and gastrointestinal stromal tumour. This paper

reviews the preclinical data supporting the development of this agent and its translation from benchtop to

bedside. It also highlights the importance of the multiple pathways that may be involved in cancer progression

and the importance of these pathways in selected malignancies.

Key words: anti-angiogenesis, multitargeted, preclinical, sunitinib, TKI

introduction

Sunitinib malate (Sutent, SU11248; Pfizer Inc.) is an oral,multitargeted receptor tyrosine kinase (RTK) inhibitor ofvascular endothelial growth factor receptors (VEGFRs),platelet-derived growth factor receptors (PDGFRs), stem cellfactor receptor (KIT), glial cell-line derived neurotrophic factor

receptor (REarranged during Transfection; RET), FMS-liketyrosine kinase-3 (FLT3) and the receptor for macrophagecolony-stimulating factor (CSF-1R). Sunitinib is approvedmultinationally for the treatment of advanced renal cellcarcinoma (RCC) and imatinib mesylate-resistant or -intolerant gastrointestinal stromal tumour (GIST). Preclinicaland/or clinical studies have implicated these RTKs as keyfactors in the progression of particular tumour types and linkedthe inhibitory effects of sunitinib on these RTKs with itsantitumour activity. In particular, genetic dysregulation of KITor PDGFR-alpha (PDGFR-a) has been associated with thepathogenesis of GIST and dysregulation of VEGFR- andPDGFR-dependent tumour angiogenesis with progression ofclear-cell RCC.

Since angiogenesis is required for continued development ofmost solid tumours, sunitinib inhibition of VEGFRs andPDGFRs expressed by vascular or perivascular endothelial cellsis predicted to produce anti-angiogenic effects on a wide varietyof solid tumours. Sunitinib can also directly inhibit the growthor survival of selected tumour types with dysregulated or

overexpressed RTK targets involved in regulation of cellproliferation or cell survival. This broad capacity of sunitinibfor both anti-angiogenic and direct antitumour effects may bean important feature of its antitumour activity, at least forcertain tumour types.

This review first examines the role of RTKs in differenttumours before exploring the preclinical data supporting the

multitargeted inhibitory activity of sunitinib and itsrelationship to its anti-angiogenic or antitumour activity. Itthen examines the data supporting the activity of sunitinib inthe clinical setting, including an introduction of preliminaryresults from clinical biomarker studies that are consistent withthe underlying mechanisms of the antitumour activity firstdemonstrated with sunitinib preclinically.

the role of RTKs in tumour progression

In recent decades, it has become clear that RTKs are linked tocellular signalling pathways that regulate critical processesimportant for cancer development and growth [1]. At least 60

of these transmembrane signaling proteins have been identifiedin humans. Structurally, RTKs consist of an extracellulardomain that serves as the ligand-binding region,a transmembrane region that traverses the cell plasmamembrane, and an intracellular domain possessing tyrosinekinase activity responsible for mediating downstream signaltransduction. Ligand binding generally causes RTKs todimerize or multimerize, which produces conformationalchanges that facilitate ATP binding and result inautophosphorylation or transphosphorylation of RTK kinasedomains. The resulting phospho-tyrosine residues serve as

*Correspondence to: Dr James G. Christensen, Pfizer Global Research and

Development, 10724 Science Center Drive, San Diego, CA 92121, USA.

Tel: +1-858-638-6336; Fax: +1-858-526-4275; E-mail: [email protected]

2007 European Society for Medical Oncology

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docking sites for adaptor proteins that trigger the intracellularsignalling cascades important for processes such as cellproliferation and survival, migration and metabolism, whichare tightly regulated during normal tissue homoeostasis andmammalian development.

The dysregulation of RTK signalling caused by mutation orectopic receptor or ligand expression has been implicated invarious aspects of tumour progression, including cell

proliferation, survival, angiogenesis and tumour dissemination.Some examples include chromosomal translocations resultingin BCR-ABL fusion proteins in chronic myeloid leukaemia[1, 2], KIT and PDGFR-a mutations in GIST [1, 3], epidermalgrowth factor receptor (EGFR) mutation/amplification in non-smallcell lung cancer (NSCLC) [47], human epidermalgrowth factor receptor 2 (HER-2) amplification in metastaticbreast cancer [8], FLT3 mutations in acute myeloid leukaemia(AML) [9, 10] and RET mutations or translocations in thyroidtumours [11]. Frequently, mutations within an RTK-encodinggene result in expression of a protein product with constitutiveor ligand-independent kinase activity that drives tumourdevelopment. These are often referred to as activating or gain-

in-function mutations and are commonly observed inKIT

orPDGFRA (the gene encoding PDGFR-a) in GIST cells, in whichthey are believed to be critical in the initiation and progressionof these tumours [1, 3].

As described above, the dysregulation of tyrosine kinasesexpressed directly by tumour cells is implicated as a key eventin the pathogenesis of a number of tumour types. However, ithas long been recognized that additional cell types also playcritical roles in tumour progression and metastasis, includingstromal, inflammatory and vascular endothelial cells andpericytes [12]. RTKs located on these cells may play significantroles in cancer development and represent relevant targetsfor anticancer treatment. In particular, VEGFR-2 and PDGFR-b located on vascular endothelial cells and pericytes,

respectively, have been shown to be important in tumour-related angiogenesis [13, 14]. In addition, anti-angiogenictherapies focused on inhibition of the function of VEGFR-2 orits cognate ligand VEGF-A have demonstrated their utility ascancer treatment in a variety of indications either as singleagents and in combination with chemotherapy. Proof ofprinciple of anti-angiogenesis as cancer therapy was firstdemonstrated with bevacizumab in patients with metastaticcolorectal cancer (mCRC). Bevacizumab, a monoclonalantibody directed against VEGF-A, was shown to significantlyprolong survival in patients with mCRC when added to5-fluorouracil-based chemotherapy [15].

Clear-cell RCC represents a highly vascularized tumour

strongly linked with dysregulation of VEGFR and PDGFRactivity [16, 17]. Most clear-cell RCC tumours are associatedwith inactivation of the von HippelLindau gene, leading toconstitutively expressed hypoxia-inducible factor alpha andinduction of VEGFs, PDGFs and other growth factors [18, 19].Excess activity of VEGFR-2 and PDGFR-b due to up-regulationof their respective ligands at least partially accounts for thehighly vascularized nature of these tumours and has beenassociated with RCC progression. Multiple preclinical studies ofinterest have demonstrated that more robust anti-angiogenicand antitumour effects resulted from inhibition of both

VEGFRs and PDGFRs, compared with inhibition of eitherVEGFRs or PDGFRs alone [2023].

In each of the cases mentioned, tumour initiation andprogression has been linked with excessive tyrosine kinaseactivity, be it due to RTK overexpression, mutations leading toligand-independent/constitutive kinase activity oroverexpression of cognate ligands for particular RTKs. Thesedata highlight RTKs (or non-receptor tyrosine kinases) as

potential targets for anticancer therapy and RTK inhibition asan attractive therapeutic strategy. Furthermore, the existingdata indicate that coordinated interaction of multiple cell typesand multiple cellular processes is involved in tumour formationand progression, and therefore point to potential benefits ofmultitargeted anticancer therapy [24]. The data presentedbelow highlight the utility of targeting multiple RTKs involvedin tumour growth and angiogenesis as a therapeutic strategy fortreatment of cancers.

sunitinib inhibition of RTK activity

in vitro

Sunitinib was first identified in a drug discovery programmedesigned to prospectively identify potent and orally bioavailablesmall-molecule inhibitors of VEGF and PDGF receptors.Initial in-vitro biochemical kinase screening assays usingrecombinant proteins demonstrated sunitinib to be a potentinhibitor of VEGFRs (VEGFR-1, -2 and -3) and PDGFR-b, withKi ranging from 0.002 to 0.017 lM [25, 26]. It wasdemonstrated that sunitinib interacts with the ATP-bindingpocket of these kinases and acts as a competitive inhibitor withATP. In-vitro biochemical studies also showed SU12662, themajor metabolite of sunitinib in vivo, to be an inhibitor ofthese RTKs with similar potency. Subsequently, sunitinib kinaseinhibitory activity was characterized across broad panels ofbiochemical assays including more than 150 tyrosine kinases or

serinethreonine kinases [25]. These studies showed thatsunitinib was generally selective for the class III and V RTKsand RET compared with other kinase families. Althougha subset of other kinases also appeared to be inhibited bysunitinib in initial kinase screens, subsequent cell-based assayssuggested that the pharmacologic activity of sunitinib ismediated by class III and V RTKs.

To further characterize and extend the understanding ofsunitinib kinase activity, cellular kinase assays were utilized toexamine its ability to inhibit ligand-dependent RTKphosphorylation in cells expressing these RTK targets. Inparticular, sunitinib was demonstrated to inhibit VEGF-stimulated VEGFR-2 phosphorylation and PDGF-stimulated

PDGFR-b phosphorylation in NIH-3T3 cells expressing thesetargets (IC500.01 lM) [26], stem-cell factor (SCF)-stimulatedphosphorylation of KIT expressed in NCI-H526 human small-cell lung cancer (SCLC) cells (IC50 0.0010.01 lM) [25], FLTligand (FL)-dependent FLT3 phosphorylation in RS4;11 or OS-AML5 AML cells expressing FLT3 (IC50 0.25 lM) [27], andM-CSF-dependent phosphorylation of CSF-1R expressed inNIH-3T3 cells (IC50 0.050.1 lM) [28]. In addition to wild-type RTK targets, cellular assays also examined the ability ofsunitinib to inhibit the tyrosine kinases in mutated RTKs withconstitutive kinase activity. Sunitinib was shown to inhibit

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constitutive FLT3 phosphorylation in MV4;11 AML cellsexpressing FLT-ITD (IC50 0.05 lM) [27] and constitutiveRET phosphorylation in TT human medullary thyroidcarcinoma cells expressing the RET C634W mutant (IC500.05lM) [25].

Of particular note, sunitinib has also been shown to inhibitconstitutive kinase activity associated with mutant forms ofKIT or PDGFR-a. This is of interest because more than 85%

of GISTs harbour mutated and constitutively active forms ofKIT or PDGFR-a receptors, which are believed to be key factorsin the pathogenesis of these tumours [29, 30]. In addition,sensitivity to imatinib (the current first-line treatment forGIST) varies for different mutant isoforms of KIT [29, 31],and acquisition of secondary KIT mutations is associatedwith acquired imatinib resistance [3235]. Sunitinib has beenshown to potently inhibit KIT autophosphorylation in isolatedGIST cells or cells bioengineered to express constitutively activeKIT exon 9 and exon 11 mutants commonly found in naveGIST [3638]. Similarly, Ikezoe and coworkers demonstratedthat sunitinib not only inhibited KIT autophosphorylation innave GIST-T1 cells with activating mutations, but also

inhibited phosphorylation of downstream effectors [39].Importantly, sunitinib also potently inhibited exon 13 andexon 14 mutant variants of KIT (i.e., V654A and T670I),which are associated with reduced binding affinity andresistance to imatinib [3638].

Sunitinib target inhibition has also been demonstrated invivo in mice implanted with tumour cells that gave rise toestablished tumours expressing high levels of PDGFR-b orVEGFR-2. Orally administered sunitinib produced dose- andtime-dependent inhibition of RTK autophosphorylation, asillustrated in Figure 1 [26]. Sunitinib also inhibited PDGFR-bactivity in Colo205 tumour xenografts, which do not directlyexpress PDGFR-b, suggesting that sunitinib was also effective ininhibiting PDGFR-b localized to stromal and perivascular

endothelial cells, where this RTK is commonly expressed [26].Other experiments also demonstrated the ability of orally

administered sunitinib to inhibit RTK targets in vivo, includinginhibition of KIT and PDGFR-b phosphorylation in athymicmice bearing NCI-H526 SCLC tumours [40] and FLT3phosphorylation in athymic mice bearing MV4;11 AMLtumours harbouring constitutively active FLT3-ITD [27]. KIT

is also known to regulate hair pigmentation and furtherevidence of KIT inhibition by sunitinib was provided by theobservation of dose-dependent decrease of hair pigmentationin mice treated with sunitinib [41].

functional consequences of sunitinib

RTK inhibition

The functional consequences of sunitinib target inhibition havebeen examined both in vitro and in vivo with a focus onexploring its anti-angiogenic and antitumour properties.Sunitinib demonstrated the ability to potently inhibit VEGF-A-stimulated proliferation of human umbilical vein endothelialcells (IC50 0.004 lM) [26] and serum-stimulated vasculartube formation of human microvascular endothelial cells (IC500.055 lM) in vitro [25, 42]. Since the ability of endothelial cellsto proliferate and form tube-like structures is critical in theformation of new blood vessels, this data indicated thatsunitinib potentially exhibited anti-angiogenic activity.Sunitinib also potently inhibited VEGF-A-stimulated vascularpermeability in mouse skin, a hallmark of VEGF-dependentangiogenesis, when orally administered to mice [26]. Theinhibition of vascular permeability by sunitinib in vivodemonstrated a strong time- and dose-dependent correlation toinhibition of VEGFR-2 phosphorylation and providedadditional evidence of its anti-angiogenic properties.

Sunitinib was also assessed for its ability to inhibit cellgrowth and proliferation of a variety of tumour cell lines invitro to provide insight into its ability to affect tumoursthrough a direct antiproliferative mechanism. Although themajority of cell lines were not directly affected by sunitinib inthese studies, there were several examples of its directantiproliferative activity. Sunitinib was demonstrated to inhibitligand-independent proliferation of primary GIST cellsexpressing KIT with activating mutations [37], MV4;11 cells

expressing FLT3-ITD mutations [27], and TT humanmedullary thyroid carcinoma cells expressing RET (C634W)mutations [25]. In each case, the sunitinib IC50 for inhibition ofproliferation was similar to the IC50 for inhibition of RTKautophosphorylation. The growth of an additional subset ofother cell lines (e.g., melanoma, glioma and breast cancer) wasalso affected at pharmacologically relevant sunitinib

Figure 1. Sunitinib dose- and time-dependent inhibition of PDGFR-b and VEGFR-2 autophosphorylation in mouse tumour xenograft models. PDGFR-b

or VEGFR-2 was immunoprecipitated from tumour lysates from mice bearing (A) SF767T human glioblastoma tumours (where each lane represents

a separate animal) or (B) A375 human melanoma tumours. Western blots were probed for phosphotyrosine (phospho-PDGFR-b or phospho-VEGFR-2) or

total PDGFR-b or VEGFR-2 protein (reproduced from Mendel et al., 2003 [26]).

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concentrations and the mechanism of growth inhibition iscurrently under investigation [25]. These data generally suggestthat sunitinib can display direct antiproliferative activity againsta subset of tumour cells and that antiproliferative activity isdependent upon presence of constitutively active RTK targets.

Sunitinib has also been examined for antitumour effects ina series of rodent tumour xenograft models using cell linesderived from various human tumours, including lung (NCI-

H460, NCI-H226, NCI-H526, NCI-H82), colorectal (Colo205,HT-29), melanoma (A375, WM-266-4), epidermoid (A431),renal (786-O) and glioblastoma (C6, SF763T) [25, 26]. In eachof these models, orally administered sunitinib significantlyinhibited tumour growth of established tumours, and in severalof the models (Colo205, HT-29, WM-266-4, A431, 786-O,NCI-H226), sunitinib produced marked regression of largeestablished tumours. Figure 2 illustrates regression ofestablished HT-29 and Colo205 tumours during sunitinibtreatment. Since several of the tumour cell lines examined inthis series of studies do not express sunitinib targets, theantitumour effects of sunitinib in these models werepresumably due to the anti-angiogenic mechanism. Orally

administered sunitinib was also shown to produce markedregression of spontaneously arising and established tumours inthe mouse MMTV-v-Ha-ras transgenic mammary model [43]and the DMBA rat carcinogenesis tumour model [25]. It is ofnote that the cytoreductive activity of sunitinib in at least sometumour models pointed to its potential for objective responsesin human cancer patients.

Because its RTK targets are also implicated in the metastaticprogression of cancer, sunitinib was evaluated for its ability toinhibit tumour cell dissemination and metastatic progression inmouse models of experimental metastasis. To evaluate theability of sunitinib to inhibit growth of established tumourmetastases in the bone, the MDA-MB-435HAL-Luc (435/HAL-Luc) mammary carcinoma model, in which bone metastases

can be evaluated by optical imaging, was utilized [28]. In thesestudies, sunitinib demonstrated significant inhibition oftumour growth in bone as measured by photon count andinhibited tumour-induced osteolysis as determined bymeasurement of collagen breakdown products and histological

analysis [28]. Similarly, sunitinib also significantly inhibitedformation of metastatic colonies of disseminated tumour cellsin mouse lung in the B16-F1 melanoma lung colonizationmodel [25]. Collectively, these data suggest that sunitinib maybe effective in treating bone metastases and metastaticprogression of certain human cancers.

Human tumour xenograft models were also utilized toexplore the antitumour mechamism of sunitinib and the role of

its therapeutic targets. Microvessel density (MVD) in sunitinib-and vehicle-treated animals has been used to assess sunitinibinhibition of angiogenesis in a variety of tumour xenograftmodels [22, 26]. As can be seen in Table 1, sunitinib treatmentreduced vascular density in established tumours by roughly70% or more in most models, and only the C6 glioma modelfailed to show significant inhibition with sunitinib versusvehicle. Tumours were grown as subcutaneous xenografts inathymic mice to mean sizes of 300400 mm3. When optimaltumour size was established, vehicle or 40 mg/kg sunitinib wasadministered once daily until the end of the study as indicated.

Furthermore, Osusky and coworkers employed the tumourvascular window model to look at sunitinib impact on in-vivo

neoangiogenesis in C57BL6J mice implanted with Lewis lungcarcinoma cells [42]. Sunitinib reduced vascular permeability,inhibited new blood vessel formation and demonstratedregression of existing vasculature compared with vehicle-treated mice.

Studies were also initiated to determine the relativecontributions of individual RTK targets of sunitinibantitumour and anti-angiogenic activity in various tumourxenograft models [22]. This was accomplished by investigatingthe anti-angiogenic and antitumour effects of agents thatinhibit VEGFR, PDGFR and KIT (sunitinib); PDGFR and KITonly (imatinib); or VEGFR only (SU10944) at dose levelsassociated with complete inhibition of intended targets.Strikingly, in all solid tumour models evaluated, the reduction

of microvessel density and antitumour efficacy of SU10944combined with imatinib was similar to that of single-agentsunitinib and was greatly superior to that of each compoundalone. These data suggest that inhibition of VEGFR, PDGFRand KIT contribute in a cooperative fashion to the antitumour

Figure 2. Regression of large established (A) HT-29 and (B) Colo205 tumour xenografts in athymic mice by sunitinib. Daily oral administration of

sunitinib at 40 mg/kg/day (HT-29) or 80 mg/kg/day (Colo205) was initiated when the tumours reached an average size of 400 mm 3 (HT-29) or 300 mm3

(Colo205) and continued to the end of the experiment. Tumour volume was measured on the indicated days, with the mean tumour volume indicated for

groups of 8 (treated) or 16 (vehicle control) animals (mean 6 SEM) [25, 26].



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and anti-angiogenic effects of sunitinib, at least in the tumourmodels examined.

A recent non-clinical study by Yao and coworkers [23] alsosupports the conclusion that dual inhibition of VEGFR andPDGFR with sunitinib is associated with greater anti-angiogenic effects compared with selective inhibition of VEGFRor PDGFR alone. Yao et al. compared the differential effectsof sunitinib or AG-028262 (a selective VEGFR inhibitor) and

CP-673,451 (a selective PDGFR inhibitor) on tumourendothelial cells or pericytes in pancreatic islet tumours fromRIP-Tag2 transgenic mice at dose levels associated withcomplete inhibition of intended targets. In these studies,sunitinib treatment was associated with a 75% reduction in thedensity per unit area of endothelial cells and a 63% reduction inthat of pericytes (Figure 3). In contrast, AG-028262 alonereduced the tumour endothelial cell density (61%) but had noeffect on the tumour pericyte population (+0.6%), while CP-673,451 alone reduced pericyte density (50%) but had noeffect on endothelial cells (5%). The combination of AG-028262 plus CP-673,451 treatment had comparable effects tosunitinib on both endothelial cells and pericytes. These resultssuggest that sunitinib causes regression of endothelial cells andpericytes by inhibiting VEGFR and PDGFR, respectively,and produces more comprehensive effects on the tumourvasculature and supporting cells than agents that target eitherVEGFR or PDGFR alone.

Based on collective non-clinical data, the potent efficacy ofsunitinib probably results from simultaneous inhibition ofindividual target receptors expressed both in cancer cells and intumour neovasculature. These data support the hypothesis thatsimultaneous inhibition of multiple critical targets thatcooperate in tumour progression by a multitargeted agentresults in robust antitumour activity and may recapitulate thecumulative antitumour efficacy of multiple single-targetinhibitors.

translating preclinical findings to the

clinical setting

Preclinical findings have been useful in identifying clinicaltumour types most likely to respond to sunitinib therapy. Inaddition, in-vivo tumour modelling studies have establishedpharmacokinetic/pharmacodynamic (PK/PD) parameters forsunitinib antitumour activity that have utility for designingoptimal clinical dosing regimens. The results of dose-responseand dose-schedule studies in preclinical models indicated that

40 mg/kg/day was a fully efficacious dose and that full efficacywas observed when a target plasma level for sunitinib plusSU12662 (its primary active metabolite) of 50 ng/mL wasmaintained for at least half of the daily dosing interval [25]. Inthe clinical setting, a dose-ranging pharmacokinetic trial ofpatients with advanced solid tumours identified 50 mg/day(administered in 6-week treatment cycles of 4 weeks on and2 weeks off) as the recommended sunitinib dose, based on thedemonstration of clinical efficacy and acceptable tolerability[44]. This trial demonstrated that a dose of 50 mg/day resultedin plasma levels exceeding the minimum target total-drug

plasma levels of 50 ng/mL, with moderate interpatientvariability and a long half-life for the full dosing interval.

Clinical trials have also demonstrated that sunitinib inhibitsits intended RTK targets in tumour cell populations at plasmaconcentrations of50 ng/mL. In a phase I study of AMLpatients, sunitinib was found to inhibit FLT3 and FLT-ITDphosphorylation in a dose-dependent manner, providingevidence of anti-FLT3 activity in patients [45]. Clinical efficacyand evidence of target inhibition by sunitinib was alsodemonstrated in patients with GIST or RCC. As discussedearlier and also in more detail in the following articles,

Table 1. Effect of sunitinib on neoangiogenesis of established tumour xenografts (adapted from Potapova et al., 2006 [22])

Tumour model Treatment day MVD Percent inhibition P value

Vehicle Sunitinib

C6 glioma 12 24.6 31.8

786-O renal 14 106.2 25.3 76 0.027

WM-266-4 melanoma 29 43.2 13.7 68 0.001

NCI-H226 lung 14 74.2 8.4 89 0.012

SF763T gliomaa

13 39.3 24.2 38

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activating mutations ofKITor PDGFRA underlie most cases ofGIST, and VEGFR- and PDGFR-dependent angiogenesis isimplicated in progression of RCC. Given the potent inhibitionof each of these RTKs by sunitinib in preclinical studies, GISTand RCC patients were chosen for sunitinib proof-of-conceptefficacy studies.

Clinical efficacy of sunitinib in patients with imatinib-resistant GIST was demonstrated in an open-label phase I/II

trial and a placebo-controlled phase III trial [38, 46]. BecauseKIT mutations are implicated in the pathogenesis of themajority of GIST cases, additional studies were initiated as partof the phase I/II trial to demonstrate the inhibition of KIT bysunitinib in tumour tissue. Immunohistochemical analyses oftumour biopsies from sunitinib-treated patients in the phaseI/II trial demonstrated near-complete inhibition of KIT activity1 week after initiation of treatment (G. Demetri, personalcommunication). In addition, a recent study reported thatGIST patients who demonstrated objective responses tosunitinib exhibited a sustained decline in plasma levels ofsoluble KIT (sKIT), while no trend was observed in patientswithout objective responses [25]. Additional analyses from the

phase I/II trial also point to the importance of KIT inhibitionfor sunitinib GIST activity. In particular, Heinrich andcoworkers showed that median progression-free survival withsunitinib was significantly longer in patients with imatinib-resistant GISTs who had primary/secondaryKIT mutationshighly sensitive to sunitinib inhibition in vitro than in patientswhose tumours contained less sensitive KIT mutations [38].This observation is the strongest evidence linking theimportance of KIT inhibition to clinical benefit in GISTpatients treated with sunitinib.

The anti-angiogenic effects of sunitinib have also beenimplicated in its activity in GIST. Patients with GISTcontaining wild-type KIT and PDGFRA who progressed duringimatinib treatment demonstrated evidence of sunitinib clinical

benefit (median overall survival 30.5 months), potentiallyimplicating other sunitinib targets or anti-angiogenic activity inthese patients [47]. A pharmacodynamic analysis of PDGFR-band VEGFR-2 inhibition and apoptosis from the phase I/II trialshowed decreases in PDGFR-b and VEGFR-2 phosphorylationand increases in endothelial and tumour cell apoptosis relativeto baseline in tumours from patients showing clinical benefitwith sunitinib; that is, partial responses or stable disease greaterthan 6 months [48, 49]. In contrast, levels of PDGFR-b andVEGFR-2 phosphorylation were higher and apoptosis wasunchanged in GISTs from patients without clinical benefit orwith progressive disease.

Anti-angiogenic effects have also been linked with sunitinib

activity in RCC. Sunitinib efficacy in patients with RCCrefractory to cytokine-based therapy was demonstrated in twophase II trials [17, 50] as well as in previously untreatedpatients in a phase III trial [51]. In one of the phase II trials,Motzer and coworkers reported that plasma levels of VEGF-Aand placental growth factor (PIGF) significantly increased inRCC patients during sunitinib therapy and tended to return tonear-baseline during off-treatment periods [17]. In addition,sunitinib treatment was associated with decreases in solubleVEGFR-2 (sVEGFR-2) and sKIT [52]. VEGF and sVEGFR-2levels were modulated to a greater extent in patients exhibiting

objective responses relative to those exhibiting progressive orstable disease [52]. Sunitinib-related increases in plasmaVEGF and decreases in plasma sVEGFR-2 have also beenreported in patients with metastatic breast cancer [53] andpatients with neuroendocrine tumour [54]. Although theexact mechanisms of modulation of VEGF-A and sVEGFR-2are unknown, these data provide evidence that sunitinib isinhibiting the target VEGFRs in patients, since similar

changes are observed clinically for other inhibitors of theVEGF pathway. Sunitinib treatment also produced decreasesin plasma sKIT levels in metastatic breast cancer patients,and decreases of more than 50% at the end of cycle 2 weresignificantly associated with treatment-related improvementsin time to progression and survival [53]. A more completeoverview of the data with respect to biomarkers will bediscussed in the following article.

conclusions

Sunitinib is an inhibitor of multiple class III and V RTKs,which are implicated in the pathogenesis of various cancers,including GIST and RCC. Preclinical studies demonstrated the

selective inhibition of target RTKs and linked RTK inhibitionwith anti-angiogenic and direct antitumour effects in in-vitrocellular assays or in-vivo tumour models. Subsequently, clinicaltrial results demonstrated the efficacy of sunitinib in patientswith several different tumour types. In addition, clinical datawere found to be consistent with the antitumour mechanismsof action of sunitinib demonstrated in preclinical studies.Future goals include understanding the molecular basis of drugsensitivity in selected populations, identifying clinically relevantbiomarkers (discussed in more detail in the following article)and determining optimal use in combination with otheranticancer therapies.

disclosuresEditorial support was provided by ACUMED and was fundedby Pfizer Inc. Dr Christensen is an employee of Pfizer Inc.,and a holder of stock in Pfizer Inc.

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