Inhibition of the Fibroblast Growth Factor Pathway and Antiangiogenesis

Published OnlineFirst September 27, 2011; DOI: 10.1158/1078-0432.CCR-11-0659

Clinical Cancer Review Research

Beyond VEGF: Inhibition of the Fibroblast Growth Factor Pathway and Antiangiogenesis

Christopher Lieu1, John Heymach2, Michael Overman1, Hai Tran2, and Scott Kopetz1

Abstract Fibroblast growth factor (FGF) signaling regulates cell proliferation, differentiation, survival, angiogenesis, and wound healing. Compelling evidence for deregulated FGF signaling in tumorigenesis continues to emerge, and a growing body of research suggests that FGF may also play an integral role in the resistance to anti-VEGF therapy. Although agents targeting FGF signaling are early in development, the potential to target both the VEGF and FGF pathways may translate into improvements in the clinical care of cancer patients. Clin Cancer Res; 17(19); 1–10. 2011 AACR.

Introduction FGFs in the development of resistance to anti-VEGF therapy. Finally, FGF as a potential therapeutic target is Fibroblast growth factors (FGF) belong to a structurally reviewed, including the current state of drug development related family of 22 molecules that interact with high- in this field. affinity tyrosine kinase FGF receptors (FGFR) to regulate multiple fundamental pathways and cellular behaviors (1). Fibroblast Growth Factor Family FGF signaling regulates events in embryonal development, including mesenchymal–epithelial signaling and the de- FGFs were first isolated as mitogens from pituitary velopment of multiple organ systems (2, 3). FGF signaling extracts and bovine brain tissue in the 1970s (9, 10). Since also regulates cell proliferation, differentiation, and surviv- initial discovery, 22 members of the FGF family have been al, as well as angiogenesis and wound healing (4). In some identified, all of which are structurally related signaling situations, FGFs can act as a negative regulator of prolifer- molecules ranging from 17 to 34 kDa in size (11). Some ation and positive regulator of differentiation (5). Given FGFs seem to be expressed exclusively during embryonic the role of FGF in multiple cellular processes, compelling development (FGF-3, 4, 8, 15, 17, and 19), and others are evidence for deregulated FGF signaling in tumorigenesis expressed in both embryonic and adult tissues (FGF-1, 2, continues to emerge (6). Studies with animal models have 5–7, 9–14, 16, 18, and 20–23; ref. 11). suggested that aberrant FGF signaling can promote tumor FGFs are structurally similar to one another, with 28 development through increased cell proliferation and sur- highly conserved and 6 identical amino acid residues (12). vival, as well as increased tumor angiogenesis (7, 8). This Most FGFs are secreted glycoproteins (FGF-3–8, 10, 15, 17– body of research indicates that FGF signaling is an impor- 19, and 21–23), but FGF-1 and FGF-2 are exported from tant pathway in tumorigenesis and tumor angiogenesis and cells by mechanisms that remain unclear. FGF-1 and FGF-2 that appropriate targeting of the FGFR tyrosine kinases may may be released from damaged cells or by exocytosis that is be an effective therapeutic intervention for cancer. independent of the endoplasmic reticulum–Golgi pathway This review describes the FGF family of growth factors (13). FGFs have strong affinities for the glycosaminoglycan and their associated receptors, as well as their association side chains of cell surface proteoglycans, which help to with tumor development, growth, and metastasis. We also sequester FGFs on the surface of the secreting cell or on describe the effect of FGFs on tumor angiogenesis, includ- nearby cells, and in the binding of FGF to the FGFR. ing key preclinical and clinical evidence about the role of FGFRs are transmembrane tyrosine kinases that contain 2 or 3 extracellular immunoglobulin-like domains and an extracellular heparin-binding sequence (14–16). The 2 Authors' Affiliations: Departments of 1Gastrointestinal Medical Oncology proximal immunoglobulin-like extracellular domains bind 2 and Thoracic/Head and Neck Medical Oncology, The University of Texas the FGF ligand, whereas the heparin-binding sequence MD Anderson Cancer Center, Houston, Texas binds a glycosaminoglycan moiety, resulting in the forma- Corresponding Author: Scott Kopetz, Department of Gastrointestinal Medical Oncology, The University of Texas MD Anderson Cancer Center, tion of a complex containing 2 FGFs, 2 FGFRs, and the 1515 Holcombe Boulevard, Unit 0426, Houston, TX 77030. Phone: 713- glycosaminoglycan moiety (17). Only 4 FGFRs are known 792-2828; Fax: 713-563-6764; E-mail: [email protected] to exist, designated FGFR-1 through FGFR-4 (Table 1). doi: 10.1158/1078-0432.CCR-11-0659 Their specificity for various ligands is altered by processes 2011 American Association for Cancer Research. that create multiple isoforms. Alternative mRNA splicing of

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Translational Relevance Fibroblast Growth Factor and Tumor Growth

Angiogenic inhibitors have shown promise in the Aberrant FGF and/or FGFR signaling has a wide range of treatment of multiple tumor types and have improved effects and can involve both tumor cells and the surround- outcomes in patients with metastatic disease. However, ing stroma. These effects include cellular proliferation; many patients will not respond to antiangiogenic ther- resistance to cell death; increased motility and invasiveness; apy, and for those that do, resistance develops quickly. increased angiogenesis; enhanced metastasis; and resis- Fibroblast growth factor (FGF) seems to play an integral tance to chemotherapy (27). Deregulation of FGF signaling role in the resistance to antiangiogenic therapy, and in tumorigenesis can result from activating mutations, agents that specifically target the FGF pathway are being FGFR gene amplifications, and chromosomal transloca- developed and tested in clinical trials. It is critical that tions, which lead to increased autocrine and paracrine investigators review the existing literature on FGF and signaling (28–30). Because aberrant signaling can promote tumor angiogenesis to understand the importance of tumorigenesis by affecting major downstream biologic this target in various malignancies, as well as to design processes, FGFs have been implicated in multiple tumor rational clinical trials that can expedite a potentially types, including prostate, astrocytoma, breast, lung, blad- meaningful therapy in patients previously treated with der, hepatocellular, and colon cancers (31–41). antiangiogenic agents. Alternative splicing of the FGFR gene has also been implicated in carcinogenesis. Carcinomas have been observed to switch expression of FGFR to the mesenchymal the FGFR gene significantly alters the ligand specificity by isoforms, enabling cells to receive signals usually restricted changing the sequence of the carboxy-terminal half of to the connective tissue, such as ectopic expression of immunoglobulin-domain III. This sequence change results typically stroma-localized FGFR1-IIIc (42, 43). The IIIc in 2 isoforms termed IIIb and IIIc (18, 19). In general, IIIb splice variant has been detected in colon carcinoma but isoforms are expressed on epithelial cells, and IIIc isoforms not in adenoma-derived cell lines, suggesting that it is are expressed on mesenchymal cells, where they mediate upregulated during tumor progression (44). During pros- tissue interactions in normal development and wound tate cancer progression, alternative splicing of FGFR2 can healing (20–22). occur in epithelial cells, leading to isoform changes from FGF signaling can also be modulated by several mem- FGFR2-IIIb into the more ligand-promiscuous FGFR2-IIIc brane proteins, including cell adhesion molecules of the (Fig. 1; refs. 8, 43). Thus, this aberrant alternative splicing cadherin and immunoglobulin superfamilies (23). The of FGFR potentially destroys the balanced interdependence best characterized of these is neural cell adhesion molecule between stroma and epithelium (8, 43, 45). (NCAM). NCAM has been shown to be a major regulator of Clinical studies of FGF have suggested a critical role of FGF-FGFR interaction in several cell types, including non- FGF signaling in clinical tumor progression. In 1 study of neural tissues (24). NCAM has been shown to act as a advanced serous ovarian adenocarcinomas, FGF-1 mRNA nonconventional ligand that can induce a specific FGFR1- copy number was found to be significantly correlated with mediated cellular migration that is different from the DNA copy number and protein expression levels (46). Both response elicited by FGF-2 (25). Furthermore, NCAM ex- FGF-1 mRNA and protein levels were associated with worse pression has been shown to reduce FGF-stimulated extra- overall survival, possibly secondary to an increase in tumor cellular signal regulated kinase (ERK)1/2 activation, cell angiogenesis and autocrine stimulation of cancer cells. In proliferation, and cell-matrix adhesion in fibroblast cell another study of 100 resected colorectal cancer surgical lines, suggesting that NCAM acts as a novel control mech- specimens, high FGF-2 expression was associated with anism for FGFR signaling (26). increased grade, stage, lymphovascular invasion, and intratumoral microvessel density (47). Significantly elevated levels of FGF-2 in circulation were found in a small cohort of patients with metastatic colorectal cancer, relative to Table 1. Receptors for the FGFR family median levels in control subjects (48). In other studies, although increased levels of FGF-2 were found in plasma Receptor Specific For from patients with different types of cancer, it was unclear whether the higher levels resulted from tumor invasion and FGFR-1b FGF-1, 2, 3, and 10 degradation of the extracellular matrix (ECM), liberating FGFR-1c FGF-1, 2, 4, 5, and 6 FGF to function as a paracrine growth factor, or whether FGFR-2b FGF-1, 3, 7, and 10 tumor cells induced FGF2 release from stromal inflamma- FGFR-2c FGF-1, 2, 4, 6, and 9 tory infiltrate (49, 50). FGFR-3b FGF-1 and 9 Loss of expression of NCAM has also been implicated in FGFR-3c FGF-1, 2, 4, 8, and 9 the progression of tumor metastasis. NCAM has been FGFR-4 FGF-1, 2, 4, 6, 8, and 9 shown to modulate neurite outgrowth and matrix adhesion b Adapted from Itoh and Ornitz (1). of cells from the Rip1-Tag2 transgenic mouse model of pancreatic neuroendocrine tumors by assembling a FGFR-4

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Inhibition of the FGF Pathway and Antiangiogenic Therapy

Stroma Normal Epithelium Stroma Cancer Epithelium FGFR ECM FGFR ECM lllb lllc FGFR lllc lllc

PDGF

FGF

FGF FGF-BP FGF FGF VEGF Ang2

Blood Blood FGFR lllc NRP1 PDGFR vessel vessel FGFR lllc

Figure 1. Changes in FGF and FGFR in the microenvironment and epithelium in normal tissue and cancer. Compared with normal tissue, alternative splicing of the FGFR gene can switch expression of FGFR from the IIIb isoform to the ligand promiscuous IIIc isoforms. Furthermore, release of FGF from the tumor and stroma may initiate tumor angiogenesis. FGF-BP–mediated reduction of the affinity of FGF-2 to heparin can induce its release from the ECM. FGF-BP, FGF binding protein; PDGF, platelet-derived growth factor; PDGFR, platelet-derived growth factor receptor. signaling complex that leads to the modulation of b1- lagen gel, VEGF and FGF-2 were able to induce cells to integrin–mediated cell-matrix adhesion (51). Ablation of invade the underlying matrix to form capillary-like tubules the expression of NCAM in this mouse model resulted in (60). Interestingly, FGF-2 was found to be twice as potent the disruption of the tumor tissue architecture, tumor- as VEGF in this model. The critical role of FGF in tumor associated lymphangiogenesis, and lymph node metastasis angiogenesis was shown through the interference of FGF (51, 52). function with a recombinant adenovirus expressing soluble Though a full review of FGF signaling and tumor growth FGFR (AdsFGFR). AdsFGFR seemed to impair the mainte- is beyond the scope of this article, please see the compre- nance of tumor angiogenesis, in contrast to inhibition of hensive review from Turner and Grose for additional the VEGF pathway that predominantly affected the initia- information (41). tion of tumor angiogenesis (61). Injection of AdsFGFR in the Rip1-Tag2 murine model of pancreatic neuroendocrine Fibroblast Growth Factors and Angiogenesis tumor resulted in repression of tumor growth, with a significant decrease in tumor vessel density. FGFs exert Angiogenesis, the formation of new blood vessels from their effects by modulating proliferation and migration the endothelium of the existing vasculature, plays a pivotal of endothelial cells, production of proteases, and promo- role in tumor growth, progression, and metastasis (53, 54). tion of integrin and cadherin receptor expression (62). Along with embryogenesis, angiogenesis was one of the In particular, FGF-1 and FGF-2 directly affect tumor first areas where FGF signaling was shown to be important angiogenesis by promoting the cellular proliferation of (55, 56). In particular, FGF-1 and FGF-2 were found to be endothelial cells. FGF-2 has been shown to induce revas- important for new blood vessel growth at the site of an cularization in various experimental models in several excision wound and have been shown to stimulate angio- species, including rabbit, chicken, and mouse (63–65). genesis in various assays (57–59). FGF-2 is a very potent In a murine model, FGF-1 knockout mice and FGF-1/ inducer of angiogenesis. In a preclinical model of micro- FGF-2 double-knockout mice had poor wound healing vascular endothelial cells grown on a 3-dimensional col- compared with normal control mice (58). FGFR-1 is most

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commonly expressed on endothelial cells, although FGFR- esis than does FGF (50). In 1 study, the angiogenic response 2 has been shown to be present under certain circum- occurred more quickly when VEGF and FGF-2 were added stances (50, 66). The presence of FGFR-3 and FGFR-4 simultaneously to microvascular endothelial cells on 3- has not been reported in the endothelium. dimensional collagen gels than the response to either cyto- FGFs can exert their angiogenic mechanism of action via kine alone, suggesting synergy between the 2 cytokines (60). paracrine signaling through their release by tumor and It has also been shown that induction of FGF-2–induced stromal cells, or through their mobilization from the angiogenesis is partly dependent on the activation of VEGF ECM (50). FGF-1, in particular, may stimulate endothelial and the presence of endogenous VEGF and VEGF-C (76). cell growth and the motility of endothelial cells through Evidence for synergy between VEGF and FGF-2 has also been paracrine signaling, which results in increased prolifera- shown in xenograft models expressing FGF-2 and VEGF in tion, survival, and motility of endothelial cells (46). FGF-2 tumor cell transfectants. Simultaneous expression of FGF-2 has been studied in xenograft models in which liposome- and VEGF resulted in fast-growing lesions with high blood mediated gene transfer was used to deliver episomal vectors vessel density and permeability (77). Interestingly, inhibi- containing antisense FGF-2 or FGFR-1 cDNAs into human tion of FGF-2 production caused a significant decrease in melanomas grown in nude mice. In contrast to the stim- tumor burden with a concomitant decrease in blood vessel ulation seen during deregulated FGF signaling, tumors density and size. This differed from the effects of VEGF injected with these antisense constructs showed arrested inhibition, which caused a decrease in pericyte organization, tumor growth or reduced tumor density because of blocked vessel patency, and permeability. This finding suggests that intratumoral angiogenesis (67). There is also evidence that FGF-2 and VEGF stimulate angiogenesis synergistically but FGF-2 increases the expression of angiopoietin-2, a potent with different effects on vessel size and function. regulator of vascular branching and angiogenesis, as well as FGF-2 may also have indirect effects on VEGF pathways. VEGF in cultured human and endothelial cells (68). In a breast cancer cell line (T47D), FGF-2 was found to FGFs may also exert their effects through autocrine activate hypoxia-induced VEGF release through augmenta- signaling in endothelial cells, stimulating the proliferation tion of the phosphoinositide 3-kinase pathway, as well as of new capillary endothelial cells (66). This induces a hypoxia-inducible factor-1a expression (78). FGF-2 has proangiogenic state that creates an environment favorable also been shown to indirectly enhance the effect of VEGF for vascular growth. Evidence for this growth is seen in a through upregulation of neurolipin-1 (NRP-1), a corecep- particular subtype of Kaposi sarcoma (KS) in which FGF-2, tor for VEGF. FGF-2 increased NRP-1 levels and enhanced produced by the KS cells, has been found to synergize with the migration of human vascular smooth muscle cells in human immunodeficiency virus type 1 Tat protein to response to VEGF. enhance endothelial cell growth and type IV collagenase There is also evidence of cross-talk between FGF-2 and expression (66, 69). These tumors were found to have platelet-derived growth factor (PDGF)–BB. FGF-2 and increased aggressiveness and frequency compared with PDGF-BB together have been shown to synergistically other forms of KS. promote tumor angiogenesis and pulmonary metastasis, Preclinical models of tumor angiogenesis have suggested as well as formation of high-density primitive vascular that FGF–binding protein (FGF-BP) serves as an angiogenic plexuses coated with pericytes and vascular smooth muscle switch molecule and can be rate limiting for tumor growth cells (79). Pericyte recruitment to coat nascent vessels (70). FGF-2 is present in most tissues under normal phys- during angiogenesis is essential for the stabilization and iologic conditions but is inactivated by being tightly bound further establishment of a tumor’s vascular network. FGF-2 to the ECM. FGF-BP binds to FGF-2 in a dose-dependent seems to upregulate PDGF receptor (PDGFR) expression, manner, which results in a marked reduction of the affinity which causes increased responsiveness to PDGF-BB, and of FGF-2 to heparin and release from the ECM (Fig. 1; refs. PDGF-BB–treated vascular smooth muscle cells become 71, 72). Exogenously administered FGF-BP has been shown responsive to FGF-2 signaling through upregulation of to stimulate FGF-2–dependent growth of both adrenal FGFR-1. Interestingly, overexpression of PDGF-BB alone carcinoma and prostate carcinoma cells. In addition, down- in tumor cells was shown to result in decreased recruitment regulation of FGF-BP expression has been shown to inhibit of pericytes and dissociation of vascular smooth muscle angiogenesis in squamous cell carcinoma xenografts (73). cells. In tumor cells subjected to prior upregulation of FGF-2 caused by VEGF receptor 2 (VEGFR2) inhibition, Fibroblast Growth Factor–2 Cross-Talk with PDGFR-b inhibition also attenuated the expression of Angiogenic Factors FGF-2 (80). In a mouse model of cervical carcinogenesis, pharmacologic blockade of PDGFR signaling with imatinib VEGF plays an integral role in the development of new slowed progression of premalignant cervical lesions and blood vessels in tumor formation, and angiogenesis inhi- impaired the growth of preexisting invasive carcinomas bitors targeting the VEGF pathway have proved to be effi- through suppression of the expression of FGF-2 and cacious in preclinical cancer models and in clinical trials (74, FGF-7, which resulted in decreased proliferation and an- 75). A significant amount of cross-talk is thought to exist giogenesis within the lesions (81). Treatment with an FGF between members of the VEGF family and FGF-2 during trap also impaired the angiogenic phenotype similar to the angiogenesis, with VEGF appearing earlier during angiogen- blockade of the PDGFR with imatinib.

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Fibroblast Growth Factor Signaling and baseline levels of VEGF. In 1 xenograft model of renal cell Resistance to Anti-VEGF Therapy carcinoma, tumors unresponsive to sunitinib treatment were sensitive to a dual inhibitor of the VEGF and FGFRs Emerging evidence has suggested that upregulation of (85). This inhibitor seemed to reduce blood vessel density FGF and FGFR may serve as a mechanism of resistance to through inhibition of FGF-2–induced angiogenesis. anti-VEGF therapy. Preclinical trials of anti-VEGF therapy Clinical evidence in colon cancer also supports the role in a murine pancreatic neuroendocrine tumor model of FGF-2 in resistance to bevacizumab-containing regi- (PNET), Rip1-Tag2, showed that tumors with an initial mens. To determine the dynamic changes in circulating response to VEGFR2 blockade expressed higher levels of factors during the emergence of therapeutic resistance, FGF-2 at the time of progression compared with stable profiles of cytokines and angiogenic factors were assessed tumors (82). When the tumors were first treated with a in 40 patients receiving a regimen of 5-fluorouracil (5-FU), VEGFR inhibitor alone and subsequently treated at the leucovorin, and irinotecan (FOLFIRI) plus bevacizumab. peak of response with an FGF trap, the combination Levels were assessed after a single dose of bevacizumab and attenuated revascularization and slowed tumor growth FOLFIRI plus bevacizumab, at the time of progression, and (Fig. 2). The dual tyrosine kinase inhibitor (TKI) of VEGFR at the nadir of radiographic response (86, 87). The inves- and FGFR, brivanib, has also been studied in this murine tigators showed that FGF-2 levels were elevated immedi- model and has proved to be efficacious following the ately prior to progression compared with baseline levels. failure of 2 VEGFR inhibitors (83). It was even more Strikingly, 30% of patients had an increase in FGF-2 to 10- effective when used in the frontline setting, suggesting that fold the upper limit of normal, suggesting significant dual inhibition of VEGFR and FGFR was able to delay heterogeneity in the FGF-2 response. In 1 study of neoad- induction of evasive resistance. Further evidence for the juvant bevacizumab with 5-FU and radiation in rectal role of alternative angiogenic factors in tumor escape from cancer, the administration of a short regimen of bevacizu- angiogenesis inhibitors has been shown in tumors in which mab in combination with chemotherapy had no significant growth was impaired by ectopic expression of the endog- effect on plasma FGF-2 levels, suggesting that elevations in enous angiogenesis inhibitors thrombospondin-1, tumsta- FGF-2 are instead a later event temporally associated with tin, and endostatin (84). In that study, renal carcinoma the development of resistance to the bevacizumab combi- cells transfected with thrombospondin-1 resisted angio- nation regimen (88). genesis inhibition by upregulating the proangiogenic fac- Elevated FGF-2 levels have also been shown in glioblas- tors FGF-2, angiopoietin 2, and PDGF-A, despite high toma patients treated with a VEGFR small molecule

160

140

120 Figure 2. VEGFR2 blockade

) transiently stops tumor growth 3 * and decreases vascularity, 100 followed by tumor progression, reinduction of angiogenesis, and reestablishment of the tumor 80 vasculature. Quantification of overall tumor burden plotted as average and SD per animal at the 60 time of initiation of treatment

Tumor burden (mm burden Tumor (t ¼ 0; purple bar), after treatment for 10 days (orange bars), and at 40 4 weeks of age (teal and green bars) for the different treatment arms. Averages from cohorts of 8 20 to 10 mice per group are plotted with their respective SD bars. (Adapted from Cancer Cell, with 0 permission from Elsevier; ref. 82.) Control Control Anti-R2 Anti-R2 Anti-R2 FGF-trap t = 0 10 days 4 weeks

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inhibitor (89). In patients who experienced tumor progres- from binding, and it had in vitro activity against lung and sion while receiving treatment, increased tumor enhance- renalcellcarcinoma(92). ment volume was associated with significant increases in The safety of FGFR inhibitors currently under develop- plasma levels of FGF-2 and stromal cell–derived factor-1. A ment seems acceptable. Preclinical toxicity studies with statistically significant positive correlation was observed these agents have not shown a reproducible class effect, between FGF-2 levels and tumor vessel size measured by aside from hypertension, which is difficult to attribute to MRI. This work provided further clinical evidence that FGF- the VEGFR or FGFR inhibitory effects of many of the agents 2 may play a role in tumor relapse in patients treated with (93, 94). Monoclonal antibodies directed against FGFR1- anti-VEGF agents (89). IIIc resulted in anorexia in mice and monkeys, which was Unlike preclinical models, clinical studies investigating caused by an FGF signaling blockade in the hypothalamus, levels of FGF can only show an association of elevated and a similar toxicity was seen in 1 phase I study (95, 96). FGF and/or FGFR with the resistance to anti-VEGF ther- Encouragingly, the concerns based on preclinical studies apy; they cannot show causation or an actual resistance about phosphate and vitamin D metabolism have not been mechanism. Furthermore, because most studies are con- reflected in the reported toxicities in the early-phase studies ducted in combination with cytotoxic chemotherapy, it is (97). difficult to discern whether the patient and tumor have Although a separate development strategy is being pur- become resistant to the chemotherapy, the antiangiogenic sued based on somatic mutation or amplification of FGFR therapy, or both. Therefore, high-quality preclinical mod- (reviewed in ref. 98), antiangiogenesis strategies targeting els will be essential for further investigation to provide a the tumor microenvironment pose additional drug devel- sound preclinical rationale for future clinical studies. opment strategies. Prior studies of anti-VEGF antibodies have shown clinical utility when the agents are used in Targeting Fibroblast Growth Factor combination with cytotoxic therapy and minimal activity as single agents with rare exceptions (75, 99). Similarly, Because FGFs have a clearly defined role in tumor FGF inhibition alone is unlikely to show a significant angiogenesis, as well as a possible role in resistance to benefit in various tumor types. Combination strategies current VEGF inhibitors, several FGF pathway inhibitors of FGF/FGFR inhibition with traditional cytotoxic therapy are currently under development. The inhibitors entering in tumor types in which proliferation and angiogenesis is the clinic are either TKIs or soluble antibody fragments FGF/FGFR–dependent will have the highest likelihood of that block ligand binding and receptor dimerization achieving a clinical benefit. (Table 2). Most of the TKIs have dual specificity for FGFR The timing of FGF/FGFR–directed therapy is also an area and VEGFR, both as a matter of design and because of the of emerging interest given the evolving evidence that the structural similarities in their kinase domains. However, FGF axis may serve as an adaptive resistance mechanism to the inhibitors vary in their relative potency for VEGFR and anti-VEGF therapy. Histopathologic evidence from a mu- FGFR, with many inhibitors having higher potency for rine PNET model suggests that the switch to an FGFR VEGFR than FGFR (90, 91). Targeting both kinases may inhibitor during treatment with a VEGFR inhibitor may also increase the toxicity associated with these com- be more effective at the time of early revascularization prior pounds, although biologically active doses have been to detectable tumor growth (83). Future clinical trials will shown with most of the compounds (41). The other class help determine whether dual inhibition of VEGFR and of compound in development, FGF ligand trap, has the FGFR will be more effective in the front-line setting or potential to block the interaction of multiple FGF ligands whether FGFR inhibition will be better instituted at the with the soluble FGFR fragments. In 1 study, such an time prior to or at progression after treatment with VEGF/ FGFR-1 antagonist was shown to prevent FGFR-1 ligands VEGFR inhibitors. However, determination of biologic

Table 2. Current FGF and FGFR-targeting agents

Drug Manufacturer Target(s) Clinical development stage

TKI258 (dovitinib; ref. 96) Novartis FGFR, PDGFR, VEGFR Phase II BIBF 1120 (Vargatef; ref. 104) Boehringer Ingelheim FGFR, PDGFR, VEGFR Phase III BMS-582664 (brivanib; ref. 105) Bristol-Myers Squibb FGFR and VEGFR Phase II and III E7080 (106) Eisai FGFR, PDGFR, VEGFR Phase I TSU-68 (SU6668; ref. 107) Taiho Pharmaceutical FGFR, PDGFR, VEGFR Phase I and II AZD4547 (108) AstraZeneca FGFR Phase I FP-1039 (109) Five Prime Therapeutics FGF ligand trap (multiple FGFs) Phase I BGJ398 (110) Novartis FGFR Phase I Astex FGFR inhibitor (111) Janssen Oncology FGFR Preclinical

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resistance to VEGF/VEGFR inhibitors is difficult in the also play a large role in angiogenesis, which is a funda- clinic, as resistance to the cytotoxic component of the mental step in the transition from a dormant to a malig- treatment may be sufficient to induce tumor growth in nant state, and FGF acts synergistically with VEGF to the absence of single-agent anti-VEGF activity. Determining increase tumor blood vessel growth and maturation. Evi- resistance to these inhibitors is a high hurdle for develop- dence is also increasing that FGF may be part of the ment of such alternate antiangiogenic therapies, and sev- mechanism of resistance to anti-VEGF agents. eral strategies are being considered in the early clinical Although agents targeting FGF signaling are still early development, including randomized discontinuation stud- in development, the potential to target both the VEGF ies and enrichment designs. and FGF pathways is near, and the combination or Biomarkers under consideration for FGF inhibitors in- sequential inhibition of these 2 critical angiogenic path- clude plasma levels of FGF family members and increased ways may translate into improvements in the clinical care FGF-axis expression in the stroma; however, their use as of cancer patients. Successful strategies will depend on surrogates for FGF/FGFR–dependent tumors remains un- appropriate selection of tumors and patients in whom clear. Integration of imaging or biomarkers of antiangio- FGF inhibition is mostly likely to inhibit angiogenesis genesis therapy likewise has hurdles of cost reproducibility and proliferation. and lack of correlation with clinical efficacy, and these approaches are not routinely integrated into most clinical Disclosure of Potential Conflicts of Interest development strategies of these agents (100, 101). Based on the VEGF inhibitor experiences, radiographic endpoints S. Kopetz: commercial research grant, AstraZeneca. The other authors disclosed no potential conflicts of interest. with antiangiogenesis strategies will be difficult, and ulti- mately, the benefits of these agents will be measured by Grant Support their ability to improve overall survival (102, 103). NIH T32 CA-009666 (C. Lieu); National Research Service Award (NRSA) Summary Research Training Grant; Conquer Cancer Foundation Young Investigators Award (C. Lieu); NIH CA-136980 (S. Kopetz); NIH/National Cancer Institute (NCI) Cancer Center Support Grant (CCSG) Core Grant CA-106672. FGFs are a family of growth factors that are involved in many pathways that can contribute to carcinogenesis and Received March 18, 2011; revised July 20, 2011; accepted July 25, 2011; affect cellular proliferation, migration, and survival. They published OnlineFirst September 27, 2011.

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OF10 Clin Cancer Res; 17(19) October 1, 2011 Clinical Cancer Research

Beyond VEGF: Inhibition of the Fibroblast Growth Factor Pathway and Antiangiogenesis

Christopher Lieu, John Heymach, Michael Overman, et al.

Clin Cancer Res Published OnlineFirst September 27, 2011.

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