The rise of the FGFR inhibitor in advanced biliary cancer: the next cover of time magazine?

Introduction

Cholangiocarcinomas (CCA) are heterogeneous epithelial tumors arising from the biliary tree with features of cholangiocyte differentiation (1). CCA is the most common primary biliary tract malignancy and the second most common primary hepatic malignancy (1,2). Overall, CCAs account for 3% of all gastrointestinal malignancies (2). The incidence of CCA has increased over the past three decades; however, the 5-year survival remains dismal at approximately 10% (3).

Based on their anatomic location within the biliary tree, CCAs are classified into intrahepatic CCA (iCCA), perihilar CCA (pCCA), and distal CCA (dCCA) subtypes (4). pCCA arises from the large bile ducts in the hepatic hilum. Proximally, pCCA is separated anatomically from the iCCA by the second-order bile ducts, and distally the cystic duct insertion serves as the point of distinction between pCCA and dCCA (5). In addition to having different anatomic origins, these subtypes also have distinct epidemiology, risk factors, pathogenesis, and treatment (1). pCCA is the most common subtype; in a large series of 564 patients with biliary tract cancer, 50% had pCCA, 42% had dCCA, and 8% had iCCA (6).

The prognosis of CCA is dismal with medial survival of less than 2 years in patients with disease not amenable to surgical resection (7). Potentially curative surgery/transplantation is an option only in early-stage disease. Patients with iCCA have the best outcome following surgical resection; the 5-year survival after R0 resection was 63% for iCCA, 30% for dCCA, and only 27% for pCCA (6). Neoadjuvant chemoradiation followed by liver transplantation is a consideration for a subset of pCCA patients who meet stringent criteria (1). In carefully selected patients, this approach has resulted in 2- and 5-year recurrence free survival rates of 78% and 65%, respectively (8).

For patients with advanced disease who are not candidates for either surgical resection or liver transplantation, systemic chemotherapy with gemcitabine and cisplatin is the practice standard (9). The combination of gemcitabine-cisplatin confers a modest survival advantage with an overall median survival of 11.7 months compared to 8.1 months with gemcitabine alone (9).

CCAs are heterogeneous tumors and medical therapy tailored to the features of each individual tumor is a promising approach. However, no targeted molecular therapies have been approved for use in biliary tract cancer. Genetic factors in CCA include chromosomal aberrations, genetic and epigenetic alterations in tumor suppressor genes and oncogenes. A recent molecular characterization of 260 biliary tract cancers identified 32 significantly altered genes, including potentially targetable genetic alterations in 40% of cases (10). Mutations in ARID1B, ELF3, PRKACA and PRKACAB occurred preferentially in pCCA and dCCA, whereas, mutations in isocitrate dehydrogenase 1 and 2 (IDH 1/2), EPHA2 and BAP1 were more frequently noted in iCCAs (10). Fibroblast growth factor receptor (FGFR) gene fusions were noted exclusively in iCCAs. KRAS, SMAD4, ARID1A, and GNAS mutations were noted in all three CCA subtypes (10). KRAS and SMAD4 mutations were noted in a whole-exome sequencing analysis of eight liver-fluke related CCAs (11). This study identified 206 mutations in 187 genes including TP53 mutations in 44.4% of cases, KRAS mutations in 16.7% of cases, SMAD4 in 16.7% of cases. In addition, somatic mutations in ten newly implicated genes including MLL3, ROBO2, GNAS, and RNF43 were identified (11). In another recent whole-exome sequencing analysis, TP53 mutations were noted to occur more frequently in liver fluke related CCA, while somatic mutations in BAP1 and ARID1A, IDH1, and IDH2 occurred with increased frequency in non-liver fluke related CCA (12). BAP1 and ARID1A are chromatin-remodeling genes and inactivating mutations of these genes were also identified in exome sequencing of 32 iCCAs (13).

Deregulation of growth factor tyrosine kinases, noted in various malignancies including CCA, plays a critical role in tumor initiation and progression. These include the FGFR pathway, ERBB family of receptor tyrosine kinases including epidermal growth factor receptor (EGFR), and hepatocyte growth factor (HGF) receptor. EGFR activation leads to activation of p44/42 mitogen-activated protein kinases (MAPKs), which has a well-established oncogenic role (14). ERBB2 overexpression, another ERBB family member, has been associated with development of biliary tract cancer in preclinical studies (15). HGF, a stroma-derived paracrine mediator and ligand for the MET receptor, promotes tumor invasiveness and metastasis (16,17). Aberrant overexpression of HGF and MET occurs in CCA and is associated with a poor prognosis (18,19).

Fibroblast growth factor (FGF) pathway

The FGF pathway consists of 22 human FGFs and four transmembrane receptor tyrosine kinases, FGFR 1–4 (20-22). FGF signaling is involved in a myriad of biological processes including regulation of developmental pathways and mesodermal patterning of the embryo, physiological functions such as regulation of angiogenesis and wound repair, and regulation of essential cell behaviors including proliferation, differentiation, survival, migration, and angiogenesis (23,24). The FGF-FGFR axis is activated with binding of FGF to FGFR and heparin sulphate proteoglycan in a specific complex on the surface of the cell (25). In this complex, a central heparin molecule links two FGFs into a dimer that bridges two FGFR chains (25). FGFR dimerization is homo-dimer driven. Once formed, this complex activates the FGFR tyrosine kinase with resultant autophosphorylation of tyrosines in the C-terminus, kinase insert, and juxtamembrane region. Phospho-FGFR then phosphorylates adapter proteins including FGFR substrate 2 and 3. This leads to activation of various intracellular signaling cascades involved in promotion of cell survival and proliferation including Ras-MAPK, phosphatidylinositol 3-kinase (PI3K)-protein kinase Akt/protein kinase B pathways, p90 ribosomal protein S6 kinase 2, signal transducers and activators of transcription, and Src (24,26).

The ubiquitous role of FGF signaling in various biological processes integral to cell survival increases susceptibility to oncogenic transformation with aberrant FGF signaling (24). Deregulated FGF signaling mediates carcinogenesis by enhancing cellular proliferation, migration, survival, and invasion and promoting tumor angiogenesis (24). Several different mechanisms underlie the oncogenic potential of FGF signaling. Genomic alteration of FGFR can result from activating mutations, receptor gene amplifications, and chromosomal translocations (24). Intragenic translocations can lead to formation of a fusion protein consisting of a transcription factor fused to an FGFR kinase domain with consequent FGFR dimerization and activation (24,27,28). Genomic aberrations lead to ligand-independent FGF signaling. Ligand-dependent signaling can also promote tumor formation and progression through autocrine or paracrine production of ligand from cancer or stromal cells respectively (24). Lastly, tumor angiogenesis is augmented by cross-talk between FGF ligands, vascular endothelial growth factors, and inflammatory mediators in the tumor stroma. Preclinical studies have demonstrated that FGFs and VEGFs can act in a synergistic manner to enhance tumor angiogenesis (29).

FGFR signaling and gene fusions in CCA

Aberrant FGF signaling has also been implicated in CCA carcinogenesis. FGFR 1, 2, and 4 are upregulated in CCA cell lines (30). An autocrine, feed-forward pathway between the oncogenic hippo signaling pathway and FGFR signaling has recently been reported. In this pathway, yes-associated protein (YAP), a transcriptional co-activator in oncogenic hippo signaling, upregulates FGFR 1, 2 and 4 (30). In turn, FGFR2 stimulation by FGF5 upregulates YAP (30). Pan-FGFR inhibition with BGJ398 promoted cell death in CCA cells and significantly reduced tumor burden in a YAP-driven genetic murine model of CCA as well as a patient-derived xenograft model with enhanced YAP expression (30). These observations indicate that FGFR inhibition has strong therapeutic potential in a subset of CCAs with activation of oncogenic hippo signaling. FGF has also been shown to promote migration of CCA cells via phosphorylation of MEK 1/2 (31). An angiogenic role of FGF signaling has also been described in CCA cells (32). Abundant FGFR4 expression occurs in all three subtypes and is associated with a poor prognosis of pCCA and iCCA (33). Specifically, FGFR4 has been reported to induce proliferation, invasion and epithelial-mesenchymal transition of CCA cells (33). Accordingly, FGFR inhibition with AP24354 promoted apoptosis while inhibiting CCA cell proliferation and invasion (33).

Gene sequencing enables discovery of “actionable” therapeutic target genes in patients with malignancy (34). Identification of “driver” target mutations may have a higher therapeutic impact. Gene fusions are an important class of driver mutations that play a vital role in certain cancer (34). For instance, the BCR-ABL fusion gene mutation is critical in chronic myeloid leukemia, and these patients tend to respond very well to imatinib, a small-molecule kinase inhibitor. The significant role of gene fusions/translocations in epithelial cancers has become increasingly evident. Several recent studies have identified FGFR2 gene fusions in CCA (34-38). Wu et al. reported the FGFR-BICCI gene fusion in two cases of CCA (34). Subsequently, in a cohort of 66 iCCA cases, the FGFR2 gene fusion was detected in 13.6% of cases (38). In addition to reporting several FGFR-2BICC1 fusions, a novel gene fusion, FGFR2-AHCYL1, was also identified in this series (38). KRAS and BRAF mutations were not present in CCA patients with FGFR2 gene translocations, signifying the potential of these gene fusions as driver mutations. A similar prevalence of FGFR2 gene fusions in iCCA was reported in a North American cohort; utilizing fluorescence in situ hybridization, Graham et al. identified FGFR2 gene fusions in 12/96 (13%) iCCA cases (36). Integrated genome-wide and whole transcriptome sequence analyses in six patients with advanced CCA identified recurrent translocations events involving the FGFR2 locus in three of six patients (37). Genomic profiling in another cohort of iCCAs identified FGFR2 gene fusions in 3 of 28 cases, including one case each of FGFR-BICC1 fusion and FGFR2-TACC3 fusion, and a novel fusion, FGFR2-KIAA 1598 (35).

Wu et al. proposed that oligomerization may serve as the common mechanism of activation of FGFR fusion proteins as a majority of the FGFR fusion partners had domains with dimerization motifs (34). Indeed, mechanistic studies demonstrated that the fusion domains provided by the FGFR binding partners interacted in vitro as oligomerization domains, supporting the notion that these partners mediate oligomerization which initiate activation of the respective FGFR kinase in tumors with the FGFR translocations (34).

Characteristics of CCA patients with FGFR2 gene fusions

From a histological standpoint, prominent intraductal cancer growth and anastomosing tubular glands with desmoplasia are present in iCCAs harboring FGFR2 translocations (36). The majority of the tumors with FGFR2 gene fusions had significantly diminished expression of cytokeratin 19, a typical marker of pancreaticobiliary tumors (36). Interestingly, the presence of FGFR2 gene translocations was associated with enhanced survival (123 vs. 37 months) in a North American study, suggesting that the presence of FGFR2 gene fusions may have prognostic utility (36). Moreover, CCA patients with FGFR2 gene fusions were younger (median age of 52 vs. 65 years) with a female preponderance (13% vs. 4%). However, in an Asian cohort of CCA patients with FGFR2 gene fusions, no survival or gender differences were noted (38). An association between hepatitis B and C virus infection and cases with FGFR2 translocations was noted in this latter cohort, whereas such an association was absent in the North American cohort. The geographic differences with distinct risk factors and etiologies likely account for the discrepancies in patient characteristics and outcomes between these two studies. FGFR2 translocations were noted in patients with de novo CCA rather than those with preexisting primary sclerosing cholangitis (PSC) (36). The only CCA subtype in which FGFR2 gene fusions have been identified is the iCCA subtype. Thus, these etiological associations are not surprising as risk factors such as hepatitis B and C are typically associated with iCCA, whereas preexisting PSC is noted mainly in pCCA patients.

In several of the cohorts, FGFR2 gene fusions were detected in resected, surgical specimens, indicating an earlier clinical stage (39). This suggests that FGFR2 translocations are an early event in carcinogenesis, and hence may serve as driver events in CCA (39).

FGFR as a therapeutic target in CCA

FGFR-selective small molecule kinase inhibitors (SMKIs)

Preclinical studies have demonstrated that only cells harboring the FGFR gene fusions were sensitive to FGFR inhibitors (34,38), indicating a role for targeted FGFR kinase inhibition in tumors harboring these fusions. Indeed, FGFR genetic aberrations appear to be the most important predictor for sensitivity to NVP-BGJ398, a pan-FGFR inhibitor (40). In vitro expression of the FGFR2 fusion kinases activates the MAPK pathway and promotes anchorage-independent growth (38). Moreover, subcutaneous transplantation of fusion kinase expressing cells resulted in tumorigenesis in vivo. These effects have been attributed to the kinase activity of the fusion kinases (38). Hence, therapeutic targeting of FGFR with SMKIs has emerged as a promising individualized approach in patients harboring FGFR2 fusions (Table 1). NVP-BGJ398 and ARQ 087 are SMKIs with specificity against FGFRs. Preclinical studies have demonstrated suppression of downstream MAPK signaling and oncogenic activity of FGFR fusion kinases with these FGFR-selective SMKIs (38). Clinical efficacy of BGJ398 is currently being investigated in a phase II study of adult patients with advanced or metastatic CCA with FGFR gene fusions or another FGFR genetic aberration (Table 1). ARQ 087 is an FGFR inhibitor with potent activity against FGFR1, 2, and 3. It is currently under clinical investigation in a phase I trial of patients with advanced solid tumors with FGFR genetic alterations including patients with iCCA harboring FGFR2 gene fusions (Table 1). JNJ-42756493 is an oral pan-FGFR selective SMKI (41). In a phase I trial of solid tumors with genetic FGFR aberrations including amplifications, mutations, and translocations, JNJ-42756493 therapy resulted in four confirmed responses and one unconfirmed partial response in patients with glioblastoma and urothelial and endometrial cancer, while 16 patients had stable disease (41). In this early phase study, the 36 patients without known FGFR alterations did not have any significant response (41). TAS-120 is a small molecule selective irreversible FGFR inhibitor being investigated in a phase I trial of patients with advanced solid tumors or multiple myeloma with or without FGF/FGFR-related abnormalities (Table 2). AZD4547 is a pan-FGFR inhibitor with demonstrated efficacy against several FGRF-driven cancers in preclinical studies (42). AZD4547 is currently being investigated in several clinical trials.

Table 1 FGFR-selective small molecule kinase inhibitors
Full table

Table 2 FGFR non-selective small molecule kinase inhibitors
Full table

CH5183284/Debio 1347 is an orally available inhibitor with selectivity against FGFR1, FGFR2, and FGFR3. CH5183284/Debio 1347 demonstrated antitumor activity against cancer cell with various FGFR genetic alterations in preclinical models including cell lines and xenograft models (43). CH5183284/Debio 1347 is currently under phase I clinical investigation in solid organ malignancy patients whose tumors have a genetic aberration of FGFR1, 2, or 3 (Table 2).

Non-selective SMKIs

Directed therapy with ponatinib, a non-selective pan-FGFR inhibitor, decreased serum levels of carbohydrate antigen 19-9 (CA 19-9), a CCA tumor marker, and induced tumor necrosis in an advanced iCCA patient with the FGFR-MGEA5 fusion (37). Further evidence of antitumor activity of FGFR inhibition was noted in an FGFR-TACC3 fusion-positive patient initially treated with pazopanib, another non-selective SMKI, with some tumor regression. After progression on pazopanib, ponatinib therapy resulted in stabilization of disease in this patient with advanced CCA (37). This promising preliminary evidence has served as the premise for several clinical studies prospectively investigating SMKIs of FGFR. Ponatinib is being assessed in a phase II study of advanced biliary cancer harboring FGFR2 gene fusions detected by either next-generation sequencing or break-apart FISH (Table 2). Another ongoing phase II trial is assessing the efficacy of ponatinib in advanced malignancies including CCA with genetic aberrations of FGFR including mutations, fusions, and/or amplifications (Table 2). Taking into account the heterogeneous nature of CCA, one can envision the advantage of combinatorial therapeutics. KRAS mutations are a frequent occurrence in CCA and one of the downstream effector pathways of KRAS is the Raf/MEK/ERK pathway (2). A phase II study of the MEK inhibitor, trametinib, in advanced biliary tract cancer revealed some benefit although most patients had progressive disease (ClinicalTrials.gov identifier: NCT01943864). The combination of pazopanib and trametinib is currently being assessed in a phase I trial of patients with advanced solid tumors including CCA (Table 2).

Antibody therapy

Monoclonal antibodies directed against FGFR2 have exhibited tumor suppressive potential in preclinical CCA models (44). The FGFR genes encode multiple structural variants through tissue-specific splicing. The FGFR2-IIIb isoform is found selectively in epithelial cells, whereas the FGFR2-IIIc isoform has selectivity for mesenchymal cells (45). RNASeq data from one study detected the FGFR2-IIIb isoform in all identified fusions (37). The FGFR2-IIIB isoform has been shown to have binding specificity for the FGF7 and FGF10 ligands. This selectivity has clinical significance as FGFR2-IIIb specific antibodies would avoid the off-target effects of SMKIs and hence would represent an attractive chemotherapeutic option in fusion-positive cases of advanced CCA. The combination of a FGFR2-IIIb specific monoclonal antibody and an FGFR SMKI would be another attractive option, one with the potential of attaining more complete FGFR signaling inhibition (39). FPA144 is a humanized monoclonal antibody directed against the FGFR2b isoform. FPA144 has demonstrated robust efficacy in preclinical tumor models of gastric cancer and is currently being assessed in a phase I clinical trial in patients with advanced solid tumors with FGFR2b overexpression or FGFR2 amplification (ClinicalTrials.gov identifier: NCT02318329).

Heat shock protein inhibitors

Heat shock protein 90 (HSP90) is a molecular chaperone which regulates the maturation and functional stability of a myriad of cellular proteins, including key regulators of cell proliferation, differentiation, and survival (46). Cancer cells can subvert this essential regulatory function to facilitate malignant transformation (46). Selective targeting of HSP90 using small molecule inhibitors has been shown to be a valid chemotherapeutic approach in fusion-driven lung cancer and represents an alternative to direct kinase inhibition (47). HSP90 and its co-chaperone CDC37 are essential in the stability of many oncogenic proteins. FGFR1OP2 gene encodes a protein of unknown function known as FGFR1 oncogene partner 2 (FOP2). The fusion protein FOP2-FGFR1 has constitutive tyrosine kinase activity. HSP90-CDC37 forms a permanent complex with FOP2-FGFR1 which protects the fusion protein from degradation and holds it in a permanently active conformation in a leukemic cell line (48). Inhibition of HSP90 function also reduces the signaling capacity of FGFR3 and induces its degradation (49). In bladder cancer harboring the FGFR3-TACC3 fusion, ganetespib, a selective HSP90 inhibitor, induced loss of expression of the fusion protein and inhibited several oncogenic signaling proteins, and induction of apoptosis (50). The combination of ganetespib and BGJ398 had enhanced efficacy compared to either agent alone. These data suggest that HSP90 inhibition may not only be an alternative but a potentially complementary approach to kinase inhibition in fusion-drive malignancies (50).

Future directions

FGFR inhibition has emerged as a viable therapeutic option in advanced biliary tract cancer particularly in patients with FGFR2 gene fusions. However, despite the purported FGFR selectivity of several newly developed inhibitors, multi-kinase activity of these inhibitors has been observed (51). The resultant off-target toxicities are a significant limitation. Future efforts should be directed at development of FGFR specific kinase inhibitors (and even FGFR2 selective SMKIs for patients with FGFR2 fusions) with minimal activity against other kinases such as vascular endothelial growth factor receptors and platelet derived growth factor receptors. Accordingly, further studies are needed to assess the efficacy of combinatorial approaches which would accomplish more complete blockade of the FGFR signaling axis. For instance, the combination of a FGFR specific monoclonal antibody and FGFR SMKI has the potential to attain comprehensive blockade of FGFR signaling. Approaches which would target FGFR signaling as well as pathways downstream of FGFR such as the PI3K-Akt-mTOR pathway also hold significant promise. Further work is also needed to assess the antitumor potential of HSP90-CDC37 inhibition in biliary tract cancer. Currently, the focus of novel clinical investigation in CCA is directed primarily at molecular aberrations present in subsets of CCA patients. However, dual-target inhibition appears to be a more viable approach than targeting individual molecular aberrations given the heterogeneous nature of CCA.

Acknowledgements

We thank Ms. Courtney Hoover for excellent secretarial support.

Funding: This work was supported by National Institutes of Health grants T32DK007198 (SI Ilyas), K12 CA90628 (MJ Borad), NIH/NCI 1DP2CA195764 (MJ Borad) and the Mayo Foundation.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

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