TAS-120

Futibatinib, an investigational agent for the treatment of intrahepatic cholangiocarcinoma: evidence to date and future perspectives

Alessandro Rizzo, Angela Dalia Ricci, Giovanni Brandi
1. Department of Experimental, Diagnostic and Specialty Medicine, S. Orsola-Malpighi University Hospital, Bologna, Italy.

Abstract
Introduction
Biliary tract cancers (BTCs) are poor prognosis malignancies usually classified in intrahepatic cholangiocarcinoma (iCCA), extrahepatic cholangiocarcinoma, gallbladder cancer, and ampulla of Vater cancer. In the last few years, novel treatment targets have been identified in iCCA patients, including fibroblast growth factor receptor (FGFR) aberrations; thus, several FGFR inhibitors are currently being developed, some of which have already suggested interesting efficacy and adequate safety in phase I and phase II trials regarding refractory iCCA.
Areas covered
This review provides an overview regarding the current scenario of FGFR2 targeted therapies in iCCA, especially focusing on the mechanism of action and clinical development of futibatinib (TAS-120), a highly selective irreversible pan-FGFR antagonist. According to the interim analysis of the FOENIX-CCA2 trial, whose results have been presented at the 2020 American Society of Clinical Oncology (ASCO) Annual Meeting, futibatinib could lead to meaningful benefit in patients affected by previously treated iCCA with FGFR2 gene fusions or other rearrangements.
Expert opinion
Because of its promising and durable activity, futibatinib has the potential to become a novel therapeutic option in the treatment of iCCAs harboring FGFR2 aberrations. Further studies are warranted to confirm the efficacy of this investigational molecule.

Introduction
Biliary tract cancers (BTCs), including intrahepatic cholangiocarcinoma (iCCA), extrahepatic cholangiocarcinoma (eCCA), gallbladder cancer (GBC), and ampulla of Vater cancer (AVC), represent the most commonly diagnosed primary biliary malignancy, accounting for approximately 3% of all gastrointestinal cancers [1, 2]. Although iCCA, eCCA, GBC, and AVC are grouped in the category of BTC, each anatomical subgroup presents unique cancer biology, with different molecular features and treatment options [3]. Despite traditionally considered rare tumors, BTC incidence and mortality have risen in the last few decades, perhaps associated with the increasing incidence of iCCA [4]. Radical resection is the mainstay of the cure for BTC, but unfortunately, the majority of patients is diagnosed with unresectable, locally advanced, or metastatic disease [5]; moreover, although the development of more aggressive surgical approaches and techniques have improved the chance of achieving radical surgical resection, there is a high relapse rate in BTC patients undergoing potentially-curative surgery [6]. Cisplatin plus gemcitabine has been established as the standard of care first-line regimen for patients with unresectable BTC following the results of the ABC-02 phase III randomized trial [7]. In this pivotal study, patients with metastatic BTC were randomized to receive the combination of cisplatin plus gemcitabine orgemcitabine single-agent as front-line treatment, with the doublet showing a statistically significant overall survival (OS) advantage over gemcitabine monotherapy (11.0 months versus 8.1 months; HR 0.64; 95% CI, 0.52-0.80; P < 0.001), which was consistent across distinct anatomical subgroups– including iCCA (HR 0.61; 95% CI 0.41-0.91). Nonetheless, the prognosis of patients affected by advanced or metastatic BTC remains unsatisfactory, with 5-year survival of less than 5% and a median OS inferior to 12 months [8, 9]. In the last few years, several genomic studies have begun to unveil the molecular landscape of BTC, uncovering several potentially actionable aberrations [10]. Thus, comprehensive genomic profiling is playing an increasing role in BTC, especially in iCCA [11]. Recent reports have suggested that nearly 50% of iCCAs could contain an actionable alteration, with fibroblast growth factor (FGF) signaling pathway representing one of the most promising targets for iCCA [12]. And recently, the role of FGFR-targeted therapies has been tested in a number of clinical trials and various agents have been evaluated or are currently under investigation, including multitarget tyrosine kinase inhibitors as well as specific anti-FGFR2 antibodies [13, 14]. Unfortunately, as reported in other driver mutations, the development of secondary polyclonal mutations in the FGFR2 kinase domain represents an important issue as a major mechanism of tumor progression and treatment resistance in patients with FGFR2 fusion-positive iCCA [15]. Thus, novel agents able to overcome resistance to ATP-competitive FGFR inhibitors are currently under development; among these, futibatinib - also named TAS-120 - recently attracted a lot of attention, with results of an interim analysis of the phase II FOENIX-CCA2 trial which have been presented at the ASCO 2020 Virtual Meeting (Box 1) [16]. This review will focus on current evidence regarding futibatinib, with a particular focus on recently published data and ongoing trials aimed at assessing the role of this novel agent in iCCA patients harboring FGFR2 fusions. Overview of the market Despite recent advancements in medical oncology, the prognosis of unresectable iCCA remains dismal, with limited treatment options currently available in this patient population [17]. Nonetheless, recent years have witnessed the identification of novel therapeutic targets including FGFR2 fusions and isocitrate dehydrogenase (IDH)-1 and IDH-2 mutations [18]. In particular, the recent phase III ClarIDHy trial comparing ivosidenib versus placebo in patients with advanced BTC with IDH1 mutations has evidenced a median PFS of 2.7 months for patients receiving ivosidenib versus 1.4 months with placebo (HR 0.37; 95% CI 0.25-0.54; p<0.001) [18]; moreover, results showed a superior median OS in patients treated with ivosidenib (10.8 months versus 9.7 months; HR 0.69, 1-sided p=0.06). Additionally, several other pathways and targets are currently being evaluated, including neurotropic tyrosine kinase receptor (NTRK) fusions, BRAF mutations, and human epidermal growth factor receptor (HER) [19]. FGFR2 fusion represents the most frequently observed FGFR aberration in iCCA - assessed by FISH or NGS - with a prevalence ranging from 6 to 25% and a mutual exclusivity with KRAS/BRAF mutations [20-27]. Interestingly, FGFR2 fusion-positive iCCA constitutes a distinct molecular subtype of BTC from an immunohistochemical, pathological and clinical point of view since several reports suggested an association with female predilection, younger age at onset and a more favorable prognosis compared to wild-type patients [21, 22]. In the last few years, several trials have evaluated the role of FGFR inhibitors (e.g. Debio 1347, infigratinib, derazantinib, erdafitinib, pemigatinib), which are rapidly emerging as novel therapeutic options in iCCAs harboring FGFR aberrations (Table 1) [23]. Infigratinib has been the first FGFR inhibitor to show interesting response rates and a manageable safety profile in a phase II trial on 61 iCCA patients with FGFR alterations whose disease progressed after gemcitabine-based first-line chemotherapy [24]. According to the results of this study, among 48 patients with FGFR2 fusions infigratinib has been reported to have an ORR and a DCR of 18.8% and 83.3%, respectively. In another phase II study, derazantinib (ARQ 087) hasshown similar levels of activity, with an ORR of 20.7%, a DCR of 82.7% and a median PFS of 5.7 months [25]. More recently, in April 2020 the US FDA approved the potent inhibitor of FGFR1, FGFR2, and FGFR3 pemigatinib in previously treated metastatic cholangiocarcinoma with FGFR2 fusions or rearrangements [26]. The accelerated approval of pemigatinib has followed the results of the FIGHT-202 trial, which assessed the role of the FGFR inhibitor in three cohorts of patients harboring different aberrations: 1) FGFR2 fusions or rearrangements; 2) other FGF/FGFR alterations; 3) no FGF/FGFR alterations [27]. In this open-label, international, phase II study, objective responses were observed in 38 out of 107 patients (35.5%) with FGFR2 fusions or rearrangements receiving pemigatinib, with a median PFS of 6.9 months and a median OS of 21.1 months. Conversely, no responses were detected in the two cohorts of patients with other FGF/FGFR alterations (n=20) and without FGF/FGFR mutations (n=18). In FIGHT-202, the safety profile of pemigatinib was manageable and similar to that reported in other trials on FGFR inhibitors, with hyperphosphataemia as the most frequently observed adverse event. Thus, pemigatinib has represented the first approved targeted treatment for iCCA in the US, something which has inaugurated the precision medicine era in BTC. Although pemigatinib showed interesting, durable responses in iCCA patients harboring FGFR2 fusions, the onset of acquired resistance has represented and still represents a major issue. In fact, the comprehension of the mechanisms underlying the resistance to FGFR inhibitors and the development of novel agents able to overcome acquired resistance is an unmet clinical need in a highly aggressive disease such as iCCA [28]. Recent studies have suggested that biopsy samples and sequencing of cell-free DNA collected at baseline and following disease progression evidenced the presence of polyclonal secondary mutations in the FGFR2 kinase domain, including the presence of the FGFR2 V564F mutation [28]. Understanding the profile of non-responding patients, together with a more comprehensive identification of mechanisms of resistance is a mandatory need in this setting. Mechanism of action FGFR pathway has been involved in the modulation of a myriad of biological processes including cell survival, proliferation, differentiation and angiogenesis [29]. Four transmembrane receptor tyrosine kinases (FGFR1, FGFR2, FGFR3 and FGFR4) and 22 ligands constitute the FGF-FGFR axis; when FGF ligands are released, the glycoproteins are able to bind to the monomeric receptor triggering dimerization and autophosphorylation of the intracellular domain [30]. Interestingly, the next step is represented by the activation of several intracellular signaling cascades including the RAS-dependent mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3KCA)/Akt/mTOR, phospholipase Cγ (PLCγ) and JAK/STAT (Figure 1) [31]. On the basis of this physiological activity, it is readily apparent that molecular aberrations of FGFR signaling (e.g. mutations, rearrangements, amplification and translocations) could play a crucial role in cancer pathogenesis, as witnessed by their detection in iCCA, urothelial carcinoma and other solid tumors [32]. In fact, FGFR aberrations could act as pro-oncogenic drivers, and consequently, are of interest as therapeutic targets [33]. Introduction to the compound Chemistry; Pharmacodynamics; Pharmacokinetics and metabolism The third-generation, irreversible FGFR inhibitor futibatinib (TAS-120) is classified as a highly selective irreversible pan-FGFR antagonist [34]. In fact, this agent is a pyrazolo[3,4-d]pyrimidine (Figure 2) derivative able to inhibit FGFR1/2/3/4 with IC50 values of 3.9/1.3/1.6/ 8.3 nM. In a pivotal study by Kalyukina and colleagues [34], the authors evidenced the covalent bond of the X- ray crystal structure of futibatinib with FGFR1, suggesting that the 4-aminogroup and the carbonyl group of E562 are linked by a covalent bond while the 3-aminogroup and the NeH group of A564 by a hydrogen bond. A hydrogen bond is also formed between the DFG-D641 NeH group and the oxygen atom of one of the methoxy groups; lastly, futibatinib forms a covalent adduct with a P-loop cysteine residue in the ATP pocket of FGFR (C492 in the FGFR2-IIIb isoform), resulting in astrong inhibition of FGFR-mediated pathways, reduced cell proliferation and increased tumor cell death in malignancies harboring FGFR1, FGFR2, FGFR3 and FGFR4 aberrations (Figure 3) [34]. Clinical trials: preliminary efficacy, safety and tolerability Firstly, in a phase I basket trial involving patients affected by different refractory advanced solid tumors, an overall response rate (ORR) of 25% and a disease control rate (DCR) of 78.6% were observed in 28 iCCA patients harboring FGFR2 fusions [35]. Interestingly, the study included patients previously treated with FGFR inhibitors; since the emergence of resistance to FGFR inhibitors is a major issue in iCCA patients receiving these agents, thus limiting the durability of response, futibatinib has been also investigated in this setting. In a recent study by Goyal and colleagues, futibatinib showed efficacy in 4 FGFR2-fusion positive iCCA patients whose disease progressed after infigratinib or Debio1347 [36]. More specifically, the authors examined circulating tumor DNA (ctDNA), serial biopsies and patient-derived cholangiocarcinoma cells, highlighting the activity of futibatinib against several FGFR2 mutations associated with resistance to infigratinib or Debio1347 [36]. These results are even more important if we consider current era of medical oncology, with ctDNA and serial biopsies which are entering into clinical practice, with a view to prolong the duration of benefit from targeted therapies and to rapidly detect the onset of resistance mechanisms [37]. Results of the first-in-human phase I dose-escalation FOENIX-1 trial have been recently published [38]. According to these results, futibatinib has shown promising activity in pretreated patients with advanced solid tumors, with partial responses (PRs) detected in 3 iCCA patients harboring FGFR2 fusions and 2 glioma patients. Moreover, the study suggested that 20 mg once daily could be defined as the maximum tolerated dose (MTD) of futibatinib; lastly, the safety profile of the FGFR inhibitor was consistent with that of other previously studied inhibitors such as infigratinib and erdafitinib. Efficacy and safety results of an interim analysis of FOENIX-CCA2 trial have been presented at the 2020 ASCO meeting [16]. In this single-arm multicenter phase II study, futibatinib was administered in iCCA patients harboring FGFR2 gene fusions or other rearrangements whose disease progressed after at least one line of therapy. Patients enrolled in the FOENIX-CCA2 trial presented an Eastern Cooperative Oncology Group Performance Status (ECOG-PS) score of 0-1, with the 44.8%, the 28.4% and the 26.9% of subjects which had previously received 1, 2 and 3 or more than 3 treatments, respectively. Futibatinib was administered orally at a dosage of 20 mg once daily, continuously (21-day cycles), until unacceptable toxicity or disease progression. Independent central radiology reviewed ORR is the primary endpoint of this study, with DCR, duration of response (DOR) and safety assessed as secondary endpoints [16]. The interim analysis presented at the ASCO 2020 Virtual Meeting has shown an encouraging and durable response to futibatinib treatment in 67 iCCA patients, with an ORR of 37.3% and a DCR of 82.1%. Interestingly, 1 patient achieved complete response (CR) and 24 PR, with a median duration of response of 8.31 months. After a median follow-up of 11.4 months, a median progression-free survival (PFS) of 7.2 months has been observed [16]. As suggested by the recent results of the FOENIX-CCA2 trial, oral futibatinib seems to have a manageable safety profile in previously treated patients, with toxicity appearing consistent for the drug class of FGFR inhibitors [16]. Given the well-known role of FGFR in the homeostasis of phosphates, treatment with futibatinib, as in the case of infigratinib, derazantinib and pemigatinib, has been associated with hyperphosphataemia, which was the commonest adverse event in FOENIX-CCA2 (80.6% of all grades, 28.4% grade 3-4). Similar to what observed with other FGFR inhibitors, hyperphosphataemia seems to occur early after start of treatment and it is generally managed with diet modifications, diuretics, phosphate binders and occasionally dose modifications / interruptions. Other frequently reported toxicities included diarrhea, dry mouth, alopecia and dry skin in 37.3%, 32.8%, 29.9% and 26.9% of patients, respectively, with no grade 3-4 adverse events (AEs) reported so far. AEs leading to dose interruptions were found in 55.2% (37/67) of patientswhile AEs resulting in dose reductions occurred in 50.7% of enrolled subjects and treatment discontinuation in only 1 patient. None of the deaths reported in the trial was considered to be related to futibatinib. Overall, the interim analysis of the FOENIX-CCA2 study has indicated that futibatinib may be an effective option in patients affected by refractory iCCA with FGFR2 gene fusions or other rearrangements [16]. Ongoing analyses will report more results regarding dose adjustments for AEs management, response assessment in patients experiencing hyperphosphatemia and patient-reported outcomes. A phase III study, the FOENIX-CCA3 trial, is currently ongoing, aimed at comparing futibatinib versus cisplatin plus gemcitabine as front-line treatment in advanced / metastatic iCCA patients harboring FGFR2 gene rearrangements (NCT04093362). The primary endpoint of the study is PFS while secondary endpoints include ORR, DCR, OS and safety. Conclusions Recent evidence regarding futibatinib suggests that this molecule has the potential to add a new option to the range of treatments in iCCA with FGFR2 fusions or rearrangements. However, some questions remain to be answered and further data are needed to confirm the encouraging and durable responses achieved with futibatinib. The comprehensive definition of the type of mutations developing during FGFR inhibition represents a high unmet clinical need in iCCA patients harboring FGFR aberrations, together with the identification of novel agents and effective combinations. Expert opinion In the last fifteen years, the increasing availability of molecular sequencing has paved the way towards a potential new era in iCCA management [39]. In light of recent findings, the most promising therapeutic options for iCCA originate from targeted therapies, including FGFR inhibitors. Although based on a small number of patients, the interim analysis of FOENIX-CCA2 provides preliminary results regarding the FGFR inhibitor futibatinib in previously treated iCCAs harboring FGFR2 fusions or rearrangements [16]. An interest element to consider in FOENIX-CCA2 is undoubtedly the inclusion of heavily pretreated patients, with 26.9% of subjects which had previously received three or more lines of treatment [16]; this percentage is particularly high compared to other previous trials regarding FGFR inhibitors such as FIGHT-202, where this subgroup of pretreated patients represented the 12% [27]. The results regarding pretreated patients in FOENIX-CCA2 are particularly interesting if we consider treatment options in iCCA, especially as third and later-line; in fact, despite limited data are currently available regarding this setting with no established third-line regimen to offer, some iCCA patients continue to have good performance status and still are candidates for third- or subsequent lines of therapy [40]. Moreover, durable responses have been detected in FOENIX-CCA2, with futibatinib showing a median duration of response of 8.31 months (6.2 – NR months), a result which suggests a low drug resistance rate for this FGFR inhibitor [16]. In terms of resistance, one of the future challenges will probably be to implement the use of ctDNA analysis in BTC, whose routine application could be extremely useful for the detection and management of therapeutic resistance, as suggested in the study by Goyal et al [36]. In fact, the use of ctDNA could allow the identification of resistance mutations in patients harboring FGFR2 aberrations, with a view to translate the experience of other malignancies (e.g. colorectal cancer, non-small cell lung cancer) in iCCA [41, 42]. Several studies have recently suggested that the resistance to various targeted therapies could be observed in ctDNA analysis even before the clinical detection of progression by imaging tools [43, 44]. A highly aggressive disease with limited treatment options such as iCCA would deserve a close monitoring of clone dynamics of resistant subclones and we believe the time is ripe for a real integration between ctDNA analysis and clinical trials, a combination that has the potential to enter into standard clinical management. Despite recent data regarding futibatinib in FOENIX-CCA2 are from being conclusive, this FGFR inhibitor has shown encouraging and durable activity. Nonetheless, further studies are warranted to confirm futibatinib activity and ongoing trials will help to clarify the role of this molecule in iCCAs harboring FGFR2 fusions. A future opportunity could be to combine targeted therapies with chemotherapy or immune checkpoint inhibitors, with a view to provide hope for more successful treatment in the near future for iCCA patients. References 1. Rizvi S, Gores GJ. Pathogenesis, diagnosis, and management of cholangiocarcinoma. Gastroenterology 2013; 145(6): 1215< 1229. 2. 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