Structural Analysis of the Human Fibroblast Growth Factor Receptor 4 Kinase

Ferroptosis-targeting drugs in breast cancer

In 2020, breast cancer surpassed lung cancer as the most common cancer in the world for the first time. Due to the resistance of some breast cancer cell lines to apoptosis, the therapeutic effect of anti-breast cancer drugs is limited. According to recent report, the susceptibility of breast cancer cells to ferroptosis affects the progress, prognosis and drug resistance of breast cancer. For instance, roblitinib induces ferroptosis of trastuzumab-resistant human epidermal growth factor receptor 2 (HER2)-positive breast cancer cells by diminishing fibroblast growth factor receptor 4 (FGFR4) expression, thereby augmenting the susceptibility of these cells to HER2-targeted therapies. In tamoxifen-resistant breast cancer cells, Fascin exacerbates their resistance by repressing solute carrier family 7 member 11 (SLC7A11) expression, which in turn heightens their responsiveness to tamoxifen. In recent years, Chinese herbs extracts and therapeutic drugs have been demonstrated to elicit ferroptosis in breast cancer cells by modulating a spectrum of regulatory factors pertinent to ferroptosis, including SLC7A11, glutathione peroxidase 4 (GPX4), acyl-CoA synthetase long chain family member 4 (ACSL4), and haem oxygenase 1 (HO-1). Here, we review the roles and mechanisms of Chinese herbal extracts and therapeutic drugs in regulating ferroptosis in breast cancer, providing potential therapeutic options for anti-breast cancer.

Fibroblast growth factor therapies in biliary tract cancers: current and future state

INTRODUCTION

Cholangiocarcinoma is the rare and aggressive tumor with poor prognosis and limited therapeutic options. Recently, there have been promising developments in molecular targeted therapies for patients following the progression of first-line chemotherapy and immunotherapy combinations. Dysregulation of fibroblast Growth Factor Receptor (FGFR) signaling is significantly associated with tumorigenesis of intrahepatic cholangiocarcinoma and has been identified as a targetable alteration. This was possible through the discovery of crucial insights into the biochemical mechanisms and pathophysiology of the FGFR pathway.

AREAS COVERED

This review summarizes the current state of FGFR targeted therapies, mechanisms of resistance, and future directions for FGFR-targeted therapies in patients with cholangiocarcinoma.

EXPERT OPINION

Currently, pemigatinib and futibatinib are FDA approved FGFR-targeted therapies that have demonstrated remarkable responses. However, there is still a significant proportion of patients whose disease remains intrinsically resistant to treatment and most patients eventually develop secondary resistance after an initial response. Additionally, unique side effects of FGFR inhibitors may limit their efficacy in clinical practice and can have detrimental effects on quality of life. Several novel FGFR inhibitors are currently being investigated to overcome resistance mechanisms and reduce toxicities.

Decoupling the dynamic mechanism revealed by FGFR2 mutation-induced population shift

The fibroblast growth factor receptor 2 (FGFR2) is a key component in cellular signaling networks, and its dysfunctional activation has been implicated in various diseases including cancer and developmental disorders. Mutations at the activation loop (A-loop) have been suggested to trigger an increased basal kinase activity. However, the molecular mechanism underlying this highly dynamic process has not been fully understood due to the limitation of static structural information. Here, we conducted multiple, large-scale Gaussian accelerated molecular dynamics simulations of five (K659E, K659N, K659M, K659Q, and K659T) FGFR2 mutants at the A-loop, and comprehensively analyzed the dynamic molecular basis of FGFR2 activation. The results quantified the population shift of each system, revealing that all mutants had a higher proportion of active-like states. Using Markov state models, we extracted the representative structure of different conformational states and identified key residues related to the increased kinase activity. Furthermore, community network analysis showed enhanced information connections in the mutants, highlighting the long-range allosteric communication between the A-loop and the hinge region. Our findings may provide insights into the dynamic mechanism for FGFR2 dysfunctional activation and allosteric drug discovery.Communicated by Ramaswamy H. Sarma.

Design, synthesis and biological evaluation of a series of novel pyrrolo[2,3-d]pyrimidin/pyrazolo[3,4-d]pyrimidin-4-amine derivatives as FGFRs-dominant multi-target receptor tyrosine kinase inhibitors for the treatment of gastric cancer

Gastric cancer is the second most lethal cancer across the world. With the progress in therapeutic approaches, the 5-year survival rate of early gastric cancer can reach > 95%. However, the prognosis and survival time of advanced gastric cancer is still somber. Therefore, more effective targeted therapies for gastric cancer treatment are urgently needed. FGFR, VEGFR and other receptor tyrosine kinases have recently been suggested as potential targets for gastric cancer treatment. We herein report the discovery of pyrrolo[2,3-d]pyrimidin/pyrazolo[3,4-d]pyrimidin-4-amine derivatives as a new class of FGFRs-dominant multi-target receptor tyrosine kinase inhibitors. SAR assessment identified the most active compounds 8f and 8k, which showed excellent inhibitory activity against a variety of receptor tyrosine kinases. Moreover, 8f and 8k displayed excellent potency in the SNU-16 gastric cancer cell line. Furthermore, 8f and 8k could inhibit FGFR1 phosphorylation and downstream signaling pathways as well as induce cell apoptosis. In vivo, 8f and 8k suppress tumor growth in the SNU-16 xenograft model without inducing obvious toxicity. These findings raise the possibility that compounds 8f and 8k might serve as potential agents for the treatment of gastric cancer.

N6-methyladenosine regulated FGFR4 attenuates ferroptotic cell death in recalcitrant HER2-positive breast cancer

Intrinsic and acquired anti-HER2 resistance remains a major hurdle for treating HER2-positive breast cancer. Using genome-wide CRISPR/Cas9 screening in vitro and in vivo, we identify FGFR4 as an essential gene following anti-HER2 treatment. FGFR4 inhibition enhances susceptibility to anti-HER2 therapy in resistant breast cancer. Mechanistically, m6A-hypomethylation regulated FGFR4 phosphorylates GSK-3β and activates β-catenin/TCF4 signaling to drive anti-HER2 resistance. Notably, suppression of FGFR4 dramatically diminishes glutathione synthesis and Fe2+ efflux efficiency via the β-catenin/TCF4-SLC7A11/FPN1 axis, resulting in excessive ROS production and labile iron pool accumulation. Ferroptosis, a unique iron-dependent form of oxidative cell death, is triggered after FGFR4 inhibition. Experiments involving patient-derived xenografts and organoids reveals a synergistic effect of anti-FGFR4 with anti-HER2 therapy in breast cancer with either intrinsic or acquired resistance. Together, these results pinpoint a mechanism of anti-HER2 resistance and provide a strategy for overcoming resistance via FGFR4 inhibition in recalcitrant HER2-positive breast cancer.

Signaling Pathway and Small-Molecule Drug Discovery of FGFR: A Comprehensive Review

Targeted therapy is a groundbreaking innovation for cancer treatment. Among the receptor tyrosine kinases, the fibroblast growth factor receptors (FGFRs) garnered substantial attention as promising therapeutic targets due to their fundamental biological functions and frequently observed abnormality in tumors. In the past 2 decades, several generations of FGFR kinase inhibitors have been developed. This review starts by introducing the biological basis of FGF/FGFR signaling. It then gives a detailed description of different types of small-molecule FGFR inhibitors according to modes of action, followed by a systematic overview of small-molecule-based therapies of different modalities. It ends with our perspectives for the development of novel FGFR inhibitors.

DW14383 is an irreversible pan-FGFR inhibitor that suppresses FGFR-dependent tumor growth in vitro and in vivo

Fibroblast growth factor receptor (FGFR) is a promising anticancer target. Currently, most FGFR inhibitors lack sufficient selectivity and have nonnegligible activity against kinase insert domain receptor (KDR), limiting their feasibility due to the serious side effects. Notably, compensatory activation occurs among FGFR1-4, suggesting the urgent need to develop selective pan-FGFR1-4 inhibitors. Here, we explored the antitumor activity of DW14383, a novel irreversible FGFR1-4 inhibitor. DW14383 exhibited equivalently high potent inhibition against FGFR1, 2, 3 and 4, with IC₅₀ values of less than 0.3, 1.1, less than 0.3, and 0.5 nmol/L, respectively. It is a selective FGFR inhibitor, exhibiting more than 1100-fold selectivity for FGFR1 over recombinant KDR, making it one of the most selective FGFR inhibitors over KDR described to date. Furthermore, DW14383 significantly inhibited cellular FGFR1-4 signaling, inducing G1/S cell cycle arrest, which in turn antagonized FGFR-dependent tumor cell proliferation. In contrast, DW14383 had no obvious antiproliferative effect against cancer cell lines without FGFR aberration, further confirming its selectivity against FGFR. In representative FGFR-dependent xenograft models, DW14383 oral administration substantially suppressed tumor growth by simultaneously inhibiting tumor proliferation and angiogenesis via inhibiting FGFR signaling. In summary, DW14383 is a promising selective irreversible pan-FGFR inhibitor with pan-tumor spectrum potential in FGFR1-4 aberrant cancers, which has the potential to overcome compensatory activation among FGFR1-4.

Molecular mechanisms of the anti-cancer drug, LY2874455, in overcoming the FGFR4 mutation-based resistance

In recent years, many strategies have been used to overcome the fibroblast growth factor receptor (FGFR) tyrosine kinase inhibitors (TKIs) resistance caused by different mutations. LY2874455 (or 6LF) is a pan-FGFR inhibitor which is identified as the most efficient TKI for all resistant mutations in FGFRs. Here, we perform a comparative dynamics study of wild type (WT) and the FGFR4 V550L mutant for better understanding of the 6LF inhibition mechanism. Our results confirm that the pan-FGFR inhibitor 6LF can bind efficiently to both WT and V550L FGFR4. Moreover, the communication network analysis indicates that in apo-WT FGFR4, αD–αE loop behaves like a switch between open and close states of the substrate-binding pocket in searching of its ligand. In contrast, V550L mutation induces the active conformation of the FGFR4 substrate-binding pocket through disruption of αD–αE loop and αG helix anti-correlation. Interestingly, 6LF binding causes the rigidity of hinge and αD helix regions, which results in overcoming V550L induced resistance. Collectively, the results of this study would be informative for designing more efficient TKIs for more effective targeting of the FGFR signaling pathway.

Fibroblast Growth Factors as Tools in the Management of Neuropathic Pain Disorders

Neuropathic pain is caused by a damage to or dysfunction of the somatosensory nervous system. The main mechanisms underlying neuropathic pain include ectopic activity in nociceptive nerves, peripheral and central sensitization, impaired inhibitory modulation, and microglial activation. Fibroblast growth factors (FGFs) make up a large family of growth factors that mediate neural development, metabolism, and function through three main key signaling pathways, including RAS/MAP kinase pathway, PI3 kinase/Akt pathway, and PLCγ. An association between the members of the FGF system and the improvement of neuropathic pain has become evident, recently. These signaling molecules may be expected to provide new drug targets for the treatment of neuropathic pain. To the best of our knowledge, it is the first study that reviews the relationship between some members of the FGF system and neuropathic pain.

Discovery of Roblitinib (FGF401) as a Reversible-Covalent Inhibitor of the Kinase Activity of Fibroblast Growth Factor Receptor 4

FGF19 signaling through the FGFR4/β-klotho receptor complex has been shown to be a key driver of growth and survival in a subset of hepatocellular carcinomas, making selective FGFR4 inhibition an attractive treatment opportunity. A kinome-wide sequence alignment highlighted a poorly conserved cysteine residue within the FGFR4 ATP-binding site at position 552, two positions beyond the gate-keeper residue. Several strategies for targeting this cysteine to identify FGFR4 selective inhibitor starting points are summarized which made use of both rational and unbiased screening approaches. The optimization of a 2-formylquinoline amide hit series is described in which the aldehyde makes a hemithioacetal reversible-covalent interaction with cysteine 552. Key challenges addressed during the optimization are improving the FGFR4 potency, metabolic stability, and solubility leading ultimately to the highly selective first-in-class clinical candidate roblitinib.

FGFR4: A promising therapeutic target for breast cancer and other solid tumors

The fibroblast growth factor receptor (FGFR) signaling pathway has long been known to cancer researchers because of its role in cell survival, proliferation, migration, and angiogenesis. Dysregulation of FGFR signaling is frequently reported in cancer studies, but most of these studies focus on FGFR1-3. However, there is growing evidence implicating an important and unique role of FGFR4 in oncogenesis, tumor progression, and resistance to anti-tumor therapy in multiple types of cancer. Importantly, there are several novel FGFR4-specific inhibitors in clinical trials, making FGFR4 an attractive target for further research. In this review, we focus on assessing the role of FGFR4 in cancer, with an emphasis on breast cancer. First, the structure, physiological functions and downstream signaling pathways of FGFR4 are introduced. Next, different mechanisms reported to cause aberrant FGFR4 activation and their functions in cancer are discussed, including FGFR4 overexpression, FGF ligand overexpression, FGFR4 somatic hotspot mutations, and the FGFR4 G388R single nucleotide polymorphism. Finally, ongoing and recently completed clinical trials targeting FGFRs in cancer are reviewed, highlighting the therapeutic potential of FGFR4 inhibition for the treatment of breast cancer.

Phase II Study of AZD4547 in Patients With Tumors Harboring Aberrations in the FGFR Pathway: Results From the NCI-MATCH Trial (EAY131) Subprotocol W

PURPOSE

NCI-MATCH is a nationwide, histology-agnostic, signal-finding, molecular profile-driven trial for patients with refractory cancers, lymphomas, or myelomas. Patients with tumors harboring actionable aberration(s) in fibroblast growth factor receptor (FGFR) 1-3 were treated with AZD4547, an oral FGFR1-3 inhibitor.

METHODS

Patients' tumors were screened by next-generation sequencing for predefined FGFR amplification, activating mutations, or fusions. Patients were treated with AZD4547, 80 mg orally twice daily until progression of disease or drug intolerance. A response rate of 16% was considered promising.

RESULTS

Between July 2016 and June 2017, 70 patients were assigned and 48 received protocol therapy and are eligible for analysis. Patients' tumors harbored FGFR1 or FGFR2 amplification (n = 20), FGFR2 or FGFR3 single-nucleotide variants (n = 19), or FGFR1 or FGFR3 fusions (n = 9). The most common primary tumors were breast (33.3%), urothelial (12.5%), and cervical cancer (10.4%).Grade 3 adverse events were consistent with those described in previous clinical trials. Confirmed partial responses were seen in 8% (90% CI, 3% to 18%) and were observed only in patients whose tumors harbored FGFR1-3 point mutations or fusions. Stable disease was observed in 37.5% (90% CI, 25.8% to 50.4%). The median progression-free survival (PFS) was 3.4 months, and the 6-month PFS rate was 15% (90% CI, 8% to 31%). For patients with tumors harboring FGFR fusions, the response rate was 22% (90% CI, 4.1% to 55%), and 6-month PFS rate was 56% (90% CI, 31% to 100%).

CONCLUSION

Preliminary signals of activity appeared to be limited to cancers harboring FGFR activating mutations and fusions, although AZD4547 did not meet the primary end point. Different FGFR somatic alterations may confer different levels of signaling potency and/or oncogene dependence.

The structure of human GCN2 reveals a parallel, back-to-back kinase dimer with a plastic DFG activation loop motif

When activated by amino acid starvation, the stress sensing protein kinase GCN2 phosphorylates the eukaryotic initiation factor 2 alpha, inhibiting translation to conserve energy and facilitate cell survival. Amino acid starvation, particularly of tryptophan and arginine, affects immune tolerance by suppressing differentiation and proliferation of T-cells via activation of GCN2 kinase. In addition, the GCN2 pathway mediates cancer survival directly within the context of metabolic stress. Here, we report the first crystal structures of the human GCN2 kinase domain (KD) in complex with two inhibitors of different size, shape, and chemical scaffold. Three novel activation loop conformations representative of different activation states of the kinase are described. In addition, a novel dimerization organization for GCN2 is observed. This arrangement is consistent with the hypothesis that the GCN2 KD forms an antiparallel inactive dimer until uncharged tRNA binds to it and triggers conformational changes that shift the equilibrium to the active parallel dimer.

Identification of a novel oncogenic mutation of FGFR4 in gastric cancer

Gastric cancer remains one of the leading causes of cancer death worldwide. Despite intensive investigations of treatments over the past three decades, the poor prognosis of patients with unresectable advanced or recurrent gastric cancer has not significantly changed, and improved therapies are required. Here, we report the identification of an oncogenic mutation in FGFR4 in a human gastric tumour that leads to constitutive activation of its product, FGFR4. The G636C-FGFR4 tyrosine kinase domain mutation was found in 1 of 83 primary human gastric tumours. The G636C mutation increased FGFR4 autophosphorylation, and activated FGFR4 downstream signalling molecules and enhanced anchorage-independent cell growth when expressed in NIH/3T3 cells. 3D-structural analysis and modelling of FGFR4 suggest that G636C destabilizes an auto-inhibitory conformation and stabilizes an active conformation, leading to increased kinase activation. Ba/F3 cell lines expressing the G636C-FGFR4 mutant were significantly more sensitive to ASP5878, a selective FGFR inhibitor, than the control. Oral administration of ASP5878 significantly inhibited the growth of tumours in mice engrafted with G636C-FGFR4/3T3 cells. Together, our results demonstrate that mutationally activated FGFR4 acts as an oncoprotein. These findings support the therapeutic targeting of FGFR4 in gastric cancer.

FGFR4 phosphorylates MST1 to confer breast cancer cells resistance to MST1/2-dependent apoptosis

Cancer cells balance with the equilibrium of cell death and growth to expand and metastasize. The activity of mammalian sterile20-like kinases (MST1/2) has been linked to apoptosis and tumor suppression via YAP/Hippo pathway-independent and -dependent mechanisms. Using a kinase substrate screen, we identified here MST1 and MST2 among the top substrates for fibroblast growth factor receptor 4 (FGFR4). In COS-1 cells, MST1 was phosphorylated at Y433 residue in an FGFR4 kinase activity-dependent manner, as assessed by mass spectrometry. Blockade of this phosphorylation by Y433F mutation induced MST1 activation, as indicated by increased threonine phosphorylation of MST1/2, and the downstream substrate MOB1, in FGFR4-overexpressing T47D and MDA-MB-231 breast cancer cells. Importantly, the specific knockdown or short-term inhibition of FGFR4 in endogenous models of human HER2⁺ breast cancer cells likewise led to increased MST1/2 activation, in conjunction with enhanced MST1 nuclear localization and generation of N-terminal cleaved and autophosphorylated MST1. Unexpectedly, MST2 was also essential for this MST1/N activation and coincident apoptosis induction, although these two kinases, as well as YAP, were differentially regulated in the breast cancer models analyzed. Moreover, pharmacological FGFR4 inhibition specifically sensitized the HER2⁺ MDA-MB-453 breast cancer cells, not only to HER2/EGFR and AKT/mTOR inhibitors, but also to clinically relevant apoptosis modulators. In TCGA cohort, FGFR4 overexpression correlated with abysmal HER2⁺ breast carcinoma patient outcome. Therefore, our results uncover a clinically relevant, targetable mechanism of FGFR4 oncogenic activity via suppression of the stress-associated MST1/2-induced apoptosis machinery in tumor cells with prominent HER/ERBB and FGFR4 signaling-driven proliferation.

Structure, activation and dysregulation of fibroblast growth factor receptor kinases: perspectives for clinical targeting

The receptor tyrosine kinase family of fibroblast growth factor receptors (FGFRs) play crucial roles in embryonic development, metabolism, tissue homeostasis and wound repair via stimulation of intracellular signalling cascades. As a consequence of FGFRs' influence on cell growth, proliferation and differentiation, FGFR signalling is frequently dysregulated in a host of human cancers, variously by means of overexpression, somatic point mutations and gene fusion events. Dysregulation of FGFRs is also the underlying cause of many developmental dysplasias such as hypochondroplasia and achondroplasia. Accordingly, FGFRs are attractive pharmaceutical targets, and multiple clinical trials are in progress for the treatment of various FGFR aberrations. To effectively target dysregulated receptors, a structural and mechanistic understanding of FGFR activation and regulation is required. Here, we review some of the key research findings from the last couple of decades and summarise the strategies being explored for therapeutic intervention.

Recent Studies on Ponatinib in Cancers Other Than Chronic Myeloid Leukemia

Ponatinib is a third line drug for the treatment of chronic myeloid leukemia patients, especially those that develop the gatekeeper mutation T315I, which is resistant to the first and the second line drugs imatinib, nilotinib, dasatinib and bosutinib. The compound was first identified as a pan Bcr-Abl and Src kinase inhibitor. Further studies have indicated that it is a multitargeted inhibitor that is active on FGFRs, RET, AKT, ERK1/2, KIT, MEKK2 and other kinases. For this reason, the compound has been evaluated on several cancers in which these kinases play important roles, including thyroid, breast, ovary and lung cancer, neuroblastoma, rhabdoid tumours and in myeloproliferative disorders. Ponatinib is also being tested in clinical trials to evaluate its activity in FLT3-ITD acute myelogenous leukemia, head and neck cancers, certain type of lung cancer, gastrointestinal stromal tumours and other malignancies. In this review we report the most recent preclinical and clinical studies on ponatinib in cancers other than CML, with the aim of giving a complete overview of this interesting compound.

Making way for suppressing the FGF19/FGFR4 axis in cancer

FGF19 is a noncanonical FGF ligand that can control a broad spectrum of physiological responses, which include bile acid homeostasis, liver metabolism and glucose uptake. Many of these responses are mediated by FGF19 binding to its FGFR4/β-klotho receptor complex and controlling activation of an array of intracellular signaling events. Overactivation of the FGF19/FGFR4 axis has been implicated in tumorigenic formation, progression and metastasis, and inhibitors of this axis have recently been developed for single agent use or in combination with other anticancer drugs. Considering the critical role of this receptor complex in cancer, this review focuses on recent developments and applications of FGF19/FGFR4-targeted therapeutics.

Computational Simulation Studies on the Binding Selectivity of 1-(1H-Benzimidazol-5-yl)-5-aminopyrazoles in Complexes with FGFR1 and FGFR4

Fibroblast growth factor receptor 1 (FGFR1) has become a potential target for the treatment of cancer. Designing FGFR1-selective inhibitors remains fundamental to the development of anti-cancer drugs because of highly sequential homology among FGFR subtypes. In present work, four inhibitors were examined with intermolecular interaction patterns with FGFR1 and FGFR4, respectively, for the exploration of binding mechanisms by applying a combined approach of computational techniques, including flexible docking, binding site analyses, electronic structure computations, molecular dynamic simulations, and binding free energy predictions. Molecular simulation-predicted binding conformations and pharmacophoric features of these molecules in the active pocket of either FGFR1 or FGFR4. MMPB(GB)SA-calculated binding free energies were accordant with the ordering of their tested potency values. Furthermore, in silico mutations of two residues (FGFR1: Tyr563 and Ser565) were also performed to check their impact on ligand binding by applying MD simulations and binding free energy calculations. The present studies may provide a structural understanding of the FGFR1-selective mechanism. The viewpoints from computational simulations would be valuable guidelines for the development of novel FGFR1-selective inhibitors.

Theoretical studies on FGFR isoform selectivity of FGFR1/FGFR4 inhibitors by molecular dynamics simulations and free energy calculations

The activation and overexpression of fibroblast growth factor receptors (FGFRs) are highly correlated with a variety of cancers. Most small molecule inhibitors of FGFRs selectively target FGFR1-3, but not FGFR4. Hence, designing highly selective inhibitors towards FGFR4 remains a great challenge because FGFR4 and FGFR1 have a high sequence identity. Recently, two small molecule inhibitors of FGFRs, ponatinib and AZD4547, have attracted huge attention. Ponatinib, a type II inhibitor, has high affinity towards FGFR1/4 isoforms, but AZD4547, a type I inhibitor of FGFR1, displays much reduced inhibition toward FGFR4. In this study, conventional molecular dynamics (MD) simulations, molecular mechanics/generalized Born surface area (MM/GBSA) free energy calculations and umbrella sampling (US) simulations were carried out to reveal the principle of the binding preference of ponatinib and AZD4547 towards FGFR4/FGFR1. The results provided by MM/GBSA illustrate that ponatinib has similar binding affinities to FGFR4 and FGFR1, while AZD4547 has much stronger binding affinity to FGFR1 than to FGFR4. A comparison of the individual energy terms suggests that the selectivity of AZD4547 towards FGFR1 versus FGFR4 is primarily controlled by the variation of the van der Waals interactions. The US simulations reveal that the PMF profile of FGFR1/AZD4547 has more peaks and valleys compared with that of FGFR4/AZD4547, suggesting that the dissociation process of AZD4547 from FGFR1 are easily trapped into local minima. Moreover, it is observed that FGFR1/AZD4547 has much higher PMF depth than FGFR4/AZD4547, implying that it is more difficult for AZD4547 to escape from FGFR1 than from FGFR4. The physical principles provided by this study extend our understanding of the binding mechanisms and provide valuable guidance for the rational design of FGFR isoform selective inhibitors.

Approaches to selective fibroblast growth factor receptor 4 inhibition through targeting the ATP-pocket middle-hinge region

A diverse range of selective FGFR4 inhibitor hit series were identified using unbiased screening approaches and by the modification of known kinase inhibitor scaffolds. In each case the origin of the selectivity was consistent with an interaction with a poorly conserved cysteine residue within the middle-hinge region of the kinase domain of FGFR4, at position 552. Targeting this region identified a non-covalent diaminopyrimidine series differentiating by size, an irreversible-covalent inhibitor in which Cys552 undergoes an SNAr reaction with a 2-chloropyridine, and a reversible-covalent inhibitor series in which Cys552 forms a hemithioacetal adduct with a 2-formyl naphthalene. In addition, the introduction of an acrylamide into a known FGFR scaffold identified a pan-FGFR inhibitor which reacted with both Cys552 and a second poorly conserved cysteine on the P-loop of FGFR4 at position 477 which is present in all four FGFR family members.

Fibroblast growth factor receptors in breast cancer

Fibroblast growth factor receptors are growth factor receptor tyrosine kinases, exerting their roles in embryogenesis, tissue homeostasis, and development of breast cancer. Recent genetic studies have identified some subtypes of fibroblast growth factor receptors as strong genetic loci associated with breast cancer. In this article, we review the recent epidemiological findings and experiment results of fibroblast growth factor receptors in breast cancer. First, we summarized the structure and physiological function of fibroblast growth factor receptors in humans. Then, we discussed the common genetic variations in fibroblast growth factor receptors that affect breast cancer risk. In addition, we also introduced the potential roles of each fibroblast growth factor receptors isoform in breast cancer. Finally, we explored the potential therapeutics targeting fibroblast growth factor receptors for breast cancer. Based on the biological mechanisms of fibroblast growth factor receptors leading to the pathogenesis in breast cancer, targeting fibroblast growth factor receptors may provide new opportunities for breast cancer therapeutic strategies.

Advances and challenges in targeting FGFR signalling in cancer

Fibroblast growth factors (FGFs) and their receptors (FGFRs) regulate numerous cellular processes. Deregulation of FGFR signalling is observed in a subset of many cancers, making activated FGFRs a highly promising potential therapeutic target supported by multiple preclinical studies. However, early-phase clinical trials have produced mixed results with FGFR-targeted cancer therapies, revealing substantial complexity to targeting aberrant FGFR signalling. In this Review, we discuss the increasing understanding of the differences between diverse mechanisms of oncogenic activation of FGFR, and the factors that determine response and resistance to FGFR targeting.

An overview of the binding models of FGFR tyrosine kinases in complex with small molecule inhibitors

The fibroblast growth factor receptor (FGFR) family receptor tyrosine kinase (RTK) includes four structurally related members, termed as FGFR1, FGFR2, FGFR3, and FGFR4. Given its intimate role in the progression of several solid tumors, excessive FGFR signaling provides an opportunity for anticancer therapy. Along with extensive pharmacological studies validating the therapeutic potential of targeting the FGFRs for cancer treatment, co-crystal structures of FGFRs/inhibitors are continuously coming up to study the mechanism of actions and explore new inhibitors. Herein, we review the reported co-crystals of FGFRs in complex with the corresponding inhibitors, main focusing our attention on the binding models and the pharmacological activities of the inhibitors.

Structural characterization of recombinant human fibroblast growth factor receptor 2b kinase domain upon interaction with omega fatty acids

The mutated recombinant kinase domain of human fibroblast growth factor receptor 2b (hFGFR2b) is overexpressed and purified, and its structural changes upon the interaction with three unsaturated fatty acids (UFAs), oleic, linoleic and α-linolenic are studied. This interaction is investigated to find out about the folding and unfolding effect of unsaturated fatty acids on the kinase domain structure of hFGFR2b. Recombinant pLEICS-01 vectors, containing the mutated coding region of hFGFR2b, are expressed in the standard Escherichia coli BL21 (DE3) host cells and purified by Ni²⁺-NTA affinity chromatography. While polyacrylamide gel electrophoresis characterizes the functionality of recombinant protein, its structural changes are studied in the presence and absence of various concentrations of oleic, α-linolenic and linoleic acids using circular dichroism (CD) and fluorescence spectroscopy. Far ultraviolet CD results show that unsaturated fatty acids do not change the secondary structure of the recombinant kinase domain of hFGFR2b. However, chemical denaturation analysis confirms that all three UFAs destabilize the tertiary structure of recombinant protein. A decrease in the fluorescence intensity without any significant red or blue shift (336±1nm) reflects a variation in the tertiary structure of protein. The direct interaction of the studied UFAs with hFGFR2b reduces the conformational stability of their kinase domains. The structural changes in hFGFR2b in the presence of UFAs may be necessary for hFGFR2b to adjust the signal transduction and regulate the key cellular processes.

Crystal Structure of the FGFR4/LY2874455 Complex Reveals Insights into the Pan-FGFR Selectivity of LY2874455

Aberrant FGFR4 signaling has been documented abundantly in various human cancers. The majority of FGFR inhibitors display significantly reduced potency toward FGFR4 compared to FGFR1-3. However, LY2874455 has similar inhibition potency for FGFR1-4 with IC50 less than 6.4 nM. To date, there is no published crystal structure of LY2874455 in complex with any kinase. To better understand the pan-FGFR selectivity of LY2874455, we have determined the crystal structure of the FGFR4 kinase domain bound to LY2874455 at a resolution of 2.35 Å. LY2874455, a type I inhibitor for FGFR4, binds to the ATP-binding pocket of FGFR4 in a DFG-in active conformation with three hydrogen bonds and a number of van der Waals contacts. After alignment of the kinase domain sequence of 4 FGFRs, and superposition of the ATP binding pocket of 4 FGFRs, our structural analyses reveal that the interactions of LY2874455 to FGFR4 are largely conserved in 4 FGFRs, explaining at least partly, the broad inhibitory activity of LY2874455 toward 4 FGFRs. Consequently, our studies reveal new insights into the pan-FGFR selectivity of LY2874455 and provide a structural basis for developing novel FGFR inhibitors that target FGFR1-4 broadly.

Biophysical characterization of drug-resistant mutants of fibroblast growth factor receptor 1

Over-expression and aberrant activation of tyrosine kinases occur frequently in human cancers. Various tyrosine kinase inhibitors (TKIs) are under clinical use, but acquisition of resistance to these drugs is a major problem. Here, we studied the interaction between two drug-resistant mutants of fibroblast growth factor receptor 1 (FGFR1), N546K and V561M, and four ATP-competitive inhibitors, ponatinib, dovitinib, PD173074 and BGJ-398. Among these protein-drug systems, the only marked reduction in affinity was that of PD173074 for the V561M mutant. We also examined the interaction of these FGFR1 variants to AMP-PNP, a nonhydrolyzable analogue of ATP, and showed that N546K showed increased affinity for the ATP analogue as compared with the wild type. These findings will help to clarify the mechanism of drug resistance in mutant tyrosine kinases.

Targeting fibroblast growth factor receptor pathway in breast cancer

PURPOSE OF REVIEW

The purpose of this manuscript is to critically review the literature published last year focusing on the rationale and potential role of fibroblast growth factor receptor (FGFR) inhibitors in breast cancer.

RECENT FINDINGS

Substantial evidence indicates that aberrant FGFR signaling is involved in the pathogenesis of breast cancer. FGFR targeting has progressed in the last years due to the development of novel agents inhibiting FGF or FGFR. One of the most investigated FGFR inhibitors is lucitanib, which has shown clinical activity in breast cancer, especially in presence of FGF aberrations. Moving forward, the design and development of FGFR4 inhibitors and covalent FGFR inhibitors may overcome resistance to first-generation FGFR inhibitors.

SUMMARY

Inhibition of FGFR signaling is under investigation in the treatment of breast cancer with increasing interest. Next steps will include the optimal selection of patients to be treated with this class of drugs and the development of new-generation FGFR inhibitors to face with the resistance issue.

The expression profiles of fibroblast growth factor 9 and its receptors in developing mice testes

An expressional lack of fibroblast growth factor 9 (FGF9) would cause male-to-female sex reversal in the mouse, implying the essential role of FGF9 in testicular organogenesis and maturation. However, the temporal expression of FGF9 and its receptors during testicular development remains elusive. In this study, immunohistochemistry was used to identify the localization of FGF9 and its receptors at different embryonic and postnatal stages in mice testes. Results showed that FGF9 continuously expressed in the testis during development. FGF9 had highest expression in the interstitial region at 17-18 d post coitum (dpc) and in the spermatocytes, spermatids and Leydig cell on postnatal days (pnd) 35-65. Regarding receptor expression, FGFR1 and FGFR4 were evenly expressed in the whole testis during the embryonic and postnatal stages. However, FGFR2 and FGFR3 were widely expressed during the embryonic testis development with higher FGFR2 expression in seminiferous tubules at 16-18 dpc and higher FGFR3 expression in interstitial region at 17-18 dpc. In postnatal stage, FGFR2 extensively expressed with higher expression at spermatids and Leydig cells on 35-65 pnd and FGFR3 widely expressed in the whole testis. Taken together, these results strongly suggest that FGF9 is correlated with the temporal expression profiles of FGFR2 and FGFR3 and possibly associated with testis development.

Fibroblast growth factor receptor 3 isoforms: Novel therapeutic targets for hepatocellular carcinoma?

Fibroblast growth factor receptors (FGFRs) are frequently up-regulated in subsets of hepatocellular carcinoma (HCC). Here, we provide mechanistic insight that FGFR3 splice variants IIIb and IIIc impact considerably on the malignant phenotype of HCC cells. The occurrence of FGFR3 variants was analyzed in human HCC samples. In hepatoma/hepatocarcinoma cell lines, FGFR3 isoforms were overexpressed by lentiviral constructs or down-modulated by small interfering RNA (siRNA; affecting FGFR3-IIIb and -IIIc) or an adenoviral kinase-dead FGFR3-IIIc construct (kdFGFR3). Elevated levels of FGFR3-IIIb and/or -IIIc were found in 53% of HCC cases. FGFR3-IIIb overexpression occurred significantly more often in primary tumors of large (pT2-4) than of small size (pT1). Furthermore, one or both isoforms were enhanced mostly in cases with early tumor infiltration and/or recurrence at the time of surgery or follow-up examinations. In hepatoma/hepatocarcinoma cells, up-regulated FGFR3-IIIb conferred an enhanced capability for proliferation. Both FGFR3-IIIb and FGFR3-IIIc suppressed apoptotic activity, enhanced clonogenic growth, and induced disintegration of the blood/lymph endothelium. The tumorigenicity of cells in severe combined immunodeficiency mice was augmented to a larger degree by variant IIIb than by IIIc. Conversely, siRNA targeting FGFR3 and kdFGFR3 reduced clonogenicity, anchorage-independent growth, and disintegration of the blood/lymph endothelium in vitro. Furthermore, kdFGFR3 strongly attenuated tumor formation in vivo.

CONCLUSIONS

Deregulated FGFR3 variants exhibit specific effects in the malignant progression of HCC cells. Accordingly, blockade of FGFR3-mediated signaling may be a promising therapeutic approach to antagonize growth and malignant behavior of HCC cells.

Structural and dynamic insights into the energetics of activation loop rearrangement in FGFR1 kinase

Protein tyrosine kinases differ widely in their propensity to undergo rearrangements of the N-terminal Asp–Phe–Gly (DFG) motif of the activation loop, with some, including FGFR1 kinase, appearing refractory to this so-called ‘DFG flip'. Recent inhibitor-bound structures have unexpectedly revealed FGFR1 for the first time in a ‘DFG-out' state. Here we use conformationally selective inhibitors as chemical probes for interrogation of the structural and dynamic features that appear to govern the DFG flip in FGFR1. Our detailed structural and biophysical insights identify contributions from altered dynamics in distal elements, including the αH helix, towards the outstanding stability of the DFG-out complex with the inhibitor ponatinib. We conclude that the αC-β4 loop and ‘molecular brake' regions together impose a high energy barrier for this conformational rearrangement, and that this may have significance for maintaining autoinhibition in the non-phosphorylated basal state of FGFR1.

Receptor tyrosine kinases are key mediators of cell proliferation that have been implicated in several disease states for which they represent promising drug targets. Here the authors determine the thermodynamic basis for the low propensity of FGFR1 to access the DFG-Phe-out conformation required to bind type-II inhibitors.

Functions of Fibroblast Growth Factor Receptors in cancer defined by novel translocations and mutations

The four receptor tyrosine kinases (RTKs) within the family of Fibroblast Growth Factor Receptors (FGFRs) are critical for normal development but also play an enormous role in oncogenesis. Mutations and/or abnormal expression often lead to constitutive dimerization and kinase activation of FGFRs, and represent the primary mechanism for aberrant signaling. Sequencing of human tumors has revealed a plethora of somatic mutations in FGFRs that are frequently identical to germline mutations in developmental syndromes, and has also identified novel FGFR fusion proteins arising from chromosomal rearrangements that contribute to malignancy. This review details approximately 200 specific point mutations in FGFRs and 40 different fusion proteins created by translocations involving FGFRs that have been identified in human cancer. This review discusses the effects of these genetic alterations on downstream signaling cascades, and the challenge of drug resistance in cancer treatment with antagonists of FGFRs.