Fibroblast growth factor receptor

The fibroblast growth factor receptors (FGFRs) are a family of receptor tyrosine kinases that bind to fibroblast growth factors (FGFs), a group of polypeptide ligands that regulate diverse cellular processes such as proliferation, differentiation, migration, and survival.^[1] There are four principal FGFR subtypes in mammals—FGFR1, FGFR2, FGFR3, and FGFR4—encoded by distinct genes, along with a fifth non-tyrosine kinase member, FGFR5 (also known as FGFRL1), which lacks an intracellular signaling domain.^[2] These receptors share a conserved structure featuring an extracellular domain with three immunoglobulin-like loops (IgI–III) for ligand binding, a single transmembrane helix, and an intracellular split tyrosine kinase domain responsible for signal transduction.^[1] Alternative splicing, particularly in the third immunoglobulin-like domain (IgIII), produces isoforms (e.g., IIIb and IIIc variants) that exhibit tissue-specific expression and ligand-binding preferences, enabling precise control of FGF signaling.^[2] FGFR activation typically occurs through high-affinity binding of one of the 18 known FGF ligands, often requiring co-receptors such as heparan sulfate proteoglycans (HSPGs) for paracrine FGFs (subfamilies FGF1, FGF4, FGF7, FGF8, FGF9) or β-Klotho/α-Klotho for endocrine FGFs (FGF19 subfamily).^[1] This binding induces receptor dimerization, followed by autophosphorylation of tyrosine residues in the kinase domain, which recruits adaptor proteins like FGFR substrate 2 (FRS2) and activates multiple downstream signaling cascades.^[2] Key pathways include the Ras-Raf-MEK-ERK (MAPK) pathway for mitogenic responses, the PI3K-AKT pathway for cell survival and metabolism, and the PLCγ pathway for calcium mobilization and PKC activation, with additional crosstalk to STAT and other signals.^[1] Negative regulators, such as Sprouty proteins and SEF, provide feedback to attenuate signaling and prevent excessive activation.^[2] Physiologically, FGFRs are indispensable for embryonic development, orchestrating organogenesis in structures like the skeleton, lungs, brain, and cardiovascular system through patterned FGF gradients.^[1] In adults, they support tissue homeostasis, wound repair, angiogenesis (e.g., via FGF2-FGFR1 in endothelial cells), and metabolic regulation, including bile acid synthesis (FGF19-FGFR4) and phosphate homeostasis (FGF23-FGFR1).^[2] Dysfunctions in FGFR signaling, often due to activating mutations, gene amplifications, or fusions, contribute to congenital disorders such as achondroplasia (FGFR3 G380R mutation) and craniosynostosis syndromes (e.g., Apert syndrome via FGFR2 mutations), as well as metabolic conditions like chronic kidney disease and obesity.^[1] In oncology, aberrant FGFR activity drives tumorigenesis in multiple cancers, including bladder (FGFR3 mutations in ~70% of low-grade cases), breast (FGFR1 amplification), and endometrial cancers, making FGFRs promising therapeutic targets, with several small-molecule inhibitors approved as of 2025, such as erdafitinib for FGFR-altered urothelial carcinoma, pemigatinib for cholangiocarcinoma, and futibatinib for FGFR2-fusion-positive cholangiocarcinoma.^[2]^[3]

Overview

Definition and Classification

Fibroblast growth factor receptors (FGFRs) are a family of transmembrane receptor tyrosine kinases (RTKs) that belong to the tyrosine kinase superfamily and play essential roles in cellular signaling by binding fibroblast growth factors (FGFs).^[4] These receptors are characterized by an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular tyrosine kinase domain that autophosphorylates upon activation to initiate downstream signaling cascades.^[4] FGFRs are integral to processes such as cell proliferation, differentiation, migration, and survival, particularly during embryonic development and tissue homeostasis.^[5] The FGFR family in humans comprises four main signaling-competent members—FGFR1, FGFR2, FGFR3, and FGFR4—each encoded by distinct genes and exhibiting tissue-specific expression patterns and ligand affinities.^[4] A fifth related member, FGFR5 (also known as FGFRL1), shares homology with the other FGFRs in its extracellular domain but lacks the intracellular tyrosine kinase domain, rendering it incapable of canonical signal transduction and classifying it as a non-signaling receptor-like protein.^[6] These receptors demonstrate varying degrees of promiscuity in ligand binding, with most FGFs capable of interacting with multiple FGFR subtypes, often requiring heparan sulfate proteoglycans as co-receptors to stabilize the complex.^[4] The discovery of FGFRs traces back to the 1980s, coinciding with the identification of FGFs as potent mitogens for fibroblasts and other cell types.^[7] In 1989, the first FGFR, designated flg (fms-like tyrosine kinase), was cloned from a human endothelial cell cDNA library and recognized as a receptor for acidic FGF (now FGF1), marking a pivotal step in understanding FGF signaling mechanisms. Subsequent cloning efforts in the late 1980s and early 1990s identified the additional family members, solidifying FGFRs as key mediators of FGF actions.^[8] FGFRs exhibit remarkable evolutionary conservation across vertebrates, with their core signaling architecture preserved from early metazoans to mammals, underscoring their fundamental role in developmental patterning and organogenesis.^[5] The expansion of the FGFR gene family occurred in two phases during metazoan evolution: an initial duplication in early metazoans to establish basic signaling, followed by vertebrate-specific duplications that increased functional diversity.^[9] This conservation highlights FGFRs' indispensable contributions to metazoan development, including mesoderm induction, limb formation, and neural crest specification.^[5]

Ligands and Binding

The fibroblast growth factor (FGF) family comprises 22 members that act as primary ligands for fibroblast growth factor receptors (FGFRs), exerting diverse effects through paracrine, endocrine, or intracrine signaling.^[5] These ligands are classified based on their modes of action: paracrine FGFs (FGF1–10, and others such as FGF16, FGF17, FGF18, FGF20, and FGF22) function locally by binding cell-surface FGFRs to regulate processes like development and tissue repair; endocrine FGFs (FGF19, FGF21, FGF23; where FGF15 denotes the rodent ortholog of human FGF19) circulate systemically as hormones to influence metabolism and mineral homeostasis, often with reduced heparin affinity; and intracrine FGFs (FGF11–14) operate intracellularly without engaging FGFRs, primarily modulating neuronal activity.^[5] Paracrine and endocrine FGFs exhibit varying binding specificities across the four FGFR subtypes (FGFR1–4), with some ligands like FGF1 showing broad affinity while others, such as FGF7 and FGF10, preferentially target specific isoforms like FGFR2b.^[1] High-affinity binding of paracrine FGFs to FGFRs requires co-receptors, particularly heparan sulfate proteoglycans (HSPGs), which are sulfated glycosaminoglycans on cell surfaces and in the extracellular matrix.^[1] HSPGs facilitate ternary complex formation (2:2:2 FGF-FGFR-HSPG stoichiometry), stabilizing interactions, promoting receptor dimerization, and enhancing signaling efficiency for paracrine FGFs like FGF1 and FGF2.^[10] In contrast, endocrine FGFs such as FGF19, FGF21, and FGF23 require alternative co-receptors like Klotho proteins for specificity, with reduced HSPG dependence to enable systemic action.^[1] Without HSPGs, FGF-FGFR binding affinity drops significantly, underscoring their role as essential mediators rather than mere scaffolds.^[11] Binding affinities vary by ligand-receptor pair, typically in the nanomolar range for direct interactions, with HSPGs increasing avidity to picomolar levels. For instance, FGF2—a canonical paracrine ligand—prefers FGFR1c and binds its extracellular domain with a dissociation constant (K_d) of approximately 60 nM, as measured by surface plasmon resonance.^[12] This moderate affinity allows graded signaling responses, while structural studies reveal that FGF2 engages both D2 and D3 domains of FGFR1 to induce conformational changes necessary for dimerization.^[13] The structural basis of ligand-receptor specificity resides in the immunoglobulin-like D2 and D3 domains of the FGFR extracellular region, which form the minimal binding unit.^[14] Key contacts occur between the FGF core and loops in D2 (e.g., βC-βE loop), while D3 accommodates ligand asymmetry through alternative splicing variants (IIIb/IIIc exons), enabling isoform-specific recognition—such as FGF10's exclusive interaction with FGFR2b via hydrogen bonds involving Asp-76 and Arg-78 residues.^[14] These domain-mediated interactions, often rotated by 40° upon binding, ensure selective signaling and prevent off-target activation across the FGF-FGFR network.^[14]

Molecular Structure

Domain Organization

Fibroblast growth factor receptors (FGFRs) are single-pass transmembrane receptor tyrosine kinases typically comprising approximately 800 amino acids in their monomeric form, organized into distinct extracellular, transmembrane, and intracellular domains that facilitate ligand recognition, membrane anchoring, and signal transduction, respectively.^[2] The canonical structure features disulfide bonds within the extracellular immunoglobulin-like domains to maintain structural integrity.^[15] The extracellular region consists of three immunoglobulin-like (Ig-like) domains, designated D1, D2, and D3. Domains D2 and D3 form the primary ligand-binding site, while D1 contributes to autoinhibition by modulating receptor conformation and ligand affinity.^[15]^[16] The transmembrane domain is a single hydrophobic alpha-helix spanning approximately 20 amino acids, which anchors the receptor in the plasma membrane and supports dimerization upon ligand binding.^[2] The intracellular portion includes a juxtamembrane segment and a split tyrosine kinase domain, characterized by two lobes connected by a hinge region, with key autophosphorylation sites such as Y653 and Y654 located in the activation loop to regulate kinase activity.^[17]^[18]

Isoforms and Variants

Fibroblast growth factor receptors (FGFRs) exhibit structural diversity primarily through alternative splicing events that generate multiple isoforms, particularly in FGFR1–3. Alternative splicing also generates isoforms lacking the D1 domain in FGFR1 and FGFR2 (e.g., two-Ig-like domain forms), which exhibit higher ligand affinity due to reduced autoinhibition.^[16] The most prominent splicing variation occurs in the third immunoglobulin-like domain (D3, also known as IgIII), where mutually exclusive exons produce IIIb and IIIc variants. The IIIb isoform incorporates exon 8, while IIIc uses exon 9, resulting in distinct amino acid sequences in the ligand-binding region that alter specificity. For instance, in FGFR2, the IIIb variant preferentially binds ligands such as FGF7 and FGF10, which are key epithelial paracrine factors, whereas the IIIc variant binds FGF1 and FGF2, facilitating broader mesenchymal interactions. FGFR4 lacks this splicing variability in D3. Additional isoform diversity arises from alternative splicing that produces secreted FGFR forms lacking the transmembrane domain, enabling them to function as soluble decoy receptors. A notable example is the soluble FGFR1 isoform, generated by exclusion of exons encoding the transmembrane and intracellular regions, which sequesters circulating FGF ligands and inhibits signaling through membrane-bound receptors. Similarly, soluble FGFR3 variants have been identified that bind specific FGFs like FGF9, modulating their availability in extracellular spaces. Post-translational modifications further contribute to FGFR isoform functionality and regulation. N-glycosylation occurs at multiple asparagine residues in the extracellular domains (D1–D3), promoting proper folding, stability, and ligand affinity while preventing aberrant intracellular trafficking. Ubiquitination, mediated by E3 ligases such as Cbl, targets activated FGFRs—particularly FGFR1—for lysosomal degradation via endocytosis, thereby attenuating signaling duration and preventing overstimulation. The prevalence of these isoforms is tissue-specific, reflecting their roles in epithelial-mesenchymal interactions. The IIIb variants predominate in epithelial cells, supporting localized paracrine signaling, while IIIc variants are more abundant in mesenchymal tissues, enabling responses to a wider array of FGFs during development and repair. This distribution underscores the splicing mechanism's importance in generating functional receptor heterogeneity without altering the core gene structure.

Genetics and Expression

Gene Family and Locations

The fibroblast growth factor receptor (FGFR) gene family in humans comprises four functional genes encoding receptor tyrosine kinases (RTKs) critical for developmental and physiological processes. These genes—FGFR1, FGFR2, FGFR3, and FGFR4—arose through evolutionary duplications from an ancestral RTK during early vertebrate genome expansions, including whole-genome duplication events that shaped the RTK repertoire.^[9]^[19] The chromosomal locations of these genes are distinct, reflecting their independent evolution post-duplication:

Gene	Chromosomal Location
FGFR1	8p11.23
FGFR2	10q26.13
FGFR3	4p16.3
FGFR4	5q35.2

^[20]^[21]^[22]^[23] The FGFR genes vary in size, with FGFR1 comprising 21 exons, FGFR2 26 exons, FGFR3 19 exons, and FGFR4 18 exons. The highly conserved intracellular tyrosine kinase domain is encoded by the latter exons, which encode the catalytic region responsible for signal transduction upon ligand binding.^[20]^[21]^[22]^[23] A fifth related gene, FGFRL1 (also known as FGFR5), is located on chromosome 4p16.3, consists of 7 exons, and lacks a functional intracellular tyrosine kinase domain and does not mediate canonical RTK signaling.^[24]^[25]^[26]

Expression Patterns and Regulation

Fibroblast growth factor receptors (FGFRs) exhibit distinct expression patterns during embryonic development and in adult tissues, reflecting their roles in organogenesis and homeostasis. FGFR1 displays ubiquitous expression throughout development, particularly in limb bud mesenchyme, brain, and kidney precursors. In contrast, FGFR2 and FGFR3 show more restricted patterns, with prominent expression in developing limbs during chondrogenic condensation and in skull osteoprogenitor cells. FGFR4 expression is notable in the pituitary gland and growth plate zones during embryogenesis, as well as in lung mesenchyme.^[1] In adult tissues, these patterns persist with tissue-specific localization. FGFR1 and FGFR2 are expressed in the brain and kidney, supporting neural and renal maintenance. FGFR3 is predominantly found in cartilage, especially in articular chondrocytes, while FGFR4 localizes to the pituitary gland and lung mesenchyme or distal epithelium. These differential expressions underscore the specialized functions of each FGFR isoform in mature organs.^[1] The expression of FGFRs is tightly regulated at transcriptional and post-transcriptional levels. FGFR promoters, such as that of FGFR2, respond to Wnt/β-catenin signaling through intermediaries like SOX9, which enhances transcription. Post-transcriptionally, microRNAs (miRNAs) modulate FGFR levels; for instance, miR-16 directly targets the 3' untranslated region (UTR) of FGFR1 mRNA, leading to its downregulation at both mRNA and protein levels.^[27]^[28] Epigenetic mechanisms also control FGFR expression. Hypermethylation of the FGFR2 promoter region has been observed in certain cancers, resulting in epigenetic silencing; treatment with DNA methyltransferase inhibitors like 5-aza-2'-deoxycytidine can restore expression.^[29]

Signaling Mechanisms

Receptor Activation

Receptor activation in fibroblast growth factor receptors (FGFRs) is initiated by the binding of fibroblast growth factor (FGF) ligands to the extracellular domains, which induces dimerization of the receptor monomers. This process requires heparan sulfate proteoglycans (HSPGs) that act as bridges between two FGFR molecules, stabilizing an asymmetric 2:2:2 complex consisting of two FGFs, two FGFRs, and the HSPG. The HSPG binds within a positively charged canyon formed by the D2 domains of the two FGFRs, facilitating direct D2-D2 contacts and secondary FGF-D2 interactions that enhance dimer stability. This ligand-induced dimerization positions the intracellular kinase domains in close proximity, enabling trans-autophosphorylation. The autophosphorylation cascade is a sequential, ordered process beginning with phosphorylation of Y653 in the activation loop, followed by Y583, Y463, Y766, and Y585, before culminating in phosphorylation of Y654 in the activation loop. This ordered progression is kinetically controlled, with phosphoryl transfer serving as the rate-limiting step, ensuring efficient progression toward full activation.^[30] Phosphorylation of Y653 induces a conformational change that opens the ATP-binding cleft in the kinase domain, dramatically increasing catalytic activity by approximately 50- to 100-fold. The additional phosphorylation at Y654 further enhances activity by about 10-fold, culminating in maximal kinase function. Allosteric regulation is mediated by the extracellular D1 domain, which autoinhibits the receptor by back-binding to the FGF-binding sites on D2 and D3 domains in the absence of ligand; FGF binding displaces the D1 domain, relieving this inhibition and permitting dimerization and activation.

Downstream Pathways

Upon activation of fibroblast growth factor receptors (FGFRs), which involves dimerization and autophosphorylation at specific tyrosine residues, several major intracellular signaling cascades are initiated, primarily through adapter protein recruitment and direct enzyme activation.^[31] These pathways, including the RAS-MAPK/ERK, PI3K-AKT, PLCγ, and STAT pathways, mediate diverse cellular responses such as proliferation, survival, differentiation, and cytoskeletal remodeling.^[32] A primary downstream effector is the adapter protein FRS2α, which is constitutively associated with the juxtamembrane region of FGFRs and becomes phosphorylated at tyrosine 466 (Y466) upon receptor activation.^[32] This phosphorylation enables FRS2α to recruit the adapter proteins Grb2 and Sos, forming a complex that activates the Ras-MAPK/ERK pathway, ultimately promoting cell proliferation through transcription factor activation like Elk-1.^[33] FRS2α also scaffolds the recruitment of Grb2-associated binder 1 (Gab1), which links to the PI3K-AKT pathway; PI3K phosphorylates PIP2 to PIP3, recruiting and activating Akt, which inhibits pro-apoptotic factors like FOXO and promotes cell survival and metabolic processes such as glucose uptake.^[31] Another key pathway involves phospholipase Cγ (PLCγ), which binds to the phosphorylated tyrosine 766 (Y766) on the C-terminal tail of FGFRs, leading to its activation.^[32] Activated PLCγ hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), triggering intracellular calcium release via IP3 and subsequent activation of protein kinase C (PKC) isoforms, which regulate calcium-dependent signaling events including cytoskeletal dynamics and gene expression.^[33] FGFRs can also directly phosphorylate and activate signal transducer and activator of transcription (STAT) proteins, particularly STAT1 and STAT3, leading to their dimerization and nuclear translocation to drive transcription of genes involved in cell differentiation and immune modulation.^[31] Additionally, FGFR signaling exhibits crosstalk with other receptor tyrosine kinases (RTKs), such as EGFR, where shared activation of the MAPK/ERK pathway can amplify proliferative signals, and ERK1/2 feedback phosphorylates FGFR2 at serine 777 to attenuate receptor activity, providing a regulatory mechanism.^[34]

Physiological Functions

Role in Development

Fibroblast growth factor receptors (FGFRs) play essential roles in embryonic development by mediating signaling that regulates cell proliferation, differentiation, migration, and patterning across multiple tissues. In early embryogenesis, FGFR1 is critical for gastrulation, where its activation promotes mesodermal cell migration and patterning; homozygous FGFR1 knockout mice exhibit defects in mesoderm migration, leading to accumulation of cells in the primitive streak, expanded axial mesoderm at the expense of paraxial mesoderm, and embryonic lethality between E7.5 and E9.5.^[35] These phenotypes underscore FGFR1's necessity for proper embryonic growth and the establishment of body axes during gastrulation.^[36] In limb development, FGFR1 facilitates limb bud establishment and outgrowth through interactions with FGF ligands produced by the apical ectodermal ridge (AER). A gradient of FGF8 signaling via FGFR1 in the limb mesenchyme maintains proliferation in the progress zone and regulates Sonic hedgehog (Shh) expression in the zone of polarizing activity (ZPA), ensuring proper proximodistal and anteroposterior patterning.^[37] Conditional inactivation of FGFR1 in limb mesenchyme results in misshapen buds with increased initial cell numbers but subsequent excess apoptosis, wider AER morphology, and digit defects such as reduced digit number or loss of specific identities like digit 3.^[37] Similarly, FGFR2 contributes to AER formation by responding to mesenchymal FGF10, establishing a positive feedback loop that sustains outgrowth.^[36] FGFR2 is vital for craniofacial development, particularly in the formation and maintenance of cranial sutures and intramembranous ossification of skull vault bones. It regulates osteoblast proliferation and differentiation in the pre-bone mesenchyme, with low-level signaling keeping suture mesenchymal cells undifferentiated to maintain patency, while balanced activation promotes appropriate bone morphogenesis and facial structure development.^[36]^[38] In angiogenesis during embryogenesis, FGF2 binds FGFR1 on endothelial cells to induce vascular endothelial growth factor (VEGF) expression through autocrine and paracrine mechanisms, promoting endothelial proliferation and capillary formation essential for vascular network establishment.^[39] This process involves FGFR1-mediated activation of downstream pathways that upregulate VEGF mRNA and protein in forming vessels.^[39]

Role in Adult Tissue Maintenance

Fibroblast growth factor receptors (FGFRs) play essential roles in maintaining adult tissues by regulating repair processes, homeostasis, and cellular interactions in various physiological contexts. In adult organisms, FGFR signaling supports tissue integrity through paracrine ligand actions that promote cell survival and proliferation without the morphogenetic emphasis seen in development. In wound healing, the interaction between FGF7 (also known as keratinocyte growth factor) and FGFR2b is critical for epithelial repair. FGF7, produced by mesenchymal cells, binds specifically to FGFR2b on keratinocytes, stimulating their migration and proliferation to facilitate re-epithelialization of skin wounds. This signaling enhances keratinocyte motility via activation of downstream pathways, including PI3K-mediated survival signals, ensuring efficient closure of cutaneous injuries. Studies in murine models have demonstrated that disruption of this axis impairs wound healing, underscoring its necessity for adult tissue regeneration. FGFR3 contributes to bone remodeling by negatively regulating osteoblast activity through paracrine signals from chondrocytes and direct effects on bone cells. In mature skeletons, FGFR3 activation inhibits bone formation to maintain homeostasis, as evidenced by chondrocyte-specific knockout studies showing increased osteoblast proliferation, bone mass, and dysregulated remodeling upon FGFR3 loss. This role is mediated by ligands such as FGF9 and involves STAT1 signaling to control differentiation, ensuring balanced bone maintenance during adulthood.^[40] Metabolic homeostasis in the liver is regulated by the FGF19-FGFR4-KLB axis, which controls bile acid synthesis. FGF19, an endocrine hormone released postprandially from the ileum, binds to FGFR4 in complex with the co-receptor β-Klotho (KLB) on hepatocytes, suppressing cytochrome P450 7A1 (CYP7A1) expression to inhibit bile acid production. This feedback mechanism prevents bile acid overload and maintains lipid metabolism, with human and rodent studies showing that FGF19 analogs mimic this effect to regulate hepatic cholesterol levels. In hematopoiesis, FGFR1 expressed on bone marrow stromal cells supports hematopoietic stem cell (HSC) niches. FGFR1 facilitates the maintenance of HSCs by promoting stromal cell-derived signals that sustain stem cell quiescence and self-renewal through interactions with ligands like FGF2. This niche-supporting function ensures steady-state blood cell production in adults, as evidenced by in vitro co-culture assays where FGFR1 inhibition disrupts HSC engraftment.

Disease Associations

Genetic Mutations and Disorders

Mutations in the fibroblast growth factor receptor (FGFR) genes, particularly FGFR2 and FGFR3, lead to a spectrum of congenital skeletal dysplasias through germline gain-of-function alterations that disrupt normal bone and cartilage development.^[41] These disorders arise from enhanced receptor signaling, which inhibits chondrocyte proliferation and differentiation, contrasting with the receptors' physiological roles in regulating skeletal growth during embryogenesis.^[42] Apert syndrome, a craniosynostosis disorder characterized by premature fusion of cranial sutures and syndactyly, is primarily caused by a heterozygous gain-of-function mutation in FGFR2, specifically the p.Ser252Trp substitution in the extracellular immunoglobulin-like domain III.^[38] This mutation, first identified in 1995, enhances ligand binding affinity and receptor dimerization, leading to constitutive activation and abnormal ossification of craniofacial bones. Crouzon syndrome, another FGFR2-related craniosynostosis condition with hypertelorism and midface hypoplasia but without syndactyly, results from various gain-of-function mutations in the same gene, often in the linker region between immunoglobulin-like domains II and III, such as p.Cys342Tyr.^[43] These alterations similarly promote ligand-independent signaling, causing premature suture closure typically evident at birth.^[41] Activating mutations in FGFR3 are responsible for several chondrodysplasias that impair endochondral ossification. Achondroplasia, the most common form of human dwarfism with an incidence of about 1 in 15,000–40,000 births, is predominantly due to the p.Gly380Arg mutation, though the p.Arg248Cys variant in the extracellular domain also contributes by stabilizing receptor dimers and overactivating downstream pathways, thereby inhibiting longitudinal bone growth through reduced chondrocyte proliferation in growth plates.^[44]^[42] Hypochondroplasia, a milder allelic disorder with short stature and macrocephaly, frequently arises from the p.Asn540Lys (N540K) mutation in the tyrosine kinase domain of FGFR3, accounting for 50–70% of cases; this substitution causes partial gain-of-function, leading to less severe inhibition of cartilage growth compared to achondroplasia.^[45]^[46] Thanatophoric dysplasia (TD), a lethal skeletal dysplasia with severe micromelia and respiratory insufficiency, results from distinct FGFR3 gain-of-function variants, including p.Arg248Cys and p.Tyr373Cys in type I, and p.Lys650Glu in type II, which induce extreme receptor hyperactivity and profound defects in endochondral ossification.^[47] These mutations prevent normal cartilage maturation, leading to bowed femurs and a narrow thorax; TD has a prevalence of approximately 1 in 20,000–50,000 births and is invariably fatal in the neonatal period due to respiratory failure.^[48]

Involvement in Cancer

Dysregulation of fibroblast growth factor receptors (FGFRs) plays a significant role in tumorigenesis across various cancers, primarily through somatic alterations such as gene amplifications, fusions, and overexpression that lead to constitutive activation of oncogenic signaling pathways. These alterations promote uncontrolled cell proliferation, survival, and metastasis by hyperactivating downstream cascades like MAPK and PI3K/AKT, independent of ligand binding in many cases. In particular, FGFR1, FGFR2, and FGFR3 alterations are frequently implicated in epithelial-derived malignancies, contributing to tumor initiation and progression.^[49] Amplification of the FGFR1 gene occurs in approximately 10% of breast cancer cases, particularly in hormone receptor-positive subtypes, where it drives ligand-independent receptor dimerization and autophosphorylation, thereby enhancing cell proliferation and conferring resistance to endocrine therapies. Similarly, FGFR1 amplification is observed in about 10% of non-small cell lung cancers, predominantly squamous cell carcinomas, where it sustains proliferative signaling and is associated with aggressive disease features such as smoking-related etiology. These amplifications result in overexpression of the receptor, amplifying mitogenic responses that fuel tumor growth.^[50]^[51]^[52] Gene fusions involving FGFR3, such as FGFR3-TACC3, are recurrent in bladder cancer, occurring in 2-6% of cases, and lead to constitutive kinase activation through the coiled-coil domain of TACC3, which promotes dimerization and persistent phosphorylation of key tyrosine residues. This fusion oncoprotein aberrantly activates downstream pathways, including MAPK, driving urothelial cell transformation and tumor progression. Overexpression of FGFR2 in gastric cancer, often mediated by epigenetic modifications such as altered promoter methylation or histone acetylation, enhances receptor-ligand interactions and sustains autocrine signaling loops that promote epithelial-mesenchymal transition and invasion. These epigenetic changes can occur independently of genetic amplifications, contributing to heterogeneous tumor phenotypes.^[53]^[54] FGFR3 mutations, including point mutations and fusions, are present in approximately 70% of low-grade, non-muscle-invasive bladder tumors and serve as a favorable prognostic marker in this subtype, correlating with lower recurrence rates and better overall survival compared to wild-type tumors. In contrast, their presence in high-grade or invasive disease often indicates a more indolent course relative to other driver alterations, though they still contribute to oncogenesis by stabilizing active receptor conformations. These prognostic implications highlight FGFR3 alterations as biomarkers for risk stratification in bladder cancer management.^[55]^[56]

Therapeutic Targeting

Small Molecule Inhibitors

Small molecule inhibitors of fibroblast growth factor receptors (FGFRs) primarily target the intracellular kinase domain to block FGFR-mediated signaling, which is implicated in various pathologies including cancer. These inhibitors are classified based on their selectivity and binding mechanisms, with early developments focusing on multi-kinase agents that incidentally inhibit FGFR alongside other targets, followed by more selective and irreversible compounds designed specifically for FGFR isoforms 1-4.^[49] Ponatinib (AP24534) represents a multi-targeted tyrosine kinase inhibitor initially developed for BCR-ABL in chronic myelogenous leukemia, where it was FDA-approved in 2012, but it also potently inhibits FGFR1-3 with IC50 values in the low nanomolar range through ATP-competitive binding to the kinase domain. This off-target FGFR inhibition has been leveraged in preclinical models of FGFR-driven tumors, such as non-small cell lung cancer and 8p11 myeloproliferative syndrome, where ponatinib suppresses cell proliferation by blocking FGFR autophosphorylation and downstream signaling.^[57]^[58] In contrast, selective FGFR inhibitors like erdafitinib are pan-FGFR agents (targeting FGFR1-4) with high potency, exhibiting IC50 values of approximately 5 nM across isoforms, and operate as reversible ATP-competitive inhibitors that bind to the inactive DFG-Din conformation of the kinase to prevent phosphorylation. Erdafitinib, approved by the FDA in 2019 for FGFR-altered advanced urothelial carcinoma, demonstrates improved selectivity over multi-kinase inhibitors, reducing off-target effects on kinases like VEGFR2 while maintaining efficacy against FGFR fusions and mutations.^[59]^[60] Covalent inhibitors, such as futibatinib (TAS-120), advance this approach by forming an irreversible bond with a conserved cysteine residue (Cys491 in FGFR2 and equivalent in FGFR3), enabling prolonged inhibition even in the presence of high ATP concentrations. The FDA granted accelerated approval to futibatinib in September 2022 for adult patients with previously treated, unresectable, locally advanced, or metastatic intrahepatic cholangiocarcinoma harboring FGFR2 fusions or other rearrangements. Developed as a next-generation FGFR1-4 inhibitor, futibatinib's acrylamide warhead targets this cysteine in the P-loop of the kinase domain, resulting in sustained suppression of FGFR signaling in preclinical FGFR-deregulated tumor models, with IC50 values below 10 nM for wild-type and mutant isoforms.^[61]^[62]^[63] Resistance to these inhibitors often arises from gatekeeper mutations in the FGFR kinase domain, such as V555M in FGFR3, which sterically hinders inhibitor binding in the ATP pocket and confers resistance to both reversible and some covalent agents like AZD4547 and dovitinib. This mutation, identified in acquired resistance models of FGFR3-driven cancers such as multiple myeloma and urothelial carcinoma, increases autophosphorylation and signaling activity, underscoring the need for next-generation inhibitors that accommodate such structural changes.^[64]^[65]

Clinical Trials and Applications

Erdafitinib, a pan-FGFR inhibitor, received accelerated FDA approval in 2019 for the treatment of adults with locally advanced or metastatic urothelial carcinoma harboring susceptible FGFR3 or FGFR2 alterations who have progressed during or following at least one line of prior platinum-containing chemotherapy. This approval was based on the phase 2 BLC2001 trial, which enrolled 99 patients with FGFR-altered advanced urothelial carcinoma and demonstrated an objective response rate (ORR) of 40%, including 3 complete responses and 37 partial responses, with a median duration of response of 6.9 months.^[66] Subsequent phase 3 THOR trial results in 2023 confirmed erdafitinib's efficacy, showing a median overall survival of 12.1 months versus 7.8 months with pembrolizumab in FGFR-altered patients previously treated with anti-PD-(L)1 therapy, supporting its conversion to full approval in 2024. Pemigatinib, another selective FGFR1-3 inhibitor, was granted accelerated FDA approval in 2020 for previously treated, unresectable locally advanced or metastatic cholangiocarcinoma with FGFR2 fusion or other rearrangement, based on the phase 2 FIGHT-202 trial.^[67] In the trial's cohort A (107 patients with FGFR2 fusions/rearrangements), pemigatinib achieved an ORR of 35%, with 3 complete responses and 34 partial responses, and a median duration of response of 7.5 months; updated 2024 analyses reported a median overall survival of 17.5 months.^[68] These findings established pemigatinib as a targeted option for this FGFR-driven subset of biliary tract cancers. Combination therapies pairing FGFR inhibitors with immune checkpoint inhibitors are under investigation to address acquired resistance and enhance antitumor immunity in FGFR-altered cancers. For instance, the phase 1b/2 NORSE trial (NCT03473743) evaluated erdafitinib plus cetrelimab (a PD-1 inhibitor) in advanced urothelial carcinoma, reporting an ORR of 49% in FGFR-altered patients without prior anti-PD-(L)1 therapy, with manageable toxicity and evidence of improved tumor microenvironment infiltration. Preclinical data and early clinical results suggest these combinations may reprogram "cold" tumors to "hot" states, boosting T-cell responses and overcoming resistance pathways like epithelial-mesenchymal transition.^[69] A key challenge in FGFR inhibitor therapy is hyperphosphatemia, a class effect from FGFR1 and FGFR3 inhibition leading to phosphate wasting, observed in up to 76% of patients on erdafitinib (grade 3 or higher in 50-60%) and similarly with pemigatinib, often requiring dose adjustments or phosphate-lowering agents.^[66] As of November 2025, multiple phase 3 trials are ongoing to expand indications and optimize regimens, including MoonRISe-1 (NCT06319820) assessing intravesical erdafitinib (TAR-210) versus chemotherapy in FGFR-altered intermediate-risk non-muscle-invasive bladder cancer.^[70] Note that the phase 3 FIGHT-302 trial (NCT03656536), which compared pemigatinib to gemcitabine-cisplatin in first-line FGFR2-altered cholangiocarcinoma, was terminated due to lack of enrollment.^[71] These efforts aim to refine therapeutic applications while mitigating toxicities through selective inhibitors and novel delivery systems.