Focal adhesion kinase (FAK), also known as protein tyrosine kinase 2 (PTK2), is a non-receptor tyrosine kinase encoded by the
PTK2 gene. It functions as a focal adhesion-associated protein kinase, playing vital roles in integrin-mediated signal transduction [
39]. FAK is a protein of 125 kD, consisting of the N-terminal FERM (F for the 4.1 protein, ezrin, radixin andmoesin) domain, the central kinase domain, two C-terminal proline-rich motifs (PR1 and PR2) and the focal adhesion targeting (FAT) domain [
39]. The central kinase domain is responsible for its enzymatic catalytic functions. Serving as a scaffold protein, the N-terminal FERM domain directly interacts with partner proteins (
e.g., integrins and growth factor receptors). It also binds to the central kinase domain to block substrate access, thus protecting FAK from activation of sarcoma protein kinase (sarcoma, Src) phosphorylation. The C-terminal FAT domain, containing multiple binding sites for protein-protein interactions, recruits FAK to focal adhesion complexes [
39]. Under resting conditions, the FERM domain and the central kinase domain form an auto-inhibitory intramolecular interaction, keeping FAK in an inactive state. Once this intramolecular interaction is disrupted, FAK becomes activated and enhances the auto-phosphorylation at Tyr397. Subsequently, FAK-dependent signaling pathways involved in cell growth, proliferation, survival and migration cellular processes are activated [
39]. Overexpression of FAK has been observed in various metastatic tumors and is associated with oncogenic signals, making FAK a significant target in anti-cancer therapy [
39]. Reported FAK inhibitors can be categorized into two types: ATP-binding site inhibitors and kinase-independent inhibitors. ATP binding site inhibitors (
e.g., TAE-226, PF-573228, PF-562271, PF-4554878 and GSK-2256098) competitively bind to the ATP binding site of FAK' kinase domain. These inhibitors exert their inhibitory activity against FAK by suppressing the phosphorylation of Tyr397 or Tyr861 sites. However, ATP-binding site inhibitors have certain limitations in inhibiting cell growth and inducing apoptosis [
39]. Other kinase-independent inhibitors and PROTAC-based degraders have been developed by targeting the scaffold function of FAK (Fig. S2A in Supporting information) [
40]. Chloropyramine hydrochloride is a protein-protein interaction inhibitor targeting the interaction between FAK and vascular endothelial growth factor receptor 3 (VEGFR-3). It binds to the VEGFR-3 binding site within the FAT domain of FAK. Preclinical studies have demonstrated that chloropyramine hydrochloride exhibits therapeutic efficacy in multiple cancer types, including pancreatic cancer, breast cancer, neuroblastoma, and advanced melanoma, by disrupting the interaction between FAK and VEGFR-3. It has been received orphan drug designation from FDA [
40]. Moreover, chloropyramine hydrochloride has shown significant synergetic anti-cancer effects against breast and pancreatic cancer in combination with chemotherapy drugs [
40,
41]. Furthermore, a variety of FAK PROTAC degraders (
e.g., PROTAC-3, A13, FC-11, B5, BI-3633 and GSK215) have been synthesized, which can directly degrade FAK protein. These degraders are designed by connecting a FAK inhibitor (such as PF562271, TAK-226 or VS-4718) with an E3 ligase ligand (CRBN or VHL) using different linkers. The linker type and length are meticulously optimized to enhance the degradation efficacy of FAK degraders [
42,
43]. These innovative degraders hold the promise of expanding the druggable landscape and exerting control over protein functions that are conventionally challenging to address with traditional small-molecule therapeutics [
44]. Collectively, efforts toward modulating the scaffolding function of FAK, whether through small molecular compounds or PROTAC-based degraders, represents a compelling and promising strategy for the development of highly selective drugs for cancer therapy.