Preclinical evaluation of a novel and highly differentiated FGFR2b-targeting ADC with low risk of ocular toxicity

Background: Fibroblast growth factor receptor-2 isoform IIIb (FGFR2b) plays a crucial role in the tumorigenesis and disease progression of several solid tumors, its overexpression is associated with poor prognosis. About 30% of gastric and gastroesophageal junction (G/GEJ) adenocarcinomas express FGFR2b with limited overlap with current biomarkers, this makes FGFR2b an attractive target. Bemarituzumab, an ADCC-enhanced FGFR2b antagonist, has demonstrated efficacy in patients with FGFR2b-overexpressing G/GEJ cancer. However, its therapeutic benefit appears to be limited by corneal adverse events, likely resulting from the strong blockade of FGFR2b ligands FGF7 and FGF10. Here we developed a novel FGFR2b-targeting ADC based on a highly differentiated antibody, ALX007, which completely spares ligand-receptor interaction and showed an encouraging ocular safety profile in preclinical models. Methods: Hit antibodies were generated by hybridoma, triaged using a proprietary structural modeling algorithm, and humanized. Antibody binding and internalization were evaluated, and blockade of FGF7 and FGF10 binding to FGFR2b-overexpressing cells was analyzed by flow cytometry. The lead antibody was conjugated to a topoisomerase I inhibitor via a stable and enzyme cleavable linker. Anti-tumor efficacy of the resulting ADC was assessed using FGFR2b-positive xenograft mouse models. PK profile and ocular toxicity were evaluated in relevant mouse models. Results: The naked antibody ALX007 demonstrated selective, high-affinity FGFR2b binding and rapid internalization in cell lines with varying levels of FGFR2b protein expression. Importantly, ALX007 did not inhibit the interaction between FGF7/FGF10 and FGFR2b even at high concentrations, distinguishing it from Bemarituzumab and several other reference antibodies with potent ligand blockade (IC 50 < 5 nM). Both structural modeling and epitope binning experiments revealed that ALX007 recognizes a novel, non-ligand-blocking epitope. Functional studies further confirmed that ALX007 did not inhibit FGF7/FGF10-induced FGFR2b phosphorylation. In the SNU-16 gastric cancer xenograft model, a single dose of the FGFR2b-targeting ADC showed sustained and superior anti-tumor efficacy compared with repeated doses of Bemarituzumab. Both the ALX007 naked antibody and its ADC exhibited significantly lower corneal dystrophy than Bemarituzumab in mice. In a human FcRn transgenic mouse model, no apparent difference in serum concentrations was observed between conjugated and total antibodies, indicating a stable antibody-linker conjugation. Conclusion: Together, these data demonstrate preclinical efficacy and low risk of ocular toxicity of a FGFR2-targeting ADC utilizing ALX007 as a highly differentiated antibody backbone. Further development for the treatment of FGFR2b-expressing tumors is warranted.