Pharmacological characterization of the novel complement C3 inhibitor BI-C

Abstract

Purpose : Geographic atrophy (GA), an advanced form of age-related macular degeneration, leads to progressive and irreversible vision loss. These preclinical studies pharmacologically characterized the novel complement C3 inhibitor BI-C, currently under clinical investigation for GA.

Methods : Kinetic parameters of BI-C binding to C3 and C3b were determined by surface plasmon resonance (SPR) including C3 single nucleotide polymorphism (SNP) variants associated with GA. The ability of BI-C to inhibit membrane attack complex (MAC) formation after classical (CP), alternative (AP), and lectin pathway (LP) activation was determined in human serum. Comparisons to reference molecule APL-1 (short-acting C3-binding synthetic cyclic peptide) were made due to expected similarity in binding behavior compared with APL-2 (Apellis; two APL-1 peptides conjugated to the ends of linear polyethylene glycol). Diffusion through the Bruch’s membrane (BrM) was investigated for BI-C (Mw 26 kDa) and an APL-2 similar molecule (Mw 42 kDa) for up to 72 hours using enriched porcine BrM mounted in Ussing chamber devices. Identification of the binding site of BI-C was achieved by analyzing the structure of the BI-C:C3c complex by cryo-electron microscopy.

Results : BI-C is a C3-binding polypeptide. BI-C showed high affinity in the picomolar range for all tested C3 ligands including the GA-associated C3 SNPs P314L and R102G, and an approximately 2-fold lower affinity for cynomolgus C3 (Table). APL-1 showed a lower affinity in the 1-digit nanomolar range, comparable to published values. Structural analysis of BI-C bound to C3c showed that BI-C binds to an epitope partly overlapping to the compstatin binding site but not limited to it. The enhanced binding affinity of BI-C to C3 is likely to result from this extended binding interface. BI-C displayed dose-dependent inhibition of MAC formation via CP (mean IC₅₀ 79 nM), AP (344 nM), and LP (67 nM). BI-C exhibited enhanced potency for inhibiting CP- and LP-mediated complement activation versus APL-1. More efficient penetration of the BrM was observed with BI-C than for the APL-2 similar molecule.

Conclusions : BI-C is a potent and specific C3 inhibitor that displays dose-dependent inhibition of MAC formation across all three complement pathways and effective penetration through BrM. In line with these promising results, BI-C is now being investigated in humans.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.

 

Kinetic parameters determined by SPR