Design, synthesis and bioactivity evaluation of novel small-molecule complement factor B inhibitors

To design and synthesize highly active complement factor B (CFB) inhibitors and verify the validity of strategies for structural optimization amid insufficient utilization of the hydrophobic binding pocket of CFB and the limited efficacy of existing inhibitors.

The marketed CFB inhibitor LNP023 was used as the lead compound, and its docking mode and binding to the co-crystal structure (PDB: 6RAV) of CFB were analyzed based on the Schrödinger molecular simulation platform. Given the insufficient utilization of the hydrophobic binding pocket of CFB and the impact of the indole ring structure on the ligand-target binding specificity and affinity, structural optimization was performed by directionally introducing such hydrophobic fragments as methylcyclopropyl around the indole ring of LNP023. Total synthesis of the target compounds was conducted before their structures were confirmed via nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) while the in vitro inhibitory activity of CFB was screened and evaluated via enzyme-linked immunosorbent assay (ELISA).

A total of 14 new compounds that had not been reported in literature were designed and synthesized, and their structures were all verified by spectroscopy. Analysis of in vitro activity suggested that the inhibition rates of compounds IB5 and IB6 on CFB at a concentration of 50 nmol/L were 44.9% and 45.4%, respectively while their half-maximal inhibitory concentrations (IC₅₀) were 53.5 nmol/L and 64.5 nmol/L, respectively.

Based on the property of the hydrophobic binding pocket of CFB protein, the strategy for directional modification of the indole ring can significantly enhance the binding effect between the inhibitor and the target so that highly active compounds IB5 and IB6 can be obtained, which verifies the hypothesis about the design of this study. The findings of this study are expected to facilitate subsequent research on structure-activity relationships and structural optimization of CFB inhibitors.