Subnanomolar Cathepsin S Inhibitors with High Selectivity: Optimizing Covalent Reversible α-Fluorovinylsulfones and α-Sulfonates as Potential Immunomodulators in Cancer.

The cysteine protease cathepsin S (CatS) is overexpressed in many tumors. It is known to be involved in tumor progression as well as antigen processing in antigen-presenting cells (APC). Recent evidence suggests that silencing CatS improves the anti-tumor immune response in several cancers. Therefore, CatS is an interesting target to modulate the immune response in these diseases. Here, we present a series of covalent-reversible CatS inhibitors based on the α-fluorovinylsulfone and -sulfonate warheads. We optimized two lead structures by molecular docking approaches, resulting in 22 final compounds which were evaluated in fluorometric enzyme assays for CatS inhibition and for selectivity towards the off-targets CatB and CatL. The most potent inhibitor in the series has subnanomolar affinity (Ki =0.08 nM) and more than 100,000-fold selectivity towards cathepsins B and L. These new reversible and non-cytotoxic inhibitors could serve as interesting leads to develop new immunomodulators in cancer therapy.