Identification of a nonbasic, nitrile-containing cathepsin K inhibitor (MK-1256) that is efficacious in a monkey model of osteoporosis

Herein, we report on the identification of nonbasic, potent, and highly selective, nitrile-containing Cathepsin K (Cat K) inhibitors that are built on our previously identified cyclohexanecarboxamide core structure. Subsequent to our initial investigations, we have found that incorporation of five-membered heterocycles as P2-P3 linkers allowed for the introduction of a methyl sulfone P3-substitutent that was not tolerated in inhibitors containing a six-membered aromatic P2-P3 linker. The combination of a five-membered N-methylpyrazole linker and a methyl sulfone in P3 yielded subnanomolar Cat K inhibitors that were minimally shifted (<10-fold) in our functional bone resorption assay. Issues that arose because of metabolic demethylation of the N-methylpyrazole were addressed through introduction of a 2,2,2-trifluoroethyl substituent. This culminated in the identification of 31 (MK-1256), a potent (Cat K IC 50 = 0.62 nM) and selective (>1100-fold selectivity vs Cat B, L, S, C, H, Z, and V, 110-fold vs Cat F) inhibitor of Cathepsin K that is efficacious in a monkey model of osteoporosis.