Azepanone-based inhibitors of human and rat cathepsin K

The synthesis, in vitro activities, and pharmacokinetics of a series of azepanone-based inhibitors of the cysteine protease cathepsin K (EC 3.4.22.38) are described. These compounds show improved configurational stability of the C-4 diastereomeric center relative to the previously published five- and six-membered ring ketone-based inhibitor series. Studies in this series have led to the identification of 20, a potent, selective inhibitor of human cathepsin K (K(i) = 0.16 nM) as well as 24, a potent inhibitor of both human (K(i) = 0.0048 nM) and rat (K(i,app) = 4.8 nM) cathepsin K. Small-molecule X-ray crystallographic analysis of 20 established the C-4 S stereochemistry as being critical for potent inhibition and that unbound 20 adopted the expected equatorial conformation for the C-4 substituent. Molecular modeling studies predicted the higher energy axial orientation at C-4 of 20 when bound within the active site of cathepsin K, a feature subsequently confirmed by X-ray crystallography. Pharmacokinetic studies in the rat show 20 to be 42% orally bioavailable. Comparison of the transport of the cyclic and acyclic analogues through CaCo-2 cells suggests that oral bioavailability of the acyclic derivatives is limited by a P-glycoprotein-mediated efflux mechanism. It is concluded that the introduction of a conformational constraint has served the dual purpose of increasing inhibitor potency by locking in a bioactive conformation as well as locking out available conformations which may serve as substrates for enzyme systems that limit oral bioavailability.