Balicatib is a potent inhibitor of cathepsin K (IC50s = 1.4, 56, and 480 nM for human, rat, and mouse forms, respectively). It is at least 100-fold selective for cathepsin K over cathepsins B, L, and S. Balicatib has a basic lipophilic nature, resulting in lysosomal trapping and increased selectivity for lysosomal cathepsin K.