Cathepsin K is a lysosomal cysteine protease found in osteoclasts whose elevated activity has been linked to the formation of osteoporosis and arthritis. 2-cyano-Pyrimidine is a cathepsin K inhibitor whose basic structure and derivatives are being studied for the treatment of osteoporosis. 2-cyano-Pyrimidine has an IC50 value of 170 nM for inhibition of cathepsin K.