STO-609 acetate (CAS 1173022-21-3) is a selective inhibitor of Ca2 /calmodulin-dependent protein kinase kinase (CaM-KK) demonstrating Ki values of 80 ng/ml for CaM-KK and 15 ng/ml for CaM-KK This compound exhibits marked specificity for CaM-KK isoforms with minimal activity against downstream CaM kinases such as CaM-KI and CaM-KIV and an IC50 of approximately 10 g/ml for CaM-KII In cell-based studies STO-609 suppresses Ca2 -induced activation of CaM-KIV and reduces endogenous CaM-KK activity In vivo STO-609 administration enhances osteoblast formation and inhibits osteoclast differentiation in mouse models highlighting its utility in studies of calcium signaling and bone metabolism