STO-609 is a selective and cell-permeable inhibitor of the Ca2+/calmodulin-dependent protein kinase kinase (CaM-KK), with Ki values of 80 and 15 ng/mL for recombinant CaM-KKα and CaM-KKβ, respectively. STO-609 inhibits AMP-activated protein kinase kinase (AMPKK) activity in HeLa cell lysates with an IC50 ~0.02 g/ml. Ki: 80 ng/mL (CaM-KKα), 15 ng/mL (CaM-KKβ)[1]

STO-609 inhibits the activities of recombinant CaM-KKα and CaM-KKβ isoforms, with Ki values of 80 and 15 ng/mL, respectively, and also inhibits their autophosphorylation activities. STO-609 is highly selective for CaM-KK without any significant effect on the downstream CaM kinases (CaM-KI and -IV), and the IC50 value of the compound against CaM-KII is 10 μg/mL. STO-609 inhibits constitutively active CaM-KKα as well as the wild-type enzyme. In transfected HeLa cells, STO-609 suppresses the Ca2+-induced activation of CaM-KIV in a dose-dependent manner. STO-609 significantly reduces the endogenous activity of CaM-KK in SH-SY5Y neuroblastoma cells at a concentration of 1μg/mL (80% inhibitory rate)[1].

[1]. Tokumitsu H, et al. STO-609, a specific inhibitor of the Ca(2+)/calmodulin-dependent protein kinase kinase. J Biol Chem. 2002 May 3;277(18):15813-8. [2]. Kukimoto-Niino M, et al. Crystal structure of the Ca••/calmodulin-dependent protein kinase kinase in complex with the inhibitor STO-609. J Biol Chem. 2011 Jun 24;286(25):22570-9.