In vitro activity: 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). In transfected HeLa cells, STO-609 suppresses the Ca2+-induced activation of CaM-KIV in a dose-dependent manner. STO-609 can permeate cells and is a competitive inhibitor of ATP.

Kinase Assay: CaM-KI (2.5 μg/mL), CaM-KII (0.75 μg/mL), CaM-KIV (9 μg/mL), and mLCK (0.6 μg/mL) are incubated with 40 μM syntide-2 or 50 μM mLC peptide (for mLCK) at 30 °C for 5 min in a solution (25 μL) containing 50 mM HEPES (pH 7.5), 10 mM Mg(Ac)2, 1 mM DTT, 50 μM [γ-32P]ATP (4500 cpm/pmol) with various concentrations of STO-609 (0–10 μg/mL)in Me2SO at a final concentration of 4%) in the presence of 1 mM CaCl2, 2 μM CaM. Protein kinase activity is measured by the phosphocellulose filter method. Specific activities of CaM-KI, CaM-KII, CaM-KIV, and mLCK in the absence of STO-609 are calculated. STO-609 is bound in the ATP-binding pocket of the CaMKKβ KD. The inhibition mechanism of STO-609 is ATP-competitive.

Cell Assay: HeLa cells were maintained in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum. Cells were subcultured in 6-cm dishes 12 h before transfection. The cells were then transferred to serum-free medium and treated with a mixture of either 3 g of pME18s plasmid DNA or 3 g of HA(hemagglutinin-tagged)-CaM-KIV and 20 μg of LipofectAMINE reagent in 2.5 ml of medium. After 20 h of incubation, the cells were further cultured in serum-free medium for 6 h in either the absence or presence of various concentrations of STO-609(0.01-10 μg/ml in Me₂SO at a final concentration of 0.5%) and then treated with or without 1 μM ionomycin for 5 min. Stimulation was terminated by the addition of 1 ml of lysis buffer, 2 mM EDTA, 2 mM EGTA, 1% Nonidet P-40, 10% glycerol, 0.2 mM phenylmethylsulfonyl fluoride, 10 mg/liter leupeptin, 10 mg/liter trypsin inhibitor, and 1 μM microcystin LR), and the cells were lysed for 30min on ice. The cell extract was collected and centrifuged at 15,000×g for 15 min, the supernatant was precleared with 40 μl of Protein G-Sepharose for 2h at 4 °C, and the supernatant was mixed with 4 g of anti-HA antibody for 3h. 40 μl of Protein G-Sepharose was then applied to the extract and incubated overnight. The immunoprecipitated resin was washed three times with 1 ml of the lysis buffer and then washed with 1 ml of kinase buffer. Protein G-Sepharose with immunoprecipitated HA-CaM-KIV was subjected to the protein kinase assay in the presence of 1 mM EGTA using syntide-2 as a substrate.