Potent selective inhibition of 7-O-methyl UCN-01 against protein kinase C. UCN-01 is a staurosporine-related compound that was isolated from the culture broth of Streptomyces sp. and shows potent and selective inhibitory activity against protein kinase C. Cellular inhibitory activity of UCN-01 against protein kinase C and cytotoxicity of UCN-01 were compared with those of staurosporine. When the mechanism of inhibitory activity was investigated in vitro, UCN-01 as well as staurosporine inhibited the activity of the catalytic domain of protein kinase C. In spite of direct inhibition against the catalytic domain of protein kinase C, cytotoxicity of UCN-01 was much lower than that of staurosporine. In addition, UCN-01 showed more selective inhibitory activity against protein kinase C than did staurosporine because of the sole structural difference at C-7. Therefore, a series of 7-O-alkyl derivatives of UCN-01 was synthesized and investigated. Interestingly, one of the compounds, the beta-methoxy derivative, showed 3-fold greater potency and 17-fold more selective inhibitory activity against protein kinase C than did UCN-01. 