Screening of natural compounds and their derivatives as potential protein kinase C inhibitors

Protein kinase C (PKC) isozymes are important signal transducers in a number of cellular responses that makes PKC inhibition a topical target for drug discovery. In this study, a set of natural compounds and their derivatives (43 in total) were screened to establish their potential inhibitory effects on PKC, exploiting kinase activity and ³H]‐phorbol ester binding assays. Statistical evaluation of the assays yielded signal‐to‐background, signal‐to‐noise, and Z′ values of 16.1, 11.3, and 0.62, respectively, for the kinase activity assay and 47.4, 15.9, and 0.73 for the binding assay, demonstrating their suitability for screening. Of the compounds investigated, the most potent PKC inhibitor was (?)‐epigallocatechin gallate (EGCG), which had an IC₅₀ of 4.8 µM. In addition, (?)‐epicatechin gallate, dodecyl gallate, and the flavonoids myricetin, quercetin, rhamnetin, luteolin, isorhamnetin, and kaempferol were effective while naringin and scopoletin demonstrated negligible effects. None of the compounds was able to significantly inhibit the binding of phorbol ester to the regulatory domain of PKCα. The highest inhibition, 59%, was observed in the case of EGCG at a concentration of 150 µM. Taken together, nine PKC inhibitors were identified, none of which was able to compete with the binding of phorbol ester to PKCα, suggesting that the mechanism of PKC inhibition could be a result of binding to the catalytic domain of PKC. Drug Dev Res 63:76–87, 2004. © 2004 Wiley‐Liss, Inc.