Modulating multidrug resistance through inhibiting of protein kinase C activity by phenothiazines

In the present study, actions of phenothiazines (PTZ) in reversing multidrug resistance (MDR) and inhibiting PKC activity were investigated. It was found that the three PTZs caused 2.49, 36.58 and 75.78 fold reversal of K562/AO2 MDR cells resistant to adriamycin, respectively, while the chemosensitizer verapamil caused 40 fold reversal in the same condition, indicating that PTZ11 is a novel reversal agent of MDR and a potential chemotherapeutic reagent for tumor therapy. PKC activity analysis in the presence of PTZs showd that PTZ6 and PTZ11 inhibited rat brain protein kinase C activity in a manner of dose-dependent. The IC₅₀ values were (489.77 ± 31.4) and (113 ± 9.64) μtmol/L, respectively. PTZ7 had no inhibition on PKC activity. Further study showed that PTZ11 could reduce PMA-mediated activation of PKC in a manner of dose-dependent, suggesting that PTZ11 might compete for the high-affinity phorbol ester binding site within PKC molecule. Recently, an X-ray structure of PMA in complex with PKC Cys2 activator-binding domain was solved. We therefore decided to explore the possible binding model of PTZ11 with PKC molecule using SYBYL 6.02 program. It was shown that the binding site of PTZ11 with PKC molecule partially overlapped with that of PMA, providing for the first time new data for designing PKC inhibitors and MDR reversal drugs.