The development of resistance to anticancer agents during treatment is a major obstacle in the chemotherapy of cancer. Cells expressing high levels of the P-glycoprotein exhibit a multidrug resistance phenotype. The P-glycoprotein is a membrane phosphoprotein which serves as a drug efflux pump to reduce intracellular drug accumulation, and hence the cytotoxicity of anticancer drugs. Several studies have shown that protein kinase activators and inhibitors may modulate the biological activity of P-glycoprotein through covalent modification by phosphorylation. Most of these drugs may have additional mechanisms of action and may alter drug accumulation within multidrug resistant cells with or without their effects on phosphorylation of P-glycoprotein. In addition, transcriptional regulation of MDR 1 gene has been found to be regulated by protein kinase A type I and protein kinase C. Therefore, these kinases may be important candidates in studies of the reversal of multidrug resistance and hence in enhancing the efficacy of anticancer drugs.

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