Abstract: This invention is directed to methods for preventing the emergence of multidrug resistance in tumor cells during cancer chemotherapy. In particular, it relates to the use of cytoplasmic calcium antagonists and calmodulin inhibitors, phosphoinositol-dependent phospholipase C inhibitors, and substances that inhibit activation of the transcription factor NF-&kgr;B to prevent the induction of expression of the multidrug resistance gene (MDR1) encoding P-glycoprotein by chemotherapeutic drugs. MDR1 expression, which results in tumor cell resistance to subsequent treatment with certain chemotherapeutic drugs, is shown herein to be induced in response to treatment with various cytotoxic agents, including such agents that are or are not substrates for P-glycoprotein-mediated efflux from cancer cells.

Abstract: This invention is directed to methods for preventing the emergence of multidrug resistance in tumor cells during cancer chemotherapy. In particular, it relates to the use of protein kinase inhibitors to prevent the induction of expression of the multidrug resistance gene (MDR1) encoding P-glycoprotein by chemotherapeutic drugs. MDR1 expression, which results in tumor cell resistance to subsequent treatment with certain chemotherapeutic drugs, is shown herein to be induced in response to treatment with various cytotoxic agents, including such agents that are and are not substrates for P-glycoprotein-mediated efflux from cancer cells. Inhibitors of protein kinases, in particular, protein kinase C, are shown herein to suppress this cellular response. In addition, such protein kinase inhibitors are also shown herein to inhibit expression of a gene encoding a multidrug resistance-associated protein (the MRP gene).