Cisplatin (CDDP) is a primary agent for the treatment of head and neck squamous cell cancers (HNSCC). During the previous funding period, our group has established that HNSCC cells resistant to CDDP are characterized by high expression of Bcl-xL and wild type p53. Based upon our data, we hypothesize that agents designed to inhibit Bcl-xL or activate p53 function can be highly effective strategies for the treatment of CDDP-resistant HNSCC. To date, we have evaluated a small-molecule inhibitor of Bcl-xL ((-)-gossypol) as a potential novel therapeutic agent for the treatment of HNSCC. Our in vitro and in vivo studies have formed the scientific basis for a future clinical trial of (-)-gossypol in collaboration with Ascenta Therapeutics and the National Cancer Institute. In this new SPORE project, we propose to test a novel therapeutic strategy to target CDDP-resistant HNSCC cells with wild-type p53 status using a potent and specific smallmolecule inhibitor of the MDM2-p53 interaction. In HNSCC cells with wild type p53, the function of p53 is effectively inhibited by its endogenous cellular inhibitor, the MDM2 (or HDM2) protein. MDM2 is transcriptionally activated by p53 and in turn, inhibits p53 activity through multiple mechanisms mediated by direct binding to p53. MDM2 thus functions as a potent and highly effective endogenous inhibitor of p53. We hypothesize that blocking of the interaction between MDM2 and p53 using a potent small-molecule inhibitor can effectively reactivate p53 function in HNSCC cells with wild type p53, which will lead to cell cycle arrest and apoptosis. Design and development of non-peptide, drug-like, small molecule inhibitors to target the MDM2-p53 interaction is an exciting and new therapeutic strategy for the treatment of CDDPresistant HNSCC with wild type p53. Based upon the high-resolution crystal structure of MDM2 in complex with the p53 peptide, we have designed a class of highly potent, non-peptide, small-molecule inhibitors of the MDM2-p53 interaction using an effective structure-based strategy. The most potent small-molecule nhibitor has a Ki value of 2 nM binding to MDM2 protein, 1000-times more potent than the natural p53 peptide. Using a potent lead compound (MI-63) and its inactive control analogue, we have already demonstrated that MI-63 is highly effective in inhibition of cell growth and induction of cell death in HNSCC cancer cell lines with wild-type p53 and displays an excellent selectivity over HNSCC cancer cell lines with mutated p53 status. Moreover, using an isogenic cell line transfected with Bcl-xL protein, we also show that overexpression of Bcl-xL has no effect upon the activity of potent MDM2 inhibitors, in contrast to CDDP. Based upon our preliminary results, we hypothesize that a potent and specific small-molecule inhibitor of the MDM2-p53 will be developed as a new and highly effective therapy for the treatment of HNSCC, either as a single agent or in combination with other therapies.
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