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Pifithrin-α (PFTα) is a small molecule inhibitor of p53. By reversibly blocking apoptosis in response to DNA damage, PFTα protects normal cells from lethal doses of γ-radiation (Komarovet al., Science, 1999;285:1733–7). We examined the effect of PFTα on the chemosensitivity of a human cancer in which cell cycle arrest, not apoptosis, is the principle cellular consequence of p53 activation. This was of interest because E6 silencing of p53 sensitizes U87MG astrocytic glioma cells to BCNU and temozolomide (TMZ), cytotoxic drugs that are modestly helpful in the treatment of aggressive astrocytic gliomas. We observed that exposure of U87MG cells to PFTα before cytotoxic chemotherapy attenuated p53-mediated induction of p21WAF1 protein levels, sensitizing U87MG cells to BCNU and TMZ. Sensitization of U87MG cells was associated with G1 arrest, delayed entry into S-phase and decreased repair of DNA damage by BCNU. Our findings suggest that in addition to protecting normal cells from the toxic effects of radiation and chemotherapy, small molecule inhibitors of p53, like PFTα, might play a role in clinical oncology by sensitizing certain resistant cancers to cytotoxic chemotherapies.