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The present study was performed to gain insight into the role of p53 and p21WAF1 on the cytotoxicity of the purine analogue cladribine (2-CdA) on cancer cells. Drug sensitivity, cell cycle distribution and drug-induced cell death were compared in three lines derived from the colorectal carcinoma HCT116: the p53+/+ cell line containing wild-type p53 and the p53-/- and p21WAF1-/- lines, in which both alleles of p53 or p21WAF1 were deleted by homologous recombination, respectively. p53-/- and p21WAF1-/- cells were significantly more resistant to the cytotoxic effects of 2-CdA than the p53+/+ cells. p53+/+ cells and p21WAF1-/-, but not p53-/- cells, displayed wt-p53 protein accumulation and arrested in S-phase after exposure to 2-CdA. mRNA analysis of the transporter hENT1 and of enzymes involved in drug metabolism did not show alterations which might explain a drug-resistant phenotype in the p53-/- or p21WAF1-/- cells. Exposure of p53+/+ cells to 2-CdA resulted in expression of p21WAF1 mRNA and protein, enhanced expression of uncleaved PARP-1, and a higher degree both of apoptosis and necrosis than in p53-/- and p21WAF1-/- cells exposed to 2-CdA. Addition of the specific PARP-1 inhibitor 3-AB to 2-CdA-treated cells rendered p53+/+ cells resistant to this drug. Bax levels were reduced in the p53-/- while they increased in the p53+/+ line and remained stable in the p21WAF1-/- cells. We conclude that p53 and p21WAF1 status of cancer cells influences their sensitivity to 2-CdA cytotoxicity. This may involve alterations in the apoptotic cascade as well as in PARP-1-dependent cell death.