Expression of p53 in Cisplatin-resistant Ovarian Cancer Cell Lines: Modulation with the Novel Platinum Analogue (1R, 2R-Diaminocyclohexane)(trans-diacetato)(dichloro)-platinum(IV)

The compound ( 1R , 2R -diaminocyclohexane)( trans -diacetato)(dichloro)platinum(IV) (DACH-acetato-Pt) is a novel platinum-based antitumor agent with clinical potential against cisplatin-resistant disease that is under development in our laboratory. In view of the central role of the wild-type p53 tumor suppressor gene in drug-induced apoptosis, we evaluated the cytotoxicity of cisplatin and DACH-acetato-Pt in a panel of cisplatin-resistant ovarian tumor models with differing p53 status. Cisplatin was relatively more effective against mutant or null p53 cell lines (continuous drug exposure IC 50 , 1.2–3.3 μ m ) than it was against those harboring wild-type p53 (IC 50 , 2.8–9.9 μ m ). In contrast, DACH-acetato-Pt was considerably more active in wild-type p53 models (IC 50 , 0.17–1.5 μ m ) than it was in mutant or null models (IC 50 , 2.7–11.3 μ m ). Inactivation of wild-type p53 function in OVCA-429 cells by the human papillomavirus type 16 (HPV 16) E6 plasmid increased resistance to DACH-acetato-Pt by 3–5-fold, which confirmed the drug’s dependence on wild-type p53 for its high cytotoxic potency. Differences between the two platinum agents were also evident in cell cycle studies: cisplatin arrested both wild-type and mutant p53 cells in G 2 -M, whereas DACH-acetato-Pt arrested wild-type p53 cells in G 1 and mutant p53 cells in G 2 -M. The G 1 arrest by DACH-acetato-Pt was abrogated in HPV 16 E6 transfectant clones of OVCA-429 cells. In agreement with effects on cell cycle progression, a 2-h pulse exposure to low concentrations (≤25 μ m ) of DACH-acetato-Pt induced marked increases in p53 and p21 Waf1/Cip1 expression in OVCA-429 cells. Cisplatin, in direct contrast, had no effect on expression of p53 or p21 Waf1/Cip1 until the drug concentration was increased to 125 μ m . In HPV 16 E6 transfectants of OVCA-429 cells, induction of p53 by the two agents was severely attenuated, and corresponding increases in p21 Waf1/Cip1 were abrogated. This suggests that p21 Waf1/Cip1 increases were p53 dependent. Collectively, the results demonstrate that DACH-acetato-Pt is very distinct from cisplatin. In particular, the greater activity of DACH-acetato-Pt in cisplatin-resistant wild-type p53 ovarian tumor models can be ascribed to its ability to more efficiently induce p53 protein and activate p53 functions.