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 (IR,2R- diaminocyclohexane)(transdiacetato)(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 IC₅₀, 1.2-3.3 μM) than it was against those harboring wild-type p53 (IC₅₀, 2.8-9.9 μM). In contrast, DACH-acetato-Pt was considerably more active in wild-type p53 models (IC₅₀, 0.17-1.5 μM) than it was in mutant or null models (IC₅₀, 2.7-11.3 μM). 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 G₂-M, whereas DACH-acetato-Pt arrested wild- type p53 cells in G₁ and mutant p53 cells in G₂-M. The G₁ 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 μM) 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 μM. 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.