Abstract 1801: APR-246, a clinical-stage mutant p53-reactivating compound, resensitizes ovarian cancer cells to platinum compounds and doxorubicin

Background: Platinum-based drugs are since decades used as first-line treatment for many solid tumors. Patients with ovarian cancer often respond well to platinum compounds but a majority of patients rapidly develop resistance and die of chemotherapy refractory disease. The mechanisms underlying resistance are multifactorial, but two of the main causes are mutations in the tumor suppressor p53 and elevated intracellular glutathione (GSH) levels. Mutations in p53 occur in about 60% of ovarian tumors. APR-246 (PRIMA-1MET) is the first clinical-stage compound that reactivates mutant p53. APR-246 is a prodrug that accumulates in cancer cells and is converted to the active form MQ, a Michael acceptor that binds to mutant p53, refolds it to wild type conformation and triggers apoptosis (Lambert et al. Cancer Cell 15, 2009). APR-246 has been tested in a Phase I/IIa clinical trial with promising results (Lehmann et al. J Clin Oncol 30, 2012), and a Phase Ib/II study with platinum-based combination therapy in recurrent p53 mutant ovarian cancer is underway. Methods: Cell viability was assessed with WST-1, MTT or FMCA assay. p53 gene status was determined by Sanger sequencing and single strand conformation analysis, and p53 protein expression by Western blotting. Intracellular GSH levels were assessed with GSH kit (Cayman). Results: We have previously shown outstanding synergistic anticancer effects with APR-246 in combination with platinum compounds in p53 mutant solid cancer cell lines, including cisplatin resistant ovarian cancer cells. Synergistic effects were also observed ex vivo as well as in vivo in mice carrying human tumor xenografts. Here we show that APR-246 not only reactivates mutant p53 but also decreases intracellular GSH levels in a dose-dependent manner, presumably via adduct formation between MQ and GSH. APR-246 resensitized cisplatin resistant p53 mutant ovarian A2780-CP20 carcinoma cells to cisplatin, as shown by a decrease in the IC50 value from 52 to 2.9 µM, similar to the IC50 in parental A2780 cells. APR-246 also restored the sensitivity of resistant A2780ADR cells to doxorubicin. A2780-CP20 and A2780ADR were developed from the parental A2780 line with wild type p53 by chronic exposure to the respective drug. Moreover, APR-246 resensitized the p53 mutant OVCAR-3 cell line, established from a drug resistant patient, to cisplatin. Conclusions: Our results show that APR-246 not only reactivates mutant p53 but also decreases intracellular glutathione levels. We propose that this unique dual mechanism of action accounts for the resensitization and strong synergistic effects with APR-246 and platinum drugs. Our results provide strong rationale for the planned clinical study in ovarian cancer and suggest that combination treatment with APR-246 and DNA damaging drugs could have broad applicability in the treatment of drug resistant p53 mutant human tumors. Citation Format: Nina Mohell, Jessica Alfredsson, Asa Fransson, Vladimir Bykov, Mikael von Euler, Klas Wiman, Ulf Bjorklund. APR-246, a clinical-stage mutant p53-reactivating compound, resensitizes ovarian cancer cells to platinum compounds and doxorubicin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1801. doi:10.1158/1538-7445.AM2014-1801