Abstract 5430: Incorporation of selenium into temozolomide elicits superior antitumor activity that involves new mechanisms of action

Abstract

Abstract Temozolomide (TMZ), an alkylating agent that exerts antitumor activity by interfering with DNA replication, is used commonly in the treatment of two deadly diseases, malignant glioma and melanoma. Although TMZ can improve overall survival of patients with cancer, the therapeutic outcomes remain unsatisfactory: nearly 3/4 of the TMZ-treated glioma patients still die within 2 years. Thus, development of better anticancer drugs remains to be an urgent task. Selenium (Se) compounds, which possess apoptosis-inducing and anti-proliferating effects, have been studied extensively as chemopreventive and chemotherapeutic agents. On the basis of previous structure-activity studies, here we tested the hypothesis that incorporating Se into TMZ would improve the anticancer properties of this drug. A TMZ-Se analog was synthesized by rationally introducing an N-ethylselenocyanate extension to the amide functionality in TMZ structure, which is known not to be involved in its mode of action. In MTT and clonogenic assays, TMZ-Se showed a superior cytotoxicity to TMZ in human glioma cell lines T98G, LN229, U251, and U87MG, and in melanoma cell lines, UACC903 and 1205Lu. TMZ-Se was also effective against a TMZ-resistant glioma cell line, CCF-STTG. In a mouse melanoma xenograft model, TMZ-Se demonstrated a more potent tumor-inhibiting activity than TMZ. To explore the mechanism underlying the superior antitumor activity of TMZ-Se, we compared the effects of TMZ and TMZ-Se on apoptosis and autophagy, two cellular processes that determine death or survival of a cell. Apoptosis was significantly increased in tumor cells treated with TMZ-Se in comparison to those treated with TMZ, as determined by flow cytometric analysis of Annexin V staining, and Western bolts of cleaved caspase-9, caspase-3 and PARP. As compared with TMZ, TMZ-Se also triggered greater autophagic response, as examined by Western blots of the autophagy markers, LC3-II and p62. Interestingly, although qRT-PCR demonstrated that mRNA level of the key autophagy gene, Beclin 1, was increased following TMZ-Se treatment, Beclin 1 protein was down-regulated in the cells subjected to same treatment. TMZ-Se-triggered autophagy appeared to be cytoprotective, as suppressing autophagy by the inhibitors, 3-MA or chloroquine, further enhanced the efficacy of TMZ-Se. Also, TMZ-Se showed stronger down-regulating effect on the amount of O6-methylguanine-DNA methyltransferase, an enzyme that plays a crucial role in protecting cells from cytotoxicity of alkylating agents. These results indicate that incorporation of Se into TMZ can render greater potency to this chemotherapeutic drug via eliciting new mechanisms of action. Thus, incorporating selenium into certain conventional anticancer drugs to improve their effectiveness might represent a new strategy of drug development that warrants further investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5430. doi:10.1158/1538-7445.AM2011-5430