Abstract P3-07-08: Temozolomide as a targeted therapy strategy for triple negative breast cancer

Abstract BACKGROUND: Approximately 15% of newly diagnosed breast cancers are classified as triple negative (TNBC). TNBCs are considered more aggressive and have a worse prognosis as no targeted therapies are currently available. These tumors are routinely treated with chemotherapy agents with only modest proven efficacy. Temozolomide (TMZ) is an oral chemotherapy agent commonly used for the treatment of brain tumors and melanoma. TMZ is an alkylating agent, and its therapeutic benefit depends on its ability to alkylate/methylate DNA, most commonly at the N-7 or O-6 positions of guanine residues. This process leads to DNA damage and subsequently triggers cell death. Cells that express the enzyme O6-Methylguanine-DNA Methyltransferase (MGMT) are able to repair damage caused by TMZ. Tumors that lack expression of MGMT, owing to methylation of the gene promoter, demonstrate a better response to TMZ treatment as a result of synthetic lethality. It was first reported in 2012 that TNBCs were more likely to be MGMT methylated, which was confirmed by another group that reported up to 64% of wild-type BRCA1 TNBC exhibited MGMT gene methylation. In 2013 it was found that basal-like breast cancers were more likely to be MGMT methylated and linked to larger tumor size. Together these findings suggest that a sub-population of TNBCs lack MGMT expression, due to promoter methylation. Currently, TMZ is not a treatment option for breast cancers given the modest efficacy of TMZ noted in breast cancer clinical trials; however, most of these trials have focused on using this agent to either treat or prevent brain metastases, due to TMZs ability to cross the blood-brain barrier. Importantly, none of these trials investigated MGMT expression or specifically TNBC populations. We hypothesize that TMZ may be a viable and efficacious treatment option for TNBCs that lack MGMT expression, due to promoter methylation. METHODS: We analyzed 12 archival specimens and 4 TNBC cell lines (HTB132, HTB26, HTB126 and HCC1806) for MGMT expression using a qRT-PCR clinical assay available from Calgary Laboratory Services. Additionally, we also looked at MGMT protein expression in the cell lines using Western Blot analysis to confirm the qRT-PCR results. Finally, we performed an in vitro assay with TNBC cell lines to determine cytotoxicity of TMZ. RESULTS: Analysis of the archival specimens found that 33% of samples analyzed had MGMT promoter methylation by qRT-PCR. Additionally, we found that HTB26 and HTB126 cell lines showed MGMT promoter methylation by qRT-pCR analysis. Western Blot analysis confirmed lack of MGMT expression in these two cell lines, and also identified another cell line (HCC1806) lacking MGMT protein that was classified as unmethylated by the qRT-PCR clinical assay. Moreover, our in vitro assay found that two cell lines (HTB26 and HCC1806) showed a noticeable response to treatment with TMZ. Interestingly, HTB126 did not show response to TMZ, suggesting that there may be another putative resistance pathway. CONCLUSIONS: Preliminary findings suggest that TMZ may be a viable targeted treatment option for TNBCs. Currently, we are investigating drug response using in vivo mouse models, as well as investigating synergistic combination therapy options. Citation Format: Kornaga EN, Gratton K, Shi Q, Yang A, Nixon NA, Roldan Urgoiti G, Morris DG. Temozolomide as a targeted therapy strategy for triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-07-08.

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