Abstract 2751: Potential of 6-mercaptopurine and 5-azacitidine in halting progression of poor prognosis residual disease in triple negative breast cancer

Breast cancer patients who have minimal residual disease (MRD) after surgery or systemic therapies are at a higher risk of relapse. The ability of MRD to efficiently switch between quiescence and proliferation, depending upon the challenges in the body, increases likelihood of relapse. With the goal of developing therapies that would halt the progression of MRD to clinical metastases, we have developed a cell culture model of such poor prognosis MRD. The model involves choosing cancer cell lines established from therapy-resistant breast cancers, such as inflammatory breast cancer, and subjecting them to glutamine deficiency to select progenitor-like cancer cells that are highly resistant, and which can metastasize to multiple organs in nude mice. Investigations of these adaptable cells (functional studies and molecular analyses) have revealed a variety of ways (genetic mutations, modifications of epigenome, transcriptome, proteome, etc.) that generate a tremendous cellular heterogeneity, and confer survival advantages under various bottlenecks in the body. For a potential therapy to be suitable at the MRD stage, it must be safe (an important criterion prior to clinical relapse), immune system-friendly, and disrupt heterogeneous progenitor-like cancer cells that evolve into clinical metastases. Here we evaluated two ribonucleoside analogs, namely 6-mercaptopurine (6-MP) and 5-azacitidine (5-AzaC), because of their potential to disrupt transcriptome and epigenome in MRD. We chose low-dose 6-MP based on its ability to induce and maintain remission in inflammatory bowel disease (IBD) and childhood acute lymphoblastic leukemia (both situations requiring a control of abnormal progenitor cells), for evaluation in our model of adaptable SUM149-MA cells (MA for metabolic adaptability). A long treatment with low-dose 6-MP inhibited progenitor-like cancer cells from proliferating thus keeping them arrested in quiescence. Here we found that that long treatment with 1 µM 5-AzaC, without a significant effect on cell proliferation, sensitized cancer cells to inhibitory effects of low dose 6-MP (1 µM). Importantly, the treatments for several weeks with low doses of 6-MP and/or 5-AzaC did not render cancer cells resistant to chemotherapeutic drugs doxorubicin or paclitaxel. In fact the cells became more sensitive to chemotherapeutic drugs upon treatment with 6-MP and/or 5-AzaC. Our analyses of protein markers of epithelial-to-mesenchymal transition (EMT) indicated that treatments with 6-MP and/or 5-AzaC do not significantly reverse EMT in our system. Our results suggest that safe and effective drugs like 6-MP (singly or combined with 5-AzaC) could halt progression of poor-prognosis MRD toward recurrence/metastasis. In addition to the direct effects on poor prognosis MRD, 6-MP could potentially modulate the immune system towards a healthy state (analogous to what it does in IBD), to control MRD. An immune modulator like 6-MP could also potentially limit autoimmune reaction type severe toxicities associated with immune checkpoint blockade therapies, thus improving their efficacy in eradicating cancer. Supported by a State of Texas Grant for Rare and Aggressive Cancers. Citation Format: Balraj Singh, Vanessa N. Sarli, Anthony Lucci. Potential of 6-mercaptopurine and 5-azacitidine in halting progression of poor prognosis residual disease in triple negative breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2751.