PSUN347 A Novel Mechanism-based Inhibitor of Ribonucleotide Reductase is Highly Effective in Preclinical Models of Endocrine Therapy-Resistant Breast Cancer

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Abstract Approximately 70% of breast cancers express Estrogen Receptor (ER), and patients with ER+ disease benefit from first-line targeted anti-hormone therapies (e. g. tamoxifen). However, at least 30% of patients develop therapeutic resistance. Second-line therapy includes CDK4/6 inhibitors, yet patients nearly uniformly develop resistance to these drugs as well, highlighting the need to identify additional targets in resistant tumors. In this regard, tamoxifen-resistant breast cancer cells and tumors display increased expression of several key cell cycle and chromosomal maintenance-related genes. This includes elevated expression of genes in the Chromosomal Instability (CIN70) signature, comprising of 70 genes that promote CIN and facilitate tumor progression. RRM2 is one such gene that encodes the small subunit of Ribonucleotide Reductase (RNR). In public RNA-seq data, RRM2 was one of the most differentially expressed CIN70 genes in tamoxifen-resistant cells and tumors, and increased expression is associated with poor patient outcomes. We hypothesized that elevated RRM2 drives CIN and subsequent tamoxifen resistance, and that targeting RRM2 or RNR activity will exacerbate CIN to intolerable levels, leading to drug-resensitization and cell death. We confirmed that RRM2 expression is elevated in multiple tamoxifen-resistant ER+ breast cancer cell lines compared to their parental counterparts. Moreover, we found that tamoxifen-resistant cells also exhibit increased CIN phenotypes as indicated by nuclear defects (micro-, multi-, and dysmorphic nuclei). As a model of intrinsic resistance to endocrine therapy, we examined RRM2 expression in triple-negative breast cancer (TNBC) cells that lack ER and found that these cells also have elevated RRM2 compared to ER+ cells. These data affirm the utility of targeting RNR for treating aggressive disease. While there are five currently approved RNR inhibitors, they cause significant patient toxicities due to their chelator mechanism of action. To address this limitation, we developed a novel mechanism-based inhibitor of the catalytic activity of RNR (RRM127) that is less toxic in mouse models. We demonstrate that RRM127 induces excessive CIN in breast cancer cell lines with high RRM2 expression. Most importantly, we have found that RRM127 suppresses growth of both cell line and patient-derived TNBC xenografts in mice. Together, these results indicate that elevated RRM2 and its associated RNR activity promote CIN and that catalytic inhibition with RRM127 has strong efficacy in preclinical models of breast cancer. Current studies are focused on identifying the mechanisms by which RRM127 blocks breast cancer cell growth. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.