Ribonucleotide Reductase as a Driver of Breast Cancer Aggressiveness - PROJECT SUMMARY Despite the vast improvement in screening techniques and therapies, breast cancer remains the second leading cause of cancer deaths in females worldwide. Approximately 30% of tumors progress to metastatic disease that is incurable. Metastatic breast cancers are highly proliferative and thus rely on rapid deoxynucleotide production. This is supported by increased activity of ribonucleotide reductase (RNR), a multimeric enzyme that is rate limiting for dNTP synthesis. RNR is an attractive therapeutic target for metastatic breast cancer, with the RNR inhibitor, gemcitabine, being FDA-approved to treat advanced disease. Inhibition of RNR causes excessive DNA damage and chromosomal instability (CIN) that inhibits breast cancer cell growth. However, gemcitabine is a nucleoside analog and, as such, has considerable off-target effects on numerous enzymes and displays severe dose-limiting toxicities (i.e. myelosuppression, hepatotoxicity). The development of a novel, non-nucleoside RNR inhibitor would potentially revolutionize patient care by inhibiting tumor growth without the toxicities associated with gemcitabine. This application describes the evaluation of TMU27a, a first-in-class RNR inhibitor that induces DNA damage and CIN in breast cancer cells and inhibits tumor growth in a xenograft breast cancer model without inducing overt toxicities. Proposed studies will expand these analyses to numerous human and mouse cell lines to investigate the impact of TMU27a on genome stability as well as its efficacy and safety compared to gemcitabine in preclinical breast cancer models. These studies will determine if TMU27a provides a foundational agent that is an effective and safe inhibitor of breast cancer growth. In addition, this project will elucidate how TMU27a binds to and inhibits RNR using cryo-electron microscopy. Preliminary data show that TMU27a tightly binds to the RRM1 subunit, and it likely binds within the catalytic site. Furthermore, TMU27a is predicted to be unable to bind in the nucleotide binding pocket of DNA polymerase lambda, a common off-target binding side of gemcitabine that contributes to its toxicity. Understanding how TMU27a inhibits RNR and blocks tumor growth will provide an avenue for designing a novel class of non-nucleoside RNR inhibitors that act with high specificity, potency, and efficacy while avoiding side effects evident with nucleoside analogs such as gemcitabine.
