Abstract 5989: Targeting RRM2 & cell cycle for breast cancer treatment
Authors: Sultana Nahid , Elford Howard L. , Faridi Jesika S. 
Published: April 2023

Abstract
Endocrine treatment is an effective first-line therapy for targeting ER positive, HER2 negative breast cancer. But success is limited by development of acquired resistance due to long-term therapy. Cyclin D1 and cyclin dependent kinase 4/6 (CDK4/6) complex causes phosphorylation and subsequent inactivation of retinoblastoma (Rb) tumor suppressor protein which promotes progression of the cell cycle from G1 to S phase. This observation led the development of the first CDK4/6 inhibitor palbociclib (Ibrance; Pfizer) which induces cell cycle arrest at G1 phase in cancer cells. Owing to intrinsic and acquired drug resistance development, success is limited despite promising clinical outcomes. This situation necessitates the development of potential combination strategies to overcome drug resistance. Ribonucleotide reductase (RR) is a rate limiting enzyme in DNA synthesis consisting of two subunits RRM1 and RRM2. Didox inhibits ribonucleotide reductase subunit 2 (RRM2) which ultimately blocks DNA synthesis. Combination of didox with palbociclib is a potential strategy to target ER positive and ER negative/triple negative breast cancer. Previously, we have shown that didox (DDX) can significantly halt malignant breast cancer cell division in combination with the anthracycline drug doxorubicin by targeting RRM2, mutant p53 and NFkB regulatory proteins. Recently, we confirmed that didox in combination with palbociclib significantly lowers ER positive and ER negative breast cancer and their palbociclib resistant counterparts’ growth compared to no treatment or palbociclib treatment. We also confirmed that ER positive MCF7 and ER negative MDA-MB-468 parental breast cancer cells IC50 of palbociclib drug is lower than their palbociclib resistant counterparts. Here, we are reporting that didox decreases cell cycle proteins alone or in combination with palbociclib in ER positive MCF7 and ER negative MDA-MB-468 parental and palbociclib resistant breast cancer cells. This finding opens a novel approach for targeting both ER positive as well as ER negative breast cancer treatment. We are also reporting that didox treatment modulates cyclin D1 (CCND1) and RRM2 expression in MCF7 and MDA-MB-468 breast cancer along with their palbociclib resistant counterparts. Additionally, we observed that didox alone or in combination with palbociclib alters the cell cycle of MCF7 and MDA-MB-468 parental and palbociclib resistant breast cancer cells. Our data presents a novel and promising approach for the treatment of ER positive and ER negative breast cancer that involves inhibition of RRM2 and cell cycle that merits further clinical investigation in animal and human models.
Citation Format: Nahid Sultana, Howard L. Elford, Jesika S. Faridi. Targeting RRM2 & cell cycle for breast cancer treatment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5989.