Preclinical Evaluation of DNA Polymerase Inhibitors Alone or in Combination with Cisplatin in Triple-Negative Breast Cancer

Abstract

Introduction: Breast cancer remains the most prevalent malignancy affecting women worldwide and in Lebanon. Among breast cancer subtypes, triple-negative breast cancer (TNBC) represents a significant clinical challenge due to its aggressiveness, metastatic potential, heterogeneity, and lack of effective targeted therapies. Despite advances in immunotherapy, chemotherapy remains the primary treatment option in TNBC. Platinum agents, such as cisplatin, remain the cornerstone of treatment for TNBC, but they are often compromised by systemic toxicity. This necessitates dose reduction, which in turn facilitates the rapid development of drug resistance. Therefore, there is a critical need for new TNBC treatments used alone or in combination with chemotherapy, to enhance their efficacy and mitigate their adverse effects. We have previously identified the adamantyl retinoid ST1926 and its novel derivative GEM144 as potent inhibitors of DNA polymerase alpha (POLA1), an enzyme essential for eukaryotic DNA replication initiation. GEM144 further acts as a dual inhibitor of histone deacetylase 11 (HDAC11) and exhibits improved pharmacological properties over the parent compound ST1926. This study evaluates the preclinical efficacy and mechanisms of action of combining cisplatin with GEM144 or ST1926 in TNBC, hypothesizing that dual targeting of DNA replication and DNA repair machinery will yield synergistic anti-tumor activities.

Methods: The study used basal-like HCC1806 and the luminal androgen receptor (LAR) MDA-MB-453 human TNBC cell lines. Cytotoxicity and cell viability were assessed using Sulforhodamine B (SRB) and trypan blue dye exclusion assays. Drug interaction was quantified using the Combination Index (CI). Cell cycle distribution and apoptosis were analyzed using flow cytometry with propidium iodide staining. In vivo efficacy was evaluated using the orthotopic Mouse Intraductal (MIND) model in NSG mice, which mimics the physiological tumor microenvironment.

Results: In vitro screening revealed that combining cisplatin with GEM144 or ST1926 synergistically suppressed cell growth in both basal-like and LAR subtypes, overcoming the resistance of TNBC cells to chemotherapy. Mechanistically, the combination treatment induced a rapid cell cycle arrest in the G0/G1 phase and a significant time-dependent accumulation of cells in the sub-G1 region, suggesting apoptotic cell death. In vivo optimization trials revealed that high dose combinations resulted in severe toxicity. However, an optimized regimen of twice daily intraperitoneal injection of GEM144 (20 mg/kg) combined with low dose cisplatin (2 mg/kg) demonstrated an acceptable safety and potent anti-tumor activity, achieving a significant 77% reduction in tumor weight in the combination treatment compared to controls.

Conclusion: This study provides the first preclinical evidence that combining the dual POLA1/HDAC11 inhibitor GEM144 with cisplatin exerts synergistic anti-tumor effects in TNBC. We successfully established an optimized dosing regimen in the clinically relevant MIND model that maximizes efficacy while minimizing toxicity, offering a promising therapeutic strategy to improve outcomes for TNBC patients.