Targeting RRM2 to enhance PARP inhibitor sensitivity in TNBC

Triple-negative breast cancer (TNBC) remains one of the most aggressive and therapeutically refractory breast cancer subtypes, primarily due to the absence of hormone receptors and HER2 expression. Poly ADP-ribose polymerase (PARP) inhibitors such as olaparib have demonstrated substantial efficacy in BRCA1/2-mutated TNBC by exploiting synthetic lethality in tumors with defective homologous recombination repair. However, the emergence of acquired resistance to olaparib significantly limits its long-term clinical utility. Ribonucleotide reductase (RR), the rate-limiting enzyme responsible for converting ribonucleotides to deoxyribonucleotides, plays a pivotal role in DNA synthesis and cell-cycle progression. Didox (3,4-dihydroxybenzohydroxamic acid) is a ribonucleotide reductase inhibitor with additional iron-chelating and free-radical scavenging activities. Our previous work identified that the RRM2 subunit is upregulated in TNBC cells and contributes to the development of chemoresistance. The present study investigates whether pharmacologic inhibition of RRM2 using Didox can potentiate olaparib activity and mitigate PARP inhibitor resistance in TNBC models. Four TNBC cell lines-MDA-MB-231, MDA-MB-468, MDA-MB-436, and HCC1937-were utilized to encompass both BRCA-wild-type and BRCA-mutant backgrounds. Cytotoxic responses to olaparib and Didox were quantified by MTS-based IC₅₀ determination, followed by combination index (CI) analysis using the Chou-Talalay method to assess potential drug synergy. Our data show that MDA-MB-436 (BRCA1-mutant) cells were highly sensitive to olaparib, whereas MDA-MB-231 and MDA-MB-468 (BRCA1-wild-type) exhibited marked resistance. Didox displayed moderate single-agent cytotoxicity, however substantially enhanced olaparib sensitivity in resistant lines. Western blot analyses revealed increased DNA damage signaling under combination treatment, indicating that RRM2 inhibition augments PARP inhibitor-induced genotoxic stress. Collectively, these findings support the therapeutic potential of adding an RR inhibitor to overcome PARP inhibitor resistance in TNBC and warrant further in-depth mechanistic and in vivo evaluation.