Overcoming Resistance in Triple-Negative Breast Cancer: A Translational Perspective on Next-Generation DNA Damage Response Inhibitors and Synthetic Lethality

Triple-negative breast cancer (TNBC), particularly when associated with breast cancer susceptibility gene 1/2 (BRCA1/2) alterations or homologous recombination deficiency (HRD), remains therapeutically challenging because DNA repair vulnerabilities coexist with molecular heterogeneity, resistance, and toxicity constraints. This narrative review synthesizes mechanistic, preclinical, clinical, and translational evidence on DNA damage response (DDR)-targeted and synthetic lethality-based strategies in TNBC. We summarize TNBC biological heterogeneity, current biomarker-guided treatment options, mechanisms of poly(ADP-ribose) polymerase (PARP) inhibition and resistance, and emerging DDR targets, including ataxia telangiectasia and Rad3-related/checkpoint kinase 1 (ATR/CHK1), WEE1, DNA-dependent protein kinase (DNA-PK), RAD51, DNA polymerase theta (POLQ), neddylation-related pathways, and targeted protein degradation. The review highlights that PARP inhibitors and platinum agents provide clinically validated examples of exploiting HRD in selected populations, whereas most next-generation DDR inhibitors remain preclinical, investigational, or in early clinical trials. Resistance mechanisms, including BRCA reversion, homologous recombination restoration, replication fork stabilization, and checkpoint adaptation, limit durable benefit. Safety, target selectivity, overlapping toxicities, and the lack of standardized functional biomarkers further constrain translation. Future progress will require prospective biomarker validation, dynamic HRD assessment, rational scheduling of combinations, and medicinal chemistry approaches that improve therapeutic index rather than a broad application of DDR inhibition across all TNBC.