Mechanisms of PARP Inhibitor Resistance: From Replication Gap Biology and Transcription-Replication Conflicts to PROTAC-Based Next-Generation Strategies.

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) have transformed precision oncology by exploiting synthetic lethality in homologous recombination (HR)-deficient cancers, with multiple FDA-approved agents targeting BRCA1/2-mutant tumors. Despite initial efficacy, resistance inevitably emerges, limiting long-term clinical benefit. This review synthesizes emerging mechanistic insights into PARPi response and resistance. Recent evidence reframes PARP inhibition cytotoxicity through a transcription-replication conflict model and identifies single-stranded DNA gaps as the primary lethal lesion in HR-deficient cells, rather than double-strand breaks. These findings suggest that resistance reflects restoration of replication gap suppression or resolution of transcription-replication stress. We further highlight DNA ligase III as a collateral vulnerability in 53BP1-deficient resistant tumors, and discuss proteolysis-targeting chimera (PROTAC)-based PARP1 degraders as a strategy to overcome resistance and induce alternative cell death pathways. Established resistance mechanisms-including BRCA1/2 reversion mutations, shieldin complex loss, RAD51 hyperactivation, and pharmacokinetic alterations-are reconsidered within this updated framework. Combination strategies with ATR inhibitors show promising clinical activity in PARPi-resistant HR-deficient ovarian cancer. Finally, we propose an integrated biomarker framework combining HRD scar assays, functional RAD51 foci analysis, replication gap profiling, and circulating tumor DNA (ctDNA) monitoring to enable dynamic resistance tracking.