Long-Term Drug Screening Identifies PARP Inhibitors as Irinotecan Combination Partners in BRCA1/2-Wild-Type Pancreatic Ductal Adenocarcinoma

Background/Objectives: Irinotecan is a component of standard first-line regimens for advanced pancreatic ductal adenocarcinoma (PDAC), yet cells that survive irinotecan treatment may retain the capacity to re-establish growth. Most anticancer drug screens use short-term growth inhibition as an initial criterion for compound selection, potentially excluding compounds with limited acute activity. We asked whether some of these excluded candidates might nevertheless show substantial long-term activity and enhance irinotecan responses in combination. Methods: We first screened 3524 compounds for short-term growth inhibition and advanced compounds with limited acute activity to a 10-day colony-growth screen. Long-term hits were narrowed to clinically relevant anticancer agents and then screened with irinotecan. Talazoparib showed the lowest combination-to-single-agent score and was selected for further evaluation. The combination was evaluated across multiple BRCA1/2-wild-type PDAC cell lines and further validated using additional PARP inhibitors and a xenograft model. RNA sequencing and γH2AX analysis were used to examine associated DNA damage-response features. Results: Of the 1877 compounds advanced from the initial 3-day screen, 334 (17.8%) met the long-term hit criterion. In short- versus long-term comparisons, irinotecan plus talazoparib produced greater growth suppression and higher Bliss synergy scores in 10-day than in 72 h assays. Enhanced long-term responses were observed across five BRCA1/2-wild-type PDAC cell lines, and similar long-term effects were observed with three additional PARP inhibitors. The combination also showed synergistic antitumor activity in an MIA PaCa-2 subcutaneous xenograft model. Transcriptomic and γH2AX analyses provided findings consistent with replication-stress and DNA-damage-response processes. Conclusions: A substantial subset of compounds with limited acute activity showed substantial long-term activity. PARP inhibition enhanced irinotecan responses and emerged as a candidate combination strategy in BRCA1/2-wild-type PDAC.