Sequential ATR and PARP Inhibition Overcomes Acquired DNA Damaging Agent Resistance in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) remains the most lethal cancer and will soon be the second most common cause of cancer related death. While regimens containing DNA damaging agents such as FOLFIRINOX and PARP inhibitors have derived clinical benefits for some patients, their efficacy invariably fails over time. This presents a significant clinical challenge, and thus there is an urgent need for novel therapeutic strategies which are able to overcome the acquisition of resistance in PDAC. Clinically relevant models of treatment resistance were generated from patient-derived cell lines by extended exposure to chemotherapy agents. Synergy scoring, clonogenicity assays, flow cytometry, immunofluorescence and transcriptomic analysis were used to investigate the efficacy of combined ATR and PARP inhibition in re-sensitising resistant PDAC to treatment. Acquisition of resistance was associated with transcriptomic shifts in cell cycle checkpoint regulation, metabolic control, DNA damage response (DDR), programmed cell death and the replication stress response. Additionally, combined treatment with the ATR inhibitor (ceralasertib), and the PARP inhibitor (olaparib) was synergistic in all models of acquired resistance. Sequential treatment using ceralasertib prior to olaparib was highly effective at low dose for DDR proficient cell lines, whereas DDR deficient models responded better when treated with olaparib first. We provide in vitro evidence of a novel therapeutic strategy to overcome acquired PARP inhibitor and platinum resistance in PDAC by using sequential exposure to ceralasertib and olaparib. A sequential regimen may be more tolerable and should be investigated clinically to circumvent dose limiting toxicity in concurrent combinations.