Abstract A080: Olaparib and the ATR inhibitor AZD6738 in relapsed, refractory cancer patients with homologous recombination (HR) repair mutations – OLAPCO

Introduction: Olaparib is a PARP inhibitor (PARPi) that provides significant clinical benefit in several BRCA-mutant cancers, including ovarian, breast, pancreas and prostate. The benefit is reduced considerably in patients with multiple prior lines of therapy and olaparib resistance has no specific treatment. PARP inhibition results in replication stress (RS) due to unrepaired single strand DNA breaks (SSB) and PARP trapping in BRCA- and other HR repair-deficient tumors. ATR has critical roles in the cellular response to SSB and RS. This makes ATR inhibitors an attractive partner with olaparib, since ATR inhibition has the potential to reverse the two major mechanisms of PARP inhibitor resistance, including restored HR or stabilization of stalled replication forks. The OLAPCO trial (NCT02576444 clinicaltrials.gov) investigated. the combination of olaparib and AZD6738, an inhibitor of ATR, in relapsed, refractory cancer patients with tumors harboring HR repair mutations and in patients with BRCA-mutated PARPi pre-treated/resistant high-grade serous ovarian cancer (HGSOC). Methods: Patients with treatment-refractory, relapsed cancer were enrolled at 4 participating centers. Germline and somatic mutations had to be deleterious by COSMIC or ClinVar for eligibility. Performance status and organ function requirements were standard for early phase trials. Olaparib was given at 300 mg bid daily and AZD6738 at 160 mg daily days 1-7 in a 28-day cycle. Patients were treated until progression. Objective response was assessed by RECIST1.1 and toxicity was assessed by CTCAE4.0. Endpoints were confirmed complete [CR] or partial [PR] response rate and clinical benefit (CB) rate (response [PR] and stable disease [SD] for > 16 weeks). Results: We enrolled 24 patients; 17 (71%) females; median age 59 (36-78) years. Patients were heavily pretreated, with a median of 4 (0-10) prior regimens. Myelosuppression, especially anemia and thrombocytopenia, was the most frequent toxicity but no patient required discontinuation. Two patients required olaparib reductions for anemia. At the time of data cut-off, 20 patients are evaluable for response assessment. One of 5 patients with ATM mutations had a CR, 2 patients have CB ongoing at 12+ months. Of 7 patients with HGSOC resistant to platinum and PARP inhibitors (1-3 prior agents), 1 achieved a PR ( -90%), 3 had a best response of SD with regression < 30% (1 ongoing at 1 year) and 3 patients had progression (PD) as best response (Table 1). No other mutation or cancer type demonstrated objective response. Conclusions: The combination of olaparib and the ATR inhibitor AZD6738 demonstrated preliminary activity in patients with tumors harboring ATM mutations and in PARPi-resistant BRCA1/2-mutated HGSOC. Activity in ATM loss with an ATR inhibitor is consistent with the expected synthetic lethality of these interwoven DNA repair pathways. The encouraging data in HGSOC patients who have already progressed on a PARP inhibitor warrants additional study to further define the potential of this regimen to reverse PARPi resistance in BRCA-mutated HGSOC and other relevant solid tumors. The durability of responses in both groups (4 >1 year) is promising. Table 1MutationATMBRCA prostate pancreasBRCA Prior PARPi HGSOCCHEK2MUS81PALB2IDH and SDHD #5471142 CR1 PR0 1 SD > 4 mos213 11 PD 33112 Inevaluable110 11 Citation Format: Joseph Paul Eder, Davendra Sohal, Haider Mahdi, Khanh Do, Vicki Keedy, Navid Hafez, Deborah Doroshow, Manuel Avedissian, Peter Mortimer, Colin Glover, Patricia LoRusso, Juliane M Juergensmeier, Geoffrey I Shapiro. Olaparib and the ATR inhibitor AZD6738 in relapsed, refractory cancer patients with homologous recombination (HR) repair mutations – OLAPCO [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A080. doi:10.1158/1535-7163.TARG-19-A080