Background: Maintenance olaparib improves PFS in gBRCA1/2m (core HRD) in mPDAC (Golan, NEJM 2019). Whether other HRD indicators, such as gene mutations other than gBRCA1/2m (non-Core HRD, Cohort B) and exceptional platinum responders (Cohort C, response > 6 months) may benefit from PARPi in mPDAC remains unanswered. We hypothesized that pembrolizumab and olaparib (POLAR) combination may improve outcome by immunogenic cell death. Methods: We conducted an open-label, non-randomized, phase 2 trial of POLAR as maintenance therapy for pts with mPDAC whose disease had not progressed for 4 months (m) in Cohort B or 6 m in C. Herein, we report on Cohorts B & C. Eligibility: ECOG 0-1, mPDAC meeting eligibility of B or C. POLAR (Pembrolizumab 200mg IV Q3W+ OLApaRib 300mg BID) until disease progression or limiting toxicity. Objective response rate (ORR), median PFS (mPFS), median overall survival (mOS), disease control rate (DCR), CA 19-9, cfDNA and baseline HRD mutational signature were analyzed. Results: Cohorts B and C enrolled N=15 each. N=25 pts evaluable by RECIST 1.1. Median follow-up 9.9 (1.3-22.8) and 11.3 (5.8-23) m, respectively. Efficacy details are shown. G3-5 AEs related to treatment: 5/14 (36%): 1 diarrhea (7%), 1 hyperglycemia (7%), 2 anemia (14%), 1 lipase increased (7%). Cohort B: 9/15 (60%) ATM, 3 CHEK2, 2 MUTYH, 1 BLM, 1 FANCC. Canonical gene mutations for mPDAC were less common for pts in Cohort B, especially in ATM PA group (n=9) vs C. Median genomic instability score (GIS) was computed and higher 28 (0-38) vs 9 (0-24) in Cohort B vs C, p=0.052. Median tumor mutation burden (TMB) was not different between B and C (3.3 and 4.1). Conclusions: Clinical activity of POLAR maintenance observed in select pts in B and C. Although PFS was modest (mPFS of 4m [2.1-5.4] in B + C), an intriguing survival signal (mOS at 14m [10-NR] from first POLAR dose) was seen in select patients without chemotherapy. Extensive correlative analyses underway to evaluate response and resistance (SPORE: 1P50CA257881-01A1). Cohort A (core HRD) actively accruing. Clinical trial information: NCT04666740.