Investigate the efficacy of immunotherapy for treatment of pancreatic adenocarcinoma (PDAC) with mismatch repair deficiency (dMMR)

415 Background: Immune checkpoint inhibitors (ICI) have been approved in solid tumors with dMMR. However, only limited data are available for PDAC with dMMR given the rarity of dMMR in PDAC. We evaluated efficacy of ICIs in PDAC with dMMR. Methods: Retrospective clinical and pathologic data were collected for patients (pts) with pancreatic adenocarcinoma from May 2017 to June 2020 at Moffitt cancer center. Results: We identified 10 pts with dMMR PDAC. The median age was 64.5 years (range: 42-86) and 4 pts were male. 4 pts had resectable disease, 3 had locally advanced and 3 had metastatic disease at initial diagnosis. MSH6 deficiency (def) was found in 2 cases, PMS2 def in 2, MLH/PMS2 def in 5, and MSH2/MSH6 in 1. 7 pts were treated with ICIs. 3 pts had locally advanced and 4 had metastatic disease when they started ICIs. 5 received Pembrolizumab (pem), 1 received ipilimumab/ nivolumab (ipi/nivo), and 1 received pem then ipi/nivo after progressive disease (PD) on pem. The median number of prior lines of chemotherapy was 1 (range 0-2). 6 pts were evaluable, and 1 had rapid disease progression after 1 dose of pem. Among 6 evaluable pts, 3 had an objective response (1: complete response and 2: partial response), and 2 had stable disease (SD). Median progression-free survival was 8.2 mo, and median overall survival was not reached with median follow-up (FU) of 6.8 mo. The median duration of response was not reached with a median FU of 22.6 mo. The pt with CR remained disease-free for up to 22 months. The pt whose treatment was switched to ipi/nivo after PD on pem achieved SD > 4mo on ipi/nivo. While on immunotherapy, one patient with ipi/nivo developed immunotherapy associated rash requiring systemic steroids, and another on pem developed hypothyroidism requiring levothyroxine. Conclusions: This series suggest ICIs can provide durable clinical efficacy in pts with dMMR PDAC.