Real-world outcomes of pembrolizumab in peritoneal mesothelioma

e21094 Background: Clinical trials evaluating pembrolizumab (P) in malignant mesothelioma (MM) have included small numbers of peritoneal MM (MPM) patients (pts), but the efficacy of P in the MPM subpopulation remains unclear. We evaluated the efficacy of P in a cohort of MPM pts treated at our institution. Methods: This retrospective cohort study identified MPM pts via ICD9/10 codes seen at our institution between 1/1/2009 and 9/1/2019. Pts were included if they received P as treatment for MPM. Median progression free survival (PFS) and median overall survival (mOS) were estimated from Kaplan-Meier curves. PFS was defined as the start of P until radiologic or clinical progression. Best overall response rate (BOR) and disease control rate (DCR) were determined retrospectively using RECIST 1.1 criteria. Results: We identified 13 non-papillary MPM pts who received P. All pts had received prior chemotherapy (12 platinum/pemetrexed, 1 pemetrexed). Median age at diagnosis was 65.6 years; 77% were white; 46% never smokers; 62% with known asbestos exposure; 70% epithelioid/15% biphasic/ 7.7% sarcomatoid/ 7.7% desmoplastic. BOR to P was 18% (Partial response (PR): 2, Progressive disease (PD): 2, Stable disease (SD): 7, Lost to follow up: 2). DCR was 81% and mPFS 5.7mo. From the start of P mOS was 20.9 mo. Only 3 pts had known PDL1 positivity (1%, 2%, 80%), 4 were negative, 6 untested. Median PFS was not statistically different between PDL1 positive and negative pts (mPFS 5.1mo vs. 5.7 mo, log rank p = 0.73, respectively). Three pts experienced a PFS of > 2 years (PDL1/BOR/% tumor change: Unknown/SD/19%, Negative/SD/-21%, 80%/PR/-70%). The patient with PDL1 80%, biphasic histology and a PR experienced skin toxicity requiring a treatment break. Outcomes for epithelioid histology vs non-epithelioid after P did not differ statistically (mPFS 5 mo vs 39 mo, log rank p = 0.14; mOS 17.5 mo vs NA, log rank p = 0.31). Conclusions: In a real world setting, P has clinically meaningful activity in a PDL1 unselected cohort of MPM pts and should be considered a treatment option for this subpopulation of MM. We did not detect a difference in outcomes based on PDL1 level or histology.