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e21093 Background: Malignant peritoneal mesothelioma (MPM) is a rare variant of malignant mesothelioma, representing < 30% of cases. Standard of care is cytoreductive surgery (CRS) with hyperthermic intraperitoneal chemotherapy (HIPEC) when feasible. The benefit of systemic chemotherapy (chemo) (Neoadjuvant- N, Adjuvant- A, or 1st-line metastatic –M) is not well established and some retrospective studies report worse outcomes with N chemo. However, our institution has favored use of N chemo prior to surgery for symptom relief and surgical optimization. We conducted a single-center retrospective cohort study of MPM patients treated at our institution to evaluate the effect of N vs. A chemo on outcomes. Methods: We identified non-papillary MPM patients via ICD9/10 codes seen at our institution between 1/1/2009 and 9/1/2019. Pts were followed until 1/1/2020. Patients without pathologic diagnosis were excluded. We explored the effect of receipt of CRS, type of systemic therapy, and histology on overall survival. Median overall survival (mOS) from diagnosis was estimated from Kaplan-Meier curves. A Cox proportional hazard model computed hazard ratios (HR) to assess the effect of the exposure on OS. Results: We identified 47 patients with non-papillary MPM: median age 62 years, 77% epithelioid histology, 74.5% white, 55.3% known asbestos exposure. CRS was performed in 24 (51%) and 18/24 (75%) received HIPEC. The majority received systemic therapy (34/47 (72%)). Among those that received chemo and surgery, N chemo was more common than A chemo (N:12 (all platinum/pemetrexed), A:7). Median OS was 52.7 months (mo) overall and 77.2 mo with surgery vs 20.2 mo without (log rank p = 0.006). Toxicity from N chemo did not prevent surgery with 8/12 successfully receiving surgery (1 surgery scheduled, 2 lost to follow up). Of the 10 pts with evaluable scans: 5 had radiographic reduction of disease (2 complete responses by RECIST 1.1), 4 stable disease and 1 with disease growth. N chemo reduced ascites in 3 out of 4 pts with baseline ascites. N chemo was not associated with worse mOS compared to A chemo (HR 0.64, 95% CI 0.1-3.0, p = 0.62). Non-epithelioid histology was not associated with a worse OS compared to epithelioid (HR 1.5, 95% CI 0.6-4.1, p = 0.4). Conclusions: N chemo was not associated with worse outcomes compared to A chemo and toxicity from N chemo did not preclude surgery. In addition, N chemo resulted in reduction of disease burden and ascites in pts with MPM.

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.