Background: Argininosuccinate synthetase 1 (ASS1)-deficient malignant pleural mesothelioma (MPM) cells are sensitive to arginine deprivation with pegylated arginine deiminase (ADI-PEG20), which also potentiates the cytotoxic effect of pemetrexed (PEM). In the phase I dose-escalation TRAP study (NCT02029690) we showed that ADI-PEG20 with first-line PEM and cisplatin (CIS) chemotherapy (ADIPEMCIS) produced a 100% disease control rate (DCR) in patients (pts; n = 9) with ASS1-deficient thoracic cancers, with no additional toxicity (Beddowes et al 2017). Here, we present the TRAP expansion cohort experience in MPM. Methods: Good performance (ECOG 0-1) MPM pts with non-resectable disease and measurable by modified RECIST, were enrolled in a phase I TRAP expansion cohort at the maximum tolerated dose (MTD) of ADIPEMCIS, using tumoral ASS1 loss as a selection biomarker. PEM (500mg/m2) and CIS (75mg/m2) were given every 3 weeks with weekly IM ADI-PEG20 (36mg/m2) for a maximum of 6 cycles with maintenance ADI-PEG20 in responding pts. Primary endpoint was tumor response rate (modified RECIST), with secondary endpoints including progression-free survival (PFS), overall survival (OS), and toxicity. We measured plasma arginine and citrulline concentrations, ADI-PEG20 antibodies, and biopsied patients on progression to explore resistance mechanisms. Results: 31 ASS1-deficient MPM pts (median age 67) were enrolled (11 epithelioid, 10 biphasic and 10 sarcomatoid) out of 92 screened pts. Plasma arginine decreased with a reciprocal increase in plasma citrulline. The partial response rate was 35.5% (95% CI 19.2%-54.6%) with a DCR of 93.5% (95% CI 78.6%-99.2%). Median PFS was 5.6 months (95% CI 4-6) and median OS was 10.1 months (95% CI 6.7-17.7). 10/31 pts (32.3%) experienced grade 3/4 treatment-related toxicities, the most common being neutropenia (16.1%). Upregulation of ASS1 expression was observed in 2/3 biopsies on progression. Conclusions: The ADIPEMCIS regimen is active in ASS1-deficient MPM pts, including non-epithelioid disease. Based on these data the ATOMIC-meso phase 2/3 trial has opened comparing ADIPEMCIS versus PEMCISPlacebo, focusing on pts with non-epithelioid MPM. Clinical trial information: NCT02029690