Background: Loss of the metabolic tumor suppressor, argininosuccinate synthetase (ASS1), a rate-limiting enzyme in arginine biosynthesis, sensitizes mesothelioma and lung carcinoma cells to apoptosis following arginine withdrawal. Recently, we showed potentiation of the cytotoxic effect of pemetrexed by the arginine depletor pegylated arginine deiminase (ADI-PEG 20) in ASS1-negative tumor cells, which was accompanied by suppression of de novo pyrimidine synthesis and the pyrimidine salvage pathway (Allen et al, Cancer Res 2014). Consequently, we have initiated a phase I study (NCT02029690) to assess the maximum tolerated dose (MTD), safety and toxicity, and preliminary efficacy of ADI-PEG 20 combined with first-line pemetrexed and cisplatin chemotherapy in patients with ASS1-deficient mesothelioma or non-squamous non-small cell lung cancer (NSCLC). Methods: Up to 47 good performance (ECOG 0-1) patients are being enrolled in a 3+3+3 phase 1 design using tumoral ASS1 loss as a selection biomarker. Weekly ADI-PEG 20 is being dose escalated (18, 27 and 36 mg/m² IM), with pemetrexed 500 mg/m² and cisplatin 75 mg/m² both given every 3 weeks, for a maximum of 18 weeks of treatment. Cohorts 1 and 2 have been completed without dose-limiting toxicity and enrollment to cohort 3 began in January 2015 at the maximum ADI-PEG 20 dose of 36mg/m². A further 10 patients with mesothelioma and 10 patients with non-squamous NSCLC will be recruited at the MTD with pharmacodynamic monitoring of response using plasma arginine and citrulline, and assessment of tumor proliferation with [(18)F]-fluoro-L-thymidine (FLT)-positron emission tomography (PET). In summary, TRAP is the first triplet chemotherapy combination study to assess the role of arginine deprivation with ADI-PEG 20 in solid cancers using ASS1 as a selection biomarker. Patient accrual at the MTD cohort is nearing completion and ADI-PEG 20 in combination with cisplatin and pemetrexed will be evaluated further in the planned expansion cohorts. Clinical trial information: NCT02029690