Background: Argininosuccinate synthetase (ASS1) loss is a biomarker to select for tumors sensitive to arginine deprivation therapy. In a phase 1 dose-escalation study of ASS1-deficient thoracic cancers, we demonstrated tolerability and a high disease control rate using weekly pegargiminase (ADI-PEG 20) combined with first-line pemetrexed (PEM) and cisplatin (CIS) chemotherapy (Beddowes et al., JCO 2017; ADIPEMCIS). Here, we report the safety and early activity of ADIPEMCIS in an expansion cohort of patients (pts) with metastatic uveal melanoma (UM). Methods: Chemotherapy naïve pts aged 18 years or older with ASS1-deficient, histologically proven metastatic UM were eligible. ADI-PEG 20 (36 mg/m² i.m.) was administered weekly together with PEM (500mg/m²) and CIS (75 mg/m²) every 3 weeks for a maximum of 18 weeks. Pts with stable disease or better were eligible to continue on ADI-PEG 20 until disease progression. Adverse events (AEs) were graded using CTCAE v4.03. Radiological response was assessed by CT or MRI every 6-8 weeks according to RECIST 1.1, alongside pharmacodynamic and immunogenicity analyses, median progression-free and overall survival (PFS and OS) estimates. Results: 10 of 14 screened pts with ASS1-ve metastatic UM received ADIPEMCIS with a median of 1 line of prior therapy (i.e. ipilimumab monotherapy; range 0 to 5). Treatment was well tolerated; neutropenic sepsis was the only grade 3 AE (n = 1 pt). The best response was stable disease with a median PFS of 3.0 months (range, 1.3 to 8.1 months) and a median OS of 11.5 months (range, 3.2 to 24.0+ months). Despite the emergence of anti-ADI-PEG 20 antibodies, plasma arginine concentrations remained low by 18 weeks with a reciprocal increase in plasma citrulline. Tumor rebiopsies at progression revealed ASS1 re-expression (n = 2/2 pts). Conclusions: ADIPEMCIS is well tolerated and has activity in metastatic UM, for which there is no established therapy. Based on recent preclinical data showing synthetic lethality of combining ADI-PEG 20 with PD-1/PD-L1 inhibition, a phase 1 study of ADI-PEG 20 with immune checkpoint blockade is planned in advanced UM. ClinicalTrials.gov identifier: NCT02029690. Clinical trial information: NCT02029690