Background: Pegylated arginine deiminase (ADI-PEG20) targets ASS1-ve tumors, including non–small-cell lung cancer (NSCLC), by potentiating pemetrexed cytotoxicity via arginine depletion. In Beddowes et al (JCO 2017) we showed a 100% disease control rate in thoracic cancers treated with ADI-PEG20, cisplatin and pemetrexed (ADIPemCis). Thus, we tested ADIPemCis in a phase I dose-expansion cohort study of patients (pts) with non-squamous NSCLC. Methods: Good performance (ECOG 0-1) advanced non-squamous NSCLC pts were enrolled 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 up to 4 cycles with maintenance ADI-PEG20 or Pem in responding pts. Primary endpoint was tumor response rate (RR by RECIST 1.1), with secondary endpoints including progression-free survival (PFS), overall survival (OS), and toxicity. We also measured plasma [arginine] and [citrulline], anti-ADI-PEG20 antibodies, and PD-L1 expression. Results: 21 of 70 screened pts (median age 60.1) were enrolled between April 15 and August 17. A confirmed partial response (PR) was observed in 55.6 % (n = 10/18 evaluable pts). Median PFS and OS were 4 months (95% CI 2.9-4.8) and 7.2 months (95% CI 5.1-18.4), respectively. 9% (n = 2/21) remain alive on subsequent therapies. 43% (n = 9/21) experienced grade 3/4 treatment-related toxicities, commonly non-febrile neutropenia. Plasma [arginine] declined rapidly and [citrulline] increased; both changes persisted at 16 weeks. 55% of pts’ tumors (n = 6/11 ) were PD-L1 < 1% by immunohistochemistry. Conclusions: The ADIPemCis regimen is active and safe in ASS1-ve NSCLC pts almost doubling the expected RR. However, the short survival compared with ASS1-agnostic historical controls indicates that ASS1 (and frequent PD-L1) loss selects for a biologically more aggressive and immunorefractory NSCLC phenotype. The iTRAP study opening Q2 of 2019 will assess the safety and tolerability of ADIPemPlatinum(Carbo) with atezolizumab in pts with ASS1-deficient non-squamous NSCLC. Clinical trial information: NCT02029690