Abstract Background: The discovery of immune checkpoint blockers (ICB) has revolutionized the systemic approach of the treatment of cancer. However, most patients receiving ICB do not derive benefit. Therefore, there is a crucial need to identify reliable predictive biomarkers of response to anti-PD-1/PD-L1 agents, both to develop precision medicine in cancer immunotherapy and to better understand mechanisms of sensitivity and resistance. One pathway that plays an important role in the regulation of immune cell reactivity is L-Arginine (Arg) metabolism, which is essential to T-cell activation. We therefore aimed at evaluating the association of baseline plasmatic level of Arg - serving as a surrogate of Arginase (Arg1) activity - and clinical benefit to ICB. Methods: Correlation with Arg levels and efficacy of ICB in the pre-clinical setting was assessed by using a syngeneic mouse model of colorectal cancer (MC38) known to be responsive to ICB. Correlation of Arg levels and clinical activity of ICB was assessed by analyzing the plasma samples obtained before treatment onset in two independent cohorts of patient with advanced cancer and included in two institutional molecular profiling programs (discovery cohort: BIP, NCT02534649, n=77; validation cohort: PREMIS, n=295, NCT03984318). In addition, using matched PBMCs-plasma samples, we analyzed the correlation between Arg level and features of PBMCs that were captured through multiplexed-flow cytometry analysis. Results: As expected, treatment of MC38-tumor bearing mice with anti-PD(L)1 antibodies demonstrated a strong anti-tumor effect with tumor rejection observed for app. 40% of mice (11 out of 28). The tumor rejection rate was significantly higher in mice with high baseline Arg level than in mice with low Arg level: 85.7% versus 23.8%, p=0.004. In both discovery and validation cohorts, low Arg level at baseline (42 <µmol/L) was significantly associated with worse clinical benefit rate, progression-free survival (PFS) and overall survival (OS). Multivariate analysis showed that low baseline Arg level isd an independent prognostic factor for both PFS and OS. Finally, PBMCs immunophenotyping showed that low Arg level was significantly associated with increased PDL1 expression in several immune cell subsets from the myeloid lineage. Conclusions: Altogether, our results demonstrate that baseline Arg levels are highly predictive of ICB efficacy. Increase in PDL1 expression in myeloid cells upon Arg deprivation could partly underly its suppressive activity. Plasmatic Arg quantification can therefore represent an attractive biomarker to tailor novel therapeutic regimens targeting the Arginase pathway in combination with ICB. Citation Format: Jean-Philippe Guegan, Florent Peyraud, Aurelien Marabelle, Nathalie Chaput, Dominique Bodet, Laure Fontan, Anthony Gaultier, Imane NAFIA, Francois-Xavier Danlos, David Planchard, Caroline Robert, Caroline Even, Mohamed Khettab, Lambros Tselikas, Luc Friboulet, Jean-Charles Soria, Fabrice Andre, Fabrice Barlesi, Alban Bessede, Antoine Italiano. Low plasma Arginine level is associated with resistance to immune checkpoint blockers in patients with advanced cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1251.