TPS9126 Background: Immunotherapy directed against the programmed death-1 / ligand-1 (PD-1/L1) axis has revolutionized the treatment of advanced non-small cell lung cancer (aNSCLC). Tumor PD-L1 is currently the only biomarker validated for predicting patient response to front line PD-1/L1 directed immunotherapy, yet 20% of patients with ≥50% PD-L1 expression die within six months of starting therapy (Reck et al. 2016). Blood-based agents such as autoantibodies and circulating inflammatory biomarkers have stratified patient outcomes on anti-PD-1/L1 immunotherapy in preliminary studies (Tarhoni, Kollipara et al. 2019; Tarhoni, Fidler et al. 2019). Moreover, a serum-based proteomic test that uses mass spectrometry and machine learning to provide three classifications (Good, Intermediate and Poor) has stratified non-treatment naïve aNSLCC patients treated with nivolumab based on their outcomes (Mueller et al. 2020) and identified a subset of patients who progressed rapidly. This study will evaluate these blood-based biomarkers as predictors of response and early progression in patients with >50% PD-L1 positive aNSCLC treated with immunotherapy regimens. Methods: This is a prospective, observational, multicenter study (NCT04676386) designed to assess biomarkers (serum and plasma) as predictive of early progression in 390 patients with aNSCLC treated with anti-PD 1/PD-L1 immunotherapy with or without platinum-based chemotherapy. Key eligibility criteria are treatment naïve aNSCLC with tumor biopsy PD-L1 tumor proportion score > 50%, Eastern Cooperative Oncology Group performance status (ECOG PS) 0-2, and ability to consent to participate. Prior to enrollment, tumor specimens will be tested for PD-L1 expression according to participating centers’ standard operating procedures. For each treatment cohort of 195 patients, enrollment will proceed in sub-cohorts to ensure a population with 20% patients with ECOG PS2 and a total of 40 patients with squamous cell carcinoma per treatment arm. Patients will be followed for a maximum of 3 years. Blood draw for biomarker assessment will be performed prior to treatment initiation, start of 3rd cycle and investigator assessed progression. Biomarker analysis will be performed retrospectively. As a secondary objective, this study will evaluate proteomic test performance in predicting early overall survival (OS) and rapid progression, and in stratifying patient survival and response. Exploratory analyses will correlate baseline and serial circulating protein analytes and autoantibodies with the proteomic test, response measures (RECIST 1.1) and toxicities. Enrollment opened in February 2021. Clinical trial information: NCT04676386.