Circulating Tumor Cell PD-L1 and Peripheral Blood CD8+ T-Cell PD-1 as Dual Liquid Biopsy Biomarkers for Immunotherapy Outcomes in Advanced Non–Small Cell Lung Cancer: A Retrospective Study

Background: Circulating tumor cells (CTCs) and peripheral T-cell immune checkpoints offer minimally invasive biomarkers for immune checkpoint inhibitor (ICI) therapy. We assessed whether a dual biomarker combining PD-L1 on CTCs and PD-1 on circulating CD8 + T cells is associated with outcomes in advanced non–small cell lung cancer (NSCLC). Methods: We retrospectively enrolled stage IIIB–IV NSCLC patients treated with anti-PD-1/PD-L1 agents (any line) from January 2022 to December 2023. Eligibility required pretreatment blood and evaluable dual-biomarker testing; all 126 included patients had successful CTC assessment and flow-cytometric CD8 + T-cell PD-1 measurement. Patients were classified as CTC PD-L1–positive if ≥ 1 CTC showed membranous and/or cytoplasmic PD-L1 staining. CD8 + PD-1high was defined by the cohort median PD-1+ fraction (35.6%). Patients were stratified by CTC PD-L1 status and CD8 + PD-1 (high/low). The primary endpoint was progression-free survival (PFS); overall survival (OS) and objective response rate (ORR) were secondary endpoints. Results: Fifty-seven patients (45.2%) were CTC PD-L1–positive. Median PFS was longest in the PD-L1 + /PD-1high group (14.2 months) versus the other three groups (8.5, 6.3, and 4.8 months; p< 0.001). ORR followed a similar gradient (41.9%, 30.8%, 22.9%, and 14.7%; p=0.021). After adjustment for baseline covariates, dual-biomarker status remained independently associated with PFS (HR=0.48, 95% CI 0.32– 0.72; p< 0.001) and OS (HR=0.53, 95% CI 0.35– 0.80; p=0.002). Conclusion: This dual liquid-biopsy approach was associated with response and survival in this retrospective advanced NSCLC cohort receiving ICIs, supporting its potential for prognostic stratification; prospective validation is needed.