Monitoring Pd-L1 Expression On Circulating Tumor Cells (Ctcs) In Patients With Nsclc Treated With Pd1 Inhibitors.

147Background: Inhibitors of the immune checkpoint PD-1/PD-L1 have become a standard of care in NSCLC. Patient selection, currently based on PD-L1 expression in tumor tissue, is limited by its temporal and spatial heterogeneity. We hypothesized that monitoring PD-L1 staining of circulating tumor cells (CTCs) could represent a valuable non-invasive biomarker. Methods: Up to 3 blood samples were prospectively collected from patients with advanced NSCLC: i) pretreatment (nivolumab), ii) first follow-up, iii) progression. CTCs were isolated from 10 mL of blood using cell size-based technology (ISET, Rarecells). PD-L1 expression was assessed by immunofluorescence on CTCs and immunohistochemistry on tissue. Results: 162 samples from 96 patients were collected. PD-L1 expression could be assessed pretreatment on tissue and CTCs in 72% and 93%, respectively; and was ≥1% in 37% and 83%; ≥5% in 35% and 79%, respectively. No correlation between tissue and CTCs PD-L1 expressions was observed (Spearman coefficient corr...