Evaluation of PD-L1 expression on circulating tumour cells in small-cell lung cancer

Background: Antibodies against the programmed death-1 (PD-1) receptor and its ligand (PD-L1) have been recently approved for small-cell lung cancer (SCLC) treatment. Circulating tumour cells (CTCs) have emerged as an appealing liquid biopsy candidate that could enhance treatment decision-making in systemic therapy for SCLC patients. Several current technologies enrich CTCs using specific surface epitopes, size, rigidity, or dielectric properties. However, they are hampered by the heterogeneity of the enriched cells from blood samples. Methods: We evaluated two CTC enrichment systems: EpCAM conjugated to magnetic beads and a microfluidic device (Parsortix, Angle plc). PD-L1 expression was evaluated on the isolated CTCs. Twentythree blood samples were collected from 21 patients with SCLC. PD-L1 expression was determined on CTCs through immunofluorescent staining. Results: CTCs were found in 14/23 (60.9%) of the samples, with 11/23 (47.8%) through EpCAMcoated magnetic beads (range, 4-1,611 CTCs/8 mL; median =5) and 11/20 (55.0%) using the Parsortix system (range, 1-165 CTCs/8 mL; median =4). Notably, a total of 17 EpCAM-negative CTCs were isolated using the Parsortix system. PD-L1 expression was detected on 268 of the 3,501 (7.7%) CTCs isolated with EpCAM-coated beads and in 33/366 (9.0%) of the CTCs isolated with the Parsortix system. No vimentin expression was observed in any of the detected CTCs. Conclusions: Overall, we identified a population of EpCAM-negative SCLC CTCs and showed that PD-L1 expression can be assessed on CTCs from SCLC patients. Comparison to tumour and treatment outcomes is needed to validate the potential of CTCs as an alternative sample for the assessment of PD-L1 expression in SCLC.