PD-L1 Expression on Circulating Tumor Cells and Platelets in Patients with Metastatic Breast Cancer

Background: Immune checkpoint inhibition (ICPi) is effective in several cancers. Expression of programmed death-ligand 1 (PD-L1) on circulating tumor or immune effector cells could provide insights into selection of patients for ICPi. Methods: Whole blood (WB) was collected at serial timepoints from metastatic breast cancer (MBC) patients and healthy donors for circulating tumor cell (CTC) and platelet PD-L1 analysis using the CellSearch® assay. CTC PD-L1 was considered positive if detected on at least 1% of the cells; platelet PD-L1 was considered positive if ≥100 platelets per CellSearch frame expressed PD-L1. Results: A total of 207 specimens from 124 MBC patients were collected. 52/124 (42%) samples at timepoint-1 (at or close to time of progressive disease) had ≥5 CTC/7.5ml WB. Of those, 21 (40%) had positive CTC PD-L1. In addition, platelet PD-L1 expression was observed in 35/124 (28%) at timepoint-1. Platelet PD-L1 was not detected in more than 70 specimens from 12 healthy donors. Platelet PD-L1 was associated with ≥5 CTC/7.5ml WB (p=0.0002), less likely in patients with higher red blood cell counts (OR=0.72, p<0.001) and a history of smoking tobacco (OR=0.76, p<0.001). Platelet PD-L1 staining was not associated with tumor marker status, recent procedures or treatments, platelet-affecting drugs, or CTC PD-L1 expression. Conclusion: PD-L1 expression was found in MBC patients on both CTC and platelets in an independent fashion. Inter-patient platelet PD-L1 expression was highly heterogeneous suggesting that it is a biological event associated with cancer in some but not all patients. Taken together, our data suggest that CTC and platelet PD-L1 expression could play a role in predicting which patients should receive ICPi and as a pharmacodynamics biomarker during treatment.