Non-invasive assessment of tumor PD-L1 status with circulating tumor cells

Now FDA-approved in the treatment of over ten malignancies, checkpoint blockade agents have transformed the treatment of cancer. These agents work by inhibiting the PD-1/PD-L1 interaction between cytotoxic T cells and tumor cells, an interaction which promotes immune evasion. PD-L1 expression levels by tumor cells, as assessed by tissue biopsy, is currently the most well-validated and clinically used predictive biomarker of response to checkpoint blockade agents (1,2). However, PD-L1 level is known to be an imperfect biomarker, and some FDA approvals do not set a PD-L1 expression level threshold to determine patient candidacy for checkpoint blockade.