PD‐L1 Expression on Circulating Tumor Cells May Be Predictive of Response to Pembrolizumab in Advanced Melanoma: Results from a Pilot Study

BACKGROUND: PD‐1 inhibitors are routinely used for the treatment of advanced melanoma. This study sought to determine whether PD‐L1 expression on circulating tumor cells (CTCs) can serve as a predictive biomarker of clinical benefit and response to treatment with the PD‐1 inhibitor pembrolizumab. METHODS: Blood samples were collected from patients with metastatic melanoma receiving pembrolizumab, prior to treatment and 6–12 weeks after initiation of therapy. Multiparametric flow cytometry was used to identify CTCs and evaluate the expression of PD‐L1. RESULTS: CTCs were detected in 25 of 40 patients (63%). Patients with detectable PD‐L1(+) CTCs (14/25, 64%) had significantly longer progression‐free survival (PFS) compared with patients with PD‐L1(−) CTCs (26.6 months vs. 5.5 months; p = .018). The 12‐month PFS rates were 76% versus 22% in the PD‐L1(+) versus PD‐L1(−) CTCs groups (p = .012), respectively. A multivariate linear regression analysis confirmed that PD‐L1(+) CTC is an independent predictive biomarker of PFS (hazard ratio, 0.229; 95% confidence interval, 0.052–1.012; p = .026). CONCLUSION: Our results reveal the potential of CTCs as a noninvasive real‐time biopsy to evaluate PD‐L1 expression in patients with melanoma. PD‐L1 expression on CTCs may be predictive of response to pembrolizumab and longer PFS. IMPLICATIONS FOR PRACTICE: The present data suggest that PD‐L1 expression on circulating tumor cells may predict response to pembrolizumab in advanced melanoma. This needs further validation in a larger trial and, if proven, might be a useful liquid biopsy tool that could be used to stratify patients into groups more likely to respond to immunotherapy, hence leading to health cost savings.