LAUSR

e14044 Background: CTCs not only constitute a cellular component of liquid biopsy for dynamic, longitudinal information of malignancies, but also pose a continuous threat to seed new metastasis. Detection of these cells can help identify patients with residual disease and poor prognosis. However, it is unknown if heterogeneous CTCs can be profiled to monitor cancer stemness and immunotherapy response for patients. Limited clinical data exist for monitoring CTCs for markers of cancer stem cells, such as CD44 or CD166, and immunotherapy therapeutic targets including PD-L1. We hypothesized that dynamic monitoring of CTC positivity for these markers would correlate with responses to immune checkpoint inhibitor treatment for patients with NSCLC. Methods: As part of a prospective analysis of peripheral blood cells, 30 patients with NSCLC undergoing immunotherapy (pembrolizumab or atezolizumab) or combination therapy with chemotherapy had blood samples collected at 3-4 week intervals for up to 24 weeks. Peripheral blood mononuclear cells were isolated and analyzed for CD45, to identify non-leukocyte putative CTCs, CD166 and CD44, to identify cancer stem cell markers among potential CTCs, and PD-L1. Patients were evaluated for response based on RECIST criteria. Statistical analysis was performed using one-way ANOVA on samples from each time point. Results: A decrease in PD-L1+ CD45- cells (CTCs) at the second interval was most strongly associated with initial response. In contrast, increasing number of PD-L1+ CD45- cells were found among patients with progressive disease as compared to those with stable disease or a partial response (p = 0.01). (1) CD45- cells, (2) CD44+; CD45- cells and (3) CD166+; CD45- cells demonstrated no significant association with clinical outcomes. Baseline PD-L1 level on CTCs did not correlate with the PD-L1 level on tissue. Conclusions: We report the feasibility and potential clinical significance of monitoring surface markers on CTCs in the blood before and after initiation of immunotherapy. Our findings that PD-L1+ CTCs increased among non-responders to immunotherapy warrant further validation. Testing is ongoing to evaluate how these changes predict long-term outcome to treatment for patients.