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Quantification of activated and exhausted immune cell populations and simultaneous characterization of circulating tumor cells (CTCs) in peripheral blood of cancer patients receiving checkpoint inhibitor therapy.

e14033 Background: Immune checkpoint inhibitors (ICIs) are becoming standard treatment options in many indications. However, a substantial portion of patients will not respond. Single biomarkers such as PD-L1 are insufficiently accurate to predict patient benefit. We sought to expand the Epic Sciences non-invasive liquid biopsy platform to identify predictive, peripheral blood biomarkers for ICIs using both molecular analyses of CTCs and immune cell changes. Methods: Blood samples from prostate, kidney, and bladder cancer patients treated with ICIs were collected at baseline and on-therapy and sent to Epic Sciences. Nucleated cells were plated on glass slides and stained with CTC (pan-CK, CD45, PD-L1 and DAPI) and immune panels for activation (CD4, CD8, Ki-67, and DAPI) and exhaustion (CD8, Ki-67, PD-1, Lag-3, Tim-3, and DAPI). Changes in populations of immune cells and circulating tumor cells were assessed using high throughput digital pathology. Results: CTCs were detected in 73% (24/33) patients, of which 12% (4/33) had PD-L1+ CTCs detected. No PD-L1+ CTCs were detected in the nine on-therapy samples tested. Of 14 patients with matched samples, 57% (8/14) patients had an increase in activated CD4+ leukocytes and 36% (5/14) patients had an increase in activated CD8+ leukocytes in on-therapy samples compared to baseline. The exhaustion assay was performed on a subset (6 of 14) of matched samples. In baseline and on-therapy samples, one patient had higher levels of exhausted CD8+ leukocytes compared to healthy donor controls, and two patients had lower levels versus controls. No change in exhausted CD8+ leukocytes was observed from baseline to on-therapy. Conclusions: We developed a liquid biopsy-based platform that can simultaneously measure biomarkers in CTCs and leukocytes from a single peripheral blood sample. Changes in activated and exhausted immune cell populations with ICI treatment were detected and PD-L1 expression on CTCs was evaluated. Efforts to further stratify immune and rare cell populations are ongoing.