PD-L1 profiling of circulating tumor cells for immune checkpoint inhibitor therapy in head and neck cancers.

Abstract

36 Background: Selection of Immune Checkpoint Inhibitor (ICI) therapies in Head and Neck cancers are based on IHC based detection of PD-L1 expression in tumor tissue. Invasive biopsy to obtain tumor tissue for IHC is associated with procedural risks, sequelae and expenses. Prior efforts at PD-L1 profiling of Circulating Tumor Cells (CTCs) have been constrained by low CTC yields. We employed a novel approach for harvesting sufficient CTCs from Head and Neck cancers that permit PD-L1 profiling by Immunocytochemistry (ICC). Methods: 15 ml peripheral blood was collected from 95 patients with head and neck squamous cell carcinomas (HNSCC). CTCs were enriched from PBMCs via an epigenetically acting stabilization process which induced apoptosis in non-malignant cells and conferred survival privilege on apoptosis-resistant CTCs, which were harvested and confirmed by immunostaining for EpCAM and pan-CK. Deep ICC profiling of CTCs was performed with organ-specific markers as well as PD-L1:22c3 and PD-L1:28-8 clones. Results: Viable CTCs could be enriched and harvested in 90 out of 95 samples (95.3%) regardless of metastatic or treatment status. Deep ICC and PD-L1 profiling could be performed in all 90 samples (100%). PD-L1 expression was quantitatively assigned as ‘Low’, ‘Moderate’ or ‘High’. 28 (31.1%) samples were positive for PD-L1:28-8 (27 Low + 1 Moderate) and 26 (28.9%) samples were positive for PD-L1:22c3 (24 Low + 2 Moderate). 15 samples were positive for both PD-L1 clones. Conclusions: CTCs in Head and Neck squamous cell carcinomas can be considered for evaluating PD-L1 expression in patients where ICI therapies are otherwise viable.