Abstract 599: Investigation of PD-L1 expression in circulating tumor cells isolated using the Parsortix system in metastatic lung and breast cancer patients
Authors: Ciccioli Mariacristina , Davis Amy , Alenkhe Ofure , Pailhes-Jimenez Anne-Sophie 
Published: July 2021

Abstract
Programmed death-ligand 1 (PD-L1) allows cancer cells to evade the host immune response when upregulated. PD-L1 antagonists have been shown to be effective immunotherapies. The FDA approved the PD-L1 inhibitor nivolumab for treatment of previously treated, non-responding metastatic non-small cell lung cancer (NSCLC) patients, independent of the PD-L1 status identified in tumour tissue, likely due to the controversial value of PD-L1 detection on tissue biopsy that may be out-of-date. Measurement of PD-L1 expression in circulating tumor cells (CTCs) may enable repeat testing to provide up-to-date PD-L1 status and the potential to monitor patients on these therapies. Whilst the epitope-dependent CELLSEARCH® system is the only technology FDA-cleared for the enumeration of CTCs, its application is limited to epithelial CTCs and has limited sensitivity in NSCLC. We developed a research use only assay for the characterisation of PD-L1 expression on CTCs isolated using the Parsortix® system, a label-independent microfluidic device that harvests CTCs of all phenotypes (epithelial and/or mesenchymal) based on size and compressibility. Peripheral blood (8 – 10mL) was drawn into K2EDTA tubes from 17 healthy volunteers, 17 metastatic breast cancer (MBC) patients and 18 metastatic NSCLC patients and processed on Parsortix systems within 24 hours. Harvested CTCs were cytospun and immunofluorescently stained for detection of cytokeratins and PD-L1. Slides were imaged using a BioView AllegroPlus imaging system.No CTCs were identified in the healthy volunteers, highlighting the specificity of the assay. CTCs (≥1) were identified in 70% of the MBC patients and 55% of the NSCLC patients. Importantly, the positivity rate observed in NSCLC patients was 2-fold higher than that in previously described studies using the CELLSEARCH system. A mean of 9 CTCs/tube of blood were identified in the CTC positive MBC patients, with a range of 1 – 128 CTCs. A mean of 11 CTCs/tube of blood were identified in the CTC positive NSCLC patients, with a range of 1 – 23 CTCs. CTC clusters, consisting of 3 – 45 cells per cluster, were observed in both NSCLC and MBC patients. High heterogeneity of PD-L1 expression was observed. The CTC positive patients were classified into three groups: patients with CTCs positive for both cytokeratins (CK+) and PD-L1 (36% in MBC and 40% in NSCLC), patients with a mixed population of CK+, PD-L1+/- CTCs (27% in MBC and 40% in NSCLC) and patients with CK+, PD-L1- CTCs only (36% in MBC and 20% in NSCLC).The optimized assay for detection of PD-L1 expression on CTCs allowed for the identification of PD-L1 positive and PD-L1 negative CTCs in a significant proportion of the NSCLC and MBC patients studied. The ability to isolate significant numbers of PD-L1 positive CTCs in blood lays the groundwork for development of dynamic PD-L1 monitoring to support personalized patient management.
Citation Format: Mariacristina Ciccioli, Amy Davis, Ofure Alenkhe, Anne-Sophie Pailhes-Jimenez. Investigation of PD-L1 expression in circulating tumor cells isolated using the Parsortix system in metastatic lung and breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 599.