Background: Blockade of programmed death receptor-1 (PD-1) pathway is effective against solid tumors including lung cancer. Although PD-ligand 1 (PD-L1) expression on tumor tissue is expected as a potent predictive biomarker, its detection remains challenging due to its dynamic and unstable status. Circulating tumor cells (CTCs) have potential as an alternative material for non-invasive and real-time diagnosis. Here, we evaluated the PD-L1 expression on CTCs in patients with lung cancer and investigated the agreement between tumor tissues and CTCs.

Material and methods: CTCs were captured and immune-stained using microcavity array system. CTCs were defined as those positive for DAPI and cytokeratin (CK) and negative for CD45. PD-L1 expression on CTCs was evaluated by addition of the process of PD-L1 immunocytochemistry. For CTCs detection, 3 ml of peripheral whole blood was collected from the patients who consented in written form and PD-L1 immunohistochemistry was performed using corresponding tumor tissues.

Results: Sixty-seven lung cancer patients were enrolled in the study between July 2015 and April 2016 at Wakayama Medical University. Patient characteristics were as follows: median age 71 (range, 39 to 86); male 72%; stage II-III/IV, 15/85%; non-small cell lung cancer (NSCLC)/small cell lung cancer (SCLC)/Other, 73/21/6%. CTCs were detected in 66 out of 67 patients (median 19; range, 0 to 115) and more than 5 CTCs were detected in 78% of patients. PD-L1-expressing CTCs were detected in 73% of patients and the proportion score (PS) of PD-L1-expressing CTCs ranged from 3% to 100%, suggesting intra-patient heterogeneity of PD-L1 expression on CTCs. Significantly more PD-L1-expressing CTCs were detected in patients without EGFR mutations than those with EGFR mutations (P = 0.0433). Tumor tissues were available from 28 patients and were immune-stained for PD-L1. Seven showed the PS of PD-L1-expressing tumor cells < 1%, 11 showed 1-49%, and 10 showed ≥ 50%. No positive correlation was observed on PD-L1 expression between tumor tissues and CTCs based on PS (R2 = 0.0034). Three adenocarcinoma cases with PD-L1-positive tumor tissue did not harbor any PD-L1-expressing CTCs and conversely, three adenocarcinoma cases with PD-L1-negative tumor tissue harbored PD-L1-expressing CTCs, showing the discrepancy between tumor tissues and CTCs. It is also noteworthy that SCLC patients had perfect agreement on PD-L1 expression between tumor tissues and CTCs.

Conclusions: PD-L1 expression was detectable on CTCs in lung cancer patients and intra-patient heterogeneity of its expression was observed. There was no agreement between tumor tissues and CTCs on PD-1 expression. Further investigation is warranted to better understand the clinical significance of PD-L1-expressing CTCs.

Citation Format: Hiroaki Akamatsu, Yasuhiro Koh, Satomi Yagi, Satoshi Kambayashi, Ayaka Tanaka, Kuninobu Kanai, Atsushi Hayata, Nahomi Tokudome, Keiichiro Akamatsu, Masayuki Higuchi, Hisashige Kanbara, Masanori Nakanishi, Hiroki Ueda, Nobuyuki Yamamoto. PD-L1 expression on circulating tumor cells and its comparison with tumor tissues in Japanese lung cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3778. doi:10.1158/1538-7445.AM2017-3778