Expression of PD-L1 on circulating breast cancer cells: Correlation with clinicopathologic data and impact on prognosis

Purpose Immune therapies, including anti-PD-L1 therapies, are widely evaluated in the treatment of solid tumors, including metastatic breast cancer (MBC). While tumor PD-L1 expression has been largely reported, with conflicting results regarding its theranostic value, few data are available regarding the prognostic value of the determination of PD-L1 expression on circulating tumor cells (CTCs), allowing a minimally invasive determination of this biomarker of interest. We previously published a validated and robust method for PD-L1 (+) -CTC detection. In this new report, we focused on the clinicopathological correlations and prognostic value of the presence of PD-L1 (+) -CTCs in an expanded MBC cohort. Methods We investigated PD-L1 (+) -CTCs as previously reported (Mazel et al., Molecular Oncology, 9 (2015); 1773-1782) using the CellSearch ® system in 72 MBC patients from our previous report and a prospective dedicated biomarker study (NCT02866149). Results Median age was 65 years (range 35-87). 69.4% of tumors were classified ER (+) /HER2 (-) , 18.1% HER2 (+) and 12.5% triple negative (TN). 40 patients (55.6%) were treated in ≥ fourth metastatic line. 58.3% of the patients presented with more than 2 metastatic sites. Archival tissue was available for 56 patients (77.8%) and 13 samples (23.2%) showed positive (≥1%) PD-L1 expression. Baseline CTCs were detected in 57 patients (79.2%), with 43 (59.7%) and 36 (50%) patients with ≥4 and ≥10 CTCs, respectively. PD-L1 (+) -CTCs were detected in 26 (36.1%) cases. No statistically significant correlation was found between PD-L1 tumor tissue expression and the presence of PD-L1 (+) -CTCs (p=0.694). At the time of analysis, with a median follow-up of 13 months, 55 patients (76.4%) had progressed and 33 (45.8%) died. Median progression-free survival (PFS) was 5 months (95%CI 3.5-8.0). In univariate analysis, TN phenotype (p=0.020), more than 3 previous metastatic treatment lines (p (+) -CTCs (p=0.047) were significantly associated with PFS, while it was not the case for the CTC count (≥4 CTCs threshold) or the PD-L1 expression in the biopsy (p=0.100 and p=0.280). Median overall survival (OS) was 20 months (95%CI 11.1-38.3). In univariate analysis, TN phenotype (p=0.007) and the CTC count, either at the ≥4 (p=0.024) or ≥10 (p=0.027) threshold were significantly associated with OS, while there was a trend for association with OS of PD-L1 (+) -CTCs detection (p=0.078). Conclusion In this heterogeneous MBC cohort, PD-L1 expression on CTCs appears independent of PD-L1 expressed in the primary tumor site. MBC patients harboring PD-L1 (+) -CTCs seems to have a worse PFS. Evaluation of the respective impact of PD-L1 expression in the tumor and by CTCs is needed in a population treated by anti-PD-1/PD-L1 antibodies in order to evaluate their respective predictive values. An update of the OS data is warranted. Citation Format: William Jacot, Martine Mazel, Caroline Mollevi, Stephane Pouderoux, Veronique D9Hondt, Laure Cayrefourcq, Angelique Ducteil, Marie Viala, Thierry Maudelonde, Severine Guiu, Catherine Alix-Panabieres. Expression of PD-L1 on circulating breast cancer cells: Correlation with clinicopathologic data and impact on prognosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4589.