Abstract P5-12-09: The ≥5% cut-off reveals tumor PD-L1 positivity as potential selection biomarker for patient enrollment into the trials evaluating anti-PD-1 or anti-PD-L1 agents in neoadjuvant treatment of triple negative breast cancer
Authors: Finck W , Passildas J , Poirier C , Kwiatkowski F , Abrial C , Durando X , Penault-Llorca F , Radosevic-Robin N 
Published: May 2019

Abstract
Background: Durable responses of triple negative breast cancer (TNBC) to pembrolizumab (anti-PD-1) or atezolizumab (anti-PD-L1) have been reported in the metastatic setting. Moreover, it is currently being hypothesized that immune checkpoint inhibitors might be more effective in the neoadjuvant setting, due to better preserved anti-tumor immunity in early TN disease. However, biomarkers predictive of response to anti-PD-1 or anti-PD-L1 agents, as well as biomarker-based strategies for testing those drugs in the neoadjuvant setting are still lacking. We evaluated PD-L1 protein expression and the composition of tumor-infiltrating lymphocytes (TILs) in untreated TNBC, in order to get a better insight into the TNBC sub-population(s) which would be suitable for neoadjuvant anti-PD-1 or anti-PD-L1 therapy evaluation. Methods: TNBC patients consecutively treated at the Jean Perrin Comprehensive Cancer Centre (Clermont-Ferrand, France), from 01/01/2010 to 12/31/2014, were included. FFPE tumor tissues were assessed for PD-L1 expression by immunohistochemical (IHC) laboratory-developed test (clone 28-8, Abcam), in tumor cells (tPD-L1) and in TILs. Positivity cut-offs evaluated were ≥1%, ≥5% and ≥10%. The amount CD8+, CD4+, FoxP3+ or PD-1+ TILs was determined by counting those cells, detected by IHC methods, within 5 consecutive HPFs (x400), from tumor invasive front toward tumor center. Clinical disease stage was determined using the TNM system. Results: One hundred and two TNBCs were assessed. There were 28.4%, 23.5% or 16.7% tPD-L1-positive cases (cs), for cut-offs ≥1%, ≥5% or ≥10%, respectively. Similarly, 32.4%, 15.7% or 5.9% of cs were positive for PD-L1 in TILs, using the same cut-offs. With ≥5% as cut-off, positivity for tPD-L1 significantly correlated with the amount of CD8+ (p=0.023), FoxP3+ (p=0.0036) or PD-1+ TILs (p=0.043). The same cut-off, applied to TILs, revealed significant correlations between PD-L1 positivity and the amount of each CD8+, CD4+ or PD-1+ TILs (p=0.025, 0.039 and 0.0042, respectively). Interestingly, when the ≥5% cut-off was used, tumors of T2 size were most frequently tPD-L1+ (11/41 cs, 26.8%), compared with the T1 (3/35 cs, 8.6%) and T3+T4 (3/18 cases, 16.7%) (p=0.04). With regards to TILs, with the ≥5% cut-off, the PD-L1+ cases belonged exclusively to the T1+T2 group (15/76), whereas the T3+T4 group was PD-L1-negative (0/18 cs). Other two cut-offs revealed only occasional correlations. Conclusion: This single-center, real-world TNBC cohort contained a high number of smaller tumors (T1-T2). The IHC-based PD-L1 assessment, with ≥5% as the positivity cut-off, revealed that approximately 1/4 of TNBC could be candidates for neoadjuvant anti-PD-1/anti-PD-L1 approaches. Combined with the amount of CD8+ and PD-1+ TILs, tumor PD-L1 positivity might make an easy-to-use composite biomarker for the 1st-level patient selection for PD-1 or PD-L1 inhibitors in neoadjuvant TNBC treatment. The 2nd level could exploit, for example, the assessment of mutation burden in tumors with low tPD-L1 or amount of CD8+ or PD-1+ TILs. Such tumors might be more frequent among larger TNBC (T3-T4).
Citation Format: Finck W, Passildas J, Poirier C, Kwiatkowski F, Abrial C, Durando X, Penault-Llorca F, Radosevic-Robin N. The ≥5% cut-off reveals tumor PD-L1 positivity as potential selection biomarker for patient enrollment into the trials evaluating anti-PD-1 or anti-PD-L1 agents in neoadjuvant treatment of triple negative breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-12-09.