Contribution of tumour and immune cells to PD‐L1 expression as a predictive biomarker in metastatic triple‐negative breast cancer: exploratory analysis from KEYNOTE‐119

Abstract The efficacy of pembrolizumab monotherapy versus chemotherapy increased with increasing programmed death ligand 1 (PD‐L1) expression, as quantified by combined positive score (CPS; PD‐L1 expression on both tumour cells and immune cells) in patients with previously treated metastatic triple‐negative breast cancer (mTNBC) in the phase 3 KEYNOTE‐119 study. This exploratory analysis was conducted to determine whether the expression of PD‐L1 on tumour cells contributes to the predictive value of PD‐L1 CPS in mTNBC. PD‐L1 expression in tumour samples was assessed using PD‐L1 IHC 22C3 pharmDx and quantified using both CPS and tumour proportion score (TPS; PD‐L1 expression on tumour cells alone). Calculated immune cell density (CID) was defined as CPS minus TPS. The ability of each scoring method (CPS, TPS, and CID) to predict clinical outcomes with pembrolizumab was evaluated. With pembrolizumab, the area under the receiver operating characteristic curve was 0.69 (95% CI = 0.58–0.80) for CPS, 0.55 (95% CI = 0.46–0.64) for TPS, and 0.67 (95% CI = 0.56–0.77) for CID. After correction for cutoff prevalence, CPS performed as well as, if not better than, CID with respect to predicting objective response rate, progression‐free survival, and overall survival. Data from this exploratory analysis suggest that, although PD‐L1 expression on immune cells alone is predictive of response to programmed death 1 blockade in mTNBC, adding tumour PD‐L1 expression assessment (i.e. CPS, which combines immune cell and tumour cell PD‐L1 expression) may improve prediction. PD‐L1 CPS thus remains an effective and broadly applicable uniform scoring system for enriching response to programmed death 1 blockade with pembrolizumab in mTNBC as well as other tumour types.