127P Exploratory biomarker (BM) analyses from IMpassion050

In IMpassion050 (NCT03726879), adding atezolizumab (A) to neoadjuvant chemotherapy (CT) and pertuzumab–trastuzumab (PH) did not increase pathological complete response (pCR) vs placebo (Pl), CT PH in patients (pts) with high-risk, HER2-positive (+) early breast cancer (EBC) either the intention-to-treat or PD-L1+ populations. Additional BM data are warranted better inform future immunotherapeutic strategies HER2+ BC. We report exploratory analyses of relationship between BMs pCR, their dynamics. Overall, 454 pts centrally confirmed HER2+, node+ EBC were randomly assigned 8 cycles A 4 PH, followed by plus until completion 1 year treatment after surgery, Pl, Pl PH. Pre-specified HER2, hormone receptor (HR), PD-L1 PIK3CA mutation status, tumour-infiltrating lymphocyte (TIL) count available at baseline; TILs surgery for residual disease. Rate difference pCR arms 95% confidence intervals (CIs) was calculated using binomial distribution. Predictive factors evaluated calculating odds ratio (OR) CI selecting those p<0.15 multivariate analysis. characteristics well balanced arms: 89% tumours HER2 immunohistochemistry (IHC) 3+; 51%, HR+ (capping 50%); 48%, PD-L1+; 28%, PIK3CA-mutated; 37% had a high stromal TIL (>10%: enriched -negative population [56% 19%]). No differential effect according subgroup observed arms. Association parameters shown table. Dynamics status will be presented. Table: 127POR95% CIp-valueN=411Treatment (reference: arm)A arm1.000.65, 1.540.9972HER2 IHC 2+/3+ staining pattern focal/heterogeneous)Homogeneous (≥80% staining)2.511.46, 4.310.0009HER2 ISH gene ≥2 <4)≥41.941.18, 3.200.0091HR oestrogen and/or progesterone receptor+)Oestrogen receptor-negative2.411.55, 3.760.0001PD-L1 immune cells [IC] 0)IC 1/2/31.611.04, 2.490.0321PIK3CA mutated)No detected1.721.07, 2.770.0264 Open table new tab There no detected addition analysed. pattern, ratio, HR, predictive line prior data.