Peony exploratory analyses of biomarker changes: From baseline to surgical samples in patients with HER2+ breast cancer.

Abstract

e15136 Background: PEONY (NCT02586025) is a randomized, multicenter, double-blind, placebo-controlled, phase III trial evaluating neoadjuvant and adjuvant pertuzumab/placebo with trastuzumab and docetaxel in HER2-positive early/locally advanced breast cancer. We previously reported that pertuzumab showed an improved efficacy vs. placebo in terms of total pathological complete response (tpCR), event-free survival and disease-free survival (DFS), as well as in pre-defined biomarker subgroups. Limited data are available on biomarker changes after neoadjuvant treatment in the Asian population. Thus, we aim to (1) describe the biomarker changes from baseline to surgical samples after HER2 targeted neoadjuvant therapy, and (2) explore the association of surgical biomarkers and DFS. Methods: This study will focus on biomarkers of tumor tissue samples collected at baseline and surgery among 218 non-tpCR patients, due to limited samples collected at disease progression. Samples were tested using Immunohistochemistry (IHC) for HER3, PTEN, CD8 and PD-L1. Both HER2/HER3 mRNA expression and PIK3CA mutation were assessed using a PCR based methodology. The statistical analyses were using R (4.1.3). Biomarkers were categorized as high/low either by median derived from baseline samples, or with well-established cutoffs, e.g. HER2 IHC of 3+ vs. 1/2+ and PD-L1 at 1%. The changes of biomarkers were examined among the paired samples using Fisher’s exact test and depicted by alluvial plots. Cox proportional hazard regression analysis was performed to examine the association between surgical biomarkers and DFS in pooled arms. Results: Since the surgical biomarkers are only evaluable among patients with residual disease, patients in this analysis had a higher stage, and were more likely to be node+. Of 166 paired samples, more patients show a decrease in HER2 and HER3 mRNA/IHC, PTEN IHC from baseline to surgery. PIK3CA mutation status remained the same among the majority of patients (n = 154, 95%). Immune-related biomarkers, PD-L1 and CD8, are more enriched in surgical samples compared to baseline. All these observations are similar between the two arms. No clear association has been shown in most of the biomarkers, except for PIK3CA, with DFS. Surgical tumor samples with PIK3CA mutation detected trended with a worse DFS than those with no PIK3CA mutation detected [HR: 1.81 (95% CI: 0.94 – 3.49)]. Conclusions: Of baseline and surgery paired samples, most biomarkers showed a change, except for PIK3CA mutation status. In pooled analyses, we only observed a trend of PIK3CA mutation detected in surgical samples with a worse DFS, which is in line with the association observed with baseline PIK3CA mutation status. No other associations were found between surgical biomarkers and DFS. These findings may suggest that despite biomarker changes patients continue to benefit From HER2 therapies and dual blockade. Clinical trial information: NCT02586025 .