Abstract PS2-07-12: Gut microbiome composition predicts pathologic complete response in patients with early-stage HER2-positive breast cancer receiving neoadjuvant HER2-targeted therapy +/- immunotherapy with pembrolizumab

Abstract Background: The gut microbiome has been shown to influence response to immune checkpoint inhibitors (ICI). This study aimed to investigate the association between gut microbiome profiles and clinical outcomes in patients with early-stage HER2-positive (HER2+) breast cancer receiving neoadjuvant HER2-targeted therapy +/- immunotherapy with pembrolizumab (K) as part of the randomized phase II neoHIP trial. Methods: Patients with stage II-III HER2+ breast cancer were randomized to 3 treatment arms: Arm A consisted of paclitaxel (T), trastuzumab (H), and pertuzumab (P), THP; Arm B of THP-K; and Arm C of TH-K. Fecal samples were collected at baseline, cycle 3 day 1, surgery, and at 30 and 60 days post-surgery and were analyzed using deep shotgun metagenomics sequencing. Pre-operative fecal samples were analyzed according to pathologic complete response (pCR) and onset of diarrhea during the neoadjuvant period. Comparisons were adjusted to control for false discovery rates (q-value). Results: 99 fecal samples (Arm A: n=32; Arm B: n=48; Arm C: n=19) from 27 patients were included in the analysis. Bacterial diversity by Jaccard analysis was significantly different across all time points for all arms (all p