Abstract 2854: Antibiotics blunt the innate immune mechanisms underlying HER2+ breast cancer therapy efficacy

Abstract Cancer patients are especially vulnerable to infections. About 20-60% of patients may receive antibiotics at some stage during their treatment, including perioperative care and neutropenic periods. Retrospective studies indicate that antibiotics can negatively affect outcomes in various cancers. HER2+ breast cancer (BC) patients often receive targeted treatments such as anti-HER2 antibodies and/or small-molecule tyrosine kinase inhibitors (TKI), which remain understudied in this context. Published work indicates that HER2+ BC patients experience more severe reductions in survival following antibiotic exposure than HER2-negative cases. Although this antibiotic effect is thought to result from host microbiota disruptions, the precise mechanisms remain unclear. To understanding how antibiotics specifically affect HER2 therapies, we examined how antibiotics influence responses to HER2-targeted therapies using an orthotopic HER2+ BC mouse model, administering an oral broad-spectrum antibiotic cocktail (ABX) to deplete the microbiota or water (H2O) as a control, and employing an anti-HER2 antibody (anti-HER2/neu) or the TKI neratinib. While antibiotic treatment alone did not influence tumor growth, it notably impaired responses to HER2 therapies. To simulate clinical scenarios, we administered oral ciprofloxacin (cipro) or amoxicillin/clavulanic acid (am/clav), common antibiotics used for cancer patients, alongside neratinib, which was most affected by ABX. Cipro reproduced the ABX effect, completely blocking neratinib response, while am/clav caused a partial impairment. Next, we characterized immune cells in tumors and the bloodstream after HER2-targeted therapy by spectral flow cytometry. In H2O mice, both HER2 therapies led to immune remodeling of the tumors, with reduced tumor-associated neutrophils and macrophages, and increased NK, CD8+, and CD4+ T cell infiltration. In contrast, NK cells were systemically decreased in untreated ABX mice compared with H2O mice and did not recover after therapy, explaining the failure to recruit these cells to the tumor site in ABX mice. To assess the role of NK cells in therapy efficacy, we depleted NK cells with an anti-asialo-GM1 antibody before and during HER2 therapies. As expected, NK cell depletion abolished the anti-HER2 response but also led to a partial reduction in neratinib efficacy, hinting that neratinib engages tumor-extrinsic innate immune mechanisms influenced by antibiotics. Overall, our findings show that antibiotic-induced disruption of the gut microbiota hampers HER2+ BC therapy by impairing the immune response both systemically and within the tumor microenvironment. Citation Format: Lily J. Parker, Payal Mitra, Nofar Erlichman, Pavani Chalasani, Rong Li, Romina E. Araya. Antibiotics blunt the innate immune mechanisms underlying HER2+ breast cancer therapy efficacy abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2854.