Association between antibiotic use and pathologic response to neoadjuvant chemotherapy in breast cancer: a...

Antibiotics (ATBs) are frequently prescribed during neoadjuvant chemotherapy (NACT) for localized breast cancer, but their impact on pathologic response is uncertain.

We conducted a retrospective multicenter cohort study of women treated with NACT between January 2009 and January 2024 at three university hospitals in Spain. ATB exposure was ≥1 systemic course within 30 days before NACT or during NACT until surgery. Pathologic response was assessed using the Residual Cancer Burden (RCB) index; endpoints were optimal response (RCB-0/I) and pathologic complete response (pCR; RCB-0). Associations were examined using multivariable logistic regression adjusting for clinical, pathologic, and treatment factors, including relative dose intensity (RDI) and ECOG performance status.

Among 1316 patients, 516 (39.2%) received antibiotics. RCB-0/I was less frequent in exposed vs unexposed patients (36.4% vs 48.6%; P < 0.001); RDI ≥85% was maintained in 83.9% of exposed patients. After adjustment, ATB exposure was associated with lower odds of RCB-0/I (OR 0.56, 95% CI 0.43-0.72; P < 0.001). In subtype-stratified models, ATB exposure was associated with lower odds of RCB-0/I across luminal, HER2-positive, and triple-negative disease (adjusted OR range, 0.54-0.56; P < 0.05), without significant ATB × subtype interaction. ATB exposure was associated with lower pCR odds (OR, 0.75; 95% CI, 0.57-0.98; P = 0.03), despite nonsignificant unadjusted rates (27.7% vs 32.4%; P = 0.08).

Antibiotic exposure shortly before or during NACT was associated with a reduced likelihood of achieving RCB-0/I and pCR after accounting for treatment delivery. These findings support antibiotic stewardship, reinforce that clinically indicated antibiotics remain essential, and require prospective validation.