Gut microbiota diversity is prognostic in metastatic hormone receptor-positive breast cancer patients receiving chemotherapy and immunotherapy.

Immune checkpoint blockade (ICB) is standard treatment in several cancer types, despite not being proven efficacious in metastatic hormone receptor-positive breast cancer (HR+ mBC). The gut microbiota is associated with patient outcome and toxicity from cancer therapy, although limited data are available for breast cancer. In the randomized phase 2b trial ICON, immunomodulating chemotherapy was investigated in combination with dual ICB in HR+ mBC. To determine whether gut microbiota could inform prognosis, we performed 16S (V3-V4) rRNA sequencing on fecal samples collected at baseline and after 8 weeks of study treatment. We showed that high alpha diversity before treatment was associated with prolonged progression-free survival (PFS; primary trial endpoint) and overall survival. Alpha diversity was lower in patients with prior chemotherapy in the metastatic setting. However, alpha diversity remained significantly associated with PFS after correcting for prior chemotherapy and other factors in bivariate analyses. High-grade immune-related toxicity was also associated with high alpha diversity. These findings suggest that high alpha diversity should be further investigated as a positive prognostic factor in HR+ mBC and approaches to increase alpha diversity could potentially improve clinical outcome.