Abstract Introduction: Postmenopausal women with metastatic hormone receptor-positive (HR+) breast cancer are treated with either aromatase inhibitors (AIs), or Faslodex (fulvestrant, ICI), in combination with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i). Despite the improvement in breast cancer outcomes, follow-up studies demonstrate the combination treatment is associated with cardiovascular toxicities. Previous studies have shown Lactobacillus probiotics supplementation shifts the gut microbiome, reduces breast tumorigenesis, and improved menopause-induced cardiometabolic syndrome. However, limited studies have investigated Lactobacillus probiotics on improving cardio-oncological outcomes. Materials/Methods: Eight-week-old female BALB/c mice (n = 60) were placed on a Western diet. At ten weeks of age, syngeneic 4T1.2ER+ breast cancer cells (1 × 106 cells) were transplanted into the left inguinal mammary fat pad. Tumors were allowed to reach 100 mm3 before initiating treatment, which was administered for 21 days. Mice were randomized by tumor volume into treatment groups: untreated control, probiotics (Probx; 2 × 108 CFU Lactobacillus spp.), Letrozole (AI)+Ribociclib (CDK4/6i), Faslodex (ICI)+CDK4/6i, Probx+AI+ CDK4/6i, or Probx+ICI + CDK4/6i. Tumor volume and weight were measured three times weekly using calipers. Vevo cardiac ultrasound was performed at the end of the study. Results: In the 4T1.2ER+ breast cancer animal model, the addition of probiotics with the combinational therapeutics (AIs or ICI with CDK4/6i) significantly reduced tumor volume validating the potential of the probiotic to increase the anti-tumor efficacy of the therapeutics. Furthermore, our cardiac data displayed both ICI+CDK4/6i and AI+CDK4/6i-treated groups significantly increased diastolic dysfunction parameters (E’E and IVRT) that were reduced in groups supplemented with Lactobacillus probiotics. We show addition of Lactobacillus probiotics reduced cardiac fibrosis induced by endocrine-targeting therapies+CDK4/6i treatment. Additionally, we assessed the expression of pro-fibrotic TGF-β/Smad-dependent pathway. We also determined the impact of therapies on oxidative stress markers in cardiac tissue. Lastly, we performed untargeted metabolomics on plasma and proteomics on cardiac tissue to determine potential molecular mechanisms mediated by Lactobacillus probiotic interventions. Conclusion: Our findings highlight CDK4/6i and endocrine-targeting therapies promote cardiac fibrotic deposition and result in diastolic dysfunction. However, Lactobacillus probiotic supplementation significantly improved treatment efficacy, ameliorated drug-induced cardiac dysfunction, and may regulate pro-fibrotic activity. Our data provides a feasible and accessible approach to address drug toxicity in the cardio-oncology setting. Citation Format: Zipporah Cornelius, Adam Wilson, Valerie Payne, Eleanor Cyrus, Kenysha YJ. Clear, David R. Soto-Pantoja, Katherine L. Cook. Shifting the gut microbiome to alleviate anti-cancer treatment-induced cardiovascular toxicity 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 5224.
Cornelius et al. (Fri,) studied this question.