Phase II clinical trial of metformin as a cancer stem cell-targeting agent in ovarian cancer.

BACKGROUND: Epidemiologic studies suggest that metformin has antitumor effects. Laboratory studies indicate metformin impacts cancer stem-like cells (CSCs). As part of a phase II trial, we evaluated the impact of metformin on CSC number, and carcinoma associated mesenchymal stem cells (CA-MSC), and clinical outcomes in non-diabetic patients with advanced stage epithelial ovarian cancer (EOC). METHODS: Thirty-eight patients with confirmed stage IIC(n = 1)/III(n = 25)/IV(n = 12) EOC were treated with either (i) neoadjuvant metformin, debulking surgery and adjuvant chemotherapy + metformin, or (ii) neoadjuvant chemotherapy and metformin, interval debulking surgery, and adjuvant chemotherapy + metformin. Metformin treated tumors, compared to historical controls, were evaluated for CSC number and chemotherapy response. Primary endpoints were (i) a greater than 2-fold reduction in ALDH+CD133+ CSC and (ii) a relapse free survival at 18 months of greater than 50%. RESULTS: Metformin was well-tolerated. Median progression-free survival was 18.0 months (95% CI 14.0-21.6) with relapse-free survival at 18 months of 59.3% (95% CI 38.6-70.5). Median overall survival was 57.9 months (95% CI 28.0 - Not Estimable). Tumors treated with metformin had a 2.4-fold decrease in ALDH+/CD133+ CSC and increased sensitivity to cisplatin ex vivo. Furthermore, metformin altered the methylation signature in CA-MSC which prevented CA-MSC driven chemoresistance in vitro. CONCLUSIONS: Translational studies confirm an impact of metformin on EOC CSC and suggest epigenetic change in the tumor stroma may drive the platinum sensitivity ex vivo. Consistent with this, metformin therapy was associated with better than expected overall survival, supporting the use of metformin in phase-III studies.