SAN ANTONIO — Adjuvant metformin lowers fasting insulin levels and improves insulin sensitivity in hyperinsulinemic nondiabetic women with early-stage breast cancer, which should in theory result in an improved 5-year overall survival rate, Dr. Pamela J. Goodwin said at the annual breast cancer symposium sponsored by the Cancer Therapy and Research Center.
The degree of insulin reduction obtained with adjuvant metformin in her small pilot study corresponds to an estimated 4% increase in 5-year overall survival. This deserves further study, said Dr. Goodwin of the University of Toronto.
She reported on a single-arm, phase II study of 22 nondiabetic women with a fasting insulin of at least 45 pmol/L who were diagnosed with early-stage breast cancer a mean of 31 months earlier. All took metformin at 1,500 mg/day for 6 months to see if the drug would lower their insulin levels. It did, by a mean 22.4% from a baseline value of 70.7 pmol/L.
Metformin improved insulin sensitivity by 25.6% according to Homeostasis Model Assessment, from a mean baseline of 2.24.
Fasting blood glucose levels didn't change in response to metformin. Neither did quality of life as reflected in a standardized measure. But body weight fell from a baseline of 75.4 kg to 73.5 kg after 6 months on metformin, and body mass index dropped from 28.1 to 27.4 kg/m2.
In Dr. Goodwin's earlier prospective cohort study involving 512 nondiabetic women with early-stage breast cancer, hyperinsulinemia was independently associated with two- to threefold increased risks of distant recurrence and death in a multivariate analysis adjusted for established treatment- and tumor-related variables (J. Clin. Oncol. 2002;20:42–51).
The study was funded by the Canadian Breast Cancer Research Alliance.