Metformin Active Surveillance Trial in Low-Risk Prostate Cancer.

[PURPOSE] Active surveillance (AS) is a standard management strategy for low-risk prostate cancer (PCa), but a significant proportion of patients ultimately experience disease progression. Metformin, a commonly prescribed antidiabetic agent, has demonstrated antitumor activity in preclinical studies and observational data, prompting investigation into its potential to delay PCa progression.

[PATIENTS AND METHODS] The Metformin Active Surveillance Trial (MAST) was a multicenter, randomized, double-blind, placebo-controlled phase III trial evaluating the efficacy of metformin in men with low-risk, localized PCa managed with AS. Eligible participants were randomly assigned 1:1 to receive either metformin (850 mg twice daily) or placebo and were followed for up to 36 months. The primary end point was time to progression, defined as therapeutic and/or pathologic progression. Progression-free survival (PFS) was assessed using Kaplan-Meier analysis and Cox proportional hazards models.

[RESULTS] A total of 408 patients were randomly assigned (205 metformin, 203 placebo). After a median follow-up of 36 months, 144 participants experienced progression (70 metformin, 74 placebo), with no significant difference in PFS (hazard ratio [HR], 1.09 [95% CI, 0.79 to 1.52]; = .59). Negative biopsy rates at 36 months were 41.0% (metformin) versus 31.1% (placebo; = .181). In prespecified subgroup analysis, metformin was associated with increased pathologic progression among obese patients (BMI ≥ 30; HR, 2.36 [95% CI, 1.21 to 4.59]; = .0092).

[CONCLUSION] Metformin did not reduce progression in men with low-risk PCa on AS. The observed adverse effect in obese patients merits further investigation.