Metformin Fails to Slow Low-Risk Prostate Cancer in Major Clinical Trial
A phase III trial of 408 men finds metformin does not delay prostate cancer progression during active surveillance — and may worsen outcomes in obese patients.
Summary
The MAST trial tested whether metformin, a widely used diabetes drug with known antitumor properties, could slow prostate cancer progression in men on active surveillance. In this multicenter, randomized, double-blind, placebo-controlled phase III trial, 408 men with low-risk localized prostate cancer received either metformin (850 mg twice daily) or placebo for up to 36 months. Results showed no significant difference in progression-free survival between groups. Unexpectedly, obese patients taking metformin showed significantly higher rates of pathologic progression compared to placebo. The trial effectively rules out metformin as a general strategy to delay low-risk prostate cancer progression, while raising important questions about its safety in obese men with this diagnosis.
Detailed Summary
Active surveillance is the standard approach for managing low-risk, localized prostate cancer, sparing patients from immediate treatment side effects while monitoring for disease progression. However, a meaningful proportion of men on active surveillance eventually progress and require intervention, creating interest in pharmacologic strategies that might delay or prevent that progression.
Metformin emerged as a promising candidate based on its antitumor mechanisms — including AMPK activation and mTOR inhibition — and observational data suggesting lower prostate cancer incidence and better outcomes in diabetic men taking the drug. The Metformin Active Surveillance Trial (MAST) was designed to rigorously test this hypothesis in a controlled setting.
The trial enrolled 408 men across multiple Canadian centers, randomizing them 1:1 to metformin 850 mg twice daily or placebo, with follow-up over 36 months. The primary endpoint was time to progression, encompassing both pathologic and therapeutic progression. After median follow-up of 36 months, 144 participants progressed — 70 in the metformin arm and 74 in the placebo arm — yielding a hazard ratio of 1.09 (95% CI, 0.79–1.52; P = .59), indicating no benefit from metformin.
A notable and concerning finding emerged in a prespecified subgroup analysis: obese men (BMI ≥ 30) taking metformin experienced significantly higher rates of pathologic progression compared to obese men on placebo (HR 2.36; P = .0092). This unexpected signal warrants further mechanistic investigation and suggests that metformin's metabolic effects may interact differently with the tumor microenvironment in obese individuals.
For clinicians and longevity-focused practitioners, this trial delivers a clear negative result — metformin should not be recommended to delay prostate cancer progression in low-risk patients on active surveillance. The obesity subgroup finding adds an important safety caveat that should inform clinical counseling.
Key Findings
- Metformin showed no significant benefit over placebo in delaying prostate cancer progression (HR 1.09, P = .59).
- Negative biopsy rates at 36 months were 41% (metformin) vs 31.1% (placebo), a non-significant difference.
- Obese patients on metformin had 2.36x higher risk of pathologic progression vs obese patients on placebo.
- 144 of 408 participants progressed over 36 months, with nearly equal distribution between arms.
- This phase III trial effectively rules out metformin as a general chemopreventive agent in low-risk prostate cancer.
Methodology
MAST was a multicenter, randomized, double-blind, placebo-controlled phase III trial across Canadian academic centers. Participants were randomized 1:1 to metformin 850 mg twice daily or placebo for up to 36 months, with progression-free survival as the primary endpoint assessed via Kaplan-Meier and Cox proportional hazards models. Prespecified subgroup analyses included BMI stratification.
Study Limitations
The study was conducted exclusively in Canadian centers, which may limit generalizability across different healthcare settings and patient populations. The abstract does not detail adherence rates or reasons for dropout, which could affect interpretation of the null result. The obesity subgroup finding, while prespecified, requires replication and mechanistic explanation before clinical conclusions can be drawn.
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