The largest randomized trial to date examining metformin in metastatic prostate cancer has revealed significant metabolic benefits for patients, despite failing to meet its primary survival endpoint. The STAMPEDE trial, published in Lancet Oncology, demonstrates that adding the diabetes medication to standard care substantially reduces the metabolic side effects of androgen deprivation therapy (ADT) in men with newly diagnosed metastatic hormone-sensitive prostate cancer (mHSPC).
Trial Design and Patient Population
The multi-arm STAMPEDE platform trial enrolled 1,874 non-diabetic men with mHSPC between 2016 and 2023 across more than 100 hospitals in the UK and Switzerland. Participants were randomly assigned 1:1 to receive either standard care alone (938 patients) or standard care plus metformin 850 mg twice daily (936 patients). The study population included patients receiving ADT alone (15%), ADT plus docetaxel (82%), or ADT plus androgen receptor pathway inhibitors (3%).
Survival Outcomes Fall Short of Significance
At a median follow-up of 60 months, metformin failed to demonstrate a statistically significant improvement in overall survival, the trial's primary endpoint. The metformin group achieved a median overall survival of 67.4 months compared to 61.8 months in the control group, yielding a hazard ratio of 0.91 (95% CI: 0.80-1.03, P = 0.15).
Secondary survival endpoints, including prostate cancer-specific survival, progression-free survival, metastatic progression-free survival, and failure-free survival, also showed no significant differences between groups.
Substantial Metabolic Benefits Observed
Despite the lack of survival benefit, metformin produced consistent and clinically meaningful improvements in metabolic parameters. After two years of treatment, patients receiving metformin gained significantly less weight than controls (2kg versus 4.4kg). The metformin group also demonstrated lower fasting glucose concentrations, reduced total cholesterol and LDL cholesterol levels, decreased glycated hemoglobin (HbA1c), and smaller waist circumference measurements.
These metabolic improvements were consistently observed at 104 weeks, suggesting sustained benefits throughout treatment. Fewer patients in the metformin group developed signs of metabolic syndrome, a constellation of conditions that increase the risk of type 2 diabetes and cardiovascular disease.
Potential Benefits in High-Risk Subgroup
While not statistically powered for definitive conclusions, the trial identified potential anti-cancer benefits in men with the highest burden of disease. This subgroup, representing the most lethal form of prostate cancer, showed improvements across all measures of anti-cancer activity, including survival, when metformin was added to standard care.
Safety Profile and Tolerability
Metformin was well-tolerated throughout the study, with diarrhea being the primary side effect reported by some participants. The drug's established safety profile from decades of diabetes treatment supported its evaluation in this cancer population.
Clinical Implications for Metabolic Management
The findings highlight the importance of addressing metabolic complications in prostate cancer patients, particularly as survival rates continue to improve with modern treatments. ADT, while essential for controlling metastatic disease, causes insulin resistance, dyslipidemia, and body composition changes that increase cardiovascular risk.
Lead researcher Silke Gillessen noted that these metabolic benefits may be particularly relevant given the increasing use of newer hormone therapy drugs, which carry additional cardiovascular risks compared to traditional ADT. The metabolic improvements observed with metformin could help mitigate these cumulative effects.
Regulatory and Access Considerations
While metformin is an inexpensive and widely available medication, its current drug license does not cover prostate cancer treatment. This regulatory limitation may affect immediate availability within healthcare systems like the NHS for men with prostate cancer, despite the demonstrated metabolic benefits.
The STAMPEDE trial, funded by Cancer Research UK and the Medical Research Council with additional support from Prostate Cancer UK and pharmaceutical collaborators, represents one of the world's largest and most successful prostate cancer research efforts. Since beginning in 2005, STAMPEDE has enrolled nearly 12,000 participants across all treatment comparisons, finishing recruitment in 2023.
