Background Preclinical studies have suggested that metformin has anti-tumour effects, likely due to blockage of mammalian target of rapamycin pathway through adenosine monophosphate-activated protein kinase and decreased insulin levels. A retrospective study showed that metformin added to everolimus to treat type 2 diabetes mellitus offered longer progression-free survival (PFS) in patients with pancreatic neuroendocrine tumours (NET). Aim(s) To evaluate the efficacy and safety of metformin monotherapy in patients with advanced/metastatic well-differentiated NET (WD-NET) of gastroenteropancreatic (GEP) or pulmonary origin. Patients and methods Single-arm phase II trial of metformin 850 mg PO twice daily until progression or intolerance for patients with progressive metastatic well-differentiated GEP or pulmonary NET. The primary endpoint was disease control rate (DCR) by RECIST 1.1 at 6 months. Secondary endpoints were response rate, PFS, toxicity and variations in glycaemic profiles (glycaemia, glycated haemoglobin and peptide C and insulin) at baseline, at 30 and 90 days. Results From 2014 to 2019, 28 patients were enrolled: median age was 50 years; 84% had non-functional NET, 86% were of GEP origin and 62% had G2 NET. At the time of last follow-up, 26 patients had progression, with 13 (46%) presenting DCR at 6 months and a median PFS of 6.3 months (95% confidence interval: 3.2–9.3). There was no objective response, but one patient with refractory carcinoid syndrome had complete symptom relief, lasting for more than 5 years. Variations in glycaemic profiles were not associated with DCR at 6 months. Diarrhoea was the most common adverse event, being grade 3 or 4 in 10% of the cases. Conclusion Metformin monotherapy offers modest anti-tumour activity in well-differentiated GEP or lung NET.

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