A Retrospective Study on the Role of Diabetes and Metformin in Colorectal Cancer Disease Survival

Background: Recent studies have suggested an effect of metformin on mortality for patients with both diabetes and colorectal cancer (CRC). However, the literature is contradictory, with both positive and negative effects being identified. We set out to determine the effect of metformin with respect to prognosis in CRC patients. Methods: After a retrospective chart review of CRC patients treated at the Cancer Centre of Southeastern Ontario, Kaplan–Meier analyses and Cox proportional hazards regression models were used to compare overall survival (os) in patients with and without diabetes. Results: We identified 1304 CRC patients treated at the centre. No significant differences between the diabetic and nondiabetic groups were observed with respect to tumour pathology, extent of metastatic disease, time or toxicity of chemotherapy, and the os rate (1-year os: 85.6% vs. 86.4%, p = 0.695; 2-year os: 73.6% vs. 77.0%, p = 0.265). In subgroup analysis, diabetic patients taking metformin survived significantly longer than their counterparts taking other diabetes treatments (os for the metformin group: 91% at 1 year; 80.5% at 2 years; os for the group taking other treatments, including diet control: 80.6% at 1 year, 67.4% at 2 years). Multivariate analysis suggests that patients with diabetes taking treatments other than metformin experience worse survival (p = 0.025). Conclusions: Our results suggest that CRC patients with diabetes, excluding those taking metformin, might have a worse CRC prognosis. Taking metformin appears to have a positive association with prognosis. The protective nature of metformin needs further evaluation in prospective analyses.