Effect of metformin on outcomes of patients treated with immune checkpoint inhibitors

Background: Immune checkpoint inhibitors have transformed the treatment landscape of cancer treatment, but only a fraction of patients responds to treatment, leading to an increasing effort to repurpose clinically approved medications to augment ICI therapy. Metformin has been associated with improved survival outcomes in patients undergoing conventional chemotherapy. However, whether metformin provides survival benefits in patients receiving immune checkpoint inhibitors (ICIs) is unknown. Methods: We performed a retrospective cohort study at two tertiary referral centers in Taiwan. All adult diabetes mellitus patients who were treated with ICIs between January 2015 and December 2021 were included. The primary and secondary outcomes were overall survival (OS) and progression-free survival (PFS), respectively. Results: In total, 878 patients were enrolled in our study, of which 86 patients used metformin and 78 patients used non-metformin diabetes medications. Compared with non-users, metformin users had a longer median OS (15.4 [IQR 5.6–not reached] vs. 6.1 [IQR, 0.8–21.0] months, P = 0.003) and PFS (5.1 [IQR 2.0—14.3] vs. 1.9 [IQR 0.7—8.6] months, P = 0.041). In a univariate Cox proportional hazard analysis, the use of metformin was associated with a reduction in the risk of mortality (HR: 0.53 [95% confidence interval: 0.35—0.81], P = 0.004) and disease progression (HR: 0.69 [95% CI 0.49—0.99], P = 0.042). The use of metformin remained associated with a lower risk of mortality after adjusting for baseline variables such as age, cancer stage, and underlying comorbidities (OS, HR: 0.55 [95% CI 0.34–0.87], P = 0.011). Similarly, the use of metformin was associated with a lower risk of disease progression. Importantly, the use of metformin before ICI initiation was not associated with a reduction in mortality (HR: 0.61 [95% CI 0.27—1.42], P = 0.25) or disease progression (HR: 0.69 [95% CI 0.33—1.43], P = 0.32). Conclusion: The use of metformin is associated with survival benefits in patients undergoing immunotherapy. Prospective clinical trials are warranted to define the role of metformin in augmenting immunotherapy.