Analysis of secondary outcomes, which included AMI, stroke, and cardiovascular death and excluded TIA, demonstrated similar results, with a cause-specific aHR of 0.78 (95% CI, 0.72-0.84) among metformin users compared with sulfonylurea users. The adjusted incidence rate difference was 5.9 (95% CI, 4.3-7.6) fewer events per 1000-person years for metformin compared with sulfonylurea users.
Conclusion. For patients with diabetes and reduced kidney function, treatment with metformin monotherapy, as compared with a sulfonylurea, was associated with a lower risk of MACE.
Commentary
There are approximately 30 million US adults with a diagnosis of type 2 diabetes (T2DM), of whom 20% also have impaired kidney function or chronic kidney disease (CKD).1 Metformin hydrochloride has remained the preferred first-line treatment for T2DM based on safety and effectiveness, as well as low cost.2 Metformin is eliminated by the kidneys and can accumulate as eGFR declines. Based on the negative clinical experience, the US Food and Drug Administration (FDA) issued a safety warning restricting metformin for patients with serum creatinine levels of 1.5 mg/dL or greater for men or 1.4 mg/dL or greater for women. The FDA recommended against starting metformin therapy in patients with CKD with eGFR between 30 and 45 mL/min/1.73 m2, although patients already taking metformin can continue with caution in that setting.1,3
There are several limitations in conducting observational studies comparing metformin to other glucose-lowering medications. First, metformin trials typically excluded patients with CKD due to the FDA warnings. Second, there is usually a time-lag bias in which patients who initiate glucose-lowering medications other than metformin are at a later stage of disease. Third, there is often an allocation bias, as there are substantial differences in baseline characteristics between metformin and sulfonylurea monotherapy users, with metformin users usually being younger and healthier.4
In this retrospective cohort study by Roumie et al, the authors used propensity score–weighted matching to reduce the impacts on time-lag and allocation bias. However, several major limitations remained in this study. First, the study design excluded those who began diabetes treatment after the onset of reduced kidney function; therefore, this study cannot be generalized to patients who already have reduced eGFR at the time of metformin initiation. Second, cohort entry and the start of follow-up was either an elevated serum creatinine or reduced eGFR less than 60 mL/min/1.73 m2. The cohort may have included some patients with an acute kidney injury event, rather than progression to CKD, who recovered from their acute kidney injury. Third, the study population was mostly elderly white men; together with the lack of dose analysis, this study may not be generalizable to other populations.