Mechanism of Metformin: A Tale of Two Sites

Abstract

Metformin (dimethylbiguanide) features as a current first-line pharmacological treatment for type 2 diabetes (T2D) in almost all guidelines and recommendations worldwide. It has been known that the antihyperglycemic effect of metformin is mainly due to the inhibition of hepatic glucose output, and therefore, the liver is presumably the primary site of metformin function. However, in this issue of Diabetes Care , Fineman and colleagues (1) demonstrate surprising results from their clinical trials that suggest the primary effect of metformin resides in the human gut. Metformin is an orally administered drug used for lowering blood glucose concentrations in patients with T2D, particularly in those overweight and obese as well as those with normal renal function. Pharmacologically, metformin belongs to the biguanide class of antidiabetes drugs. The history of biguanides can be traced from the use of Galega officinalis (commonly known as galega) for treating diabetes in medieval Europe (2). Guanidine, the active component of galega, is the parent compound used to synthesize the biguanides. Among three main biguanides introduced for diabetes therapy in late 1950s, metformin (Fig. 1 A ) has a superior safety profile and is well tolerated. The other two biguanides, phenformin and buformin, were withdrawn in the early 1970s due to the risk of lactic acidosis and increased cardiac mortality. The incidence of lactic acidosis with metformin at therapeutic doses is rare (less than three cases per 100,000 patient-years) and is not greater than with nonmetformin therapies (3). Major clinical advantages of metformin include specific reduction of hepatic glucose output, with subsequent improvement of peripheral insulin sensitivity, and remarkable cardiovascular safety, but without increasing islet insulin secretion, inducing weight gain, or posing a risk of hypoglycemia. Moreover, metformin has also shown benefits in reducing cancer risk and improving cancer prognosis (4,5), as well as counteracting the cardiovascular complications …