Differentiating members of the thiazolidinedione class: a focus on efficacy

The thiazolidinediones (TZDs) or 'glitazones' are a new class of drug used for the treatment of type 2 diabetes. Although their precise mechanism of action is not known, TZDs target insulin resistance directly and thus tackle an underlying cause of the disease. Two TZDs are indicated for use in type 2 diabetes in the USA, pioglitazone and rosiglitazone. A third, troglitazone, has been associated with significant hepatotoxicity and has been withdrawn from use. In clinical trials, all three TZDs effectively lower blood glucose levels as monotherapy and in combination therapy with sulfonylureas, metformin and insulin. To date, head-to-head comparative studies with these agents have not been performed. It is difficult, therefore, to make direct comparisons of their efficacy since other variables, including baseline glucose levels and study design, can have a significant impact on treatment outcome. Despite this and in light of unique safety issues characterized with certain TZDs, it is useful to look closely at the efficacy data for these agents. It is not sufficient to assume that 'all glitazones are the same' because the studies have not yet been done to support this statement. This article will review what is known about the relative efficacy of the TZDs.