Incretin-based therapies for type 2 diabetes mellitus: New therapeutic mechanisms

BETA-CELL FUNCTION STILL A CHALLENGE

Another aspect of T2DM highlighted by the UKPDS is the degree of pancreatic beta-cell function loss—typically about 50% or more—at the time of clinical diagnosis, and the steady decline in function thereafter.⁷ This, as much as the understandable fatigue with lifestyle modification that normal humans experience, accounts for the frequent failure of oral antihyperglycemic monotherapy or dual therapy to maintain satisfactory glycemic control over the years. Relieving hyperglycemia at the time of diagnosis by any means usually leads to a temporary improvement in beta-cell function, but the possibility of slowing or even reversing the long-term decline has been an elusive therapeutic goal.

Although direct quantitative assessment of beta-cell function in humans is difficult in routine practice or outside of strict research protocols, a randomized study comparing different monotherapies for T2DM showed that over several years, the rise in HbA1c was more gradual with rosiglitazone than with glyburide or metformin; this suggests that, at least compared with metformin and sulfonylureas, the TZDs may have some longer-term benefit with respect to beta-cell function.⁸

That incretin-based treatments may help preserve or improve beta-cell function has been suggested by animal data.⁹ Proving that that is the case in humans will be much more challenging. A recent randomized study in patients with T2DM already taking metformin showed that addition of exenatide for 1 year resulted in improved beta-cell function, assessed by C-peptide responses to glucose and to arginine during a combined euglycemic-hyperinsulinemic and hyperglycemic clamp procedure. The improvement was evident compared with baseline function and with patients randomized to receive insulin glargine in addition to metformin for a year.¹⁰ However, 4 weeks after exenatide and glargine were discontinued, the beta-cell function had reverted to the pretreatment level and was not significantly different in the two groups of patients. Moreover, 3 months after treatment discontinuation, the HbA1c levels, which had decreased during the year to a similar extent in both groups, had returned to pretreatment levels. The investigators acknowledged that it was impossible in their study to “discriminate between acute and long-term effects of exenatide on beta-cell function.”¹⁰ So, in my opinion, the challenge remains to show that meaningful long-term effects on beta-cell function can be achieved with incretin-based therapy.

That said, there is no doubt that the incretin-based therapies bring a new dimension to our ability to treat diabetes. The articles in this supplement will provide both the specialist and nonspecialist with a better understanding of these relatively new therapies.