Combination therapy with DPP-4 inhibitors and insulin in patients with type 2 diabetes mellitus: what is the evidence?

As type 2 diabetes mellitus (T2DM) progresses, most patients will require insulin replacement therapy. Whether oral antidiabetic drug (OAD) therapy should be retained when initiating insulin is still debated. While the rationale to keep metformin with insulin is strong (mostly as an insulin-sparing agent to limit weight gain), the evidence is less clear for other OADs. In particular, the question now comes up what the expected benefit could be of combining the newer agents, such as the dipeptidyl peptidase-4 (DPP-4) inhibitors with insulin. Additionally, when metformin is no longer a treatment option, as in the case of patients with severe renal impairment, insulin is often used as monotherapy, with little evidence of benefit in maintaining other OADs. In this specific situation, it is also of interest to evaluate the potential benefit of combined treatment with a DPP-4 inhibitor and insulin. Among the classic limitations of insulin therapy in patients with T2DM, hypoglycemia remains a major barrier to glycemic control, along with weight gain exacerbation. The oral DPP-4 inhibitors improve glycemic control by increasing the sensitivity of the islet cells to glucose, and thus are not associated with an increased risk for hypoglycemia and are weight neutral. In addition to the expected benefits associated with limiting insulin dose and regimen complexity, the specific advantages the DPP-4 inhibitor drug class on hypoglycemia and weight gain could justify combining DPP-4 inhibitors with insulin; additionally, a DPP-4 inhibitor may be of special value to decrease glycemic excursions that are not properly addressed by basal insulin therapy and metformin use, even after optimizing titration of the basal insulin. However, given the common original perception that treatment with DPP-4 inhibitors may be less beneficial with increasing disease progression because of the loss of β-cell function, the potential relevance of these agents in the setting of advanced T2DM treated with insulin was not necessarily anticipated. Promising data from studies on the use of these new agents in insulin-treated patients with T2DM have started to emerge. Our article provides a comprehensive overview of the currently available evidence from controlled randomized clinical trials and we discuss the potential role of DPP-4 inhibitors in the this setting. Further clinical experience will allow to fully assess the positioning of these agents in insulin-treated T2DM populations.