The streak of positive phase 3 trial results for the novel “twincretin” tirzepatide when treating patients with type 2 diabetes continued in a report in The Lancet on results from the SURPASS-3 trial, which compared weekly subcutaneous injections of tirzepatide against daily treatment with insulin degludec in patients inadequately controlled on metformin alone or on metformin plus a sodium-glucose cotransporter 2 inhibitor.
Despite positive results in SURPASS-3, as well as in four other pivotal trials that are in the process of releasing full results, the safety and efficacy picture of tirzepatide still includes several as-yet unresolved issues, including the true incidence rate of gastrointestinal adverse effects, the role these effects play in weight loss during tirzepatide treatment, and the drug’s effect on important endpoints beyond weight loss and glycemic control such as cardiovascular outcomes and renal function, said two Australian experts who coauthored a comment on the new SURPASS-3 report.
Tirzepatide is called a “twincretin” because the molecule acts as both a glucagonlike peptide–1 receptor agonist, the drug class that includes semaglutide (Ozempic, Rybelsus, Wegovy) and liraglutide (Saxenda, Victoza), and also as a glucose-dependent insulinotropic polypeptide (GIP). Trial results reported to date suggest that tirzepatide “might be more potent than available GLP-1 receptor agonists,” based on evidence of superior glycemic control it produced relative to semaglutide in results from the SURPASS-2 phase 3 trial reported in August 2021, wrote Christopher K. Rayner, MD, and Michael Horowitz, MD, in their comment.
Uncertainty about gastrointestinal adverse effects
“Limitations of SURPASS-3 include the relatively small number of Asian and Black” patients enrolled, “and an open-label design that carries a risk for bias” when tallying the incidence of gastrointestinal adverse effects, which the trial recorded based on self-reports by enrolled patients.
A better design would use validated questionnaires geared to discerning gastrointestinal symptoms like the ones used in trials involving patients with functional gastrointestinal disorders, wrote Dr. Rayner, a professor of gastroenterology at the University of Adelaide, and Dr. Horowitz, a professor at the same institution and also director of the endocrine and metabolic unit at Royal Adelaide Hospital.
This approach would “allow for more robust evaluation of whether gastrointestinal symptoms are associated with increased weight loss,” they proposed, a possible partial explanation for the weight loss of some patients treated with a GLP-1 receptor agonist.
Additional outstanding questions about tirzepatide include the contribution resulting from the drug’s stimulation of the GIP receptor, as well as the role of GLP-1 receptor stimulation by tirzepatide in slowing gastric emptying. And they also cite the still-unreported effects of tirzepatide on cardiovascular events, fatty liver disease, and kidney function, and its longer-term effects with chronic treatment beyond a year.
All five of the recently completed SURPASS trials ran for 40-52 weeks.
Tirzepatide surpasses insulin degludec’s glycemic control
SURPASS-3 enrolled 1,444 patients with type 2 diabetes at 122 sites in 13 countries during 2019. The study’s primary endpoint was mean change in hemoglobin A1c from baseline after 52 weeks on treatment. The results showed that the A1c reduction with tirzepatide treatment significantly exceeded the drop produced by insulin degludec by 0.59%, 0.86%, and 1.04%, respectively, across the three tirzepatide dosages tested in a dose-response fashion, according to the recent publication.
The most common treatment-emergent adverse effects were gastrointestinal, which decreased with continued treatment, and tirzepatide produced fewer episodes of hypoglycemia, compared with insulin degludec (Tresiba).
In addition to full reports now out from SURPASS-2 and SURPASS-3, researchers also recently published full primary results from SURPASS-1. Results from SURPASS-5 appeared in a poster presented at the American Diabetes Association scientific sessions in June 2021 but have not yet been published in a full report, and the primary results from SURPASS-4are expected in a report during the European Association for the Study of Diabetes in September 2021.
SURPASS-3 and the other trials of tirzepatide were funded by Lilly, the company developing the drug. Dr. Rayner has been an adviser to Allergen and Glyscend, and has received research funding from Sanofi and Novartis. Dr. Horowitz has received symposia fees from Lilly, as well as from AstraZeneca and Boehringer Ingelheim.