This article was originally published December 10 on Medscape editor-in-chief Eric Topol’s Substack ”Ground Truths.”
– achieving a substantial amount of weight loss without serious side effects. Many attempts to get there now fill a graveyard of failed drugs, such as fen-phen in the 1990s when a single small study of this drug combination in 121 people unleashed millions of prescriptions, some leading to serious heart valve lesions that resulted in withdrawal of the drug in 1995. The drug rimonabant, an endocannabinoid receptor blocker (think of blocking the munchies after marijuana) looked encouraging in randomized trials. However, subsequently, in a trial that I led of nearly 19,000 participants in 42 countries around the world, there was a significant excess of depression, neuropsychiatric side-effects and suicidal ideation which spelled the end of that drug’s life.
In the United States, where there had not been an antiobesity drug approved by the Food and Drug Administration since 2014, Wegovy (semaglutide), a once-weekly injection was approved in June 2021. The same drug, at a lower dose, is known as Ozempic (as in O-O-O, Ozempic, the ubiquitous commercial that you undoubtedly hear and see on TV) and had already been approved in January 2020 for improving glucose regulation in diabetes. The next drug on fast track at FDA to be imminently approved is tirzepatide (Mounjaro) following its approval for diabetes in May 2022. It is noteworthy that the discovery of these drugs for weight loss was serendipitous: they were being developed for improving glucose regulation and unexpectedly were found to achieve significant weight reduction.
Both semaglutide and tirzepatide underwent randomized, placebo-controlled trials for obesity, with marked reduction of weight as shown below. Tirzepatide at dose of 10-15 mg per week achieved greater than 20% body weight reduction. Semaglutide at a dose of 2.4 mg achieved about 17% reduction. These per cent changes in body weight are 7-9 fold more than seen with placebo (2%-3% reduction). Note: these levels of percent body-weight reduction resemble what is typically achieved with the different types of bariatric surgery, such as gastric bypass.
Another way to present the data for the two trials is shown here, with an edge for tirzepatide at high (10-15 mg) doses, extending to greater than 25% body-weight reduction
The results with semaglutide were extended to teens in a randomized trial (as shown below), and a similar trial with tirzepatide is in progress.
How do these drugs work?
These are peptides in the class of incretins, mimicking gut hormones that are secreted after food intake which stimulate insulin secretion.
These two drugs have in common long half-lives (about 5 days), which affords once-weekly dosing, but have different mechanisms of action. Semaglutide activates (an agonist) the glucagonlike peptide–1 receptor, while tirzepatide is in a new class of dual agonists: It activates (mimics) both the GLP-1 receptor and GIP receptors (Gastric inhibit polypeptide is also known as glucose-dependent insulinotropic polypeptide.) The potency of activation for tirzepatide is fivefold more for GIPR than GLP1. As seen below, there are body wide effects that include the brain, liver, pancreas, stomach, intestine, skeletal muscle and fat tissue. While their mode of action is somewhat different, their clinical effects are overlapping, which include enhancing satiety, delaying gastric emptying, increasing insulin and its sensitivity, decreasing glucagon, and, of course, reducing high glucose levels. The overlap extends to side effects of nausea, vomiting, abdominal pain, constipation and diarrhea. Yet only 4%-6% of participants discontinued the drug in these trials, mostly owing to these GI side effects (and 1%-2% in the placebo group discontinued the study drug for the same reasons).
In randomized trials among people with type 2 diabetes, the drugs achieved hemoglobin A1c reduction of at least an absolute 2 percentage points which led to their FDA approvals (For semaglutide in January 2020, and for tirzepatide in May 2022). The edge that tirzepatide has exhibited for weight-loss reduction may be related to its dual agonist role, but the enhancement via GIP receptor activation is not fully resolved (as seen below with GIP? designation). The Amgen drug in development (AMG-133) has a marked weight loss effect but inhibits GIP rather than mimics it, clouding our precise understanding of the mechanism.
The gut-brain regulation of food intake with the many gut hormones (including leptin, gherlin, PYY, amylin) and targets in the body and brain regions.
Nevertheless, when the two drugs were directly compared in a randomized trial for improving glucose regulation, tirzepatide was superior to semaglutide, as shown below. Of note, both drugs achieved very favorable effects on lipids, reducing triglycerides and LDL cholesterol and raising HDL cholesterol, along with reduction of blood pressure, an outgrowth of the indirect effect of weight reduction and direct metabolic effects of the drugs.
While there has been a concern about other side effects besides the GI ones noted above, review of all the trials to date in these classes of medication do not reinforce a risk of acute pancreatitis. Other rare side effects that have been noted with these drugs include allergic reactions, gallstones (which can occur with a large amount of weight loss), and potential of medullary thyroid cancer (so far only documented in rats, not people), which is why they are contraindicated in people with Type 2 multiple endocrine neoplasia syndrome.