Weight loss drugs like Ozempic and Wegovy are used by over 15 million adults in the U.S., or 4.5% of the population. Despite their effectiveness, they have drawbacks. Their effect may not last after discontinuing use, and side effects including osteoporosis and muscle loss have raised concerns about long-term harms. They also induce nausea, which can make it difficult to stay the course of treatment.
Now Tufts researchers led by Krishna Kumar, Robinson Professor of Chemistry, have designed a new, next-generation compound with hopes that it could be more effective with fewer side effects, which they report in a paper in the Journal of the American Chemical Society.
While weight loss drugs currently on the market and in development target one, two, or even three hormone receptors related to glucose metabolism and the desire to eat, the Tufts team has identified a fourth target that could potentially further enhance the control strategy.
“Obesity is linked to over 180 different disease conditions, including cancer, cardiovascular disease, osteoarthritis, liver disease, and type 2 diabetes, and affects over 650 million people worldwide,” said Kumar. “What drives us is the idea that we can design a single drug to treat obesity and simultaneously mitigate the risk of developing a long list of health problems plaguing society.”
How the Drugs Work
After we eat a meal, our gut and brain trigger a hormonal “fuel gauge” that regulates levels of glucose and tells us when we have had enough to eat.
The hormone glucagon-like peptide 1 (GLP-1) is released to help stimulate the production of insulin and the uptake of glucose in muscle and other tissues. With the cells now loaded with fuel, the level of glucose in the blood returns to normal. Ozempic uses GLP-1 with slight modifications to increase its availability in the bloodstream. Its success in controlling blood glucose has prompted the American Diabetes Association to recommend it and other GLP-1-based drugs as the new first line injectable treatments for diabetes, ahead of insulin.
But GLP-1 also acts directly on the brain, making us feel full after having a meal, and it slows down the rate that the stomach contents are emptied into the intestines, creating a more evenly paced release of nutrients and glucose into the bloodstream. That’s why it has also become extremely popular as a weight loss treatment.
It’s still not a perfect drug strategy for weight loss, though. “The biggest problem with GLP-1 drugs is that they have to be injected once a week, and they can induce a very strong feeling of nausea,” said Kumar. “As much as 40% of people using these drugs give up after the first month.”
A second hormone released after eating is glucose-dependent insulinotropic peptide (GIP). It also makes us feel full after a meal. GIP looks a lot like GLP-1, so rather than administer two drugs, researchers created one peptide that incorporates structural elements of both — what’s called in drug development a chimera. That drug, called Mounjaro or Zepbound (the brand names for tirzepatide), has the added benefit of significantly reducing nausea. As a more tolerable treatment, it may overtake Ozempic in the weight loss market.
“And then there is a third hormone, glucagon,” said Kumar. “Paradoxically, it actually increases blood glucose, but at the same time increases the expenditure of energy in cells of the body, raises body temperature, and suppresses appetite.” By adding glucagon to the mix, GLP-1 and GIP end up neutralizing its glucose-enhancing effect, leaving the remaining functionalities of all three hormones working together to enhance weight loss.
Glucagon is also similar in structure to GLP-1 and GIP, so drug developers created a single chimera peptide that incorporates elements of all three hormones, which can be recognized by their three separate receptors. That drug, called retatrudide, is currently in clinical trials that indicate even greater achievable weight loss (up to 24%) compared to the original GLP-1 drugs (6-15%).
Going for the Weight Loss Gold Standard with a Fourth Target
“The goal that people are trying to shoot for is bariatric surgery,” said Kumar. That’s a surgical procedure significantly reducing the size of the stomach, which can achieve long-lasting weight loss up to 30%. “For individuals with persistent obesity and potential deadly associated conditions, it becomes a necessary but invasive treatment.”
Current injectable weight loss drugs still fall short of that gold standard, so the Tufts chemists are focused on a drug redesign that could match the 30% weight loss outcome.
“There is one more hormone we wanted to bring in to complete a weight control quartet,” said Tristan Dinsmore, a graduate student in the Kumar lab and the lead author of the study. “It’s called peptide YY (PYY). This molecule is also secreted by the gut after we eat a meal, and its job is to reduce appetite and slow the process of emptying food from the stomach, but via different mechanisms than either GLP-1 or GIP. It may also be involved in directly ‘burning off’ fat.”
PYY is from a separate and structurally unrelated class of hormones than the first three, so blending its structure into a chimeric peptide that also mimics GLP-1, GIP, and glucagon was not easy. Instead, the Tufts team was able to join two peptide segments end-to-end, creating a new ‘tetra-functional’ clinical candidate.
“One of the limitations of the current drugs is that individual variation, possibly including how people express target receptors or respond to their corresponding hormones, can lead to lesser than desired weight loss outcomes in many patients,” said Martin Beinborn, visiting scholar in the Department of Chemistry. “By hitting four different hormone receptors at the same time, we hope to improve the chances of averaging out such variation toward the goal of achieving greater and more consistent overall effectiveness.”
“A second issue is that patients tend to regain weight after discontinuing currently available GLP-1 related drugs,” said Beinborn, who notes that lifestyle changes should ideally be a complement to medication treatment. This two-pronged approach will not only support reaching and keeping one’s target weight, but may also help preserve bone and muscle mass.
“Recent studies indicate that weight rebound after drug discontinuation is delayed with the newer, more effective GLP-1 mimetics,” he said. “Extending from this observation, one may speculate that multi-chimeras along the lines of the one we discovered could get us closer to the bariatric surgery standard of lasting weight loss.”
