Tirzepatide: A novel obesity drug ushers in a new era of weight loss — because this one works

A new diet drug is under fast-track FDA review and some financial analysts predict it could break records, with annual sales of as much as $48 billion. According to a recent clinical study, patients treated with high-dose tirzepatide lost 21 percent of their body weight (an average of 52 pounds, or 23.6 kilograms), more than any other weight-loss drug. Curiously, tirzepatide wasn't designed to treat obesity; in fact, it mimics a hormone traditionally thought to cause weight gain.

obesity epidemic

Adipocytes (adipocytes) secrete hormones that regulate metabolism, affect satiety, and trigger inflammation. Obesity occurs when these cells accumulate more lipids than they can handle, which causes them to malfunction. Molecules released by overloaded fat cells can lead to a cascade of metabolic and inflammatory problems that increase a person's risk of other serious diseases and conditions, such as diabetes, high blood pressure, cardiovascular disease, cancer, asthma and hypercholesterolemia.

In order to reduce the stress on the lipid-laden cells (and thereby repair metabolic and inflammatory dysfunction), obese individuals typically need to lose at least 5% to 10% of their body weight. Traditional interventions to achieve this reduction are lifestyle changes (eg, better diet and more exercise). However, these changes don't apply to everyone. And even when they do work, they rarely do so quickly, and when it comes to metabolic and inflammatory dysfunction, the sooner you fix it, the better.

Tirzepatide: A Diabetes Drug That Causes Weight Loss

A handful of medications can reduce 5 to 10 percent of your body weight, but like lifestyle changes, they don't work for everyone. For example, orlistat, approved in 1999, only works in about half of patients. However, this trend has changed in recent years. Semaglutide, which was approved in 2021, helped 86% of patients lose at least 5% of their body weight, with an average weight loss of 15% (vs. 2.4% on placebo). Since its approval, semaglutide (marketed under the brand name Wegovy) has been hailed as a “revolutionary breakthrough” in the fight against obesity. The newer tirzepatide, however, eclipsed semaglutide: 91% of patients lost at least 5% of their body weight, with an average weight loss of 21% in the highest-dose group (compared with 3.1% in the placebo group).

Surprisingly, tirzepatide was not originally designed to treat obesity. It is also the first drug to mimic a pair of hormones released from the gut that stimulate postprandial insulin production: glycogen-like protein-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Because these two molecules stimulate insulin (prompting the body's cells to absorb glucose, thereby lowering blood sugar levels), the researchers suspected that tirzepatide would work well in treating type 2 diabetes, and those assumptions were correct.

Clinical trials have shown that the dual-target drug helped about 50% of patients achieve long-term blood sugar control. However, the trials also revealed a big surprise: Tirzepatide provided more weight loss than the leading weight-loss drug. In other words, tirzepatide appears to be effective in treating two of the world's most common diseases: obesity and diabetes. This is a bit surprising, considering GIP was once considered the “fat hormone.”

Tirzepatide is like a new synthetic hormone

Of the two hormones that tirzepatide mimics, GLP-1 is by far the most studied. It is a powerful tool for weight loss because it reduces appetite and food intake; it is a powerful tool for diabetes management because it stimulates insulin production. Some popular weight loss drugs (such as semaglutide) and weight loss drugs share structure with GLP-1 and stimulate GLP-1 receptors.

GIP, on the other hand, is a bit of a mystery. Although tirzepatide was discovered a decade before GLP-1, it was the first drug to realize its therapeutic potential. GLP-1 suppresses appetite and food intake, whereas GIP does not. Instead, numerous studies have shown that GIP promotes obesity, earning it the nickname “the obesity hormone.” For example, people with a genetic defect in the GIP receptor are more likely to have lean body mass. Therefore, scientists generally assumed that blocking GIP receptors would lead to weight loss; however, tirzepatide stimulates GIP receptors.

Unsurprisingly, scientists don't fully understand tirzepatide's dramatic weight loss effects. One theory is that tirzepatide acts like a new synthetic hormone that triggers slightly different cellular processes than the natural hormone it was designed to mimic. Cells in the gut secrete GLP-1 and GIP as two separate molecules that can interact with their respective receptors independently of each other. Tirzepatide, on the other hand, is a single molecule that binds to both receptors. In addition, the tirzepatide molecule has specialized regions that allow it to remain stable longer than the naturally occurring hormone. These structural changes may cause dual active drugs to act differently than the independent actions of the two natural hormones.

But tirzepatide's maker, Eli Lilly, didn't need to understand why the drug worked in order to bring it to market. The company plans to apply for drug approval in April 2023.

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