GLP-1 Receptor Agonists: How the Drug Class Developed

The Incretin Effect

The story begins with a simple observation from the 1960s. Researchers noticed that oral glucose produced a much larger insulin response than the same amount of glucose delivered intravenously. Something in the gut was amplifying the pancreatic signal. They called this the “incretin effect,” and it took another two decades to identify the hormones responsible: GIP (glucose-dependent insulinotropic polypeptide, discovered in 1970) and GLP-1 (glucagon-like peptide-1, characterized in the 1980s).

GLP-1 is released by L-cells in the small intestine after eating. It stimulates insulin secretion (glucose-dependent, so it does not cause hypoglycemia on its own), suppresses glucagon, slows gastric emptying, and acts on hypothalamic circuits to reduce appetite. Native GLP-1 is rapidly degraded by the enzyme DPP-4, giving it a half-life of only 2-3 minutes. The entire history of GLP-1 drug development is, in essence, a story of overcoming that limitation.

First Generation: Exenatide and Liraglutide

Exenatide (Byetta), approved in 2005, was the first GLP-1 receptor agonist to reach the market. It was based on exendin-4, a peptide found in the saliva of the Gila monster lizard, which is structurally similar to human GLP-1 but resistant to DPP-4 degradation. The drug required twice-daily injection and produced modest weight loss of approximately 3-4% in clinical trials. An extended-release formulation (Bydureon) reduced dosing to once weekly in 2012.

Liraglutide (Victoza for diabetes, 2010; Saxenda for obesity, 2014) represented a step forward. Novo Nordisk modified human GLP-1 by adding a fatty acid side chain that promotes albumin binding, extending the half-life to approximately 13 hours. This allowed once-daily dosing. In the SCALE obesity trials, liraglutide 3.0mg produced about 8% weight loss over 56 weeks. Meaningful, but still well below what surgical interventions could achieve.

The first generation proved the concept. GLP-1 receptor agonism could treat both diabetes and obesity through a single mechanism. But the effect sizes left significant room for improvement.

Second Generation: Semaglutide

Semaglutide was the drug that transformed public perception of GLP-1 therapeutics. Approved for diabetes in 2017 (Ozempic) and for obesity in 2021 (Wegovy), it built on liraglutide’s design with further modifications that extended the half-life to approximately 168 hours, enabling true once-weekly dosing.

The STEP clinical trial program established semaglutide 2.4mg as a genuine rival to bariatric surgery. STEP 1 (NEJM, 2021) showed 14.9% mean body weight loss at 68 weeks. STEP 5 extended the observation to 104 weeks and demonstrated sustained efficacy.

Then came the SELECT trial. Published in NEJM in 2023, SELECT enrolled over 17,000 participants with overweight or obesity and established cardiovascular disease but without diabetes. Semaglutide reduced major adverse cardiovascular events (MACE) by 20% compared to placebo. This was the first time a GLP-1 agonist had demonstrated hard cardiovascular outcomes benefit in a non-diabetic population. It expanded the therapeutic rationale for the entire drug class from metabolic control into cardiovascular prevention.

For the full pharmacology, see our semaglutide research profile.

The SELECT trial did for GLP-1 agonists what the statin trials did for cholesterol-lowering drugs: it established cardiovascular risk reduction as a primary clinical benefit, independent of its metabolic effects.

Third Generation: Tirzepatide and the Dual Agonist Approach

Tirzepatide (Mounjaro for diabetes, 2022; Zepbound for obesity, 2023) introduced a new design philosophy: why target one receptor when you can target two? Eli Lilly’s molecule activates both GIP and GLP-1 receptors, with approximately five-fold selectivity for GIP. The results were immediately notable.

The SURMOUNT-1 trial (NEJM, 2022) showed 22.5% mean weight loss with tirzepatide 15mg over 72 weeks. More than half of participants on the highest dose lost at least 20% of their body weight, a threshold that had previously been the exclusive domain of bariatric surgery. The SURPASS diabetes program showed superior HbA1c reduction compared to semaglutide in a head-to-head trial (SURPASS-2).

Our tirzepatide research profile covers the dual mechanism and full trial data.

The GIP receptor’s contribution to tirzepatide’s effects was something of a surprise. For years, GIP was considered a secondary incretin. Some researchers had even proposed GIP receptor antagonism as an obesity treatment, arguing that blocking GIP would reduce fat storage. Tirzepatide proved the opposite approach correct: agonism at both incretin receptors produces synergistic metabolic benefits. The reason appears to involve GIP’s distinct effects on fat metabolism, energy expenditure, and possibly direct central nervous system actions that differ from GLP-1 signaling.

Fourth Generation: Retatrutide and Triple Agonism

Retatrutide adds a third receptor target: the glucagon receptor (GCGR). Glucagon is traditionally viewed as a counter-regulatory hormone that raises blood glucose, which might seem counterproductive in a metabolic drug. But glucagon also increases energy expenditure, promotes hepatic fat oxidation, and suppresses appetite through central mechanisms distinct from GLP-1.

The Phase 2 trial (NEJM, 2023) enrolled 338 adults with obesity. At the highest dose (12mg), participants lost approximately 24% of body weight over 48 weeks. The trajectory had not plateaued at study end, suggesting that longer treatment could produce even greater weight loss. Liver fat reduction was particularly striking, with some participants achieving near-complete resolution of hepatic steatosis.

Our retatrutide research profile covers the triple agonist mechanism in detail.

Phase 3 trials for retatrutide are underway. If the Phase 2 results hold, it will represent another step-change in efficacy. The addition of glucagon receptor agonism may also provide advantages for metabolic-associated steatohepatitis (MASH), given glucagon’s role in hepatic fat metabolism.

Beyond Weight Loss: Expanding Indications

The clinical reach of GLP-1 agonists is expanding rapidly. Active trials are exploring their use across conditions that were not part of the original development programs.

The neurological connections are among the most intriguing. GLP-1 receptors are expressed throughout the brain, including regions involved in reward, memory, and neuroinflammation. Observational data from large diabetic populations has shown reduced incidence of Parkinson’s and Alzheimer’s disease among GLP-1 agonist users. Randomized trials are now testing whether this association is causal. If it is, the market and public health implications would dwarf even the obesity applications.

Unsolved Problems

For all their success, incretin peptides have not solved every aspect of metabolic disease. Several challenges persist across the entire class.

Lean mass preservation is another concern. Weight loss from GLP-1 agonists includes both fat and lean tissue, with lean mass comprising roughly 25-40% of total weight lost in most trials. In older adults or those with sarcopenia risk, this loss of muscle and bone mass could offset some metabolic benefits. Combination approaches pairing GLP-1 agonists with resistance training or anabolic agents are being explored, but no standardized protocol exists.

The gastrointestinal side effect burden, while manageable with slow dose titration, remains a barrier. Nausea affects 30-50% of patients in the first weeks of treatment across the class. Roughly 5-10% discontinue treatment due to GI intolerance. Higher-efficacy compounds like retatrutide will need to demonstrate that their additional receptor activity does not amplify these effects beyond acceptable thresholds.

The GLP-1 class has solved the efficacy problem for obesity pharmacotherapy. What remains is the durability problem, the lean mass problem, and the access problem. Those will define the next decade of research.

The Competitive Landscape

Beyond the Novo Nordisk and Eli Lilly duopoly, dozens of companies are developing GLP-1-based therapeutics. Oral semaglutide at higher doses (Rybelsus 25mg and 50mg) aims to match injectable efficacy. Amgen’s MariTide combines GLP-1 agonism with anti-GIP antibody activity, testing the opposite of tirzepatide’s dual-agonist approach. Viking Therapeutics, Structure Therapeutics, and others have candidates in Phase 2 or Phase 3.

Small molecule GLP-1 agonists are also in development, which could eliminate the need for injection entirely while potentially reducing manufacturing costs. The economic pressure to bring cheaper alternatives to market is substantial, given that current branded GLP-1 drugs cost $800 to $1,300 per month in the United States.

What the Future Looks Like

The trajectory of incretin peptide research follows a remarkably consistent arc: each generation adds receptor targets, improves pharmacokinetics, and delivers more weight loss. There is no obvious reason that arc would stop at three receptors. Research on amylin co-agonism, peptide YY combinations, and other multi-target approaches is already underway.

The more fundamental shift may be in how we think about metabolic disease. GLP-1 agonists have demonstrated that obesity is a treatable chronic condition, responsive to pharmacological intervention with the same rigor we apply to hypertension or hyperlipidemia. The clinical data is too consistent and too robust to dismiss. Whether the health systems of the world can afford to treat everyone who would benefit remains the largest open question.

For detailed profiles on the current and next-generation incretin peptides, see our research pages on semaglutide, tirzepatide, and retatrutide.

This article is for educational and informational purposes only. It is not intended as medical advice and should not be used to diagnose, treat, or prevent any condition. Always consult with a qualified healthcare professional before making health-related decisions. Clinical trial data referenced here is sourced from peer-reviewed publications and may not reflect the most current findings.