From Gila Monster to Triple Agonist: The Evolution of GLP-1 Drugs

In 1992, endocrinologist Dr. John Eng was studying the saliva of the Gila monster, a venomous lizard native to the American Southwest. He discovered a peptide called exendin-4 that powerfully activated the human GLP-1 receptor but resisted the enzymatic degradation that destroyed the body's own GLP-1 within minutes. That discovery launched a pharmaceutical revolution that would eventually produce semaglutide, tirzepatide, and retatrutide -- medications that have fundamentally changed how we treat obesity (Jastreboff et al., NEJM 2023).

Chapter 1: The Gila Monster (1992)

The Gila monster (Heloderma suspectum) is one of only two venomous lizards in North America. It eats infrequently -- sometimes only 3-4 large meals per year. Dr. Eng hypothesized that its saliva might contain substances that regulate blood sugar during these extreme feast-fast cycles. He was right. Exendin-4, a 39-amino acid peptide in Gila monster saliva, activated the human GLP-1 receptor with similar potency to human GLP-1 but with a critical advantage: it was resistant to DPP-4, the enzyme that destroys natural GLP-1 in minutes.

Chapter 2: Exenatide -- First Generation (2005)

A synthetic version of exendin-4, called exenatide (brand name Byetta), became the first GLP-1 receptor agonist approved by the FDA in 2005. It required twice-daily injections and produced modest weight loss (2-3 kg) alongside blood sugar improvements. A weekly formulation (Bydureon) followed. Exenatide proved the concept but was limited by short duration and modest efficacy.

Chapter 3: Liraglutide -- Better Engineering (2010)

Novo Nordisk took a different approach: instead of using the Gila monster peptide, they modified human GLP-1 itself. By attaching a fatty acid chain (C16 palmitic acid) that bound to albumin, they created liraglutide -- a once-daily GLP-1 analogue with a 13-hour half-life. Approved for diabetes as Victoza (2010) and for obesity as Saxenda (2014), liraglutide produced approximately 8% weight loss. The fatty acid acylation technology would prove pivotal for future drugs.

Chapter 4: Semaglutide -- The Breakthrough (2017-2021)

Semaglutide refined liraglutide's approach with a longer fatty acid chain (C18) and amino acid modifications that further extended the half-life to approximately 7 days, enabling once-weekly dosing. Approved for diabetes as Ozempic (2017) and for obesity as Wegovy (2021), semaglutide produced 15-17% weight loss -- a transformative result that proved GLP-1 medications could rival bariatric surgery in efficacy. Semaglutide became a cultural phenomenon, creating the "Ozempic era."

Chapter 5: Tirzepatide -- Adding GIP (2022)

Eli Lilly's tirzepatide added a second receptor target: GIP (glucose-dependent insulinotropic polypeptide). As a dual GLP-1/GIP agonist, tirzepatide produced 20-22% weight loss, surpassing semaglutide. Approved for diabetes as Mounjaro (2022) and obesity as Zepbound (2023), it proved that multi-receptor targeting could break through the GLP-1-only ceiling.

Chapter 6: Retatrutide -- The Triple Agonist (2023-Present)

Retatrutide added a third receptor: glucagon. By combining GLP-1 appetite suppression, GIP metabolic enhancement, and glucagon-driven energy expenditure and fat oxidation, the triple agonist produced up to 24% weight loss in Phase 2 -- the highest ever achieved with a pharmaceutical. Phase 3 trials (TRIUMPH program) are underway, with potential approval expected in late 2027 or 2028.

What Comes Next?

The trajectory is clear: each generation targets more pathways and produces more weight loss. Researchers are now exploring amylin analogues, peptide YY agonists, and other targets that could push weight loss even further. Oral formulations that eliminate injections entirely are in development. The Gila monster could not have imagined where its saliva would lead.

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