Retatrutide (product code LY3437943) is an investigational once-weekly injectable peptide developed by Eli Lilly. It activates three receptors simultaneously: GIP, GLP-1, and glucagon. Its biggest difference from approved drugs in the same class is its ability to activate the glucagon receptor.
One molecule, three receptors
GLP-1 and GIP receptor activity works the same way as approved drugs: slowing gastric emptying, suppressing appetite, and improving glucose metabolism. Most of the weight loss from semaglutide and tirzepatide comes from patients eating less.
The glucagon receptor arm is the new part. In obese animals, increased glucagon activity further increases energy expenditure on top of the reduced energy intake from the other two receptors. At least in mice, retatrutide increases both energy expenditure and reduces energy intake.
Potency is not equal
Retatrutide’s potency at the three receptors is not equal. Relative to each receptor’s natural hormone, its potency at the glucagon receptor is about 0.3× that of glucagon, about 0.4× that of GLP-1 at the GLP-1 receptor, and about 8.9× that of GIP at the GIP receptor. Its activity at the GIP receptor is significantly higher than at the other two, a balance that appears carefully designed.
Half-life design
Retatrutide is a laboratory-synthesized single peptide chain designed to survive about six days in the body. Unnatural amino acids in the sequence resist the enzymes that would otherwise break it down, and a fatty acid tail attached at position 17 binds to albumin in the blood, giving it a half-life long enough for weekly injection.
Doesn’t glucagon raise blood sugar?
This is the obvious objection. Glucagon is a hormone that prompts the liver to release glucose. Deliberately activating its receptor in a drug designed to lower blood sugar seems contradictory. But in practice, the glucose-lowering effects of the GLP-1 and GIP arms are enough to compensate. In the Phase II obesity trial, no clinically meaningful hypoglycemia was reported in any dose group.
Conclusion
Retatrutide has a clear mechanism and elegant design, but it remains in Phase III and has not been approved in any country. Understanding its triple mechanism is the first step toward a rational view.