GLP-1 Drugs Slash Weight in Genetic Obesity Even Without MC4R Pathway

Mutations in the POMC-MC4R pathway—which governs satiety signaling in the hypothalamus—cause severe, early-onset obesity that is notoriously difficult to treat. The only approved targeted therapy, setmelanotide, achieves only about 5.2% weight reduction in affected patients, far below the efficacy seen with newer GLP-1 receptor agonists in general obesity populations. Because GLP-1 analogs are thought to act partly through hypothalamic circuits that include POMC and AgRP neurons upstream of MC4R, a critical question was whether these drugs could still work when MC4R itself is absent.

Researchers at Axcelead Drug Discovery Partners used 35-week-old male MC4R knockout (KO) mice—a validated model of severe genetic obesity—and administered subcutaneous injections of semaglutide (0.5 mg/kg/day), tirzepatide (0.05 mg/kg/day), or retatrutide (0.05 mg/kg/day) once daily for 21 days. Vehicle-treated MC4R KO mice were markedly obese at baseline, weighing roughly 62 g versus 35 g for wild-type controls, with nearly three times the fat mass, markedly elevated insulin, ALT, AST, and cholesterol, and significantly increased food intake.

All three drugs produced robust, statistically significant weight loss. Tirzepatide (a dual GIP/GLP-1 agonist) showed the greatest effect at 31.6% body weight reduction, followed by retatrutide (a triple GLP-1/GIP/glucagon agonist) at 24.1%, and semaglutide (a GLP-1 agonist) at 19.7%. Cumulative food intake over 19 days dropped from 97.3 g in vehicle-treated KO mice to 29.7 g with tirzepatide, 43.9 g with retatrutide, and 52.2 g with semaglutide. Body composition analysis via Echo-MRI confirmed significant reductions in both fat mass and lean mass across all treatment groups—mirroring the pattern seen in human clinical trials. X-ray CT further documented reductions in visceral and subcutaneous adipose tissue.

Metabolic improvements were also comprehensive. All three drugs significantly reduced plasma insulin, improved HOMA-IR (a marker of insulin resistance), lowered total cholesterol, and normalized liver injury markers AST and ALT. Liver and heart hypertrophy—characteristic features of MC4R KO mice—were also significantly ameliorated. Hepatic gene expression analysis showed suppression of fatty acid synthesis genes (Scd1 and Fasn) in treated mice, while inflammation-related genes (Tnf, Il1b, Il6, Ccl2) were not significantly changed. Metabolic chamber data showed all drugs reduced energy expenditure; notably, only tirzepatide produced a significant decrease in the respiratory quotient (RQ), suggesting a shift toward preferential fat oxidation.

The ordering of efficacy—tirzepatide > retatrutide > semaglutide—closely mirrors the hierarchy seen in human clinical trials, supporting the translational validity of MC4R KO mice as a preclinical model. The authors note that since GLP-1 analogs appear to act upstream of MC4R (suppressing AgRP/NPY neurons and activating POMC/CART neurons at a level that does not require downstream MC4R signaling), their efficacy is preserved even in complete MC4R deficiency. This has direct implications for conditions like Prader-Willi syndrome, POMC-null mutations, and MSH deficiency, where patients have few effective pharmacological options.