Setmelanotide

Setmelanotide is a cyclic octapeptide agonist of the melanocortin-4 receptor (MC4R), a central regulator of hunger and energy balance. Formerly known as RM-493, it was developed by Rhythm Pharmaceuticals. MC4R sits downstream of leptin signalling in the hypothalamus: leptin stimulates neurons that produce pro-opiomelanocortin (POMC), which is processed into alpha-melanocyte-stimulating hormone, the natural MC4R agonist. Genetic disruption anywhere along this pathway causes severe early-onset obesity.

Setmelanotide was approved by the FDA in 2020 and subsequently by the EMA, marketed as Imcivree, for obesity due to genetically confirmed POMC, PCSK1 or leptin receptor deficiency, with later approval extended to Bardet-Biedl syndrome. It is a leading example of precision medicine in metabolic research: rather than targeting obesity broadly, it restores signalling in a specific defective pathway. Earlier MC4R agonists had been hampered by cardiovascular effects in clinical studies, and setmelanotide's development renewed interest in the receptor. Research focuses on monogenic obesity, hypothalamic satiety signalling and patient stratification by genotype.

Setmelanotide binds and activates MC4R, a Gs-coupled GPCR expressed in hypothalamic nuclei, particularly the paraventricular nucleus. Under normal conditions, leptin stimulates POMC neurons, and POMC-derived alpha-MSH activates MC4R to reduce hunger and increase energy expenditure. In POMC, PCSK1 or leptin receptor deficiency, alpha-MSH production or its upstream signal is lost, so MC4R is under-stimulated. Setmelanotide acts as a replacement agonist downstream of these defects, restoring receptor signalling. It also has activity at the melanocortin-1 receptor on melanocytes, which accounts for skin hyperpigmentation reported in clinical studies.