MC4R agonist · rare forms of obesity
A small ring-shaped peptide, built from eight amino acids and modeled on the hormone alpha-MSH, that switches on the melanocortin-4 receptor, a brain signal that curbs hunger. It is a prescription injection for specific rare forms of severe obesity in which this pathway is broken by a gene defect, Bardet-Biedl syndrome, or damage to the hypothalamus.
In the hypothalamus, the hormone leptin reports how much energy the body has stored. That message is relayed through POMC-producing nerve cells, which release a signal that activates the melanocortin-4 (MC4) receptor and dampens hunger. When a gene in this chain (POMC, PCSK1 or LEPR) is faulty, in Bardet-Biedl syndrome, or after a tumor or injury damages the hypothalamus, the MC4 receptor is under-stimulated and hunger can be relentless. Setmelanotide binds the MC4 receptor directly, standing in for the missing upstream signal; the label says this is thought to lower food intake and raise energy use. It is about 20 times less active at the related MC1 receptor, but the activity it keeps there drives melanin production, which explains the skin darkening many patients notice. The label says it would not be expected to work in general obesity.
The approvals rest on small trials in people, all funded by the manufacturer. In two single-arm phase 3 trials, 8 of 10 participants with POMC deficiency and 5 of 11 with LEPR deficiency lost at least 10% of body weight at about one year, and hunger scores fell2.
In Bardet-Biedl syndrome, a trial with a 14-week placebo-controlled phase followed by open-label treatment found that about a third of participants aged 12 or older lost at least 10% of body weight after 52 weeks on the drug; results in the smaller Alström syndrome group were inconclusive3. The largest trial, TRANSCEND, randomized 120 people aged 4 to 66 with acquired hypothalamic obesity: BMI fell 16.5% with setmelanotide but rose 3.3% with placebo at 52 weeks, and hunger eased, though serious adverse events were more frequent (28% vs 8%)4. These results apply only to the specific conditions studied1.
FDA-approved in 2020 as Imcivree (Rhythm Pharmaceuticals). The label covers obesity due to POMC, PCSK1 or LEPR deficiency or Bardet-Biedl syndrome (from age 2) and acquired hypothalamic obesity (from age 4); it is not indicated for common obesity.
Per the FDA label, the most common effects include skin darkening, injection-site reactions, nausea, headache, diarrhea, abdominal pain, vomiting, depression and spontaneous penile erections. Warnings cover changes in sexual arousal (an erection lasting over four hours needs emergency care), depression and suicidal thoughts, serious allergic reactions including anaphylaxis, darkening of existing moles and new moles (skin exams are advised), and, in acquired hypothalamic obesity, adrenal insufficiency and sodium imbalance.
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