Here's a win for a small group of patients who have had very few options: the U.S. Food and Drug Administration just gave the green light to the first-ever treatment for a rare weight disorder called acquired hypothalamic obesity.

The therapy is Rhythm Pharmaceuticals Inc.'s (RYTM) drug Imcivree (setmelanotide). The FDA expanded its approval on Thursday so it can now be used to treat this specific condition in adults and kids aged 4 and older. The drug is also approved for other rare genetic obesity disorders.

So, what is acquired hypothalamic obesity? It's a rare disease where people experience rapid and sustained weight gain because of damage or dysfunction in a part of the brain called the hypothalamus. This often happens after tumors in that area or their treatment. The hypothalamus is like the body's master control center for things like hunger and energy use, and when it's injured, that system can go haywire.

"Imcivree is now the first and only FDA-approved therapy for acquired HO...," said David Meeker, Chairman, CEO, and President of Rhythm. The company estimates about 10,000 people in the U.S. are living with it.

The approval didn't come out of thin air. It's backed by data from a big global study called the Phase 3 TRANSCEND trial, which involved 142 patients. The results were pretty clear. After 52 weeks, patients on the drug saw their body mass index drop by an average of 15.8%. Meanwhile, patients on a placebo saw their BMI actually increase by 2.6%. That works out to a statistically significant 18.4% placebo-adjusted reduction.

It's not all smooth sailing for Rhythm's broader obesity program, though. Earlier this week, the company reported that another trial, called EMANATE, which was testing setmelanotide in patients with other rare, genetically driven obesities, did not meet its primary goals. However, a deeper look at the data (a post-hoc analysis) suggested the drug did help patients with specific genetic variants, showing statistically significant BMI reductions at the 52-week mark. So, there might still be a path forward for some patients in that group.

Investors seemed focused on the positive news. Rhythm Pharmaceuticals (RYTM) shares were up 8.34% at $97.84 during premarket trading on Friday.