Rhythm RM-718: -11.6% BMI Drop in Hypothalamic Obesity

Rhythm Pharmaceuticals (RYTM) reported preliminary Phase 2 data for RM-718, its once-weekly MC4R-specific agonist, showing a mean -11.6% BMI reduction from baseline at 16 weeks (n=7) in patients with acquired hypothalamic obesity. Rhythm simultaneously posted Q2 2026 IMCIVREE (setmelanotide) revenue of $71.3 million, up 19% sequentially from the first quarter.

RYTM closed at $105.78 on August 3 with a market cap of approximately $7.05 billion. The stock has rallied roughly 40% over the past three months as the IMCIVREE launch in acquired HO gained traction: the drug generated over 150 patient start forms since its March 2026 FDA approval in the indication.

The MC1R-sparing bet

Setmelanotide, Rhythm’s daily injectable MC4R agonist, is FDA-approved for HO, Bardet-Biedl syndrome, POMC deficiency, and LEPR deficiency. The drug works. The problem is generalized skin hyperpigmentation, which affects roughly 60% to 80% of patients and is often the reason they discontinue treatment.

RM-718 is engineered to spare the MC1 receptor, the pathway responsible for melanin production and the resulting darkening of skin and nevi. The Phase 2 data suggest the design is working: across 11 enrolled patients, only two cases of mild injection-site hyperpigmentation were observed, with zero cases of the generalized darkening that limits setmelanotide adherence.

The -11.6% BMI reduction at 16 weeks is consistent with setmelanotide (-10.1% at 16 weeks across Phase 2 and 3 trials, n=64) and bivamelagon (-10.1% at 14 weeks, 600 mg dose), Rhythm’s oral MC4R agonist. Rhythm plans to enroll 10 to 15 Prader-Willi syndrome patients in the RM-718 study by the end of 2026.

Beat yourself before someone else does

Rhythm is voluntarily developing a replacement for its own flagship product. If RM-718 matches setmelanotide’s efficacy with weekly injection convenience and without the hyperpigmentation liability, it consolidates Rhythm’s hold on the MC4R pathway for rare obesity disorders rather than ceding the next-generation opportunity to a competitor.

The commercial logic is straightforward. IMCIVREE generated $71.3 million in Q2 2026, with US revenue of $51.0 million accounting for 72% of the total. That is roughly $285 million annualized. A next-generation MC4R agonist with better tolerability and weekly dosing could expand the addressable market by converting patients who reject or discontinue setmelanotide because of the skin darkening.

What to watch

The sample is small: n=7 at the 16-week mark in an open-label study. Rare obesity trials have a history of producing BMI reductions that shrink in randomized, placebo-controlled settings. Rhythm has not disclosed a registrational timeline for RM-718, and a Phase 3 program in a disease affecting roughly 5,000 to 10,000 US patients will be slow to enroll.

For investors, the Q2 revenue trajectory is the nearer-term focus. $71.3 million with 19% sequential growth puts IMCIVREE on a path to roughly $300 million annualized, supporting the $7 billion valuation but not justifying a premium without pipeline expansion. RM-718 and bivamelagon together represent Rhythm’s multi-asset bet on owning the MC4R pathway end-to-end.

The guide to reading clinical trial press releases explains what to look for in early-phase readouts like this one. The Rhythm press release provides the full data.

What It Means for the Trade

The setmelanotide launch trajectory — $71.3 million in Q2, up 19% sequentially, over 150 patient start forms in acquired HO since March — is doing the valuation work right now, and the roughly 40% three-month rally reflects that commercial acceleration more than seven open-label patients. RM-718 is the optionality layer: if weekly dosing without hyperpigmentation converts the patients who refuse or abandon setmelanotide (60-80% of whom get the skin darkening), the MC4R franchise stops being one drug and becomes a platform with three assets — setmelanotide, RM-718, and oral bivamelagon. The catalyst calendar is thin and that is the trading problem: no registrational timeline disclosed for RM-718, Prader-Willi enrollment running to end of 2026, and the next quarterly print is the only dated event. A $7 billion market cap on roughly $285 million annualized revenue means the pipeline bets are where multiple expansion or compression happens.

Mechanism and Market Context

The melanocortin-4 receptor is the central conduit of the leptin-melanocortin pathway regulating satiety and energy expenditure; hypothalamic obesity is what happens when tumors or surgery damage the hypothalamus upstream of it, leaving a brain that cannot register fullness. MC4R agonism restores the downstream signal regardless of the upstream injury, which is why the mechanism works across acquired HO, BBS, POMC, and LEPR deficiency. The design problem setmelanotide never solved is receptor selectivity: hitting MC1R produces melanin, hence the generalized hyperpigmentation that drives discontinuation. RM-718’s MC1R-sparing pharmacology — two mild injection-site cases in 11 enrolled patients, zero generalized darkening — is the specific engineering claim, and the -11.6% BMI at 16 weeks matching setmelanotide’s -10.1% suggests efficacy did not get sacrificed for tolerability. Prader-Willi syndrome, the next cohort, would be a distinct and larger rare-obesity population.

Risks and What Could Go Wrong

1. n=7, open-label, 16 weeks — the exact profile that historically shrinks in randomized settings; treat the BMI number as unconfirmed.
2. No registrational timeline means years of capital committed before a filing, with quarterly revenue as the only interim proof point.
3. If RM-718 cannibalizes setmelanotide revenue without expanding the market, the net franchise gain is smaller than the multiple implies.
4. Bivamelagon, the oral backup, duplicates the bet; a safety or efficacy failure in either asset re-rates the whole MC4R-platform thesis.