Rhythm Pharmaceuticals has reported preliminary clinical results showing that RM-718, its investigational once-weekly melanocortin-4 receptor agonist, produced an average 11.6% reduction in body mass index after 16 weeks among seven evaluable patients with acquired hypothalamic obesity.
The August 4, 2026 update provides an early efficacy signal for a drug intended to offer less frequent dosing and potentially less hyperpigmentation than Rhythm Pharmaceuticals’ approved therapy, IMCIVREE, or setmelanotide. Eleven patients entered the open-label portion of the study, but only seven had completed 16 weeks and were included in the headline body mass index analysis.
RM-718 was generally tolerated in the preliminary dataset, with injection-site reactions, nausea and vomiting reported as the most common adverse events. Two patients discontinued treatment because of adverse events, one following injection-site induration and another because of nausea. Two mild cases of hyperpigmentation occurred at the injection site, while the company reported no generalised hyperpigmentation.
The results are encouraging because acquired hypothalamic obesity can cause rapid and sustained weight gain after injury or dysfunction involving the hypothalamus. However, the absence of a placebo group, the very small number of patients reaching the 16-week assessment and the short follow-up period mean the findings should be treated as an initial signal rather than comparative proof.
Rhythm Pharmaceuticals already markets setmelanotide for acquired hypothalamic obesity in the United States, making RM-718’s development challenge unusually clear. The weekly candidate must ultimately demonstrate that its dosing convenience and receptor selectivity offer a meaningful advantage without sacrificing the weight reduction established by daily setmelanotide.
What did the preliminary RM-718 hypothalamic obesity trial results show?
Eleven patients with acquired hypothalamic obesity enrolled in Part C of the continuing RM-718 clinical programme. The reported efficacy analysis included seven patients who had completed 16 weeks of weekly treatment.
Those seven participants recorded an average body mass index reduction of 11.6% from baseline. Rhythm Pharmaceuticals compared that result with average reductions of approximately 10.1% previously observed over similar periods with setmelanotide and bivamelagon, another melanocortin-4 receptor agonist in its development portfolio.
These figures come from separate studies involving different patients, trial designs and treatment conditions. They should not be interpreted as evidence that RM-718 is superior, equivalent or non-inferior to either therapy. Only a prospectively designed comparative trial could answer that question reliably.
Eight patients remained on active treatment as of July 16, including two who had not reached week 16. Two participants had discontinued because of adverse events, while another withdrew from the trial’s extension period. The imbalance between the 11 patients enrolled and the seven included in the primary preliminary efficacy figure makes the eventual results from all participants particularly important.
The update did not disclose individual patient responses, body mass index ranges, hunger-score changes or the proportion of participants achieving predefined reductions such as 5%, 10% or 15%. Those details will be necessary to understand whether the reported average represents broadly consistent improvement or is disproportionately influenced by a small number of strong responders.
How does the once-weekly RM-718 injection work?
RM-718 is designed to activate the melanocortin-4 receptor, commonly known as MC4R. The MC4R pathway in the brain contributes to the regulation of hunger, satiety, energy expenditure and body weight.
Acquired hypothalamic obesity can develop when tumours, surgery, radiation, trauma, stroke or inflammation damage the hypothalamus and disrupt signalling through this pathway. The resulting weight gain may be severe, rapid and resistant to conventional dietary and behavioural interventions.
The investigational medicine is administered through a weekly subcutaneous injection. Its clinical study includes multiple components evaluating RM-718 in healthy people with obesity and in patients whose obesity is associated with impaired MC4R signalling. The registered programme includes single-dose, repeated-dose and longer treatment components, with weekly dosing extending for 16 weeks in Part C and 26 weeks in Part D.
Rhythm Pharmaceuticals describes RM-718 as an MC4R-specific agonist designed to spare the melanocortin-1 receptor. This distinction could be clinically important because activation of the melanocortin-1 receptor on melanocytes increases melanin production and can lead to skin darkening.
The United States Food and Drug Administration’s clinical pharmacology review of setmelanotide explained that MC4R activation is associated with hunger, satiety and energy expenditure, whereas melanocortin-1 receptor activation contributes to pigmentation independently of ultraviolet-light exposure.
RM-718 is therefore attempting to preserve the desired metabolic effect while reducing an unwanted pigmentation-related effect associated with less selective melanocortin receptor activity.
Why does limited hyperpigmentation matter for RM-718?
Hyperpigmentation is a recognised and frequent adverse reaction associated with setmelanotide. In the pivotal acquired hypothalamic obesity study reviewed by the Food and Drug Administration, skin hyperpigmentation occurred in 55.3% of setmelanotide-treated patients compared with 8.3% of placebo recipients. Nausea and vomiting were also more frequent in the active-treatment group.
The two RM-718 hyperpigmentation events reported in the preliminary update were mild and limited to injection sites. No generalised pigmentation was observed among the small group treated to date.
That finding is consistent with the candidate’s proposed receptor-selective design, but the dataset is too small to establish a lower incidence or severity than setmelanotide. A side effect that occurs in a minority of patients might not appear in a group of only 11 participants.
Longer exposure is equally important because pigmentation changes may develop or become more noticeable over time. The continuing study should clarify whether hyperpigmentation remains uncommon as more patients receive RM-718 for longer periods.
A reduction in generalised hyperpigmentation could improve treatment acceptability, particularly for patients who are reluctant to begin or continue therapy because of visible changes in skin colour. The clinical value would need to be considered alongside gastrointestinal tolerability, injection reactions, efficacy and the convenience of weekly administration.
How does RM-718 compare with approved IMCIVREE setmelanotide?
Setmelanotide became the first therapy approved in the United States specifically for acquired hypothalamic obesity in March 2026. The indication covers adults and children aged four years and older and is intended to reduce excess body weight and maintain long-term weight reduction.
The approval was supported by a randomised, double-blind and placebo-controlled Phase 3 trial. The Food and Drug Administration’s statistical review found a placebo-adjusted body mass index difference of approximately 18.65% after 52 weeks, with the analysis demonstrating a robust treatment effect across examined demographic subgroups.
Setmelanotide is injected once daily and titrated according to age, weight and tolerability. RM-718 is being developed as a weekly injection, potentially reducing the annual number of injections from 365 to about 52.
Convenience could be especially relevant because acquired hypothalamic obesity frequently affects children and adolescents who may already face complicated care following treatment for brain tumours or other hypothalamic injuries. A weekly regimen could reduce administration burden for patients and caregivers.
Yet convenience alone will not establish RM-718’s role. Setmelanotide has completed a large controlled trial and received regulatory approval, whereas RM-718 currently has preliminary open-label evidence from seven patients at the central 16-week assessment.
Rhythm Pharmaceuticals will need to determine whether RM-718 should eventually replace setmelanotide for some patients, serve people who cannot tolerate daily treatment or become an alternative within additional rare MC4R-pathway diseases.
What does the safety profile reveal at this early stage?
The company described RM-718 as generally well tolerated, but the reported discontinuations require context. Two of 11 enrolled patients stopped treatment because of adverse events, representing a meaningful proportion in a small study even though the events were injection-site induration and nausea rather than severe systemic complications.
Nausea and vomiting are biologically plausible adverse reactions within this drug class and were also common during setmelanotide’s development. Injection-site reactions may take on greater significance for a long-acting formulation if local exposure or depot characteristics differ from those of a daily injection.
The eventual safety analysis should describe the severity, duration and recurrence of gastrointestinal events, whether symptoms were associated with dose escalation and whether supportive medication or treatment interruptions were required.
Investigators will also need to monitor sexual adverse reactions, mood changes and other warnings associated with setmelanotide. An MC4R-selective design intended to reduce melanocortin-1 receptor activity does not automatically eliminate effects mediated through the candidate’s principal pharmacological target.
The small preliminary dataset has not identified a new major safety signal, but it cannot rule out uncommon or delayed risks.
Why are comparisons with bivamelagon important but limited?
Bivamelagon is an oral MC4R agonist that Rhythm Pharmaceuticals acquired as another potential treatment for rare neuroendocrine obesity disorders. The company cited an average 10.1% body mass index reduction after 14 weeks among seven patients receiving a 600-milligram dose when placing the RM-718 data in context.
This creates a three-way development strategy involving approved daily injectable setmelanotide, investigational oral bivamelagon and investigational weekly injectable RM-718.
Each option could offer a different balance of efficacy, convenience, tolerability and patient preference. An oral medicine avoids injections but may require daily dosing and could have different absorption or gastrointestinal considerations. A weekly injection reduces dosing frequency while retaining direct and predictable drug delivery. Setmelanotide has the strongest clinical and regulatory evidence but requires daily administration.
For Rhythm Pharmaceuticals, maintaining several candidates provides flexibility but also creates portfolio questions. The company will eventually need to establish which product is best suited to each disease, age group and treatment setting.
The early cross-trial similarities in body mass index reduction do not resolve those questions. Direct comparisons, longer follow-up and more extensive safety data will determine whether the candidates are complementary or competing assets.
What are the limitations of the reported 11.6% BMI reduction?
The first limitation is the sample size. An average calculated from seven people is sensitive to individual outcomes and cannot provide a precise estimate of the effect expected across the broader patient population.
The second limitation is the open-label design. Patients and investigators knew that active treatment was being administered, and there was no concurrent placebo group against which to measure natural variation, trial participation effects or changes in supportive care.
The third limitation is the short treatment period. Sixteen weeks may be sufficient to detect an early weight signal, but acquired hypothalamic obesity requires long-term management. A useful therapy must maintain its effect without introducing cumulative toxicity or unacceptable treatment burden.
The fourth limitation involves incomplete reporting. The announcement did not provide confidence intervals, statistical testing, hunger outcomes, changes in body composition or complete results for every enrolled participant.
Finally, body mass index is an important but incomplete outcome. Future studies should examine fat mass, lean mass, metabolic health, hunger, quality of life, physical functioning and the durability of weight reduction.
What happens next in the RM-718 clinical programme?
The registered RM-718 study is expected to evaluate approximately 150 participants across its various components and is scheduled to continue beyond the current preliminary analysis. The programme includes patients with hypothalamic obesity and Prader-Willi syndrome as well as earlier assessments in healthy participants with obesity.
Longer-term data from the patients already treated will help determine whether the initial body mass index reduction deepens, plateaus or diminishes. Results from additional participants will also provide a more reliable estimate of discontinuations, gastrointestinal events, injection-site reactions and pigmentation changes.
Rhythm Pharmaceuticals has not identified the design or timing of a registration-enabling RM-718 trial. Decisions about later-stage development will probably depend on the complete Phase 1 and Phase 2 evidence and consultations with regulators.
Because setmelanotide is already approved, future RM-718 development may need to do more than demonstrate activity against baseline. Regulators, clinicians and payers may expect evidence explaining why another MC4R agonist is needed and which patients would benefit most from it.
RM-718’s weekly dosing is attractive, but seven patients cannot establish differentiation
The 11.6% average body mass index reduction gives Rhythm Pharmaceuticals a credible reason to continue developing RM-718. An early response of that magnitude suggests that weekly MC4R activation may produce clinically relevant weight loss in acquired hypothalamic obesity.
The low level of reported hyperpigmentation is also encouraging because it aligns with the candidate’s MC1R-sparing design. Together, weekly dosing and reduced pigmentation could create a more convenient and acceptable alternative to daily setmelanotide.
The evidence remains preliminary. The efficacy analysis includes only seven patients, two of the 11 enrolled patients discontinued because of adverse events and no placebo or active comparator was included.
The real strategic question is not whether RM-718 can reduce body mass index. The initial data suggest that it can. The challenge is proving that the candidate offers a more attractive overall treatment profile than an approved drug targeting the same biological pathway.
A larger controlled study must confirm the consistency of weight reduction, the durability of benefit and whether limited hyperpigmentation persists across a broader population. It should also show how hunger, body composition and quality of life change alongside body mass index.
RM-718 has passed its first clinical credibility test, but it has not yet passed the differentiation test. That will determine whether Rhythm Pharmaceuticals is building a genuine next-generation therapy or merely a more convenient variation on an already validated mechanism.
