Rhythm’s obesity drug scores ‘better than expected’ weight loss in rare genetic disease

Rhythm's Imcivree Shows Promising Phase 2 Results in Prader-Willi Syndrome

Rhythm Pharmaceuticals announced positive Phase 2 results for its weight loss injection, Imcivree (setmelanotide), in patients with Prader-Willi syndrome. At the six-month analysis, patients experienced a 3.06% mean decrease in body mass index, with adults seeing a 3.11% reduction and children a 3% drop. Body scans further revealed that the weight loss was primarily due to a 4.19% reduction in fat mass, accompanied by a 0.74% increase in lean mass. These outcomes were described as "better than expected" by Stifel analysts, marking a rebound for Imcivree after a recent late-stage disappointment. The drug also showed "clinically meaningful" improvements in hyperphagia, suggesting multiple potential paths to market.

• The Phase 2 study demonstrated a mean BMI reduction of 3.06% across all patients, with similar efficacy observed in both adult (3.11% reduction) and pediatric (3% reduction) populations. Crucially, body scans indicated that this weight loss was driven by a 4.19% decrease in fat mass, while lean mass concurrently increased by 0.74%, suggesting a favorable change in body composition.
• Beyond weight loss, Imcivree also showed "clinically meaningful" improvements in hyperphagia, or extreme hunger, as measured by the validated Hyperphagia Questionnaire for Clinical Trials score. This positive signal for hyperphagia is considered encouraging by analysts and suggests a potential alternative or complementary path to market, even though it is a more subjective endpoint than BMI.
• These positive Phase 2 results for Imcivree in Prader-Willi syndrome represent a significant rebound for Rhythm Pharmaceuticals, following a recent Phase 3 failure in March 2026 for other rare obesities. Imcivree, an MC4 receptor agonist, is Rhythm's only marketed drug, with $194.8 million in sales in 2025, and is already approved for several conditions associated with obesity, including Bardet-Biedl Syndrome and hypothalamic obesity.

Imcivree's Encouraging Phase 2 Outcomes in Prader-Willi Syndrome

A 2026 randomized, double-blind withdrawal study evaluated diazoxide choline extended-release (DCCR) tablets — recently FDA-approved for hyperphagia in adults and children aged four years and older with Prader-Willi syndrome (PWS) — in 77 participants who had previously completed earlier randomized and open-label DCCR studies. Over 16 weeks, participants were randomized 1:1 to once-daily DCCR or placebo. The primary endpoint, change in Hyperphagia Questionnaire for Clinical Trials (HQ-CT) total score, demonstrated statistically significant worsening in the placebo arm versus DCCR (LS mean change: 7.6 [SE 1.09] vs. 2.6 [SE 1.12]; P=0.0022), confirming the superiority of continued DCCR treatment. Weight outcomes also favored DCCR, with the placebo cohort gaining more weight and showing greater BMI z-score increases (LS mean weight difference: −1.6 kg [95% CI: −3.1, −0.1]; LS mean z-score difference: −0.09 [−0.17, −0.01]). Clinical Global Impression of Severity and Improvement scores trended in favor of DCCR but did not reach statistical significance. From a safety perspective, adverse event rates were comparable between arms, and no serious adverse events were recorded in the DCCR group.

A 2025 retrospective, multicenter study examined the glycemic and renal effects of sodium-glucose cotransporter-2 inhibitors (SGLT2is) in 48 adults with PWS and comorbid type 2 diabetes, of whom 24 received SGLT2is over a six-month follow-up period. Efficacy outcomes were encouraging: HbA1c — which was higher at baseline in the SGLT2i group — improved significantly (P < 0.05), while remaining stable in controls, and the albumin-to-creatinine ratio decreased significantly with no notable weight change. However, the safety profile warrants close attention: adverse events were reported in 37.5% of treated patients, including acute kidney injury in 8.3%, underscoring the need for rigorous renal function monitoring in this population. A separate 2025 systematic review and meta-analysis — encompassing five randomized controlled trials identified across PubMed, Cochrane, EMBASE, and Web of Science — assessed the effects of probiotic supplementation in PWS. Probiotics significantly increased the abundance of the Bifidobacterium genus (SMD 1.21; 95% CI: 0.02–2.39) and demonstrated a favorable trend on social engagement following 12 weeks of intake (SMD −0.68; 95% CI: −1.14 to −0.21; p=0.004), though no significant effect on behavioral problems was observed. Importantly, probiotic use was not associated with a significant increase in gastrointestinal adverse events, supporting an acceptable safety profile.

A 2026 retrospective review assessed the respiratory impact of growth hormone therapy in 56 children with PWS who underwent polysomnography before and after treatment initiation between January 2011 and June 2024. Children were stratified by age (< 2 years vs. ≥ 2 years), with the older cohort exhibiting more severe obstructive sleep apnea (OSA) pre-treatment (p < 0.05), while no between-group differences in central sleep apnea (CSA) were observed at baseline. Post-growth hormone, OSA developed in 21% of children under two years and 20% of those aged two or older, while CSA developed in 11% and 13%, respectively; CSA resolved in 21% of younger children and 6% of older children. The study's key conclusion was that very young children with PWS do not appear to carry a disproportionately elevated risk of developing OSA or CSA following growth hormone initiation, lending support to early treatment in this age group with appropriate polysomnographic monitoring.

Addressing Key Challenges in Prader-Willi Syndrome Treatment

Current treatment approaches for Prader-Willi syndrome (PWS) face significant hurdles spanning pharmacological efficacy, safety tolerability, procedural risk, and the critical importance of treatment timing. Despite progress in understanding the syndrome's complex pathophysiology, no single intervention has demonstrated comprehensive disease modification. The landscape reflects a field still navigating between established therapies with recognized limitations and investigational approaches requiring further validation.

• Liraglutide efficacy limitations: A 2022 study demonstrated that liraglutide failed to meet its coprimary endpoints for BMI standard deviation score reduction at weeks 16 and 52 in both adolescents and children with PWS. Changes in weight-related parameters between treatment arms were not statistically significant, and the most commonly reported adverse events were gastrointestinal in nature. The only signal of potential benefit was observed in hyperphagia scores at week 52, where adolescents receiving liraglutide showed lower hyperphagia total and drive scores versus untreated controls — an finding the authors noted warrants further investigation.

• Adverse event burden with DCCR: In the DESTINY PWS Phase 3 study, treatment-emergent adverse events with diazoxide choline extended-release tablet (DCCR) included hypertrichosis, peripheral edema, and hyperglycemia, resulting in discontinuation in 7.2% of participants — a non-trivial tolerability consideration for a chronic treatment population.

• Variable and controversial efficacy of rhGH: Although recombinant human growth hormone (rhGH) has been licensed for PWS since 1985 and remains the only approved pharmacological therapy, its growth response is variable and largely indication-dependent. For newer height-related indications, efficacy data remain controversial, and treatment decisions must carefully weigh potential risks against anticipated benefits.

• Sleep-related complications following rhGH initiation: Post-rhGH therapy, obstructive sleep apnea (OSA) developed in approximately 21% of children under 2 years and 20% of children aged 2 years and older. Central sleep apnea (CSA) dynamics were also altered: CSA resolved in 21% of children under 2 years but only 6% of older children, while new-onset CSA developed in 11% and 13% of the respective age groups — underscoring the need for vigilant respiratory monitoring during rhGH treatment.

• Treatment timing as a critical determinant of outcome: Evidence from a 2023 multicenter study highlights that earlier rhGH initiation yields meaningfully greater neurodevelopmental benefit. Improvements in general quotient and locomotor sub-quotient were significantly greater in infants treated before 9 months of age compared to those treated at 9 months or older, and motor quotients showed a negative correlation with age at treatment initiation — reinforcing that delayed intervention may irreversibly limit therapeutic gain.

• Limited efficacy of deep brain stimulation (DBS) in PWS-related obesity: A systematic review of DBS across 12 patients identified a mean BMI reduction of only 3.9 points overall, with PWS patients showing a delta of −2.3 compared to −10.0 in those with primary obesity. Moderate adverse events occurred in 33% of cases, including manic symptoms, electrode fracture, and seizure, with mild complications in an additional 41.6% of patients. The authors emphasized the need for prospective studies with longer follow-up periods to adequately characterize both efficacy and appropriate patient selection criteria.

Designing the Phase 2 Study for Imcivree in PWS

The PWS clinical trial landscape spans multiple therapeutic modalities, with study designs ranging from short-term randomized controlled trials to decade-long longitudinal studies. Key endpoints have evolved from growth and cognitive measures in early GH trials to hyperphagia-specific instruments and behavioral composites in more recent pharmacological studies.

Imcivree's Prader-Willi Success: A New Horizon for Genetic Obesity

The recent positive Phase 2 results for Imcivree (setmelanotide) in Prader-Willi syndrome (PWS) represent a pivotal moment for both patients and Rhythm Pharmaceuticals. For individuals living with PWS, a rare genetic form of obesity marked by insatiable hunger and severe weight gain, effective treatment options have been profoundly limited. The data showing not only a significant reduction in body mass index but also a favorable shift in body composition, with decreased fat mass and a concurrent increase in lean mass, offers a quality of weight loss that is highly desirable for long-term health. Crucially, the reported 'clinically meaningful' improvements in hyperphagia directly address the most challenging aspect of PWS, promising a better quality of life for patients and their caregivers, as studies indicate improved quality of life with setmelanotide treatment in other genetic obesities.

This success provides a much-needed boost for Imcivree, an MC4R agonist, following a prior late-stage setback. It underscores the drug's potential to address specific, severe genetic obesities where the MC4R pathway is implicated, further solidifying its role in precision medicine for obesity. While the expansion into PWS is promising, it is important to acknowledge the known side effects of MC4R agonism, such as skin hyperpigmentation and potential gastrointestinal issues like nausea and diarrhea, which have been observed in other indications and require careful monitoring. Furthermore, the precise identification of eligible patients through genetic testing remains paramount, as the drug's efficacy is tied to specific genetic defects, sometimes requiring in vitro assays to confirm pathogenicity. This development positions Imcivree as a targeted intervention in a landscape increasingly dominated by broader anti-obesity medications, highlighting the ongoing need for tailored therapies in rare genetic conditions and the potential for early intervention to mitigate severe outcomes.