SKY-0515

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by an expanded CAG repeat in HTT. Alleles of 40 or more repeats are fully penetrant, and alleles of 36 to 39 repeats show reduced penetrance. A 2022 meta-analysis of 33 studies estimated a pooled global prevalence of 4.88 per 100,000 and a pooled incidence of 0.48 cases per 100,000 person-years. In the US, a commercial claims analysis of over 67 million enrollees reported an age-adjusted diagnostic frequency of 6.52 per 100,000 persons, and the Huntington's Disease Society of America estimates approximately 41,000 symptomatic Americans and more than 200,000 at risk of inheriting the disease.

Mean age of onset is approximately 45 years, and median survival after onset is 15 to 18 years. In 878 individuals with known ages at motor onset and death, median disease duration was 15 years. In a UK primary care cohort, median survival from first recorded diagnosis was 12.4 years, and risk of death was 4.51 times that of matched controls. Among 147 brain bank records that listed a cause of death, pneumonia was recorded in 81 (55%). In a Norwegian registry, suicide accounted for 2.3% of deaths in people with the disease compared with 1.3% in the general population.

Morbidity spans motor, cognitive, and psychiatric domains. Neuropsychiatric symptoms were present in 98% of 52 participants in one cohort, and suicidal ideation was endorsed by 169 of 2,106 mutation carriers (8.0%) at baseline in the European REGISTRY study. Falls were reported by 27 of 45 participants (60%) with early to midstage disease, and 228 of 3,070 participants (7.4%) with clinically definite disease in a US database resided in skilled nursing facilities. Late-stage disease is characterized by total dependence, mutism, and incontinence.

No placebo-controlled efficacy results are available for SKY-0515. In the SKY-0515-001 Part C study, 26 participants with HD were randomized to 3 mg (10 participants), 9 mg (10), or placebo (6) for 84 days and then received blinded 4 mg or 9 mg SKY-0515 for an additional 12 months. At day 84, blood mHTT protein was reduced by 62% with 9 mg and by 29% with 3 mg, and blood HTT and PMS1 mRNA were reduced by 62% and 26% with 9 mg. Reductions in blood mHTT of up to 69% were reported at 12 months, and average reductions of more than 60% in mHTT and 25% in PMS1 mRNA at 9 mg were reported through 15 months. In healthy volunteers, blood HTT mRNA was reduced by up to 67% after a single 16 mg dose and by 72% after 14 days of 9 mg daily.

Clinical outcomes have been compared only with overlap-weighted external natural-history controls from the Enroll-HD, 2CARE, and CREST-E datasets, not with placebo. At month 15, the designated primary timepoint (15 treated participants), cUHDRS changed by +0.94 points with SKY-0515 and by -0.65 points in the external control (difference +1.59; 95% CI 1.09 to 2.09; p<0.001), with differences favoring SKY-0515 on TFC (+0.98), TMS (-9.38), SDMT (+4.15), and SWRT (+4.18). The cUHDRS differences at month 3 (+0.66; p=0.191) and month 6 (+0.78; p=0.051) were not statistically significant. The sources also disagree on the month 12 result and on which timepoint is primary. The September 2026 fifteen-month announcement reports a month 12 cUHDRS difference of +0.79 (18 treated participants; 95% CI 0.11 to 1.48; p=0.023) and designates month 15 as the primary timepoint. The June 2026 poster reports a difference of +1.25 at "the primary 12-month endpoint" versus propensity score-weighted natural-history controls, pooling the 4 mg and 9 mg groups and including participants who switched from placebo (15 participants at month 12). Because the two analyses use different weighting methods and participant sets, their estimates are not directly comparable. These results are reported only in company announcements and congress posters.

In participants with HD, the most common related TEAEs (reported by two or more participants) were headache, diarrhea, and nausea, with the majority mild or moderate. The sources differ on serious adverse event attribution. A February 2026 poster listed four severe or serious events and described one, a prolonged headache, as "possibly drug related". A June 2026 poster reported that three participants had at least one serious adverse event (grade 3 mania; grade 3 tooth infection; grade 3 pneumonia and pleural effusion), none considered related to study drug, and the fifteen-month announcement reported no treatment-related SAEs. Plasma NfL through 12 months and CSF NfL through 9 months were reported as generally stable. In healthy volunteers, no severe or serious adverse events were observed at single doses up to 16 mg or multiple doses up to 9 mg/day for 14 days.

The randomized, placebo-controlled phase 2/3 FALCON-HD trials have not reported results. FALCON-HD 004-ANZ enrolled 144 participants in Australia and New Zealand. Planned enrollment in FALCON-HD 004-WW is 400 participants in the registry record and in a July 2026 company announcement ("up to an additional 400"), and approximately 600 participants in the September 2026 fifteen-month announcement.

No price, SKY-0515-specific sales forecast, cost-effectiveness analysis, budget impact model, or health technology assessment has been published. The company has stated that launch could occur as early as 2027. The only market estimate that names SKY-0515 is a market research announcement that estimates the seven-major-market HD market at approximately USD 320 million in 2025, with a projected 14% compound annual growth rate from 2026 to 2036, and lists SKY-0515 among pipeline candidates; the announcement does not report product-level forecasts.

The available cost evidence is disease-level. In U.S. Medicare fee-for-service claims (3,688 beneficiaries with Huntington's disease), mean annual total healthcare costs were $20,475 in early-stage, $29,733 in middle-stage, and $56,657 in late-stage disease; in U.S. Medicaid claims (595 beneficiaries), the corresponding means were $22,797, $55,294, and $95,251. In the Huntington's Disease Burden of Illness study (five European countries and the United States), mean annual direct medical costs were €12,663 (n = 2,094) and mean annual indirect costs were €47,576 (n = 436); mean annual total costs were €42,477 in early-stage, €64,688 in mid-stage, and €86,177 in advanced-stage disease, and in a subsample of 307 participants indirect costs accounted for 84%, 80%, and 66% of total costs by stage.

Modeling evidence exists for other investigational huntingtin-lowering therapies. A U.S. Markov model from a modified societal perspective estimated incremental cost-effectiveness ratios of $2.28 million per QALY gained for tominersen and $285,703 per QALY gained for AMT-130 versus standard of care. Because no established cost data exist for either product, the model assumed drug costs by analogy with spinal muscular atrophy therapies (an annual tominersen cost of $318,750 and a one-time AMT-130 cost of $1,806,250), and the probability that tominersen was cost-effective was 0% throughout the acceptability curve. A separate decision-modeling framework found that hypothetical disease-modifying treatments reduced lifetime costs in a prefunctional decline population but increased costs in populations with active functional decline, and its authors stated that differences in mechanism, route (including oral), and dosing frequency made it difficult to assign a treatment price. In August 2026, the Institute for Clinical and Economic Review announced an assessment of AMT-130 that is planned to include a five-year health care system budget impact analysis; SKY-0515 is not an intervention in that draft scope. Budget impact for SKY-0515 therefore cannot be estimated from public data as of September 2026.

SKY-0515 is an investigational, once-daily oral small-molecule RNA splicing modulator designed to lower HTT mRNA, and with it both mutant and wild-type huntingtin protein, and to lower PMS1, a modifier of somatic CAG repeat expansion. It is not approved for any disease in any country. In the SKY-0515-001 Part C study, blood mHTT protein was reduced by 62% with 9 mg and by 29% with 3 mg at day 84, mean mHTT reductions of approximately 69% at 9 mg were reported during the extension, and PMS1 mRNA was reduced by 25% to 26%. Independent commentary noted that there is no direct evidence that the PMS1 reductions slow somatic CAG repeat expansion in the brains of people with HD.

Clinical efficacy has not been demonstrated against placebo. The placebo-controlled period of Part C lasted 84 days in 26 participants and reported no placebo comparison of clinical outcomes. The reported cUHDRS benefit (+1.59 points at month 15 in 15 treated participants) comes from an extension period in which all participants received SKY-0515, compared with overlap-weighted external natural-history controls; an external control cannot account for unmeasured differences between groups. Reported tolerability through 15 months is favorable, but posters differ on whether one serious prolonged headache was possibly drug related. The randomized, placebo-controlled phase 2/3 FALCON-HD trials have not reported results; FALCON-HD 004-ANZ has an estimated primary completion of May 2027, and FALCON-HD 004-WW, with a 72-week cUHDRS primary endpoint, of December 2028.

In Australia, the TGA determined that SKY-0515 meets the eligibility criteria for the provisional approval pathway, and Skyhawk submitted an application for provisional approval; provisional registration is time-limited and is not full approval. In the United States, SKY-0515 is limited to clinical trial use under an accepted IND for FALCON-HD, and no FDA marketing application or expedited program designation was identified. The company has projected approval in other major markets during 2027 and launch as early as 2027. No price or budget impact estimate is available.

Current standard of care is symptomatic, and no approved therapy delays onset or slows progression. Three VMAT2 inhibitors (tetrabenazine, deutetrabenazine, and valbenazine) are FDA-approved for chorea associated with Huntington's disease; each label carries a boxed warning for depression and suicidality. In 12-week placebo-controlled trials, the chorea score treatment effect was 3.5 units with tetrabenazine (84 participants), 2.5 units with deutetrabenazine, and 3.2 units with valbenazine (125 participants in the full-analysis set). The 2022 MDS evidence-based review found that the data support only VMAT2 inhibitors for specific motor symptoms and found no eligible evidence for treatments of depression, psychosis, irritability, apathy, or suicidality. Use of antipsychotics, antidepressants, and rehabilitation rests mainly on expert consensus in the EHDN and DGN guidelines. Among other investigational disease-modifying approaches, dosing in the phase 3 tominersen trial was stopped in 2021, and in an ad hoc week 69 analysis the cUHDRS change with the every-8-week regimen was significantly worse than with placebo (-0.54 points; adjusted P=0.001); a BLA for accelerated approval of neurosurgically administered AMT-130 was submitted in September 2026 on the basis of a comparison with an external control, after the FDA had stated, following a January 2026 Type A meeting, that such a comparison was not sufficient as primary evidence of effectiveness. No direct comparison of SKY-0515 with any symptomatic or investigational therapy has been reported.