SKY-0515

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.