Summary

No head-to-head clinical comparison of SKY-0515 with an approved symptomatic therapy or with another huntingtin-lowering agent has been reported. The investigational disease-modifying candidates differ in route: SKY-0515 and votoplam are oral small-molecule splicing modifiers, tominersen is an intrathecal antisense oligonucleotide, and AMT-130 is a one-time gene therapy delivered by stereotactic neurosurgery into the striatum. Efficacy claims for AMT-130, votoplam, and SKY-0515 beyond 12 months rest on comparisons with propensity-weighted, propensity score-matched, or overlap-weighted natural history cohorts rather than concurrent randomized controls. Sponsor announcements describe SKY-0515 mHTT lowering as the greatest demonstrated in patients; independent commentary noted that the SKY-0515 and votoplam studies differ too much in length, size, and data presentation for a true comparison. In the placebo-controlled phase 3 GENERATION HD1 trial of intrathecal tominersen (791 participants), treatment was stopped in March 2021; in an ad hoc week 69 analysis, the cUHDRS change with 120 mg every 8 weeks was significantly worse than with placebo (-0.54 points; adjusted P=0.001), neither cUHDRS nor TFC change with the every-16-week regimen differed significantly from placebo, and more adverse events occurred with the every-8-week regimen. Among approved symptomatic agents, no blinded head-to-head RCT of VMAT2 inhibitors exists; two indirect comparisons of the TETRA-HD and First-HD trials reached different overall conclusions on safety, and an open-label randomized trial of 179 participants compared tetrabenazine, tiapride, and olanzapine.

SKY-0515 in relation to huntingtin-only lowering: independent trials review

“SKY-0515 is designed to reduce both HTT and PMS1 proteins, the latter implicated in somatic CAG repeat expansion, a mechanistic process increasingly recognised as a key driver of neuronal loss in HD.” (opens the source at this quote in a new tab)
“This important phase II/III clinical trial of SKY-0515 will assess whether HTT and PMS1 lowering can offer greater therapeutic benefit than HTT reduction alone in early-stage HD.” (opens the source at this quote in a new tab)

Magnitude of HTT lowering relative to other agents: statements in sponsor announcements

“SKY-0515’s HTT reduction has the highest dynamic range I’ve seen from any therapeutic modality” (opens the source at this quote in a new tab)

“SKY-0515 continues to reduce mHTT protein to the greatest extent demonstrated by any therapeutic tested to date in patients” (opens the source at this quote in a new tab)

“SKY-0515 continues to reduce mHTT protein to the greatest extent demonstrated in patients, with clinical and biomarker data showing the drug is well tolerated at all doses tested.” (opens the source at this quote in a new tab)

HTT lowering targets in other trials: independent commentary

“Astute readers may notice that 70% lowering is a fair bit higher than the current target in other HTT lowering trials of around 50%. Currently, most companies have targeted the 30-50% range for HTT lowering drugs.” (opens the source at this quote in a new tab)

Route of administration compared with antisense oligonucleotides and gene therapy

“Unlike earlier approaches such as antisense oligonucleotides ( ASOs) given by spinal tap, or virus-based gene therapies delivered by brain surgery, SKY-0515 is a small-molecule compound which can be taken by mouth and can spread throughout the body. This is very similar to votoplam, another splice modulator in development for HD by Novartis and PTC Therapeutics.” (opens the source at this quote in a new tab)

Investigational disease-modifying candidates: differences in route and regimen

“Furthermore, the DMTs currently under development vary in mechanism of action (small molecule, ASO, gene therapy), route of administration (oral, intrathecal, intraparenchymal), and once vs recurrent therapy; these contingencies make it difficult at this time to assign a DMT price in our exploratory analysis.” (opens the source at this quote in a new tab)

cUHDRS component pattern compared with votoplam

“In PTC/Novartis’s data, for example, people taking votoplam seemed specifically to do better on thinking tests; in this study, people taking SKY-0515 seemed to do better on all of the tests.” (opens the source at this quote in a new tab)
“That’s a novel observation worth highlighting, but the studies are too different in length, size, and the way the data is presented to make a true comparison.” (opens the source at this quote in a new tab)

External natural-history comparisons across HTT-lowering trials

“Astute HDBuzz readers will have noticed that this is a similar type of comparison that was made for the AMT-130 clinical trial data which was contentious among some HD researchers.” (opens the source at this quote in a new tab)

AMT-130 (ifezuntirgene inilparvovec): administration and regulatory submissions

“Treated patients receive a single administration through targeted, MRI-guided, convection-enhanced stereotactic neurosurgical delivery directly into the striatum (caudate and putamen).” (opens the source at this quote in a new tab)
“The BLA and MAA are supported by the previously announced three-year data analysis from the Phase I/II clinical study of ifezuntirgene inilparvovec, compared to a propensity score-matched external control derived from the Enroll-HD natural history database.” (opens the source at this quote in a new tab)

AMT-130: 36-month topline results versus external controls

“The prespecified analysis included 29 treated participants (17 high-dose, 12 low-dose), with 12 patients per cohort reaching 36 months of follow-up. Outcomes were compared with propensity score-matched external controls from the Enroll-HD natural history dataset (n = 940 for high dose; n = 626 for low dose). At 36 months, high-dose AMT-130 met the primary endpoint, demonstrating a statistically significant 75% slowing of disease progression versus controls, as measured by the cUHDRS” (opens the source at this quote in a new tab)

AMT-130: FDA position after the January 2026 Type A meeting

“The FDA stated that it cannot agree that data from the Phase I/II studies, compared to an external control, are sufficient to provide the primary evidence of effectiveness required to support a marketing application for AMT-130. The FDA strongly recommended uniQure conduct a prospective, randomized, double-blind, sham surgery-controlled study.” (opens the source at this quote in a new tab)

AMT-130: FDA position after the June 2026 Type B meeting

“In addition, the FDA seeks to align on the confirmatory study design prior to the BLA submission, including consideration of concurrent control on standard-of-care therapy instead of a sham procedure.” (opens the source at this quote in a new tab)

Tominersen: phase 3 GENERATION HD1 dosing discontinued

“The iDMC made its recommendation based on the investigational therapy’s potential benefit/risk profile for study participants.” (opens the source at this quote in a new tab)
“Study participants were randomised to either 120 mg every-2-months or 120 mg every-4-months intrathecal injections of tominersen, or placebo. The study has recruited 791 participants from 18 countries around the world.” (opens the source at this quote in a new tab)

Tominersen: GENERATION HD1 design and primary end points

“in which 791 participants with manifest Huntington’s disease were randomly assigned in a 1:1:1 ratio to receive intrathecal tominersen, at a dose of 120 mg every 8 weeks or every 16 weeks, or placebo” (opens the source at this quote in a new tab)
“The primary end point was the change from baseline in the score on the composite Unified Huntington’s Disease Rating Scale (cUHDRS) or Total Functional Capacity (TFC) scale at week 101.” (opens the source at this quote in a new tab)
“Biomarker end points included levels of mutant HTT in CSF, levels of neurofilament light protein (NfL) in CSF, and ventricular volume.” (opens the source at this quote in a new tab)

Tominersen: GENERATION HD1 ad hoc week 69 clinical results

“On the basis of an overall benefit–risk assessment by an independent data monitoring committee, treatment was stopped in March 2021; therefore, an ad hoc analysis of the results at week 69 was carried out” (opens the source at this quote in a new tab)
“In the every-8-week tominersen group, the mean change from baseline in the score on the cUHDRS at week 69 was significantly worse than that in the placebo group (−0.54 points, adjusted P=0.001), but the mean change from baseline in the score on the TFC scale did not differ significantly (−0.40 points, adjusted P=0.09).” (opens the source at this quote in a new tab)
“In the every-16-week tominersen group, the mean changes from baseline in the scores on the cUHDRS and the TFC scale did not differ significantly from those in the placebo group (−0.16 points, adjusted P=0.84; and −0.04, adjusted P=1.00, respectively)” (opens the source at this quote in a new tab)
“Nonetheless, these results must be interpreted with caution, given the ad hoc nature of the analysis.” (opens the source at this quote in a new tab)

Tominersen: GENERATION HD1 biomarkers, adverse events, and post hoc subgroup analyses

“The changes in mutant HTT levels in CSF were consistent with dose regimen–dependent decreases and concomitant increases in ventricular volume” (opens the source at this quote in a new tab)
“The absence of whole-brain volume loss in the context of ventricular expansion and the absence of a correlation between the change in CSF NfL levels and change in ventricular volume suggest that increases in ventricular volume do not reflect increased atrophy” (opens the source at this quote in a new tab)
“A transient increase was seen in CSF NfL levels in the every-8-week tominersen group at week 21; although neuroaxonal damage may be implicated, increased levels of leukocytes and total protein in CSF in some participants suggest an inflammatory component” (opens the source at this quote in a new tab)
“More adverse events occurred in the participants in the every-8-week tominersen group than in those in the placebo group and the every-16-week tominersen group” (opens the source at this quote in a new tab)
“The results of post hoc analyses support the hypothesis that younger participants (who have less disease burden) may benefit from tominersen therapy” (opens the source at this quote in a new tab)
“this hypothesis is currently being tested in a phase 2 dose-finding trial (GENERATION HD2; NCT05686551).” (opens the source at this quote in a new tab)

Tominersen: phase 2 GENERATION HD2

“GENERATION HD2 is a phase II, randomised, double-blind, placebo-controlled, dose-finding clinical trial comparing 60 mg tominersen, 100 mg tominersen and placebo.” (opens the source at this quote in a new tab)
“The clinical trial remains ongoing, and definitive conclusions regarding clinical benefit will only be possible once the data are unblinded and fully analysed. This is anticipated to be late 2026.” (opens the source at this quote in a new tab)

Votoplam: oral splicing modifier, 24-month extension versus natural history

“In the interim analysis following 24 months of votoplam treatment, there was evidence of dose-dependent benefit in slowing progression on the Composite Unified Huntington's Disease Rating Scale (cUHDRS) relative to a propensity weighted natural history cohort in Stage 2 participants, with 52% and 28% slowing for 10 mg and 5 mg participants, respectively.” (opens the source at this quote in a new tab)
“Votoplam (formerly PTC518) is a small molecule splicing modifier that acts via a unique mechanism to promote the inclusion of a novel pseudoexon containing a premature termination codon, thus triggering Huntingtin (HTT) mRNA degradation and subsequent reduction in HTT protein levels.” (opens the source at this quote in a new tab)

Votoplam: route and 12-week PIVOT-HD primary endpoint

“PTC518 (also known as votoplam), an orally administered huntingtin mRNA splice modulator.” (opens the source at this quote in a new tab)
“The study met its primary endpoint, showing dose-dependent reductions in blood HTT protein levels at week 12, along with a favourable safety and tolerability profile across all dose levels and disease stages.” (opens the source at this quote in a new tab)

Pridopidine: phase 3 PROOF-HD and EMA opinion

“The primary and key secondary endpoints, change in total functional capacity (TFC) and composite Unified Huntington's Disease Rating Scale (cUHDRS) score at week 65, were not met in the overall population.” (opens the source at this quote in a new tab)

“Prilenia and Ferrer announced that the European Medicines Agency's Committee for Medicinal Products for Human Use had recommended refusal of marketing authorisation for pridopidine in HD.” (opens the source at this quote in a new tab)

Modeled comparison of investigational agents with standard of care: tominersen and AMT-130

“Compared with SoC, Tominersen increased costs by $2.11 million and QALYs by 0.93, resulting in an ICER of $2.28 million per QALY gained, whereas AMT-130 increased costs by $1.49 million and QALYs by 5.23, resulting in an ICER of $285,703 per QALY gained in the base case.” (opens the source at this quote in a new tab)

Approved VMAT2 inhibitors: indirect tolerability comparison of deutetrabenazine and tetrabenazine

“Compared with tetrabenazine, deutetrabenazine was associated with a significantly lower risk of moderate to severe AEs and neuropsychiatric AEs including agitation, akathisia, depression, depression/agitated depression, drowsiness/somnolence, insomnia, and parkinsonism in both adjusted and unadjusted analyses (p < 0.05 for each).” (opens the source at this quote in a new tab)

Approved VMAT2 inhibitors: GRADE-based indirect comparison

“There is low-quality evidence that tetrabenazine and deutetrabenazine do not differ in efficacy or safety.” (opens the source at this quote in a new tab)

Symptomatic antichoreic agents: open-label randomized comparison of tetrabenazine, tiapride, and olanzapine

“We conducted a pragmatic, randomized, open-label trial across 11 centers of the French-Speaking Huntington's Network, enrolling 179 patients with Huntington's disease.” (opens the source at this quote in a new tab)
“Tetrabenazine was most frequently associated with mood disorders and sedation, while olanzapine caused occasional weight gain and mild increases in LDL and total cholesterol.” (opens the source at this quote in a new tab)

Direct head-to-head clinical comparison with votoplam, AMT-130, tominersen, WVE-003, or another active therapy

No evidence found.