Skyhawk CEO hopes FDA’s new uniQure position will translate to Huntington’s pill

Skyhawk's Oral SKY-0515 Shows Positive Functional and Biomarker Data in Huntington's Phase 1/2

Skyhawk Therapeutics revealed 12-month functional data from a Phase 1/2 trial of its oral drug, SKY-0515, for Huntington's disease. The data showed "favorable and consistent trends" on all composite Unified Huntington’s Disease Rating Scale (cUHDRS) components compared to natural history controls, with patients experiencing improvements in functional and motor scores. Additionally, SKY-0515 led to dose-dependent reductions of up to 69% in mutant huntingtin (mHTT) protein and up to 26% in PMS1 mRNA. Patient and physician surveys also indicated positive outcomes, with no patients reporting worsening disease and many reporting improvement. The drug demonstrated an "extraordinary" safety profile and is orally administered.

• Skyhawk's SKY-0515 demonstrated significant functional and motor improvements in Huntington's disease patients after 12 months in a Phase 1/2 trial. Data showed "favorable and consistent trends" across all cUHDRS components compared to natural history controls. Notably, patient and physician surveys reinforced these findings, with 65% of patients reporting improvement and no patients or physicians reporting disease worsening, a rare outcome in this progressive neurodegenerative condition.
• The investigational drug, SKY-0515, achieved dose-dependent reductions in key biomarkers associated with Huntington's disease progression. Patients treated with SKY-0515 showed up to a 69% reduction in mutant huntingtin (mHTT) protein, the primary driver of the disease. Additionally, reductions of up to 26% were observed in PMS1 mRNA, a crucial factor in somatic CAG repeat expansion linked to disease progression, indicating a direct impact on the underlying pathology.
• SKY-0515, an orally administered small molecule RNA splicing modifier, exhibited an "extraordinary" safety profile and a patient-friendly 30-hour half-life, offering an advantage over more invasive therapies like intrathecal gene therapies. Following recent FDA reversals regarding uniQure's gene therapy, Skyhawk is engaging with the FDA about a potentially accelerated approval pathway for SKY-0515, with a pivotal placebo-controlled Phase 2/3 FALCON-HD study already enrolling over 140 patients.

Positive 12-Month Functional Data for SKY-0515 in Huntington's Disease

Recent clinical investigation in Huntington's disease (HD) has spanned disease-modifying biologics, small molecules, and symptomatic therapies, with mixed outcomes across efficacy and safety endpoints. The studies summarized below represent a cross-section of late-stage and early-phase evidence published through 2025–2026.

• PROOF-HD (Pridopidine, NCT04556656): This randomized, double-blind, placebo-controlled Phase 3 trial evaluated pridopidine, a selective Sigma-1 receptor agonist, at 45 mg twice daily. Both co-primary endpoints were missed in the overall population: TFC least-squares mean difference was −0.18 (95% CI −0.49 to 0.14; P = 0.26) and cUHDRS was −0.11 (95% CI −0.40 to 0.18; P = 0.45). A sensitivity analysis excluding participants who received antidopaminergic medications at any time showed a consistent pattern favoring pridopidine across TFC and cUHDRS measures. Pridopidine demonstrated a favorable safety and tolerability profile at the studied dose.

• NestaCell® Phase I Study (hDPSCs, NCT02728115): This first-in-human, open-label, single-centre Phase I trial evaluated intravenous human dental pulp stem cells (hDPSCs) in six male HD patients across two dose cohorts (1 million cells/kg and 2 million cells/kg). Treatment cycles consisted of three monthly infusions followed by cycles every six months, with total infusions ranging from 4 to 26 over five years. No adverse events were observed during 48-hour ICU monitoring or within 15 days post-infusion. Of 41 total TEAEs, 35 were considered unrelated to hDPSCs; 6 treatment-related TEAEs involved transient changes in hair pigmentation or regrowth. One patient discontinued following a diagnosis of lung cancer arising from a pre-existing pulmonary nodule; genetic analysis of the excised tumor showed no evidence of investigational product engraftment. Minor, clinically insignificant fluctuations in CD4/CD8 counts and cytokine levels were noted. Preliminary efficacy analyses indicated potential stabilisation of disease progression, particularly on UHDRS Total Motor Score and Total Functional Capacity.

• VIBRANT-HD (Branaplam, NCT05111249): This Phase 2b randomized study evaluated branaplam, an orally available HTT mRNA splicing modulator, using an innovative design with staggered cohorts and prospective safety monitoring via neurofilament light chain (NfL) measurements and nerve conduction studies. Of 21 participants in the initial cohort receiving 56 mg weekly, 18 (85.7%) exhibited at least one sign or symptom of peripheral neuropathy, triggering early study termination alongside dose-modelling findings. Elevated NfL levels observed in most participants reversed following treatment discontinuation. Despite early termination, branaplam became the first splicing modulator to demonstrate reductions in CSF mutant HTT levels in individuals with HD.

• VMAT2 Inhibitors Meta-Analysis (3 RCTs, N = 302): A meta-analysis of placebo-controlled RCTs evaluating tetrabenazine, deutetrabenazine, and valbenazine identified significant improvement in UHDRS Total Maximal Chorea (TMC) scores (MD −2.98; 95% CI −4.21 to −1.75; P = 0.009; I² = 0%) and Clinical Global Impression of Change (CGI-C; OR 5.36; 95% CI 2.94–9.76; P = 0.007; I² = 0%). Adverse event rates were not significantly different from placebo (OR 1.89; 95% CI 0.47–7.70; P = 0.19), and depression scale scores were not meaningfully affected (SMD −0.40; 95% CI −1.20 to 0.41; P = 0.17). A complementary PRISMA-registered systematic review confirmed that all three agents reduced TMC scores, with tetrabenazine showing the greatest chorea reduction but the highest adverse event rate, deutetrabenazine offering favorable tolerability with modest improvements in physical functioning and swallowing, and valbenazine demonstrating an intermediate safety profile.

• Laquinimod Systematic Review (10 studies, 2638 records screened): This systematic review of preclinical and clinical evidence for laquinimod, an oral immunomodulator, found that preclinical studies demonstrated improved motor function, reduced neuroinflammation, and promoted myelination—likely via microglial modulation, NF-κB suppression, and increased BDNF expression—though effects on myelin integrity and inflammatory markers were inconsistent. In vitro studies showed limited, variable cytokine modulation in HD patient-derived cells. Clinical trials did not demonstrate significant improvements in motor or functional outcomes, with only one study reporting minor cognitive and behavioral benefits. Long-term safety data remain limited.

Targeting Genetic Drivers: SKY-0515's Approach to Huntington's Disease

Huntington's disease (HD) is a monogenic autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in exon 1 of the huntingtin (HTT) gene on the short arm of chromosome 4. This expansion encodes an abnormally elongated polyglutamine (polyQ) tract in the huntingtin protein, which drives protein misfolding, toxic oligomer formation, and the accumulation of ubiquitinated intranuclear inclusion bodies in striatal neurons. Critically, somatic instability of the CAG repeat — with gains of up to 1,000 additional repeats occurring in striatal cells early in the disease course, likely preceding pathological cell loss — is thought to contribute significantly to the progressive and cell-selective nature of HD pathogenesis. Repeat length strongly correlates with age of onset and clinical severity, and differential alternative splicing of associated proteins such as tau (including changes in exons 2, 3, and 10 in the putamen) further amplifies downstream pathology through increased tau hyperphosphorylation and aggregation.

At the molecular and cellular level, mutant huntingtin (mHTT) disrupts multiple interdependent pathways. Transcriptional dysregulation arises as polyQ-expanded mHTT sequesters nuclear transcription factors and transcriptional coactivators — including CA150, whose expression is markedly elevated in HD brain tissue — compromising gene expression programs essential for neuronal survival. Concurrently, mHTT impairs both the ubiquitin-proteasome system (UPS) and autophagy-lysosomal pathways, reducing synaptic UPS activity and inducing autophagic granule formation under conditions of oxidative stress. Disturbed calcium signalling represents a convergent pathogenic node: mHTT sensitises medium spiny neurons (MSNs) to excitotoxicity by facilitating NR2B-subtype NMDA receptor currents and type 1 inositol 1,4,5-trisphosphate receptor (InsP₃R1) activity, resulting in supranormal intracellular Ca²⁺ responses, rapid mitochondrial membrane potential loss, and glutamate-induced apoptosis. Mitochondrial dysfunction and reactive oxygen species generation further compound energy metabolic failure, with early bioenergetic alterations — including localised changes in cerebral glucose utilisation — observable in premanifest HD carriers.

Selective neurodegeneration of striatal MSNs, despite ubiquitous mHTT expression, reflects the convergence of these pathways in a particularly vulnerable cell population. Glial dysfunction contributes non-cell-autonomously: HD astrocytes exhibit reduced BDNF and GDNF secretion, lower intracellular glutathione levels, and impaired proteostasis marked by elevated ubiquitination and p62, rendering the neuronal microenvironment increasingly hostile. An emerging mechanism involves the unconventional secretion of mHTT via extracellular vesicles, facilitated by a HSPB1–p62/SQSTM1 cargo-loading complex; the resulting HTT-containing vesicular structures are biologically active, internalised by recipient cells, and capable of seeding oligomerisation of wild-type huntingtin — providing a molecular basis for prion-like propagation of pathology. Notably, gene silencing studies demonstrate that while established neuronal loss is irreversible, halting mHTT expression prevents further degeneration and permits full motor recovery, confirming that ongoing mHTT production — rather than static inclusion bodies — is the principal driver of progressive dysfunction.

SKY-0515 Navigates the Evolving Huntington's Disease Treatment Landscape

Over the past five years, the Huntington's disease (HD) treatment landscape has been substantially shaped by the pursuit of gene-silencing strategies targeting the root cause of the disease. Clinical trial approaches have centred on RNA-targeted knockdown of HTT through splice modulation, siRNA, and antisense oligonucleotides (ASOs), reflecting the field's pivot toward disease-modifying rather than purely symptomatic intervention. However, recent high-profile ASO trial failures have underscored the gap between preclinical promise and clinical efficacy, reinforcing that no therapy capable of modifying HD disease progression has yet received regulatory approval. In parallel, preclinical innovation continues: an acyclic serinol nucleic acid (SNA)-modified siRNA targeting CAG repeats has demonstrated selective silencing of polyQ-encoding alleles without affecting wild-type counterparts in mouse models, and a novel in-silico fragment scanning approach has identified GLYN122 as a candidate compound that directly binds mutant huntingtin (mHTT), induces autophagy in neurons, and improves motor symptoms in the R6/2 mouse model.

Beyond gene-directed strategies, cell replacement therapies — initially based on fetal neural tissue transplantation and more recently on alternative stem cell sources — have been evaluated in both animal models and HD patients, with stem cells demonstrating capacity to generate the target neuron phenotype and supply trophic support to degenerating host tissue. Genetic tools including RNA interference and CRISPR/Cas9 have advanced through animal model and human-derived cell studies, though genome editing has not yet reached clinical testing specifically in HD. Modulators of the adenosine system have also emerged as a candidate avenue, given that polymorphic variation in ADORA2A influences age at onset in HD. The Enroll-HD platform, now operating across 159 clinical sites in 21 countries with nearly 25,000 participants recruited, has materially strengthened the infrastructure available to support interventional trial recruitment and biomarker development across this pipeline.

On the symptomatic side, recent trial data highlight the persistent difficulty of demonstrating pharmacological benefit over placebo in HD. A 2026 double-blind, randomised, crossover trial evaluating dextromethorphan/quinidine (DM/Q) 20/10 mg for HD-associated irritability enrolled 20 participants and found that both active treatment and placebo reduced mean Irritability Scale scores (32% and 27.5%, respectively) and the irritability subscale of the PBA-s (42% vs. 33%), with no statistically significant between-group differences across motor, behavioural, or cognitive endpoints. Notably, no FDA-approved medication currently targets irritability specifically in HD. A separate 2026 open-label pilot study of neuronavigated intermittent theta burst stimulation (iTBS) rTMS targeting the left dorsolateral prefrontal cortex in 11 HD patients with comorbid major depressive disorder reported a 43% reduction in Hamilton Depression Rating Scale-17 scores at post-treatment and improvements in attention, working memory, processing speed, and verbal learning — representing the first clinical evaluation of rTMS in this comorbid population. Collectively, these data illustrate a field advancing on multiple mechanistic fronts while remaining without an approved disease-modifying therapy.

Oral SKY-0515 Data Hints at New Era for Huntington's Treatment

The recent disclosure of 12-month functional data for Skyhawk Therapeutics' oral drug, SKY-0515, in Huntington's disease (HD) marks a potentially pivotal moment for patients grappling with this devastating neurodegenerative condition. The Phase 1/2 trial results indicate that SKY-0515 not only demonstrates an 'extraordinary' safety profile but also elicits 'favorable and consistent trends' across key functional and motor metrics, as measured by the composite Unified Huntington’s Disease Rating Scale (cUHDRS). This suggests a genuine impact on disease progression, a critical goal in HD treatment.

Perhaps most compelling is the drug's ability to achieve dose-dependent reductions of up to 69% in mutant huntingtin (mHTT) protein, the direct cause of HD. This biomarker reduction, coupled with reported patient and physician improvements, positions SKY-0515 as a potential disease-modifying therapy. The convenience of an oral administration further enhances its appeal, offering a significant advantage over more invasive or complex delivery methods.

However, as with all early-phase data, important considerations remain. The findings, while encouraging, stem from a relatively small cohort and require rigorous validation in larger, longer-duration Phase 3 trials. Furthermore, the observed reduction in PMS1 mRNA, a gene linked to the mismatch repair pathway, warrants deeper investigation. While a finding, its precise contribution to the drug's therapeutic effect in HD, or any potential broader implications for cellular function, is not fully elucidated by current literature, which suggests that compromised mismatch repair can lead to increased mutation rates. Finally, while comparisons to natural history controls are valuable, the drug's relative efficacy and safety against other investigational or approved therapies will need to be established through direct comparative studies. Despite these points for future exploration, the data for SKY-0515 offers a beacon of hope, potentially ushering in a new era of accessible, disease-modifying treatment for Huntington's disease.