Oral HTT-Lowering Drug Branaplam Shows Promise but Halted Due to Peripheral Neuropathy in Phase 2 Huntington's Disease Trial
核心洞察
Branaplam, the first oral splicing modulator tested in Huntington's disease (搜索) patients, successfully reduced mutant huntingtin (搜索) protein levels by 25% in cerebrospinal fluid after 17 weeks of treatment.
The VIBRANT-HD phase 2b trial was terminated early after 85.7% of participants receiving branaplam 56 mg weekly developed signs of peripheral neuropathy (搜索), with 76.2% showing elevated neurofilament light chain (搜索) levels.
Despite safety concerns, the study demonstrated proof-of-concept that oral drugs can effectively lower huntingtin (搜索) levels in the brain, paving the way for development of more selective second-generation HTT (搜索)-lowering therapies.
The first oral drug designed to lower huntingtin (搜索) (HTT (搜索)) protein levels in Huntington's disease (搜索) patients has demonstrated biological efficacy but was halted due to safety concerns, according to results from the VIBRANT-HD phase 2b trial published in Nature Medicine.
Branaplam, originally developed by Novartis for spinal muscular atrophy (搜索), works by modifying HTT (搜索) mRNA splicing to promote inclusion of a pseudoexon containing premature stop codons. This mechanism leads to degradation of HTT mRNA before protein translation, resulting in decreased production of both normal and mutant huntingtin (搜索) (mHTT).
Trial Design and Early Termination
The VIBRANT-HD study was designed as a randomized, double-blind, placebo-controlled trial with an innovative staggered cohort approach and comprehensive safety monitoring plan. The study planned to enroll 75 participants across multiple dose levels, starting with branaplam 56 mg weekly in cohort 1.
However, after enrolling only 26 participants in cohort 1, the study was suspended on August 5, 2022, due to emerging safety signals. Following a four-month review of unblinded data, the study team, endorsed by the independent Data Monitoring Committee and Steering Committee, made the decision to permanently discontinue treatment and halt branaplam development for Huntington's disease (搜索).
Efficacy Results Demonstrate HTT Lowering
Despite early termination, the study achieved its primary biological objective. Branaplam 56 mg weekly lowered mHTT levels in cerebrospinal fluid by 22.3% at week 9 and 26.6% at week 17, compared to placebo reductions of 6.4% and 1.4%, respectively. The placebo-corrected reduction of 25.2% at week 17 aligned closely with translational pharmacokinetic-pharmacodynamic modeling predictions.
Blood biomarker analysis confirmed the expected mechanism of action, showing increased HTT (搜索) pseudoexon inclusion and decreased HTT mRNA expression. These findings provided the first proof-of-concept that an oral drug can effectively lower HTT levels in the human brain through the intended splicing modulation pathway.
Safety Concerns Drive Study Termination
The primary safety concern was peripheral neuropathy (搜索), which affected 85.7% of participants receiving branaplam. Multiple assessment methods revealed the extent of nerve involvement:
- Neurofilament light chain (NfL) elevation: 76.2% of branaplam-treated participants showed increased serum NfL levels, typically beginning at week 9, compared to no participants in the placebo group
- Nerve conduction abnormalities: 52.4% of participants had ≥50% reduction in sural sensory nerve action potential amplitude, and 14.3% showed similar reductions in tibial compound motor action potential amplitude
- Neurological examination findings: 61.9% of participants developed new findings of reduced motor function, absent reflexes, or decreased sensory function
- Patient-reported symptoms: 28.6% of participants reported neuropathy symptoms through questionnaires
Importantly, these effects showed reversibility after treatment discontinuation. During the one-year follow-up period, NfL levels returned to baseline, and most neuropathy signs and symptoms either resolved completely or improved significantly.
Brain Imaging Reveals Ventricular Changes
MRI analysis revealed a transient increase in lateral ventricle volume in branaplam-treated participants compared to placebo at week 17. This effect appeared to partially reverse after treatment discontinuation, with subsequent progression rates similar to those observed in a matched external control cohort from the TRACK-HD observational study. The clinical significance of this finding remains unclear, though similar observations have been reported with other HTT (搜索)-lowering investigational therapies.
Preclinical Studies Support NfL as Safety Biomarker
Parallel studies in nonhuman primates provided crucial validation for the clinical safety monitoring approach. In cynomolgus monkeys treated with branaplam for up to 39 weeks, dose-related increases in NfL levels preceded histological evidence of peripheral neuropathy (搜索). Animals receiving ≥3 mg/kg twice weekly showed elevated plasma NfL levels as early as week 4, with most showing increases by weeks 13-17.
Histopathological examination revealed sensory peripheral neuropathy (搜索) findings in the spinal cord, dorsal root ganglia, and peripheral nerves. Importantly, these changes showed partial to full resolution during a 16-17 week recovery period after treatment cessation, supporting the reversible nature of branaplam-induced neuropathy observed in the clinical trial.
Modeling Analysis Rules Out Lower Doses
To determine whether safer dosing regimens might still provide clinical benefit, researchers employed translational pharmacokinetic-pharmacodynamic modeling using data from the BacHD mouse model adapted to humans. The analysis evaluated whether lower or less frequent doses could achieve the target 30% CSF mHTT reduction considered necessary for clinical benefit.
The modeling results were definitive: while some alternative regimens could reduce branaplam exposure and potentially improve safety, none would achieve the minimum 30% mHTT lowering threshold. The 56 mg weekly dose represented the minimum effective regimen, making dose reduction an unviable strategy for maintaining therapeutic potential.
Clinical Implications and Future Directions
The VIBRANT-HD results provide several important insights for Huntington's disease (搜索) drug development. The study successfully demonstrated that oral HTT (搜索)-lowering is scientifically feasible, offering a significant advantage over intrathecal injection approaches currently in development.
The comprehensive safety monitoring strategy, including the use of NfL as an early biomarker, proved effective in detecting toxicity signals before irreversible damage occurred. This approach has informed the design of subsequent HTT (搜索)-lowering trials and established NfL as a valuable safety tool for the field.
The peripheral neuropathy (搜索) observed with branaplam appears to result from off-target splicing effects rather than HTT (搜索) lowering itself, as the drug does not modulate HTT mRNA splicing in the animal species that developed neuropathy. This finding underscores the importance of developing more selective splicing modulators.
Study Limitations and Strengths
The early termination resulted in an imbalanced study design, with only 5 participants in the placebo arm compared to 21 in the branaplam arm, and shorter follow-up for placebo participants (33 weeks versus 69 weeks). This limitation was partially addressed through post-hoc inclusion of propensity score-matched controls from the TRACK-HD observational study.
Despite these constraints, the study's innovative design successfully achieved its primary safety objective of detecting early toxicity signals while generating valuable proof-of-concept data for oral HTT (搜索) lowering.
The knowledge gained from VIBRANT-HD continues to inform the development of second-generation oral HTT (搜索)-lowering therapies, with companies including Skyhawk Therapeutics (搜索) and Novartis advancing more selective splicing modulators through clinical development.