Arbaclofen extended-release tablets for spasticity in multiple sclerosis: open-label extension study

Baclofen, a racemic γ-aminobutyric acid B receptor agonist, is commonly used for the management of multiple sclerosis–related spasticity but is associated with frequent dosing and poor tolerability. Arbaclofen, the active R-enantiomer of baclofen, exhibits 100- to 1000-fold greater specificity for the γ-aminobutyric acid B receptor compared with the S-enantiomer and ∼5-fold greater potency compared with racemic baclofen. Arbaclofen extended-release tablets allow a dosing interval of 12 h and have shown a favourable safety and efficacy profile in early clinical development. A 12-week, randomized, placebo-controlled Phase 3 trial in adults with multiple sclerosis–related spasticity demonstrated that arbaclofen extended-release 40 mg/day significantly reduced spasticity symptoms compared with placebo and was safe and well tolerated. The current study is an open-label extension of the Phase 3 trial designed to evaluate the long-term safety and efficacy of arbaclofen extended-release. In a 52-week, open-label, multicentre study, adults with a Total Numeric-transformed Modified Ashworth Scale score ≥2 in the most affected limb received oral arbaclofen extended-release titrated over 9 days up to 80 mg/day based on tolerability. The primary objective was assessment of arbaclofen extended-release safety and tolerability. Secondary objectives included an assessment of efficacy using the Total Numeric-transformed Modified Ashworth Scale-most affected limb, the Patient Global Impression of Change and Expanded Disability Status Scale. Of 323 patients enrolled, 218 (67.5%) completed 1 year of treatment. Most patients (74.0%) achieved an arbaclofen extended-release maintenance dose of 80 mg/day. At least one treatment-emergent adverse event was reported by 278 patients (86.1%). The most common adverse events were [n patients (%)]: urinary tract disorder [112 (34.7)], muscle weakness [77 (23.8)], asthenia [61 (18.9)], nausea [70 (21.7)], dizziness [52 (16.1)], somnolence [41 (12.7)], vomiting [29 (9.0)], headache [24 (7.4)] and gait disturbance [20 (6.2)]. Most adverse events were of mild–moderate severity. Twenty-eight serious adverse events were reported. One death occurred during the study, a myocardial infarction that was considered by investigators as unlikely to be related to treatment. Overall, 14.9% of patients discontinued due to adverse events, primarily muscle weakness, multiple sclerosis relapse, asthenia and nausea. Evidence of improvement in multiple sclerosis–related spasticity was observed across arbaclofen extended-release dosages. Arbaclofen extended-release treatment (up to 80 mg/day) was well tolerated and reduced symptoms of spasticity in adult patients with multiple sclerosis for 1 year. Clinical Trial Identifier: ClinicalTrials.gov, NCT03319732