Pharmacologic profile, adverse reactions
Valbenazine and its 2 metabolites lack affinity for receptors other than VMAT2, leading to an absence of orthostasis in clinical trials.1,2 In the phase-II trial, 76% of participants receiving valbenazine (n = 51) were titrated to the maximum dosage of 75 mg/d. Common adverse reactions (incidence ≥5% and at least twice the rate of placebo) were headache (9.8% vs 4.1% placebo), fatigue (9.8% vs 4.1% placebo), and somnolence (5.9% vs 2% placebo).1 In the phase-III trial, participants were randomized 1:1:1 to valbenazine, 40 mg (n = 72), valbenazine, 80 mg (n = 79), or placebo (n = 76). In the clinical studies the most common diagnosis was schizophrenia or schizoaffective disorder, and 40% and 85% of participants in the phase-II and phase-III studies, respectively, remained on antipsychotics.1,2 There were no adverse effects with an incidence ≥5% and at least twice the rate of placebo in the phase-III trial.2
When data from all placebo-controlled studies were pooled, only 1 adverse effect occurred with an incidence ≥5% and twice that of placebo, somnolence with a rate of 10.9% for valbenazine vs 4.2% for placebo. The incidence of akathisia in the pooled analysis was 2.7% for valbenazine vs 0.5% for placebo. Importantly, in neither study was there a safety signal related to depression, suicidal ideation and behavior, or parkinsonism. There also were no clinically significant changes in measures of schizophrenia symptoms.
The mean QT prolongation for valbenazine in healthy participants was 6.7 milliseconds, with the upper bound of the double-sided 90% confidence interval reaching 8.4 milliseconds. For those taking strong 2D6 or 3A4 inhibitors, or known 2D6 poor metabolizers, the mean QT prolongation was 11.7 milliseconds (14.7 milliseconds upper bound of double-sided 90% CI). In the controlled trials, there was a dose-related increase in prolactin, alkaline phosphatase, and bilirubin. Overall, 3% of valbenazine-treated patients and 2% of placebo-treated patients discontinued because of adverse reactions.
As noted above, there were no adverse effects with an incidence ≥5% and at least twice the rate of placebo in the phase-III valbenazine trial. Aggregate data across all placebo-controlled studies found that somnolence was the only adverse effect that occurred with an incidence ≥5% and twice that of placebo (10.9% for valbenazine vs 4.2% for placebo).2 As a comparsion, rates of sedation and akathisia for tetrabenazine were higher in the pivotal Huntington’s disease trial: sedation/somnolence 31% vs 3% for placebo, and akathisia 19% vs 0% for placebo.8
How it works
Tetrabenazine, a selective VMAT2 inhibitor, is the only agent that has demonstrated significant efficacy and tolerability for TD management; however, its complex metabolism generates numerous isomers of the metabolites α-DH-TBZ and β-DH-TBZ, of which only 2 are significantly active (Table 3). By choosing an active isomer (NBI-98782) as the metabolite of interest because of its selective and potent activity at VMAT2 and having a metabolism not solely dependent on CYP2D6, a compound was generated (valbenazine) that when metabolized slowly converts into NBI-98782.
