A proof-of-concept study with SOM3355 (bevantolol hydrochloride) for reducing chorea in Huntington's disease

AIMS

The study's aim is to investigate the efficacy and safety of SOM3355 (bevantolol hydrochloride), a β₁ -adrenoreceptor antagonist with recently identified vesicular monoamine transporter type 2 inhibitory properties, as a repositioned treatment to reduce chorea in Huntington's disease (HD).

METHODS

A randomized, placebo-controlled proof-of-concept study was performed in 32 HD patients allocated to 2 arms of 4 sequential 6-week periods each. Patients received placebo and SOM3355 at 100 and 200 mg twice daily in a crossover design. The primary endpoint was improvement by at least 2 points in the total maximal chorea score in any active drug period compared with the placebo period.

RESULTS

The primary endpoint was met in 57.1% of the patients. Improvements ≥3, ≥4, ≥5 and ≥6 points vs. placebo treatment were observed in 28.6, 25.0, 17.9 and 10.7% of the patients, respectively. A mixed-model analysis found a significant improvement in the total maximal chorea score of -1.14 (95% confidence interval, -2.11 to -0.16; P = .0224) with 200 mg twice daily SOM3355 treatment compared with placebo treatment. These results were paralleled by Clinical and Patient Global Impression of Change ratings (secondary endpoints). An elevation in plasma prolactin levels by 1.7-1.9-fold was recorded (P < .005), probably reflecting the effect on the dopamine pathway, consistent with vesicular monoamine transporter type 2 inhibition. The most frequent adverse events during SOM3355 administration were mild to moderate.

CONCLUSION

Within the limits of this study, the results suggest that SOM3355 reduces chorea in patients with HD and is well-tolerated. Larger studies are necessary to confirm its therapeutic utility as an antichoreic drug. EudraCT number: 2018-000203-16 and ClinicalTrials.gov Identifier: NCT03575676.

Deutetrabenazine for treatment of involuntary movements in patients with tardive dyskinesia

Introduction: Tardive dyskinesia (TD) is a hyperkinetic movement disorder that arises as a complication of exposure to dopamine receptor blocking agents. Vesicular monoamine transporter type 2 (VMAT2) inhibitors reduce dyskinesia by decreasing transport of monoamines, including dopamine, into presynaptic vesicles, leaving unpackaged dopamine to be metabolized by monoamine oxidase. Deutetrabenazine was adapted from an earlier VMAT2 inhibitor, tetrabenazine, by substituting three deuterium isotopes in place of three hydrogen isotopes at the site of metabolic degradation to improve upon the pharmacokinetics of the parent compound. Areas covered: The authors reviewed the pivotal trials examining the safety and efficacy of deutetrabenazine, as well as long-term data from an open-label extension. Also reviewed were posters and oral presentations, as well as information from the product label and the United States Food and Drug Administration. Expert opinion: Deutetrabenazine is effective at decreasing dyskinesia in TD, but drug selection and cost-effectiveness between existing VMAT2 inhibitors are evolving areas of study. Other areas of investigation include novel anti-dyskinetic agents and use of deep brain stimulation.

Synthesis of Tetrabenazine and Its Derivatives, Pursuing Efficiency and Selectivity

Tetrabenazine is a US Food and Drug Administration (FDA)-approved drug that exhibits a dopamine depleting effect and is used for the treatment of chorea in Huntington’s disease. Mechanistically, tetrabenazine binds and inhibits vesicular monoamine transporter type 2, which is responsible for importing neurotransmitters from the cytosol to the vesicles in neuronal cells. This transportation contributes to the release of neurotransmitters inside the cell to the synaptic cleft, resulting in dopaminergic signal transmission. The highly potent inhibitory activity of tetrabenazine has led to its advanced applications and in-depth investigation of prodrug design and metabolite drug discovery. In addition, the synthesis of enantiomerically pure tetrabenazine has been pursued. After a series of research studies, tetrabenazine derivatives such as valbenazine and deutetrabenazine have been approved by the US FDA. In addition, radioisotopically labeled tetrabenazine permits the early diagnosis of Parkinson’s disease, which is difficult to treat during the later stages of this disease. These applications were made possible by the synthetic efforts aimed toward the efficient and asymmetric synthesis of tetrabenazine. In this review, various syntheses of tetrabenazine and its derivatives have been summarized.