Overview
11 DESCRIPTION Tetrabenazine is a monoamine depletor for oral administration. The molecular weight of tetrabenazine is 317.43 g/mol; the pKa is 6.51. Tetrabenazine is a hexahydro-dimethoxy-benzoquinolizine derivative and has the following chemical name: cis rac –1,3,4,6,7,11b-hexahydro-9,10-dimethoxy-3-(2-methylpropyl)-2H-benzo[a]quinolizin-2-one.
Mechanism of Action
12 CLINICAL PHARMACOLOGY 12.1 Mechanism of Action The precise mechanism by which tetrabenazine exerts its anti-chorea effects is unknown but is believed to be related to its effect as a reversible depletor of monoamines (such as dopamine, serotonin, norepinephrine, and histamine) from nerve terminals. Tetrabenazine reversibly inhibits the human vesicular monoamine transporter type 2 (VMAT2) (K i ≈ 100 nM), resulting in decreased uptake of monoamines into synaptic vesicles and depletion of monoamine stores. Human VMAT2 is also inhibited by dihydrotetrabenazine (HTBZ), a mixture of α-HTBZ and β-HTBZ. α- and β-HTBZ, major circulating metabolites in humans, exhibit high in vitro binding affinity to bovine VMAT2.
Indications
- 1 INDICATIONS AND USAGE Tetrabenazine tablets are indicated for the treatment of chorea associated with Huntington's disease. Tetrabenazine tablets are a vesicular monoamine transporter 2 (VMAT) inhibitor indicated for the treatment of chorea associated with Huntington's disease ( 1 )
Common Doses
- 12.5 mg
- 25 mg
- 37.5 mg
- 50 mg
Dosage
2 DOSAGE AND ADMINISTRATION Individualization of dose with careful weekly titration is required. The 1 st week's starting dose is 12.5 mg daily; 2 nd week, 25 mg (12.5 mg twice daily); then slowly titrate at weekly intervals by 12.5 mg to a tolerated dose that reduces chorea. ( 2.1 , 2.2 ) Doses of 37.5 mg and up to 50 mg per day should be administered in three divided doses per day with a maximum recommended single dose not to exceed 25 mg. ( 2.
Black Box Warning
WARNING: DEPRESSION AND SUICIDALITY Tetrabenazine can increase the risk of depression and suicidal thoughts and behavior (suicidality) in patients with Huntington’s disease. Anyone considering the use of tetrabenazine must balance the risks of depression and suicidality with the clinical need for control of chorea. Close observation of patients for the emergence or worsening of depression, suicidality, or unusual changes in behavior should accompany therapy.
Contraindications
- 4 CONTRAINDICATIONS Tetrabenazine tablets are contraindicated in patients: Who are actively suicidal, or in patients with untreated or inadequately treated depression [see Warnings and Precautions ( 5.1 )] . With hepatic impairment [see Use in Specific Populations ( 8.6 ), Clinical Pharmacology ( 12.3 )] . Taking monoamine oxidase inhibitors (MAOIs). Tetrabenazine tablets should not be used in combination with an MAOI, or within a minimum of 14 days of discontinuing therapy with an MAOI [see Drug Interactions ( 7.3 )] . Taking reserpine.
Side Effects
- 6 ADVERSE REACTIONS The following serious adverse reactions are described below and elsewhere in the labeling: Depression and Suicidality [see Warnings and Precautions ( 5.1 )] Neuroleptic Malignant Syndrome (NMS) [see Warnings and Precautions ( 5.4 )] Akathisia, Restlessness, and Agitation [see Warnings and Precautions ( 5.5 )] Parkinsonism [see Warnings and Precautions ( 5.6 )] Sedation and Somnolence [see Warnings and Precautions ( 5.7 )] QTc Prolongation [see Warnings and Precautions ( 5.8 )] Hypotension and Orthostatic Hypotension [see Warnings and Precautions ( 5.
Interactions
- 7 DRUG INTERACTIONS 7.1 Strong CYP2D6 Inhibitors In vitro studies indicate that α-HTBZ and β-HTBZ are substrates for CYP2D6. Strong CYP2D6 inhibitors (e.g., paroxetine, fluoxetine, quinidine) markedly increase exposure to these metabolites. A reduction in tetrabenazine dose may be necessary when adding a strong CYP2D6 inhibitor (e.g., fluoxetine, paroxetine, quinidine) in patients maintained on a stable dose of tetrabenazine.