Direct injection high-performance liquid chromatography of tetrabenazine and its metabolite in plasma of humans and rats

Liquid chromatography-tandem mass spectrometric assay for the determination of tetrabenazine and its active metabolites in human plasma: a pharmacokinetic study

A simple, rapid and sensitive liquid chromatography-tandem mass spectrometric (LC-MS/MS) assay method has been developed and fully validated for the simultaneous quantification of tetrabenazine and its active metabolites α-dihydrotetrabenazine and β-dihydrotetrabenazine in human plasma. Tetrabenazine d7 was used as internal standard (IS). The analytes were extracted from 200 μL aliquots of human plasma via solid-phase extraction using C18 solid-phase extraction cartridges. The reconstituted samples were chromatographed on a Zorbax SB C18 column using a 60:40 (v/v) mixture of acetonitrile and 5 mm ammonium acetate as the mobile phase at a flow rate of 0.8 mL/min. The API-4000 LC-MS/MS in multiple reaction-monitoring mode was used for detection. The calibration curves obtained were linear (r(2) ≥ 0.99) over the concentration range of 0.01-5.03 ng/mL for tetrabenazine and 0.50-100 ng/mL for α-dihydrotetrabenazine and β-dihydrotetrabenazine. Method validation was performed as per Food and Drug Administration guidelines and the results met the acceptance criteria. The method is precise and sensitive enough for its intended purpose. A run time of 2.5 min for each sample made it possible to analyze more than 300 plasma samples per day. The proposed method was found to be applicable to clinical studies.

Spectrofluorimetric determination of tetrabenazine after photochemical derivatization in basic medium

Photochemical derivatization is proposed for the spectrofluorimetric determination of tetrabenazine (TBZ). A central composite design was used to adjust experimental conditions (60 min of UV in a 0.45 mol L(-1) NaOH solution) enabling the improvement of the analyte signal-to-blank ratio of one order of magnitude, when compared to the TBZ original fluorescence. Limit of quantification was 4.7×10(-8) mol L(-1) but the detection power can be improved at least 10 times using solid phase extraction that also allows the separation of the analyte from matrix components, enabling the analysis of biologic fluids. Linear range covered at least three orders of magnitude. The combined uncertainty of the determination (at a 5×10(-6) mol L(-1)) was 16%. Recoveries of TBZ in the analyses of a pharmaceutical formulation were in agreement with the ones obtained using a HPLC method. Recovery in saliva (5×10(-7) mol L(-1) of TBZ) was 90±3% (n=3). The procedure minimizes the use of toxic chemical derivatization reagents and the generation of hazardous waste.

Whole body [11C]-dihydrotetrabenazine imaging of baboons: biodistribution and human radiation dosimetry estimates

PURPOSE

Vesicular monoamine transporter type 2 abundance quantified using the radiotracer [(11)C]-dihydrotetrabenazine (DTBZ) has been used to study diagnosis and pathogenesis of dementia and psychiatric disorders in humans. In addition, it may be a surrogate marker for insulin-producing pancreatic beta cell mass, useful for longitudinal measurements using positron emission tomography to track progression of autoimmune diabetes. To support the feasibility of long-term repeated administrations, we estimate the biodistribution and dosimetry of [(11)C]-DTBZ in humans.

METHODS

Five baboon studies were acquired using a Siemens ECAT camera. After transmission scanning, 165-210 MBq of [(11)C]-DTBZ were injected, and dynamic whole body emission scans were conducted. Time-activity data were used to obtain residence times and estimate absorbed radiation dose according to the MIRD model.

RESULTS

Most of the injected tracer localized to the liver and the lungs, followed by the intestines, brain, and kidneys. The highest estimated absorbed radiation dose was in the stomach wall.

CONCLUSIONS

The largest radiation dose from [(11)C]-DTBZ is to the stomach wall. This dose estimate, as well as the radiation dose to other radiosensitive organs, must be considered in evaluating the risks of multiple administrations.

Characterization of [11C]tetrabenazine as an in vivo radioligand for the vesicular monoamine transporter

[11C]Tetrabenazine ([11C]TBZ) is a new in vivo radioligand for positron emission tomographic (PET) imaging of vesicular monoamine transporters. The in vivo distribution, metabolism and pharmacological specificity of [11C]TBZ has been determined in rodents. Regional mouse brain retention of [11C]TBZ is highest in brain regions with greatest monoaminergic innervation (striatum, hypothalamus) and can be reduced with ligands for the monoamine vesicular transporter (TBZ, ketanserin) but not haloperidol, a dopamine D2 receptor antagonist. Chromatographic analysis of rat blood demonstrated rapid metabolism of [11C]TBZ to radiolabeled metabolites (alpha- and beta -[11C]dihydrotetrabenazine) resulting from reduction of the 2-keto group. These metabolites, as well as a third potential metabolite, 9-O-desmethylTBZ, have been synthesized in unlabeled form and all three were shown to be capable of greatly reducing in vivo accumulation of [11C]TBZ in mouse striatum and hypothalamus. Whole body biodistribution of radioactivity after [11C]TBZ injection was determined in rats, and the data used to calculate the expected human dosimetry from this radiotracer. These studies demonstrated that [11C]TBZ can be safely administered for in vivo PET imaging and semi-quantitative determination of vesicular monoamine transporters in living human brain, but quantitative pharmacokinetic modeling of radioactivity distribution will be complicated by the presence of pharmacologically active metabolites.

Treatment of tardive dyskinesia: preliminary report on use of tetrabenazine

Tetrabenazine has been used for treatment of tardive dyskinesia sporadically over the past twenty years. Dose has usually been decided empirically without assaying blood levels. This report describes 23 cases treated successfully with tetrabenazine. Our method of measuring levels of tetrabenazine and its metabolites in biological samples is described briefly.

Concentration-effect relationships of tetrabenazine and dihydrotetrabenazine in the rat

To investigate the pharmacodynamics of tetrabenazine [1,3,4,6,7,11b-hexahydro-3-isobutyl-9,10-dimethoxy-2-H-benzo(a)quinol izine-2- one; 1], the drug of choice in the control of Huntington's chorea and tardive dyskinesia, and its major metabolite, dihydrotetrabenazine (2), ip doses of 3 mg/kg of the drug and the metabolite were administered to rats. Animals were decapitated at 0.0, 0.5, 2, 5, and 12 h. Brain and serum concentrations of 1 and 2, and brain concentrations of dopamine, norepinephrine, and serotonin were measured. Time courses of 1 and 2 in the brain (site of action) were parallel to those in serum, indicating that the brain is a part of the central compartment for both compounds. Despite its greater polarity, 2 was able to readily cross the blood-brain barrier. The monoamine depletions at any time following the administration of 2 were at least equal to or greater than those observed following the administration of the same dose of 1. After both compounds, the maximum serum and brain concentrations and maximum depletions were observed with the first sample at 0.5 h, with dopamine and serotonin being the most and least affected, respectively. The brain levels of the amines returned to the control values a maximum of 12 h after ip injections of either 1 or 2. Sigmoidal relationships were found between the monoamine levels and the corresponding log brain or serum concentrations of 1 or 2. The concentration-response curves of 2 following the administration of 1 and 2 were superimposable, suggesting that the observed activity after tetrabenazine injection is mainly, if not entirely, due to the formation of the active metabolite.(ABSTRACT TRUNCATED AT 250 WORDS)