Syntheseversuche in der Emetin-Reihe. 3. Mitteilung. 2-Hydroxy-hydrobenzo[a]chinolizine

Differences in Dihydrotetrabenazine Isomer Concentrations Following Administration of Tetrabenazine and Valbenazine

Background

Tetrabenazine (TBZ) activity is thought to result from four isomeric dihydrotetrabenazine (HTBZ) metabolites ([+]-α-HTBZ, [−]-α-HTBZ, [+]-β-HTBZ, [−]-β-HTBZ). Each isomer has a unique profile of vesicular monoamine transporter 2 (VMAT2) inhibition and off-target binding. Previously published data only report total isomer (α) and (β) concentrations. We developed a method to quantify the individual HTBZ isomers in samples from patients with Huntington’s disease receiving TBZ. For comparison, concentrations of [+]-α-HTBZ, the single active metabolite shared by valbenazine (VBZ) and TBZ, were determined in samples from patients with tardive dyskinesia receiving VBZ.

Methods

A liquid chromatography–tandem mass spectrometry (LC-MS/MS) method was developed and validated for quantitation of the four individual HTBZ isomers. Concentrations were determined in serum from patients with Huntington’s disease administered TBZ and plasma from patients with tardive dyskinesia administered VBZ once daily.

Results

In patients administered TBZ, [−]-α-HTBZ and [+]-β-HTBZ were the most abundant HTBZ isomers while [−]-β-HTBZ and [+]-α-HTBZ were present as minor metabolites. Only [+]-α-HTBZ was observed in patients administered VBZ.

Conclusions

Based on relative abundance and potency, [+]-β-HTBZ appears to be the primary contributor to VMAT2 inhibition by TBZ, a finding in contrast with the generally held assertion that [+]-α-HTBZ is the major contributor. [−]-α-HTBZ, the other abundant TBZ metabolite, has much lower VMAT2 inhibitory potency than [+]-β-HTBZ, but increased affinity for other CNS targets, which may contribute to off-target effects of TBZ. In contrast, pharmacological activity for VBZ is derived primarily from [+]-α-HTBZ. Individual HTBZ isomer concentrations provide a more clinically relevant endpoint for assessing on- and off-target effects of TBZ than total isomer concentrations.

Synthesis of (±)-Tetrabenazine by Visible Light Photoredox Catalysis

(±)-Tetrabenazine was synthesized in six steps from commercially available compounds. The key cyclization substrate was assembled rapidly via Baylis-Hillman and aza-Michael reactions. Annulation of the final ring was achieved through visible light photocatalysis, wherein carbon-carbon bond formation was driven by the oxidation of a tertiary amine. Solvent played a critical role in the photoredox cyclization outcome, whereas methanol led to a mixed ketal, acetonitrile/water (10:1) gave direct cyclization to (±)-tetrabenazine and occurred more rapidly.

Asymmetric synthesis of tetrabenazine and dihydrotetrabenazine

The enantioselective synthesis of (+)-tetrabenazine (TBZ) and (+)-dihydrotetrabenazine (DTBZ), agents of significant interest for therapeutic and molecular imaging applications, has been completed in 21% (TBZ) and 16% (DTBZ) overall yield and in >97% ee from the starting dihydroisoquinoline. The synthesis utilizes Sodeoka's palladium-catalyzed asymmetric malonate addition to set the initial stereocenter followed by a number of diastereoselective transformations to incorporate the remaining asymmetric centers.

Vesicular monoamine transporter 2: role as a novel target for drug development

In the central nervous system, vesicular monoamine transporter 2 (VMAT2) is the only transporter that moves cytoplasmic dopamine (DA) into synaptic vesicles for storage and subsequent exocytotic release. Pharmacologically enhancing DA sequestration by VMAT2, and thus preventing the oxidation of DA in the cytoplasm, may be a strategy for treating diseases such as Parkinson's disease. VMAT2 may also be a novel target for the development of treatments for psychostimulant abuse. This review summarizes the possible role of VMAT2 as a therapeutic target, VMAT2 ligands reported in the literature, and the structure-activity relationship of these ligands, including tetrabenazine analogs, ketanserin analogs, lobeline analogs, and 3-amine-2-phenylpropene analogs. The molecular structure of VMAT2 and its relevance to ligand binding are briefly discussed.

A simple synthesis of [11C]dihydrotetrabenazine (DTBZ)

[11C]Dihydrotetrabenazine (2-hydroxy-3-isobutyl-9-[11C]methoxy-10 -methoxy-1,2,3,4,6,7,- hexahydro-11bH-bezo[alpha]-quinolizine) ([11C]DTBZ) was synthesized by reacting the 9-hydroxy precursor in DMSO with gas-phase [11C]methyl iodide on a column of alumina impregnated with KOH. The reaction was instantaneous at room temperature. This column was then connected to the inlet of a short column containing basic alumina. Elution with cyclohexane removed radioactive contaminants. The radioactive product was then eluted with a few milliliters ether containing 1% ethanol. The [11C]DTBZ was obtained in isolated yields of > 200 mCi and specific activities > 1600 Ci/mmol.

Binding of alpha-dihydrotetrabenazine to the vesicular monoamine transporter is stereospecific

The two enantiomers of alpha-dihydrotetrabenazine were separated using chiral high performance liquid chromatography. The (+)-isomer showed high affinity in vitro (Ki = 0.97 +/- 0.48 nM) for the vesicular monoamine transporter (VMAT2) in rat brain striatum, whereas the (-)-isomer was inactive (Ki = 2.2 +/- 0.3 microM). Each isomer was then synthesized in carbon-11 labeled form, and regional brain biodistributions in mice determined after intravenous injection. Only (+)-alpha-dihydrotetrabenazine showed selective and specific accumulations in regions of dense monoaminergic innervation (e.g., striatum, hypothalamus), which could be blocked by coinjection of unlabeled tetrabenazine. Binding of alpha-dihydrotetrabenazine to the vesicular monoamine transporter is thus stereospecific.

The pharmacokinetics of tetrabenazine and its hydroxy metabolite in patients treated for involuntary movement disorders

The pharmacokinetics of tetrabenazine and a metabolite, hydroxytetrabenazine, have been investigated in seven patients being treated for involuntary movement disorders. Tetrabenazine had a very low oral systemic availability (mean 0.049 +/- 0.032 SD). First-pass metabolism to hydroxytetrabenazine was extensive, and the systemic availability for this metabolite was high (mean 0.81 +/- 0.30 SD). Since hydroxytetrabenazine has been reported to be as active as tetrabenazine in depleting brain amines, and is present at much higher plasma concentrations than the parent drug, it is likely that this metabolite is the more important therapeutic moiety.

Acido-basic properties of the catecholamine uptake inhibitors tetrabenazine and dihydrotetrabenazine

Tetrabenazine (2-oxo-3-isobutyl-9,10-dimethoxy-1,2,3,4,6,7 hexahydro-11 bH-benzo (a) quinolizine) and dihydrotetrabenazine (2-hydroxy derivative) are inhibitors of catecholamine uptake by the chromaffin granules of adrenal medulla. In the 6.6 - 8.8 pH range, inhibition by tetrabenazine was pH-independent whereas dihydrotetrabenazine efficiency increased up to pH 8.3. The fluorescence and the buffer-octanol partition coefficient of these drugs was affected by the pH. Analysis of the pH-dependency of these effects indicated the existence of an acido-basic transition characterized by a pKa of 6.0 for tetrabenazine and 7.5 for dihydrotetrabenazine and associated with protonation of the tertiary amine of these molecules. For both compounds, the neutral form was less fluorescent and more soluble in octanol. Comparison of the uptake inhibition constants IC50 and of the neutralization curves showed that this form was the biologically active one. This result implies that the monoamine carrier of chromaffin granule membrane binds either to only deprotonated amines or to the molecules present only in the lipidic phase where the neutral form is largely predominant.

Synthesis of potential antineoplastic agents XXVI: 1,3,4,6,7,11b-hexahydro-9,10-dimethoxy-2H-benzo[a]2-quinolizinone derivatives

A number of 3-alkyl-1,3,4,6,7,11b-hexahydro-9,10-dimethoxy-2H-benzo[a]2-quinolizinones and 2-substituted 3-ethyl-1,3,4,6,7,11b-hexahydro-9,10-dimethoxy-2H-benzo[a]quinolizines were prepared and submitted for antineoplastic and anticonvulsant screening.