Lihsueh Lee

Binding of α-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.

Kinetic Evaluation of [11C]Dihydrotetrabenazine by Dynamic PET: Measurement of Vesicular Monoamine Transporter

(+)-α-[11C]Dihydrotetrabenazine (DTBZ) binds to the vesicular monoamine transporter (VMAT2) located in presynaptic vesicles. The purpose of this work was to evaluate various model configurations for analysis of [11C]DTBZ with the aim of providing the optimal measure of monoamine vesicular transporter density obtainable from a single dynamic PET study. PET studies on seven young normal volunteer subjects, ages 20–35, were performed following i.v. injection of 666 ± 37 MBq (18 ± 1 mCi) of (+)-α-[11C]DTBZ. Dynamic acquisition consisted of a 15-frame sequence over 1 h. Analysis methods included both creation of pixel-by-pixel functional images of transport (K1) and binding (DVtot) and nonlinear least-squares analysis of volume-of-interest data. Pixel-by-pixel calculations were performed for both two-compartment weighted integral calculations and slope-intercept estimations from Logan plots. Nonlinear least-squares analysis was performed applying model configurations with both two-compartments, estimating K1 and DVtot, and three compartments, estimating K1-k4. For the more complex configuration, we examined the stability of various binding-related parameters including k3 (konBmax′), k3/k4 (Bmax′/Kd), DVsp [(K1/k2)(k3/k4)], and DVtot [K1/k2(1 + k3/k4)]. The three-compartment model provided significantly improved goodness-of-fit compared to the two-compartment model, yet did not increase the uncertainty in the estimate of the DVtot. Without constraining parameters in the three-compartment model fits, DVtot was found to provide a more stable estimate of binding density than either k3, k3/k4, or DVsp. The two-compartment least-squares analysis yielded approximately 10% underestimations of the total distribution. However, this bias was found to be very consistent from region to region as well as across subjects as indicated by the correlation between two- and three-compartment DVtot estimates of 0.997. We conclude that (+)-α-[11C]DTBZ and PET can provide excellent measures of VMAT2 density in the human brain.

Positron emission tomography imaging of (2R,3R)-5-[18F]fluoroethoxybenzovesamicol in rat and monkey brain: a radioligand for the vesicular acetylcholine transporter

INTRODUCTION The regional brain distribution of (2R,3R)-5-[(18)F]fluoroethoxy-benzovesamicol ((-)-[(18)F]FEOBV), a radioligand for the vesicular acetylcholine transporter (VAChT), was examined in vivo in mice, rats and rhesus monkeys. METHODS Regional brain distributions of (-)-[(18)F]FEOBV in mice were determined using ex vivo dissection. MicroPET imaging was used to determine the regional brain pharmacokinetics of the radioligand in rat and rhesus monkey brains. RESULTS In all three species, clear heterogeneous regional brain distributions were obtained, with the rank order of brain tissues (striatum>thalamus>cortex>cerebellum) consistent with the distribution of cholinergic nerve terminals containing the VAChT. CONCLUSIONS (-)-[(18)F]FEOBV remains a viable candidate for further development as an in vivo imaging agent for positron emission tomography (PET) studies of the VAChT in the human brain.