Douglas M. Jewett

A simple synthesis of [11C]methyl triflate

[11C]Methyl triflate ([11C]methyl trifluoromethanesulfonate) was formed in high yield when [11C]methyl iodide in a nitrogen carrier was passed at 200 degrees C through a column containing graphitized carbon impregnated with 50% by weight of silver triflate.

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.

In vivo Muscarinic Cholingeric Receptor Imaging in Human Brain with [11C]Scopolamine and Positron Emission Tomography

Cerebral muscarinic cholinergic receptors were imaged and regionally quantified in vivo in humans with the use of [11C]scopolamine and positron emission tomography. Previous studies in experimental animals have suggested the utility of radiolabeled scopolamine for in vivo measurements, on the bases of its maintained pharmacologic specificity following systemic administration and the exclusion of labeled metabolites from the brain. The present studies describe the cerebral distribution kinetics of [11C]scopolamine in normal subjects following intravenous injection. Scopolamine is initially delivered to brain in a perfusion-directed pattern. After 30 to 60 min, activity is lost preferentially from cerebral structures with low muscarinic receptor density including the cerebellum and thalamus. Activity continues to accumulate throughout a 2 h postinjection period in receptor-rich areas including cerebral cortex and the basal ganglia. The late regional concentration of [11C]scopolamine does not, however, accurately parallel known differences in muscarinic receptor numbers in these receptor-rich areas. Tracer kinetic analysis of the data, performed on the basis of a three-compartment model, provides receptor binding estimates in good agreement with prior in vitro measurements. Kinetic analysis confirms significant contributions of ligand delivery and extraction to the late distribution of [11C]scopolamine, reconciling the discrepancy between receptor levels and tracer concentration. Finally, a novel dual-isotope method for rapid chromatographic processing of arterial blood samples in radiotracer studies is presented. The combination of rapid chromatography and compartmental analysis of tracer distribution should have broad utility in future in vivo studies with short-lived radioligands.

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.

Buprenorphine-Induced Changes in Mu-Opioid Receptor Availability in Male Heroin-Dependent Volunteers: A Preliminary Study

A principle of opioid pharmacotherapy is that high medication doses should occupy fractionally more opioid receptors that mediate heroin effects. In this preliminary study we examined in vivo μ opioid receptor (μOR) binding in three healthy opioid-dependent volunteers during maintenance on 2 and 16 mg sublingual buprenorphine (BUP) liquid, and after detoxification (0 mg) under double-blind, placebo-controlled conditions, and once in matched controls. Binding measures were obtained with the μOR-selective radioligand [11C]carfentanil (CFN) and PET 4 hrs after BUP administration. BUP induced dose-dependent reductions in μOR availability, 36–50% at 2 mg and 79–95% at 16 mg relative to placebo. Heroin abusers also had greater μOR binding potential in the inferofrontal cortex and anterior cingulate regions during placebo, compared to matched controls. Further studies are warranted to examine the relationship of μOR availability with BUP therapeutic actions, and the clinical implications of increased μOR binding during withdrawal.

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.

A gas-solid-phase microchemical method for the synthesis of acetyl hypofluorite

Abstract The useful electrophilic fluorinating agent, acetyl hypofluorite, was obtained by passing F2 diluted in N2 through columns containing complexes of alkali metal acetates with acetic acid. Acetyl hypofluorite obtained from KOAc(HOAc)1.5 and 0.14% F2 in N2 was reacted with tri- O -acetyl-D-glucal in CCl3F to obtain tetra- O -acetyl-2-deoxy-2-fluoro-D-glucose in 68% yield based on F2. These conditions are appropriate to the radiosynthesis of 18F-labeled 2-deoxy-2-fluoro-D-glucose for positron emission tomography. Important variables were the metal cation used, the ratio of HOAc to MOAc and the water content of the complex.

A simple synthesis of [11C]carfentanil using an extraction disk instead of HPLC

[11C]Carfentanil was prepared without the need for purification by HPLC. The tetrabutylammonium salt of the precursor carboxylate was reacted with [11C]methyl triflate in DMSO. The resulting [11C]carfentanil was trapped on an Empore extraction disk and washed to remove precursor and most radioactive contaminants. The product was eluted by a small volume of ethanol, mixed with water and passed through a small column containing fibrous anion exchanger to remove remaining radioactive contaminants.

Absolute configuration of (+)‐α‐dihydrotetrabenazine, an active metabolite of tetrabenazine

Chiral column liquid chromatography and enantiospecific enzymatic hydrolysis were utilized to separate the enantiomers of α- and β-dihydrotetrabenazine and α-9-O-desmethyldihydrotetrabenazine, three benzo[a]quinolizines derived from the amine-depleting drug tetrabenazine. An X-ray crystal structure analysis of (−)-α-9-O-desmethyldihydrotetrabenazine gave an absolute structure of that compound as the 2S, 3S, 11bS isomer. Therefore, (−)-α-dihydrotetrabenazine also has the 2S, 3S, 11bS absolute configuration. (+)-α-Dihydrotetrabenazine, the single biologically active isomer from the metabolic reduction of tetrabenazine, thus has the absolute configuration of 2R, 3R, 11bR. For further in vitro and in vivo studies of the vesicular monoamine transporter, it is now possible to use the single enantiomer of radiolabeled α-dihydrotetrabenazine. Chirality 9:59–62, 1997.

Synthesis of 1-[11c]methylpiperidin-4-yl propionate ([11c]pmp) for in vivo measurements of acetylcholinesterase activity

Synthesis of 1-[11C]methylpiperidin-4-yl propionate ([11C]PMP), an in vivo substrate for acetylcholinesterase, is reported. An improved preparation of 4-piperidinyl propionate (PHP), the immediate precursor for radiolabeling, was accomplished in three steps from 4-hydroxypiperidine by (a) protection of the amine as the benzyl carbamate, (b) acylation with propionyl chloride, and (c) deprotection of the carbamate by catalytic hydrogenation. The final product was obtained in an overall 82% yield. Reaction of the free base form of PHP with [11C]methyl trifluoromethanesulfonate at room temperature in N,N-dimethylformamide, followed by high performance liquid chromatography (HPLC) purification, provided [11C]PMP in 57% radiochemical yield, > 99% radiochemical purity, and > 1500 Ci/mmol at the end of synthesis. The total synthesis time from end-of-bombardment was 35 min. [11C]PMP can thus be reliably prepared for routine clinical studies of acetylcholinesterase in human brain using positron emission tomography.