Striatal [11C]dihydrotetrabenazine and [11C]methylphenidate binding in Tourette syndrome

OBJECTIVE

Tourette syndrome (TS) is a common neurodevelopmental disorder marked by tics and behavioral comorbidities. Clinical pharmacology suggests that dopaminergic signaling abnormalities are part of the pathophysiology of TS. Prior molecular imaging studies of nigrostriatal dopaminergic terminal markers report conflicting results. Our goal was to characterize the distribution of nigrostriatal dopaminergic terminals in subjects with TS.

METHODS

Thirty-three adult subjects with TS were studied with PET using [11C]dihydrotetrabenazine (DTBZ), a ligand for the type 2 vesicular monoamine transporter, and with [11C] methylphenidate (MP), a ligand for the plasmalemmal dopamine transporter. Subjects were characterized with standard rating instruments for tic severity, obsessive-compulsive behaviors, and attentional deficits.

RESULTS

We found no differences between subjects with TS and control subjects in DTBZ and MP binding in any striatal region. There was no correlation between binding measures and clinical variables. Ventral striatal DTBZ and MP binding distributions in subjects with TS were normal.

CONCLUSIONS

We found no evidence of increased striatal dopaminergic innervation in Tourette syndrome (TS). Discrepancy between our present results and those of other studies may be explained by heterogeneity of TS.

Increased ventral striatal monoaminergic innervation in Tourette syndrome

BACKGROUND

Excessive striatal dopaminergic innervation is suggested to underlie Tourette syndrome (TS). Prior imaging and postmortem studies yield conflicting data.

METHODS

The authors used PET with the type 2 vesicular monoamine transporter ligand [(11)C]dihydrotetrabenazine (DTBZ) to quantify striatal monoaminergic innervation in patients with TS (n = 19) and control subjects (n = 27). Compartmental modeling was used to determine blood to brain ligand transport (K(1)) and tissue to plasma distribution volume (a measure of ligand binding) during continuous infusion of DTBZ. TS data were compared with control data using predefined regions of interest and on a voxel by voxel basis.

RESULTS

There were no significant differences in ligand binding or ligand transport between patients with TS and control subjects in the dorsal striatum. With voxel by voxel analysis, there was increased DTBZ binding in the right ventral striatum.

CONCLUSIONS

Previously reported differences between patients with TS and control subjects in dorsal striatal dopamine terminal markers may reflect medication-induced regulation of terminal marker expression or be the result of intrinsic differences in striatal dopaminergic synaptic function. Increased right ventral striatal DTBZ binding suggests that abnormal ventral striatal dopaminergic innervation may underlie tics.

Imaging the vesicular monoamine transporter

Quantitative measures of striatal VMAT2 binding sites appear to represent an excellent surrogate of nigrostriatal projection integrity in experimental animal models. Importantly, there does not appear to be a significant effect of dopaminergic drugs or of lesion compensatory effect on the expressed level of VMAT2 binding sites in the striatum. Highly precise and specific measures of human VMAT2 are possible with PET employing the novel tracer (+)11C-DTBZ. This methodology appears particularly suited to the objective measure of PD severity and of its progression. Such measures will be indispensable in the search for disease-modifying effects of PD therapy.

Dual-[11C]tracer single-acquisition positron emission tomography studies

The ability to study multiple physiologic processes of the brain simultaneously within the same subject would provide a new means to explore the interactions between neurotransmitter systems in vivo. Currently, examination of two distinct neuropharmacologic measures with positron emission tomography (PET) necessitates performing two separate scans spaced in time to allow for radionuclide decay. The authors present results from a dual-tracer PET study protocol using a single dynamic-scan acquisition where the injections of two tracers are offset by several minutes. Kinetic analysis is used to estimate neuropharmacologic parameters for both tracers simultaneously using a combined compartmental model configuration. This approach results in a large reduction in total study time of nearly 2 hours for carbon-11-labeled tracers. As multiple neuropharmacologic measures are obtained at nearly the same time, interventional protocols involving a pair of dual-tracer scans become feasible in a single PET session. Both computer simulations and actual human PET studies were performed using combinations of three different tracers: [11C]flumazenil, N-[11C]methylpiperidinyl propionate, and [ 11 C]dihydrotetrabenazine. Computer simulations of tracer-injection separations of 10 to 30 minutes showed the feasibility of the approach for separations down to 15 to 20 minutes or less. Dual-tracer PET studies were performed in 32 healthy volunteers using injection separations of 10, 15, or 20 minutes. Model parameter estimates for each tracer were similar to those obtained from previously performed single-injection studies. Voxel-by-voxel parametric images were of good quality for injections spaced by 20 minutes and were nearly as good for 15-minute separations, but were degraded noticeably for some model parameters when injections were spaced by only 10 minutes. The authors conclude that dual-tracer single-scan PET is feasible, yields accurate estimates of multiple neuropharmacologic measures, and can be implemented with a number of different radiotracer pairs.

The role of species-dependent metabolism in the regional brain retention of 18F-labeled muscarinic acetylcholine receptor ligands

We have developed PET radioligands for the muscarinic acetylcholine receptor designed to be sensitive to endogenous acetylcholine changes. These radioligands were based on the piperidyl and pyrrolidyl benzilate scaffold and include (R)-N-(2-[18F]fluoroethyl)-3-piperidyl benzilate (1b), (R)-N-(2-[18F]fluoroethyl)-3-pyrrolidyl benzilate (2b), and N-(2-[18F]fluoroethyl)-4-piperidyl benzilate (3b). In the mouse, intravenous injection of 2b produced a heterogeneous receptor-mediated regional retention of radioactivity, whereas in the rat a homogeneous brain distribution was observed. Analyses of blood and brain extracts showed a radiolabeled metabolite for 2b which was formed to a much greater extent in mice than rats. This metabolite may have a higher receptor binding affinity than authentic 2b, and thus be responsible for the apparent receptor-mediated binding in the mouse brain. Our findings emphasize the importance of metabolite analysis in multiple species when developing novel radiopharmaceuticals for in vivo use.

Regional mu opioid receptor regulation of sensory and affective dimensions of pain

The endogenous opioid system is involved in stress responses, in the regulation of the experience of pain, and in the action of analgesic opiate drugs. We examined the function of the opioid system and mu-opioid receptors in the brains of healthy human subjects undergoing sustained pain. Sustained pain induced the regional release of endogenous opioids interacting with mu-opioid receptors in a number of cortical and subcortical brain regions. The activation of the mu-opioid receptor system was associated with reductions in the sensory and affective ratings of the pain experience, with distinct neuroanatomical involvements. These data demonstrate the central role of the mu-opioid receptors and their endogenous ligands in the regulation of sensory and affective components of the pain experience.

Assessment of muscarinic receptor concentrations in aging and Alzheimer disease with [11C]NMPB and PET

Cerebral cholinergic deficits have been described in Alzheimer disease (AD) and as a result of normal aging. At the present time, there are very limited options for the quantification of cholinergic receptors with in vivo imaging techniques such as PET. In the present study, we examined the feasibility of utilizing [11C]N-methyl-4-piperidyl benzilate (NMPB), a nonselective muscarinic receptor ligand, in the study of aging and neurodegenerative processes associated with cholinergic dysfunction. Based on prior data describing the accuracy of various kinetic methods, we examined the concentration of muscarinic receptors with [11C]NMPB and PET using two- and three-compartment kinetic models. Eighteen healthy subjects and six patients diagnosed with probable AD were studied. Pixel-by-pixel two-compartment model fits showed acceptable precision in the study of normal aging, with comparable results to those obtained with a more complex and less precise three-compartment model. Normal aging was associated with a reduction in muscarinic receptor binding in neocortical regions and thalamus. In AD patients, the three-compartment model appeared capable of dissociating changes in tracer transport from changes in receptor binding, but suffered from statistical uncertainty, requiring normalization to a reference region, and therefore limiting its potential use in the study of neurodegenerative processes. After normalization, no regional changes in muscarinic receptor concentrations were observed in AD.

Acetylcholinesterase inhibition increases in vivo N-(2-[18F]fluoroethyl)-4-piperidyl benzilate binding to muscarinic acetylcholine receptors

Although the inhibition of acetylcholinesterase remains the primary treatment of Alzheimer's disease, little is known of the results of increased acetylcholine levels on muscarinic receptor occupancy or function. Using N-(2-[18F]fluoroethyl)-4-piperidyl benzilate ([18F]FEPB), a moderate affinity (Ki = 1.7 nmol/L) nonsubtype-selective muscarinic receptor antagonist, the authors examined the sensitivity of equilibrium in vivo radioligand binding in rat brain with changes in endogenous acetylcholine levels produced by treatments with acetylcholinesterase inhibitors. Phenserine administration 30 minutes before resulted in a dose-dependent into muscarinic cholinergic receptors, reaching a maximum increase of 90% in the striatum at a dose of 5 mg/kg intraperitoneally. Constant infusion of physostigmine at a dosage of 250 microg/kg/min produced an identical increase in radioligand binding. This agonist-induced increase of in vivo mAChR radioligand binding offers a new method for monitoring of the efficacy of acetylcholinesterase inhibitors or other drugs to enhance acetylcholine actions at the muscarinic receptors.

Radiolabeled cholinesterase substrates: in vitro methods for determining structure-activity relationships and identification of a positron emission tomography radiopharmaceutical for in vivo measurement of butyrylcholinesterase activity

There is currently great interest in developing radiolabeled substrates for acetylcholinesterase and butyrylcholinesterase that would be useful in the in vivo imaging of patients with Alzheimer's disease. Using a simple in vitro spectrophotometric assay for determination of enzymatic cleavage rates, the structure-activity relationship for a short series of 1-methyl-4-piperidinyl esters was investigated. Relative enzymatic hydrolysis rates for the well-characterized 1-methyl-4-piperidinyl acetate, propionate, and i-butyrate esters were in agreement with literature values. The 4 and 5 carbon esters of 1-methyl-4-piperidinol were specific for butyrylcholinesterase and cleaved in the rank order n-valerate > n-butyrate >> 2-methylbutyrate, iso-valerate. These spectrophotometric results were also in agreement with in vitro hydrolysis rates in mouse blood and with in vivo regional retention of radioactivity in mouse brain of 11C-labeled analogs. Brain uptake and apparent enzymatic rate constants for 1-[11C]methyl-4-piperidinyl n-butyrate and n-valerate were calculated from in vivo measurements in M. nemistrina using positron emission tomography. Based on higher brain uptake of radioactivity and superior pharmacokinetics, 1-[11C]methyl-4-piperidinyl n-butyrate was identified as a new radiopharmaceutical for the in vivo measurement of butyrylcholinesterase activity.

Vesicular monoamine transporter concentrations in bipolar disorder type I, schizophrenia, and healthy subjects

BACKGROUND

Previous analyses of vesicular monoamine transporter (VMAT2) binding in euthymic bipolar disorder type I (BDI) patients have shown increases of this presynaptic marker in the thalamus and ventral midbrain. To assess the diagnostic specificity of those findings, we compared VMAT2 concentrations between euthymic BDI patients, patients diagnosed with schizophrenia (SCH), and age-matched healthy volunteers.

METHODS

Binding sites for VMAT2 were quantified with (+)-alpha-[11C]DTBZ (dihydrotetrabenazine) and positron emission tomography. Fifteen euthymic BDI and 12 SCH patients and 15 group-matched healthy controls were studied. [11C]DTBZ tracer transport and binding potentials were examined in the thalamus and ventral midbrain with factorial analyses of variance and post hoc Tukey's honestly significantly different tests.

RESULTS

Analysis of variance detected diagnosis effects in binding potentials in both brain regions. Binding of VMAT2 in the thalamus was higher in BDI patients than in control subjects and SCH patients. Conversely, ventral brainstem binding was nearly identical between BDI and SCH patients and were higher than in the control group.

CONCLUSIONS

The patterns of regional VMAT2 expression, and by extension, the concentration of monoaminergic synaptic terminals, differ between BDI, SCH, and a control group. These findings may relate to both similarities and differences in the presentation or clinical course of these syndromes and require further examination.

Synthesis, (18)F-labeling, and biological evaluation of piperidyl and pyrrolidyl benzilates as in vivo ligands for muscarinic acetylcholine receptors

A series of 31 compounds based on the piperidyl or pyrrolidyl benzilate scaffold were prepared from methyl benzilate and 4-piperidinol, (R)-(+)-3-piperidinol, or (R)-(+)-3-pyrrolidinol. Amine substituents included alkyl and aralkyl groups. In vitro K(i) values ranged from 0.05 nM to >100 nM. (R)-N-(2-Fluoroethyl)-3-piperidyl benzilate (3-FEPB, 22, K(i) = 12.1 nM) and N-(2-fluoroethyl)-4-piperidyl benzilate (4-FEPB, 8, K(i) = 1. 83 nM) were selected for radiolabeling with fluorine-18. Using alkylation with 2-[(18)F]fluoroethyl triflate, 3-[(18)F]FEPB (42) and 4-[(18)F]FEPB (43) were produced in 7-9% radiochemical yield and >97% radiochemical purity. For in vivo studies, retention was moderate in mouse brain for 42; however, blocking with scopolamine showed that uptake was not muscarinic cholinergic receptor-mediated. Conversely, 43 exhibited high, receptor-mediated retention in mouse brain, with significant clearance after 1 h. These results suggest that 43 could have applications as an in vivo probe for measuring endogenous acetylcholine levels.

High vesicular monoamine transporter binding in asymptomatic bipolar I disorder: sex differences and cognitive correlates

OBJECTIVE

It has been hypothesized that anomalies in monoaminergic function underlie some of the manifestations of bipolar disorder. In this study the authors examined the possibility that trait-related abnormalities in the concentration of monoaminergic synaptic terminals may be present in patients with asymptomatic bipolar disorder type I.

METHOD

The concentration of a stable presynaptic marker, the vesicular monoamine transporter protein (VMAT2), was quantified with (+)[(11)C]dihydrotetrabenazine (DTBZ) and positron emission tomography. Sixteen asymptomatic patients with bipolar I disorder who had a prior history of mania with psychosis (nine men and seven women) and individually matched healthy subjects were studied. Correlational analyses were conducted to examine the relationship between regional VMAT2 binding, cognitive function, and clinical variables.

RESULTS

VMAT2 binding in the thalamus and ventral brainstem of the bipolar patients was higher than that in the comparison subjects. VMAT2 concentrations in these regions correlated with performance on measures of frontal, executive function. In addition, sex differences in VMAT2 binding were detected in the thalamus of the bipolar patients; the male patients had higher binding than the women. No sex differences in binding were observed in the healthy comparison group.

CONCLUSIONS

These initial results suggest that higher than normal VMAT2 expression and, by extension, concentration of monoaminergic synaptic terminals, may represent a trait-related abnormality in patients with bipolar I disorder and that male and female patients show different patterns. Also, VMAT2 concentrations may be associated with some of the cognitive deficits encountered in euthymic bipolar disorder.

Limited donepezil inhibition of acetylcholinesterase measured with positron emission tomography in living Alzheimer cerebral cortex

Based on surrogate assays of peripheral red blood cells, reports state that widely prescribed doses of donepezil hydrochloride provide nearly complete inhibition of cerebral cortical acetylcholinesterase activity in the treatment of Alzheimer's disease (AD). To test this, direct positron emission tomography measures of cerebral acetylcholinesterase activity were made in AD patients before and after treatment with donepezil (5 and 10 mg/day) for at least 5 weeks and compared with similar measures in normal controls who were untreated or after acute administration of another AChE inhibitor, physostigmine salicylate (1.5 mg/hr). After physostigmine, acetylcholinesterase inhibition averaged 52% in normal cerebral cortex. After donepezil, cerebral cortical inhibition in AD brain averaged only 27%. Clinical trials of this donepezil dose schedule are not testing the effect of nearly complete cerebral cortical inhibition.

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 mu opioid receptor (muOR) 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 muOR-selective radioligand [11C]carfentanil (CFN) and PET 4 hrs after BUP administration. BUP induced dose-dependent reductions in muOR availability, 36-50% at 2 mg and 79-95% at 16 mg relative to placebo. Heroin abusers also had greater muOR 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 muOR availability with BUP therapeutic actions, and the clinical implications of increased muOR binding during withdrawal.

Absolute configuration of (+)-alpha-dihydrotetrabenazine, an active metabolite of tetrabenazine

Chiral column liquid chromatography and enantiospecific enzymatic hydrolysis were utilized to separate the enantiomers of alpha- and beta-dihydrotetrabenazine and alpha-9-O-desmethyldihydrotetrabenazine, three benzo[a]quinolizines derived from the amine-depleting drug tetrabenazine. An X-ray crystal structure analysis of (-)-alpha-9-O-desmethyldihydrotetrabenazine gave an absolute structure of that compound as the 2S, 3S, 11bS isomer. Therefore, (-)-alpha-dihydrotetrabenazine also has the 2S, 3S, 11bS absolute configuration. (+)-alpha-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 alpha-dihydrotetrabenazine.

Decreased striatal monoaminergic terminals in Huntington disease

OBJECTIVE

To evaluate the integrity of the dorsal striatal dopaminergic innervation in rigid and choreic Huntington disease (HD).

BACKGROUND

Some patients with HD have an akinetic-rigid phenotype. It has been suggested that nigrostriatal in addition to striatal pathology is present in this subgroup. The authors sought to determine whether in vivo measures of striatal vesicular monoamine transporter type-2 (VMAT2) binding could distinguish patients with akinetic-rigid (HDr) from typical choreiform (HDc) HD.

METHODS

Nineteen patients with HD (mean age 48 +/- 16 years) and 64 normal controls (mean age 50 +/- 14 years) underwent (+)-alpha-[11C]dihydrotetrabenazine (DTBZ) PET imaging. DTBZ blood to brain ligand transport (K1) and tissue to plasma distribution volume (DV) in the caudate nucleus, anterior putamen, and posterior putamen were normalized to the occipital cortex.

RESULTS

The normalized striatal specific DV was reduced in HDr (n = 6) when compared with controls: caudate nucleus -33% (p < 0.001), anterior putamen -56% (p < 0.0001), and posterior putamen -75% (p < 0.0001). Patients with HDc (n = 13) also had reduced striatal DV: caudate nucleus -6% (NS), anterior putamen -19% (p < 0.01), and posterior putamen -35% (p < 0.0001). Patients with HDr had significantly lower striatal (+)-alpha-[11C]DTBZ binding than HDc patients. After correction for tissue atrophy effects, normalized DV differences were less significant, with values somewhat increased in the caudate, slightly reduced in the anterior putamen, and moderately decreased in the posterior putamen. There were no significant regional differences in K1 reductions among caudate, anterior, and posterior putamen in HD.

CONCLUSIONS

Reduced striatal VMAT2 binding suggests nigrostriatal pathology in HD, most severely in the HDr phenotype. Striatal DV reductions were most prominent in the posterior putamen, similar to PD.

Synthesis and in vivo evaluation of (E)-N-[(11)C]Methyl-4- (3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine) as a nicotinic receptor radioligand

(E)-N-[(11)C]Methyl-4-(3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine), a high affinity (K(i) = 16 nM) CNS-selective nicotinic agonist, was prepared by the [(11)C]alkylation of the desmethyl precursor with [(11)C]methyl trifluoromethanesulfonate. In vivo distribution studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution and no evidence of specific binding to nicotinic cholinergic receptors. Identical results were obtained in an imaging study performed in a monkey brain. Therefore, despite literature reports supporting the use of metanicotine as a cognition enhancing nicotinic agonist, (E)-N-[(11)C]methyl-4-(3-pyridinyl)-3-butene-1-amine does not appear to be a suitable candidate for in vivo imaging studies of nicotinic acetylcholine receptors in the mammalian brain.

Rapid and differential losses of in vivo dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) radioligand binding in MPTP-treated mice

The dose- and time-dependent changes of in vivo radioligand binding to the neuronal membrane dopamine transporter (DAT) and vesicular monoamine transporter type 2 (VMAT2) were examined in mouse brain after MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) administrations. Regional brain distribution studies were done in male C57BL/6 mice using simultaneous injections of d-threo-[(3)H]methylphenidate (DAT) and (+)-alpha-[(11)C]dihydrotetrabenazine (VMAT2). Single (55 mg/kg i.p. ) or multiple (4 x 10 mg/kg i.p., 1-hour intervals) administration of MPTP caused significant reductions in [(3)H]methylphenidate and [(11)C]dihydrotetrabenazine specific striatal binding, measured 14 days later. The single high dose of MPTP produced greater losses of [(11)C]dihydrotetrabenazine binding than did the multiple MPTP dosing regimen. Using the single high dose of MPTP, changes of in vivo binding of the two radioligands were determined at 1, 3, and 14 days after neurotoxin injection. At 1 day, there are large losses of [(3)H]methylphenidate binding (DAT) but no changes in [(11)C]dihydrotetrabenazine binding to the VMAT2 site in the striatum. At 3 and 14 days, there were >50% losses of binding of both bot radioligands, but significantly (P < 0.001) greater losses of VMAT2 binding of [(11)C]dihydrotetrabenazine. These studies indicate that the losses of the neuronal membrane and vesicular transporters are not always equal, and do not occur in the same time frame, after administration of the neurotoxin MPTP.

Assessment of extrastriatal vesicular monoamine transporter binding site density using stereoisomers of [11C]dihydrotetrabenazine

Previous studies have demonstrated the utility of [11C]dihydrotetrabenazine ([11C]DTBZ) as a ligand for in vivo imaging of the vesicular monoamine transporter system. The (+)-isomer has a high affinity (approximately 1 nmol/L) for the vesicular monoamine transporter (VMAT2) binding site, whereas the (-)-isomer has an extremely low affinity (approximately 2 micromol/L). Efforts to model dynamic (+)-[11C]DTBZ data demonstrate the difficulty in separating the specific binding component from the free plus nonspecific component of the total positron emission tomography (PET) measure. The authors' previous PET work, as well as in vitro studies, indicate that there is little specific VMAT2 binding in neocortical regions. However, precise determination of in vivo binding levels have not been made, leaving important questions unanswered. At one extreme, is there sufficient specific binding in cortex or other extrastriate regions to be estimated reliably with PET? At the other extreme, is there sufficiently little binding in cortex so that it can be used as a reference region representing nonsaturable tracer uptake? The authors address these questions using paired studies with both active (+) and inactive (-) stereoisomers of [11C]DTBZ. Six normal control subjects were scanned twice, 2 hours apart, after injections of 16 mCi of (+)- and (-)-[11C]DTBZ (order counter-balanced). Three-dimensional PET acquisition consisted of 15 frames over 60 minutes for each scan. Arterial samples were acquired throughout, plasma counted, and corrected for radiolabeled metabolites. Analysis of specific binding was assessed by comparison of total distribution volume measures from the (+)- and (-)-[11C]DTBZ scans. The authors' findings indicate that only approximately 5% of the cortical signal in (+)-[11C]DTBZ scans results from binding to VMAT2 sites. The strongest extrastriatal signal comes from the midbrain regions where approximately 30% of the PET measure results from specific binding. The authors conclude that (1) the density of VMAT2 binding sites in cortical regions is not high enough to be quantified reliably with DTBZ PET, and (2) binding does appear to be low enough so that cortex can be used as a free plus nonspecific reference region for striatum.

Kinetic modeling of N-[11C]methylpiperidin-4-yl propionate: alternatives for analysis of an irreversible positron emission tomography trace for measurement of acetylcholinesterase activity in human brain

N-[11C]Methylpiperidin-4-yl propionate ([11C]PMP) is a substrate for hydrolysis by acetylcholinesterase (AChE). This work evaluates kinetic analysis alternatives for estimation of relative AChE activity using dynamic positron emission tomography (PET) studies of [11C]PMP. The PET studies were performed on three groups of subjects: (1) 12 normal volunteer subjects, aged 20 to 45 years, who received a single intravenous injection of 16 to 32 mCi of [11C]PMP; (2) six subjects, aged 21 to 44 years, who received two 16-mCi injections of [11C]PMP (baseline and visual stimulation, respectively); and (3) five subjects, aged 24 to 40 years, who received two 16-mCi injections separated by 200 minutes (baseline and after a 1-hour constant infusion of 1.5 mg of physostigmine, respectively). Dynamic acquisition consisted of a 17-frame sequence over 80 minutes. All analysis methods were based on a first-order kinetic model consisting of two tissue compartments with the parameter k3, representing PMP hydrolysis, being the index of AChE activity. Four different schemes were used to estimate k3: (1) an unconstrained non-linear least-squares fit estimating blood-brain barrier transport parameters, K1 and k2, in addition to the hydrolysis rate constant k3; (2) and (3), two methods of constraining the fit by fixing the volume of distribution of free tracer (DVfree); and (4), a direct estimation of k3 without use of an arterial input function based on the shape of the tissue time-activity curve alone. Results showed that k3 values from the unconstrained fitting and no input methods were estimated with similar accuracy, whereas the two methods using DVfree constraints yielded similar results. The authors conclude that the optimal analysis method for [11C]PMP differs as a function of AChE activity. All four methods gave precise measures of k3 in regions with low AChE activity (approximately 10% coefficient of variation in cortex), but surprisingly, with unconstrained methods yielding estimates with lower variability than constrained methods. In regions with moderate to high AChE activity, constrained methods were required to yield meaningful estimates and were superior to the unconstrained methods.

Striatal presynaptic monoaminergic vesicles are not increased in Tourette's syndrome

BACKGROUND

Abnormal function of striatal dopaminergic synapses is suggested to underlie Tourette's syndrome (TS).

OBJECTIVE

To determine dorsal striatal dopaminergic innervation in TS. Prior in vitro and in vivo studies of dopamine reuptake transporter binding sites suggest increased striatal dopaminergic innervation in TS.

METHODS

We used in vivo measures of striatal vesicular monoamine transporter type-2 (VMAT2) binding to quantify striatal dopaminergic innervation in TS. Eight TS patients (mean age 30+/-9 years) and 22 age-comparable normal controls (age 34+/-8 years) underwent PET imaging with the VMAT2 ligand (+)-alpha-[11C]dihydrotetrabenazine (DTBZ). Compartmental modeling was used to quantify blood-to-brain ligand transport and VMAT2 binding site density from the tissue-to-plasma distribution volume (DV) during continuous (+)-alpha-[11C]DTBZ infusion. DTBZ DV in dorsal striatal regions was expressed relative to the occipital cortex to estimate relative specific VMAT2 binding (binding potential).

RESULTS

We found no significant differences in VMAT2 binding potential between patients and controls in the caudate nucleus, anterior putamen, or posterior putamen. There were no significant differences in striatal VMAT2 binding between patients with (n = 5) or without (n = 3) features of obsessive-compulsive disorder.

CONCLUSIONS

There is no evidence for increased binding to the VMAT2 in TS striatum and that dorsal striatal dopaminergic innervation density is normal in TS. The previously reported changes in dopamine transporter binding sites may reflect medication effect and/or altered synaptic activity or regulation of dopamine transporter expression in nigrostriatal neurons.

Simplified methods for in vivo measurement of acetylcholinesterase activity in rodent brain

Simplified methods for in vivo studies of acetylcholinesterase (AChE) activity in rodent brain were evaluated using N-[11C]methylpiperidinyl propionate ([11C]PMP) as an enzyme substrate. Regional mouse brain distributions were determined at 1 min (representing initial brain uptake) and 30 min (representing trapped product) after intravenous [11C]PMP administration. Single time point tissue concentrations (percent injected dose/gram at 30 min), tissue concentration ratios (striatum/cerebellum and striatum/cortex ratios at 30 min), and regional tissue retention fractions (defined as percent injected dose 30 min/percent injected dose 1 min) were evaluated as measures of AChE enzymatic activity in mouse brain. Studies were carried out in control animals and after dosing with phenserine, a selective centrally active AChE inhibitor; neostigmine, a peripheral cholinesterase inhibitor; and a combination of the two drugs. In control and phenserine-treated animals, absolute tissue concentrations and regional retention fractions provide good measures of dose-dependent inhibition of brain AChE; tissue concentration ratios, however, provide erroneous conclusions. Peripheral inhibition of cholinesterases, which changes the blood pharmacokinetics of the radiotracer, diminishes the sensitivity of all measures to detect changes in central inhibition of the enzyme. We conclude that certain simple measures of AChE hydrolysis rates for [11C]PMP are suitable for studies where alterations of the peripheral blood metabolism of the tracer are kept to a minimum.

In vivo mapping of cerebral acetylcholinesterase activity in aging and Alzheimer's disease

OBJECTIVE

To validate an in vivo method for mapping acetylcholinesterase (AChE) activity in human brain, preparatory to monitoring inhibitor therapy in AD.

BACKGROUND

AChE activity is decreased in postmortem AD brain. Lacking a reliable in vivo measure, little is known about central activity in early AD, when the disease is commonly targeted by AChE inhibitor drug therapy.

METHODS

Intravenous N-[11C]methylpiperidin-4-yl propionate ([11C]PMP) served as an in vivo AChE substrate. AChE activity was defined using cerebral PET for tracer kinetic estimates of the local rate of [11C]PMP hydrolysis in 26 normal controls and 14 patients with AD. Eleven AD patients also had concomitant in vivo cerebral measures of vesicular acetylcholine transporter (cholinergic terminal) density and glucose metabolism.

RESULTS

Cerebral AChE activity measures 1) were independent of changes in tracer delivery to cerebral cortex; 2) agreed with reported postmortem data concerning normal relative cerebral distributions, absence of large age-effect in normal aging, and deficits in AD; 3) correlated in AD cerebral cortex with concomitant in vivo measures of cholinergic terminal deficits, but not with metabolic deficits; and 4) agreed quantitatively with predicted level of cerebral AChE inhibition induced by physostimine.

CONCLUSIONS

This in vivo PET method provided valid measures of central AChE activity in normal subjects and AD patients. Applied in early AD, it should facilitate inhibitor treatment by confirming central inhibition, optimizing drug dosage, identifying likely responders, and testing surrogate markers of therapeutic response.

Synthesis and evaluation of 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2- benzisoxazole as an in vivo radioligand for acetylcholinesterase

6-Methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzisoxazole is a high affinity (K(i) = 8.2 nM) reversible inhibitor of acetylcholinesterase (AChE). The carbon-11 labeled form was prepared in high (>97%) radiochemical purity and with specific activities of 37+/-20 GBq/micromol at end of synthesis, by the alkylation of the desmethyl precursor with [11C]methyl trifluoromethanesulfonate in N,N-dimethyl-formamide at room temperature. In vivo studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution. Thus, despite in vitro and in vivo activity as an AChE inhibitor, 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzis oxa zole does not appear to be a good candidate for in vivo imaging studies of AChE in the mammalian brain.

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.

Syntheses of carbon-11 labeled piperidine esters as potential in vivo substrates for acetylcholinesterase

A series of carbon-11 labeled N-methylpiperidinyl esters were prepared as potential in vivo substrates for acetylcholinesterase (AChE). Target compounds were designed based on the structure of N-[11C]methylpiperidin-4-yl propionate, an ester currently used to measure AChE enzymatic activity in the human brain, to examine the structure-activity relationship for in vivo enzymatic hydrolysis. Changes in steric bulk and in the ester order ("reverse" esters) were made. Addition of methyl groups was made to both the acid side chain (synthesis of N-[11C]methylmethylpiperidin-4-yl isobutyrate) and to the piperidine ring (syntheses of N-[11C]methyl-4-methylpiperidin-4-yl propionate, N-[11C]methyl-4-methylpiperidin-4-yl acetate, and N-[11C]methyl-3-methylpiperidin-4-yl propionate). Alterations of the order of the ester heteroatoms was accomplished through syntheses of the N-[11C]methyl-2,3- and 4-piperidinecarboxylic acid ethyl esters. Finally, an additional piperidine-based ester (N-[11C]methylpiperidin-2-yl)methyl propionate was also prepared. All carbon-11-labeled esters were prepared by N-[11C]methylation reactions, using the desmethyl precursors and no-carrier-added [11C]methyltriflate, and were obtained in decay-corrected yields (not optimized) of 10-40% and high specific activities.

[18F]fluoroethoxy-benzovesamicol, a PET radiotracer for the vesicular acetylcholine transporter and cholinergic synapses

Loss of cholinergic transmission in the cortex and hippocampus is a characteristic feature of Alzheimer's disease, and visualization of functional cholinergic synapses in the brain with PET could be a useful method for studying this degenerative condition in living humans. We investigated [18F]fluoroethoxybenzovesamicol, (-)-[18F] FEOBV,(-)-(2R,3R)-trans-2-hydroxy-3-(4-phenylpiperidino)-5-(2-[18F ]fluoroethoxy)-1,2,3,4-tetralin, a high affinity positron emitting ligand for the vesicular acetylcholine transporter, as a potential in vivo cholinergic synapse mapping agent. Rodent biodistribution, dosimetry, stereospecificity of biological effects, pharmacologic blocking studies, in vivo rodent brain autoradiography and metabolites were examined. (-)-[18F]FEOBV brain uptake following intravenous injection was robust, with 2.65% dose/brain in mice at 5 min, and the regional localization matched the known distributions of presynaptic cholinergic markers at later times. Both the cholinergic localization and curare-like effects of FEOBV were associated with the "(-)"-enantiomer exclusively. (-)-[18F]FEOBV regional brain distribution in rodents was changed little by pretreatment with haloperidol, (+)-3-PPP, or E-2020, indicating FEOBV, unlike other vesamicol analogs, did not interact in vivo with dopamine or sigma receptor systems. Autoradiography of rat brain 3 h following i.v. injection of (-)-[18F]FEOBV showed high localization in brain areas rich in presynaptic cholinergic elements. Metabolic defluorination in rodents was modest, and analysis of brain tissue following tracer administration found FEOBV as the only extractable radioactive species. (-)-[18F]FEOBV dosimetry calculated from rat data estimate 10 mCi doses can be given to humans. These studies show FEOBV maps cholinergic areas with high specificity in vivo, and may provide a noninvasive means to safely and accurately gauge the functional integrity of cholinergic synapses in man using PET.

N-[11C]methylpiperidine esters as acetylcholinesterase substrates: an in vivo structure-reactivity study

A series of simple esters incorporating the N-[11C]methylpiperidine structure were examined as in vivo substrates for acetylcholinesterase in mouse brain. 4-N-[11C]Methylpiperidinyl esters, including the acetate, propionate and isobutyrate esters, are good in vivo substrates for mammalian cholinesterases. Introduction of a methyl group at the 4-position of the 4-piperidinol esters, to form the ester of a teritary alcohol, effectively blocks enzymatic action. Methylation of 4-N-[11C]methylpiperidinyl propionate at the 3-position gives a derivative with increased in vivo reactivity toward acetylcholinesterase. Esters of piperidinecarboxylic acids (nipecotic, isonipecotic and pipecolinic acid ethyl esters) are not hydrolyzed by acetylcholinesterase in vivo, nor do they act as in vivo inhibitors of the enzyme. This study has identified simple methods to both increase and decrease the in vivo reactivity of piperidinyl esters toward acetylcholinesterase.

Reduced MPTP neurotoxicity in striatum of the mutant mouse tottering

The effects of MPTP treatment (4 x 10 mg/kg, 2-h intervals) on in vivo striatal binding of (+)-alpha-[3H]dihydrotetrabenazine ((+)-[3H]DTBZ) to the vesicular monoamine transporter type 2 (VMAT2) were examined in wild type (+,+) and tottering (tg/tg) mice of the C57BL/6J strain. The tottering mutant has been previously characterized as having hyperinnervation of noradrenergic terminals in the brain, with increased concentrations of norepinephrine and increased numbers of VMAT2 binding sites. In wild-type mice, MPTP caused a significant decrease in specific striatal (+)-[3H]DTBZ binding in both males (-71%) and females (-57%), consistent with dopaminergic terminal losses. In the tottering mice, the neurotoxic effects of MPTP were diminished, with smaller losses of (+)-[3H]DTBZ binding observed both in males (-45%) and females (-26%). These results are consistent with the hypothesis that vesicular storage (as a result of hyperinnervation) offers neuroprotection toward MPTP toxicity, although the confounding effects of increases in norepinephrine concentrations or changes in calcium ion channel function (both also characteristics of the tottering mutant) cannot be ruled out. The tottering mutant does, however, offer another animal model to examine the biochemical features responsible for MPTP toxicity.

In vivo radiotracers for vesicular neurotransmitter transporters

The vesicular monoamine transporter is a specific presynaptic protein involved in the transport of monoamines from the cytosol to storage vesicles of monoaminergic nerve terminals. Recently, radioligands for this transporter have been developed and utilized for in vivo positron emission tomographic (PET) imaging of monoaminergic nerve terminals in the human brain. In this review, the characteristics of vesicular transport and storage that provided the impetus for development of these radioligands are presented and discussed.

Equilibrium versus compartmental analysis for assessment of the vesicular monoamine transporter using (+)-alpha-[11C]dihydrotetrabenazine (DTBZ) and positron emission tomography

This work compares equilibrium to kinetic analysis of positron emission tomography data for the assessment of vesicular monoamine transporter (VMAT2) binding density using (+)-alpha-[11C]dihydrotetrabenazine ((+)-alpha-[11C]DTBZ). Studies were performed for 80 minutes after intravenous administration of 18 +/- 1 mCi (+)-alpha-[11C]DTBZ on 9 young control subjects, 20 to 45 years of age. A 9-mCi bolus was injected over the first minute of the study, whereas the remaining 9 mCi were infused at a constant rate over the following 79 minutes. Steady-state was reached in both blood and brain by approximately 30 minutes after initiation of the study. Nonlinear least-squares analysis using two- and three-compartment models, weighted integral analysis using a two-compartment configuration, and Logan plot analysis all yielded kinetic estimates of the total tissue distribution volume, DVtot(kin). These results were compared with equilibrium distribution volume estimates, DVtot(eq), calculated from the tissue to metabolite corrected arterial plasma concentration ratio after 30 minutes. Kinetic modeling results from this study were in close agreement with prior bolus-injection (+)-alpha-[11C]DTBZ studies. In the current study, coefficients of variation in DVtot(kin) (19% to 23% across regions) and DVtot(eq) (18% to 22%) were nearly identical. Equilibrium estimates of DVtot were slightly lower than kinetic estimates, averaging 5% +/- 9% lower (P = 0.04, paired t test) in regions of high binding density (caudate and putamen), but only 2% +/- 6% (P = 0.09) in lower binding density regions (cortex, thalamus, cerebellum). DVtot(eq) estimates, however, still correlated highly with DVtot(kin) estimates (r = 0.977-0.989). Steady-state conditions can be achieved in both tissue and blood by 30 minutes, and the tissue-to-blood ratios of (+)-alpha-[11C]DTBZ at equilibrium yield DVtot(eq) measures that are in close agreement with DVtot(kin) estimates. Thus, a simple, easily tolerated protocol using a loading bolus followed by continuous infusion can provide excellent measures of VMAT2 binding.

Simplifying the dosimetry of carbon-11-labeled radiopharmaceuticals

A two time-point sacrifice method is proposed as an alternative to conventional multiple time-point sacrifice methods to determine the organ cumulated activity of 11C-labeled radiopharmaceuticals.

METHODS

Rat biodistribution data for 10 11C-labeled radiopharmaceuticals were analyzed to determine organ cumulated activity. Data were obtained at four sacrifice intervals 2-5 min, 10-15 min, 30-45 min and 1-1.5 hr postinjection. The organ absorbed dose per unit administered radioactivity (mGy/MBq) was calculated using all four data points and combinations of limited data. The objective was to determine if a limited sampling technique would provide sufficient accuracy in estimating absorbed dose.

RESULTS

Residence times calculated using two time-points acquired during the first half-life of 11C were either equivalent or positively biased compared to using all sacrifice times. Overall, 87% of the residence times assessed were conservative compared to the multipoint method. For bladder organs, a consistent negative bias was observed with the reduced sacrifice method.

CONCLUSION

Analysis of animal biodistributions using a reduced sacrifice protocol provides results in good agreement with and generally conservative to results using all sacrifice intervals. Correction factors are required for the urinary bladder and gallbladder when using the simplified technique due to bias. The bladder was often the limiting organ in determining human administered activity.

Intrastriatal neurotoxin injections reduce in vitro and in vivo binding of radiolabeled rotenoids to mitochondrial complex I

The in vivo and in vitro bindings of radiolabeled rotenoids to mitochondrial complex I of rat striatum were examined after unilateral intrastriatal injections of quinolinic acid or 1-methyl-4-phenylpyridinium salt (MPP+). Quinolinic acid produced significant, similar losses of in vivo binding of [11C]dihydrorotenol ([11C]DHROL: 40%) and in vitro binding of [3H]dihydrorotenone ([3H]DHR: 53%) in the injected striatal at 13 days after the injection of neurotoxin. MPP+ reduced in vivo binding of [11C]DHROL up to-55%) as measured 1.5 to 6 h after its administration. Reductions of in vivo [11C]DHROL binding after either quinolinic acid or MPP+ injections did not correlate with changes in striatal blood flow as measured with [14C]iodoantipyrine. These results are consistent with losses of complex I binding sites for radiolabeled rotenoids, produced using cell death (quinolinic acid) or direct competition for the binding site (MPP+). Appropriately radiolabeled rotenoids may be useful for in vivo imaging studies of changes of complex I in neurodegenerative diseases.

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.

Time-dependent recovery of in vivo binding sites after drug dosing: a method for radiotracer evaluation

The recovery of in vivo binding sites for (+/-)-alpha-[11C] methoxytetrabenazine, a radioligand for the monoamine vesicular transporter (VMAT2), was determined in mouse brain at various times following a pharmacological dose of tetrabenazine. Concentrations of in vivo radioligand binding sites progressively increased and had reached control values by 8.5 h, and this recovery was consistent with the pharmacokinetics of the competing drug tetrabenazine and its active metabolite, dihydrotetrabenazine. This study demonstrates a simple experimental protocol of using a single dose of a reversible competing drug and time-dependent measurements of in vivo binding of a radioligand. This protocol is suitable for testing the sensitivity of an in vivo radiotracer for measurement of varying concentrations of in vivo binding sites.

Decreased striatal monoaminergic terminals in olivopontocerebellar atrophy and multiple system atrophy demonstrated with positron emission tomography

We used [11C]dihydrotetrabenazine, a new ligand for the type 2 vesicular monoamine transporter (VMAT2), with positron emission tomography to study striatal monoaminergic presynaptic terminals in 4 patients with multiple system atrophy, 8 with sporadic olivopontocerebellar atrophy, and 9 normal control subjects. Specific binding in the striatum was significantly reduced in the multiple system atrophy patients as compared with the normal control group, with average reductions of 61% in the caudate nucleus (p = 0.002) and 58% in the putamen (p = 0.009). Smaller reductions were found in the sporadic olivopontocerebellar atrophy group, averaging 26% in the caudate nucleus (p = 0.05) and 24% in the putamen (p = 0.11). Mean blood-to-brain [11C]dihydrotetrabenazine transport (K1) was significantly different between groups only in the cerebellum, with values for the sporadic olivopontocerebellar atrophy group diminished compared with the normal control group. Cerebellar K1 was not significantly decreased in the multiple system atrophy group. The finding of reduced striatal VMAT2 in sporadic olivopontocerebellar atrophy patients suggests nigrostriatal pathology, indicating that some may later develop symptomatic extrapyramidal disease.

Presynaptic monoaminergic vesicles in Parkinson's disease and normal aging

We present development and human application of a method for determining the regional cerebral density of the type 2 vesicular monoamine transporter (VMAT2) using positron emission tomography (PET) and [11C]dihydrotetrabenazine (DTBZ). Previous animal studies indicate striatal VMAT2 density is linearly related to the integrity of substantia nigra dopamine neurons and is not subject to drug- or lesion-compensatory regulation. In the present studies, kinetic compartmental modeling was employed to estimate blood-brain [11C]DTBZ transport (K1) and VMAT2 binding site density (tissue-to-plasma DTBZ distribution volume, DV) from the cerebral and plasma DTBZ time courses after intravenous tracer injection. In controls, we found reductions of putamen DTBZ DVwith advancing age, corresponding to losses of 0.77% per year in specific VMAT2 binding. Parkinson's disease (PD) patients had reduction in specific DTBZ DV in the putamen (-61%) and in the caudate nucleus (-43%). There was no overlap of lowest specific putamen DTBZ DV between individual elderly controls and PD patients. The present results indicate the suitability of [11C]DTBZ PET for objective quantification of nigrostriatal integrity, including evaluation of PD progression and its possible therapeutic modification.

Kinetic evaluation of [11C]dihydrotetrabenazine by dynamic PET: measurement of vesicular monoamine transporter

(+)-alpha-[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 (+)-alpha-[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 (+)-alpha-[11C]DTBZ and PET can provide excellent measures of VMAT2 density in the human brain.

Effects of dopaminergic drug treatments on in vivo radioligand binding to brain vesicular monoamine transporters

The effects of various dopaminergic drug treatments on the in vivo regional brain distribution of high-affinity radioligands ([11C]dihydrotetrabenazine and [11C]methoxytetrabenazine) for the rat brain vesicular monoamine transporter (VMAT2) were determined. Acute treatments with reserpine (2 mg/kg i.p.), tetrabenazine (10 mg/kg i.v.) or related benzoisoquinolines significantly reduced radiotracer binding in vivo. In contrast, radiotracer distributions remained unchanged after treatments with other dopaminergic drugs, whether given by single injection (haloperidol, 1 mg/kg i.p., pargyline 80 mg/kg), repeatedly (pargyline, 80 mg/kg s.c., 14 days), or by continuous infusion (deprenyl, 10 mg/kg/day, 5 days; L-DOPA methyl ester 100 mg/kg/day, 5 days). Repeated injections of tetrabenazine (5 mg/kg i.p., twice daily, 3 days) did not alter in vivo radioligand binding measured after allowing drug washout from the brain. These studies support the proposal that in vivo PET imaging of VMAT2 radioligands in patients with extrapyramidal movement disorders will not be affected by concurrent use of L-DOPA or deprenyl.

Synthesis of carbon-11- and fluorine-18-labeled 1-methyl-4-piperidyl-4'-fluorobenzoate and their biodistribution in mice

Carbon-11- and fluorine-18-labeled forms of 1-methyl-4-piperidyl-4'-fluorobenzoate were prepared as potential in vivo substrates for brain acetylcholinesterase. The 1-methyl-4-piperidyl-4'-[18F]fluorobenzoate was prepared by aromatic nucleophilic substitution using the nitro precursor and no-carrier added [18F]fluoride ion. The 1-[11C]methyl-4-piperidyl-4'-fluorobenzoate was synthesized by N-[11C]methylation of the appropriate nor-methyl precursor. Biodistribution studies in mice showed high brain uptake of these radiotracers followed by a fast washout with no significant retention of radioactivity in areas of high acetylcholinesterase enzymatic activity. This is contrasted with 1-[11C]methyl-4-piperidylacetate, which is rapidly trapped in brain tissues through hydrolysis by AChE. Further in vivo and in vitro studies demonstrated that 1-methyl-4-piperidyl-4'-fluorobenzoate was not a substrate for AChE, and thus not suitable as an in vivo radiotracer for studying this enzyme in the brain.

Stereotaxic summation analysis of human cerebral benzodiazepine binding maps

Summation analysis strategies are recognized throughout diverse scientific fields as powerful means of differentially enhancing experimental signals over random fluctuations (noise). Such techniques, applied to emission tomographic cerebral blood flow scans, reveal subtle alterations in neuronal activity during specific behavioral states. In the present work, we extend the principles of intersubject image summation analysis to the evaluation of emission tomographic ligand-binding studies. A general methodology is presented that may be applied to a wide variety of binding site determinations. The procedure consists of anatomic standardization of individual brains to a common stereotaxic orientation, followed by statistical analyses of group versus group or individual versus group differences. We develop and evaluate performance of our technique with the use of positron emission tomographic [11C]flumazenil scans from normal volunteers, depicting the regional cerebral distribution of benzodiazepine binding sites.

In vivo studies of acetylcholinesterase activity using a labeled substrate, N-[11C]methylpiperdin-4-yl propionate ([11C]PMP)

Two esters, N-[11C]methylpiperidyl acetate ([11C]AMP) and N-[11C]methylpiperidyl propionate ([11C]PMP), were synthesized in no-carrier-added forms and evaluated as in vivo substrates for brain acetylcholinesterase (AChE). After peripheral injection in mice, each ester showed rapid penetration into the brain and a regional retention of radioactivity (striatum > cortex, hippocampus > cerebellum) reflecting known levels of AChE activity in the brain. Regional brain distributions after [11C]PMP administration showed better discrimination between regions of high, intermediate, and low AChE activities. Chromatographic analysis of blood and brain tissue extracts showed rapid and nearly complete hydrolysis of [11C]PMP within 10 min after injection. For both [11C]AMP and [11C]PMP, retention of radioactivity in all regions was reduced by pretreatment with diisopropylfluorophosphate (DFP), a specific irreversible AChE inhibitor. DFP treatment also significantly increased the proportions of unhydrolyzed ester in both blood and brain. Radioactivity localization in brain after peripheral injection was thus dependent on AChE-catalyzed hydrolysis to the hydrophilic product N-[11C]methylpiperidinol. PET imaging of [11C]AMP or [11C]PMP distributions in monkey brain showed clear accumulation of radioactivity in areas of highest AChE activity (striatum, cortex). These esters are thus in vivo substrates for brain AChE, with potential applications as in vivo imaging agents of enzyme action in the human brain. [11C]PMP, the ester with a slower rate of hydrolysis, appears to be the better candidate radiotracer for further development.

In vitro and in vivo studies of benzisoquinoline ligands for the brain synaptic vesicle monoamine transporter

Tetrabenazine is a high-affinity inhibitor of the vesicular monoamine transporter in mammalian brain. As part of a program to develop in vivo imaging agents for these transporters in human brain, a series of 2-alkylated dihydrotetrabenazine ligands was synthesized and evaluated in vitro and in vivo for binding to the brain vesicular monoamine transporter. Additions of organometallic reagents to tetrabenazine produced 2-methyl, 2-ethyl, 2-n-propyl, 2-isopropyl, and 2-isobutyl derivatives of dihydrotetrabenazine. The stereochemistry and conformation of the addition products were thoroughly verified by two-dimensional NMR techniques. All of these alkyl derivatives displayed in vitro affinity for the vesicular monoamine transporter binding site in rat brain using competitive assays with the radioligand [3H]methoxytetrabenazine. Except for the isopropyl derivative, all compounds when tested at 10 mg/kg iv showed an ability to inhibit in vivo accumulation of the radioligand [11C]methoxytetrabenazine in the mouse brain striatum. Derivatives with small alkyl groups (methyl, ethyl) were more effective than those with large groups (propyl, isobutyl). These studies suggest that large groups in the 2-position of the benzisoquinoline structure will significantly diminish both in vitro and in vivo binding of these compounds to the vesicular monoamine transporter.

In vivo imaging of the brain vesicular monoamine transporter

In the search for an in vivo marker of monoamine nerve terminal integrity, we investigated methoxytetrabenazine (MTBZ) as a tracer of the brain synaptic vesicular monoamine transporter (VMAT2).

METHODS

The biodistribution, metabolism and in vivo specificity of MTBZ binding were first evaluated in rodents and the human dosimetry was estimated. Subsequently, the human brain distribution of VMAT2 binding was determined in normal volunteers following administration of [11C]MTBZ. Brain regional time-activity curves were obtained, and parametric transport and binding images were calculated using arterial blood sampling and a two-compartment tracer kinetic model.

RESULTS

Regional rat brain localization of [3H]MTBZ 15 min postinjection was consistent with the known monoamine nerve terminal density, which demonstrated the highest activity in the striatum, lateral septum, substantia nigra pars compacta, the raphe nuclei and the locus coeruleus. At this time, chromatography revealed over 82% of brain activity, but less than 47% of plasma activity corresponded to authentic MTBZ. In vivo [11C]MTBZ binding in the mouse brain was inhibited by coinjection of excess unlabeled dihydrotetrabenazine. In humans [11C]MTBZ had high initial brain uptake and rapid clearance from all regions, with longest retention in areas of high VMAT2 concentration. Parametric quantification of VMAT2 density revealed the highest distribution volume in the putamen and caudate with lower values in cerebral cortex and cerebellum.

CONCLUSION

Carbon-11-MTBZ is a suitable ligand for PET quantification of the vesicular monoamine transporter in the human brain.

[3H]methoxytetrabenazine: a high specific activity ligand for estimating monoaminergic neuronal integrity

The properties as well as the distribution of high specific activity alpha-[O-methyl-3H]methyoxytetrabenazine binding to the synaptic vesicular monoamine transporter were studied autoradiographically in rat brain sections. Saturation analysis revealed [3H]methoxytetrabenazine interaction with a homogeneous population of striatal sites (Hill coefficient 1.00 +/- 0.05), with an apparent equilibrium dissociation binding constant of 3.9 +/- 0.4 nM and a maximal binding capacity of 1.2 +/- 0.1 fmol/micrograms protein. Highest levels of [3H]methoxytetrabenazine binding sites were observed in regions richly innervated by the monoamine systems. In the presence of 1 microM concentrations of a variety of competing drugs, only reserpine significantly inhibited [3H]methoxytetrabenazine binding. The presynaptic nigrostriatal location of [3H]methoxytetrabenazine binding was demonstrated by unilateral lesion of the median forebrain bundle with 6-hydroxydopamine. The resulting decrease of striatal [3H]methoxytetrabenazine binding showed an excellent correlation with tyrosine hydroxylase-positive neuron density in the substantia nigra pars compacta (r2 = 0.96; P < 0.001). The present studies demonstrate that in vitro [3H]methoxytetrabenazine binding is a reliable, quantitative marker of the synaptic vesicular monoamine transporter. Further, it is indicated that [3H]methoxytetrabenazine binding provides an accurate assessment of monoamine neuronal losses and may thus be of great value in future studies of neurodegenerative diseases.

Synthesis of carbon-11-, fluorine-18-, and iodine-125-labeled GABAA-gated chloride ion channel blockers: substituted 5-tert-butyl-2-phenyl-1,3-dithianes and -dithiane oxides

A series of substituted 5-tert-butyl-2-phenyl-1,3-dithianes and 5-tert-butyl-2-phenyl-1,1,3,3-tetraoxo-1,3-dithianes was synthesized as ligands for the GABAA receptor complex-associated neuronal chloride ion channels. The in vitro binding affinities of these compounds for the GABA-gated chloride ion channel were determined by their ability to compete with [3H]TBOB for binding to rat brain slices. Of the eight compounds tested, trans-5-tert-butyl-2-(4-cyanophenyl)-2-methyl-1,1,3,3-tetraoxo+ ++-1,3-dithiane, 9b, trans-5-tert-butyl-2-(4-fluorophenyl)-1,1,3,3-tetraoxo-1,3-dithian e, 10, and trans-5-tert-butyl-2-(4-iodophenyl)-2-methyl-1,1,3,3-tetraoxo-1,3- dithiane, 11, showed moderately high binding affinities (Ki = 41, 180, and 105 nM, respectively). Four radioligand candidates from this series, 5-tert-butyl-2-(4-cyanophenyl)-2-[11C]methyl-1,3-dithiane, [11C]6, 5-tert-butyl-2-(4-[18F]fluorophenyl)-1,3-dithiane, [18F]7, 5-tert-butyl-2-(4-[18F]-fluorophenyl)-1,1,3,3-tetraoxo-1,3- dithiane, [18F]10, and 5-tert-butyl-2-(4-[125I]iodophenyl)-2-methyl-1,1,3,3- tetraoxo-1,3-dithiane, [125I]11, have been successfully prepared for evaluation as in vivo imaging agents useful for positron emission tomography and single photon emission computed tomography. Preliminary in vivo studies indicate significant uptake into mouse brain for [18F]7, [18F]10, and [125I]11.