SPECT imaging of dopamine and serotonin transporters with [123I]beta-CIT: pharmacological characterization of brain uptake in nonhuman primates

Differential kinetics of [123I]beta-CIT binding to dopamine and serotonin transporters

Iodine-123-labelled 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid ([123I]beta-CIT) labels both the dopamine transporter (DAT) and the serotonin transporter (5-HTT) and this ligand is able to clarify pathological changes in both dopaminergic and serotonergic systems. However, the differential kinetics of beta-CIT binding to DAT and 5-HTT has not been clarified fully. In this study we examined time-activity curves of [123I]beta-CIT in individual regions in the rat brain. Using cerebellum as the reference region, k3 and k4 values were estimated by a two-compartment kinetic analysis. In the striatum, the kinetics was slowest among all brain areas. In this area specific binding reached its peak 4 h after the injection. In the hypothalamus, specific binding reached its peak 1 h after the injection and its amount did not change until 4 h after the injection. In the occipital cortex, the binding and washout of the ligand were fastest among all brain regions. Estimated k3 values were 0.040+/-0.003 in the striatum, 0.019+/-0.002 in the hypothalamus and 0.082+/-0.011 in the occipital cortex (min-1, mean +/-SD). Estimated k4 values were 0.0034+/-0.0005 in the striatum, 0.0071+/-0.0009 in the hypothalamus and 0.083+/-0.013 in the occipital cortex (min-1, mean +/-SD). Therefore binding kinetics of [123i]beta-cit in the region rich in dat is apparently different from that in the region rich in 5-HTT. These results will provide fundamental data to image both DAT and 5-HTT in one series of examinations with [123I]beta-CIT.

Pharmacological modifications of dopamine transmission do not influence the striatal in vivo binding of [3H]mazindol or [3H]cocaine in mice

We have considered the in vivo striatal binding of two ligands of the neuronal dopamine uptake complex: [3H]cocaine and [3H]mazindol. The [3H]cocaine tracer dose labelled the dopamine uptake complex in striatum but not the noradrenaline complex in cerebellum. On the contrary, the [3H]mazindol tracer dose induced a marked labelling of the noradrenaline uptake complex in cerebellum; its prevention by desipramine (5 mg/kg) increased simultaneously the cerebral bioavailability and thereby the striatal labelling of the dopamine transporter. In mice submitted to treatments modifying dopaminergic transmission either to decrease it (gammabutyrolactone, 750 mg/kg, i.p.) or to increase it (L-DOPA, 200 mg/kg, i.p., dexamphetamine, 4 mg/kg, s.c., or their combination), only dexamphetamine pretreatment significantly reduced [3H]cocaine and [3H]mazindol binding. Thus it appears that the level of dopamine transmission would not interfere with the in vivo quantification of striatal dopamine uptake sites assessed with either ligands.

Affinities of methylphenidate derivatives for dopamine, norepinephrine and serotonin transporters

We have synthesized several derivative of dl-threo-methylphenidate (Ritalin) bearing substituents on the phenyl ring. IC50 values for binding these compounds to rat brain monoamine transporters were assessed using [3H]WIN 35,428 (striatal membranes, dopamine transporters, DAT), [3H]nisoxetine (frontal cortex membranes, norepinephrine transporters, NET) and [3H]paroxetine (brain stem membranes, 5HT transporters, 5HTT). Affinities (1/Ki) decreased in the order: DAT > NET >> 5HTT. Substitution at the para position of dl-threo-methylphenidate generally led to retained or increased affinity for the dopamine transporter (bromo > iodo > methoxy > hydroxy). Substitution at the meta position also increased affinity for the DAT (m-bromo > methylphenidate; m-iodo-p-hydroxy > p-hydroxy). Substitution at the ortho position with bromine considerably decreased affinity. Similar IC50 values for binding of o-bromomethylphenidate to the dopamine transporter were measured at 0, 22 and 37 degrees. N-Methylation of the piperidine ring of methylphenidate also considerably reduced affinity. The dl-erythro isomer of o-bromomethylphenidate did not bind to the DAT (IC50 > 50,000 nM). Affinities at the dopamine and norepinephrine transporters for substituted methylphenidate derivatives were well correlated (r2=0.90). Abilities of several methylphenidate derivatives to inhibit [3H]dopamine uptake in striatal synaptosomes corresponded well with inhibition of [3H]WIN 35, 428 binding. None of the compounds examined exhibited significant affinity to dopamine D1 or D2 receptors (IC50 > 500 or 5,000 nM, respectively), as assessed by inhibition of binding of [3H]SCH 23390 or [123I]epidepride, respectively, to striatal membranes.

Displacement of RTI-55 from the dopamine transporter by cocaine

The cocaine analog 3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid methyl ester (RTI-55 or beta CIT) has a higher affinity for the dopamine transporter and may be potentially useful in interfering with cocaine's actions in brain. However, imaging studies have demonstrated displacement of tracer doses of [123I]RTI-55 by a subsequent dose of cocaine. Similar displacement of pharmacological doses of RTI-55 might compromize therapy with RTI-55 in cocaine abuse. The reduction in dopamine transporter availability, assessed in vivo in mouse striatum using [3H]cocaine, caused by pretreatment with RTI-55 was significantly mitigated by subsequent administration of cocaine. In a similar experiment using a tracer dose of [123I]RTI-55 significant reductions of striatal radioligand binding by pretreatment with cocaine or RTI-55 were not observed. These results suggest that: (1) cocaine can displace pharmacological doses of RTI-55 from striatum, and (2) radioligands used to assess binding site occupancy should have a lower affinity than the occupying drug.

3 Beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester: a new precursor for the preparation of [123I]RTI-55

Here we report the synthesis of 3 beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester, a new, easily prepared precursor for the synthesis of radioiodinated versions of RTI-55. In contrast to the 3 beta-(p-trimethylstannylated) precursor for radioiodination, the aryl silane is prepared directly by the 1,4 addition of p-trimethylsilylphenylmagnesium iodide to anhydroecgonine methyl ester in 49% yield (Scheme 1). Radioiododesilylation proceeded smoothly (approx. 85% radioincorporation after 15 min at room temperature) using 1 microgram of the novel precursor in trifluoroacetic acid and chloramine-T as oxidant. Isolated radiochemical yields for 125I and 123I radiolabelings were 72 and 48%, respectively, with a calculated specific activity for [123I]RTI-55 of > 2000 mCi/mumol.

Euphorigenic doses of cocaine reduce [123I]beta-CIT SPECT measures of dopamine transporter availability in human cocaine addicts

The in vivo potency of euphorigenic doses of intravenous cocaine for displacing [123I]beta-CIT ([123I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane) binding to striatal dopamine transporters (DAT) was assessed in human cocaine addicts using single photon emission computed tomography (SPECT). Cocaine-dependent subjects (n = 6) were injected with [123I]beta-CIT and imaged 24 h later under equilibrium conditions. Sequential cocaine infusions (0.28 +/- 0.03 and 0.56 +/- 0.07 mg/kg) produced significant (P < 0.0005) reductions in the specific to non-specific equilibrium partition coefficient, V3" (6 +/- 6 and 17 +/- 3%), a measure proportional to DAT binding potential. Regression analysis of the logit transformed data enabled reliable determination of the Hill coefficient (0.51) and 50% displacement (ED50) dose of cocaine (2.8 mg/kg). These preliminary data suggest that cocaine produces behavioral effects in humans at measurable levels of DAT occupancy.

Evaluation of [123I] beta-CIT binding with SPECT in controls, early and late Parkinson's disease

The main neuropathological feature in Parkinson's disease (PD) is a severe degeneration of the dopaminergic neurons in the substantia nigra resulting in a loss of dopamine in the striatum. Recently, a new radioligand (beta-CIT) for single photon emission computed tomography (SPECT) became available for in vivo imaging of the dopamine transporter on nerve endings of dopaminergic neurons in the striatum. The present results demonstrate that [123I]-beta-CIT SPECT allows a discrimination between early and late PD patients. In our opinion, these preliminary data suggest that [123I]-beta-CIT SPECT should be used from now on in longitudinal studies (such as the DATATOP study) in which the effects of (putative) neuroprotective interventions in PD are monitored.

PET examination of the monoamine transporter with [11C]beta-CIT and [11C]beta-CFT in early Parkinson's disease

The monoamine transporter was studied in 4 healthy controls and 5 patients with early Parkinson's disease (PD), who had not received any antiparkinsonian medication, by means of positron emission tomography (PET) using two novel ligands, [11C]beta-CIT and [11C]beta-CFT. Both ligands showed highest uptake in the striatum. There was intermediate accumulation of activity in the thalamus and midbrain, which was more marked for [11C]beta-CIT than for [11C]beta-CFT. In the cortical areas, uptake of both ligands was not different from that seen in the cerebellum. In the controls, the putamen-to-cerebellum and caudate-to-cerebellum ratios for [11C]beta-CFT were higher than those for [11C]beta-CIT (putamen: 3.15 +/- 0.39 for [11C]beta-CFT, and 1.84 +/- 0.10 for [11C]beta-CIT; caudate: 3.15 +/- 0.31 for [11C]beta-CFT, and 1.95 +/- 0.17 for [11C]beta-CIT). Reduction from mean control value in PD patients was greater for [11C]beta-CFT (45% in the putamen contralateral to the predominant symptoms, P < 0.001) than for [11C]beta-CIT (20%, P > 0.05). [11C]beta-CFT uptake in the caudate nucleus was also diminished in PD patients (to 80% of the control mean, P < 0.05), whereas [11C]beta-CIT was within normal range (reduced to 90% of the control mean). These results indicate that both [11C]beta-CIT and [11C]beta-CFT are useful PET ligands to study brain monoamine transporter in healthy controls and in patients with PD. However, [11C]beta-CFT seems superior to [11C]beta-CIT in this respect.

[O-methyl-11C]beta-CIT-FP, a potential radioligand for quantitation of the dopamine transporter: preparation, autoradiography, metabolite studies, and positron emission tomography examinations

beta-CIT-FP [N-(3-fluoropropyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane] is a cocaine analogue with a high affinity for the dopamine transporter. [O-methyl-11C]beta-CIT-FP ([11C]beta-CIT-FP) was prepared by O-alkylation of the free acid with [11C]methyl iodide. The total radiochemical yield of [11C]beta-CIT-FP was 50 to 60% with an overall synthesis time of 30 min. The radiochemical purity was > 99%, and the specific radioactivity at time of injection was about 37 GBq/mumol (1000 Ci/mmol). Autoradiographic examination of [11C]beta-CIT-FP binding in human brain postmortem demonstrated specific binding in the caudate nucleus and putamen. Positron emission tomography (PET) examination of [11C]beta-CIT-FP in a Cynomolgus monkey demonstrated accumulation in the striatum with a striatum-to-cerebellum ratio of about 8 after 60 min. Equilibrium in the striatum was attained within 70 to 90 min. The radioactivity ratios of thalamus/cerebellum and neocortex/cerebellum were about 2 and 1.5, respectively. In a displacement experiment, radioactivity in the striatum but not in the cerebellum was reduced after injection of beta-CIT, indicating that striatal radioactivity following injection of [11C]beta-CIT-FP is associated with dopamine transporter sites and that the binding is reversible. The fraction of the total radioactivity in plasma representing [11C]beta-CIT-FP determined by high-performance liquid chromatography (HPLC) was 84% at 15 min and 50% at 95 min. [11C]beta-CIT-FP should be a useful PET radioligand for the quantitation of dopamine transporters in the human brain in vivo.

beta-CIT SPECT demonstrates blockade of 5HT-uptake sites by citalopram in the human brain in vivo

The cocaine analogue 2-beta-carbomethoxy-3-beta-(4-iodophenyl)-tropane (beta-CIT) is a potent ligand for both dopamine- and serotonin uptake sites which in its 123I labeled form can be used for single photon emission computerized tomography (SPECT). It was demonstrated previously by SPECT-studies in non-human primates that 123I-beta-CIT binds to dopamine transporters in the striatum and to serotonin transporters in hypothalamus and midbrain. The aim of the present study was to compare 123I-beta-CIT binding in the brain stem of normal controls and a group of subjects under treatment with the selective serotonin reuptake inhibitor (SSRI) citalopram. 123I-beta-CIT-SPECT was performed in 12 depressed patients under 20 mg (n = 5), 40 mg (n = 6) and 60 mg (n = 1) citalopram daily, in one untreated depressed patient and in 11 controls at regular time intervals up till 24 hours p.inj. A highly significant reduction of beta-CIT binding was found in an area including mesial thalamus, hypothalamus, midbrain and pons in patients under citalopram compared to controls (44.1 +/- 14.4 vs. 82.3 +/- 18.6cpm's/mCi x kg body weight; specific binding 4 hrs p.inj.; p = 0.0001). No differences were seen between the high and low dose group and no changes were found in the striatum. 123I-beta-CIT binding in the brain stem and striatum in one untreated depressed patient fell within the range of control values. To our knowledge this is the first report directly demonstrating the effect of a selective serotonin uptake inhibitor in the brain in humans in vivo. SPECT measurements of serotonin uptake sites in patients with depression and other psychiatric disorders might provide better insights into the pathophysiology of these disorders and into mechanisms of drug action.

Decreased single-photon emission computed tomographic [123I]beta-CIT striatal uptake correlates with symptom severity in Parkinson's disease

Previous studies have utilized single-photon emission computed tomography (SPECT) to demonstrate decreased [123I]beta-CIT striatal uptake in idiopathic Parkinson disease (PD) patients. The present study extends this work by examining SPECT outcome measures in a larger group of PD patients with varying disease severity. Twenty-eight L-dopa-responsive PD patients (Hoehn-Yahr stages 1-4) and 27 healthy controls had SPECT scans at 18 to 24 hours after injection of [123I]beta-CIT. Specific to nondisplaceable striatal uptake ratios (designated V3") were correlated with Hoehn-Yahr stage and Unified Parkinson's Disease Rating Scale (UPDRS) subscores. Linear discriminant function analyses utilizing striatal uptakes, putamen-to-caudate ratios, and ipsilateral-contralateral asymmetry indices were performed. Decreased striatal tracer uptake (V3") was correlated with total UPDRS score for both contralateral and ipsilateral striatum. Putamen uptake was relatively more reduced than caudate with mean putamen:caudate ratios of 0.50 +/- 0.17 and 0.82 +/- 0.09 for PD patients and controls, respectively. Ipsilateral:contralateral asymmetry was significantly greater in PD patients than controls. Discriminant function analysis utilizing V3" for ipsilateral and contralateral caudate and putamen correctly classified all 55 cases. These data demonstrate marked differences in [123I]beta-CIT SPECT measures in healthy controls and PD patients. The significant correlation of SPECT measures with motor severity suggests [123I]beta-CIT may be a useful marker of disease severity in PD.

Binding of d-threo-[11C]methylphenidate to the dopamine transporter in vivo: insensitivity to synaptic dopamine

The regional distribution of [11C]d-threo-methylphenidate in mouse brain was very similar to that of [3H]WIN 35,428 ((-)-2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane), and the two radioligands were displaced from striatum similarly after administration of the potent cocaine analog RTI-55 ((-)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane). However, while striatal [3H]WIN 35,428 increased between 5 and 30 min, striatal [11C]d-threo-methylphenidate halved. Thus [11C]d-threo-methylphenidate binds similarly to but more reversibly than [3H]WIN 35,428. The methyl ester of L-DOPA (L-3,4-dihydroxyphenylalanine; 200 mg/kg) plus benserazide plus clorgyline, which markedly elevates rat striatal extracellular dopamine (Wachtel and Abercrombie, 1994, J. Neurochem. 63, 108), decreased the mouse striatum-to-cerebellum ratio for [11C]d-threo-methylphenidate at 30 min by 13% (P < 0.05). In positron emission tomographic (PET) baboon studies [11C]d-threo-methylphenidate binding was insensitive to drugs expected to lower endogenous dopamine. These experiments suggest that normal synaptic dopamine does not compete for binding with [11C]d-threo-methylphenidate, and will not affect PET measures of dopamine transporter availability.

Synthesis of two radiofluorinated cocaine analogues using distilled 2-[18F]fluoroethyl bromide

Two fluorinated congeners of cocaine, 2'-fluoroethyl (1R-2-exo-3-exo)-8-methyl-3-(4-methylphenyl)-8-azabicyclo[3.2.1]octane- 2-carboxylate (FETT) and its 4-chlorophenyl analogue (FECT) were synthesized. Radiolabelling with 18F was achieved by O-[18F]fluoroalkylation of the corresponding carboxylic acid salts with distilled 2-[18F]fluoroethyl bromide in DMF. After HPLC purification, yields of radiochemically pure, formulated products were 22-30% (not corrected for decay) in a synthesis time of 60-70 min. The use of distilled 2-[18F]fluoroethyl bromide was indispensable for the reliable production of pure products.

IPT: a novel iodinated ligand for the CNS dopamine transporter

An iodinated cocaine derivative, N-(3'-iodopropen-2'-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl) tropane (IPT), was evaluated as a probe for in vitro and in vivo labeling of dopamine (DA) and serotonin (5-HT) transporters in Sprague-Dawley rat brain. Saturation analysis of [125I]IPT in rat striatal homogenates, in two different buffer solutions, Tris-HCl and phosphate, demonstrated a one-site binding with affinities (Kd) of 0.25 +/- 0.02 and 0.16 +/- 0.02 nM and densities (Bmax) of 939 +/- 161 and 1,982 +/- 137 fmol/mg protein, respectively. Competition by known DA transporter ligands showed a rank order of RTI-55 > IPT > GBR12909 > mazindol > (-)cocaine. Binding to 5-HT transporter sites was evaluated in rat cortical homogenates. Saturation experiment results showed a single site with a Kd value of 1.2 +/- 0.2 nM and a Bmax value of 100 +/- 20 fmol/mg protein. The rank order of potency of several monoamine uptake inhibitors (paroxetine > fluoxetine > mazindol > R-nisoxetine > GBR12909) suggests that [125I] IPT labels 5-HT transporters in rat cortical homogenates. Both ex vivo and in vitro autoradiographic studies revealed high densities of [125I]IPT binding sites in the caudate nucleus, putamen, olfactory tubercle and nucleus accumbens, areas known to be rich in dopaminergic innervation. Moderate accumulation of activity was also observed in the substantia nigra. The dorsal raphe, a region with a high density of 5-HT innervation, was labeled using in vitro autoradiography with [125I]IPT, but the labeling using ex vivo autoradiography was less prominent at 30 min postinjection and not noticeable at 60 min postinjection.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the dopamine transporter in nonhuman primate brain: homogenate binding, whole body imaging, and ex vivo autoradiography using [125I] and [123I]IPCIT

IPCIT [2 beta-carboisopropoxy-3 beta-(4-iodophenyl)tropane; also designated RTI-121] is the isopropyl ester of beta-CIT [2 beta-carbomethoxy-3 beta-(4-iodophenyl) tropane]. Although beta-CIT binds to dopamine (DA), serotonin (5-HT) and norepinephrine (NE) transporters, IPCIT has been reported to be selective for the DA transporter. IPCIT was labeled with 125I and its receptor binding to membranes prepared from baboon striatum was compared with that of [125I] beta-CIT. These studies confirmed the relative selectivity of IPCIT for the DA transporter in comparison to 5-HT and NE transporters. The nonspecific binding of [125I]IPCIT was almost four times greater than that of [125I] beta-CIT. The biodistribution of IPCIT was examined in two baboons with whole body imaging for 24-30 h after administration of 3 mCi of 123I-labeled tracer. The brain uptake peaked within the first hour at 9.2% of the injected dose and the majority of activity in the body cleared through the hepatobiliary system. The distribution of activity within the brain was examined with ex vivo autoradiography in one monkey injected with [123I]IPCIT. Activity was concentrated in the caudate and putamen and had values of 5 and 7 microCi/cm3 per microCi/g, respectively. The distribution in brain regions receiving moderately dense serotonergic innervation (e.g. superior colliculus and thalamus) had levels of activity equivalent to that in cerebellum. This study confirmed the in vitro and in vivo selectivity of IPCIT for the DA transporter but also showed that [125I]IPCIT had higher in vitro nonspecific binding than [125I] beta-CIT.

Rate of binding of various inhibitors at the dopamine transporter in vivo

The rate of entry of drugs into brain is thought to be a factor in their abuse liability. In this investigation, we have examined the rate of entry and binding at dopamine transporters in mouse striatum for a variety of dopamine transporter inhibitors. The method utilized was based on measuring the displacement of 3H-WIN 35,428 from striatal dopamine transporter sites in vivo at different times. Eleven cocaine analogs (RTI-31, RTI-32, RTI-51, RTI-55, RTI-113, RTI-114, RTI-117, RTI-120, RTI-121, WIN 35,065-2, and WIN 35,428) as well as other dopamine uptake site blockers (bupropion, nomifensine, and methylphenidate) were compared with (-) cocaine for their rates of displacement of 3H-WIN 35,428 binding in vivo. The drugs that displayed the fastest occupancy rates were bupropion, (-) cocaine, nomifensine, and methylphenidate. RTI-51, RTI-121, RTI-114, RTI-117, RTI-120, RTI-32, RTI-55, and RTI-113, showed intermediate rates, whereas RTI-31, WIN 35,065-2, and WIN 35,428 exhibited the slowest rates of displacement. While many of the cocaine analogs have proven to be behaviorally and pharmacologically more potent than (-) cocaine, their rates of entry and binding site occupancy were slower than that for (-) cocaine. Earliest times of transporter occupancy by the different drugs were correlated (although weakly) with their degree of lipophilicity (r = 0.59; P < 0.02). Kinetic effects and metabolism of the compounds could complicate the interpretations of these data.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipophilic metabolite of [123I]beta-CIT in human plasma may obstruct quantitation of the dopamine transporter

I-123 or C-11 labelled 2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane (beta-CIT) is a recently developed radioligand for the study of dopamine and serotonin reuptake sites in humans with single photon emission tomography (SPET) or positron emission tomography (PET). Determination of the radioligand metabolite pattern is fundamental for a quantitative analysis of radioligand binding. The metabolism of [123I]beta-CIT was determined by a gradient HPLC method in plasma samples of six human subjects. Two metabolites of [123I]beta-CIT were found, a polar and a lipophilic. At 4 h after [123I]beta-CIT injection the percentages of parent compound and polar and lipophilic metabolites were 23 +/- 3% (mean +/- SD), 33 +/- 11%, and 44 +/- 8%, respectively. The lipophilic metabolite might pass the blood-brain barrier and account for a fraction of free and nonspecifically bound radioactivity in brain. The existence of a lipophilic metabolite of [123I]beta-CIT may obstruct the use of simple ratio methods for quantitation of the dopamine transporter in brain.

Long-lasting inhibition of in vivo cocaine binding to dopamine transporters by 3 beta-(4-iodophenyl)tropane-2-carboxylic acid methyl ester: RTI-55 or beta CIT

Cocaine analogs such as 3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid methyl ester (RTI-55 or beta CIT) with a higher affinity for the dopamine transporter (DAT) may be potentially useful in interfering with cocaine's actions in brain. This study evaluates the time course of the effects of RTI-55 on cocaine binding in baboon brain using PET and [11C]cocaine. [11C]Cocaine binding was measured prior to, and 90 minutes, 24 hours, 4-5 days and 11-13 days after RTI-55 (0.3 mg/kg i.v.). Parallel studies with [3H]cocaine and RTI-55 (0.5 mg/kg i.v. or 2 mg/kg i.p.) were performed in the mouse. RTI-55 significantly inhibited [11C]cocaine binding at 90 minutes and 24 hours after administration. The half-life for the clearance of RTI-55 from the DAT was estimated to be 2 to 3 days in the baboon brain. In the mouse brain, RTI-55 significantly inhibited [3H]cocaine binding at 60 and 180 minutes after administration and recovery was observed at 12 hours. These results document long-lasting inhibition of cocaine binding by RTI-55 and corroborate that binding kinetics of RTI-55 in striatum observed in imaging studies with [123I]RTI-55 represents binding to DATs.

Regional brain uptake and pharmacokinetics of [123I]N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane esters in baboons

Four N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane ester (beta-CIT-FE), N-fluoropropyl, methyl ester (beta-CIT-FP), N-fluoroethyl, isopropyl ester (IP-beta-CIT-FE), and N-fluoropropyl, isopropyl ester (IP-beta-CIT-FP)] were labeled with 125I and evaluated in baboons by dynamic SPECT regional brain imaging, measurement of pharmacokinetics in arterial plasma, and whole body imaging. The labeled tracers were prepared by iododestannylation of the corresponding 4-(trimethylstannyl)phenyl compounds in radiochemical yield 63-96% and radiochemical purity > 96%. Regional SPECT brain imaging was carried out over a period of 5 h with a Strichman 810X Brain Imager to assess regional uptake in the striatum and midbrain compared to reference regions in the occipital cortex and cerebellum; arterial blood samples were taken for analysis of metabolites by solvent extraction and HPLC. The methyl esters showed higher total and specific peak uptake in the striatum than the isopropyl esters. Midbrain uptake was uniformly lower than striatal uptake and washed out more rapidly. beta-CIT-FE had more rapid striatal kinetics than beta-CIT-FP, with specific striatal washout rates of 10-14 vs 4-6% peak/h. Biodistribution of [123I] beta-CIT-FE and [123I] beta-CIT-FP measured by whole body conjugate imaging demonstrated major uptake in the brain, liver, and GI tract, with excretion occurring through both the renal and hepatobiliary routes. Absorbed radiation does estimates based on the MIRD schema indicated highest dose rates to the urinary bladder wall and lower large intestine wall (0.7 and 0.6 rad/mCi for [123I] beta-CIT-FE and 0.7 and 0.9 rad/mCi for [123I]beta-CIT-FP, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of [123I]beta-CIT and [123I]IPCIT as single-photon emission tomography radiotracers for the dopamine transporter in nonhuman primates

Single-photon emission tomographic (SPET) imaging with the radiotracer [123I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane ([123I]beta-CIT) has been reported to be a useful in vivo measure of dopamine (DA) transporters. However, in addition to its high DA transporter affinity, beta-CIT also binds with high affinity to serotonin (5-HT) transporters. 2 beta-Carboisopropoxy-3 beta-(4-iodophenyl)tropane (IPCIT) has been demonstrated by in vitro studies to have higher selectivity for the DA transporter. We compared [123I]beta-CIT and [123I]IPCIT SPET imaging and plasma metabolite analyses in baboons to evaluate the potential advantages of [123I]IPCIT for quantitative in vivo measurements of DA transporter densities. Both tracers had low levels (2% of total plasma 123I activity) of lipophilic radiolabeled metabolites at 420 min. [123I]IPCIT had significantly higher binding to plasma proteins. The average percent free (nonprotein bound) [123I]beta-CIT and [123I]IPCIT were 52% +/- 7% and 14% +/- 6%, respectively. Region of interest uptake data were normalized by injected dose and body weight. Consistent with the high density of 5-HT transporters in the midbrain and the lower 5-HT transporter affinity of IPCIT, the normalized peak specific midbrain uptake of [123I]beta-CIT (1.7 +/- 0.5) was higher than that of [123I]IPCIT (0.4 +/- 0.2). Consistent with its greater lipophilicity, [123I]IPCIT had higher nonspecific uptake, such that normalized cerebellar uptake of [123I]IPCIT was about twice that of [123I]beta-CIT. The ratio of specific to nonspecific uptake in striatum was greater for [123I]beta-CIT compared to [123I]IPCIT; however, striatal binding potentials and distribution volumes were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Graphical, kinetic, and equilibrium analyses of in vivo [123I] beta-CIT binding to dopamine transporters in healthy human subjects

The in vivo kinetics of the dopamine (DA) transporter probe 123I-labeled 2 beta-carboxymethoxy-3 beta-(4-iodophenyl) tropane ([123I] beta-CIT) in striatum was investigated with single-photon emission computerized tomography (SPECT) in five healthy human subjects. The aim of this study was to derive an adequate measure of the DA transporter density that would not be affected by regional cerebral blood flow or peripheral clearance of the tracer. SPECT data were acquired on the day of injection (day 1) from 0 to 7 h and on the following day (day 2) from 19 to 25 h. Arterial sampling on day 1 was used to measure the input function. Graphical, kinetic, and equilibrium analyses were evaluated. Graphical analysis of day 1 data, with the assumption of negligible dissociation of the tracer-receptor complex (k4 = 0), was found to be blood flow-dependent. A three-compartment kinetic analysis of day 1 data were performed using a three (k4 = 0)- and a four (k4 > 0)-parameter model. The three-parameter model estimated the konBmax product at 0.886 +/- 0.087 min-1. The four-parameter model gave a binding potential (BP) of 476 ml g-1, a value consistent with in vitro measurements. The stability of the regional uptake on day 2 allowed direct measurement of the specific to nonspecific equilibrium partition coefficient (V3" = k3/k4 = 6.66 +/- 1.54). Results of day 1 kinetic analysis and day 2 equilibrium analysis were well correlated among subjects. Simulations indicated that the error associated with the day 2 equilibrium analysis was acceptable for plasma tracer terminal half-lives > 10 h. We propose the equilibrium analysis on day 2 as the method of choice for clinical studies since it does not require multiple scans or the measurement of the arterial plasma tracer concentrations.

Cocaine accumulates in dopamine-rich regions of primate brain after i.v. administration: comparison with mazindol distribution

Pharmacological and neurochemical evidence suggest that brain dopamine systems, and the dopamine transporter in particular, contribute significantly to the behavioral effects and reinforcing properties of cocaine. The first objective of this study was to determine whether the brain distribution of cocaine supports these conclusions. A high resolution neuroanatomical map of cocaine disposition in brain after i.v. administration was developed. [3H]Cocaine ([3H](-)-cocaine) was administered to squirrel monkeys (Saimiri sciureus) at a trace dose (0.001 mg/kg) and at doses at or above the threshold for producing behavioral effects (0.1 mg/kg, 0.3 mg/kg). After 15 min, ex vivo autoradiography revealed the highest accumulation of [3H]cocaine in dopamine-rich brain regions, including the caudate nucleus, putamen, and nucleus accumbens/olfactory tubercle. The norepinephrine-rich locus coeruleus, the hippocampus, and amygdala also accumulated large quantities of [3H]cocaine. Moderately high levels were found in the stria terminalis, medial septum, substantia nigra, and other regions. Lowest levels were found in the cerebellum. A high and positive correlation was established for the brain distribution of [3H]cocaine administered at trace or at behaviorally relevant doses (r: 0.94; P < 0.001). To determine whether radioactivity represented [3H]cocaine or its metabolic products, tissue extracts from brain regions with high levels of cocaine were subjected to thin layer chromatography using two solvent systems. In caudate-putamen, nucleus accumbens, cortex, and hippocampus, radioactivity comigrated with standard [3H]cocaine. In substantia nigra, less than 70% of the radioactivity comigrated with [3H]cocaine, suggesting that cocaine metabolites are generated more rapidly in the substantia nigra than in other brain regions. The second objective was to determine the brain distribution of mazindol, a potent norepinephrine and dopamine transport inhibitor with low abuse liability in humans. The disposition of intravenously administered [3H]mazindol in brain (0.001 mg/kg, 0.007 mg/kg) was surveyed by ex vivo autoradiography. In sharp contrast to [3H]cocaine distribution, the highest accumulation of [3H]mazindol was localized in the norepinephrine-rich pineal gland, discrete regions of the hypothalamus (paraventricular nucleus, supraoptic nucleus), and the locus coeruleus. Moderately high levels were detected in the caudate-putamen, nucleus accumbens, and other regions. The following conclusions were drawn: (1) Although dopamine-rich brain regions are principal targets of cocaine after i.v. administration to the nonhuman primate, other prominent targets of cocaine (locus coeruleus, hippocampus, and amygdala) may contribute to the acute and chronic effects of cocaine.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics and in vivo specificity of [11C]dl-threo-methylphenidate for the presynaptic dopaminergic neuron

dl-threo-Methylphenidate (Ritalin) was labeled with carbon-11 (t1/2:20.4 minutes) in order to measure its pharmacokinetics, to evaluate it as a radiotracer for the presynaptic dopaminergic neuron, and to examine its sensitivity to the loss of dopaminergic neurons. Positron emission tomographic (PET) studies were carried out in the baboon to determine specificity for the presynaptic dopaminergic neuron and in humans to assess sensitivity to neuronal loss. Studies with [11C]dl-threo-methylphenidate ([11C]MP) in baboon demonstrated high regional uptake in the striatum. Peak uptake (0.04%/cc) occurred at 5-15 minutes post-injection. The half-time for clearance from peak uptake for [11C]MP was 60 minutes and the ratio between the radioactivity in the striatum and that in the cerebellum (ST/CB) ranged from 2.2 to 2.6 at 40 minutes. Repeated measures in the same baboon showed < or = 8% variability in the ST/CB ratio. Pretreatment with unlabeled methylphenidate (0.5 mg/kg) or GBR12909 (1.5 mg/kg) 30 minutes prior to [11C]MP injection markedly reduced the striatal but not the cerebellar uptake of [11C]MP, demonstrating the saturable and specific binding of [11C]MP to a site on the dopamine transporter in the brain. In both cases, the ratio of striatum to cerebellum (ST/CB) after pretreatment was reduced by about 43%. The ratios of distribution volumes at the steady-state for the striatum to cerebellum (ST/CB) for these two separate studies in the same baboon were reduced by 37 and 38%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

SPECT quantification of [123I]iomazenil binding to benzodiazepine receptors in nonhuman primates: II. Equilibrium analysis of constant infusion experiments and correlation with in vitro parameters

In vivo benzodiazepine receptor equilibrium dissociation constant, KD, and maximum number of binding sites, Bmax, were measured by single photon emission computerized tomography (SPECT) in three baboons. Animals were injected with a bolus followed by a constant i.v. infusion of the high affinity benzodiazepine ligand [123I]iomazenil. Plasma steady-state concentration and receptor-ligand equilibrium were reached within 2 and 3 h, respectively, and were sustained for the duration (4-9 h) of the experiments (n = 15). At the end of the experiments, a receptor saturating dose of flumazenil (0.2 mg/kg) was injected to measure nondisplaceable activity. Experiments were carried out at various levels of specific activity, and Scatchard analysis was performed for derivation of the KD (0.59 +/- 0.09 nM) and Bmax (from 126 nM in the occipital region to 68 nM in the striatum). Two animals were killed and [125I]iomazenil Bmax and KD were measured at 22 and 37 degrees C on occipital homogenate membranes. In vitro values of Bmax (114 +/- 33 nM) and 37 degrees C KD (0.66 +/- 0.16 nM) were in good agreement with in vivo values measured by SPECT. This study demonstrates that SPECT can be used to quantify central neuroreceptors density and affinity.

Occupancy of the serotonin transporter by fluoxetine, paroxetine, and sertraline: in vivo studies with [125I]RTI-55

[125I]RTI-55 was used tracer doses to label serotonin (5-HT) transporters in vivo in the mouse brain. Fluoxetine, paroxetine, and sertraline, potent antidepressants and selective inhibitors of serotonin transporter sites, were administered in various doses and at various times. The doses and times that result in significant binding of the drugs to transporters correspond to doses and times where they are reported to have physiological effects. Estimates of occupancy rate and duration of binding to serotonin transporters were made. The rate of occupancy of the 5-HT transporter site was fastest for sertraline, intermediate for paroxetine and slowest for fluoxetine. Similarly, the duration of occupancy was significantly shorter for sertraline and paroxetine (approximately 10 h) than for fluoxetine (approximately 50 h). The results indicate that in competition studies, [125I]RTI-55 can be used to identify doses of drugs that are physiologically effective, to determine their relative rate of occupancy, and most importantly, to measure the residency time on the central serotonin transporter in vivo.

Studies with differentially labeled [11C]cocaine, [11C]norcocaine, [11C]benzoylecgonine, and [11C]- and 4'-[18F]fluorococaine to probe the extent to which [11C]cocaine metabolites contribute to PET images of the baboon brain

The psychostimulant drug of abuse, cocaine (benzoylecgonine methyl ester), is rapidly metabolized by cleavage of its two ester groups, to give benzoylecgonine (BE) and ecgonine methyl ester, and by N-demethylation, to give N-norcocaine (NC). The recent use of [N-methyl-11CH3]cocaine to image brain cocaine binding sites with positron emission tomography (PET) raises the question of whether PET images partially reflect the distribution and kinetics of labeled cocaine metabolites. We prepared [O-methyl-11CH3]cocaine by methylation of the sodium salt of BE with [11C]CH3I, and showed that PET baboon brain scans, as well as regional brain kinetics and plasma time-activity curves corrected for the presence of labeled metabolites, are nearly identical to those seen with [N-methyl-11CH3]cocaine. This strongly suggests that 11C metabolites do not significantly affect PET images, because the metabolite pattern is different for the two labeled forms of cocaine. In particular, nearly half the 11C in blood plasma at 30 min was [11C]CO2 when [N-methyl-11CH3]cocaine was administered, whereas [11C]CO2 was not formed from [O-methyl-11CH3]cocaine. Only a trace of [11C]NC was detected in plasma after [O-methyl-11CH3]cocaine administration. Nearly identical brain PET data were also obtained when 4'-[N-methyl-11CH3]fluorococaine and 4'-[18F]fluorococaine (prepared by nucleophilic aromatic substitution from [18F]fluoride- and 4'-nitrococaine) were compared with [N-methyl-11CH3]cocaine. In vitro assays with rat brain membranes showed that cocaine and 4'-fluorococaine were equipotent at the dopamine reuptake site, but that 4'-fluorococaine was about 100 times more potent at the 5-hydroxytryptamine reuptake site.(ABSTRACT TRUNCATED AT 250 WORDS)

Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate in vitro binding to dopamine and serotonin transporters under "physiological" conditions

Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate ([123I]beta-CIT) is a single photon emission computed tomographic radiotracer for in vivo labeling of dopamine (DA) and serotonin (5-HT) transporters. Single photon emission computed tomographic experiments in nonhuman primates showed that [123I]beta-CIT in vivo binding to DA transporters had a much slower washout than binding to 5-HT transporters. This observation was not predicted from previously published in vitro studies. These studies, performed at 22 degrees C in nonphysiological buffer, reported similar affinity of [125I]beta-CIT for DA and 5-HT transporters. We now report [125I]beta-CIT binding parameters to fresh rat membranes at 22 degrees C and 37 degrees C, in a buffer mimicking the composition of cerebrospinal fluid. At both temperatures, binding to DA transporters was best fit by a two-site model, whereas binding to 5-HT transporters was compatible with one population of sites. At 22 degrees C, [125I]beta-CIT showed similar affinity to high-affinity DA (0.39 nM) and 5-HT transporter sites (0.47 nM). Increasing the incubation temperature from 22 degrees C to 37 degrees C reduced binding to DA transporters by 60%, whereas binding to 5-HT transporters was only marginally affected. In vitro kinetic experiments failed to detect significant differences in on or off rates that could explain the observed in vivo kinetics. These experiments thus failed to explain [125I]beta-CIT in vivo uptake kinetics, suggesting the existence of specific factors affecting the in vivo situation.

PET study of [11C]beta-CIT binding to monoamine transporters in the monkey and human brain

The cocaine congener beta-CIT has been labeled with 11C for positron emission tomographic (PET) studies of the dopamine transporter. In the present autoradiographic study on human brain sections and PET study on monkey and human [11C]beta-CIT accumulated markedly in the striatum. [11C]beta-CIT binding in the striatum was selective to the dopamine transporter. The binding in the thalamus was on an intermediate level and was displaced by compounds having affinity for norepinephrine and serotonin transporters. The neocortical binding was on a low level and could be displaced only by citalopram, a serotonin uptake inhibitor. A high dose of cocaine intravenously (7 mg/kg) induced a 50% occupancy of specific [11C]beta-CIT binding to the dopamine transporter in the striatum. This dose is much higher than the doses of 0.25-0.5 mg/kg i.v. for cocaine arousal in human subjects. The finding indicates that cocaine arousal may be induced at a low dopamine transporter occupancy of a few percent. [11C]beta-CIT should be a useful radioligand to explore cocaine actions in humans and to follow the pathophysiological process in vivo by PET in neurodegenerative diseases of the striatum.

Single photon emission computed tomographic imaging demonstrates loss of striatal dopamine transporters in Parkinson disease

[123I][(1R)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane] ([123I]beta-CIT) labels dopamine transporters and is, therefore, a marker of neurons that degenerate in Parkinson disease. Single photon emission computed tomography imaging with [123I]beta-CIT showed that radioactivity in striatal regions in healthy subjects increased during a 2-day imaging study, whereas that in Parkinsonian patients peaked earlier at reduced levels relative to healthy subjects. Kinetic analyses of radioactivity in plasma and brain suggest that this decrease was due to an approximately 65% loss of target sites in patients compared with healthy subjects; greater losses occurred in putamen than in caudate. All patients showed lateralized differences in striatal uptake, with greater losses in the striatum contralateral to the side of the body with initial symptoms. These preliminary results suggest that [123I]beta-CIT is a marker for the loss of striatal dopamine terminals in patients with Parkinson disease. Single photon emission computed tomographic imaging with [123I]beta-CIT may be useful for early diagnosis of the disorder, for monitoring the progression of the disease, and for distinguishing the idiopathic disorder from other Parkinsonian syndromes with more widespread pathology.

SPECT imaging of dopamine and serotonin transporters with [123I]beta-CIT. Binding kinetics in the human brain

Single photon emission computerized tomography (SPECT) studies in non-human primates have previously shown that the cocaine derivative [123I]-2-beta-carbomethoxy-3-beta-(4-iodophenyl)-tropane ([123I]beta-CIT) labels dopamine transporters in the striatum and serotonin transporters in the hypothalamus-midbrain area. Here, we report on the regional kinetic uptake of [123I]beta-CIT in the brain of 4 normal volunteers and 2 patients with Parkinson's disease. In healthy subjects striatal activity increased slowly to reach peak values at about 20 hours post injection. In the hypothalamus-midbrain area peak activities were observed at about 4 hours with a slow decrease thereafter. Low activity was observed in cortical and cerebellar areas. The striatal to cerebellar ratio was about 4 after 5 hours and 9 after 20 hours. In 2 patients with idiopathic Parkinson's disease striatal activity was markedly decreased while the activity in hypothalamus-midbrain areas was only mildly diminished. Uptake into cortical and cerebellar areas appeared to be unchanged in Parkinson's disease. Consequently, in Parkinson's disease the striatal to cerebellar ratio was decreased to values around 2.5 after 20 hours. These preliminary methodological studies suggest that [123I]beta-CIT is a useful SPECT ligand for studying dopamine and possibly also serotonin transporters in the living human brain.