PET imaging of brain acetylcholinesterase using [11C]CP-126,998, a brain selective enzyme inhibitor

PET and [(11)C]CP-126,998, an N-benzylpiperidinebenzisoxazole, were used to image brain acetylcholinesterase (AChE) distribution in healthy controls before and after administration of 5 mg donepezil p.o., a reversible AChE inhibitor. Logan plots were used to compute distribution volumes (V(T)). The V(T) of [(11)C]CP-126,998 was highest in the basal ganglia and cerebellum and lowest in the cerebral cortex, thalamus, amygdala, and hippocampus. The regional V(T) values correlated well with AChE concentration measured in vitro. Donepezil, given 4 h before PET scanning, induced a substantial inhibition of [(11)C]CP-126,998 binding (43-62%) in all brain regions when compared to the baseline PET study. The results of this study indicate that PET imaging of [(11)C]CP-126,998 may be useful in quantifying the distribution of regional brain AChE. This new PET radiotracer may potentially be employed in the diagnosis and treatment of patients with disorders of cholinergic neurotransmission, such as Alzheimer's disease.

In vivo labeling of endothelin receptors with [(11)C]L-753,037: studies in mice and a dog

Endothelin (ET) is a potent mammalian vasoconstrictive peptide and a pressor agent. Its 3 isoforms, ET-1, ET-2, and ET-3, mediate several physiologic actions in several organ systems, binding to 2 major receptor subtypes: ET(A) and ET(B). This study was undertaken to evaluate [(11)C]L-753,037 [(+)-(5S,6R,7R)-2-butyl-7-[2-((2S)-2-carboxy-propyl)-4-methoxyphenyl]-5-(3,4-methylenedioxyphenyl)cyclopenteno [1,2-beta]pyridine-6-carboxylate), a new mixed ET receptor A and B antagonist, as a tracer for in vivo labeling of ET receptors in mice and a dog.

METHODS

[(11)C]L-753,037 was synthesized, purified, and formulated from a normethyl precursor, L-843,974, and [(11)C]H(3)I. The tracer was studied for its in vivo kinetics, biodistribution, and ET receptor binding characteristics in mice. In the dog, PET imaging was performed to evaluate binding of [(11)C]L-753,037 to ET receptors in the heart. Specificity of binding was studied in the heart with the selective ET(A) antagonist L-753,164.

RESULTS

Kinetic studies in mice showed highest tracer uptake at 5 min after injection in liver (25.0 percentage injected dose per gram [%ID/g]), kidneys (18.7 %ID/g), lungs (15.2 %ID/g), and heart (5.6 %ID/g). Initial high uptake in liver, lungs, and kidneys was followed by rapid washout during the next 10 min and a very slow clearance during the time of observation (2 h after injection). By contrast, the radioactivity in the heart remained constant over 2 h. Administration of both ET(A) (L-753,164) and mixed ET(A)/ET(B) (L-753,137) receptor antagonists resulted in dose-dependent inhibition of [(11)C]L-753,037 binding in mouse heart, lungs, and kidneys but not in the liver. Radioactivity in the brain was very low, indicating that the tracer does not cross the blood-brain barrier. In the dog, a dynamic PET study of the heart showed high tracer accumulation at 55-95 min after injection. Injection of L-753,164 at 30 min before [(11)C]L-753,037 administration led to a significant reduction in tracer binding. [(11)C]methyl triphenyl phosphonium was used as a tracer for reference images of the dog heart muscle.

CONCLUSION

The results suggest that [(11)C]L-753,037 binds to ET receptors in vivo and is, therefore, a promising candidate for investigation of these receptors and their occupancy by ET receptor antagonists using PET.

5-[I-125/123]lodo-3(2(S)-azetidinylmethoxy)pyridine, a radioiodinated analog of A-85380 for in vivo studies of central nicotinic acetylcholine receptors

The in vivo biodistribution profile of the novel nicotinic acetylcholine receptor (nAChR) radioligand 5-[I-125/123]Iodo-3(2(S)-azetidinylmethoxy)pyridine, [I-125/123]-5-IA, in mouse brain was examined. This radiotracer displayed good brain penetration (3.1% of the injected dose (ID) in whole brain at 15 min post-radioligand injection). Radioligand distribution was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus (14.9 %ID/g at 60 min), moderate uptake in cortex (8.5 %ID/g at 60 min), and lowest uptake in the cerebellum (2.4 %ID/g at 60 min). Pretreatment with several different nAChR agonists (A-85380, (-)-nicotine, cytisine) significantly inhibited [I-125]-5-IA binding in all brain regions studied (P < 0.01) demonstrating the high specificity of the radioligand for nAChRs. Blocking doses of the muscarinic antagonist scopolamine and the non-competitive nAChR channel blocker mecamylamine had no significant effect on radioactive uptake supporting the in vitro selectivity of [I-125]-5-IA for the nAChR component of the cholinergic system. [I-125]-5-IA binding sites were shown to be saturable with unlabeled 5-IA. With a relatively low acute toxicity (LD50 > 3 mg/kg via intravenous injection in mice) and high in vivo specificity and selectivity, 5-IA labeled with the imaging radionuclide I-123 may prove useful for single photon emission computed tomography (SPECT) studies of nAChRs in human subjects.

Biodistribution of [18F] SR144385 and [18F] SR147963: selective radioligands for positron emission tomographic studies of brain cannabinoid receptors

ABSTRACT. [(18)F] SR144385 and [(18)F] SR147963 were synthesized in a multistep reaction in which fluorine-18 was introduced by nucleophilic halogen displacement on a bromo precursor. The fluorine-18-labeled intermediate was deprotected and coupled with the appropriate alkyl amine to give the final products. Both radioligands had appropriate regional brain distribution for cannabinoid receptors with a target to nontarget ratio of 1.7 for [(18)F] SR147963 and 2.5 for [(18)F] SR144385 at 60 and 90 min postinjection, respectively. The uptake of both tracers was blocked with a 1 mg/kg dose of SR141716A.

Radiolabeled neuronal nitric oxide synthase inhibitors: synthesis, in vivo evaluation, and primate PET studies

The objectives of this study were to synthesize neuronal nitric oxide synthase (NOS-I)-selective imaging agents based on the 2 potent, selective inhibitors AR-R 17443 [N-(4-((2-((phenylmethyl) (methyl)-amino)ethyl)phenyl)-2-thiophenecarboximidamide)] and AR-R 18512 [(N(2-methyl-1,2,3,4-tetrahydroisoquinoline-7-yl)-2-thiophenecarboxim idamide)] in positron-emitting form and to evaluate regional brain uptake in rodents and primates.

METHODS

[11C]AR-R 17443 and [11C]AR-R 18512 were produced by N-alkylation of the corresponding desmethyl precursors using [11C]iodomethane. Regional brain uptake of [11C]AR-R 17443 and [11C]AR-R 18512 was assayed in rats and NOS-I knockout mice, and PET was performed in baboons. Tracer kinetic modeling used a 2-compartment plasma and brain tissue model.

RESULTS

Yields of [11C]AR-R 17443 and [11C]AR-R 18512 ranged from 8% to 16% at the end of synthesis, with specific activities of 50-178 GBq/micromol (1,350-4,800 Ci/mmol) at the end of synthesis. In rat cerebellum and cortex at 30 min after injection, [11C]AR-R 17443 showed 1.01 +/- 0.01 and 1.63 +/- 0.12 percentage injected dose per gram (%ID/g) uptake, respectively, whereas [11C]AR-R 18512 showed 0.88 +/- 0.01 and 1.30 +/- 0.07 %ID/g uptake, respectively. Attempts to block tracer uptake by pretreatment with the NOS-I-selective inhibitor 7-nitroindazole or the corresponding unlabeled inhibitor (or desmethyl precursor to AR-R 17443 of similar potency) were unsuccessful. A small but significant (20%) decrease in cerebellar uptake of [11C]AR-R 18512 was present in NOS-I knockout mice compared with control mice. PET of [11C]AR-R 18512 in baboons with concurrent regional cerebral blood flow (rCBF) determination before and after administration of blocker showed dose-related decreases in cerebellar uptake that were greater than or equal to decreases in rCBF. Plasma metabolites accounted for 27% of total activity at 30 min after injection. Kinetic modeling of binding potentials revealed a distribution volume of 334 in cerebral blood that dropped 51% after blocker administration.

CONCLUSION

Rodent studies for [11C]AR-R 17443 and [11C]AR-R 18512 showed little evidence of specific NOS-I binding. In baboons, we detected a higher uptake of [11C]AR-R 18512 in the cerebellum than in the cortex (approximately 5%, accounting for decreased rCBF because of blockade), indicating minimal specific binding. Analogs of higher affinity are likely required if this class of agents is to prove viable for PET.

Toxicodynamics and long-term toxicity of the recreational drug, 3, 4-methylenedioxymethamphetamine (MDMA, 'Ecstasy')

The recreational drug, (+/-)3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy'), is a potent serotonin (5-HT) neurotoxin in animals. Whether humans who use MDMA incur 5-HT neural injury is unknown. The present studies utilized positron emission tomography (PET) in conjunction with the 5-HT transporter ligand, [11C]McN-5652 to assess the status of brain 5-HT neurons in human MDMA users. Like nonhuman primates treated with neurotoxic doses of MDMA, humans with a history of MDMA use showed lasting decrements in global brain [11C]McN-5652 binding, with decreases in [11C]McN-5652 binding positively correlated to the extent of previous MDMA use. These results suggest that human MDMA use results in brain 5-HT neurotoxicity.

6-[18F]Fluoro-A-85380: an in vivo tracer for the nicotinic acetylcholine receptor

6-[18F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380 or 6-[18F]FA), a new tracer for positron emission tomography, was synthesized by no-carrier-added [18F] fluorination of 6-iodo-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)methoxy)pyridine followed by acidic deprotection. 6-[18F]FA followed the regional densities of brain nicotinic acetylcholine receptors (nAChRs) reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 6-FA. A preliminary toxicology study of the 6-FA showed a relatively low biological effect.

[11C]-methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl)-methyl]-1- piperazinecarboxylate ([11C]GR89696): synthesis and in vivo binding to kappa opiate receptors

GR89696, racemic methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl) methyl]-1-piperazinecarboxylate, a kappa opioid receptor ligand, was labeled with [11C]methyl chloroformate. The radiochemical yield was 20% with an observed specific radioactivity of 75.5 GBq/micromol at end of synthesis (2,040 mCi/micromol). Five minutes after intravenous administration, 5.4% of the injected dose accumulated in mouse whole brain. Brain region to cerebellar ratios increased over time with ratios at 90 min of 7.8, 5.6, and 4.5 for the hypothalamus, olfactory tubercle, and striatum, respectively. The uptake of [11C]GR89696 correlated with known kappa opioid receptor densities and was inhibited by kappa opioid selective drugs.

Kinetic analysis of [11C]McN5652: a serotonin transporter radioligand

The impulse response function of a radioligand is the most fundamental way to describe its pharmacokinetics and to assess its tissue uptake and retention pattern. This study investigates the impulse response function of [11C](+)McN5652, a radioligand used for positron emission tomography (PET) imaging of the serotonin transporter (SERT) in the brain. Dynamic PET studies were performed in eight healthy volunteers injected with [11C](+)McN5652 and subsequently with its pharmacologically inactive enantiomer [11C](-)McN5652. The impulse response function was calculated by deconvolution analysis of regional time-activity curves, and its peak value (f(max)), its retention value at 75 minutes (fT), and its normalized retention (f(rel) = fT/f(max)) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DV(T)) and its specific binding component (DV(S)). Both the noncompartmental (fT,f(rel)) and the compartmental parameters (DV) were investigated with and without correction for nonspecific binding by simple subtraction of the corresponding value obtained with [11C](-)McN5652. The impulse response function obtained by deconvolution analysis demonstrated high tracer extraction followed by a slow decline in the form of a monoexponential function. Statistical analysis revealed that the best compartmental model in terms of analysis of variance F and condition number of the parameter variance-covariance matrix was the one that was based on a single tissue compartment with parameters k1 and k2 and that also included the parameter of regional cerebral blood volume (BV). The parameter f(rel) demonstrated low between-subject variance (coefficient of variation [CV] = 19%), a midbrain to cerebellum ratio of 1.85, and high correlation with the known density of SERT (r = 0.787 where r is the coefficient of linear correlation between the parameter and the known density of SERT). After correction for nonspecific binding, f(rel) demonstrated further improvement in correlation (r = 0.814) and midbrain to cerebellum ratio (3.09). The variance of the distribution volumes was acceptable when the logarithmic transform lnDV was used instead of DV (17% for the three-parameter model), but correlation of this compartmental parameter was slightly less (r = 0.652 for the three-parameter model) than the correlation of the noncompartmental f(rel) with the known density of SERT, and the midbrain to cerebellum ratio was only 1.5 (uncorrected) and 1.8 (corrected). At the expense of increasing variance, the correlation was increased after correction for nonspecific binding using the inactive enantiomer (r = 0.694; CV = 22%). These results indicate that the kinetics of [11C](+)McN5652 can best be described by a one-tissue compartment model with three parameters (k1, k2, and BV), and that both the noncompartmental parameter f(rel) and the compartmental distribution volumes have the potential for quantitative estimation of the density of SERT. Further validation of the radioligand in experimental and clinical situations is warranted.

Enhanced uptake of [11C]TPMP in canine brain tumor: a PET study

In vitro studies have demonstrated the membrane potential-dependent enhanced uptake of phosphonium salts, including [3H]triphenylmethylphosphonium (TPMP), into mitochondria of carcinoma and glioma-derived tumor cells, suggesting the potential use of phosphonium salts as tracers for tumor imaging. This study characterizes the in vivo uptake of [11C]TPMP in canine brain glioma using PET.

METHODS

Dynamic paired PET studies of [11C]TPMP followed by [68Ga]ethylenediaminetetraacetic acid (EDTA) were performed 4 d before and 9 d after tumor cell inoculation. Graphical analysis was used to evaluate [11C]TPMP retention in tumor tissue. Distribution of tracer uptake was compared with tumor histological sections.

RESULTS

[11C]TPMP exhibited enhanced uptake and prolonged retention in tumor cells. Patlak plot was linear over the 20- to 95-min postinjection period (r = 0.97 +/- 0.1). [68Ga]EDTA exhibited a gradual washout from the tumor tissue. The tumor-to-normal brain uptake ratio at 55 to 95 min postinjection was 47.5 for [11C]TPMP and 8.1 for [68Ga]EDTA. Qualitative comparison with histological sections indicated that [11C]TPMP enhanced uptake was restricted to the tumor area.

CONCLUSION

The enhanced uptake and prolonged retention in tumor suggest [11C]TPMP as a promising means for imaging of gliomas in dogs. The need for studies in humans is indicated.

Effect of tracer metabolism on PET measurement of [11C]pyrilamine binding to histamine H1 receptors

The present study was carried out to investigate the time course of [11C]pyrilamine metabolism and the degree of entry of metabolites into the brain. PET studies were performed in seven healthy volunteers and arterial plasma concentrations of [11C]pyrilamine and its labeled metabolites were determined. After intravenous injection, [11C]pyrilamine metabolized gradually in the human body, with less than 10% of plasma activity being original radioligand at 60 min. Tracer metabolism markedly affected the input function and the calculated impulse response function of the brain. Rat experiments demonstrated that although metabolites of [11C]pyrilamine might enter the brain, they were not retained for prolonged periods of time. At 30-90 min after injection of [11C]pyrilamine, less than 1% of the radioactivity in the brain was originating from metabolites of [11C]pyrilamine. Based on the rat data, the contribution of 11C-labeled metabolites to total [11C]pyrilamine radioactivity in the human brain was estimated and found to be negligible. These results suggest that the metabolites of [11C]pyrilamine do not accumulate within the cerebral extravascular space and that there is minimal metabolism of [11C]pyrilamine by brain tissue itself. Therefore, [11C]pyrilamine metabolites can be neglected in kinetic analysis, using either a compartmental or a noncompartmental model, of the [11C]pyrilamine binding to histamine H1 receptors.

Radioligands for the study of the 5-HT transporter in vivo

Loss of 5-HT transporter (SERT) sites has been implicated in various neurodegenerative diseases and users of some amphetamine derivatives such as MDMA. Therefore, the development of suitable radioligands for neuroimaging of the SERT in the human brain is important. A large number of drugs have been labeled with 11C, 18F or (123)I over the last ten years in order to achieve such radioligands. Despite these attempts most of the compounds were found unsuitable because of low target-to-nontarget ratios. Some cocaine-derived radioligands allow SERT imaging of the human brain using positron emission tomography (PET) although they have a limited selectivity. Among the various specific 5-HT uptake inhibitors only [(123)I]iodo-nitroquipazine for single photon emission computed tomography (SPECT) and [11C](+)McN5652 for PET appear to meet the criteria of a useful radioligand. There is still a need for the development of new radioligands for SERT imaging. Advances in tracer synthetic methodologies may bring further progress in this field.

Positron emission tomographic evidence of toxic effect of MDMA ("Ecstasy") on brain serotonin neurons in human beings

BACKGROUND

(+/-)3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") is a popular recreational drug that selectively damages brain serotonin (5-HT) neurons in animals at doses that closely approach those used by humans. We investigated the status of brain 5-HT neurons in MDMA users.

METHODS

We enrolled 14 previous users of MDMA who were currently abstaining from use and 15 controls who had never used MDMA. We used positron emission tomography (PET) with the radioligand carbon-11-labelled McN-5652, which selectively labels the 5-HT transporter. We analysed whether there were differences in 5-HT transporter binding between abstinent MDMA users and participants in the control group. Blood and urine samples were taken and tested to check for abstinence.

FINDINGS

MDMA users showed decreased global and regional brain 5-HT transporter binding compared with controls. Decreases in 5-HT transporter binding positively correlated with the extent of previous MDMA use.

INTERPRETATION

Quantitative PET studies with a ligand selective for 5-HT transporters can be used to assess the status of 5-HT neurons in the living human brain. We show direct evidence of a decrease in a structural component of brain 5-HT neurons in human MDMA users.

Synthesis and evaluation of N-[11C]methylated analogues of epibatidine as tracers for positron emission tomographic studies of nicotinic acetylcholine receptors

Four halogen-substituted analogues of N-methylepibatidine, a nicotinic acetylcholine receptor (nAChR) ligand, were synthesized. They were (+/-)-exo-N-methyl-2-(2-halogeno-5-pyridyl)-7-azabicyclo[2. 2.1]heptanes, where halogeno = F (1a), Cl (2a), Br (3a), I (4a). (+/-)-N-Ethylepibatidine (2b) also was synthesized. The compounds 1a, 2a, 3a, and 4a and their corresponding normethyl analogues 1, 2, 3, and 4 inhibited the in vitro binding of [3H]epibatidine to nAChRs to a similar degree, with affinities in the 27-50 pM range. The binding affinity of N-ethylepibatidine (2b), however, was substantially lower. The N-[11C]methyl derivatives of 1, 2, and 3 were synthesized from high-specific radioactivity [11C]methyl iodide using a high-temperature/high-pressure technique. The corresponding radiolabeled compounds [11C]1a, [11C]2a, and [11C]3a were administrated to mice intravenously. The pattern of regional distribution of the three tracers in the mouse brain following intravenous administration matched those of [3H]epibatidine, [3H]norchloroepibatidine, and (+/-)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([18F]FPH), which are highly specific nAChR probes. The initial brain uptake of the 11C analogues and the acute toxicity of the corresponding authentic nonlabeled compounds appeared to be related to their lipophilicity.

2-[18F]Fluoro-A-85380, an in vivo tracer for the nicotinic acetylcholine receptors

The in vivo brain regional distribution of 2-[18F]fluoro-A-85380, a novel tracer for positron emission tomographic (PET) studies, followed the regional densities of brain nAChRs reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 2-fluoro-A-85380. A preliminary toxicology study of the 2-fluoro-A-85380 showed a relatively low biological effect. 2-[18F]Fluoro-A-85380 holds promise as a useful radiotracer for imaging of nAChRs with PET.

Nicotine induced up-regulation of nicotinic receptors in CD-1 mice demonstrated with an in vivo radiotracer: gender differences

Up-regulation of brain nicotinic receptors (nAChRs) by chronic nicotine treatment has chiefly been demonstrated by in vitro binding assays. Here, we report that up-regulation of nAChRs in CD-1 mice can be detected using a specific in vivo radioligand for nAChRs, [125I]IPH. After 10 days of (-)-nicotine administration, male and female mice demonstrated a significant elevation of [125I]IPH binding in all brain regions studied. [125I]IPH uptake also displayed significant gender differences with male animals showing a more pronounced increase in [125I]IPH accumulation.

Investigation of angiotensin II/AT1 receptors with carbon-11-L-159,884: a selective AT1 antagonist

Antagonists of the angiotensin II AT1 receptor subtype have been recently introduced for treatment of arterial hypertension and for pharmacological studies of these receptors. The purpose of this work was to label such an antagonist with 11C and test the applicability of the radioligand for PET studies.

METHODS

The potent and selective nonpeptide AT1 antagonist L-159,884 was labeled with 11C and injected intravenously into six dogs. Renal accumulation and kinetics of the radioligand were imaged with PET at baseline and after receptor blockade with 1 mg/kg MK-996. Time-activity curves were derived from the renal cortex and were analyzed by the Gjedde-Patlak plot to obtain the influx rate constant of the radioligand.

RESULTS

There was selective radioligand binding in the kidneys, mainly located in the cortex. Within the time interval between 95 and 115 min postinjection, the radioactivity retained in the kidneys was 109 +/- 27 and 42 +/- 4 nCi/ml/mCi of the injected dose for the control and inhibition studies, respectively. The influx rate constant of the radioligand decreased from a baseline of 0.0298 +/- 0.0156 to a post-MK-996 value of 0.0098 +/- 0.0052.

CONCLUSION

These results demonstrate distinct binding of 11C-L-159,884 in the renal cortex with a specific binding component suitable for quantitative PET imaging of angiotensin II/AT1 receptors.

Selective in vivo binding of [3H]naltriben to delta-opioid receptors in mouse brain

Naltriben (NTB) is a selective antagonist for the putative delta2-opioid receptor. We have determined the regional kinetics and pharmacological profile of [3H]naltriben in vivo in mouse brain. After i.v. administration to CD1 mice, [3H]naltriben uptake and retention were high in striatum, cortical regions and olfactory tubercles, and low in superior colliculi and cerebellum. Robust rank order correlation was found between [3H]naltriben uptake in discrete brain regions and prior delta-opioid receptor binding determinations in vitro and in vivo. [3H]Naltriben binding in vivo was saturable, and was blocked by the delta-opioid receptor antagonist naltrindole, but not by the mu-opioid receptor antagonist cyprodime or the K-opioid receptor agonist (trans)-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]ben zeneacetamide mesylate (U50,488H). (E)-7-Benzylidenenaltrexone (BNTX), a selective antagonist for the putative delta1-opioid receptor, was 9.6- to 12.9-fold less potent than naltriben as an inhibitor of [3H]naltriben binding. Thus, the sites labeled by [3H]naltriben in vivo may correspond to the delta2-opioid receptor subtype. Such assignment is not definitive, particularly considering the 4-fold higher brain uptake of naltriben as compared to (E)-7-benzylidenenaltrexone. Moreover, the regional distribution of [3H]naltriben in brains from CXB-7/BY (CXBK) mice, a strain that shows supraspinal delta1- but not delta2-opioid receptor agonist effects, was quite similar to that found for CD1 mice.

Autoradiographic and SPECT imaging of cerebral opioid receptors with an iodine-123 labeled analogue of diprenorphine

The feasibility of imaging cerebral opioid receptors by single photon emission computed tomography (SPECT) has been established in baboon using a novel analog of diprenorphine (DPN) radiolabeled with iodine-123. The radioligand, [123I]-O-IA-DPN (C6-O-[123I]iodoallyl-DPN), was prepared in good yield (80%) with high radiochemical purity (>97%) and high specific radioactivity (>2,400 mCi/micromol). In ex vivo autoradiographic studies, with and without naltrexone blockade, [123I]-O-IA-DPN specifically labeled opioid receptors throughout the mouse brain. Nonmetabolized radioligand accounted for >90% of the signal observed in extracts of whole mouse brain. SPECT imaging trials showed that [123I]-O-IA-DPN selectively localized in regions of baboon brain known to have high densities of opioid receptors, such as striatum, thalamus, and temporal cortex. A much lower level of radioligand uptake and retention was noted for cerebellum, a region with few opioid binding sites. Pretreatment with naltrexone (6.5 pmol/kg) blocked [123I]-O-IA-DPN binding in all brain regions. Using naltrexone blockade to define the nonspecific component for a given region of interest, total to nonspecific binding ratios increased linearly (r > or = 0.98) over the SPECT study with maximal values for striatum (9.8), thalamus (7.1), and temporal cortex (6.9) reached at the last time point investigated (3.5 h). Specific binding for these regions, assessed as the difference between regional SPECT activity for the control and blocked states, proved irreversible over the observation period. By the end of the time course, specific [123I]-O-IA-DPN binding was >85% of total radioactivity in regions rich in opioid receptors and 62% of total radioactivity in cerebellum. The aggregate data are consistent with visualization of multiple opioid receptor types. Thus, [123I]-O-IA-DPN should prove useful for SPECT studies within the constraints imposed by a lack of innate selectivity for a single type of brain opioid receptor.

In vivo detection of short- and long-term MDMA neurotoxicity--a positron emission tomography study in the living baboon brain

The present study evaluated short- and long-term effects of MDMA (3,4-methylenedioxymethamphetamine) in the baboon brain using PET and [11C](+)McN 5652, a potent 5-HT transporter ligand, as well as [11C]RTI-55, a cocaine derivative which labels both 5-HT and dopamine transporters. Following baseline PET scans with [11C](+)McN5652, [11C](-)McN5652 (the inactive enantiomer of the active enantiomer [11C](+)McN5652) and [11C]RTI-55, a baboon was treated with MDMA (5 mg/kg, s.c., twice daily for four consecutive days). PET studies at 13, 19, and 40 days post-MDMA revealed decreases in mean radioactivity levels in all brain regions when using [11C](+)McN 5652, but not with [11C](-)McN5652 or [11C]RTI-55. Reductions in specific [11C](+)McN5652 binding (calculated as the difference in radioactivity concentrations between (+) and (-)[11C]McN5652) ranged from 44% in the pons to 89% in the occipital cortex. PET studies at 9 and 13 months showed regional differences in the apparent recovery of 5-HT transporters, with increases in some brain regions (e.g., hypothalamus) and persistent decreases in others (e.g., neocortex). Data obtained from PET studies correlated well with regional 5-HT axonal marker concentrations in the CNS measured after sacrifice of the animal. The results of these studies indicate that PET imaging of the living nonhuman primate brain with [11C](+)McN5652 can detect changes in regional 5-HT transporter density secondary to MDMA-induced neurotoxicity. Using PET, it should also be feasible to use [11C](+)McN5652 to determine whether human MDMA users are also susceptible to MDMA's neurotoxic effects.

Regional distribution of 203PbCl2 in the mouse after intravenous injection

Lead is a known neurotoxicant, and concentrations of lead in the CNS after acute exposure to low doses have not been studied in detail. In this investigation, the temporal distribution of lead ([203Pb]), injected intravenously at no-carrier-added (NCA) (0.6-0.7 mumol/kg) and at carrier-added (7.0-62.3 mumol/kg) levels was determined in mice. Concentrations of [203Pb] were measured in major organs and in different regions of the brain. Ex-vivo autoradiographic visualization was used to confirm and extend the brain distribution findings.

RESULTS

The highest concentrations of NCA [203Pb] were observed initially in the kidneys (52% of the injected dose (ID)/g at 1 hr) and liver (10.5% ID/g at 30 min). Uptake into bone occurred gradually, reaching 25% ID/g at 24 hr. In accord with previous reports, excretion of the tracer was very slow. Approximately 80% of total ID remained in the body after 24 hr and 68% at 48 hr. Interestingly, in the mouse brain, the highest levels of [203Pb] were noted in the area of the hypothalamus. At all times between 30 min and 16 hr postinjection, and at all Pb dose levels injected, the accumulation of [203Pb] in the hypothalamic region exceeded that in all other brain regions examined. Autoradiography performed at the 16 hr time point confirmed the high uptake and strong retention of [203Pb] by the hypothalamus. These studies afford new insight into the distribution of acutely administered lead in the brain, and may have implications for the understanding of some of the neurotoxic effects of lead.

High-affinity no-carrier-added 99mTc-labeled chemotactic peptides for studies of inflammation in vivo

Nalpha-for-Nle-Leu-Phe-Nle-Tyr-Lys, a chemotactic peptide that binds with high affinity to the chemoattractant receptor on granulocytes and monocytes, was labeled with 99mTc using the diaminedithiol (DADT) chelating system to coordinate the Tc. 99mTc labeling of the DADT-coupled peptide was accomplished in 84% overall yield (room temperature for 10 min) using [99mTc]glucoheptonate as the donor of prereduced Tc. HPLC analysis showed two major 99mTc-labeled peptide peaks, 99mTc-DADT-Pep-I and 99mTc-DADT-Pep-II, were obtained in a ratio of 1:0.85. Using an iodoacetamide-derivatized gel to remove unlabeled peptide from the 99mTc labeling mixtures, essentially no-carrier-added (nca) high-specific activity 99mTc-labeled chemotactic peptides were obtained. The 99Tc analogues of the peptides were synthesized (72% yield) in a similar fashion and correlated with 99mTc complexes I and II by HPLC. In vitro competitive receptor binding assays of the isolated 99Tc analogues were performed against the tritiated chemotactic peptide [3H]N-for-Met-Leu-Phe ([3H]fMLF) using isolated granulocytes. The 99Tc-derivatized peptides showed similar binding affinities to the chemoattractant receptor as the unlabeled Nalpha-for-Nle-Leu-Phe-Nle-Tyr-Lys. The nca 99mTc-labeled peptides gave high contrast images of experimental inflammation in rabbits without causing neutropenia. Thus, it is feasible to attach the Tc-DADT chelate to low-molecular weight receptor binding chemotactic peptides and retain substantial binding to the receptor. Chemotactic peptides labeled with 99mTc via the DADT ligand system have the potential for imaging focal sites of inflammation without toxic effects, an important consideration in the successful utilization of chemotactic peptide agonists.

Design, synthesis, and initial evaluation of high-affinity technetium bombesin analogues

Potent antagonists of bombesin-like peptides have shown great potential for applications in cancer therapy. A 99mTc-labeled agent capable of identifying patients who could benefit from these emerging therapies would have a great impact on patient management. This study involves the synthesis and initial evaluation of technetium diaminedithiolate analogues derived from the potent bombesin analogue Pyr-Gln-Lys-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2 (Lys3-bombesin). We coupled two diaminedithiol (DADT) bifunctional chelating agents (BCAs 1 and 2) to the Lys3 residue at the N-terminal region that is not required for binding to the receptor. 99mTc labeling was performed by ligand exchange on addition of [99mTc]glucoheptonate to a solution of the adduct at room temperature. Two products were obtained from each adduct on analysis by HPLC. The major to minor product ratios of the 99mTc-labeled analogues were 3:1 for products from BCA 1 and 9:1 for the products from BCA 2. Macroscopic amounts of the 99Tc analogues were similarly prepared using [99Tc]glucoheptonate. In this case, the major to minor ratios were 2:1 for the products from both BCAs. For initial evaluation of the binding of the Tc-labeled peptides to bombesin receptors, the 99Tc analogues were used in vitro in competitive binding assays in rat brain cortex membranes against [125I-Tyr4]bombesin. Results of the in vitro assays showed that the inhibition constants (Ki) of the major and minor products were 3.5+/-0.7 and 3.9+/-1.5 nM, respectively, for the products from BCA 1; and 7.4+/-2.0 and 5.2+/-1.5 nM for the products derived from BCA 2, respectively. The high affinity exhibited by these technetium analogues is an indication of their potential for use in non-invasive in vivo biochemical characterization of cancers that possess receptors for bombesin.

Imaging nicotinic acetylcholine receptors with fluorine-18-FPH, an epibatidine analog

Nicotinic acetylcholine receptors (nAChRs) have been implicated in a variety of central processes, such as learning and memory and analgesia. These receptors also mediate the reinforcing properties of nicotine in tobacco products and are increased in postmortem samples of brains of smokers. On the other hand, brains of individuals who have died from dementia of the Alzheimer type show abnormally low densities of nAChRs. In this study, the distribution and kinetics of [(+/-)-exo-2-(2-[18F] fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane (18F-FPH), a high-affinity nAChR agonist, was evaluated in a baboon using PET.

METHODS

After intravenous injection of 5 mCi [185 MBq] 18F-FPH into a 25-kg anesthetized baboon, sequential quantitative tomographic data were acquired over a period of 150 min. Regions of interest were placed and time-activity curves were generated. Brain kinetics of the radiotracer were calculated, and the in vivo regional binding in the baboon brain was compared with the known in vitro regional distribution of nAChRs in the rat and human brain.

RESULTS

Brain activity reached a plateau within 60 min after injection of the tracer, and the binding was reversible. Elimination of 18F-FPH was relatively rapid from the cerebellum (clearance t[1/2] = 3 hr), intermediate from the hypothalamus/midbrain (t[1/2] = 7 hr) and slow from the thalamus (t[1/2] = 16 hr). Radioactivity due to 18F-FPH at 130 min postinjection was highest in the thalamus and hypothalamus/midbrain, intermediate in the neocortex and hippocampus and lowest in the cerebellum. Subcutaneous injection of 1 mg/kg cytisine 45 min after injection of the radiotracer reduced brain activity at 130 min by 67%, 64%, 56% and 52% of control values in the thalamus, hypothalamus/midbrain, hippocampus and cerebellum, respectively. The regional binding of 18F-FPH at 130 min was highly correlated with the known densities of nAChR measured in vitro in human (r = 0.81) and rat brain (r = 0.90).

CONCLUSION

These results demonstrate the feasibility of imaging nAChRs in vivo. Fluorine-18-FPH appears to be a suitable tracer to study nAChRs in the human brain.

A simple probe measures the pharmacokinetics of[125I]RTI-55 in mouse brain in vivo

A simple external radiation detector system was used to assess brain dopamine and serotonin transporters in mice in vivo using [125I]RTI-55. The results were compared to ex vivo dissection data. Methyl 3beta-(4-iodophenyl) tropane-2beta-carboxic acid methyl ester (RTI-55 or beta-CIT), a high-affinity cocaine antagonist, binds to presynaptic dopamine and serotonin transporters in the brain. Radiotracer accumulation increased for the first 150 min after intravenous injection and then remained constant. When unlabeled RTI-55 was injected, either before or 60 min after radiotracer administration, a significant decrease in tracer accumulation was observed. [125I]RTI-55 binding was also displaced by blocking doses of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine dihydrochloride (GBR 12909) and paroxetine. The results were similar to the ex vivo dissection data. The results demonstrate the feasibility of using the probe detector system to study the presynaptic transporter system in vivo in the mouse brain. The technique is applicable to other cerebral neurotransmitter systems.

[125/123I]IPH: a radioiodinated analog of epibatidine for in vivo studies of nicotinic acetylcholine receptors

Tomographic imaging of central nicotinic acetylcholine receptors (nAChRs) via single photon emission computed tomography (SPECT) has been hampered by the lack of a radioligand with suitable in vivo binding characteristics. Therefore, a novel analog of epibatidine, (+/-)-exo-2-(2-iodo-5-pyridyl)-7-azabicyclo[2.2.1]heptane (IPH), labeled with [(125)I] or [(123)I] was evaluated as an in vivo marker of central nicotinic acetylcholine receptors (nAChRs). [(125)I]IPH showed substantial brain penetration (4.2% of the injected dose at 30 min) and a cerebral biodistribution in mice consistent with the in vivo labeling of nAChRs (% injected dose/gram of thalamus, superior colliculi >> cerebellum). [(125)I]IPH binding sites were shown to be saturable with unlabeled IPH (ED50 approximately 1 microg/kg). The uptake of [(125)I]IPH was blocked significantly by the nicotinic agonists, cytisine, lobeline, and (-)-nicotine, but not by the noncompetitive nAChR antagonist, mecamylamine. Antagonists of muscarinic (scopolamine), serotonin (ketanserin), and opioid (naloxone) receptors had no significant effect on [(125)I]IPH binding. A preliminary SPECT imaging study with [(123)I]IPH in a baboon showed [(123)I]IPH to localize in nAChR-rich areas of brain (thalamus > frontal cortex > cerebellum). [(123)I]IPH binding in baboon brain was also displaced (35-45% displacement) by a challenge dose of cytisine showing that a well-characterized nicotinic agonist effectively competes for [(123)I]IPH binding sites. [(123)I]IPH seems well suited for imaging studies of nAChRs and, to our knowledge, is the first SPECT agent that has allowed for the visualization of nAChRs in primate brain.

Fluorine-18-FPH for PET imaging of nicotinic acetylcholine receptors

Visualization of central nicotinic acetylcholine receptors (nAChRs) with modern PET or SPECT imaging techniques has been hampered by the lack of a radioligand with suitable in vivo binding characteristics (i.e., high target-to-nontarget ratios and kinetics appropriate for the half-life of the tracer and imaging modality used). This paper describes in vivo binding, kinetics and pharmacology of a highly potent 18F-labeled analog of epibatidine, (+/-)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([18F]FPH), in the mouse brain with the view towards application of this tracer for PET imaging of nAChR in human brain.

METHODS

Fluorine-18-FPH was administered intravenously to mice, and time-activity curves were determined for several regions in the brain and other organs. Saturation and pharmacology of [18F]FPH binding was demonstrated in vivo by preinjecting unlabeled FPH or other drugs with known pharmacological action before [18F]FPH was injected. The effect of the drugs on [18F]FPH accumulation was evaluated.

RESULTS

[18F]FPH was rapidly incorporated into the mouse brain; peak activity (2.4% of the injected dose) was measured at 5 min after intravenous administration, followed by washout to 1.1% injected dose (ID) at 60 min. Highest concentrations of 18F occurred at 15 min in areas known to contain high densities of nAChR ¿e.g., thalamus [9.7% of injected dose per gram tissue (ID/g¿] and superior colliculus (8.3% ID/g)]. Accumulation of the 18F tracer in hippocampus, striatum, hypothalamus and cortical areas was intermediate (5.0, 5.6, 4.2 and 5.6% ID/g, respectively) and low in the cerebellum (2.8% ID/g). The distribution of [18F]FPH in the mouse brain matched that of other in vivo nAChR probes such as 3H-labeled epibatidine or norchloroepibatidine, [3H](-)-nicotine and [3H]cytisine and that of nAChR densities determined in postmortem autoradiographic studies in rodents. Preinjection of blocking doses of unlabeled epibatidine, (-)-nicotine, lobeline and cytisine significantly inhibited [18F]FPH binding in thalamus and superior colliculus, but not in cerebellum, whereas drugs that interact with binding sites other than acetylcholine recognition sites of nAChR (e.g., mecamylamine, scopolamine, N-methylspiperone and ketanserin) had no effect on [18F]FPH accumulation in any of the brain regions examined.

CONCLUSION

Fluorine-18-FPH labels nAChR in vivo in the mouse brain. Because of its high uptake into the brain and high ratios of specific-to-nonspecific binding, this radioligand appears to be ideally suited for PET imaging of nAChR in the mammalian brain.

N-substituted phenyltropanes as in vivo binding ligands for rapid imaging studies of the dopamine transporter

Variously substituted phenyltropanes are proven as superb binding ligands for the dopamine transporter (DAT). In this study, we examine four N-substituted phenyltropanes which are derivatives of RTI-55 as in vivo binding ligands in mice. In this series, the methyl group on the nitrogen was replaced by a propyl (RTI-310), an allyl (RTI-311), a butyl (RTI-312), or a fluoropropyl (RTI-313) group. The in vitro binding potencies of these compounds at rat striatal DAT varied somewhat but were about 1 nM. While these compounds did not display marked selectivity for the dopamine transporter, they were more selective than RTI-55. Injection of the radiolabeled compound into mice resulted in striatal-to-cerebellar ratios that varied from about 4.5-6.5. The ratios peaked most rapidly for RTI-311 and RTI-313, at about 20 min. Pharmacological inhibition studies indicated that these compounds were binding to DATs in the striatum, as expected. These findings suggest that some compounds of this type may be excellent in vivo binding ligands for rapid imaging studies of the DAT.

Fenfluramine-induced loss of serotonin transporters in baboon brain visualized with PET

The present study sought to determine whether or not Positron Emission Tomography (PET) with the newly developed positron emitting serotonin (5-HT) transporter ligand, (+)[11C]McN-5652, could be used to detect fenfluramine-induced 5-HT neurotoxicity in the brain of living primates (baboons). Six PET imaging studies were performed: three before treatment with fenfluramine (5 mg/kg, s.c., twice daily for 4 days) and three after (18, 45, and 81 days after treatment). The dose of fenfluramine used in this study (5 mg/kg) is known to produce 5-HT neurotoxicity in primates, and to be approximately two times higher than a dose of fenfluramine reported to produce small and inconsistent weight loss in baboons (2 mg/kg). Following fenfluramine treatment, marked lasting reductions in regional brain specific binding of (+)[11C]McN-5652 were found by means of PET. Findings with PET corresponded well with post-mortem neurochemical findings indicative of serotonergic neurotoxicity (lasting depletions of regional brain 5-HT, 5-HIAA, and 5-HT uptake sites). These results suggest that PET imaging with (+)[11C]McN-5652 will be useful for evaluating the 5-HT neurotoxic potential of fenfluramine and related drugs in living humans.

Imaging of delta opioid receptors in human brain by N1'-([11C]methyl)naltrindole and PET

Recently, we have developed the positron emitting radiotracer N1'-([11C]methyl)naltrindole ([11C]MeNTI) and demonstrated its high selectivity for delta opioid receptors in the mouse brain [Lever et al. (1992) Eur. J. Pharmacol., 216:449-450]. In the present study, we examined the selectivity of [11C]MeNTI for the delta opioid receptor in the human brain, using positron emission tomography (PET). The regional kinetics and distribution as well as the pharmacology confirmed the selectivity of [11C]MeNTI for delta opioid receptor in the human brain. First, the regional kinetics of [11C]MeNTI are in accordance with the density of the delta opioid receptor. Rapid washout in receptor-poor areas and prolonged retention in receptor-rich areas were observed. Second, the regional distribution of [11C]MeNTI correlated well (r = 0.91) with the in vitro distribution of delta opioid sites but not with mu or kappa site densities (r < or = 0.008 or r < or = 0.014, respectively). [11C]MeNTI binding was highest in regions of the neocortex (insular, parietal, frontal, cingulate, and occipital), caudate nucleus, and putamen. Binding was intermediate in the amygdala and lowest in the cerebellum and thalamus. Third, studies using the competitive antagonist naltrexone demonstrated the inhibition of [11C]MeNTI binding. Naltrexone inhibition of [11C]MeNTI binding was most effective in delta receptor-rich regions, and its inhibitory potency correlated well (r = 0.88) with the regional distribution of delta opioid sites. [11C]MeNTI is the first radioligand which selectively labels delta opioid receptors in vivo in the human brain following systemic administration. The availability of [11C]MeNTI will enable a receptor specific analysis of the role of [11C]MeNTI receptors in normal and abnormal human brain.

Effect of dopaminergic drugs on the in vivo binding of [3H]WIN 35,428 to central dopamine transporters

[11C]WIN 35,428 (also designated [11C]CFT) is now being used in several positron emission tomography (PET) centers to image dopamine (DA) transporter sites in the mammalian brain. Whether and to what extent in vivo WIN 35,428 binding is influenced by intra- and extrasynaptic dopamine levels are largely unknown. The purpose of the present study was to evaluate the effects of various drugs, known to affect DA levels and release, on the binding of [3H]WIN 35,428 to central DA transporters in the mouse brain. D-Amphetamine, which releases DA from neurons and blocks the DA transporter directly, inhibited striatal [3H]WIN 35,428 binding in dose-dependent manner. Similarly, alpha-methyl-DL-p-tyrosine, an inhibitor of tyrosine hydroxylase, blocked [3H]WIN 35,428 binding, possibly via competitive inhibition by the metabolite p-hydroxyamphetamine. Specific binding of [3H]WIN 35,428 was insensitive to changes in synaptic DA levels caused by pretreatment of the animals with high doses of D2 receptor agonists (apomorphine, bromocriptine), antagonists (haloperidol) or the inhibitor of dopaminergic neuron firing gamma-butyrolactone (GBL). High doses (> 50 mg/kg) of L-DOPA (in combination with benserazide), however, reduced [3H]WIN 35,428 binding significantly, yet for a relatively short time (approximately 2.5 h). Chronic treatment with L-deprenyl elicited no changes in in vivo [3H]WIN 35,428 accumulation in the striatum. Neurotoxic damage of DA neurons caused by administration of high doses of amphetamine was detected in the striatum by a significant reduction in [3H]WIN 35,428 binding 7 days after cessation of amphetamine treatment. Thus, [3H]WIN 35,428 binding was only affected by neurotoxic loss of neurons, by administration of uptake inhibitors, or by some treatments which significantly elevate DA levels. Compounds which inhibit DA release or deplete DA acutely do not increase [3H]WIN 35,428 binding, suggesting that normal or "resting" levels of DA are not sufficient to alter [3H]WIN 35,428 binding in vivo. These findings are important for our understanding of the function and regulation of the DA transporter, as well as the in vivo binding of the radioligand [3H/11 C]WIN 35,428. Moreover, they will be important for the interpretation of PET studies in which [11C]WIN 35,428 is used to assess the integrity of dopaminergic neurons.

In vivo labeling of delta opioid receptors in mouse brain by [3H]benzylidenenaltrexone, a ligand selective for the delta 1 subtype

(E)-7-Benzylidenenaltrexone (BNTX) is a selective ligand for the putative delta 1 (delta 1) opioid receptor. To explore the feasibility of labeling delta 1 sites in vivo; we determined the cerebral distribution of radioactivity after systemic administration of [3H]BNTX to CD1 mice. Uptake was highest in striatum and lowest in cerebellum throughout the 4 hr time course. Specific radioligand binding, approximated as the difference in radioactivity concentrations between striatum and cerebellum, peaked at 0.32 percent injected dose/g at 30 min and comprised a modest 23% of total striatal radioactivity. For seven brain regions, radioactivity concentrations correlated with delta site densities known from prior in vitro studies (rS = 0.79, p = 0.03), and also with the uptake of N1'-([11C]methyl)naltrindole in vivo (rS = 0.78, p = 0.04) in mice. Specific binding in striatum, olfactory tubercles and cortical regions was saturable by BNTX, and was inhibited stereoselectively by the optical isomers of naloxone. Naltrindole and naltriben (NTB), delta antagonists, blocked 65-99% of [3H]BNTX specific binding at a dosage of 5.0 mumol/kg. Similar doses of the mu antagonist cyprodime, or the kappa agonist U50,488H, did not inhibit binding. Adjusted for the four-fold greater brain penetration of NTB relative to BNTX, dose-response studies suggested that delta 1 selective BNTX (ED50 = 1.51 mumol/kg) was 50% more potent than delta 2 selective NTB (ED50 = 0.56 mumol/kg) in blocking specific [3H]BNTX binding in striatum. In CXBK mice, a strain with functional delta 1 but not delta 2 receptors in antinociceptive assays, radioligand uptake and distribution proved similar to that in CD1 mice. In sum, [3H]BNTX labels murine delta opioid receptors in vivo with a low extent of specific binding. The data is consistent with, but not conclusive for, selective labeling of the delta 1 subtype.

Synthesis and in vivo studies of a selective ligand for the dopamine transporter: 3 beta-(4-[125I]iodophenyl) tropan-2 beta-carboxylic acid isopropyl ester ([125I]RTI-121)

A selective ligand for the dopamine transporter 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid isopropyl ester (RTI-121) has been labeled with iodine-125 by electrophilic radioiododestannylation. The [125I]RTI-121 was obtained in good yield (86 +/- 7%, n = 3) with high radiochemical purity (> 99%) and specific radioactivity (1210-1950 mCi/mumol). After i.v. administration of [125I]RTI-121 to mice, the rank order of regional brain tissue radioactivity (striatum > olfactory tubercles > > cortex, hippocampus, thalamus, hypothalamus, cerebellum) was consistent with dopamine transporter labeling. Specific in vivo binding in striatum and olfactory tubercles was saturable, and was blocked by the dopamine transporter ligands GBR 12,909 and (+/-)-nomifensine. By contrast, binding was not reduced by paroxetine, a serotonin transporter inhibitor, or desipramine, a norepinephrine transporter inhibitor. A variety of additional drugs having high affinities for recognition sites other than the neuronal dopamine transporter also had no effect. The [125I]RTI-121 binding in striatum and olfactory tubercles was inhibited by d-amphetamine in dose-dependent fashion. Nonmetabolized radioligand represents 85% of the signal observed in extracts of whole mouse brain. Thus, [125I]RTI-121 is readily prepared, and is a useful tracer for dopamine transporter studies in vivo.

In vivo labeling of angiotensin II receptors with a carbon-11-labeled selective nonpeptide antagonist

Angiotensin II (ANG II) initiates a variety of physiological effects by binding to high affinity receptors. Two ANG II receptor subtypes, AT1 and AT2, have recently been identified. This study was undertaken to evaluate [11C]L-159,884, an AT1 subtype selective nonpeptide antagonist, as a potential PET tracer.

METHODS

Carbon-11-L-159,884 was prepared by alkylation of the nor precursor with [11C]methyliodide and was studied for its in vivo binding characteristics, biodistribution and kinetics in mice. The effects of PD-123319, an AT2-selective ANGII antagonist, as well as those of alpha- and beta-adrenergic drugs on [11C]L-159,884 binding were investigated also.

RESULTS

Administration of the AT1 antagonists resulted in dose-dependent inhibition of [11C]L-159,884 binding in the kidneys, the organ with the highest density of AT1 receptors. Inhibition was also observed in the lungs and the heart. Adrenergic drugs did not influence [11C]L-159,884 binding to AT1 receptors. Kinetic studies showed rapid tracer uptake in the liver, kidneys, lungs and heart. Excretion of the radioactivity occurred primarily through the intestinal tract (> 20% in 90 min), with less than 8% excreted through the urine.

CONCLUSION

The results suggest that [11C]L-159,884 binds in vivo to AT1 receptors in mouse kidneys, lungs and heart. This radiotracer appears to be a promising candidate for studying ANG II receptors in vivo by PET.

In vivo labeling of neuronal nicotinic acetylcholine receptors with radiolabeled isomers of norchloroepibatidine

In vitro binding studies have shown that epibatidine and its norchloro analogues have high affinities for the cholinergic nicotinic receptor. In this study, the in vivo binding characteristics of [3H](+)norchloroepibatidine (NCEPB) and [3H](-)NCEPB in mice were examined. After injection of [3H](+)NCEPB, radioactivity levels in all brain regions examined increased and then decreased, with different regions accumulating different levels of radioactivity. The regional distribution of radioactivity at later times paralleled the distribution of nicotinic receptor binding in vitro. The ratio of radioactivity in the superior colliculus to that in the cerebellum, a reflection or estimate of total: non-specific binding, was strikingly high at 8 h (about 35). Drugs known to bind to nicotinic receptors reduced [3H](+)NCEPB binding, while atropine and spiperone, muscarinic and dopaminergic drugs respectively, did not. [3H](-)NCEPB had a similar regional distribution to [3H](+)NCEPB. Pretreatment with increasing doses of non-radioactive (-)NCEPB resulted in saturation of in vivo binding of [3H](-)NCEPB. These data indicate that [3H](+) and (-)NCEPB are useful in vivo binding ligands for the cholinergic nicotinic receptor.

Positron emission tomography of 5-HT reuptake sites in the human brain with C-11 McN5652 extraction of characteristic images by artificial neural network analysis

The goal of this work was to identify the distribution of serotonin transporters in the human brain with [11C](+)McN5652/PET. Four healthy volunteers were studied. To determine non-specific binding, a PET study was also performed with the inactive enantiomer [11C](-)McN5652 as well as with [11C](+)McN5652 after pretreatment with fluoxetine. For pattern extraction the PET data sets were analyzed by a back-propagation neural network. Two pharmacokinetic patterns and two characteristic images were separated; one representing specific binding, the other representing non-specific binding. The specific binding image showed characteristic distribution of serotonin transporters with [11C](+)McN5652. The pattern images demonstrated an improvement in image quality compared to the original PET images (reduced variance, higher region-to-cerebellum ratio, good correlation with known density of serotonin transporters). The non-specific binding images extracted from [11C](-)McN5652/PET were similar to those of [11C](-)McN5652 and [11C](-)McN5652 with fluoxetine. Thus, PET studies obtained with [11C](+)McN5652 largely represent the regional distribution of the serotonin transporters and the inactive enantiomer [11C](-)McN5652 shows the distribution of its nonspecific binding.

Sympathetic tone assessed by washout of iodine 125-labeled metaiodobenzylguanidine from the murine left ventricle--influence of immobilization stress and inhibition of the renin-angiotensin system

BACKGROUND

Because it is not metabolized as is norepinephrine (NE), most of the metaiodobenzylguanidine (MIBG) taken up by the heart is considered to be lost subsequently by release concomitant with sympathetic stimulation. Therefore we examined whether the washout of MIBG is influenced by sympathetic tone, which we modulated by using immobilization stress or activation of the renin-angiotensin system (RAS).

METHODS AND RESULTS

In 175 male ddY mice, left ventricular radioactivity was counted 30 minutes or 4 hours after injection of 74 kBq (2 microCi) of iodine 125- or iodine 131-labeled MIBG (125I- or 131I-MIBG). The washout rates of MIBG were determined under immobilization stress or under sodium loading or restriction in combination with losartan (10 mg/kg) or cilazapril (1 mg/kg) pretreatment. Immobilization enhanced the washout of 125I-MIBG (80.9% vs 57.9% in the control animals); this was determined to be related to washout from the neuronal compartment, because the nonneuronal component assessed through desipramine pretreatment was not affected. Pretreatment with losartan or cilazapril decreased the facilitation of 125I-MIBG washout in sodium-restricted mice (40.9% and 33.7%, respectively, vs 43.5% in the control animals), but not in sodium-loaded mice.

CONCLUSIONS

Measurement of MIBG washout may be feasible for determining the changes in sympathetic tone caused by immobilization stress or activation of the RAS.

Positron emission tomography of 5-HT transporter sites in the baboon brain with [11C]McN5652

[11C]McN5652 is a new radioligand specific for 5-hydroxytryptamine (5-HT; serotonin) transporters. In this study we used [11C]McN5652 to image the 5-HT transporter sites in baboon brain by positron emission tomography (PET). Dynamic PET studies were performed in three Papio anubis baboons. The animals were injected intravenously first with 11C-labeled (+)-McN5652([11C](+)McN5652), then with pharmacologically inactive enantiomer 11C-labeled (-)-McN5652 ([11C](-)McN5652); two animals received a third study with [11C](+)McN5652 after pretreatment with the specific 5-HT uptake site inhibitor fluoxetine (5 mg/kg). Initial uptake into the brain was similar for both [11C](+)McN5652 and [11C](-)McN5652. At later times (45-120 min after injection), only [11C](+)McN5652 showed a distribution characteristic for 5-HT uptake sites. In contrast, in studies with [11C](-)McN5652 and in those with [11C](+)McN5652 after 5-HT uptake site blockade with fluoxetine, 11C radioactivity concentrations were significantly lower and the distribution pattern was relatively even. The differences between [11C](+)-and (-)McN5652 were calculated for the time interval 95-125 min postinjection and used to estimate specific binding. Specific binding correlated well (r = 0.95, p < 0.001) with the known density of 5-HT uptake sites in human brain. These results indicate that [11C](+)McN5652 is suitable for PET imaging of 5-HT uptake sites in primate brain.

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)

In vivo labeling of nicotinic acetylcholine receptors in brain with [3H]epibatidine

In vivo imaging of nicotinic acetylcholine receptors in brain has been hampered by lack of an adequate radioligand. In the present study, [3H]epibatidine was administered to mice intravenously, and its time-course in brain regions and sensitivity to blockade by nicotinic drugs were studied. The distribution of radioligand accumulation in brain, and the pharmacological characteristics of binding indicate that radiolabeled forms of epibatidine would be exceptionally promising ligands for the study of nicotinic acetylcholine receptors in vivo.

Positron emission tomography imaging of serotonin transporters in the human brain using [11C](+)McN5652

This paper presents the first Positron Emission Tomography (PET) images of the serotonin (5-hydroxytryptamine, 5-HT) transporter in the living human brain. PET imaging was performed in three healthy subjects after administration of [11C](+)McN5652 (the (+) enantiomer of trans-1,2,3,5,6,10 beta-hexahydro- 6-[4-(methylthio) phenyl]pyrrolo-[2,1-a] -isoquinolone), a radioligand previously shown to selectively label the 5-HT transporter in vivo in the mammalian (mouse and baboon) brain. To demonstrate the specificity of [11C](+)McN5652 binding, additional images were obtained in the same subjects after injection of [11C](-)McN5652, the pharmacologically inactive enantiomer, and, in two of the subjects, with [11C](+)McN5652 after pretreatment with the 5-HT uptake site blocker fluoxetine. Highest accumulation of [11C](+)McN5652 was observed in the midbrain, putamen, caudate nucleus, hypothalamus, and thalamus, regions known to contain high densities of 5-HT transporters. In these areas [11C](+)McN5652 concentrations rose steadily over 120 min. In contrast, with [11C](-)McN5652 and when the [11C](+)McN5652 binding was inhibited with fluoxetine, radioactivity concentrations declined after reaching a maximum (at 20 to 30 min). Inhibition studies with fluoxetine suggest that only with [11C](+)McN5652, there is specific binding. In the cerebellum, a region relatively void of 5-HT transporters, both [11C](+)McN5652 with and without fluoxetine block and [11C](-)McN5652 were released at approximately the same rate. The results of the studies indicate that [11C](+)McN5652 labels 5-HT transporter sites in the human brain. Quantitative PET imaging studies with this new tracer should provide valuable information on the status of these sites in health and disease.

In vivo labeling of sigma receptors in mouse brain with [3H]4-phenyl-1-(4-phenylbutyl)piperidine

4-Phenyl-1-(4-phenylbutyl)piperidine (4-PPBP) is a very potent ligand for sigma (Sigma) receptors. The present study was undertaken to evaluate [3H]4-PPBP as a radioligand for in vivo labeling of cerebral sigma receptors. After intravenous administration of [3H]4-PPBP to mice, there is high uptake of radioactivity in the brain. The regional distribution of radioactivity in the brain 2 h after intravenous injection of [3H]4-PPBP parallels the in vitro binding of the radioligand in rat brain (pons/medulla > cerebellum > or = prefrontal cortex > or = parietal cortex > hypothalamus > olfactory tubercle > or = thalamus > hippocampus > striatum). Pretreatment with haloperidol (2 mg/kg) significantly decreases the radioactivity measured in the brain 30-120 min after injection of [3H]4-PPBP. Pretreatment with unlabeled 4-PPBP or ifenprodil also significantly decreases radioactivity in the brain 2 h after injection of [3H]4-PPBP, in a dose-dependent manner. The in vivo binding of [3H]4-PPBP in the brain also is significantly inhibited by SL 82.0715, BMY 14802, 1,3-di-o-tolylguanidine (DTG), and (+)-enantiomers of pentazocine, SKF 10,047, and 3-PPP, but not by the corresponding (-)-enantiomers, consistent with stereoselectivity of inhibition obtained in in vitro binding studies. In contrast, pretreatment with dizocilpine and spiperone does not inhibit in vivo binding of [3H]4-PPBP. The results indicate that [3H]4-PPBP would be a suitable radioligand for in vivo labeling of sigma receptors in brain.

An improved method for the synthesis of radiolabeled McN5652 via thioester precursors

An improved procedure that facilitates routine production and increases the radiochemical yield of [11C]McN5652 (trans-1,2,3,5,6,10b-hexahydro-6-[4-([11C]methylthio)-phenyl]pyrrolo- [2,1-alpha]-isoquinoline) is presented. Specifically, thiol acetate, butyrate, and benzoate derivatives of McN5652 were prepared as the precursors for the [11C]McN5652 synthesis. These thioesters offer greater stability than the previously used thiol precursor (desmethyl McN5652) and enable a single batch of material to be used for multiple radiolabelings. Hydrolysis of the thioester functionality (tetrabutylammonium hydroxide, 10 min) unmasked the free thiol which, without purification, was reacted with [11C]iodomethane in DMF at 40-45 degrees C for 1 min. The average decay-corrected radiochemical yield for [11C]McN5652 was 26% with an average specific activity of 2290 mCi/mumol (end of synthesis). This facile radiolabeling method, utilizing the butyrate thioester of McN5652, was also employed in the preparation of [3H](+)- and (-)McN5652 [trans-1,2,3,5,6S (or 6R),10bR, (or 10bS)-hexahydro-6-[4-([3H]methylthio)phenyl]pyrrolo-[2,1,alpha]- isoquinoline] from [3H]iodomethane.

[11C]A-69024: a potent and selective non-benzazepine radiotracer for in vivo studies of dopamine D1 receptors

[11C]A-69024, (+/-)-1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-[11C]methyl- 1,2,3,4-tetrahydroisoquinoline, is a specific and selective dopamine D1 radiotracer. The in vivo biodistribution of this novel radioligand in mice showed a high uptake in the striatum (6.7% ID/g) at 5 min, followed by clearance with a half-life of 16.1 min. As a measure of specificity, the striatal/cerebellar ratio reached a maximum of 7.4 at 30 min post-injection. Radioactivity in the striatum was reduced to the level of the cerebellum by pre-administration of the D1 antagonist SCH 23390 (1 mg/kg). Pretreatment of mice with spiperone (D2), 7-hydroxydipropylaminotetralin (7-OH-DPAT) (D3), clozapine (D4), ketanserin (5-HT2/5-HT2C), mazindol (monoamine reuptake), prazosin (alpha 1), and haloperidol (D2/sigma) had no inhibitory effect on [11C]A-69024 uptake in the striatum. The dextrorotatory enantiomer of the dopamine antagonist butaclamol inhibited striatal uptake, while the less active isomer (-)-butaclamol did not. [11C]A-69024 binding was inhibited by unlabeled A-69024 in a dose dependent manner (ED50 = 0.3 mg/kg) in the striatum while no change occurred in the cerebellum. [11C]A-69024 warrants further investigation as a PET ligand for examination of central dopamine D1 receptors in humans.

Transient increase in the in vivo binding of the benzodiazepine antagonist [3H]flumazenil in deafferented visual areas of the adult mouse brain

Flumazenil is an imidazobenzodiazepine, an antagonist of central benzodiazepine (BDZ) receptors. BDZ binding sites are a modulatory component located on the gamma-aminobutyric acid (GABA) receptor macromolecule. We studied the effect of monocular enucleation on [3H]flumazenil binding in deprived and intact visual areas and nonvisual areas of the adult mouse brain under in vivo conditions. [3H]flumazenil binding was examined at seven time points up to 56 days postenucleation. In some monocularly deprived mice, changes in local blood flow accompanied with the BDZ receptor response were evaluated by coinjection of [3H]flumazenil and 99mTc-HMPAO. Monocular enucleation produced a transient increase in [3H]flumazenil binding in the deprived visual cortex and superior colliculus. At 17 days postenucleation, [3H]flumazenil binding in the anterior and posterior portions of the visual cortex and the superior colliculus increased by 28%, 15% and 23%, respectively, and declined to control levels at 45 days postenucleation. The increase in [3H]flumazenil was accompanied with a decrease in blood flow. Alterations in BDZ receptors and blood flow were selective to deprived visual structures. The regional correlation between the metabolic deficit and the BDZ response provides further support that the increase in BDZ receptor binding is confined to regions of reduced neuronal activity. [11C]flumazenil is an excellent radiotracer for in vivo imaging of benzodiazepine receptors in human brain using positron emission tomography (PET). This study suggests the suitability of [11C]flumazenil for in vivo PET study of BDZ receptor response to deafferentation of visual structures in human brain.

Myocardial perfusion with [11C]methyl triphenyl phosphonium: measurements of the extraction fraction and myocardial uptake

The present study describes extraction fraction and uptake measurements of the [11C]methyl triphenyl phosphonium (11C-MTP), a promising positron emission tomography (PET) agent for cardiac imaging. PET imaging was performed in mongrel dogs. Under physiological flow conditions 11C-MTP uptake reached a maximum within the first 10 minutes after injection and remained constant during the entire observation period of 80 minutes. Over the same time period, the heart/blood ratio was 46-106:1, and the heart/lung ratio 14:1. Following permanent occlusion of the left anterior descending coronary artery, 11C-MTP uptake in the normally perfused myocardium also reached a maximum at 10 minutes after injection, whereas in the infarcted area there was no significant accumulation of 11C-MTP. For a time period of 80 minutes the noninfarcted/infarcted myocardium ratio was 12:1. Extraction was measured in anesthetized dogs with a double isotope method using 99mTc-HSA as the reference tracer. The extraction fraction was 91% at a flow of 69 mL/min/100g. As flow increased to five-fold (342 mL/min/100g) following administration of adenosine, extraction fell to 61%. Following coronary artery occlusion, the 11C-MTP content in the myocardium was highly correlated (r = 0.93, p < 0.01; y = 10.46 + 0.92x) with the microsphere determined regional myocardial blood flow.