Production of 3-N-[11C]methylspiperone with high specific activity and high radiochemical purity for PET studies: suppression of its radiolysis

Special radionuclide production activities – recent developments at QST and throughout Japan

National Institutes for Quantum Science and Technology (QST), formerly known as the National Institute of Radiological Sciences (NIRS), has been engaged in work on radiopharmaceutical science using cyclotrons since 1974. Eight pioneering researchers founded the basis of this field of research at NIRS, and to the present, many researchers and technicians have accumulated both scientific and technical achievements, as well as inherited the spirit of research. Besides, in recent years, we have developed production systems with AVF-930 cyclotron for various ‘non-standard’ radioisotopes applied in both diagnosis and therapy. Here, we review the past 50 years of our activities on radioisotope and radiopharmaceutical development, as well as more recent activities.

Design, Synthesis, Radiosynthesis and Biological Evaluation of Fenretinide Analogues as Anticancer and Metabolic Syndrome-Preventive Agents

Fenretinide (4-HPR) is a synthetic derivative of all-trans-retinoic acid (ATRA) characterised by improved therapeutic properties and toxicological profile relative to ATRA. 4-HPR has been mostly investigated as an anti-cancer agent, but recent studies showed its promising therapeutic potential for preventing metabolic syndrome. Several biological targets are involved in 4-HPR's activity, leading to the potential use of this molecule for treating different pathologies. However, although 4-HPR displays quite well-understood multitarget promiscuity with regards to pharmacology, interpreting its precise physiological role remains challenging. In addition, despite promising results in vitro, the clinical efficacy of 4-HPR as a chemotherapeutic agent has not been satisfactory so far. Herein, we describe the preparation of a library of 4-HPR analogues, followed by the biological evaluation of their anti-cancer and anti-obesity/diabetic properties. The click-type analogue 3 b showed good capacity to reduce the amount of lipid accumulation in 3T3-L1 adipocytes during differentiation. Furthermore, it showed an IC₅₀ of 0.53±0.8 μM in cell viability tests on breast cancer cell line MCF-7, together with a good selectivity (SI=121) over noncancerous HEK293 cells. Thus, 3 b was selected as a potential PET tracer to study retinoids in vivo, and the radiosynthesis of [¹⁸ F]3b was successfully developed. Unfortunately, the stability of [¹⁸ F]3b turned out to be insufficient to pursue imaging studies.

In Vivo PET Measurements with [11C]PE2I to Evaluate Fetal Mesencephalic Transplantations to Unilateral 6-OHDA-Lesioned Rats

Positron emission tomography (PET) is a useful tool to assess and visualize neurotransmissions in vivo. In this study, we performed repeated PET scans with [11C]PE2I, a tracer of the dopamine transporter, to evaluate the alteration of the expression of dopamine (DA) transmission component after a fetal mesencephalic transplantation. The fetal mesencephalic cells were transplanted into the striatum of unilateral 6-OHDA-lesioned rats. PET scans with [11C]PE2I were performed to evaluate the DA transporter before and 2 and 4 weeks after the transplantation. Rotation behavior tests, in vitro autoradiography, measurements of DA contents in the striatum by high-performance liquid chromatography (HPLC), and tyrosine hydroxylase (TH) immuno-histological examinations were performed at the same time points and examined for their relationship to changes in the dopamine transporter. The number of ipsilateral rotations induced by methamphetamine injections decreased. DA contents in the striatum measured with HPLC significantly increased. In the PET study, the binding potential of [11C]PE2I increased at 4 weeks. The results of the in vitro autoradiography study corresponded with those of the PET study. The degrees of the change in the binding potentials correlated with those of the numbers of rotations in the behavioral study and the DA contents in the striatum. In the histological examination, TH-positive cells with axons were observed at 2 and 4 weeks after the transplantation. As the dopamine transporter exists only in the axon terminal of DA neurons, these results suggested that PET measurements of [11C]PE2I binding indicated not only survival, but maturity and functioning of the transplanted cells. Repeated PET measurements of DA transporters are a useful tool in assessing the effectiveness of neural transplantations.

Dopamine receptor mapping with PET imaging in Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disorder characterised pathologically by the loss of dopaminergic neurons in the substantia nigra pars compacta. These neurons project to the striatum, and their loss leads to alterations in the activity of the neural circuits that regulate movement. The striatal output of the circuit related to the control of movement is mediated by two pathways: the direct striatal pathway, which is mediated through facilitation of D1 receptors, and the indirect striatal pathway, mediated through D2 receptors. Positron emission tomography (PET) molecular imaging is a powerful in vivo technique in which using selective dopaminergic radioligands has been employed to investigate the dopaminergic system in humans. In this article we aim to review the role of PET imaging in understanding the postsynaptic dopaminergic mechanisms in PD. PET studies have allowed us to gain important insights into the functions of the dopaminergic system, the mechanisms of drug-induced motor and non-motor complications, and the placebo effect in PD.

Radioligands for the dopamine receptor subtypes

The actions of the predominant neurotransmitter in the brain, dopamine, are mediated by the postsynaptic dopamine receptors. The five dopamine receptor subtypes and their regulation have been associated with a large variety of psychiatric diseases. Therefore, positron emission tomography (PET) imaging studies using suitable and selective (18) F-labeled and (11) C-labeled dopamine receptor radioligands could provide valuable knowledge on the impact of receptor density on the pathogenesis and evolvement of neuropsychiatric and neurological diseases. This special issue subchapter provides a summary of the most important (18) F-labeled and (11) C-labeled radioligands for PET imaging of the dopamine receptor subtypes, their radiochemistry, and characteristics from in vitro and in vivo applications, considering not only the already established PET ligands but also the recently published preclinical work.

Use of positron emission tomography for real-time imaging of biodistribution of green tea catechin

The aim of this study was to achieve real-time imaging of the in vivo behavior of a green tea polyphenol, catechin, by positron emission tomography (PET). Positron-labeled 4″ -[(11)C]methyl-epigallocatechin gallate ([(11)C]Me-EGCG) was orally administered to rats, and its biodistribution was imaged for 60 min by using a small animal PET system. As the result, images of [(11)C]Me-EGCG passing through the stomach into the small intestines were observed; and a portion of it was quantitatively detected in the liver. On the other hand, intravenous injection of [(11)C]Me-EGCG resulted in a temporal accumulation of the labeled catechin in the liver, after which almost all of it was transferred to the small intestines within 60 min. In the present study, we succeeded in obtaining real-time imaging of the absorption and biodistribution of [(11)C]Me-EGCG with a PET system.

Fully automated production of iodine-124 using a vertical beam

A fully automated system for the production of iodine-124, based on techniques of vertical-beam irradiation and dry distillation, was developed. The system, coupled with a capsulated target, was able to irradiate the (124)TeO(2) target up to 29 μA for 1-4h, which yielded iodine-124 with an almost constant yield of 6.9 MBq/μAh at the end of bombardment. All procedures were performed automatically and repeatedly. The newly developed system would be suitable for routine, large-scale productions of iodine-124.

Radiosynthesis of (N-[11C]methyl)benperidol for PET investigation of D2 receptor binding

A procedure for the reliable production of the novel PET D2 receptor ligand (N-[11C]methyl)benperidol ([11C]NMB) is described. The method is based on N-methylation of benperidol using [11C]methyl iodide from the PETtrace MeI MicroLab, with product purification via normal-phase HPLC. Batch yields of 2.5±0.9 GBq (68±25 mCi) of [11C]NMB are routinely (n=16) obtained, with a radiochemical purity >98% and an average specific activity of over 40.7 TBq (1100 Ci)/mmol. The overall radiosynthesis, purification, and pre-release quality control testing is accomplished within 50 minutes. The procedure described herein produces a radiopharmaceutical product that is suitable for D2 receptor studies using PET or microPET imaging.

Radiolysis of an aqueous [11C]iomazenil solution

Radiolysis of [11C]iomazenil was investigated in an aqueous solution. Preparation of [11C]iomazenil with [11C]CH3I produced a good radiochemical yield, however, its radiochemical purity was always less than 90 without additives. This poor radiochemical purity was attributed to radiolysis of [11C]iomazenil. The degradation of [11C]iomazenil via radiolysis was markedly facilitated in the presence of HCOONa (strong selective hydroxyl radical and hydrogen atom scavenger). On the other hand, the radiolysis was suppressed effectively in the presence of NaNO3 (selective hydrated electron scavenger), suggesting participation of hydrated electrons in the radiolysis of [11C]iomazenil. LC/MS analysis of the degradation product suggested that the major degradation product by radiolysis had a molecular weight of 285 corresponding to the deiodinated compound. The present study showed that a decrease in the radiochemical purity of [11C]iomazenil was attributed to the radiolysis via the deiodination reaction by hydrated electron.

Studies into radiolytic decomposition of fluorine-18 labeled radiopharmaceuticals for positron emission tomography

Radiolytic decomposition of high specific concentration radiopharmaceuticals is an undesired side-effect that can hamper development of novel PET tracers. This was particularly evident in a series of carbon-11 and fluorine-18 labeled mono- and dimethyl-substituted aryl amines, where rapid decomposition was observed in isolation and formulation steps. We tested a number of additives that inhibit radiolysis and can be safely added to the synthesis procedures (purification and isolation) and reformulation steps to provide suitable clinical formulations. Ethanol and sodium ascorbate are established anti-oxidant stabilizers that completely inhibit radiolytic decomposition and are amenable to human use. Herein, we also demonstrate for the first time that nitrones are non-toxic radical scavengers that are capable of inhibiting radiolysis.

Time course of dopamine D2 receptor occupancy by clozapine with medium and high plasma concentrations

Most antipsychotics were thought to induce antipsychotic action at an excess of 70% striatal dopamine D2 receptor occupancy, while the clinical dose of clozapine was reported to show less than 60% occupancy. High-dose clozapine could occupy as high as 80% of striatal dopamine D2 receptor in monkey PET studies. Although the time course of dopamine D2 receptor occupancy is an important property of antipsychotics, that by clozapine has not been investigated in a clinical setting. We measured the time course of extrastriatal dopamine D2 receptor occupancy with different doses of clozapine and evaluated whether the measured occupancies fitted the binding theory. Three consecutive PET scans with [11C]FLB 457 were performed for two patients with schizophrenia, chronically taking 600 mg/day and 200 mg/day of clozapine, respectively. Series of occupancies were also measured in combination with fluvoxamine or paroxetine in one patient. Dopamine D2 receptor occupancies were also simulated using individual clozapine plasma data and previously determined in vivo ED50 value. The occupancy of one patient with high plasma concentration (1207 ng/ml at peak time) was around 75% at peak and around 60% after 26 h. Another patient with medium plasma concentration (649 ng/ml at peak time) showed less than 50% occupancy at peak, decreasing to 15% after 25 h. The measured occupancy values fitted well with the simulated occupancy values. At high plasma concentration, clozapine can induce high extrastriatal dopamine D2 receptor occupancy in the human brain, and this finding fitted well with the theoretical estimation.

Binding kinetics of11c-n-methyl piperidyl benzilate (11c-nmpb) in a rhesus monkey brain using the cerebellum as a reference region

The binding kinetics of 11C-N-methyl piperidyl benzilate (11C-NMPB) in rhesus monkey brain were studied using animal positron emission tomography (PET) (SHR2000). This study is intended to assess the validity of the method using the cerebellum as a reference region, and to evaluate the effects of anesthesia on 11C-NMPB binding. Two monkeys, anesthetized with ketamine, received intravenous 11C-NMPB alone (370-760 MBq, < 1 microg/kg) or mixed with varying doses of nonradioactive NMPB (3 microg/kg, 10 microg/kg, 30 microg/kg) and were subjected to PET scans for 60 minutes. Regions of interest (ROI) were drawn on reconstructed PET images and a time-activity curve was obtained for each region. 11C-NMPB accumulated densely in the striatum and cerebral cortex with time. In contrast, the tracer accumulation significantly decreased with increased doses of nonradioactive NMPB. In the cerebellum, on the other hand, the accumulation of 11C-NMPB remained low and the tracer was slowly eliminated from the brain following the injection. 11C-NMPB binding in the cerebellum was barely affected by the increased dose of nonradioactive NMPB. We thus concluded that the specific 11C-NMPB binding was negligible in the cerebellum, and performed simplified evaluation of 11C-NMPB binding in each brain region by a graphical method using the cerebellum as a reference region. PET was conducted 26 times, in total both in ketamine-anesthetized and awake monkeys (n = 3 each). Measurements of 11C-NMPB binding showed good run-to-run reproducibility within individual animals. When 11C-NMPB binding was compared between ketamine-treated and awake animals, a significant increase in 11C-NMPB binding was observed in the striatum but not in other brain regions of ketamine-treated animals.

Stability of 11C-labeled PET radiopharmaceuticals

The stability of widely used 11C-radiopharmaceuticals was investigated. The purity of many 11C-radiopharmaceuticals was found to be low even at the end of synthesis and to lower with time by radiolysis. Examinations of the effects of radical scavengers showed that the radiopharmaceuticals were classified into four groups: Group 1 showing excellent stability; Group 2 showing suppressed radiolysis in the presence of a selective scavenger of hydroxyl radicals; Group 3 showing suppression in the presence of a selective scavenger of hydrated electrons; Group 4 showing suppression in the presence of both scavengers. Using appropriate additives according to these classes, 11C-radiopharmaceuticals were prepared with excellent radiochemical purity even with high levels of radioactivity and specific activity.

Decomposition of an aqueous solution of [11C]Ro 15-4513: implication of hydrated electrons in the radiolysis of [11C]Ro 15-4513

An aqueous solution of [(11)C]Ro 15-4513 underwent decomposition to give a few radioactive degradation products. The major degradation product corresponded to that of authentic Ro 15-4513 with (60)Co radiolysis, which showed a molecular weight of 301 (M+H). The structure of this degradation product was estimated to have an amine group instead of an azido group on Ro 15-4513. Radiolysis of Ro 15-4513 was not suppressed by the addition of selective hydroxyl radical scavenger (MeOH, HCOO(-)), in contrast, it was suppressed effectively in the presence of a selective hydrated electron scavenger (NaNO(3)), suggesting that hydrated electrons would play an important role in the radiolysis of [(11)C]Ro 15-4513.

Automated synthesis of the ultra high specific activity of [11C]Ro15-4513 and its application in an extremely low concentration region to an ARG study

We have designed and constructed an automated device for the production of ultra-high specific activity (11)C-labeled compounds via [(11)C]CH(3)I synthesized by the single pass I(2) method. The optimum condition for the production of [(11)C]CH(3)I was determined to be 630 degrees C for oven-1 (reaction column), 50 degrees C for oven-2 (iodine column) and 50 ml/min for the He gas flow rate, and gave the maximum conversion ratio of [(11)C]CH(3)I, 44%. [(11)C]Ro15-4513, known as an inverse agonist of the benzodiazepine receptor, was produced under optimized conditions. An i.v. injectable [(11)C]Ro15-4513 solution of 1500 +/- 490 MBq (n = 6) with specific activity 4700 +/- 2500 GBq/micromol and a radiochemical purity of 98.2 +/- 2% was obtained automatically within 25 minutes (from EOB) by irradiating nitrogen gas containing 5% H(2) with 18 MeV protons (14.2 MeV on target) at 20 microA for 20 minutes. The highest specific activity of 9700 GBq/micromol (at EOS) could be achieved, although the radiochemical purity was 92.4%. By the use of the ultra-high specific activity [(11)C]Ro15-4513, the super high affinity binding sites in the rat brain hippocampus could be clearly visualized even at the extremely low concentration of 0.66 pM Ro15-4513 by in vitro autoradiography.

Development of an automated system for synthesizing 18F-labeled compounds using [18F]fluoroethyl bromide as a synthetic precursor

An automated system was developed to synthesize 18F-labeled compounds using [18F]fluoroethyl bromide ([18F]FEtBr) as a synthetic precursor. The apparatus makes possible the following sequence of processes: (1) production of an aqueous solution of [18F]fluoride ([18F]F-), (2) recovery of [18F]F- from target chamber, (3) drying of [18F]F-, (4) formation and distillation of [18F]FEtBr into a trapping vessel, (5) alkylation of target compounds with [18F]FEtBr, (6) High performance liquid chromatography purification and (7) formulation. [18F]FEtBr, the synthetic precursor for fluoroethylation, was labeled via nucleophilic displacement of 2-trifluoromethanesulfonyloxy ethylbromide (BrCH2CH2OTf) with [18F]F- and was purified from the reaction mixture by distillation. After the conditions for forming [18F]FEtBr and drying [18F]F- were optimized, [18F]FEtBr was obtained in a radiochemical yield of 71 +/- 13% (n = 21, based on [18F]F-, corrected for decay) and a radiochemical purity of 98 +/- 1.4% at end of the syntheses (EOS). Using this automated system, [18F]fluoroethylspiperone ([18F]FEtSP) was prepared by reacting spiperone with [18F]FEtBr in a radiochemical yield and purity of 56 +/- 12% (n = 5, based on [18F]FEtBr, corrected for decay) and 97 +/- 1.5% with a specific activity of 310 +/- 120 GBq/mumol at EOS. The total synthesis time was 55 +/- 2.3 min from the end of bombardment and the developed system has proved to be reliable and reproducible.

Positron emission tomography and ex vivo and in vitro autoradiography studies on dopamine D2-like receptor degeneration in the quinolinic acid-lesioned rat striatum: comparison of [11C]raclopride, [11C]nemonapride and [11C]N-methylspiperone

With [11C]raclopride,[11C]nemonapride and [11C]N-methylspiperone, degeneration of dopamine D2-like receptors in the unilaterally quinolinic acid-lesioned rats was evaluated by positron emission tomography (PET) and ex vivo and in vitro autoradiography. PET showed a decreased uptake of [11C]raclopride in the lesioned striatum, but an increased uptake of [11C]nemonapride and [11C]N-methylspiperone despite a decreased binding in vitro. Ex vivo autoradiography showed an increased accumulation of the three ligands in the cortical region overlying the injured striatum, probably enlarging PET signals. PET has the limited potential for evaluating the receptor degeneration in the present animal model.

Effect of endogenous dopamine on extrastriatal [¹¹C]FLB 457 binding measured by PET

Central dopaminergic systems are known to be implicated in the pathophysiology of schizophrenia and recent in vivo dopamine receptor imaging studies have focused on the measurement of extrastriatal dopamine receptor. However, there are only a limited number of ligands that can measure the low-density D2 receptor in extrastriatal regions and their sensitivity to endogenous dopamine in extrastriatal regions has not yet been fully examined. In this study, the effect of endogenous dopamine on the extrastriatal binding of [11C]FLB 457 was examined in the rhesus monkey after facilitation with 1 mg/kg of methamphetamine (MAP) and was compared with the effect on the striatal binding of [11C]raclopride. The indices of receptor binding were obtained by four methods using cerebellum as a reference region. The bindings of [11C]FLB 457 in the frontal cortex, temporal cortex, and thalamus were not significantly changed after MAP treatment, while the striatal binding of [11C]raclopride was decreased by more than 20%. These results suggest that [11C]FLB 457 is not sensitive to endogenous dopamine in the extrastriatal regions of rhesus monkeys, despite a sufficient dose of MAP to decrease the binding of [11C]raclopride in the striatum.

Different sensitivities to competitive inhibition of benzodiazepine receptor binding of 11C-iomazenil and 11C-flumazenil in rhesus monkey brain

The in vivo binding kinetics of 11C-iomazenil were compared with those of 11C-flumazenil binding in rhesus monkey brain. The monkey was anesthetized with ketamine and intravenously injected with either 11C-iomazenil or 11C-flumazenil in combination with the coadministration of different doses of non-radioactive flumazenil (0, 5 and 20 microg/kg). The regional distribution of 11C-iomazenil in the brain was similar to that of 11C-flumazenil, but the sensitivity of 11C-iomazenil binding to competitive inhibition by non-radioactive flumazenil was much less than that of 11C-flumazenil binding. A significant reduction in 11C-flumazenil binding in the cerebral cortex was observed with 20 microg/kg of flumazenil, whereas a relatively smaller inhibition of 11C-iomazenil binding in the same region was observed with the same dose of flumazenil. These results suggest that 11C-flumazenil may be a superior radiotracer for estimating benzodiazepine receptor occupancy in the intact brain.

Development and evaluation of muscarinic cholinergic receptor ligands N-[11C]ethyl-4-piperidyl benzilate and N-[11C]propyl-4-piperidyl benzilate: a PET study in comparison with N-[11C]methyl-4-piperidyl benzilate in the conscious monkey brain

The muscarinic cholinergic receptor ligands N-[(11)C]ethyl-4-piperidyl benzilate (4-EPB) and N-[(11)C]propyl-4-piperidyl benzilate (4-PPB) were developed and evaluated in comparison with N-[(11)C]methyl-4-piperidyl benzilate (4-MPB) in the conscious monkey brain using positron emission tomography (PET). Time-activity curves of [(11)C]4-EPB, unlike [(11)C]4-MPB, showed peaks within 91 min in regions rich in muscarinic receptors. [(11)C]4-PPB showed no specific binding even in the regions rich in these receptors. These observation demonstrated that increases in [(11)C]alkyl chain length could alter the kinetic properties of receptor ligands for PET.

Positron emission tomography of radioligand binding in porcine striatum in vivo: haloperidol inhibition linked to endogenous ligand release

The ligands N-methylspiperone and haloperidol both bind to D(2)-like dopamine receptors. The competitive nature of the binding over a wide range of haloperidol concentrations and the effect on dopamine release have never been tested in vivo. We determined the competitive interaction between 3-N-[(11)C]methylspiperone ([(11)C]NMSP) and haloperidol binding to striatal dopamine D(2)-like receptors with positron emission tomography (PET) of pig brain. [(11)C]NMSP tomography was performed with haloperidol at five different plasma concentrations maintained constant by programmed infusion. Kinetic parameters of ligand competition for binding in the striatum were determined by deconvolving time-activity curves of the striatum and cerebellum from metabolite-corrected arterial plasma [(11)C]NMSP and haloperidol concentrations. Two types of [(11)C]NMSP-binding sites were evident in the striatum, both saturable by haloperidol administration. The preponderant or primary sites bound [(11)C]NMSP irreversibly, as dopamine D2-like receptors, while the secondary sites bound [(11)C]NMSP reversibly, as do serotonin S2 receptors. Woolf-Hanes plots revealed the predicted approximately linear relationships between the binding indices and the haloperidol plasma concentration. For the irreversible binding sites, this relationship indicated a 50% inhibitory concentration of haloperidol of 2 nM in plasma and a maximum binding capacity of 64 pmol cm(-3) in striatum. For the reversible binding sites, the relationship indicated a 50% inhibitory plasma concentration of haloperidol of 1 nM and a maximum binding capacity of 4.5 pmol cm(-3). Second-order polynomial Eadie-Hofstee-Scatchard plots were consistent with increased competition from an endogenous ligand of the irreversibly binding sites only with increasing doses of haloperidol. At the highest haloperidol dose, this hypothetical endogenous ligand had risen 6-7-fold. We contend that this reveals the release of dopamine by high concentrations of haloperidol.

Development of an automated synthesis apparatus for L-[3-11C] labeled aromatic amino acids

We have developed an automated synthesis apparatus for L-[3-11C]-labeled aromatic amino acids by improved multi-enzymatic reactions. Use of a specially designed reaction vessel and of CH2Cl2 as a solvent has greatly simplified the overall operation, proving to be suitable for automated synthesis, and also has given better yields and higher specific activities than formerly reported values.

Automatic synthesis of L-[beta-11C]amino acids using an immobilized enzyme column

We have developed a system for the automatic synthesis of L-[beta-11C]amino acids for i.v. injection by means of enzyme-mediated reactions from 11CO2 via 11CH3I and D,L-[beta-11C]alanine as labeled intermediates. This system, which incorporates an ultrafilter cartridge sterilized by electron beam irradiation and a column packed with immobilized enzymes, was effective for eliminating enzymes and endotoxins that may contaminate the product. Using this system, 1.3 +/- 0.5 GBq of 5-hydroxy-L-[beta-11C]tryptophan with a radiochemical purity of 97.1 +/- 0.6% and a specific activity of 39.6 +/- 8 GBq/mumol a pH value of 4 could be obtained in about 32 min (n = 3, at EOS). No endotoxin, enzyme, or bacteria was detected in the product. L-[beta-11C]dihydroxyphenylalanine (L-[beta-11C]DOPA) was also synthesized using this system.

Comparison of three PET dopamine D2-like receptor ligands, [11C]raclopride, [11C]nemonapride and [11C]N-methylspiperone, in rats

We studied the tracer kinetics of three dopamine D2-like receptor ligands, [11C]raclopride ([11C]RAC), [11C]nemonapride ([11C]NEM) and [11C]N-methylspiperone ([11C]MSP), in anesthetized rats by tissue dissection, ex vivo ARG and PET in order to clarify their characteristics for PET imaging. The in vivo affinity of the three ligands for the striatum ([11C]MSP > [11C]NEM > [11C]RAC) obeyed the in vitro affinity for dopamine D2 receptors. The affinity of [11C]RAC and [11C]MSP for the cerebellum was very low, but the affinity of [11C]NEM for the cerebellum was compatible to that for the cortex and was not to be ignored. Also the affinity of [11C]MSP for the cortex was relatively high. [11C]RAC showed the highest selectivity. The striatal PET image with [11C]RAC was clearer than that with [11C]NEM or [11C]MSP, but the activity decreased much faster than that measured by tissue dissection because of the partial volume effect. The striatal activity with [11C]NEM remained high and that with [11C]MSP gradually increased. [11C]RAC and [11C]MSP, but not [11C]NEM, showed a high accumulation in the periorbital region.

Dopamine D2 and serotonin S2 receptors in susceptibility to methamphetamine psychosis detected by positron emission tomography

Positron emission tomography (PET) was used to assess the role of dopamine D2 receptors in the striatum and serotonin S2 receptors in the frontal cortex in the susceptibility to methamphetamine-induced psychosis. Subjects were six men who had previously experienced methamphetamine psychosis (methamphetamine subjects) and 10 age- and sex-matched control subjects. The radiotracer used was 11C-N-methylspiperone. Although binding availability, assessed by dynamic analysis, in the two regions did not differ between the two groups, the ratio of binding availability in the striatum to that in the frontal cortex significantly decreased in the methamphetamine subjects as compared with the control subjects. These findings suggest that an imbalance in the activity of these two receptors may be related to the susceptibility to methamphetamine psychosis.

On-line [11C]methylation using [11C]methyl iodide for the automated preparation of 11C-radiopharmaceuticals

A novel method for the efficient preparation of 11C-radiopharmaceuticals by on-line [11C]methylation using [11C]methyl iodide has been developed and applied to a rapid, convenient automated system. [11C]Methyl iodide is first trapped in a short column, containing an adsorber and coated substrate, which is connected to an HPLC injector. DMF is then introduced. Alternatively the substrate is added with the DMF. A whole reaction mixture can be easily injected onto an HPLC column for purification by switching the injector valve immediately after the reaction. Thus, radiochemical yields in the preparation of 11C-labeled doxepin, benztropine, cyproheptadine and N-methylspiperone have been improved remarkably and the synthetic procedure simplified.

PET radioligands for dopamine receptors and re-uptake sites: chemistry and biochemistry

This report, based on the past experience of European centres, offers practical guidance on the chemistry and biochemistry of PET radioligands used for the in vivo imaging of dopamine receptors and re-uptake sites. It mainly summarizes methods for the preparation of D1 and D2 receptor ligands labelled with positron-emitting radioisotopes. Some of these ligands (11C-labelled SCH23390, raclopride and nomifensine, 18F-labelled butyrophenones, [76Br]bromolisuride), which have been found useful in PET clinical investigations, have been emphasized. This report is intended as an introduction and guideline for new PET-groups who want to start research in the dopaminergic neurotransmission imaging field.