Dopamine D1 receptor imaging in the rodent and primate brain using the isoquinoline +-[11C]A-69024 and positron emission tomography.

In vivo pharmacokinetic and brain binding characteristics of (+)-[(11)C]A-69024, a high-affinity-D1-selective dopamine receptor antagonist, were assessed with micro-PET and beta-microprobes in the rat and PET in the baboon. The biodistribution of (+)-[(11)C]A-69024 in rats and baboons showed a rapid brain uptake (reaching a maximal value at 5 and 15 min postinjection in rats and baboons, respectively), followed by a slow wash out. The region/cerebellum concentration ratio was characterized by a fourfold higher uptake in striatum and a twofold higher uptake in cortical regions, consistent with in vivo specific binding of the radiotracer in these cerebral regions. Furthermore, this specific (+)-[(11)C]A-69024 binding significantly correlated with the reported in vitro distribution of dopamine D1-receptors. Finally, the specific uptake of the tracer in the striatum and cortical regions was completely prevented by either a pretreatment with large doses of nonradioactive (+/-)A-69024 or of the D1-selective antagonist SCH23390, resulting in a similar uptake in the reference region (cerebellum) and in other brain regions. Thus, (+)-[(11)C]A-69024 appears to be a specific and enantioselective radioligand to visualize and quantify brain dopamine D1 receptors in vivo using positron emission tomography.

Quantification of cerebral nicotinic acetylcholine receptors by PET using 2-[18F]fluoro-A-85380 and the multiinjection approach.

The multiinjection approach was used to study in vivo interactions between alpha4beta2(*) nicotinic acetylcholine receptors and 2-[(18)F]fluoro-A-85380 in baboons. The ligand kinetics was modeled by the usual nonlinear compartment model composed of three compartments (arterial plasma, free and specifically bound ligand in tissue). Arterial blood samples were collected to generate a metabolite-corrected plasma input function. The experimental protocol, which consisted of three injections of labeled or unlabeled ligand, was aiming at identifying all parameters in one experiment. Various parameters, including B'(max) (the binding sites density) and K(d)V(R) (the apparent in vivo affinity of 2-[(18)F]fluoro-A-85380) could then be estimated in thalamus and in several receptor-poor regions. B'(max) estimate was 3.0+/-0.3 pmol/mL in thalamus, and ranged from 0.25 to 1.58 pmol/mL in extrathalamic regions. Although K(d)V(R) could be precisely estimated, the association and dissociation rate constants k(on)/V(R) and k(off) could not be identified separately. A second protocol was then used to estimate k(off) more precisely in the thalamus. Having estimated all model parameters, we performed simulations of 2-[(18)F]fluoro-A-85380 kinetics to test equilibrium hypotheses underlying simplified approaches. These showed that a pseudo-equilibrium is quickly reached between the free and bound compartments, a favorable situation to apply Logan graphical analysis. In contrast, the pseudo-equilibrium between the plasma and free compartments is only reached after several hours. The ratio of radioligand concentration in these two compartments then overestimates the true equilibrium value, an unfavorable situation to estimate distribution volumes from late images after a bolus injection.

[18F]FPhEP and [18F]F2PhEP, two new epibatidine-based radioligands: evaluation for imaging nicotinic acetylcholine receptors in baboon brain.

The radioligand 2-[(18)F]fluoro-A-85380 has been developed for imaging alpha(4)beta(2) nAChRs with PET. However, it has slow kinetics and a large fraction of bound activity is nondisplaceable. In an attempt to address these problems, two epibatidine-based alpha(4)beta(2) nicotinic antagonists, coded FPhEP and F(2)PhEP, were evaluated in vivo in baboons. They were radiolabeled with fluorine-18 from the corresponding N-Boc-protected bromo-derivatives and the no-carrier-added K[(18)F]F-Kryptofix(222) complex. Radiochemically pure [(18)F]FPhEP or [(18)F]F(2)PhEP was obtained in 80 min in amounts of 1.11-2.22 GBq (111-185 GBq/micromol). After injection of 215 MBq of [(18)F]FPhEP or [(18)F]F(2)PhEP, dynamic PET data were acquired. Thalamic radioactivity peaked at 20 min (4.9% +/- 0.2% ID/100 mL tissue) for [(18)F]FPhEP. For [(18)F]F(2)PhEP, the peak was at 45 min (3.3% +/- 0.1% ID/100 mL tissue). Regional distribution of both radiotracers was in accordance with the known distribution of nAChRs. In presaturation experiments, nicotine, cytosine, or FPhEP reduced brain radioactivity of [(18)F]FPhEP. In a displacement experiment with nicotine only a small amount of [(18)F]F(2)PhEP was dislodged. In spite of a moderate to high in vitro affinity, both ligands do not fulfill the widely adopted criteria for a PET radioligand.

[11C]LBT-999: a suitable radioligand for investigation of extra-striatal dopamine transporter with PET.

A new tropane derivative, (E)-N-(4-fluorobut-2-enyl)-2beta-carbomethoxy-3beta-(4'-tolyl)nortropane (LBT-999), was evaluated in baboons as a carbon-11 radioligand for studies of the dopamine transporter (DAT) using positron emission tomography (PET). Brain uptake was high in the striatum (17 and 13% ID/100 mL tissue in the putamen and the caudate, respectively), moderate in the midbrain and thalamus (5 and 3% ID/100 mL tissue, respectively), and low in the cortex and cerebellum (2% ID/100 mL tissue) at 30 min post injection. The striatum-to-cerebellum ratio was high (30 at 110 min post injection). Specific binding was completely blocked following pretreatment with the DAT antagonists GBR12909 (5 mg/kg i.v.) or PE2I (1 mg/kg i.v.). The [(11)C]LBT-999 uptake was decreased by these antagonists in the putamen (-79 and -92%, respectively), caudate (-80 and -91%, respectively), midbrain (-73 and -78%, respectively), and thalamus (-34 and -46%, respectively). The serotonin transporter (SERT) antagonist citalopram (5 mg/kg i.v.) or the norepinephrine transporter antagonist maprotiline (5 mg/kg i.v.) had no effect on LBT specific binding. Pharmacological challenge with PE2I (1 mg/kg i.v.) induced a rapid and almost complete decrease of the specific binding in the putamen (-97%), caudate (-96%), midbrain (-96%), and thalamus (-81%), confirming the reversibility of [(11)C]LBT-999 binding. The high brain uptake of [(11)C]LBT-999 together with its low nonspecific binding (reflected by the very high brain structure-to-cerebellum ratio) indicate that this radiotracer is an excellent candidate for in vivo quantification of the DAT, especially in extrastriatal structures, such as the midbrain.

Synthesis and radiolabeling of N-[4-[4-(2-[11C]methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide -- a potential radiotracer for D3 receptor imaging with PET.

FAUC346 (N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), an in vitro D(3)-selective ligand, and its normethyl derivative have been synthesized from commercially available 1-(2-substituted-phenyl)piperazines. FAUC346 has been labeled using [(11)C]methyl triflate in acetone containing aqueous NaOH (5 Eq) at -10 degrees C for 1 min, purified on semipreparative reverse-phase high-performance liquid chromatography (HPLC) and formulated as an intravenous injectable solution using a Sep-Pak Plus C(18) device. Up to 5.5 GBq of [(11)C]FAUC346 (N-[4-[4-(2-[methyl-(11)C]methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), with a specific radioactivity of 45-75 GBq/micromol, could be obtained in 30-35 min, including HPLC purification and formulation starting from 44.4 GBq of [(11)C]carbon dioxide. Preliminary pharmacological evaluation of [(11)C]FAUC346 in rat brain clearly demonstrated in vivo selectivity for D(3) receptors and the absence of radiolabeled metabolite within the brain. These encouraging results, however, could not be confirmed in nonhuman primates; therefore, this radioligand does not appear to have the required pharmacological profile for a positron emission tomography probe for imaging D(3) receptors.

Synthesis and radiosynthesis of [(18)F]FPhEP, a novel alpha(4)beta(2)-selective, epibatidine-based antagonist for PET imaging of nicotinic acetylcholine receptors.

FPhEP (1, (+/-)-2-exo-(2'-fluoro-3'-phenyl-pyridin-5'-yl)-7-azabicyclo[2.2.1]heptane) belongs to a recently described novel series of 3'-phenyl analogues of epibatidine, which not only possess subnanomolar affinity and high selectivity for brain alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs), but also were reported as functional antagonists of low toxicity (up to 15 mg/kg in mice). FPhEP (1, K(i) of 0.24 nM against [(3)H]epibatidine) as reference as well as the corresponding N-Boc-protected chloro- and bromo derivatives (3a,b) as precursors for labelling with fluorine-18 were synthesized in eight and nine steps, respectively, from commercially available N-Boc-pyrrole (overall yields=17% for 1, 9% for 3a and 8% for 3b). FPhEP (1) was labelled with fluorine-18 using the following two-step radiochemical process: (1) no-carrier-added nucleophilic heteroaromatic ortho-radiofluorination from the corresponding N-Boc-protected chloro- or bromo derivatives (3 a,b-1mg) and the activated K[(18)F]F-Kryptofix(222) complex in DMSO using microwave activation at 250 W for 1.5 min, followed by (2) quantitative TFA-induced removal of the N-Boc-protective group. Radiochemically pure (>99%) [(18)F]FPhEP ([(18)F]-1, 2.22-3.33 GBq, 66-137 GBq/micromol) was obtained after semi-preparative HPLC (Symmetry C18, eluent aq 0.05 M NaH(2)PO(4)/CH(3)CN, 80:20 (v:v)) in 75-80 min starting from a 18.5 GBq aliquot of a cyclotron-produced [(18)F]fluoride production batch (10-20% nondecay-corrected overall yield). In vitro binding studies on rat whole-brain membranes demonstrated a subnanomolar affinity (K(D) 660 pM) of [(18)F]FPhEP ([(18)F]-1) for nAChRs. In vitro autoradiographic studies also showed a good contrast between nAChR-rich and -poor regions with a low non-specific binding. Comparison of in vivo Positron Emission Tomography (PET) kinetics of [(18)F]FPhEP ([(18)F]-1) and [(18)F]F-A-85380 in baboons demonstrated faster brain kinetics of the former compound (with a peak uptake at 20 min post injection only). Taken together, the preliminary data obtained confirm that [(18)F]FPhEP ([(18)F]-1) has potential for in vivo imaging nAChRs in the brain with PET.

Acute effects of physostigmine and galantamine on the binding of [18F]fluoro-A-85380: a PET study in monkeys.

2-[18F]fluoro-3-[2S-2-azetidinylmethoxy]pyridine ([18F]fluoro-A-85380) is an alpha4beta2 subtype selective nicotinic cholinergic agonist with potential suitability for studying changes in endogenous acetylcholine synaptic concentration. Physostigmine, a potent AChE inhibitor, and galantamine, an allosteric modulator of nAChRs, are widely used for the treatment of Alzheimer's disease. Before studying patients with this neurodegenerative disease, positron emission tomography (PET) studies in monkeys were performed to assess the impact of these two compounds on the radiotracer distribution volumes. Physostigmine was administered i.v. at two dosages: 150 microg/kg/h and 37.5 microg/kg/h for 160 min. Galantamine was administered i.v. at two dosages: 2 or 4 mg over 20 min. For PET data analysis, a model with one tissue (radioactivity of the parent compound in plasma and radioactivity in brain tissue) compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. The higher dose of physostigmine produced a 40%, 23%, and 30% reduction of distribution volumes in the putamen, the temporal, and frontal cortices, respectively. The lower dose of physostigmine produced a reduction of 33%, 31%, and 24% in the same structures, respectively. Galantamine (4 mg or 2 mg) produced no significant change of distribution volumes in the basal ganglia, the temporal and frontal cortex. The effects of physostigmine, a more potent AChE inhibitor than galantamine, could be interpreted as a desensitization of nAChRs, due to a prolonged exposure to high synaptic concentration of acetylcholine or as a competition with acetylcholine.

Acute inhibition of cardiac monoamine oxidase A after tobacco smoke inhalation: validation study of [11C]befloxatone in rats followed by a positron emission tomography application in baboons.

The in vivo characteristics of [11C]befloxatone were assessed in myocardium of rats and monkeys. A complete multicompartmental model was developed to quantify monkey cardiac monoamine oxidase A (MAO-A) binding sites using positron emission tomography (PET) and was applied to assess the acute effects of inhalation of tobacco smoke. Unknown compounds contained in tobacco smoke inhibit brain MAO. In vitro, befloxatone inhibits selectively, competitively, and reversibly MAO-A in human tissues. [11C]Befloxatone (1.85 MBq) was i.v. injected into rats. Animals were sacrificed, dissected, and samples were assessed for radioactivity. Another group of rats was pretreated with clorgyline (10 mg/kg i.v.). Monkeys were injected with [11C]befloxatone (222-370 MBq), and the chest was imaged with PET for 2 h. Presaturation and displacement experiments were performed using unlabeled befloxatone. For quantification of myocardial binding sites (Bmax), [11C]befloxatone was first injected as a tracer dose (2.7-9.3 nmol) and 20 min later injected as a mixture of labeled and unlabeled befloxatone (labeled, 10.3-41.9 nmol; unlabeled, 407-765 nmol). In rodents, cardiac uptake was high (3.39 +/- 0.5% injected dose/g tissue) and strongly inhibited (80%) by clorgyline. In monkeys, administration of unlabeled befloxatone displaced 85% of cardiac radioactivity. Bmax was found to be 208 +/- 13 pmol ml(-1) tissue. Inhalation of tobacco smoke decreased Bmax: 150 +/- 6.2 pmol ml(-1), whereas nicotine did not. [11C]Befloxatone allows a good visualization of the heart. Cardiac MAO-A Bmax was quantified and a clear effect of acute inhalation of tobacco smoke was evidenced. Therefore, a single cigarette can interfere with the cardiac turnover of catecholamines.

In vivo imaging of human cerebral nicotinic acetylcholine receptors with 2-18F-fluoro-A-85380 and PET.

UNLABELLED: 2-(18)F-fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine (2-(18)F-fluoro-A-85380) is a PET radioligand that is specific for nicotinic acetylcholine receptors (nAChRs) and has a high affinity for the alpha(4)beta(2) subtype. The purpose of this study was to evaluate different strategies to quantify 2-(18)F-fluoro-A-85380 binding in healthy nonsmoking human volunteers. METHODS: After intravenous injection of 189 +/- 30 MBq (0.8-5.7 nmol) of 2-(18)F-fluoro-A-85380, the first dynamic PET scan was acquired over 150 min. The second 30-min PET scan was performed 60 min later. Time-activity curves were generated from volumes of interest. 2-(18)F-Fluoro-A-85380 volume of distribution (DV) was quantified using compartmental kinetic analysis and Logan graphical analysis. In the kinetic analysis, the 1-tissue compartment model (1TCM) and the 2-tissue (2TCM) compartment model were applied. The most appropriate kinetic model was determined using the Akaike Information Criterion. The effect of reducing the PET study duration on the reliability of the DV values computed by the kinetic and the graphical analyses was evaluated. RESULTS: Time-activity curves were better described by the 2TCM. The DV values ranged from 5.2 +/- 0.5 in the occipital cortex, 6.2 +/- 0.2 in the frontal cortex, and 7.3 +/- 0.4 in the putamen to 15.4 +/- 2.1 in the thalamus. These regional DV values were consistent with the distribution of nAChRs in the human brain. Logan graphical analysis provided slightly lower DV values than those of the 2TCM (from -3.5% in the occipital cortex to -6.6% in the thalamus). The minimal study duration required to obtain stable DV estimates in all regions was similar for the 2 methods: 140 min for the 2TCM and 150 min for the Logan analysis. DV estimates obtained with the 2TCM were more stable than those calculated by the Logan approach for the same scan duration. CONCLUSION: These results show that 2-(18)F-fluoro-A-85380 can be used to assess nAChRs binding in the human brain with PET.

Synthesis of a [2-pyridinyl-18F]-labelled fluoro derivative of (-)-cytisine as a candidate radioligand for brain nicotinic alpha4beta2 receptor imaging with PET.

In recent years, there has been considerable effort to design and synthesize radiotracers suitable for use in Positron Emission Tomography (PET) imaging of the alpha4beta2 neuronal nicotinic acetylcholine receptor (nAChR) subtype. A new fluoropyridinyl derivative of (-)-cytisine (1), namely (-)-9-(2-fluoropyridinyl)cytisine (3, K(i) values of 24 and 3462 nM for the alpha4beta2 and alpha7 nAChRs subtypes, respectively) has been synthesized in four chemical steps from (-)-cytisine and labelled with fluorine-18 (T(1/2): 119.8 min) using an efficient two-step radiochemical process [(a). nucleophilic heteroaromatic ortho-radiofluorination using the corresponding N-Boc-protected nitro-derivative, (b). TFA removal of the Boc protective group]. Typically, 20-45 mCi (0.74-1.67 GBq) of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3, 2-3 Ci/micromol or 74-111 GBq/micromol) were easily obtained in 70-75 min starting from a 100 mCi (3.7 GBq) aliquot of a cyclotron-produced [18F]fluoride production batch (20-45% non decay-corrected yield based on the starting [18F]fluoride). The in vivo pharmacological profile of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) was evaluated in rats with biodistribution studies and brain radioactivity monitoring using intracerebral radiosensitive beta-microprobes. The observed in vivo distribution of the radiotracer in brain was rather uniform, and did not match with the known regional densities of nAChRs. It was also significantly different from that of the parent compound (-)-[3H]cytisine. Moreover, competition studies with (-)-nicotine (5 mg/kg, 5 min before the radiotracer injection) did not reduce brain uptake of the radiotracer. These experiments clearly indicate that (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) does not have the required properties for imaging nAChRs using PET.

Biodistribution and radiation dosimetry of 18F-fluoro-A-85380 in healthy volunteers.

UNLABELLED: This study reports on the biodistribution and radiation dosimetry of 2-(18)F-Fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine ((18)F-fluoro-A-85380), a promising radioligand for the imaging of central nicotinic acetylcholine receptors (nAChRs). METHODS: Whole-body scans were performed in 3 healthy male volunteers up to 2 h after intravenous injection of 137-238 MBq (18)F-fluoro-A-85380. Transmission scans (3 min per step, 8 or 9 steps according to the height of the subject) in 2-dimensional mode were used for subsequent correction of attenuation of emission scans. Emission scans (1 min per step) were acquired over 2 h. Venous blood samples were taken up to 2 h after injection of the radiotracer. Urine was freely collected up to 2 h after injection of the radiotracer. For each subject, the percentage of injected activity measured in regions of interest over brain, intestine, stomach, bladder, kidneys, and liver were fitted to a monoexponential model, as an uptake phase followed by a monoexponential washout, or to a biexponential model to generate time-activity curves. Using the MIRD method, ten source organs were considered in estimating radiation absorbed doses for organs of the body. RESULTS: Injection of (18)F-fluoro-A-85380 was clinically well tolerated and blood and urine pharmacologic parameters did not change significantly. The primary routes of clearance were renal and intestinal. Ten minutes after injection, high activities were observed in the bladder, kidneys, and liver. Slow uptake was seen in the brain. The liver received the highest absorbed dose. The average effective dose of (18)F-fluoro-A-85380 was estimated to be 0.0194 mSv/MBq. CONCLUSION: The amount of (18)F-fluoro-A-85380 required for adequate nAChR imaging results in an acceptable effective dose equivalent to the patient.

Long-lasting occupancy of central nicotinic acetylcholine receptors after smoking: a PET study in monkeys.

The aim of this study was to compare the degree of occupancy of central nicotinic acetylcholine receptors (nAChR) in isoflurane anaesthetized baboon brain following inhalation of tobacco smoke (one cigarette containing 0.9 mg nicotine) or i.v. nicotine (0.6 mg i.v.). [18F]Fluoro-A-85380 and positron emission tomography (PET) were used to assess the distribution volumes (DV) of the radiotracer in selected brain areas using a one-compartment model. Eighty minutes after nicotine i.v., DV was reduced by 50 and 66% in the thalamus and putamen, respectively. Six hours after nicotine, a reduction in DV (27% in the thalamus) was still observed. Eighty minutes after inhalation of tobacco smoke, DV was decreased by 52 and 65% in the thalamus and putamen, respectively. Previous PET experiments have demonstrated a short-lasting interaction of [11C]nicotine with nAChRs. Thus, we hypothesized that a metabolite of nicotine with high affinity and long half-live (several hours) could bind at nAChRs. Eighty minutes after a high dose of nornicotine (0.5 mg i.v.), DV was reduced by 53 and 31% in thalamus and putamen, respectively. No significant effect was observed following 0.15 mg nornicotine. Therefore, nornicotine could contribute to the long-lasting occupancy of central nAChRs after smoking.

Temporal pole hypometabolism may be linked to a reduction of grey matter in temporal lobe epilepsy.

In order to gain further insight into the pathophysiology of temporal pole hypometabolism, we decided to perform a voxel-based automated analysis of structural MRI in epileptic patients with or without temporal pole hypometabolism. After fully automated segmentation of cerebral grey matter from structural T1-weighted MRI scans, we applied the automated technique of statistical parametric mapping (SPM) to the analysis of grey matter of nine control subjects, and 18 patients with right medial temporal lobe epilepsy with (n = 13) or without (n = 5) significant temporal pole hypometabolism. Group comparisons between subject controls and epileptic patients with temporal pole hypometabolism showed a reduction of grey matter located into the superior part of the right temporal pole, the right hippocampus and the left parahippocampal gyrus. Epileptic patients without temporal pole hypometabolism did not exhibit temporal pole grey matter abnormalities. These findings suggest that a reduction of temporal pole neocortical grey matter might contribute to temporal pole hypometabolism in temporal lobe epilepsy.

Pharmacological evaluation of a Br-76 analog of epibatidine: a potent ligand for studying brain nicotinic acetylcholine receptors.

[(76)Br]-Norchlorobromoepibatidine ([(76)Br]BrPH) is a specific and high affinity radioligand for the nicotinic acetylcholine receptors (nAChRs). In vitro, on rat thalamus membranes [(76)Br]BrPH bound to two sites with apparent affinities of 8 pM and 3 nM. The density of binding sites were 1.9 and 70 fmol/mg protein, respectively. In vivo, in biodistribution and autoradiographic studies in rats the regional distribution of [(76)Br]BrPH paralleled the neuroanatomical localization of nAChRs. Two hours postinjection, the highest concentration in the brain was found in thalamus and colliculi (4% ID/g). Competition experiments with specific nicotinic, muscarinic, dopaminergic, and serotoninergic drugs confirmed that the in vivo binding of [(76)Br]BrPH was consistent with neuronal nicotinic receptors. PET imaging of [(76)Br]BrPH in baboon demonstrated a rapid and high uptake in the brain. Peak uptake occurred at 30-40 min for the thalamus. Due to the constant washout in the cerebellum, the thalamus to cerebellum ratio was 5 at 2 h postinjection. Subcutaneous injection of cytisine (1 mg/kg), 3 h postinjection of [(76)Br]BrPH reduced the radioactivity concentration in thalamus and cortex by 58 and 50%, respectively, as observed 1 h later. Cytisine pretreatment (5 mg/kg s.c.) inhibited completely the radioligand accumulation in the thalamus. Chronic MPTP pretreatment resulted in reduction of [(76)Br]BrPH uptake in all brain regions except in cerebellum. These preliminary results suggest that [(76)Br]BrPH has the potential to be a useful radioligand for studying the pharmacology of nicotinic acetylcholine receptors in preclinical experiments.

Synthesis of a fluorine-18-labelled derivative of 6-nitroquipazine, as a radioligand for the in vivo serotonin transporter imaging with PET.

Considerable efforts have been engaged in the design, synthesis and pharmacological characterization of radioligands for imaging the serotonin transporter, based on its implication in several neuropsychiatric diseases, such as depression, anxiety and schizophrenia. In the 5-halo-6-nitroquipazine series, the fluoro derivative has been designed for positron emission tomography (PET). The corresponding 5-iodo-, 5-bromo- and 5-chloro N-Boc-protected quipazines as labelling precursors, as well as 5-fluoro-6-nitroquipazine as a reference compound have been synthesized. 5-[(18)F]Fluoro-6-nitroquipazine has been radiolabelled with fluorine-18 (positron-emitting isotope, 109.8 min half-life) by nucleophilic aromatic substitution from the corresponding N-Boc protected 5-bromo- and 5-chloro-precursors using K[(18)F]F-K(222) complex in DMSO by conventional heating (145 degrees C, 2 min) or microwave activation (50 W, 30-45 s), followed by removal of the protective group with TFA. Typically, 15-25 mCi (5.5-9.2 GBq) of 5-[(18)F]fluoro-6-nitroquipazine (1-2 Ci/micromol or 37-72 GBq/micromol) could be obtained in 70-80 min starting from a 550-650 mCi (20.3-24.0 GBq) aliquot of a cyclotron [(18)F]F(-) production batch (2.7-3.8% non decay-corrected yield based on the starting [(18)F]fluoride). Ex vivo studies (biodistribution in rat), as well as PET imaging (in monkey) demonstrated that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) readily crossed the blood brain barrier and accumulated in the regions rich in 5-HT transporter (frontal- and posterial cortex, striata). However, the low accumulation of the tracer in the thalamus (rat and monkey) as well as the comparable displacement of the tracer observed with both citalopram, a -HT re-uptake inhibitor and maprotiline, a norepinephrine re-uptake inhibitor (rat), indicate that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) does not have the suggested potential for PET imaging of the serotin transporter (SERT).

(+)-[76Br]A-69024: a non-benzazepine radioligand for studies of dopamine D1 receptors using PET.

(+)-[76Br]A-69024 is a specific and enantioselective dopamine D1 receptor radioligand. The Bmax of (+)-[76Br]A-69024 measured in vitro on rat striatum membranes was 320 +/- 25 fmoles/mg protein with an apparent dissociation constant of Kd = 0.6 +/- 0.1 nM. The inactive enantiomer, (-)-[76Br]A-69024, displayed no affinity in the same assay. In vivo, the biodistribution (+)-[76Br]A-69024 in rats showed a rapid and high uptake in the striatum (1% ID/g), followed by a slow wash out. The striatum/cerebellum concentration ratio (index of specific binding) reached a maximum value of 10 at 60 minutes post injection. A tissue to cerebellum ratio of 2.8 and 1.5 was also observed for frontal and posterior cortex respectively. With the pharmacologically inactive enantiomer, (-)-[76Br]A-69024, the brain uptake was determined to be non specific since a striatum/cerebellum ratio of approximately 1 was observed throughout the time course of the experiment. The selectivity of (+)-[76Br]A-69024 uptake was demonstrated in competition experiments. The specific uptake in the striatum and cortical regions was completely prevented after administration of the D1 antagonist SCH 23390. Pre-treatment of rats with unlabelled (+)A-69024 also displayed the same regional inhibition of (+)-[76Br]A-69024 uptake. Pre-administration of rats with spiperone (D2) and ketanserin (5-HT2/5-HT2C) showed no inhibitory effect on (+)-[76Br]A-69024 uptake in any brain region. Using (+)-[76Br]A-69024, PET study in baboon demonstrated a preferential accumulation of the radioactivity in the striatum, frontal and posterior cortex which was displaced to the level of the cerebellum by SCH 23390. These results suggest that (+)-[76Br]A-69024 may deserve further investigation as a potential radioligand for studying striatal and cortical dopamine D1 receptors using PET.