Karl-Johan Fasth

Preparation of 11C-labelled SCH 23390 for the in vivo study of dopamine D-1 receptors using positron emission tomography

The dopamine D-1 receptor antagonist, SCH 23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepin-7-ol), was labelled by alkylation of the desmethyl compound SCH 24518 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-5-phenyl-1H-3-benzazepin-7- ol) with [11C]methyl iodide. A multivariate optimization method, Simplex, was employed to obtain the optimal radiochemical yield. Both straight-phase and reversed-phase preparative HPLC were investigated in the purification of [11C]SCH 23390. Reaction in acetone with subsequent straight-phase LC separation resulted in 80% radiochemical yield, based on [11C]methyl iodide, with a total synthesis time of 35-40 min and a radiochemical purity greater than 99%. The average specific activity was on the order of 11.1 GBq/mmol. The 11C-labelled SCH 23390 was used to visualize the dopamine D-1 receptor-rich areas of a monkey brain by positron emission tomography. The data obtained showed a rapid distribution of radioactivity into the brain and a conspicuous accumulation of [11C]SCH 23390 in the striatum.

Tacrine restores cholinergic nicotinic receptors and glucose metabolism in alzheimer patients as visualized by positron emission tomography

Three patients with Alzheimers disease, a 68-year-old woman with mild dementia and 2 men (aged 64 and 72 years) with moderate dementia were treated orally with the cholinesterase inhibitor tacrine (tetrahydroaminoacridine), 80 mg daily, for several months. The patients were investigated using positron emission tomography (PET) prior to, and after 3 weeks and 3 months of treatment. The PET studies involved a multi-tracer system consisting of [18F]-fluoro-deoxy-glucose (18F-FDG) (tracer for glucose metabolism); 11C-butanol (cerebral blood flow) and (S)(-)- and (R)(+)-[N-11C-methyl]-nicotine (nicotinic receptors; cholinergic neural activity). Tacrine treatment increased the uptake of 11C-nicotine to the brain. Significant reduced difference in uptake between the two enantiomers (S)(-)- and (R)(+)11C-nicotine was observed in the frontal and temporal cortices after tacrine treatment in all three patients. The kinetic analysis indicated increased binding of (S)(-)11C-nicotine in brain compatible with a restoration of nicotinic cholinergic receptors. The most pronounced effect was observed after 3 weeks and 3 months treatment in the patient with mild dementia. An increase in cerebral glucose utilization was found in the 68-year-old patient with mild dementia but also slightly in the 64-year-old man with moderate dementia when treated with tacrine for 3 months. Tacrine administration did not affect cerebral blood flow. The PET data obtained after 3 weeks of tacrine treatment was paralleled by improvement in neuropsychological performance. This study shows in vivo by PET neurochemical effects induced in brain by treatment with tacrine to Alzheimer patients. Intervention with tacrine in the early course of the disease might be necessary for clinical improvement.

Manganese induced brain lesions inMacaca fascicularis as revealed by positron emission tomography and magnetic resonance imaging

A series of positron emission tomography scans was made on two monkeys during a 16-month period when they received manganese(IV)oxide by subcutaneous injection. The distribution of [11C]-nomifensine uptake, indicating dopamine terminals, was followed in both monkey brains. The brain distributions of [11C]-raclopride, demonstrating D2 dopamine receptors, and [11C]-l-dopa, as a marker of dopamine turnover, were followed in one monkey each. The monkeys developed signs of poisoning namely unsteady gait and hypoactivity. The [11C]-nomifensine uptake in the striatum was reduced with time and reached a 60% reduction after 16 months exposure. This supports the suggestion that dopaminergic nerve endings degenerate during manganese intoxication. The [11C]-l-dopa decarboxylation was not significantly altered indicating a sparing of [11C]-l-dopa decarboxylation during manganese poisoning. A transient decrease of [11C]-raclopride binding occurred but at the end of the study D2-receptor binding had returned to starting values. The magnetic resonance imaging (MRI) revealed that the manganese accumulated in the globus pallidus, putamen and caudate nucleus. There were also suggestions of gliosis/edema in the posterior limb of the internal capsule. MRI might be useful to follow manganese intoxication in humans as long as the scan is made within a few months of exposure to manganese, i. e. before a reversal of the manganese accumulation.

Brain networks affected by synchronized sleep visualized by positron emission tomography.

Nineteen lightly sleep-deprived healthy volunteers were examined with H215O and positron emission tomography (PET). Scanning was performed during wakefulness and after the subjects had fallen asleep. Sleep stage was graded retrospectively from electroencephalogram (EEG) recordings, and scans were divided into two groups: wakefulness or synchronized sleep. Global flow was quantified, revealing no difference between sleep and wakefulness. A pixel-by-pixel—blocked one-way analysis of variance (ANOVA) was performed after correcting for differences in anatomy and global flow. The sum of squares of the z-score distribution showed a highly significant (P < 0.00001) omnibus difference between sleep and wakefulness. The z-score images indicated decreased flow in the thalamus and the frontal and parietal association cortices and increased flow in the cerebellum during sleep. A principal component (PC) analysis was performed on data after correction for global flow and block effects, and a multivariate analysis of variance (MANOVA) on all PC scores revealed significant (P = 0.00004) differences between sleep and wakefulness. Principal components 2 and 5 correlated to sleep and revealed distinct networks consisting of PC 2, cerebellum and frontal and parietal association cortices, and PC 5, thalamus.

Middle Cerebral Artery Occlusion and Reperfusion in Primates Monitored by Microdialysis and Sequential Positron Emission Tomography

Background and Purpose— In a previous investigation concerning the hemodynamic and metabolic changes over time displayed by sequential positron emission tomography (PET) in a middle cerebral artery (MCA) occlusion/reperfusion primate model, a metabolic threshold for irreversible ischemia could be identified (reduction of metabolic rate of oxygen[CMRO2] to ≈60% of the contralateral hemisphere). To evaluate the potential of microdialysis (MD) as an instrument for chemical brain monitoring, the aim of this subsequent study was to relate the chemical changes in MD levels directly to the regional metabolic status (CMRO2 above or below the metabolic threshold) and the occurrence of reperfusion, as assessed by PET. Methods— Continuous MD (2 probes in each brain) and sequential PET measurements were performed during MCA occlusion (2 hours) and 18 hours (mean) of reperfusion in 8 monkeys (Macaca mulatta). Energy-related metabolites (lactate, pyruvate, and hypoxanthine) and glutamate were analyzed. The MD probe regions were divided into 3 categories on the basis of whether CMRO2 was below or above 60% of the contralateral region (metabolic threshold level) during MCA occlusion and whether reperfusion was obtained: severe ischemia with reperfusion (n=4), severe ischemia without reperfusion (n=4), and penumbra with reperfusion (n=5). Results— The lactate/pyruvate ratio, hypoxanthine, and glutamate showed similar patterns. MD probe regions with severe ischemia and reperfusion and probe regions with severe ischemia and no reperfusion displayed high and broad peaks, respectively, during MCA occlusion, and the levels almost never decreased to baseline. Penumbra MD probe regions displayed only slight transient increases during MCA occlusion and returned to baseline. Conclusions— This experimental study of focal ischemia showed that the extracellular changes of energy-related metabolites and glutamate differed depending on the ischemic state of the brain during MCA occlusion and depending on whether reperfusion occurred. If MD proves to be beneficial in clinical practice, it appears important to observe relative changes over time.

Amphetamine effects on dopamine release and synthesis rate studied in the Rhesus monkey brain by positron emission tomography

SummaryPositron emission tomography (PET) was used in a multitracer protocol to evaluate D-amphetamine induced effects on dopamine biosynthesis rate and release in propofol anesthetized Rhesus monkeys.l-[β-11C]DOPA was used as biochemical probe to study the brain dopamine biosynthesis rate whilst dopamine release was followed by the binding displacement of the [11C]-radiolabelled dopamine receptor antagonists, raclopride and N-methylspiperone. Studies were performed with either a constant rate intravenous infusion of D-amphetamine aiming at plasma concentrations of 0.2 to 25 ng/ml or with intravenous bolus doses of 0.1 and 0.4 mg/ kg. Decreased binding of the dopamine receptor antagonists was measured in both modes of D-amphetamine administration but notably [11C]N-methylspiperone was less able to sense D-amphetamine induced release of dopamine. At plasma concentrations aimed above 1 ng/ml a levelling off of the binding of [11C]raclopride at 68 ± 8.1% of the baseline value indicated that displacement was only possible from a fraction of the binding sites. Amphetamine was observed to increase the rate constant forl-[β-11C]DOPA utilization in the brain. This was most likely due to an acutely induced subsensitivity of presynaptic dopamine receptors.l-[β-11C]DOPA and [11C]raclopride were found suitable to indicate changes in dopamine synthesis rate and release respectively using PET and can be used to mirror drug-induced changes of brain dopaminergic function.

Rapid decrease in amino acid metabolism in prolactin-secreting pituitary adenomas after bromocriptine treatment: a PET study.

Four patients with prolactin-secreting pituitary adenomas were examined with positron emission tomography using L-[11C]methionine to monitor the effect of dopamine agonist treatment on the amino acid metabolism in the tumors. Within the first few hours after intramuscular injection of bromocriptine retard (50 mg) the amino acid metabolism decreased by 40%. Two of the patients were reexamined 7 and 9 days later and showed a 70% reduction in the metabolism of the adenomas. This metabolic effect was later accompanied by significant tumor shrinkage in all adenomas. It is suggested that bromocriptine has a general and rapid effect on the protein synthesis of the prolactin-secreting pituitary adenoma cells.

Methodological aspects for in vitro characterization of receptor binding using 11C-labeled receptor ligands: a detailed study with the benzodiazepine receptor antagonist [11C]Ro 15-1788.

As a complement to in vivo studies with positron emission tomography (PET), it is desirable to perform in vitro characterization of newly developed 11C tracers. In this report we describe the technique for determination of receptor-ligand kinetics utilizing ligands labeled with the short-lived radionuclide 11C. The limitations and advantages are discussed. The benzodiazepine antagonist [11C]Ro 15-1788 was used as a model substance, and the use of storage phosphor plates for quantification of radioactivity was validated. Storage phosphor plates showed an excellent linear range (approximately 10[3]) and acceptable resolution (approximately 0.5 mm). Receptor-ligand kinetics, including depletion, association and dissociation, saturation and displacement were evaluated with good results through the use of short-lived radiotracers and storage phosphor plates.

Effects of the substituted (S)-3-phenylpiperidine (−)-OSU6162 on PET measurements in subhuman primates: Evidence for tone-dependent normalization of striatal dopaminergic activity

(−)‐OSU6162 is a substituted (S)‐3‐phenylpiperidine derivative which exhibits some affinity to the dopamine D2 receptor family. In vivo, the compound displays a unique normalizing profile on psychomotor activity by an intriguing mixture of stimulatory and inhibitory properties. In the present investigation, some of the effects of (−)‐OSU6162 on central dopaminergic function were studied by positron emission tomography (PET) and L‐[11C]DOPA in anaesthetized female rhesus monkeys. (−)‐OSU6162 displayed a dopaminergic tone‐dependent effect with a reduction in the striatal L‐[11C]DOPA influx rate in monkeys with high baseline values and an increased striatal L‐[11C]DOPA influx rate in animals with low baseline values. Infusion of (−)‐OSU6162 for a whole day resulted in a stable effect with no evidence of tolerance. (−)‐OSU6162 also stabilized dopaminergic function by attenuating the upregulation of the striatal L‐[11C]DOPA influx rate which has previously been shown to occur following 6R‐BH4 or 6R‐BH4 + L‐tyrosine infusions. This “Protean” effect of (−)‐OSU6162 on the striatal dopaminergic function corresponds to previous behavioral observations in intact animals and demonstrates a true functional correlation to the measures obtained with L‐[11C]DOPA and PET. The normalizing and stabilizing profile of (−)‐OSU6162 should be of value in treating a variety of disorders where an underlying dysregulation or disruption of dopaminergic function can be assumed. Synapse 28:280–287, 1998.

Receptor imaging technique with 11C-labeled receptor ligands in living brain slices: its application to time-resolved imaging and saturation analysis of benzodiazepine receptor using [11C]Ro15-1788

Recently we developed a novel imaging technique using positron emitter-labeled compounds as probes and a storage phosphor screen as a detector. This approach makes it possible to follow a variety of biochemical processes with spatial information in living brain slices. Further technical development is reported here in terms of time-resolved imaging and receptor characterization in a real equilibrium state. The method was validated by use of [11C]Ro15-1788, a benzodiazepine receptor antagonist. Fresh brain slices were incubated with [11C]Ro15-1788 in oxygenated Krebs-Ringer solution at 37 degrees C, in a specially designed chamber. By placing the chamber on a storage phosphor screen, we could obtain two-dimensional images of radioactivity in the slices. Time-resolved imaging was made at 5 min intervals, revealing that it took 60 min to reach equilibrium binding. The dissociation process was observed by adding an excess amount of unlabeled Ro15-1788 to the chamber, 25 min was required for the full dissociation. In the equilibrium state, i.e. in the presence of free radio-ligand, Scatchard plot analysis was performed on the cerebral cortex (Kd = 7.4 nM, Bmax = 146 fmol/mg tissue) and striatum (Kd = 7.5 nM, Bmax = 107 fmol/mg tissue), suggesting the presence of a single component of binding site in these two regions. The present method, for the first time, made it possible to study a ligand-receptor interaction in living brain slices with temporal and spatial resolutions. This technique should prove useful for studies of receptor function under physiological conditions.