Positron emission tomography studies of brain receptors

Positron emission tomography: ligand imaging

Since it was first used to image the brain in 1976, positron emission tomography (PET) has been utilized in a wide range of neurologic and psychiatric applications. From cerebral metabolism to receptor concentration, various PET imaging techniques involving a host of radiopharmaceuticals have provided insight into countless facets of both the normal and diseased brain. Although the majority of these radiopharmaceuticals are still limited to the realm of research, one PET ligand in particular has gained widespread clinical use: (18)F-fluorodeoxyglucose, a radiolabeled analog of glucose, has become an exceedingly prevalent clinical tool for the measurement of metabolism in organs throughout the body, including the brain. In recent years, a number of novel PET ligands have also made it through the US Food and Drug Administration approval process and been used clinically. However, gaining approval is by no means the only challenge facing these radiopharmaceuticals. Traversing the blood-brain barrier is a formidable obstacle in drug delivery, and accurately modeling tracer kinetics and correcting for the partial-volume effect are among the difficult tasks that remain once the ligand reaches its intended target. Even so, the use of PET imaging in neurology and psychiatry can be expected to expand in the coming years as novel radiopharmaceuticals continue to be developed.

Imaging in cell-based therapy for neurodegenerative diseases

Fetal cell transplantation for the treatment of Parkinson's and Huntington's diseases has been developed over the past two decades and is now in early clinical testing phase. Direct assessment of the graft's survival, integration into the host brain and impact on neuronal functions requires advanced in vivo neuroimaging techniques. Owing to its high sensitivity, positron emission tomography is today the most widely used tool to evaluate the viability and function of the transplanted tissue in the brain. Nuclear magnetic resonance techniques are opening new possibilities for imaging neurochemical events in the brain. The ultimate goal will be to use the combination of multiple imaging modalities for complete functional monitoring of the repair processes in the central nervous system.

Correlation between quantitative imaging and behavior in unilaterally 6-OHDA-lesioned rats

We evaluated correlation between neurochemical and functional alterations of the nigrostriatal dopaminergic system in rat brains lesioned with 6-hydroxydopamine (6-OHDA), that model hemi-Parkinson's disease (PD), by using three different quantitative in vivo and in vitro methods. Rats unilaterally lesioned with different doses of 6-OHDA underwent two types of in vivo experiments: (1) a rotational behavioral study with methamphetamine (MAP) or apomorphine (APO); and (2) a positron emission tomography (PET) study with [11C]PE2I (radioligand for dopamine transporters) or [11C]raclopride (radioligand for dopamine D2 receptors). An in vitro autoradiographic study with the same radioligands was also conducted. The number of rotations after the MAP or APO injection increased with increased doses of 6-OHDA. The in vitro and in vivo binding of [11C]PE2I dose-dependently decreased in response to the 6-OHDA injections, while that of [11C]raclopride dose-dependently increased. There was a significant negative hyperbolic correlation between the number of rotations after MAP injection and the binding of [11C]PE2I. In contrast, there was a significant positive linear correlation between the number of rotations after APO injections and the binding of [11C]raclopride. These results robustly reveal a molecular pharmacological basis of parkinsonian symptoms in animal models of PD, and indicate the utility and validity of in vivo PET measurements in assessing pre- and post-synaptic dopaminergic functions.

The role of PET imaging in the management of patients with central nervous system disorders

PET will continue to play a critical role in both clinical and research applications with regard to CNS disorders. PET is useful in the initial diagnosis of patients presenting with CNS symptoms and can help clinicians determine the best course of therapy. PET studies can also be useful for studying the response to therapy. From the research perspective, the various neurotransmitter and other molecular tracers currently available or in development will provide substantial information about pathophysiologic process in the brain. As such applications become more widely tested, their introduction into the clinical arena will further advance the use of PET imaging in the evaluation and management of CNS disorders.

Clinical pharmacology of psychotropic drugs

No Abstract

Binding potency of 6-nitroquipazine analogues for the 5-hydroxytryptamine reuptake complex

The in-vitro inhibition constants (Ki) of nine structural analogues of the potent 5-hydroxytryptamine (5-HT)-uptake inhibitor, 6-nitroquipazine, were determined to assess the structure-affinity relationship of these derivatives. The goal of these studies was to determine those positions on 6-nitroquipazine that could be derivatized without significantly decreasing the affinity of the drug for the binding site, so that radiolabels such as 123I, 76Br or 18F might be appended for in-vivo imaging studies of the 5-HT reuptake system. Using bromine as a steric probe, the rank order of potency of bromine-substituted 6-nitroquipazine analogues for inhibiting the binding of [3H]paroxetine to the 5-HT reuptake binding site was: 8- < 3- < 7- < 4- < 5-bromo. The in-vitro equipotent molar ratio (EPMR, Ki (analogue)/Ki(6-nitroquipazine)) of the 5-bromo analogue was 0.57, indicating that this analogue had greater affinity for the 5-HT reuptake complex than 6-nitroquipazine. Derivatization at the 5-position with fluorine and iodine also resulted in potent compounds with EPMR values of 1.1 and 0.83, respectively. Substitution of quipazine with bromo, cyano, and formyl groups at the 6-position produced less potent compounds than the 6-nitro group. Based upon the high affinities of the 5-bromo-, 5-fluoro- and 5-iodo-6-nitroquipazines for the 5-HT reuptake complex, these compounds are candidates for radiolabeling for in-vivo studies of the 5-HT reuptake site.

Selective synthesis of racemic 1-11C-labelled norepinephrine, octopamine, norphenylephrine and phenylethanolamine using [11C]nitromethane

A new and simple method for the selective condensation of no-carrier-added [11C]nitromethane (1) with various substituted (protected) benzaldehydes to [beta-11C]beta-nitrophenethyl alcohols was developed. This method which utilizes tetrabutylammonium fluoride in THF as a catalyst gave a condensation yield of 80-90% and a selectivity of 80-90% for [11C]nitroalcohol vs [11C]nitrostyrene formation within 2 min. Reduction of these [11C]nitroalcohols with Raney nickel in formic acid gave the corresponding [11C]aminoalcohols in a yield of 60-90%. Boron tribromide was used for the cleavage of 4-methoxy and 3,4-(methylenedioxy) phenol protecting groups. After HPLC-purification, racemic 1-11C-labelled norepinephrine (7), phenylethanolamine (4), norphenylephrine (5) and octopamine (6) were prepared in a 12-30% decay corrected total radiochemical yield (20-50% counted from 1) with an overall synthesis time of 40-70 min from end of bombardment (EOB). The radiochemical purity was > 98% and the specific radioactivity 700-1500 Ci/mmol (26-56 GBq/mumol).

Kinetics of the uptake of [3H]paroxetine in the rat brain

Paroxetine, an antidepressant with a high affinity for serotonin (5-HT) re-uptake sites, is a potential tracer of these sites. We determined the kinetic properties of [3H]paroxetine in rat brain in vivo. Relative to [14C]iodo-antipyrine, the brain uptake index (BUI) of [3H]paroxetine was 60-70%. The unidirectional blood clearance of [3H]paroxetine were 0.05-0.12 ml g-1 min-1, lower than expected from the BUI values. The steady state volume of distribution was 3.5 ml hg-1 in the diencephalon and 1.8 ml g-1 in the cerebellum, suggesting a binding potential of unity. Autoradiographs at four hours after [3H]paroxetine injection (300 microCi, i.p.) revealed heterogeneous binding consistent with the calculated binding potentials. Binding was nearly absent from cerebellum and was highest in the dorsal raphe, superior colliculus, dorsal hypothalamus, and entorhinal cortex, but did not reach equilibrium in four hours of tracer circulation. The specific binding relative to vermis was displaced by pretreatment with fluoxetine (10 mg/kg, i.p.).