SPECT imaging with the D(4) receptor antagonist L-750,667 in nonhuman primate brain

D2 and D4 dopamine receptor mRNA distribution in pyramidal neurons and GABAergic subpopulations in monkey prefrontal cortex: implications for schizophrenia treatment

D2 and D4 dopamine receptors play an important role in cognitive functions in the prefrontal cortex and they are involved in the pathophysiology of neuropsychiatric disorders such as schizophrenia. The eventual effect of dopamine upon pyramidal neurons in the prefrontal cortex depends on which receptors are expressed in the different neuronal populations. Parvalbumin and calbindin mark two subpopulations of cortical GABAergic interneurons that differently innervate pyramidal cells. Recent hypotheses about schizophrenia hold that the root of the illness is a dysfunction of parvalbumin chandelier cells that produces disinhibition of pyramidal cells. In the present work we report double in situ hybridization histochemistry experiments to determine the prevalence of D2 receptor mRNA and D4 receptor mRNA in glutamatergic neurons, GABAergic interneurons and both parvalbumin and calbindin GABAergic subpopulations in monkey prefrontal cortex layer V. We found that around 54% of glutamatergic neurons express D2 mRNA and 75% express D4 mRNA, while GAD-positive interneurons express around 34% and 47% respectively. Parvalbumin cells mainly expressed D4 mRNA (65%) and less D2 mRNA (15-20%). Finally, calbindin cells expressed both receptors in similar proportions (37%). We hypothesized that D4 receptor could be a complementary target in designing new antipsychotics, mainly because of its predominance in parvalbumin interneurons.

SPECT imaging with the serotonin transporter radiotracer [123I]p ZIENT in nonhuman primate brain

INTRODUCTION

Serotonin dysfunction has been linked to a variety of psychiatric diseases; however, an adequate SPECT radioligand to probe the serotonin transporter system has not been successfully developed. The purpose of this study was to characterize and determine the in vivo selectivity of iodine-123-labeled 2beta-carbomethoxy-3beta-(4'-((Z)-2-iodoethenyl)phenyl)nortropane, [(123)I]p ZIENT, in nonhuman primate brain.

METHODS

Two ovariohysterectomized female baboons participated in nine studies (one bolus and eight bolus to constant infusion at a ratio of 9.0 h) to evaluate [(123)I]p ZIENT. To evaluate the selectivity of [(123)I]p ZIENT, the serotonin transporter blockers fenfluramine (1.5, 2.5 mg/kg) and citalopram (5 mg/kg), the dopamine transporter blocker methylphenidate (0.5 mg/kg) and the norepinephrine transporter blocker nisoxetine (1 mg/kg) were given at 8 h post-radiotracer injection.

RESULTS

In the bolus to constant infusion studies, equilibrium was established by 4-8 h. [(123)I]p ZIENT was 93% and 90% protein bound in the two baboons and there was no detection of lipophilic radiolabeled metabolites entering the brain. In the high-density serotonin transporter regions (diencephalon and brainstem), fenfluramine and citalopram resulted in 35-71% and 129-151% displacement, respectively, whereas methylphenidate and nisoxetine did not produce significant changes (<10%).

CONCLUSION

These findings suggest that [(123)I]p ZIENT is a favorable compound for in vivo SPECT imaging of serotonin transporters with negligible binding to norepinephrine and dopamine transporters.

Synthesis of 5- and 6-substituted 2-(4-dimethylaminophenyl)-1,3-benzoxazoles and their in vitro and in vivo evaluation as imaging agents for amyloid plaque

A series of novel 5- and 6-substituted 2-(4-dimethylaminophenyl)-1,3-benzoxazoles was synthesized and their potential as imaging probes for Alzheimer's Disease (AD)-related amyloid plaque was evaluated in vitro and in vivo. In vitro binding affinities for Abeta1-40 peptide of several of these compounds were in the low-nanomolar range . The lowest K(i) of 9.3nM was found for N-(2-(4-(dimethylamino)phenyl)-1,3-benzoxazol-5-yl)-4-iodobenzamide (1e). Its (123)I-radiolabeled form ([(123)I]1e) was subsequently prepared by iododestannylation of the corresponding tributylstannyl precursor and evaluated in vivo in a baboon model using SPECT imaging. Contrary to our expectations, 1e did not cross the blood-brain barrier (BBB) to any significant extent.

GD. Synthesis, radiolabeling and baboon SPECT imaging of 2beta-carbomethoxy-3beta-(3'-[(123)I]iodophenyl)tropane ([(123)I]YP256) as a serotonin transporter radiotracer

To develop a potential SPECT probe to evaluate the integrity of the serotoninergic system (5-HTT) whose dysfunction is linked to several disease conditions such as Parkinson's disease, Alzheimer's disease and depression, we report the synthesis, radiolabeling and in vivo baboon imaging of 2beta-carbomethoxy-3beta-(3'-[(123)I]iodophenyl) tropane (YP256, 6). The radiolabeling was performed by iododestannylation using sodium [(123)I]iodide and peracetic acid. Although the ligand displayed high selectivity for 5-HTT over dopamine transporter in vitro, SPECT imaging in baboons did not reveal selective 5-HTT accumulation in brain in vivo.

Human tobacco smokers in early abstinence have higher levels of beta2* nicotinic acetylcholine receptors than nonsmokers

Nicotine, the addictive chemical in tobacco smoke, initiates its actions in brain through nicotinic acetylcholine receptors (nAChRs). In particular, nAChRs containing beta2-subunits (beta2*-nAChRs) the most prevalent subtype, mediate the reinforcing properties of nicotine. We hypothesized that abnormal numbers of beta2*-nAChRs during early abstinence contribute to the perpetuation of addiction to tobacco smoking. Using molecular imaging, specifically single-photon emission computed tomography with the nAChR agonist radiotracer [123I]5-IA-85380 ([123I]5-IA), we imaged beta2*-nAChR availability in human smokers. First, using nonhuman primates treated chronically with nicotine, we estimated the time interval necessary for smokers to abstain from smoking so that residual nicotine would not interfere with [123I]5-IA binding to the beta2*-nAChR as approximately 7 d. Thus, we imaged human smokers at 6.8 +/- 1.9 d (mean +/- SD) of abstinence. Abstinence was confirmed by daily assessments of urinary cotinine and expired carbon monoxide levels. In smokers, [123I]5-IA uptake was significantly higher throughout the cerebral cortex (26-36%) and in the striatum (27%) than in nonsmokers, suggesting higher beta2*-nAChR in recently abstinent smokers. Beta2*-nAChR availability in recently abstinent smokers correlated with the days since last cigarette and the urge to smoke to relieve withdrawal symptoms but not the severity of nicotine dependence, severity of nicotine withdrawal, or the desire to smoke. Higher brain beta2*-nAChR during early abstinence indicates that, when smokers quit smoking, they do so in the face of a significant increase in the receptors normally activated by nicotine. Greater beta2*-nAChR availability during early abstinence may impact the ability of smokers to maintain abstinence.

Synthesis and evaluation of fluorine-substituted 1H-pyrrolo[2,3-b]pyridine derivatives for dopamine D4 receptor imaging

Seven fluorine-substituted 1H-pyrrolo[2,3-b]pyridine derivatives were synthesized based on a lead ligand, 3-[[4-(4-iodophenyl)piperazin-1-yl]-methyl]-1H-pyrrolo[2,3-b]pyridine (L-750,667) and evaluated as potential dopamine D(4) receptor imaging agents by positron emission tomography (PET). Binding affinities of these ligands for the dopamine D(2), D(3), and D(4) receptor subtypes were measured in vitro. Most ligands showed high and selective binding for the D(4) receptor. Ligand 7 had high affinity for the D(4) receptor, whereas ligands 1, 2, and 6 showed high selectivity for the D(4) receptor. LogP values were calculated for the ligands in this series and ligand 6 had the lowest lipophilicity. (18)F-labeled ligand 7 demonstrated a uniform regional brain distribution and a rapid washout in mice, probably due to nonspecific binding. Based on their in vitro binding properties and calculated logP values, ligand 6 appears to have the most promise for dopamine D(4) receptor imaging.

The dopamine D4 receptors and mechanisms of antipsychotic atypicality

The dopamine D4 receptor (D4) is a target for most common neuroleptic medications. After its initial discovery, it was found to possess the highest affinity of all dopamine receptor subtypes for the archetypical, atypical, antipsychotic clozapine. Nevertheless, initial clinical trials have not provided evidence that this receptor is a primary target for antipsychotic drugs. Considering the accumulated in vivo evidence that at least a subgroup of psychotic patients have altered dopamine signaling, all dopamine receptor subtypes likely contribute to the phenotypic expression of schizophrenia. New insights into the function of this receptor and its role in the modulation of excitatory signaling support the view that this dopamine receptor may affect attention and cognition. In this review, the authors outline some recent developments that provide insight into D4 receptor physiology, function and its possible relationship to schizophrenia treatment.

Synthesis of potent and selective dopamine D4 antagonists as candidate radioligands

A series of dopamine D(4) antagonists was synthesized and evaluated as potential candidates for development as positron emission tomography (PET) radioligands. All new compounds display high affinity and selectivity for the D(4) receptors and compounds 5b, 5d, and 5e were identified as candidates for radioligand development.