Neuroreceptor imaging of stress and mood disorders

Synthesis and Evaluation of Mefway Analogs as Ligands for Serotonin 5HT1A Receptors

¹⁸F-Mefway (N-{2-[4-(2'-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(4'-¹⁸F-fluoro-methylcyclohexane)carboxamide) was developed and evaluated for use as a PET ligand for imaging 5-HT_1A receptors. Ongoing studies of ¹⁸F-Mefway have shown it to be an effective PET radiotracer. We have synthesized isomers of Mefway by changing the position of the methyl-group in attempts to evaluate stability for imaging purposes. 2-Methyl-, 3-methyl-, and 4-methyl-cyclohexane-1-carboxylic acids and 3-carbomethoxy-, 4-carbomethoxycyclohexane-1-carboxylic acids were coupled with WAY-100634 to provide the methylcyclohexyl derivatives (2-, 3- and 4-methyl). Mefway and 3-Mefway analogs were prepared by reduction of carbomethoxy-derivatives followed by fluorination. In vitro binding affinities for the methylated derivatives in rat brain homogenates was found to be 10.4 nM (2-methyl), 77 nM (3-methyl) and 21.5 nM (4-methyl). Binding affinity of 3-Mefway and 4-Mefway was found to be 17.4 nM and 6.26 nM, respectively. Our results suggest that 3-methyl/3-fluoromethyl substituent has approx. 3-fold lower affinities compared to the 4-methyl/4-fluoromethyl substituent.

Evaluation of serotonin 5-HT(1A) receptors in rodent models using [¹⁸F]mefway PET

INTRODUCTION

Serotonin 5-HT(1A) receptors have been investigated in various CNS disorders, including epilepsy, mood disorders, and neurodegeneration. [¹⁸F]Mefway (N-{2-[4-(2'-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(cis/trans-4'-[¹⁸F]fluoromethylcyclohexane)-carboxamide) has been developed as a suitable positron emission tomography (PET) imaging agent for these receptors. We have now evaluated the suitability of [¹⁸F]trans-mefway in rat and mouse models using PET and computerized tomography (CT) imaging and corroborated with ex vivo and in vitro autoradiographic studies.

METHODS

Normal Sprague-Dawley rats and Balb/C mice were used for PET/CT imaging using intravenously injected [¹⁸F]trans-mefway. Brain PET data were coregistered with rat and mouse magnetic resonance imaging template and regional distribution of radioactivity was quantitated. Selected animals were used for ex vivo autoradiographic studies to confirm regional brain distribution and quantitative measures of binding, using brain region to cerebellum ratios. Binding affinity of trans-mefway and WAY-100635 was measured in rat brain homogenates. Distribution of [¹⁸F]trans-4-fluoromethylcyclohexane carboxylate ([¹⁸F]FMCHA), a major metabolite of [¹⁸F] trans-mefway, was assessed in the rat by PET/CT.

RESULTS

The inhibition constant, K(i) for trans-mefway was 0.84 nM and that for WAY-100635 was 1.07 nM. Rapid brain uptake of [¹⁸F]trans-mefway was observed in all rat brain regions and clearance from cerebellum was fast and was used as a reference region in all studies. Distribution of [¹⁸F]trans-mefway in various brain regions was consistent in PET and in vitro studies. The dorsal raphe was visualized and quantified in the rat PET but identification in the mouse was difficult. The rank order of binding to the various brain regions was hippocampus > frontal cortex > anterior cingulate cortex > lateral septal nuclei > dorsal raphe nuclei.

CONCLUSION

[¹⁸F]trans-Mefway appears to be an effective 5-HT(1A) receptor imaging agent in rodents for studies of various disease models.

Cardiac syndrome X in women: the role of oestrogen deficiency

Cardiac syndrome X (CSX), defined as typical exertional chest pain, a positive response to stress testing, and normal coronary arteriograms, encompasses different pathogenic subgroups. Both cardiac and non-cardiac mechanisms have been suggested to play a pathogenic role, and it has been shown that the syndrome is associated with myocardial ischaemia in at least a proportion of patients. Radionuclide myocardial perfusion defects, coronary sinus oxygen saturation abnormalities and pH changes, myocardial lactate production and stress-induced alterations of cardiac high energy phosphate have been reported in CSX patients, suggesting an ischaemic origin for their symptoms. Microvascular abnormalities often caused by endothelial dysfunction appear to be responsible for myocardial ischaemia in these patients. CSX is more prevalent in women than in men, and the majority of women with CSX are peri- or post-menopausal. Thus oestrogen deficiency has been suggested to have a pathogenic role in CSX. Additional factors such as abnormal pain perception may also contribute to the genesis of chest pain in patients with angina and normal coronary angiograms. The management of this syndrome is difficult because of the heterogeneity of pathogenic mechanisms and uncertainties as to its origin. This article discusses the problem of CSX in women, the potential pathogenic role of oestrogen deficiency, and practical clinical management.