An efficient synthesis, PET evaluation and application of [18F]MEFWAY designed for imaging brain serotonin 1A receptors in rodents

Abstract

The changes of serotonin 1A (5-HT1A) receptor density in the central nervous system are related to various psychiatric disorders such as depression, anxiety and schizophrenia. Thereby in vivo imaging of brain 5-HT1A receptors has been an important subject in neuroscience. Recently, 4-([18F]fluoranylmethyl)-N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexane-1-carboxamide ([18F]MEFWAY) was developed as a positron emission tomography (PET) radioligand for quantifying the 5-HT1A receptors. Ex vivo autoradiography of [18F]MEFWAY represented a high affinity and selectivity to the 5-HT1A receptors. However, there is no report on testing its biochemical and pharmacological characteristics in vivo due to the low synthetic yield of its precursor. Here, I increased the synthetic yield of the [18F]MEFWAY precursor and studied the effectiveness of [18F]MEFWAY in vivo. First of all, I established an efficient synthetic method for the [18F]MEFWAY precursor, which consists of the improved acid chloride coupling reaction to activate the carboxylic acid and optimized reduction condition to suppress the breakdown of the amide bond. This new protocol dramatically improves the synthetic yield compared to the previous method (<8% → 45%). After F-18 was incorporated into the precursor, metabolic stability against defluorination and efficacy of [18F]MEFWAY were assessed in the rat brain. Unexpectedly, [18F]MEFWAY showed severe skull uptake due to the defluorination. It is known that cytochrome P450 2E1 is a major metabolizing enzyme that causes in vivo defluorination of 18F-based radioligands and can be suppressed by antifungal drugs (i.e. miconazole and fluconazole). The skull uptake of [18F]MEFWAY was suppressed by the pretreatment of miconazole and fluconazole up to 68% or 80%, respectively. Then, I confirmed the specificity of [18F]MEFWAY to 5-HT1A receptors by the pre-block and displacement experiments using a highly selective 5-HT1A antagonist.Finally, I tested the utility of [18F]MEFWAY in animal disease models. Unilateral 6-OHDA lesion and forced swimming were respectively used to evoke Parkinson''s disease model and the acute model for depression. In the unilateral 6-OHDA lesion rats, binding potential in the hippocampus was significantly reduced bilaterally compared with that of the control. The acute model for depression also exhibited significant decrement of binding potential in the hippocampus. These results suggested that [18F]MEFWAY is a useful PET radioligand to examine the changes of 5-HT1A receptor density in vivo.