Synthesis and Evaluation of Mefway Analogs as Ligands for Serotonin 5HT_1A 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.