Radiosynthesis and evaluation of N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(4-fluorophenyl)thiazol-2-yl]-1-[11C]carboxamide for in vivo positron emission tomography imaging of fatty acid amide hydrolase in brain

Abstract

We developed a novel positron emission tomography (PET) radiotracer N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(4-fluorophenyl)thiazol-2-yl]-1-[¹¹C]carboxamide ([¹¹C]DPFC, [¹¹C]1) for in vivo imaging of fatty acid amide hydrolase (FAAH) in rat brain. Compound 1 showed a high binding affinity for FAAH (IC₅₀: 3.3 nM). [¹¹C]1 was synthesized by reaction of 5-amino-3,4-dimethylisoxazole (2) with [¹¹C]phosgene ([¹¹C]COCl₂), followed by reaction with 4-(4-fluorophenyl)-2-(piperazin-1-yl)thiazole (3), with a 9 ± 4% radiochemical yield (decay-corrected, n = 9) based on [¹¹C]CO₂. A biodistribution study in mice showed a high uptake of radioactivity in FAAH-rich organs, including the lung, liver, and kidney. PET summation images of rat brains showed high radioactivity (>2 SUV) in the cerebellar nuclei and frontal cortex. This pattern was consistent with the known regional distribution pattern of FAAH in the rodent brain. Pretreatment with the FAAH-selective inhibitor URB597 significantly reduced the whole brain uptake of [¹¹C]1. At 30 min after the radiotracer injection, more than 95% of the total radioactivity was found to be irreversible in the brain homogenate of rats. Our results indicate that [¹¹C]1 is a promising PET tracer for in vivo visualization of FAAH in living brains.