Synthesis and preclinical evaluation of [11C-carbonyl]PF-04457845 for neuroimaging of fatty acid amide hydrolase

Abstract

Fatty acid amide hydrolase (FAAH) has a significant role in regulating endocannabinoid signaling in the central nervous system. As such, FAAH inhibitors are being actively sought for pain, addiction, and other indications. This has led to the recent pursuit of positron emission tomography (PET) radiotracers targeting FAAH. We report herein the preparation and preclinical evaluation of [(11)C-carbonyl]PF-04457845, an isotopologue of the potent irreversible FAAH inhibitor. PF-04457845 was radiolabeled at the carbonyl position via automated [(11)C]CO(2)-fixation. Ex vivo brain biodistribution of [(11)C-carbonyl]PF-04457845 was carried out in conscious rats. Specificity was determined by pre-administration of PF-04457845 or URB597 prior to [(11)C-carbonyl]PF-04457845. In a separate experiment, rats injected with the title radiotracer had whole brains excised, homogenized and extracted to examine irreversible binding to brain parenchyma. The title compound was prepared in 5 ± 1% (n = 4) isolated radiochemical yield based on starting [(11)C]CO(2) (decay uncorrected) within 25 min from end-of-bombardment in >98% radiochemical purity and a specific activity of 73.5 ± 8.2 GBq/μmol at end-of-synthesis. Uptake of [(11)C-carbonyl]PF-04457845 into the rat brain was high (range of 1.2-4.4 SUV), heterogeneous, and in accordance with reported FAAH distribution. Saturable binding was demonstrated by a dose-dependent reduction in brain radioactivity uptake following pre-treatment with PF-04457845. Pre-treatment with the prototypical FAAH inhibitor, URB597, reduced the brain radiotracer uptake in all regions by 71-81%, demonstrating specificity for FAAH. The binding of [(11)C-carbonyl]PF-04457845 to FAAH at 40 min post injection was irreversible as 98% of the radioactivity in the brain could not be extracted. [(11)C-carbonyl]PF-04457845 was rapidly synthesized via an automated radiosynthesis. Ex vivo biodistribution studies in conscious rodents demonstrate that [11C PF-04457845 is a promising candidate radiotracer for imaging FAAH in the brain with PET. These results coupled with the known pharmacology and toxicology of PF-04457845 should facilitate clinical translation of this radiotracer.