Synthesis and Preclinical Evaluation of ¹⁸F-labeled Ketoprofen Methyl Esters for Cyclooxygenase-1 Imaging in Neuroinflammation

Abstract

Cyclooxygenase (COX) is a rate-limiting enzyme in the synthesis of pro-inflammatory prostanoids from arachidonic acid. In vivo imaging of COX by positron emission tomography (PET) is a potentially powerful tool for assessing the inflammatory response to injury, infection, and disease. We previously reported on a promising PET probe for COX imaging, ¹¹C-labeled ketoprofen methyl ester, which can detect COX-1 activation in models of neuroinflammation and neurodegenerative disorders. In the current study, we aimed to design a fluorine-substituted benzoyl group of ketoprofen (FKTP) and to evaluate its racemate and enantiomers (¹⁸F-labeled ketoprofen methyl ester [¹⁸F]FKTP-Me) as PET pro-radiotracers, potential radiopharmaceuticals for in vivo PET study of COX-1. Methods: We performed nucleophilic aromatic ¹⁸F-fluorination in order to obtain the desired racemic radiolabeled probe (RS)-[¹⁸F]FKTP-Me at a radiochemical yield of 11–13%. Subsequent high performance liquid chromatography separation with a chiral column yielded the desired enantiomerically pure (R)- and (S)-[¹⁸F]FKTP-Me. We examined the in vivo properties of (RS)-, (R)-, and (S)-[¹⁸F]FKTP-Me in PET studies using rats in which hemispheric inflammation was induced by intrastriatally injecting a lipopolysaccharide. Results: Racemic (RS)-[¹⁸F]FKTP-Me and enantiomeric (R)- or (S)-[¹⁸F]FKTP-Me were synthesized with radiochemical and chemical purities of >99%. The metabolite analysis revealed that the racemic (RS)-[¹⁸F]FKTP-Me crossed the blood-brain barrier and entered the brain, where it was subsequently hydrolyzed to its pharmacologically active acid form. PET images revealed a high accumulation of (R)-, (S)-, and (RS)-[¹⁸F]FKTP in the inflamed regions in rat brain. Moreover, the accumulated radioactivity of (S)-[¹⁸F]FKTP-Me was higher than that of (RS)-[¹⁸F]FKTP-Me and (R)-[¹⁸F]FKTP-Me, which was correlated with the stereospecific inhibitory activity of FKTP against COX-1. Conclusion: Based on the results of this study, we conclude that racemic (RS)-[¹⁸F]FKTP-Me and its enantiomers could act as pro-radiotracers of neuroinflammation in rat brain by the association of their hydrolyzed acid forms with COX-1 in inflamed regions. In particular, (S)-[¹⁸F]FKTP-Me demonstrated suitable properties as a COX-1-specific probe in PET imaging of neuroinflammation.