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 proinflammatory prostanoids from arachidonic acid. In vivo imaging of COX by 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 proradiotracers, potential radiopharmaceuticals for in vivo PET study of COX-1. Methods: We performed nucleophilic aromatic 18F-fluorination 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 more than 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: From the results of this study, we conclude that racemic (RS)-[18F]FKTP-Me and its enantiomers could act as proradiotracers 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.

• Authors: Miho Shukuri, Aya Mawatari, Shuhei Takatani, Tsuyoshi Tahara, Michiko Inoue, Wakiko Arakaki, Masahiro Ohno, Hisashi Doi, Hirotaka Onoe
• Journal: Journal of nuclear medicine : official publication, Society of Nuclear Medicine (J Nucl Med) Vol. 63 Issue 11 Pg. 1761-1767 (11 2022) ISSN: 1535-5667 [Electronic] United States
• PMID: 35332095 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
• Copyright: © 2022 by the Society of Nuclear Medicine and Molecular Imaging.

Chemical References

• Cyclooxygenase 1
• Ketoprofen
• ketoprofen methyl ester
• Radiopharmaceuticals

Topics

• Animals
• Rats
• Cyclooxygenase 1 (metabolism)
• Ketoprofen (metabolism)
• Neuroinflammatory Diseases
• Positron-Emission Tomography (methods)
• Radiopharmaceuticals (chemistry)