Evaluation of [¹⁸F]3F4AP in non-human primates: a PET tracer for K⁺ channels to image brain demyelination

Objectives: ¹⁸F-labeled 3-fluoro-4-aminopyridine, [¹⁸F]3F4AP, is a PET tracer for K⁺ channels developed to image multiple sclerosis (MS) lesions. [¹⁸F]3F4AP is a radiofluorinated analog of the MS drug 4-aminopyridine (4-AP, dalfampridine). 4-AP and 3F4AP bind to voltage-gated potassium (K⁺) channels (K_v1 family). During demyelination, axonal K⁺ channels, which are normally buried beneath the myelin sheath, become exposed, migrate through the demyelinated segment and increase in expression. Consequently, binding of [¹⁸F]3F4AP is increased in areas of demyelination. Recent studies have shown that this PET radioligand localizes to demyelinated areas in rodent models of MS and to less-myelinated areas in the brain of healthy rhesus monkeys¹. The goals of this study are to further evaluate the biodistribution of this tracer in non-human primates to obtain human dosimetry estimates and characterize the pharmacokinetic properties of the tracer.

Methods: [¹⁸F]3F4AP was produced through radiofluorination of 3-nitroisonicotinic methyl ester followed by hydrolysis of the ester and conversion of the carboxylic acid to the amine as recently described². Whole body dynamic PET/CT imaging was performed on a Siemens mCT scanner. Focused brain PET imaging was conducted on a High Resolution Research Tomograph (HRRT) scanner. Data were analyzed with VivoQuant. Results: [¹⁸F]3F4AP was produced with high purity (>98%) and high molar activity (2-3 Ci/μmol at EOS) in 90 min with a ~15% non-decay-corrected yield. [¹⁸F]3F4AP was found to distribute primarily to the kidneys, liver, brain, bone marrow, eyes and pituitary gland. [¹⁸F]3F4AP shows high penetration into the brain (SUV_peak ~3.5) followed by slow to moderate washout. Within the brain, binding of [¹⁸F]3F4AP is highest in areas rich in K⁺ channels such as the putamen and hippocampus and low in heavily myelinated areas such as the corpus callosum and the cerebellar white matter. Unusual retention in the kidneys was noticed, which is likely due to binding to K⁺ channels in this organ. Prominent uptake, presumed to be nonspecific binding to melanin pigment, was observed in the eyes. No bone uptake due to defluorination occurred. Conclusion: [¹⁸F]3F4AP is the first PET tracer for K⁺ channels. This radioligand, developed to image demyelinated lesions in MS, has high penetration into the brain, excellent metabolic stability and good overall properties for PET imaging. Further exploration in humans is warranted. Support: NIH/NIBIB, NINDS Intramural Research Program, Adelson Foundation and the Chicago Innovation Exchange from The University of Chicago. References: 1. Brugarolas P, Sánchez-Rodríguez J, Tsai HM, Basuli F, Cheng SH, Zhang X, Caprariello AV, Lacroix J, Freifelder R, Murali D, DeJesus O, Miller R, Swenson RE, Chen CT, Herscovitch P, Reich DS, Bezanilla F, Popko B[asterisk]. Development of a PET radioligand for potassium channels to image CNS demyelination. Sci. Rep. (2017). doi: 10.1038/s41598-017-18747-3 2. Basuli F, Zhang X, Brugarolas P, Reich DS, Swenson RE. An efficient new method for the synthesis of [18F]3-fluoro-4-aminopyridine via Yamada-Curtius rearrangement. J. Labelled. Comp. Radiopharm. (2017). doi: 10.1002/jlcr.3560