Development of a PET radioligand for potassium channels to image CNS demyelination

Abstract

Central nervous system (CNS) demyelination represents the pathological hallmark of multiple sclerosis (MS) and contributes to other neurological conditions. Quantitative and specific imaging of demyelination would thus provide critical clinical insight. Here, we investigated the possibility of targeting axonal potassium channels to image demyelination by positron emission tomography (PET). These channels, which normally reside beneath the myelin sheath, become exposed upon demyelination and are the target of the MS drug, 4-aminopyridine (4-AP). We demonstrate using autoradiography that 4-AP has higher binding in non-myelinated and demyelinated versus well-myelinated CNS regions, and describe a fluorine-containing derivative, 3-F-4-AP, that has similar pharmacological properties and can be labeled with 18F for PET imaging. Additionally, we demonstrate that [18F]3-F-4-AP can be used to detect demyelination in rodents by PET. Further evaluation in Rhesus macaques shows higher binding in non-myelinated versus myelinated areas and excellent properties for brain imaging. Together, these data indicate that [18F]3-F-4-AP may be a valuable PET tracer for detecting CNS demyelination noninvasively.

Evaluation of the potassium channel tracer [18F]3F4AP in rhesus macaques

“…Receptor panel screen: Samples of non-radiolabeled 4AP and 3F4AP were sent to the psychoactive drug screening program (PDSP) from the NIH/NIMH at the University of North Carolina 36 and Monkey 2) were scanned on a Siemens mCT PET/CT scanner for 4h as previously described 27 . Procedural details are provided in the SI.…”

“…3-fluoro-4-aminopyridine, 3F4AP, is a fluorinated analog of 4AP that binds with similar affinity as 4AP to K v 1 channels expressed in Xenopus oocytes as well as explanted optic nerves from mice 27,28 . It has been recently shown by autoradiography that [ 14 C]4AP and [ 14 C]3F4AP accumulate in demyelinated areas in rodent brain after intravenous administration 27 .…”

“…3-fluoro-4-aminopyridine, 3F4AP, is a fluorinated analog of 4AP that binds with similar affinity as 4AP to K v 1 channels expressed in Xenopus oocytes as well as explanted optic nerves from mice 27,28 . It has been recently shown by autoradiography that [ 14 C]4AP and [ 14 C]3F4AP accumulate in demyelinated areas in rodent brain after intravenous administration 27 . This is due to the increased level of expression of K v 1 channels in those areas as well as the increased accessibility of the drug to the channels given the lack of myelin 19,20,29,30 .…”

“…This is due to the increased level of expression of K v 1 channels in those areas as well as the increased accessibility of the drug to the channels given the lack of myelin 19,20,29,30 . It has also been reported that 3F4AP can be labeled with fluorine-18 [31][32][33] and that radiofluorinated [ 18 F]3F4AP can be used to detect chemically-induced demyelinated lesions in rat brains using PET 27 . The high sensitivity of this tracer towards demyelination makes it a promising candidate for imaging many neurological diseases where there is demyelination including MS, TBI, SCI, stroke, AD, and others 30 .…”

Synthesis of meta-substituted [18F]3-fluoro-4-aminopyridine via direct radiofluorination of pyridine N-oxides

“…For the current project, we were faced with the challenge of developing a labeling method for [ 18 F]3-fluoro-4-aminopyridine ( 10 ). 3-fluoro-4-aminopyridine is a fluorine-containing derivative of 4-aminopyridine, a clinically approved drug for multiple sclerosis that binds to potassium channels in the central nervous system, currently under investigation for imaging demyelination 10–12 . In this report, we describe how we synthesized this compound and discuss our findings on the different reactivity of 3-bromo-4-nitropyridine and 3-bromo-4-nitropyridine N-oxide towards fluorination.…”