Synthesis of K+ channel radioligand [18F]5-methyl-3-fluoro-4-aminopyridine and PET imaging in mice

[¹⁸F]3-fluoro-4-aminopyridine ([¹⁸F]3F4AP) is the first positron emission tomography (PET) radioligand that targets voltage-gated potassium (K⁺) channels in the brain for imaging demyelination. [¹⁸F]3F4AP exhibits high brain penetration, favorable kinetics for PET imaging, and high sensitivity to demyelinating lesions. However, recent studies in awake human subjects indicate lower metabolic stability than in anesthetized animals, resulting in reduced brain uptake. Therefore, there is a need for novel radioligands for K⁺ channels with suitable pharmacological properties and enhanced metabolic stability. Recent in vitro studies demonstrate that 5-methyl-3-fluoro-4-aminopyridine (5Me3F4AP) exhibits comparable binding affinity to K⁺ channels, pK_a, logD, and membrane permeability as 3F4AP, and a slower enzymatic metabolic rate, suggesting its potential as a K⁺ channel PET tracer. In this study, we describe the radiochemical synthesis of [¹⁸F]5Me3F4AP using an isotope exchange method from the corresponding 3-fluoro-5-methyl-4-nitropyridine N-oxide, followed by a palladium on carbon mediated hydrogenation of the nitro and N-oxide groups. This method yielded [¹⁸F]5Me3F4AP with high purity and acceptable molar activity. PET/CT studies using naïve mice demonstrate that [¹⁸F]5Me3F4AP effectively crosses the blood–brain barrier and has comparable kinetics to [¹⁸F]3F4AP. These findings strongly suggest that [¹⁸F]5Me3F4AP is a promising candidate for neuroimaging applications and warrant further studies to investigate its sensitivity to lesions and in vivo metabolic stability.