Chemical and biophysical characterization of novel potassium channel blocker 3-fluoro-5-methylpyridin-4-amine

4-aminopyridine (4AP) is a potassium (K+) channel blocker used clinically to improve walking in people with multiple sclerosis (MS). 4AP binds to exposed K+ channels in demyelinated axons, reducing the leakage of intracellular K+ and enhancing impulse conduction. Multiple derivatives of 4AP capable of blocking K+ channels have been reported including three radiolabeled with positron emitting isotopes for imaging demyelinated lesions using positron emission tomography (PET). Here, we describe 3-fluoro-5-methylpyridin-4-amine (5Me3F4AP), a novel K+ channel blocker with potential application in PET. 5Me3F4AP has comparable potency to 4AP and the PET tracer 3-fluoro-4-aminopyridine (3F4AP). Compared to 3F4AP, 5Me3F4AP is more lipophilic (logD = 0.664 {+/-} 0.005 vs. 0.414 {+/-} 0.002) and slightly more basic (pKa = 7.46 {+/-} 0.01 vs. 7.37 {+/-} 0.07). In addition, 5Me3F4AP appears to be more permeable to an artificial brain membrane and more stable towards oxidation by the cytochrome P450 enzyme family 2 subfamily E member 1 (CYP2E1), responsible for the metabolism of 4AP and 3F4AP. Taken together, 5Me3F4AP has promising properties for PET imaging warranting additional investigation. Significance StatementThe PET tracer [18F]3-fluoro-4-aminopyridine ([18F]3F4AP) binds to K+ channels in demyelinated axons and has shown promise for imaging demyelinated lesions in animal models. However, its use in humans may be compromised due to rapid metabolism. Thus, a novel 3F4AP derivative amenable to labeling with fluorine-18 was designed and evaluated in vitro. The results indicate that 5-methyl-3F4AP exhibits high binding affinity, good physicochemical properties and slower oxidation by CYP2E1 than 3F4AP, making it a promising candidate for further PET studies.