Transdermal Delivery of 4‐Aminopyridine, a Potassium Channel Blocker, Accelerates Motor Functional Recovery with Improved Nerve Myelination and Conduction Velocity after Sciatic Nerve Crush Injury in Mice
Authors: Talukder M.A. Hassan , Clark Andrew R , Hsu Chia , Turpin Loel C , Noble Mark , Elfar John C 
Published: June 2021

4‐Aminopyridine (4‐AP), a broad‐spectrum potassium channel blocker and FDA‐approved drug for symptomatic treatment of multiple sclerosis, has been shown to improve neuromuscular function in patients with diverse demyelinating disorders. We recently repurposed 4‐AP and demonstrated that systemic 4‐AP administration enhances global functional recovery of the affected limb, promotes remyelination of the nerve and improves the nerve conduction velocity in a mouse model of traumatic peripheral nerve injury. However, despite its beneficial effects, the clinical ability of 4‐AP to restore function is limited because of its narrow therapeutic window, the need for frequent dosing throughout the day, and potential adverse effects. Although oral or injection routes are commonly used, the therapeutic benefits with transdermal delivery of drugs are well documented to provide a sustained circulating blood levels with enhanced patient compliance and importantly without the need for multiple daily oral dosing or injections of a drug with a short half‐life. There is no current data on the transdermal delivery of 4‐AP (plasma half‐life ~3.5 hours and the effect of therapeutic window 4–7 hours post dosing). We asked whether 4‐AP could be used as a transdermal agent with an eye towards its pharmacokinetics and effects on motor function and neuronal recovery after peripheral nerve injury. Mice were assigned to moderate sciatic nerve crush injury and the effects of acute and chronic treatments with transdermal 4‐AP and vehicle (DMSO) were investigated. Using Franz diffusion cells, the skin permeability of 4‐AP (40 mg/ml) in 0.5 mL water or DMSO was determined through mouse skin. Pharmacokinetic parameters of 4‐AP in serum were determined by HPLC method at specified time points after applying 7.5μL of 4‐AP in DMSO (10 mg/ml or 20mg/ml) to the lower back skin of anesthetized mice. 4‐AP showed similar skin permeability coefficients in water (0.014 mm/hr) and DMSO (0.011 mm/hr). The maximum concentrations of 4‐AP after 75 μg and 150 μg administrations were 6.93 μM and 9.8 μM, respectively, and the time to maximum blood concentration was 60 min for both dosages. The half‐life of 4‐AP was 171.4 min and 160.0 min for 75 μg and 150 μg administration, respectively. Importantly, while both acute and chronic 4‐AP (150 μg) treatments significantly improved sciatic function index, chronic 4‐AP‐treated mice had greater nerve conduction velocity, fewer degenerating axons and thicker myelin sheaths than vehicle‐treated controls. To the best of our knowledge, this is the first study to show that topical 4‐AP, absorbed through the skin, enhances in vivo global motor function recovery with decreased axonal degeneration and increased myelination of axons.Support or Funding InformationThis work was supported by grants from the NIH (K08 AR060164‐01A) and DOD (W81XWH‐16‐1‐0725) in addition to institutional support from the University of Rochester and Pennsylvania State University Medical Centers.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.