Design Principles of Ionic Liquids for Transdermal Drug Delivery

Ionic liquids (ILs) and deep eutectic solvents have shown great promise in drug delivery applications. Choline-based ILs, in particular choline and geranic acid (CAGE), have been used to enhance the transdermal delivery of several small and large molecules. However, detailed studies outlining the design principles of ILs for transdermal drug delivery are still lacking. Using two model drugs of differing hydrophilicities, acarbose and ruxolitinib and 16 ILs, the dependence of skin penetration on the chemical properties of ILs is examined. First, the impact of ion stoichiometry on skin penetration of drugs is assessed using CAGE, which evidences that a molar ratio of 1:2 of choline to geranic acid yields the highest delivery. Subsequently, variants of CAGE are prepared using anions with structural similarity to geranic acid and cations with structural similarity to choline at a ratio of 1:2. Mechanistic studies reveal that the potency of ILs in enhancing transdermal drug delivery correlates inversely with the inter-ionic interactions as determined by 2D NMR spectroscopy. Using this understanding, a new IL is designed, and it provides the highest delivery of ruxolitinib of all ILs tested here. Overall, these studies provide a generalized framework for optimizing ILs for enhancing skin permeability.