Breaking Permeation Barriers of Charged Molecules with Excess Permeation Enhancers.

Permeation enhancers (PEs) are a class of amphiphilic molecules that promote the passive permeation of drug molecules through cellular membranes. There is currently a lack of understanding of the mechanism of action for permeation enhancers. Two PEs are studied in this manuscript, sodium caprate (C10) and salcaprozate sodium (SNAC), which are commonly used in coformulations and clinical trials. All-atom molecular dynamics simulations are carried out to probe their interactions with a membrane made of POPC. The permeation of water and three charged small molecule drugs (ketoprofen, propranolol, and salicylic acid) through a PE-embedded POPC membrane is studied. The results show that 1) the lipid bilayers can be saturated with PEs up to 250 mol % without losing structural integrity, 2) PEs embedded in the membrane facilitate the insertion of water into the tail groups in the center of the membrane, and 3) charged molecules, which are known to have a prohibitively high free energy barrier of permeation, have a significantly decreased free energy barrier of permeation in the presence of high concentration of PEs. Hydrogen bond contacts made between PEs and water molecules with the charged molecules when they are in the center of the membrane are primarily responsible for the decrease in the free energy barrier. This study provides insight about the permeation of larger molecules, which may have multiple charged states, namely peptides.