Effect of vehicles on the maximum transepidermal flux of similar size phenolic compounds.

In principle, maximum transepidermal fluxes of solutes should be similar for different vehicles, except when the solute or vehicle modifies the skin. Here we estimated maximum flux, stratum corneum solubility, diffusivity and permeability coefficient for a range of similarly sized phenolic compounds with varying lipophilicity from polar and lipophilic vehicles. Maximum flux and other skin transport parameters through human epidermis were obtained from lipophilic vehicles (mineral oil (MO) and isopropyl myristate (IPM)) and compared with values from water and propylene glycol (PG)-water solutions. Solvent uptake and changes in stratum corneum infrared spectroscopy and multiphoton microscopy imaging were also investigated. Maximum fluxes for MO and water were similar but IPM has a higher value for more polar phenols due to a higher diffusivity and PG-water had a higher flux due to higher solubility in the stratum corneum. Whereas maximum flux for various phenols was directly related to solubility in the stratum corneum independent of vehicle, increasing phenol lipophilicity increased and decreased permeability coefficient for aqueous solvents and lipophilic solvents, respectively. The maximum fluxes for phenols with a similar molecular size and varying lipophilicity were comparable between water and MO vehicles but higher for IPM and PG-water mixtures.