Absorption of solvent-deposited weak electrolytes and their salts through human skin in vitro

Permeation of a weak acid (benzoic acid) and a weak base (propranolol) in various stages of ionization through human skin in vitro was measured from 0 to 72 h following solvent deposition of radiolabeled doses ranging from 11 to 11,000 nmol/cm2 and 1.93–1930 nmol/cm2, respectively. For the twenty combinations tested for each compound, mean permeation into the receptor fluid over 72 h ranged from 1.5 to 40.7 percent of dose for benzoic acid and 1.3–35.5 percent of dose for propranolol. For all but the lowest doses, permeation increased with increasing fraction of nonionized permeant in the dose solution. Generally, this trend became stronger as the dose increased. Recovery of radioactivity averaged 94.3 ± 5.5% for propranolol and was independent of ionization state and dose. Recovery of radioactivity for benzoic acid ranged from 40 to >100%, increasing with fraction nonionized and with dose. These effects can be qualitatively explained in terms of the low permeability of ionized species through stratum corneum, the volatility of free benzoic acid, and a buffer capacity of the stratum corneum deposition region on the order of 10–20 nmol/cm2.