In Vitro Evaluation of Skin Absorption and Metabolism

Abstract

In vitro methods using human and animal skin have frequently been used to measure the absorption/diffusion of chemicals through skin. Since some compounds have recently been shown to be metabolized during the percutaneous absorption process, we are examining the importance and feasibility of determining metabolic activity in skin when making absorption measurements. The viability of skin can be readily maintained for at least 24 hr in flow-through diffusion cells. A Hepes-buffered Hank's balanced salt solution is, in some cases, superior as a receptor fluid to minimum essential medium containing 10% fetal bovine serum. The balanced salt solution is sufficient to maintain viability as assessed by glucose metabolism and electron microscopy and it does not interfere with quantitation of metabolites. We have found enzyme activity in skin to be 10-100 times less than that in liver and insufficient for detectable metabolism of caffeine, salicylic acid, or chlorophenothane (DDT) during absorption studies using fuzzy rat skin. However, 6.6% of the absorbed butylated hydroxytoluene was converted to ethyl acetate-extractable metabolites. Activities of aryl hydrocarbon hydoxylase, glutathione transferase, and 7-ethoxycoumarin de-ethylase have been measured in skin of animal models for comparison with enzyme activity in human skin.