A rapid method for measuring drug enrichment in epidermis.

A rapid and simple method is described for measuring the enrichment of small molecules in epidermal tissue. To measure such an enrichment, a small tissue sample (2-10 mg) is allowed to equilibrate with a buffered solution of a labelled substance for periods of 12-36 h. The concentration of the radioactive molecule in the tissue is measured as a decrease of radioactivity in the solution. Concentration measurements in the tissue itself can be performed, but are not required to detect enrichment in the tissue or to assess its magnitude. The specific density of appendage free human epidermis has been determined and was found to be 1.20 g/cm3. Using this value, tissue weight can be translated into volume and concentraton changes in the solution can be recalculated to yield the concentration of the substance in the tissue itself. Close agreement was found between the calculated tissue concentration and the values actually measured, following digestion of the epidermis with NaOH and measuring the activity in the tissue digest. The enrichment of five substances in human epidermis was measured: alpha-estradiol, thiopyronincce, 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), and theophylline. Of these substances, the first four are concentrated by human epidermis and the concentrations reached within the tissue are 10-500 times higher than the concentration of the same substance in the surrounding buffer. The enrichment data has been analysed in an attempt to distinguish between reversible affinity binding to specific tissue sites and partitioning of the substances between buffer and tissue components (lipids, membranes, etc.). In the case of thiopyronin and 8-MOP, reversible binding is indicated with dissociation constants of 10(-7) M and 10(-5) M, respectively, while partitioning distribution could account for the behavior of 5-MOP and alpha-estradiol. The method can be used either as a rapid screening method or as a quantitative analysis for the characterization of tissue enrichment with specific drugs.