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Distribution of rhEGF in the skin, plasma and several organ tissues following topical application of 125I-rhEGF (0.4mcCi) solution in 25% Pluronic F-127 on 154mm 2 normal and damaged (burned and stripped) skins of hairless mice was examined. The radioactivity in the stripped skin tissues increased as a function of time, and was 10-20 times higher than that in the normal and burned skins. The fractions of intact drug in the skin tissues were 40-60% for the normal and burned skins, and 60-80% for the stripped skin. It indicates that the stratum corneum layer behaves as a barrier for the dermal penetration of the drug. The radioactivity in the plasma was much higher for the stripped skin than for the normal and burned skins. However, the concentration of intact drug in the stripped skin was comparable to those in the normal and burned skins indicating most severe degradation (or metabolism) of the drug in the stripped skin. As a result, the fraction of intact drug in the plasma was lowest for the stripped skin (<10%). Body organ distribution of the drug was much higher for the stripped skin. The concentration in the stomach. Both in total radioactivity and intact drug, showed more than 10-times higher value than in the other organs (liver, kidney and spleen). The fraction of intact drug in each organ tissue was below 10-20%. And generally lowest for the stripped skin. The lowest fraction of the drug for the stripped skin could not be explained by the activity of the aminopeptidases in the skin since it was lower for the stripped skin than for the normal skin. Thereover, the fraction of intact drug appears to be determined by the balance between dermal uptake and systemic elimination of the drug, for example. The mechanism of dermal uptake of rhEGF was examined by topical applying 200mcl of 25% Pluronic F-127 solution containing 0.4 mcCi of 125I-rhEGF and 0.14mcCi of 14C-inulin (a marker of passive diffusion). The radioactivity of 125I-rhEGF at each sampling time point (0.5, 1, 2, 4 and 8hr) was correlated (p<0.05) with the corresponding radioactivity of 14C-inulin. It appears to indicate the rhEGF may be uptaken into the skins mainly by the passive diffusion. This hypothesis was supported by the constant specific binding of EGF to the skin homogenates regardless of the skin models. Receptor mediated endocytosis (RME) appears to contribute negligibly, if any, to the overall uptake process.
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