Background and objective: Cutaneous drug application is used for both local drug therapy and systemic treatment. For both types of treatment, the drug concentration profile in, and transport across, the skin is important. To evaluate skin penetration of topically-applied drugs we recently used cutaneous microdialysis. The aim of this study was the use of this method for studying acyclovir and salicylic acid. Method: Five per cent acyclovir cream was applied on intact and tape-stripped skin of healthy volunteers and 5% salicylic acid ointmentonto intact skin of other volunteers. Microdialysis probes with 2 kDa molecular weight cut-off were inserted intradermally and were perfused with Ringer solution. Drug concentrations were measured by high-performance liquid chromatography. Results: Following topical application of 5% acyclovir cream onto intact skin of eight healthy volunteers, no drug was determinable in the skin (cutaneous microdialysate) in any of the subjects studied. After partial removal of the stratum corneum the penetration of this drug into skin increased markedly. The mean maximum skin concentration was about 2AE5 lmol/L after 2AE4 ± 0AE7 h. Topically applied salicylic acid penetrated intact skin with a maximum concentration in the cutaneous microdialysate of 7AE57 ± 3AE90 lmol/L after 5AE3 ± 0AE4 h.Conclusion: Cutaneous microdialysis is a valuable method for estimating skin concentration of topically-applied drug. It allows evaluation after application to a small skin area, of about 2 cm 2 , thereby reducing the risk of systemic toxicity. The method may be helpful for evaluating the influence of skin condition on the transport process.
In selected cases skin concentrations should be determined rather than those in blood plasma when studying the distribution of orally administered drugs. Evaluation of acyclovir concentrations in the skin cannot be replaced by the calculation of the theoretical peripheral compartment.
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