The letter by el-Azhary and Goldberg raises issues regarding the metabolic fate of minoxidil following topical application. Could metabolism and/or binding of minoxidil or its metabolites in the skin have affected the results of my study and led to erroneous conclusions regarding the values determined for total minoxidil absorption?The technique I used is that most commonly used for the study of percutaneously administered drugs, as popularized by Feldmann and Maibach.1,2 In essence, the method uses the measurement of total urinary excretion of radioactivity following topical application as a means to calculate total percutaneous absorption. No distinction is made between the excretion of the parent drug or the metabolite, since the appearance of either in the urine can only occur as a consequence of percutaneous absorption. Whether the parent drug is metabolized within the skin systemically or remains unmetabolized is unimportant to the calculation of total absorption.
Franz TJ. Percutaneous Absorption of Minoxidil-Reply. Arch Dermatol. 1986;122(1):15-16. doi:10.1001/archderm.1986.01660130017008