To the Editor.—
In the February issue of the Archives, Franz1 described the percutaneous absorption of minoxidil labeled with carbon 14 following the application to the scalp in 12 healthy adult male subjects. Based on the amount of radioactivity excreted in the urine and feces, he concluded that only a small fraction of the total dose of applied minoxidil was absorbed. We found the study to be very interesting and informative. However, two questions were raised in our minds that require further clarification:
What is the metabolic fate, distribution, and tissue binding of minoxidil or its metabolites following percutaneous absorption? The skin is known to be a viable membrane capable of metabolizing a wide variety of drugs.2 Measurement of urinary radioactivity following percutaneous application does not account for metabolism by the skin, nor does it account for any tissue binding of minoxidil and/or its metabolites. Since 53% to 57% of the applied minoxidil dose was not recovered, it would have been helpful if the author had determined or cited a reference as to the distribution and metabolic fate of minoxidil following percutaneous absorption. This issue becomes relevant if the drug is bound to tissue.
el-Azhary R, Goldberg LH. Percutaneous Absorption of Minoxidil. Arch Dermatol. 1986;122(1):15. doi:10.1001/archderm.1986.01660130017007