Minoxidil: inhibition of proliferation of keratinocytes in vitro.

Previous studies have suggested that minoxidil stimulates growth of keratinocytes, possibly in a manner similar to the action of epidermal growth factor. Using both a short-term assay, thymidine incorporation, and a longer term assay, cell counting, to assess proliferative growth, we tested the activity of minoxidil in human keratinocyte cultures grown in 0.1 mM Ca(++). Minoxidil failed to stimulate growth in these assays. At concentrations of 5-10 micrograms per ml, minoxidil showed half-maximal inhibition of both EGF- and placental extract-stimulated thymidine incorporation. Minoxidil also inhibited proliferative growth in the presence or absence of placental extract. Direct measurement of the ability of minoxidil to compete for binding to the EGF receptor indicated that minoxidil probably does not bind to the EGF receptor. Minoxidil was not toxic, as keratinocytes continued to survive and grow, although at a slower rate, in the presence of minoxidil.