January 1995
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Minoxidil, a potent antihypertensive, is moderately effective in the treatment of hair loss when it is applied to the scalp as a 2% solution in 60% ethanol, 20% propylene glycol and 20% water. Important questions remain concerning both the mechanism of delivery and the pathway of penetration of this drug from its ternary solvent system. Due to limited solubility of minoxidil in water, we examined the relative deposition and penetration influences of binary combinations of ethanol and propylene glycol. Generally, irrespective of the volume applied, the penetration of minoxidil through hairless mouse skin in vitro increased with increasing ethanol fraction with a maximum penetration at 90% ethanol at 24 h. The formulation for maximum penetration depends on the duration of the experiment. Decreasing the application volume had a variable effect which depended on formulation. Penetration from all the formulations was enhanced upon occluding the skin, with greatest increase evident in solutions with higher volatile fraction. Pretreating the skin with pure ethanol increased penetration from minoxidil solutions in propylene glycol, but not to the same levels as penetration from minoxidil solutions in ethanol. Saturated solutions in the different formulations showed increased penetration signifying the contribution of thermodynamics to the penetration. Loss of solvent by evaporation and subsequent concentration of minoxidil in the formulation film on the skin surface was also evaluated. Comparison of selected formulations in vivo showed similar trends with application volume and ethanol fraction to in vitro experiments, but did not show an increase in penetration after pretreatment with ethanol.