Topically applied minoxidil in baldness.

Minoxidil, an orally administered, peripheral vasodilator used to treat hypertension, causes hypertrichosis in more than 80% of users. The drug reduces elevated blood pressure by decreasing peripheral vascular resistance. Chemically, minoxidil i s 2,4-diamino-6-piperidinopyrimidine-3-oxide; it is soluble in water to the extent of approximately 2 mg/ml, i s more readily soluble in propylene glycol or ethanol, and is nearly insoluble in acetone, chloroform, or ethyl acetate. Following oral administration of minoxidil and in association with the reduction in peripheral vascular resistance, cardiac output is augmented, salt and water are retained, and plasma renin activity rises.',2 When taken orally, minoxidil is at least 90% absorbed from the human gastrointestinal tract. After one 5-mg tablet of minoxidil, blood levels of the parent drug reach a maximum of 156.6 ng/ml within the first hour and decline rapidly thereafter to 1.5 ng/ml within 24 hours (Table 1). The average plasma half-life in humans is 4.2 hours for the combination of parent drug and its metabolites. About 90% of administered drug is metabolized, predominantly by conjugation with glucuronic and parent drug and metabolites are principally excreted in the urine. Minoxidil does not bind to plasma proteins, and its renal clearance corresponds to the glomerular filtration rate. In the absence of functional renal tissue, minox-