Minoxidil: Mechanisms of Action on Hair Growth

A.G. Messenger; J. Rundegren

Disclosures

The British Journal of Dermatology. 2004;150(2) 

In This Article

Response of the Hair Follicle to Minoxidil

There are a number of ways in which a drug may stimulate hair growth; it may increase the linear growth rate of hair, increase the diameter of the hair fibre, alter the hair cycle, either shortening telogen or prolonging anagen, or act through a combination of these effects. Present evidence suggests that minoxidil acts mainly on the hair cycle; it may also increase hair diameter.

Mori and Uno[4] studied the effect of topical application of minoxidil on spontaneous hair cycles in the rat from birth to 80 days of age. Minoxidil had no effect on the duration of anagen, but telogen was shortened. The telogen phase of the third cycle lasted approximately 20 days in untreated animals, whereas follicles re-entered anagen after only 1-2 days in telogen in minoxidil-treated animals. The same shortening of telogen by minoxidil treatment was also seen in the fourth cycle. The effect of minoxidil on hair growth has been studied extensively in the stump-tailed macaque, a primate that develops postadolescent scalp hair loss closely resembling human androgenetic alopecia. Topical minoxidil prevents the development of scalp hair loss in periadolescent macaques and promotes regrowth of hair in balding animals. Histological studies showed that treatment with minoxidil causes an increase in the proportion of follicles in anagen, a reduction in telogen follicles, and an increase in hair follicle size.[5]

Little is known of the effect of minoxidil on normal human hair growth and studies have been limited mainly to the response of androgenetic alopecia to topical minoxidil. In male pattern balding (male androgenetic alopecia) there is a gradual reduction in the duration of anagen and a prolongation of the latent period of the hair cycle (the time between shedding of the telogen hair and the onset of the next anagen).[6] Hair follicles also become miniaturized.[7] There is some controversy over whether female androgenetic alopecia is the same entity as male balding. Nevertheless, the follicular changes are very similar,[8,9] if not identical, although prolongation of the latent period has not yet been demonstrated in women. Clinical trials of topical minoxidil in male and female hair loss all show a remarkably rapid increase in hair growth, measured by hair counts or hair weight. The increase is evident within 6-8 weeks of starting treatment and has generally peaked by 12-16 weeks (Fig. 1). It seems improbable that a response of this rapidity can be accounted for by reversal of follicular miniaturization, and a more likely explanation is that minoxidil triggers follicles in the latent part of telogen into anagen. The hypertrichosis that develops in humans taking minoxidil orally, and occasionally following topical use, may affect the forehead as well as other sites such as the limbs. The increased length of hair at these sites suggests that minoxidil also prolongs the duration of anagen in humans.

Results of two clinical trials of minoxidil topical solution in the treatment of male androgenetic alopecia using different methods for measuring the response. Both methods show a rapid increase in hair growth which has reached a plateau by 12-16 weeks. (A) Comparison of mean percentage change in interval hair weight per square centimetre for three treatment groups: 5% minoxidil, 2% minoxidil and placebo. Vertical line at 96 weeks indicates cessation of treatment. Adapted from Price et al.[65] (B) Mean change from baseline in nonvellus hair counts (per square centimetre) in men treated with 5% minoxidil solution (TMS), 2% minoxidil and placebo. From Olsen et al.[66]

Results of two clinical trials of minoxidil topical solution in the treatment of male androgenetic alopecia using different methods for measuring the response. Both methods show a rapid increase in hair growth which has reached a plateau by 12-16 weeks. (A) Comparison of mean percentage change in interval hair weight per square centimetre for three treatment groups: 5% minoxidil, 2% minoxidil and placebo. Vertical line at 96 weeks indicates cessation of treatment. Adapted from Price et al.[65] (B) Mean change from baseline in nonvellus hair counts (per square centimetre) in men treated with 5% minoxidil solution (TMS), 2% minoxidil and placebo. From Olsen et al.[66]

The results of histological studies in humans are less conclusive than in the macaque. Abell[10] found a trend towards an increase in anagen/telogen ratios after 12 months of minoxidil treatment in balding men, but the main change was an increase in mean hair diameter. This was most apparent at 4 months and mean diameter had declined at 12 months. He suggested this might be due to later recruitment of small diameter hairs into anagen. Headington and Novak[11] reported that minoxidil treatment caused hypertrophy of follicles but, although there was an increase in mean hair diameter in minoxidil-treated balding men after 12 weeks, a similar increase occurred in control subjects. Care should be taken in interpreting change in mean hair diameters. This does not necessarily imply that individual hair follicles become larger, as an increase in mean diameter may also occur through preferential recruitment of large diameter hairs in a latent phase of the hair cycle.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....