Topical calcitriol enhances normal hair regrowth but does not prevent chemotherapy-induced alopecia in mice.

Using a murine model that mimics chemotherapy-induced alopecia (CIA) in humans particularly well, we show here that in contrast to previously reported CIA-protective effects in neonatal rats, topical calcitriol does not prevent CIA in adolescent mice but enhances the regrowth of normally pigmented hair shafts. When, prior to injecting 1 X 120 mg/kg cyclophosphamide i.p., 0.2 microg calcitriol or vehicle alone were administered topically to the back skin of C57BL/6 mice with all hair follicles in anagen, no significant macroscopic differences in the onset and severity of CIA were seen. However, hair shaft regrowth after CIA, which is often retarded and patchy, thus displaying severe and sometimes persistent pigmentation disorders, was significantly accelerated, enhanced, and qualitatively improved in test compared with control mice. Histomorphometric analysis suggests that this is related to the fact that calcitriol-pretreated follicles favor the "dystrophic catagen pathway" of response to chemical injury, ie., a follicular repair strategy allowing for the unusually fast reconstruction of a new, undamaged anagen hair bulb. Thus, it may be unrealistic to expect that topical calcitriol can prevent human CIA, but topical calcitriols may well enhance the regrowth of a normal hair coat.