Pharmacological disruption of hair follicle pigmentation by cyclophosphamide as a model for studying the melanocyte response to and recovery from cytotoxic drug damage in situ.

Here we show that cyclophosphamide induces disruption of follicular melanogenesis, which is characterized by abnormal transfer of pigment granules to ectopic hair bulb locations, extrafollicular melanin incontinence, disordered formation of melanosomes, and inhibition of melanosome transfer into precortical keratinocytes. This is in contrast to dexamethasone-induced termination of follicle melanogenesis, which activates premature but predominantly normal catagen development. Cyclophosphamide-induced pigmentation disruption was accompanied by significant alterations of biochemical and biophysical markers of melanogenesis, compared to control mice treated either with vehicle or with topical dexamethasone. Electron paramagnetic resonance spectroscopy shows a decline in the melanin signal and predominant eumelanin production. Tyrosine hydroxylase activity of tyrosinase and dihydroxyphenylalanine oxidation drop rapidly, while DOPAchrome tautomerase activity increases and dihydroxyindole carboxylic acid conversion factor activity remains unchanged in cyclophosphamide-treated mice compared to controls. These observations emphasize the key role of tyrosinase as opposed to postdihydroxyphenylalanine oxidase steps in normal and pathological termination of melanogenesis and shows that tyrosinase is the most sensitive target of the melanogenic apparatus for pharmacological regulation. Follicle pigmentation recovers only during the subsequent hair cycle, i.e., after a new anagen hair bulb has been constructed, which points to the existence of a relatively chemoresistant melanoblast-like cell population residing in the noncycling part of the hair follicle.