Incorporation of methoxyphenamine into white hair in the early stage after intake: Effect of hair pigmentation on drug distribution in the hair root.

PURPOSE: Hair-root analysis provides information on recent drug exposure, which can serve as a useful complementary approach to urine- and blood-based testing. This study investigated the early-stage incorporation behavior of a model drug into white hair (from 30 min to 72 h after intake). METHODS: Black and white hair specimens were provided from participants after a single oral dose of methoxyphenamine (MOP), a non-regulated analog of methamphetamine. Drug distribution in hair was determined by 1-mm segmental analysis of single-hair specimens using liquid chromatography-tandem mass spectrometry. RESULTS: In white hair, as in black hair, MOP was first detected in the hair bulb region 30 min after intake, and subsequently in the upper dermis zone 1 h after intake. However, marked differences between black and white hair were observed in the subsequent time-course changes in drug distribution. In the hair bulb region, MOP concentrations in white hair began to decrease earlier than in black hair, and the maximum concentrations in white hair were only about one-twentieth to one-tenth of those in black hair. In the upper dermis zone, a gradual decrease in drug concentration was observed only in white hair after 12 h from intake, although the difference attributable to hair color was smaller than that observed in the hair bulb region. CONCLUSIONS: Hair-root analysis is applicable to white hair; however, concentrations of basic drugs are substantially lower than those in black hair. The drug incorporation behavior revealed in this study is crucial for the proper interpretation of hair testing results.