Influences of hair dyeing on the distribution shapes of zolpidem and methoxyphenamine in hair

In order to investigate the influences of hair dyeing on the distribution shapes of drugs in hair, different hair dyeing processes ("semi-permanent coloring without bleaching" and "permanent coloring with bleaching") were performed in vitro on black hair specimens collected from two subjects (Asians) who took a single dose of zolpidem (ZP, 10 mg of ZP tartrate) or methoxyphenamine (MOP, 50 mg of MOP hydrochloride). Under the following three different dyeing conditions, (1) semi-permanent coloring, (2) permanent coloring (once), (3) permanent coloring (twice), drug distributions in single hair specimens were investigated using a 2-mm segmental analysis by liquid chromatography-tandem mass spectrometry. Distribution shapes of drugs changed significantly only under the permanent coloring (twice) condition, resulting in reduced peak concentration and extended distribution width. There was, however, no significant difference in the amounts of drugs in hair between non-treated and dyed specimens, suggesting the drugs hardly leaked out of hair or were only slightly degraded during dyeing. In addition, while assuming contact with aqueous environment such as daily hair washing after dyeing, dyed hair specimens were individually immersed in ultrapure water for 20 hours, then the outflow of drugs in ultrapure water as well as the distribution shapes of drugs remaining in hair were determined. The drug outflow after permanent coloring (once and twice) was significantly larger than those after semi-permanent coloring, and the outflow ratios, [outflow]/([outflow] + [amount remaining in hair]), ranged over 9.8-24% (n = 3) for ZP and 68-71% (n = 3) for MOP after permanent coloring (once), and 54-72% (n = 3) for ZP and 86-91% (n = 3) for MOP after permanent coloring (twice). The distribution shapes of drugs after 20 h of immersion tended to flatten as outflow ratios increased, resulting in no change in the shapes after semi-permanent coloring, and complete collapse of their shapes after permanent coloring (twice). Thus, the present results indicated that hair dyeing involving bleaching and subsequent contact with aqueous environment after dyeing could significantly influence distribution shapes of drugs in hair.