Hair analysis for drugs of abuse. III. Movement and stability of methoxyphenamine (as a model compound of methamphetamine) along hair shaft with hair growth.

This paper describes the movement of methoxyphenamine (MOP, a model compound of methamphetamine) along the hair shaft at the rate of hair growth and the stability of drugs in hair for several months. Five healthy subjects (3 males and 2 females) took 50 mg of methoxyphenamine orally once a day for 7 days. Scalp hairs from the posterior vertex were collected every 2 weeks or every 8 weeks after the first dosage. The hairs were cut into 1-cm sections and extracted with methanol-5N HCl (20:1). MOP in the extract was determined by gas chromatography/mass spectrometry (GC/MS) with tetradeuterium labeled MOP as an internal standard. The drug moved along hair shaft at the rate of 2.8-3.2 mm/week, according to hair growth, without diffusion. When drug bands were extrapolated according to the sections in which drug was detected, the bands were approximately 5 mm wide, equivalent to 1.7-2.4 periods of 7-day hair growth. In the case of identical doses, the drug level was highest in the root side and lowest in the distal side. In our data, we found that the drug level in hair had decreased approximately 50% five months later. The ratio of drug levels in corresponding sections correlated well to the ratio of doses, except where the hair shafts had been damaged or the drugs had decomposed.