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Nakahara Y, Kikura R, Takahashi K. 
“Hair analysis for drugs of abuse XX. Incorporation and behaviors of seven methamphetamine homologs in the rat hair root”. 
Life Sci. 1998;63(10):883-93.
To elucidate drug disposition in hair, the incorporation and retention behavior of 7 phenethylamines in the rat hair root were investigated: methamphetamine(MA), 3,4-methylenedioxymethamphetamine(MDMA), benzphetamine(BZP), ephedrine(EP), N,N-dimethylamphetamine(DMA), p-nitro-methamphetamine(NO2MA), and N-acetylmethamphetamine(AcMA). On day 10 after shaving the hair on the back of the rats, drug was intraperitoneally administered at a single dose of 10 mg/kg to Long Evans rats (which were male and 6 weeks of age), possessing black and white hair, and the back hair that grew was collected by plucking with hair nippers at 0.083 h (5 min), 0.25 h, 0.5 h, 1 h, 2 h, 4 h, 6 h, 9 h, 24 h, 33 h and 48 h. After washing the plucked hairs three times with 0.1% sodium dodecylsulfate, the amount of drug in each of the hair root samples was analyzed by a selected ion monitoring of gas chromatography mass spectrometry (GC-MS) analysis. The times at which the concentration of each drug in the hair root samples reached the peak concentration, ranged between 3.30 and 41.51 ng/mg. For each drug, the point of time at which the largest positive incremental change in drug concentration was seen, ranged between 5 min and 1 h, for all of the drugs except for AcMA which was hardly incorporated in the rat hair. The data showed that there are mainly 4 modes in which a drug becomes incorporated into the black hair root: rapid and prolonged incorporation (NO2MA, MDMA), rapid and short incorporation(MA, DMA), slow and prolonged incorporation(BZP, EP), slow and short incorporation, which includes hardly any incorporation (AcMA). As all seven drugs were hardly incorporated into the white hair, it was concluded that the combination of melanin and basic compounds is essential for a drug to become incorporated into hair. Our results suggest that a portion of the drugs in the hair root is accumulated in the hair shaft, and the remaining portion is redistributed outside the hair shaft. The second finding is that the concentration of drug incorporated into hair mainly depends on two processes--(1) the drug incorporation into hair and the drug retention in hair.
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