Disappearance of R/S-methamphetamine and R/S-amphetamine from human scalp hair after discontinuation of methamphetamine abuse

Methamphetamine (MA) and amphetamine (AM) are widely abused drugs. These compounds contain a chiral center, and their enantiomers exhibit different pharmacologic, pharmacokinetic, and metabolic properties due to differences in binding affinities to their receptor sites. Until now, there was a lack of information on the decline in the concentration of drugs in hair after abstinence. A simple procedure for the chiral separation and determination of methamphetamine (MA) and its metabolite amphetamine (AM) enantiomers by LC-MS/MS in hair samples has been developed and fully validated. The LODs and LLOQs were 0.02 ng/mg and 0.05 ng/mg for all analytes, respectively. This method was successfully applied to both real hair specimens from chronic MA users and after the discontinuation of MA. The concentration of total MA, and total AM in fifty-eight authentic hair specimens ranged from 7.8 ng/mg to 521.0 ng/mg, and from 0.3 ng/mg to 84.0 ng/mg, respectively. Both enantiomers of MA and/or AM were detected in seven of fifty-eight authentic hair specimens. Hair specimens were from thirteen women with a known history of MA abuse, who went to a rehabilitation center and ceased consuming MA (for 4-5 months). The S-isomers of MA and AM were detected in all the 5-6 cm segments. Both enantiomers of MA were detected in nine of the 5-6 cm segments and the enantiomers of AM were found in only five of the nine samples. Assuming a hair growth rate of 1 cm/month, the mean hair elimination half-lives of S-MA, R-MA, S-AM, and R-AM were 0.64(95% CI, 0.46-0.96), 0.58(95% CI, 0.41-0.93), 0.62(0.49-0.88), and 0.50 months (95% CI, 0.42-0.56), respectively. With the developed method, R/S-MA and R/S-AM could be detected in the hair of former drug users for approximately 4 months after abstinence. S-MA is the most commonly found analyte in hair segments and is principally used by abusers. Our results suggest that to evaluate the discontinuation of MA abuse after a 6-month period of abstinence, a 3-cm proximal hair segment should be free of MA at the SOHT proposed cut-off level.