Individual variations of amitriptyline biotransformation examined in scalp hair samples.

The identification of amitriptyline, nortriptyline and its hydroxy-metabolites including a subsequent measurement of concentration profiles in hair samples was carried out to evaluate the administration history of amitriptyline. Analyses were carried out using liquid chromatography-tandem mass spectrometry, which permits simultaneous identification of all relevant substances in hair at low target concentrations (limit of detection better than 0.5 pg/mg). Standard hair sample preparation was applied for the estimation of average substance concentration in a hair bundle, while segmentation of individual hairs was utilised to examine accurate concentration profiles. Replication of analyses demonstrated a good reproducibility of individual hair concentration profiles, which proved to coincide for all relevant compounds. Significant variations of metabolite ratios (e.g. nortriptyline to amitriptyline and E10- to Z10-hydroxynortriptyline) between individuals suggest a correlation between hair concentration and metabolic phenotype. Different concentration ratios of certain metabolites in hair are highly correlated, indicating a systematic association between demethylation and stereo-specificity of hydroxylation. The trans isomers of hydroxy-metabolites become significantly more prevalent with increasing degree of demethylation of amitriptyline or hydroxylation of amitriptyline.