Determination of exogenous melatonin and its 6-hydroxy metabolite in human plasma by liquid chromatography-mass spectrometry.

Melatonin has recently garnered interest as a possible treatment for sleep disorders, and this has created a desire for appropriate pharmacokinetic studies. No method has yet been published that can measure the concentrations of both melatonin and its main metabolite, 6-hydroxymelatonin, in plasma or serum. Therefore, a liquid chromatography-mass spectrometry (LC/MS) method including enzymatic hydrolysis and one-step liquid-liquid extraction was developed for this purpose. The mean extraction recovery was 59% for melatonin and 42% for 6-hydroxymelatonin. The mean precision of the method as calculated from the interassay coefficient of variation of quality control samples at high and low concentrations was 18% for 6-hydroxymelatonin and 15% for melatonin. The inaccuracy was always less than 2%. The limit of detection was approximately 2 ng/mL for 6-hydroxymelatonin (signal/noise ratio = 4) and less than 2 ng/mL for melatonin (signal/noise ratio at 2 ng/mL = 10). The method was applied to the analysis of plasma from a healthy volunteer who had received a single oral dose of 25 mg melatonin.