Quantitative determination of methylnaltrexone in human serum using liquid chromatography–tandem mass spectrometry

Methylnaltrexone (MNTX) is a novel peripherally acting μ-opioid antagonist that prevents peripheral side effects of opioid drugs such as constipation without affecting the analgesia. We developed a selective and sensitive assay to measure MTNX concentrations in human serum.The drug was measured after protein precipitation with perchloric acid using naltrexone as internal standard and liquid chromatography–tandem mass spectrometry (LC–MS/MS) for detection. The chromatography was performed isocratically on a RP18 column using 25 mM ammonium acetate buffer (pH 4)/acetonitrile (90%/10%; flow rate 200 μl/min) as mobile phase. The MS/MS analysis was performed in positive ionization mode monitoring the m/z transitions 356.4/284.2 for MNTX and 342.4/324.2 for naltrexone.The method was validated according to selectivity, linearity, accuracy, precision, recovery, matrix effects and stability. The validation range for MNTX in serum was 0.5–250 ng/ml. The developed LC–MS/MS was shown to be valid and successfully applied to measure serum-concentration–time curves of MNTX in a pilot study in healthy volunteers.