Simultaneous determination of oxymorphone and its active metabolite 6-OH-oxymorphone in human plasma by high performance liquid chromatography–tandem mass spectrometry

A selective high performance liquid chromatography–tandem mass spectrometric (LC–MS/MS) method for the simultaneous determination of oxymorphone and its active metabolite 6-OH-oxymorphone in human plasma was developed and validated using oxymorphone-d3 as the internal standard. Chromatographic conditions were optimized to separate oxymorphone from the other metabolite, oxymorphone-3-glucuronide, which may convert to oxymorphone in MS ion source, resulting in inaccurate quantitation of oxymorphone. Solid phase extraction (SPE) was used to extract oxymorphone and 6-OH-oxymorphone from plasma. SPE offered the advantage of being able to remove the unwanted metabolite, oxymorphone-3-glucuronide, through the wash step during the extraction. The developed method was precise and reproducible as shown by good linearity of calibration curves (correlation coefficients ≥0.9968 for oxymorphone and ≥0.9967 for 6-OH-oxymorphone) with high intraday assay and interday assay precision (CV% ≤11.0% for oxymorphone and ≤12.6% for 6-OH-oxymorphone) over a range of 35/25 – 5000/5000 pg/mL for oxymorphone/6-OH-oxymorphone. The method has been successfully applied to analyze oxymorphone and 6-OH-oxymorphone in plasma from 19 healthy volunteers in a bioequivalence study. A total of 1026 samples were analyzed. Good linearity (average correlation coefficient 0.9988 for oxymorphone and 0.9966 for 6-OH-oxymorphone) was achieved with calibration curves and high precision (CV% ≤5.9% for oxymorphone and ≤10.9% for 6-OH-oxymorphone) was obtained with QCs.