Determination of morphine, morphine-3-glucuronide, and morphine-6-glucuronide in plasma after intravenous and intrathecal morphine administration using HPLC with electrospray ionization and tandem mass spectrometry

High-performance liquid chromatography (HPLC) coupled to atmospheric pressure ionization (API) mass spectrometry (MS) has become a useful technique in the direct analysis of low concentrations of conjugated opiate metabolites. Previous methods using HPLC with traditional detection methods do not have the sensitivity to detect low concentrations of most conjugated drug metabolites. Methods using gas chromatography-mass spectrometry (GC-MS) require hydrolysis and derivatization of the sample followed by an indirect quantitation of conjugated metabolites. Recently, several reports have described direct analysis of opiates and their glucuronide conjugates by HPLC and API-MS. These methods report lower limits of detection than GC-MS methods and quantitation in the low nanogram-per-milliliter range for the glucuronide metabolites of morphine. This report describes an HPLC-electrospray-MS-MS method capable of detecting subnanogram concentrations of morphine (MOR) and its 3- and 6-glucuronide metabolites (M3G and M6G, respectively). The assay has a dynamic range of 250-10,000 pg/mL for M3G and M6G and 500-10,000 pg/mL for MOR. Inter- and intra-assay precision and accuracy varied by less than 8% for all analytes at 750-, 2500-, and 7500-pg/mL concentrations. This assay was used for the determination of MOR, M3G, and M6G in human plasma after intravenous (IV) and intrathecal (IT) administration of MOR and its effects on the ventilatory response to hypoxia. Peak plasma concentrations of MOR and M6G were measured I h after IV administration of MOR. Peak concentrations of M3G were measured 2 h after IV administration of MOR. After IT administration of MOR, peak concentrations of M3G were measured 8 h postdose. MOR was not detected in plasma of patients administered MOR IT. Subnanogram concentrations of M6G were measured in the plasma of five of nine patients administered MOR IT.