Determination of Meperidine and Normeperidine in Serum by Gas Chromatography/Mass Spectrometry

The method presented describes a sensitive and specific quantitative assay for the simultaneous determination of meperidine and its major metaboUte, normeperidine, in human serum without derivatization. The drugs and internal standard, phencyclidine, are isolated from serum by a basic extraction and back extraction process. The final extract, with methylene chloride as solvent, is quantitated by gas chromatography/mass spectrometry (GC/MS) in the chemical ionization, selected ion monitoring mode of operation. Minimum detectable quantities are 0.17 ng meperidine/mL serum and 0.50 ng normeperidine/mL serum. The coefficient of variation for the quantitative assay is approximately 5%. I n t r o d u c t i o n Since its introduction in 1939 as a synthetic narcotic analgesic (I), meperidine has become one of the primary agents used to alleviate obstetrical pain during the intrapartum period (2). The only known pharmacologically active metabolite of meperidine is normeperidine (3), which is twice as potent a convulsant and one-half as potent an analgesic as the parent drug (4). Both of these agents have been shown to cross the placenta (5). Infants born to mothers who received meperidine during labor have a tendency for lower Apgar scores, increased time to spontaneous respiration (6,7), depressed neonatal oxygen saturation (8), higher arterial partial pressure of carbon dioxide, and lower pH of arterial blood (9). Variability of heart rate (6) and depressed responses to psychophysiologic tests(10) have also been demonstrated in these neonates. In order to characterize the pharmacokinetics of meperidine in the pregnant patient and to correlate physiologic manifestations with blood levels of parent drug and metabolite, a sensitive, reproducible, and rapid method of analysis is needed. The analytical methods curi'ently available do not offer the sensitivity or specificity demanded by this clinical problem. For example, the spectrophotometric method for analysis of these compounds as a methyl orange complex cannot detect less than 0.3 /~g meperidine/mL; in addition, amines other than meperidine and normeperidine react similarly (11). Gas chromatographic (GC) analyses using a flame ionization detector also have insufficient sensitivity, the typical limits of detection being 0.025 ~g/mL for both drugs (12, 13). Derivatization is required in some procedures to use the more sensitive electron capture detector (14) or to oCercome the typically poor chromatographic qualities of normeperidine on most GC columns (15). The major problem affecting these derivatizations is that meperidine and normeperidine often form the same derivatives (14). The specificity of radioimmunoassay procedures currently available is reduced because of cross-reactivity between meperidine and normeperidine (16). The method presented offers simultaneous, sensitive, and specific analysis of meperidine and normeperidine without derivatization. E x p e r i m e n t a l Meperidine and normeperidine were obtained through the courtesy of Wyeth Laboratories (Philadelphia, PA). Phencyclidine was obtained from Applied Science Labs, lnc. (State College, PA). Anhydrous diethyl ether was obtained from Mallinckrodt, Inc. (St. Louis, MO) and methylene chloride, distilled in glass, was obtained from Burdick and Jackson Laboratories, lnc. (Muskegon Ml). All other chemicals were reagent grade. All glassware was acid washed with a mixture of 120 g sodium dichromate, I liter H2SO4 and 1 liter H20. Reacti-vials (Pierce Chemical Company, Rockford, Ik) were silanized with 5% dimethyldichlorosilane (Supelco, Inc., Bellefonte, PA) in toluene. Extraction tubes were silanized by the method of BriStell, et al. (17).