Quantitative analysis of cimetidine in human plasma using LC/APCI/SRM/MS Keyang Xu, 1 Vinod K. Arora, 1 Ajai K. Chaudhary, 1 ² Robert B. Cotton 2 and Ian A. Blair 1 * 1 Center for Cancer Pharmacology, Department of Pharmacology, University of Pennsylvania, Philadelphia, PA 19104-4318, USA 2 Department of Pediatrics, Vanderbilt University, School of Medicine, Nashville, TN 37232-2370, USA Received 1 September 1998; accepted 7 December 1998 ABSTRACT: A quantitative method was developed and validated for rapid and sensitive analysis of cimetidine in human plasma. The method involved the use of liquid chromatography (LC) coupled with atmospheric pressure chemical ionization (APCI) and selected reaction monitoring (SRM) mass spectrometry (MS). A cimetidine analog, SKF92374, was used as the internal standard. Separation of cimetidine and the internal standard was accomplished using a reverse-phase HPLC column (C18). The eluted components were ionized by the APCI source and subsequently detected by a highly selective triple quadrupole mass spectrometer in the SRM mode. Linear standard curves were obtained from 5 ng/mL (lower limit of quantitation) to 10,000 ng/mL. The results demonstrated excellent precision (%RSD 1.1–8.9%) and accuracy (94.7–108.0%) over this range. In addition, the amount of plasma sample needed for analysis was small (50 mL), and the plasma pretreatment (analyte recovery >94%) was simple and time saving. This assay was used to evaluate cimetidine levels in premature infants following intravenous infusion of cimetidine. Copyright # 1999 John Wiley & Sons, Ltd. INTRODUCTION Lung cytochrome P450 activity has been linked to the production of reactive oxidant species and of potent arachidonic acid metabolites that could lead to oxidant- induced lung injury (Hazinski et al., 1995). Hazinski et al. (1989) reported that cimetidine [Fig. 1(a)], a histamine (H2) receptor antagonist, reduced acute hyperoxic lung injury in lambs. This was due to inhibition of cytochrome P450 activity in lamb lung by cimetidine. Cimetidine has been suspected to have potential utility in settings where this biochemical effect would confer similar therapeutic benefit in humans, such as in treatment of newborn premature infants at risk for bronchopulmonary dys- plasia, a disorder characterized by chronic respiratory insufficiency (Cotton et al., 1996). Therefore, it is important to have cimetidine levels closely monitored in the analysis of treatment effects. Currently available methodologies for quantification of cimetidine in human plasma include using HPLC with ultraviolet (UV) detection (reported by Larsen et al., 1979; Abdel-Rahim et al., 1985; Chiou et al., 1986; Strong and Spino, 1987; Russel et al., 1994; Kelly et al., 1995; Hempenius et al., 1998), and using capillary electrophoresis (CE) with UV (reported by Luks ˇa and Josic ´, 1995). Also, a report by Jenko et al. (1983) addressed the qualitative characterization of cimetidine and its degradation products based on the combination of HPLC, high-performance thin-layer chromatography (HPTLC) and fast atom bombardment (FAB) mass spectrometry (MS). However, these reported HPLC/UV and CE/UV methods exhibit one or more limitations: (1) Figure 1. Structures of (a) cimetidine and (b) internal standard (SKF92374). BIOMEDICAL CHROMATOGRAPHY Biomed. Chromatogr. 13: 455–461 (1999) *Correspondence to: I. A. Blair, Center for Cancer Pharmacology, Department of Pharmacology, University of Pennsylvania, Philadel- phia, PA 19104-4318, USA. ²Current address: Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, IN 46285, USA. Contract/grant sponsor: NHLBI; contract/grant number: R01- HL56697. Abbreviations used: CID, collision induced dissociation; APCI, atmospheric pressure chemical ionization; CE, capillary electro- phoresis; FAB, fast atom bombardment; HPTLC, high-performance thin-layer chromatography; LLQ, lower limit of quantitation; LQC, lower quality control; MQC, middle quality control; SRM, selected reaction monitoring; UQC, upper quality control. Copyright  1999 John Wiley & Sons, Ltd. CCC 0269–3879/99/070455–07 $17.50 ORIGINAL RESEARCH