High-Throughput Liquid Chromatography–Tandem Mass Spectrometry Assay for Plasma Theophylline and Its Metabolites

Abstract

Theophylline is metabolized to 1,3-dimethyluric acid (1,3-DMU), 3-methylxanthine (3MX), and 1-methylxanthine (1MX) mainly by CYP1A2, and 1MX is rapidly converted to 1-methyluric acid (1MU) by xanthine oxidase (1)(2)(3). Individuals differ in terms of their rates of theophylline metabolism and the resulting serum concentrations (1); moreover, some theophylline metabolites, such as 3MX, are known to have bronchodilator activity. Thus, we need to determine the plasma concentrations of theophylline and its metabolites simultaneously to ensure the safe use of theophylline, especially in patients with renal insufficiency, in whom serious side effects can occur if metabolites are allowed to accumulate. Many HPLC methods have been used to measure theophylline and its metabolites. However, most of these techniques require the avoidance of caffeine or require a longer separation time because of the interfering effect of caffeine metabolites such as 1,7-dimethylxanthine (1,7-DMX), 3,7-dimethylxanthine, and theophylline (1,3-dimethylxanthine) (4). Recently, liquid chromatography–mass spectrometry and liquid chromatography–tandem mass spectrometry (LC-MS/MS) methods to determine theophylline and caffeine metabolites have been developed (5)(6). However, these methods require two runs per sample, with negative and positive ionization, and the total HPLC run time is ∼60 min. These methods therefore are not suitable for the high-throughput determinations of theophylline and its metabolites required for routine therapeutic drug monitoring. In the present study, we developed a simple and rapid LC-MS/MS method for the simultaneous determination of plasma theophylline and its metabolites and applied this method to phenotyping of CYP1A2. We prepared stock solutions containing 1 g/L theophylline and 100 mg/L each of its metabolites in methanol. We constructed calibration curves with seven concentrations ranging from 0 to 20 mg/L theophylline and 0 to 2 mg/L each of the metabolites. A plasma sample (100 μL) was mixed with 100 μL of internal standard (IS) solution (1 μg of …