Metabolism of theophylline by cDNA‐expressed human cytochromes P‐450.

Abstract

1 Theophylline metabolism was studied using seven human cytochrome P-450 isoforms (CYPs), namely CYPlAl, 1A2, 2A6, 2B6, 2D6, 2E1 and 3A4, and microsomal epoxide hydroxylase (EH), expressed in human B-lymphoblastoid cell lines. 2 At a high theophylline concentration of 10 mm four CYPs (lAl, 1A2, 2D6, 2E1) catalyzed the metabolism of theophylline. 3 Theophylline had the highest affinity (apparent Km range 0.2-1.0 mM) for the CYPIA subfamily and the kinetics of metabolic formation mediated by CYP1A2indicated substrate-inhibition (K1 range 9-16 mM). 4 CYP1A2 catalyzed the demethylation of theophylline as well as its hydroxylation, and was associated with the highest intrinsic clearance (1995 1 h-' per mol CYP) to 1,3-dimethyluric acid (DMU). Therefore, this isoform can be considered to be the most important enzyme involved in theophylline metabolism in vitro. 5 CYP2El was responsible for a relatively high intrinsic clearance by 8-hydroxylation (289 1 h-' per mol CYP). The apparent Km value of this reaction was about 15 mm, suggesting that CYP2E1 may be the low-affinity high-capacity isoform involved in theophylline metabolism. 6 The affinity of theophylline for CYPlAl was comparable with that of its homologue 1A2. When induced, the participation of CYPlAl in theophylline metabolism may be important. 7 CYP2D6 played only a minor role and CYP3A4 was not active in the in vitro metabolism of theophylline. 8 Our findings confirm the major role of CYP1A2 in theophylline metabolism and explain why in vivo the elimination kinetics of theophylline are non-linear and in vitro theophylline metabolism by human liver microsomes does not obey monophasic kinetics. 9 The data suggest also that not only tobacco smoking but also chronic alcohol intake may influence theophylline elimination in man as ethanol induces CYP2E1.Keywords theophylline metabolism CYP1A2 CYP2E1 in vitro cDNA expressed microsomes

Rofecoxib Is a Potent, Metabolism-Dependent Inhibitor of CYP1A2: Implications for in Vitro Prediction of Drug Interactions

“…It is largely responsible for the metabolism of many important xenobiotics including caffeine, theophylline, clozapine, olanzapine, duloxetine, tacrine, riluzole, lidocaine, zolmitriptan, and tizanidine (Berthou et al, 1991;Bertilsson et al, 1994;Ha et al, 1995;Madden et al, 1995;Ring et al, 1996;Wang et al, 2000;Granfors et al, 2004a) and is partially involved in the metabolism of, for example, tricyclic antidepressants and the Risomer of warfarin Kaminsky and Zhang, 1997;Olesen and Linnet, 1997). In addition, CYP1A2 plays a central role in the metabolism of certain endogenous compounds such as melatonin, 17␤-estradiol, and uroporphyrinogen (Aoyama et al, 1990;Lambrecht et al, 1992;Facciola et al, 2001).…”

Animal Models of Acute Moderate Hypoxia Are Associated with a Down-Regulation of CYP1A1, 1A2, 2B4, 2C5, and 2C16 and Up-Regulation of CYP3A6 and P-glycoprotein in Liver

“…In humans, at therapeutic concentrations, the biotransformation of theophylline is primarily catalyzed by CYP1A2, whereas CYP2D6, 2E1, and 3A4 exhibit low affinity and variable capacity (Ha et al, 1995). Rabbits exposed to a fractional concentration of inspired O 2 (FiO 2 ) of 10% for 24 h demonstrate a reduced clearance of theophylline and decreased expression of CYP1A1 and 1A2, although the expression of CYP3A6 is increased (Kurdi et al, 1999).…”

“…Patients with pulmonary insufficiency who show an impairment of their general state present an increased incidence of adverse effects, e.g., grand mal seizures, associated with the administration of "usual" doses of theophylline. Theophylline clearance is significantly reduced in patients with acute cardiogenic pulmonary edema (Piafsky et al, 1977) and in patients with worsening airway obstruction, severe bronchial obstruction, congestive heart failure, and pneumonia (Vozeh et al, 1978).In humans, at therapeutic concentrations, the biotransformation of theophylline is primarily catalyzed by CYP1A2, whereas CYP2D6, 2E1, and 3A4 exhibit low affinity and variable capacity (Ha et al, 1995). Rabbits exposed to a fractional concentration of inspired O 2 (FiO 2 ) of 10% for 24 h demonstrate a reduced clearance of theophylline and decreased expression of CYP1A1 and 1A2, although the expression of CYP3A6 is increased (Kurdi et al, 1999).…”

Effect of alosetron on theophylline pharmacokinetics

“…This discrepancy may be partially explained by incomplete dependence of theophylline on CYP1A2 for elimination. Caffeine demethylation to 1,7-DMX [10] and theophylline metabolism to 1,3-DMU, 3-MX, and 1-MU [6] constitute similar dose-fractions (< 0.8) metabolized with similarly high af®nities (K m < 0.3 mM) by CYP1A2. However, in contrast to the singular dependence of in vivo caffeine demethylation on CYP1A2, the high concentrations of theophylline in this study (6±9 mg ml 33±50 mM) may have allowed a greater contribution from other enzymes with lower af®nity, such as CYP2E1, which mediates hydroxylation of theophylline to 1,3-DMU (K m < 15 mM) [6,10,11].…”

“…Caffeine demethylation to 1,7-DMX [10] and theophylline metabolism to 1,3-DMU, 3-MX, and 1-MU [6] constitute similar dose-fractions (< 0.8) metabolized with similarly high af®nities (K m < 0.3 mM) by CYP1A2. However, in contrast to the singular dependence of in vivo caffeine demethylation on CYP1A2, the high concentrations of theophylline in this study (6±9 mg ml 33±50 mM) may have allowed a greater contribution from other enzymes with lower af®nity, such as CYP2E1, which mediates hydroxylation of theophylline to 1,3-DMU (K m < 15 mM) [6,10,11]. Although incomplete dependence of theophylline on CYP1A2 may partially explain the observed in vitro/in vivo discrepancy, more is needed to account for the complete lack of inhibition of CYP1A2-dependent theophylline metabolism.…”