Cytochrome P450-dependent metabolism of gefitinib.

The in vitro metabolism of [(14)C]-gefitinib (1-3 microM) was investigated using human liver microsomes and a range of expressed human cytochrome P450 enzymes, with particular focus on the formation of O-desmethyl-gefitinib (M523595), the major metabolite observed in human plasma. High-performance liquid chromatography with ultraviolet light, radiochemical and mass spectral analysis, together with the availability of authentic standards, enabled quantification and structural identification of metabolites. On incubation with pooled human liver microsomes, [(14)C]-gefitinib underwent rapid and extensive metabolism to a number of metabolites, although M523595 was only a minor microsomal product. Formation of most metabolites was markedly decreased by ketoconazole, but M523595 production was inhibited only by quinidine. Gefitinib was metabolized extensively by expressed CYP3A4, producing a similar range of metabolites to liver microsomes, but M523595 was not formed. CYP1A2, 2C9 and 2C19 produced no measurable metabolism of gefitinib, while CYP3A5 produced a range of metabolites similar to CYP3A4, but to a much lower degree. In contrast, CYP2D6 catalysed rapid and extensive metabolism of gefitinib to M523595. While formation of M523595 was CYP2D6 mediated, the overall metabolism of gefitinib was dependent primarily on CYP3A4, and this was not obviously diminished in liver microsomes from CYP2D6 poor metabolizers.

In vitro metabolism of gefitinib in human liver microsomes.

The in vitro metabolism of gefitinib was investigated by incubating [14C]-gefitinib, as well as M537194, M387783 and M523595 (the main metabolites of gefitinib observed in man), at a concentration of 100 microM with human liver microsomes (4 mg ml(-1)) for 120 min. These relatively high substrate and microsomal protein concentrations were used in an effort to generate sufficient quantities of metabolites for identification. HPLC with ultraviolet light, radiochemical and mass spectral analysis, together with the availability of authentic standards, enabled quantification and structural identification of a large number of metabolites. Although 16 metabolites were identified, metabolism was restricted to three regions of the molecule. The major pathway involved morpholine ring-opening and step-wise removal of the morpholine ring and propoxy side chain. O-demethylation of the quinazoline methoxy group was a quantitatively less important pathway, in contrast to the clinical situation, where O-desmethyl gefitinib (M523595) is the predominant plasma metabolite. The third metabolic route, oxidative defluorination, was only a minor route of metabolism. Some metabolites were formed by a combination of these processes, but no metabolism was observed in other parts of the molecule. Incubation of gefitinib produced ten identified metabolites, but the use of the three main in vivo metabolites as additional substrates enabled a more comprehensive metabolic pathway to be constructed and this has been valuable in supporting the more limited data available from the human in vivo study.

Sulphation and glucuronidation of xamoterol in the dog: dose dependence and site of sulphation.

1. Xamoterol has been administered both intravenously and orally over a 100-fold dose range to male beagle dogs. 2. Over the dose range examined, sulphation was not saturable, with the proportion of the dose excreted as the sulphate conjugate remaining constant. 3. Extensive first-pass sulphation of an oral dose of xamoterol occurred in the intestine with approximately 50% of sulphation occurring during absorption. 4. The intestine is not a major site of sulphation for circulating xamoterol. 5. The liver is not believed to play an important role in the first-pass sulphation of xamoterol.

The metabolism of ICI 118,587, a partial agonist of beta 1-adrenoceptors, in mice, rats, rabbits, dogs, and humans.

The absorption, metabolism, and excretion of 14C-labeled xamoterol (ICI 118,587) has been examined in mice, rats, rabbits, dogs, and humans. There was incomplete absorption by all species after oral administration, ranging from 9% by humans to 36% by dogs. Most of the absorbed radioactivity was eliminated within 24 hr of administration and the renal route predominated. Conjugates of the parent compound were the only observed metabolites in urine, the phenolic glucuronide being the principal animal metabolite and the phenol sulfate being the only human metabolite. There were marked interspecies variations in metabolite patterns and dogs were the only animal species in which the sulfate metabolite was detected. Comparison of the urinary metabolite patterns also showed higher output of the conjugates after oral administration than after intravenous administration, indicating that first pass metabolism was taking place. Little significant change in absorption or metabolism was seen over a range of oral doses; in rats, some saturation of the glucuronide-conjugating mechanism was observed but the sulfate-conjugating mechanism showed little, if any, diminished capacity at high dose levels in dogs. The use of fast atom bombardment mass spectroscopy for the determination of the molecular weight of conjugates is described.