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.

ICI 153,110 and ICI 170,777, two pyridyl diazinone cardiotonic agents, produced a different profile of effects on hepatic microsomal mixed function oxidase enzymes following multiple oral dosing to rats and dogs; these differences may be related to the molecular dimensions of the two molecules. ICI 153,110 significantly increased levels of total P450, ethoxycoumarin O-deethylase and ethoxyresorufin O-deethylase in rat microsomes, indicating an induction profile (P448) similar to that of beta-naphthoflavone. This was supported by gel electrophoresis (SDS-PAGE) of microsomal proteins; a similar type of induction was observed in dog microsomes. In contrast, ICI 170,777 produced no changes indicating enzyme induction in either rat or dog. Instead, ICI 170,777 appeared to inhibit specifically the activity of aldrin epoxidase in the rat. Inhibitory activity was also indicated in the rat by prolongation of pentobarbitone sleeping time following single oral doses of either ICI 153,110 or ICI 170,777. The time-course of this effect appeared to correlate more closely with the profile of circulating metabolites, although both parent compounds were found to produce type II spectral changes on interaction with control rat microsomes. The molecular dimensions (area/depth2) of the compounds supported the finding that only ICI 153,110 should interact with or induce P448 isozymes.

1. Oral absorption and bioavailability of the orally active cardiotonic agent, (6RS)-6-methyl-5-(pyrid-4-yl)-3H,6H-1,3,4-[6-14C]thiadiaz in-2-one (MPTD) (5 mg/kg), in rat and baboon were high. Peak blood concentrations of MPTD and total radioactivity were reached by 1.5-4 h when MPTD accounted for 60-70% of total radioactivity. In both species, elimination of MPTD from blood was rapid (t 1/2 = 3-4 h), although total nonspecific radioactivity was eliminated more slowly. 2. Radioactivity was rapidly eliminated by both species mainly into urine. In rat, about 3% dose was collected as 14CO2 and 2% remained in the carcass after 4 days. Recovery from baboon was incomplete (78-86%). 3. Examination of urine indicated extensive metabolism of MPTD showing a marked species difference. In baboon, MPTD was metabolized largely by glucuronidation at the pyridyl nitrogen to yield a quaternary ammonium conjugate and only about 1% of the dose was excreted unchanged. In rat, the major urinary component was unchanged MPTD and no glucuronide conjugate was found. Both species formed the pyridine N-oxide of MPTD as well as a number of unidentified minor components. 4. Distribution of radioactivity in rat was rapid and extensive. In general, elimination from tissues was also rapid, although radioactivity was eliminated much more slowly from the nasal and bronchiolar epithelium and from the preputial gland.