Stereoselective metabolism of tazofelone, an anti-inflammatory bowel disease agent, in rats and dogs and in human liver microsomes.

Incubation of (R)-tazofelone and (S)-tazofelone in rat, dog, and human liver microsomes demonstrated that the (R)-tazofelone enantiomer was more rapidly metabolized, with two diastereomeric sulfoxides as the major metabolites formed in all three species. The two diasteresomers epimerized at physiological pH, therefore total sulfoxide formation rates were measured. The formation of the total sulfoxide metabolites followed Michaelis-Menten kinetics. The K(m), Vmax, and intrinsic formation clearance (Vmax/K(m)) values were determined in rat, dog, and human liver microsomes. The intrinsic formation clearance of sulfoxide from (R)-tazofelone exceeded that of (S)-tazofelone in all three species. In vivo studies in rats and dogs dosed orally and intravenously confirmed the stereoselective metabolism of tazofelone observed in vitro. Plasma concentrations of (S)-tazofelone exceeded (R)-tazofelone in rats and dogs by a factor of 3 to 4. In rat portal plasma, both enantiomers were of approximately equal concentration after oral dosing, indicating similar absorption. The half-lives of tazofelone and total sulfoxides in rats were 3.5 and 2.8 h, respectively. In dogs, the half-lives of tazofelone and total sulfoxides were 2.2 and 5.5 h, respectively. Plasma clearance was 2.3 l/h in rats and 1.4 l/h in dogs, and the volumes of distribution were 12 and 4.5 l, respectively, in rats and dogs. Both enantiomers were highly bound to plasma proteins to a similar extent in both species.

Cytochrome P-450 complex formation by dirithromycin and other macrolides in rat and human livers.

Some macrolide antibiotics cause clinical drug interactions, resulting in altered metabolism of concomitantly administered drugs, via formation of an inactive cytochrome P-450 complex. In the present study, the formation of a cytochrome P-450 type I binding spectrum and a metabolic intermediate complex by troleandomycin and dirithromycin was assessed in liver microsomes obtained from untreated rats and phenobarbital- or dexamethasone-pretreated rats. Troleandomycin produced a type I binding spectrum and metabolic intermediate complex in microsomes from dexamethasone- and phenobarbital-pretreated rats. Dirithromycin did not produce a detectable type I binding spectrum but formed a small cytochrome P-450 metabolic intermediate complex (6% of that formed by troleandomycin) in microsomes from dexamethasone-pretreated rats only. The formation of a cytochrome P-450 type I binding spectrum and a metabolic intermediate complex by troleandomycin, erythromycin, dirithromycin, and erythromycylamine was also assessed in microsomes prepared from human livers. Troleandomycin and erythromycin formed a type I binding spectrum and a metabolic intermediate complex which were larger in microsomes from subjects on barbiturate therapy than in microsomes from subjects with no recent barbiturate exposure. Erythromycylamine did not form a detectable type I binding spectrum with any of the human microsomal samples, but a small metabolic intermediate complex was formed with microsomes from a subject on phenobarbital, phenytoin, and propranolol therapy. Dirithromycin did not form a detectable type I binding spectrum or a metabolic intermediate complex in any human liver sample. Preclinical quantitation of the human metabolic intermediate complex may be helpful in predicting the possibility of clinical drug interactions of new drug candidates.

Effects of hepatic ischemia-reperfusion injury on the hepatic mixed function oxidase system in rats.

Hepatic ischemia induced in vivo by ligation of the left hepatic lobe of rats for up to 2 hr had no effect on cytochrome P-450, cytochrome c reductase, or lobe histology; however, cytochrome b5 increased with ischemia duration. Ethylmorphine demethylation decreased 35% after 2 hr of ischemia. Reperfusion of tissue previously made ischemic for up to 2 hr was associated with appreciable necrosis as well as decreases in cytochrome P-450, cytochrome b5, cytochrome c reductase, and ethylmorphine demethylation. Serum alanine transaminase and aspartate transaminase concentrations were increased by reperfusion of previously ischemic tissue. Reperfusion of the previously ischemic lobe for 18 hr was associated with a greater loss of cytochromes P-450 and b5, cytochrome c reductase, and ethylmorphine demethylation than reperfusion for 1 hr. The total decrease in cytochrome P-450 and b5 content was equal to the decrease in total microsomal heme content, although cytochrome P-450 decreased more than cytochrome b5. Ethoxyresorufin deethylation by hepatic microsomes from 3-methylcholanthrene-treated rats was decreased by ischemia-reperfusion; however, pentoxyresorufin dealkylation by hepatic microsomes from phenobarbital-treated rats was not, suggesting specific cytochrome P-450 isozyme loss. In vitro NADPH-dependent lipid peroxidation in hepatic microsomes from control and phenobarbital- and 3-methylcholanthrene-treated rats resulted in a selective decrease of ethoxyresorufin but not pentoxyresorufin dealkylation, similar to that observed in livers subjected to ischemia-reperfusion in vivo. These data suggest that cytochrome P-450, ethylmorphine demethylation, and ethoxyresorufin deethylation are more susceptible to ischemia-reperfusion injury than cytochrome b5 or pentoxyresorufin dealkylation.