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Cytochrome P450-dependent metabolism of monensin in hepatic microsomes from chickens and turkeys.

Monensin is a polyether ionophore used as feed additive to prevent coccidiosis in poultry. Monensin is safe and effective for the target species when used at recommended dosages but the therapeutic index of this ionophore is rather narrow and several accidental poisonings are quoted in the literature involving horses, pigs, cattle, turkeys, sheep, goats and rabbits (Langston et al., 1985). These recommended dosages, in addition to the safety and efficiency of use, are established to prevent the presence of unwanted residues (with the respect of a withdrawal time) because of the cardiovascular system (Pressman, 1980) perturbations provoked by coccidiostats even at low dose. To predict residual concentrations of monensin in the organs of birds and then to assess the scenarios of contamination of food as a risk for the consumer, a physiologically based pharmacokinetic (PBPK) model describing the disposition of monensin in chickens and turkeys is currently under investigation. The aim of the present study is to quantitate the hepatic metabolism of the parent compound, using hepatic microsomes. The description of the metabolic clearance is essential in the model to evaluate its part in the total body clearance of monensin. Especially as its metabolites are not of toxicological interest, it is interesting to follow monensin disappearance. A metabolic balance (Davison, 1984) established in colostomized and bile-cannulated chickens indicated a fairly rapid excretion of the radioactivity mainly in the faeces (about 96%) and to a limited extent in the urine (about 1%). Extensive biotransformation studies of monensin in the rat, chicken and turkey have been conducted in order to identify the metabolites (Donoho et al., 1978). Recently (Anonymous, 2004), metabolites in chicken have been quantified using HPLC ⁄ESI-MS. These data indicate that monensin gives rise to nine metabolites. It has been decided to measure monensin depletion rather than formation of metabolites as usually presented in comparable studies because of the large number of metabolites. Furthermore, according to the parsimony principle, it simplifies the PBPK model and it is sufficient to follow the behaviour of the parent compound, which is of particular interest as a marker of total residues. All these reasons explain the choice of the substrate depletion approach (Obach & Reed-Hagen, 2002). The metabolites result from demethylation and a subsequent oxidation, decarboxylation, and from mono or dihydroxylation at different positions of the rings, which correspond essentially to the O-demethylation and simultaneous hydroxylation. These data also indicate that the metabolic pathways are very similar in chicken, turkey and rat. From a quantitative point of view, all metabolites identified in the chicken liver represented each less than 10% of the labelling and unchanged monensin contributed to about 5% only (Anonymous, 2004). Livers were removed from Ross chickens of 7 weeks of age and BUT9 turkeys of 12 weeks old. Six birds of each species were used to obtain three samples of two pooled livers per species. Each sample was obtained with two livers, one from a bird of each sex.