Effect of oral administration of clinically relevant doses of dexamethasone on regulation of cytochrome P450 subfamilies in hepatic microsomes from dogs and rats.

Kewei Zhang, Masanori Kuroha, Yuko Shibata, Eiichi Kokue, Minoru Shimoda

Index: Am. J. Vet. Res. 67(2) , 329-34, (2006)

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Abstract

To evaluate the effect of oral administration of dexamethasone (DEX) at clinically relevant doses on metabolic activities of cytochrome P450 (CYP) isoenzymes in dogs and rats.15 healthy 1-year-old male Beagles and 20 healthy 10-week-old male Wistar rats.Hepatic microsomes were harvested from dogs treated orally with DEX at 2.5 and 7.5 mg for 5 days and from rats treated orally with DEX at 0.75, 6, and 48 mg/kg for 5 days. 7-ethoxyresorufin, tolbutamide, bufuralol, and midazolam were used as CYP1A, CYP2C, CYP2D, and CYP3A substrates, respectively. Concentrations of metabolites formed by CYPs were measured by use of high-performance liquid chromatography, except for the resorufin concentrations measured by use of a fluorometric method. Reaction velocity-substrate concentration data were analyzed to obtain maximum reaction velocity (Vmax) and Michaelis-Menten constant (Km).Values of Vmax for midazolam 4-hydroxylation were significantly decreased by treatment with DEX at 2.5 and 7.5 mg in dogs, although values of Km were not affected. Values of Vmax for bufuralol 1'-hydroxylation were also decreased by treatment with DEX. In rats, values of Vmax for midazolam 4- hydroxylation were significantly decreased by treatment with DEX at 0.75 and 6 mg/kg but significantly increased at 48 mg/kg. Other reactions were not affected by treatment with DEX.Our results indicate that DEX downregulates the CYP3A subfamily when administered at clinically relevant doses to dogs. The effect of downregulation of CYP3A in dogs treated with DEX should be considered to avoid adverse effects from coadministration of drugs.