Involvement of phenobarbital and SKF 525A in the hepatotoxicity of antifungal drugs itraconazole and fluconazole in rats.

Itraconazole and fluconazole are potent wide spectrum antifungal drugs. Both of these drugs induce hepatotoxicity clinically. The mechanism underlying the hepatotoxicity is unknown. The purpose of this study was to investigate the role of phenobarbital (PB), an inducer of cytochrome P450 (CYP), and SKF 525A, an inhibitor of CYP, in the mechanism of hepatotoxicity induced by these two drugs in vivo. Rats were pretreated with PB (75 mg/kg for 4 days) prior to itraconazole or fluconazole dosing (20 and 200 mg/kg for 4 days). In the inhibition study, for 4 consecutive days, rats were pretreated with SKF 525A (50 mg/kg) or saline followed by itraconazole or fluconazole (20 and 200 mg/kg) Dose-dependent increases in plasma alanine aminotransferase (ALT), gamma-glutamyl transferase (gamma-GT), and alkaline phosphatase (ALP) activities and in liver weight were detected in rats receiving itraconazole treatment. Interestingly, pretreatment with PB prior to itraconazole reduced the ALT and gamma-GT activities and the liver weight of rats. No changes were observed in rats treated with fluconazole. Pretreatment with SKF 525A induced more severe hepatotoxicity for both itraconazole and fluconazole. CYP 3A activity was inhibited dose-dependently by itraconazole treatment. Itraconazole had no effects on the activity of CYP 1A and 2E. Fluconazole potently inhibited all three isoenzymes of CYP. PB plays a role in hepatoprotection to itraconazole-induced but not fluconazole-induced hepatotoxicity. SKF 525A enhanced the hepatotoxicity of both antifungal drugs in vivo. Therefore, it can be concluded that inhibition of CYP may play a key role in the mechanism of hepatotoxicity induced by itraconazole and fluconazole.

Effect of griseofulvin on the toxicity of signal grass (Brachiaria decumbens) in sheep.

The effect of griseofulvin treatment on signal grass (Brachlaria decumbens) toxicity was studied in 27 male Wiltshire Indigenous Malaysian crossbred sheep. Grazing on signal grass generally decreased the activity of the drug metabolizing enzymes in livers and kidneys. Griseofulvin oral administration of 5 mg/kg body weight for 5 consecutive days every other week for 10 w increased the hepatic concentration of cytochrome P-450 and the activity of phase II drug metabolizing enzymes (UDP-glucuronyltransferase and glutathione-S-transferase) while it decreased the hepatic and increased the renal activity of phase I enzymes aminopyrine-N-demethylase and aniline-4-hydroxylase. Griseofulvin did not protect sheep against B decumbens toxicity as 5/7 animals treated with griseofulvin and grazed on B decumbens showed signs of the plant toxicity.

Effect of phenobarbitone treatment against signal grass (Brachiaria decumbens) toxicity in sheep.

The effect of phenobarbitone against signal grass (Brachiaria decumbens) toxicity was studied in 26 male crossbred sheep. Grazing on signal grass significantly decreased the concentration of cytochrome P-450 and the activity of drug metabolizing enzymes, viz. aminopyrine-N-demethylase, aniline-4-hydroxylase, UDP- glucuronyltransferase and glutathione-S-transferase in liver and kidneys of affected sheep.Oral administration of phenobarbitone (30 mg/kg body weight) for five consecutive days before grazing on B. decumbens pasture, and thereafter, for three consecutive days every two weeks, resulted in significant increases in hepatic and renal activities of drug-metabolizing enzymes. The induction of drug metabolizing activity in sheep grazing on signal grass group was found to be lower than in animals given phenobarbitone alone. Induction by phenobarbitone provided a degree of protection against the toxic effects of B. decumbens as indicated by the delay in the appearance of signs of toxicity. Furthermore, these were much milder compared to those in the sheep not treated with phenobarbitone. The present study suggests that phenobarbitone-type cytochrome P-450 isoenzyme-induction may increase resistance against signal grass (B. decumbens) toxicity in sheep.

The effect of signal grass (Brachiaria decumbens) on drug-metabolizing enzymes in sheep and comparison with normal cells.

The effect of Brachiaria decumbens (signal grass) on drug-metabolizing enzymes was studied in sheep. After 14 d of grazing a pure signal grass pasture, significant declines were observed in hepatic aminopyrine N-demethylase and aniline 4-hydroxylase (phase I biotransformation) and in conjugative enzymes UDP-glucuronyltransferase and glutathione S-transferase. Kidney enzymes were significantly decreased except for UDP-glucuronyltransferase. Enzyme activities were also compared for normal sheep and cattle livers and kidneys. Lower activities were found in cattle, indicating that factors other than biotransformation are responsible for the clincial tolerance of cattle to B. decumbens toxicity.