Hepatic cytochrome P450 2B induction by ethyl/phenyl-substituted congeners of phenobarbital in the B6C3F1 mouse.

The abilities of structural congeners of phenobarbital to induce immunoreactive hepatic cytochrome P450 2B (CYP2B) protein and associated catalytic activity (benzyloxyresorufin O-dealkylation) in the male B6C3F1 mouse were examined. Interspecies differences in inducing ability were examined through comparison of the results with induction data obtained previously with the male F344/NCr rat. The congeners were administered in the diet for 2 weeks at concentrations equimolar to 500 ppm of the prototype CYP2B inducer, phenobarbital. Of the series of compounds tested, phenobarbital was the most effective inducer of benzyloxyresorufin O-dealkylation and immunoreactive CYP2B protein, with 2-ethyl-2-phenylsuccinimide, 5-ethyl-5-phenylhydantoin, primidone, and glutethimide being only 19-42% as effective. 5-Ethyl-5-phenyloxazolidinedione and the ring-opened and decarboxylated congeners, N-(2-ethyl-2-phenylacetyl)urea and 2-ethyl-2-phenylmalonamide, displayed minimal induction of these catalytic activities. Dose-response experiments performed with 5-ethyl-5-phenylhydantoin indicated that the intrinsic CYP2B-inducing activity of this congener was as great as that of phenobarbital in the mouse, although a fourfold greater dietary concentration of this hydantoin (2000 ppm) was required to elicit a response equivalent to that caused by 500 ppm phenobarbital. When extent of induction was related to serum total xenobiotic concentration rather than to administered dietary concentration, the potencies of the two congeners were determined to be more similar (58 vs. > or = 78 microM for phenobarbital and 5-ethyl-5-phenylhydantoin, respectively).

Comparison of the behavioral teratogenic potential of phenytoin, mephenytoin, ethotoin, and hydantoin in rats.

Pregnant Sprague-Dawley CD rats were orally administered either phenytoin (PHT, 200 mg/kg), mephenytoin (MPH, 100 mg/kg), ethotoin (ETH, 600 mg/kg), hydantoin (HYD, 1,200 mg/kg) or vehicle (propylene glycol) on days 7-18 of gestation. Mean (+/- S.E.) maternal serum concentrations of PHT, MPH, and ETH 1 hour after dosing on gestational day 18 were 16.0 +/- 3.3, 10.7 +/- 3.0, and 65.2 +/- 10.45, respectively, and free fractions were 16%, 18%, and 11% respectively. The free fraction for PHT is similar, but was lower for both MPH and ETH than that seen in humans. Preweaning mortality for PHT, MPH, ETH, HYD, and controls was 25%, 6.3%, 12.5%, 2.0% and 0.8%, respectively. The MPH and ETH-exposed animals weighed approximately 6.6% less than controls throughout the study; the other groups did not differ significantly. PHT offspring showed increased early locomotor activity. Only PHT-exposed animals (27%) exhibited abnormal circling behavior after weaning. PHT-circlers accounted for higher levels of activity in an open-field test and for longer straight channel swimming times. PHT-circlers and noncirclers differed from one another and controls on performance of a complex (Cincinnati) maze and on the development of the air-righting reflex. Offspring prenatally exposed to MPH showed an early delay in air-righting. ETH and HYD offspring were not consistently different from controls in behavior. The data suggest the following ordinal relationship among the drugs for behavioral teratogenesis: PHT much greater than MPH greater than ETH congruent to HYD congruent to CON. The effects of PHT are consistent with previous findings. Data on the other drugs suggest that other hydantoins do not possess the behavioral teratogenic efficacy of PHT and that PHT may be unique in its effects on CNS development.

P-450 enzyme induction by 5-ethyl-5-phenylhydantoin and 5,5-diethylhydantoin, analogues of barbiturate tumor promoters phenobarbital and barbital, and promotion of liver and thyroid carcinogenesis initiated by N-nitrosodiethylamine in rats.

Male F344/NCr rats, 6 wk old, were fed 500 ppm of phenobarbital (PB) or equimolar doses of either 5-ethyl-5-phenylhydantoin (EPH) or 5,5-diethylhydantoin (EEH) in diet for 2 wk and hepatic cytochrome P-450-mediated alkoxyresorufin O-dealkylase and aminopyrine N-demethylase activities were determined. Both PB and EPH greatly increased P-450-mediated enzyme activities in rat liver while EEH was ineffective. To evaluate the hydantoins as tumor promoters, 5-wk-old male F344 rats were given a single i.p. injection of 75 mg N-nitrosodiethylamine/kg body weight. Beginning 2 wk later, they were placed either on normal diet or diet containing 500 ppm of PB or equimolar doses of EPH or EEH for the remaining experimental period. Control groups received an i.p. injection of saline followed by each of the test diets. Animals were sacrificed at either 52 or 78 wk. PB and EPH significantly enhanced the development of hepatocellular foci and hepatocellular adenomas at 52 wk and hepatocellular carcinomas at 78 wk in N-nitrosodiethylamine-initiated rats. Neither the incidence of hepatocellular neoplasms nor the number and size of hepatocellular foci was significantly increased by EEH. At 78 wk, both PB and EPH enhanced the development of thyroid follicular cell neoplasms in N-nitrosodiethylamine-initiated rats while no such enhancement was observed with EEH. Thus, EPH, a long-acting sedative/anticonvulsant, like the structurally similar PB, promoted hepatocellular and thyroid follicular cell carcinogenesis and induced the PB-inducible form(s) of cytochrome P-450 (P-450b) in rats. In contrast, EEH unlike barbital failed to promote hepatocellular and thyroid follicular cell carcinogenesis and also failed to induce PB-inducible form(s) of cytochrome P-450 in rats.

Relation of in vivo drug metabolism to stereoselective fetal hydantoin toxicology in mouse: evaluation of mephenytoin and its metabolite, nirvanol.

Anticonvulsant therapy during pregnancy with the hydantoin phenytoin carries a risk of teratologic sequelae and developmental retardation known as the fetal hydantoin syndrome. The putative teratogen is a highly reactive arene oxide intermediate produced during phenytoin metabolism. By using two structurally similar hydantoins, mephenytoin and its in vivo demethylated metabolite nirvanol, we examined the possibility for a stereoselective dissociation of fetal hydantoin toxicity from maternal anticonvulsant activity. The d-isomers (S-enantiomers) of mephenytoin and nirvanol are p-hydroxylated via an arene oxide and hence were suspect teratogens. The l-isomers (R-enantiomers), being eliminated primarily via alternate pathways, were expected to be less toxic. Equimolar doses of the d- or l-isomers of mephenytoin or nirvanol were administered to pregnant Swiss ICR mice i.p. on gestational day 10. An 8-fold increase in fetal resorptions and a significant 11% decrease in fetal body weight was observed in the group treated with l-nirvanol, whereas no or minimal toxicity was observed in the other treatment groups. The blood clearance for d-nirvanol was double that for l-nirvanol, with correspondingly higher urinary concentrations of arene oxide-mediated metabolites. ANIH shift during metabolism supported the presence of an arene oxide intermediate. Contrary to our expectations, only l-nirvanol, the isomer producing less arene oxide, demonstrated fetal toxicity, thus raising some doubt concerning the arene oxide as the putative hydantoin teratogen. Further animal studies with the d-isomers of mephenytoin and nirvanol may lead to an anticonvulsant of choice during pregnancy.

Paradoxical intoxication--a complication of anticonvulsant administration.

A new syndrome, paradoxical intoxication, has been defined in which high levels of hydantoins, and in one instance carbamazepine, produced an increase in seizures with little or no evidence of intoxication; a decrease in these levels produced an improvement in seizure control. This syndrome occurs often but not exclusively in those people who are less astute in assessing their neurologic status and therefore may experience unexpectedly higher blood levels of their anticonvulsants. Instances have been documented with serum levels above 40 mugm/ml for phenytoin or mephenytoin alone, or above 50 mugm/ml of combined hydantoins phenytoin and mephenytoin, and when the level is in the range of 20 mugm/ml and above for carbamazepine. Possible mechanisms underlying the syndrome are reviewed. Appropriate therapy is a reduction of the dose of the drug in question.