Mephenytoin

Risk of hepatotoxicity from a single toxic dose or prolonged use of acetaminophen may be increased and therapeutic efficacy may be decreased in patients regularly taking other hepatic enzyme-inducing agents such as phenytoin. /Hydantoin anticonvulsants/

Concurrent use of alcohol or CNS depression-producing medications with hydantoin anticonvulsants may enhance CNS depression. Chronic use of alcohol may decrease serum concentrations and effectiveness of hydantoins; concurrent use of hydantoin anticonvulsants with acute alcohol intake may increase serum hydantoin concentrations. /Hydantoin anticonvulsants/

Concurrent use of amiodarone with phenytoin and possibly with other hydantoin anticonvulsants may increase plasma concentrations of the hydantoin, resulting in increased effects and/or toxicity. /Hydantoin anticonvulsants/

Concurrent use with coumarin- or indandione-derivative anticoagulants, chloramphenicol, cimetidine, disulfiram, influenza virus vaccine, isoniazid, methylphenidate, phenylbutazone, ranitidine, salicylates, or sulfonamide may increase serum concentrations of hydantoin anticonvulsants because of decreased metabolism, thereby increasing the /hydantoins'/ effects and/or toxicity. Dosage adjustments of the anticonvulsant may be necessary. In addition, the anticoagulant effect of coumarin- or indandione-derivative anticoagulants may be increased initially, but decrease with continued concurrent use. /Hydantoin anticonvulsants/

For more Interactions (Complete) data for MEPHENYTOIN (23 total), please visit the HSDB record page.

/HUMAN EXPOSURE STUDIES/ /Investigators/ tested the ability of 194 unrelated, healthy Jordanian volunteers to metabolize S-mephenytoin. Mephenytoin (100 mg) was coadministered with debrisoquin (10 mg) orally and urine was collected for 8 hours. Mephenytoin metabolism was tested according to three measures: the amount of 4-hydroxymephenytoin, the S/R enantiomeric ratio, and the presence of a polar, acid-labile metabolite in urine collected for 8 hours after the dose. The S/R ratio and the presence of the acid-labile metabolite were determined in the urine of 16 patients who had low amounts of 4-hydroxymephenytoin (log hydroxylation index > or = 1). On examination of these three parameters of oxidation status, nine subjects were found to be poor metabolizers of mephenytoin by all three parameters. Thus 4.6% (95% confidence interval of 1.6% to 7.6%) of Jordanian subjects studied were poor metabolizers of mephenytoin. According to the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling the poor metabolizer status of mephenytoin was predicted to be 0.215% (95% confidence interval of 0.146% to 0.283%). These results are on the same order of magnitude as those determined in European white populations and constitute the first report in Arab populations.

/SIGNS AND SYMPTOMS/ Maculopapular, morbilliform, scarlatiniform, urticarial, purpuric (associated with thrombocytopenia), and nonspecific skin rashes have been reported. Exfoliative dermatitis, erythema multiform (Stevens-Johnson syndrome), and toxic epidermal neurolysis and fatal dermatitides have been described on rare occaisions. Skin pigmentation and rashes associated with a lupus erythematosis syndrome have also been reported.

/SIGNS AND SYMPTOMS/ Clinical effects of overdose include ataxia (clumsiness or unsteadiness) or staggering walk, blurred or double vision, severe confusion, severe dizziness or drowsiness, dysarthria (stuttering) or slurred speech, hyperreflexia, nausea and vomiting, nystagmus (continuous, uncontrolled back-and-forth and/or rolling eye movements), tremor, and unusual tiredness or weakness. /Hydantoin anticonvulsants/

/CASE REPORTS/ ... A 26-year-old female overdosed on approximately 12 g of mephenytoin and an unknown amount of valproic acid. She became comatose, developing pulmonary aspiration and pancreatitis with fever. Her Intensive Care Unit treatment was prolonged with slow resolution over 10 days.

For more Human Toxicity Excerpts (Complete) data for MEPHENYTOIN (9 total), please visit the HSDB record page.

Drug Induced Liver Injury Rank (DILIrank 2.0)

Mephenytoin

Ambiguous-DILI-concern

5

Adverse reactions

DOI:10.1016/j.drudis.2016.02.015

/LABORATORY ANIMALS: Developmental or Reproductive Toxicity/ 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 hr 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 incr 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.

/LABORATORY ANIMALS: Developmental or Reproductive Toxicity/ ... Equimolar doses of the d- or l-isomers of mephenytoin or nirvanol were administered to pregnant Swiss ICR mice ip 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.

Because acute attacks of porphyria may be precipitated by anticonvulsants, a therapeutic dilemma arises when seizures complicate hepatic porphyria. The list of unsafe agents includes barbiturates, primidone, phenytoin, mephenytoin, ethotoin, ethosuximide, methsuximide, phensuximide, and trimethadione. ...

The urinary metabolic profile of mephenytoin and its oxidative metabolites indicates significant stereoselective metabolism of its two enantiomers. Also, polymorphic oxidation, which is present in about 2 to 5% of the Caucasian population, has been demonstrated by an impaired ability to 4-hydroxylate this anticonvulsant. In order to determine the consequences of such metabolism, the plasma concentration/time profiles of the enantiomers of mephenytoin and its N-demethylated metabolite, phenylethylhydantoin (PEH), were investigated after a single p.o. dose of racemic mephenytoin in normal subjects with different metabolizing ability for mephenytoin [extensive metabolizer (EM) vs. poor metabolizer (PM) phenotypes]. In the EM subjects, the disposition of S- and R-mephenytoin was markedly different with a 100- to 200-fold difference in mean oral clearance (4.7 vs. 0.027 liters/min) and a 30- to 40-fold difference in elimination half-life (2.1 vs. 76 hr). In these same subjects, R-PEH concentrations significantly accumulated over several days and then very slowly declined with an apparent half-life of about 200 hr. Plasma levels of S-PEH were essentially negligible. In contrast, the stereoselective elimination of mephenytoin was reduced markedly in subjects of the PM phenotype, with the disposition of the S-enantiomer being the same as that for R-mephenytoin, which in turn was similar to that observed for this enantiomer in EMs. Almost comparable plasma levels of S- and R-PEH were also present in PMs. Only a small amount (less than 5%) of unchanged mephenytoin was excreted in the urine regardless of phenotype.

Following equal doses of phenytoin, total plasma phenytoin concentrations are lower in chronic uremic patients than in non-uremic patients which suggests an altered metabolic disposition of the drug in patients with uremia. /Phenytoin/

While data specific to mephenytoin were not located(SRC, 2007), the literature suggests that some pharmaceutically active compounds originating from human and veterinary therapy are not eliminated completely in municipal sewage treatment plants and are therefore discharged into receiving waters(1). Wastewater treatment processes often were not designed to remove them from the effluent(2). Selected organic waste compounds may be degrading to new and more persistent compounds that may be released instead of or in addition to the parent compound(2).

Since there is no specific antidote for overdose with hydantoin anticonvulsants, treatment is symptomatic and supportive and may include the following: ... Multiple oral doses of charcoal and cathartic may shorten the duration of symptoms. Supportive care - oxygen, vasopressors, and assisted ventilation may be necessary for CNS, respiratory, or cardiovascular depression. Patients in whom intentional overdose is confirmed or suspected should be referred for psychiatric consultation. Following recovery, careful evaluation of blood-forming organs is advisable. /Hydantoin anticonvulsants/

Maintain an open airway and assist ventilation if necessary. Administer supplemental oxygen. Treat stupor and coma if they occur. Protect the patient from self-injury caused by ataxia. If seizures occur, consider an alternate diagnosis and treat with other usual anticonvulsants. If hypotension occurs with intravenous phenytoin administration, immediately stop the infusion and administer intravenous fluids and pressors if necessary. There are no specific antidotes. Decontamination: administer activated charcoal orally if conditions are appropriate. Gastric lavage is not necessary after small to moderate ingestions if activated charcoal can be given promptly. Elimination: Repeat-dose activated charcoal may enhance phenytoin elimination but is not necessary and may increase the risk of aspiration pneumonitis in drowsy patients. There is no role for diuresis, dialysis, or hemoperfusion.

/SRP:/ Basic treatment: Establish a patent airway (oropharyngeal or nasopharyngeal airway, if needed). Suction if necessary. Watch for signs of respiratory insufficiency and assist ventilations if needed. Administer oxygen by nonrebreather mask at 10 to 15 L/min. Monitor for pulmonary edema and treat if necessary ... . Monitor for shock and treat if necessary ... . Anticipate seizures and treat if necessary ... . For eye contamination, flush eyes immediately with water. Irrigate each eye continuously with 0.9% saline (NS) during transport ... . Do not use emetics. For ingestion, rinse mouth and administer 5 ml/kg up to 200 ml of water for dilution if the patient can swallow, has a strong gag reflex, and does not drool ... . Cover skin burns with dry sterile dressings after decontamination ... . /Poisons A and B/

/SRP:/ Advanced treatment: Consider orotracheal or nasotracheal intubation for airway control in the patient who is unconscious, has severe pulmonary edema, or is in severe respiratory distress. Positive-pressure ventilation techniques with a bag valve mask device may be beneficial. Consider drug therapy for pulmonary edema ... . Consider administering a beta agonist such as albuterol for severe bronchospasm ... . Monitor cardiac rhythm and treat arrhythmias as necessary ... . Start IV administration of D5W /SRP: "To keep open", minimal flow rate/. Use 0.9% saline (NS) or lactated Ringer's if signs of hypovolemia are present. For hypotension with signs of hypovolemia, administer fluid cautiously. Watch for signs of fluid overload ... . Treat seizures with diazepam or lorazepam ... . Use proparacaine hydrochloride to assist eye irrigation ... . /Poisons A and B/

LD50 Guinea pig oral 380 mg/kg

LD50 Rabbit oral 430 mg/kg

LD50 Cat 190 mg/kg

LD50 Mouse ip 317 mg/kg

For more Non-Human Toxicity Values (Complete) data for MEPHENYTOIN (7 total), please visit the HSDB record page.

Therap Cat: Anticonvulsant

Antiepileptic drug

MEDICATION

MEDICATION (VET)

Propiophenone + hydrogen cyanide + ammonium bicarbonate + dimethyl sulfate (Bucherer- Bergs reaction/methylation).

Prepared from 5,5-ethylphenylhydantoin by the action of 1 mol dimethylsulfate: Fr pat. 769,667; Swiss pat. 166,004 (both 1934 to Sandoz). By treating phenylmethylureidoacetonitrile with HCl: Swiss pat. 179,692 (1935 to Sandoz).

METHOIN

SEDANTOINAL