Action And Clinical Pharmacology: Phenytoin is an anticonvulsant drug which can be useful in the treatment of epilepsy. The primary site of action appears to be the motor cortex where spread of seizure activity is inhibited. Possibly by promoting sodium efflux from neurons, phenytoin tends to stabilize the threshold against hyperexcitability caused by excessive stimulation or environmental changes capable of reducing membrane sodium gradient. This includes the reduction of post-tetanic potentiation at synapses. Loss of post-tetanic potentiation prevents cortical seizure foci from detonating adjacent cortical areas. Phenytoin reduces the maximal activity of brain stem centres responsible for the tonic phase of tonic-clonic (grand mal) seizures.
The plasma half-life in man after oral administration of phenytoin averages 22 hours, with a range of 7 to 42 hours. Steady-state therapeutic levels are achieved at least 7 to 10 days after initiation of therapy with recommended doses of 300 mg/day.
When serum level determinations are necessary, they should be obtained at least 7 to 10 days after treatment initiation, dosage change, or addition or subtraction of another drug to the regimen so that equilibrium or steady-state will have been achieved. Trough levels obtained just prior to the patient’s next scheduled dose, provide information about clinically effective serum level range and confirm patient compliance. Peak drug levels, obtained at the time of expected peak concentration, indicate an individual’s threshold for emergence of dose-related side effects. For phenytoin, peak serum levels occur 4 to 12 hours after administration.
In most patients maintained at a steady dosage, stable phenytoin serum levels are achieved. There may be wide interpatient variability in phenytoin serum levels with equivalent dosages. Patients with unusually low levels may be noncompliant or hypermetabolizers of phenytoin. Unusually high levels result from liver disease, congenital enzyme deficiency or drug interactions which result in metabolic interference. The patient with large variations in phenytoin serum levels, despite standard doses, presents a difficult clinical problem. Serum level determinations in such patients may be particularly helpful. As phenytoin is highly protein bound, free phenytoin levels may be altered in patients whose protein binding characteristics differ from normal.
Most of the drug is excreted in the bile as inactive metabolites which are then reabsorbed from the intestinal tract and excreted in the urine. Urinary excretion of phenytoin and its metabolites occurs partly with glomerular filtration but more importantly by tubular secretion. Because phenytoin is hydroxylated in the liver by an enzyme system which is saturable at high serum levels small incremental doses may increase the half-life and produce very substantial increases in serum levels, when these are in or above the upper therapeutic range. The steady-state level may be disproportionately increased, with resultant intoxication, from an increase in dosage of 10% or more.
Indications And Clinical Uses: For the control of generalized tonic-clonic and psychomotor (grand mal and temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. Phenytoin serum level determinations may be necessary for optimal dosage adjustments (see Dosage).
Contra-Indications: In those patients who are hypersensitive to phenytoin or other hydantoins.
Manufacturers’ Warnings In Clinical States: Abrupt withdrawal of phenytoin in epileptic patients may precipitate status epilepticus. When, in the judgment of the clinician, the need for dosage reduction, discontinuation, or substitution of alternative anticonvulsant medication arises, this should be done gradually. However, in the event of an allergic or hypersensitivity reaction, rapid substitution of alternative therapy may be necessary. In this case, alternative therapy should be an anticonvulsant drug which does not belong to the hydantoin chemical class.
Cases of acute hepatoxicity, including infrequent cases of acute hepatic failure, have been reported with phenytoin. These incidents have been associated with a hypersensitivity syndrome characterized by fever, skin eruptions, and lymphadenopathy, and usually occur within the first 2 months of treatment. Other common manifestations include jaundice, hepatomegaly, elevated serum transaminase levels, leukocytosis, and eosinophilia. The clinical course of acute phenytoin hepatotoxicity ranges from prompt recovery to fatal outcomes. In these patients with acute hepatotoxicity, phenytoin should be immediately discontinued and not readministered.
There have been a number of reports suggesting a relationship between phenytoin and the development of lymphadenopathy (local or generalized) including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and Hodgkin’s Disease. Although a cause and effect relationship has not been established, the occurrence of lymphadenopathy indicates the need to differentiate such a condition from other types of lymph node pathology. Lymph node involvement may occur with or without symptoms and signs resembling serum sickness, e.g., fever, rash and liver involvement. In all cases of lymphadenopathy, follow-up observation for an extended period is indicated and every effort should be made to achieve seizure control using alternative anticonvulsant drugs.
Acute alcoholic intake may increase phenytoin serum levels while chronic alcoholic use may decrease serum levels.
In view of isolated reports associating phenytoin with exacerbation of porphyria, caution should be exercised in using this medication in patients suffering from this disease.
Pregnancy: A number of reports suggests an association between the use of antiepileptic drugs by women with epilepsy and a higher incidence of birth defects in children born to these women. Data are more extensive with respect to phenytoin and phenobarbital, but these are also the most commonly prescribed antiepileptic drugs; less systematic or anecdotal reports suggest a possible similar association with the use of all known anticonvulsant drugs.
The reports suggesting a higher incidence of birth defects in children of drug-treated epileptic women cannot be regarded as adequate to prove a definite cause and effect relationship. There are intrinsic methodologic problems in obtaining adequate data on drug teratogenicity in humans. Genetic factors or the epileptic condition itself may be more important than drug therapy in leading to birth defects. The great majority of mothers on anticonvulsant medication deliver normal infants. It is important to note that anticonvulsant drugs should not be discontinued in patients in whom the drug is administered to prevent major seizures, because of the strong possibility of precipitating status epilepticus with attendant hypoxia and threat to life. In individual cases where the severity and frequency of the seizure disorder are such that the removal of medication does not pose a serious threat to the patient, discontinuation of the drug may be considered prior to and during pregnancy, although it cannot be said with any confidence that even minor seizures do not pose some hazard to the developing embryo or fetus. The prescribing physician will wish to weigh these considerations in treating or counseling epileptic women of childbearing potential.
In addition to the reports of the increased incidence of congenital malformations, such as cleft lip/palate and heart malformations in children of women receiving phenytoin and other antiepileptic drugs, there have more recently been reports of a fetal hydantoin syndrome. This consists of prenatal growth deficiency, microcephaly and mental deficiency in children born to mothers who have received phenytoin, barbiturates, alcohol, or trimethadione. However, these features are all interrelated and are frequently associated with intrauterine growth retardation from other causes.
There have been isolated reports of malignancies, including neuroblastoma, in children whose mothers received phenytoin during pregnancy.
An increase in seizure frequency during pregnancy occurs in a high proportion of patients, because of altered phenytoin absorption or metabolism. Periodic measurement of serum phenytoin levels is particularly valuable in the management of a pregnant epileptic patient as a guide to an appropriate adjustment of dosage. However, postpartum restoration of the original dosage will probably be indicated.
Neonatal coagulation defects have been reported within the first 24 hours in babies born to epileptic mothers receiving phenobarbital and/or phenytoin. Vitamin K has been shown to prevent or correct this defect and has been recommended to be given to the mother before delivery and to the neonate after birth.
Precautions: General: The liver is the chief site of biotransformation of phenytoin. Patients with impaired liver function, elderly patients, or those who are gravely ill may show early signs of toxicity.
A small percentage of individuals who have been treated with phenytoin have been shown to metabolize the drug slowly. Slow metabolism may be due to limited enzyme availability and lack of induction; it appears to be genetically determined.
Toxic hepatitis, liver damage, and hypersensitivity syndrome have been reported and may, in rare cases, be fatal (See Adverse Effects).
Phenytoin should be discontinued if a skin rash appears (see Warnings section regarding drug discontinuation). If the rash is exfoliative, purpuric, or bullous or if lupus erythematosus or Stevens-Johnson syndrome or toxic epidermal necrolysis is suspected, use of this drug should not be resumed and alternative therapy should be considered (see Adverse Effects). If the rash is of a milder type (measles-like or scarlatiniform), therapy may be resumed after the rash has completely disappeared. If the rash recurs upon reinstitution of therapy, further phenytoin medication is contraindicated.
Literature reports suggest that the combination of phenytoin, cranial irradiation and the gradual reduction of corticosteroids may be associated with the development of erythema multiforme, and/or Stevens-Johnson syndrome, and/or toxic epidermal necrolysis. In any of the above instances, caution should be exercised if using structurally similar compounds (e.g., barbiturates, succinimides, oxazolidinediones and other related compounds) in these same patients.
While macrocytosis and megaloblastic anemia have occurred, these conditions usually respond to folic acid therapy. If folic acid is added to phenytoin therapy, a decrease in seizure control may occur.
Hyperglycemia, resulting from the drug’s inhibitory effects on insulin release, has been reported. Phenytoin may also raise the serum glucose level in diabetic patients.
Osteomalacia has been associated with phenytoin therapy and is considered to be due to phenytoin’s interference with Vitamin D metabolism.
Phenytoin is not indicated for seizures due to hypoglycemic or other metabolic causes. Appropriate diagnostic procedures should be performed as indicated.
Phenytoin is not effective for absence (petit mal) seizures. If tonic-clonic (grand mal) and absence (petit mal) seizures are present, combined drug therapy is needed.
Serum levels of phenytoin sustained above the optimal range may produce confusional states referred to as delirium, psychosis, or encephalopathy, or rarely irreversible cerebellar dysfunction. Accordingly, at the first sign of acute toxicity, serum drug level determinations are recommended. Dose reduction of phenytoin therapy is indicated if serum levels are excessive; if symptoms persist, termination of phenytoin therapy is recommended (see Warnings).
Information for the Patient: Patients taking phenytoin should be advised of the importance of adhering strictly to the prescribed dosage regimen, and of informing their physician of any clinical condition in which it is not possible to take the drug orally as prescribed, e.g., surgery, etc.
Patients should also be cautioned on the use of other drugs or alcoholic beverages without first seeking their physician’s advice.
Patients should be instructed to call their physician if skin rash develops.
The importance of good dental hygiene should be stressed in order to minimize the development of gingival hyperplasia and its complications.
Do not use capsules which are discolored.
Laboratory Tests: Phenytoin serum level determinations may be necessary to achieve optimal dosage adjustments.
Drug Interactions: There are many drugs which may increase or decrease serum phenytoin levels or which phenytoin may affect. Determinations of serum phenytoin concentrations are especially helpful when possible drug interactions are suspected. The most commonly occurring drug interactions are listed below:
Drugs which may increase phenytoin serum levels include: acute alcohol intake, cimetidine, dicumarol, disulfiram, ethosuximide, methylphenidate, omeprazole, phenothiazines, ticlopidine and topiramate. Coadministration with topiramate reduces serum topiramate levels by 59%, and has the potential to increase phenytoin levels by 25% in some patients. The addition of topiramate therapy to phenytoin should be guided by clinical outcome. The following drug classes are also included.
Drugs Which May Increase Phenytoin Serum Levels Drug Classes, Drugs in Each Class
Analgesic/Anti-inflammatory Agents, phenylbutazone salicylates
Antibacterial Agents, chloramphenicol erythromycin isoniazid sulfonamides
Antifungal Agents, amphotericin B fluconazole ketoconazole miconazole itraconazole
Benzodiazepines/Psychotropic Agents, chlordiazepoxide diazepam trazodone
Calcium Channel Blockers/ Cardiovascular Agents, amiodarone diltiazem nifedipine
Oral Hypoglycemic Agents, tolbutamide
Serotonin Reuptake Inhibitors, fluoxetine
Drugs which may decrease phenytoin serum levels include: antibacterial agents/fluoroquinolones (such as ciprofloxacin and rifampin), carbamazepine, chronic alcohol abuse, diazoxide, reserpine, sucralfate, theophylline and vigabatrin. Coadministration with vigabatrin reduces serum phenytoin levels by 20 to 30%. This may be clinically significant in some patients and may require dosage adjustment.
Molindone HCl contains calcium ions which interfere with the absorption of phenytoin. Ingestion times of phenytoin and calcium preparations, including antacid preparations containing calcium should be staggered to prevent absorption problems.
Drugs Which May Either Increase or Decrease Phenytoin Serum Levels Drug Classes, Drugs in Each Class
Anticonvulsants, carbamazepine phenobarbital sodium valproate valproic acid Antineoplastic Agents Benzodiazepines , chlordiazepoxide Phenothiazines , Psychotropic Agents , diazepam
Similarly, the effect of phenytoin on carbamazepine, phenobarbital, valproic acid and sodium valproate serum levels is unpredictable.
Although not a true drug interaction, tricyclic antidepressants may precipitate seizures in susceptible patients and phenytoin dosage may need to be adjusted.
Drugs whose blood levels and/or effects may be altered by phenytoin include: clozapine, corticosteroids, coumarin anticoagulants, cyclosporine, diazoxide, furosemide, lamotrigine, paroxetine, theophylline, topiramate and vitamin D. Coadministration with topiramate reduces serum topiramate levels by 59%, and has the potential to increase phenytoin levels by 25% in some patients. The addition of topiramate therapy to phenytoin should be guided by clinical outcome. Coadministration with lamotrigine doubles the plasma clearance and reduces the elimination half-life of lamotrigine by 50%. This clinically important interaction requires dosage adjustment. The following drug classes are also included.
Drugs Whose Blood Levels and/or Effects May Be Altered by Phenytoin Drug Classes, Drugs in Each Class
Antibacterial Agents, doxycycline praziquantel rifampin tetracycline
Calcium Channel Blockers/ Cardiovascular Agents, digitoxin nicardipine nimodipine quinidine verapamil
Hormones, estrogens oral contraceptives
Neuromuscular Blocking Agents, pancuronium vecuronium
Opioid Analgesics, methadone
Oral Hypoglycemic Agents, chlorpropamide glyburide tolbutamide
Drug-Enteral Feeding/Nutritional Preparations Interaction: Literature reports suggest that patients who have received enteral feeding preparations and/or related nutritional supplements have lower than expected phenytoin plasma levels. It is therefore suggested that phenytoin not be administered concomitantly with an enteral feeding preparation.
More frequent serum phenytoin level monitoring may be necessary in these patients.
Drug/Laboratory Test Interactions : Phenytoin may cause decreased serum levels of protein-bound iodine (PBI). It may also produce lower than normal values for dexamethasone or metyrapone tests. Phenytoin may cause increased serum levels of glucose, alkaline phosphatase, and gamma glutamyl transpeptidase (GGT). Phenytoin may affect blood calcium and blood sugar metabolism tests.
Carcinogenesis: See Warnings.
Pregnancy: See Warnings.
Lactation : Infant breast-feeding is not recommended for women taking this drug because phenytoin appears to be secreted in low concentrations in human milk.
Children: See Dosage.
Adverse Reactions: CNS: The most common manifestations encountered with phenytoin therapy are referable to this system and are usually dose-related. These include nystagmus, ataxia, slurred speech, decreased coordination and mental confusion. Dizziness, insomnia, transient nervousness, motor twitchings, and headaches have also been observed. There have also been rare reports of phenytoin induced dyskinesias, including chorea, dystonia, tremor and asterixis, similar to those induced by phenothiazine and other neuroleptic drugs.
A predominantly sensory peripheral polyneuropathy has been observed in patients receiving long-term phenytoin therapy.
Gastrointestinal: nausea, vomiting, constipation, toxic hepatitis, and liver damage (see Precautions).
Integumentary: Dermatological manifestations sometimes accompanied by fever have included scarlatiniform or morbilliform rashes. A morbilliform rash (measles-like) is the most common; other types of dermatitis are seen more rarely. Other more serious forms which may be fatal have included bullous, exfoliative or purpuric dermatitis, lupus erythematosus, Stevens-Johnson syndrome and toxic epidermal necrolysis (see Precautions).
Hemopoietic: Hemopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin. These have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression. While macrocytosis and megaloblastic anemia have occurred, these conditions usually respond to folic acid therapy. Lymphadenopathy including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and Hodgkin’s Disease have been reported (see Warnings).
Connective Tissue: Coarsening of the facial features, enlargement of the lips, gingival hyperplasia, hypertrichosis and Peyronie’s Disease.
Immunologic: Hypersensitivity syndrome (which may include, but is not limited to symptoms such as arthralgias, eosinophilia, fever, liver dysfunction, lymphadenopathy or rash), systemic lupus erythematosus, periarteritis nodosa, and immunoglobulin abnormalities. Several individual case reports have suggested that there may be an increased, although still rare, incidence of hypersensitivity reactions, including skin rash and hepatotoxicity, in black patients.
Symptoms And Treatment Of Overdose: Symptoms: The lethal dose of phenytoin in pediatric patients is not known. The lethal dose in adults is estimated to be 2 to 5 g. The initial symptoms are nystagmus, ataxia, and dysarthria. Other signs are tremor, hyperreflexia, somnolence, drowsiness, lethargy, slurred speech, blurred vision, nausea, vomiting. The patient may become comatose and hypotensive. Death is due to respiratory and circulatory depression.
There are marked variations among individuals with respect to phenytoin plasma levels where toxicity may occur. Nystagmus on lateral gaze, usually appears at 80 µmol/L (20 µg/mL), ataxia at 119 µmol/L (30 µg/mL). Dysarthria and lethargy appear when the serum concentration is >159 µmol/L (40 µg/mL), but a concentration as high as 198 µmol/L (50 µg/mL) has been reported without evidence of toxicity. As much as 25 times the therapeutic dose has been taken to result in a serum concentration over >396 µmol/L (100 µg/mL) with complete recovery.
Treatment: Treatment is nonspecific since there is no known antidote.
The adequacy of the respiratory and circulatory systems should be carefully observed and appropriate supportive measures employed. Hemodialysis can be considered since phenytoin is not completely bound to plasma proteins. Total exchange transfusion has been used in the treatment of severe intoxication in pediatric patients.
In acute overdosage the possibility of other CNS depressants, including alcohol, should be borne in mind.
Dosage And Administration: Serum phenytoin concentrations should be monitored when switching a patient from the sodium salt to the free acid form.
Dilantin capsules, are formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Dilantin-30 Pediatric and Dilantin-125 Suspensions and Dilantin Infatabs. Because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product formulated with the free acid to a product formulated with the sodium salt and vice versa.
General: Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments. The clinically effective serum level is usually 40 to 80 µmol/L (10 to 20 µg/mL). Serum blood level determinations are especially helpful when possible drug interactions are suspected. With recommended dosage, a period of 7 to 10 days may be required to achieve therapeutic blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than 7 to 10 days.
Adults: Patients who have received no previous treatment may be started on one 100 mg extended phenytoin sodium capsule 3 times daily, and the dose then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be 3 to 4 capsules (300 to 400 mg) daily. An increase to 6 capsules daily may be made, if necessary.
Children: Initially, 5 mg/kg/day in 2 or 3 equally divided doses, with subsequent dosage individualized to a maximum of 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old may require the minimum adult dose (300 mg/day). Pediatric dosage forms available include a 30 mg extended phenytoin sodium capsule, a 50 mg palatably flavored Infatab, or an oral suspension form containing 30 mg of phenytoin in each 5 mL.
Alternative Dose: Once-a-day dosage for adults with 300 mg of extended phenytoin sodium capsules may be considered if seizure control is established with divided doses of three 100 mg capsules daily. Studies comparing divided doses of 300 mg with a single daily dose of this quantity indicated that absorption, peak plasma levels, biologic half-life, difference between peak and minimum values, and urinary recovery were equivalent. Once-a-day dosage offers a convenience to the individual patient or to nursing personnel for institutionalized patients, and is intended only to be used for patients requiring this amount of drug daily. A major problem in motivating noncompliant patients may also be lessened when the patient can take all of his medication once-a-day. However, patients should be cautioned not to inadvertently miss a dose. Only extended phenytoin sodium capsules are recommended for once-a-day dosing.
Availability And Storage: 30 mg: Each Coni-Snap white capsule with pink cap, imprinted Parke-Davis and P-D 30 in black ink, contains: extended phenytoin sodium 30 mg. Nonmedicinal ingredients: lactose, magnesium stearate, sugar and talc; capsule shell: D&C Yellow No. 10, FD&C Red No. 3, gelatin and titanium dioxide. Energy: 3.0 kJ (0.7 kcal).
100 mg: Each Coni-Snap white capsule with orange cap, imprinted Parke-Davis and P-D 100 in black ink, contains: extended phenytoin sodium 100 mg. Nonmedicinal ingredients: lactose, magnesium stearate, sugar and talc; capsule shell: FD&C Yellow No. 6, gelatin and titanium dioxide. Energy: 2.6 kJ (0.6 kcal). Sodium:
Store at controlled room temperature 15 to 30°C. Protect from light and moisture.
DILANTIN® CAPSULES Parke-Davis Phenytoin Sodium Anticonvulsant
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