PHENOBARBITAL SODIUM

The barbiturates are nonselective central nervous system (CNS) depressants which are primarily used as sedative hypnotics and also anticonvulsants in subhypnotic doses. The barbiturates and their sodium salts are subject to control under the Federal Controlled Substances Act (CIV). Barbiturates are substituted pyrimidine derivatives in which the basic structure common to these drugs is barbituric acid, a substance which has no central nervous system activity. CNS activity is obtained by substituting alkyl, alkenyl or aryl groups on the pyrimidine ring. Phenobarbital Sodium Injection, USP is a sterile solution for intramuscular or slow intravenous administration as a long-acting barbiturate. Each mL contains phenobarbital sodium either 65 mg or 130 mg, alcohol 0.1 mL, propylene glycol 0.678 mL and benzyl alcohol 0.015 mL in Water for Injection; hydrochloric acid added, if needed, for pH adjustment. The pH range is 9.2-10.2. Chemically, phenobarbital sodium is 2,4,6(1 H ,3 H ,5 H )-Pyrimidinetrione,5-ethyl-5-phenyl-, monosodium salt and has the following structural formula: C12 H 11 N 2 NaO 3 MW 254.22 The sodium salt of phenobarbital occurs as a white, slightly bitter powder, crystalline granules or flaky crystals; it is soluble in alcohol and practically insoluble in ether or chloroform. Chemical Structure

Parenteral a. Sedative. Sedation is obtainable within an hour, and in adequate dosage, the duration of action is more than six hours. Included in the more common conditions in which the sedative action of this class of drugs is desired are anxiety-tension states, hyperthyroidism, essential hypertension, nausea and vomiting of functional origin, motion sickness, acute labyrinthitis, pylorospasm in infants, chorea and cardiac failure. Phenobarbital is also a useful adjunct in treatment of hemorrhage from the respiratory or gastrointestinal tract. Phenobarbital controls anxiety, decreases muscular activity and lessens nervous excitability in hyperthyroid patients. However, thyrotoxic individuals occasionally react poorly to barbiturates. b. Hypnotic, for the short-term treatment of insomnia, since it appears to lose its effectiveness for sleep induction and sleep maintenance after 2 weeks (see CLINICAL PHARMACOLOGY ). c. Preanesthetic. d. Long-term anticonvulsant, (phenobarbital, mephobarbital and metharbital) for the treatment of generalized tonic-clonic and cortical focal seizures. And, in the emergency control of certain acute convulsive episodes, e.g., those associated with status epilepticus, cholera, eclampsia, cerebral hemorrhage, meningitis, tetanus, and toxic reactions to strychnine or local anesthetics. Phenobarbital sodium may be administered intramuscularly or intravenously as an anticonvulsant for emergency use. When administered intravenously, it may require 15 or more minutes before reaching peak concentrations in the brain. Therefore, injecting phenobarbital sodium until the convulsions stop may cause the brain level to exceed that required to control the convulsions and lead to severe barbiturate-induced depression. e. Phenobarbital is indicated in pediatric patients as an anticonvulsant and as a sedative, including its preoperative and postoperative use.

Dosages of barbiturates must be individualized with full knowledge of their particular characteristics and recommended rates of administration. Factors to consider are the patient's age, weight and condition. Suggested doses of phenobarbital sodium for specific indications follow: Pediatric Dosage Recommended by the American Academy of Pediatrics (intended as a guide) Preoperative Sedation: 1 to 3 mg/kg IM or IV Anticonvulsant: 4 to 6 mg/kg/day for 7 to 10 days to blood level of 10 to 15 mcg/mL or 10 to 15 mg/kg/day IM or IV Status Epilepticus: 15 to 20 mg/kg over 10 to 15 minutes IV Adult Dosage (intended as a guide) Daytime Sedation: 30 to 120 mg daily in 2 to 3 divided doses IM or IV Bedtime Hypnosis: 100 to 320 mg IM or IV Preoperative Sedation: IM only — 100 to 200 mg 60 to 90 minutes before surgery Acute Convulsions: 20 to 320 mg IM or IV, repeated in 6 hours as necessary Parenteral routes should be used only when oral administration is impossible or impractical. Intramuscular injection of the sodium salts of barbiturates should be made deeply into a large muscle and a volume of 5 mL should not be exceeded at any one site because of possible tissue irritation. Injection into or near peripheral nerves may result in permanent neurological deficit. After intramuscular injection of a hypnotic dose, the patient's vital signs should bemonitored. Subcutaneous administration is not recommended (see CONTRAINDICATIONS ). Intravenous Administration Intravenous injection is restricted to conditions in which other routes are not feasible, either because the patient is unconscious (as in cerebral hemorrhage, eclampsia or status epilepticus), or because the patient resists (as in delirium) or because prompt action is imperative. Slow IV injection is essential, and patients should be carefully observed during administration. This requires that blood pressure, respiration and cardiac function be maintained, vital signs be recorded and equipment for resuscitation and artificial ventilation be available. The rate of intravenous injection for adults should not exceed 60 mg/min for phenobarbital sodium. When given intravenously, do not use small veins, such as those on the dorsum of the hand or wrist. Preference should be given to a larger vein to minimize the risk of irritation with the possibility of resultant thrombosis. Avoid administration into varicose veins because circulation there is retarded. Inadvertent injection into or adjacent to an artery has resulted in gangrene requiring amputation of an extremity or a portion thereof. Careful technique, including aspiration, is necessary to avoid inadvertent intraarterial injection. (See below.) Treatment of Adverse Effects Due to Inadvertent Error in Administration Extravasation into subcutaneous tissues causes tissue irritation. This may vary from slight tenderness and redness to necrosis. Recommended treatment includes the application of moist heat and the injection of 0.5% procaine solution into the affected area. Intraarterial injection of any barbiturate must be avoided. The accidental intraarterial injection of a small amount of the solution may cause spasm and severe pain along the course of the artery. The injection should be terminated if the patient complains of pain or if other indications of accidental intraarterial injection occur, such as a white hand with cyanosed skin or patches of discolored skin and delayed onset of hypnosis. The consequences of intraarterial injection of phenobarbital can vary from transient pain to gangrene. It is not possible to formulate strict rules for management of such accidents. The following procedures have been suggested: 1) release of the tourniquet or restrictive garments to permit dilution of injected drug, 2) relief of arterial spasm by injecting 10 mL of a 1% procaine solution into the artery and, if considered necessary, brachial plexus block, 3) prevention of thrombosis by early anticoagulant therapy and 4) supportive treatment. Anticonvulsant Use A therapeutic anticonvulsant level of phenobarbital in the serum is 10 to 25 μg/mL. To achieve the blood levels considered therapeutic in children, higher per-kilogram dosages are generally necessary for phenobarbital and most other anticonvulsants. In children and infants, phenobarbital at loading doses of 15 to 20 mg/kg produces blood levels of about 20 μg/mL shortly after administration. In status epilepticus, it is imperative to achieve therapeutic blood levels of a barbiturate (or other anticonvulsants) as rapidly as possible. When administered intravenously, phenobarbital sodium may require 15 minutes or more to attain peak concentrations in the brain. If phenobarbital sodium is injected continuously until the convulsions stop, the brain concentration will continue to rise and can eventually exceed that required to control the seizures. Because a barbiturate-induced depression may occur along with a postictal depression once the seizures are controlled, it is important, therefore, to use the minimal amount required and to wait for the anticonvulsant effect to develop before administering a second dose. Phenobarbital has been used in the treatment and prophylaxis of febrile seizures. However, it has not been established that prevention of febrile seizures influences the subsequent development of epilepsy. Special Patient Population Dosage should be reduced in the elderly or debilitated because these patients may be more sensitive to barbiturates. Dosage should be reduced for patients with impaired renal function or hepatic disease. Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.

Barbiturates are contraindicated in patients with known barbiturate sensitivity. Barbiturates are also contraindicated in patients with a history of manifest or latent porphyria, marked impairment of liver functions or with severe respiratory distress where dyspnea or obstruction is evident. Large doses are contraindicated in nephritic subjects. Barbiturates should not be administered to persons with known previous addiction to the sedative-hypnotic group since ordinary doses may be ineffectual and may contribute to further addiction. Intraarterial administration is contraindicated. Its consequences vary from transient pain to gangrene. Subcutaneous administration produces tissue irritation, ranging from tenderness and redness to necrosis and is not recommended. (See DOSAGE AND ADMINISTRATION, Treatment of Adverse Effects Due to Inadvertent Error in Administration .) Phenobarbital Sodium Injection contains the preservative benzyl alcohol and is not recommended for use in neonates. There have been reports of fatal 'gasping syndrome' in neonates (children less than one month of age) following the administration of intravenous solutions containing the preservative benzyl alcohol. Symptoms include a striking onset of gasping respiration, hypotension, bradycardia, and cardiovascular collapse. Habit Forming Barbiturates may be habit forming. Tolerance and psychological and physical dependence may occur with continued use (see DRUG ABUSE AND DEPENDENCE and CLINICAL PHARMACOLOGY ). Patients who are psychologically dependent on barbiturates may increase the dosage or decrease the dosage interval without consulting a physician and may subsequently develop a physical dependence on barbiturates. To minimize the possibility of overdosage or the development of dependence, the prescribing and dispensing of sedative-hypnotic barbiturates should be limited to the amount required for the interval until the next appointment. Abrupt cessation after prolonged use in the dependent person may result in withdrawal symptoms, including delirium, convulsions and possibly death. Barbiturates should be withdrawn gradually from any patient known to be taking excessive dosage over long periods of time (see DRUG ABUSE AND DEPENDENCE ). Dermatologic Reactions Exfoliative dermatitis and Stevens-Johnson syndrome, possibly fatal, are rare hypersensitivity reactions to phenobarbital. Physicians should be alert to signs which may precede the onset of barbiturate- induced cutaneous lesions, and the drug should be discontinued whenever dermatological reactions occur. Intravenous Administration Too rapid administration may cause severe respiratory depression, apnea, laryngospasm, hypertension or vasodilation with fall in blood pressure. When administered intravenously, it may require 15 or more minutes before reaching peak concentrations in the brain. Therefore, injecting phenobarbital sodium until the convulsions stop may cause brain levels to exceed that required to control the convulsions and lead to severe barbiturate-induced depression. Acute or Chronic Pain Caution should be exercised when barbiturates are administered to patients with acute or chronic pain, because paradoxical excitement could be induced or important symptoms could be masked. However, the use of barbiturates as sedatives in the postoperative surgical period and as adjuncts to cancer chemotherapy is well established. Use in Pregnancy Barbiturates can cause fetal harm when administered to a pregnant woman. Retrospective, case-controlled studies have suggested a connection between the maternal consumption of barbiturates and a higher than expected incidence of fetal abnormalities. Phenobarbital may cause major fetal malformations. Following oral or parenteral administration, barbiturates readily cross the placental barrier and are distributed throughout fetal tissues with highest concentrations found in the placenta, fetal liver and brain. Fetal blood levels approach maternal blood levels following parenteral administration. Withdrawal symptoms occur in infants born to mothers who receive barbiturates throughout the last trimester of pregnancy (see DRUG ABUSE AND DEPENDENCE ). Phenobarbital should be used during pregnancy only when clearly indicated. If phenobarbital is used during pregnancy or if the patient becomes pregnant while taking the drug, the patient should be apprised of the potential hazard to the fetus. Use in Children Phenobarbital has been reported to be associated with cognitive defects in children taking it for complicated febrile seizures. Synergistic Effects The concomitant use of alcohol or other CNS depressants may produce additive CNS depressant effects.

The following adverse reactions and their incidence were compiled from surveillance of thousands of hospitalized patients. Because such patients may be less aware of certain of the milder adverse effects of barbiturates, the incidence of these reactions may be somewhat higher in fully ambulatory patients. Nervous System Somnolence, agitation, confusion, hyperkinesia, ataxia, CNS depression, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia, anxiety, dizziness, thinking abnormality Respiratory System Hypoventilation, apnea Cardiovascular System Bradycardia, hypotension, syncope Digestive System Nausea, vomiting, constipation Dermatologic Reactions Exfoliative dermatitis, Stevens-Johnson syndrome, toxic epidermic necrolysis. Other Reported Reactions Headache; injection site reactions; hypersensitivity reactions, including but not limited to angioedema and skin rashes; fever; liver damage and megaloblastic anemia following chronic phenobarbital use.

Most reports of clinically significant drug interactions occurring with the barbiturates have involved phenobarbital. However, the application of these data to other barbiturates appears valid and warrants serial blood level determinations of the relevant drugs when there are multiple therapies. Anticoagulants Phenobarbital lowers the plasma levels of dicumarol (bishydroxycoumarin) and causes a decrease in anticoagulant activity as measured by the prothrombin time. Barbiturates can induce hepatic microsomal enzymes resulting in increased metabolism and decreased anticoagulant response of oral anticoagulants (e.g., warfarin, acenocoumarol, dicumarol and phenprocoumon). Patients stabilized on anticoagulant therapy may require dosage adjustments if barbiturates are added to or withdrawn from their dosage regimen. Corticosteroids Barbiturates appear to enhance the metabolism of exogenous corticosteroids probably through the induction of hepatic microsomal enzymes. Patients stabilized on corticosteroid therapy may require dosage adjustments if barbiturates are added to or withdrawn from thier dosage regimen. Griseofulvin Phenobarbital appears to interfere with the absorption of orally administered griseofulvin, thus decreasing its blood level. The effect of the resultant decreased blood levels of griseofulvin on therapeutic response has not been established. However, it would be preferable to avoid concomitant administration of these drugs. Doxycycline Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued. This mechanism is probably through the induction of hepatic microsomal enzymes that metabolize the antibiotic. If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely. Phenytoin, Sodium Valproate, Valproic Acid The effect of barbiturates on the metabolism of phenytoin appears to be variable. Some investigators report an accelerating effect, while others report no effect. Because the effect of barbiturates on the metabolism of phenytoin is not predictable, phenytoin and barbiturate blood levels should be monitored more frequently if these drugs are given concurrently. Sodium valproate and valproic acid appear to decrease barbiturate metabolism; therefore, barbiturate blood levels should be monitored and appropriate dosage adjustments made as indicated. Central Nervous System Depressants The concomitant use of other central nervous system depressants, including other sedatives or hypnotics, antihistamines, tranquilizers or alcohol, may produce additive depressant effects. Monoamine Oxidase Inhibitors (MAOIs) MAOIs prolong the effects of barbiturates probably because metabolism of the barbiturate is inhibited.

Storage Store at 20°-25°C(68°-77°F), excursions permitted to 15°-30°C(59°-86°F) [See USP Controlled Room Temperature]. Do not use if solution is discolored or contains a precipitate. To report SUSPECTED ADVERSE REACTIONS, contact the FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.