Tolbutal

Uses

Isoniazid is used for the treatment of all forms of tuberculosis in which organisms are susceptible.

Tolbutal is also used to associated treatment for these conditions: Active Tuberculosis, Mycobacterium kansasii infection, Late phase Tuberculosis

How Tolbutal works

Isoniazid is a prodrug and must be activated by bacterial catalase. Specficially, activation is associated with reduction of the mycobacterial ferric KatG catalase-peroxidase by hydrazine and reaction with oxygen to form an oxyferrous enzyme complex. Once activated, isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bacteriocidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms. Specifically isoniazid inhibits InhA, the enoyl reductase from Mycobacterium tuberculosis, by forming a covalent adduct with the NAD cofactor. It is the INH-NAD adduct that acts as a slow, tight-binding competitive inhibitor of InhA.

Dosage

Tolbutal dosage

Adult:

• Active tuberculosis: 5 mg/kg/day. Max: 300 mg/day or 15 mg/kg up to 900 mg/day, 2 or 3 times wkly.
• Latent tuberculosis: 300 mg/day for 6 mth. Nontuberculous mycobacterial infections 5 mg/kg/day for at least 12 mth of culture-negative sputum. Max: 300 mg/day.

Child:10-15 mg/kg/day, max 300 mg/day q 12-24 hourlyWith directly observed biweekly therapy, dosage is 20-30 mg/kg, max 900 mg/dose twice weekly

Side Effects

Peripheral neuropathy (dose-related incidence, 10-20% incidence with 10 mg/kg/d), Loss of appetite, Nausea, Vomiting, Stomach pain, Weakness 1-10%, Dizziness, Slurred speech, Lethargy, Progressive liver damage (increases with age; 2.3% in pts > 50 yo), Hyperreflexia, Agranulocytosis, Anemia, Megaloblastic anemia, Thrombocytopenia, Systemic lupus erythematosus, Seizure

Toxicity

LD₅₀ 100 mg/kg (Human, oral). Adverse reactions include rash, abnormal liver function tests, hepatitis, peripheral neuropathy, mild central nervous system (CNS) effects. In vivo, Isoniazid reacts with pyridoxal to form a hydrazone, and thus inhibits generation of pyridoxal phosphate. Isoniazid also combines with pyridoxal phosphate; high doses interfere with the coenzyme function of the latter.

Precaution

Renal or hepatic impairment; convulsive disorders; history of psychosis; patients at risk of neuropathy or pyridoxine deficiency eg, diabetic, alcoholic, malnourished, uraemic, infected with HIV. Careful monitoring of hepatic function is necessary for black and hispanic women. Check hepatic function before and during treatment. Pregnancy and lactation.

Interaction

Inhibit the hepatic metabolism of antiepileptics (e.g. carbamazepine, ethosuximide, primidone, phenytoin), benzodiazepines (e.g. diazepam, triazolam), chlorzoxazone, theophylline, disulfiram, sometimes leading to increased toxicity. Increased metabolism of enflurane, resulting in potentially nephrotoxic levels of fluoride. Increased concentrations and enhanced effects or toxicity of clofazimine, cycloserine and warfarin. Reduced absorption with Al-containing antacids. Increased risk of peripheral neuropathy with zalcitabine and stavudine.

Elimination Route

Readily absorbed following oral administration; however, may undergo significant first pass metabolism. Absorption and bioavailability are reduced when isoniazid is administered with food.

Half Life

Fast acetylators: 0.5 to 1.6 hours. Slow acetylators: 2 to 5 hours.

Elimination Route

From 50 to 70 percent of a dose of isoniazid is excreted in the urine within 24 hours.

Pregnancy & Breastfeeding use

Pregnancy Category C. Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks

Lactation: distributed into milk but safe for nursing infants

Contraindication

Acute liver disease or history of hepatic damage during INH therapy; hypersensitivity.

Innovators Monograph

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