Mechanism of Action

Isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bactericidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms.

Resistance
Resistance to isoniazid occurs because of mutations in the katG, inhA, kasA and ahpC genes. Resistance in M. tuberculosis develops rapidly when isoniazid monotherapy is administered.

Susceptibility Testing

For specific information regarding susceptibility test criteria and associated test methods and quality control standards recognized by FDA for this drug, please see: www.fda.gov/STIC.

General

All drugs should be stopped and an evaluation made at the first sign of a hypersensitivity reaction. If isoniazid therapy must be reinstituted, the drug should be given only after symptoms have cleared. The drug should be restarted in very small and gradually increasing doses and should be withdrawn immediately if there is any indication of recurrent hypersensitivity reaction.

Use of isoniazid should be carefully monitored in the following:

1. Daily users of alcohol. Daily ingestion of alcohol may be associated with a higher incidence of + isoniazid hepatitis.
2. Patients with active chronic liver disease or severe renal dysfunction.
3. Age greater than 35.
4. Concurrent use of any chronically administered medication.
5. History of previous discontinuation of isoniazid.
6. Existence of peripheral neuropathy or conditions predisposing to neuropathy.
7. Pregnancy.
8. Injection drug use.
9. Women belonging to minority groups, particularly in the postpartum period.
10. HIV seropositive patients.

Laboratory Tests

Because there is a higher frequency of isoniazid associated hepatitis among certain patient groups, including Age greater than 35, daily users of alcohol, chronic liver disease, injection drug use and women belonging to minority groups, particularly in the post-partum period, transaminase measurements should be obtained prior to starting and monthly during preventative therapy or more frequently as needed. If any of the values exceed three to five times the upper limit of normal, isoniazid should be temporarily discontinued and consideration given to restarting therapy.

Drug Interactions

Food
Isoniazid should not be administered with food. Studies have shown that the bioavailability of isoniazid is reduced significantly when administered with food. Tyramine- and histamine-containing foods should be avoided in patients receiving isoniazid. Because isoniazid has some monoamine oxidase inhibiting activity, an interaction with tyramine-containing foods (cheese, red wine) may occur. Diamine oxidase may also be inhibited, causing exaggerated response (e.g., headache, sweating, palpitations, flushing, hypotension) to foods containing histamine (e.g., skipjack, tuna, other tropical fish).

Carcinogenesis and Mutagenesis

Isoniazid has been shown to induce pulmonary tumors in a number of strains of mice. Isoniazid has not been shown to be carcinogenic in humans. (Note: a diagnosis of mesothelioma in a child with prenatal exposure to isoniazid and no other apparent risk factors has been reported). Isoniazid has been found to be weakly mutagenic in strains TA 100 and TA 1535 of Salmonella typhimurium(Ames assay) without metabolic activation.

Pregnancy

Teratogenic Effects
Pregnancy Category C
Isoniazid has been shown to have an embryocidal effect in rats and rabbits when given orally during pregnancy. Isoniazid was not teratogenic in reproduction studies in mice, rats and rabbits. There are no adequate and well-controlled studies in pregnant women. Isoniazid should be used as a treatment for active tuberculosis during pregnancy because the benefit justifies the potential risk to the fetus. The benefit of preventive therapy also should be weighed against a possible risk to the fetus. Preventive therapy generally should be started after delivery to prevent putting the fetus at risk of exposure; the low levels of isoniazid in breast milk do not threaten the neonate. Since isoniazid is known to cross the placental barrier, neonates of isoniazid treated mothers should be carefully observed for any evidence of adverse effects.

Nonteratogenic Effects
Since isoniazid is known to cross the placental barrier, neonates of isoniazid-treated mothers should be carefully observed for any evidence of adverse effects.

Nursing Mothers

The small concentrations of isoniazid in breast milk do not produce toxicity in the nursing newborn; therefore, breast feeding should not be discouraged. However, because levels of isoniazid are so low in breast milk, they cannot be relied upon for prophylaxis or therapy of nursing infants.

Nervous System Reactions

Peripheral neuropathy is the most common toxic effect. It is dose-related, occurs most often in the malnourished and in those predisposed to neuritis (e.g., alcoholics and diabetics) and is usually preceded by paresthesias of the feet and hands. The incidence is higher in “slow inactivators”.

Other neurotoxic effects, which are uncommon with conventional doses, are convulsions, toxic encephalopathy, optic neuritis and atrophy, memory impairment and toxic psychosis.

Hepatic Reactions

See boxed warning. Elevated serum transaminase (SGOT; SGPT), bilirubinemia, bilirubinuria, jaundice and occasionally severe and sometimes fatal hepatitis. The common prodromal symptoms of hepatitis are anorexia, nausea, vomiting, fatigue, malaise and weakness. Mild hepatic dysfunction, evidenced by mild and transient elevation of serum transaminase levels occurs in 10 to 20 percent of patients taking isoniazid. This abnormality usually appears in the first 1 to 3 months of treatment but can occur at any time during therapy. In most instances, enzyme levels return to normal and generally, there is no necessity to discontinue medication during the period of mild serum transaminase elevation. In occasional instances, progressive liver damage occurs, with accompanying symptoms. If the SGOT value exceeds three to five times the upper limit of normal, discontinuation of the isoniazid should be strongly considered. The frequency of progressive liver damage increases with age. It is rare in persons under 20, but occurs in up to 2.3 percent of those over 50 years of age.

Gastrointestinal Reactions

Nausea, vomiting, epigastric distress, and pancreatitis.

Hematologic Reactions

Agranulocytosis; hemolytic, sideroblastic or aplastic anemia, thrombocytopenia; and eosinophilia.

Hypersensitivity Reactions

Fever, skin eruptions (morbilliform, maculopapular, purpuric or exfoliative), lymphadenopathy, vasculitis, toxic epidermal necrolysis, and drug reaction with eosinophilia syndrome (DRESS).

Metabolic and Endocrine Reactions

Pyridoxine deficiency, pellagra, hyperglycemia, metabolic acidosis and gynecomastia.

Miscellaneous Reactions

Rheumatic syndrome and systemic lupus erythematosus-like syndrome.

Signs and Symptoms

Isoniazid overdosage produces signs and symptoms within 30 minutes to 3 hours after ingestion. Nausea, vomiting, dizziness, slurring of speech, blurring of vision and visual hallucinations (including bright colors and strange designs) are among the early manifestations. With marked overdosage, respiratory distress and CNS depression, progressing rapidly from stupor to profound coma, are to be expected, along with severe, intractable seizures. Severe metabolic acidosis, acetonuria and hyperglycemia are typical laboratory findings.

Treatment

Untreated or inadequately treated cases of gross isoniazid overdosage, 80 mg/kg to 150 mg/kg, can cause neurotoxicity 6 and terminate fatally, but good response has been reported in most patients brought under adequate treatment within the first few hours after drug ingestion.

General

Obtain blood samples for immediate determination of gases, electrolytes, BUN, glucose, etc.; type and cross-match blood in preparation for possible hemodialysis.

Rapid Control of Metabolic Acidosis

Patients with this degree of INH intoxication are likely to have hypoventilation. The administration of sodium bicarbonate under these circumstances can cause exacerbation of hypercarbia. Ventilation must be monitored carefully, by measuring blood carbon dioxide levels and supported mechanically, if there is respiratory insufficiency.

Dialysis

Both peritoneal and hemodialysis have been used in the management of isoniazid overdosage. These procedures are probably not required if control of seizures and acidosis is achieved with pyridoxine, diazepam and bicarbonate.

Along with measures based on initial and repeated determination of blood gases and other laboratory tests as needed, utilize meticulous respiratory and other intensive care to protect against hypoxia, hypotension, aspiration, pneumonitis, etc.

For Treatment of Tuberculosis

Isoniazid is used in conjunction with other effective anti-tuberculous agents. Drug susceptibility testing should be performed on the organisms initially isolated from all patients with newly diagnosed tuberculosis. If the bacilli becomes resistant, therapy must be changed to agents to which the bacilli are susceptible.

Usual Oral Dosage (depending on the regimen used):

Adults
5 mg/kg up to 300 mg daily in a single dose; or

15 mg/kg up to 900 mg/day, two or three times/week

Children
10 mg/kg to 15 mg/kg up to 300 mg daily in a single dose; or

20 mg/kg to 40 mg/kg up to 900 mg/day, two or three times/week

Patients with Pulmonary Tuberculosis Without HIV Infection
There are 3 regimen options for the initial treatment of tuberculosis in children and adults:

Option 1
Daily isoniazid, rifampin and pyrazinamide for 8 weeks followed by 16 weeks of isoniazid and rifampin daily or 2 to 3 times weekly. Ethambutol or streptomycin should be added to the initial regimen until sensitivity to isoniazid and rifampin is demonstrated. The addition of a fourth drug is optional if the relative prevalence of isoniazid-resistant Mycobacterium tuberculosis isolates in the community is less than or equal to four percent.

Option 2
Daily isoniazid, rifampin, pyrazinamide and streptomycin or ethambutol for 2 weeks followed by twice weekly administration of the same drugs for 6 weeks, subsequently twice weekly isoniazid and rifampin for 16 weeks.

Option 3
Three times weekly with isoniazid, rifampin, pyrazinamide and ethambutol or streptomycin for 6 months.

*All regimens given twice weekly or 3 times weekly should be administered by directly observed therapy

[see also Directly Observed Therapy (DOT)].

The above treatment guidelines apply only when the disease is caused by organisms that are susceptible to the standard antituberculous agents. Because of the impact of resistance to isoniazid and rifampin on the response to therapy, it is essential that physicians initiating therapy for tuberculosis be familiar with the prevalence of drug resistance in their communities. It is suggested that ethambutol not be used in children whose visual acuity cannot be monitored.

Patients with Pulmonary Tuberculosis and HIV Infection
The response of the immunologically impaired host to treatment may not be as satisfactory as that of a person with normal host responsiveness. For this reason, therapeutic decisions for the impaired host must be individualized. Since patients co-infected with HIV may have problems with malabsorption, screening of antimycobacterial drug levels, especially in patients with advanced HIV disease, may be necessary to prevent the emergence of MDRTB.

For Preventative Therapy of Tuberculosis

Before isoniazid preventive therapy is initiated, bacteriologically positive or radiographically progressive tuberculosis must be excluded. Appropriate evaluations should be performed if extra pulmonary tuberculosis is suspected.

Adults over 30 kg: 300 mg per day in a single dose.

Infants and Children: 10 mg/kg (up to 300 mg daily) in a single dose. In situations where adherence with daily preventative therapy cannot be assured, 20 mg/kg to 30 mg/kg (not to exceed 900 mg) twice weekly under the direct observation of a health care worker at the time of administration 8.

Continuous administration of isoniazid for a sufficient period is an essential part of the regimen because relapse rates are higher if chemotherapy is stopped prematurely. In the treatment of tuberculosis, resistant organisms may multiply and the emergence of resistant organisms during the treatment may necessitate a change in the regimen.

For following patient compliance: the Potts-Cozart test 9, a simple colorimetric 6 method of checking for isoniazid in the urine, is a useful tool for assuring patient compliance, which is essential for effective tuberculosis control. Additionally, isoniazid test strips are also available to check patient compliance.

Concomitant administration of pyridoxine (B 6) is recommended in the malnourished and in those predisposed to neuropathy (e.g., alcoholics and diabetics).

References

1. Murphy, R., et al: Annuals of Internal Medicine; 1990: November 15; volume 113: 799-800.
2. Burke, R.F., et al: Res Commun Chem Pathol Pharmacol; 1990: July; vol. 69: 115-118.
3. Fleenor, M.F., et al: Chest (United States) Letter; 1991; June; 99 (6): 1554.
4. Baciewicz, A.M. and Baciewicz, Jr. F.A.: Arch Int Med 1993: September; volume 153: 1970-1971.
5. Jonviller, A.P., et al: European Journal of Clinical Pharmacol (Germany), 1991: 40 (2) p198.
6. American Thoracic Society/Centers for Disease Control: Treatment of Tuberculosis and Tuberculosis Infection in Adults and Children. Amer. J. Respir Crit Care Med. 1994; 149: p1359-1374.
7. Hoglund P., et al: European Journal of Respir Dis (Denmark) 1987: February; 70 (2) p110-116.
8. Committee on Infectious Diseases American Academy of Pediatrics: 1994, Red Book: Report of the Committee on Infectious Diseases; 23 edition; p487.
9. Schraufnagel, DE; Testing for Isoniazid; Chest (United States) 1990: August; 98 (2) p314-316.

To report SUSPECTED ADVERSE REACTIONS, contact Teva at 1-888-838-2872 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.