Ethambutol is a water-soluble, heat-stable compound that acts by inhibition of arabinosyl transferase enzymes that are involved in cell wall biosynthesis. Nearly all strains of M. tuberculosis and M. kansasii and most strains of Mycobacterium avium-intracellulare are sensitive to ethambutol. Drug resistance relates to point mutations in the gene (EmbB) that encodes the arabinosyl transferases that are involved in mycobacterial cell wall synthesis.
Orally administered ethambutol is well absorbed (70-80%) from the gut, and peak serum concentrations are obtained within 2 to 4 hours of drug administration; it has a half-life of 3 to 4 hours. Ethambutol is widely distributed in all body fluids, including the cere-brospinal fluid, even in the absence of inflammation. A majority of the unchanged drug is excreted in the urine within 24 hours of ingestion. Up to 15% is excreted in the urine as an aldehyde and a dicarboxylic acid metabolite. Ethambutol doses may have to be modified in patients with renal failure.
Ethambutol has replaced aminosalicylic acid as a first-line antitubercular drug. It is commonly included as a fourth drug, along with isoniazid, pyrazinamide, and rifampin, in patients infected with MDR strains. It also is used in combination in the treatment of M. avium-intracellular infection in AIDS patients.
The major toxicity associated with ethambutol use is retrobulbar neuritis impairing visual acuity and red-green color discrimination; this side effect is dose related and reverses slowly once the drug is discontinued. Mild GI intolerance, allergic reaction, fever, dizziness, and mental confusion are also possible. Hyperuricemia is associated with ethambutol use due to a decreased renal excretion of urates; gouty arthritis may result.
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