Ethambutol, an antitubercular drug used to treat tuberculosis (TB), was developed in the 1960s and approved for medical use in the late 1960s. Its history is marked by its effectiveness in inhibiting the growth of Mycobacterium tuberculosis, particularly as part of combination therapy to prevent drug resistance. Ethambutol acts as a bacteriostatic agent, targeting the synthesis of the mycobacterial cell wall by inhibiting arabinosyl transferases. Its development included rigorous clinical evaluation to establish safe dosing regimens and monitor for ocular toxicity, the most significant adverse effect associated with the drug. Ethambutol remains an important component of first-line TB treatment regimens worldwide, often administered alongside isoniazid, rifampicin, and pyrazinamide.
BRAND NAMES
Myambutol – widely used in many countries.
Ethambutol Teva – a formulation by Teva Pharmaceuticals.
MECHANISM OF ACTION
Ethambutol is an antitubercular agent that acts by inhibiting the synthesis of the mycobacterial cell wall. It specifically targets arabinosyl transferases, enzymes responsible for polymerizing arabinogalactan, a critical component of the cell wall in Mycobacterium tuberculosis.
PHARMACOKINETICS
Absorption
Ethambutol is well absorbed orally, with about 80% of the administered dose entering systemic circulation. Peak plasma concentrations are usually reached 2–4 hours after oral ingestion. Its absorption is not significantly affected by food, so it can be taken with or without meals.
Distribution
Ethambutol has a moderate volume of distribution, typically around 1–2 L/kg. This indicates that the drug distributes well into body fluids and tissues, including lungs, liver, kidneys, and spleen, which are relevant sites for Mycobacterium tuberculosis infection.
Metabolism
Ethambutol is partially metabolized in the liver. Approximately 20–30% of the administered dose undergoes hepatic metabolism, primarily through oxidation to form inactive metabolites.
Elimination
Ethambutol is primarily eliminated unchanged by the kidneys through glomerular filtration and tubular secretion. About 50% of the drug is excreted unchanged in the urine, with the remainder as inactive metabolites.
PHARMACODYNAMICS
Ethambutol is a bacteriostatic antitubercular agent that specifically targets Mycobacterium tuberculosis and some atypical mycobacteria. Its activity is concentration-dependent at the site of infection, but it primarily works by inhibiting the synthesis of arabinogalactan, a key component of the mycobacterial cell wall.
ADMINISTRATION
Ethambutol is administered orally, usually in the form of tablets or an oral suspension, and is well absorbed regardless of food intake. It is typically given once daily at a dose of 15–25 mg/kg, depending on the patient’s weight and treatment regimen.
DOSAGE AND STRENGTH
Ethambutol is available in oral tablet formulations, commonly in strengths of 100 mg, 200 mg, 400 mg, and 800 mg. The usual adult dosage is 15–25 mg/kg body weight once daily, depending on the phase of tuberculosis treatment and clinical condition.
DRUG INTERACTIONS
Ethambutol has relatively few clinically significant drug interactions, but certain precautions are important. Aluminum-containing antacids can reduce its absorption, so they should be taken at least 2–4 hours apart. Ethambutol is routinely used in combination with other antitubercular drugs such as isoniazid, rifampicin, and pyrazinamide.
FOOD INTERACTIONS
Ethambutol has minimal food interactions and can be taken with or without food without significantly affecting its absorption. Food does not interfere with its bioavailability, making administration flexible and convenient for patients.
CONTRAINDICATIONS
Ethambutol is contraindicated in patients with known hypersensitivity to the drug or any of its components. It should not be used in individuals with pre-existing optic neuritis, as the drug can worsen visual impairment and lead to irreversible vision loss.
SIDE EFFECTS
Optic neuritis: blurred vision, decreased visual acuity, loss of red–green color vision
Peripheral neuropathy: tingling or numbness in extremities (rare)
Gastrointestinal effects: nausea, vomiting, abdominal discomfort, loss of appetite
Central nervous system effects: headache, dizziness, confusion.
TOXICITY
Ethambutol toxicity is primarily dose-related, with the most significant adverse effect being optic neuritis, which presents as decreased visual acuity, blurred vision, and loss of red–green color discrimination. This toxicity is usually reversible if the drug is discontinued early, but prolonged exposure or high doses may lead to permanent vision loss. The risk is higher in patients receiving doses above recommended levels or those with renal impairment, due to drug accumulation.
