etambutol

Ethambutol, sometimes spelled etambutol, is a first-line antimycobacterial agent used in the treatment of active tuberculosis. It is typically included in the initial four-drug regimen (rifampin, isoniazid, pyrazinamide, ethambutol) for drug-susceptible TB and may be continued or stopped based on susceptibility testing and the patient’s clinical response.

Mechanism and role: Ethambutol inhibits arabinosyl transferases involved in the synthesis of arabinogalactan, an essential component of the mycobacterial cell wall, thereby limiting bacterial growth. It is generally considered bacteriostatic rather than bactericidal and is used to reduce the emergence of drug resistance when combined with other TB drugs.

Pharmacokinetics and dosing: Ethambutol can be given by mouth or intravenously. It is well absorbed orally, with a plasma half-life of about 3 to 4 hours in individuals with normal renal function, and is primarily excreted unchanged by the kidneys, requiring dose adjustment in renal impairment. Typical dosing is 15 to 25 mg/kg per day, given once daily or divided, with a maximum around 2.5 g per day depending on guidelines. It is usually employed for the initial two months of therapy or until susceptibility is confirmed.

Adverse effects and precautions: The major adverse effect is optic neuritis, presenting as decreased visual acuity and impaired red-green color discrimination; this risk is dose-related and higher in renal impairment, and symptoms often improve after stopping the drug but can be permanent with prolonged exposure. Other adverse effects are generally mild. Patients should have periodic visual monitoring, and therapy should be discontinued if vision changes occur. Ethambutol is typically used with other TB drugs; dose adjustments are necessary in renal disease, and caution is advised during pregnancy and lactation. Brand names include Myambutol.