Ethambutol is an antimycobacterial agent that is most commonly used in combination with other drugs in the treatment of tuberculosis . It is also used as part of a combination regimen in the therapy of Mycobacterium avium complex (MAC) infections in patients with or without concomitant infection with human immunodeficiency virus (HIV) .
The American Thoracic Society (ATS), Centers for Disease Control (CDC), and Infectious Disease Society of America (IDSA) statement on the treatment of tuberculosis, as well as other ATS guidelines, can be accessed through the ATS web site.
MECHANISM OF ACTION
The mechanism of action of ethambutol is not completely known. There is evidence that the drug exerts its bacteriostatic activity by virtue of inhibition of arabinosyl transferase, an enzyme that polymerizes arabinose into arabinan and then arabinogalactan, a mycobacterial cell wall constituent.
Organisms with minimal inhibitory concentrations (MICs) exceeding 5 mcg/mL are generally considered resistant to ethambutol. M. tuberculosis resistance to ethambutol is due to random spontaneous genetic mutations, occurring at a rate of approximately 1 in 107 organisms; mutations most commonly result in increased production of the enzyme arabinosyl transferase, which overwhelms the inhibitory effects of ethambutol [2,3]. (See "Epidemiology and molecular mechanisms of drug-resistant tuberculosis".)
Primary (pretreatment) resistance rates of M. tuberculosis to ethambutol vary widely. Although a report from New York City cited primary resistance as high as 14 percent, a global survey performed in 35 countries between 1994 and 1997 found a primary ethambutol resistance rate of 1 percent .