Issues in Mycobacterium tuberculosis Complex (MTBC) Drug Susceptibility Testing: Ethambutol (EMB)

1 Issues in MTBC Drug Susceptibility Testing: Ethambutol BACKGROUND Ethambutol (EMB) is an antituberculosis drug used as part of the first line treatment against Mycobacterium tuberculosis complex (MTBC).1 A primary role of EMB when used in initial combination antituberculosis therapy is to minimize the risk of drug resistance development to companion first line drugs, particularly isoniazid.2 Resistance to EMB will not decrease the effectiveness or increase the length of treatment for MTBC susceptible to the other first-line drugs. Within current dosing recommendations, EMB is bacteriostatic against actively replicating bacteria but can be bactericidal when serum concentrations are over 10 μg/mL.3 Ethambutol acts through disruption of MTBC cell wall synthesis, targeting and inhibiting the function of arabinosyl transferases, encoded by the embCAB operon, and responsible for biosynthesis of the cell wall components arabinogalactan and lipoarabinomannan.4–6 This disruption may lead to increased permeability of the cell wall.1,7 Ethambutol has also been found to inhibit RNA biosynthesis.8