Ethambutol is a bacteriostatic antimycobacterial agent, which obstructs the formation of cell wall by inhibiting arabinosyl transferases.Target: AntibacterialEthambutol directly affects two polymers, arabinogalactan (AG) and lipoarabinomannan (LAM) in Mycobacterium smegmatis. In M. smegmatis, Ethambutol inhibits synthesis of arabinan completely and inhibits AG synthesis most likely as a consequence of this; more than 50% of the cell arabinan is released from the bacteria following Ethambutol treatment, whereas no galactan is released. Ethambutol main targets against embB gene product in M. avium. Ethambutol induces 60% changes in the embB gene in M. tuberculosis resistant mutants [1]. Ethambutol is effective against actively growing microorganisms of the genus Mycobacterium, including M. tuberculosis. Nearly all strains of M. tuberculosis and M. kansasii as well as a number of strains of the M. aviumcomplex (MAC) are sensitive to Ethambutol. [1] Ethambutol is potency against M. tuberculosis (H37Rv) with MIC of 0.5 μg/mL in vitro [2]. Ethambutol is efficient on treatment of mycobacterial-infected macrophages. When M. tuberculosis infected macrophages are treated with 6 μg/mL Ethambutol, the log CFUs following treatment for 3 days is 4.17, while value in control group is 4.8. The MICs for M. avium (MTCC 1723) and M. smegmatis (MTCC 6) are 15 μg/mL and 0.18 μg/mL, respectively. Ethambutol is efficient in animal model. 100 mg/kg Ethambutol given orally 15 days post i.v. infection 1 ×/week for 5 weeks, induces a lower log CFU compared with untreatment (4.59 vs 5.07) [3].

References:
[1]. Ethambutol. Tuberculosis (Edinb), 2008. 88(2): p. 102-5.
[2]. Rastogi, N., V. Labrousse, and K.S. Goh, In vitro activities of fourteen antimicrobial agents against drug susceptible and resistant clinical isolates of Mycobacterium tuberculosis and comparative intracellular activities against the virulent H37Rv strain
[3]. Kaur, D. and G.K. Khuller, In vitro, ex-vivo and in vivo activities of ethambutol and sparfloxacin alone and in combination against mycobacteria. Int J Antimicrob Agents, 2001. 17(1): p. 51-5.