Ethambutol

Question 1

Ethambutol

Answer 1

EMB

First line drug for both active TB and MAC infections

Used in initial four drug regimen
Usually given for duration of MAC therapy

Question 2

Ethambutol MOA

Answer 2

Inhibits enzyme aribinosyl transferase III (which blocks production of arabinogalactan)

Microbial activity limited to mycobacteria because arabinogalactan is only a component of mycobacterial cell wall

Question 3

Ethambutol Spectrum

Answer 3

M. tuberculosis
M. avium-intracellulare complex
M. kansasii

Question 4

Ethambutol Adverse Effects

Answer 4

Characteristic adverse effect is optic neuritis (often manifesting as decreased visual acuity or inability to differentiate red from green)
Dose dependent and duration dependent 
Generally reversible 
Monitoring required to detect problem 
Remember: ethambutol- eyes

Use in children less than 5 not recommended; generally not able to reliably perform the vision tests needed for monitoring

Rash and drug fever occur uncommonly

Question 5

Ethambutol Important Facts

Answer 5

Very well tolerated
It is the one component of RIPE active TB regimen not associated with hepatotoxicity

If patient cannot take rifampin during continuation phase (after 2 months) of active TB therapy, can substitute with ethambutol; duration of therapy has to be extended compared to rifampin + isoniazid

One of the primary first line drugs for MAC infections (along with a macrolide + rifabutin)

Question 6

Ethambutol Good For

Answer 6

First line for both active TB and MAC infections

Second line in patients unable to tolerate rifampin during continuation phase