M. tuberculosis and other mycobacteria, including M. avium
Ethambutol inhibits arabinotransferases involved in the biosynthetic pathway of mycobacterial cell wall
EMB generally is bacteriostatic at the doses that can be achieved in humans. As such, keeping the serum concentrations above the MIC for the entire dosing interval would be desirable from a theoretical standpoint.
Cmax: 2-6 mg/L; Tmax: about 2-3 hours; Bioavailability: at least 50%, possibly higher; not directly measured against IV dosage form. Protein binding: 20-30%
Optic neuritis is dose and concentration-dependent, being uncommon at 25 mg per kg daily, more common at higher daily doses, and more common if standard doses are given to patients with renal dysfunction. Patients should be questioned regarding visual problems, and color vision tested using Ishihara plates. Renal clearance of urates is decreased by ethambutol.
PO: 400 mg tablets
Usual dose: 25 mg per kg daily, 50 mg per kg in twice-weekly regimens
Hepatic failure: No specific recommendations, but EMB is partially metabolized so patients should be followed for possible visual changes
Renal failure: Adjustment is required for patients in renal failure or on hemodialysis. The usual daily dose should be given only 3 times weekly.
Caution in renal failure
None clearly established
Generally considered safe.
Toxic: baseline visual acuity and red-green color discrimination
Myambutol (Wyeth), generics