ethambutol
Brand: Myambutol
Prototype Drug
Drug Class: antimycobacterial agent
Drug Family: antibiotic
Subclass: arabinosyltransferase inhibitor
Organ Systems: infectious-disease
Mechanism of Action
Inhibits arabinosyltransferases involved in arabinan biosynthesis, a critical component of mycobacterial cell wall arabinogalactan and lipoarabinomannan; bacteriostatic; primarily used in TB regimens to prevent resistance emergence.
arabinosyltransferases (EmbA, EmbB, EmbC)
Indications
- active tuberculosis (part of standard 4-drug initial regimen: RIPE)
- MAC infections (in combination)
Contraindications
- ethambutol hypersensitivity
- optic neuritis (relative)
- children too young to reliably monitor visual acuity
Adverse Effects
Common
- GI upset
- headache
Serious
- optic neuritis (dose- and duration-dependent; reversible if caught early)
- color vision loss (red-green discrimination)
- peripheral neuropathy (less common)
- hyperuricemia
Pharmacokinetics (ADME)
| Absorption | 75–80% oral bioavailability; unaffected by food |
| Distribution | widely distributed; 50% enters red blood cells; poor CSF penetration (unless inflamed) |
| Metabolism | hepatic partial oxidation |
| Excretion | renal (50% unchanged); dose adjustment required in renal impairment |
| Half-life | 3–4 hours |
| Onset | 2–4 hours |
| Peak | 2–4 hours |
| Duration | 24 hours |
| Protein Binding | 20–30% |
| Vd | moderate |
Drug Interactions
| Drug / Agent | Mechanism | Severity |
|---|---|---|
| aluminum-containing antacids | reduce ethambutol absorption; separate by 4 hours | moderate |
Nursing Considerations
- Baseline visual acuity testing (Snellen chart) and color vision testing (Ishihara plates) are mandatory before starting therapy and should be repeated monthly during therapy.
- Instruct patients to immediately report any change in vision, blurred vision, color vision disturbance, or eye pain — early discontinuation of ethambutol is essential to prevent permanent optic nerve damage.
- Reduce dose in renal impairment; monitor BUN and creatinine — ethambutol accumulates in kidney disease.
- Ethambutol is the 'E' in the standard RIPE regimen (Rifampin, Isoniazid, Pyrazinamide, Ethambutol); it can typically be discontinued once drug susceptibility results are confirmed.
Clinical Pearls
- Optic neuritis from ethambutol is dose-dependent and usually reversible if the drug is stopped promptly; irreversible vision loss can occur with continued use, making visual monitoring mandatory.
- Color vision loss (specifically red-green discrimination) often precedes visual acuity changes, making color vision testing a sensitive early monitoring tool.
Safety Profile
Pregnancy generally-safe
Lactation use-with-caution
Renal Adjustment Required
Hepatic Adjustment Not required
TDM Not required
Concordance Terms
Cross-referenced clinical concepts — click any term to see all content where it appears.