Ethambutol: Anti-Tuberculosis Pharmacology, Arabinosyl Transferase (embB) Inhibition & Optic Neuritis Monitoring

Ethambutol Hydrochloride (ATC Code J04AK02) is a bacteriostatic first-line antimycobacterial agent essential to the standard four-drug RIPE regimen (Rifampicin, Isoniazid, Pyrazinamide, and Ethambutol) for active Mycobacterium tuberculosis and disseminated Mycobacterium avium complex (MAC) infections.

Mechanism of Action: Arabinosyl Transferase Inhibition

Ethambutol selectively inhibits membrane-associated arabinosyl transferases (encoded by the embCAB operon, specifically embB codon 306) in actively dividing mycobacteria. Blocking polymerization of D-arabinose into arabinogalactan disrupts assembly of the mycolic acid–arabinogalactan–peptidoglycan cell wall complex, increasing cell wall permeability to companion drugs such as rifampicin and isoniazid.

Ophthalmic Safety & Renal Dosing Protocol

• Retrobulbar Optic Neuritis Screening: Dose-dependent optic neuropathy manifests as decreased visual acuity, central scotoma, or loss of red-green color discrimination. Baseline and monthly Snellen visual acuity and Ishihara color plate testing are mandatory, especially when dosing exceeds 15 mg/kg/day or therapy extends beyond 2 months.
• Renal Clearance Adjustment: Approximately 80% of ethambutol is excreted unchanged by the kidneys; in patients with eGFR < 30 mL/min/1.73m² or on hemodialysis, dosing intervals must be extended to 15–20 mg/kg every 48 hours (three times weekly post-dialysis).