Generic Name

Ethambutol

Mechanism

• Inhibits peptide‑transferase 1 (Alr) involved in arabinyl‑transferase activity.
• Impedes synthesis of the mycobacterial arabinogalactan component of the cell wall, leading to weakened cell integrity and bacterial death.
• Active primarily against *slow‑growing* mycobacterial species; less effective as a monotherapy.

Pharmacokinetics

• Absorption: Rapid oral uptake; peak plasma levels in 30–60 min.
• Distribution: Good penetration into most tissues, including lungs; limited distribution to the CNS.
• Metabolism: Mainly renally excreted unchanged (~95% in urine).
• Half‑life: 4–5 h in adults with normal renal function.
• Renal adjustment: Requires dose reduction in decreased eGFR or dialysis.

Indications

• First‑line treatment of active pulmonary and extrapulmonary tuberculosis.
• Component of RIPE regimen (rifampin, isoniazid, pyrazinamide, ethambutol).
• Used for drug‑resistant TB in combination regimens in select cases.

Contraindications

• Contraindicated in patients with pre‑existing optic neuropathy or severe visual impairment.
• Warnings:
• Ocular toxicity (visual acuity loss, red‑green color discrimination).
• Hepatotoxicity rare but possible—monitor liver enzymes.
• Renal impairment necessitates dose adjustment.

Dosing

• Route: Oral.
• Administration tips: Take with food to reduce GI upset; avoid splitting tablets if dose >1 g.

Adverse Effects

• Common (≥5%)
• Visual disturbances (color vision, central scotoma).
• Rash, arthralgia.
• Gastrointestinal upset (nausea, diarrhea).
• Serious (≤1%)
• Permanent optic neuropathy → stop immediately.
• Hepatotoxicity; liver function tests required.
• Renal failure in very high doses.

Monitoring

• Visual acuity & color vision: baseline, 2–4 weeks, then every 4–6 weeks.
• Kidney function: serum creatinine, eGFR at baseline & every 2–4 weeks.
• Liver enzymes (ALT/AST) at baseline and periodically.
• Drug interaction checks: e.g., rifampin may lower ethambutol levels; adjust if needed.

Clinical Pearls

1. Early Ocular Cues – Red‑green color confusion often precedes full visual loss; patients should report any color perception changes immediately.

2. “QID” Is Key – Ethambutol’s short half‑life necessitates four times daily dosing; missed doses drastically reduce efficacy.

3. Renal Safety Net – Because 95 % excreted unchanged, a simple eGFR calculation suffices for dose adjustment; avoid over‑correction which may increase toxicity.

4. Combined Pharmacodynamic Synergy – In RIPE, ethambutol’s non‑mycolic‑acid inhibition complements rifampin’s RNA polymerase blockade, lowering resistance emergence.

5. Patient Education – Emphasize routine vision checks; instruct caregivers to monitor children’s reading and recognition tasks.

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• *Ethambutol* is a cornerstone anti‑tuberculosis agent; when used correctly and monitored meticulously, it offers high efficacy with an acceptable safety profile.