ETHAMBUTOL
ETHAMBUTOL
INTRODUCTION:
- One of the first-line anti-tubercular drug.
- Given in combination with Isoniazid, Rifampin & Pyrazinamide.
MECHANISM OF ACTION:
- Selectively bacteriostatic active against Mycobacterial group including Mycobacterium avium complex (MAC).
- Inactive against other bacterial types.
- Inhibits mycobacterial arabinosyl transferases (Encoded by embAB gene).
- Arabinosyl transferases essential for polymerization reaction of arabinogalactan, an essential component of mycobacterial cell wall.
RESISTANCE:
- Due to mutations resulting in Emb gene product overexpression or within emb B structural gene.
PHARMACOKINETICS:
- Absorption
- Bioavailability: ~80%
- Peak plasma time: 2-4 hr
- Distribution
- Widely throughout body.
- Concentrated in kidneys, lungs, saliva, & RBC’s.
- CSF:blood level ratio:
- 0% (normal meninges); 25% (inflamed meninges)
- Protein binding: 20-30%
- Widely throughout body.
- Metabolism
- Hepatic (20%) to inactive metabolite.
- Not a hepatotoxic antitubercular drug.
- Elimination
- Half-life elimination: 2.5-3.6 hr;
- In end-stage renal disease – 7-15 hr.
- Hence, requires dose adjustment in renal conditions.
- Excretion: ~50% urine; ~50% faeces as unchanged drug.
- Pregnancy & Lactation
- Pregnancy Category: B
- Lactation: Enters breast milk; use with caution
USES:
Used along with other medications to treats number of infections including:
- Tuberculosis,
- Mycobacterium avium complex (MAC
- Mycobacterium kansasii.
- Under RNTCP, for diagnosis of MDR TB, currently, drug sensitivity testing is done for Rifampicin, Isoniazid and Ethambutol.
CONTRAINDICATIONS:
- Optic neuritis.
- In young children unable to report visual effects.
- Hypersensitivity.
MONITORING PARAMETERS
- Ophthalmologic exam
- Platelet count
- Serum creatinine/BUN
- Serum uric acid
ADVERSE REACTION:
- Acute gout/hyperuricemia.
- Due to interference with urate excretion.
- Abdominal pain
- Anaphylaxis.
- Confusion.
- Disorientation.
- Fever.
- Headache.
- LFT abnormalities.
- Malaise & nausea.
- Optic neuritis – Most important.
- Is a dose & duration of therapy dependent toxicity.
- Symptoms may include decreased acuity, colour blindness(green colour) or visual defects.
- Usually are reversible with discontinuation.
- Peripheral neuritis.
- Rash.
DRUG INTERACTIONS:
- Aluminium hydroxide – Reduce rate or extent of ethambutol absorption.
- At least 4 hours should elapse between doses of aluminium hydroxide-containing antacids & ethambutol.
- BCG vaccine:
- Ethambutol may interfere with development of an immune response following Bacillus Calmette-Guerin vaccine, BCG.
- Vaccine is live & sensitive to commonly used antituberculosis agents (e.g., isoniazid, ethambutol, rifampin).
Exam Important
- Ethambutol is not a hepatotoxic antitubercular drug.
- Ethambutol is bacteriostatic.
- Under the RNTCP, for diagnosis of MDR TB, currently, drug sensitivity testing is done for Rifampicin, Isoniazid and Ethambutol.
- In ethambutol toxicity, there will be loss of ability to see green colour.
- Retrobulbar neuritis is seen with ethambutol.
- Hyperuricemia is seen with ethambutol.
- Ethambutol is avoided in children as it causes visual disturbance
- Antitubercular drug causing gout is ethambutol.
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