Ethambutol

MECHANISM OF ACTION:
  • Ethambutol inhibits mycobacterial arabinosyl transferases. 
  • Ethambutol is bacteriostatic
  • Arabinosyl transferases are involved in the polymerization reaction of arabinoglycan, an essential component of the mycobacterial cell wall.
  • Resistance to ethambutol is due to mutations resulting in overexpression of Emb gene products or within the emb B structural gene.
PHARMACOKINETICS:
  • Absorption
  1. Bioavailability: ~80%
  2. Peak Plasma Time: 2-4 hr
  • Distribution
  1. Widely throughout body; concentrated in kidneys, lungs, saliva, and red blood cells
  2. CSF: blood level ratio: 0% (normal meninges); 25% (inflamed meninges)
  3. Protein binding: 20-30%
  • Metabolism
  1. Hepatic (20%) to inactive metabolite
  2. Ethambutol is not a hepatotoxic antitubercular drug
  • Elimination
  1. Half-life elimination: 2.5-3.6 hr; 7-15 hr (end-stage renal disease)
  2. Excretion: ~50% urine; ~50% feces as unchanged drug.
  3. Pregnancy & Lactation
  • Pregnancy Category: B:
  1. Lactation: enters breast milk; use with caution
USES:
Ethambutol is used along with other medications to treat a number of infections including:
  • Tuberculosis, 
  • Mycobacterium avium complex, and 
  • Mycobacterium kansasii.
  • Under the RNTCP, for diagnosis of MDR TB, currently drug sensitivity testing is done for Rifampicin, Isoniazide and Ethambutol
CONTRAINDICATIONS:
  • Optic neuritis
  • Hypersensitivity
MONITORING PARAMETERS
  • Ophthalmologic exam
  • Platelet count
  • Serum creatinine/BUN
  • Serum uric acid
ADRS
  • Acute gout or hyperuricemia, Abdominal pain, Anaphylaxis, Confusion, disorientation, Fever, Headache, LFT abnormalities, Malaise, Nausea
  • Optic neuritis; symptoms may include decreased acuity, color blindness(green color) or visual defects (usually reversible with discontinuation)
  • Peripheral neuritis
  • Rash
DRUG INTERACTIONS:
  • Aluminum hydroxide can reduce the rate or extent of ethambutol absorption. 
  • At least 4 hours should elapse between doses of aluminum hydroxide-containing antacids and ethambutol.
  • Ethambutol may interfere with the development of an immune response following Bacillus Calmette-Guerin vaccine, BCG. 
  • The vaccine is a live vaccine and is sensitive to commonly used antituberculosis agents (e.g., isoniazid, ethambutol, rifampin)
Exam Question
  • 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, Isoniazide and Ethambutol
  • In ethambutol toxicity there will be loss of ability to see green color
  • 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 Antitubercular drug causing gout is

Don't Forget to Solve all the previous Year Question asked on Ethambutol