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What are tetracyclines? 

The tetracyclines are broad-spectrum antibiotics. Its effectiveness against a wide range of microorganisms. The drugs have four cyclic rings in their structure so-called tetracyclines.

Classifications

  • Group I: Tetracycline, chlortetracycline, oxytetracycline
  • Group II: Demeclocycline, lymecycline
  • Group III: Doxycycline, minocycline

Mechanism of action of Tetracyclines

The drug is actively taken up by susceptible bacteria and bind reversibly to the 30S ribosomal subunit.

Prevent binding of aminoacyl tRNA to mRNA-ribosome complex and further prevent the addition of amino acid to the growing peptide chain. That leads to inhibit bacterial protein synthesis (bacteriostatic).

tetracyclines
MOA

Pharmacokinetics

In drugs are taken orally, the absorption of drugs should be affected by food and multivalent cations (calcium, iron, aluminum, etc.).

Yogurt decreases the absorption of tetracyclines (due to cations like calcium and magnesium). The drugs cross the placenta and affect the fetus if administered to a pregnant female.

It undergoes enterohepatic circulation. It is excreted mainly in the urine except for doxycycline. Doxycycline is excreted in the feces and thus it can mostly be used in the presence of renal failure. The half-life of doxycycline and minocycline is longer than other tetracyclines.

Adverse effects

There are various side effects are present but some common adverse drugs reaction are shown in below

  • Gastrointestinal effects: nausea, vomiting, epigastric distress, abdominal discomfort, and diarrhoea
  • Phototoxicity
    • Demeclocycline and doxycycline
    • It may produce a sunburn-like reaction in the skin on exposure to the sunlight and may also produce pigmentation of nails.
  • Hepatotoxicity
    • Acute hepatic necrosis with fatty changes due to patients receiving high doses intravenously.
  • Renal toxicity
    • Demeclocycline may produce nephrogenic diabetes insipidus by blocking the action of antidiuretic hormone (ADH) on the collecting duct of the kidney.
  • Fanconi syndrome: expired drugs tetracycline may damage the proximal renal tubules
  • Superinfection
    • Occurs with organisms like Proteus, candida, pseudomonas, etc.
  • Increased intracranial pressure (pseudotumor cerebri)
  • Hypersensitivity reactions (skin rashes, fever, urticaria, exfoliative dermatitis, etc.)
  • Dentition and Bone defects (contraindicated in pregnancy and children).

Uses of tetracyclines

  • Rickettsial infection
  • Mycoplasma pneumoniae
  • Chlamydial infections
    • Lymphogranuloma venereum
    • Psittacosis
  • Cholera
  • Brucellosis
  • Plague
  • Anthrax and leptospirosis
  • Lyme disease
  • Granuloma inguinale
  • Acne
  • Malaria
  • Amoebiasis
  • Leprosy
  • SIADH
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