Tetracyclines are a group of broad-spectrum bacteriostatic antibiotics active against Gram-positive, Gram-negative, and intracellular organisms, e.g. Chlamydia, mycoplasmas, rickettsiae, and protozoan parasites.
The first tetracycline chlortetracycline was isolated from Streptomyces aureofaciens, a soil organism. Since then, a number of other tetracyclines have been developed.
Tetracyclines differ in their pharmacological properties, rather than their spectrum of cover, although minocycline has a slightly broader spectrum.
• First generation—tetracycline, chlortetracycline, oxytetracycline, demeclocycline, lymecycline, and metacycline.
• Second generation—doxycycline and minocycline.
• Third generation (glycylcyclines)—tigecycline.
MECHANISM OF ACTION.
The tetracyclines arrest bacterial growth by inhibiting their protein synthesis.
• Chlamydial infections—trachoma,psittacosis, salpingitis, urethritis, and lymphogranuloma venereum (LGV).
• Rickettsial infections.
• Q fever.
• Brucellosis (doxycycline with either streptomycin or rifampicin).
• Lyme disease (B.burgdorferi).
• Mycoplasma spp. infections.
• Infective exacerbations of COPD (due to their activity against H.influenzae).
• Also used in acne(pimples), destructive (refractory) periodontal diseases, sinusitis, chronic prostatitis, pelvic inflammatory disease (PID), melioidosis.
• Tetracyclines are usually given orally. Absorption of tetracycline and oxytetracycline is reduced by milk, antacids, and some salts. Doxycycline and minocycline are highly bioavailable.
• They are sometimes divided into three groups on the basis of their half-lives:short-acting (tetracycline, oxytetracycline), intermediate-acting
(demeclocycline), and long-acting (doxycycline, minocycline).
• They are widely distributed and show good tissue penetration.
• Tetracycline is eliminated in the urine. Minocycline is metabolized in the
liver. Doxycycline is mainly eliminated in the faeces.
TOXICITY AND SIDE EFFECTS OF TETRACYCLINES
• Nausea, vomiting, diarrhoea, dysphagia,and oesophageal irritation are common.
• Photosensitivity reactions are common and appear to be toxic, rather than allergic.
• Prolonged minocycline administration can cause skin, nail, and scleral pigmentation.
• Deposition occurs in growing bones and teeth, so tetracyclines should not be given to children less than 12 years or pregnant/breastfeeding women.
• Hepatotoxicity due to fatty change may be fatal.
• Tetracyclines exacerbate renal(kidney) impairment. All tetracyclines (except minocycline and doxycycline) should be avoided in renal failure.
.Demeclocycline causes nephrogenic diabetes insipidus and is used as a treatment for inappropriate antidiuretic hormone (ADH) secretion.
• Vertigo is unique to minocycline.
• Benign intracranial hypertension has been described with all tetracyclines.
• Superinfection— mucocutaneous candidiasis is common. C.difficile colitis may occur.
• Allergic reactions (rashes, urticaria, anaphylaxis) are uncommon.
NOTE: Tetracycline popularly called ‘red and yellow capsule’ is one of the most abused antibiotics in Nigeria. Many persons erroneously believe that tetracycline cures diarrhoea or running stomach or purging and hence it is a popular over-the-counter medication with little or no now efficacy in treating other diseases because of developed bacterial resistance.
Be informed that tetracycline does not cure stomach pain or diarrhoea. Do not patronize quacks.
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This article was adapted from the Oxford Handbook on Infectious Diseases and Treatment.
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