Tetracyclins

Tetracyclines are a class of broad-spectrum antibiotics produced by soil actinobacteria, most notably Streptomyces species, and include compounds such as tetracycline, doxycycline, minocycline, and oxytetracycline. They are primarily bacteriostatic and exert their effect by reversibly binding to the 30S subunit of the bacterial ribosome, which blocks the attachment of aminoacyl-tRNA to the ribosomal A site and thereby inhibits protein synthesis.

The spectrum of activity is broad, covering many Gram-positive and Gram-negative bacteria, as well as atypical organisms such as Mycoplasma, Chlamydia, and Rickettsia, and several spirochetes including Borrelia. They are often used for infections where these pathogens are suspected, as well as for certain intracellular infections. Resistance can limit effectiveness and arises mainly through efflux pumps, ribosomal protection proteins, or enzymatic inactivation of the drug.

Pharmacokinetics vary among agents. Tetracycline has variable oral bioavailability and interactions with food and dairy can reduce absorption. Doxycycline and minocycline have higher and more reliable oral absorption and are less affected by food. All tetracyclines can chelate with multivalent cations (calcium, iron, magnesium), reducing absorption and efficacy. They are available in oral forms and, for some agents, as intravenous preparations.

Common indications include acne, certain respiratory and soft-tissue infections, sexually transmitted infections caused by Chlamydia, Lyme disease, and other rickettsial diseases. Notable adverse effects include gastrointestinal upset, photosensitivity, and, in children and during pregnancy, effects on teeth and bone growth that can cause dental discoloration and enamel hypoplasia. They can also cause esophagitis if not taken with sufficient fluids. Long-term use may lead to resistance or secondary infections.