Cyclines

Cyclines are a class of broad-spectrum antibiotics commonly referred to as the tetracyclines. They are active against many Gram-positive and Gram-negative bacteria as well as atypical pathogens, and have been used since the mid-20th century in both human and veterinary medicine. The most widely used members include doxycycline, minocycline, tetracycline, and oxytetracycline.

Tetracyclines act as bacteriostatic agents by reversibly binding to the 30S ribosomal subunit in bacteria. This prevents the attachment of aminoacyl-tRNA to the ribosome A site and impairs protein synthesis, inhibiting bacterial growth. Their activity can be reduced by the presence of divalent cations such as calcium and magnesium, which can interfere with oral absorption and overall effectiveness when taken with dairy products or supplements.

Clinical use and spectrum vary, but tetracyclines are commonly prescribed for acne, respiratory and urinary tract infections, sexually transmitted infections, and infections caused by intracellular or atypical organisms such as Chlamydia, Mycoplasma, Rickettsia, and Borrelia. Doxycycline is frequently chosen for malaria prophylaxis in travelers and for certain tick-borne diseases.

Safety, contraindications, and drug interactions are important considerations. Tetracyclines are generally avoided during pregnancy and in children under about eight years old because they can cause permanent teeth discoloration and enamel hypoplasia. Photosensitivity, gastrointestinal upset, and liver or kidney effects may occur in some patients. Because they chelate with multivalent cations, they are often separated from iron, calcium, magnesium, and aluminum-containing products to optimize absorption.

Resistance to tetracyclines can arise through efflux pumps and ribosomal protection proteins, among other mechanisms. Responsible use and adherence to dosing guidelines remain important to preserve their effectiveness.