- Doxycycline is a second-generation tetracycline antibiotic with a molecular formula of C22H24N2O8 and a molecular mass of 444.44 g/mol.
- Its structure features a hydronaphthacene core with multiple hydroxyl and dimethylamino groups, as well as a unique 6-methyl and 6-hydroxy substitution pattern, which enhances its stability, bioavailability, and spectrum of activity compared to earlier tetracyclines. This structural modification classifies it as a member of the tetracycline family.
- The antibiotic exerts its bacteriostatic action by inhibiting bacterial protein synthesis. It specifically binds to the 30S ribosomal subunit, preventing the attachment of aminoacyl-tRNA to the ribosomal acceptor site and blocking the elongation of peptide chains during translation. This mechanism effectively halts bacterial growth, particularly in rapidly dividing organisms. Doxycycline’s ability to penetrate bacterial cells and achieve high intracellular concentrations contributes to its broad-spectrum activity.
- Doxycycline demonstrates broad-spectrum activity against a wide range of gram-positive and gram-negative bacteria, as well as atypical pathogens. It is particularly effective against Rickettsia, Chlamydia, Mycoplasma pneumoniae, Borrelia burgdorferi (the causative agent of Lyme disease), and Vibrio cholerae. It also exhibits activity against certain protozoa, such as Plasmodium species (used in malaria prophylaxis), and is effective in treating acne due to its action against Cutibacterium acnes.
- The pharmacokinetic profile of doxycycline includes excellent oral bioavailability (approximately 90-100%), rapid absorption, and extensive tissue distribution. It achieves high concentrations in tissues, including the prostate, lungs, and skin, making it effective for treating respiratory infections, sexually transmitted infections, and skin conditions. Unlike earlier tetracyclines, doxycycline is less affected by food and dairy products, which can interfere with absorption. It undergoes partial hepatic metabolism and is excreted primarily through bile and feces, with a relatively long half-life of 18 to 22 hours, allowing for once- or twice-daily dosing.
- Despite its efficacy, doxycycline is associated with potential adverse effects, including gastrointestinal disturbances (e.g., nausea, vomiting), photosensitivity, and, rarely, hepatotoxicity or esophageal irritation. Its use is contraindicated in pregnant women and children under 8 years of age due to the risk of tooth discoloration and inhibition of bone growth.
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