ABSTRACT
The tetracyclines are a group of natural and semisynthetic antibiotics derived from species of Streptomyces. Chlortetracycline was the first tetracycline used clinically in 1948, followed by oxytetracycline in 1950 and tetracycline in 1952. Chlortetracycline and oxytetracycline are both natural compounds, and tetracycline is a semisynthetic derivative of chlortetracycline. The so-called second-generation tetracyclines, minocycline and doxycycline, are semisynthetic products with improved therapeutic utility, although they are not utilized in veterinary medicine. The tetracyclines all share a common hydronaphtracene nucleus and are amphoteric zwitterions which will readily form water-soluble salts with acids and bases. The antimicrobial spectrum of the tetracyclines is broad and includes a broad range of both Gram-positive and Gram-negative bacteria, chlamydia, rickettsiae, actinomycetes and protozoa. Tetracyclines are absorbed after oral administration. Tetracyclines are eliminated through a combination of renal and biliary routes, the exact proportion of which being dependent upon the lipophilicity of the compound. Evidence for hepatic biotransformation suggests negligible metabolism.
