These antibiotics are partially absorbed from the stomach and upper gastrointestinal tract. Food impairs absorption of all tetracyclines except doxycycline and minocycline. Absorption of doxycycline and minocycline is improved with food. Since the tetracyclines form insoluble chelates with calcium (such as are found in many antacids), magnesium, and other metal ions, their simultaneous administration with milk (calcium), magnesium hydroxide, aluminum hydroxide, or iron will interfere with absorption. Because some of the tetracy-clines are not completely absorbed, any drug remaining in the intestine may inhibit sensitive intestinal microorganisms and alter the normal intestinal flora.
The tetracyclines are distributed throughout body tissues and fluids in concentrations that reflect the lipid solubility of each individual agent. Minocycline and doxycycline are the most lipid soluble, while oxytetracy-cline is the least lipid soluble. The tetracyclines penetrate (but somewhat unpredictably) the uninflamed meninges and cross the placental barrier. Peak serum levels are reached approximately 2 hours after oral administration; cerebrospinal fluid (CSF) levels are only one-fourth those of plasma.
The various congeners differ in their half-lives and their protein binding ability (Table 47.1). Significant differences in serum half-life allow the grouping of the tetracyclines into subclasses: short acting (tetracycline, chlortetracycline, and oxytetracycline), intermediate acting (demeclocycline and methacycline), and long acting (minocycline and doxycycline).
The tetracyclines are metabolized in the liver and are concentrated in the bile. Bile concentrations can be up to five times those of the plasma. Doxycycline, minocycline, and chlortetracycline are excreted prima rily in the feces. The other tetracyclines are eliminated primarily in the urine by glomerular filtration. Obviously, these tetracyclines have greater urinary antibacterial activity than those (e.g., doxycycline) that are excreted by nonrenal mechanisms.
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