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The tetracyclines are a group of medications with common basic chemical structures, antimicrobial activity, and pharmacologic properties.

ANTIMICROBIAL ACTIVITY

Tetracyclines are inhibitors of protein synthesis and are bacteriostatic for many gram-positive and gram-negative bacteria. They reliably inhibit mycoplasmas, rickettsiae, chlamydiae, spirochetes, and some protozoa (eg, amoebas, malaria). Their antipneumococcal activity is superior to the macrolides, and almost all H influenzae are inhibited. Tetracyclines also have some activity against some vancomycin-resistant enterococci (VRE). Doxycycline and minocycline are potential options for treatment of S aureus infections, including some methicillin-resistant strains. Tetracycline-resistant staphylococci often retain susceptibility to doxycycline and minocycline, and doxycycline-resistant S aureus can still be susceptible to minocycline. Tetracyclines have little utility in the treatment of gram-negative aerobic infection. However, minocycline is reliable in its activity against S maltophilia and multidrug-resistant Acinetobacter. Receipt of doxycycline in combination with ceftriaxone in the treatment of community-acquired pneumonia has been associated with a reduction in the rate of associated antibacterial C difficile disease. The antibacterial activity of doxycycline against C difficile is likely the mechanism of action for this finding.

PHARMACOKINETICS & ADMINISTRATION

Oral bioavailability varies depending on the specific tetracycline. Absorption is impaired by dairy products, aluminum hydroxide gels (antacids), and chelation with divalent cations, eg, Ca2+ or Fe2+. Chelation is less problematic with doxycycline and minocycline when compared with tetracycline. However, doses of all tetracyclines should be staggered at least 2 hours before or after receipt of multivalent cations. Oral bioavailability is only moderate with tetracycline and best with doxycycline and minocycline (95% or more). Lipid solubility of minocycline and doxycycline accounts for their penetration into the cerebrospinal fluid, prostate, tears, and saliva.

Tetracyclines are primarily metabolized in the liver and excreted in bile. Doxycycline requires no dosage adjustment in kidney disease; in contrast, other tetracyclines should be avoided or given in reduced dosage.

For patients unable to take oral medication, some tetracyclines (doxycycline, minocycline) are formulated for parenteral administration in doses similar to the oral preparations.

CLINICAL USES

Tetracyclines are medications of choice for infections with Chlamydia, Chlamydophila, Mycoplasma, Rickettsia, Ehrlichia, and Vibrio organisms and for some spirochetal infections. Sexually transmitted diseases in which chlamydiae often play a role—endocervicitis, urethritis, proctitis, and epididymitis—should be treated with doxycycline for 7–14 days. Pelvic inflammatory disease is often treated with doxycycline plus cefoxitin or cefotetan. Other chlamydial infections (psittacosis, lymphogranuloma venereum, trachoma) and sexually transmitted diseases (granuloma inguinale) also respond to doxycycline. Other uses include treatment of acne, respiratory infections, Lyme disease and relapsing fever, brucellosis, glanders, tularemia (often in combination with streptomycin), cholera, mycoplasmal pneumonia, actinomycosis, nocardiosis, malaria, infections caused by M marinum and Pasteurella species (typically after an animal bite), and as malaria prophylaxis (including multidrug-resistant P falciparum). They also have been used in combination with other medications for amebiasis, falciparum malaria, and recurrent ...

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