Tetracyclines are adequately but incompletely absorbed from the gastrointestinal tract. Approximately 65 percent of a short-acting tetracycline is bound to plasma proteins; the plasma protein binding for intermediate- and long-acting analogues is usually greater.

Penetration of the tetracyclines into most body fluids and tissues is excellent. Tetracyclines are distributed in varying degrees into bile, liver, lung, kidney, prostate, urine, cerebrospinal fluid, synovial fluid, mucosa of the maxillary sinus, brain, sputum, and bone. Tetracyclines cross the placenta and enter the fetal circulation and amniotic fluid.

Following a single oral dose, peak plasma concentrations are achieved in two to four hours.

Tetracyclines are concentrated by the liver in the bile. They are excreted in both the urine and feces at high concentrations in a biologically active form. Since renal clearance of tetracyclines is by glomerular filtration, excretion is significantly affected by the state of renal function. (See WARNINGS.)

Microbiology

The tetracyclines are primarily bacteriostatic and are thought to exert their antimicrobial effect by the inhibition of protein synthesis. The tetracyclines have a similar antimicrobial spectrum of activity against a wide range of gram-positive and gram-negative organism. Crossresistance of these organisms to tetracyclines is common. In addition, gram-negative bacilli made tetracycline-resistant, may also show cross-resistance to chloramphenicol.

GRAM-NEGATIVE BACTERIA:

Bartonella bacilliformis
Brucella species
Calymmatobacterium granulomatis
Campylobacter fetus
Francisella tularensis
Haemophilus ducreyi
Haemophilus influenzae
Listeria monocytogenes
Neisseria gonorrhoeae
Vibrio cholerae
Yersinia pestis

Because many strains of the following groups of gram-negative microorganisms have been shown to be resistant to tetracyclines, culture and susceptibility testing are especially recommended:

Acinetobacter species
Bacteroides species
Enterobacter aerogenes
Escherichia coli
Klebsiella species
Shigella species

GRAM-POSITIVE BACTERIA:

Enterococcus group [Enterococcus faecalis, (formerly Streptococcus faecalis) and Enterococcus faecium (formerly Streptococcus faecium)

Streptococci viridans group
Streptococcus pneumoniae
Streptococcus pyogenes

Because many strains of these gram-positive microorganism have been shown to be resistant to tetracycline, culture and susceptibility testing are recommended. Up to 44 percent of strains of Streptococcus pyogenes and 74 percent of Enterococcus faecalis (formerly Streptococcus faecalis) have been found to be resistant to tetracycline drugs. Therefore, tetracyclines should not be used for treatment of streptococcal disease unless the organism is known to be susceptible.

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