CLINICAL PHARMACOLOGY
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.
OTHER MICROORGANISMS:
Actinomyces species
Bacillus anthracis
Balantidium coli
Borrelia recurrentis
Chlamydia psittaci
Chlamydia trachomatis
Clostridium species
Entamoeba species
Fusobacterium fusiforme
Mycoplasma pneumoniae
Rickettsiae
Propionibacterium acnes
Treponema pallidum
Treponema pertenue
Ureaplasma urealyticum
Susceptibility Testing
Diffusion Techniques
Quantitative methods that require measurement of zone diameters give the most precise estimate of the susceptibility of bacteria to antimicrobial agents.
One such standard procedure1 that has been recommended for use with
disks to test susceptibility of microorganisms to tetracycline uses the 30-mcg
tetracycline disk. Interpretation involves the correlation of the zone diameters
obtained in the disk test with the minimum inhibitory concentration (MIC) for
tetracycline.
Reports from the laboratory giving results of the standard single- disk susceptibility test with a 30-mcg tetracycline disk should be interpreted according to the following criteria:
Zone Diameter (mm) |
Interpretation |
≥ 19 |
Susceptible |
15-18 |
Intermediate |
≤ 14 |
Resistant |
A report of "Susceptible" indicates that the pathogen is likely to be inhibited
by generally achievable blood levels. A report of "Intermediate" suggests that
the organism would be susceptible if high dosage is used or if the infection
is confined to tissues or fluids in which high antibiotic (or antimicrobial)
levels are attained. A report of "Resistant" indicates that achievable concentrations
are unlikely to be inhibitory and other therapy should be selected.
Standardized procedures require the use of laboratory control organisms. The 30-mcg tetracycline disk should give the following zone diameters:
Organism |
|
Zone Diameter (mm) |
E. coli |
ATCC 25922 |
18-25 |
S. aureus |
ATCC 25923 |
19-28 |
Dilution Techniques
Use a standardized dilution method2 (broth, agar, microdilution)
or equivalent with tetracycline powder. The MIC values obtained should be interpreted
according to the following criteria:
MIC (mcg/mL) |
Interpretation |
≤ 4.0 |
Susceptible |
> 4.0 < 16 |
Intermediate |
≥ 16 |
Resistant |
As with standard diffusion techniques, dilution methods require the use of laboratory control organisms. Standard tetracycline pow- der should provide the following MIC values:
Organism |
|
MIC (mcg/mL) |
E. coli |
ATCC 25922 |
1-4 |
S. aureus |
ATCC 29213 |
0.25-1 |
E. faecalis |
ATCC 29212 |
8-32 |
P. aeruginosa |
ATCC 27853 |
8-32 |
Animal Pharmacology and Animal Toxicology
Hyperpigmentation of the thyroid has been produced by members of the tetracycline
class in the following species: in rats by oxytetracycline, doxycycline, tetracycline
PO4 and methacycline; in minipigs by doxycycline, minocycline, tetracycline
PO4 and methacycline; in dogs by doxycycline and minocycline; in
monkeys by minocycline.
Minocycline, tetracycline PO4, methacycline, doxycycline, tetracycline
base, oxytetracycline HCl and tetracycline HCl were goitrogenic in rats fed
a low iodine diet. This goitrogenic effect was accompanied by high radioactive
iodine uptake. Administration of minocycline also produced a large goiter with
high radioiodine uptake in rats fed a relatively high iodine diet.
Treatment of various animal species with this class of drugs has also resulted
in the induction of thyroid hyperplasia in the following: in rats and dogs (minocycline),
in chickens (chlortetracycline), and in rats and mice (oxytetracycline). Adrenal
gland hyperplasia has been observed in goats and rats treated with oxytetracycline.
REFERENCE
1. National Committee for Clinical Laboratory Standards, Perfor- mance Standards for Antimicrobial Disk Susceptibility Tests- Fourth Edition. Approved Standard NCCLS Document M2-A4, Vol. 10, No. 7 NCCLS, Villanova, PA, April 1990.
2. National Committee for Clinical Laboratory Standards, Meth- ods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically-Second Edition. Approved Standard NCCLS Document M7-A2, Vol. 10, No. 8 NCCLS, Villanova, PA, April 1990.