Tetracyclines are readily absorbed and are bound to plasma proteins in varying degrees. They are concentrated by the liver in the bile and excreted in the urine and feces at high concentrations in a biologically active form. Doxycycline is virtually completely absorbed after oral administration.

Following a 200 mg dose of doxycycline monohydrate, 24 normal adult volunteers averaged the following serum concentration values:

Excretion of doxycycline by the kidney is about 40%/72 hours in individuals with normal function (creatinine clearance about 75 mL/min). This percentage excretion may fall as low as 1-5%/72 hours in individuals with severe renal insufficiency (creatinine clearance below 10 mL/min). Studies have shown no significant difference in serum half-life of doxycycline (range 18-22 hours) in individuals with normal and severely impaired renal function.

Hemodialysis does not alter serum half-life.

Microbiology

The tetracyclines are primarily bacteriostatic and are thought to exert their antimicrobial effect by the inhibition of protein synthesis. The tetracyclines, including doxycycline, have a similar antimicrobial spectrum of activity against a wide range of gram-positive and gramnegative microorganisms. Cross-resistance of these microorganisms to tetracyclines is common.

Doxycycline has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS section.

Aerobic Gram-Positive Microorganisms

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

Bacillus anthracis
Listeria monocytogenes
Staphylococcus aureus*

*Doxycycline is not the drug of choice in the treatment of any type of staphylococcal infection.

Up to 44 percent of strains of Streptococcus pyogenes and 74 percent of Streptococcus faecalis have been found to be resistant to tetracycline drugs. Therefore, tetracyclines should not be used to treat streptococcal infections unless the microorganism has been demonstrated to be susceptible.

Streptococcus pneumoniae

Aerobic Gram-Negative Microorganisms

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

Anaerobic Microorganisms

Other Microorganisms

Susceptibility Tests

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MIC's). These MIC's provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MIC's should be determined using a standardized procedure. Standardized procedures are based on a dilution method1,3 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of tetracycline powder. The MIC values should be interpreted according to the following criteria for indicated aerobic microorganisms other than Haemophilus species, Neisseria gonorrhoeae, and Streptococcus pneumoniae:

When testing Haemophilus spp.^a

When testing Neisseria gonorrhoeae ^b

When testing Streptococcus pneumoniae ^c

^a. Interpretative criteria applicable only to tests performed by broth microdilution method using Haemophilus Test Medium (HTM).1,3

^b. Interpretative criteria applicable only to tests performed by agar dilution method using GC agar base with 1% defined growth supplement.1,3

^c. Interpretative criteria applicable only to tests performed by broth microdilution method using cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood.1,3

A report of "Susceptible" indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. A report of "Intermediate" indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.

Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard tetracycline powder should provide the following MIC values:

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2,3 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 30-mcg tetracycline or 30-mcg doxycycline to test the susceptibility of microorganisms to doxycycline.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30-mcg tetracycline-class disk or the 30-mcg doxycycline disk should be interpreted according to the following criteria for indicated aerobic microorganisms other than Haemophilus species, Neisseria gonorrhoeae, and Streptococcus pneumoniae:

When testing Haemophilus spp.^a

When testing Neisseria gonorrhoeae ^b

Zone diameters ≤ 19 mm may indicate a plasmid-mediated tetracycline-resistant Neisseria gonorrhoeae (TRNG) isolate. These TRNG strains should be confirmed by the dilution test (MIC ≥ 16 mcg/mL). When testing Streptococcus pneumoniae ^c

^a. Interpretative criteria applicable only to tests performed by disk diffusion method using a 30-mcg tetracycline-class disk and using Haemophilus Test Medium (HTM).2,3

^b. Interpretative criteria applicable only to tests performed by disk diffusion method using a 30-mcg tetracycline-class disk and using GC agar base with 1% defined growth supplement.2,3

^c. Interpretative criteria applicable only to tests performed by disk diffusion method using a 30-mcg tetracycline-class disk and using Mueller-Hinton agar with 5% defibrinated sheep blood and incubated in 5% CO2.2,3

Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for tetracycline or doxycycline, respectively.

As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 30-mcg tetracycline-class disk or the 30-mcg doxycycline disk should provide the following zone diameters in these laboratory test quality control strains:

Anaerobic Techniques

For anaerobic bacteria, the susceptibility to tetracycline as MIC's can be determined by standardized test methods.4 The MIC values obtained should be interpreted according to the following criteria:

Interpretation is identical to that stated above for results using dilution techniques.

As with other susceptibility techniques, the use of laboratory control microorganisms is required to control the technical aspects of the laboratory standardized procedures. Standardized tetracycline powder should provide the following MIC values:

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.

Last reviewed on RxList: 1/24/2012
This monograph has been modified to include the generic and brand name in many instances.