Tetracyclines are readily absorbed and are bound to plasma proteins in varying degree. They are concentrated by the liver in the bile, and excreted in the urine and feces at high concentrations and in a biologically active form. Doxycycline is virtually completely absorbed after oral administration.
Following a 200 mg dose, normal adult volunteers averaged peak serum levels of 2.6 mcg/mL of doxycycline at 2 hours, decreasing to 1.45 mcg/mL at 24 hours. Excretion of doxycycline by the kidney is about 40% per 72 hours in individuals with normal function (creatinine clearance about 75 mL/min). This percentage excretion may fall as low as 1 to 5% per 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 to 22 hours) in individuals with normal and severely impaired renal function.
Hemodialysis does not alter serum half-life.
Results of animal studies indicate that tetracyclines cross the placenta and are found in fetal tissues.
Microbiology
Mechanism of Action
Doxycycline inhibits bacterial protein synthesis by binding the 30S ribosomal subunit. Doxycycline has bacteriostatic activity against a broad range of Gram-positive and Gram-negative bacteria.
Resistance
Cross resistance with other tetracyclines is common.
Antimicrobial Activity
Doxycycline has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section of the package insert for doxycycline hyclate tablets.
Gram-Negative Bacteria
Acinetobacter species
Bartonella bacilliformis
Brucella species
Klebsiella species
Klebsiella granulomatis
Campylobacter fetus
Enterobacter aerogenes
Escherichia coli
Francisella tularensis
Haemophilus ducreyi
Haemophilus influenzae
Neisseria gonorrhoeae
Shigella species
Vibrio cholerae
Yersinia pestis
Gram-Positive Bacteria
Bacillus anthracis
Listeria monocytogenes
Streptococcus pneumoniae
Anaerobic Bacteria
Clostridium species
Fusobacterium fusiforme
Propionibacterium acnes
Other Bacteria
Nocardiae and other aerobic Actinomyces species
Borrelia recurrentis
Chlamydophila psittaci
Chlamydia trachomatis
Mycoplasma pneumoniae
Rickettsiae
Treponema pallidum
Treponema pallidum subspecies pertenue
Ureaplasma urealyticum
Parasites
Balantidium coli
Entamoeba species
Plasmodium falciparum*
*Doxycycline has been found to be active against the asexual erythrocytic forms of Plasmodium falciparum, but not against the gametocytes of P. falciparum. The precise mechanism of action of the drug is not known.
Susceptibility Testing Methods
When available, the clinical microbiology laboratory should provide cumulative reports of in vitro susceptibility test results for antimicrobial drugs used in local hospital and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.
Dilution Techniques
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method1,2,4 (broth or agar). The MIC values should be interpreted according to criteria provided in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized test method.1,3,4 This procedure uses paper disks impregnated with 30 mcg doxycycline to test the susceptibility of microorganisms to doxycycline. The disk diffusion interpretive criteria are provided in Table 1.
Anaerobic Techniques
For anaerobic bacteria, the susceptibility to doxycycline can be determined by a standardized test method5. The MIC values obtained should be interpreted according to the criteria provided in Table 1.
Table 1: Susceptibility Test Interpretive Criteria for Doxycycline and TetracyclineBacteriaa | Minimal Inhibitory Concentration (mcg/mL) | Zone Diameter (mm) | Agar Dilution (mcg/mL) |
| S | I | R | S | I | R | S | I | R |
Acinetobacter spp. Doxycycline Tetracycline | ≤4 ≤4 | 8 8 | ≥16 ≥16 | ≥13 ≥15 | 10-12 12-14 | ≤9 ≤11 | - - | - - | - - |
Anaerobes Tetracycline | - | - | - | - | - | - | ≤4 | 8 | ≥16 |
Bacillus anthracisb Doxycycline Tetracycline | ≤1 ≤1 | - - | - - | - - | - - | - - | - - | - - | - - |
Brucella speciesb Doxycycline Tetracycline | ≤1 ≤1 | - - | - - | - - | - - | - - | - - | - - | - - |
Enterobacteriaceae Doxycycline Tetracycline | ≤4 ≤4 | 8 8 | ≥16 ≥16 | ≥14 ≥15 | 11-13 12-14 | ≤10 ≤11 | - - | - - | - - |
Franciscella tularensisb Doxycycline Tetracycline | ≤4 ≤4 | - - | - - | - - | - - | - - | - - | - - | - - |
Haemophilus influenzae Tetracycline | ≤2 | 4 | ≥8 | ≥29 | 26-28 | ≤25 | - | - | - |
Mycoplasma pneumoniaeb Tetracycline | - | - | - | - | - | - | ≤2 | - | - |
| | | | | | | | | |
Nocardiae and other aerobic Actinomyces speciesb Doxycycline | ≤1 | 2-4 | ≥8 | - | - | - | - | - | - |
Neisseria gonorrhoeaec Tetracycline | - | - | - | ≥38 | 31-37 | ≤30 | ≤0.25 | 0.5-1 | ≥2 |
Streptococcus pneumoniae Doxycycline Tetracycline | ≤0.25 <1 | 0.5 2 | >1 >4 | >28 >28 | 25-27 25-27 | <24 <24 | - - | - - | - - |
Vibrio cholerae Doxycycline Tetracycline | ≤4 ≤4 | 8 8 | ≥16 ≥16 | - - | - - | - - | - - | - - | - - |
Yersinia pestis Doxycycline Tetracycline | ≤4 ≤4 | 8 8 | ≥16 ≥16 | - - | - - | - - | - - | - - | - - |
Ureaplasma urealyticum Tetracycline | - | - | - | - | - | - | ≤1 | - | ≥2 |
a Organisms susceptible to tetracycline are also considered susceptible to doxycycline. However, some organisms that are intermediate or resistant to tetracycline may be susceptible to doxycycline. b The current absence of resistance isolates precludes defining any results other than “Susceptible”. If isolates yielding MIC results other than susceptible, they should be submitted to a reference laboratory for further testing. c Gonococci with 30 mcg tetracycline disk zone diameters of <19 mm usually indicate a plasmid-mediated tetracycline resistant Neisseria gonorrhoeae isolate. Resistance in these strains should be confirmed by a dilution test (MIC ≥ 16 mcg per mL). |
A report of Susceptible (S) indicates that the antimicrobial drug is likely to inhibit growth of the microorganism if the antimicrobial drug reaches the concentration usually achievable at the site of infection. A report of Intermediate (I) 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 product is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant (R) indicates that the antimicrobial drug is not likely to inhibit growth of the microorganism if the antimicrobial drug reaches the concentrations usually achievable at the infection site; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test1,2,3,4,5,6,7. Standard doxycycline and tetracycline powders should provide the following range of MIC values noted in Table 2. For the diffusion technique using the 30 mcg doxycycline disk the criteria noted in Table 2 should be achieved.
Table 2: Acceptable Quality Control Ranges for Susceptibility Testing for Doxycycline and TetracyclineQC Strain | Minimal Inhibitory Concentration (mcg/mL) | Zone Diameter (mm) | Agar Dilution (mcg/mL) |
Enterococcus faecalis ATCC 29212 Doxycycline Tetracycline | 2-8 8-32
| - -
| - -
|
Escherichia coli ATCC 25922 Doxycycline Tetracycline | 0.5-2 0.5-2
| 18-24 18-25
| - -
|
Eggerthella lenta ATCC 43055 Doxycycline | 2-16
| -
| -
|
Haemophilus influenzae ATCC 49247 Tetracycline | 4-32
| 14-22
| - |
Neisseria gonorrhoeae ATCC 49226 Tetracycline | -
| 30-42
| 0.25-1
|
Staphylococcus aureus ATCC 25923 Doxycycline Tetracycline | - -
| 23-29 24-30
| - -
|
Staphylococcus aureus ATCC 29213 Doxycycline Tetracycline | 0.12-0.5 0.12-1
| - -
| - -
|
Streptococcus pneumoniae ATCC 49619 Doxycycline Tetracycline | 0.015-0.12 0.06-0.5
| 25-34 27-31
| - -
|
Bacteroides fragilis ATCC 25285 Tetracycline | -
| -
| 0.12-0.5
|
Bacteroides thetaiotaomicron ATCC 29741 Doxycycline Tetracycline | 2-8 -
| - -
| - 8-32
|
Mycoplasma pneumoniae ATCC 29342 Tetracycline | 0.06-0.5
| -
| 0.06-0.5
|
Ureaplasma urealyticum ATCC 33175 Tetracycline | -
| -
| ≥8
|