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 gram-negative 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 AND USAGE 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 pyogenesand 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

Bartonella bacilliformis

Brucella species

Calymmatobacterium granulomatis

Campylobacter fetus

Francisella tularensis

Haemophilus ducreyi

Haemophilus influenzae

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 recommended:

Acinetobacterspecies

Enterobacteraerogenes

Escherichia coli

Klebsiellaspecies

Shigella species

Anaerobic Microorganisms

Actinomycesisraelii

Clostridium species

Fusobacteriumfusiforme

Other Microorganisms

Borreliarecurrentis

Chlamydia psittaci

Chlamydia trachomatis

Mycoplasma pneumoniae

Rickettsiae

Treponemapallidum

Treponemapertenue

Susceptibility Tests

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 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 Haemophilusspecies, Neisseria gonorrhoeae, and Streptococcus pneumoniae:

When testing Haemophilusspp.a

When testing Neisseria gonorrhoeaeb

When testing Streptococcus pneumoniaec

a Interpretative criteria applicable only to tests performed by broth microdilution method using HaemophilusTest 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

Reporting Results

Recommended interpretive criteria are based on usual dosage regiment and routes of administration in the U.S. Susceptible, intermediate, or resistant interpretations of zone diameter measurements are reported and defined as follows:

1. Susceptible (S)

The “susceptible” category implies that isolates are inhibited by the usually achievable concentration of antimicrobial agent when the recommended dosage is used for the site of infection.

2. Intermediate (I)

The “intermediate” category includes isolates with antimicrobial agent MICs that approach usually attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates. The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated (e.g., quinolones and β-lactams in urine) or when a higher than normal dosage of a drug can be used (e.g., β-lactams). This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major disappearances in interpretations, especially for drugs without narrow pharmaceutical margins.

3. Resistant (R)

The “resistant” category implies that isolates are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules, and/or that demonstrate zone diameters that fall in the range where specific microbial resistance mechanisms (e.g., beta-lactamases) are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.

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 procedure2,3requires 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 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 Haemophilusspecies, Neisseria gonorrhoeae, and Streptococcus pneumoniae:

When testing Haemophilusspp.a

When testing Neisseria gonorrhoeaeb

Gonococci with 30 mcg tetracycline disk zone diameters ≤19 mm usually indicate a plasmid-mediated tetracycline-resistant N. gonorrhoeae(TRNG) isolate. These strains should be confirmed by the dilution test (MIC ≥16 mcg/mL).

When testing Streptococcus pneumoniaec

a Interpretative criteria applicable only to tests performed by disk diffusion method using a 30-mcg tetracycline-class disk and using HaemophilusTest Medium (HTM) and incubated with 5% CO2.2,3

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 and incubated with 5% CO2.2,3

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:

a Range applicable only to tests performed by disk diffusion method using a 30-mcg tetracycline-class disk and using HaemophilusTest Medium (HTM) and incubated with 5% CO2.2,3

b Range 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 and incubated with 5% CO2.2,3

c Range 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

Anaerobic Techniques

For anaerobic bacteria, the susceptibility to tetracycline as MICs 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:

General

As with other antibiotic preparations, use of this drug may result in overgrowth of non-susceptible organisms, including fungi. If superinfection occurs, the antibiotic should be discontinued and appropriate therapy instituted.

Bulging fontanels in infants and benign intracranial hypertension in adults have been reported in individuals receiving tetracyclines. These conditions disappeared when the drug was discontinued.

Incision and drainage or other surgical procedures should be performed in conjunction with antibiotic therapy when indicated.

Prescribing doxycycline tablets in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.

Information for Patients

All patients taking doxycycline should be advised:

•

to avoid excessive sunlight or artificial ultraviolet light while receiving doxycycline and to discontinue therapy if phototoxicity (e.g., skin eruptions, etc.) occurs. Sunscreen or sunblock should be considered (see WARNINGS).

•

to drink fluids liberally along with doxycycline to reduce the risk of esophageal irritation and ulceration (see ADVERSE REACTIONS).

•

that the absorption of tetracyclines is reduced when taken with foods, especially those which contain calcium. However, the absorption of doxycycline is not markedly influenced by simultaneous ingestion of food or milk (see Drug Interactions).

•

that the absorption of tetracycline is reduced when taking bismuth subsalicylate (see Drug Interactions).

•

not to use outdated or poorly stored doxycycline.

•

that the use of doxycycline might increase the incidence of vaginal candidiasis.

Diarrhea is a common problem caused by antibiotics which usually ends when the antibiotic is discontinued. Sometimes after starting treatment with antibiotics, patients can develop watery and bloody stools (with or without stomach cramps and fever) even as late as two or more months after having taken the last dose of the antibiotic. If this occurs, patients should contact their physician as soon as possible.

Patients should be counseled that antibacterial drugs including doxycycline should only be used to treat bacterial infections. They do not treat viral infections (e.g., the common cold). When doxycycline is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by doxycycline or other antibacterial drugs in the future.

Laboratory Tests

In venereal disease when coexistent syphilis is suspected, a dark-field examination should be done before treatment is started and the blood serology repeated monthly for at least four months.

In long-term therapy, periodic laboratory evaluations of organ systems, including hematopoietic, renal, and hepatic studies should be performed.

Drug Interactions

Because tetracyclines have been shown to depress plasma prothrombin activity, patients who are on anticoagulant therapy may require downward adjustment of their anticoagulant dosage.

Since bacteriostatic drugs may interfere with the bactericidal action of penicillin, it is advisable to avoid giving tetracyclines in conjunction with penicillin.

Absorption of tetracyclines is impaired by antacids containing aluminum, calcium, or magnesium, and iron-containing preparations.

Barbiturates, carbamazepine, and phenytoin decrease the half-life of doxycycline.

The concurrent use of tetracycline and methoxyflurane has been reported to result in fatal renal toxicity.

Concurrent use of tetracycline may render oral contraceptives less effective.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term studies in animals to evaluate the carcinogenic potential of doxycycline have not been conducted.

However, there has been evidence of oncogenic activity in rats in studies with related antibiotics, oxytetracycline (adrenal and pituitary tumors) and minocycline (thyroid tumors). Likewise, although mutagenicity studies of doxycycline have not been conducted, positive results in in vitro mammalian cell assays have been reported for related antibiotics (tetracycline, oxytetracycline). Doxycycline administered orally at dosage levels as high as 250 mg/kg/day had no apparent effect on the fertility of female rats. Effect on male fertility has not been studied.

Pregnancy

Labor and Delivery

The effect of tetracyclines on labor and delivery is unknown.

Nursing Mothers

Tetracyclines are excreted in human milk, however, the extent of absorption of tetracyclines, including doxycycline, by the breastfed infant is not known. Short-term use by lactating women is not necessarily contraindicated; however, the effects of prolonged exposure to doxycycline in breast milk are unknown.d Because of the potential for adverse reactions in nursing infants from doxycycline, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother (see WARNINGS).

Pediatric Use

See WARNINGS and DOSAGE AND ADMINISTRATION sections.

Gastrointestinal

Anorexia, nausea, vomiting, diarrhea, glossitis, dysphagia, enterocolitis, and inflammatory lesions (with monilial overgrowth) in the anogenital region. Hepatotoxicity has been reported.(b)(4) These reactions have been caused by both the oral and parenteral administration of tetracyclines. Rare instances of esophagitis and esophageal ulcerations have been reported in patients receiving capsule and tablet forms of drugs in the tetracycline class. Most of these patients took medications immediately before going to bed (see DOSAGE AND ADMINISTRATION).

Skin

Maculopapular and erythematous rashes, Stevens-Johnson syndrome, toxic epidermal necrolysis and erythema multiforme have been reported.. Exfoliative dermatitis has been reported but is uncommon. Photosensitivity is discussed above (see WARNINGS).

Renal Toxicity

Rise in BUN has been reported and is apparently dose related (see WARNINGS).

Hypersensitivity Reactions

Urticaria, angioneurotic edema, anaphylaxis, anaphylactoid purpura, serum sickness, pericarditis, and exacerbation of systemic lupus erythematosus.

Blood

Hemolytic anemia, thrombocytopenia, neutropenia, and eosinophilia have been reported with tetracyclines.

Other

Bulging fontanels in infants and intracranial hypertension in adults (see PRECAUTIONS, General).

When given over prolonged periods, tetracyclines have been reported to produce brown-black microscopic discoloration of the thyroid gland. No abnormalities of thyroid function are known to occur.

Adults

The usual dose of oral doxycycline is 200 mg on the first day of treatment (administered 100 mg every 12 hours or 50 mg every 6 hours) followed by a maintenance dose of 100 mg/day. The maintenance dose may be administered as a single dose or as 50 mg every 12 hours. In the management of more severe infections (particularly chronic infections of the urinary tract), 100 mg every 12 hours is recommended.

For pediatric patients above eight years of age

The recommended dosage schedule for pediatric patients weighing 100 pounds or less is 2 mg/lb of body weight divided into two doses on the first day of treatment, followed by 1 mg/lb of body weight given as a single daily dose or divided into two doses, on subsequent days. For more severe infections up to 2 mg/lb of body weight may be used. For pediatric patients over 100 pounds the usual adult dose should be used.

Uncomplicated gonococcal infections in adults (except anorectal infections in men)

100 mg, by mouth, twice a day for 7 days. As an alternate single visit dose, administer 300 mg stat followed in one hour by a second 300 mg dose.

Acute epididymo-orchitis caused by N. gonorrhoeae

100 mg, by mouth, twice a day for at least 10 days.

Primary and secondary syphilis

300 mg a day in divided doses for at least 10 days.

Nongonococcal urethritis caused by C. trachomatis and U. urealyticum

100 mg, by mouth, twice a day for at least 7 days.

Acute epididymo-orchitis caused by C. trachomatis

100 mg, by mouth, twice a day for at least 10 days.

Inhalational anthrax (post-exposure)

ADULTS: 100 mg of doxycycline, by mouth, twice a day for 60 days.

CHILDREN: weighing less than 100 pounds (45 kg): 1 mg/lb (2.2 mg/kg) of body weight, by mouth, twice a day for 60 days. Children weighing 100 pounds or more should receive the adult dose.

When used in streptococcal infections, therapy should be continued for 10 days.

Administration of adequate amounts of fluid along with capsule and tablet forms of drugs in the tetracycline class is recommended to wash down the drugs and reduce the risk of esophageal irritation and ulceration (see ADVERSE REACTIONS). If gastric irritation occurs, doxycycline may be given with food. Ingestion of a high fat meal has been shown to delay the time to peak plasma concentrations by an average of one hour and 20 minutes. However, in the same study, food enhanced the average peak concentration by 7.5% and the area under the curve by 5.7%.