CLINICAL PHARMACOLOGY
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%/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.
The mean Cmax and AUC0-∞ of doxycycline are reduced by 24% and 13%, respectively, following single dose administration of DORYX tablets with a high fat meal. The clinical significance of this decrease is unknown.
When Doryx Tablets are sprinkled over applesauce and taken with or without water, extent of doxycycline absorption is equivalent, but absorption rate is increased slightly.
Hemodialysis does not alter serum half-life.
Results of animal studies indicate that tetracyclines cross the placenta and are found in fetal tissues.
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 organisms. Cross-resistance of these organisms to tetracycline is common.
Gram-Negative Bacteria
Neisseria gonorrhoeae
Calymmatobacterium granulomatis
Haemophilus ducreyi
Haemophilus influenzae
Yersinia pestis ( formerly Pasteurella pestis)
Francisella tularensis ( formerly Pasteurella tularensis)
Vibrio cholerae ( formerly Vibrio comma)
Bartonella bacilliformis
Brucella species
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:
Escherichia coli
Klebsiella species
Enterobacter aerogenes
Shigella species
Acinetobacter species ( formerly Mima species and Herellea species )
Bacteroides species
Gram-Positive Bacteria
Because many strains of the following groups of gram-positive microorganisms 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 Streptococcus faecalis have been found to be resistant to tetracycline drugs. Therefore, tetracycline should not be used for streptococcal disease unless the organism has been demonstrated to be susceptible.
Streptococcus pyogenes
Streptococcus pneumoniae
Enterococcus group (Streptococcus faecalis and Streptococcus faecium)
Alpha-hemolytic streptococci (viridans group)
Other Microorganisms
Rickettsiae
Chlamydia psittaci
Chlamydia trachomatis
Mycoplasma pneumoniae
Ureaplasma urealyticum
Borrelia recurrentis
Treponema pallidum
Treponema pertenue
Clostridium species
Fusobacterium fusiforme
Actinomyces species
Bacillus anthracis
Propionibacterium acnes
Entamoeba species
Balantidium coli
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 Tests: 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 organisms to doxycycline uses the 30 mcg tetracycline-class disk or the 30 mcg doxycycline disk. Interpretation involves the correlation of the diameter obtained in the disk test with the minimum inhibitory concentration (MIC) for tetracycline or doxycycline, respectively.
Reports from the laboratory giving 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:
| Zone Diameter (mm) | Interpretation |
| tetracycline | doxycycline | |
| ≥19 | ≥16 | Susceptible |
| 15-18 | 13-15 | Intermediate |
| ≤14 | ≤12 | 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 and fluids in which high 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-class disk or the 30 mcg doxycycline disk should give the following zone diameters:
| Organism | Zone Diameter (mm) |
| tetracycline | doxycycline |
| E. coli ATCC 25922 | 18-25 | 18-24 |
| S. aureus ATCC 25923 | 19-28 | 23-29 |
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 | Susceptible |
| 8 | Intermediate |
| ≥16 | Resistant |
As with standard diffusion methods, dilution methods require the use of laboratory control organisms. Standard tetracycline powder should provide the following MIC values:
| Organism | MIC (mcg/mL) |
| E. coli ATCC 25922 | 1.0-4.0 |
| S. aureus ATCC 29213 | 0.25-1.0 |
| E. faecalis ATCC 29212 | 8-32 |
| P. aeruginosa ATCC 27853 | 8-32 |
ANIMAL PHARMACOLOGY AND 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.
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