DORYX- doxycycline hyclate capsule, delayed release pellets
Warner Chilcott (US), Inc.

To reduce the development of drug-resistant bacteria and maintain the effectiveness of DORYX® and other antibacterial drugs, DORYX should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.


DORYX Capsules contain specially coated pellets of doxycycline hyclate, a broad-spectrum antibiotic synthetically derived from oxytetracycline, in a delayed-release formulation for oral administration.

The structural formula for doxycycline hyclate is

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with a molecular formula of C22 H24 N2 O8 , HCl, ½ C2 H6 O, ½ H2 O and a molecular weight of 512.9. The chemical designation for doxycycline hyclate is [4S (4aR, 5S, 5aR, 6R, 12aS)]-4-(dimethylamino)-1,4,4a, 5,5a, 6, 11,12a-octahydro-3, 5,10,12,12a-pentahydroxy-6-methyl-1, 11-deoxonapthtacene-2-carboxamide monohydrochloride, compound with ethyl alcohol (2:1), monohydrate.

Doxycycline hyclate is a yellow crystalline powder soluble in water and in solutions of alkali hydroxides and carbonates. Doxycycline has a high degree of lipid solubility and a low affinity for calcium binding. It is highly stable in normal human serum. Doxycycline will not degrade into an epianhydro form.

Inert ingredients in the capsule formulation are: lactose; microcrystalline cellulose; povidone; starch wheat; magnesium stearate; cellulosic polymer coating. The capsule shell and/or band contains FD and C blue No. 1; FD and C yellow No. 6; D and C yellow No. 10; gelatin; silicon dioxide; sodium lauryl sulfate; titanium dioxide.


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.

Hemodialysis does not alter serum half-life.

Results of animal studies indicate that tetracyclines cross the placenta and are found in fetal tissues.


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

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 30mcg 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.

Standardised 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 techniques, 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
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