Tetracyclines are broad-spectrum antibiotics. They may be classi-fied as:
§ intermediate-acting compounds such as demeclocycline hydrochloride
§ long-acting compounds, such as doxycycline hyclate and minocycline hydrochloride.
Tetracyclines are absorbed from the stomach and small intestine when taken orally.
Tetracyclines are distributed widely into body tissues and fluids, concentrated in bile, and excreted primarily by the kidneys. Doxy-cycline is also excreted in stool. Minocycline undergoes entero-hepatic recirculation.
All tetracyclines are primarily bacteriostatic, meaning they inhibit the growth or multiplication of bacteria. They penetrate the bacte-rial cell by an energy-dependent process. Within the cell, they bind primarily to a subunit of the ribosome, inhibiting the protein syn-thesis needed to maintain the bacterial cell.
Tetracyclines provide a broad spectrum of activity against:
· grampositive and gram-negative aerobic and anaerobic bacteria
· some protozoa.
The long-acting compounds doxycycline and minocycline provide more action against various organisms than other tetracyclines.
Tetracyclines are used to treat Rocky Mountain spotted fever, Q fever, and Lyme disease. They’re the drugs of choice for treating nongonococcal urethritis caused by Chlamydia and Ureaplasma.
Combination therapy with a tetracycline and streptomycin is the most effective treatment for brucellosis.
Tetracyclines in low dosages effectively treat acne because they can decrease the fatty acid content of sebum.
Tetracyclines can reduce the effectiveness of hormonal contracep-tives, which may result in breakthrough bleeding or ineffective contraception. The patient taking hormonal contraceptives should use a reliable, secondary method of contraception. Tetracyclines may also decrease the bactericidal action of penicillin.
§ Other interactions commonly affect the ability of tetracyclines to move through the body.
§ Aluminum, calcium, and magnesium antacids reduce the ab-sorption of oral tetracyclines.
§ Iron salts, bismuth subsalicylate, and zinc sulfate reduce the ab-sorption of tetracyclines. Reduced absorption can be prevented by separating doses of tetracyclines and these agents by 2 to 3 hours.
§ Barbiturates, carbamazepine, and phenytoin increase the me-tabolism and reduce the antibiotic effect of doxycycline.
These drugs, with the exception of doxycycline and minocycline, may also interact with milk and milk products, which bind with the drugs and prevent their absorption. To prevent decreased ab-sorption, administer the tetracycline 1 hour before or 2 hours after meals. (See Adverse reactions to tetracyclines.)