Tetracycline

Pharmacotherapeutics

Oral tetracycline (Sumycin) is FDA approved for many types of infection, however, it has acquired substantial resistance patterns so it is not used for all of its indications. It is used for infections caused by Borrellia burgdorferi, Rickettsia species, Mycoplasma pneumoniae, and Chlamydia trachomatis. It is also used to treat brucellosis, cholera, anthrax, Lyme disease, and H. pylori infection.

Pharmacokinetics

Tetracycline is administered orally because it is no longer available for parenteral administration. In the fasting state, tetracycline is about 75% to 77% absorbed. Absorption takes place mainly in the stomach and upper intestine. As the dosage is increased, the percentage absorbed decreases. Absorption is decreased in the presence of food, iron preparations, and antacids containing calcium, magnesium, and aluminum salts. Tetracycline is widely distributed into body fluids, including CSF. Tetracycline tends to concentrate in bone, liver, tumors, spleen, and teeth. It crosses the placenta and is distributed in breast milk. Tetracycline is about 65% bound to plasma protein and does not appear to undergo hepatic metabolism; however, it undergoes enterohepatic circulation and is excreted in the feces by way of the bile. The primary excretion route is the kidney. About 60% of a dose is excreted unchanged from both routes. The serum half-life of tetracycline is between 6 and 12 hours in adults with normal renal function but is greatly increased in patients with severely impaired renal function.

Contraindications and Precautions

Tetracycline us contraindicated in patients with a known allergy to tetracyclines or to tartrazine (specific oral preparations contain tartrazine) and during pregnancy and lactation. Tetracycline should be used with caution in children younger than 8 years and in patients with hepatic and renal dysfunction.

Adverse Effects

The common adverse effects of tetracycline therapy involve the GI tract and the CNS; they include nausea, vomiting, diarrhea, abdominal or epigastric discomfort, headache, dizziness, and photosensitivity. Rare but serious adverse effects include anaphylaxis, angioedema, blood dycrasias, damage to the teeth (in children < 8 years), and hepatotoxicity or nephrotoxicity. Like other broad-spectrum antibiotics, tetracycline has a high potential to cause superinfection, such as candidiasis.

Drug Interactions

The effectiveness of penicillin G decreases if it is taken concurrently with tetracyclines. If this combination is used, the dosage of the penicillin will have to be increased. Oral contraceptives may be less effective if taken with tetracycline. Patients taking oral contraceptives should be advised to use an additional form of birth control while receiving tetracycline. Tetracycline forms an insoluble chelate with aluminum, bismuth, calcium, iron, magnesium, and zinc salts, which are frequently an ingredient in antacids. This chelate decreases the absorption of either tetracycline or the salt. See table 40-12 on pg. 857 for more drug interactions.

Pharmacodynamics

The tetracyclines are bacteriostatic; they inhibit or retard the growth of bacteria but do not kill them. They retard bacterial growth by inhibiting protein synthesis in sensitive bacteria and by preventing cell division and replication. Like other antibiotics, their effect on the body is indirect.