Pharmacotherapeutics of the newer tetracyclines

The newer tetracyclines are defined as those tetracyclines available in the United States but not approved for veterinary use. These include demeclocycline, methacycline, doxycycline, and minocycline. Of these, doxycycline and minocycline appear to offer advantages that would render them useful in certain situations in veterinary medicine. Their major advantage lies in their greater lipid solubility relative to other tetracyclines. This characteristic probably accounts for their enhanced antimicrobial effectiveness for some organisms, more efficient absorption after oral administration, and enhanced distribution in the body. The principal excretory organ for doxycycline is the intestine, where the drug diffuses through the intestinal mucosa into the intestinal tract. This unique characteristic makes this drug useful in cases of preexisting renal dysfunction and may render this drug superior to other tetracyclines in the treatment of intestinal infections. Doxycycline is used in other countries for respiratory tract and intestinal tract diseases of poultry. The usefulness of doxycycline and minocycline in food-producing animals may be limited because of persistent drug residues. Minocycline has, in large doses, been used with streptomycin in the elimination of the carrier state of canine brucellosis. The superiority of doxycycline and minocycline, relative to other tetracyclines, in their distribution to areas of he body such as the eye, brain, cerebrospinal fluid, and prostate gland suggests that trials of their efficacy in tetracycline-sensitive infections of these areas are indicated. Pharmacokinetic studies designed to determine optimal dosage schedules have not been made for domestic animals. These determinations are necessary to evaluate most effectively the usefulness of the newer tetracyclines in veterinary medicine.