Demeclocycline absorption

Tetracyclines mainly differ in their absorption after oral administration and their elimination. Absorption after oral administration is approximately 30% for chlortetracycline 60-70% for tetracycline, oxytetracycline, demeclocycline, and methacycline and 95-100% for doxycycline and minocycline. Tigecycline is poorly absorbed orally and must be administered intravenously. A portion of an orally administered dose of tetracycline remains in the gut lumen, modifies intestinal flora, and is excreted in the feces. Absorption occurs mainly in the upper small intestine and is impaired by food (except doxycycline and... 

Tetracyclines are classified as short-acting (chlortetracycline, tetracycline, oxytetracycline), intermediate-acting (demeclocycline and methacycline), or long-acting (doxycycline and minocycline) based on serum half-lives of 6-8 hours, 12 hours, and 16-18 hours, respectively. Tigecycline has a half-life of 36 hours. The almost complete absorption and slow excretion of doxycycline and minocycline allow for once-daily dosing. 

The oral dosage for rapidly excreted tetracyclines, equivalent to tetracycline hydrochloride, is 0.25-0.5 g four times daily for adults and 20-40 mg/kg/d for children (8 years of age and older). For severe systemic infections, the higher dosage is indicated, at least for the first few days. The daily dose is 600 mg for demeclocycline or methacycline, 100 mg once or twice daily for doxycycline, and 100 mg twice daily for minocycline. Doxycycline is the oral tetracycline of choice because it can be given as a once-daily dose and its absorption is not significantly affected by food. All tetracyclines chelate with metals, and none should be orally administered with milk, antacids, or ferrous sulfate. To avoid deposition in growing bones or teeth, tetracyclines should be avoided in pregnant women and children less than 8 years of age. 

TETRACYCLINES ANTIVIRALS-DIDANOSINE i efficacy of tetracycline, and possibly demeclocycline, doxy-cycline, lymecycline, minocycline and oxytetracycline, with buffered didanosine Absorption may be affected by the buffered didanosine formulation, which t gastric pH Avoid co-administration with buffered didanosine preparations. Consider changing to enteric-coated didanosine tablets... 

Demeclocycline lacks the C-6-methyl of tetracycline and is produced by a genetically altered strain of Streptomyces aureofaciens. Because it is a secondary alcohol, it is more chemically stable than tetracycline against dehydration. Food and milk co-consumption decrease absorption by half, although it is 60 to 80% absorbed by fasting adults. It is the tetracycline most highly associated with phototoxicity and has been shown to produce dose-dependent, reversible diabetes insipidus with extended use. 

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