[Pharmacokinetics of tetracyclines and glycylcyclines]

Glycylcyclines were derived from the chemical structure of tetracyclines. After they started to be used in clinical practice, tetracycline group came into focus as a whole. First-generation tetracyclines feature low lipophilia and they are usually available in peroral form only, except rolitetracycline. Moreover, their absorption is highly variable and incomplete, ranging usually from 25 % to 60 % (absorption of tetracycline ranges from 77 % to 88 %). The majority of first-generation tetracyclines are not metabolized (though 5 % of tetracycline is metabolized to a less active metabolite). Instead, they are most often eliminated by renal excretion. Second-generation tetracyclines are 3 to 5 times more lipophilic, which enhances their tissue penetration. Doxycycline, the most common member of this group, features more than 80 % bioavailability. Bile concentration of doxycycline is 10 to 25 times higher as compared with its serum concentration. High concentrations of doxycycline are found also in kidneys, liver and bowel. Primarily, doxycycline is excreted in bile to feces. Part of doxycycline is inactivated in the liver and 40 % of it is excreted by kidneys in urine. Tigecycline is administered intravenously and it shows high tissue penetration, especially in bones, skin, liver and lungs. Less than 20 % of tigecycline is metabolized before it is excreted. Primarily, it is eliminated by biliary/fecal excretion in unchanged form. Small part of tigecycline is eliminated as metabolites. Secondary routes of elimination are glucuronidation and renal excretion.