Pharmacokinetics and metabolism of anthracyclines

The anthracycline antibiotics constitute a major series of anti-cancer drugs, the best known and most widely used being doxorubicin. Among hundreds of analogues, only a few have reached routine clinical use. Their main metabolic feature is the reduction of a ketone group to an hydroxyl group, giving an -ol derivative generally less active than the parent compound. Anthracyclines are characterized by a rapid distribution phase and a slow elimination phase. The successive half-lives of doxorubicin in plasma are about 5 minutes, 1 hour and 30 hours. Its total plasma clearance is about 30 l/hr/m2, and its total volume of distribution at steady state is approximately 15 l/kg. Anthracyclines are excreted mostly through bile, and special care must be taken with their use in patients with hepatic dysfunction. The new anthracyclines of clinical interest in solid tumours (epirubicin, pirarubicin) are more lipophilic than doxorubicin and have a higher volume of distribution and an increased total plasma clearance. Idarubicin is active in leukaemia rather than against solid tumours, and an oral form is available. Because of their high tissue fixation, these new anthracyclines are of particular interest for locoregional therapy, especially through the hepatic artery. Myelosuppression is the dose-limiting toxicity of anthracyclines and is related to drug exposure, so that pharmacokinetic-pharmacodynamic relationships have been clearly established for these drugs. A new subfamily, characterized by a morpholino group, presents very original features such as direct covalent linking to DNA after cytochrome P450 activation. These molecules are active at 100-fold lower concentrations than the conventional anthracyclines and are currently in clinical trials.