12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
The cytotoxic effect of doxorubicin hydrochloride on malignant cells and its toxic effects on various organs are thought to be related to nucleotide base intercalation and cell membrane lipid binding activities of doxorubicin. Intercalation inhibits nucleotide replication and action of DNA and RNA polymerases. The interaction of doxorubicin with topoisomerase II to form DNA-cleavable complexes appears to be an important mechanism of doxorubicin hydrochloride cytocidal activity.
Pharmacokinetic studies conducted in patients with various types of tumors have shown that doxorubicin follows multiphasic disposition after intravenous injection. In four patients, doxorubicin demonstrated dose-independent pharmacokinetics across a dose range of 30 mg/m2 to 70 mg/m2.
The distribution half-life is approximately 5 minutes. Steady-state distribution volume ranges from 809 L/m2 to 1214 L/m2. Binding of doxorubicin and its major metabolite, doxorubicinol, to plasma proteins is 75% and is independent of plasma concentration of doxorubicin up to 1.1 µg/mL.
Doxorubicin does not cross the blood brain barrier.
Plasma clearance is ranges from 324 mL/min/m2 to 809 mL/min/m2. The terminal half-life is 20 hours to 48 hours.
Doxorubicin is a substrate of CYP3A4, CYP2D6, and P-gp.
Enzymatic reduction at the 7 position and cleavage of the daunosamine sugar yields aglycones which are accompanied by free radical formation, the local production of which may contribute to the cardiotoxic activity of doxorubicin hydrochloride.
Disposition of doxorubicinol in patients is formation rate limited, with the terminal half-life of doxorubicinol being similar to doxorubicin. The relative exposure of doxorubicinol, i.e., the ratio between the AUC of doxorubicinol and the AUC of doxorubicin is approximately 0.5.
Plasma clearance is predominately by metabolism and biliary excretion. Approximately 40% of the dose appears in the bile in 5 days, while only 5% to 12% of the drug and its metabolites appear in the urine during the same time period. In urine, <3% of the dose was recovered as doxorubicinol over 7 days.
Systemic clearance of doxorubicin is significantly reduced in obese women with ideal body weight greater than 130%. There was a significant reduction in clearance without any change in volume of distribution in obese patients when compared with normal patients with less than 115% ideal body weight.
Following administration of doses ranging from 10 mg/m2 to 75 mg/m2 of doxorubicin hydrochloride to 60 patients ranging from 2 months to 20 years, doxorubicin clearance averaged 1443 ± 114 mL/min/m2. Further analysis demonstrated that clearance in 52 patients ranging from 2 to 20 years (1540 mL/min/m2) was increased compared with adults. However, clearance in infants younger than 2 years of age (813 mL/min/m2) was decreased compared with older patients (ranging from 2 to 20 years) and approached the range of clearance values determined in adults [see Use in Specific Populations (8.4)].
A published clinical study involving 6 men and 21 women with no prior anthracycline therapy reported a significantly higher median doxorubicin clearance in men compared to women (1088 mL/min/m2 versus 433 mL/min/m2). However, the terminal half-life of doxorubicin was longer in men compared to women (54 versus 35 hours).