12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Topoisomerase I relieves torsional strain in DNA by inducing reversible single-strand breaks. Topotecan binds to the topoisomerase I-DNA complex and prevents re-ligation of these single-strand breaks. The cytotoxicity of topotecan is thought to be due to double-strand DNA damage produced during DNA synthesis, when replication enzymes interact with the ternary complex formed by topotecan, topoisomerase I, and DNA. Mammalian cells cannot efficiently repair these double-strand breaks.
12.3 Pharmacokinetics
Following administration of topotecan at doses of 0.5 to 1.5 mg/m2 (0.3 to 1 times the recommended dose) administered as a 30-minute infusion, area under the curve (AUC) increases approximately proportional with dose.
Elimination
The terminal half-life of topotecan is 2 to 3 hours following intravenous administration.
Metabolism
Topotecan undergoes a reversible pH-dependent hydrolysis of its pharmacologically active lactone moiety. At pH ≤ 4, the lactone is exclusively present, whereas the ring-opened hydroxy-acid form predominates at physiologic pH. Topotecan is metabolized to an N-demethylated metabolite in vitro. The mean metabolite: parent AUC ratio was about 3% for total topotecan and topotecan lactone following intravenous administration.
Excretion
The overall recovery of total topotecan and its N-desmethyl metabolite in urine and feces over 9 days averaged 73% ± 2% following an intravenous dose. Mean values of 51% ± 3% as total topotecan and 3% ± 1% as N-desmethyl topotecan were excreted in the urine. Fecal elimination of total topotecan accounted for 18% ± 4% while fecal elimination of N-desmethyl topotecan was 1.7% ± 0.6%. An O-glucuronidation metabolite of topotecan and N-desmethyl topotecan has been identified in the urine.
Specific Populations
No clinically significant differences in the pharmacokinetics of topotecan were observed based on age, sex, or hepatic impairment following intravenous administration.
Patients with Renal Impairment
Compared to patients with CLcr > 60 mL/min (calculated by the Cockcroft-Gault method using ideal body weight), plasma clearance of topotecan lactone decreased by 33% in patients with CLcr 40 to 60 mL/min and decreased by 65% in patients with CLcr 20 to 39 mL/min. The effect on topotecan pharmacokinetics in patients with CLcr < 20 mL/min is unknown [see Dosage and Administration (2.4)].