Phase I and pharmacologic study of topotecan: A novel topoisomerase I inhibitor

Purpose: A phase I and pharmacologic study was undertaken to determine the maximum-tolerated dose (MTD), describe the principal toxicities, and characterize the pharmacologic behavior of topotecan, which is a semisynthetic analog of camptothecin with broad preclinical antitumor activity and the first topoisomerase I-targeting agent to enter clinical development in the United States since studies of sodium camptothecin over 2 decades ago. Patients and Methods: Thirty-minute infusions of topotecan were administered daily for 5 consecutive days every 3 weeks to patients with advanced solid malignancies at doses ranging from 0.5 to 2.5 mg/m²/d. Results: At doses of 1.5 and 2.0 mg/m², grade 3 and 4 neutropenia occurred in most courses; however, neutropenia was brief and rarely associated with fevers or treatment delays. Neutropenia was more severe in patients with extensive prior treatment than in minimally pretreated patients, but these differences were not substantial. At 2.5 mg/m², topotecan induced profound and prolonged neutropenia that was frequently associated with fever and treatment delays in minimally pretreated patients. Topotecan also induced mild depressions in the hematocrit level in the majority of courses; however, precipitous drops requiring transfusional therapy occurred in 14% of courses and suggested a drug-induced hemolytic effect. Unlike sodium camptothecin, hemorrhagic cystitis was not observed. Thrombocytopenia, skin rash, diarrhea, and vomiting occurred infrequently and were modest in severity. Responses were observed in non-small-cell lung carcinoma and platinum-refractory ovarian carcinoma. Drug disposition in plasma was described by a biexponential model, with renal elimination accounting for 38.7% of drug disposition. Topotecan was rapidly hydrolyzed in vivo to a less active, open-ring form. Conclusions: Neutropenia is the dose-limiting toxicity, and 1.5 mg/m2 is the recommended starling dose of topotecan for both minimally and heavily prefreated patients in future phase II trials, with escalation to 2.0 mg/m² if treatment is well tolerated. Non-small-cell lung and platinum-refractory ovarian carcinomas should be among those evaluated in phase II trials of topotecan.