Purpose: Based on preclinical data that demonstrated synergy between alkylating agents and topoisomerase (topo) I poisons, we determined the maximum-tolerated dose (MTD) of topotecan, using a 5 day bolus schedule, that could be given in combination with a single, fixed dose of cyclophosphamide. Pharmacodynamics of this combination were explored by analyzing biochemical effects of treatment in peripheral-blood mononuclear cells (PBMCs).
Patients and methods: Patients with refractory cancer were treated with cyclophosphamide 600 mg/m2 on day 1, followed by topotecan given as a 30-minute infusion for 5 consecutive days. Cycles were repeated every 3 weeks. Once the MTD was defined, granulocyte colony-stimulating factor (G-CSF) was added to the regimen in an attempt to escalate further the dose of topotecan. Plasma concentrations of topotecan were determined during the first treatment cycle by high-performance liquid chromatography. PBMCs were sampled at baseline and throughout the 5-day treatment period for analysis of topo I protein concentrations and to determine drug-induced DNA fragmentation.
Results: Twenty-six patients were treated with topotecan at doses that ranged from 0.5 mg/m2/d to 1.2 mg/ m2/d for a total of 74 cycles. Reversible neutropenia was dose-limiting, with mild to moderate suppression of the other blood-cell elements commonly occurring. Transfusions of RBCs and platelets were required in 24% and 7% of treatment cycles, respectively. The most prominent nonhematologic toxicities were fatigue and weight loss. Compared with previously published data in which topotecan was administered alone, cyclophosphamide did not appear to alter the pharmacokinetics of topotecan. Significant increases in topo I concentration were identified in PBMCs following the administration of cyclophosphamide on day 1 and there was a significant decrease in topo 1 during the 5-day course of treatment (P < .01, sign test). DNA fragmentation as a result of drug treatment was identified in 11 of 15 (73%) cycles analyzed.
Conclusion: For previously treated patients, the recommended dose of topotecan in this schedule is 0.75 mg/m2/d without growth factor support and 1.0 mg/ m2/d if it is administered with G-CSF. Biochemical changes in cells induced by exposure to camptothecins can be measured in vivo and these effects may have important implication in the design of combination therapies and the optimal scheduling of this class of agents.