Purpose: To determine the maximum-tolerated systemic exposure (MTSE) and exposure-limiting toxicity of continuous infusion topotecan in children with recurrent acute leukemia.
Patients and methods: Patients received escalating levels of topotecan systemic exposure as measured by steady-state topotecan lactone concentration (Css). Samples obtained within the first 24 hours were measured by high-pressure liquid chromatography (HPLC) for topotecan. A two-compartment model was fit to the data using a Bayesian algorithm. Css was calculated for each patient; if it differed by more than 20% of target, a new dosage was begun within 6 hours. Follow-up concentrations were obtained as well as serial plasma samples postinfusion. Toxicity and evidence of activity were assessed after each course.
Results: Thirteen boys and five girls received 23 courses of topotecan. Target Css ranged from 1.0 to 5.3 ng/mL (topotecan doses, 0.5 to 3.3 mg/m2/d). Nineteen of 23 courses were within +/- 20% of target after adjustment (range, 77% to 139%). The MTSE was 4.0 ng/mL, and mucositis was exposure-limiting at 5.3 ng/mL. A significant relation between topotecan lactone Css and the severity of mucositis was observed. Myelosuppression was experienced but was not considered exposure-limiting. One complete response and one partial response were noted.
Conclusion: The MTSE for continuous infusion topotecan was 4.0 ng/mL. Responses were noted at Css comparable to those producing responses in a severe combined immunodeficiency (SCID) mouse model. Further studies of topotecan are warranted.