Topotecan administered intravenously over 30 minutes daily for 5 consecutive days for 2 consecutive weeks, with a two-day rest given in between the five-day treatment blocks. The topotecan dose started at 1.8 mg/m2/dosage and adjusted to attain a target systemic exposure of 80 plus or minus 10 ng-hr/ml.each cycle consists of 28 days and subsequent cycles can be administered upon hematological recovery. Patients with a CR, PR, or SD, can continue to receive up to a total of six cycles. Patients with PD are removed from the study.
Secondary Objectives include:
* To describe the anti-tumor activity of topotecan in children with recurrent Wilms tumor of anaplastic histology.
* To assess the relation between CYP3A4/5 genotype and the pharmacokinetics and pharmacodynamics of topotecan.
* To assess the relation between ABCG2 genotype and the pharmacokinetics and pharmacodynamics of topotecan.
