In spite of the overall success of treating Wilms tumor, certain patients still have poor clinical outcomes. The sub-optimal outcomes for patients with anaplastic histology and recurrent Wilms tumor warrant the identification of new therapeutic agents. The objective of this trial is to estimate the response rate to two cycles of intravenous topotecan in children with recurrent Wilms tumor of favorable histology that is refractory to standard curative therapy.

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.

Inclusion Criteria:

• Favorable histology Wilms tumor that has recurred or progressed after primary treatment and at least one standard salvage treatment regimen OR anaplastic histology Wilms tumor that has recurred or progressed after primary treatment
• Age< 21 years of age at the time of study entry
• Adequate bone marrow function
• Adequate liver function
• Adequate renal function
• Adequate performance status

Exclusion Criteria:

• Subject is pregnant
• Subject is lactating
• Renal tumors other than Wilms tumors