 This is a Phase 1, open-label, single-center, dose-escalating study in pediatric patients with refractory or recurrent solid tumors. Patients will be registered into 1 of 2 strata, depending upon the presence bone marrow metastases or previous treatment with intensive myelosuppression therapy. Patients will receive Karenitecin along with cyclophosphamide daily for 5 consecutive days, every 21 days 1 treatment cycle. Treatment may continue for up to 20 cycles, as long as there is continued evidence of clinical benefit and an absence of unacceptable toxicity. 