Phase I dose-escalation trial of clofarabine followed by escalating doses of fractionated cyclophosphamide in children with relapsed or refractory acute leukemias

Background: By inhibiting DNA repair, clofarabine (CLO) may augment cyclophosphamide (CY)-induced DNA damage and apoptosis. We performed a Phase I study for refractory and/or relapsed (R/R) leukemia in children to determine maximum-tolerated dose (MTD) of time-sequential CLO followed by CY. Procedure: Thirteen patients with (R/R) ALL (n=8) and AML (N=5), median age 9 years (range: 2-12 years), were treated with escalating doses of CLO on days 1, 2, 3 and 8, 9, 10 and CY 200mg/m²/day on days 0 and 1 then 400mg/m²/day on days 2, 3, 8, 9, and 10. Ten patients were treated at dose level 1 (DL1) (CLO 20mg/m²/day) and three patients at DL2 (CLO 30mg/m²/day). The average number of prior chemotherapies was 8.9. DNA damage testing was performed before treatment on day 0, and 2hours after CY on day 0, before sequential CLO, CY treatment on day 1, and 2hours after CLO followed by CY on day 1. Results: Two patients at DL2 had dose-limiting toxicities (DLTs) that included hypotension with cardio-respiratory failure (1) and hepato-renal failure (1). Complete remission (CR) was achieved in 2/11 (18.2%) and partial remission (PR) in 2/11 (18.2%) for an overall response (OR) of 36.4%. The use of CLO before CY augmented CY-induced DNA damage in leukemic blasts compared to CY alone. Conclusion: In pediatric patients with R/R leukemia, 20mg/m²/day is the MTD for CLO in timed sequential combination with CY. Increased DNA damage with the use of this combination suggests a mechanism for the sequential timing of these two chemotherapeutic agents.