Mitoxantrone-induced DNA damage in leukemia cells is enhanced by treatment with high-dose arabinosylcytosine

In a phase II study, patients with chronic myelogenous leukemia in blast crisis (CML-BC) were treated with intravenous (IV) mitoxantrone (5 mg/m²) per day given over 30 min x 5 days and high-dose arabinosylcytosine (ara-C) (3 g/m² IV q 12 h x 6).The effect of this treatment on DNA damage was studied in the leukemia cells of four patients using the alkaline elution technique modified to measure DNA in unlabeled human cells. A fluorescence assay using Hoechst 33258 dye was applied for the determination of eluted DNA.After a single infusion of mitoxantrone, neither frank nor protein-associated single-strand breaks (SSB) were observed.Even repeated treatment with mitoxantrone on 3 consecutive days did not induce significant SSB. However, after the combined sequential infusion of ara-C and mitoxantrone the DNA elution pattern changed, showing significant DNA damage.SSB remained apparent after 24 h and increased with subsequent doses of ara-C and mitoxantrone. Studies of other patients treated with ara-C alone did not reveal significant SSB (n=5).Following mitoxantrone infusion the median peak concentrations of intracellular ara-CTP (the triphosphate of ara-C) exceeded 900 μM,a value greater than that observed in CML-BC patients receiving ara-C alone (230 μM, n=15, P<0.02). The present study shows the applicability of the alkaline elution method for the assay of DNA damage in vivo. The enhanced DNA damage after combined treatment with mitoxantrone and high-dose ara-C suggests a synergistic drug effect.