Gore, Steven D.

AbstractTopotecan (TPT) is a topoisomerase I (topo I) poison that has shown promising antineoplastic activity in solid tumors and acute leukemia. In the present study, a band depletion assay was used to evaluate the ability of TPT to stabilize topo I-DNA adducts in human leukemia cell lines and in clinical leukemia samples ex vivo. This assay showed that 50% of the cellular topo I in HL-60 human myelomonocytic leukemia cells became covalently bound to DNA at an extracellular TPT concentration of 4 μmol/L. In contrast, in 13 clinical specimens of human leukemia harvested before treatment of patients with TPT, the TPT concentration required to stabilize 50% of the cellular topo I in topo I-DNA complexes ranged from 3 to greater than 100 μmol/L (median, 30 μmol/L). Flow microfluorimetry showed that cellular TPT accumulation varied over only a twofold range and failed to provide evidence for transport-mediated resistance in the clinical samples. These observations raise the possibility that formation of topo I-DNA adducts is diminished in many specimens of refractory/relapsed acute leukemia by a mechanism that might alter topo I sensitivity to TPT.

Abstract Purpose: Incorporation of cytarabine into DNA activates checkpoint kinase 1 (Chk1), which stabilizes stalled replication forks, induces S-phase slowing, and diminishes cytarabine cytotoxicity. The selective Chk1 inhibitor SCH 900776 abrogates cytarabine-induced S-phase arrest and enhances cytarabine cytotoxicity in acute leukemia cell lines and leukemic blasts in vitro. To extend these findings to the clinical setting, we have conducted a phase I study of cytarabine and SCH 900776. Experimental Design: Twenty-four adults with relapsed and refractory acute leukemias received timed sequential, continuous infusion cytarabine 2 g/m2 over 72 hours (667 mg/m2/24 hours) beginning on day 1 and again on day 10. SCH 900776 was administered as a 15- to 30-minute infusion on days 2, 3, 11, and 12. The starting dose of SCH 900776 was 10 mg/m2/dose. Results: Dose-limiting toxicities consisting of corrected QT interval prolongation and grade 3 palmar-plantar erythrodysesthesia occurred at 140 mg flat dosing (dose level 5, equivalent to 80 mg/m2). Complete remissions occurred in 8 of 24 (33%) patients, with 7 of 8 at 40 mg/m2 or higher. SCH 900776 did not accumulate at any dose level. Marrow blasts obtained pretreatment and during therapy showed increased phosphorylation of H2Ax after SCH 900776 beginning at 40 mg/m2, consistent with unrepaired DNA damage. Conclusions: These data support a randomized phase II trial of cytarabine +/− SCH 900776 at a recommended flat dose of 100 mg (equivalent to 56 mg/m2) for adults with poor-risk leukemias. The trial (SP P05247) was registered at www.clinicaltrials.gov as NCT00907517. Clin Cancer Res; 18(24); 6723–31. ©2012 AACR.