SNS-595 a Novel S Phase Active Cytotoxic Acts Synergistically with Cytarabine To Reduce Bone Marrow Cellularity and Circulating Neutrophils.

Abstract

SNS-595 is a novel naphthyridine analog, a class of compounds not previously used for cancer treatment. SNS-595 is currently in an escalating-dose phase 1 trial in acute leukemia and several phase 2 studies in solid tumors. SNS-595 has a unique mechanism of action; causing double-strand breaks during S phase that are solely repaired through a DNA-PK-dependent pathway. This damage induces DNA damage-repair signals through both p53 and p73 pathways during DNA synthesis and is accompanied by a rapid onset of apoptosis and an irreversible G2 arrest. This mechanism suggests that SNS-595 could interact in combination favorably with other DNA damaging agents that utilize different mechanisms of damage signaling and repair. SNS-595 demonstrates potent anti-proliferative effects on human leukemic cell lines in vitro and is active in LM-3 Jck and CCRF-CEM hematologic xenograft models. Previously, we have shown that administration of SNS-595 results in a dose dependent loss in bone marrow cellularity and reduction in circulating neutrophils in mice (Proc. Amer. Assoc. Cancer. Res. 47: 4726, 2006). These data suggest that SNS-595 activity in bone marrow and circulating blood may provide a therapeutic rationale for the treatment of acute leukemias and advanced chronic myelogenous leukemia. In this study we evaluated the effects of SNS-595 on peripheral white blood cells and bone marrow cellularity as a single agent and in combination with cytarabine (Ara-C) and daunorubicin (DNR). SNS-595 dosed once a day at 10 mg/kg IV as a single agent and Ara-C dosed three times a day as a single agent at 20 mg/kg SC, on day 0 and 4, resulted in 44% and 12% reductions in cellularity, respectively. In contrast, combination dosing of SNS-595 once a day at 10 mg/kg IV with Ara-C dosed three times a day at 20 mg/kg SC resulted in a 93% reduction in cellularity. Normal cellularity recovered on Day 12 (8 days post last dose). Furthermore, the SNS-595/Ara-C combination reduced cellularity more than Ara-C dosed at the MTD of 60 mg/kg three times a day SC (42% reduction in cellularity). Circulating leukocyte levels were also reduced including neutrophils that dropped from 1359 cells/ml to 587 cells/ml for SNS-595 dosed at 10 mg/kg IV and 1212 cells/ml for Ara-C dosed three times a day at 20 mg/kg SC on day 8. The co-administration of these two doses resulted in a near elimination of circulating neutrophils (29 cells/ml). Leukocyte counts subsequently returned to normal levels by Day 18 (14 days post last dose). In conclusion, co-administration of SNS-595 and Ara-C reversibly ablates murine bone marrow cells and reversibly depletes circulating neutrophils. These data indicate that SNS-595 has the potential to combine effectively with Ara-C for the treatment of acute leukemias and suggests that additional studies in patients with advanced hematologic malignancies are warranted.