Anti-tumor activity of novel nuclear export inhibitors (NEIs) in multiple murine leukemia models

5688 CRM1 is an essential protein required to export a set of ‘cargo’ proteins which bear a leucine-rich nuclear export sequence (NES) from the nucleus to the cytoplasm. Examples of cancer-related proteins bearing CRM1-dependent NESs include p53, BCR-Abl, and FOXO-3a. Nuclear sequestration of these CRM1 ‘cargo’ proteins upon inhibition of CRM1 can lead to cell growth inhibition in normal cells or apoptosis in cancer cells. The identification of CRM1 as a novel target in cancer came from the study of Leptomycin B (LMB) and its analogs. These molecules are potent cytotoxic agents which belong to a group of polyketide natural products known as nuclear export inhibitors (NEIs). LMB and its analogs covalently bind to the nuclear exportin CRM1 and thereby inhibit its ability to shuttle cargo proteins to the cytoplasm. The development of LMB was limited by the significant toxicity observed in previous studies (Newlands, et al., 1996, Br. J. Can., 74: 648-649). We have recently developed a series of NEI analogs with significantly improved biological and pharmacological properties over LMB. Upon examination of the activity of these molecules in leukemia cell lines, we found that NEIs were very potent in vitro in ALL (Molt-4, CCRF-CEM, Jurkat) and AML (MV-4-11) cell lines. Exposure to NEIs led to a rapid and dramatic induction of multiple markers of apoptosis (caspases, Annexin V binding). Based on these results, we examined the activity of two NEI analogs, KOS-1815 and KOS-2464, in murine leukemia xenograft models. Both of these drugs, when dosed on a weekly schedule, significantly inhibited the growth of MV-4-11 (T/C = ~22% for KOS-2464 and ~26% for KOS-1815) and Molt-4 (T/C = ~38% for KOS-2464 and ~29% for KOS-1815) xenografts. In addition, a single administration of KOS-2464 or KOS-1815 induced regression of large (>1000 mm3) CCRF-CEM xenografts. KOS-2464 was very well-tolerated at efficacious doses in all of these leukemia models, while KOS-1815 induced a ~5 to 10% transient body weight loss upon dosing. Finally, we established a Molt-4 orthotopic leukemia model in mice. Weekly dosing of KOS-1815 and KOS-2464 for three weeks, beginning eight days after cancer cell implantation, doubled survival of these leukemic mice. Median survival increased from 25 days for the control group to 50+ days for the drug-treated groups (56 days for KOS-2464 and 50 days for KOS-1815). Studies are ongoing to gain further insight into the mechanism by which these molecules exert their activity in leukemia models. Taken together, the current study shows that KOS-2464, our lead NEI analog, with its novel mechanism of action and wide therapeutic window, has significant potential as a novel anti-cancer agent, especially for the treatment of leukemia.