Preclinical Development of Small-Molecule CRM1 Inhibitors as Novel Therapy for the Treatment of Myeloma and Other Hematological Malignancies

CRM1 (XPO1) is a key nuclear export protein which controls multiple tumor suppressor proteins (TSP) and cell proliferation pathways including p53, PI3K/AKT, Wnt/ß-catenin and NF-kB. Further, mislocalization of proteins can abrogate TSP functions and render chemotherapies ineffective. For example, multiple myeloma (MM) cells that are grown at high densities (to mimic the in vivo situation) become resistant to topoisomerase 2 (topo2) inhibitors (e.g., doxorubicin) simply because topo2α is exported from the nucleus. Forcing the nuclear expression of chemotherapy targets, TSP and growth regulatory proteins by CRM1 inhibition can restore drug sensitivity and restore checkpoint control/genome surveying functions. These events lead to apoptosis or autophagy in cancer cells while sparing normal cells.

CRM1 inhibitors were synthesized and nuclear distribution studies were performed in U2OS cells transfected with HIV-rev GFP proteins. Cell proliferation studies were performed in multiple myeloma (MM), leukemia and lymphoma cell lines and in human peripheral blood mononuclear cells (PBMCs). Toxicology studies were performed in several mouse strains. Antitumor activity is assessed in a xenograft model of human MM.1 cells growing in scid-mice.

The lead CRM1 inhibitor, KPT-0127, blocks CRM1 mediated nuclear export of HIV-Rev-GFP, FOXO, and p53 with an IC50 of ~300 nM. KPT-0127 is selectively cytotoxic to various hematological cell lines with EC50s in the 0.02–1.0 μM range, and shows limited cytotoxicity in similar studies in normal cell lines (NIH-3T3, MRC5, HUVECs) and PBMCs (EC50 >5-20 μM). KPT-0127 increases the nuclear localization of IkB in HUT-78 leukemia cells and human PBMCs, and inhibits TNF-α secretion in LPS stimulated U937 macrophage derived cells, likely through inhibition of NF-kB signaling. In MM cells grown at high densities which actively export topo2 to the cytoplasm via CRM1, blocking CRM1-dependent transport strongly enhanced topo2 nuclear localization and augmented the apoptotic effects of doxorubicin in <24 hours showing clear synergy between KPT-0127 and the anthracycline. Combination of sublethal concentrations of bortezomib plus KPT-0127 in MM1.S and MM.1R myeloma cells (as well as in Jurkat and HS-Sultan) induced synergistic cytotoxicities. In mice, KPT-0127 given by subcutaneous (SC) injection of 30–100 mg/kg leads to serum levels exceeding the effective CRM1 inhibitory concentration for at least 4 hours. In 5 day repeated dose toxicology studies, SC administration of 100 mg/kg KPT-0127 (QD × 5) was well tolerated in mice with no cutaneous or obvious systemic clinical findings. Modest neutrophilia and mild lymphopenia were seen and no neurologic signs resulted from treatment with KPT-0127. Administration of KPT-0127 SC to mice bearing HCT-116 colon cancer results in dose-dependent antitumor activity. Xenograft studies with MM.1 myeloma cells are underway and will be presented at the meeting.

The sensitivity of tumors with multiple TSP and oncogenic abnormalities, including p53 mutations/deletions and PTEN deficiency/AKT activation, to killing with KPT-0127 likely reflects the ability to affect multiple critical and non-redundant regulatory pathways. CRM1 inhibition also forces topo 2 to remain in the nucleus, and increases levels of nuclear IkB antagonizing NF-kB function, thereby reducing the likelihood of resistance development. These results support the development of small molecule, drug-like CRM1 inhibitors for the treatment of MM and other hematological cancers. IND-enabling CMC and toxicology work are expected to begin in early 2011.

Shacham:Karyopharm: Employment, Equity Ownership, Patents & Royalties. Nir:Karyopharm: Consultancy. Draetta:Karyopharm: Consultancy. Sandanayaka:Karyopharm: Employment. Shechter:Karyopharm: Employment. Kauffman:Karyopharm: Consultancy, Equity Ownership, Patents & Royalties.