CRM1/XPO1 Represents a Promising Therapeutic Target for Treatment of Chronic Lymphocytic Leukemia

Abstract 232

Exportin 1 (CRM1, XPO1) is a nuclear exporter that promotes the transit of tumor suppressor proteins (TSPs) including p53, I-κB, and FOXO3A out of the nucleus, thereby preventing their activity and contributing to disrupted apoptosis and enhanced proliferation. Recently, whole-genome sequencing in patients with CLL allowed the identification of recurrent mutations in a highly conserved region of CRM1 that can potentially affects its gene function, suggesting a direct role for CRM1 in the pathogenesis of CLL (Puente XS, et al: Nature 75:101, 2011). However the role of CRM1 and the consequences of its mutation in the development of CLL have yet to be explored. CRM1 has been shown to be up-regulated in hematologic and various solid tumors, making it a highly attractive molecular target impacting multiple pro apoptotic pathways. KPT-SINEs are new, potent and irreversible small molecule selective inhibitors of nuclear export developed by Karyopharm that specifically and irreversibly bind to CRM1 and block the function of this protein. CLL is characterized by disrupted apoptosis caused both by co-dependent stromal elements and aberrant activation of several survival-promoting signaling/transcriptional pathways including PI3K/Akt, NF-kB, and p53. Because of the distinct subtypes of CLL and multiple signaling pathways dysregulated, a therapeutic agent targeting a single biological pathway is unlikely to be effective. Thus, pursuit of CRM1 inhibition as a novel strategy aimed to restore multiple death pathways is crucial and has broad implications for many types of patients. Our preliminary work demonstrated CRM1 is over-expressed in CLL cells compared to normal B cells at a protein (3 fold, p<0.005) and mRNA level (2.6 fold p=0.014). Inhibition of CRM1 by KPT-185 induced apoptosis in primary patient CLL cells in a dose and time dependent manner (EC₅₀<500nM) while limited cytotoxicity against normal PBMC and isolated B, NK and T cells was observed (EC₅₀ values >20 μM). Additionally, KPT-185 treatment of NK cells had no effect on their function as measured by ability of NK cells to mediate antibody dependent (ADCC) as wekk as direct cytotoxicity. The effect of KPT-185 on T function is currently under evaluation. Nuclear accumulation of FOXO3, p53 and IkB was also observed in primary CLL cells in a time dependent manner as shown by western blot and confocal microscopy. The evaluation of activated target genes is currently ongoing. Given the importance of microenvironmental stimuli on survival of CLL cells and response to therapy, we evaluated the ability of KPT-185 to induce cytotoxicity of CLL cells in the presence or absence of soluble factors such as CPG, CD40L, BAFF, TNF-α, IL-6, or IL-4, which are known to reduce the spontaneous apoptosis associated with CLL cells. KPT-185 treatment abrogated the protection induced by each of these factors suggesting that KPT-SINEs can disrupt signaling from the microenvironment that lead to in vivo CLL cell survival and potentially drug resistance. Interestingly the cytotoxic effect elicited by KPT-185 was enhanced in CPG activated cells (p=0.02). We also tested the ability of KPT-185 to kill CLL cells under coculture conditions with Hs5 stromal cell line. Coculture of CLL cells alone for 48 hours on the Hs5 stromal cell line resulted in a marked reduction of spontaneous apoptosis suggesting a strong protective effect elicited (P<0.001) by the stromal cells. Interestingly the cytotoxic effect mediated by KPT-185 was enhanced under coculture conditions (p=0.013). KPT-185 was also proven to be effective on murine TCL1+ cells (EC₅₀<500nM) in vitro. The in vivo efficacy of this compound and other structurally related analogs is currently being assessed in an ongoing study in theTCL1 mouse model of CLL. In conclusion CRM1 represents a novel target that has not been adequately explored in CLL. KPT-SINEs are a class of promising therapeutic agents with proven selective in vitro activity in CLL cells providing the rationale for developing small molecule, drug-like CRM1 inhibitors for the treatment of this disease.