Next Generation XPO1 Inhibitor Shows Improved Efficacy and In Vivo Tolerability in Hematologic Malignancies

Restoring nuclear localization of tumor suppressors by blocking exportin 1 (XPO1) holds promise as a new therapeutic paradigm in many cancers, including chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML). Oral selective inhibitor of nuclear export (SINE) compounds that covalently modify XPO1 were recently discovered and are exciting new compounds to implement this strategy. Selinexor, the clinical lead SINE, has made progress in Phase I/II clinical trials and is generally well tolerated, but limited to twice weekly dosing, with supportive care. The discovery of novel SINE compounds with improved tolerability is therefore of considerable clinical interest and represents a significant contribution beyond the targeted therapies currently available for hematologic malignancies and a variety of other cancers where upregulation of XPO1 is observed. Presented herein is the discovery of KPT-8602, the next generation SINE compound that shows lower brain penetration, improved tolerability allowing continuous dosing, and improved efficacy beyond any current XPO1 inhibitor. Results: Our crystallography data revealed that KPT-8602 binds covalently to XPO1 through a Michael acceptor that is activated by an electron withdrawing pyrimidyl moiety, allowing a 2-fold increased reversible interaction with XPO1 compared to earlier SINE compounds. The crystal structure of the KPT-8602-XPO1 complex showed interactions between XPO1 and the activating pyrimidyl group. In vitro pull-downs using immobilized GST-nuclear export sequences and purified recombinant XPO1 demonstrated greater reversible binding of KPT-8602 compared to KPT-330 (selinexor). In vivo toxicology studies demonstrated that KPT-8602 possesses lower brain penetration compared to KPT-330 allowing for continuous dosing and improved tolerability. Our results also showed that KPT-8602 induced a comparable level of cytotoxicity as well as inhibition of cell proliferation compared to KPT-330 in primary CLL tumors and in a representative panel of DLBCL cell lines. Furthermore, KPT-8602 inhibited proliferation and induced apoptosis in AML cell lines and primary AML blasts while inducing nuclear accumulation of p53 and NPM1. We hypothesized that these improved pharmacological parameters would allow daily KPT-8602 to abrogate disease progression in CLL and AML animal models. The Eµ-TCL1-C57BL/6 transplant model of CLL was used to evaluate the therapeutic benefit of continuous dosing of KPT-8602. Eµ-TCL1-engrafted mice were treated with KPT-8602 given daily or 2x/week. The KPT-8602 daily cohort had significantly improved survival with a median overall survival of 70 vs 50 days (vs vehicle 33 days), compared to those treated only 2x/week with KPT-8602 (p=0.001). Mice treated with KPT-330 2x/week showed a similar survival to mice treated with KPT-8602 2x/week. Mice given daily KPT-8602 had significantly smaller spleens and reduced circulating leukemic cells compared to all the other groups (p<0.001 for all comparisons). We recently showed the combination of KPT-330 and ibrutinib (bruton tyrosine kinase inhibitor) increases survival compared to ibrutinib alone in Eµ-TCL1-engrafted mice. We therefore tested the effect of KPT-330+ibrutinib or KPT-8602+ibrutinib in a cohort of Eµ-TCL1-engrafted mice compared to any agent alone. Notably, the combination of KPT-8602 to ibrutinib was able to further improve the survival induced by KPT-330+ibrutinib (p=0.008). We next assessed the activity of KPT-8602 in a human leukemia xenograft model of AML where NOD/SCID γ mice were inoculated with MV4-11 cells obtained from spleens of primary MV4-11 xenografts. Recipient mice given KPT-8602 5x/week showed strikingly better outcomes with a median survival of 58 vs 35 days compared to KPT-8602 2x/week (p<0.0001) and KPT-330 2x/week (p<0.0001). Conclusion: Consistent with the in vitro finding of a two-fold enhanced reversibility of XPO1-KPT-8602 binding and lower brain penetration, our data indicate that KPT-8602 allows a prolonged frequent dosing schedule, which leads to an excellent therapeutic benefit and less toxicity in animal models of CLL and AML. These data suggest that KPT-8602 represents a new treatment paradigm and warrants further evaluation in clinical trials of patients with hematologic malignancies, particularly in combination with ibrutinib or other second generation Btk inhibitors.