Nuclear Export (Karyopherin) Inhibitors: A Novel Therapeutic Strategy for Treating Blast Crisis Chronic Myelogenous Leukemia (CML) and Philadelphia-Positive (Ph⁺) Acute Lymphoblastic Leukemia (ALL) Through Interference with hnRNP Nucleocytoplasmic Shuttling and Rescue of Protein Phosphatase 2A (PP2A) Tumor Suppressor Activity,

Abstract 3758

Despite the remarkable response in chronic phase CML, tyrosine kinase inhibitor (TKI)-based therapies do not induce long-term response in myeloid or lymphoid blast crisis CML (CML-BC) and Ph⁺ ALL, and are unable to kill quiescent Ph⁺ hematopoietic stem cells. We reported that CML disease progression is characterized by a BCR-ABL1 dose- and kinase-dependent increase in the expression/activity of nucleocytoplasmic-shuttling of heterogeneous nuclear ribonucleoproteins (hnRNPs) A1, E2 and K, which are essential post-transcriptional and translational modulators of critical regulators of cell proliferation, survival and differentiation of CD34⁺ CML-BC and/or CD34⁺/CD19⁺ Ph⁺ ALL progenitors including SET/PP2A, E2F3, Bcl-xL, C/EBPa, miR-328, and c-Myc. The karyopherin (a class of proteins involved in regulating nuclear import/export) chromosome region maintenance 1 (CRM1) controls the nuclear export of several hnRNPs, and because growth/survival of CML-BC and Ph⁺ ALL progenitors requires the aberrant cytoplasmic activity of hnRNPs A1, E2 and/or K, we have explored the therapeutic potential of CRM1 inhibitors in p210 and p190 BCR-ABL1⁺ cell line models of CML-BC and Ph⁺ ALL, respectively.

Thus, the cytokine-dependent myeloid 32Dcl3 and lymphoid BaF3, and the derived cytokine-independent 32D-p210^BCR-ABL1 and Baf3-p190^BCR-ABL1 mouse progenitors were exposed for 48 hr to different concentrations (0–10 μM) of the CRM1 selective & potent inhibitors of nuclear export (SINE) KPT-185 and KPT-207. MTT viability assays (n=3) revealed that KPT-185 and KPT-207 decreased viability ∼80% in 32D-p210^BCR-ABL1 cells (KPT-185≥207) at concentrations ranging from 150–350 nM. Similar results were obtained in Baf3-p190^BCR-ABL1 cells as the EC₅₀ was 300nM for KPT-185 and KPT-207. The KPT-SINE not only induced killing, but also affected cytokine-independent growth of BCR-ABL1⁺ cells: proliferation was inhibited 89% and 81% by KPT-185 and KPT-207, respectively. Notably, growth and survival of non-transformed 32Dcl3 and BaF3 cells was not affected (70–100% viable cells) at concentrations (150–350 nM) of KPT-185 and KPT-207 that impair survival of BCR-ABL1⁺ cells.

Mechanistically, we found that KPT-SINE CRM1 inhibitors altered the nuclear/cytoplasmic ratio of hnRNPs important for BCR-ABL1 leukemogenesis. Indeed, in KPT-207-treated (1 μM; 48h) BCR-ABL1⁺ cells hnRNP A1, E2 and K accumulated in the cytoplasm. Likewise, KPT-185 treatment (1 μM; 48h) resulted in hnRNP A1 being sequestered in the nucleus whereas hnRNP E2 distribution was not altered and, unexpectedly, levels of hnRNP K expression were markedly decreased in both subcellular compartments.

Interestingly, treatment of BCR-ABL1⁺ cells with KPT-185 and KPT-207 (1 μM; 48h) resulted in 75% and 50% suppression of BCR-ABL1 expression and kinase activity, respectively. Furthermore, KPT-207 also reduced Myc expression in 32D-p210^BCR-ABL1 cells; this is consistent with the potential interference of KPT-207 with the proliferation/survival signals triggered by the BCR-ABL1/MAPK/hnRNP K/Myc pathway in CML-BC progenitors.

Because both KPT-185 and KPT-207 significantly alter hnRNP A1 localization in addition to impairing BCR-ABL1 expression/activity, and suppression of PP2A tumor suppressor activity is essential for both p210 and p190 BCR-ABL1-driven leukemogenesis, it is highly plausible that KPT-207 and KPT-185 negatively regulate the BCR-ABL1-dependent and hnRNP A1-mediated induction of the PP2A inhibitor SET thereby rescuing PP2A phosphatase activity, which in turn decreases BCR-ABL1 expression and kinase activity, and triggers in vitro and in vivo apoptosis of primary CML-BC and Ph⁺ ALL progenitors. Indeed, treatment with KPT-207 and KPT-185 (250 nM; 48h) restored PP2A activity in 32D-p210^BCR-ABL1 cells to levels similar to those detected in non-transformed 32Dcl3 cells.

Although further investigation of KPT-207 and KPT-185 mechanism of action and assessment of their biologic/therapeutic effects in CML-BC and Ph⁺ ALL mouse models and primary leukemic and normal progenitors is currently ongoing, it is safe to conclude that selective nuclear export (SINE CRM1) inhibitors represent potentially powerful therapeutic tools that, if used alone or in combination with TKIs, might lead to sustained complete molecular remission in CML-BC and Ph⁺ ALL patients.