OP449, a Novel SET Antagonist, Is Cytotoxic To Leukemia Cells and Enhances Efficacy Of Tyrosine Kinase Inhibitors In Drug-Resistant Myeloid Leukemias

Background The SET oncoprotein, an inhibitor of the protein phosphatase 2A (PP2A), is overexpressed in leukemia cells, preventing PP2A from performing its regulatory role in deactivating signaling proteins by dephosphorylation. Restoration of PP2A activity in both chronic myeloid leukemia (CML) and acute myeloid leukemia (AML) cells to normal levels through shRNA-mediated knockdown of SET results in reduced leukemogenesis. Given the central role of PP2A and SET in regulating various kinase-dependent and -independent downstream signaling pathways, we evaluated the efficacy of SET antagonism in CML and AML cell lines as well as primary patient cells using OP449, a novel, specific, cell-penetrating SET antagonist. Results Treatment of human and murine CML cells with OP449 resulted in dose-dependent increase in PP2A activity and selective inhibition of cell growth (IC50: 0.60 to 1.11 μM), while parental Ba/F3 cells exhibited no measurable cytotoxicity. OP449-mediated decrease in the viability of leukemia cells was significantly rescued by co-treatment with okadaic acid, a PP2A inhibitor, confirming efficacy is mediated through PP2A activation. OP449 was also 3 to 8-fold more potent than FTY720 (a known activator of PP2A) and induced dephosphorylation/degradation of BCR-ABL1, AKT, and STAT5. Importantly, OP449 demonstrated activity against the ABL1 tyrosine kinase inhibitor-resistant BCR-ABL1T315I mutant and the BCR-ABL1E255V/T315I compound mutant (IC50: 1.62 and 1.97 μM, respectively). Consistent with cell line findings, OP449 also inhibited growth of primary cells from CML blastic phase patients harboring either wildtype BCR-ABL1 or BCR-ABL1T315I while normal CD34+ cells exhibited minimal effect. Further, treatment of CML cell lines and primary CD34+ CML cells with OP449 in combination with the ABL1 tyrosine kinase inhibitors showed significantly increased cytotoxicity as compared to each compound alone. For example, treatment of primary CD34+ CML cells with 2.5 μM OP449 or 200 nM nilotinib alone each resulted in a 50% reduction in colony formation, while combination of OP449 and nilotinib at these concentrations reduced colony formation by approximately 87%, suggesting synergistic reduction of clonogenicity (combination index: 0.195). Similar to our findings in CML cells, OP449 increased PP2A activity and suppressed growth in a dose-dependent manner in AML cell lines and primary patient samples harboring various different genetic lesions including FLT3-ITD, CSF1R overexpression, NRASQ61L, and JAK3A572V. Additionally, synergistic inhibition of these cells was observed when OP449 was combined with relevant tyrosine kinase inhibitors and chemotherapy. For example, treatment of MOLM-14 cells (FLT3-ITD) with 2.5 μM OP449 or 1 nM AC220 alone reduced cell viability by 58% and 75%, respectively; combined treatment reduced cell growth 96% (combination index: 0.723). Similarly, treatment of HL-60 cells (NRASQ61L) with 1 μM OP449 or 250 nM cytarabine alone reduced cell viability by 40% and 60%, respectively, whereas combined treatment led to a 94% reduction in viability (combination index: 0.630). Mechanistically, AML patient samples showed significantly increased SET expression compared to normal CD34+ cells, and treatment of AML cells with OP449 reduced phosphorylation of downstream ERK, STAT5, AKT and S6 ribosomal protein signaling. Finally, to evaluate OP449 antitumor efficacy in vivo, we tested OP449 (5 mg/kg intraperitoneally every 3 days) in xenograft mice bearing human HL-60 cell derived tumors. OP449 significantly inhibited tumor growth measured over time and resulted in a >2-fold reduction in tumor burden at the end of the experiment compared to vehicle-treated controls (Day 18: 1.14±0.06 g vs. 0.45±0.08 g, respectively; p<0.001). These results demonstrate the in vivo efficacy of OP449 in a murine leukemia model. Conclusions SET antagonism is selectively cytotoxic to CML and AML cells harboring various genetic lesions and drug-resistant mutations. Our results demonstrate that combined targeting of SET and tyrosine kinases provides more efficient and selective inhibition of leukemia cell growth for a broad range of oncogenic lesions as compared to normal cells. Taken together, our findings suggest a novel therapeutic paradigm of SET antagonism in combination with tyrosine kinase inhibitors for the treatment of CML and AML patients with drug resistance. Disclosures: Agarwal: Oncotide Pharmaceuticals: Research Funding. Tyner:Incyte Corporation: Research Funding. Vitek:Oncotide Pharmaceuticals: Employment. Christensen:Oncotide Pharmaceuticals: Employment. Druker:Ambit Biosciences: Consultancy, PI or co-investigator on Novartis clinical trials. OHSU and Dr. Druker have a financial interest in MolecularMD. OHSU has licensed technology used in some of these clinical trials to MolecularMD. Potential conflicts of interest are managed by OHSU., PI or co-investigator on Novartis clinical trials. OHSU and Dr. Druker have a financial interest in MolecularMD. OHSU has licensed technology used in some of these clinical trials to MolecularMD. Potential conflicts of interest are managed by OHSU. Other; Bristol-Myers Squibb/Novartis: Currently PI or co-I on Novartis & Bristol-Myers Squibb clinical trials. His institution has contracts with these companies to pay for patient costs, nurse and data manager salaries, and institutional overhead. He does not derive salary, nor does his lab Other; Oncotide Pharmaceuticals: Research Funding, Subaward from NIH STTR, Subaward from NIH STTR Other.