Anti-Leukemic Effects of Venetoclax on Philadelphia Chromosome Positive Leukemia Cells

Introduction: Chronic myeloid leukemia (CML) is characterized by the t(9:22) translocation known as the Philadelphia chromosome (Ph). Although ABL tyrosine kinase inhibitors (ABL TKI) such as imatinib, dasatinib and nilotinib have improved CML treatment, such therapies cannot cure patients with Philadelphia chromosome (Ph)-positive leukemia because of leukemia stem cells. Moreover, some patients develop BCR-ABL point mutations including T315I and become resistant to ABL TKI therapy. These leukemia stem cells are contained within a niche in the bone marrow and are often impervious to current treatments. Therefore, new approach against BCR-ABL mutant cells and LSCs may improve the outcome of Ph-positive leukemia patients. B cell lymphoma 2 (BCL-2) protein families are key regulator of apoptosis and highly promising targets for the development of anti-cancer treatment. Venetoclax, also known as ABT-199 is a selective, orally bioavailable BCL-2 inhibitor. Venetoclax is investigated in a pivotal phase 3 clinical trial against hematological malignancies such as chronic lymphocytic leukemia (CLL) and approved for the treatment of patients with CLL.

Materials and methods: In this study, we investigated whether venetoclax could suppress Ph-positive leukemia cells including T315I mutation and primary samples.

Results: BCL-2 expression was found in Ph-positive leukemia cells including primary samples, however, BCL-2 expression was reduced in K562 cells. We found 72 h venetoclax treatment inhibited the growth of Ba/F3 BCR-ABL and KCL-22 cells in a dose dependent manner. However, venetoclax activity was reduced in K562 cells. We examined the intracellular signaling after treatment of venetoclax. Phosphorylation of BCR-ABL and Crk-L was not reduced. However, activity of caspase 3, poly (ADP-ribose) polymerase (PARP) was increased. We next investigated the efficacy between ABL TKI and venetoclax by using these cell line. Combined treatment of Ba/F3 BCR-ABL cells with imatinib and venetoclax caused significantly more cytotoxicity than each drug alone. Apoptotic cells were also increased. Phosphorylation of BCR-ABL, Crk-L was reduced and cleaved caspase 3 and PARP activity was increased after imatinib and venetoclax treatment. We investigated the venetoclax activity against T315I positive cells. Venetoclax potently induced cell growth inhibition of Ba/F3 T315I mutant cells in a dose dependent manner. Combined treatment of Ba/F3 T315I mutant cells with ponatinib and venetoclax caused significantly more cytotoxicity than each drug alone. Apoptotic cells were also increased. Phosphorylation of BCR-ABL, Crk-L was reduced and cleaved PARP was increased after ponatinib and venetoclax treatment. To assess the activity of ponatinib and venetoclax, we examined tumor formation in mice model. We injected subcutaneously 1×10⁷ Ba/F3 T315I mutant cells in nude mice. A dose of 20 mg/kg/day p.o of ponatinib and 50 mg/kg/day p.o of venetoclax inhibited tumor growth and reduced tumor volume compared with control mice. In the immunohistochemical analysis, we found that tumors in mice treated with ponatinib and venetoclax exhibited an increase in apoptotic cells. We also found that co-treatment with ponatinib and venetoclax increased mouse survival. The treatments were well tolerated with no animal health concerns observed. We also found that the treatment of venetoclax exhibits cell growth inhibition against CD34 positive CML samples.

Conclusion: The results of our study indicate that the BCL-2 inhibitor venetoclax may be a powerful strategy against ABL TKI resistant cells including T315I mutation and enhance cytotoxic effects of ABL TKI against those Ph-positive leukemia cells.