Abstract 2938: BMI-1 inhibition by PTC-209 induces mitochondrial apoptosis in acute myeloid leukemia cells

Leukemia stem cells play important roles in leukemia initiation, progression, and relapse, and represent a critical target for therapeutic intervention. BMI-1 is essential for the self-renewal of normal hematopoietic and leukemia stem cells. High expression of BMI-1 has been associated with poor prognosis in AML. PTC-209 is a novel transcriptional inhibitor of BMI-1, which has been reported to exhibit antitumor activity against cancer-initiating cells in colorectal cancer. We investigated anti-leukemia effects of the BMI-1 inhibitor PTC-209 on AML cells. A total of 8 AML cell lines (MOLM-13, OCI-AML3, MV4-11, NB4, HL-60, U-937, MOLM-14, OCI-AML2) were treated with different concentrations of PTC-209 for 48 hours and the IC50 values (concentration at which cell growth is inhibited by 50% at 48 hours of exposure) and the ED50 values (effective concentration inducing 50% killing as measured by Annexin V positivity) were determined. The IC50 values were 0.38 ± 0.07 μM (mean ± SEM), indicating high anti-proliferation effect. The ED50 values ranged from 1.2 to 2.6 μM in 7 cell lines (1.97 ± 0.22 μM) and the remaining cell line (NB4) showed relative resistance to PTC-induced apoptosis (> 10 μM). PTC-209 exhibited dose- and time-dependent anti-proliferative and cytotoxic activities, by inducing G1-S transition delay and apoptosis. PTC-209 induced conformational change of BAX (i.e., BAX activation), loss of mitochondrial membrane potential (MMP), caspase-3 activation and DNA fragmentation in addition to phosphatidylserine (PS) externalization, indicating mitochondrial apoptosis. PTC-209 induced Annexin V in primary AML cells (23.4 ± 4.6% after 48-hour treatment with 1 μM PTC-209, n = 23) in a dose-dependent manner, which is more prominent in immature CD34+CD38- population (33.7 ± 6.0%; P = 0.0036). In accordance with its high anti-leukemia activity, PTC-209 strongly reduced cellular BMI-1 levels in CD34+CD38- AML cells. Higher reduction of BMI-1 expression was positively correlated with higher susceptibility to PTC-209 in CD34+CD38- AML cells (r = 0.88; P = 0.047). The apoptotic activity was observed independent of p53 or FLT3 mutational status of the leukemia cells. Our data suggest that BMI-1 inhibition by small molecule inhibitors may be developed into a novel therapeutic strategy for AML. (Drs M. Andreeff and S. Kimura are co-senior authors with equal contribution to the work) Citation Format: Kensuke Kojima, Yuki Nishida, Aya Maeda, Dhruv Chachad, Hiroaki Kitamura, Jo Ishizawa, Michael Andreeff, Shinya Kimura. BMI-1 inhibition by PTC-209 induces mitochondrial apoptosis in acute myeloid leukemia cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2938. doi:10.1158/1538-7445.AM2015-2938