Leukemia stem cells are resistant to chemotherapeutic agents and contribute to disease relapse. BMI-1, a part of polycomb repressive complex 1 (PRC1) is essential for the self-renewal of normal hematopoietic and leukemia stem cells. We have reported that acute myeloid leukemia (AML) patients with higher levels of BMI-1 have worse overall survival and that the translational BMI-1 inhibitor PTC-209 induces apoptosis in patient-derived CD34+CD38low/- AML cells. PTC596 is a second-generation BMI-1 inhibitor that accelerates BMI-1 degradation. Based on preclinical data showing excellent anti-tumor activities against solid cancers, PTC596 is currently in PH1 clinical development. We here investigated preclinical activities of PTC596 against acute leukemias in vitro and in vivo. A total of 6 AML (MOLM-13, OCI-AML3, U-937, MV4-11, MOLM-14 and HL60) and 3 acute lymphoblastic leukemia (ALL) (Reh, NALM6 and MOLT-4) cell lines were exposed to PTC596 for 48 hours. PTC596 exhibited dose- and time-dependent anti-proliferative and cytotoxic (apoptotic) activities. The IC50 values (concentration at which cell growth is inhibited by 50% at 48 hours of exposure) were 30.7 ± 4.1 nM (mean ± SEM) for AML and 37.7 ± 8.0 nM for ALL. The ED50 values (effective concentration inducing 50% killing as measured by Annexin V/PI induction) were 60.3 ± 6.7 nM for AML and 77.8 ± 11.5 nM for ALL. PTC596 induced apoptosis in AML cells irrespective of their p53 status. 72-hour treatment of MOLM-13 and OCI-AML3 cells with 100nM PTC596 reduced total BMI-1 protein levels by 87% and 61%, respectively. Forced overexpression of BMI-1 in K562 cells abrogated PTC596-induced apoptosis (∼ 80% at 50 nM PTC596; P < 0.05). 16-hour treatment of MOLM-13 cells with 200nM PTC596 resulted in reduced levels of non-phosphorylated BMI-1 by 85%, along with increased levels of phosphorylated BMI-1. PTC596 reduced the levels of ubiquitylated histone H2A, which indicates a deactivation of the PRC1 complex function. The anti-leukemia activity of PTC596 was tested in xenograft (K562) and syngeneic (L210) mouse models. PTC596 at a daily dose of 20 mg/kg significantly delayed an enlargement of the implanted tumor of K562 by ∼55%. PTC596 significantly prolonged survival of mice injected with syngeneic L210 leukemia cells (27 versus 37 days; 76% extension of lifespan with a median survival time; P < 0.05). Collectively, BMI-1 inhibition by PTC596 offers a novel therapeutic strategy for acute leukemias.

Citation Format: Yuki Nishida, Aya Maeda, Liangxian Cao, Melissa Dumble, Shinya Kimura, Thomas W. Davis, Kensuke Kojima. Targeting BMI-1 by the novel BMI-1 inhibitor PTC596 in acute leukemia. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A130.