Abstract 1884: Treatment with Hedgehog inhibitor PF-913 attenuates leukemia-initiation potential in acute myeloid leukemia cells

Abstract

Abstract Aberrant activation of the Hedgehog (Hh) signaling pathway is involved in a variety of cancers, and required for maintenance of the leukemic stem cell (LSC) populations in several experimental systems. Cumulative evidence suggests that dormant self-renewing LSC contribute to acute myeloid leukemia (AML) propagation and relapse by evading conventional chemotherapies that target cycling cells. PF-04449913 (PF-913) is a novel oral small molecule inhibitor that selectively binds and targets Smoothened (SMO), a membrane protein regulating the Hh pathway. Treatment with PF-913 has shown promising results regarding safety, tolerability, and early signs of efficacy in patients with hematologic malignancies including AML. However, the detailed mechanisms and biomarkers remain to be elucidated in AML therapy. Using the co-culturing system with HS-5 stromal cells, the colony assay system, and the immunodeficient NOD/SCID/IL2rγnull (NOG) mouse model serially xenotransplanted with primary AML cells, we examined the effects of PF-913 on LSC population and AML propagation. In primary AML cells, the Hh signaling pathway was activated more in CD34-positive cells than CD34-negative cells. Ex vivo-treatment with PF-913 inhibited proliferation and induced minimal cell death in leukemia cell lines and primary AML cells. However, in vivo-treatment with PF-913 attenuated leukemia-initiation potential in AML cells through the serial transplantation system (0% human-CD45+ cells in bone marrow cells derived from the 2nd recipient mice), while limiting reduction of tumor burden in the primary leukemia system. Also in the colony-assay system using primary AML cells, treatment with PF-913 reduced serially colony formation. In leukemia cell lines and primary AML cells, treatment with PF-913 decreased the quiescent (Hoechst-33342low/Pyronin-Ylow) cell population as well as down-regulated mRNA encoding downstream effector GLIs and GLI-targeting molecules in the canonical Hh pathway. In the in vivo-NOG mouse system, comprehensive Gene Set Enrichment Analysis (GSEA) revealed that PF-913 treatment modulated cell cycle regulation and self-renewal signaling in primary AML cells. Moreover, combined treatment with PF-913 abrogated resistance to Ara-C in AML cell lines co-cultured with HS-5 stromal cells and sensitized AML cells to Ara-C in the cutaneous tumor system. Our findings imply that selective Hh inhibitor, PF-913 treatment can attenuate the leukemia-initiation potential in AML cells by modulation of cell cycle regulation and self-renewal signaling, and can also improve AML therapy through sensitizing dormant LSC to chemotherapy and overcoming the resistance in the bone marrow microenvironment. Citation Format: Yosuke Minami, Nobuaki Fukushima, Anil Sadarangani, Hironobu Minami, Catriona Jamieson, Tomoki Naoe. Treatment with Hedgehog inhibitor PF-913 attenuates leukemia-initiation potential in acute myeloid leukemia cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1884. doi:10.1158/1538-7445.AM2014-1884