Efficacy of Vorinostat In a Murine Model of Polycythemia Vera

Abstract 629

The JAK2V617F mutation is present in most patients with polycythemia vera (PV) and 50–60% patients with essential thrombocythemia (ET), and primary myelofibrosis (PMF). Inhibitors of histone deacetylases (HDAC), which regulate gene transcription, cell cycle progression and programmed cell death, are emerging as a new class of potential anticancer drugs. A number of HDAC inhibitors are in clinical trials for various neoplasms. Vorinostat (also known as SAHA or Zolinza) was the first FDA approved HDAC inhibitor for clinical use in treating cutaneous T cell lymphoma (CTCL). In this study, we have investigated the efficacy of vorinostat in hematopoietic cells expressing JAK2V617F and in a murine model of PV. We found that proliferation of Ba/F3-EpoR cells expressing JAK2V617F or human erythroleukemia (HEL) cells harboring JAK2V617F mutation was significantly reduced upon treatment with vorinostat at 0.5–1.0 μM. However, proliferation of wild type JAK2 expressing BCR-ABL-transformed K562 cells was only modestly affected by vorinostat. Vorinostat treatment also resulted in marked apoptosis in HEL cells but not in wild-type JAK2 expressing K562 or UT7 cells as determined by annexin V staining and flow cytometry. Moreover, vorinostat treatment (at 1.0 μM) completely inhibited the clonogenic growth of Ba/F3-EpoR-JAK2V617F and HEL cells in a colony assay. Clonogenic growth of K562 cells was only modestly affected by vorinostat at 1.0 μM. Biochemical analyzes revealed significant inhibition of phosphorylation of JAK2, STAT5, Akt, Erk1/2 in JAK2V617F-positive HEL cells but not in wild-type JAK2-expressing K562 cells. Expression of JAK2 and STAT5 was also significantly downregulated by vorinostat treatment in HEL cells but not in K562 cells. In addition, we observed a marked reduction in c-Myc and Pim-1, and cleavage of caspase 3 and PARP in HEL cells, but not in K562 cells, upon treatment with vorinostat. These results suggest that vorinostat selectively targets JAK2V617F mutated cells and inhibits the activation/expression of JAK2 and STAT5. We reported previously the generation of an inducible JAK2V617F knock-in mouse (Akada et al., Blood 2010; 115: 3589–3597). Expression of the JAK2V617F in the knock-in mice reproducibly produced all the features of human PV. We have utilized this JAK2V617F knock-in mouse model to test the in vivo effects of vorinostat. We found that vorinostat treatment (200 mg/kg/day by IP injection) for 2 weeks significantly reduced the increase in red blood cells (RBC), hemoglobin and hematocrit as well as spleen size in mice expressing JAK2V617F compared with vehicle-treatment. Furthermore, vorinostat markedly inhibited the clonogenic growth of primary erythroid progenitors expressing JAK2V617F without exhibiting any significant toxicity towards wild-type JAK2-expressing normal hematopoietic progenitors in mice. Thus, our in vitro and in vivo results suggest that vorinostat may have therapeutic potential for treatment of PV.