Givinostat and hydroxyurea synergize in vitro to induce apoptosis of cells from JAK2V617F myeloproliferative neoplasm patients

We investigated whether clinically achievable concentrations of the histone deacetylase (HDAC) inhibitors givinostat and hydroxyurea induce synergistic cytotoxicity in Jak2V617F cells in vitro and through which possible mechanism. Givinostat and hydroxyurea at low doses potentiated the pro-apoptotic effects of each other in the Jak2V617F HEL and UKE1 cell lines. Givinostat induced 6.8%-20.8% and hydroxyurea (HU) 20.4%-42.4% cell death alone and 35.8%-75.3% in combination. The effect was statistically significant using the median effect Chou-Talalay method, resulting in a combination index less than 1, indicating synergy. Givinostat alone induced cell cycle arrest of the cell lines in G0/G1 and hydroxyurea in S phase, whereas both drugs together led to a G1 block. At the molecular level, hydroxyurea counteracted the induction of p21CDKN1A by Givinostat and potentiated caspase 3 activation, explaining at least in part the increased apoptosis observed in presence of both compounds. We also verified the effect of the same drugs in colony assays of freshly isolated Jak2V617F polycythemia vera cells. In this case, low doses of the compounds were additive to each other. These results suggest that combined treatment with givinostat and hydroxyurea is a potential strategy for the management of Jak2V617F myeloproliferative neoplasms.