Anti-leukemic activity of valproic acid and imatinib mesylate on human Ph+ ALL and CML cells in vitro.

The armamentarium of anti-leukemic drugs has increased substantially since anti-leukemic activities were recently found for a variety of non-classical cytostatic drugs, among them the histone deacetylase (HDAC) inhibitor valproic acid (VPA). This study investigated the effect of VPA on proliferation and apoptosis of human Philadelphia chromosome-positive (Ph+) acute lymphatic (ALL) and chronic myeloid leukemia (CML) cells and on colony formation of human chronic-phase CML progenitor cells. Strong anti-proliferative and pro-apoptotic effects of VPA were observed on human ALL and CML cell lines at concentrations achievable in vivo. These effects were most pronounced in ALL cell lines as well as in primary ALL cells. Notably, VPA revealed enhanced activity with imatinib mesylate, nilotinib, the farnesyl transferase inhibitor SCH66336, interferon-alpha and cytosine arabinoside. VPA inhibited the growth of colony-forming cells from 12 Ph+ chronic-phase CML patients but also of those from normal healthy controls in a dose-dependent fashion. HDAC-inhibiting activity of VPA was confirmed on ALL and CML cells. In conclusion, VPA, whether alone or in combination with other non-classical anti-leukemic compounds, exerts significant anti-leukemic effects on human ALL and CML cells.