OncoHist^®, an rh Histone 1.3, Is Cytotoxic to Acute Myeloid Leukemia Cells and Results in Altered Downstream Signaling

Acute myeloid leukemia (AML) is a heterogeneous myeloid stem cell disorder; subtypes, which can be defined histologically, cytogenetically, and genetically, have varying prognoses and clinical characteristics. However, all but those younger patients with favorable biological features are inherently resistant to available cytotoxic chemotherapeutic agents; novel and less toxic therapies are required. Studies have shown that exposure of cancer cells, including leukemic blasts, to histone H1 produce cytotoxicity. The mechanism of cytotoxicity is believed to be an increase in focal membrane permeability induced by histone H1 binding to phosphotidyl serine moieties. However, the precise mechanism and functional effects of histone H1 on AML cells is currently unclear. The present study was aimed at investigating the effect of recombinant human Histone 1.3, OncoHist^® (Xenetic Biosciences), on the proliferation of AML cell lines and on primary AML cells. We demonstrate that purified OncoHist® exerts growth inhibition and induces necrosis by flow cytometry assessment of staining for propidium iodide (PI) of AML cell lines (MOLM14, MV4-11, U937, HL60) as well as primary AML (n=3, including one with a p53 mutation) cells independent of lineage, stage and maturation with an IC₅₀ of 2-5 mM. An AML cell line (MOLM13) with a FLT3 activating mutation (length or internal tandem duplication) which was developed to express resistance to the FLT3 inhibitor midostaurin/PKC412 was sensitive to OncoHist^®-induced growth arrest measured by trypan blue staining and death (flow for PI) with about 70-80% necrosis at 5mM. Furthermore, 2uM OncoHist^® treatment of MV4-11 and MOLM-14 cells for 15 minutes was associated with inhibition of the FLT3 downstream effectors phospho-AKT and phospho-extra cellular regulated kinase (phospho-ERK) as assessed by immunoblotting. Moreover, treatment of MOLM14 cells with OncoHist^® in combination with cytarabine was associated with a significant synergistic inhibition of growth as measured by Alamar blue staining with a CI index of 0.35. Our findings support the development of OncoHist^® alone and in combination with chemotherapy for the treatment of AML. A Phase I trial of OncoHist^® for the treatment of refractory/relapsed AML is planned.