Identification of 6-Benzylthioinosine as a Myeloid Leukemia Differentiation–Inducing Compound

Abstract

As the pathophysiology of acute myelogenous leukemia (AML) involves a block of myeloid maturation, a desirable therapeutic strategy is to induce leukemic cell maturation to increase the efficacy and to avoid the side effects of traditional chemotherapeutics. Through a compound library screen, 6-benzylthioinosine (6BT) was identified as a promising differentiation-inducing agent. 6BT induces monocytic differentiation of myeloid leukemia cell lines such as HL-60 and OCI-AML3, as well as primary patient samples as evidenced by morphology, immunophenotyping, and nitroblue tetrazolium reduction. Not only can 6BT induce differentiation but a subset of AML cell lines such as MV4-11 and HNT34 instead undergo 6BT-mediated cell death. Despite inducing cell death in some leukemic cells, 6BT exhibits extremely low toxicity on several nonmalignant cells such as fibroblasts, normal bone marrow, and endothelial cells. This toxicity profile may relate to the function of 6BT as an inhibitor of the nucleoside transporter, ent1, which is thought to prevent it from entering many cell types. In contrast, 6BT likely enters at least some leukemic cell lines as shown by its requirement for phosphorylation for its differentiation activity. 6BT is also able to synergize with currently used myeloid differentiation agents such as ATRA and decitabine. Early studies indicate that the mechanism of action of this compound may involve ATP depletion that leads to growth inhibition and subsequent differentiation. Besides in vitro activity, 6BT also shows the ability to impair HL-60 and MV4-11 tumor growth in nude mice. 6BT is a promising new monocytic differentiation agent with apparent leukemic cell-specific activity. [Cancer Res 2008;68(11):4369-76]

ATP Depletion Triggers Acute Myeloid Leukemia Differentiation through an ATR/Chk1 Protein-dependent and p53 Protein-independent Pathway

“…Although p53 is a major downstream effector of DNA damage signaling, the fact that p53 is not essential for 6BT-mediated differentiation is consistent with the fact that pifithrin does not impair 6BT-mediated differentiation and that 6BT can induce differentiation in p53-null AML cell lines (4). As the majority of cancer chemotherapeutics depend upon functional p53 for their optimal activity, this finding suggests that 6BT may be particularly useful for patients whose leukemic cells do not express p53 or exhibit mutant p53 (16,17).…”

“…1C). Although limited DNA damage is induced compared with a standard DNA-damaging agent, doxorubicin, 6BT treatment at the doses used does not lead to significant cytotoxicity (4). In addition, to further confirm DNA damage induction by 6BT, the comet assay, which is another sensitive method to measure DNA damage, was performed (Fig.…”

“…During our characterization of its activities, we identified that 6BT induces ATP depletion (4). Utilizing 6BT, we demonstrate here that ATP depletion can induce DNA damage signaling in AML cells and that this DNA damage signaling can play a major role in inducing AML differentiation.…”

Axl, a prognostic and therapeutic target in acute myeloid leukemia mediates paracrine crosstalk of leukemia cells with bone marrow stroma

“…Subcutaneous human AML FLT3 ITD 1 MV4-11 xenografts 45 displayed inhibition of AML tumor growth and survival of systemic FLT3 WT Hoxa9/Meis1-bearing AML mice 26 was significantly prolonged upon treatment with BGB324 ( Figure 5A-B). Remarkably, the highest dose caused regression of established MV4-11 tumors by 50.4 6 22.9% ( Figure 5A).…”

Antineoplastic activity of ouabain and pyrithione zinc in acute myeloid leukemia

“…Several high-throughput screening approaches have been undertaken to identify novel antileukemic agents for the treatment of AML. One of these strategies consists of culturing AML cells in the presence of compounds from chemical libraries and measuring cytotoxic effects by the decline of metabolic activity, by performing cell death assays (for instance with cytofluorometry) (Edwards et al, 2009;Kepp et al, 2011) or by assessing cellular differentiation, which also constitutes a therapeutic endpoint (Wald et al, 2008).…”