Synergism between arsenic trioxide and aclacinomycin in acute myeloid leukemia

Acute myeloid leukemia (AML) is a genetically heterogeneous disease with aggressive treatment options comprised of different treatment phases depending on disease stage [1], such as remission induction therapy aimed to eliminate actively proliferating tumor cells and post-remission therapy aimed to prevent relapse. Despite advances in the understanding of the disease and treatment options, the expected remission rate is inversely related to age, particularly for patients over the age of 60 [2]. Therefore, there is increased interest in the investigation of novel combination therapies that could result in better outcomes. Arsenic trioxide (ATO) is a potent pro-differentiating and pro-apoptotic drug used in the treatment of acute promyelocytic leukemia (APL) cells [3]. It is usually given in combination with all-trans-retinoic acid (ATRA), either during induction therapy or in the course of the consolidation phase [4]. The limitation in the use of ATO as an anti-cancer agent is the multitude of toxic side-effects, such as gastrointestinal and cardiac toxicity, particularly at higher doses and/or upon long-term exposure. This agent has been shown to affect a plethora of intercellular signal pathways, including being a potent inducer of apoptosis by elevating intracellular H2O2 or increasing expression of BAX as two of the possible mechanism of actions [5]. Additionally, combination of ATO with other compounds enhances clinical therapeutic outcomes [6]. Aclacinomyin (ACM) is an anti-tumor antibiotic with cytotoxic effects due to its DNA intercalating activity and topoisomerase I and II enzyme inhibition [7]. In solid tumors, ACM was found to act as a radiosensitizer [8]. In hematopoietic malignancies, ACM induced erythroid differentiation of the erythroleukemia CML cell line K562 [9]. In a recent clinical trial on refractory AML patients, a drug combination including ACM achieved higher levels of complete response and improved overall survival with acceptable cytotoxic effects [10]. In this issue of Leukemia and Lymphoma, Ye et al. [11] identify a synergistic effect by the combination of low dose Arsenic Trioxide with the antibiotic aclacinomycin in human models of AML. They show that a combination of Arsenic trioxide with aclacinomycin leads to increased apoptosis compared to drug alone and their effects are synergistic. This is achieved even at low ATO (0.4 mM) and ACM (10 nM) concentrations with a combination index (CI)  0.5 in acute myeloid leukemia cell lines (KG-1a and HL-60). The effect was also evidenced by the decrease of pro-apoptotic markers Bcl-2, c-IAP and XIAP and increase of the pro-apoptotic proteins caspase-3 and SMAC. Similarly G0/G1 arrest and S phase induction was observed, adding cell cycle arrest as an additional mechanism of suppression exerted by both drugs. Interestingly, compared to proliferative AML cells, minimal apoptosis was recorded in peripheral blood mononuclear cells (PBMNCs) with the combination treatments. For the first time, Ye et al. [11] show synergistic anti-cancer events upon combination of ATO with aclacinomycin. Combination therapy could prove useful in both treatment phases of AML. The enhanced apoptosis observed by combination of aclacinomycin with ATO is potentially applicable to the induction phase of treatment, while the involvement of both drugs in differentiation of AML cells could be applied to prevent relapse by differentiating the remaining stem cells. As previous studies have shown that both drugs separately induce differentiation of AML cells [12,13], the synergy reported by Ye et al. [11] establishes the foundation of further research in the combinational effect of ATO and aclacinomycin, not only in the pro-apoptotic effects, but also any potential differentiation of AML cells, particularly in the case of ATRA-resistance development. Clearly more experiments and data are needed to better understand the mechanism of action of either drug alone and in combination with each other. The inter-play of cell death and cell cycle pathways could be further explored, since the authors suggested AMC-induced apoptosis may be regulated independently of apoptosis-related signals and more associated directly with the cell cycle arrest process. Furthermore, other cell death pathways could be investigated since ATO has been shown to induce autophagy in leukemic cells; however, minimal studies have examined the role of AMC in