Natural Products and Acute Myeloid Leukemia: A Review Highlighting Mechanisms of Action

Recent findings have shown great potential of alternative interventions such as immunotherapy and natural products for acute myeloid leukemia (AML). This study aims to review the anti-AML effect of various natural compounds. Natural compounds were classified into five groups: alkaloids, carotenoids, nitrogen-containing compounds, organosulfur compounds or phenolics based on each compound’s chemical properties. Fifty-eight studies were collected and reviewed in this article. Phenolics are the most abundant group to have an apoptotic effect over AML cells, while other groups have also shown significant apoptotic effects. Some compounds induced apoptosis by regulating unique mechanism like human telomerase reverse transcriptase (hTERT) or laminin receptor (67LR), while others modified caspases, poly (adp-ribose) polymerase (PARP) and p53. Further study is required to identify side-effects of potent compounds and the synergistic effects of combination of two or more natural compounds or existing conventional anti-AML drugs to treat this dreadful disease.


Introduction
Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells that infiltrate the bone marrow, blood, and other tissues [1]. Current standard intervention for AML consists of chemotherapy and stem cell transplantation. The chemotherapies include daunorubicin, idarubicin, cytarabine, and doxorubicin (showed 60% to 80% of cure rate in adults) [2]. Moreover, new drugs including vosaroxin, CPX-351, sapacitabine, SGI-110 and midostaurin are being developed for AML. Many of them are categorized into cytotoxic agents, small-molecule inhibitors, or targeted therapies [3]. Recent studies highlighted that midostaurin accompanied with standard chemotherapy significantly prolonged overall event-free survival for AML patients with a FMS-like tyrosine kinase 3 (FLT3) mutation [4]. While daunorubicin, idarubicin, cytarabine and doxorubicin remain the standard therapy for AML, the drug has been reported with a few cardiotoxic effects, a well-known risk factor for congestive heart failure [5]. AML is an incurable disease with relapse possibilities and conventional therapies have shown side-effects such as hepatotoxicity, myelosuppression, and tumor lysis syndrome [6]. Thus, we focused on discovering new potent materials for AML from natural products.
Natural products are extracted from abundant living organism sources. They include bioactive compounds that have the potential for prevention or treatment of major diseases and have been used as therapeutic medicine in human history [7][8][9]. Recently, they have continued to provide key scaffolds for drug development [8]. Many of these have been identified to demonstrate diverse biological actions, including anti-cancer activities [10,11]. Polyphyllin D from Paris polyphylla is well known for its induction of endoplasmic reticulum (ER) stress and mitochondria-mediated apoptotic pathways against lung cancer cells [12]. Resveratrol is well-known natural product that has anti-cancer effect in human cancer [13,14]. Withaferin A is a steroidal lactone from the ayurvedic plant Withania somnifera, which is reported to possess effective general anti-cancer activities [15]. Substantial studies highlight the anti-neoplastic effect of natural products, suggesting that they have the potential to become novel interventions for acute myeloid leukemia [16]. Furthermore, natural products or their bioactive compounds not only trigger apoptosis, but also lower the resistance against chemotherapies via in leukemic cells [17,18]. We classified natural compounds as alkaloids, carotenoids, nitrogen-containing compounds, organosulfur compounds, and phenolics in accordance with an early study ( Figure 1) [7]. Carotenoids and phenolics, among these, are widely studied as chemotherapy interventions. Recent studies shed more light on the role of naturally derived products in reducing oxidative stress induced by free radicals, which is involved in a wide range of chronic diseases [19]. A well-known example of natural compound is curcumin, a yellow spice which is categorized as a phenolic compound or more specifically, a polyphenol. Curcumin is known to induce apoptosis and autophagy in prostate cancer cells [20]. In this review, potent anti-AML natural compounds were classified and reviewed by a mechanism of actions.

Methods
We searched the MEDLINE database for relevant experimental literature published between September 2013 and September 2018 elucidating the apoptotic effects of natural products on specific prostate cell lines. Our search algorithm was designed based on the following criteria: each cell line including HL-60, U937, KG-1, Kasumi-1, THP-1 and their variants; related keywords such as 'apoptosis' and 'natural product' were used in the search formula. After completing the initial search, we removed duplicates, non-English literature, and studies handling multi-compound natural products such as extracts. We only took single compounds into consideration so that we could thoroughly understand how each compound exerts cytotoxicity on appropriate cells through certain signal pathways and induce apoptosis. For data unity, we have only included in vitro studies. We have classified the results into five categories, based on the phytomedicinal classification mentioned above [7]. A total of fifty-eight studies demonstrating fifty-five compounds were reviewed. We included seven alkaloids, four carotenoids, one nitrogen-containing compound, three organosulfur compounds and forty-three phenolics ( Figure 2).

Methods
We searched the MEDLINE database for relevant experimental literature published between September 2013 and September 2018 elucidating the apoptotic effects of natural products on specific prostate cell lines. Our search algorithm was designed based on the following criteria : each cell line including HL-60, U937, KG-1, Kasumi-1, THP-1 and their variants; related keywords such as 'apoptosis' and 'natural product' were used in the search formula. After completing the initial search, we removed duplicates, non-English literature, and studies handling multi-compound natural products such as extracts. We only took single compounds into consideration so that we could thoroughly understand how each compound exerts cytotoxicity on appropriate cells through certain signal pathways and induce apoptosis. For data unity, we have only included in vitro studies. We have classified the results into five categories, based on the phytomedicinal classification mentioned above [7]. A total of fifty-eight studies demonstrating fifty-five compounds were reviewed. We included seven alkaloids, four carotenoids, one nitrogen-containing compound, three organosulfur compounds and forty-three phenolics ( Figure 2).

Discussion
There are FDA approved drugs for acute myeloid leukemia (AML), which include midostaurin, liposomal cytarabine, enasidenib, gemtuzumab, and ogozamicin [82,83]. Current frontline treatment of AML is the "7 + 3" induction regimen, which includes a 7-day continuous intravenous cytarabine infusion and three daily doses of daunorubicin, especially for patients under 60 [84]. The regimen has remained as the standard cure for AML since 1973 [85,86]. Recent studies have suggested that a higher dose of cytarabine (Ara-C) and nucleoside analogue doublets may have a better effect. However, overall effect and safety of the treatment are still debatable [85,87,88]. The chemotherapies for AML can cause serious side-effects and relapse [89,90].
Targeting leukemic cells with natural products has been a big branch among various alternative attempts to treat AML. Complete cure with lower risk of relapse requires specific targeting of leukemic stem cells, and avoiding toxicity on normal hematopoietic cells. A review by Siveen et al. highlighted the various natural products that potentially target leukemic stem cells of AML. Natural products allow much lower relapse rates for AML patients [91]. Early studies have discovered many cases in which natural products exert cytotoxicity on AML, but a systematic categorization of such compounds has never been elucidated. Classifying natural products based on chemical properties can help to further understand the linkage between the chemical root of each substance and its mode of action in targeting and eliminating AML cells. Most collected natural products in this review belong to the phenolic group because phenolics are one of the most widespread substances among plants. They are involved in many different activities, including anti-Alzheimer's disease, anti-hyperglycemia, and cancer [92][93][94]. Notwithstanding such facts, it is meaningful that many natural products, well known for their significant clinical effects, such as coumarin, curcumin, ginsenosides Rh2 and shikonin, were all phenolics. Phenolic compounds could be considered to possess advantageous structures for effectively triggering apoptotic signal pathways on specific AML cells. Some compounds in other groups have also shown great potential, although it is difficult to compare their clinical effects side by side due to different controls in experiments. Some potent compounds for treating AML include aromatic tumerone and fucoxanthin in carotenoids and asterosaponin in organosulfur compounds. While supplementary carotenoid intakes are common worldwide, further studies regarding anti-AML activities and cytotoxicity of carotenoid compounds in human are necessary [95].
One significant drawback of our study is that we have only included single compounds in the analysis. Single compounds that do not belong to any of the five categories were also excluded. Further studies with regards to the synergistic effects of conventional drugs with natural products for AML treatment are strongly required. Synergistic anti-AML effects of natural product combinations and extracts should also be studied. The interactions of phytomedicines have garnered huge interest, especially because separating single compound from extracts is difficult and natural products exert strong efficacy using low amounts of core active substance mixtures [93]. At the same time, multi-target therapies have been gaining more interest in cancer, as single-targeted therapies have shown their limitations [94,95]. Such systemic view will help researchers understand how natural products offer significant cytotoxic activities to target cells without harming adjacent environments, using low dose bioactive compounds and thereby lowering adverse effects in clinical cases [96]. Taken together, the natural products with anti-AML efficacies were categorized, and the mechanisms were reviewed in this study. These natural products have the potential to be future alternative therapies for this incurable disease, AML.

Conclusions
This study reviewed the cytotoxic effects of natural products on AML cells. We categorized the natural products and their mechanisms of actions based on a phytomedicinal classification. Natural products are promising in that they have the potential to treat AML. Further animal and clinical studies should be conducted to make use of such potential materials.