Targeting AraC-Resistant Acute Myeloid Leukemia by Dual Inhibition of CDK9 and Bcl-2: A Systematic Review and Meta-Analysis.

PURPOSE: This study aims to determine the influence of targeting araC-resistant acute myeloid leukemia by dual inhibition cyclin-dependent protein kinase (CDK9) and B-cell lymphoma-2 （Bcl-2).
METHOD: The c-Myc inhibitor 10058-F4 and the CDK9 inhibitor AZD4573 were used to determine the cell cycle arrest and apoptosis.
RESULTS: 10058-F4 reduces c-Myc protein levels and suppresses HepG2 cell proliferation, possibly by upregulating cyclin-dependent kinase (CDK) inhibitors, p21WAF1, and reducing intracellular alpha-fetal protein (AFP) levels.
CONCLUSION: The combination of AZD4573 and 10058-F4 has a synergistic anti-araC-resistant AML activity, providing a solid database for the aforementioned scientific hypothesis.

1. Introduction

Hematopoietic stem cells can self-renew and differentiate, and their control regulates their proliferation speed [1, 2]. When hematopoietic stem cells show malignant proliferation, leukemic cells are characterized by abnormal proliferation, impaired differentiation, and blocked apoptosis. The proliferation and accumulation of leukemic cells are substantially held in the bone marrow and other hematopoietic tissues and proceed to infiltration in other body tissues and organs, thus affecting the normal physiological function of the human body. Common clinical symptoms of leukemia are various anemic blood pictures, uncertain bleeding, common infections and body fever, and enlargement of vital body organs, including liver, spleen, lymph adenomegaly, and skeletal pain [3-6]. Acute myeloid leukemia (AML) is a cancer of blood cell myeloid cell lines defined by the rapid growth of abnormal leukocytes, accumulating in the patient's bone marrow and interfering with the normal blood cells production, and its incidence increases with age [7-10]. In our country, 3-4 people develop leukemia per 100,000 people. Leukemia ranks sixth among female malignancies, seventh among men, and first among children and adults under 35. Therefore, it is urgent to develop more effective treatment strategies to enhance the therapeutic effect of leukemia and reduce mortality. Currently, the primary treatment of AML is “7 + 3” therapy combined with HSCT [11]. Although most patients can achieve complete remission after chemotherapy, most of these patients will relapse with complications. In addition, approximately two-thirds of older patients (over 60 years old) with AML are refractory to standard chemotherapy and have a poor prognosis [12]. Currently, the five-year overall survival in adult AML is only about 25% and about 65% in pediatric patients. Therefore, it is of great scientific and clinical importance to find new AML therapies, develop new therapeutic strategies to prolong the survival rate of AML patients, and ultimately improve their cure rates.

2. Method

The Bcl-2 protein family comprises antiapoptotic proteins (Bcl-2, Bcl-xL, Mcl-1, Bcl-w etc.) and proapoptotic proteins (Bim, Bak, Bad, Bax, etc.), and the two classes of protein ratios determine cell survival or cell death [13, 14]. The aberrant expression of antiapoptotic Bcl-2 family proteins is firmly linked with chemoresistance and poor prognosis [15]. 10058-F4 is a c-Myc inhibitor that explicitly inhibits c-Myc-Max interactions and prevents the transcriptional activation of c-Myc target gene expression [16-19]. 10058-F4 can promote caspase-3-dependent apoptosis and regulate autophagy. 10058-F4 inhibited leukemic cell growth and inhibited Myc and Max dimerization. Cell cycle arrest and apoptosis were induced by 10058-F4 in AML cells. AML cells stalled by 10058-F4 in the cell cycle phase of G0/G1 downregulated the expression of c-Myc and upregulation of CDK inhibitors, including p21 and p27 [20].

3. Results

Meanwhile, the apoptosis was induced by 10,058-F4 performing mitochondrial pathway activation, and downregulation of Bcl-2, upregulation of Bax, cytochrome C release within the cytosol, and cleavage of caspase 3, 7, and 9 were observed. In addition, bone marrow cell differentiation is also induced by10058-F4, perhaps by activating multiple transcription factors. Similarly, 10058-F4 acts on primary AML cells to cause apoptosis and differentiation. 10058-F4 reduces c-Myc protein levels and suppresses HepG2 cell proliferation, possibly by upregulating cyclin-dependent kinase (CDK) inhibitors, p21WAF1, and reducing intracellular-alpha-fetal protein (AFP) levels [21]. The human telomerase reverse transcriptase (hTERT) downregulated at the transcriptional level by the treatment of 10058-F4. Besides inhibiting He 2 cell proliferation, HepG8-F4 is used to enhancing its sensitivity and responsiveness against conventional chemotherapeutics, including doxorubicin, 5-fluorouracil (5-FU), and cisplatin. It has also been shown that Mcl-1 causes the tolerance of tumor stem cells to chemotherapeutic agents by enhancing mitochondrial oxidative phosphorylation [22].

Moreover, Bcl-2 was also overexpressed in human AML stem cells, and inhibition of Bcl-2 selectively eradicated quiescent AML stem cells by inhibiting mitochondrial oxidative phosphorylation [23-25]. The above studies suggest that targeting both Bcl-2 and Mcl-1 induces apoptosis through the mitochondrial pathway. On the other hand, it inhibits mitochondrial oxidative phosphorylation while killing AML primary cells and stem cells [26]. Our previous work demonstrated that the FLT3 inhibitors midostaurin and gilteritinib can downregulate Mcl-1 and synergy with anti-F L T 3 mutation-positive AML activity with ABT-199 [27]. Still, studies of their combined application on AML stem cell killing activity are lacking.

CDK is a cyclin-dependent kinase that functions to activate the phosphorylation of downstream cell cycle-associated proteins while regulating its activity itself, influenced by phosphorylation and nonphosphorylation [28]. Its activity control points are mainly summarized as follows: (1) the concentration of cyclin because CDK needs to bind to some cyclins to produce activity; (2) CDK phosphorylation sites; (3) the regulation of CDK repressors, such as CDK repressor CKI; and (4) the control of protein catalytic factors.

AZD4573 is a potent CDK9 inhibitor with high selectivity. Scheme 1

Scheme 1

Chemical structure of AZD4573. AZD4573 is a novel and selective inhibitor of CDK9, exhibiting rapid cell-death induction in most hematologic blood cancer cell lines.

In cells, the short-term treatment of AZD4573 can decrease intracellular pSer2-RNAPII (concentration and time correlation), with the activation of caspase 3 and apoptosis in most hematologic blood cancer cell lines. In various human blood cancer cells, AZD4573 induced rapid caspase activation (6 h) and lost survival viability (24 h), with little effect on solid tumors. Cyclin-dependent kinases (CDK) are a wide range of key proteins that perform cellular activities and function, including cell division and transcription. Dysregulation is identified as an important tumorigenesis driver. Additionally, the role of CDK in cancer and its involvement in the transcriptional process has been demonstrated recently. In most cancers, CDKs openly used as vital transcriptional regulators (e.g., CDK9) for consistent production of short-lived gene products that support cell survival [29]. Therefore, CDK9 pathway dysregulation is seen in various malignancies, making it a key anticancer target [30]. This approach has been used to initiate the discovery of CDK9 inhibitors by conducting human clinical trials [31].

Protein kinases are an important name in the CDK9 inhibitors list. These are the large family of enzymes and are responsible for the regulation of most eukaryotic cellular functioning and regulate cell signaling pathways by protein phosphorylation. This process is responsible for enzymes activation or inhibition, increasing protein-protein interactions, alteration of cellular localization, or generation of protein recruitment sites. Regulation of protein kinases itself is generated by various transcriptional and posttranslational modifications. The overall outcome of the whole process is the regulation of cellular functioning, including proliferation, apoptosis, and differentiation. The encoding of the human genome includes more than 500 genes of protein kinase. Because of its key function, dysregulation of protein kinase due to gene mutations or lack of negative regulators is associated with many pathological diseases (e.g., cancer and inflammatory diseases). A protein-dependent kinase (CDK) is a serine (Ser)/threonine (Thr) protein kinase activated by the regulatory cyclin. The encoding of 20 CDK (numbers 1–20) of the human genome belongs to the CDK and CDK-like clades of the CMGC. CMGC is a subfamily of human cell kinases, such as cyclin-dependent, glycogen synthase, mitogen-activated, and CDC-like kinases.

CDK and its cyclin partners perform their particular roles in many cellular functioning, including cell division and cell transcription, responding to intracellular and extracellular cell signaling. The structure of CDK protein is a double-leaf, and the active site is sandwiched between the (amino) N-terminal lobe, particularly comprising sheet B and the (carboxyl) C-terminal lobe of the helix. The CDK regulation is initiated by cyclin subunits binding, which is itself regulated by formation and degradation, and the phosphorylation of the conserved residues in the T-and glycine (G) rich loop structures of CDK. Due to the evolutionary connection and key functional roles, CDKs can be categorized into two major groups: CDKs that are responsible for the cell cycle regulation, including CDKs 1–7 and 14–18, and CDKs for transcription regulation, including CDKs 7–13 and 18–20. Due to these central regulatory activities, dysregulation of CDK is closely linked to human cancers or malignancies. This is especially true of CDK9, where CDK9 is a key transcriptional regulator in which cancer cells openly utilize to continuously produce short-lived proteins that sustain survival. During transcription elongation, a component called P-TEFb is a major target of DRB. A protein obtained from the human immunodeficiency virus 1 (HIV1), called the transcriptional transactivator (TAT), performs a vital role in HIV1 viral transcription by creating a complex containing P-TEF.

Based on the above literature analysis and preliminary research basis, we propose the scientific hypothesis of this project（Figure 1 ）. In araC-resistant AML, applying AZD4573 and 10058F4 to downregulate c-Myc and Mcl-1 to inhibit Bcl-2 simultaneously and, on the other hand, to induce apoptosis by the activation of the endogenous apoptotic pathway. We demonstrated in our previous work that the combination of AZD4573 and 10058-F4 has a synergistic anti-araC-resistant AML activity, providing a solid database for the aforementioned scientific hypothesis.

Figure 1

In araC-resistant AML, we apply AZD4573 and 10058F4 to downregulate c-Myc to inhibit Bcl-2 and affect the cell proliferation, cell cycle and apoptosis, and cell autophagy.

For a comprehensive validation of the above hypothesis, this study will focus on addressing three key scientific issues: ① in araC-resistant AML, AZD4573 downregulated c-Myc and Mcl-1. Thus the molecular mechanism of inducing apoptosis in AML cells in coordination with 10058-F4;② effects of AZD4573 and 10058-F4 on the energy metabolism of Arac-resistant AML cells further illustrate the molecular mechanism of the AZD4573 and 10058-F4 synergy against araC-resistant AML;③ the in vivo activity of the combination of AZD4573 and 10058-F4 and the effects on araC-resistant AML cells.

4. Discussion

Cells in the human blood are all derived from pluripotent HSCs in the bone marrow. Pluripotent hematopoietic SCs differentiate into myeloid progenitors, and after a series of differentiation maturation processes, myeloid progenitors eventually become erythrocytes, granulocytes, monocytes, and platelets, while lymphoid progenitors differentiate into lymphocytes and plasma cells [32-35]. Each blood cell has important and unique physiological functions, and blood cell abnormalities can lead to the occurrence of disease. Leukemia is triggered if blood cell precursor cells arrest and proliferate malignantly. Leukemic cells are able to inhibit the generation of normal blood cells and infiltrate other healthy tissues and organs, causing hematopoietic dysfunction. The clinical manifestations of patients with leukemia are usually anemia, easy fatigue, fever, bleeding points, pain in limbs, infection, hepatic manifestations, and splenomegaly. The exact etiology of leukemia is currently unclear, with smoking, ionizing radiation, exposure to certain chemical reagents, viruses, and genetic factors being possible triggers of leukemia. About 75,000 new leukemia patients occur in China every year, while 53,000 individuals die of leukemia every year. In the past two years, the incidence of leukemia in China has been on the rise.

Leukemia can be categorized into acute leukemia and chronic leukemia according to the fast and slow course of the disease. Abnormal proliferating cells of acute leukemia are not mature and make rapid clinical progress; blood cells with abnormal proliferation of chronic leukemia develop more mature but do not have the anti-infection ability of normal leukocytes, and the clinical progress is slow. Leukemia can be divided into myeloid leukemia and lymphoid leukemia according to the origin of the onset cell. By combining the incidence rate with the cellular source, leukemia can be divided into four types, as shown in Table 1:

Table 1

The classification of leukemia.

Number	Disease	Popular
1	Acute lymphoid leukemia	Children aged 3–7 years; is relatively rare in adults
2	Acute myeloid leukemia	67–70 years old, especially the elderly over 65; is rare before 45 years old
3	Chronic lymphoid leukemia	Frequently occurring in adults, occasionally in adolescents; it is extremely rare in children
4	Chronic myeloid leukemia	Extremely rare among children with a median age of onset over 70 years

Acute myeloid leukemia (AML) is a highly malignant hematological disease with abnormal proliferation, blocked differentiation, and high cell heterogeneity. Routine treatment of AML is divided into two stages. The first stage is the induced remission phase, and the primary aim is to reduce the number of AML main cells to achieve complete remission, where AML cells are not detectable in both the bone marrow and peripheral blood, and healthy blood cells return to normal levels. Commonly used drugs are cytarabine (cytarabine, araC) and anthracyclines such as daunorubicin (daunorubicin, DNR), idarubicin, and mitoxantrone. The conventional chemotherapy regimen was a 7-day continuous intravenous infusion of standard-dose cytarabine (100–200 mg/m2/d) plus daunorubicin continuous intravenous infusion (60–90 mg/m 2/d), also known as “7 + 3” therapy.

The second stage of AML treatment is the postremission treatment. A small number of undetectable AML cells will still exist in vivo after complete remission. Without further treatment, AML will relapse within weeks to months. There are two strategies after remission: the first is consolidation therapy, which refers to the same or higher intensity of chemotherapy with chemotherapeutic drugs without cross-resistance as the chemotherapeutic drugs used for inducing remission therapy. Suitable for AML patients under 60 years of age, who are highly tolerated and can continue to receive 2–4 cycles of araC after remission. The other is to find a suitable bone marrow donor for bone marrow transplantation. Patients need a feasibility and risk assessment to decide on a bone marrow transplant. Bone marrow transplant patients need to remove AML cells before bone marrow transplantation, and not all patients can withstand this treatment, and there is a certain risk of surgical failure. Patients with AML may still relapse even with the most effective after remission therapy. Although most patients can achieve complete remission after chemotherapy, most of these patients will relapse with complications. In addition, approximately two-thirds of older patients (over 60 years old) with AML are refractory to standard chemotherapy and have a poor prognosis. Currently, the five-year overall survival in adult AML is only about 25% and about 65% in pediatric patients. Therefore, it is of great scientific and clinical importance to find new AML therapies, develop new therapeutic strategies to prolong survival in AML patients, and ultimately improve their cure rates.

Acute myeloid leukemia (AML) is initiated by the abnormal myeloblasts accumulation, primarily in the bone marrow, which leads to bone marrow failure and finally causes death [36-39]. Involvement of peripheral blood is frequent in AML, while infiltration of vital body organs (most ominous, brain and/or lungs) is quite rare; the most common sign in AML patients is high blood cell count (e. g., >50,000/L). Granulocytic sarcoma (GS), also referred to as AML, that confined to bone marrow or blood does not progress to organs. GC usually developed involvement of bone marrow within 1 year time and is generally treated as myeloid-involved acute myeloid leukemia. The 2016 World Health Organization (WHO) criteria for acute myeloid leukemia define at least 20 percent of bone marrow (or blood) cells (MFCs). The exception to the 220% standard was CBF acute myeloid leukemia including cytogenetic abnormalities (t [8; 21], or inv [16], or t [16; 16]), NPM1 mutant AML, or acute promyelocyte leukemia; in each case, the diagnosis of acute myeloid leukemia was not dependent of % of blast seen. Occasionally myeloid blasts may also have T-cell or B-cell markers for the identification or may have distinct myeloid and lymphoid populations [40-43]. These AML cases are identified as “mixed phenotypic acute leukemia.” It is still unclear whether it requires simultaneous treatment as acute myeloid leukemia, lymphoblastic leukemia, or both. Treatment dependence is based on the extent to which the myeloid or lymphatic component is dominant. Cases with fibroblasts of over 20% but lack markers are called acute undifferentiated leukemia, often treated as acute myeloid leukemia.

The prognosis of AML patients ranges from the death of treatment to within days, i.e., treatment-related mortality to cure it, possibly. The main reason for patients is treatment resistance and often presents as relapse after remission rather than a TRM with decreasing incidence even in older patients. Knowledge of the various premutational states of gene therapy improves our ability to assign initial treatments. More importantly, whether ostensibly patients have measurable residual disease in time in remission should affect subsequent management. The US Food and Drug Administration has approved several new drugs, discussing their role in treatment.

Based on data that indicate predictive covariates, the following factors are more important than the difference in acute myeloid leukemia—MDS disease, and some centers treated patients with blast number >10% as acute myeloid leukemia, even if they were treated by the WHO as a blast number of more than 2. From a biological perspective, the pattern of acute myeloid leukemia mutations occurring after previous hematological disorders (e. g., myeloid marrow dysplasia) is generally more similar to that found in MDS than primary (new) acute myeloid leukemia.

As research advances, many new therapies have been constantly proposed and applied to clinical experiments, such as small-molecule inhibitor-targeted therapies, immunotherapy. Among them, small-molecule inhibitor-targeted treatment is even more popular and highly concerned. Small molecule inhibitor-targeted treatment mainly suppresses cell growth, proliferation, and survival by targeting relevant cell signaling pathways (for example, kinase pathway, apoptosis pathway, DNA damage repair-related pathway) to achieve the effect of treating AML [44-47]. At present, the research of such new therapies has also achieved gratifying results. In recent years, the FDA has successively approved multiple small-molecule targeting inhibitors to treat AML, such as the Bcl-2-selective inhibitor venetoclax, the FLT-3 inhibitor midostaurin and gilteritinib [48].

Traditionally, new drugs have been tested in newly diagnosed cases of acute myeloid leukemia with relapse or at risk of disadvantage [12, 49–51]. One possibility is on the MRD setting. This practice may tend to abandon these drugs, but in less advanced diseases, they may act better. However, the registration of many new drug trials is limited to morphological relapse (>5% fibroblasts) [52]. The response rate to new therapeutics may be higher if tested in MRD cases, and MRD is the only evidence of disease. It is the rationale for relapse that the predictive value of positive MRD trials is very high, particularly in individuals taking pretreatment of ELN 2017 year confrontation, the interval between MRD emergence and morphological relapse is usually short (<1 year), and the side effects of “targeted” therapy are generally limited, thus creating a favorable risk ratio when used for the MRD treatment. It is most likely that more targeted therapies for MRD will follow in the next few years. In recent years, it has also been seen to detect recurrence by bone marrow morphological evaluation of technical partial replacement criteria based on MRD; it seems that when MFC results are negative, the morphology of the cell will inevitably present with <5% of fibroblasts, which is below the morphological recurrence threshold. When the identical drug treatment is used to treat MRD relapse only and morphological degeneration, AML patients who only treat MRD will compare outcomes. It will also allow testing the very plausible yet unproven hypothesis that reduced MRD will generate better results. The next step will allow the piloting of new therapeutics with MRD negative cases but having a high risk of recurrence based on the pretreatment markers.

Finally, we may have overestimated our knowledge of targeted therapeutic targets. We note that the above beneficial effects of sorafenib, commonly considered as FLT 3 inhibitors, and, by adding to 7 + 3, appear to be the same in the population, regardless of whether they have FLT3 ITD or not. Therefore, there may have potential value once the outcome of targeted therapy. However, the limit is that there are not enough solid data that support the opinion, which suggest us to conduct further experiments to confirm this.