Acute Myeloid Leukemia: Review and Current Update

Sign and Symptoms

The signs and symptoms of AML are related to variety of other, less serious diseases. To feel a loss of well-being due to the underproduction of normal bone marrow cells is common for people with AML. The person may more prone to have shortness of breath while having normal physical activities. AML patients also have, pale complexion from anemia, Signs of bleeding caused by a really low platelet count, including:

• Black-and-blue marks or bruises on the skin
• Looks of pinhead-sized red spots on the skin, called “petechiae”

• Abnormally Prolonged bleeding from minor cuts
• Mild fever
• Swollen gums
• Frequent minor infections, such as perianal sores
• Loss of appetite and weight loss
• Discomfort in bones or joints
• Enlarged spleen and liver etc. Bleeding: Bleeding causes low platelet count predisposes patients to further bleed. Bleeding within the brain or lung is serious and may be fatal. However, such bleeding is typically preceded by minor bleeding, like nose bleeds, blood within the urine or bruises. Infection: Severe infection can occur at the time of diagnosis but becomes more common and sometimes more serious during treatment, when the bone marrow is totally suppressed. Because of AML or its treatment, if the Neutrophils cell count becomes or remains low, serious infection most commonly occurs and becomes cause of death from AML. Myeloid Sarcoma: Very uncommonly, a bunch of AML cells, called a “myeloid sarcoma,” originates outside the marrow. A myeloid sarcoma may occur in almost any a part of the body. After the initial myeloid sarcoma diagnosis AML signs might not appear within blood and marrow for few weeks or months. A myeloid sarcoma diagnosis is like a diagnosis of AML, and is treated with chemotherapy instead of local therapy. Treatment can also include allogeneic or autologous somatic cell transplant. “chloroma,” “granulocytic sarcoma,” “myeloblastoma” or “monocytoma” are other names for myeloid sarcoma.

Pathophysiology of Acute Myeloid Leukemia

It appears as a denovo malignancy in previously healthy individuals in majority of cases. Regardless of various etiologies, pathogenesis of AML involves the abnormal proliferation and differentiation of a clonal population of myeloid stem cells. The formation of chimeric proteins (RUNX1-RUNX1T1 and PML-RARA, respectively) are due to well-characterized chromosomal translocations, such as t (8:21) in core-binding factor AML (CBF-AML) or t (15:17) in acute promyelocytic leukemia (APL), which changes to the abnormal maturation process of myeloid precursor cells. In the development of AML instead of addition to large chromosomal rearrangements, molecular changes have also been implicated. In fact, More than 97% cases represent genetic mutations often in the absence of any large chromosomal abnormality [5].

Table1: The molecular lesions in acute myeloid leukemia (AML) associated with malignant characteristics.

The molecular pathogenesis of AML is complex, but multiple genetic defects are depicted in the (Table 1) which shows the molecular lesions in acute myeloid leukemia (AML) associated with malignant characteristics. There is a different correlation in some cases between the molecular defect and the biological process, while in other cases there may be an undetermined explanation for the molecular basis of the disease. By a more explained understanding of these chemical links we anticipate that more precise and specific therapies for AML can be developed. Whereas by current investigational therapies, the differentiation block and proliferative activity of AML is being targeted, by current investigational therapies we try to restore genomic stability, try to induce specific apoptosis of leukemia cells and try to restore cell cycle checkpoint control, should be given strong consideration in the development of future experimental therapeutic efforts [6].

Classification of Acute Myeloid Leukemia

Acute Myeloid Leukemias are divided by World Health Organization (WHO) [7] and the WHO classification is Table 2 and also FAB French-American-British [8] classification is also there for AML which is in Table 2 below.

Myeloblastic leukaemia with minimal differentiation is difficult to diagnose from the morphology only because the blasts represent both lymphoblastic and Myeloblastic features. They account for only 5% of adult AMLs.

In acute myeloblastic leukaemia without maturation, cells are monomorphic, very commonly with myeloid blasts in peripheral blood and no cells beyond the myeloblast stage of differentiation. Leukocytes are present at diagnosis.

Acute promyelocytic leukaemia is usually presents with low peripheral blood leucocyte levels, which makes difficult to diagnose it. A very characteristic morphology called atypical promyelocytes (hypergranular cells), majority of cells have. The cytogenic change characteristic of AML-M3 is t (15;17), which leads to the fusion of the PML (promyelocytic leukaemic) and retinoic acid receptor-alpha (RAR) genes, resulting in the PML-RAR transcript.

Microgranular variant of AML (M3) this accounts for between 15% and 20% of all cases of M3. Prognosis is poorer, and leukocytosis is usually featured. Sparsely granulated leukaemia cells are morphologically is characteristic of it. The cytoplasm is more basophilic, due to a lower concentration of azurophilic granules.

Acute myelomonocytic leukaemia (M4) accounts for 20% of AML. Clinically, patients present hyperplasia of gums. This type of leukaemia has a granulocyte component and a monocyte component in varying proportions and different degrees of maturation. Chloroacetate esterase is Cytochemically positive and monocytes are positive for naphthol-AS-d-acetate esterase or -Alfa-naphthyl-butyrate- esterase.

M4Eo variant: about 5% of cells are abnormal eosinophils then M4Eo confirmed, also presenting with monocytic nuclei and atypical granules.

(M5) consumes almost 15% of all cases of AML. Leukaemic cells are of a monocytic lineage (monoblasts and promonocytes), divided into: M5a (acute monoblastic leukaemia), in which monoblasts predominate; and M5b (acute monocytic leukaemia), in which a high proportion of promonocytes and monocytes are found, in addition to monoblasts. Monoblastic form is usually presents with t (9;11), t (6;11) and t (8;16) genetic translocations and 11q23 rearrangements (AML with recurrent genetic abnormalities according to the World Health Organization (WHO).

(M6) erythroid sub-type is a proliferation of dysplastic erythroid elements with a proliferation of blasts of myeloid origin. The WHO classification divides 2 sub-types: erythroleukemia (M6a), described as a mixed proliferation of myeloid and erythroid blasts, and can be secondary to a previous myelodysplastic syndrome. (M6a) accounts for 5-6% of all AMLs. The other subtype is the pure erythroid variant (M6b), which occurs when dyserythropoiesis is prominent.

Immunophenotyping

Immunophenotypic analysis of the reactivity of leukaemic cells with monoclonal antibodies has proved useful and nowadays essential in the diagnosis of acute leukaemias [13]. This was first shown to be relevant in the characterization and classification of acute lymphoblastic leukaemias of various cell types as there is no specific and reliable cytochemical marker to recognize lymphoblasts. Subsequently, immunological markers have also been shown to be important in the diagnosis of acute myeloblastic leukaemias, particularly when the nature of the blasts cannot be defined by morphology and cytochemistry [14]. Examples of these “undifferentiated” acute leukaemias include those with poorly differentiated myeloblasts (AML-M0),>5 or those derived from early erythroid and megakaryocyte precursors. Some AMLs with recurrent genetic abnormalities are associated with characteristic immunophenotypic features. For example, AMLs with t (8;21) frequently express the lymphoid markers cluster of differentiation (CD), CD19 or, to a lesser extent, CD7; they may also express CD56. AMLs with Inv16 frequently express the T lineage–associated marker CD214; and AMLs with NPM1 mutation typically have high CD33 but absent or low CD34 expression [14].

• MO- Immunophenotype
• CD13 +
• CD33 +
• CD11b +
• CD11c +
• CD14 +
• CD15 +.
• M1- Immunophenotype
• MPO +
• CD13 +
• CD33 +
• CD117+
• CD34 +/-.
• M2- Immunophenotype
• MPO +
• CD34 +/-
• CD13 +
• CD15 +
• HLA-DR +/-
• Sudan black +
• CD117 +/-
• M3- Immunophenotype
• CD13 +
• CD33 +
• HLA-DR -
• CD34 -
• M4- Immunophenotype
• CD13 +
• CD15 +
• CD33 +
• CD11b +
• CD11c +
• CD14 +
• CD64 +
• CD4 +.
• M5- Immunophenotype
• CD14 +
• CD68 +
• CD4 +
• CD11c +
• HLA-DR +
• CD64
• M6- Immunophenotype
• CD13 +
• CD33 +
• CD15 +
• Glycophorin A +
• Glycophorin C +.
• M7- Immunophenotype
• CD41 +
• CD61 +
• CD42 +
• CD13 +
• CD33 +
• CD34.

Cytogenetics

Cytogenetics has become a compulsory for diagnoses of patient suspected of having AML, because research has shown that approximately 55% of adults having AML have chromosomal abnormality which either falls on the WHO category: “AML with recurrent genetic abnormalities” and “AML with myelodysplasia-related features” [15]. A minimum of 20 metaphase cells analyzed from bone marrow is considered mandatory to establish the diagnosis of a normal karyotype, and recommended to define an abnormal karyotype and may be diagnosed from blood specimens [16].

The following are some of the cytogenetic abnormalities encountered in AML with their genetic consequences due to chromosomal translocation. Cytogenetic abnormalities regardless of blast count:

• AML with t(8;21) (q22; q22); RUNX1-RUNX1T1
• AML with Inv16 (p13.1q22) or t (16;16)(p13.1;q22); CBFB-MYH11 .
• APL with t (15;17) (q24.1; q21.1); PML-RARA. 8;21 translocation in AML.

This balanced translocation t(8;21) (q22; q22); RUNX1- RUNX1T1 has been reported in adult patients that have been newly diagnosed with AML. It is also a very common disorder among children diagnosed with AML [17]. It results from fusion between RUNX1 or core binding factor alpha 2 gene on chromosome 21 with the RUNX1T1 gene on chromosome 8 to form RUNX1-RUNX1T1 chimeric product that regulate the transcription of number genes that are important in hematopoietic stem and progenitor cells growth, differentiation and function [18]. Kozu T, et al. [19] also demonstrates this type of translocation in AML-M2 and AML-M4. Adults with t (8;21) type of AML has a favorable prognosis while in children the prognoses are poor.

Induction Therapy

Since 1926, the importance of intensive induction chemotherapy remains unchanged. The intensive anthracycline and cytarabine regimen, “7 + 3”, induction therapy is the standard of care for young adults and fit elderly patients (harboring NPM1 mutations and CBF leukemia). With seven days of continuous cytarabine infusion (100mg/ m2/daily for one week (days 1 through 7) either daunorubicin (60 or 90mg/m2 on days 1, 2 and 3) or idarubicin (10-12mg/ m2 on days 1, 2 and 3) is given. The target of induction chemotherapy is to achieve morphologically complete remission (CR). CR is defined as:

• <5% blast in bone marrow aspirat4e sample with marrow spicules and with a count of ≥200nucleated cells (no blast with Auer rods or persistence of extramedullary disease.
• Absolute Neutrophills Count (ANC) >1000/ul
• Platelets ≥ 100,000/ul.

Using standard induction therapy AML patients less than 60 years of age achieve CR in 65%-73% while only 38%-62% of patients over 60 years of age with AML achieve CR [20, 21, 22]. It has been shown in several trials that higher dose of anthracycline (90 versus 45 mg/m2) in both younger and older fit adults (from 60 to 65 yrs.) results in higher CR rates [21, 22]. Daunorubicin is highly toxic taking care with it and its wide use of the 60-mg/m2 dose as a newer “standard,” boosts up the United Kingdom (UK) National Cancer Research Council (NCRC) to conduct a prospective randomized trial with the goal to compare daunorubicin at 60 vs. 90 mg/m2 in the induction of 1206 AML patients [23]. In this study there was no benefit of using higher dosing (90 mg/m2) over 60mg/m2 across all subgroups [23]. In this trial, however there are some caveats to consider. More precisely, due to multiple courses of anthracycline, the cumulative dose of anthracyclines in the low dose arm (60 mg/m2) was equivalent in the United Kingdom National Cancer Research Institute (UK NCRC) trial to the higher dose (90 mg/m2) of the other clinical trials. Instead of that UK NCRC trial has a shorter follow up period also [24]. Thus, it is clear that 45 mg/m2 of daunorubicin seems insufficient and 60 mg/m2 is not inferior to 90 mg/m2 with less associated toxicity. Midostaurin (FLT3 Inhibitor), added to standard induction therapy to patients found to have a FLT3 mutation [25]. In the adult population characterizing fitness is important when deciding treatment strategy. In particular, appropriate therapy in the elderly AML patient should be determined depending on the “patient-specific fitness” independent of age [26]. The use of hypomethylating agents including decitabine and azacitidine has shown useful in older adults, deemed not fit for intensive induction therapy especially harboring complex karyotype without NPM1 mutations, [26, 27, 28]. Both agents have activity in AML as initial induction therapy and in the relapsed setting. Several phase II and III studies using azacitidine and decitabine have been conducted [27, 28, 29]. In a study of 82 patients who received azacitidine as part of a compassionate use program showed CR/incomplete CR in 11 of the 35 untreated patients (31%). The median overall response duration was found to be 13 months with the one-year and two-years overall survival rates of 58% and 24%, respectively [29]. With 10 days of low-dose decitabine at 20 mg/m2 intravenous over 1 h, Blum et al. showed an even higher CR rate of 47% and overall response rate of 64% [28]. This treatment was well tolerated with CR achieved in 52% of subjects presenting with CN- AML and also in 50% of those with complex karyotypes [28]. Decitabine usually require a median of two to four cycles of therapy to have an optimal response in older patients for induction. Even before the diagnosis is confirmed, Patients with suspected acute promyelocytic leukemia (APL) should be treated with all-trans retinoic acid (ATRA). Because early use of ATRA decreases the risk coagulopathy, development of disseminated intravascular coagulation (DIC) and mortality. For patients with low-to-intermediate-risk APL with white blood cell count in control outcomes are excellent with the use of ATRA with arsenic (ATO) [29]. The ATO-ATRA combination showed CR rates in all 77 patients (100%) and in 75 of 79 patients (95%) in the ATRA-idarubicin group in non-inferiority study. In the comparison of ATO-ATRA arm with those in the ATRA–chemotherapy arm the two-year event-free survival and Overall Survival (OS) rates were significantly improved [29]. For rapid control of leukocytosis in high-risk patients (Abnormal WBC count), chemotherapy with idarubicin should be initiated once the diagnosis is confirmed in addition to ATO-ATRA therapy. It is highly recommended that WBC, fibrinogen level, prothrombin time and partial thromboplastin time be monitored at least twice daily with aggressive transfusion support during induction treatment. The recommendation of prophylactic steroids are also there, in particular to prevent differentiation syndrome, when using ATRA/ATO combination for induction therapy in patients with high WBC count [29, 30].

Consolidation Strategies

To prevent relapse and eradicate minimal residual leukemia (MRD) in the bone marrow after induction consolidation or post-induction therapy is given to achieve cure. Using real-time PCR or Next Generation Sequencing (NGS) techniques assessment of minimal residual disease is increasingly used to track treatment response and has been shown to be superior than morphology alone in predicting impending relapse [31, 32]. Generally, for consolidation, there are two main strategies, chemotherapy (including targeted agents) and hematopoietic stem cell transplantation [20]. Out of those, both strategies could be implied or one out of two could be used but most commonly they are used in combination depending on the type of leukemia, the fitness of the patient and the availability of a stem cell donor. Intermediate-dose cytarabine 1.5 g/m2 twice daily on days 1, 3 and 5 given in three to four cycles is an effective and established regimen used post induction chemotherapy, to prolong remission and improve survival in favorable risk young adults (60 year of age). With high dose cytarabine no positive effect was noted and sometimes irreversible neurotoxicity noted [33], therefore 500-1000 mg/m2 is standardly used. After achieving CR in particular fit patients with intermediate risk or high risk disease, allogeneic hematopoietic stem cell transplantation remains the most effective long term therapy for AML. Instead of that, several patients never become eligible for transplant. It is standard practice to give post induction chemotherapy to maintain CR and keep the leukemia burden low while waiting for transplant. As consolidation therapy increases risk of morbidity and mortality, which may hinder eventual curative transplant so the decisions regarding consolidation rather than moving straight to transplant should be individualized. Age should no longer be used as the sole criteria for transplant eligibility, recent evidence unanimously confirms that [34], On Pre-transplant performance status, co- morbidities and current remission decides the transplant eligibility. Hematopoietic Cell Transplantation Comorbidity Index (HCT-CI) is most widely recognized and validated tool for assessing comorbidity for such transplant [34]. The clinical outcome becomes worse with the increase of score.

Based on patient fitness, conditioning regimen should be decided and transplant options given. Instead of a higher risk of relapse, long term outcomes of reduced-intensity allogeneic hematopoietic stem cell transplant in patients who were ineligible for myeloablative transplant are promising. The results of a prospective multicenter phase II trial conducted by the Alliance for Clinical Trials in Oncology (formerly Cancer and Leukemia Group B) and the Blood and Marrow Transplant Clinical trial Network showed reduced intensity conditioning-based hematopoietic stem cell transplant (HSCT) to be an effective strategy for suitable older patients with an available matched donor with a disease-free survival and OS at two years after transplant of 42% and 48%, respectively [35]. Therefore more common and more clinically accepted are reduced intensity transplant.

Relapsed Disease

Of the patients who relapse, only a small fraction achieve successful second remission using salvage chemotherapy followed by allogeneic stem cell transplant with curative intent [20]. Studies examining clonal evolution of relapse show that relapse can occur from expansion of major or minor clones present at diagnosis or through newly acquired mutations over time [36]. So, especially in light of novel targeted therapies clinical trials are the preferred treatment approach. Early relapse (occurring within the first six months after CR1) portends a poor overall survival. Salvage regimes include intermediate dose cytarabine (500-1500 mg/m2 intravenously every 12h on days 1-3); MEC (Mitoxantrone 8 mg/m2 on days 1–5, Etoposide 100 mg/m2 on days 1-5, and Cytarabine 100 mg/m2 on days 1-5) or lastly, FLAG- IDA (Fludarabine 30 mg/m2 , intravenously on days 1-5 (20 mg/m2 in patient >60 years old), Cytarabine 1500 mg/m2 (500–1000 mg/m2 in patients >60 year) intravenously, 4 h after fludarabine infusion, on days 1-5; Idarubicin 8 mg/m2 , intravenously, on days 3-5; Granulocyte colony-stimulating factor 5 µg/kg, subcutaneously, from day 6 to white-cell count >1 g/L (FLAG-IDA) [37]. The likelihood of achieving a second CR is best in patients with a long first remission, younger age and in those with favorable cytogenetics, In cases of APL, re- induction with ATO ±ATRA remains the standard therapy. CR rates with single agent ATO are good nearly 85% [38].

Novel Targets

Fms-Like Tyrosine Kinase 3 (FLT3) Inhibitors: At 2015 American Society of Hematology (ASH) Plenary session presented the ‘FLT3-mutated AML’, the largest randomized, phase III clinical trial showing the benefit of midostaurin added to induction chemotherapy (RATIFY trial) in which patients receiving midostaurin had significantly longer median OS than those receiving placebo: 74.7 versus 25.6 months (p=0.0076) [39]. Quizartinib and Crenolanib are Second generation agents, promising to have better potency and fewer side effects, still undergoing clinical investigation. Better blast count clearance has been shown in one trial, using quizartinib (AC220), also noted the development of secondary resistance. The major challenge in treating patients with a single FLT3 inhibitor is drug resistance. Within the kinase domain of FLT3-ITD point mutations identified which lead to resistance are found N676, F691, and D835 respectively [40]. Isocitrate Dehydrogenase (IDH) Inhibitors: The IDH1 inhibitor AG-120 and the IDH2 inhibitor AG-221 have demonstrated promising response rates in patients with AML in two separate phase I clinical trials [41, 42]. On the analysis more interestingly the duration of the responses for AG-221 and AG-120 were more than 15 and 11 months, and still ongoing. The responses lasted longer than six months are about 76%. Based on these Promising figures, for patients with AML, the Food and Drug Administration (FDA) have granted the medication an orphan drug designation. Nuclear Exporter Inhibitors: The much excitement has brought by the anti-leukemic efficacy of reversible inhibitors of the major nuclear export receptor, chromosome region maintenance 1 (CRM1, also termed XPO1). The export and inactivation of several tumor suppressors such as p53, p73, FOXO1, RB1 and p21 (CDKN1A) among others is maintained by the CRM1 major nuclear exporter protein [43]. The up- regulation of CRM1 has been shown in a range of solid tumors and hematological malignancies, including AML [44]. With novel CRM1 inhibitors (Selinexor), preclinical studies indicate that treatment of AML cell lines, patient samples and AML xenografts induces strong anti-leukemic effects [45]. Phase I/II clinical trials are currently ongoing to assess the safety, tolerability and activity of selinexor in AML patients based on these studies. Immune Therapies: So many new strategies for treating leukemias like novel antibody therapies are currently under development, Monoclonal antibodies which include CD33 (Gemtuzumab ozogamicin) and bi specific antibodies for eg:- AMG 330 (anti-CD33 and CD3) [46]. On the majority of AML blasts CD123 has been found to be expressed but also found on normal hematopoietic cells. Preclinical data shows that targeting CD123 via CARTs (chimeric antigen receptor targets) results in rejection of human AML and myeloablation in the mouse models [47].