Toward Individualized Therapy in Acute Myeloid Leukemia
A Contemporary Review
Tapan M. Kadia, MD; Farhad Ravandi, MD; Jorge Cortes, MD; Hagop Kantarjian, MD
T
he phrase “divide and conquer” is thought to be derived from the Latindivide et impera, divide and rule. This refers to the maxim in politics or war to break up large, complex opponents into smaller, more manageable (and defeat-able) entities. This technique might apply as well to the treatment of acute myeloid leukemia (AML). Genomic and molecular charac-terization has uncovered the widespread heterogeneity in AML and has added to our understanding of the disease biology. We now realize that we are dealing with not just 1 overarching disease but with multiple distinct entities. Our approach, therefore, must be to divide them along the lines of biological diversity and treat each with an individualized approach that takes advantage of the new discoveries. With the rapid accessibility to leukemia specimens and with validated methods of mutation detection, treatments can be chosen based on the molecular abnormalities, the driver mutations, pathogenic pathways, and the predicted response to therapy.While there is a perception of slow progress in the treatment of AML overall, the opposite is true in several biologically well-defined subtypes, within which there have been steady gains in cure rates and overall survivals. The present review focuses on a chang-ing paradigm of management in which we recognize the heteroge-neity of AML prior to treatment and identify the appropriately di-rected therapies that will change the natural history of these diseases. The new management paradigm involves optimally classifying AML cases on the basis of their biology and then intensely focusing re-search into each subtype toward effecting a cure. Herein, we first briefly summarize the existing genetic classification systems that are widely used and consider the significance of newer genetic abnor-malities that have been discovered through next-generation DNA sequencing. We then describe AML as several distinct entities and review how recent clinical studies are shaping the current manage-ment of each and helping to improve outcomes.
Cytogenetic and Molecular Classification
The prognostic relevance of recurrent cytogenetic abnormalities on AML outcome is well established.1-3For about 2 decades, pretreat-ment cytogenetic abnormalities have been used to help classify AML into prognostic categories and direct treatment strategies. Those with favorable karyotypes are treated with curative intent using high-dose chemotherapy as postremission consolidation. Those with ad-verse karyotypes are known to have much lower cure rates and should be referred for allogeneic stem cell transplantation (SCT), in-vestigational approaches, and long-term maintenance strategies. While there is good consensus for the categorization of favorable and adverse karyotypes in AML, there is inconsistency in defining the prognostic impact of several less common, but recurrent cyto-genetic abnormalities that make up the intermediate-risk group. In addition, the implications of recurrent mutations in a number of genes such asNPM1,FLT3, andCEBPα in cytogenetically normal AML (CN-AML) are still being refined.4,5Currently, 2 major classifica-tions are commonly used to categorize AML into prognostic sub-sets: the United Kingdom Medical Research Council (MRC-C) and European Leukemia Net (ELN-C) systems (Table 1).2,3
The MRC-C is based on the outcomes of about 6000 patients younger than 60 years and divides patients into 3 categories by pre-treatment cytogenetics: favorable, intermediate, and adverse.3 Ow-ing to its large cohort size, the MRC-C had statistical power to refine the previous systems and individually report the outcomes of most of the recurrent cytogenetic abnormalities seen in AML, including those previously overlooked owing to small numbers. The ELN-C prog-nostic classification system expands on existing knowledge of cyto-genetic subgroups by including the prognostic influence of recur-rent somatic mutations in AML.2Incorporating the mutation status ofNPM1,FLT3, andCEBPα in patients with CN-AML, the ELN-C pro-Acute myeloid leukemia (AML) is a heterogeneous disease in its clinical presentation,
response to therapy, and overall prognosis. For decades, pretreatment karyotype evaluation has served to identify subgroups for risk-adapted postremission therapy, but the initial treatment approach has been largely unchanged. With continued advances in the genetic and epigenetic characterization of AML, we have discovered even more diversity and are starting to understand the biological underpinnings of these multiple disease entities. Newer therapies are being developed to address the pathophysiology within these individual AML subsets. This review categorizes AML into biologically defined groups based on currently available data and describes the evolving treatment approaches within these groups. Identifying the genetic abnormalities and biological drivers prior to AML treatment will be important as we work to individualize therapy and improve outcomes.
JAMA Oncol. 2015;1(6):820-828. doi:10.1001/jamaoncol.2015.0617 Published online April 30, 2015.
Author Affiliations:Department of Leukemia, University of Texas MD Anderson Cancer Center, Houston. Corresponding Author:Tapan Kadia, MD, Assistant Professor, Department of Leukemia, MD Anderson Cancer Center, 1515 Holcombe Blvd, Unit 0428, Houston, TX 77401 ([email protected]). Review
posed 4 prognostic categories: favorable, intermediate-1, intermedi-ate-2, and adverse. With the additional data of mutational status, se-lected patients with CN-AML formerly grouped within the ambiguous intermediate-risk category could then be appropriately identified as higher or lower risk and treated accordingly.
Next-generation, whole-genome sequencing has uncovered sev-eral recurrent somatic mutations that better define the landscape of AML genomics. A recent study by the Cancer Genome Atlas Re-search Network6analyzed the genomes of 200 cases of de novo AML and reported several important findings. Unlike most solid tumors, AML genomes appear to have a limited number of mutations, with an average of 13 mutated genes per case. Of these, an average of only 5 are in genes recurrently mutated in AML, suggesting a role in the disease biology. The top 10 genes mutated at higher than 5% fre-quency includeFLT3,NPM1,DNMT3a,IDH1,IDH2,TET2,RUNX1,p53, NRAS,CEBPα, andWT1.6Based on functional analysis and known path-ways, the genetic abnormalities can be grouped into categories based on biological function: (1) myeloid transcription-factor fusions or mu-tations, (2)NPM1mutations, (3) tumor-suppressor gene mutations, (4) epigenome-modifying gene mutations, (5) activated signaling-pathway gene mutations, (6) cohesin-complex gene mutations, and (7) spliceosome-complex gene mutations.6,7Finally, from analysis of mutual exclusivity and cooccurrence between these genetic abnor-malities, patterns of interplay between pathways were identified that may help delineate further subsets of AML and provide more insight into disease biology.
The roles of many of these genes in AML pathogenesis are still being investigated. Their prognostic significance is also not yet clear, often confounded by small numbers and the heterogeneity of pa-tient characteristics and treatment approaches. The challenge will be to study the implications of each genetic abnormality in the con-text of uniform treatment, alone and integrated with other known mutations. Studies by Schlenk et al8and Patel et al,9for example, provide a framework to gain further insight into newer mutations in a systematic fashion. Herein we highlight our current understand-ing of treatment subsets in AML, organized by underlyunderstand-ing biology and therapeutic strategy.
Acute Promyelocytic Leukemia
Acute promyelocytic leukemia (APL) is defined by the t(15;17) cyto-genetic abnormality, which leads to the PML-RARA (promyelocytic leukemia–retinoic acid receptor alpha) fusion gene and oncopro-tein. The PML-RARA protein acts as a dominant negative inhibitor of the wild-type retinoic acid receptor, resulting in differentiation block and the clinicopathologic picture of APL.10The treatment of APL, which has evolved from nonspecific chemotherapy to selec-tive nonchemotherapy-based regimens, represents one of the first examples of individualized therapy targeted to a genetically de-fined AML subset. This has led to an improvement in cure rates from 30% to 90%.
The major breakthroughs in APL therapy were the discovery of the activity of all-trans-retinoic acid (ATRA) and the activity of ar-senic trioxide (ATO) in APL.11,12ATRA overcomes the differentia-tion arrest in APL.13ATO binds to PML, accelerates the degradation of PML-RARA, and is likely the most active single agent in APL. An additional important observation is the high expression of CD33 on APL and the significant clinical activity of the anti-CD33 antibody-drug conjugate, gemtuzumab ozogamicin (GO).14Based on these findings, combination studies of ATRA, ATO, and chemotherapy were developed.15-17
Investigators at our institution developed a nonchemotherapy-based regimen combining ATRA and ATO for remission induction, fol-lowed by ATRA and ATO consolidation. Patients with leukocytosis at diagnosis or following therapy, and those who remained molecularly positive for PML-RARA 3 months into consolidation or later received
At a Glance
•Two major classifications are commonly used to categorize acute myeloid leukemia (AML) into prognostic subsets: the United Kingdom Medical Research Council (MRC-C) and European Leukemia Net (ELN-C) systems.
•AML genomes have a limited number of mutations, with an average of 13 mutated genes per case.
•The top 10 genes mutated at higher than 5% frequency include
FLT3,NPM1,DNMT3a,IDH1,IDH2,TET2,RUNX1,p53,NRAS,
CEBPα, andWT1.
•Early recognition of pretreatment karyotype is an important prognostic and predictive factor and has led to improved outcomes in subsets of patients with core-binding factor AML and acute promyelocytic leukemia.
Table 1. Comparison of the Revised MRC-C and ELN-C Classification Systems for AML
Group Cytogenetic Abnormality MRC-C
Favorable t(15;17)(q22;q21), t(8;21)(q22;q22), inv(16)/t(16;16)(p13;q22)
Intermediate All cytogenetic abnormalities not classified as favorable or adverse
Adverse abn(3q) [excluding t(3;5)], inv(3)/t(3;3)(q21;q26) add(5q), del(5q), -5, -7, add(7q)/del(7q) t(6;11)(q27;q23), t(10;11)(p11 ~ 13;q23), t(11q23)[excluding t(9;11), and t(11;19)], t(9;22)(q34;q11)
−17/abn(17p),
Complex (≥4 unrelated abnormalities)
ELN-C
Favorable t(8;21)(q22;q22), inv(16)/t(16;16)(p13;q22), NPM1(+) andFLT3ITD WT (normal karyotype), mutatedCEBPα (normal karyotype) Intermediate-1 NPM1(+) andFLT3ITD(+) (normal karyotype),
NPM1WT andFLT3ITD(+) (normal karyotype), NPM1WT andFLT3ITD WT (normal karyotype) Intermediate-2 t(9;11)(p22;q23), Those cytogenetic abnormalities
not classified as favorable or adverse Adverse inv(3)/t(3;3)(q21;q26),
t(6;9)(p23,q34),
t(v;11)(v;23),MLLrearranged −5 or del(5q), −7, abnormal (17p), complex karyotype
Abbreviations: AML, acute myeloid leukemia; ELN-C, European Leukemia Net classification; ITD, internal tandem duplication; MRC-C, United Kingdom Medical Research Council classification; WT, wild type.
additional GO therapy at 6 mg/m2.18The combination demon-strated a complete remission (CR) rate of 92% and a 3-year survival rate of 85%. This study was the basis of a European consortium ran-domized study comparing the nonchemotherapy ATRA-ATO regi-men with standard AIDA regiregi-men (combination of ATRA and idarubicin).19Among 162 patients with low- and intermediate-risk APL treated, the CR rate was 100% with ATRA-ATO and 95% with AIDA. The 2-year event-free survival was 97% vs 86% (P= .02). The ATRA-ATO regimen was also associated with significantly better survival (P= .02), lower incidences of hematologic toxic effects, and fewer infections.19This represents a major advance in the individualized treatment of patients with APL and can be considered the new stan-dard of care in newly diagnosed low-risk APL.
Although cure rates have increased significantly with the intro-duction of ATRA and ATO, rates of early mortality remain high, par-ticularly in patients older than 55 years and those treated outside major academic centers.20Life-threatening hemorrhage from se-vere coagulopathy, delay of ATRA-based therapy, and complica-tions of differentiation syndrome account for the majority of induc-tion deaths. Better supportive care, early recogniinduc-tion of APL, and better education of “first responders” such as primary care physi-cians and emergency department personnel are needed to in-crease awareness and provide disease-modifying therapy. The de-velopment of guidelines and their dissemination through a partnered network of academic and community hospitals can help reduce rates of early death and has been previously pioneered in developing countries.21
Core-Binding Factor Leukemia
The 2 subtypes of core-binding factor (CBF) AML include the t(8; 21) cytogenetic abnormality, resulting in the formation of theRUNX1/ RUNX1T1fusion gene and inversion of chromosome 16 (inv 16) (and its variant translocation t(16;16)), which results in the formation of theCBFB/MYH11fusion gene. Treatment of CBF AML was associ-ated with high CR rates (80%-90%) and cure rates of 40% to 60% in earlier studies using 3 or 4 cycles of high-dose cytarabine consolidations.22
Recognizing the sensitivity of CBF AML to intensive chemo-therapy has led to the refinement of the standard chemochemo-therapy regi-mens with more potent combinations (fludarabine, high-dose ara-C [cytarabine] [HiDAC], idarubicin, and GO), improved schedules, and more consolidations, leading to cure rates of 80% to 90%.23-29 Brad-stock et al24reported 1 of the first studies to demonstrate the ben-efit of HiDAC-based induction followed by HiDAC consolidation, which led to relapse-free survival and overall survival rates of 76% and 88%, respectively, among patients with CBF AML.24At our in-stitution, the use of fludarabine and HiDAC (FLAG) with GO or with idarubicin resulted in CR rates of 90% and estimated 5-year sur-vival rates of 80%.29These results were recently confirmed by the AML15 trial,30which found an 8-year overall survival rate of 95% in patients with CBF AML treated with FLAG and idarubicin. Further-more, in separate analyses, the addition of GO was found to be the most significant factor associated with improved survival.25,26The survival benefit of GO among a subset of patients with CBF AML was also confirmed by a Southwest Oncology Group study in the United States.28These studies demonstrate a steady improvement in
long-term overall survival with the incorporation of higher doses of ara-C and fludarabine in induction and consolidation. Patients with CBF AML should therefore be offered high-dose therapy with curative intent, and allogeneic SCT should be reserved for second CRs and beyond.
While GO has shown significant benefit in CBF AML, its lack of commercial availability limits its use to compassionate use protocols in the United States. However, the effective activity of GO has prompted exploration of monoclonal antibodies for use in treat-ment of this disease and has led to the developtreat-ment of newer anti-body constructs for treatment. A new anti-CD33 antianti-body-drug con-jugate, SGN-33a, is currently undergoing early clinical investigation in patients with AML.31In an ongoing phase 1 dose-escalation study of SGN-33a, patients with relapsed or minimally pretreated AML have been treated with SGN-33a, 5 to 60 μg/kg intravenously every 3 weeks without yet reaching a maximum tolerated dose.32Among 38 evalu-able patients, 16 (42%) have had clearance of bone marrow blasts.
Radioimmunotherapy using an actinium isotope–labeled mono-clonal antibody to CD33 is also under investigation.33A new bi-specific T-cell engager (BiTE) antibody construct is engineered to spe-cifically recognize CD3 on an immune-effector T lymphocyte as well as CD33 on AML blasts. Engagement of both antigens by a BiTE monoclonal antibody brings an AML blast in close proximity with an immune-effector cell, triggers the formation of an immune syn-apse thus inducing T-cell activation and lysis of the target cell. Fol-lowing on the success of the CD3-CD19 BiTE, blinatumumab,34in acute lymphoblastic leukemia, the CD3-CD33 BiTE AMG330 is cur-rently in preclinical development. Other AML-specific antigens such as CD123 are also being explored with different monoclonal anti-body approaches.
Among patients with CBF AML, several studies have sug-gested that the presence ofc-KITmutation or persistence of mini-mal residual disease (MRD) may be associated with a higher inci-dence of relapse and with worse outcome.35-37Studies of dasatinib, aKITinhibitor, inc-KIT-mutated and wild-type CBF AML are ongoing.38In cases of CR, MRD is monitored routinely, and treat-ment is adjusted if MRD remains positive. Among such patients, al-logeneic SCT may be beneficial.39,40
NPM1
-Mutated and
CEBP
α-Mutated AML
TheNPM1gene encodes for nucleophosmin, a nuclear phosphopro-tein that shuttles between the nucleus and cytoplasm.41NPM1 mu-tations lead to aberrant cytoplasmic localization of the protein and are found in 50% of patients with normal-karyotype AML and in 60% of patients withFLT3internal tandem duplication (ITD) mutations.42 In the absence of concurrentFLT3ITD mutations,NPM1mutations confer a favorable prognosis.8,41-43Patients with normal-karyotype AML andNPM1mutation without anFLT3mutation are now classified as having a favorable prognosis and should be treated accordingly.2,44Those withNPM1mutations and eitherIDH1orIDH2 mutations, withoutFLT3mutations may have a particularly favor-able prognosis.9Patients with bothNPM1andFLT3ITD mutations have a worse prognosis, reflecting the adverse effect ofFLT3ITD.
TheCEBPα gene encodes for the CCAAT–enhancer binding pro-teinα, a transcription factor. Loss-of-function mutations inCEBPα occur in about 10% of patients with AML and normal karyotype and
were reported to be associated with a favorable prognosis com-pared with wild-typeCEBPα. More recent analyses have indicated that a biallelic loss ofCEBPα is important in conferring this favor-able prognosis.8,45-47
The recent World Health Organization (WHO) classification of myeloid neoplasms considers these subtypes as provisional dis-tinct entities.44NPM1,FLT3, andCEBPα mutations should now be routinely tested in patients with AML to define prognosis and to choose appropriate therapy. Our approach to patients with NPM1-mutated orCEBPα-mutated AML (in the absence ofFLT3 muta-tions) is similar to that in CBF AML, to pursue induction and consoli-dation using HiDAC-based regimens with curative intent.
FLT3
-Mutated AML
The FMS-like tyrosine kinase 3 (FLT3) and its ligand are important for the normal proliferation of hematopoietic precursors. Activat-ing mutations in theFLT3gene occur in about 30% of patients with AML.8,48,49Internal tandem duplications of the juxtamembrane do-main, and point mutations in the tyrosine kinase domain (TKD) af-fecting amino acid D835, lead to ligand-independent constitutive ac-tivation ofFLT3signaling.50FLT3ITD–mutated AML is associated with inferior survival compared with wild-typeFLT3; the prognostic sig-nificance ofFLT3D835 is less clear.8,48,49,51,52
In addition to theFLT3mutation type, the allelic ratio of FLT3-mutant genes (ratio of FLT3-mutantFLT3allele to wild-typeFLT3allele by polymerase chain reaction) may affect prognosis.51,52The exact threshold and prognostic relevance of theFLT3-mutant allelic ratio needs to be refined; but for now, it appears that a high FLT3-mutant allelic ratio may characterize an AML subtype that is depen-dent onFLT3signaling and potentially more sensitive to FLT3-inhibitor therapy.53
Several FLT3 tyrosine kinase inhibitors (midostaurin, sorafenib, and quizartinib) are in clinical development. These are mostly ac-tive inFLT3ITD AML, but not inFLT3TKD mutants. Early studies sug-gest that development of anFLT3TKD mutation may be a resis-tance measure or escape mechanism in patients being treated with these drugs.48
The available FLT3 inhibitors are being evaluated, alone or in combination with chemotherapy. The combination of sorafenib with idarubicin and HiDAC in newly diagnosed AML demonstrated a CR rate of 95% inFLT3-mutated disease vs 84% inFLT3 wild type AML (P= .23).54,55There were no significant differences in survival or dis-ease-free survival between the 2 groups. Stone et al56evaluated treatment with midostaurin combined with daunorubicin and cy-tarabine. The CR rate was 80% overall and 92% inFLT3-mutated AML. Two-year survival was 62% inFLT3-mutated AML.56Similar outcomes betweenFLT3 ITD and wild-typeFLT3cohorts in these studies suggest that the FLT3 inhibitors may negate the adverse ef-fect of theFLT3mutation.
In a study conducted by a German AML study group (SORAML study),57276 younger patients (ⱕ65 years) were randomized to re-ceive treatment with daunorubicin and cytarabine with or without sorafenib. After a median follow-up of 3 years, the addition of sorafenib was found to be associated with a significant prolonga-tion of 3-year event-free survival (40% vs 22%;P= .01) and a trend for better 3-year overall survival (63% vs 56%;P= .38),
particu-larly among patients withFLT3ITD.57In contrast, Serve et al58 in-vestigated this same treatment approach in 201 older patients with AML and reported trends for a lower CR rate (48% vs 60%,P= .12), a higher early death rate (17% vs 7%;P= .05), and no improve-ment in event-free survival or overall survival with sorafenib.
These studies suggest an improvement in outcome with the ad-dition of an FLT3 inhibitor in AML therapy, with the possible excep-tion of increased toxic death rate in older patients receiving the com-bination. Lower-intensity therapy in combination with an FLT3 inhibitor may be a better option for this subset of patients. For in-stance, a phase 2 trial combining sorafenib with azacytidine in pa-tients with multiply relapsedFLT3ITD–mutated AML demon-strated an overall response rate of 46%.59This regimen was well tolerated and is under evaluation in newly diagnosed older pa-tients withFLT3ITD–mutated AML.
Newer, more selective FLT3 inhibitors are in development. A po-tent FLT3 inhibitor, quizartinib (AC220), was studied in relapsed or refractory AML cases,60and CR or CR with incomplete blood count recovery rates of 54% were observed in patients withFLT3ITD and 32% in patients withoutFLT3ITD. The median survival in patients with FLT3ITD–positive AML was 25 weeks.60These encouraging results with single-agent quizartinib have prompted ongoing combination studies.61Studies with next-generation inhibitors such as creno-lanib, which have activity againstFLT3TKD mutations, are ongoing.62 The longer-term impact of FLT3 inhibitors on overall survival in patients with FLT3-mutated AML will need to be determined. In a historical comparison with previous non–FLT3 inhibitor regimens at our institution,63the addition of FLT3 inhibitors to chemotherapy since 2006 appears to have improved survival in both front-line and salvage AML settings. Whether their use can obviate the role of al-logeneic SCT in this setting remains to be seen, but their clinical ac-tivity is clear, and their optimal utilization is being studied. Our ap-proach in patients withFLT3-mutated AML with higher allelic burdens (>0.25) is to offer enrollment in clinical trials of FLT3 inhibitors, when available, to monitor allelic ratios during treatment and to refer eli-gible patients for allogeneic SCT in first remission. As we obtain more data on the safety and activity of newer FLT3 inhibitors in AML and develop combinations to overcome resistance, we can expect FLT3 inhibitors to become part of standard regimens for long-term re-mission maintenance after consolidation and also after allogeneic SCT to improve outcomes.
RAS-Activated AML
RAS is a guanosine triphosphate (GTP)-dependent second messen-ger protein that couples signals from receptor tyrosine kinases with downstream signaling networks. Mutations inRASaffect its inher-ent GTP-ase function and lead to aberrant, constitutive down-stream signaling.RASmutations are present in 10% to 25% of pa-tients with AML and are overrepresented in those with the inv(16) karyotype.64,65There does not appear to be an independent prog-nostic role forRASmutations in AML, but recent data suggest that patients with these mutations may benefit from postremission con-solidation with high-dose ara-C.64,65MutatedRASleads to dysregu-lated downstream signaling through Mek (mitogen activated pro-tein kinase [MAPK] kinase) and may serve as a driver in AML. Inhibition of this pathway may be therapeutic. In a phase 1/2 trial of
patients with relapsed or refractory AML, the Mek inhibitor tra-metinib produced an overall response rate of 28%, including 12% CRs in a cohort enriched for patients with activatingRASmutations.66 Proteomic analysis of patients withRAS-mutated AML has dem-onstrated simultaneous upregulation of the RAS-MAPK pathway as well as the phosphatidylinositol 3-kinase-protein kinase B (PI3K-AKT) signaling pathway, providing a rationale for dual-pathway inhibition.64Studies combining Mek inhibitors with PI3K and AKT inhibitors inRAS-mutated AML are in progress (NCT01907815). In a recent report examining myeloid malignancies with karyotypic ab-normalities affecting theEVI1locus (inv(3), t(3;3)), 98% of the cases were found to have mutations that led to activated RAS or receptor-tyrosine-kinase signaling.67This may provide an additional oppor-tunity for directed therapy with Mek inhibition alone or in combi-nation with PI3K-AKT inhibition.
Epigenetically Targeted Therapy in AML
Epigenetics refers to biochemical modifications to chromatin such as DNA methylation, histone methylation, or histone acetylation that do not alter the primary DNA sequence but play an important role in genomic regulation at the level of gene transcription. Acute my-eloid leukemia is an epigenetically regulated disease. This is evi-denced empirically by the clinical activity of hypomethylating agents in this disease and directly by the identification of recurrent so-matic mutations in genes of epigenetic modifiers.
Hypomethylating agents such as 5-azacytidine (5-AZA) and decitabine have shown significant activity in AML, providing an im-portant option for older patients who are not candidates for inten-sive chemotherapy. In a subset analysis of a phase 3 randomized study of older patients with AML (20%-30% bone marrow blasts),685-AZA was found to be associated with an improvement in survival com-pared with conventional care (24.5 vs 16 months;P= .001). The fol-low-up trial,69which randomized 488 patients 65 years or older with AML and greater than 30% bone marrow blasts to 5-AZA vs conven-tional care, showed improved survival with 5-AZA (median survival 10.4 vs 6.5 months;P= .08).69Similarly, in a randomized phase 3 trial of 435 patients (median age, 73 years) with AML randomized to decitabine or standard treatment choice,70decitabine treatment was associated with an improvement in survival (median, 7.7 months vs 5 months;P= .037), an effect that was maintained in patients with higher bone marrow blast counts. While treatment with hypometh-ylating agents has demonstrated a decrease in DNA methylation, this has not yet correlated directly with treatment outcomes.
Additional evidence of the contribution of epigenetic abnormali-ties to the pathogenesis of AML is supported by the discovery of re-current somatic mutations in genes of proteins involved in DNA meth-ylation and histone modification. The enzyme DNA methyltransferase (DNMT) is responsible for catalyzing the addition of a methyl group to cytosine, leading to DNA methylation. Mutations inDNMT3aare among the most common mutations in de novo AML.6,71,72While ab-errant DNA methylation may lead to AML pathogenesis, the func-tional significance ofDNMT3amutations and their correlation with re-sponse to hypomethylating-agent therapy is still being determined. A small study of patients with newly diagnosed AML treated with decitabine suggested improved CR rates in patients withDNMT3a mutations,73but this has not been further confirmed.
Mutations inTET2are also common in AML and have fur-thered our understanding of the epigenome.TET2catalyzes the oxi-dation of 5-methyl cytosine to 5-hydroxymethylcytosine, eventu-ally leading to loss of methylation.7,74,75A loss-of-function mutation forTET2would therefore predict for a net increase in DNA meth-ylation and a potential role for hypomethylating agents.76
IDH
-Mutated AML and
IDH
-Directed Therapies
Mutations inIDH1andIDH2create a neomorphic enzyme activity that leads to the aberrant production of the oncometabolite 2-hydroxy-glutarate (2-HG), which leads to the inhibition of enzymes involved in epigenetic function and may be sufficient in causing leukemia.77-79 Strategies to targetIDH-mutant AML are being investigated with promising results. A small-molecule inhibitor of IDH2, AG-221, has dem-onstrated a decrease in 2-HG production, which has translated into objective clinical responses.80In a phase 1 study of 32 evaluable pa-tients withIDH2-mutated AML treated once or twice daily with doses from 30 to 200 mg, the overall response rate was 63%, including 8 CRs, 1 CR without total platelet recovery, and 1 CR with incomplete blood count recovery.80Specific IDH1 inhibitors are also in develop-ment. In a phase 1 trial of the IDH1 inhibitor AG-120 in relapsed or re-fractoryIDH1-mutated AML, responses were seen in 7 (50%) of 14 evaluable patients, including 4 CRs.81Both studies are ongoing with dose escalation and no maximum tolerated dose reached.Another approach that has identified B-cell lymphoma 2 (BCL-2) as a synthetic lethal partner forIDH2-mutated AML suggests a po-tential role for BCL-2 inhibitors in this setting.82Indeed, prelimi-nary results of the BCL-2 inhibitor ABT-199 in patients with AML dem-onstrated significant clinical activity in a subset of patients with mutantIDH2.83If the results are confirmed, these may represent im-portant, rationally targeted therapies for this subset of AML cases, complete with 2-HG as a biomarker for appropriate dosing and early signal of activity. Combination therapies with IDH2 inhibitors and BCL-2 inhibitors, if safe, could represent a promising area of clinical development forIDH2-mutated AML. Recently, mutations inWT1 have been also implicated in a DNA hypermethylation phenotype in AML84and may be predictive of response to hypomethylation-based therapy.
Targeting the Chromatin-Modification Machinery
Posttranslational modification of histone H3 within the chromatin is involved in the regulation of gene transcription. This is coordi-nated by proteins that can add, remove, and “read” the posttrans-lational modifications—which may include methylation or acetylation.7Changes in the pattern of chromatin modification can alter entire programs of gene expression that may be involved in growth and differentiation. Recurrent abnormalities within several of the genes encoding these proteins highlight their important role in AML pathogenesis. TheMLLgene, frequently involved in AML chromosomal translocations, is a histone 3 lysine 4 methyltransfer-ase. By virtue of its translocations,MLL, within the fusion protein, retains its DNA-binding capacity but loses its normal histone 3 ly-sine 4 methyltransferase activity.7,85Instead, it gains the ability to recruit an associated histone 3 lysine 79 methyltransferase,DOT1L.7,85The MLL fusion proteins can thereby recruit DOT1L to MLL target genes and direct an aberrant gene expression program that drives leukemogenesis. Small-molecule inhibitors of DOT1L showed promising activity in preclinical models and are currently in clinical trials in AML (NCT01684150).7,85,86EZH2, another recurrently mu-tated gene in AML, encodes a histone 3 lysine 27 methyltransfer-ase usually characterized by loss-of-function mutations.87ASXL1, thought to be a “reader” of histone lysine methylation, is also mu-tated in myeloid malignancies. Further understanding the contri-bution of these mutated genes to leukemogenesis may help un-lock the processes of chromatin modification and result in the development of newer therapies.
Many of the discoveries of aberrant proteins may not yield obviously druggable targets. Innovative methods such as RNA interference screens88and synthetic lethal approaches82may be used to find vulnerabilities that can be exploited. For example, one such RNA interference screen targeting known chromatin regulators led to the discovery of BET bromodomain–containing protein 4 (BRD4) as an important protein required for AML dis-ease maintenance.88The BRD4 protein functions as an epigen-etic reader of acetylated histones and activates transcription of large genetic programs, most prominently ofMYC.88,89 A small-molecule inhibitor of BRD4, JQ1, recapitulated the effects of BRD4 knockdown and led to a breakdown ofMYCtranscription
Table 2. Summary of Selected Targets in AML, Prognostic Implications, and Potential Targeted Therapy
Target
Prognostic Implications
Potential
Therapies Comment
PML-RARA Favorable ATRA and ATO Randomized trial confirming efficacy and superior safety profile of ATRA + ATO compared with standard chemotherapy + ATRA in low-risk patients
CD33 NA Gemtuzumab
ozogamicin
Demonstrated improved outcomes in subsets of patients with CBF leukemias and APL; recently withdrawn from US commercial use, but several recent reports highlight its efficacy
SGN-33a New antibody-drug conjugate in early studies in AML
FLT3 FLT3ITD carries an
unfavorable prognosis, directly related to its allelic ratio. The prognostic implication ofFLT3 D835 mutations are less clear, but they may serve as a mechanism of resistance to FLT3 inhibitors
Sorafenib Approved for solid tumors, but several trials demonstrating its efficacy as a potent FLT3 inhibitor; recent randomized trial in younger patients with AML demonstrated improved event-free survival
Midastaurin Ongoing studies including randomized trial determining its role
Quizartinib Ongoing single-agent and combination studies demonstrating activity in relapsed AML
Crenolanib Ongoing single-agent studies in FLT3-mutated AML; may have activity inFLT3ITD andFLT3TKD mutations RAS No known effect on prognosis;
patients with AML andRAS mutations may benefit from HiDAC-based consolidation chemotherapy
Trametinib Approved for melanoma, but has been shown to have modest single-agent activity inRASmutated AML; combination studies with PI3K inhibitors and AKT inhibitors are currently under way; may have a role in AML with other mutations activating the RAS-MAPK pathway
DNMT3a Associated with an adverse prognosis
Hypomethylating agents
Preliminary studies suggesting increased sensitivity ofDNMT3a-mutated AML to hypomethylating agents; separate study suggests survival benefit with high-dose daunorubicin
IDH1and IDH2
Prognostic significance of these mutations is still not clear, with different studies having inconsistent conclusions; effect likely depends on associated mutational landscape and therapy
AG221 New small-molecule inhibitor ofIDH2 demonstrating blunting of 2-HG production and objective responses inIDH2-mutated AML
ABT-199 Potent BH3-mimetic, inhibitor of bcl-2; shown preliminary efficacy in IDH2-mutated AML
AG-120 New small-molecule inhibitor ofIDH1 that is in early clinical trials TET2 May be associated with an
adverse prognosis in AML; further studies are needed to validate this
Hypomethylating agents?
Small studies suggesting increased sensitivity ofTET2-mutated AML to hypomethylating agents; further validation in large cohorts is needed
MLL (11q23)
Generally associated with an adverse prognosis in AML; MLLinvolvement with the translocations t(9;11) and t(11;19), which are associated with an intermediate prognosis
DOT1L Small-molecule inhibitors of the DOT1L histone methyltransferase may inhibit the aberrant methyltransferase activity associated withMLLtranslocations; early clinical trials are under way
BET inhibitors Small-molecule inhibitors of BET may be active inMLL-translocated leukemia
Abbreviations: AML, acute myeloid leukemia; APL, acute promyelocytic leukemia; ATO, arsenic trioxide; ATRA, all-trans-retinoic acid; CBF, core binding factor; FLT3, fms-like tyrosine kinase-3; HiDAC, high-dose ara-C (cytarabine); ITD, internal tandem duplication; NA, not applicable;
PML-RARA, promyelocytic leukemia–retinoic acid receptor alpha; TKD, tyrosine kinase domain; 2-HG, 2-hydroxyglutarate.
and robust antileukemia activity.88,89This may be important in several subsets of AML cases, particularly those with mutations inMLLorp53.89,90More recent data have implicated the BET proteins in a broader role, as regulators of a core transcriptional program that initiates leukemogenesis in several non-MLL subsets, including those with mutatedNPM1.91The role of BRD4 inhibitors as targeted antileukemic therapy will likely expand and can be the subject of a more in-depth review. Currently, small-molecule inhibitors of BRD4 based on JQ1 are in clinical trials in AML and have preliminarily demonstrated responses in heavily pretreated patients (NCT01943851 and NCT01713582).92
Conclusions
As we gain further insight into its biology, AML appears to be a highly heterogeneous group of diseases with different drivers and different vulnerabilities (Table 2). The challenge going forward is to identify these differences and develop therapies that address the drivers and vulnerabilities. Through collaborative science and intense focus on each individual subtype, we can continue to develop tailored therapies for specific targets and have a smaller fraction of patients treated with the “1 size fits all” anthracycline and cytarabine approach that has defined AML therapy for decades.
ARTICLE INFORMATION
Accepted for Publication:February 25, 2015. Published Online:April 30, 2015.
doi:10.1001/jamaoncol.2015.0617.
Author Contributions:Dr Kadia had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design:Kadia, Ravandi, Cortes, Kantarjian.
Acquisition, analysis, or interpretation of data:
Kadia, Ravandi, Cortes.
Drafting of the manuscript:Kadia, Ravandi, Kantarjian.
Critical revision of the manuscript for important intellectual content:Kadia, Ravandi, Cortes.
Administrative, technical, or material support:
Kadia, Cortes.
Study supervision:Cortes, Kantarjian. Conflict of Interest Disclosures:Dr Kadia has received research funding from GlaxoSmithKline and Celgene and has served on advisory boards for Sunesis Inc and Novartis Inc. Dr Ravandi has received research funding from Seattle Genetics, Sunesis, Merck, Novartis, Cellarant, Actinium, and Celgene and has served on advisory boards for Amgen, Seattle Genetics, Sunesis, and Astra Zeneca. Dr Cortes has received research funding from Ariad, Ambit, Astellas, Arog, and Novartis and has served as a consultant for Novartis and Astellas. Dr Kantarjian has received research funding from Agios Pharmaceuticals, Amgen, and Pfizer. Funding/Support:This work was supported in part by the National Institutes of Health (NIH) through MD Anderson Cancer Center Support Grant CA016672.
Role of the Funder/Sponsor:The NIH had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
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