Courtney D. DiNardo

Keywords:Acute myeloid leukemia;AML;Complete response;CR;fms-related tyrosine kinase 3;FLT3;Isocitrate dehydrogenase-1/2;IDH1/2;Overall survival;OS

ABSTRACT
The therapeutic landscape for acute myeloid leukemia (AML) has changed dramatically with the approval of targeted agents, including venetoclax, midostaurin, gilteritinib, ivosidenib, and en- asidenib, among others. However, older patients with AML continue to experience poorer out- comes and are in ongoing need of more effective and less toxic regimens. This review examines the efficacy of novel therapeutics and promising combination approaches to further improve outcomes in the treatment of patients with AML.

Introduction
Acute myeloid leukemia (AML) is a heterogeneous hematologic malignancy that affects approximately 4.3 adults per 100,000 per year in the United States. AML accounts for over 10,000 deaths annually and the median age at diagnosis is around 68 years [1]. The 5-year survival rate for younger patients (18–60 years) is approximately 40%, while it is a dismal 10% for patients above the age of 60 years [2]. Until recently, the treatment of AML relied on a combination of cytarabine and anthracycline, in a standard intensive regimen (7 + 3 chemotherapy), to which most older patients respond poorly. Within the past two years, the therapeutic arma- mentarium for AML has expanded, with the approval of several agents including CPX-351 (dual-drug liposomal encapsulation of cytarabine and daunorubicin), gemtuzumabozogamicin ±induction for CD33 + AML (humanized anti-CD33 monoclonal antibody conjugated with calicheamicin), venetoclax (selective inhibitor of B-cell lymphoma-2 or BCL-2) in combination with either a hy- pomethylating agent (HMA) or low-dose cytarabine (LDAC), glasdegib (inhibitor of the hedgehog signaling pathway) plus LDAC, midostaurin (fms-related tyrosine kinase 3 or FLT3 inhibitor)plus induction/consolidation chemotherapy, gilteritinib (FLT3 in- hibitor), ivosidenib (isocitrate dehydrogenase-1 or IDH1 inhibitor),and enasidenib (IDH2 inhibitor). As a result, treatment for AML has been revolutionized and recurrent mutations such as NPM1, FLT3, CEBPA and IDH1/IDH2 are increasingly used to guide therapy decisions [3]. Indeed these advances are expected to improve outcomes for patients; however, several unmet needs prevail. To this end, these approved therapies are currently being investigated in combination with other approved as well as emerging agents for different treatment settings and distinct patient subsets in AML. This paper reviews accumulating experience from a selection of these studies that potentially could address some of the outstanding medical needs of our diverse patients with AML.

Venetoclax is an oral selective inhibitor of the anti-apoptotic protein BCL-2 which has demonstrated impressive synergistic anti-CG, cytogenetic risk; CR, complete response; CRi, complete response with incomplete blood count recovery; HMA, hypomethylating agent; LDAC, low-dose ara-C; Ven, venetoclax.leukemic activity in preclinical studies when combined with an HMA. In clinical trials, combining venetoclax with an HMA such as azacitidine among older patients with AML has also been associated with synergistic effects; including a median overall survival of 16.9 months, which is impressive compared to historical expectations with an HMA alone (~10.4 months) [4–6]. Similarly, in older (≥60 years) patients with AML, venetoclax plus LDAC cellular bioimaging demonstrated rapid and durable remissions that were associated with a median OS of 13.5 months, which also compares favorably to historical expectations of LDAC alone (~5 months) [7,8]. These results have led to the recent accelerated approval of venetoclax in combination with either an HMA or LDAC for newly diagnosed AML or AML ineligible for intensive chemotherapy in adults aged 75 years and older [9]. However, these approved venetoclax combinations are not typically considered curative, and subgroup analyses have suggested molecular determinants of more durable remissions (mutations in IDH1, IDH2, NPM1) versus less durable (TP53 mutations or complex cytogenetics) responses (Table 1) [6,8].

In addition to combinations with lower-intensity therapy, the safety and efficacy of venetoclax in combination with intensive chemotherapy is now under investigation. For example, venetoclax in combination with dose-reduced intensive chemotherapy (5+2 regimen of cytarabine + idarubicin) is being studied for the treatment of fit, older patients with newly diagnosed AML who are eligible for intensive chemotherapy [10]. As observed, interim results showed an overall complete remission (CR)/CRi (CR with incomplete blood count recovery) rate of 71%, which was even higher (95%) inpatients with newly diagnosed AML, and lower (42%) in those with secondary AML [10]. Survival data from this study is not yet available. Additionally, a combination of venetoclax and the FLAG-IDA (fludarabine, cytarabine, idarubicin, granulocyte colony–stimulating factor) regimen is under investigation for the treatment offit patients with either newly diagnosed or relapsed/refractory (R/R) AML who are eligible for intensive therapy. Interim analysis from the first cohort of relapsed/refractory AML patients demonstrated that 75% of patients achieved CR/CRi, and 50% of responding patients were able to proceed to SCT. Accrual to the phase 2 portion of the trial is now ongoing [11].

Idasanutlin is an oral murine double minute 2 (MDM2) antagonist that blocks MDM2 interaction with p53 and results in acti- vation of p53 (in the setting of wild-type TP53) resulting in cell cycle arrest and apoptosis, preventing tumor growth. Idasanutlin is being investigated for the treatment of R/R AML in combination with venetoclax [12] or with araC [13] in ongoing trials, with encouraging preliminary results from early phase studies.TP53 mutations are commonly detected among elderly patients with AML and especially in patients with secondary or therapy- related AML [14], and are associated with particularly poor outcomes. There are no currently approved strategies to target mutant TP53, however there is currently significant interest in APR-246, which is a small molecule that selectively binds to mutant p53 protein, leading to reactivation. Outcomes from a phase 1b trial studying the efficacy of APR-246 in combination with azacitidine indicated 82% CR among patients with TP53 mutant MDS or oligoblastic AML, and this combination is now being investigated in a randomized phase 2 study [15].Ivosidenib and enasidenib have been approved as single agents for the treatment of mutant IDH1 and mutant IDH2 R/R AML, respectively. These agents as monotherapy have been associated with a CR rate of approximately 20%, an overall response rate (ORR) of approximately 40%, and a median OS of 9 months [16,17]. These IDH1/2 inhibitors are now being investigated in combination with induction therapy (7 + 3) among adult patients with newly diagnosed mutant IDH1 or mutant IDH2 AML. As shown in Table 2, the CR/CRi rates with 7 + 3 + either ivosidenib or enasidenib are > 70%, ORR > 85%, and 1-year OS is ≥ 75% for both groups, thereby providing the basis for a randomized phase 3 trial [18]. In the older and intensive chemotherapy ineligible population, several combinations with venetoclax or the IDH1/2 inhibitors have been evaluated in the frontline setting. Table 3 shows that many of these combinations are associated with improved outcomes. Additionally, a phase 1b/2 combination study is ongoing to evaluate the effect of ivosidenib and venetoclax with or without azacitidine among patients with IDH1 mutant AML [NCT03471260]. Overall, the treatment paradigm for elderly patients with IDH1/2 mutations is evolving at a rapid pace [19].

Another area of key interest and scientific development over the past decade has been immune-based therapies for AML. Currently, several antibody-drug conjugates (ADC) and bispecific antibodies are under investigation in early phase trials. These include IMGN632 (CD123 ADC), IMGN779 (CD33 ADC), flotetuzumab (CD123xCD3 dual-affinity re-targeting molecule), XmAb 14045 (CD3xCD123), AMG330 (CD3xCD33), AMV564 (CD3xCD33), cusatuzumab (CD70 plus azacitidine), ASP1235 (ADC against FLT3), MCLA-117(CD3xCLL1),among others [20]. The checkpoint inhibitors (ie, PD1,PDL1, CTLA4 inhibitors) are also under active evaluation in AML and recently results from the combination of the PD1 inhibitor nivolumab with azacitidine have been presented. Compared with historical HMA combinations among patients with R/R AML, nivolumab + azacitidine demonstrated encouraging median OS particularly in first salvage patients (10.5 months versus 5.4 months), consistent with other immunotherapy studies where the less pre-treated a patient is, the less exhausted their T-cell repertoire and the increased efficacy of immunotherapy-based therapy. This was further exemplified Human Tissue Products through correlative work demonstrating that responding patients to nivolumab + azacitidine had significantly higher tumor infiltrating CD3 and CD8 T cells at baseline in their bone marrow aspirates [21]. Thus, immune-based therapies also represent a promising approach for patients with AML.

Conclusions
The recent progress in the AML treatment landscape has heralded a new era of personalized medicine. Despite these advances, older patients with AML continue to have Apilimod order limited options and are still associated with poorer outcomes. Ongoing trials of combi- nation regimens that include approved agents such as venetoclax, ivosidenib, and enasidenib, along with other emerging and less toxic approaches, are expected to expand choices and improve outcomes for these difficult-to-treat patients.