Mixed phenotype acute leukemia

INTRODUCTION — Mixed phenotype acute leukemia (MPAL) refers to acute leukemia that displays an ambiguous pattern of antigen expression (ie, reflecting more than one hematopoietic lineage), to a degree that it cannot be unequivocally assigned to one lineage. MPAL, as defined by the World Health Organization (WHO), includes categories that were previously referred to as bilineage leukemia and biphenotypic acute leukemia [1].

This topic will discuss clinical manifestations, diagnosis, and treatment of MPAL in adults and children.

Related topics that include further details about specific aspects of acute leukemia are presented separately:

●(See "Acute myeloid leukemia in adults: Overview".)

●(See "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia".)

●(See "Overview of the clinical presentation and diagnosis of acute lymphoblastic leukemia/lymphoma in children".)

TERMINOLOGY — Nomenclature of mixed phenotype acute leukemia (MPAL) has evolved. The 2016 World Health Organization (WHO) definition of MPAL includes cases that were previously referred to as bilineage leukemia (two distinct populations of blasts that can be assigned to different lineages) and biphenotypic acute leukemia (a single blast cell population that expresses antigens of more than one lineage on the same cell) [1,2].

After establishing the diagnosis of MPAL, each case of MPAL should be classified as either:

●Philadelphia chromosome-positive (Ph+) MPAL also known as MPAL with t(9;22)(q34.1;q11.2); BCR-ABL1, is characterized by detection of the Philadelphia chromosome, t(9;22)(q34.1;q11.2), and/or the associated BCR-ABL1 rearrangement. (See 'Ph+ MPAL' below.)

●Philadelphia chromosome-negative (Ph-negative) MPAL, all other categories of MPAL are considered Ph-negative MPAL. (See 'Ph-negative MPAL' below.)

In addition, all cases should be categorized according to 2016 WHO criteria (table 1) [1]. (See 'Classification of MPAL' below.)

EPIDEMIOLOGY — MPAL is rare, but the true incidence is uncertain due to changes in disease nomenclature and diagnostic criteria. MPAL accounts for <3 percent of acute leukemia according to retrospective studies [3-6]. A population-based study reported 0.35 cases of MPAL/1,000,000 person-years, based on United States Surveillance, Epidemiology, and End Results registry (SEER) data [7].

MPAL occurs more frequently in adults than in children, but it is seen in patients of all ages and has a bimodal age distribution, with peaks at ages 19 and ≥60 years [1,7,8]. A male preponderance (estimated as 1.5:1) is reported [8]. No familial cases of MPAL have been reported.

MPAL B/myeloid, not otherwise specified (NOS) accounts for approximately two-thirds of MPAL; Philadelphia chromosome-positive MPAL approximately one-quarter of cases; and the remainder comprise MPAL with t(v;11q23.3); KMT2A-rearranged; MPAL T/myeloid, NOS; and rare cases of triphenotypic MPAL (ie, B- plus T-lymphoid plus myeloid) [8-10].

CLINICAL PRESENTATION — Patients with MPAL generally present with clinical findings that reflect pancytopenia, such as fatigue, weakness, infections, and/or bleeding (eg, epistaxis, gingival bleeding, ecchymoses). Other findings may relate to accumulation of leukemic cells in peripheral blood, bone marrow, or extramedullary sites (eg, abdominal fullness from splenomegaly, bone pain). The clinical presentation does not distinguish MPAL from other forms of acute leukemia. (See "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia", section on 'Clinical presentation'.)

PATHOLOGIC FEATURES — Pathologic features of MPAL include:

Peripheral blood — The white blood cell count in MPAL at presentation is variable, and can range from <1000 cells/microL with rare circulating leukemic blasts to >100,000 blasts/microL. Anemia and thrombocytopenia are almost invariably present.

The blood smear may reveal a morphologically uniform blast cell population, or two distinct populations; these morphologic patterns reflect bone marrow findings, as described below.

Bone marrow morphology — By definition, the bone marrow blast count is ≥20 percent, but may exceed 95 percent. Residual normal hematopoiesis is proportionately reduced, but morphologically unremarkable.

The morphologic features of MPAL leukemic blasts are not diagnostic. Typically, one of two patterns is seen:

●Leukemic blasts in MPAL are morphologically uniform, often with no distinguishing features, or

●Leukemic blasts are dimorphic (ie, two distinct populations), generally with smaller lymphoid blasts and larger myeloid blasts/monoblasts

Either morphologic pattern can be seen with any of the World Health Organization categories of MPAL. (See 'Classification of MPAL' below.)

The morphology and immunophenotype of MPAL blasts may evolve over time, so that relapsed disease or leukemia that persists after treatment can demonstrate a lineage switch and appear as pure acute lymphoblastic leukemia (ALL) or pure acute myeloid leukemia (AML).

EVALUATION OF MPAL — Diagnostic evaluation of MPAL requires morphologic, immunophenotypic, and cytogenetic testing, as described below. (See 'Diagnostic evaluation' below.)

After establishing the diagnosis of MPAL, each case should be classified, as described below. (See 'Classification of MPAL' below.)

Pretreatment evaluation of the patient with MPAL should include an initial diagnostic lumbar puncture, as described below. (See 'Pretreatment evaluation' below.)

Diagnostic evaluation — Diagnosis of MPAL requires demonstration of leukemic blasts that express markers of more than one hematopoietic lineage and exclusion of other specific categories of leukemia, as described below. (See 'Differential diagnosis' below.)

Diagnostic evaluation of MPAL requires all of the following:

●Morphology – ≥20 percent leukemic blasts in bone marrow and/or documentation of extramedullary leukemia. (See "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia", section on 'Diagnosis'.)

●Ambiguous lineage – Leukemic blasts cannot be unequivocally assigned to a single hematopoietic lineage. (See 'Lineage ambiguity' below.)

●Cytogenetics – Chromosome analysis, as described below. (See 'Cytogenetics' below.)

Lineage ambiguity — Diagnosis of MPAL requires demonstration of myeloid differentiation plus a lymphoid phenotype by flow cytometry, with or without other lineage-defining techniques.

Flow cytometry — Multiparameter flow cytometry is the preferred method for immunophenotyping MPAL and enables characterization of small populations of blasts with aberrant phenotype. Flow cytometry may reveal bilineal or biphenotypic patterns of antigen expression. (See 'Terminology' above.)

World Health Organization (WHO) definitions of lineage-positivity in MPAL include (table 1):

●B cell – Strong expression of CD19 (at least as bright as normal B cells) together with at least one other B cell marker (eg, CD10, cytoplasmic CD79a, CD22); if CD19 expression is dim or absent, ≥2 or ≥3 other B cell markers, respectively, must be expressed.

●T cell – Strong expression of cytoplasmic CD3 (with some blasts at least as bright as normal residual T lymphocytes); surface CD3 is usually negative.

●Myeloid – Myeloperoxidase (MPO) expression or ≥2 monocytic markers (CD11c, CD14, CD36, CD64).

In the absence of MPO expression or evidence of monocytic differentiation, expression of the myeloid markers, CD13 and/or CD33, is not sufficient for a diagnosis of MPAL, and must be supplemented by cytochemical studies, as described below.

Other techniques — Other techniques can provide supportive evidence of hematopoietic lineage.

●Enzyme cytochemistry

•Myeloperoxidase (MPO) – MPO cytochemistry is a robust method for assigning myeloid lineage; MPO positivity in >3 percent blasts constitutes evidence for myeloid differentiation.

•Nonspecific esterase (NSE) – NSE positivity can be used as evidence of monocytic differentiation (as an alternative to the monocytic cell surface markers described for flow cytometry, above)

●Immunohistochemistry – Immunohistochemistry may provide supportive evidence, but in the absence of lineage-defining results from flow cytometry or enzyme cytochemistry, the diagnosis of MPAL with immunohistochemistry should be made with caution, because immunohistochemistry does not permit distinction between small numbers of residual normal cells and the leukemic cells.

Cytogenetics — Chromosomal analysis is required to detect t(9;22) (the Philadelphia chromosome [Ph]) or t(v;11q23.3), and may also detect additional numerical and/or structural aberrations. (See "Tools for genetics and genomics: Cytogenetics and molecular genetics", section on 'Detecting cytogenetic abnormalities'.)

Cytogenetic analysis is also required to exclude other leukemia diagnoses. (See 'Differential diagnosis' below.)

Fluorescence in situ hybridization (FISH) can define the category of MPAL, but it does not conclusively exclude other leukemia diagnoses on the basis of cytogenetic findings. (See 'Differential diagnosis' below.)

No chromosomal or genetic abnormality is exclusively associated with MPAL. Aside from BCR-ABL1 and KMT2A, no chromosomal/genetic abnormality is present with high enough frequency to permit further clinical classification [4,8,9].

Cytogenetic abnormalities are reported in 59 to 91 percent of MPAL cases [4,8,9,11]. Cytogenetic abnormalities that have been reported in more than one case of MPAL include: del(6p), t(6;14), 12p11.2 abnormalities, del(5q), structural abnormalities of chromosome 7, numerical abnormalities including trisomy of chromosomes 4 and 8, and polysomy 8. However, complex karyotype and myelodysplasia-related AML-defining cytogenetics (eg, monosomy 5 or 7) should exclude the diagnosis of MPAL, since they would satisfy an alternative AML diagnosis (ie, AML with myelodysplasia-related changes), as discussed below. (See 'Differential diagnosis' below.)

Molecular evaluation — RT-PCR (reverse transcriptase polymerase chain reaction) or next generation sequencing may be useful for defining the category of MPAL, based on detection of BCR-ABL1 or KMT2A. (See "Tools for genetics and genomics: Cytogenetics and molecular genetics", section on 'Detecting known mutations'.)

The pattern of gene expression in MPAL appears to be distinct from that of other acute leukemias [12], but analysis of gene expression is not a diagnostic criterion and is not required for the diagnosis of MPAL.

MPAL is associated with a high frequency of somatic mutations in genes encoding epigenetic regulators, tumor suppressors, and transcription factors. Whole exome sequencing of 23 pediatric and adult MPAL specimens reported that one-third had mutations in epigenetic regulators (eg, DNMT3A, IDH2, EZH2); mutations were also identified in the RAS pathway, TP53, NOTCH1, and other genes [13]. Studies that used targeted sequencing reported similar findings [4,14].

Classification of MPAL — After establishing the diagnosis, each case of MPAL should be classified as:

●Either Philadelphia chromosome (Ph)-positive MPAL or Ph-negative MPAL, based on cytogenetic findings (see 'Cytogenetics' above)

and

●Assigned a WHO category (table 2), as described below. (See 'Ph+ MPAL' below and 'Ph(-) categories' below.)

Pretreatment evaluation — Pretreatment evaluation of acute leukemia is discussed separately. (See "Acute myeloid leukemia: Induction therapy in medically fit adults", section on 'Pretreatment'.)

Lumbar puncture (LP) — We suggest performing an initial diagnostic LP (including cytology and flow cytometry) for all patients with MPAL. The first dose of intrathecal (IT) chemotherapy should be administered at the time of the initial diagnostic LP, as described below. (See 'CNS management' below.)

The following cautions should be noted:

●For patients with focal neurologic findings, altered mental status, recent seizure, or papilledema on retinal exam, computed tomography (CT) should be performed prior to LP to assess possible causes of elevated intracranial pressure. (See "Lumbar puncture: Technique, contraindications, and complications in adults".)

●For patients with a platelet count ≤20,000/microL, we suggest platelet transfusion immediately prior to the procedure. There is controversy regarding what constitutes a "safe" platelet count, as discussed separately. (See "Involvement of the central nervous system (CNS) with acute myeloid leukemia (AML)", section on 'Lumbar puncture (LP)'.)

●LP should be performed regardless of the peripheral blood leukemic blast count.

●LP should be performed before beginning leukemia therapy (or as soon as possible), but we suggest not delaying the initiation of chemotherapy in order to perform the LP.

An LP should be performed at any time during or after treatment of acute leukemia, if new neurologic symptoms appear. Further management of the central nervous system (CNS) is discussed below. (See 'CNS management' below.)

DIFFERENTIAL DIAGNOSIS — Diagnosing MPAL requires excluding other types of leukemia, as described above. (See 'Diagnostic evaluation' above.)

Distinguishing MPAL from other types of leukemia is challenging and requires expert pathology support. A prerequisite for the diagnosis of MPAL is the absence of cytogenetic findings or clinical context that would define an alternative leukemia diagnosis (table 2), as described in the sections below.

MPAL is difficult to distinguish from other leukemias because:

●The morphologic features are generally nondiagnostic

●Leukemic blasts may be either bilineal or biphenotypic

●The immunophenotype is, by definition, ambiguous

●There is no pathognomonic cytogenetic or molecular finding

●Lineage switching may occur spontaneously or after treatment

Morphologic, immunologic, and cytogenetic criteria for the diagnosis of MPAL are described above. (See 'Diagnostic evaluation' above.)

Acute undifferentiated leukemia — Acute undifferentiated leukemia (AUL) blast cells are morphologically bland and generally indistinguishable from those of MPAL.

Distinguishing between these entities depends on immunophenotype. AUL blasts do not express lineage-specific antigens by flow cytometry or immunohistochemistry; in contrast, MPAL blasts express both myeloid and lymphoid antigens. There is no cytogenetic or molecular finding that distinguishes these leukemias.

Acute myeloid leukemia — Acute myeloid leukemia (AML) may share morphologic, immunophenotypic, and genetic findings with MPAL.

AML that can be classified on the basis of cytogenetic/molecular findings, dysplasia of other hematopoietic lineages, or clinical context should not be categorized as MPAL (table 2). (See "Acute myeloid leukemia: Classification".)

Examples include:

●AML with other category-defining cytogenetic or genetic features should be so-classified:

•AML with t(8;21)(q22;q22.1);RUNX1-RUNX1T1 which, in some cases, can express B-lymphoid or stem cell antigens in addition to myeloid antigens

•AML with inv(16)(p13.1q22) or t(16;16)(p13.1;q22);CBFB-MYH11

•AML with biallelic mutations of CEBPA

●AML with myelodysplasia-related changes is distinguished from MPAL by dysplasia in erythroid precursors and/or megakaryocytes, and/or characteristic myelodysplastic syndrome (MDS)-associated cytogenetic abnormalities (eg, loss of chromosome 7 or del(7q), del(5q) and t(5q))

●AML that arises after chemotherapy and/or radiation therapy should be classified as therapy-related AML

FGFR1 rearrangement — Myeloid/Lymphoid neoplasms with FGFR1 rearrangement may be morphologically and immunophenotypically indistinguishable from MPAL.

Detection of FGFR1 rearrangement (with or without associated chromosomal abnormalities) distinguishes this disorder from MPAL.

Acute lymphoblastic leukemia — Acute lymphoblastic leukemia (ALL) may be morphologically indistinguishable from MPAL.

Leukemic blasts of ALL express lymphoid antigens, but can be distinguished from MPAL because they generally do not express myeloid antigens. An exception is B-lymphoblastic leukemia/lymphoma with t(12;21)(p13.2;q22.1); ETV6-RUNX1, which can express both lymphoid and myeloid antigens; however, such cases should be classified as B-lymphoblastic leukemia based on the diagnostic cytogenetic findings.

Mutations of PAX5 and IL7R may be seen in ALL and have not been reported in MPAL, but this finding is not a diagnostic feature that can be used to distinguish between ALL and MPAL [15,16].

Early T cell precursor ALL (ETP-ALL) can express multiple myeloid antigens but can be distinguished from MPAL, T/myeloid by the lack of myeloperoxidase (MPO) expression.

CML with mixed blast crisis — Chronic myeloid leukemia (CML) in mixed blast crisis can be indistinguishable from Philadelphia chromosome-positive (Ph+) MPAL on the basis of morphology, immunophenotype (ie, expression of both myeloid and lymphoid antigens), and cytogenetic features: t(9;22) and/or BCR-ABL1.

The BCR-ABL1 isoform is helpful, but does not unequivocally distinguish between CML in mixed blast crisis and MPAL. CML usually expresses the p210 isoform, but it can express p190, which is the isoform most commonly expressed in MPAL; conversely, MPAL can express the BCR-ABL1 p210 isoform.

Ultimately, the clinical context (ie, findings of prior chronic or accelerated phase of CML) may be needed to distinguish between these entities.

Distinguishing Ph+ MPAL from CML in mixed blast crisis has important therapeutic implications, as follows:

●CML with mixed blast crisis is typically treated with a tyrosine kinase inhibitor (TKI) plus AML-like remission induction therapy, followed by allogeneic transplantation. (See "Treatment of chronic myeloid leukemia in blast crisis", section on 'Myeloid blast crisis'.)

●In contrast, Ph+ MPAL is usually treated with a TKI plus ALL-like remission induction therapy, followed by allogeneic transplantation, as described below. (See 'Treatment of Ph+ MPAL' below.)

PH-NEGATIVE MPAL — MPAL that does not exhibit the Philadelphia chromosome (Ph) and/or BCR-ABL1 is considered Ph-negative MPAL.

Ph(-) categories — The 2016 World Health Organization (WHO) classification includes the following categories of Ph-negative MPAL based on cytogenetic/genetic findings and immunophenotype, as described above. (See 'Evaluation of MPAL' above.)

Integrative genomic analyses indicate that the various categories of Ph(-) MPAL have distinct genetic and epigenetic features [17,18]. As examples, rearrangement of ZNF384 is common in MPAL B/myeloid, whereas biallelic WT1 alterations are common in T/myeloid MPAL. The DNA methylation signatures also differ with regard to lineage committed gene expression; this may provide an opportunity to stratify therapy for Ph(-) MPAL into AML-type and ALL-type treatment in the future.

MPAL with t(v;11q23.3); KMT2A-rearranged — MPAL in which t(v;11q23.3) and/or associated KMT2A rearrangements are detected.

This category of MPAL is more common in the pediatric population than in adults [9]. KMT2A-rearrangement was seen in <10 percent of patients with MPAL in the two largest series [4,8].

MPAL with t(v;11q23.3) is typically bilineal (two distinct populations of blasts), and only rarely biphenotypic (co-expression of lymphoid and myeloid markers on the same cell). Most cases have B-lymphoblasts and the myeloid blasts are usually monoblastic [19]. Phenotypic switch and recurrence with a single phenotype has been reported.

MPAL with t(v;11q23.3) must be distinguished from acute B- or T-lymphoblastic leukemia and acute myeloid leukemia (AML). (See 'Differential diagnosis' above.)

MPAL B/myeloid, NOS — MPAL that lacks defining cytogenetic/genetic findings (ie, no detectable Ph chromosome or t(v;11q23.3)) and has a B-lymphoblast immunophenotype, as described above. (See 'Flow cytometry' above.)

The B/myeloid subtype comprises about two-thirds of MPAL cases [8].

MPAL T/myeloid, NOS — MPAL that lacks defining cytogenetic/genetic findings and has a T-lymphoblast immunophenotype, as described above. (See 'Flow cytometry' above.)

One study reported worse outcome for the T/myeloid subtype compared to B/myeloid [20].

Mutations of DNMT3A and FLT3 have been reported in T/myeloid MPAL [13,21].

Treatment of Ph(-) MPAL — There is no consensus regarding optimal therapy of Ph-negative MPAL.

Ph(-) remission induction — There have been no prospective clinical trials and only limited retrospective studies of Ph-negative MPAL. Populations in various studies may differ because of changes in disease nosology, as described above. (See 'Terminology' above.)

We suggest remission induction therapy with an acute lymphoblastic leukemia (ALL)-like regimen rather than an acute myeloid leukemia (AML)-like regimen, based on observational data that indicate more favorable outcomes and less toxicity. This suggestion is in agreement with a consensus statement from the Children's Oncology Group (COG) [22].

Our approach to treatment of Ph-negative MPAL is guided by age:

●For patients <40 years old, we treat with a pediatric-type ALL regimen, as described separately. (See "Treatment of acute lymphoblastic leukemia/lymphoma in children and adolescents".)

●For patients ≥40 years old, we treat with an adult-type ALL regimen, as described separately. (See "Induction therapy for Philadelphia chromosome negative acute lymphoblastic leukemia in adults", section on 'Chemotherapy'.)

All patients should receive prophylaxis for central nervous system (CNS) involvement, as described below. (See 'CNS management' below.)

Immediately after completing remission induction therapy (ie, day 29), bone marrow examination should be performed, including assessment for measurable residual disease (MRD; also referred to as minimal residual disease) by flow cytometry or polymerase chain reaction (PCR)-based assay. (See "Detection of measurable residual disease in acute lymphoblastic leukemia/lymphoblastic lymphoma", section on 'Methods for detecting MRD'.)

All patients with Ph-negative MPAL should undergo human leukocyte antigen (HLA) typing early in the course of disease in anticipation of possible allogeneic hematopoietic cell transplantation (HCT).

Using ALL-like remission induction therapy, complete remission (CR) rates of 61 to 85 percent and median overall survival (OS) of 15 to 8 months are reported [23].

Most studies suggest that superior clinical outcomes are associated with ALL-like therapy, when compared with AML-like regimens, although results are not entirely consistent across all studies. As an example, compared with AML-like therapy, ALL-like remission induction therapy achieved a higher CR rate (85 versus 41 percent) in a study of 100 pediatric and adult patients with MPAL [8]. Other studies also reported superior remission rates with ALL-like therapy versus AML-like approaches [24-27]. However, a single institution study reported similar outcomes with ALL-like and AML-like induction regimens (CR 90 versus 84 percent, respectively) among 36 patients with MPAL; CR rates for adults and children were 87 and 85 percent, respectively [28].

Compared with ALL-like regimens, AML-like regimens are generally more toxic (eg, more prolonged and profound cytopenias, infections, cardiac complications, but less neuropathy), but no studies have directly compared toxicity of these regimens in the setting of MPAL. Hybrid ALL/AML remission induction approaches for MPAL achieve comparable outcomes to ALL-like therapy, but are associated with greater toxicity [4,20,24,25,29,30].

For patients who are medically frail, acceptable options include participation in a clinical trial or approaches that focus on palliation of symptoms, as described separately.

CNS management — All patients with MPAL should undergo an initial diagnostic lumbar puncture (LP), as described above. (See 'Lumbar puncture (LP)' above.)

The first dose of intrathecal (IT) chemotherapy should be administered at the time of the initial diagnostic LP (usually day 1 of remission induction therapy), and IT chemotherapy should be repeated on day 8.

Each IT treatment should include 12 to 15 mg preservative-free methotrexate (MTX) or 30 to 50 mg of cytarabine, with or without 50 mg hydrocortisone (HC).

Four additional IT treatments should be administered. Our suggestions for the schedule are informed by the initial diagnostic LP:

●Positive CSF – Initial LP: >5 white blood cells (WBC)/microL or cytospin positive for lymphoblasts:

•IT chemotherapy on days 15 and 22 of remission induction and days 1 and 8 after completing remission induction therapy

●Negative CSF – Initial LP: ≤5 WBC/microL and cytospin negative for lymphoblasts:

•IT chemotherapy on days 1, 8, 15, and 22 following recovery from remission induction therapy

Ph(-) post-remission therapy — There are no prospective studies of post-remission management of Ph-negative MPAL. Our suggestions are based on limited retrospective studies that are discussed below.

For patients whose day 29 bone marrow reveals CR and MRD <10^-2, our suggestions are:

●For patients who are medically eligible, we suggest allogeneic hematopoietic cell transplantation (HCT) in first CR (CR1) rather than observation or consolidation chemotherapy, based on superior outcomes and acceptable toxicity.

Eligibility for allogeneic HCT is discussed separately. (See "Determining eligibility for allogeneic hematopoietic cell transplantation".)

Management regarding allogeneic HCT is determined by the patient's age, and is discussed separately. (See "Post-remission therapy for Philadelphia chromosome negative acute lymphoblastic leukemia in adults", section on 'Allogeneic transplantation' and "Treatment of acute lymphoblastic leukemia/lymphoma in children and adolescents", section on 'Consolidation/intensification'.)

●For patients who are not eligible for allogeneic HCT, we suggest treatment with consolidation chemotherapy that is guided by the patient's age, as described separately. (See "Post-remission therapy for Philadelphia chromosome negative acute lymphoblastic leukemia in adults", section on 'Chemotherapy' and "Treatment of acute lymphoblastic leukemia/lymphoma in children and adolescents", section on 'Consolidation/intensification'.)

In general, allogeneic HCT is associated with improved outcomes compared with consolidation chemotherapy, and outcomes are similar to patients who undergo allogeneic HCT for AML and ALL [29,31-33].

Informative studies of post-remission management in MPAL include:

●A single center study of 66 patients with MPAL reported that, compared with consolidation chemotherapy, allogeneic HCT in CR1 was associated with superior outcomes (77 versus 16 percent three-year OS) [24]. Rates of relapse and nonrelapse mortality (NRM) following HCT were 21 and 10 percent, respectively. A smaller study reported comparable results [14].

●A study of 95 patients with MPAL from the Center for International Blood and Marrow Transplant Research (CIBMTR) registry reported that at three years, OS, NRM, relapse rate, and leukemia-free survival (LFS) were 67, 15, 29, and 56 percent, respectively [32]. Outcomes were comparable for patients age 20 to 40 years versus >40 years, and for patients transplanted in CR1 versus second remission.

We suggest not treating Ph-negative MPAL with maintenance chemotherapy, and there is no evidence that it provides a benefit in this setting.

Surveillance after post-remission therapy is described below. (See 'Surveillance' below.)

PH+ MPAL — Philadelphia chromosome-positive (Ph+) MPAL has the characteristic cytogenetic finding of t(9;22)(q34.1;q11.2) and/or expression of BCR-ABL1. Additional cytogenetic abnormalities may also be detected.

Ph+ MPAL represents approximately one-quarter of MPAL, and is more common in adults than in pediatric populations [8-10]. Ph+ MPAL is often associated with dimorphic morphology of leukemic blasts, and most cases have a B/myeloid phenotype. Most cases of Ph+ MPAL express the p190 BCR-ABL1 transcript [8-10].

Some series report that patients with Ph+ MPAL have a high incidence of central nervous system (CNS) involvement at presentation [30,34-36].

Ph+ MPAL was historically considered an adverse feature, but its prognostic impact is currently uncertain because of the routine use of tyrosine kinase inhibitors (TKIs). (See 'Prognostic factors' below.)

Ph+ MPAL must be distinguished from a mixed blast crisis of chronic myeloid leukemia (CML) because of important therapeutic differences, as described above. (See 'CML with mixed blast crisis' above.)

Treatment of Ph+ MPAL — There is no consensus regarding optimal management of Ph+ MPAL.

Our suggestions for management of Ph+ MPAL are largely informed by studies of Ph+ ALL. A retrospective analysis reported that outcomes for 13 patients with Ph+ MPAL were comparable to those of 27 patients with Ph+ ALL, including complete remission (CR; 100 versus 85 percent, respectively), five-year overall survival (OS; 55 versus 53 percent), and disease-free survival (DFS; 46 versus 42 percent) [37].

Our approach follows:

●Remission induction therapy should include a TKI plus an age-based Ph+ ALL remission induction regimen, as described separately. (See "Induction therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults", section on 'Remission induction therapy' and "Treatment of acute lymphoblastic leukemia/lymphoma in children and adolescents", section on 'Ph+ B-ALL/LBL'.)

We favor dasatinib because it penetrates the CNS, but an alternative TKI may be selected based on toxicity profile and comorbid illness. (See "Induction therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults", section on 'Choice of TKI'.)

Immediately after completing remission induction therapy (ie, day 29), bone marrow examination should be performed, including assessment of BCR-ABL1 for measurable residual disease (MRD; also referred to as minimal residual disease) by RT-PCR (reverse transcriptase polymerase chain reaction). (See "Detection of measurable residual disease in acute lymphoblastic leukemia/lymphoblastic lymphoma", section on 'Methods for detecting MRD'.)

All patients with Ph+ MPAL should undergo human leukocyte antigen (HLA) typing early in the course of disease in anticipation of possible allogeneic hematopoietic cell transplantation (HCT).

●CNS management should begin with the first dose of intrathecal (IT) chemotherapy given at the time of the initial diagnostic lumbar puncture (LP). Further details of CNS management are described above. (See 'CNS management' above.)

●Post-remission therapy:

•For patients who are medically eligible, we suggest allogeneic HCT in first CR (CR1) rather than consolidation chemotherapy plus a TKI, use of a TKI alone, or observation. This suggestion is based on superior outcomes of allogeneic HCT in Ph+ ALL and acceptable toxicity, as described separately. (See "Post-remission therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults".)

Eligibility for allogeneic HCT is discussed separately. (See "Determining eligibility for allogeneic hematopoietic cell transplantation".)

Management regarding allogeneic HCT is determined by the patient's age, and is discussed separately. (See "Post-remission therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults" and "Treatment of acute lymphoblastic leukemia/lymphoma in children and adolescents", section on 'Ph+ B-ALL/LBL'.)

•For patients who are not eligible for allogeneic HCT, we suggest consolidation chemotherapy plus a TKI.

●Maintenance therapy – There is no proven benefit for TKI maintenance therapy in Ph+ MPAL following either allogeneic HCT or consolidation chemotherapy.

We suggest maintenance therapy with dasatinib (or other TKI), based on the potential to prolong remission and acceptable toxicity in the setting of Ph+ ALL, as described separately. (See "Post-remission therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults", section on 'Maintenance therapy'.)

The duration of maintenance therapy is determined by the status of MRD in bone marrow after HCT or consolidation chemotherapy:

•MRD <0.01 percent (<10^-4) – Treat with TKI for 12 additional months

•MRD ≥0.01 percent (≥10^-4) – Duration of therapy is guided by MRD monitoring, as follows:

-Treat with TKI indefinitely for MRD ≥0.01 percent

-Treat with TKI for ≥12 months beyond two consecutive results with MRD <0.01 percent

Surveillance after post-remission therapy is described below. (See 'Surveillance' below.)

SURVEILLANCE — There is no consensus regarding an optimal protocol of surveillance for either Philadelphia chromosome (Ph)-positive or Ph-negative MPAL.

Our approach to surveillance for relapse and complications of therapy is informed by studies of acute lymphoblastic leukemia, and is discussed separately. (See "Post-remission therapy for Philadelphia chromosome negative acute lymphoblastic leukemia in adults", section on 'Follow-up' and "Post-remission therapy for Philadelphia chromosome positive acute lymphoblastic leukemia in adults".)

PROGNOSTIC FACTORS — Limited data are available, but factors associated with less favorable outcomes in MPAL are similar to those found in other categories of acute leukemia, including:

●Older age [7,8,10,12,29,30,34,35,38,39]

●High white blood cell count at diagnosis [20,28,30]

●Adverse cytogenetics [8,9,27,35] and KMT2A rearrangement [28]

●Failure to achieve remission [24,30,33,38,39]

●T/myeloid MPAL had less favorable outcomes than B/myeloid MPAL, according to one report [20]

Philadelphia chromosome-positive (Ph+) has been considered a poor prognostic feature in the past, but its prognostic significance in the era of routine use of tyrosine kinase inhibitors is uncertain [8].

RELAPSED/REFRACTORY MPAL — For patients with MPAL who do not achieve complete remission (refractory MPAL) or who relapse after an initial response, we favor participation in a clinical trial.

Outside of the context of a clinical trial, salvage therapy as used for either acute myeloid leukemia (AML) or acute lymphoid leukemia (ALL) is an acceptable approach.

We favor an AML-like regimen for patients who failed to achieve remission with an ALL-like remission induction regimen. If refractory or relapsed MPAL undergoes a lineage switch, selection of salvage therapy may be guided by immunophenotypic findings.

Treatment strategies for relapsed/refractory AML and ALL are discussed separately. (See "Treatment of relapsed or refractory acute lymphoblastic leukemia in adults" and "Treatment of relapsed or refractory acute myeloid leukemia".)

For patients with Philadelphia chromosome-positive MPAL who relapse or fail to achieve remission we suggest mutation analysis of BCR-ABL1 to guide the choice of TKI, as discussed separately. (See "Treatment of chronic myeloid leukemia in chronic phase after failure of initial therapy", section on 'Initial management'.)

Patients who achieve a remission with salvage therapy should be considered for allogeneic hematopoietic cell transplantation. (See "Post-remission therapy for Philadelphia chromosome negative acute lymphoblastic leukemia in adults", section on 'Allogeneic transplantation'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Acute myeloid leukemia".)

SUMMARY AND RECOMMENDATIONS

●Mixed phenotype acute leukemia (MPAL) refers to acute leukemia in which blast cells express an ambiguous pattern of antigens (ie, more than one hematopoietic lineage) to a degree that it cannot be unequivocally assigned to a single lineage. (See 'Terminology' above.)

●Diagnosis of MPAL is based on morphologic, immunologic, and cytogenetic findings defined by World Health Organization (WHO) 2016 criteria (table 1). (See 'Diagnostic evaluation' above.)

●The morphologic appearance of MPAL is not diagnostic, and distinguishing MPAL from other forms of leukemia requires expert interpretation of immunologic, cytogenetic, and molecular findings, as discussed above. (See 'Differential diagnosis' above.)

●After establishing the diagnosis of MPAL, each case should be classified as:

•Either Philadelphia chromosome (Ph)-positive MPAL or Ph-negative MPAL, based on cytogenetic findings (see 'Cytogenetics' above)

and

•Assigned a WHO category (table 2) as described above (see 'Ph+ MPAL' above and 'Ph(-) categories' above)

●Pretreatment evaluation of all patients with MPAL should include a diagnostic lumbar puncture. (See 'Lumbar puncture (LP)' above.)

●Ph-negative MPAL – Ph-negative MPAL comprises all cases of MPAL in which neither t(9;22) nor BCR-ABL1 is detected.

•We suggest remission induction therapy with an acute lymphoblastic leukemia (ALL)-like regimen, rather than acute myeloid leukemia (AML)-like induction therapy, because of more favorable outcomes and less toxicity (Grade 2C), as discussed above. (See 'Ph(-) remission induction' above.)

We suggest incorporating central nervous system (CNS) prophylaxis into induction therapy (Grade 2C) based on experience with Ph-negative ALL, as described above. (See 'CNS management' above.)

•Following achievement of remission, we suggest allogeneic hematopoietic cell transplantation (HCT) in first complete remission (CR1) rather than observation or consolidation chemotherapy, based on superior outcomes and acceptable toxicity (Grade 2B). For patients who are not eligible for allogeneic HCT, we treat with consolidation chemotherapy. (See 'Ph(-) post-remission therapy' above.)

●Ph-positive (Ph+) MPAL – Suggestions for management of Ph+ MPAL (ie, presence of t(9;22) and/or BCR-ABL1) are largely informed by experience with Ph+ ALL.

We recommend incorporation of a tyrosine kinase inhibitor (TKI) into all phases of treatment (Grade 1B). We generally use dasatinib because it penetrates the CNS. (See 'Treatment of Ph+ MPAL' above.)

•Remission induction therapy – For remission induction therapy, we treat with an age-based Ph+ ALL regimen, rather than an AML-like induction therapy, because of more favorable outcomes and less toxicity (Grade 2C).

•CNS prophylaxis – Treatment should include CNS prophylaxis (Grade 2C). (See 'CNS management' above.)

•Post-remission management – Following achievement of remission, we suggest allogeneic HCT in CR1 rather than consolidation chemotherapy plus a TKI, treatment with a TKI alone, or observation (Grade 2C).

•Maintenance therapy – We treat with dasatinib maintenance therapy after either allogeneic HCT or consolidation chemotherapy with duration of therapy guided by BCR-ABL1 measurable residual disease (MRD; also referred to as minimal residual disease) (Grade 2C), as described above. (See 'Treatment of Ph+ MPAL' above.)

●For patients with relapsed or refractory MPAL, we favor participation in a clinical trial.

Outside of the context of a clinical trial, we favor AML-like salvage therapy for MPAL that was refractory to an ALL-like remission induction regimen. Selection of salvage therapy for relapsed disease may be guided by the immunophenotype of the blasts. Mutation analysis of BCR-ABL1 should be performed for Ph+ MPAL. (See 'Relapsed/Refractory MPAL' above.)