Mixed phenotype acute leukemia (MPAL) is a rare and aggressive hematologic malignancy classified as an acute leukemia of ambiguous lineage. Rather than displaying the characteristics of a single blood cell type, MPAL blasts express markers of more than one lineage simultaneously — most often myeloid and lymphoid — or comprise two distinct blast populations each belonging to separate lineages. Recognized subtypes include MPAL with the Philadelphia chromosome translocation t(9;22)(q34.1;q11.2), MPAL with KMT2A rearrangements involving 11q23.3, and acute biphenotypic leukemia. The condition arises from acquired, somatic alterations in a hematopoietic precursor cell and does not follow a heritable familial pattern. Its biological complexity distinguishes it from conventional acute myeloid and lymphoblastic leukemias and presents significant diagnostic and therapeutic challenges for treating teams.

The presenting features of mixed phenotype acute leukemia broadly resemble those of other acute leukemias, arising from the displacement of normal bone marrow elements by rapidly proliferating blast cells. Anemia produces fatigue, pallor, and exertional breathlessness. Deficient normal white blood cell production leads to susceptibility to bacterial, viral, and fungal infections, which may manifest as persistent fever. Thrombocytopenia results in bleeding from mucosal surfaces, easy bruising, and petechiae. Bone pain — particularly in the sternum, spine, and long bones — reflects marrow infiltration. Hepatosplenomegaly and peripheral lymphadenopathy are observed in a subset of cases. In individuals with markedly elevated blast counts, hyperviscosity or leukostasis can produce respiratory compromise and neurological changes. Given the nonspecific nature of these features, MPAL is typically identified only after laboratory and bone marrow evaluation initiated for unexplained cytopenias or constitutional symptoms.

Mixed phenotype acute leukemia is caused by acquired somatic genetic and chromosomal alterations within a hematopoietic progenitor cell. The normal process of blood cell development involves progressive restriction to a single lineage; in MPAL, this developmental commitment is disrupted, resulting in blast populations that retain or aberrantly re-express markers from multiple lineages. Two chromosomal rearrangements define specific subtypes: the BCR-ABL1 fusion gene arising from the t(9;22)(q34.1;q11.2) translocation, and rearrangements of the KMT2A locus at 11q23.3. Cases lacking these defining translocations are categorized as NOS (not otherwise specified), and their molecular drivers remain an active area of investigation. No germline or heritable genetic predisposition has been established as a cause of MPAL. As with other acute leukemias, prior exposure to certain chemotherapeutic agents or ionizing radiation may contribute to leukemogenesis in a subset of adults.

Diagnosis of mixed phenotype acute leukemia requires integration of bone marrow morphology, multiparameter flow cytometry, cytogenetics, and molecular testing. The 2022 World Health Organization classification specifies immunophenotypic thresholds for assigning myeloid lineage (myeloperoxidase expression or monocytic differentiation markers), B-lymphoid lineage (strong CD19 with co-expression of at least one additional B-cell marker), and T-lymphoid lineage (cytoplasmic CD3). A case qualifies as MPAL when a single blast population satisfies criteria for more than one lineage, or when two morphologically and immunophenotypically distinct blast populations are each assignable to different lineages. Conventional karyotyping and fluorescence in situ hybridization detect the Philadelphia chromosome and KMT2A rearrangements. RNA sequencing and targeted mutational panels are increasingly employed to characterize fusion transcripts and co-occurring mutations, which have prognostic relevance and may guide therapeutic selection.

Management of mixed phenotype acute leukemia involves specialist teams encompassing hematologists, oncologists, and, where applicable, hematopoietic stem cell transplant physicians. Because MPAL does not align with either standard acute myeloid or acute lymphoblastic leukemia, induction strategy is not universally established. In clinical practice, ALL-type induction chemotherapy is frequently selected as the initial approach. For MPAL with the Philadelphia chromosome, concurrent tyrosine kinase inhibitor therapy is incorporated given the BCR-ABL1 driver. Allogeneic stem cell transplantation in first complete remission is considered for many patients in light of elevated relapse risk. Forty-four active clinical trials are enrolling pediatric and adult patients with MPAL, investigating novel immunotherapeutic agents, targeted compounds, gene therapy approaches, and transplantation strategies. Participation in clinical trials through specialized leukemia centers represents an important pathway to access emerging therapies.

Outcomes in mixed phenotype acute leukemia are generally less favorable than those observed in most other acute leukemias, reflecting biological heterogeneity and the challenges of selecting effective induction therapy. Complete remission is achievable for many patients, but relapse rates are elevated compared to lineage-defined leukemias. MPAL with Philadelphia chromosome positivity may benefit from the addition of targeted kinase inhibitors, with some improvement in outcomes. Pediatric patients tend to fare better than adults. Allogeneic stem cell transplantation in first remission has been associated with improved long-term disease control in eligible patients who achieve remission. The rarity of MPAL limits the availability of large prospective outcome data, and individual prognosis is best assessed by treating hematologists with access to complete disease characterization including cytogenetics and molecular profile.

The research landscape for mixed phenotype acute leukemia reflects the challenges of studying a rare, biologically heterogeneous malignancy. Among 129 classified publications, case reports and case series predominate, supplemented by review articles and emerging clinical trial publications. Forty-four active clinical trials investigate a broad array of interventions, including targeted kinase inhibitors, immunotherapy, gene therapy, and transplantation optimization, with both academic and commercial sponsors represented. Pediatric cooperative groups — including the Children's Oncology Group and St. Jude Children's Research Hospital — are active contributors to the trial portfolio. Ongoing biomarker and gene therapy research is represented in the published literature. Multi-institutional registries and cooperative group infrastructure are critical to generating evidence in a disease whose rarity precludes single-center prospective trials.