Mixed phenotype acute leukemia: A review of diagnostic and treatment approaches

Acute leukemias generally have a distinct blast cell lineage, either myeloid (in acute myeloid leukemia [AML]), or lymphoid (in acute lymphoblastic leukemia [ALL]), but in rare cases lineage cannot be clearly assigned. Patients are classified with mixed phenotype acute leukemia (MPAL) if antigens from two or more lineages are expressed, either on the same blast cells or as separate blast populations. To date, diagnostic definitions for MPAL have been subjective, and research into optimal therapy has been lacking due to past exclusion from clinical trials. Current research efforts are providing more insight into the biology of this disease and helping to inform therapeutic decision-making.

A review of the current diagnostic and treatment approaches for MPAL has been published by Thomas Alexander and Etan Orgel in Current Oncology Reports¹ and is summarized below.

MPAL classification

Variable approaches to lineage assignment have led to inconsistency in defining MPAL. Table 1 outlines the classification of MPAL according to two predominant systems, the European Group for the Immunological Classification of Leukemias (EGIL), and the World Health Organization (WHO). Both systems characterize MPAL according to immunophenotype, but there are inconsistencies in MPAL definitions between the two criteria. The WHO classification is generally considered more restrictive than the EGIL scoring system, identifying fewer cases of MPAL.

Table 1. MPAL classification systems*

There are two main controversies surrounding the current classification of MPAL. Firstly, bilineal MPAL, which is characterized by at least two distinct phenotypic cell populations, is not currently distinguished from biphenotypic MPAL, which has a single phenotypic population expressing markers from multiple lineages on the same cells. Some researchers argue that there may be a pathological relevance to this distinction.

The second controversy surrounds cases with typical B-cell ALL (B-ALL) markers and a low percentage of myeloperoxidase (MPO)-positive blasts. From a technical perspective, variability in the specificity of flow cytometric detection antibodies may result in the misclassification of B-ALL as MPAL; the WHO 2016 clarifications have thus emphasized that a diagnosis of MPAL may not be appropriate if weak MPO expression is the only indicator of myeloid differentiation. Moreover, it has been queried whether children with B-ALL markers and isolated MPO expression should be classified as MPAL, since prognosis for these children has been reported to be superior to that for children with B-cell/myeloid MPAL diagnosed with additional myeloid markers, and inferior to children with B-ALL without MPO expression.

Disease biology

New insights into the genomic landscape of MPAL have been gained in recent years, including genetic differences between pediatric and adult patients.¹

• Adults with MPAL frequently harbor mutations that are also common in other adult acute leukemias, such as mutations in DNMT3A, IDH1, and IDH2. BCR-ABL translocation is found in around 15% of cases, whereas KMT2A rearrangements are infrequent.
• Broadly speaking, pediatric B-cell/myeloid MPAL has similar DNA mutations and gene expression profiles to pediatric B-ALL. For example, children with B-cell/myeloid MPAL (not otherwise specified) frequently have ZNF384 rearrangements, and they have a similar genomic profile to B-ALL with ZNF384 rearrangements.
• Pediatric T-cell/myeloid MPAL has a very similar mutational and gene expression profile to early T-cell precursor ALL, the only distinction being MPO expression in T-cell/myeloid MPAL. A shift in treatment approach towards risk-adapted ALL therapy, which has provided a good prognosis for early T-cell ALL, may therefore be warranted for T-cell/myeloid MPAL.

Treatment of MPAL

Establishing optimal treatment regimens for MPAL has been limited due to the inconsistent and evolving disease definition, resulting in a lack of uniformly diagnosed and treated patient populations. For most cases, MPAL disease biology is considered more like ALL than AML, favoring an ALL-directed treatment approach.

The questions that physicians commonly face when treating adult and pediatric patients with MPAL  include:

• Which initial therapy should be selected?
• Is there a role for hematopoietic stem cell transplant (HSCT)?
• What salvage therapy should be used after treatment failure?

Frontline therapy

Currently, an ALL-directed regimen is the recommended approach for initial therapy in either pediatric or adult patients with MPAL. The following evidence was presented in support of this:

• There is a growing body of data indicating that most patients with MPAL benefit from initial treatment with ALL-directed therapy, irrespective of age. A large retrospective study of patients with pediatric MPAL of any subtype reported five-year event free survival of 80% after ALL therapy, compared with ≤50% for patients receiving AML therapy or a hybrid-type regimen.¹
• ALL regimens benefit from less acute and chronic morbidity than AML regimens. However, it should be noted that in a large meta-analysis, there was no difference in overall survival between ALL and AML regimens for MPAL patients.¹

Intensified therapy is warranted for adult patients with MPAL due to inferior outcomes compared with patients with ALL. At present, there are little data to guide risk-stratified treatment approaches for MPAL, and the absence of a clearly defined high-risk population with inferior outcomes has hampered the testing of targeted therapies in the frontline setting.

HSCT consolidation

Survival for adult patients with MPAL is generally poor after treatment with chemotherapy alone. Improved survival demonstrated in small case studies of adult patients with MPAL receiving HSCT supports the current recommendation for HSCT consolidation in adult patients with MPAL in first complete remission.

Conversely, survival rates are high for pediatric patients with early response to chemotherapy, justifying the restricted use of HSCT for pediatric patients with relapsed and/or refractory disease.

Salvage therapy

Data guiding optimal salvage approaches after initial treatment failure for MPAL are lacking, and the identification of therapy failure presents a challenge due to the immunophenotypic complexity of the disease. The following observations were noted:

• Evidence to support a change from ALL- to AML-directed therapy, or vice versa, is limited, and further data to evaluate this strategy are being obtained.
• Salvage with HSCT consolidation is considered the best option for long-term survival for patients in morphologic remission who have not received HSCT in first complete remission.
• Targeted therapy, including blinatumomab, nelarabine, and nilotinib/lenalidomide, is being used in some cases to avoid the toxic effects of chemotherapy.
• There are published case reports describing the use of chimeric antigen receptor (CAR) T-cell therapy, but this approach has not been extensively tested due to concerns over lineage switching.

Summary

ALL-directed therapy is currently recommended for patients with pediatric or adult MPAL. With significant progress being made in the understanding of disease biology, it is anticipated that diagnostic criteria for MPAL will be refined, and treatment approaches for this heterogeneous population of patients will continue to evolve.