Acute myeloid leukemia (AML) is characterized by high risk for relapse and a 5-year overall survival (OS) rate of 30%. Prognostic factors for AML include age, presence of leukemic blasts in the bone marrow (BM), cytogenetic risk, mutational status, and performance status. Patients in morphologic remission (fewer than 5% leukemic blasts in the BM after initial therapy) may still harbor high levels of leukemic blasts, posing a high relapse risk.1
Mounting evidence indicates that the presence of measurable residual disease (MRD), defined as the persistence of leukemic blasts at levels below morphologic detection, may act as an independent predictor of relapse. MRD has been shown to be a stronger predictor of relapse than mutations at diagnosis and may be used to guide treatment for AML.
Standard morphologic examination is limited by low sensitivity and high interpersonal variability, prompting the development of novel detection methods that can detect leukemic blasts at submicroscopic levels. The application of reliable testing methods to treatment remains an unmet need in patients with AML.2
Techniques for Measuring MRD
Novel detection methods such as multicolor flow cytometry (MFC), polymerase chain reaction (PCR), and next-generation sequencing (NGS) are increasingly being used by hematologists to predict relapse.
MFC is the prevailing method for rapid and accurate determination of MRD; using MFC, immunophenotypic shifts expressed on cell surface antigens at diagnosis and during the course of AML can be detected. Most commonly, the patient’s leukemia-associated immunophenotype (LAIP) is determined at diagnosis and used to track residual leukemia cells. The LAIP method allows detection of 10-3 to 10-5 leukemic blasts. However, MFC requires an extensive panel of monoclonal antibodies and is limited by cost, the requisite skills, lack of sensitivity, and risk of false negatives.1
Molecular methods amplify genetic abnormalities in AML using various PCR and NGS techniques. Optimized real-time quantitative PCR is the test of choice for mutations in chimeric fusions genes created through balanced chromosomal rearrangements, such as CBFB-MYH11, RUNX1-RUNX1T1, or NPM1, as this method is more sensitive than MFC and can detect 10-4 to 10-6 leukemic blasts.
Although limited by insufficient sensitivity and specificity, NGS holds promise as a tool for MRD detection. Advances in technology may offer higher sensitivity for the quantification of RNA transcripts in patients with RUNX1-RUNX1T1-, CBF-MYH11- or NPM1-mutated leukemia.2
Applications of MRD Monitoring
Leukemic blast levels of less than 10-5 to 10-7 cells in the BM are strong predictors of relapse, but the exact leukemic blast level required to cause relapse is unknown. The evidence suggests that MRD monitoring can improve treatment outcomes and direct transplantation decisions.