PB1814: RARE PHENOTYPE WITH CD33 ABSENT IN ACUTE MYELOID LEUKEMIA MIMICKING MRD POSITIVITY: A CASE REPORT

Abstract

Background: Treatment of acute myeloid leukemia (AML) remains difficult to this day, with high rates of relapse that lead to poor long-term prognosis and low cure rates and are usually driven by residual leukemic cells that can be identified as measurable residual disease (MRD). Multiparametric Flow Cytometry (MFC) has been a fast and effective tool for detection of MRD, but disease heterogeneity makes MRD assessment a clinical challenge, especially in cases with rare phenotype, needing clinical and technical expertise to be accurately performed. Aims: To show a rare case of AML with absent CD33 expression and the importance of accurately interpreting MDR in AML. Methods: We report a clinical case of AML with absent CD33 assessed for FLT3 and NPM1 mutation, RUNX1/RUNX1T1 and CBFB-MYH11 rearrangements, karyotype and an 8-color panel immunophenotyping (Table 1). Empty gating strategy was used to identify LAIPs (Leukemia Associated-Immunophenotype), different from normal and leukemic stem cells, compared to healthy BM from diagnosis and follow up. Assays were performed at the Hematology Laboratory of Ribeirão Preto the Medical School, University of São Paulo, in accordance to Local Ethical Boards. Results: A 21-year-old male with AML without maturation, presented anemia (7.9 g/dL), thrombocytopenia (26 000/µL) and a total white blood cell count of 5710/μL with 84% of blasts in BM. MFC revealed CD45dim and low side scatter, CD117+, CD34+, HLA-DR+, CD38+, CD123+, CD11c+, CD64+, CD15+, CD133+, CD14-, CD7-, CD11b-, CD4-, CD56-, CD19-, CD41a-, NG2- and, unexpectedly, completely negative CD33. No metaphases or molecular abnormality was found. Treatment consisted of 2 induction cycles (cycle 1: 3 days of Daunorubicin 60 mg/m2 and 7 days of cytarabine 200 mg/m2; cycle 2: 6 days of cytarabine 1 g/m2 twice a day). Consolidation consisted of 1 or 2 cycles of 6 days of cytarabine 1 g/m2 twice a day. BM were obtained at diagnosis, after 1st and 2nd induction, and 3, 9, 12 and 15 months after consolidation of chemotherapy. A specific phenotype (CD117+/CD34+/CD33wk/CD13+ ≥ 0.1%) was preserved from diagnosis to all follow up points: 0,506%, 0,681%, 0,402%, 0,101%, 0,158% and 0,122% (Figure 1). Complete remission was achieved by day 30 after 1st induction and no change in the outcome was reported, even though an altered maturational phenotype persisted with complete absent CD33 expression, during and after treatment, suggesting a very rare polymorphism of CD33 receptor that mimicked MRD positivity. Image:Summary/Conclusion: Post-analytical phase of MFC is the most vulnerable to wrong interpretations. CD33 is a myeloid antigen expressed on malignant blasts in AML and had been reported as a potential target therapy. CD33low blasts are associated to a more mature AML and its high expression is related to adverse landscapes in AML, highlighting the importance of its evaluation. Standard care for AML enrolls MRD monitoring and failure to achieve an MRD-negative, CR or detected MRD during or after therapy is associated with relapse. Still, MRD analysis must embrace a set of standards in order to prevent post-analytic errors. Here we present a case report of AML with persistent absent CD33 expression since diagnosis that we found to be mimicking MRD positivity. Our data emphasizes the importance of clinical and technical expertise in MFC analysis and correlation of diagnostic and follow-up interpretation, especially in a heterogeneous disease such as AML, bringing up the need of standardization and constant training of the team.