Quantitation of Leukemic Stem Cell Populations Predicts Clinical Outcome in Acute Myeloid Leukaemia

Abstract 638

Relapse is a major cause of treatment failure in adults with Acute Myeloid Leukemia (AML) and new treatments are required. DNA methyltransferase inhibitors (DNMTi) (e.g. 5-Azacitidine (AZA)) alone or in combination with histone deacetylase inhibitors (HDI) (e.g. sodium valproate, (VAL)) represent a potentially important therapeutic advance. However, their clinical utility is limited by the short median response duration and relapse is almost universally observed. It has been postulated that chemoresistant leukemic stem/progenitor cells (LSC) contribute towards disease relapse but this hypothesis has never been examined clinically. We therefore correlated clinical outcomes in 79 patients with AML or myelodysplasia (MDS) ineligible for myelosuppressive chemotherapy who were treated with combined AZA and VAL with serial immunophenotypic and functional analysis of bone marrow stem/progenitor cells(1). Clinical and marrow response was assessed at 1, 3 and 6 months and clinical responses assessed according to Cheson criteria. 64 patients had AML and 57 had received prior intensive chemotherapy. 49 patients completed ≥3 cycles of AZA. Major clinical responses (MCR) (defined as acquisition of CR, CRi or PR) were observed in 26/79 patients (14 CR/Cri, 12 PR). MCR was achieved with a median of 2 cycles of therapy (range 1 to 4). 52% of patients who received ≥3 cycles of AZA achieved an MCR. In patients achieving a CR/CRi median survival was 18.2 months compared to 5 months for patients who either failed to respond or only achieved a PR (p=<0.001). There was no correlation between mutational status in DNMT3A, IDH1 or IDH2 genes and achievement of an MCR. These data confirm significant clinical activity of combined DNMTI and HDI therapy in high risk AML and MDS. In contrast to previous studies, suggest that the survival benefit associated with this treatment approach is correlated with marrow response. Abnormal immunophenotypic stem/progenitor compartments were documented in all patients at diagnosis. A greater reduction in the expanded LSC-containing compartments was seen in patients who achieved a CR and only in these patients were normal functional myeloid progenitors detected in-vitro. Persistence of leukemic clones in FACS-purified stem/progenitor populations was documented in all evaluable patients including those achieving CR. Expansion of this leukemic stem/progenitor population was shown to predict impending morphological relapse. Similar studies in 7 adults treated with myelosuppressive chemotherapy, showed clearance of residual leukemic stem/progenitor cells in 3 patients, all of whom remain in molecular remission. In contrast, expanded LSC-containing populations were detected in the other 4 patients, who either relapsed or had refractory disease. Taken together, this study demonstrates, for the first time, that epigenetic therapies fail to eliminate leukemic stem cell-containing populations. Similarly in patients treated with myelosuppressive chemotherapy persistence of leukemic stem/progenitor populations correlates with disease relapse. Consequently, immunophenotypic quantitation of leukemic stem cell-containing populations may represent a novel biomarker of response in AML.