Inhibition of mutant IDH1 promotes cycling of acute myeloid leukemia stem cells.

Approximately 20% of acute myeloid leukemia (AML) patients carry mutations in IDH1 or IDH2 that result in over-production of the oncometabolite D-2-hydroxyglutarate (2-HG). Small molecule inhibitors that block 2-HG synthesis can induce complete morphological remission; however, almost all patients eventually acquire drug resistance and relapse. Using a multi-allelic mouse model of IDH1-mutant AML, we demonstrate that the clinical IDH1 inhibitor AG-120 (ivosidenib) exerts cell-type-dependent effects on leukemic cells, promoting delayed disease regression. Although single-agent AG-120 treatment does not fully eradicate the disease, it increases cycling of rare leukemia stem cells and triggers transcriptional upregulation of the pyrimidine salvage pathway. Accordingly, AG-120 sensitizes IDH1-mutant AML to azacitidine, with the combination of AG-120 and azacitidine showing vastly improved efficacy in vivo. Our data highlight the impact of non-genetic heterogeneity on treatment response and provide a mechanistic rationale for the observed combinatorial effect of AG-120 and azacitidine in patients.

Bcor loss perturbs myeloid differentiation and promotes leukaemogenesis.

The BCL6 Corepressor (BCOR) is a component of a variant Polycomb repressive complex 1 (PRC1) that is essential for normal development. Recurrent mutations in the BCOR gene have been identified in acute myeloid leukaemia and myelodysplastic syndrome among other cancers; however, its function remains poorly understood. Here we examine the role of BCOR in haematopoiesis in vivo using a conditional mouse model that mimics the mutations observed in haematological malignancies. Inactivation of Bcor in haematopoietic stem cells (HSCs) results in expansion of myeloid progenitors and co-operates with oncogenic KrasG12D in the initiation of an aggressive and fully transplantable acute leukaemia. Gene expression analysis and chromatin immunoprecipitation sequencing reveals differential regulation of a subset of PRC1-target genes including HSC-associated transcription factors such as Hoxa7/9. This study provides mechanistic understanding of how BCOR regulates cell fate decisions and how loss of function contributes to the development of leukaemia.

PD-1 blockade enhances elotuzumab efficacy in mouse tumor models.

Elotuzumab, a humanized monoclonal antibody that binds human signaling lymphocytic activation molecule F7 (hSLAMF7) on myeloma cells, was developed to treat patients with multiple myeloma (MM). Elotuzumab has a dual mechanism of action that includes the direct activation of natural killer (NK) cells and the induction of NK cell-mediated antibody-dependent cellular cytotoxicity. This study aimed to characterize the effects of elotuzumab on NK cells in vitro and in patients with MM and to determine whether elotuzumab antitumor activity was improved by programmed death receptor-1 (PD-1) blockade. Elotuzumab promoted NK cell activation when added to a coculture of human NK cells and SLAMF7-expressing myeloma cells. An increased frequency of activated NK cells was observed in bone marrow aspirates from elotuzumab-treated patients. In mouse tumor models expressing hSLAMF7, maximal antitumor efficacy of a murine immunoglobulin G2a version of elotuzumab (elotuzumab-g2a) required both Fcγ receptor-expressing NK cells and CD8+ T cells and was significantly enhanced by coadministration of anti-PD-1 antibody. In these mouse models, elotuzumab-g2a and anti-PD-1 combination treatment promoted tumor-infiltrating NK and CD8+ T-cell activation, as well as increased intratumoral cytokine and chemokine release. These observations support the rationale for clinical investigation of elotuzumab/anti-PD-1 combination therapy in patients with MM.