Isocitrate dehydrogenase mutations in myeloid malignancies.

Alterations to genes involved in cellular metabolism and epigenetic regulation are implicated in the pathogenesis of myeloid malignancies. Recurring mutations in isocitrate dehydrogenase (IDH) genes are detected in approximately 20% of adult patients with acute myeloid leukemia (AML) and 5% of adults with myelodysplastic syndromes (MDS). IDH proteins are homodimeric enzymes involved in diverse cellular processes, including adaptation to hypoxia, histone demethylation and DNA modification. The IDH2 protein is localized in the mitochondria and is a critical component of the tricarboxylic acid (also called the 'citric acid' or Krebs) cycle. Both IDH2 and IDH1 (localized in the cytoplasm) proteins catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate (α-KG). Mutant IDH enzymes have neomorphic activity and catalyze reduction of α-KG to the (R) enantiomer of 2-hydroxyglutarate, which is associated with DNA and histone hypermethylation, altered gene expression and blocked differentiation of hematopoietic progenitor cells. The prognostic significance of mutant IDH (mIDH) is controversial but appears to be influenced by co-mutational status and the specific location of the mutation (IDH1-R132, IDH2-R140, IDH2-R172). Treatments specifically or indirectly targeted to mIDH are currently under clinical investigation; these therapies have been generally well tolerated and, when used as single agents, have shown promise for inducing responses in some mIDH patients when used as first-line treatment or in relapsed or refractory AML or MDS. Use of mIDH inhibitors in combination with drugs with non-overlapping mechanisms of action is especially promising, as such regimens may address the clonal heterogeneity and the multifactorial pathogenic processes involved in mIDH myeloid malignancies. Advances in mutational analysis have made testing more rapid and convenient, and less expensive; such testing should become part of routine diagnostic workup and repeated at relapse to identify patients who may benefit from treatments that target mIDH.

Life after transplant: are we becoming high maintenance in AML?

Allogeneic hematopoietic cell transplantation (allo-HCT) for patients with AML is increasingly able to impact the historically poor outcomes in this disease. Nonetheless, even with transplant, the rates of post-HCT relapse are unacceptably high, and remain a great challenge in the treatment of patients with AML. Maintenance therapies after allo-HCT, given to patients at high risk of relapse or with evidence of minimal residual disease (MRD), may provide a way to reduce relapse rates and improve survival. New therapies may offer acceptable toxicity profiles in the post-HCT setting, and investigations are ongoing using hypomethylating agents, histone deacetylase inhibitors, immunomodulatory drugs, targeted tyrosine kinase inhibitors, drug-antibody conjugates and cellular therapies. Future directions in the field of post-HCT therapies may include better risk stratification with MRD, as well as the exploitation of novel mechanisms such as immune checkpoint inhibition and modified chimeric antigen receptor (CAR) T cells. In this mini review, we discuss the current landscape of clinical research in post-HCT maintenance therapies, as well as future therapeutic strategies of interest. Although there is great potential for post-HCT agents to improve AML outcomes, these will need to be evaluated prospectively through well-designed randomized clinical trials.