In this issue of Blood, Perner and colleagues show that 2 new DOT1L inhibitors with favorable administration routes and pharmacokinetic properties yield promising antileukemic activity in patient-derived xenograft (PDX) models of MLL-rearranged acute myeloid leukemia (AML).1 

Acute leukemia arises when hematopoietic progenitor cells become locked into an immature state of perpetual proliferation. Several genetic alterations can impose this differentiation block, including chromosomal translocations affecting the MLL (MLL1, KMT2A) gene observed in ∼10% of acute leukemias.2,3  MLL rearrangements encode novel protein fusions of truncated MLL with a variety of partner proteins, resulting in aberrant transcriptional regulation.2,3  MLL-fusion proteins have a bad reputation based in part on their apparent oncogenic autonomy: they are far more frequently found in infant and pediatric leukemia compared with adult disease, and MLL-fusion leukemias often have a paucity of additional genetic alterations.2  In keeping with this, expression of MLL-fusions causes rapid, aggressive leukemia in mice.2  The extraordinary oncogenicity of MLL-fusions is associated with poor prognosis4 ; hence, there is much interest in developing leukemia therapies targeting MLL-fusions or their cofactors.

One such cofactor is the enzyme DOT1L (KMT4), which methylates lysine 79 of histone H3 (H3K79).4  As a component of MLL-fusion protein complexes, DOT1L facilitates aberrant transcription of MLL-fusion target genes.4  Proof of principle for in vivo therapeutic targeting of DOT1L was originally established in 2011 in mouse models of MLL-fusion–driven leukemia, where genetic DOT1L deletion caused leukemia differentiation and apoptosis.5-7  DOT1L ablation in mice compromised normal hematopoiesis but was relatively well tolerated, indicating a potential therapeutic window.5-7  Soon after, Daigle et al described pinometostat (EPZ-5676), a selective inhibitor of DOT1L methyltransferase activity.8  In rodents xenografted with a human MLL-rearranged AML cell line, pinometostat triggered sustained leukemia regression associated with reduced H3K79 methylation. Although this DOT1L inhibitor was well tolerated, it required continuous IV infusion due to rapid clearance.8  Pinometostat was also well tolerated in a recent first-in-human study but again it required continuous IV infusion.9  It had modest clinical effects, inducing complete remission in 2 of 51 MLL-fusion leukemia patients.9 

The current study from Perner et al builds on a 2019 report from some of the same authors that identified 2 novel DOT1L inhibitors.10  Although these new compounds each have markedly different structures and target binding modes, Perner et al show specific DOT1L methyltransferase inhibition comparable to pinometostat. However, in contrast to previous DOT1L inhibitors, these new compounds are effectively administered by oral or intraperitoneal routes and produce stable plasma drug levels that are well tolerated by immunocompromised mice.1  Furthermore, in PDX mouse models of primary MLL-rearranged AML, the new compounds potently inhibit DOT1L-mediated H3K79 methylation, triggering leukemia differentiation and significantly reducing disease burden.1 

By demonstrating that DOT1L inhibition can be achieved in AML in vivo with relative ease using these new compounds, Perner et al set the scene for further interrogation of DOT1L dependency in preclinical AML models. It is unclear whether the modest single-agent DOT1L inhibitor efficacy observed in the original AML patient trial9  reflects suboptimal dosing and target inhibition, distinct sensitivity of different MLL-fusion proteins to DOT1L inhibition, and/or other factors. The new compounds characterized by Perner and colleagues should help resolve these questions. The field is now poised to define and refine the antileukemic potency and tolerability of DOT1L inhibitors in preclinical models and then hopefully in leukemia patients, either as monotherapies or in combination with other antileukemic agents.

Conflict-of-interest disclosure: R.A.D. has been a member of advisory boards for Celgene.

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