Abstract 1770: A new DOT1L inhibitor with in vivo activity in mouse models of MLL-translocated leukemia

Rearrangements in the mixed lineage leukemia (MLL) gene define a distinct, aggressive form of acute leukemia with poor prognosis. The MLL gene encodes for a SET domain histone methyl transferase that catalyzes the methylation of histone 3 lysine 4 (H3K4) at specific gene loci, thus regulating transcription of developmental genes including HOX genes. In disease-linked translocations, the catalytic SET domain is lost and the remaining part fused to a variety of partners, e.g. AF4, AF9, AF10 and ENL. These MLL fusion proteins directly interact with disruptor of telomeric silencing 1-like protein (DOT1L), the only known H3K79 methyltransferase. The H3K79me2 mark is broadly associated with active transcription. Mislocated enzymatic activity of DOT1L causes local H3K79 hypermethylation, misexpression of leukomogenic genes, e.g. HOXA9 and MEIS1, and the aberrant maintenance of a stem cell-like state. Current treatment options of MLL are limited to chemotherapy and allogeneic hematopoietic stem cell transplantation, and outcomes remain poor. The first and only clinical DOT1L inhibitor Pinometostat (EPZ-5676), an S-Adenosyl Methionine (SAM) competitive inhibitor, showed only modest activity and emerging resistance in adult acute leukemia. Pinometostat is no oral drug, but requires administration as continuous infusion to achieve sufficient exposure and sustained target inhibition. So far, there is no approved DOT1L inhibitor and the need remains to develop effective DOT1L inhibitors. We report the identification of new SAM-competitive, structurally SAM-unrelated DOT1L inhibitors with subnanomolar biochemical potency. Compounds with nanomolar cellular activity and good exposure in mouse were tested in tumor xenograft models. Compounds 8 and 9 showed excellent blood exposure after a single dose. However, repeated dosing led to reduced exposure due to cytochrome P450 (Cyp450) 3A4 induction, insufficient pharmacodynamics (PD) and lack of efficacy. Further modification resulted in compound 10 that could be administered orally and was stable upon repeated dosing. A 300 mg/kg dose covered efficacious blood exposure for 24 h, but was not tolerated by tumor-bearing mice. Unfortunately, a 6-fold reduced dose did not achieve sufficient PD modulation and efficacy. Additional activities led to compound 11, which lost its oral bioavailability and had to be administered subcutaneously. Nevertheless, compound 11 achieved tumor growth inhibition in the MV4-11 and Molm-13 xenograft models in mice. In conclusion, we progressed in making DOT1L inhibitors with improved PK properties, but further optimization is required to generate a viable clinical candidate. The limited efficacy obtained with compound 11 and lack of efficacy observed with subcutaneous administration of EPZ-5676 around the maximally tolerated dose illustrate the difficulty to achieve a sustained level of DOT1L inhibitor in vivo to suppress DOT1L activity sufficiently. Furthermore, H3K79me2 and efficacy in vivo seem disconnected. Citation Format: Andreas Weiss, Frederic Stauffer, Clemens Clemens, Henrik Mobitz, Christian Ragot, Kim S. Beyer, Keith Calkins, Claudio Thoma, Daniel Guthy, Michael Kiffe, Bernard Van Eerdenbrugh, William R. Sellers, Francesco Hofmann, Ralph Tiedt, Christoph Gaul. A new DOT1L inhibitor with in vivo activity in mouse models of MLL-translocated leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1770.