Development of DOT1L-Targeted Protein Degraders for Treating MLL-r Leukemia.

DOT1L is aberrantly recruited in MLL-r leukemias and serves as a critical oncogenic driver. Substantial previous work has developed catalytic inhibitors like pinometostat, which showed limited clinical efficacy. Recent work has defined nonenzymatic functions of DOT1L in promoting leukemia progression. Because these functions cannot be blocked by enzymatic inhibitors, we developed novel DOT1L-targeting PROTACs (Proteolysis-Targeting Chimeras ) to degrade the protein and thereby block all of its functions. We describe PROTACs DOT1L705 and DOT1L808 as potent and highly selective DOT1L degraders with DC values of 0.33 μM and 5 nM, respectively. The effect of DOT1L705 on cell viability is highly dependent on the MLL-r status of leukemia cell lines, and it retains activity against menin inhibitor-resistant cells. In vivo studies with DOT1L808 showed its ability to achieve complete tumor regression in an orthotopic leukemia model without overt toxicity. These results establish protein degradation as a promising therapeutic strategy for MLL-rearranged leukemias.