Synthesis, Structure–Activity Relationships, and Antitumor Activities of Quinoxiline-Containing Inhibitors of the Protein–Protein Interactions between Transcription Coactivator AF9/ENL and DOT1L/AF4

Mixed lineage leukemia (MLL) gene rearrangements cause ∼75% of acute leukemia in infants and 5–10% in children and adults with poor clinical outcomes. Protein–protein interactions (PPI) between frequent MLL fusion partners AF9/ENL and AF4 or histone methyltransferase DOT1L are drug targets for MLL-rearranged (MLL-r) leukemia. Sixty-seven quinoxiline compounds were synthesized and tested for their ability to inhibit such PPIs. Compounds 16, 17, 59, and 63 were found to be potent inhibitors with IC₅₀ values of 0.35–1.5 μM. Structure–activity relationships are discussed. Potent inhibitors can suppress the expression of MLL target genes Myc and Meis1 and selectively block the proliferation of MLL-r and several other leukemia cells with EC₅₀ values as low as 0.84 μM. Compound 17 exhibited significant antitumor activities in a mouse model of MLL-r leukemia without overt toxicities. It also showed favorable pharmacokinetics in mice. These results indicate that compound 17 is a promising pharmaceutical lead for the treatment of MLL-r leukemia.