Description_x000D_ Biochem/physiol Actions_x000D_ MI-503 is a highly potent menin-MLL interaction inhibitor (cell-free IC50 = 14.7 nM against 1 nM FITC-MLL(4-13) peptide; cellular IC50 = 220 nM using HEK293 MLL-AF9 fusion transfectants) that targets menin MLL-binding site with high affinity (KD = 9.3 nM by ITC). MI-503 selectively inhibits mixed lineage leukemia (MLL) oncogenic fusion-driven cancer progression in cultures (GI50 from 250 to 570 nM; GI50 >5 μM against cells without MLL gene translocation) and is efficacious against human MLL leukemia MV4;11 tumor growth in mice in vivo (by 80-100%; 60 mg/kg/day i.p.) with good pharmacokinetic properties, oral bioavailability (73% post 30 mg/kg p.o.), and no adverse effects to the animals.