Here we report the development of (1R, 2S, 4R)-4-({[4-(5,6-dimethoxypyridazin-3-yl)phenyl]methyl}amino)-2-{methyl[6-(2,2,2-trifluoroethyl)thieno[2,3-d]pyrimidin-4-yl]amino}cyclopentan-1-ol (DS-1594a), a novel, potent, orally available small-molecule inhibitor of the Menin-MLL1 interaction with well-optimized drug properties for the treatment of hematologic malignancy patients, including patients with acute leukemia with MLL1-r and NPM1c mutations. The gene discussed is MEN1; the disease is acute leukemia.