While bronchodilators and inhaled corticosteroids are the mainstay of asthma treatment, up to 50% of asthmatics remain uncontrolled. Many studies show that the cysteinyl leukotriene cascade remains highly activated in some asthmatics, even those on high-dose inhaled or oral corticosteroids. Hence, inhibition of the leukotriene C4 synthase (LTC4S) enzyme could provide a new and differentiated core treatment for patients with a highly activated cysteinyl leukotriene cascade. Starting from a screening hit (<b>3</b>), a program to discover oral inhibitors of LTC4S led to (1<i>S</i>,2<i>S</i>)-2-({5-[(5-chloro-2,4-difluorophenyl)(2-fluoro-2-methylpropyl)amino]-3-methoxypyrazin-2-yl}carbonyl)cyclopropanecarboxylic acid (AZD9898) (<b>36</b>), a picomolar LTC4S inhibitor (IC<sub>50</sub> = 0.28 nM) with high lipophilic ligand efficiency (LLE = 8.5), which displays nanomolar potency in cells (peripheral blood mononuclear cell, IC<sub>50,free</sub> = 6.2 nM) and good in vivo pharmacodynamics in a calcium ionophore-stimulated rat model after oral dosing (in vivo, IC<sub>50,free</sub> = 34 nM). Compound <b>36</b> mitigates the GABA binding, hepatic toxicity signal, and in vivo toxicology findings of an early lead compound <b>7</b> with a human dose predicted to be 30 mg once daily.