A potent and selective inhibitor of PI3Kδ (Ki = 79 pM); has >1,000-fold selectivity for PI3Kδ over isoforms PI3Kα, PI3Kβ, and PI3Kγ (Kis = 501, 630, and 501 nM, respectively); >100-fold selective for PI3Kδ over a panel of 250 kinases; inhibits IFNγ and IL-2 production (IC50s = 1.9 and 3.16 nM, respectively) in a human lung parenchyma assay; protects against eosinophil recruitment (ED50 = 35 μg/kg) in the brown Norway rat acute ovalbumin model of Th2-driven lung inflammation,