PI3Kδ-IN-21 (Compound 31) is a selective inhibitor for phosphoinositide 3-kinases δ ( PI3Kδ ), with an IC 50 of 13.6 nM. PI3Kδ-IN-21 inhibits proliferation and differentation of T cells through PI3K/AKT/mTOR signaling pathway. PI3Kδ-IN-21 exhibits good pharmacokinetic characters in rat model, and attenuates the experimental autoimmune encephalomyelitis in myelin oligodendrocyte glycoprotein (MOG)-induced EAE model.