A selective PI3K inhibitor that targets the β isoform more potently than PI3Kδ, PI3Kγ, or PI3Kα (IC50s = 0.69, 13.6, 47.8, and 136 nM, respectively); blocks Akt signaling and tumor growth in a large number of cancer cell lines and suppresses the growth of PTEN-deficient xenograft tumors in mice