We have made a novel series of pyrazolo [1, 5-a] pyridines as PI3 kinase inhibitors, and demonstrated their selectivity for the p110α isoform over the other Class Ia PI3 kinases. We investigated the SAR around the pyrazolo [1, 5-a] pyridine ring system, and found compound 5x to be a particularly potent example (p110α IC50 0.9 nM). This compound inhibits cell proliferation and phosphorylation of Akt/PKB, a downstream marker of PI3 kinase activity, ...