A potent inhibitor of PI3K未 that is 猢?00 fold selective for the remaining three Class I PI3K isoforms and additional kinases is described. The hypothesis for selectivity is illustrated through structure activity relationships and crystal structures of compounds bound to a K802T mutant of PI3K纬. Pharmacokinetic data in rats and mice support the use of class="boldFont">3 as a useful tool compound to use for in vivo studies.