At least 11 compounds in the clinic have been designed to be isoform-selective inhibitors of phosphoinositide 3-kinase (PI3K), although most appear to inhibit two or three isoforms with similar potency. The IC50 value indicates the concentration of compound needed to inhibit PI3K signaling by 50% and serves as a rough measure of the relative selectivity of an inhibitor for one isoform over another. Lower IC50 values generally indicate greater selectivity. NA = not available; Sources: BCIQ: BioCentury Online Intelligence; company presentations; S.J. Shuttleworth et al., Current Medicinal Chemistry, 2011