Idelalisib — A PI3Kδ Inhibitor for B-Cell Cancers

The phosphoinositide 3-kinase (PI3K) signal transduction pathway is highly active in tumors and drives many of the hallmarks of cancer. Efforts to develop PI3K inhibitors for oncology have been complicated by the existence of four distinct PI3K catalytic isoforms with partially overlapping functions. In addition, diverse resistance mechanisms and on-target toxic effects have been observed with many PI3K inhibitors.1,2 In this issue of the Journal, two clinical teams report that idelalisib, a selective inhibitor of the delta isoform of PI3K, provides impressive efficacy and has an acceptable side-effect profile when used to treat patients with certain B-cell cancers.3, . . .