The phosphatidylinositol 3-kinase (PI3K)/Akt signal transduction pathway plays critical roles in cell growth and proliferation making it an attractive target for anticancer agents. Ins(1,3,4,5,6)P5 is one of the many inositol phosphate isomers that act as small, soluble second messengers in the transmission of cellular signals. It can be interconverted with Ins(3,4,5,6)P4 by a 1-kinase/1-phosphatase cycle, as well as with Ins(1,4,5,6)P4 in a 3-kinase/3-phosphatase cycle. Ins(1,3,4,5,6)P5 inhibits the phosphorylation and kinase activity of Akt/PKB, inducing apoptosis in ovarian, lung, and breast cancer cells. It exhibits antiangiogenic activity in vitro, blocking capillary tube formation of HUVEC, as well as antitumor effects against cancer xenografts in nude mice. Ins(1,3,4,5,6)P5 binds to the PH domain of Grp1 with a Kd value of 590 nM.