LY303511 (2-piperazinyl-8-phenyl-4H-1-benzopyran-4-one) acts via phosphatidylinositol 3-kinase-independent pathways to inhibit cell proliferation via mammalian target of rapamycin (mTOR)- and non- mTOR-dependent mechanisms.

Mammalian target of rapamycin ( mTOR), a serine/ threonine kinase, regulates cell growth and proliferation in part via the activation of p70 S6 kinase ( S6K). Rapamycin is an antineoplastic agent that, in complex with FKBP12, is a specific inhibitor of mTOR through interaction with its FKBP12-rapamycin binding domain, thereby causing G 1 cell cycle arrest. However, cancer cells often develop resistance to rapamycin, and alternative inhibitors of mTOR are desired. 2-(4- Morpholinyl)-8- phenyl-4H- 1- benzopyran- 4- one ( LY294002) blocks mTOR kinase activity, but it also inhibits phosphatidylinositol 3- kinase ( PI3K), an enzyme that regulates cellular functions other than proliferation. We hypothesized that a close structural analog, 2- piperazinyl8- phenyl- 4H- 1- benzopyran- 4- one ( LY303511) might inhibit mTOR- dependent cell proliferation without unwanted effects on PI3K. In human lung epithelial adenocarcinoma A549) cells, LY303511, like rapamycin, inhibited mTOR- dependent phosphorylation of S6K, but not PI3K- dependent phosphorylation of Akt. LY303511 blocked proliferation in A549 as well as in primary pulmonary artery smooth muscle cells, without causing apoptosis. In contrast to rapamycin, LY303511 reduced G(2)/ M progression as well as G(2)/M- specific cyclins in A549 cells. Consistent with an additional mTOR- independent kinase target, LY303511 inhibited casein kinase 2 activity, a known regulator of G1 and G(2)/M progression. In addition to its antiproliferative effect in vitro, LY303511 inhibited the growth of human prostate adenocarcinoma tumor implants in athymic mice. Given its inhibition of cell proliferation via mTOR- dependent and independent mechanisms, LY303511 has therapeutic potential with antineoplastic actions that are independent of PI3K inhibition.