XL388 is a selective small-molecule ATP-competitive inhibitor of mTOR kinase complexes mTORC1 and mTORC2. XL388 inhibited cellular phosphorylation of mTOR complex 1 (p-p70S6K, pS6, and p-4E-BP1) and mTOR complex 2 (pAKT (S473)) substrates. In vitro, XL388 inhibits the viability of solid and hematopoietic tumor cell lines when assayed as a single agent. XL388 also synergizes with chemotherapeutics in cell‐based assays to block cell viability. When dosed orally once daily in mice, XL388 shows robust anti‐tumor activity in multiple xenograft models. Exelixis has discontinued the development of XL388. The company intends to pursue a collaboration partner or other external opportunities for the development of XL388.