Onatasertib (CC223) is an orally available mTOR inhibitor with potential antitumor activity. Onatasertib inhibits the activity of mTOR, which may result in the induction of tumor cell apoptosis and a decrease in tumor cell proliferation. Onatasertib is a potent mTOR kinase inhibitor (IC50: 16 nM), with >150-fold sensitivity than the related lipid kinase PI3Kα (IC50: 4 μM).