ETP-46464 is a potent and selective inhibitor of the DNA damage response kinase Ataxia telangiectasia-mutated (ATM) - and Rad3-related (ATR) with an IC50 of 25 nM. ETP-46464 shows moderate activity against PI3K and is a potent inhibitor of mTor, but is selective for ATR versus ATM and DNA -dependent protein kinase (DNA -PKcs) even at high doses. ETP-46464 inhibited phosphorylation of Chk1, the downstream kinase of ATR, but did not affect ionizing radiation–induced γH2AX formation, which is jointly controlled by ATM and DNA-PKc.