An orally available inhibitor of ataxia telangiectasia and Rad3 related (ATR) kinase, with potential antineoplastic activity. Upon oral administration, ATR kinase inhibitor M1774 selectively inhibits ATR activity and blocks the downstream phosphorylation of the serine/threonine protein kinase checkpoint kinase 1 (CHK1). This prevents ATR-mediated signaling, which results in the inhibition of DNA damage checkpoint activation, the disruption of DNA damage repair, and the induction of tumor cell apoptosis. ATR, a serine/threonine protein kinase upregulated in a variety of cancer cell types, plays a key role in DNA repair, cell cycle progression and survival. It is activated by DNA damage caused during DNA replication-associated stress.