An orally available inhibitor of ataxia telangiectasia and Rad3 related (ATR) kinase, with potential antineoplastic activity. Upon oral administration,gartisertib selectively inhibits ATR activity and blocks the downstream phosphorylation of the serine/threonine protein kinase 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.