Abstract 3343: The pre-clinical in vitro and in vivo activity of AZD6738: A potent and selective inhibitor of ATR kinase.

Abstract AZD6738 is a potent and selective orally bioavailable kinase inhibitor of ataxia telangiectasia and rad3 related (ATR). Here we report the pre-clinical in vitro and in vivo and biological profile of AZD6738. ATR is a serine/threonine protein kinase involved in DNA damage response signalling caused by DNA replication associated stress. Activation of ATR at stalled replication forks leads to suppression of replication fork origin firing, promotes repair and S/G2-cell cycle checkpoints to prevent premature mitosis and maintain genomic integrity. Failure to resolve damage leads to genomic instability and if sufficiently high, cell death. Stalled replication forks may collapse leading to formation of DNA double stranded breaks and activation of the ataxia telangiectasia mutated (ATM) kinase. ATM works in conjunction with ATR to efficiently resolve replication associated DNA damage creating a co-dependency with loss of one leading to a greater reliance on the other to maintain genomic stability. ATM is frequently inactivated across B-cell malignancies, head and neck, breast and lung cancers through chromosomal deletion, promoter hypermethylation or mutation. ATM-deficient tumours are hypothesised to be more reliant on ATR for survival and specific inhibition of ATR may lead enhanced anti-tumour activity while minimizing normal tissue toxicity. AZD6738 demonstrates this preclinical profile. AZD6738 inhibits the phosphorylation of direct downstream substrate CHK1 while increasing the phosphorylation of ATM-dependent substrate CHK2 and DNA damage marker γH2AX. This is associated with impaired S-phase cell cycle progression with prolonged inhibition causing cell death, indicative of replication fork stalling, collapse and irreversible damage. AZD6738 is active as a single agent across cancer cell line panels but shows enhanced sensitivity in cell lines with ATM-pathway defects. AZD6738 when used in combination with DNA damaging inducing agents’ gemcitabine, cisplatin or ionising radiation (IR) shows enhanced synergistic cell killing activity. In vivo, AZD6738 monotherapy treatment leads to significant anti-tumour activity in ATM-deficient but not ATM-proficient xenograft models at equivalent, tolerated doses. When AZD6738 is used in combination with carboplatin or IR anti-tumour growth inhibitory activity or regression is observed. AZD6738 in vivo activity is also associated with a persistent increased γH2AX staining in tumour tissue but only a transient increase in normal bone marrow or gut tissue suggesting a favourable therapeutic index can be achieved. Early pre-clinical safety studies support these findings with target related bone marrow suppression, neutropenia and GI tract impact only at high-doses. These data merit further investigation of AZD6738 as a monotherapy or in combination with chemo or radiotherapy. Citation Format: Sylvie M. Guichard, Elaine Brown, Rajesh Odedra, Adina Hughes, Dan Heathcote, Jen Barnes, Alan Lau, Steve Powell, Clifford D. Jones, Willem Nissink, Kevin M. Foote, Philip J. Jewsbury, Martin Pass. The pre-clinical in vitro and in vivo activity of AZD6738: A potent and selective inhibitor of ATR kinase. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3343. doi:10.1158/1538-7445.AM2013-3343