Fulvestrant is an estrogen receptor (ER) antagonist administered to breast cancer patients by monthly intramuscular injection. Given its present limitations of dosing and route of administration, a more flexible orally available compound has been sought to pursue the potential benefits of this drug in patients with advanced metastatic disease. Here we report the identification and characterization of AZD9496, a non-steroidal small molecule inhibitor of ERα which is a potent and selective antagonist and down-regulator of ERα in vitro and in vivo in ER-positive models of breast cancer. Significant tumour growth inhibition was observed as low as 0.5 mg/kg dose in the estrogen-dependent MCF-7 xenograft model, where this effect was accompanied by a dose-dependent decrease in PR protein levels demonstrating potent antagonist activity. Combining AZD9496 with PI3K pathway and CDK4/6 inhibitors led to further growth inhibitory effects compared to monotherapy alone. Tumour regressions were also seen in a long-term estrogen-deprived breast model, where significant down-regulation of ERα protein was observed. AZD9496 bound and down-regulated clinically relevant ESR1 mutants in vitro and inhibited tumour growth in an ESR1-mutant patient-derived xenograft model that included a D538G mutation. Collectively, the pharmacological evidence showed that AZD9496 is an oral, non-steroidal, selective estrogen receptor antagonist and down-regulator in ER-positive breast cells that could provide meaningful benefit to ER-positive breast cancer patients. AZD9496 is currently being evaluated in a Phase 1 clinical trial.
Copyright ©2016, American Association for Cancer Research (AACR) (Free AACR Whitepaper).

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