Not All Selective Estrogen Receptor Degraders Are Equal - Preclinical Comparison Of Azd9833, Azd9496 And Fulvestrant

Anti-hormonal therapy has been the mainstay treatment for estrogen receptor (ER) +ve breast cancer for g40 years. The selective ER degrader (SERD) fulvestrant has demonstrated clinical benefit over aromatase inhibitors and the selective ER modulator tamoxifen. However, fulvestrant9s efficacy may be limited by the intramuscular route of administration. To overcome this, AstraZeneca has developed orally bioavailable SERDs, the first generation compound AZD9496 and the next generation oral SERD AZD9833. Here we report the preclinical comparison of fulvestrant, AZD9496 and AZD9833. All three compounds induced equal ERα degradation in the ER+ breast cancer cell line MCF7. Importantly, analysis of other ER+ cell lines revealed AZD9833 phenocopied ERα degradation induced by fulvestrant, while AZD9496 only reduced ERα levels to 54% of that achieved by fulvestrant. Activity in the endometrial Ishikawa cell line demonstrated induction of progesterone receptor, an ER target gene, by AZD9496 but not AZD9833. RNAseq analysis of ER+ breast cancer cell lines treated with the three agents with and without estradiol did not reveal differences between the compounds; all estradiol-induced gene expression changes were completely reversed by the SERDs and no ER agonism was detected. By contrast, AZD9833 and fulvestrant produced an equivalent maximal anti-proliferative effect in MCF-7 and CAMA-1 cells, while AZD9496 induced a significantly inferior anti-proliferative effect. The pure anti-estrogen nature of AZD9833 and the partial agonist potential of AZD9496 was confirmed in vivo in a juvenile rat uterine assay. Like fulvestrant, AZD9833 caused a significant reduction in uterine weight and endometrial thickness, while AZD9496 produced a significant increase in both parameters. In MCF7 xenografts, both oral SERDs caused equivalent anti-tumour and pharmacodynamic effects to a supraclinical exposure of fulvestrant. However, in the patient derived xenograft CTC174 harbouring an ESR1 D538G mutation, AZD9833 caused equivalent anti-tumour and ERα degradation effects to supraclinical fulvestrant. By contrast, AZD9496 produced significantly poorer maximal anti-tumour and pharmacodynamic effects in this model compared to fulvestrant. These data demonstrate that AZD9833 phenocopied fulvestrant preclinically, as a SERD and pure anti-estrogen. It is, therefore, a suitable candidate to test the hypothesis that increasing the exposure of a SERD above what is achieved with fulvestrant can provide additional patient benefit. AZD9496, on the other hand, demonstrated inferior ERα degradation in several ER+ breast cancer cell lines and partial agonism. This may explain the result of the clinical window of opportunity study, comparing AZD9496 to fulvestrant, in which AZD9496 failed to demonstrate superior degradation of ERα, downregulation of PR expression and reduction in Ki67 positivity. Citation Format: Mandy Lawson, Natalie Cureton, Michelle DuPont, Oona Delpuech, Dawn Trueman, Pei Zhang, Azadeh Cheraghchi-Bashi-Astaneh, Sladjana Gagrica, Gareth Maglennon, Daniel Sutton, Bairu Zhang, Jonathan Cairns, Jamie Scott, Teresa Klinowska, Christopher J. Morrow. Not all selective estrogen receptor degraders are equal - Preclinical comparison of AZD9833, AZD9496 and fulvestrant [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4379.