Abstract P3-04-07: The new oral SERD AZD9496 is efficacious in antagonizing ER and circumventing resistance to endocrine therapy (2017)

Abstract Background: The selective estrogen receptor (ER) degrader (SERD) fulvestrant (Ful) is a potent ER antagonist that upon binding to ER induces its degradation. Ful has shown clinical efficacy in metastatic disease upon progression on previous endocrine therapies and superior activitycompared to an aromatase inhibitor as first line therapy when given at a high dose, 500mg. However, major clinical limitations of Ful are its low bioavailability and its route of administration. Here, we assess the efficacy and the mechanism of action of the new oral SERD AZD9496 compared to Ful in our panel of endocrine-sensitive and -resistant (EndoR) in vitro and in vivo models. Methods: The effects of AZD9496 and Ful were studied in vitro in various ER+ MCF7, ZR75-1, T47D, 600MPE, and MDAMB415 parental lines and in MCF7 and T47D derivatives made resistant (R) to estrogen deprivation (ED), tamoxifen (Tam), or Ful. Cell growth, Western blot, Q-RT-PCR, and ERE-reporter assays were conducted to assess treatment efficacy as well as ER levels and activity. Xenografts of parental MCF7 cells were established in ovariectomized nude mice with exogenous estrogen (E2). Mice were then randomized to continued E2 or ED, with and without AZD9496 or Ful. Mice bearing transplantable MCF7 EDR and TamR xenografts were randomized to continue original treatment or to switch to Ful or AZD9496, and tumor size was followed. Expression of classic and nonclassic/indirect ER-regulated genes was evaluated in RNA extracts of short-term-treated xenografts using the BioMark FLUIDIGM platform. Results: AZD9496 inhibited cell growth (50-100%) of all ER+ parental cells and greatly, though not fully, degraded ER protein levels. AZD9496 also potently reduced ER-dependent exogenous and endogenous gene/protein expression in presence and absence of E2. In parental MCF7 xenograft-bearing mice, 10 days of AZD9496 resulted in a greater inhibition of tumor growth and in a greater reduction of levels of ER-dependent targets in comparison to Ful in the presence of E2. The effects of the 2 SERDs were similar in the absence of E2. In EndoR models that retain ER, AZD9496 inhibited cell growth in vitro by degrading ER, similar to Ful. Both SERDs also delayed tumor growth of EDR and TamR xenografts and effectively reduced levels of ER and ER-induced proteins, though no tumor regression was observed in the TamR model. Notably, AZD9496 failed to inhibit growth of FulR cells and xenografts. Expression analysis showed that the 2 SERDs potently inhibited classic ER activity, while simultaneously increasing expression of some genes known to be regulated by the nonclassic/indirect ER activity, including genes involved in escape pathways of endocrine resistance. Conclusions: The oral SERD AZD9496 is a potent antiestrogen that antagonizes and degrades ER. AZD9496, like Ful, inhibits ER-dependent transcription and tumor growth in both naïve and resistant EDR and TamR models, but shows cross-resistance in FulR models. Both AZD9496 and Ful failed to completely reduce ER protein expression and to induce TamR tumor regression, suggesting that additional strategies to reduce ER levels and to enhance the inhibition of ER signaling and/or of co-operating survival mechanisms may be needed to improve treatment outcome. Citation Format: Nardone A, Weir H, De Angelis C, Cataldo ML, Fu X, Shea MJ, Mitchell T, Trivedi M, Chamness GC, Osborne CK, Schiff R. The new oral SERD AZD9496 is efficacious in antagonizing ER and circumventing resistance to endocrine therapy [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-07.