Abstract 5641: Effects of G1T48, a novel orally bioavailable selective estrogen receptor degrader (SERD), and the CDK4/6 inhibitor, G1T38, on tumor growth in animal models of endocrine resistant breast cancer

Abstract

Abstract Background: The combination of targeting the CDK4/6 and ER signaling pathways with palbociclib and fulvestrant is a proven therapeutic strategy for the treatment of ER positive breast cancer. However, the poor physicochemical properties of fulvestrant require monthly intramuscular injections to patients, which limit the pharmacokinetic and pharmacodynamic activity of the compound. Therefore, an orally available compound that more rapidly reaches steady state may lead to a better clinical response in patients. Here we report the preclinical characterization of G1T48, a novel, orally bioavailable, non-steroidal small molecule inhibitor of ERα, which is a potent, selective antagonist that down regulates ERα in vitro and in vivo in ER-positive models of breast cancer. Methods: Breast cancer cells expressing clinically relevant ESR1 mutations (ER-Y537S, ER-D538G) were threated with G1T38, G1T48, and mechanistically distinct SERMs/SERDs and cellular proliferation was assessed by measuring DNA content (Hoechst dye). Ovariectomized nu/nu mice bearing xenograft tumors of clinically relevant tamoxifen (TamR) and aromatase (long term estrogen deprived) resistant ER+ breast cancer were treated with G1T38 and G1T48, alone or in combination, with clinically relevant comparators. Time to progression and tumor volume were assessed over a 4 week dosing period. Results: G1T38 and G1T48 significantly inhibited cellular proliferation of MCF7 breast cancer cells bearing endocrine resistant ER mutations, ER-Y537S and ER-D538G. G1T48 treatment led to dramatic reductions in ER protein levels. Importantly, tumor growth inhibition was observed in mouse models of sensitive and resistant human breast cancer when G1T48 was dosed as a single agent or in combination with G1T38, a potent, selective CDK4/6 inhibitor. Conclusions: G1T38, a novel CDK4/6 inhibitor, and G1T48, a novel SERD, either alone or in combination, demonstrated highly potent inhibition of tumor growth in animal models of tamoxifen and aromatase resistance. G1T48 also demonstrated activity in models of endocrine resistance mediated by ER mutation. G1T48 is currently completing IND enabling studies. Citation Format: Suzanne E. Wardell, Alexander P. Yllanes, Jennifer G. Baker, Robert M. Baldi, Taylor K. Krebs, Jessica Sorrentino, John Bisi, Jay Strum, John D. Norris. Effects of G1T48, a novel orally bioavailable selective estrogen receptor degrader (SERD), and the CDK4/6 inhibitor, G1T38, on tumor growth in animal models of endocrine resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5641. doi:10.1158/1538-7445.AM2017-5641