Abstract 2300: Brain penetrant CDK4/6 inhibitor PRT3645 demonstrates anti-tumor activity and enhances survival in glioblastoma and breast cancer brain metastasis models
Authors: Juvekar Ashish , Zhang Yang , Buesking Andrew , Wang Min , Rominger Dave , Rager Joseph , Ruepp Stefan , Gallagher Kirsten , Zou Yue , Cowart Miles , Wu Xiaowei , Pawley Sarah , Holmes Ryan , Gowen-MacDonald William , Vaddi Kris , Combs Andrew , Ruggeri Bruce , Scherle Peggy 
Published: June 2022

Abstract
Cell cycle deregulation is a hallmark of cancer and CDK inhibitors, specifically inhibiting CDK4/6 and blocking cells transition from the G1 to the S phase of the cell cycle are the first and only class of highly specific CDK inhibitors approved for cancer treatment to date. CDK4/6 inhibitors have transformed the treatment paradigm of estrogen receptor-positive (ER+), HER2- breast cancer with three CDK4/6 inhibitors currently FDA approved.
Brain metastasis commonly arises in patients with breast, lung, melanoma and other cancer types, is associated with poor survival outcomes and poses distinct challenges in clinical management. Due to advances in imaging technologies, the detection of brain metastases is increasing and there is a dearth of novel therapies to combat brain metastatic cancers and impact patient survival.
Here, we describe a novel brain penetrant CDK4/6 inhibitor, PRT3645 exhibiting single digit nanomolar biochemical potency against CDK 4/6 and &gt;2000-fold selectivity against other CDK family members (CDK1, CDK2, and CDK9). In cellular assays, PRT3645 inhibits cellular phosphorylation of RB with low nanomolar activity. Consistent with this, PRT3645 treatment resulted in concentration-dependent inhibition of cell proliferation in glioblastoma (GBM) cell lines and in HER2- and HER2+ breast cancer lines (EC50 values &lt; 125 nM). Furthermore, PRT3645 demonstrated additive in vitro activity with fulvestrant or tucatinib in ER+ and HER2+ breast cancer lines.
PRT3645 exhibits favorable in vitro safety pharmacology and ADME profiles, including brain exposure in rodents at steady state, and demonstrates oral bioavailability across rodents, dog and nonhuman primates. In vivo, oral PRT3645 was well tolerated and highly efficacious in a dose-dependent manner in subcutaneous xenograft models of GBM and breast cancer and in orthotopic human breast cancer brain metastasis (BCBM) and GBM models in mice as a monotherapy. PRT3645 showed tumor regression as single agent in the MCF7 ER+ breast cancer model and a combinatorial benefit with the estrogen receptor blocker, fulvestrant. In a HER2+ BT474-luc orthotopic model, similarly efficacious single agent activity of PRT3645 was achieved, as well as a significant combinatorial benefit on tumor growth and median survival when administered with the brain penetrant HER2 kinase inhibitor, tucatinib. PRT3645 was highly efficacious in a U87-luc GBM orthotopic model and demonstrated enhanced median survival benefit when combined with an orally active brain penetrant PRMT5 inhibitor.
In summary, PRT3645 demonstrates an excellent balance of potency, selectivity, PK parameters across species, brain penetrance and favorable tissue distribution relative to brain exposure, and currently has advanced into IND-enabling preclinical studies.
Citation Format: Ashish Juvekar, Yang Zhang, Andrew Buesking, Min Wang, Dave Rominger, Joseph Rager, Stefan Ruepp, Kirsten Gallagher, Yue Zou, Miles Cowart, Xiaowei Wu, Sarah Pawley, Ryan Holmes, William Gowen-MacDonald, Kris Vaddi, Andrew Combs, Bruce Ruggeri, Peggy Scherle. Brain penetrant CDK4/6 inhibitor PRT3645 demonstrates anti-tumor activity and enhances survival in glioblastoma and breast cancer brain metastasis models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2300.