Abstract 5710: The brain penetrant CDK4/6 inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of breast cancer, CRC and NSCLC

Abstract Cell cycle deregulation is a hallmark of cancer and the hyperactivation and overexpression of cyclin-dependent kinase (CDK) s are often drivers of cancer pathogenesis. CDK4/6 are critical mediators of cellular transition into S phase and important for cancer initiation, growth, and survival. Previously we reported a novel brain penetrant CDK4/6 inhibitor, PRT3645, that exhibits single digit nanomolar biochemical potency against CDK4/6 and 2000-fold selectivity against other CDKs. PRT3645 inhibits cellular phosphorylation of Rb with nanomolar potency. Furthermore, PRT3645 exhibits favorable in-vivo safety pharmacology and ADME properties, including increased brain penetration, and demonstrates oral bioavailability across rodents, dog and non-human primates. Here, we explored the therapeutic potential of PRT3645 in tumor models harboring actionable genomic alterations but lacking effective targeted treatments. Specifically, we explored the combination of PRT3645 with a selective estrogen receptor degrader (SERD), an approved treatment for patients with HR+/HER2-, ESR1-mutated advanced or metastatic breast cancer. In an ESR1 mutated breast cancer PDX model, PRT3645 not only exhibited monotherapy activity but also induced significant tumor regression in combination with a SERD through inhibiting ER-mediated signaling. We also investigated PRT3645 single agent activity and combination with a clinically validated MEK inhibitor in a BRAF class III mutant colorectal CDX model. PRT3645 was well-tolerated and demonstrated significant anti-tumor efficacy, which was further enhanced by combination therapy. To further explore comprehensive inhibition of cell cycle progression and overcome potential resistance to CDK4/6 inhibition, we investigated dual inhibition of CDK4/6 and CDK2. CDK2 is a crucial player in regulating the cell cycle by controlling the late G1/S transition, promoting DNA replication, and contributing to the DNA repair processes. Additionally, CDK2 activation has been identified as a potential mechanism of resistance to CDK4/6 inhibition. We evaluated CDK2 and CDK4/6 dual inhibition in CDKN2A loss or CCNE amplified cancer cell lines and observed enhanced proliferation suppression, cell cycle arrest and senescence induction, underscoring the potential of this combination for improved therapeutic efficacy across various cancer types. In summary, PRT3645 displays an excellent balance of potency, selectivity, pharmacokinetic parameters across species as well as brain penetrance. In preclinical studies, PRT3645 was highly efficacious when combined with other targeted therapies, offering potential benefits across a wide spectrum of cancer types. In addition, the concept of dual inhibition of CDK4/6 and CDK2 presents a rational and promising approach to enhance the efficacy of cancer therapy. Citation Format: Yue Zou, Kirsten Gallagher, Srijita Dhar, Andrew Busking, Sarah Pawley, Ryan Holmes, Xiaowei Wu, Min Wang, Joseph Rager, Tom Emm, Stefan Ruepp, Miles Cowart, Andrew Combs, Kris Vaddi, Sandy Geeganage, Peggy Scherle, Sang Hyun Lee. The brain penetrant CDK4/6 inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of breast cancer, CRC and NSCLC abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5710.