Abstract LB-287: Combination therapies with CDK4/6 inhibitors to treat KRAS-mutant pancreatic cancer
Authors: Goodwin Craig M. , Javaid Sehrish , Waters Andrew M. , Papke Bjoern , Yang Runying , Pierobon Mariaelena , Freed Daniel M. , Roberts Patrick J. , Cox Adrienne D. , Wood Kris C. , Petricoin Emanuel F. , McRee Autumn J. , Der Channing J. 
Published: August 2019

Abstract
Pancreatic ductal adenocarcinoma (PDAC) patients have a dismal five-year survival rate of just eight percent in the advanced metastatic setting. Outcomes with standard chemotherapy regimens are less than ideal; therefore, the development of targeted therapies for the treatment of PDAC is a significant unmet clinical need. The two most frequent genetic events in PDAC (mutational activation of KRAS and loss of the tumor suppressor CDKN2A) converge on activation of the kinases CDK4 and CDK6, which promote G1 cell cycle progression. While clinically relevant agents directly targeting KRAS in PDAC remain elusive, pharmacologic restoration of CDKN2A function by inhibition of CDK4/6 may be an effective anti-KRAS therapeutic strategy. Supporting this hypothesis, we found that CDK4/6 inhibitors (e.g., palbociclib, abemaciclib, lerociclib) elicited single-agent activity in a subset of tested PDAC cell lines. However, using Reverse Phase Protein Array (RPPA), we observed widespread compensatory changes that overcome CDK4/6 inhibition, including increased anti-apoptotic protein expression, PI3K-mTOR and ERK MAPK signaling. Concurrent treatment with the potent and selective ERK1/2 inhibitor (ERKi) SCH772984 reversed resistance and increased sensitivity to palbociclib. This combination caused synergistic reduction in anchorage-dependent cell growth, and increased apoptosis, G1 arrest, and senescence. In an organoid model of pancreatic cancer, cell viability was synergistically reduced and caspase activation enhanced. We have also evaluated the combination of ERKi with palbociclib or the clinical-stage CDK4/6 inhibitor lerociclib (G1T38) in mouse models of PDAC, supporting our initiation of a phase Ib clinical trial of the ERK inhibitor ulixertinib/BVD-523 in combination with palbociclib in advanced metastatic pancreatic cancer (NCT03454035). Next, we sought to identify additional genes that regulate sensitivity to CDK4/6 inhibition. Using a CRISPR/Cas9 loss-of-function screen, we individually silenced expression of 2500 genes from the “druggable genome” in combination with palbociclib in six KRAS-mutant pancreatic and two KRAS-mutant colorectal cancer cell lines. We identified a wide array of genes that enhanced growth suppression in combination, centered around diverse signaling nodes including PI3K-AKT-mTOR signaling, cell cycle regulation and mitosis, SRC family kinase signaling, cell metabolism and biosynthesis, chromosome regulation and maintenance, and DNA damage & repair pathways, suggesting novel small molecule combinations to overcome de novo or acquired CDK4/6 inhibitor resistance in the clinic. We also identified genes whose loss imparts a survival advantage, suggesting possible resistance mechanisms to single-agent CDK4/6 inhibition. In total, these data suggest that CDK4/6 inhibitors alone, or in combination, may benefit PDAC patients clinically.
Citation Format: Craig M. Goodwin, Sehrish Javaid, Andrew M. Waters, Bjoern Papke, Runying Yang, Mariaelena Pierobon, Daniel M. Freed, Patrick J. Roberts, Adrienne D. Cox, Kris C. Wood, Emanuel F. Petricoin, Autumn J. McRee, Channing J. Der. Combination therapies with CDK4/6 inhibitors to treat KRAS-mutant pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-287.