Identification of resistance mechanisms to direct KRAS inhibition in pancreatic cancer

Project Summary/Abstract Pancreatic cancer is the 3rd leading cause of cancer death in the USA and the need for effective therapies is dire. KRAS is mutationally activated in ~95% of pancreatic ductal adenocarcinoma (PDAC), the major pancreatic cancer subtype. Cell culture and mouse model analyses provide strong validation for the role of mutant KRAS in the maintenance of PDAC, and the NCI has identified development of KRAS-targeted therapies as one of four major priorities for the field. Sotorasib was recently FDA-approved as the first direct KRAS inhibitor for use in KRASG12C-mutant lung cancer. However, the clinical utility of sotorasib and all other clinical candidates in PDAC is limited for two main reasons. First, sotorasib targets KRASG12C, a mutation that represents less than 2% of KRAS mutations in PDAC. KRAS inhibitors that target other KRAS mutations are currently under preclinical development. Second, patients initially responsive to KRASG12C inhibitors all soon relapse. Early studies have begun to identify genetic events that drive resistance, with expected components of RAS signaling identified in patients. However, no clear mechanisms have been identified in about half of relapsed patients. A full delineation of these mechanisms will be needed to develop effective combinations that can prolong patient response to KRAS-targeted therapies. This proposal builds on the emerging concept in understanding KRAS driver mechanisms, that different mutations cause distinct consequences on KRAS function. Consequently, there are mutation-specific driver functions that can be exploited to develop mutation-selective combination therapies. These studies focus on two lesser-studied, atypical KRAS mutations in PDAC – KRASG12R, which is unexpectedly enriched, and KRASQ61H, which is unexpectedly rare. The proposed studies will utilize direct pharmacologic inhibitors of KRASG12R and KRASQ61H in PDAC. System-wide unbiased genetic loss-of-function CRISPR/Cas9 oncogenic signaling pathway libraries will be used to identify therapeutic resistance mechanisms to KRAS inhibition. Preliminary data strongly implicate both known and novel resistance mechanisms. Finally, because KRASG12R and KRASQ61H-mutant pancreatic cancers exhibit distinct functional differences, these studies will likely identify both common and distinct resistance mechanisms to KRAS inhibition, which will lead to distinct KRAS inhibitor combination approaches. This proposal also describes the development of KrasG12R and KrasQ61H syngeneic, orthotopic pancreatic cancer mouse models. These new models will be used to define mutation- specific cancer functions, and to enable assessment of the influence of the tumor microenvironment on the consequences of KRAS inhibition alone and in combination. The overarching goals of this proposal are to further elucidate how two atypical KRAS mutations drive PDAC growth and to inform identification of combinations that can increase the clinical effectiveness of mutation-specific anti-KRAS strategies in PDAC.