ABSTRACT:As many as 90% of human pancreatic ductal adenocarcinoma (PDAC) tumors harbor gain-of-function mutations in theKRASoncogene. Recently, inhibitors of the most common KRAS mutation, KRASG12D, have entered the clinical arena. However, early evidence suggests that as monotherapy, KRASG12Dinhibitors such as MRTX1133 at best provide brief periods of disease stabilization. Hence, there is a growing interest in understanding the mechanisms through which tumors acquire resistance to KRAS inhibition. In the present study, we generatedin vitromodels of MRTX1133 resistance and subjected parental and drug-resistant cell lines to RNA sequencing. This suggested that MRTX1133-resistant tumor cells undergo a global shift toward histone acetylation. Inhibition of the histone acetyltransferase EP300 reversed the drug-resistant phenotypein vitro, which subsequent RNA sequencing experiments determined was associated with the suppression of pro-survival FOSL1 signaling. Accordingly,siFOSL1reversed the MRTX1133-resistant phenotype with similar effects on pro-survival signaling. Given the lack of clinically useful EP300 or FOSL1 inhibitors, we next explored whether inhibitors of the acetylation scanning BET proteins would be similarly effective. The addition of BET inhibitors re-sensitized several resistant cell lines to MRTX1133 and impaired FOSL1-mediated survival signalingin vitro. In murine models of MRTX1133-resistant PDAC, BET inhibition cooperated with MRTX1133 to markedly extend overall survival. As BET inhibitors are currently under clinical testing, the combination of MRTX1133 and BET inhibitors warrants further investigation, particularly in tumors that have developed resistance to KRAS inhibition.
SIGNIFICANCE:Here, we demonstrate that BET inhibition is effective in PDAC with acquired resistance to KRAS inhibitors. As BET inhibitors are under clinical testing, the combination of KRAS and BET inhibitors warrants consideration in PDAC patients.