Background: Mutant-activated RAS genes are the most frequently mutated gene family associated with cancer (almost 30% of all cancers contain a mutant RAS gene). KRAS is the predominant isoform mutated in cancer and is the isoform exclusively mutated in pancreatic ductal carcinoma (PDAC). Since almost all PDAC cases harbor a mutant RAS, it is arguably the most RAS-addicted tumor type. There is now considerable evidence implicating mutant KRAS as a driver of PDAC. Recently, several mutant KRAS-targeted therapies (sotorasib and adagrasib) have been developed and show promise in PDAC patients. We have developed a direct inhibitor of RAS with a predicted unique interaction region capable of directly binding to wild-type H- and K-RAS, but which shows preferential binding for KRAS G12D and G12C mutants. This novel inhibitor disrupts the RAS effector domain and blocks the ability of RAS to signal through its effectors. Methods: We used in silico virtual library screening to identify an initial candidate inhibitor which was effective at inhibiting the 3D growth of PDAC cells without affecting their growth in 2D. Subsequent iterative rounds of medicinal chemistry was then performed to identify a series of derivatives with enhanced activity, as determined by 3D growth inhibition assays and effects on Ras signaling, as determined by Western blot analysis of phosphor-ERK, phosphor-Akt and activation of Ral A. Results: Our series of RAS inhibitors effectively block PDAC cell growth in 3D without impacting their 2D proliferation and suppress the interaction of KRAS with its effector cRAF. The inhibitors also effectively inhibit RAS signaling in mutant RAS containing PDAC cells. Since our novel compounds are predicted to bind to RAS at a different site to either AMG-510 (G12C specific inhibitor) and MRTX-1133 (G12D specific inhibitor), we tested the co-operativity of our compounds with these existing agents. Our compounds enhanced the anti-proliferative effects of both MRTX-113 and AMG-510 in mutant KRAS G12D and G12C PDAC cells, respectively. Conclusions: We have developed a series of novel RAS inhibitors that directly bind preferentially to mutant KRAS that may serve as new mutant KRAS-targeted therapeutics, and that may also have the potential to enhance the efficacy or suppress the resistance of AMG-510 and MRTX-1133.