Sequence-defined oligophosphoesters for selective inhibition of the KRAS G12D/RAF1 interaction

Rat Sarcoma (RAS) genes are the most frequently mutated genes in cancer, with KRAS being the most predominant oncogene, yet they have proved extremely difficult to drug because they operate primarily through protein-protein interactions (PPIs) which lack an obvious pocket for small molecules. Sequence-defined synthetic oligomers could combine the precision and customisability of synthetic molecules with the size requirements to address entire protein-protein interaction surfaces. We have adapted the phosphoramidite chemistry of oligonucleotide synthesis to produce a library of nearly one million non-nucleosidic oligophosphoester sequences – phosphoestamers - and used a fluorescent-activated bead sorting (FABS) process to select oligomers that inhibit the interaction between KRASG12D (the most prevalent, and undrugged, mutant) and RAF, a downstream effector of RAS whose activation results in cell proliferation. Hits were identified using tandem mass spectrometry, and validation showed effective inhibition with IC50 values as low as 25 nM, and excellent selectivity for the mutant over the wild type form. These findings could lead to new drugs against cancers driven by mutant RAS, and provided proof-of-principle for the phosphoestamer platform against PPIs in general. ### Competing Interest Statement The authors have declared no competing interest.