Abstract IA24: Drugging the undruggable: Small-molecule inhibition of Ras oncoprotein

Ras is a nucleotide-dependent switch that converts from an inactive GDP-bound state to an active GTP-bound state when activated by guanine nucleotide exchange factors, such as SOS. Active RasGTP then binds to and activates downstream signaling effectors. Ras is the most frequently mutated oncogene and hyperactive mutant Ras constitutively signals to effectors to promote cell survival, proliferation and metastasis. Thus, Ras oncoprotein has been considered by the cancer community to be one of the most important oncology drug targets. Despite the enormous interest and extensive exploratory efforts in industry and academia, small molecules that bind to Ras in a well-defined manner and exert inhibitory effects have not been uncovered to date. We report here the identification and characterization of small-molecule inhibitors of the Ras oncoprotein.

To explore a new means of directly targeting Ras, we used a fragment-based lead discovery approach via an NMR-based screen. Hits from the fragment screen were characterized for their interactions with Ras by NMR and X-ray crystallography and for their effects on Ras activation and signaling in reconstituted biochemical assays in vitro and in cellular assays in vivo. From the fragment-based screen, we identified a group of small molecules that each bind to a common site adjacent to the switch I/II regions in the Ras protein. X-ray crystallography studies of three compound-Ras complexes indicate that the binding site can be expanded upon ligand binding. Nucleotide exchange factors, notably SOS, are required to convert inactive RasGDP to active RasGTP. We determined that the compound-binding site is located at the interface of Ras and SOS. A subset of our Ras-binding molecules indeed inhibited SOS-mediated nucleotide exchange. Further mechanistic studies revealed that through steric hindrance the compounds block the formation of the Ras-SOS complex, a key intermediate of the exchange reaction. At the cellular level, our compounds inhibit the formation of active RasGTP and prevent Ras signaling to downstream effectors. To define the potential clinic utility of these compounds, we performed biological characterization of Ras-driven tumors and identified a subset of Ras mutant tumors that depend on nucleotide exchange factors for the activation of Ras, suggesting a specific profile for the use of exchange inhibitors.

We conclude that the compounds act as competitive inhibitors of nucleotide exchange to prevent the activation of Ras. The discovery of a binding pocket on Ras with functional significance represents a breakthrough finding that will offer a new direction for therapeutic intervention of Ras. Our findings provide new opportunities to target the “undruggable” Ras oncoprotein.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr IA24.