TARGETING RAS SIGNALING USING RAS MIMETICS
Oncogenic activation of RAS genes occurs via point mutations in 20-30% of all human cancers. Studies aimed at understanding the biochemical and biological mechanisms that govern the function of these oncogenic proteins have shown that oncogenic RAS not only activates proliferative signals but that it also mediates signals responsible for cell survival, metastasis and evasion of apoptosis through interaction with a plethora of effector proteins by a highly conserved mechanism that involves the switch I and switch II regions of RAS and the RAS-binding domains (RBDs) of its effector proteins. It is now estimated that over 100 mammalian effector proteins share this binding motif, which plays a critical role in their association with RAS. Interestingly, in spite of low sequence homology, RBDs adopt similar, ubiquitin-like fold tertiary structures whereby the molecular interactions with RAS are mediated by side-by-side alignment of ß-sheets that are stabilized by multiple polar and hydrophobic interactions.
We have synthesized a library of approximately 10,000 compounds as a part of our translational research program, and one of these small molecules, rigosertib (ON01910.Na, Estybon®) functions as a inhibitor of the RAS-RBD protein:protein interactions. Rigosertib, is a novel benzyl styryl sulfone that inhibits the growth of a wide variety of human tumor cells in vitro, impairs tumor growth in vivo with minimal toxicity and has recently completed Phase III clinical trials for myelodysplastic syndrome (MDS). Our results show that this compound acts as a RAS-mimetic and binds to the RBDs of the RAF family proteins (A, B and c-RAF) and disrupts their ability to bind to RAS. Disruption of the RAS-RAF interaction blocks the activation of RAF kinase activity, inhibits the RAS-RAF-MEK signaling and prevents RAF from associating with and activating PLK1. In addition to RAF family proteins, rigosertib also binds to the RBDs of other RAS effectors, such as Ral-GDS and class I PI3Ks (a,ß,g,d), and therefore likely inhibits multiple RAS-driven signaling pathways. Our laboratory is currently focused on identifying the RBD-containing proteins to which rigosertib binds. We have also developed and are characterizing a number of analogs that bind to the RAF RBD but inhibit RAS-mediated signaling by a mechanism that distinct from that of rigosertib. This novel strategy of inactivating RAS signaling based on disruption of RAS interaction with downstream effectors paves the way for a new class of drugs for the treatment of RAS activated cancers.