Is the RAS Mimetic, Rigosertib a Tubulin Binding Agent?

Cancer, the second leading cause of death in the US is in desperate need of improved therapeutics.  Rigosertib, a novel small molecule has shown some promising results in several cancer models and functions as a RAS mimetic that blocks downstream kinase activation.  Additionally, there is some evidence that Rigosertib may also operate by binding tubulin and promoting its destabilization; thereby, affecting proper cell division.  In this study by Baker et al.  the group determined that a contaminating intermediate found in commercial grade formulations of Rigosertib, and not the active drug itself functioned as the tubulin destabilizing agent.  Commercial grade Rigosertib (95% pure) and clinical grade Rigosertib (>99.9% pure) were analyzed by NMR and mass spectrometry and it was determined that the commercial grade contained 5% of the late-stage intermediate impurity, ON01500.  Critical tubulin polymerization assays were performed using Cytoskeleton’s BK011P and determined that ON01500 but not Rigosertib promoted actin depolymerization at the effective treatment doses.  The group further confirmed that b-tubulin mutant cell lines were not resistant to clinical grade Rigosertib and showed similar kinetics to cells expressing empty vector.   Collectively, in combination with other structural and cell studies, they concluded that Rigosertib does not bind tubulin and alter its dynamics at clinically relevant doses.  This study highlights the utility of the Cytoskeleton’s tubulin polymerization assay (Cat. # BK011P) to effectively identify molecules that alter tubulin polymerization.

**It should be noted that in the same issue of Molecular Cell Jost et al. reported the exact opposite finding, which was, Rigosertib is a microtubule destabilizing agent.