Mechanism of action of rigosertib does not involve tubulin binding

Rigosertib is a novel benzyl styryl sulfone that inhibits the growth of a wide variety of human tumor cells in vitro and in vivo and is currently in Phase III clinical trials. We recently provided structural and biochemical evidence to show that rigosertib acts as a RAS-mimetic by binding to Ras Binding Domains (RBDs) of the RAF and PI3K family proteins and disrupts their binding to RAS. In a recent study, Jost et al (2017) attributed the mechanism of action of rigosertib to microtubule-binding. In these studies, rigosertib was obtained from a commercial vendor. We have been unable to replicate the reported results with clinical grade rigosertib, and hence compared the purity of clinical grade and commercially sourced rigosertib. We find that the commercially sourced rigosertib contains approximately 5% ON01500, a potent inhibitor of tubulin polymerization. Clinical grade rigosertib, which is free of this impurity, does not exhibit tubulin binding activity. In vivo, cell lines that express mutant lower case Greek beta-tubulin (TUBBL240F) were also reported to be resistant to the effects of rigosertib. However, our studies showed that both wild-type and TUBBL240F-expressing cells failed to proliferate in the presence of rigosertib at concentrations that are lethal to wild-type cells. Morphologically, we find that rigosertib, at lethal concentrations, induced a senescence-like phenotype in the small percentage of both wild-type and TUBBL240F-expressing cells that survive in the presence of rigosertib. Our results suggest that TUBBL240F expressing cells are more prone to undergo senescence in the presence of rigosertib as well as BI2536, an unrelated ATP-competitive pan-PLK inhibitor. The appearance of these senescent cells could be incorrectly scored as resistant cells in flow cytometric assays using short term cultures.