Scientists from the Tanenbaum group have developed a method capable of identifying the mechanism of action of small molecule anti-cancer drugs. This novel screening method was applied and validated for rigosertib, a drug in advanced clinical development. The results were recently published in Molecular Cell.

In the past years, scientists have been able to access increasing amounts of data on the molecular and genetic basis of various diseases. This has made the development of drugs more specific. However, for many drugs, even those currently used in the clinic, the mechanism of action remains unclear. To elucidate this, there is a need for novel methods.

Combination screening
It has been known that genetic screening can shed light on the function of drugs, by assessing which genes increase or decrease the activity of the drug. For this, a combination of inhibiting or activating genes proves to be the ideal method. Now, with the advent of CRISPR-based screening platforms, it has become feasible to combine genome-wide knockdown (CRISPRi) and overexpression (CRISPRa) screens for drug activity.

Mechanism of action
Marvin Tanenbaum and his colleagues applied a combination of these genetic screenings to rigosertib, an anti-cancer drug that is currently in clinical development. Despite extensive clinical trials, the mechanism of action of this small molecule is still elusive, impeding clinical development of the drug. With this novel method, the authors discovered that rigosertib destabilizes the microtubules – tubular proteins forming the cell’s skeleton – causing the cell to die. Tanenbaum and colleagues demonstrated that assertion by comparing rigosertib’s genetic profile to that of other microtubule-destabilizing compounds. Also, the scientists proved that rigosertib has a direct effect on microtubule growth and, vice versa, they demonstrated that cells with a mutated form of tubulin – the protein of which microtubules consist – were left unharmed by the drug.

Blueprint
With their research, Tanenbaum and his colleagues show that their CRISPR-based screening method of inhibition and activation is robust, reproducible and easily implementable. This paper not only sheds light on rigosertib’s mechanism of action, but may serve as a blueprint for the genetic profiling of many other small molecule drugs to come.

Dr. Marvin Tanenbaum is group leader at the Hubrecht Institute and International Research Scholar of the Howard Hughes Medical Institute.

Combined CRISPRi/a-Based Chemical Genetic Screens Reveal that Rigosertib Is a Microtubule-Destabilizing Agent. 
Marco Jost, Yuwen Chen*, Luke A. Gilbert*, Max A. Horlbeck*, Lenno Krenning*, Grégory Menchon*, Ankit Rai*, Min Y. Cho, Jacob J. Stern, Andrea E. Prota, Martin Kampmann, Anna Akhmanova, Michel O. Steinmetz, Marvin E. Tanenbaum#, Jonathan S. Weissman#. Molecular Cell 2017. Volume 68, Issue 1, 5 Oct 2017, pp 210–223.