Domainex employed a number of techniques and its extensive experience of fragment-based drug design to identify hits which exhibited strong RAS binding. These were then developed into leads with increased binding affinity and improved physical properties.
Activating RAS mutations are associated with approximately 30% of cancers; therefore, blocking RAS-effector interactions with biological reagents has beneficial effects in cancer models. Our collaborator, Professor Terry Rabbitts of the University of Oxford, commissioned Domainex to help with the identification of small molecules which bind potently to activated RAS and inhibit binding of effectors such as PI3K and Raf. This work was unprecedented when it was initiated.
A fragment hit (Abd-1) that demonstrated selective binding to GTP-bound (i.e. the active form) of RAS was identified from an SPR screen of a fragment library of 656 compounds.
Our medicinal chemists explored the SAR around Abd-1 to generate Abd-2, which had increased RAS-binding affinity and improved aqueous solubility. These superior properties facilitated structural biology studies and enabled the binding mode to be confirmed by X-ray crystallography.
Domainex utilised its experience in medicinal chemistry and, with structural and binding analysis, we were able to develop the fragment-hit into lead molecules using a structure-guided approach. RAS-binding was enhanced, and a solubilising substituent was introduced to enhance the physical properties in order to assist with the X-ray crystallography.
Small H-bonding groups proved to be ineffective in forming additional interactions with RAS, but an 8-pyridyl substituent showed a significant improvement in potency. X-ray crystallography showed that this induced a conformational change (see Figure 6):
- Asp-54 rotation opens up a lipophilic feature to accommodate aromatic group
- Arg-41 moves closer to pocket
Figure 6: 8-Pyridyl substitution induced a conformational change
The conformational change presented an opportunity for growth towards the Switch 1 region, accessible via substitution on the pyridyl ring, which led to increased binding affinity.
Finally, removal of the solubilising group increased ligand efficiency, resulting in the cell-permeable RAS PPI inhibitor, Abd-7. The Rabbitts group showed that Abd-7 disrupted the interaction of RAS with a range of effector proteins, and modulated signalling downstream of RAS, eliciting a reduction in phosphorylation of downstream proteins AKT and ERK.1
Domainex utilised its extensive experience of fragment- and structure-based drug design to elaborate a fragment hit into a lead molecule with efficient binding to RAS. This and related compounds were shown to inhibit the RAS-effector interaction and tumour viability in cellular assays.
1. Small molecule inhibitors of RAS-effector protein interactions derived using an intracellular antibody fragment. Camilo E. Quevedo, Abimael Cruz-Migoni, Nicolas Bery, Ami Miller, Tomoyuki Tanaka, Donna Petch, Carole J.R. Bataille, Lydia Y.W. Lee, Phillip S. Fallon, Hanna Tulmin, Matthias T. Ehebauer, Narcis Fernandez-Fuentes, Angela J. Russell, Stephen B. Carr, Simon E.V. Phillips and Terence H. Rabbitts., Nat Commun 9, 3169 (2018).
A new class of RAS-effector protein-protein interaction inhibitors
20th December 2018