Drugging the Fbw7 E3 Ligase with a Fragment-Based Approach ^†

Fbw7 is an important E3 ligase and one of the most commonly deregulated proteins in human cancers. Six percent of cancers have mutations in the fbw7 gene. On one hand, the loss of activity of the mutated Fbw7 results in a loss of its tumor suppressor function and an upregulation of the natural and oncogenic substrate proteins, such as c-Myc, cyclin-E, and Notch. On the other hand, the inhibition of Fbw7 has been proposed as an approach to sensitize cancer stem cells to chemotherapies. Given the key role of Fbw7 in tumorgenesis, a small molecule directly targeting Fbw7 would have a large impact on the clinic. However, so far, no potent small molecules that directly bind to Fbw7 have been reported, in part because modulating their activity and regulation requires targeting protein–protein interactions.

Our goal is to identify and characterize fragments that bind to the Fbw7 E3 ligase and can be further developed as chemical probes. These fragments may turn on or off the activity of the protein. Fbw7 binders could serve as anchors to develop disease-specific PROTAC molecules, leading to proximity-induced ubiquitylation and subsequent degradation of proteins of interest. Our group has built a library of around 700 fragments. Surface plasmon resonance (SPR) has been carried out. Potential fragment-hits have been identified, and they are being validated using orthogonal biophysical techniques. Furthermore, in order to elucidate the binding mode of the fragments, it is crucial to perform X-ray crystallography. The crystal structure of fragments binding to the protein will not only show the key points for the interaction, but it can also provide the starting point for a rational design to grow the molecules in order to improve their affinity and specificity.