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Stabilising Protein-Protein Interactions: A Target-Guided Fragment-Based Approach

Periodic Reporting for period 1 - TaGFrag (Stabilising Protein-Protein Interactions: A Target-Guided Fragment-Based Approach)

Reporting period: 2016-10-01 to 2018-09-30

The TaGFrag project aimed to discover novel, synthetically tractable small-molecule stabilisers of the therapeutically important 14-3-3σ – estrogen receptor alpha (ERα) protein-protein interaction (PPI). PPI stabilisation is an emerging strategy in drug discovery with the potential to deliver new medicines for the treatment of diseases such as cancer which pose a major societal challenge in Europe and beyond. Despite this potential it has been an under-explored approach in pharmaceutical research. This action set out to develop a rational target-guided chemical biology approach combining the concepts of fragment-based drug discovery (FBDD) with bioorthogonal synthesis. It sort to provide valuable tool or lead compounds to aid the maturation of this interaction as a clinical target and provide a rational and efficient approach for the discovery of PPI stabilisers in general.

Early results prompted a change in focus to the 14-3-3σ – p53 PPI which was also of great pharmaceutical interest because of its potential as a drug target for cancer treatment. FBDD screening led to the identification of novel fragment compounds binding to the 14-3-3σ – p53 PPI interface, a crucial first step toward functional molecules. The development of new bioorthogonal chemistry approaches led to the discovery of fusicoccin A (FC-A) as the first small molecule stabiliser of this PPI thus providing an important starting point for rational drug design. During the project it was also observed that different length peptides used to mimic the p53 binding motif had very different binding properties. This had significant fundamental implications for how PPI stabilisation is studied in vitro and so was also pursued as a research avenue.
As part of a broad collaboration fragment screening resulted in the identification of a family of structurally related binders with the potential to stabilise the 14-3-3σ – p53 PPI. Fragments of interest were evaluated for binding but at this early stage affinities were below detection limits. The combined data directed the design of second generation fragments by collaboration partners. This project remains ongoing and will be the subject of a publication that will aim to showcase this PPI as a tractable drug target in due course.

Early development of the biorthogonal chemistry-based discovery approaches led to the unexpected observation that the natural product fusicoccin A (FC-A) stabilises the 14-3-3σ - p53 PPI. This project element therefore evolved into a systematic investigation into the molecular basis for this finding. These results have been published: Doveston et al., FEBS Lett. 2017, 591, 2449.

As part of this study it was discovered that the length of peptide (used to mimic the p53 binding partner) had a dramatic effect on binding affinity. This result had significant implications for the planned biorthogonal chemistry research elements as well as the reliability of other assays in development. In order to establish a thorough rational for these observations this also became the focus of a systematic study the outcomes of which are the subject of a manuscript in preparation for publication.
The results obtained through this action represent significant progress beyond the state-of-the-art. The FBDD work has provided the first examples of small molecules binging at the interface of a 14-3-3σ PPI and therefore the potential to be developed into functional stabilisers. FC-A is the first example of a small molecule stabilising the 14-3-3σ – p53 PPI and this finding can now be exploited in rational drug design approaches. Finally, the discovery of variation in the binding profiles of peptide mimics signals the need for better fundamental understanding of how we study PPI systems in vitro. These results are important milestones in PPI stabiliser development. Specifically, they show significant progress toward demonstrating 14-3-3σ – p53 stabilisation as a potential therapeutic modality in cancer treatment and signal the need for further research effort in this area.
Stabilisation of the 14-3-3 - p53 Protein-Protein Interaction