Abstract A143: Coupling fragment-based screening with targeted protein degradation and genetic rescue to identify and explore the function of a non-canonical pocket on eIF4E
Authors: Sharp Swee Y , Martella Marianna , Milton Christopher I , Ward George , Vinkovic Mladen , Richardson Caroline , Woodhead Andrew , Clarke Paul A 
Published: December 2023

Abstract
The translation initiation factor eIF4E binds the mRNA m7G-cap to initiate recruitment and binding of eIF4F components such as eIF4G. eIF4E exhibits oncogenic activity in in vitro and in vivo assays and high eIF4E expression in cancers is associated with poor prognosis and resistance to chemotherapeutics and targeted agents. Targeting eIF4E has long been considered a promising anticancer strategy but this target has remained undruggable using conventional screening approaches. In an accompanying submission we describe a fragment-based screening approach that discovered a non-canonical binding site on eIF4E with unknown functional relevance. Lead compounds with potency of 100nM were generated and provided tools for probing the function of the non-canonical site. Here we have used the tool compounds in combination with a targeted protein degradation (dTAG) to explore the function of this non-canonical site. H1299 NSCLC clones expressing an eIF4E-FKBP12F36V fusion protein but lacking endogenous eIF4E were established. Treatment of clones with a heterobifunctional dTAGV-1 molecule, that selectively binds to FKBP12F36V and recruits the VHL E3 ligase, induced rapid degradation of the eIF4E-FKBP12F36V protein leading to inhibition of cell growth and reduced expression of the MCL1 protein biomarker. We then expressed wild-type or eIF4E mutants and determined their ability to rescue the phenotype associated with eIF4E-loss following treatment with dTAGV-1. Expression of wild-type eIF4E rescued the phenotype associated with eIF4E-loss. The W73F mutation, that is reported to block canonical eIF4G binding, disrupted binding to eIF4G in cell lysate assays, but surprisingly the W73F mutant retained the ability to rescue the eIF4E degradation phenotype. Similar to our lead compound, mutations that impacted on the non-canonical pocket (L85R and L134R) disrupted the eIF4E:eIF4G interaction. However, L85R remained able to rescue the cellular and molecular responses, in contrast to the L134R mutant that could not rescue eIF4E loss. Interestingly, a W73F/L85R double mutant had a synergistic effect with a loss of function. Despite disrupting eIF4E:eIF4G binding and protein synthesis in lysates, treatment of H1299 cells with our lead compound did not inhibit proliferation. In contrast, compound treatment of the W73F mutant recapitulated the effects of combining W73F with the L85R non-canonical pocket mutation. In summary, targeting the non-canonical pocket was sufficient to disrupt the eIF4E:eIF4G binding in lysate assays. However, the differential impact of compounds or mutation on eIF4G binding in cells suggest that the eIF4E:eIF4G interaction in intact cells may be more complex than that predicted by the in vitro models. This observation may also explain the challenges associated with developing cellularly potent inhibitors of the eIF4E:eIF4G interaction. Our approach demonstrates the power of coupling fragment-based screening with target-degradation and genetic rescue approaches to find and explore novel functional pockets on difficult to drug proteins.
Citation Format: Swee Y Sharp, Marianna Martella, Christopher I Milton, George Ward, Mladen Vinkovic, Caroline Richardson, Andrew Woodhead, Paul A Clarke. Coupling fragment-based screening with targeted protein degradation and genetic rescue to identify and explore the function of a non-canonical pocket on eIF4E [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A143.