(394c) Targeted Degradation of Secreted and Cell Surface Proteins through the LRP-1 Pathway.

Upregulation or deregulation of proteins is a common cause for disease pathogenesis, in diseases ranging from cancers (where EGFR and PD-L1 are commonly upregulated) to neurogenerative diseases (such as amyloid-β aggregation in Alzheimer’s disease) and thus targeted protein degradation is a potent therapeutic approach. Several such approaches have been described in the past, both for intracellular and extracellular proteins, including PROTACs, which mark cytosolic proteins for degradation through ubiquitinoylation, and LYTACs, which target specific proteins to the lysosome using glycopeptide ligands and antibodies against the target proteins. However, both systems are based on antibodies against either the target protein or targeting the lysosomal pathway, leading to large biological molecules with complicated syntheses and purifications. A small molecule drug with the ability to mark specific proteins for degradation would have the advantage of easier synthesis and delivery compared to previously described such constructs.

In previous work we described the LRP-1 mediated endocytosis of TG2 substrates, in which gluten peptides are endocytosed after the formation of a ternary complex between the peptide, TG2 and α₂-macroglobulin (α₂M). The complex is recognized by LRP-1, the canonical α₂M receptor and shuttled to the lysosome. Based on this work, we designed and tested a series of small bifunctional molecules which bind to target proteins and harbour a TG2 epitope or inhibitor. The TG2 epitope allows the complex that forms between the target protein and the bifunctional molecule to be endocytosed through the LRP-1 endocytosis pathway. We described three prototypical systems in which target proteins are endocytosed and degraded through the LRP-1 pathway. First, we targeted a secreted, extracellular protein. We showed that the protein was effectively endocytosed within 90 minutes and degraded within 2 hours. Next, we sought to degrade a representative cell surface protein. We selected cubilin which is present on the surface of NRK cells and showed that in the presence of our designed bifunctional molecule it disappears from the cell surface within 3h in an α₂M, TG2 and LRP-1 dependent manner. Finally, we demonstrated that this approach for protein degradation can be applied to pharmacologically relevant proteins, by showing the degradation of other cell surface receptors. Similarly to cubilin, these were degraded within 4h using a bifunctional molecule that binds both the receptor and TG2, in the presence of α₂M. These data exhibit the potential of the LRP-1/TG2 pathway to be used to selectively degrade a variety of proteins. Further discovery and optimization of the prototypical bifunctional molecules could lead to molecules with significant therapeutic value.