Developing ROTACs as a novel tool for cancer therapy

Targeted protein degradation (TPD) is a rapidly progressing field that has broadened the scope of therapeutic targets to include historically undruggable proteins and overcome drug resistance. In contrast to degradation of cytosolic targets, the TPD toolbox to target extracellular proteins is rather limited. To overcome this limitation, we took advantage of R-spondin's ability to target diverse cargo to degradation and developed ROTACs (R-spondin Targeting Chimera). ROTACs are bispecific WNT- and BMP signaling-disabled R-spondin chimeras, which leverage the specificity of these stem cell growth factors for ZNRF3/RNF43 E3 transmembrane ligases to target transmembrane protein degradation.

As proof of concept, we targeted the immune checkpoint protein programmed death ligand 1 (PD-L1), a prominent cancer therapeutic target, with a WNT- and BMP signaling-disabled bispecific RSPO2 chimera, R2PD1. R2PD1 binds PD-L1 and at picomolar concentration induces its lysosomal degradation. In melanoma cell lines, R2PD1 induces between 50-90% PD-L1 protein degradation. Moreover, R2PD1 reactivates cytotoxic T cells and inhibits tumor cell proliferation more potently than Atezolizumab, a well-established PD-L1 in clinical use. We suggest that signaling-disabled ROTACs represent a paradigm to target cell surface proteins for degradation in a range of applications. Hence, signaling-disabled RSPO chimeras represent a novel avenue to target cell surface proteins for degradation. We currently develop novel ROTACs for targeted cancer therapy.