A synthetic KLHL20 ligand to validate CUL3^KLHL20 as a potent E3 ligase for targeted protein degradation

Targeted protein degradation (TPD) has risen as a promising therapeutic modality. Leveraging the catalytic nature of the ubiquitin-proteasome enzymatic machinery, TPD exhibits higher potency to eliminate disease-causing target proteins such as oncogenic transcription factors that may otherwise be difficult to abrogate by conventional inhibitors. However, there are challenges that remain. Currently, nearly all degraders engage CUL4^CRBN or CUL2^VHL as the E3 ligase for target ubiquitination. While their immediate efficacies are evident, the narrowed E3 ligase options make TPD vulnerable to potential drug resistance. In addition, E3 ligases show differential tissue expression and have intrinsic limitations in accessing varying types of disease-relevant targets. As the success of TPD is closely associated with the ability of E3 ligases to efficiently polyubiquitinate the target of interest, the long-term outlook of TPD drug development will depend on whether E3 ligases such as CUL4^CRBN and CUL2^VHL are accessible to the targets of interest. To overcome these potential caveats, a broad collection of actionable E3 ligases is required. Here, we designed a macrocyclic degrader engaging CUL3^KLHL20 for targeting BET proteins and validated CUL3^KLHL20 as an E3 ligase system suitable for TPD. This work thus contributes to the expansion of usable E3 ligases for potential drug development.