Development of FBXO22 Degraders and the Recruitment Ligand 2-Pyridinecarboxyaldehyde (2-PCA)

Targeted protein degradation (TPD) is a promising therapeutic strategy that requires the discovery of small molecules that induce the proximity between E3 ubiquitin ligases and proteins of interest. FBXO22 is an E3 ligase that is overexpressed in many cancers and implicated in tumorigenesis. While FBXO22 was previously identified as capable of recognizing ligands bearing a primary amine degron, further investigation and development of recruitment ligands are required to enable its broader utility for TPD. Here, we describe the discovery of chemical probes that can either selectively degrade FBXO22 or recruit this ligase for TPD applications. First, we described AHPC(Me)-C6-NH₂ as a potent and selective FBXO22 degrader (DC₅₀ = 77 nM, D_max = 99%) that is suitable for interrogating the effects of FBXO22 loss of function. Further, we discovered that the simple hexane-1,6-diamine acts as a minimal FBXO22 self-degrader, whereas shorter C4 (putrescine) to C5 (cadaverine) analogs, found in mammalian cells, do not induce degradation. Finally, we found that 2-pyridinecarboxaldehyde (2-PCA) functions as a novel electrophilic degron capable of forming a reversible thioacetal with cysteine 326 for recruiting FBXO22. Conjugating 2-PCA to various ligands successfully induced the FBXO22-dependent degradation of BRD4 and CDK12. Collectively, these chemical probes will facilitate the study of FBXO22 biology and broaden its applicability in the TPD.