To further elucidate the degradation efficiency of the synthesized class IIb HDAC degraders, AP1, AP3, AP4, and AP6 were selected for the determination of their DC50 values for HDAC6 and HDAC10 in MM1.S cells (Figure ). As depicted in Table , all four compounds demonstrated potent DC50 values towards HDAC6 and HDAC10. Notably, AP4 emerged as the most potent degrader for both HDAC6 and HDAC10, with DC50 values of 1.3 nM and 1.7 nM, respectively. Furthermore, it is noteworthy that all four compounds exhibited stronger degradation potency towards HDAC6 compared to HDAC10. The most significant difference, reaching 3.1-fold (DC50, HDAC10 / DC50, HDAC6), was observed for AP3 featuring a bicyclic warhead and a pomalidomide-based CRBN ligand. In contrast, the lowest difference of 1.3-fold was observed with AP4, comprising tubastatin A as the POI ligand and a phenylglutarimide derivative-based CRBN ligand. Consistent with the experiments at 1 and 5 µM (Figure ), the phenylglutarimides AP4 and AP6 showed a "hook effect". In contrast, the pomalidomide-based PROTACs AP1 and AP3 did degrade HDAC6 and HDAC10 in a concentration-dependent manner.