To assess the HDAC-degrading capabilities of 17a-d and 18a-d, we treated multiple myeloma cells (MM.1S) with our test compounds with concentration of 0.5 µM and 5 µM. Compound A6, a selective HDAC6 degrader identified in our previous study, was included as a control. 17 MM1.S cells were treated for 24 hours before HDAC6 levels were determined by immunoblot analysis, as depicted in Table and Figure (Supporting Information). Interestingly, significant HDAC6 degradation was shown by all of the CRBN-engaging compounds 17a-d, but no degradation of HDAC6 was exhibited by VHL-based compounds 18a-d. The most potent degradation of 91% was achieved by 17c at 0.5 µM. Additionally, 17a-c showed increased levels of HDAC6 at 5 µM compared to the levels at 0.5 µM. This effect was not observed for 17d. Here, at 5 µM the HDAC6 degradation was higher (90%) than at 0.5 µM (85%). It was also investigated whether HDAC1-3 could be degraded by the compounds. As shown in Figure , no degradation of HDAC1-3 was observed for any compound. [a] mean ± SD, results from at least three individual experiments. Structure-activity relationships. The CRBN-targeting compounds 17a and 17c exhibited the strongest degradation of HDAC6 at 0.5 µM with Dmax values of 86% and 91%, respectively. Both compounds contain alkyl spacers, which seem to be more efficient in inducing HDAC6 degradation, since the PEG spacer-containing compounds 17b and 17d showed slightly lower degradation activities of 75% and 85%. Furthermore, our previously developed HDAC6 degrader A6 contains an alkyl spacer, suggesting that this spacer type may enhance HDAC6 degradation via CRBN-recruiting PROTACs. When comparing the connecting units, the triazole-based degraders were superior to the carboxamide-based compounds (see Table , 17a vs 17c, and 17b vs 17d). The VHL-based compounds 18a-d showed no significant degradation activity, preventing the identification of structure-degradation relationships.