Consequently, we tested AP1-AP6 in viability assays against the multiple myeloma cell line MM.1S as well as the breast cancer cell line MCF-7. Both cell lines were selected because they are widely used to characterize HDAC6 degraders , and serve as representative models of hematological and solid cancers. Interestingly, all compounds showed low or no inhibition of cell viability ( antiproliferative activity after 72 h is not a result of instability of the degraders. To rule out that the limited antiproliferative effects in MCF-7 cells comes from absence of degradation, we treated MCF-7 cells for 24 h with 1 or 5 μM of AP1-AP6 and analyzed the HDAC6 and HDAC10 levels by immunoblot analysis (Figure , Supplementary Information). Consistent with the results obtained in MM.1S cells, AP1, AP3, AP4 and AP6 were identified as potent degraders of HDAC6 and HDAC10, while AP2 and AP5 were less effective. Again, the phenylglutarimides AP4 and AP6 showed a pronounced "hook effect" at 5 μM.