show anticancer activity. All PROTACs were first tested for their antiproliferative activity against the human multiple myeloma cell line MM.1S using a cell viability assay (Figure ). The published selective HDAC6 PROTAC A6 was included as a control (Figure ), which recruits CRBN with a thalidomide-based ligand. A6 is analogous to 1j, only differing in the E3 ligase ligand. With the exception of the three PEG spacer-bearing PROTACs (1d-f) and the γaminobutyric acid derivative 1h, all compounds showed single-digit micromolar half maximal effective concentration (EC50) values against MM.1S. Notably, 1g, 1j and 1k outperformed ricolinostat, a HDAC6 preferential inhibitor currently in clinical development. In contrast, A6 did not show antiproliferative effects against the multiple myeloma cell line.