HDAC6-IN-18 (Compound 4) is a first irreversible HDAC6 isoform selective inhibitor with potent anti-multiple myeloma activity. HDAC6-IN-18 has HDAC6 inhibitory activity in RPMI8266, U266 and MM.1S cells with IC 50 values of 0.17, 0.7 and 0.42 μM, respectively.