Clin. used pan and class I HDACi cause severe side effects, whereas class IIa HDACi are less cytotoxic.Here, we present the synthesis and anticancer effects of a series of 5-(trifluoromethyl)-1,2,4-oxadiazole (TFMO)-based amides and alkoxyamides derived from the previously reported class IIa HDACi YAK540.The most active class IIa inhibitor 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent showed nanomolar inhibition of the class IIa enzymes 4, 5, 7 (IC50 HDAC4: 12 nM) and high selectivity (selectivity index >318 for HDAC4) over non-class IIa HDACs.Instead of a hydroxamic acid group, 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent has a trifluoromethyloxadiazolyl (TFMO) moiety as a non-chelating Zinc-binding group (ZBG).Applying the Chou-Talalay-method we found an increased synergistic cytotoxic effect of 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent in combination with bortezomib in THP1 cells. 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent in combination with bortezomib enhanced expression of p21 leading to increased caspase-induced apoptosis.Eventually, growth inhibition by 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent of the head-neck cancer cell line Cal27 was increased upon HDAC4 overexpression in Cal27 in cell culture and using the in-vivo chorioallantoic membrane model.The class IIa HDACi 1,2,4-oxadiazole derivative containing N',N'-diethylethane-1,2-diamine substituent outperforms previously described HDAC class IIa inhibitor YAK540 regarding anticancer effects and may constitute a novel option compared to pan and class I HDACi in anticancer combination treatments.