The two potent prodrugs 5 and 6 were both 2-nitroimidazole rather than nitrobenzyl compounds, which is in line with previous findings on the better activation of the heterocyclic prodrugs. Interestingly, compound 4 and compound 10 show little effect on NTR-THP1 cells at 10 µM (14 and 19% inhibition, Table ), even so compound 4 is a nitroimidazole derivative. Potentially, the core structure (from compound I) is binding weakly to NTR and subsequently prodrug activation is hindered. However, regarding compound 5 and 6, the cellular effects are in the range of the parent HDAC inhibitor, implying almost full conversion (Table ). Whereas the parent HDAC inhibitors have essentially the same activity on both cell lines, we find a selectivity window of 6-12 fold for the prodrugs (Table ). The inhibition in wild-type THP1 cells resulted either from the intrinsic toxicity of the carbamates or from some background reductive activation in the non-transfected cells. Thus, while the selectivity could still be improved, compound 6 is a prodrug with very good bioreductive properties, which leads to highly potent activity against a leukemic cell line and is a great starting point for further optimization. To analyze the therapeutic window of the developed prodrugs, the cytotoxic properties of the prodrugs and the parent inhibitors were determined using a MTT assay protocol against the murine macrophage cell line (J774A.1) after incubation for 72 hours. To quantify cytotoxic activity, the half-maximal inhibitory concentration (IC50) was determined. The results, summarized in Fig. and Table , consistently showed that the synthesized prodrugs exhibited significantly higher IC50 values compared to the corresponding parent HDAC inhibitors.