KDM5-C70 is an ethyl ester derivative of KDM5-C49, which is a potent and selective inhibitor of Jumonji AT-Rich Interactive Domain 1 (JARID1) histone demethylases. The highly polar carboxylate group of KDM5-C49 restricts its cellular permeability; therefore KDM5-C70 was developed as a pro-drug, masking the polarity of the acid group of the KDM5-C49, for cellular assays and in vivo use. It has IC50 <1 uM for KDM5B (or PLU1) and UTX in histone lysine demethylase AlphaLISA assays, very good selectivity over JMJD1B, JMJD2A, JMJD2B and JMJD3. It has IC50 <1 uM for inhibiting demethylation of H3K4 in U2OS human osteosarcoma cell line. KDM5-C70 also suppresses human breast adenocarcinoma MCF7 tumor cell proliferation with IC50 < 1 uM. KDM5-C70 has shown antiproliferative effects in myeloma cells, leading to genome-wide elevation of H3K4me3 levels. KDM5-C70 treatment increased global H3K4me3 levels in HeLa cells. In this case, KDM5-C70 led to decreased 3′UTR length of DICER1. Although the cellular potency of KDM5-C70 is weak and about one to two orders of magnitude beyond the desired cellular potency characterizing high quality chemical probes, the correlated antiproliferative phenotype and the favorable in vitro selectivity profile suggest that further optimization of this scaffold could potentially lead to valuable tools for interrogating KDM5 phenotypes in human biology.