The activity of a series of imidazo[2,1-b]thiazole guanylhydrazones as inhibitors of p90 ribosomal S6 kinase 2 (RSK2) is described. It was found that a small subset of compounds show both potent inhibition of RSK2 kinase activity and tumor cell growth in vitro. Detailed study of one of the most active compounds indicates a high degree of selectivity for inhibition of RSK2 compared to a spectrum of other related kinases. Selective inhibition of the MCF-7 breast tumor cell line compared to MCF-10A non-transformed cells, as well as selective inhibition of the biomarker GSK3 provides evidence that the compounds can affect the RSK2 target in cells.