Biological evaluation of 4,5,7-trisubstituted Indeno[1,2-<em>b</em>]indoles reveals a potent inhibitor of protein kinase CK2 in tumor cells with diverse anti-cancer effects and preferential cytoplasmic localization

The highly pleiotropic and constitutively active serine/threonine protein kinase CK2 is considered a key target in cancer. The indeno[1,2-b]indole scaffold was previously shown to provide derivatives exhibiting strong CK2 inhibition and satisfactory drug-like characteristics. In this work, we evaluated one 4,5,7-trisubstituted indeno[1,2-b]indole derivative for its intracellular inhibition of CK2 activity and the accompanying effects on proliferation, migration and apoptosis in cancer cells. The compound 5-isopropyl-4-methoxy-7-methyl-5,6,7,8-tetrahydro-indeno[1,2-b]indole-9,10-dione (5a-2) strongly inhibited CK2 activity in vitro with IC50 value of 25 nM and in cultured A431, A549 and LNCaP cell lines (> 75% inhibition at 20 µM). The intracellular inhibition of CK2 by 5a-2 was comparable to that induced by the reference CK2 inhibitor CX-4945, though the latter exhibited > 6-fold higher inhibitory potency toward CK2 in vitro (IC50 = 3.7 nM). A possible explanation for this discrepancy is the significantly higher intracellular concentrations of 5a-2 compared to CX‑4945 following their cellular uptake. Compared to CX-4945, 5a-2 induced similar anti-proliferative, weaker pro-apoptotic but stronger anti-migratory effects on cancer cells. These variations can be partly attributed to the observed differences in the subcellular localization of both compounds whereby 71% of the uptaken 5a-2 molecules were found in the cytoplasm while 49% of intracellular CX-4945 was detectable in the nuclear fraction. Our study emphasizes the potential of indeno[1,2-b]indole as an interesting framework for developing potent CK2 inhibitors and highlights the significance of subcellular distribution in dictating preferential cellular effects of CK2 inhibitors.