Ten protein kinase C (PKC) isozymes play divergent roles in signal transduction. Because of sequence similarities, it is particularly difficult to generate isozyme-selective small molecule inhibitors. In order to identify such a selective binder, we derived a pharmacophore model from the peptide EAVSLKPT, a fragment of PKCε that inhibits the interaction of PKCε and receptor for activated C-kinase 2 (RACK2). A database of 330 000 molecules was screened in silico, leading to the discovery of a series of thienoquinolines that disrupt the interaction of PKCε with RACK2 in vitro. The most active molecule, N-(3-acetylphenyl)-9-amino-2,3-dihydro-1,4-dioxino[2,3-g]thieno[2,3-b]quinoline-8-carboxamide (8), inhibited this interaction with a measured IC₅₀ of 5.9 μM and the phosphorylation of downstream target Elk-1 in HeLa cells with an IC₅₀ of 11.2 μM. Compound 8 interfered with MARCKS phosphorylation and TPA-induced translocation of PKCε (but not that of PKCδ) from the cytosol to the membrane. The compound reduced the migration of HeLa cells into a gap, reduced invasion through a reconstituted basement membrane matrix, and inhibited angiogenesis in a chicken egg assay.

It was found by virtual screening that 3-amino-1H-pyrazolo[3,4-b]quinolines could have wide protein kinase inhibitory activity. Amides of titled amines and thioureas were synthesized regioselectively. 3-Amino-7-methoxy-1H-pyrazolo[3,4-b]quinoline demonstrated in vitro significant inhibitory activity on bacterial serine/threonine protein kinases (inhibition of resistance to kanamycin in Streptomyces lividans regulated by protein kinases). The studies of Structure Activity Relationship (SAR) showed that the substitution of the NH₂ group and 1-NH of pyrazole ring or aromatic ring at the position 6 decreased or removed inhibitory activity.