Indolactam and Benzolactam Compounds as New Medicinal Leads with Binding Selectivity for C1 Domains of Protein Kinase C Isozymes

Protein kinase C (PKC) isozymes (α, βI, βII, γ, δ, ε, η, Θ) are major receptors of tumor promoters and also play a crucial role in cellular signal transduction via the second messenger, 1,2-diacyl-sn-glycerol (DG). Each isozyme of PKC is involved in diverse biological events, indicating that it serves as a novel therapeutic target. Since PKC isozymes contain two possible binding sites of tumor promoters and DG (C1A and C1B domains), the design of agents with binding selectivity for individual PKC C1 domains is a pressing need. We developed a synthetic C1 peptide library of all PKC isozymes for high-throughput screening of new ligands with such binding selectivity. This peptide library enabled us to determine that indolactam and benzolactam compounds bound to the C1B domains of novel PKC isozymes (δ, ε, eegr, Θ) in some selective manner, unlike phorbol esters and DG. Simpler in structure and higher in stability than the other potent tumor promoters, a number of indolactam and benzolactam derivatives have been synthesized to develop new PKC isozyme modulators by several groups. We focused on the amide function of these compounds because recent investigations revealed that both the amide hydrogen and carbonyl oxygen of indolactam-V (ILV) are involved in hydrogen bonding with the C1B domains of PKCd. Synthesis of several conformationally fixed analogues of ILV led to the conclusion that the trans-amide restricted analogues with a hydrophobic chain at an appropriate position (2,7) are promising leads with a high binding selectivity for novel PKC isozyme C1B domains. We also developed a new lactone analogue of benzolactam-V8 (17) which shows significant binding selectivity for the C1B domains of PKCe and PKCh. Furthermore, our synthetic approach with the PKC C1 homology domains clarified that diacylglycerol kinase b and g are new targets of phorbol esters.