Exploring kinase inhibition properties of 9H-pyrimido[5,4-b]- and [4,5-b]indol-4-amine derivatives

In previous works 6,5,6-fused tricyclic analogues of 4-aminoquinazolines were identified as kinase inhibitors, in the micromolar to the nanomolar range of IC₅₀ values. In an effort to expand the chemical space and to highlight novel efficient kinase inhibitors, the central five-member ring of such tricyclic skeleton, i.e. thiophene or furan, was replaced by a pyrrole to afford pyrimidoindol-4-amine derivatives. Such compounds are considered as analogues of harmine, a natural alkaloid that still generates a lot of work in the hope of developing therapies for Alzheimer's disease (AD) and Down syndrome (DS). Here, we describe simple and convenient synthetic routes to 9H-pyrimido[5,4-b]indol-4-amine derivatives and their 9H-pyrimido[4,5-b]indole isomers. The chemistry described in this paper was mainly carried out under microwave irradiation in an eco-friendly approach. Kinase inhibition of the products obtained was evaluated on an array of four Ser/Thr kinases (CDK5/p25, CK1, DYRK1A and GSK3), all members of the CMGC kinase family, chosen for their strong implication in various cellular regulation processes. As a result, we have identified promising compounds targeting CK1 and DYRK1A and displaying micromolar and submicromolar IC₅₀ values.