Structure-activity relationships in a series of antiplasmodial thieno[2,3-b]pyridines

Malaria is one of the most prevalent tropical infectious diseases. Since recently cases of artemisinin resistance were reported, novel anti-malarial drugs are required which differ from artemisinins in structure and biological target. The plasmodial glycogen synthase kinase-3 (PfGSK-3) was suggested as a new anti-malarial drug target. 4-Phenylthieno[2,3-b]pyridines were previously identified as selective PfGSK-3 inhibitors with antiplasmodial activity. The present study aims at identifying a molecular position on this scaffold for the attachment of side chains in order to improve solubility and antiplasmodial activity. Furthermore, the role of axial chirality in the compound class for antiplasmodial activity and PfGSK-3 inhibition was investigated.

4-Phenylthieno[2,3-b]pyridines with substituents in 4-position of the phenyl ring were docked into the ATP binding site of PfGSK-3. The compounds were synthesized employing a Thorpe reaction as final step. The enantiomers of one congener were separated by chiral HPLC. All derivatives were tested for inhibition of asexual erythrocytic stages of transgenic NF54-luc Plasmodium falciparum. Selected compounds with promising antiplasmodial activity were further evaluated for inhibition of HEK293 cells as well as inhibition of isolated PfGSK-3 and HsGSK-3. The kinetic aqueous solubility was assessed by laser nephelometry.

The para position at the 4-phenyl ring of the title compounds was identified as a suitable point for the attachment of side chains. While alkoxy substituents in this position led to decreased antiplasmodial activity, alkylamino groups retained antiparasitic potency. The most promising of these congeners (4h) was investigated in detail. This compound is a selective PfGSK-3 inhibitor (versus the human GSK-3 orthologue), and exhibits improved antiplasmodial activity in vitro as well as better solubility in aqueous media than its unsubstituted parent structure. The derivative 4b was separated into the atropisomers, and it was shown that the (+)-enantiomer acts as eutomer.

The attachment of alkylamino side chains leads to the improvement of antiplasmodial activity and aqueous solubility of selective PfGSK-inhibitors belonging to the class of 4-phenylthieno[2,3-b]pyridines. These molecules show axial chirality, a feature of high impact for biological activity. The findings can be exploited for the development of improved selective PfGSK-3 inhibitors.