Exploration of benzofuran as scaffold for the design of potent and selective PfGSK-3 inhibitors

Abstract

Control and prevention of malaria in endemic regions rely heavily on vector management (insecticide-treated bed nets and indoor residual spraying) and the limited number of effective antimalarial drugs. Unfortunately, the malaria parasite is highly adaptable and continuously develop resistance to available antimalarial drugs and insecticides. Plasmodium falciparum, the malaria species responsible for the majority of malaria-related deaths, in particular is resistant to all antimalarial drugs including the first-line artemisinin-based combination therapies. This necessitates the continual generation of new antimalarial drugs with novel modes of action. Plasmodial protein kinases have been explored as targets for next-generation antimalarial drugs due to their involvement in various critical processes throughout the life cycle of the malaria parasite. Significant progress has been made thus far with regard to drug development for this target group. However, drug discovery programs for some essential plasmodial kinases have been limited. One such kinase is the Plasmodium falciparum glycogen synthase kinase-3 (PfGSK-3). PfGSK-3 was identified as the parasitic orthologue of human glycogen synthase kinase (HsGSK-3) and although its exact biological functions is still unknown, it has been demonstrated to be essential for the survival of erythrocytic P. falciparum parasites. Despite the high degree of sequence and structural similarity between PfGSK-3 and the two HsGSK-3 orthologues (HsGSK-3α and HsGSK-3β), studies have shown that it is indeed possible to achieve PfGSK-3 selective inhibition. To date, PfGSK-3 selective inhibitors have been limited to the 4-phenylthieno[2,3-b]pyridine class. The present study explored benzofurans as an alternative chalcone-based scaffold for the design of potent and selective PfGSK-3 inhibitors. A series of benzofuran derivatives (5a-v) with varied structural substituents on both ring A and B were synthesised and characterised through nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), high-performance liquid chromatography (HPLC) and melting point (mp) analyses. The benzofuran derivatives were then evaluated as potential inhibitors of recombinantly expressed and purified PfGSK-3 and HsGSK-3β. Of the seven compounds that demonstrated inhibitory activity, five compounds (5k, 5m, 5p, 5r, 5s) preferentially inhibited PfGSK-3 with IC50 values ranging from 0.00049 – 1.692 μM. Compounds 5p and 5m were the most potent PfGSK-3 inhibitors of the series (IC50: 0.00049 μM and 0.00168 μM, respectively), while compound 5r was the most selective inhibitor with a 559-fold selectivity towards PfGSK-3. The structure-activity relationships indicated that C6-OCH3 substitution on ring A promotes

selectivity towards PfGSK-3, while the effect of ring B substitution on activity, in decreasing order was: C4’-CN > C4’-F > C3’-OCH3 > C3’,4’-diCl. In conclusion, the current study evaluated benzofurans to gain further insight into the structure-activity relationships around this scaffold for the design of potent and selective PfGSK-3 inhibitors. The benzofuran derivatives proved to be highly potent and selective PfGSK-3 inhibitors. As these compounds have potential in the treatment of multidrug-resistant malaria, they are promising hits for future medicinal chemistry efforts.