Towards Novel Antiplasmodial Agents-Design, Synthesis and Antimalarial Activity of Second-Generation β-Carboline/Chloroquine Hybrids.

As the resistance of to the existing antimalarials increases, there is a crucial need to expand the antimalarial drug pipeline. We recently identified potent antimalarial compounds, namely harmiquins, hybrids derived from the β-carboline alkaloid harmine and 4-amino-7-chloroquinoline, a key structural motif of chloroquine (CQ). To further explore the structure-activity relationship, we synthesised 13 novel hybrid compounds at the position -9 of the β-carboline ring and evaluated their efficacy in vitro against 3D7 and Dd2 strains (CQ sensitive and multi-drug resistant, respectively). All compounds exhibit persistent antimalarial activity against both strains of . The most interesting derivatives had low nanomolar activity against both strains (IC () = 4.7 ± 1.3 nM against 3D7 and 6.5 ± 2.5 nM against Dd2; IC () = 4.6 ± 0.6 nM against 3D7 and 10.5 ± 0.4 nM against Dd2). Resistance indices (RIs) ranged from 0.9 to 5.3 compared to CQ (RI = 14.4), highlighting their superior consistency in activity against both strains. The cytotoxicity screening performed on HepG2 revealed over 3 orders of magnitude higher IC for most of the compounds, with SIs from 711.0 to 8081.8. Spectroscopic studies explored the affinities of newly synthesised compounds for DNA, RNA, and HSA. Both tested hybrids, and , were intrinsically fluorescent in an aqueous medium, characterised by remarkable Stokes shifts of emission maxima (Δ = +103 and +93 nm for and , respectively). Fluorimetric experiments revealed that compound , with its shorter and more flexible linker, exhibited at least an order of magnitude higher affinity toward ds-DNAs versus ds-RNA and two orders of magnitude higher affinity toward GC-DNAs compared to . The behaviour of the investigated compounds upon binding to HSA is very similar, showing a strong hypsochromic shift of the emission maximum (almost Δ = -70 nm) and demonstrating their effectiveness as fluorimetric probes for distinguishing between DNA/RNA and proteins.