Advancing N,N′-disubstituted diamines in antimalarial drug development: Structure-activity relationships and mechanistic studies

Previously we reported the synthesis of thirty N,N′-disubstituted diamines, several of which exhibited potent activity against apicomplexan parasites, including Plasmodium falciparum and Toxoplasma gondii. Building on this, we expanded the series with fifty-four new compounds, which were evaluated against the same parasites. These analogs were synthesized via a one-pot reductive amination of aliphatic diamines with various aromatic aldehydes. The new library showed great potential, with several compounds achieving a pIC₅₀ greater than 6.0 against P. falciparum and with an acceptable selectivity index range (SI ≥ 10). Additionally, some compounds exhibited activity against T. gondii, although no clear correlation in activity was observed between the two apicomplexan parasites which survive in different intracellular niches. A comprehensive structure-activity relationship (SAR) analysis was performed, encompassing both the previously reported collection and the new analogs. Furthermore, selected hits were tested against resistant P. falciparum strains, demonstrating comparable activity to that observed with sensitive strains. To gain insights into the mechanism of action, we examined morphological changes in the parasite for selected hits, observing distinct alterations that suggested diverse mechanisms across the compounds. Finally, we evaluated the in vivo activity of three selected hits, though substantial parasite clearance was not observed. This outcome highlights the opportunity to optimize SAR to enhance permeability, solubility, and bioavailability, or it may suggest a parasitostatic rather than parasiticidal mechanism of action. In conclusion, this work highlights the potential of N,N′-disubstituted diamines in antimalarial drug discovery. Future efforts will focus on improving in vivo efficacy and further elucidating the mechanism of action.