Aminoquinoline-Pyrimidine-Based Alkyl-Piperazine TetheredHybrids: Synthesis, Antiplasmodial Activity, and Mechanistic Studies

Though great progress has been made to eliminate malaria globally, effective and inexpensive strategies to design new antimalarials are still required due to the problem of drug resistance to the currently used antimalarials. Herein, in continuation of our efforts to improve the therapeutic efficacy of 4-aminoquinoline-pyrimidine (4-AQ-Py) based molecular hybrids, a series of 4-AQ-Py hybrids linked through diamine-piperazine (flexible and rigid) linkers was synthesized and assessed for <i>in vitro</i> antiplasmodial activity. In the <i>in vitro</i> assay, these hybrids exhibited excellent potency and selectivity index against both the chloroquine (<b>CQ</b>)-sensitive (D6) and <b>CQ</b>-resistant (W2) strains of Plasmodium falciparum. Compound <b>7i</b> was found to be the most potent (5-fold more active than <b>CQ</b>) against the D6 strain, while compound <b>7e</b> displayed the most potency (53-fold more potent than <b>CQ</b>) against the W2 strain. Furthermore, nine compounds (<b>7d, 7f</b>–<b>i</b>, <b>7l</b>, and <b>7o</b>–<b>q</b>) showed better antiplasmodial activity than the reference drug artemisinin (<b>ART</b>) against the D6 strain, and compared to <b>ART</b>, seven compounds (<b>7d</b>–<b>e, 7i</b>–<b>k</b>, and <b>7p</b>–<b>q</b>) demonstrated better activity against the W2 strain. All the synthesized hybrids were found noncytotoxic against the mammalian VERO cell lines. Two potent compounds, <b>7e</b> and <b>7i,</b> were evaluated for their <i>in vivo</i> antiplasmodial activity against P. berghei-infected mouse models. Additionally, one of the best active compounds, <b>7i</b>, was tested for heme binding, and docking studies were conducted with <i>Pf</i>-DHFR to determine the primary mechanism of action of these hybrids.