Repositioning the antitubercular candidate SQ109 as potent antimalarial with polypharmacology

Abstract

Elimination of malaria requires effective targeting of the asexual replicative (asexual blood stage, ABS) and transmissible stages of the most lethal human malaria parasite, Plasmodium falciparum. Targeting gametocytes for transmission-blocking activity is particularly desirable as they are amenable to pharmacological intervention [1]. The search for new transmission-blocking antimalarial compounds has generally been biased towards compounds targeting biology important to the ABS parasites with screening campaigns prioritising hits based on ABS activity and gametocyte activity merely adding dual activity value to these drugs [2]. This approach poorly identifies equipotent or gametocyte-targeted antimalarial candidates for formulation into combination therapies for dual-stage activities. Alternative strategies used de novo, parallel screening against multiple life cycle stages, to identify the antitubercular clinical candidate MMV687273 (SQ109) with preferential activity against P. falciparum late-stage gametocytes [3] with comparatively poor activity against ABS parasites. Although combining compounds with gametocyte and ABS activity reduces the risk of resistance transmission, the pursuit of compounds solely focused on blocking transmission cannot currently be prioritised over the discovery of compounds with additional ABS activity given the urgent need for effective clinical treatments for malaria. Therefore, identifying compounds with dual-stage activity remains equally important. SQ109, therefore provides a new chemical starting point for further development and optimisation for dual-stage active compounds.

Therefore, this study aimed to design and synthesise analogues of SQ109 to identify features required to consolidate curative and transmission-blocking compounds in structure activity relationship analysis, and to see if improvements can be made to ABS activity. Here, we confirmed SQ109’s antiplasmodial activity and evaluated the structure activity relationship of the SQ109 scaffold to design dual-stage active antiplasmodial compounds. We show that equipotent activity (IC50) in the 100 – 300 nM range could be attained for both ABS parasites and sexual stages, with the activity of most compounds retained against a multidrug-resistant strain. The dual-stage activity profile relies on high lipophilicity due to the adamantane headgroup, and antiplasmodial activity is critically dependent on the diamine linker. We additionally validated that this series could block transmission to mosquitoes, marking these compounds as novel chemotypes with dual-stage, antiplasmodial activity. These results indicate that this series holds promise as an effective treatment, with no apparent chemical features that could impede its development into a drug.

SQ109 is a second-generation ethylenediamine antitubercular that completed phase IIb clinical trials for tuberculosis, and potently targets multidrug-resistant Mycobacterium tuberculosis [1, 4]. Its microbial target was identified as mycobacterial membrane protein large 3 [5], involved in cell wall biosynthesis. Additionally, SQ109 shows polypharmacology, whereby it triggers multiple biological responses and/or has multiple targets [6, 7]; as an example it was implicated in the disruption of the proton motive force [8] by acting as a protonophore uncoupler in mycobacteria. This activity has been proposed to be a significant driver of the mode of action of SQ109 against organisms that lack a mycobacterial membrane protein large 3 counterpart, such as Trypanosoma cruzi and Leishmania spp [9, 10], where effects on Ca2+, H+ and sterol homeostasis have also been found. No mycobacterial membrane protein large 3 homologues are present within the genome of P. falciparum parasites, and the Niemann–Pick type C1-related H+/lipid symporter, which belongs within the same resistance-nodulation-division superfamily of transporters as mycobacterial membrane protein large 3, was excluded as a potential mycobacterial membrane protein large 3-like target [3]. Therefore, with the mode of action of SQ109 in P. falciparum parasites essentially unknown, guided by its activity in other organisms, this study aimed to determine the mode of action of SQ109 and investigate if polypharmacology of SQ109 exists in P. falciparum parasites. If the polypharmacology of SQ109 held true for P. falciparum parasites, such a compound when used in combination, would have the added advantage of protecting compounds with ABS activity from resistance spreading events. Here, we proved polypharmacology of SQ109 in P. falciparum parasites, where its effects are based on physical properties rather than direct protein inhibition. These attributes of SQ109 are highly favourable for the development of more resistance-resistant antimalarials.