Anti-schistosomal activities of quinoxaline-containing compounds: From hit identification to lead optimisation

Abstract

Schistosomiasis is a neglected disease of poverty that is caused by infection with blood fluke species contained within the genus Schistosoma . For the last 40 years, control of schistosomiasis in endemic regions has predominantly been facilitated by administration of a single drug, praziquantel. Due to limitations in this mono-chemotherapeutic approach for sustaining schistosomiasis control into the future, alternative anti-schistosomal compounds are increasingly being sought by the drug discovery community. Herein, we describe a multi-pronged, integrated strategy that led to the identification and further exploration of the quinoxaline core as a promising anti-schistosomal scaffold. Firstly, phenotypic screening of commercially available small molecules resulted in the identification of a moderately active hit compound against Schistosoma mansoni (1, EC 50 = 4.59 μM on schistosomula). Secondary exploration of the chemical space around compound 1 led to the identification of a quinoxaline-core containing, non-genotoxic lead (compound 22). Compound 22 demonstrated substantially improved activities on both intra-mammalian (EC 50 = 0.44 μM, 0.20 μM and 84.7 nM, on schistosomula, juvenile and adult worms, respectively) and intra-molluscan (sporocyst) S. mansoni lifecycle stages. Further medicinal chemistry optimisation of compound 22, resulting in the generation of 20 additional analogues, improved our understanding of the structure-activity relationship and resulted in considerable improvements in both anti-schistosome potency and selectivity (e.g. compound 30; EC 50 = 2.59 nM on adult worms; selectivity index compared to the HepG2 cell line = 348). Some derivatives of compound 22 (e.g. 31 and 33) also demonstrated significant activity against the two other medically important species, Schistosoma haematobium and Schistosoma japonicum . Further optimisation of this class of anti-schistosomal is ongoing and could lead to the development of an urgently needed alternative to praziquantel for assisting in schistosomiasis elimination strategies.

Recent Advances in Anti-Schistosomiasis Drug Discovery

“…Again, the in vivo potency of these compounds was underwhelming, with highest WBR among them 46.3% for compound 35 [67]. In a separate contemporaneous study, other quinoxaline analogs of 36 bearing nitro, amine and amide functionalities were screened for both phenotypic and motility effects on schistosomula [68]. Compared to compound 27, compounds 36, 37 and 38 showed significantly greater efficacy against the adult worms; the latter two compounds also showed excellent activity against S. japonicum and S. haematobium adults [68].…”

Perspective on Schistosomiasis Drug Discovery: Highlights from a Schistosomiasis Drug Discovery Workshop at Wellcome Collection, London, September 2022

“…Projects at a much earlier phase are being conducted by the Universities of Dundee, Aberystwyth, and Cardiff. Aberystwyth/Cardiff Universities have reported a compound series that demonstrates potent activity against schistosomes (schistosomula, juveniles, and adults; S. mansoni, S. japonicum, and S. haematobium), 31 Our analysis of currently published anti-schistosomal compounds indicates that these tend to be lipophilic with low polar surface areas. It is uncertain whether these properties are a prerequisite for anti-schistosomal activity (due to the parasite's heptalaminate surface membranes) 41 or if they are a consequence of the compound collections that have been screened.…”

“…As an approach to increasing assay throughput, adult worm screens are conducted using mixtures of compounds followed by hit deconvolution to identify the active compound(s). This approach of combining observational analysis and measuring worm motility via WormAssay has proven consistent in identifying hits and generating useful structure–activity relationships. , The importance of testing compounds against endemic schisto­some strains was stressed. There is currently a large collection of clinical isolates being collated by the Natural History Museum in London.…”

“…Using an automated, high-content imaging platform which quantifies both schisto­somula phenotype and motility, large collections of compounds can be rapidly progressed for anti-schisto­somal triaging. This platform was originally developed by Quentin Bickle and colleagues, but has been further developed in Aberystwyth to drive the identification and progression of new compounds for entering the drug discovery pipeline . A drawback to this higher throughput approach is that compounds targeting the schisto­somula stage do not always show activity against adult worms.…”

Drug Repurposing and De Novo Drug Discovery of Protein Kinase Inhibitors as New Drugs against Schistosomiasis

“…Accordingly, this area has also been broadly addressed for schistosomiasis, in which several different classes of synthetic compounds have been created, optimized, and studied, such as biaryl alkyl carboxylic acids [42,43], trioxolanes [44][45][46], aryl ozonides [47], tetraazamacrocyclics [48], etc. [49][50][51][52][53][54][55][56]. Although most of these synthetic derivatives show satisfactory antiparasitic properties against Schistosoma in vitro and/or in vivo, there is still a wide knowledge gap regarding ligand-target interactions.…”