Orally active antischistosomal early leads identified from the open access malaria box

Abstract

Background: Worldwide hundreds of millions of schistosomiasis patients rely on treatment with a single drug, praziquantel. Therapeutic limitations and the threat of praziquantel resistance underline the need to discover and develop next generation drugs.

Methodology: We studied the antischistosomal properties of the Medicines for Malaria Venture (MMV) malaria box containing 200 diverse drug-like and 200 probe-like compounds with confirmed in vitro activity against Plasmodium falciparum. Compounds were tested against schistosomula and adult Schistosoma mansoni in vitro. Based on in vitro performance, available pharmacokinetic profiles and toxicity data, selected compounds were investigated in vivo.

Principal findings: Promising antischistosomal activity (IC50: 1.4-9.5 µM) was observed for 34 compounds against schistosomula. Three compounds presented IC50 values between 0.8 and 1.3 µM against adult S. mansoni. Two promising early leads were identified, namely a N,N'-diarylurea and a 2,3-dianilinoquinoxaline. Treatment of S. mansoni infected mice with a single oral 400 mg/kg dose of these drugs resulted in significant worm burden reductions of 52.5% and 40.8%, respectively.

Conclusions/significance: The two candidates identified by investigating the MMV malaria box are characterized by good pharmacokinetic profiles, low cytotoxic potential and easy chemistry and therefore offer an excellent starting point for antischistosomal drug discovery and development.

Figure 1. Screening flow.

Screening was conducted at the Swiss TPH (screening cascade I; steps A–E and Quality control): Primary screening steps (yes/no filters) of 100 µM and 33.3 µM resulted in 179 and 72 hits, respectively. Active compounds (n = 72) moved on to Step C and IC₅₀ values were evaluated on NTS. Thirty-four compounds showed activities with IC₅₀ values <10 µM and pre-screening was conducted on adult schistosomes (Step D). Active compounds (n = 16) with schistosomicidal effects at 33 µM compound concentration were further characterized (step E). The quality control represents randomly selected compounds from compounds classified as non-active from the pre-screening steps (step A/B) on NTS which were re-evaluated in step B. In parallel, all compounds (n = 400) were studied at the LSHTM in London (screening cascade II): step A, all 400 compounds were screened on S. mansoni adults at 15 µM. Step B: 44 compounds were active and these were then tested for IC₅₀ determination on adult worms. From both screening cascades, five compounds were selected for in vivo testing based on pharmacodynamic and pharmacokinetic properties as well as toxicity.

Figure 2. Chemical structures of the five lead compounds selected for in vivo studies.

Figure 3. Adult worm IC₅₀ values of five in vivo candidates over time post drug exposure.

Values were determined 1, 2, 4, 7, 10, 24, 48 and 72>33.3 µM are indicated with (//). PZQ: praziquantel.