Date Published: August 12, 2016
Publisher: Public Library of Science
Author(s): Alessandra Guidi, Cristiana Lalli, Emerald Perlas, Giulia Bolasco, Martina Nibbio, Edith Monteagudo, Alberto Bresciani, Giovina Ruberti, Robert M Greenberg. http://doi.org/10.1371/journal.pntd.0004928
Abstract: BackgroundSchistosomiasis, one of the world’s greatest human neglected tropical diseases, is caused by parasitic trematodes of the genus Schistosoma. A unique feature of schistosome biology is that the induction of sexual maturation as well as the maintenance of the differentiation status of female reproductive organs and egg production, necessary for both disease transmission and pathogenesis, are strictly dependent on the male. The treatment and most control initiatives of schistosomiasis rely today on the long-term application of a single drug, praziquantel (PZQ), mostly by campaigns of mass drug administration. PZQ, while very active on adult parasites, has much lower activity against juvenile worms. Monotherapy also favors the selection of drug resistance and, therefore, new drugs are urgently needed.Methods and FindingsFollowing the screening of a small compound library with an ATP-based luminescent assay on Schistosoma mansoni schistosomula, we here report the identification and characterization of novel antischistosomal properties of the anti-anginal drug perhexiline maleate (PHX). By phenotypic worm survival assays and confocal microscopy studies we show that PHX, in vitro, has a marked lethal effect on all S. mansoni parasite life stages (newly transformed schistosomula, juvenile and adult worms) of the definitive host. We further demonstrate that sub-lethal doses of PHX significantly impair egg production and lipid depletion within the vitellarium of adult female worms. Moreover, we highlighted tegumental damage in adult male worms and remarkable reproductive system alterations in both female and male adult parasites. The in vivo study in S. mansoni-patent mice showed a notable variability of worm burdens in the individual experiments, with an overall minimal schistosomicidal effect upon PHX treatment. The short PHX half-life in mice, together with its very high rodent plasma proteins binding could be the cause of the modest efficacy of PHX in the schistosomiasis murine model.Conclusions/SignificanceOverall, our data indicate that PHX could represent a promising starting point for novel schistosomicidal drug discovery programmes.
Partial Text: Schistosomiasis, one of the world’s greatest neglected tropical diseases, contributes to the global morbidity with 4,026,000 DALYs (disability-adjusted life years) . Schistosomiasis is caused by a parasitic trematode of the genus Schistosoma. Three species (S. mansoni, S. haematobium and S. japonicum) account for the majority of human infections. Among human parasitic diseases, schistosomiasis ranks second behind malaria in socio-economic terms, public health importance and prevalence in the developing world. More than 780 million people are at risk and it is estimated that there are approximately 261 million infected people in 78 countries, of whom 85% reside in sub-Saharan Africa . A vaccine is unavailable to date; in addition the treatment and most control initiatives rely on the long-term application of a single drug, praziquantel (PZQ) mostly by campaigns of mass drug administration. PZQ has high efficacy, excellent tolerability, few and transient side effects, ease of distribution and competitive cost [3,4]. However while very active on adult parasites, PZQ has much lower activity against juvenile worms both in vitro and in vivo  a fact that can partially explain the low cure rates in high transmission areas where patients are likely to harbor juvenile and adult parasites concurrently [6,7]. Furthermore, the use of a single drug to treat a large population of infected people and people at risk worldwide, appears particularly worrisome when considering the threat of drug resistance. Alarmingly, it is possible to induce resistance of S. mansoni and S. japonicum to PZQ in mice under laboratory conditions [8–11] and also after exposure to PZQ during the intramolluscan phase . In addition, reduced susceptibility to PZQ in field isolates of S. mansoni has been sporadically reported [4,13–16]. For all the above reasons, the search for new antischistosomal agents represents a compelling priority.
Since the 1970s, PZQ has been the drug of choice for the treatment of schistosomiasis, being highly effective after a single oral dose against all species of schistosomes. A number of positive features make PZQ an excellent drug, especially with regard to safety, efficacy, cost and ease of distribution . However, the efficacy of PZQ is dependent on the age of the infection, on the sex of worms and on their paired or unpaired status. The ED50 of PZQ against juvenile S. mansoni worms in mice (4 weeks after infection) is at least 30 times higher than that observed for adult worms . The low sensitivity of immature worms (between 1–5 weeks after infection) is actually the most serious problem in the clinical use of PZQ. For this reason re-treatments are often necessary. A large number of compounds have been identified as potential antischistosomal agents in the last few decades, but none so far have represented a suitable alternative to PZQ.