Josué de Moraes

*Supervisão de Vigilância em Saúde, SUVIS Casa Verde, Secretaria de Saúde da Cidade de São Paulo, Rua Ferreira de Almeida, São Paulo, SP Brazil* 

### **1. Introduction**

332 Current Topics in Tropical Medicine

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> Schistosomiasis, or bilharzias, is a neglected disease that remains a considerable public health problem in tropical and subtropical regions. This parasitic disease is the most important human helminth infection in terms of morbidity and mortality and is a growing concern worldwide. It is estimated that more than 200 million people have been infected and that 779 million are at risk of infection, resulting in 280,000 deaths annually (van der Werf et al., 2003; Steinmann et al., 2006). Schistosomiasis is caused by blood-dwelling uke worms of the genus *Schistosoma* and is endemic in African, Asian and South American countries. The main disease-causing species are *S. mansoni*, *S. haematobium*, and *S. japonicum. S. mansoni* is the most widely distributed, affecting people in Africa, the Middle East, South America, and the Caribbean, while *S. japonicum* is confined to China, Indonesia, and the Philippines. *S. haematobium* is found in Africa and the Middle East. The adult worms colonise the veins of either the portal system (*S. mansoni* and *S. japonicum*) or the urinary bladder plexus (*S. haematobium*) and can live for years or even decades in human hosts; thus, the disease runs a chronic and debilitating course. Egg production is responsible for both the transmission of the parasite and the aetiology of the disease. Schistosomal species are distinguished by differences in their morphology, both in their parasite stages and in their eggs; further species distinction is made by the species of intermediate host snails that support transmission of the parasite (Gryseels et al., 2006).

> The global strategy for the control of schistosomiasis is by chemotherapy. Systematic searching for chemotherapeutic drugs began almost a century ago, and the development of praziquantel (PZQ) in 1970 was essential for a reduction in morbidity and mortality due to schistosomiasis. Currently, treatment is still based on the use of PZQ, but the long-term application of PZQ results in decreased efficiency and serious concerns regarding the onset of resistance. In addition, PZQ has no prophylactic properties and is ineffective against larval stages of parasites (schistosomula), meaning that for effective treatment and sustainable control, PZQ must be given on a regular basis. Thus, it is prudent to search for novel therapeutics, and recent discussions have focused on reawakening the need to search for alternatives to PZQ (Caffrey, 2007; Doenhoff et al., 2008; Fenwick et al., 2003; Hagan et al., 2004; Keiser & Utzinger, 2007).

Antischistosomal Natural Compounds: Present Challenges for New Drug Screens 335

2001, 2003). As there is currently no available vaccine for this disease in people (Bergquist et

Chemotherapy against schistosomiasis was reviewed extensively by Cioli et al. (1995), with an emphasis on compounds that were used in the past. Additionally, Cioli (1998) summarised some interesting laboratory studies on potential antischistosomal compounds and the possible emergence of praziquantel-resistant schistosomes. More recently, Ribeirodos-Santos et al. (2006) reviewed results from a comprehensive search of the scientific literature for substances and compounds tested for schistosomiasis therapy over the past century. The authors gathered information on the therapeutic action in humans or animal

Briefly, antimonial compounds were introduced in 1918, and this group of drugs has been the major point of schistosome chemotherapy for approximately 50 years. However, they cause numerous side effects, such as nausea, vomiting, diarrhoea, anorexia, and cardiovascular, hepatic, and dermatological disturbances. Lethality from cardiac syncope and anaphylactic shock was also reported. Emetine, a drug used to treat amoebiasis, was employed in the second decade of the past century, but the doses required against schistosomiasis were at the very limit of toxicity. The introduction of 2,3-dehydroemetine reduced the toxicity of the parent compound, but patients had to be hospitalised over a month for treatment. Thus, the use of 2,3-dehydroemetine as an antischistosomal agent was abandoned (Cioli et al., 1995). Only in the 1960s was there a breakthrough in the treatment of schistosomiasis, with the rise of metrifonate, nitrofurans, lucanthone, niridazole, hycanthone, and, finally, oxamniquine. In the 1970s, several schistosomicidal drugs emerged, such as tubercidin, amoscanate, PZQ and its benzodiazepine derivative Ro11- 3128, and oltipraz. Nevertheless, the therapeutic doses of most of these drugs were found to cause major side effects. PZQ, an isoquinoline-pyrazine derivative, immediately proved to be superior to any other schistosomicidal drug and quickly became the drug of choice in most endemic areas (Cioli et al., 1995; Fenwick & Webster, 2006). Because of the reliance on a single drug for the treatment and control of schistosomiasis and the considerable concern regarding the development of PZQ resistance, it is timely to review potential alternatives,

**2.1 Antischistosomals: Natural product and natural product-derived compounds**  The use of natural products for curative and therapeutic purposes has a long history, and compounds derived from natural products have made a big impact on the pharmaceutical industry (Newman, 2003; Newman & Cragg, 2007). In addition to microbes and plants, there has been growing interest in other living organisms, such as arthropods and amphibians, as important sources of biologically active compounds (Kayser et al., 2003). However, the potential for using living beings as sources of new antischistosomal drugs is still poorly explored. In recent decades, there has been a growing interest in the scientific community to search for extracts and pure compounds, especially those derived from plants, that exhibit potential schistosomicidal properties, as one alternative method to the conventional chemical

Plants have been traditionally used in the treatment of different diseases, including schistosomiasis, especially in Africa and Asia (Ndamba et al., 1994). In general, medicinal plants are prepared by traditional healers, who have empirical knowledge and

al., 2008), chemotherapy may now be at a crucial point.

models and the mechanisms of action of over 40 drugs.

with an emphasis on natural products.

control.

Natural products, mainly plants, have been the source of medicines for thousands of years. The discovery of pure compounds as active principles in plants was first described at the beginning of the 19th century, and the art of exploiting natural products has become part of the molecular sciences (Kayser et al., 2003). Several extracts or bioactive constituents from living organisms have been used in many communities worldwide against parasitic diseases, including schistosomiasis, and in the past decades, natural products have attracted renewed interest (Kayser et al., 2003; Mølgaard et al., 2001; Ndamba et al., 1994; Sanderson et al., 2002; Tagboto & Townson, 2001).

*In vitro* screening systems are useful and affordable ways to discover potential anthelmintic candidates for *in vivo* tests (Keiser, 2010; Ramirez et al., 2007; Yousif et al., 2007). Because a molecular-target approach is still rarely employed in schistosomicidal drug discovery, a more common strategy has been the complementary approach of whole-organism phenotypic screening *in vitro* to measure compound efficacy (Keiser, 2010; Ramirez et al., 2007; Yousif et al., 2007). In this context, screening for natural products that are active against schistosome is important in the establishment of future strategies for new antischistosomal drug discovery to control schistosomiasis (Yousif et al., 2007).

Considerable efforts are ongoing to develop novel schistosomicidal agents. As a result, many natural compounds with promising antischistosomal properties have been identified (Braguine et al., 2009; de Moraes et al., 2011; Magalhães et al., 2009, 2010; Moraes et al., 2011; Mølgaard et al., 2001; Parreira et al., 2010; Sanderson et al., 2002). The efficacy of these new compounds against schistosome is defined using three strategies: a) curative, by killing the adult worm; b) prophylactic, by killing schistosomula; and c) suppressive, by inhibiting worm egg-laying. Thus, several parameters, such as motor activity, tegumental changes, and oviposition, are often evaluated as indicators of biological activity and toxicity in studies with schistosome species.

This chapter reviews the present state of *in vitro* drug screening strategies used to discover new compounds active against *S. mansoni*, the most important species infecting humans, with an emphasis on natural products. Also highlighted are the best practices and challenges for drug screenings. Furthermore, information is provided about toxicity, susceptible *Schistosoma* stages, and other interesting laboratory studies on potential antischistosomal compounds, both natural products and natural product-derived compounds.
