Review Special Focus Issue: Schistosomiasis
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Medicinal Chemistry
Natural products with antischistosomal activity
In recent years, natural product groups have been gaining prominence as possible sources of new drugs for schistosomiasis. This review attempts to update the antischistosomal natural compounds, or natural product-derived compounds, from the mid-1980s. Some of the main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) with in vitro and/or in vivo antischistosomal properties are discussed. Less thoroughly, due to scarcity of data in the literature, molecules from animals (e.g., peptides) are also described. Special mention of the anthelmintic activity against different parasitic stages of schistosomes is made; the mechanism of action of most of the metabolites is discussed, and a number of bioassay procedures are listed.
Background Schistosomiasis is a neglected tropical disease caused by blood-dwelling digenetic trematodes of the genus Schistosoma. It remains a major problem in more than 70 countries, and the disease is the second most important human parasitic disease after malaria, with more than 200 million people infected and approximately 800 million people living at risk of infection [1] . Five species of Schistosoma parasitize humans, namely S. mansoni, S. haematobium, S. japonicum, S. intercalatum and S. mekongi; the first three species have the widest geographic distribution, whereas infections with the last two species only occur locally [2,3] . Morbidity due to schistosomiasis includes hepatic and intestinal fibrosis (S. mansoni, S. japonicum, S. intercalatum and S. mekongi), and ureteric and bladder fibrosis and calcification of the genitourinary tract (S. haematobium). The distribution of the different species depends mainly on the ecology of the snail hosts. Schistosome species are distinguished by differences in their morphology, both in their parasite stages and in their eggs [3,4] . Schistosomes have a complex life cycle involving a phase of sexual reproduction by adult worms in humans (definitive host) and
10.4155/FMC.15.23 © 2015 Future Science Ltd
Josué de Moraes Núcleo de Pesquisa em Doenças Negligenciadas (NPDN/FACIG), Avenida Guarulhos,1844, Vila Augusta, 07025–000, Guarulhos, SP, Brazil [email protected]
an asexual phase in a specific freshwater snail (intermediate host). Schistosomes, unlike most parasitic flatworms, which are hermaphrodites, are dioecious and in the definitive host they start their sexual cycle. Infection occurs when humans come into contact with fresh water that contains free-swimming larval forms of the parasite (cercariae). Cercariae penetrate the intact human skin and transform into schistosomula. Schistosomula travel through the bloodstream for several days before they differentiate into male and female worms and unite [3,4] . Schistosomiais is responsible for over 280,000 human deaths per annum in subSaharan Africa alone [5] . However, the disease is better known for its chronicity and debilitating morbidity [3,6] . Pathology is mainly caused by tissue deposition of eggs. Organs typically affected include the urinary tract, the bowel and the liver, depending on the species of schistosome. Eggs that are trapped in the tissues provoke immunogenic inflammatory, granulomatous and fibrotic reactions that cause intestinal, hepatosplenic or urinary disease to develop over many years [3,4] . Schistosomes cause nonspecific but disabling systemic morbidities including anemia, malnutrition and impaired childhood development, as a
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part of
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Review de Moraes result of the effect of continued inflammation on normal growth, iron metabolism, and cognitive and physiological capacities. Moreover, ectopic schistosomiasis can lead to unexpected morbidities (e.g., migration of parasite or eggs to the central nervous system, eggs in the perialveolar capillary beds and eggs in the reproductive organs) [3–7] . Furthermore, schistosomiasis often occurs alongside other infectious diseases, such as malaria, tuberculosis and especially HIV/AIDS [3,8] . Coinfection of schistosomes with bacteria, protozoa and helminths increases the severity of the prolonged form of the disease in humans, which has implications for patient treatment and recovery [9] . Such unspecific and multifactorial morbidity is difficult to measure and to dissociate from other poverty-related health problems. The chronic and debilitating nature of the disease results in high costs in public health and economic productivity in developing countries [4,10] . The global burden of schistosomiasis has been estimated to exceed 70 million disability-adjusted life years [7] . Despite the public health importance of schistosomiasis and the risk that the disease might further spread and intensify, schistosomiasis control programmes are based mainly on chemotherapy [2,7] . Until the 1970s, the treatment of schistosomiasis was almost as difficult and toxic as it is still today for other neglected tropical diseases such as trypanosomiasis and leishmaniasis. In the early 1980s, the treatment and control of schistosomiasis relied on a single drug, namely praziquantel (PZQ) [11] . This drug is safe and effective against all Schistosoma species and has been used for the last 40 years [12] . Although PZQ is safe and well tolerated, it is not free of problems. For example, evidence of emerging drug resistance and low efficacy of PZQ has been reported [13,14] . Moreover, PZQ acts against adult schistosome worms, but has poor activity against the immature schistosome; hence, retreatment is necessary to kill those parasites that have since matured. Additionally, PZQ tablets are large, taste bitter, and to date, no readily available pediatric formulation exists [12,15] . Having a single drug to treat a disease that affects millions of people is a real concern and, consequently, it is imperative to develop new effective antischistosomal drugs. In order to provide new hit and lead compounds, which can be used in drug development to control schistosomiasis, the search for anthelmintic compounds, mainly from natural sources, has been intensified in recent years [16] . Natural products 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 [17] . Natural products have
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come from various source materials including terrestrial plants, terrestrial microorganisms, marine organisms, and terrestrial vertebrates and invertebrates. The scientific evaluation of medicinal plants used in the preparation of folk remedies has provided modern medicine with effective pharmaceuticals for the treatment of diseases caused by parasites [18–21] . Many drugs are derived from plants and others that are synthetic analogues are built on prototype compounds isolated from plants. Atropine, aspirin, colchicine, digoxin, ephedrine, morphine, pilocarpine, reserpine, tubocurarine and vincristine are a few important examples of what medicinal plants have given us in the past [17,22] . In addition, the antiparasitic drugs artemisinin and chloroquine are examples of plant-derived products, and amphotericin B and ivermectin are important antiparasitics isolated from Streptomyces microorganisms. Many other natural products of diverse molecular structure have revealed antiparasitic potency in the laboratory and represent interesting lead structures for the development of new and urgently needed antiparasitics [18] . In this context, currently natural products and natural product-derived compounds are gaining prominence as possible sources of new drugs in the control and treatment of schistosomiasis. Natural products are a source of compounds with diversified structural arrangements possessing interesting biological activities [16–27] . This review attempts to update the antischistosomal natural compounds isolated from plants, or natural product-derived compounds, from the mid-1980s, the date when more formal and constant research on natural metabolites with schistosomicidal activity was initiated. Some of the main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) with in vitro and/or in vivo antischistosomal properties are discussed. Due to scarcity of data in the literature, molecules from animals, especially peptides, are also described in less detail. Natural products with antischistosomal properties The importance of natural products in modern medicine has been described in a number of earlier reviews and reports [16–27] . For example, an analysis of the origin of the drugs developed between 1981 and 2002 showed that natural products or natural productderived drugs comprised 28% of all new chemical entities launched onto the market. In addition, 24% of these were synthetic or natural mimic compounds, based on the study of pharmacophores related to natural products [25–27] . The use of drugs derived from plants, fungi, bacteria and marine organisms has a long tradition
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Natural products with antischistosomal activity
in medicine [17–27] . Among these, medicinal plants contain a wide variety of active principles that have been exploited against schistosomiasis, and in recent decades, natural products have attracted renewed interest [16] . Several extracts or essential oils from plants have been tested against schistosomes, especially during in vitro screening. For example, Yousif et al. [28] screened extracts originated from 281 native and cultivated plant species growing in Egypt. Despite the importance of plant extracts and essential oils, isolated compounds (secondary metabolites) or derivatives are currently gaining prominence as possible sources of new drugs in the control and treatment of schistosomiasis. Some of the most interesting antischistosomal compounds are derivatives of artemisinin, such as artemether, artesunate and dihydroartemisinin and the aminoalcohol/quinoline mefloquine [29] . The main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) and animals (e.g., peptides) with promising antischistosomal properties in vitro and/or in vivo are summarized in Table 1, and will be discussed below. Except for artemisinins and mefloquine, which are approved therapeutic agents, all others are still considered only as hit or lead compounds. Major groups of antischistosomal compounds from plants Terpenes
Among plant secondary metabolites, terpenes are a structurally most diverse group [70] . They occur as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30) and tetraterpenes (C40). The terpenes with antischistosomal activity are: rotundifolone [66] , (+)-limonene epoxide [56] , and carvacryl acetate, a derivative of carvacrol (monoterpenes) [39] ; artemisinin and its derivatives [30,31] , nerolidol [59] , and budlein-A and its derivatives [38] (sesquiterpenes); phytol [60] (diterpene), betulin [37] and its triphenylphosphonium derivatives [69] , balsaminol F, karavilagenin C [36] (triterpenes) (Figure 1) . Importantly, among the group of terpenes, in vitro and in vivo antischistosomal properties have been described only for artemisinin and its derivatives, phytol and triphenylphosphonium derivatives, whereas it was reported in vitro schitosomicidal activity for other terpenes (Table 1) . The biological properties of terpenes on different parasitic diseases are still unclear. The mechanisms of action seem to be related to the interaction with the parasite redox cycling system [71] . The mechanism by which terpenes exert their schistosomicidal effect may
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be that, due to inherent lipophilicity, some terpenes will easily cross plasmatic membranes and, consequently, may also interact with intracellular molecules of parasites. In fact, some of the compounds described herein are capable of causing morphological changes in the integument of worms, and a relationship has been observed between tegumental damage and the death of worms [39,56,59–60] . Monoterpenes
Rotundifolone (1) is a monoterpene found in the essential oil of many plant species. Recently, in vitro assays have demonstrated that rotundifolone isolated from essential oil of Mentha x villosa (Lamiaceae) caused mortality of S. mansoni adult worms at a concentration of 70 μg/ml [66] . (+)-Limonene epoxide (2) is an oxygenated constituent found in essential oils, which can be easily prepared in one step and with a good yield from limonene. In vitro test using S. mansoni adult worms presented schistosomicidal activity when tested at a concentration of 25 μg/ml [56] . In addition, morphological alterations on the schistosome surface were detected with (+)-limonene epoxide at concentrations of 25–75 μg/ml. The mechanism by which (+)-limonene epoxide exerts its schistosomicidal effects remains unclear; however, all schistosomes died accompanied by severe destruction of the worm body, and a relationship between tegumental damage and the death of worms was seen. In vitro antischistosomal properties have also been described for carvacryl acetate (3), a semisynthetic derivative of carvacrol [39] . Carvacryl acetate at 6.25 μg/ml affected parasite motility and viability. Moreover, microscopy pictures revealed morphological alterations on the tegumental surface of adult worms, where some tubercles appeared to be swollen with numerous small blebs emerging from the tegument Key terms Natural product-derived compounds: Prepared by chemical synthesis from natural materials. In other words, usually a semisynthetic modification. Pharmacophore: Description of molecular features which are necessary for molecular recognition of a ligand by a biological macromolecule. According to IUPAC, one pharmacophore is ‘the set of steric and electronic features necessary to ensure optimal supramolecular interactions with a structure of a specific biological target and to trigger (or block) its biological response.’ Secondary metabolites: Substances which often are produced by plants as defense mechanisms. Several pharmaceuticals are based on plant chemical structures, and secondary metabolites are widely used as medicines and flavorings. They include alkaloids, terpenes, quinones, flavonoids, neolignans, etc.
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4
6
36
16
13
9
3
35
40
41
Artemisinin
Artesunate
Aspidin
Balsaminol F
Betulin
Budlein-A
Carvacryl acetate
Curcumin
Kalata B1
Kalata B2
Peptide
Peptide
Peptide
Polyphenol
Monoterpe
Sesquiterpene lactone
Triterpene
Triterpene
Phloroglucinol/ phenol
Sesquiterpene lactone
Sesquiterpene lactone
Sesquiterpene lactone
Compound class
Frog of the genus Phyllomedusa (Hylidae)
Oldenlandia affinis (R&S) DC. (Rubiaceae)
Oldenlandia affinis (R&S) DC. (Rubiaceae)
Curcuma longa L. (Zingiberaceae)
Derived from carvacrol
Viguiera (Asteraceae)
Schefflera vinosa (Cham. & Schltdl.) Frodin (Araliaceae)
Momordica balsamina L. (Cucurbitaceae)
Dryopteris genus (Dryopteridaceae)
Derived from artemisinin
Artemisia annua L. (Asteraceae)
Derived from artemisinin
Origin
N
∼18
In vivo against immature and adult worms
∼15
N
N
N
ND
∼30 50
N
12.5
N
ND
∼15
100
N
ND
∼100–780
10
N
ND
∼335–1000
NR
Source
Antischistosomal activity (μM)
In vitro against adult ∼1 worms and In vivo against immature and adult worms
In vitro against adult worms
In vitro and in vivo against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro and in vivo against immature worms
In vivo against immature worms
In vitro and in vivo against immature worms
Antischistosomal properties
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
42
5
Artemether
Dermaseptin 01
Compound ID
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically.
[46,47]
[45]
[45]
[40–44]
[39]
[38]
[37]
[36]
[35]
and references therein
[30–32,34]
[30,31] and references therein
[30–33] and references therein
Ref.
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31
29
17
26
Hesperidin
Kaempferol
Karavilagenin C
β-lapachone
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Aminoalcohol/ quinolines
Monoterpene
Neolignan
Naphthoquinone
Triterpene
Flavonoid
Flavanone glycoside
Phloroglucinol/ phenol
Alkaloid imidazole
Sesquiterpene lactone
Sesquiterpene lactone
Phloroglucinol/ phenol
Compound class
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms and In vivo against immature worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms and in vitro and in vivo against immature and adult worm
In vitro against adult worms
In vitro against immature and adult worms
In vitro against adult worms
In vivo against immature and adult worms
In vitro against adult worms
Antischistosomal properties
N
∼165
ND
N ND
∼55–95
ND
N
∼30
100
N
N
N
N
ND
ND
N
Source
100
165
10
>500
50
NR
10
Antischistosomal activity (μM)
analogue of quinine In vitro and in vivo ∼15–30 against immature and adult worms
many plants
Many plants. Enantiomers are semi synthetics
Derived from lapachol
Momordica balsamina L. (Cucurbitaceae)
Styrax pohlii Pohl (Styracaceae)
Citrus fruits
Dryopteris genus (Dryopteridaceae)
Pilocarpus microphyllus Stapf ex (Rutaceae)
Derived from budlein-A
Derived from artemisinin
Dryopteris genus (Dryopteridaceae)
Origin
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
24
37
Flavaspidic acid
Mefloquine
20
Epiisopiloturine
2
10
4α,5-dihydrobudlein A
(+)-limonene epoxide
7
Dihydroartemisinin
32–34
39
Desaspidin
(±)-licarin A and its enantiomers
Compound ID
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
[57,58]
[56]
[55]
[53,54]
[36]
[52]
[50,51]
[35]
[49]
[38]
and references therein
[30–31,48]
[35]
Ref.
Natural products with antischistosomal activity
Review
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12
19
18
25
1
30
21
22
23
Phytol
Piperamide 1
Piplartine
Plumbagin
Rotundifolone
Sativan
Sanguinarine
Solamargine
Solasonine
Flavonoid
Steroidal alkaloid (glycoalkaloids)
Steroidal alkaloid (glycoalkaloids)
In vitro against adult worms
In vitro against immature and adult worms
In vitro against adult worms
In vitro and in vivo against adult worms
In vitro against adult worms
In vitro against adult worms
Antischistosomal properties
Astragalus englerianus Ulbr. (Fabaceae or Leguminosae)
Roupala montana Aubl. (Proteaceae)
In vitro against adult worms
Solanum lycocarpum In vitro against adult A. St. Hil. worms (Solanaceae)
Solanum lycocarpum In vitro against adult A. St. Hil. worms (Solanaceae)
In vitro against adult worms
In vitro against adult worms
Mentha x villosa In vitro against adult Hudson (Lamiaceae) worms
Plumbago spp. (Plumbaginaceae) and other families (e.g., Droseraceae and Ebenaceae)
Piper tuberculatum Jacq. (Piperaceae)
Piper amalago L. (Piperaceae)
many plants
many plants
Dryopteris genus (Dryopteridaceae)
Origin
Benzophenanthridine Sanguinaria spp. alkaloid (Papaveraceae)
Isoflavonoid
Monoterpene
Naphthoquinone
Alkaloid amide
Alkaloid amide
Diterpene alcohol
Sesquiterpene
Phloroglucinol/ phenol
Compound class
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
27
8
Nerolidol
Quercetin 3-O-β-dglucoside
38
Compound ID
Methylene-bis-aspidinol
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
NA
50
32
10
N
N
N
N
N
N
∼425 150
N
10
N
∼10
N
∼170 N
N
∼30
100
N
Source
100
Antischistosomal activity (μM)
[37]
[68]
[68]
[64]
[67]
[66]
[64,65]
[62,63]
[61]
[60]
[59]
[35]
Ref.
Review de Moraes
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[37]
[38]
[69]
N
ND
ND
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around the tubercles. Furthermore, experiments performed using carvacryl acetate at sublethal concentrations (ranging from 1.562 to 6.25 μg/ml) showed an inhibitory effect on the daily egg output of paired adult schistosomes. It should be noted that since carvacrol is more toxic than many esters, the synthesis of carvacryl acetate was performed to obtain a derivative of carvacrol with improved pharmacological profile and less toxicity.
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
2 In vitro against immature and adult worms.In vivo against adult worms Derived from betulin Triterpene Triphenylphosphonium salts 14 15
200 In vitro against adult worms derived from budlein-A 11 4α,5–11β,13tetrahydrobudlein A
Sesquiterpene lactone
100 In vitro against adult worms Schefflera vinosa (Cham. & Schltdl.) Frodin (Araliaceae) 28 Quercetin 3-O-β-drhamnoside
Flavonoid
Antischistosomal properties Origin Compound ID
Compound class
Review
Sesquiterpenes
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
Antischistosomal activity (μM)
Source
Ref.
Natural products with antischistosomal activity
Some of the most interesting antischistosomal natural compounds are artemisinin (4) and its derivatives such as artemether (5), artesunate (6) and dihydroartemisinin (7) [30,31] . They are highly effective in the treatment and control of malaria, and have also been shown to exhibit antischistosomal properties; although safe, artemisinins are active only against the immature stages of the parasites. Artemisinin is a sesquiterpene lactone with an endoperoxide group, which was isolated from the leaves of Artemisia annua L. This plant has been used for centuries in Chinese traditional medicine as an antidote to many different ailments [30,72–73] . Artemisinin has been used as an antimalarial since the early 1970s, and its antischistosomal activity was discovered in 1980 by a group of Chinese scientists. In 1982, antischistosomal properties were confirmed for artemether, the methyl ether derivative of artemisinin. The precise mechanism of action of the artemisinins against schistosomes is unknown. However, it appears to involve an interaction with heme, which cleaves the endoperoxide bridge of the drug to produce carbon-centered free radicals that then alkylate parasite proteins. Additionally, artemisinin derivatives induce morphological alternations and damage to the tegument of schistosomes [29–30,73–74] . Interestingly, Vennerstrom et al. [75] synthesized several synthetic trioxolane derivatives incorporating the critical endoperoxide pharmacophore of artemisinins, among which ozonide OZ-277 has proved to be more effective than artemisinin against malaria. Currently, the rational design of structurally simpler analogs of artemisinins has led to the synthesis of a large number of peroxides such as trioxaquines (1,2,4-trioxanes) [32,76] and ozonides (1,2,4-trioxolanes) [77,78] , some of which have displayed interesting antischistosomal activities. For example, ozonide OZ418 was identified as a promising lead compound possessing high activity on both juvenile and adult schistosome infections in mice (worm burden reductions 80–90%) [78] . In general, these compounds are characterized by structural simplicity, ease of synthesis, metabolic stability and improved pharmacokinetic parameters [75,77] . Thus, this provides confidence to move forward with the synthesis of novel
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Review de Moraes antischistosomal compounds based on pharmacophore model design. For clarity, it is important to mention that these trioxaquines and ozonides are fully synthetic compounds, whereas artemether, artesunate and dihydroartemisinin are semisynthetic compounds. Nerolidol (8), a sesquiterpene present in the essential oils of several plants, is found in many foods and was approved by the US FDA [79,80] . When tested in vitro against S. mansoni adult worms, nerolidol at 31–62 μM reduced the worm motor activity and caused the death of parasites, with male worms being more sensitive than female [59] . Moreover, microscopy
H3C
H3C
O
O
O
1
2 CH3
CH3
CH2 H CH3
H3C
CH3 O
O
CH3
O
8
HO
CH3
H
11
CH3
CH2
H3C
O
14
O
H3C
CH3 HO
H 3C
CH3
CH3
H3C
CH3
15
CH3 O
CH3
CH3 HO
CH3
CH3
CH3
16
CH3
CH3 HO
OH H3C CH3
CH3
H3C
CH3
O
H3C CH3
CH
H3C CH3 H3 C
CH3
O
CH3
HO CH3
O
CH3
HO
CH2
Br Ph3P
O
OH
CH3
13
+
H
CH3
O
Br- Ph3P+
H3C CH3
-
H
10
CH3
12
HO
CH3
O
O
O
CH2
CH3
CH3
CH3
O
H
H3C
CH3
CH3
O
9
CH3
CH3
O
CH2
O
OH
6
O
O
O
H3C
O
O
CH2 HO
O
O
CH3 CH3
H CH3
H O
OH
5
O
O
CH3
7 OH
H
O
H3C
CH3
O
O
O
4
CH3
HO
O H O
CH3 O
O O
H
3
H CH3
H3C
H
O
O
O
O
O H
CH3
H CH3
H3C
O
O O
H3C
CH2
H3C
CH3
H CH3
H3C
H CH3
H3C
O CH3
O
O H3C
CH3
analysis revealed morphological alterations on the tegument of worms such as disintegration, sloughing and erosion of the surface, and a correlation between viability and tegumental damage was observed. Budlein-A (9), a sesquiterpene lactone isolated from Viguiera spp (Asteraceae), has been reported to exert in vitro schistosomicidal activity against S. mansoni adult worms at 12.5 μM [38] . However, at this concentration, budlein-A was highly toxic to human cells. In this context, structural modification of budlein-A by the Stryker’s reaction furnished derivatives. All the derivatives were less toxic than the starting budlein-A and two
H3C CH3
O
CH3
CH3 O CH3
17
Figure 1. Antischistosomal terpenes. Rotundifolone (1); (+)-limonene epoxide (2); carvacryl acetate (3); artemisinin (4) and its derivatives artemether (5), artenusate (6), and dihydroartemisinin (7); nerolidol (8); budlein-A (9); 4α,5-dihydrobudlein A (10); 4α,5–11β,13-tetrahydrobudlein A (11); phytol (12); betulin (13) and its triphenylphosphonium salts (14 and 15); balsaminol F (16); karavilagenin C (17).
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Natural products with antischistosomal activity
compounds, 4α,5-dihydrobudlein A (10) and 4α,5– 11β,13-tetrahydrobudlein A (11), which led to 100% mortality of schistosomes at 50 and 200 μM, respectively [38] . These two derivatives differ only in terms of the C-11/C-13 bond type: double and single, respectively. Diterpenes
One the most interesting terpenes is phytol (12), a molecule from chlorophyll widely used as a food additive and in medicinal fields [81,82] . Phytol shows promising antischistosomal properties against adult S. mansoni in vitro and in laboratory studies, with mice harbouring adult S. mansoni [60] . In vitro assays demonstrated that phytol affected parasite motility, viability, and egg production and it induced severe tegumental damage in schistosomes. Additionally, various parasitological criteria indicated the in vivo antischistosomal effects of a single dose of phytol (40 mg/kg) administered orally to mice: it caused significant reductions in worm load, faeces egg load and the frequency of egg developmental stages. Interestingly, adult female parasites are more susceptible to the phytol than male worms for both in vitro and in vivo studies. The mechanism by which phytol exerts its antishistosomal effect is not clear. However, confocal laser scanning microcopy studies revealed tegumental damage in adult schistosomes, especially in female worms, and a correlation between viability and tegumental damage was described [60] . Triterpenes
Betulin (13) is an abundant naturally occurring triterpene [83] . A recent study has showed that betulin isolated from Schefflera vinosa (Araliaceae), a plant from the Brazilian Savannah, was effective in vitro against S. mansoni adult worms at a concentration of 100–200 μM [37] . More recently, Spivak et al. [69] have synthesized triphenylphosphonium derivatives of betulin and betulinic acid and results showed in vitro antischistosomal of activity against newly transformed schistosomula and adult worms of S. mansoni at low micromolar concentrations (
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Medicinal Chemistry
Natural products with antischistosomal activity
In recent years, natural product groups have been gaining prominence as possible sources of new drugs for schistosomiasis. This review attempts to update the antischistosomal natural compounds, or natural product-derived compounds, from the mid-1980s. Some of the main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) with in vitro and/or in vivo antischistosomal properties are discussed. Less thoroughly, due to scarcity of data in the literature, molecules from animals (e.g., peptides) are also described. Special mention of the anthelmintic activity against different parasitic stages of schistosomes is made; the mechanism of action of most of the metabolites is discussed, and a number of bioassay procedures are listed.
Background Schistosomiasis is a neglected tropical disease caused by blood-dwelling digenetic trematodes of the genus Schistosoma. It remains a major problem in more than 70 countries, and the disease is the second most important human parasitic disease after malaria, with more than 200 million people infected and approximately 800 million people living at risk of infection [1] . Five species of Schistosoma parasitize humans, namely S. mansoni, S. haematobium, S. japonicum, S. intercalatum and S. mekongi; the first three species have the widest geographic distribution, whereas infections with the last two species only occur locally [2,3] . Morbidity due to schistosomiasis includes hepatic and intestinal fibrosis (S. mansoni, S. japonicum, S. intercalatum and S. mekongi), and ureteric and bladder fibrosis and calcification of the genitourinary tract (S. haematobium). The distribution of the different species depends mainly on the ecology of the snail hosts. Schistosome species are distinguished by differences in their morphology, both in their parasite stages and in their eggs [3,4] . Schistosomes have a complex life cycle involving a phase of sexual reproduction by adult worms in humans (definitive host) and
10.4155/FMC.15.23 © 2015 Future Science Ltd
Josué de Moraes Núcleo de Pesquisa em Doenças Negligenciadas (NPDN/FACIG), Avenida Guarulhos,1844, Vila Augusta, 07025–000, Guarulhos, SP, Brazil [email protected]
an asexual phase in a specific freshwater snail (intermediate host). Schistosomes, unlike most parasitic flatworms, which are hermaphrodites, are dioecious and in the definitive host they start their sexual cycle. Infection occurs when humans come into contact with fresh water that contains free-swimming larval forms of the parasite (cercariae). Cercariae penetrate the intact human skin and transform into schistosomula. Schistosomula travel through the bloodstream for several days before they differentiate into male and female worms and unite [3,4] . Schistosomiais is responsible for over 280,000 human deaths per annum in subSaharan Africa alone [5] . However, the disease is better known for its chronicity and debilitating morbidity [3,6] . Pathology is mainly caused by tissue deposition of eggs. Organs typically affected include the urinary tract, the bowel and the liver, depending on the species of schistosome. Eggs that are trapped in the tissues provoke immunogenic inflammatory, granulomatous and fibrotic reactions that cause intestinal, hepatosplenic or urinary disease to develop over many years [3,4] . Schistosomes cause nonspecific but disabling systemic morbidities including anemia, malnutrition and impaired childhood development, as a
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part of
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Review de Moraes result of the effect of continued inflammation on normal growth, iron metabolism, and cognitive and physiological capacities. Moreover, ectopic schistosomiasis can lead to unexpected morbidities (e.g., migration of parasite or eggs to the central nervous system, eggs in the perialveolar capillary beds and eggs in the reproductive organs) [3–7] . Furthermore, schistosomiasis often occurs alongside other infectious diseases, such as malaria, tuberculosis and especially HIV/AIDS [3,8] . Coinfection of schistosomes with bacteria, protozoa and helminths increases the severity of the prolonged form of the disease in humans, which has implications for patient treatment and recovery [9] . Such unspecific and multifactorial morbidity is difficult to measure and to dissociate from other poverty-related health problems. The chronic and debilitating nature of the disease results in high costs in public health and economic productivity in developing countries [4,10] . The global burden of schistosomiasis has been estimated to exceed 70 million disability-adjusted life years [7] . Despite the public health importance of schistosomiasis and the risk that the disease might further spread and intensify, schistosomiasis control programmes are based mainly on chemotherapy [2,7] . Until the 1970s, the treatment of schistosomiasis was almost as difficult and toxic as it is still today for other neglected tropical diseases such as trypanosomiasis and leishmaniasis. In the early 1980s, the treatment and control of schistosomiasis relied on a single drug, namely praziquantel (PZQ) [11] . This drug is safe and effective against all Schistosoma species and has been used for the last 40 years [12] . Although PZQ is safe and well tolerated, it is not free of problems. For example, evidence of emerging drug resistance and low efficacy of PZQ has been reported [13,14] . Moreover, PZQ acts against adult schistosome worms, but has poor activity against the immature schistosome; hence, retreatment is necessary to kill those parasites that have since matured. Additionally, PZQ tablets are large, taste bitter, and to date, no readily available pediatric formulation exists [12,15] . Having a single drug to treat a disease that affects millions of people is a real concern and, consequently, it is imperative to develop new effective antischistosomal drugs. In order to provide new hit and lead compounds, which can be used in drug development to control schistosomiasis, the search for anthelmintic compounds, mainly from natural sources, has been intensified in recent years [16] . Natural products 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 [17] . Natural products have
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come from various source materials including terrestrial plants, terrestrial microorganisms, marine organisms, and terrestrial vertebrates and invertebrates. The scientific evaluation of medicinal plants used in the preparation of folk remedies has provided modern medicine with effective pharmaceuticals for the treatment of diseases caused by parasites [18–21] . Many drugs are derived from plants and others that are synthetic analogues are built on prototype compounds isolated from plants. Atropine, aspirin, colchicine, digoxin, ephedrine, morphine, pilocarpine, reserpine, tubocurarine and vincristine are a few important examples of what medicinal plants have given us in the past [17,22] . In addition, the antiparasitic drugs artemisinin and chloroquine are examples of plant-derived products, and amphotericin B and ivermectin are important antiparasitics isolated from Streptomyces microorganisms. Many other natural products of diverse molecular structure have revealed antiparasitic potency in the laboratory and represent interesting lead structures for the development of new and urgently needed antiparasitics [18] . In this context, currently natural products and natural product-derived compounds are gaining prominence as possible sources of new drugs in the control and treatment of schistosomiasis. Natural products are a source of compounds with diversified structural arrangements possessing interesting biological activities [16–27] . This review attempts to update the antischistosomal natural compounds isolated from plants, or natural product-derived compounds, from the mid-1980s, the date when more formal and constant research on natural metabolites with schistosomicidal activity was initiated. Some of the main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) with in vitro and/or in vivo antischistosomal properties are discussed. Due to scarcity of data in the literature, molecules from animals, especially peptides, are also described in less detail. Natural products with antischistosomal properties The importance of natural products in modern medicine has been described in a number of earlier reviews and reports [16–27] . For example, an analysis of the origin of the drugs developed between 1981 and 2002 showed that natural products or natural productderived drugs comprised 28% of all new chemical entities launched onto the market. In addition, 24% of these were synthetic or natural mimic compounds, based on the study of pharmacophores related to natural products [25–27] . The use of drugs derived from plants, fungi, bacteria and marine organisms has a long tradition
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Natural products with antischistosomal activity
in medicine [17–27] . Among these, medicinal plants contain a wide variety of active principles that have been exploited against schistosomiasis, and in recent decades, natural products have attracted renewed interest [16] . Several extracts or essential oils from plants have been tested against schistosomes, especially during in vitro screening. For example, Yousif et al. [28] screened extracts originated from 281 native and cultivated plant species growing in Egypt. Despite the importance of plant extracts and essential oils, isolated compounds (secondary metabolites) or derivatives are currently gaining prominence as possible sources of new drugs in the control and treatment of schistosomiasis. Some of the most interesting antischistosomal compounds are derivatives of artemisinin, such as artemether, artesunate and dihydroartemisinin and the aminoalcohol/quinoline mefloquine [29] . The main metabolites obtained from plants (e.g., terpenes, alkaloids, phenolic compounds and peptides) and animals (e.g., peptides) with promising antischistosomal properties in vitro and/or in vivo are summarized in Table 1, and will be discussed below. Except for artemisinins and mefloquine, which are approved therapeutic agents, all others are still considered only as hit or lead compounds. Major groups of antischistosomal compounds from plants Terpenes
Among plant secondary metabolites, terpenes are a structurally most diverse group [70] . They occur as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30) and tetraterpenes (C40). The terpenes with antischistosomal activity are: rotundifolone [66] , (+)-limonene epoxide [56] , and carvacryl acetate, a derivative of carvacrol (monoterpenes) [39] ; artemisinin and its derivatives [30,31] , nerolidol [59] , and budlein-A and its derivatives [38] (sesquiterpenes); phytol [60] (diterpene), betulin [37] and its triphenylphosphonium derivatives [69] , balsaminol F, karavilagenin C [36] (triterpenes) (Figure 1) . Importantly, among the group of terpenes, in vitro and in vivo antischistosomal properties have been described only for artemisinin and its derivatives, phytol and triphenylphosphonium derivatives, whereas it was reported in vitro schitosomicidal activity for other terpenes (Table 1) . The biological properties of terpenes on different parasitic diseases are still unclear. The mechanisms of action seem to be related to the interaction with the parasite redox cycling system [71] . The mechanism by which terpenes exert their schistosomicidal effect may
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be that, due to inherent lipophilicity, some terpenes will easily cross plasmatic membranes and, consequently, may also interact with intracellular molecules of parasites. In fact, some of the compounds described herein are capable of causing morphological changes in the integument of worms, and a relationship has been observed between tegumental damage and the death of worms [39,56,59–60] . Monoterpenes
Rotundifolone (1) is a monoterpene found in the essential oil of many plant species. Recently, in vitro assays have demonstrated that rotundifolone isolated from essential oil of Mentha x villosa (Lamiaceae) caused mortality of S. mansoni adult worms at a concentration of 70 μg/ml [66] . (+)-Limonene epoxide (2) is an oxygenated constituent found in essential oils, which can be easily prepared in one step and with a good yield from limonene. In vitro test using S. mansoni adult worms presented schistosomicidal activity when tested at a concentration of 25 μg/ml [56] . In addition, morphological alterations on the schistosome surface were detected with (+)-limonene epoxide at concentrations of 25–75 μg/ml. The mechanism by which (+)-limonene epoxide exerts its schistosomicidal effects remains unclear; however, all schistosomes died accompanied by severe destruction of the worm body, and a relationship between tegumental damage and the death of worms was seen. In vitro antischistosomal properties have also been described for carvacryl acetate (3), a semisynthetic derivative of carvacrol [39] . Carvacryl acetate at 6.25 μg/ml affected parasite motility and viability. Moreover, microscopy pictures revealed morphological alterations on the tegumental surface of adult worms, where some tubercles appeared to be swollen with numerous small blebs emerging from the tegument Key terms Natural product-derived compounds: Prepared by chemical synthesis from natural materials. In other words, usually a semisynthetic modification. Pharmacophore: Description of molecular features which are necessary for molecular recognition of a ligand by a biological macromolecule. According to IUPAC, one pharmacophore is ‘the set of steric and electronic features necessary to ensure optimal supramolecular interactions with a structure of a specific biological target and to trigger (or block) its biological response.’ Secondary metabolites: Substances which often are produced by plants as defense mechanisms. Several pharmaceuticals are based on plant chemical structures, and secondary metabolites are widely used as medicines and flavorings. They include alkaloids, terpenes, quinones, flavonoids, neolignans, etc.
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4
6
36
16
13
9
3
35
40
41
Artemisinin
Artesunate
Aspidin
Balsaminol F
Betulin
Budlein-A
Carvacryl acetate
Curcumin
Kalata B1
Kalata B2
Peptide
Peptide
Peptide
Polyphenol
Monoterpe
Sesquiterpene lactone
Triterpene
Triterpene
Phloroglucinol/ phenol
Sesquiterpene lactone
Sesquiterpene lactone
Sesquiterpene lactone
Compound class
Frog of the genus Phyllomedusa (Hylidae)
Oldenlandia affinis (R&S) DC. (Rubiaceae)
Oldenlandia affinis (R&S) DC. (Rubiaceae)
Curcuma longa L. (Zingiberaceae)
Derived from carvacrol
Viguiera (Asteraceae)
Schefflera vinosa (Cham. & Schltdl.) Frodin (Araliaceae)
Momordica balsamina L. (Cucurbitaceae)
Dryopteris genus (Dryopteridaceae)
Derived from artemisinin
Artemisia annua L. (Asteraceae)
Derived from artemisinin
Origin
N
∼18
In vivo against immature and adult worms
∼15
N
N
N
ND
∼30 50
N
12.5
N
ND
∼15
100
N
ND
∼100–780
10
N
ND
∼335–1000
NR
Source
Antischistosomal activity (μM)
In vitro against adult ∼1 worms and In vivo against immature and adult worms
In vitro against adult worms
In vitro and in vivo against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms
In vitro and in vivo against immature worms
In vivo against immature worms
In vitro and in vivo against immature worms
Antischistosomal properties
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
42
5
Artemether
Dermaseptin 01
Compound ID
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically.
[46,47]
[45]
[45]
[40–44]
[39]
[38]
[37]
[36]
[35]
and references therein
[30–32,34]
[30,31] and references therein
[30–33] and references therein
Ref.
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31
29
17
26
Hesperidin
Kaempferol
Karavilagenin C
β-lapachone
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Aminoalcohol/ quinolines
Monoterpene
Neolignan
Naphthoquinone
Triterpene
Flavonoid
Flavanone glycoside
Phloroglucinol/ phenol
Alkaloid imidazole
Sesquiterpene lactone
Sesquiterpene lactone
Phloroglucinol/ phenol
Compound class
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms and In vivo against immature worms
In vitro against adult worms
In vitro against adult worms
In vitro against adult worms and in vitro and in vivo against immature and adult worm
In vitro against adult worms
In vitro against immature and adult worms
In vitro against adult worms
In vivo against immature and adult worms
In vitro against adult worms
Antischistosomal properties
N
∼165
ND
N ND
∼55–95
ND
N
∼30
100
N
N
N
N
ND
ND
N
Source
100
165
10
>500
50
NR
10
Antischistosomal activity (μM)
analogue of quinine In vitro and in vivo ∼15–30 against immature and adult worms
many plants
Many plants. Enantiomers are semi synthetics
Derived from lapachol
Momordica balsamina L. (Cucurbitaceae)
Styrax pohlii Pohl (Styracaceae)
Citrus fruits
Dryopteris genus (Dryopteridaceae)
Pilocarpus microphyllus Stapf ex (Rutaceae)
Derived from budlein-A
Derived from artemisinin
Dryopteris genus (Dryopteridaceae)
Origin
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
24
37
Flavaspidic acid
Mefloquine
20
Epiisopiloturine
2
10
4α,5-dihydrobudlein A
(+)-limonene epoxide
7
Dihydroartemisinin
32–34
39
Desaspidin
(±)-licarin A and its enantiomers
Compound ID
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
[57,58]
[56]
[55]
[53,54]
[36]
[52]
[50,51]
[35]
[49]
[38]
and references therein
[30–31,48]
[35]
Ref.
Natural products with antischistosomal activity
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12
19
18
25
1
30
21
22
23
Phytol
Piperamide 1
Piplartine
Plumbagin
Rotundifolone
Sativan
Sanguinarine
Solamargine
Solasonine
Flavonoid
Steroidal alkaloid (glycoalkaloids)
Steroidal alkaloid (glycoalkaloids)
In vitro against adult worms
In vitro against immature and adult worms
In vitro against adult worms
In vitro and in vivo against adult worms
In vitro against adult worms
In vitro against adult worms
Antischistosomal properties
Astragalus englerianus Ulbr. (Fabaceae or Leguminosae)
Roupala montana Aubl. (Proteaceae)
In vitro against adult worms
Solanum lycocarpum In vitro against adult A. St. Hil. worms (Solanaceae)
Solanum lycocarpum In vitro against adult A. St. Hil. worms (Solanaceae)
In vitro against adult worms
In vitro against adult worms
Mentha x villosa In vitro against adult Hudson (Lamiaceae) worms
Plumbago spp. (Plumbaginaceae) and other families (e.g., Droseraceae and Ebenaceae)
Piper tuberculatum Jacq. (Piperaceae)
Piper amalago L. (Piperaceae)
many plants
many plants
Dryopteris genus (Dryopteridaceae)
Origin
Benzophenanthridine Sanguinaria spp. alkaloid (Papaveraceae)
Isoflavonoid
Monoterpene
Naphthoquinone
Alkaloid amide
Alkaloid amide
Diterpene alcohol
Sesquiterpene
Phloroglucinol/ phenol
Compound class
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
27
8
Nerolidol
Quercetin 3-O-β-dglucoside
38
Compound ID
Methylene-bis-aspidinol
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
NA
50
32
10
N
N
N
N
N
N
∼425 150
N
10
N
∼10
N
∼170 N
N
∼30
100
N
Source
100
Antischistosomal activity (μM)
[37]
[68]
[68]
[64]
[67]
[66]
[64,65]
[62,63]
[61]
[60]
[59]
[35]
Ref.
Review de Moraes
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[37]
[38]
[69]
N
ND
ND
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around the tubercles. Furthermore, experiments performed using carvacryl acetate at sublethal concentrations (ranging from 1.562 to 6.25 μg/ml) showed an inhibitory effect on the daily egg output of paired adult schistosomes. It should be noted that since carvacrol is more toxic than many esters, the synthesis of carvacryl acetate was performed to obtain a derivative of carvacrol with improved pharmacological profile and less toxicity.
Identification number of the compound is according to the order appearing in the text. Antiparasitic activity is according to the in vitro studies. N: Natural product; NA: Not achieved when tested up to 100 μM; ND: Derived from a natural product; NR: Not reported.
2 In vitro against immature and adult worms.In vivo against adult worms Derived from betulin Triterpene Triphenylphosphonium salts 14 15
200 In vitro against adult worms derived from budlein-A 11 4α,5–11β,13tetrahydrobudlein A
Sesquiterpene lactone
100 In vitro against adult worms Schefflera vinosa (Cham. & Schltdl.) Frodin (Araliaceae) 28 Quercetin 3-O-β-drhamnoside
Flavonoid
Antischistosomal properties Origin Compound ID
Compound class
Review
Sesquiterpenes
Constituent
Table 1. Antischistosomal compounds from natural source organized alphabetically (cont.).
Antischistosomal activity (μM)
Source
Ref.
Natural products with antischistosomal activity
Some of the most interesting antischistosomal natural compounds are artemisinin (4) and its derivatives such as artemether (5), artesunate (6) and dihydroartemisinin (7) [30,31] . They are highly effective in the treatment and control of malaria, and have also been shown to exhibit antischistosomal properties; although safe, artemisinins are active only against the immature stages of the parasites. Artemisinin is a sesquiterpene lactone with an endoperoxide group, which was isolated from the leaves of Artemisia annua L. This plant has been used for centuries in Chinese traditional medicine as an antidote to many different ailments [30,72–73] . Artemisinin has been used as an antimalarial since the early 1970s, and its antischistosomal activity was discovered in 1980 by a group of Chinese scientists. In 1982, antischistosomal properties were confirmed for artemether, the methyl ether derivative of artemisinin. The precise mechanism of action of the artemisinins against schistosomes is unknown. However, it appears to involve an interaction with heme, which cleaves the endoperoxide bridge of the drug to produce carbon-centered free radicals that then alkylate parasite proteins. Additionally, artemisinin derivatives induce morphological alternations and damage to the tegument of schistosomes [29–30,73–74] . Interestingly, Vennerstrom et al. [75] synthesized several synthetic trioxolane derivatives incorporating the critical endoperoxide pharmacophore of artemisinins, among which ozonide OZ-277 has proved to be more effective than artemisinin against malaria. Currently, the rational design of structurally simpler analogs of artemisinins has led to the synthesis of a large number of peroxides such as trioxaquines (1,2,4-trioxanes) [32,76] and ozonides (1,2,4-trioxolanes) [77,78] , some of which have displayed interesting antischistosomal activities. For example, ozonide OZ418 was identified as a promising lead compound possessing high activity on both juvenile and adult schistosome infections in mice (worm burden reductions 80–90%) [78] . In general, these compounds are characterized by structural simplicity, ease of synthesis, metabolic stability and improved pharmacokinetic parameters [75,77] . Thus, this provides confidence to move forward with the synthesis of novel
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Review de Moraes antischistosomal compounds based on pharmacophore model design. For clarity, it is important to mention that these trioxaquines and ozonides are fully synthetic compounds, whereas artemether, artesunate and dihydroartemisinin are semisynthetic compounds. Nerolidol (8), a sesquiterpene present in the essential oils of several plants, is found in many foods and was approved by the US FDA [79,80] . When tested in vitro against S. mansoni adult worms, nerolidol at 31–62 μM reduced the worm motor activity and caused the death of parasites, with male worms being more sensitive than female [59] . Moreover, microscopy
H3C
H3C
O
O
O
1
2 CH3
CH3
CH2 H CH3
H3C
CH3 O
O
CH3
O
8
HO
CH3
H
11
CH3
CH2
H3C
O
14
O
H3C
CH3 HO
H 3C
CH3
CH3
H3C
CH3
15
CH3 O
CH3
CH3 HO
CH3
CH3
CH3
16
CH3
CH3 HO
OH H3C CH3
CH3
H3C
CH3
O
H3C CH3
CH
H3C CH3 H3 C
CH3
O
CH3
HO CH3
O
CH3
HO
CH2
Br Ph3P
O
OH
CH3
13
+
H
CH3
O
Br- Ph3P+
H3C CH3
-
H
10
CH3
12
HO
CH3
O
O
O
CH2
CH3
CH3
CH3
O
H
H3C
CH3
CH3
O
9
CH3
CH3
O
CH2
O
OH
6
O
O
O
H3C
O
O
CH2 HO
O
O
CH3 CH3
H CH3
H O
OH
5
O
O
CH3
7 OH
H
O
H3C
CH3
O
O
O
4
CH3
HO
O H O
CH3 O
O O
H
3
H CH3
H3C
H
O
O
O
O
O H
CH3
H CH3
H3C
O
O O
H3C
CH2
H3C
CH3
H CH3
H3C
H CH3
H3C
O CH3
O
O H3C
CH3
analysis revealed morphological alterations on the tegument of worms such as disintegration, sloughing and erosion of the surface, and a correlation between viability and tegumental damage was observed. Budlein-A (9), a sesquiterpene lactone isolated from Viguiera spp (Asteraceae), has been reported to exert in vitro schistosomicidal activity against S. mansoni adult worms at 12.5 μM [38] . However, at this concentration, budlein-A was highly toxic to human cells. In this context, structural modification of budlein-A by the Stryker’s reaction furnished derivatives. All the derivatives were less toxic than the starting budlein-A and two
H3C CH3
O
CH3
CH3 O CH3
17
Figure 1. Antischistosomal terpenes. Rotundifolone (1); (+)-limonene epoxide (2); carvacryl acetate (3); artemisinin (4) and its derivatives artemether (5), artenusate (6), and dihydroartemisinin (7); nerolidol (8); budlein-A (9); 4α,5-dihydrobudlein A (10); 4α,5–11β,13-tetrahydrobudlein A (11); phytol (12); betulin (13) and its triphenylphosphonium salts (14 and 15); balsaminol F (16); karavilagenin C (17).
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Natural products with antischistosomal activity
compounds, 4α,5-dihydrobudlein A (10) and 4α,5– 11β,13-tetrahydrobudlein A (11), which led to 100% mortality of schistosomes at 50 and 200 μM, respectively [38] . These two derivatives differ only in terms of the C-11/C-13 bond type: double and single, respectively. Diterpenes
One the most interesting terpenes is phytol (12), a molecule from chlorophyll widely used as a food additive and in medicinal fields [81,82] . Phytol shows promising antischistosomal properties against adult S. mansoni in vitro and in laboratory studies, with mice harbouring adult S. mansoni [60] . In vitro assays demonstrated that phytol affected parasite motility, viability, and egg production and it induced severe tegumental damage in schistosomes. Additionally, various parasitological criteria indicated the in vivo antischistosomal effects of a single dose of phytol (40 mg/kg) administered orally to mice: it caused significant reductions in worm load, faeces egg load and the frequency of egg developmental stages. Interestingly, adult female parasites are more susceptible to the phytol than male worms for both in vitro and in vivo studies. The mechanism by which phytol exerts its antishistosomal effect is not clear. However, confocal laser scanning microcopy studies revealed tegumental damage in adult schistosomes, especially in female worms, and a correlation between viability and tegumental damage was described [60] . Triterpenes
Betulin (13) is an abundant naturally occurring triterpene [83] . A recent study has showed that betulin isolated from Schefflera vinosa (Araliaceae), a plant from the Brazilian Savannah, was effective in vitro against S. mansoni adult worms at a concentration of 100–200 μM [37] . More recently, Spivak et al. [69] have synthesized triphenylphosphonium derivatives of betulin and betulinic acid and results showed in vitro antischistosomal of activity against newly transformed schistosomula and adult worms of S. mansoni at low micromolar concentrations (
