Introduction

Praziquantel has a very broad spectrum of activity against trematodes and cestodes, and it has advantages covering high efficacy, excellent tolerability, few and transient side effects, simple administration, and competitive cost (Andrews et al. 1983; Redman et al. 1996; Grover et al. 2001; Cioli and Pica-Mattoccia 2003; Keiser and Utzinger 2007; Huang and Xiao 2008). The drug is equally suited for both individual and large scale treatment, especially for schistosomiasis (Kumar and Gryseels 1994; Stephenson and Wiselka 2000; Chen 2005; Doenhoff and Pica-Mattoccia 2006; Ribeiro-dos-Santos et al. 2006; Caffrey 2007; Danso-Appiah et al. 2008; Doenhoff et al. 2009; Dömling and Khoury 2010; Fried and Abruzzi 2010). Since praziquantel was developed (Gönnert and Andrews 1977; Seubert et al. 1977), it has replaced all other schistosomicidal agents to become the only anti-schistosomal drug of choice for treatment against all the major species of schistosome (WHO 2002). However, praziquantel exhibits different sensitivities to various developmental stages of schistosomes. It is of great interest to understand the causation related to the mechanism of drug actions (Xiao et al. 1985a, 1987a; You et al. 1986; Yang et al. 1987; Flisser et al. 1989). Previously many studies were carried out with attempt to investigate the mechanism of actions of praziquantel against schistosomes (Mehlhorn et al. 1981, 1982; Cunha and Noël 1997; Milhon et al. 1997; Harder 2002; Cioli and Pica-Mattoccia. 2003; Ribeiro-dos-Santos et al. 2006; Angelucci et al. 2007; Pica-Mattoccia et al. 2007; Tallima and El Ridi 2007; Troiani et al. 2007; Aragon et al. 2009); however, there are still plenty of mysteries that have not been disclosed. This paper reviews the researches on the effects and mechanism of praziquantel against schistosomes briefly and highlights some issues in the further studies.

An oddity mode

Praziquantel has different effects on various developmental stages of schistosomes, which presents an oddity intermitted mode, i.e., praziquantel can kill the adult worms effectively, but has few effects on eggs; can kill miracidia quickly, but cannot kill sporocysts in the host snails; can kill cercariae easily, but has less effect on schistosomula apart from very early stage like skin-stage (0 day or 3 h old) schistosomula (Fig. 1). This oddity phenomenon should have potential mechanisms; however, there are few researches in a penetrating way yet.

Fig. 1
figure 1

Different effects of praziquantel on various developmental stages of schistosomes. Plus sign schistosomicidal effect; minus sign no schistosomicidal effect (at the dose of schistosomicidal adult worms). Asterisk there is effect on very early developmental stage of schistosomula

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Researches on effects and mechanisms of praziquantel against schistosomes

Adult worms

Praziquantel can kill schistosomal adult worms quickly and effectively. The mechanism now known is that at the molecular level, praziquantel disrupts Ca2+ homeostasis in adult worms, which induces mainly two aspects. Firstly, there appears a spasmodic contraction of worm musculatures and then the worm's body becomes immobilize with feeble movement of oral sucker, which results in hepatic shift of schistosomes from mesenteric veins to the liver. Secondly, the tegument of the worm suffers severe destruction revealed in extensive swelling, erosion, vacuolization, and peeling which results in exposure of the worm surface antigen and disruption of the concomitant immune mechanism occurring between the host and schistosomes. Meanwhile, the damage to the tegument also affects the functions of absorption, excretion, and secretion of the worm, which may be involved in the carbohydrate metabolism, nucleic acid metabolism, and ATP metabolism of the parasites (Andrews et al. 1983; Andrews 1985; Xiao 2007). The half-life of praziquantel is only a few hours, so the destruction of the worms needs the continuous participation of the host immune system. Otherwise, the damaged worms may recover. For example, while rabbits were administrated orally with praziquantel at a single dose of 40 mg/kg, the concentration of praziquantel in the serum of portal vein could be 30 μg/ml. When the adult worms of Schistosoma japonicum were incubated in the culture medium containing the serum, the worms contracted immediately, and the worm surface appeared with vacuolar changes, its oral sucker maintained shrinking for 2–3 days. When the adult worms were incubated in the aforementioned medium for 2 to 24 h and then moved into the medium not containing the serum, the worms protracted and their activities could recover in some degrees or totally. The vacuolar changes on the surface of the worms could be reduced or might disappear. While the sera or neutrophilic granulocytes of the immunized rabbit were added into the medium, the adult worms died within a day (Xiao et al. 1985a). It is obvious that the action of praziquantel against schistosomes has some immune dependence and immune synergy (Huang et al. 2006). The Ca2+ homeostasis in the cells of schistosomal adult worms is regulated by some cellular components, such as voltage-gated calcium channels ((VGCCs); Greenberg 2005). In an in vitro experiment, the calcium channel blockers in human therapy, i.e., nicardipine, nifedipine, nimoldipine, diltiazem, and verapamil, were tested. Schistosoma mansoni adult worms were incubated for 1 h with each of these inhibitors, then praziquantel, at a concentration of 3 × 10−6 M that normally could kill the majority of adult worms, was added and the incubation continued overnight in the presence of the two drugs. On the following morning, the worms were washed, re-suspended in a drug-free medium and the incubation continued for 7–10 days. The results indicated that nicardipine and nifedipine conferred an incomplete but significant protection against the schistosomicidal effects of praziquantel, allowing the survival of about half of the worms. All other compounds tested did not achieve significant protections (Pica-Mattoccia et al. 2007). These results imply that the schistosome calcium channels may be involved in or partially involved in the mechanism of action of praziquantel. Now, the research on the calcium channels of schistosomes, especially on VGCC β-subunits, becomes the highlight to elucidating the targets of action of praziquantel against schistosomes (Richards et al. 2004; Jeziorski and Greenberg 2006; Nogi et al. 2009). However, cytochalasin D, an actin-depolymerizing agent, does not block Ca2+ influx into the schistosomes, and the pretreatment with cytochalasin D renders the schistosomal adult worms completely refractory to the effects of praziquantel (Pica-Mattoccia et al. 2007, 2008; Doenhoff et al. 2008). Therefore, there is a considerable potential for further research on the mechanism of action of praziquantel against schistosomes.

Eggs

Webbe and James (1977) reported that praziquantel had no ovicidal properties in experimental hamsters infected with Schistosoma haematobium, S. japonicum, S. mansoni, Schistosoma intercalatum, and Schistosoma mattheei, respectively. Oogram examinations revealed that praziquantel at a dose of 300 mg/kg which could kill the majority of adult schistosome worms in experimental mice, exerted no obvious effects on the development of schistosome ova, nor did so on hatching (Xiao et al. 1980; Shao et al. 1980). Matsuda et al. (1983) also reported the action of praziquantel on S. japonicum eggs in mice and in vitro. In a long-term oogram observation of mouse intestine, lasting from 1 to 56 days after oral administration of 100 mg/kg four times in 1 day, many empty egg shells were observed from the next day. Immature eggs, however, were not killed, but developed into miracidia and later degenerated into granulated and calcified eggs. In the oograms of mouse, from 5 to 240 min after administered with praziquantel at a single oral dose of 100 mg/kg, miracidium hatching occurred 5 min in the tissues posttreatment. In addition, miracidia in eggs from cut pieces of intestine began to hatch 5 min after exposure to praziquantel at least at a concentration of 1 ng/ml in vitro and reached maximum hatching at 30 min and died in several minutes after hatching. These results showed that praziquantel cannot kill the schistosome eggs but can stimulate the immature eggs to develop and mature eggs to hatch. It has been shown that the hatching rates (0–48.1%) of the eggs of S. japonicum were significantly lower than those of S. mansoni (0–91.0%) while exposed to the same concentration of praziquantel, and it is suggested that the eggs of S. japonicum are more sensitive to praziquantel than those of S. mansoni (Liang et al. 2001a).

Praziquantel can suppress the formation of schistosomal egg granulomas. Yang et al. (1984) reported that after praziquantel short administration in a daily dose of 300 mg/kg for 3 days, the average diameter of pulmonary S. japonicum egg granulomas in the sensitized mice was significantly reduced, and the numbers of granulocytes, eosinocytes, and fibroblasts within the egg granulomas were also markedly decreased. Botros et al. (1984) also reported that after the short treatment of praziquantel in a daily dose of 150 mg/kg for 3 days, the size of the pulmonary S. mansoni egg granulomas in the unsensitized mice was reduced. Recently, Huang et al. (2011) reported that the administration of praziquantel, especially the prolonged administration (in a daily dose of 150 mg/kg for 5 days weekly until 8-week's sacrificing of the animals), suppressed obviously the formation of schistosomal egg granulomas, including reduction in the areas of granulomas and suppression of the inflammatory cells and the hyperplasia of fibroblasts within granulomas.

Miracidia

Praziquantel can kill miracidia of schistosomes while the miracidia have hatched from the eggs. For example, free-swimming miracidia were rapidly killed by the drug at a concentration of 1 μg/ml and even lower concentration (0.01 μg/ml) remained partially effective (Andrews 1978). In another in vitro experiment, S. japonicum eggs were incubated in the water containing praziquantel at concentrations of 0.001–0.1 μg/ml; no miracidia were found in the upper level of the water, but there were a lot of metamorphosis, unusually active or dead miracidia in the sediment. While the concentration of praziquantel was 0.1 μg/ml, there were unusually active miracidia observed in the middle and upper levels of the water. Therefore, praziquantel cannot suppress, or even can stimulate, the hatching of mature eggs, but while the miracidia have hatched, praziquantel can kill them or affect their shape, activity, and vitality immediately (Xiao et al. 1980). It has been reported that miracidia of S. japonicum being exposed to 5 × 10−6 M praziquantel immediately contracted in the middle part of their bodies, giving the shapes of an unequal dumbbell or calabash, with the greater mass at the anterior end. In 10−6 M praziquantel over 1 min, the morphological change rate of miracidia of S. japonicum was 100%, while in 5 × 10−7 M praziquantel over 5 min, it was 97% (Liang et al. 2001a).

Sporocysts

Praziquantel cannot kill mother sporocysts and daughter sporocysts of schistosomes but can prevent infected host snails from shedding cercariae. It was reported that S. mansoni-infected snails, shedding cercariae, were exposed to praziquantel at 3 × 10−6 M and 3 × 10−5 M respectively for 24 h before being transferred to normal water. The snails were checked individually for shedding of cercariae before being dissected on day 7. A very few cercariae were shed by snails treated with 3 × 10−6 M praziquantel and none from snails treated with 3 × 10−5 M praziquantel. However, all snails were filled with daughter sporocysts and developing cercariae on the dissected day (Coles 1979). In another experiment, at concentrations of 3 × 10−7, 3 × 10−6, and 3 × 10−5 M, respectively for 24 or 48 h, the mother and daughter sporocysts, and young cercarial embryos of S. japonicum were not affected but nearly mature cercariae were killed and dissociated (Yi and Combes 1987). However, it has been shown that when S. mansoni-infected Biomphalaria glabrata snails were exposed to praziquantel, although the sporocysts remained alive, there was a marked contraction of the musculature, and there occurred a change on the tegument of the sporocyst, but the change could recover (Mattos et al. 2006).

Cercariae

Praziquantel can kill the mature or nearly mature cercariae of schistosomes in the host snails and prevent infected host snails from shedding cercariae and can kill the cercariae that were release from the host snails immediately. In an in vitro experiment, after 2-h incubation in praziquantel at a concentration of 0.01 μg/ml, the swimming ability of S. mansoni cercariae was impaired in such a way that the number of cercariae penetrating in the tail immersion test and developing to schistosomula was reduced by half. In a solution of praziquantel of 0.03 μg/ml, cercariae lost their ability to swim within 10 min. The morphological damage to cercariae incubated in 0.1 μg/ml of praziquantel was clearly evident. Incubation in 1 μg/ml of praziquantel reduced the infection rate by 80% (Andrews 1978). S. japonicum cercariae are very sensitive to praziquantel. In vitro experiments, when treated with praziquantel solution, cercariae were affected obviously if the concentration was adequate. Initially, the tail continuously oscillated from right to left, the body remaining immobile, then the tail was lost. In the concentrations of 3 × 10−5 and 3 × 10−6 M, all the cercariae died after 40 and 60 min, respectively (Xiao et al. 1985b; Yi and Combes 1987). Another experiment revealed that when schistosome cercariae were exposed to 4 × 10−7 M over 100 min, 95.82% of the cercariae from S. japonicum isolates shed their tails; when schistosome cercariae were exposed to 10−5 M over 40 min, 96.75% of the cercariae shed their tails (Liang et al. 2001a).

Schistosomula

In vitro experiments, the minimal effective concentrations (MECs) of praziquantel inducing increased motor activity and muscular contraction or paralysis or both of S. mansoni were higher (0.01 μg/ml) in days-3, -7, and -14 schistosomula than those (0.005 μg/ml) in days-0, -28 schistosomula, and day-35 and -42 adult worms. Day-3 lung forms were more resistant than other stages when either drug-induced tegumental vesiculation (MEC, 1 μg/ml) or recovery from drug exposure was tested (Xiao et al. 1985c). In experimental S. mansoni-infected mice and hamsters, praziquantel is effective against the invading stages and slightly less against schistosomula up to an age of 7 days. It is less effective against 2- to 4-week-old juveniles (Gönnert and Andrews 1977; Sabah et al. 1986). For S. japonicum, in vitro, MECs of praziquantel were 0.01 μg/ml in days-0, -3, -7, -11, -14, and -21 schistosomula, which were twice higher than that (0.005 μg/ml) in adult worms. In experimental S. japonicum-infected mice, praziquantel, in the dosage that can kill most of adult worms, is effective against 3-h-old schistosomula but less effective or ineffective against 3-, 7-, 14-, and 21-day-old juveniles (Yue et al. 1985; Xiao et al. 1987b, 2009.

Further research in the future

From the aforementioned data, it is concluded that the effects of praziquantel on various developmental stages of schistosomes are different and the phenomenon should have deep mechanisms. It is hope that with the completion of sequencing of the S. japonicum and S. mansoni genomes (Schistosoma japonicum Genome Sequencing and Functional Analysis Consortium 2009; Berriman et al. 2009), as well as the rapid advances in -omics and molecular biology (Hu et al. 2003; de Oliveira et al. 2004; Liu et al. 2006, 2007, 2009; Brejová et al. 2009; Chuan et al. 2010; Webster et al. 2010; Piao et al. 2011), the issue would be solved in the future. The mechanisms of action of praziquantel against schistosomes are very complex and may involve many targets. May the structure, especially the structure of tegument, of different developmental stages of schistosomes be different? May the cell organs be different, resulting in their physiological functions being different? The causation of the different effects of praziquantel on various developmental stages may be the differences in protein components, enzymes, and other chemical components among the different developmental stages of schistosomes. May the allocation and distribution of VGCCs and their subunits be different among the various developmental stages? Are other calcium-permeable channels and their sites and actions different? Other explanations, such as possible differences among the various developmental stages of schistosomes in the distribution and extent of praziquantel-sensitive sites, receptors, Ca2+-pump, Na+–Ca2+-exchange proteins, and so on should be considered. The comparison of praziquantel-susceptible and -resistant developmental stages of the parasite could reveal more about the mechanism of praziquantel activity (Liang et al. 2001b, c, 2002, 2010).

In addition, when praziquantel acts on schistosome adults in vivo, the inflammation cells like neutrophilic granulocytes and eosinocytes can be attracted to the worms and attack the worms, but after the host is given with praziquantel treatment, the inflammation cell reaction around schistosomal eggs is relieved obviously and the formation of egg granulomas is suppressed. The causation of the contradictory phenomenon that praziquantel serves to accelerate the response of host to schistosome adult worms but to suppress the response to schistosome eggs is unclear and interesting; more researches are therefore required.

In conclusion, it may be a key for the development of new anthelmintic drugs or other medicines to explore the mechanisms of praziquantel against schistosomes and the causations of praziquantel having different effects on various developmental stages of schistosomes, or it may be no worth doing so, especially only for getting economic values. However, as we all know, most of historical important scientific discoveries do not start from a particular value goal but only start from the curiosity of human beings.