New Uses for Old Drugs and Their Application in Helminthology

Abstract

Parasitic infection research, performed on both humans and domestic animals, has been mostly focused on vaccines, diagnostic methods, epidemiology, and the evolutionary origins of parasites, thanks to the emergence of genomics and proteomics. However, the basic biology of the host-parasite interactions of several medical or veterinary important parasites has not been fully studied. Limited information has been obtained on the intricate neuroimmunoendocrine effects of host-parasite interplay in particular; therefore, the consequences of these interactions, and their possible therapeutic applications, are in need of thorough research. The current manuscript attempts to review the available literature regarding the host-parasite neuroimmunoendocrine network and to discuss how this basic research can be used to design new treatments using hormones, antihormones, and hormone analogs as a novel therapy against parasitic diseases. In addition, these studies may also contribute in identifying alternative treatments for parasitic diseases in the future. The complex immune-endocrine network may also help in explaining the frequently conflicting results observed in infections with regards to host sex and age and offer helpful insight into other research avenues besides parasite treatment and control strategies. Finally, several natural products isolated from plants, used in traditional medicine, offer an alternative approach for natural products in the preparation of inexpensive and effective antiparasitic drugs.

1. Introduction

Parasitic infections rank amongst the most significant causes of morbidity and mortality in the world, yet economic and other factors have contributed to a lack of innovation in treating these diseases. Nitazoxanide (NTZ), a pyruvate ferredoxin oxidoreductase inhibitor, is a new antiparasitic drug notable for its activity in treating common intestinal helminths. The availability of a product with this spectrum of activity raises interesting new possibilities for treating intestinal parasitic infections [1]. Recent studies have shown that NTZ inhibits pyruvate ferredoxin oxidoreductase (PFOR), a vital enzyme of central intermediary metabolism in protozoan. In contrast to the nitroimidazoles, NTZ appears to interact directly with PFOR (i.e., NTZ is not dependent on reduced ferredoxin), and the products of NTZ activation do not induce mutations in DNA. This distinct mechanism of action is important in explaining the therapeutic efficacy of this drug against organisms displaying high level of resistance to metronidazole [2].

The use of hormones, or hormone antagonists, as immunoregulators or as agents to prevent colonization, growth, or reproduction of parasites, may be potentially useful in the treatment of a large variety of parasitic diseases, particularly those in which hormones are known to have a strong controlling effect. The discovery of new antiparasitic drugs is a very expensive process that has resulted in few drugs being commercialized over an extended period of time (Figure 1).

Figure 1.

The cost and time of the drug development process. It does start with targeting the disease. Then it goes throughout the basic research until it gets the lead compound. If there is a failure, and the compound is not promissory, then it goes back. Then the process continues until a marketable drug is obtained. The accumulated cost is around 300 million dollars to get to the final product. Today, these costs are being greatly decreased. The time since the basic compound is found in the marketable drug is around 20 years. However, it may be more if there are failures in the development of the same. In the case of helminthology, few compounds are being discovered since praziquantel, mebendazole, and albendazole were discovered.

Since new drugs must be targeted against parasite survival interactions and be selective and unimpaired by known resistance mechanisms, the knowledge gained by studying physiological regulation of the host-parasite interaction could make it less expensive and faster, to produce antiparasitic drugs. Recent advances in genomic technology offer us the opportunity to identify, validate, and develop constructs of parasite key molecules that could be regulated by hormones, for testing drugs such as tamoxifen, RU-486, fadrozole, or flutamide (all of them hormone agonists) that could result in the identification of antiparasitic drug targets. This would also give new uses to old drugs that are already on the market. Understanding how the host’s neuroimmunoendocrine system can, under certain circumstances, favor the colonization of a parasite and how the characterization of the parasite’s hormone receptors involved might assist the design of hormonal analogs and drugs that affect the parasite exclusively [3]. Most of the current research on parasitic infections that affect humans and domestic animals has been focused on vaccines, diagnostic methods, epidemiology, new drug design, and recently, with the advancement of genomics and proteomics, on the evolutionary origins of parasites. However, the design of new treatments using hormones, antihormones, and hormone analogs as a possible novel therapy during parasitic diseases has been recently proposed. The pharmaceutical industry is now currently investing a higher sum of resources in the development of new antiparasitic drugs. We and other research groups (focusing mainly on sex-associated susceptibility to infection, the direct effects of adrenal and sex steroids as well as the study of parasite genomes) have suggested the study of known drugs, whose formulae have been redesigned, to test possible antiparasitic function. In this respect, animal models are highly convenient in the study of infectious diseases and the design and test of new drugs. It is desirable that these drugs are also tested in in vitro systems, where parasite growth, reproduction, and viability can be evaluated in response to pharmacological treatment [3]. An in vitro approach is convenient when seeking to define the molecular mechanisms by which a drug affects a parasite without including host-parasite interaction parameters (Figure 2).

Figure 2.

The current drug discovery process specifically in helminthology. In this case, we focus on proteases as an example. But it is only the step of finding the lead compound. It can take up to 3 years to find the lead compound, and up to 2 million dollars to obtain it. Then, it has to go throughout all the further steps pointed out in Figure 1.

In the present manuscript, we highlight the novel use of known drugs (currently used in cancer treatment and other proliferative disorders) to treat parasitic infections, i.e., cysticercosis, trichinellosis, ascariasis, schistosomiasis, toxocariasis, onchocerciasis, and others helminthic infections. Parasite fecundity is extremely important in the biological course of infection, therefore, it is worth considering some of the well-described antiproliferative drugs, which may also have inhibitory effects upon parasite reproduction, even if pathogens are inside the host cell. The genome of several parasites is currently being sequenced, which enables the knowledge acquisition on the molecular mechanisms involved in the infectious process, as well as in the design of different transcriptome maps, that could potentially explain the interaction and expression of the involved genes in parasite colonization and reproduction [3]. Hormonal effects are the keystone for parasite development. Experimental evidence, previously obtained by our group, suggests that the scolex evagination of Taenia solium cysticerci is stimulated by progesterone, however, other authors refer to the opposite effect for progesterone, which inhibits the reproduction and parasite molting in Trichinella spiralis [4, 5]. The target genes for progesterone action remain to be identified, and we must wonder if commercial progestin would inhibit parasite reproduction and differentiation. Although the knowledge of host-parasite interactions has grown over the last few years, there are still many unanswered questions that would allow us to fully unravel these host-parasite events and consider the complex neuroimmunoendocrine network involved in this pathogenic relationship. With this in mind, it is important to understand the biological role of sex steroids and the use of their inhibitors, alongside other drugs, aimed to inhibit cellular proliferation in the parasite. An experimental approach could clarify these points and further contribute to elucidating the host’s biological factors that control, or facilitate infection. Research on drugs utilized to treat different diseases could well allow the discovery of their active role in the regulation of parasite gene transcription during proliferation. Only a few novel classes of antiparasitic drugs have emerged over the last few decades thus reflecting the difficulties associated with bringing a safe and effective molecule to the market. Moreover, the screening paradigm has shifted from an empirical whole parasite screening to mechanism-based high-throughput screening. This approach requires a heavy investment in molecular parasitology and in depth understanding of the basic biology of parasites, as well as considerable infrastructure for the screening assays. Add to this the fact that the drug discovery process is interactive with high attrition, and the animal health industry, by necessity, must focus on discovering medicines for diseases that will provide a profit in return. In this regard, the rapid progression of genomics has unlocked a plethora of tools dedicated to target identification, validation, and screening, resulting in revolutionizing mechanism-based screening methods for antiparasitic drug discovery [3]. Therefore, the use of sexual hormones, their analogs, and other immune-regulatory factors are receiving more attention concerning new therapeutic strategies in the prevention and outcome of parasitic diseases. As an example, the treatment with testosterone or dihydrotestosterone in a model of murine cysticercosis, prior to infection, reduced the parasite load by 50% and 70%, respectively. This effect was mediated by significant lymphocyte proliferation recovery and enhanced IL-2 and IFN-γ production in the infected mice [6], suggesting the possible use of androgens to activate the host’s immune system and increase resistance against lethal infections [7]. Estrogens and progestins, particularly estradiol and progesterone, contribute to either susceptibility or resistance to parasitic disease during pregnancy [8]. Usually, these sexual hormones are associated with immunosuppression leading to susceptibility to infection, as demonstrated in the murine cysticercosis model [9, 10]. However, parasiticidal activity was observed in murine trichinellosis. In vitro and in vivo experiments performed on Trichinella spiralis newborn larvae (NBL) in pregnant rats showed that progesterone can induce activation of peritoneal cells to destroy NBL in an antibody-independent manner. This observation opened up the possibility for the use of progesterone to treat trichinellosis but not cysticercosis [11, 12]. Sexual hormone precursors, analogs, antagonists, or inhibitors can also be used to modify the immune response induced by specific parasites to affect the outcome of infection. For example, exogenous administration of dehydroepiandrosterone confers resistance to several intracellular metazoan and protozoan parasites [13, 14, 15]. Concerning Taenia crassiceps in specific, this effect is not mediated through over induction of the Th1 response. Instead, the antiparasitic effect of dehydroepiandrosterone targets the reproduction, growth, viability, and infectivity of the parasite [16]. Regarding sexual steroid analogs, the synthetic androstane steroid 16α-bromoepiandrosterone (HE2000) has shown positive immune effects in experimental infection as malaria and tuberculosis, even infection with human immunodeficiency virus [17], due to its anti-inflammatory properties and the induction of innate and adaptive cellular immunity [13, 17, 18, 19].

In other studies, the inhibition of sexual hormones induced the recovery of a specific cellular immune response. Recently, the use of phytoestrogens as antiparasitic drugs has increased. Genistein, an isoflavone isolated from soybean, exhibits significant metacestodicidal activity in vitro but also binds to the ER and induces estrogenic effects. Furthermore, modified synthetic genistein derivatives have shown improved metacestodicidal activity [20].

2. Resistance to antihelminthic drugs

The development of resistance to antihelminthic drugs is an increasing problem that compromises livestock productivity and threatens the success of treatment in humans [21]. The intensive use of drugs in the livestock industry has led to widespread resistance to all current antihelminthic drugs [22]. Notably, resistance to antihelminthic drugs occurred rather quickly after their introduction to animals. The first widely administered antihelminthic, phenothiazine, was introduced into the market in the 1940s, and resistant populations were reported by 1957. In 1961, thiabendazole was released by 1964, resistance to this compound had been reported. Similar trends occurred with the release of levamisole in 1968, ivermectin in 1981, and moxidectin in 1991 [21]. Resistance to these was reported in 1979, 1988, and 1995, respectively [23]. Parasite populations are genetically heterogeneous, and this genetic diversity leads to a variable response to drugs. Although the impact of parasitic diseases could be reduced dramatically by improved sanitation for humans and pasture control in domestic animals, such methods are not sufficient to eradicate helminths [21], which are treated with a variety of drugs, i.e., macrocyclic lactones, benzimidazoles, imidazothiazoles, and praziquantel [24].

3. Alternative drugs for parasitology

The fact that hormones have a direct effect upon parasites opens the possibility for designing new strategies for parasitic control and hormonal therapy based on: (1) the knowledge of which hormone has direct restrictive actions on pathogen growth, reproduction, and/or differentiation, independently of the immune system; (2) the design of hormonal analogs that exclusively affect the parasite, diminishing any collateral effects upon the host; and (3) the improvement of drugs that competitively bind to parasite receptors, thus blocking gene expression as well as other important cellular processes of the invading organism.

The pharmaceutical industry invests ~25 million dollars annually for the development of new antiparasitic drugs. However, some of these drugs are being commercialized almost every fifteen years. These new antiparasitic drugs focus on interfering with the parasite’s survival; however, they also must be safe for the host and avoid cross-resistance with other existent drugs. Furthermore, the development of new drugs is an expensive and very slow process since these drug candidates must first be tested in experimental animal models where high antiparasitic efficiency and low toxicity for the host must be evaluated before they can be tested in humans. This process takes at least 5−10 years, being the main reason why the pharmaceutical industry and medical research have stopped this task. Presently, the current age of parasite genomics promises to reduce both the cost and time of antiparasitic drug development, again with an impact on the pharmaceutical industry and medical research. However, the genome of several parasites is still being sequenced, and the uses and applications derived from that knowledge are thought to be applicable in at least another five to ten years.

With the results obtained at our laboratory and elsewhere, our research group has sought the possibility of using old drugs (unrelated to parasite infections) with renewed formulae to test their antiparasitic potential in experimental infection models in vitro and in vivo. In addition, the dire need of developing countries to control or eradicate parasitic infections led us to test certain drugs currently approved by the Food and Drug Administration (FDA) for human use as a strategy to reduce the impact of these parasitic diseases, but also reducing the costs and time in which a new drug is generated.

Considering the fact that parasite reproduction is extremely important in the biological course of the infection, it is possible that some of the well-described antiproliferative drugs could also have inhibitory effects on parasite reproduction (Table 1).

Compound	Current status	Possible parasiticidal use	References
DHEA	Complementary health therapy	Schistosomiasis and taeniasis	[14, 15, 16, 25, 26, 27, 28, 29]
Tamoxifen	Cancer drug	Cysticercosis	[26, 30]
Doxycycline	Antibiotics	Filarial diseases	[31, 32, 33, 34, 35, 36, 37, 38]
Amiodarone	For irregular heartbeat treatment	Schistosomiasis	[39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]
Paclitaxel	Cancer drug	Echinococcus protoscoleces and metacestodes	[50, 51]
Docetaxel	Cancer drug	Alveolar echinococcosis	[51]
Cisplatin	Cancer drug	Schistosomiasis	[52]
Genistein	Reduce symptoms of menopause	Metacestodicidal effect	[53, 54, 55, 56, 57, 58, 59, 60, 61, 62]

Table 1.

Antiproliferative compounds with parasiticidal effects upon parasites.

The challenge remains, however, to identify novel chemical entities with the required properties to deliver a safe and effective antiparasitic drug. At present, we have data suggesting that steroids can exert a wide spectrum of effects (suppression or induction) on the host’s immune system during the course of infection and also affect the viability of metazoan parasites. In this regard, the use of sexual hormones, their analogs, and other immunoregulatory factors is being focused to develop alternative therapeutic strategies to prevent parasitic diseases.

The hypothesis that sex steroids regulate the expression of genes important in either susceptibility or resistance to infection has been explored by testing antihormonal and antiproliferative drugs.

3.1 Dehydroepiandrosterone

We have also shown the protective effects of progesterone on neutered mice infected with Taenia crassiceps cysticerci. Neutered male and female mice treated with progesterone were completely protected from the parasite in comparison with untreated-infected, infected Gx, and vehicle-treated infected mice. These results showed higher protective levels than any other reported in the literature yet, including vaccination. Notably, no variation was observed in this experimental system, which otherwise, showed large differences in parasite numbers among mice. The fact that progesterone was being metabolized to DHEA further supports our data indicating that progesterone levels were not as high as expected and, in contrast, DHEA levels were greatly increased. Thus, it seems that the observed effects were the result of adrenal conversion of progesterone metabolism to DHEA. This hypothesis was confirmed when the administration of DHEA prior to infection reduced the parasite load by 50% when compared with untreated mice. Interestingly, this protective effect was not associated with the host’s immune response as there was no effect on the mRNA levels of interleukin (IL)-2, interferon (IFN), IL-4, or IL-10; notably, in vitro treatment of Taenia crassiceps with DHEA reduced reproduction, motility, and viability in a dose- and time-dependent manner. These results indicate that DHEA has a direct and strong negative modulation effect on murine cysticercosis [16]. DHEA has been demonstrated as a strong parasiticidal molecule in several systems. In another study, exogenous DHEA administration was shown to upregulate the immune system, specifically the cellular immune response, by increasing the number and function of natural killer cells [25]. However, our findings do not support this notion since IL-2 mRNA levels do not change in response to DHEA treatment [16]. The lack of any DHEA effect on cytokine expression, regardless of its dramatic effect on parasite load and reproduction in vivo and on survival in vitro, supports the hypothesis that DHEA exerts its protective properties by directly affecting the parasite. To the best of our knowledge, this effect is consistent with the known effects of DHEA on the survival of protozoan parasites [14, 15, 16].

For example, it has been suggested that in human schistosomiasis, DHEA is the cause of the puberty-associated drop in susceptibility. This idea has been reinforced by experiments in which the treatment of mice with the bloodstream form of DHEA (DHEA-s) protected them from infection with Schistosoma mansoni [15].

In this manuscript, we extend these findings on the role of DHEA in protecting mice against Taenia crassiceps infection. Our findings of decreased DHEA levels in mice as the infection progresses agree with previous results in a S. mansoni-baboon model, in which baboons with primary infections showed decreasing levels of DHEA as the infection progressed, compared with uninfected and re-exposed baboons [63].

Our results showing that DHEA treatment protects mice against Taenia crassiceps infection support and extend the notion that androgens are an important factor involved in limiting Taenia crassiceps colonization in immunocompetent hosts. Previous immunological experiments have suggested that testosterone and dihydrotestosterone, two potent androgens (such as DHEA), negatively regulate parasite reproduction in mice of both sexes, presumably by interfering with the thymus-dependent cellular immune mechanisms that inhibit parasite growth (Th2) and enhancing those that facilitate it (Th1) [63], but also by directly affecting parasite motility, survival, and reproduction [16].

It has been shown that administration of tamoxifen (an antiestrogen) increases the cellular immune response, which protects against the parasite but also has a direct parasiticidal effect on the parasite’s reproduction, motility, and survival. These activities lead to a reduction of 80% and 50% of parasite burden in female and male mice infected with Taenia crassiceps, respectively. Also, increased mRNA levels of interleukin (IL)-2 (Th1) and IL-4 (Th2) and a decreased expression of estrogen receptors (ER) (ER-α and ER-β) were observed. In all, these features indicate that the treatment of cysticercosis with tamoxifen could well be a new therapeutic possibility [26]. In other cases, the inhibition of sexual hormones could induce recovery of the specific cellular immune response. In murine cysticercosis, 17β-estradiol (E₂) positively regulates parasite reproduction in hosts of both genders, obstructing the Th1 response and facilitating the Th2 immune response [27, 28]. Administration of fadrozole, an aromatase inhibitor, suppressed the production of 17β-estradiol in males and females interfering with the enzyme P450 aromatase, which converts testosterone to E2 in ovary and testes [29]. This led to a 70% reduction in parasite burden, an increase in IL-6 serum levels, and a shift of the Th2 to the Th1 immune response [9], opening the possibility of a new therapeutic approach against several infections.

3.2 Tamoxifen

Tamoxifen is one of the most prescribed drugs used in cases of estrogen-dependent breast cancer in the world. A selective modulator of estrogen receptors, its mechanism of action is to prevent estrogen binding in cancer cells, thus halting replication and cancer progression. Indicated in the treatment or prevention of breast cancer, it is administered continuously over 5 years with daily doses of 20–40 mg [30]. The use of Tamoxifen in parasitic diseases, such as Taenia crassiceps, has also been attempted, showing that tamoxifen administration produced an 80% parasite load reduction in female mice and a weaker effect of 50% in male mice [26]. This protective effect was associated, in both genders, with increased mRNA levels of IL-2 (a cytokine associated with protection against cysticerci) and IL-4 (no effect on infection). In vitro, treatment of Taenia crassiceps with tamoxifen reduced both reproduction rate and loss of motility. These results indicate that tamoxifen treatment is a new therapeutic possibility in the treatment of cysticercosis because it can act at both ends of the host-parasite interaction, i.e., increasing the protective cellular immune response against the parasite and directly affecting the parasite’s reproduction and survival capabilities [26].

3.3 Antibiotics

Antibiotics do not have any antiparasitic effects against helminths; however, different therapeutical approach has been developed in the last two decade for filarial diseases. Filarial nematodes (Onchocerca volvulus, Wuchereria bancrofti and Brugia spp) infect over 138 million individuals worldwide, causing morbidity, disability, and economic hardship and are distributed mainly in tropical and subtropical regions. The majority of infections are caused by Onchocerca volvulus, which causes human Onchocerciasis (river blind-ness) in sub-Saharan Africa, Latin America, and the Arabian Peninsula [31, 32]. After Onchocerciasis Control Programme (OCP) (1974−2002) using mainly insecticides for vector control, subsequently the ivermectin, a microfilaricidal drug, was distributed on large scale since 1989 in all communities where onchocerciasis was endemic. The ivermectin mass treatment reduced the burden of parasite infection since it can produce “embryostatic” effect, which temporarily prevents the release of microfilariae and temporary parasites’ sterility [33]. Unfortunately, the drug has been administered for years and some Onchocerca volvulus populations are less responsive to ivermectin, which could be explained by genetic drift [34].

In the last twenty years, key drug trials have been performed with a new chemotherapeutical approach to antifilarial therapy, targeting the essential Wolbachia endosymbiotic bacteria present in many filariae that is important for their viability and fertility [35]. The objectives of the anti-Wolbachia (A-WOL) research programmed by the Bill and Melinda Gates Foundation (BMGF) proposed to evaluate antibiotics such as doxycycline, rifampicin, and azithromycin in Onchocerca volvulus infected population to find out the most effective dose for large scale use [36]. After extensive research, it was demonstrated that doxycycline had the better larval burden reduction since it affects development embryonic stages as well as the development from L3 into adult worms [37, 38].

3.4 Amiodarone

Amiodarone is an antiarrhythmic medication that affects heartbeat rhythm. This compound has been tested against different protozoan parasites such as Trypanosoma cruzi [39, 40, 41, 42], Acanthamoeba castellanii [43], Leishmania spp. [42, 44, 45, 46], and Plasmodium [47].

In the case of helminth parasites, this compound has been tested against S. mansoni. Porto et al., (2021), by electron microscopy analysis, reported that amiodarone affects the viability of schistosomes in vitro with effective concentrations of 50% and 90% values ranging from 8 to 50 μM. Also, amiodarone was tested in a murine model of schistosomiasis for both early and chronic S. mansoni infections using a single oral dose of 400 mg/kg or 100 mg/kg daily for five consecutive days. They report that Amiodarone had a low efficacy in chronic infection, with the worm and egg burden reduction ranging from 10 to 30%. In contrast, this compound caused a significant reduction in worm and egg burden in early infection (>50%) [48]. Similarly, Talaam et al., (2021), evaluated the possible effect of amiodarone against S. mansoni. In this experiment, amiodarone showed complete inhibition of cercaria motility after 18 hours. In the case of schistosomula, after 24 hours with amiodarone, the inhibition of motility was complete. In adult parasites, amiodarone inhibited the motility after 20 hours of incubation was not complete, providing mean motility scores of 0.3 and 1.0 for the male and female, respectively [49]. In in vivo experiments, mice were prophylactically treated with amiodarone (50 mg/kg) by 4 days of once-daily intraperitoneal injection, starting 1 day prior to infection, and then euthanized six days postinfection to recover the schistosomula from the lungs. The results show a worm burden reduced to 14.7%. In the case of therapeutic treatment, the mice at week six after infection were treated intraperitoneally with amiodarone (50 mg/kg) for 4 days, and subsequently, they were sacrificed 14 days after the last treatment, the parasite load showed a decrease to 29.2% [49].

3.5 Paclitaxel

Paclitaxel (Taxol) is a drug used in the treatment of breast, ovarian, lung, bladder, prostate, melanoma, esophageal, and other types of solid tumor cancers. It has also been used in Kaposi’s sarcoma.

The use of this drug against helminths has been little tested. Pensel et al., (2014), tested the in vitro effect of this compound against germinal cells, protoscoleces and cysts of Echinococcus granulosus, and parasites responsible for echinococcosis in humans. They report that the use of paclitaxel at a concentration of 1, 5, and 10 μg/ml inhibited the growth of Echinococcus granulosus cells in a time-dependent manner. In addition, paclitaxel had a direct effect against protoscoleces in a dose- and time-dependent manner. At 30 days postexposure with 10 and 5 μg/ml paclitaxel, viability of protoscoleces decreased to approximately 60% and the treatment with 1 μg/ml also showed protoscolicidal effect, with 75.3% of parasites remaining viable in culture. Finally, in an in vitro cyst incubation it was shown that paclitaxel resulted in dramatic alterations within 3 to 5 days after initiation of treatment [50]. In another experiment, Huang et al. (2018) evaluated the effect of paclitaxel on growth and proliferation of Echinococcus multilocularis metacestodes. They exposed metacestode tissues in vitro to paclitaxel (2, 5, and 10 μM) for one week and, thereafter, were injected into the peritoneum of Meriones unguiculatus. After, magnetic resonance imaging and simultaneous positron emission tomography were applied to monitor in vivo growth of drug-exposed Echinococcus multilocularis. The in vivo growth of metacestodes was suppressed until 3 months postinfection, thereafter, parasite tissues enlarged up to 3 cm³ [51].

3.6 Docetaxel

Docetaxel (taxotere) is a chemotherapeutic drug administered as a treatment for some types of cancer, such as breast, prostate, and non-small cell lung cancer, but it also may be used for many other types of cancers.

The only report where the effect of docetaxel against a helminth has been evaluated was carried out by Huang et al. (2018). They report that at three months postinfection, docetaxel (at 10 μM, 5 μM and 2 μM) inhibited in vivo growth and proliferation of Echinococcus multilocularis, and at 5 months postinfection, only in the 2 μM docetaxel exposure group 0.3 cm³ of parasite tissue was found [51]. Moreover, in Meriones infected with Echinococcus multilocularis metacestodes previously exposed to docetaxel, in vivo grown parasite tissues weighed 0.2 g and in vitro cultured Echinococcus multilocularis metacestodes exposed to docetaxel did not produce vesicles until 7 weeks post-drug exposure. With the above, they suggest that the use of this drug can work as an alternative option for the treatment of alveolar echinococcosis [51].

3.7 Cisplatin

Cisplatin is a first-generation platinum-containing drug, used in the treatment of various solid tumors. This drug prevents or inhibits cell maturation and proliferation.

The effect of cisplatin against helminths was tested by Eldeed et al., (2018), in an in vivo and in vitro experiments where they tested a single dose of cisplatin against S. mansoni. In in vitro experiments, they report that a single dose of cisplatin (10 to 200 μg/ml) for 24 or 48 hours demonstrated as reduction in viability of the treated worms after 24 hours and, especially, after 48 hours. Moreover, the survival rate of the treated worms decreased gradually in a concentration-dependent manner [52]. On the other hand, in in vivo experiments in which female mice were injected subcutaneously with cercariae of S. mansoni and administered cisplatin at a dose of 8 mg/kg/day for 3 days beginning on day 42 postinfection, to which samples were collected 2 weeks after the last dose of treatment, they reported that cisplatin significantly reduced the number of living ova, while the number of dead eggs significantly increased. Furthermore, the number of worms recovered was less compared to the control group [52]. The examination for the tegument of adult male S. mansoni recovered from infected mice showed erosion, necrosis, and severe damage to the tegument surface, abnormal dropped spines from the tegument surface, vacuolization of the subtegumental cells, and disorganization of muscle layers after treatment with cisplatin [52]. Finally, hepatic histological analysis of S. mansoni-infected mice shows that cisplatin treatment decrease granuloma size. In liver function tests, alanine aminotransferase was decreased in infected animals treated with cisplatin compared to their infection control [52].

3.8 Natural products

Recently, the use of phytoestrogens with antiparasitic activity has increased. One of them, genistein, an isoflavone isolated from soybean, exhibits significant metacestodicidal activity in vitro, but also binds to the ER and induces estrogenic effects. Furthermore, synthetic genistein derivatives have shown an improved metacestodicidal effect [53].

Parasitic diseases remain a major public health problem affecting hundreds of millions of people, particularly in tropical developing countries. The limited availability and affordability of pharmaceutical medicines mean that the majority of the world’s population depends on traditional medical remedies, and it is estimated that some 20,000 species of higher plants are used clinically throughout the world [54]. In medieval times, plants with reputed antihelminthic properties were often mixed with mineral salts (arsenic, copper, etc.) or more esoteric materials (blood, feces, fluids from reptiles, wild animals, etc.) to form quite bizarre and often hazardous concoctions – for both parasites and hosts alike. With time, trial, and error, such preparations were refined in an attempt to at least moderate the undesirable consequences to the host, but with the advent of safer and more effective synthetic antihelminthic compounds, they rapidly disappeared from the veterinary antihelminthic market. Nevertheless, it is of interest to note that the WHO has recently estimated that 80% of the population of developing countries rely on traditional medicine, mostly plant drugs, for their primary health care needs. Higher plants represent a rich source of new molecules with pharmacological properties, which are lead compounds for the development of new drugs. During the last decades, the renewed interest in researching natural products has led to the introduction of several important drugs, such as the anticancer drugs vinblastine and taxol or the antimalarial agent artemisinin. Success in natural products research is conditioned by careful plant selection, based on various criteria such as chemotaxonomic data, information from traditional medicine, field observation, or even random collection. One main strategy in the isolation of new lead compounds consists of so-called bioactivity-guided isolation, in which pharmacological or biological assays are used to target the isolation of bioactive compounds. One major drawback of this strategy is the frequent isolation of known metabolites. The tropical fruit Carica papaya and its seeds have proven antihelminthic and anti-amoebic activities [55]. To determine the effectiveness of air-dried C. papaya seeds on human intestinal parasitosis, 60 asymptomatic Nigerian children with stool microscopic evidence of intestinal parasites received immediate doses (20 mL) of either an elixir composed of air-dried C. papaya seeds and honey (CPH) or honey alone (placebo) in two randomized treatment groups. Repeat stool microscopic examinations were conducted 7 days post-intervention for intestinal parasites. Significantly more subjects given CPH elixir than those given honey had their stools cleared of parasites [23 of 30 (76.7%) vs. five of 30 (16.7%); z = 4.40, P = .0000109]. There were no harmful effects. The stool clearance rate for the various types of parasites encountered was between 71.4% and 100% following CPH elixir treatment compared with 0−15.4% with honey. Thus, air-dried C. papaya seeds are efficacious in treating human intestinal parasites and without significant side effects. Their consumption offers a cheap, natural, harmless, readily available monotherapy, and preventive strategy against intestinal parasitosis, especially in tropical communities. Further and large-scale intervention studies to compare C. papaya with standard antiparasitic preparations are desirable [55]. For example, schistosomiasis, a widespread helminthic disease whose treatment is chemotherapy based, the drug of choice being praziquantel. Since resistance to praziquantel has been discovered in the exposed parasites, alternative drugs must be considered. Myrrh is an oleo-gum resin from the stem of the plant Commiphora molmol [56]. This study was performed on 204 patients with schistosomiasis. The drug was administered at a dose of 10 mg/kg of body weight/day for three days, inducing a cure rate of 91.7%. Re-treatment of cases who did not respond with a dose of 10 mg/kg of body weight/day for six days gave a cure rate of 76.5%, increasing the overall cure rate to 98.09%. The drug was well tolerated, and side effects were mild and transient. Twenty cases provided biopsy samples six months after treatment and none of them showed living ova [56]. Other treatments involve hand infusions and decoctions of the leaves, roots, and inflorescences of the herbaceous shrub Chenopodium ambrosioides (American wormseed, goosefoot, epazote, paico); additional related species, indigenous to the New World, have been used for centuries as dietary condiments and as traditional antihelminthics by native peoples in the treatment of intestinal worms [57]. Commercial preparations of Chenopodium oil and its active constituent, ascaridol, obtained by steam distillation, have been and continue to be used with considerable success in mass treatment campaigns. Ethnopharmacological studies in a community of Mayan subsistence farmers in Chiapas, Mexico, confirmed that decoctions containing up to 300 mg of dry plant material (kg/body weight) were widely used and traditionally highly regarded in the treatment of ascariasis. However, therapeutic doses of up to 6000 mg (kg/body weight) of powdered, dried plant material had no significant antihelminthic effect on the adults of Necator, Trichuris, or Ascaris. Gas-liquid chromatographic analyses of plant samples used consistently demonstrated the presence of ascaridol in the expected amounts. Possible origins of subjective belief in the efficacy of C. ambrosioides may be related to the positive association of spontaneous or peristalsis-induced passage of senescent worms immediately following a therapeutic episode [57]. It is also possible that, in the past, varieties of the plant containing much more ascaridol were used. The results of these controlled field studies did not sustain any widely held traditional beliefs nor did they support the value of the therapeutic practices regarding this plant. It is, therefore, essential that all indigenous ethnomedical practices be objectively evaluated for efficacy and safety using the appropriate protocols before being considered for their adaptation or promotion in health care programs [57].

Naphthoquinones are naphthalene-derived compounds that can be found in some plants. These products possess antibacterial, antifungal, antitumoral, and antiparasitic properties. Aranda-López et al., (2021), evaluated in vitro anti-helminth effect of a pure naphthoquinone (naphthoquinone 4a) in a model of murine cysticercosis caused by Taenia crassiceps. Naphthoquinone 4a causes paralysis in the cysticerci membrane from day 3 of the in vitro treatment. Moreover, it induces changes in the shape, size, and appearance of the cysticerci and a decrease in the reproduction rate depending on the duration of the treatment and the concentration of the compound [58]. Wang et al., (2017), evaluated the effect of 1,4 naphthoquinone against Caenorhabditis elegans nematodes and eggs and report that 1,4 naphthoquinone kills more than 50% of nematodes and inhibits more than 50% of eggs hatching at a dose of 50 μg/ml. This effect is mediated by stimulating oxidative stress (increase reactive oxygen production, superoxide dismutase activity, and the heat-shock transcription factor (HSF)-1 pathway). In addition, they showed that the lethality caused by naphthoquinone was related to the Insulin/IGF signaling (IIS) pathway, and the effect on IIS pathway-related genes (age-1, sod-3, mtl-1, ctl-2, daf-12) indicated that 1,4-naphthoquinone could activate this pathway and suppress the expression of DAF-16 target genes [59]. El-Beshbishi et al., (2019), in an in vitro study tested the use of artemisinin-naphthoquinone phosphate combination against Schistosoma haematobium and its vector Bulinus truncates. They report that naphthoquinone treatment at a dose of 1 μg/ml of Schistosoma haematobium worms for 24 hours reduces worm motility, while the dose of 20 μg/ml results in 25–100% mortality of adult flukes within 48–72 hours. Moreover, the incubation of miracidia and cercaria with artemisinin-naphthoquinone phosphate at a concentration of 7.5 μg/ml killed all the free larval stages within 40 and 15 min, respectively. Finally, the exposure of Bulinus truncatus adult snails to 20 ppm of the combined regimen caused a mortality rate of 100% within 24 hours [60]. In an experiment realized by Cha et al., (2019), where they evaluated the nematicidal activity of three naphthoquinones (1,4-naphthoquinone, juglone, and plumbagin) against the pine wood nematode (Bursaphelenchus xylophilus), showed that lethal concentration 50 (LC₅₀) at 48 hours of exposure was 100 ppm for 1,4 naphthoquinone, 57 ppm for juglone, and 104 ppm for plumbagin. In in vivo test, they report that mortality of Bursaphelenchus xylophilus was significantly affected by the presence of the three naphthoquinones at concentrations above 62.5 ppm. In the semi-in vivo assay, the population of inoculated Bursaphelenchus xylophilus was significantly decreased at two weeks after treatment with juglone when compared with the effects of treatment with 1,4-naphthoquinone and plumbagin. The mechanism by which mortality occurs was associated with the generation of reactive oxygen species by naphthoquinones that cause oxidative stress in the parasite [61]. Rufener et al., (2018), tried in vitro and in vivo buparvaquone (a second-generation naphthoquinone with action on hemoprotozoa) against Echinococcus multilocularis. Their results show that buparvaquone has an Inhibitory Concentration 50 (IC₅₀) of 2.87 μM against in vitro cultured Echinococcus multilocularis metacestodes. Moreover, transmission electron microscopy revealed that treatment with buparvaquone impaired parasite mitochondria early on, and additional tests showed that had a reduced activity under anaerobic conditions. Furthermore, buparvaquone show an inhibition effect of the cytochrome bc₁ complex in Echinococcus multilocularis germinal layer cells. On the other hand, in a in vivo experiment using mice with secondary alveolar echinococcosis were treated with buparvaquone (100 mg/kg per dose, three doses per week, four weeks of treatment), the treatment failed to reduce the parasite burden [62].

Acknowledgments

Grant IN-209719 from Programa de Apoyo a Proyectos de Innovación Tecnológica (PAPIIT), Dirección General de Asuntos del Personal Académico (DGAPA), Universidad Nacional Autónoma de México (UNAM). Grant FC2016-2125 from Fronteras en la Ciencia, Consejo Nacional de Ciencia y Tecnología (CONACYT), both to Jorge Morales-Montor. Grant IA-206220 and Víctor H del Río Araiza, from PAPIIT, DGAPA, UNAM. Grant CB-2015-2101 from Ciencia Básica, Consejo Nacional de Ciencia y Tecnología (CONACYT) (number: 255173) and grant Programa de Apoyo a la Investigación Científica y Tecnológica (PAICYT) (number: SA211-15) to Romel Hernández-Bello.

Conflict of interest

The authors declare no conflict of interest.