**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**).

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


#### **Table 1.**

*Antiproliferative compounds with parasiticidal effects upon parasites.*

#### *New Uses for Old Drugs and Their Application in Helminthology DOI: http://dx.doi.org/10.5772/intechopen.106176*

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 [14–16].

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 thymusdependent 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* 
