The anti‐Trypanosoma activities of medicinal plants: A systematic review of the literature

Abstract Background The existing drug treatments for trypanosomiases are limited and suffer from shortcomings due to their toxicity and the emergence of resistant parasites. Developing anti‐trypanosomal compounds based on natural products is a promising way of fighting trypanosomiases. Objectives This study aims to identify through scientific review a large variety of medicinal plants (anti‐trypanosomal) used worldwide and scientifically shown to display anti‐trypanosomal effects. Methods To collect data, the anti‐trypanosomal activities of Africa, Asia, the Middle East, South America, North America, Europe and Oceania medicinal plants have been checked by considering the published paper. Results Based on collected data, 77 natural molecules were reported in the literature. Of which 59 were from the African region, 11 from Asia, 3 from Europe and 4 from Latin America. These active components belong to alkaloids, triterpenoids, lactone, quinoids, flavonoids, iridoids, lignans, steroids, lipids, oxygenated heterocycles, benzenoids, proteins, coumarins, phenylpropanoids and peptides. We also specified the prosperous plants with unique anti‐trypanosomal activities. Conclusions However, there is a need for further studies on the ability of the isolated compounds to ameliorate the trypanosome‐induced pathological alterations and also the elucidation of their modes of actions and activities against other trypanosome species.

as sleeping sickness) and Chagas disease, caused by Trypanosoma brucei and Trypanosoma cruzi. According to the World Health Organization (PAHO, 2016) data, T. cruzi infects about 5-6 million people worldwide and causes approximately 10,000 deaths per year (WHO, 2015). For HAT, its incidence is now at a historic low, with fewer than 1000 cases reported in 2018 (WHO, 2018).
A small number of trypanocidal drugs have shown efficacy against the two species of parasites. These include two approved drugs that can treat Chagas Disease during its acute phase (Benznidazole and Nifurtimox) (Sepúlveda-Robles et al., 2019). The recommended drugs to treat the HAT include suramin (EC: 205-658-4), pentamidine (EC: 205-424-1), melarsoprol  and Fexinidazole Winthrop (Dickie et al., 2020). Fexinidazole is a DNA synthesis inhibitor for the Neglected Diseases initiative (DNDi) for the oral treatment of HAT and Chagas' disease, which shows activity against Trypanosoma brucei gambiense and T. b. rhodesiense as well as preceeds through Phase II clinical trial based on FDA definition (Deeks, 2019). The other drugs, including Nifurtimox, are in Phase III clinical trials, and Eflornithine  has not yet entered into clinical trial stages. However, the first three drugs have limitations, including poor efficacy, potential adverse effects and the development of resistance by the parasites (Wilkinson and Kelly, 2009). Oral fexinidazole is a valuable first-line treatment option in the early stages of (stage 1 or early stage 2) African Trypanosoma brucei gambiense (Kande Betu Ku Mesu et al., 2021). Eflornithine is a standard treatment for second-stage therapy, and nifurtimox-eflornithine combination therapy is a proper combination for first-line use in HAT control programs (Priotto et al., 2009).
Additionally, DNDi has developed another oral therapy, acoziborole, suitable for the treatment of both stage 1 and stage 2 disease in a single dose (Dickie et al., 2020).
Also, the neglected disease status means a little economic benefit for developing novel drugs in this field (Dickie et al., 2020). There is little interest in developing drugs against these diseases because they are neglected. However, they are called 'neglected diseases' because pharmaceutical companies have little interest in investing in them, as fexinidazole has recently met that need for T. brucei gambiense (Kande Betu Ku Mesu et al., 2021). However, melarsoprol is very toxic and is still being used against T. brucei rhodesiense (Fairlamb and Horn, 2018), and resistance may still arise against fexinidazole, so new lead compounds for drugs against these parasites remain essential. Thus, there has been a considerable need to find new trypanocidal agents with better efficacy and safety profiles.
Natural products are valuable sources for discovering and developing effective medicines against various diseases Newman and Cragg, 2016;Nezaratizade et al., 2021;Tajbakhsh et al., 2021a;Tajbakhsh et al., 2021b). The WHO report highlighted that a quarter of currently useful drugs had been derived from traditional plants. For many parts of the world, especially where trypanosomiases are prevalent in Africa, India, China, the Middle East and South Asia, traditional medicines with local preparations are the predominant means of therapy (Ahmad Khan and Ahmad, 2019). These countries are also endowed with tremendous medicinal plant resources, some of which have shown efficacy under in vitro and/or in vivo conditions. At present, the available reviews in this field report anti-trypanosomal activity for particular regions, such as the African region (Ibrahim et al., 2014;Lawal et al., 2015), Myanmar (Asia) (Bawm, 2010) and Saudi Arabia . These exciting but somehow dated but interesting publications reported a lot of medicinal plants and some isolated active compounds. Finally, the current, up-to-date review covers natural products isolated from plants used worldwide and active against trypanosomiases.

Ethnopharmacology of anti-trypanosomal medicinal plants in Africa continent
Since the primitive period, herbs have been a valuable source of medication for both human and livestock diseases (Odhiambo et al., 2011).
During these thousand years of observation, it has been found that different parts of herbs possess healing properties. With the advancement in pharmaceutical and medical sciences, phytoconstituents were subsequently confirmed to be accountable for the curative characteristics of plants. Nowadays, high-tech methods have resulted in the isolation and elucidation of these phytoconstituents. Some of these phytoconstituents have served as lead compounds to develop chemotherapeutic drugs against diseases, whether infectious or noninfectious (Kasilo et al., 2010).
On the one hand, some modern drugs have their ethnopharmacological sources. Nevertheless, despite technological advances, the discovery of new drugs faces a primary innovation deficit that unfavourably impacts the pharmaceutical industry. On the other hand, current studies demonstrate that entry barriers have decreased for introducing a new drug (DiMasi & Paquette, 2004;Patwardhan, 2005). Seventy-five per cent of the approved anti-infectious disease drugs from 1981 to 2002 are natural origins (Newman et al., 2003), while 61% of all new chemical compounds presented as drugs during the same period could be considered natural products (Gupta et al., 2005).
Aside from this significant role of medicinal herbs in drug discovery, the use of local herbal products provides the only option for therapeutic purposes for African populations. The primary reason for this issue is the lack of a sound health care system in some parts of the continent, which causes the population's vulnerability to many infectious diseases (Elujoba et al., 2005). Eighty per cent of the African population depends almost entirely on herbal medicinal products for their primitive health care needs (Kasilo et al., 2010).
One of the significant infections that severely affect humans and animals in Africa is African trypanosomiasis, also called 'sleeping sickness' in humans or 'Nagana' in animals. (Atawodi 2005;Welburn et al., 2009). It is one of the most neglected parasitic diseases that affect human health and dramatically reduces Africa's livestock productivity (Atawodi 2005;Welburn et al., 2009). Preliminary estimates show that almost 70 million people distributed over 1.55 million km 2 in Africa are at risk of this infectious disease (Simarro et al., 2012). In addition, animal trypanosomiases, or Nagana, are distributed over nearly 25 million km 2 in Africa, where livestock productivity fell by 50%. The important species in this disease include Trypanosoma vivax, Trypanosoma congolense, Trypanosoma evansi and Trypanosoma brucei (Mbaya et al., 2009). Currently, the African trypanosomiases chemotherapy remains abandoned due to the available approved drugs with some concerns, including parasite resistance, toxicity, poor availability, high cost and parenteral root of administration (Ibrahim et al., 2014). Fortunately, the continent has vast resources of medicinal plants that are traditionally used to cure this disease. This is evident in the tendency to use ethnobotanical science to manage disease in different parts of Africa (Atawodi et al., 2002;Ntie-Kang et al., 2013). It is important to note that studies have confirmed the impact of these African herbal remedies as antitrypanosomal agents under in vitro and/or in vivo models. Hence, a critical review of these studies (anti-trypanosomal) African medicinal plants in the African continent and anti-trypanosomal plants in other continents is required to provide a comprehensive record to specify gaps in knowledge about the basic strategies to address such gaps.

Search strategy
Literature about medicinal plants (with anti-trypanosomal activity) was collected online from published articles using the keywords: 'Trypanosoma AND medicinal plant' , 'Trypanosoma AND natural product' from 1960 to May 2020. These keywords were entered into the primary scientific databases, such as PubMed, Science Direct, Scopus and Google scholar. The articles obtained were included based on the reliability of their source. Some articles were found by examining the bibliography of other publications or by directly accessing the webpage of the journal.

Data extraction
The information such as the species and family of the plant, the type of extraction, the active compound(s) if isolated, the strain of

Ethical approval statement
An ethics statement is not applicable because this study is based exclusively on published literature.
We explained 264 and 215 plants, respectively, which were assessed for anti-trypanosomal activity. Due to the high amount of data available for African region anti-trypanosomal plants, only the plants with the minimum inhibitory concentration of the bioactive compound were scrutinised (Table 1 and Figure 3).

Asia plants
Asia plants assessed for anti-trypanosomal activity with EC50 values for inhibition of parasites and cytotoxicity are shown in Table 2. 2) and β-amyrin palmitate (EC50 = 60.8) (Bawm, 2010). The extracts were mostly evaluated on Trypanosoma evansi due to its prevalence in Asia (Dyary et al., 2014). Considering the potency criteria asserted by Pink et al. (2005), it was expressed that the isolated compounds with an EC50> 20 μg/ml were not considered effective drugs. Thus, the seven isolated compounds may not be considered lead drugs. There is a need to pursue investigations that isolate more   Figure 5).    According to the standards of the National Cancer Institute (NCI), a crude extract can be considered active for an EC50 ≤ 20 μg/ml (Cordell et al., 1993). Hence, most plant extracts (more than 50%) showed activity below 20 μg/ml. We highlighted the plant extracts that have the most activity below 1 μg/ml, which include Kanahia laniflora, Arctium Artemisinin is an endoperoxide sesquiterpene lactone isolated from Artemisia annua, one of the well-known antiparasitic and antitumoural chemotherapeutic agents (Rocha et al., 2005). brucei brucei and T. cruzi (Loo et al., 2017).

Critical assessment of the literature information embodied in the present study
Africa, Asia and the Middle East flore provide many promising plants, but further in vivo studies are required to confirm their application as anti-trypanosomal agents. It is worth noting that before in vivo studies, the in vitro biological activity should be accompanied by cytotoxicity studies against mammalian cells, followed by pharmacokinetic studies.
On the other hand, some literature was not entered into this systematic review based on mesh terms. In West Africa and South America, Trypanosoma vivax is at the helm of the majority of trypanosome infections in cattle and other ruminants. This pathogen is not well established in laboratory animals, and investigation into pathogenic isolates has been restricted by the difficulty of its in vitro establishment. In this study, very few compounds were screened against Trypanosoma vivax (Isoun andIsoun, 1974' Cortez et al., 2006).