Compounds from African Medicinal Plants with Activities Against Selected Parasitic Diseases: Schistosomiasis, Trypanosomiasis and Leishmaniasis

Parasitic diseases continue to represent a threat on a global scale, particularly among the poorest countries in the world. This is particularly because of the absence of vaccines, and in some cases, resistance against available drugs, currently being used for their treatment. In this review emphasis is laid on natural products and scaffolds from African medicinal plants (AMPs) for lead drug discovery and possible further development of drugs for the treatment of parasitic diseases. In the discussion, emphasis has been laid on alkaloids, terpenoids, quinones, flavonoids and narrower compound classes of compounds with micromolar range activities against Schistosoma, Trypanosoma and Leishmania species. In each subparagraph, emphasis is laid on the compound subclasses with most promising in vitro and/or in vivo activities of plant extracts and isolated compounds. Suggestions for future drug development from African medicinal plants have also been provided. This review covering 167 references, including 82 compounds, provides information published within two decades (1997–2017).


Introduction
Parasites are considered as organisms that obtain their food by eating other organisms or their products in nature. These diseases continue to be a cause of considerable morbidity and mortality globally [1][2][3][4], including Trypanosomiasis (African sleeping sickness and Chagas disease) [5][6][7], Leishmaniasis [8] and Schistosomiasis [9,10]. They threaten almost one-third of the world's population, the most numerous incidents being recorded in over 100 tropical and developing countries and territories, Fig. 1 [11][12][13]. The African region recorded the most death-related cases, especially amongst infants below the age of 5 and pregnant women. Schistosomiasis, caused by parasites of the Schistosoma genus are responsible for about 200 million sickness cases and about 280,000 death-related incidents annually worldwide [9,10,14]. Only one drug (praziquantel) has been proven to be effective in the treatment of human Schistosomiasis, with no vaccine available or in development so far [15][16][17][18][19][20][21]. Serious concerns about drug selectivity and resistance were raised in 2013 when over 30 million people were treated in Sub-Saharan Africa [20]. Moreover, observed resistance and reduced efficiency of praziquantel in laboratory strains have prompted the search for alternative therapeutic strategies [20][21][22][23][24][25][26][27].
Trypanosomiasis, which represents several diseases caused by parasites of the genus Trypanosoma, is also of interest [5,[27][28][29]. This disease, which is much arguably the most important disease of man and domesticated animals, accounts for over 8 million reported annual cases globally, especially in the tropical regions of Latin America and Africa [30,31]. Besides, great socioeconomic effects on the endemic areas by this disease are forecast if inadequate attention (both at the communal, national, and international levels) is not given [7,29,[32][33][34]. Leishmaniasis is caused by parasites of the Leishmania type, which is also transmitted by certain types of sandflies [35,36]. The diseases are reported by the WHO to be responsible for about 1 million new cases leading to approximately 30,000 deaths annually on a global scale. The major cause is linked to environmental changes and affects mainly the very poor populations [37,38]. These three diseases represent a real burden to the lives of millions of persons and their domesticated animals. The trio is capable of inflicting long-term disability and social stigmatisation, which can ultimately lead to a highly unproductive population and eventually result in economic loss and the slowdown of a country's development.
With the absence of any vaccine targeting any parasites and resistance against the already existing anti-parasitic drugs, research efforts have been employed and encouraged towards the search for new, cheaper, potent and effective drugs to treat these diseases. Medicinal plants represent a potential source of new drugs. This is because natural products (NPs) from organisms such as animals, fungi and the higher plants have been known to be good sources of pharmacologically active compounds against several ailments, including parasitic infections. Moreover, NPs are believed to have significant advantages as lead molecules over synthetic molecules [39][40][41][42][43][44][45][46][47][48][49]. The criteria for choosing a particular natural product for studies are either based on the pre-existing knowledge on the traditional use of the source species in therapy (ethnobotanical knowledge) or the search for structurally related molecules with known pharmacologically active agents from chemical databases [49][50][51][52][53][54]. The African continent is highly diverse ethnobotanically. This might explain why about 80% of the population tends to rely on medicinal plants as a primary source of healthcare [55][56][57][58][59][60][61][62][63][64][65][66][67]. It is our goal to provide evidence of the efficacy and potency of plants used in traditional medicine against parasitic infections. The systematic documentation of the plant-based chemical constituents of African traditional medicine and attempting to using in silico procedures to investigate their modes of action are ongoing efforts [44-46, 52, 53], particularly on the isolated compounds from African medicinal plants (AMPs) with evaluated in vitro and/or in vivo activities against Trypanosomiasis [68][69][70][71][72][73][74], Schistosomiasis, Leishmaniasis [72][73][74] and other parasitic diseases [4]. However, the most recent review dates about 3 years back and was focused only on plants collected from Nigeria. Thus, an updated review that covers the entire continent for these three parasitic diseases is required now. The information presented herein was retrieved by searching literature from major international journals on natural products and medicinal chemistry, alongside available M.Sc. and Ph.D. theses and online databases [54,75]. The information gathered is discussed under the main compound classes, as presented below and summarised in Tables 1, 2, and 3.

Naphthylisoquinolines
The leaves, stem bark and roots of Ancistrocladus sp.

Quinolines
Other trypanocidal alkaloids include the quinolines (Fig. 4); waltheriones E-L (19-26), 8-deoxoantidesmone (27) and antidesmone (28) from Waltheria indica (Malvaceae) [93]. This plant is used in traditional medicine for the treatment of several ailments, including malaria [63,[100][101][102][103][104]. The dichloromethane root extract showed activities against T. cruzi (IC 50 = 0.74 lg/mL), T. b. brucei (2.3% survival at 20 lg/mL) and T. b. rhodesiense (IC 50-= 17.4 lg/mL) [93]. With the exception of waltherione L (26), with a slightly higher IC 50 (3.1 lM), the isolated compounds all displayed potent growth inhibition toward the amastigote form of T. cruzi (the Tulahuen C4 strain), with IC 50 values lower than that of the reference drug benznidazole (IC 50 = 2.9 lM). Structure-activity relationships (SARs) provide suggestions that, a methoxy group, bound to the nitrogen atom is important for activity (e.g., as in compounds 22, 24 and 25). This group at this position increased the lethality of T. cruzi. Furthermore, the absolute configuration (5R) (as in compounds 23, 26, 27) seems to result in a decrease of activity, while the presence of an N-oxide function (as in compound 26) is detrimental for T. cruzi inhibitory activity (Fig. 5). Finally, a comparison of the IC 50 values of the isolated compounds against T. brucei sp. and T. cruzi highlighted selective toxicity towards the latter. This suggests that these molecules (or the waltherione scaffold) is a potential starting point for new safe antitrypanocidal drug development, although antidesmone (28) has already been patented for its potential as an antiprotozoal drug since 2003 [93,105,106].

Triterpenoids
The anti-schistosomal activity of Asparagalin A (53, Fig. 10 (39) Acetyl tigloyl 3,7-dihydroxy-5-tigloyloxycarvotacetone (40) H tigloyl 3-acetoxy-5,7-dihydroxycarvotacetone (41) A c e t y l H Fig. 7 Carvotacetones with potent antileishmanial activities 3β,13β-dihydroxy-urs-11-en-28-oic acid (47  stipularis (Asparagaceae) has been evaluated [128]. It was found that this compound was able to significantly reduce the ability of adult female worms to lay eggs. It was further shown that the compound had some suppressive effect on egg-laying capacity in a dose-dependence manner [137]. Elaeodendron schlechteranum (Celastraceae) is the source of tingenin B or 22b-hydroxytingenone (54) [129]. This compound has displayed a broad range of activities, e.g., against T. cruzi (IC 50 \ 0.57 lM), T. brucei (\ 0.57 lM), L. infantum (1.67 lM), and P. falciparum (0.83 lM), confirming the claim of the applicability of the plant in traditional medicine to treat various non-infectious diseases [63,138]. Albeit, being highly cytotoxic to MRC-5 cells (CC 50 0.45 lg/mL), compound 54 indicates a poor selectivity to normal cells. Further studies on this compound could be considered in order to suggest less toxic and more selective analogues for the development of novel antiparasitics. The bisnortriterpenes from Salacia madagascariensis (Celastraceae); isoiguesterin (55) and 20-epi-isoiguesterinol (56) showed potent activities against Leishmania sp. [130]. Meanwhile, isoiguesterin (55) and 20-epi-isoiguesterinol (56) displayed comparable activities with chloroquine and artemisinin against the D6 clone, being more potent and selective against L. donovani (a species known to cause visceral Leishmaniasis). When compared with amphotericin B, used currently in the treatment of Leishmaniasis, compounds 55 and 56 show great potential for future selective drug development against Leishmania. Keetia leucantha (synonym: Plectronia leucantha Krause) is a West African tree of the Rubiaceae, used to treat a variety of infections, including parasitic infections [139,140]. Ursolic acid (57) and oleanolic acid (58), along with other constituents were isolated from the leaves of this plant. An investigation of the antitrypanosomal activities of essential oil, the dichloromethane extract and isolated compounds on T. b. brucei bloodstream forms (Tbb BSF) and procyclic forms (Tbb PF) [131] showed that ursolic acid (57) and oleanolic acid (58) were the most bioactive tested compounds [131]. Ursolic acid displayed IC 50 values of 5.48 and 14.25 lM, respectively, on Tbb BSF and Tbb PF, while oleanolic acid displayed an IC 50 value of 16.00 lM on Tbb BSF. This could explain why the plant is effective in the traditional treatment of related parasitic ailments. Another identified triterpenoid was polycarpol or lanosta-7,9(11),24-trien-3b,15a-diol (59) from Piptostigma preussi (Annonaceae) [132]. The compound showed antitrypanosomal activity with an ED 50 value of 5.11 lM on T. brucei cells. An investigation of its mode of action showed that the compound acted by inhibiting T. brucei  Fig. 10 Triterpenoids with antiparasitic activities glycolytic enzymes GAPDH and PFK (glycolytic pathway enzymes validated by WHO as good targets for the development of drugs against trypanosomiasis), with IC 50 values of 650 and 180 lM, respectively. The glycolytic enzymes GAPDH are responsible for ATP production and have been reported to be vital for the survival of Trypanosomatids [141,142]. From the stem bark of Vernonia guineensis (Asteraceae), vernoguinosterol (60) and vernoguinoside (61), exhibited interesting trypanocidal activity with IC 50 values in the range 4.60-7.67 lM [133].

Other Compound Classes
Other compound classes from AMP with reported activities on Leishmaniasis and Trypanosomiasis are shown in Figs. 11, 12, 13, 14, and 15, while a summary of the reported molecules is given in Table 3 (compounds 62-82).

Phytosterols
22-Hydroxyclerosterol (81) and clerosterol (82) , Fig. 15, were isolated from the stem bark of Allexis cauliflora (Violaceae) [155]. These compounds were evaluated for trypanocidal activities, and the activity of compound 81 (ED 50 = 1.12 lM) was far better than that of compound 82 (ED 50 = 134.34 lM). These results prompted an investigation of their cytotoxic activities. It was observed that compound 81 inhibited mammalian cells at quite a similar concentration (ED 50 = 1.56 lM), while compound 82 had no effect. This difference in activity could be attributed to the presence of the hydroxyl group at C-22 in the side chain of compound 81 which is absent in compound 82. Additionally, it was observed that compound 81 was more active and selective on the parasite enzyme glycolytic enzymes (PGI and GAPDH), when compared with compound 82.

Conclusions
Parasitic diseases continue to represent a menace on a global scale and require attention due to lack of vaccines and reported resistance against available drugs for their treatment. This review focuses on different natural compounds and scaffolds that could lead drug discovery research groups into reasonable starting points for further development of fast, effective and affordable novel molecules for the treatment of parasitic diseases. Drug discovery and development now place efforts on the search for new moieties or chemical scaffolds of natural/semisynthetic origin and in the development of phytomedicines. As a means to facilitate accessibility of information, our research team has as one of its goals, to develop free online natural products libraries from African flora (http://africancompounds.org/). In this paper, an attempt has been made to draw together original research works on natural products from AMP with micromolar range activities against Schistosoma, Trypanosoma and Leishmania species. The compounds presented herein have demonstrated a diverse range of activities against different forms of Trypanosomiasis, Schistosomiasis and Leishmaniasis, with some scaffolds and molecules showing great potential as starting points for further development into drugs. We recently collected a dataset of several hundred bioactive plant based metabolites from AMPs with activities against Trypanosoma sp. (Afrotryp) [68]. It becomes interesting to  perform in silico prediction of binding modes and binding free energy calculations of some of the compounds against some selected targets.