Anti-trypanosomal activity of nigerian plants and their constituents.

African trypanosomiasis is a vector-borne parasitic disease causing serious risks to the lives of about 60 million people and 48 million cattle globally. Nigerian medicinal plants are known to contain a large variety of chemical structures and some of the plant extracts have been screened for antitrypanosomal activity, in the search for potential new drugs against the illness. We surveyed the literatures on plants and plant-derived products with antitrypanosomal activity from Nigerian flora published from 1990 to 2014. About 90 plants were identified, with 54 compounds as potential active agents and presented by plant families in alphabetical order. This review indicates that the Nigerian flora may be suitable as a starting point in searching for new and more efficient trypanocidal molecules.

1. Introduction

African trypanosomiasis is a parasitic disease caused by a protozoan of the genus Trypanosoma. Trypanosoma vivax (T. vivax), Trypanosoma congolense (T. congolense) and to a lesser extent Trypanosoma brucei brucei (T. b. brucei) are the main species responsible for African animal trypanosomosis (AAT) called nagana in West Africa while T. b. rhodesiense and T. b. gambiense cause sleeping sickness (human African trypanosomiasis, HAT). Surra and Dourine are caused by the other trypanosome species T. evansi and T. equiperdum respectively. The disease is transmitted by a bite of the vector—tsetse fly (Glossina species) [1].

In Nigeria, trypanosomiasis seems to be re-emerging as an important livestock disease, assuming major clinical importance in small ruminants and extending to previously designated tsetse-free zones [2,3]. Apart from the old Gboko endemic focus remaining active, there have been reports of the disease outbreak in many other communities in Nigeria [4,5].The prevalence rate in different breed of animals in Nigeria for the past few years have been studied and ranged from 8.4% to 15.53% [6,7].

In Africa, the annual loss in livestock production and mixed agriculture alone due to the disease is valued at 5 billion US dollars. In 1995, WHO Expert Committee estimated that 60 million people were at risk with an estimated 300,000 new cases per year in Africa, with fewer than 30,000 cases diagnosed and treated. In 2004, the number of new reported cases fell to 17,616 and WHO considered in that due to increased control, estimated cumulative rate to be between 50,000 and 70,000 cases. In 2009, the number of new cases reported dropped below 10,000 (9878) for the first time in 50 years and the estimated number of actual cases is currently 30,000. This trend has been maintained in 2012, with 7216 cases reported [8,9,10].

The current chemotherapy of HAT relies on only six drugs (suramin, pentamidine, melarsoprol, eflorinithine, arsobal and mel B), five of which were developed more than 30 years ago. Others such as homidium, isometamidium and diminazene aceturate are used in animal infections. Each of these drugs has one or more of these challenges: expensive, highly toxic, need parenteral administration and parasites increasing resistance. However, tireless effort being made by WHO, private partners and local governments to eliminate HAT is yielding significant success. The Drugs for Neglected Diseases initiative (DNDi) is developing fexinidazole to a new oral drug for HAT with a good chance of success. It has entered Phase II/III clinical study in patients with late-stage sleeping sickness. It is hoped that fexinidazole would solve the problems and limitations of current chemotherapeutic options [11,12,13].

Several reviews on medicinal plants used in treatment of trypanosomiasis have been published [14,15,16,17,18,19,20,21]. It is estimated that 66%–85% of the World’s population depends directly on plants as medicine and search for drugs derived from plants has accelerated in recent years [22,23,24,25,26,27,28,29,30]. Nigeria, located in West Africa on the Gulf of Guinea, has a rich biodiversity. There are many reports documenting the potentials of medicinal plants in Nigeria against several diseases except trypanosomiasis [31,32]. This paper documents works on Nigerian medicinal plants and derived products as source of trypanocidal agents which could be further investigated for the development of better drug molecules for the disease. We present the plants which are 84 by their plant families (40) in alphabetical order.

2. Acanthaceae, Amaryllidaceae, Anacardiaceae, Annonaceae, Apocynaceae, Araceae, Asclepiadaceae, Asteraceae and Burseraceae

Plants produce a great diversity of substances that could be active in many fields of medicine. Natural products from plant are proven template for new drug development [33]. The plants in these families are summarized in Table 1 and the compounds isolated from them are shown in Figure 1. Peristrophe bicalyculata (Acanthaceae) is found almost throughout India, Afghanistan and Africa. The herb is used against tuberculosis, snake poison, in bone fracture, sprain, fever, cold, and cough treatments [34]. 50 mg/kg of Cold water whole plant extract of P. bicalyculata immobilized 90% of T. b. brucei in vitro after one hour of incubation, while the methanol extract of the plant showed a dose-dependent suppressive property in mice infected with T. evansi [35,36]. Nok and Williams described that the extract obtained from Allium sativum (Amaryllidaceae) completely eliminated trypanosomes in mice on administering 120 mg/kg live weight at 4 days post-treatment. A. sativum is thought to have caused cell death in trypanosomes by inhibiting the synthesis of membrane lipids of the cell [37,38]. The aqueous methanol root extracts of Lannea kerstingii and Mangifera indica from Anacardiaceae and petroleum ether root extract of Annona senegalensis (Annonaceae) at 4 mg/mL, stopped motility of T. brucei in vitro within an hour of incubation [39]. In another study, Adeiza et al. tested in vitro trypanocidal activity of A. senegalensis and found that the crude extract immobilized T. evansi at 10 mg/mL [40,41]. A fraction obtained from an aqueous leaf extract of Holarrhena africana (Apocynaceae) completely cleared T. b. rhodesiense at a dose of 40 mg/kg bw i.p. in infected mice for 5 days post treatment [42]. Hexane and methanol extracts of Spondias mombim (Anacardiaceae) root yielded Compounds 1 and 2 respectively and compounds 3 and 4 were obtained from the ethyl acetate seed extract of Monodora myristica (Annonaceae). Compounds 1, 3 and 4 were active in vitro against T. b. brucei with minimum inhibition concentration (MIC—μg/mL) of 25, 12.5 and 25 respectively [43]. The leaf extract of Lannea welwistchii showed trypanocidal activity (MIC = 6.3 mg/mL) against T. b. brucei [44]. Haematostaphis barteri is used by traditional medical practitioners in the north-eastern Nigeria to treat and manage trypanosomiasis [45]. Using short assay duration of 30 min, 0.5 mg/mL of H. barteri aqueous extract immobilize T. b. brucei and T. congolense [46].

Table 1

Plants from Nigeria with activity against African trypanosomes.

Family	Species	Traditional Uses	Plant Part	Ref.
Acanthaceae	Peristrophe bicalyculata	skin diseases, antidote for snake poison, diabetes	WP	[34]
Amaryllidaceae	Allium sativum	diabetes, tetanus, swellings	WP	[37]
Anacardiaceae	Lannea kerstingii	diarrhoea, cancer	R	[39]
	Mangifera indica	clearing digestion and acidity due to pitta	R	[39]
	Spondias mombim	used as febrifuge and diuretic	R	[43]
	Lannea welwistchii	diarrhoea, dysentery, dropsy	L	[44]
	Haematostaphis barteri	trypanosomiasis	SB	[45]
Annonaceae	Monodora myristica	stomachic, headaches, sores	SD	[43]
	Anonna senegalensis	food and pneumonia	L, R, SB	[40]
Apocynaceae	Carissa spinarum	analgesic	R	[47]
	Adenium obesum	arrow poison for hunting	R	[39]
	Carrisa edulis	rheumatism, stomach disorder	R, B, L	[49]
	Holarrhena africana	dysentery, diarrhoea, snakebite, infertility, malarial, diabetics	L	[42]
	Saba florida	rheumatism, antidote against vomiting, diarrhoea and food poison	L	[50]
Araceae	Anchomanes difformis	diabetes, diarrhoea	R	[48]
Asclepiadaceae	Gongronema latifolium	diabetes, high blood pressure	L, SB	[52]
Asteraceae	Tridax procumbens	inflammatory, microbial and protozoal diseases	WP	[53]
	Artemisia maritima	worm, stomachic infusion	WP	[57]
Burseraceae	Boswellia dalzielii	wound healing, diarrhea, syphilis, induce vomiting	L, SB, Re	[59]

R = root, RB = root bark, SB = stem bark, L = leaves, WP = whole plant, Re = rhizome, B = bark.

Figure 1

Chemical structures of compounds 1–8.

Carissa spinarum, also known as the conkerberry or bush plum, is a large shrub that belongs to the Apocynaceae. Its ethanol root extract has been shown to have in vivo activity against T. b. brucei at ≥100 mg/kg body weight in infected mice [47]. Using a one hour exposure time, methanol extracts of Adenium obesum stem bark (Apocynaceae) and Anchomanes difformis rhizome (Araceae) stopped 50% of the motility of T. b. brucei in vitro at 4 mg/mL [39,48]. In vivo trypanocidal activity of Carrisa edulis (Apocynaceae) against T. congolence infection in rats was investigated using a methanol root extract. Oral treatment at different doses did not significantly clear the parasitemia, however, animals treated with 100 mg/kg/day survived longer than those treated with 200 mg/kg/day and the infected control group [49]. Saba florida (Apocynaceae) is traditionally eaten as an antidote against vomiting, diarrhoea and food poisoning [50]. S. florida aqueous methanol leaf extract (400 mg/kg) exhibited in vivo activity by clearing T. b. brucei in infected rats after 7 days [51]. Gongronema latifolium (Asclepiadaceae) has been reported to stop motility of T. congolense after about 10 min of in vitro treatment using 400 mg/kg of the whole plant’s methanol extract [52]. The ethyl acetate and methanol extracts of Tridax procumbens (Asteraceae), in contrast to extracts obtained with other solvents, were trypanocidal towards T. b. brucei at 200 mg/kg [53]. Further investigation of the ethyl acetate extract of T. procumbens led to the isolation of four flavonoids; 3-hydroxyflavone (5), quercetin (6), 7,8-dihydroxyflavone (7) and catechin (8). Compounds 6 and 7 were described to exhibit trypanocidal activity in vitro and in vivo as pure compounds without affecting normal human cell [54,55,56]. In vitro studies showed T. b. brucei was immobilized by 0.4 mg/mL of Artemisia maritime (Asteraceae) chloroform and petroleum ether extracts using short assay duration of less than one hour [57]. Boswellia dalzielii (Burseraceae), a tree of the Savannah forest of Nigeria, is used for the treatment of wound, diarrhoea, syphilis and to induce vomiting [58,59]. Freiburghaus et al. found that the trypanocidal activity of B. dalzielii varies according to extraction medium and part of plant used. Based on this, Atawodi et al. tested activities of extracts of different parts of B. dalzielii against T. b. brucei. His results revealed that methanol leaf, stem and root bark extracts of the plant at 10 mg/mL significantly immobilized the trypanosome [60,61].

3. Capparaceae, Celastraceae, Clusiaceae, Combretaceae, Cucurbitaceae, Ebenaceae and Euphorbiaceae

The plants in the families listed above are summarized in Table 2 and potential active isolates are represented in Figure 2. Leaves of Crateva adansonii (Capparaceae) are used to treat ear infections while its root is employed to treat syphilis, jaundice and yellow fevers [62]. The ethyl acetate and hexane crude extract of the plant demonstrated moderate in vitro activity (MIC 12.5 μg/mL) against T. b. brucei [63]. Two phytoconstituents (oleanolic acid (9) and 4-epi-hederagenin (10)), which were not tested for activity, had also been isolated from C. adansonii and could account for the activity of the plant. The seed extract of Bucholzia coriacea (Capparaceae) is locally used in treatment of feverish conditions in Eastern Nigeria [64]. T. b. brucei was cleared in infected mice after administering 1000 mg/kg of aqueous and methanol seed extracts of B. coriacea i.p. for five consecutive days [65,66]. MIC of 0.625 μg/mL has been reported for both compounds 11 and 12 isolated from the hexane-ethyl acetate fraction of Maytenus laevis (Celastraceae) root [44,67]. The highest in vivo trypanocidal activity of Garcinia kola (Clusiaceae) seeds was observed in the alkaloid fraction which brought about 92.25% reduction in parasitaemia at 100 mg/kg in T. b. brucei infected rats. The antitrypanosomal property of the alkaloids from G. kola has been suggested to be due to DNA intercalation in combination with protein biosynthesis inhibition [68].

Table 2

Plants from Nigeria with activity against African trypanosomes.

Family	Species	Traditional Uses	Plant Part	Ref.
Capparaceae	Crateva adansonii	stomach troubles, syphilis, jaundice and yellow fevers	L	[62]
	Buchholzia coriacea	feverish, malaria	SD	[64]
Celastraceae	Maytenus laevis	anti-inflammatory, analgesic	R	[67]
Clusiaceae	Garcinia kola	purgative, antiparasitic, antimicrobial	SD	[68]
Combretaceae	Anogeissus leiocarpus	trypanosomiasis, babesiosis	R, SB	[69]
	Terminalia avicennioides	cancer, fungal, bacterial infections	B	[70]
	Terminalia superba	furniture making and musical instrument	B	[44]
Cucurbitaceae	Mormordica balsamina	used to treat wound	WP	[48]
Ebenaceae	Diospyros mespiliformis	styptic to staunch bleeding, leprosy	L	[39]
Euphorbiaceae	Euphorbia poisonii	latex used as pesticides	B	[43]
	Alchornea cordifolia	eye treatment, venereal diseases	S	[43]

R = root, RB = root bark, SB = stem bark, L = leaves, S = stem, WP = whole plant, Re = rhizome, B = bark, SD = seed.

Figure 2

Chemical structures of compounds 9 to 15.

Anogeissus leiocarpus (Combretaceae) is a tree widely distributed in northern Nigeria. The aqueous methanol bark extract of A. leiocarpus had the highest in vitro antitrypanosomal activity out of all the other parts of the plant. 200 mg/kg of the extract made T. b. brucei immotile after 10 min incubation. Furthermore, the extract (200 mg/kg) was analyzed in vivo using T. b. brucei infected rats. Although it did not clear parasitemia in experimental rats after seven days, the rats survived longer than the infected control group [69]. The hexane-ethyl acetate extract of Terminalia avicennioides (Combretaceae) bark inhibited T. b. brucei activity in vitro with MIC of 2.5 μg/mL [43,70,71]. Compound 13 was isolated from T. avicennioides but did not show activity. Methanol extracts of Terminalia superba (Combretaceae) root and stem were effective with MIC value of 3.1 mg/mL each against T. b. brucei in vitro [44]. Using a short assay duration of about an hour revealed that 10 mg/mL of both Momordica balsamina (Cucurbitaceae) and Diospyros Mespiliformis (Ebenaceae) methanol extract drastically reduced motility of T. b. brucei [39,48]. Two plants of Euphorbiaceae family whose crude extracts showed antitrypanosomal activity, yielded two active compounds; compound 14 (Euphorbia poisonii) and compounds 15 (Alchornea cordifolia). Compounds 14 and 15 had activity with (MIC of 1.56 μg/mL and < 0.2 μg/mL respectively) against T. b. brucei [43].

4. Fabaceae, Fagaceae, Hymenocardiaceae, Lamiaceae, Lauraceae, Loganiaceae, Lythraceae, Malvaceae and Melastomataceae

The compounds isolated from plants of the families listed above are shown in Figure 3 and summarized in Table 3. Northern and South-Western Nigerians use Acacia nilotica (Fabaceae) to treat dysentery, tuberculosis and diabetes. Methanol extract (400 mg/kg) of A. nilotica stem bark cleared T. b. brucei in infected mice within eight days [72,73]. In vitro activities of petroleum ether extract of Afzelia africana leaves, aqueous extract of Parkia clappertoniana root, aqueous extractof Piliostigma reticulatum leaves, chloroform extract of Prosopis africana stem bark, methanol extract of Afrormosia laxiflora leaves, chloroform extract of Erythrophleum suaveolus stem bark, methanol extract of Lonchocarpus laxiflorus stem bark and chloroform extract of Swartzia madagascariensis root (all from the Fabaceae family) against Trypanosoma species have been described using short assay durations of less than an hour [39,48,74,75,76]. Antia et al. further demonstrated that methanol extract of A. africana leaves and stems were active against T. b. brucei in vitro with MLC (mg/mL) values of 3.1 and 12.5, respectively [44]. Senna occidentalis (Fabaceae) is a weed distributed throughout the tropical and subtropical regions of the World. It has been reported as a remedy for bacterial and malaria infections [77,78,79,80,81]. The ethanol extract of S. occidentalis leaf at 6.66 mg/mL concentration, eliminated T. b. brucei in vivo in 10 min in infected rats. Acute anaemia recorded in the T. b. brucei infected rat is a consistent feature of Trypanosoma infection and the treatment with the extract was able to significantly (p < 0.05) ameliorate the disease-induced anaemia. Methanol extract of Quercus borealis (Fagaceae) leaves exhibited a significant trypanocidal activity in vitro towards T. evansi by reducing the average mean trypanosomes counts from initial concentration (40.00 ± 0.00) at 250 μg/mL and completely killing of trypanosomes at 9 h of incubation at the same concentration [82]. Denise and Barret proposed that constituents of Q. borealis exhibited trypanocidal action by intercalation with DNA of the parasite, blockage of glycolysis pathway and interference with flagella [83].

Figure 3

Chemical structures of compounds 16 to 21.

Table 3

Plants from Nigeria with activity against African trypanosomes.

Family	Species	Traditional Uses	Plant Part	Ref.
Fabaceae	Acacia nilotica	used to treat cancers and/or tumours of ear, eye	SB	[72,73]
	Afzelia Africana	trypanosomiasis, convulsion, hernia	WP	[74]
	Parkia clappertoniana	dental caries, conjunctivitis	R	[75]
	Piliostigma reticulatum	ulcer, boils, wounds, cancer, syphilis and diarrhoea	L	[76]
	Prosopis Africana	used to prepare food in Northern Nigeria	SB	[77]
	Afrormosia laxiflora	epilepsy and psychosis	L	[39]
	Erythrophleum suaveolus	arthritis, rheumatism, dropsy, swelling, eye treatment, laxative	SB	[39]
	Lonchocarpus laxiflorus	dermatitis, headache, intestinal worm, jaundice, ulcer, anthelmintic	SB	[48]
	Swartzia madagascariensis	poison arrow and fishing, insecticide	R	[48]
	Senna occidentalis	bacterial and malaria infections	L	[ 89]
Fagaceae	Quercus borealis	dyspnea, nausea, emesis, diarrhoea and muscular pain	L	[78]
Hymenocardiaceae	Hymenocardia acida	Hypertension	R, SB	[80]
Lamiaceae	Ocimum gratissimum	the oil is medicine for respiratory tract infections, diarrhoea, eye problem, skin diseases	L	[88]
	Hyptis spicigera	Cold, insecticides	L	[59]
Lauraceae	Cassytha filiformis	food and infectious diseases	L, S	[48]
Loganiaceae	Anthocleista vogelii	purgative, diuretic, ulcer, stomach-ache	R, SB	[87]
	Strychnos spinosa	taken by lactating women to stimulate breast milk production	L	[96]
Loranthaceae	Tapinanthus globiferus	hypertension epilepsy, relief pain, tinnitus and trypanosomiasis	L, SB	[99]
Lythraceae	Lawsonia inermis	used to adorn women’s bodies as part of social and holiday celebration	L	[39]
	Punica granatum	diarrhoea, dysentery	L	[100]
Malvaceae	Bombax buonopozense	to treat edema	SB	[102]
	Adansonia digitata	to treat cancer	SD	[101]
	Sterculia setigera	used as a thickener and emulsifier	R	[39]
Melastomataceae	Heterotis rotundifolia	malaria, rheumatism, diarrhoea	WP	[102]
	Dissotis rotundifolia	trypanosomiasis treatment	L	[104]

R = root, RB = root bark, SB = stem bark, L = leaves, S = stem, WP = whole plant, Re = rhizome, B = bark, SD = seed.

In folkloric medicine of Idoma people of North Central Nigeria, Hymenocardia acida (Hymenocardiaceae) is used alone or in combination to treat trypanosomiasis and other fever related diseases [84]. Di-(2-ethylhexylphthalate (DEHP), friedelan-3-one, betulinic acid, lupeol, β-sitosterol, stigmasterol, oleic acid and homoorientin were isolated from the plant [85,86,87,88,89,90]. Aqueous ethanol extract of H. acida stem bark was active against T. b. brucei in vitro at MIC of 2.5 mg/mL in half an hour. At this concentration (2.5 mg/mL), morphology of red blood cells was unaffected [91]. None of the compounds from H. acida was tested for antitrypanosomal activity. The ethyl acetate extract of Ocimum gratissimum (Lamiaceae) leaves showed high antitrypanosomal in vitro activity (IC50 of 2.08 ± 0.01 μg/mL) and a high selective index of 29 [92]. In vitro analysis of chloroform extract of Sterculia setigera root revealed that 4 mg/mL of the extract only slightly reduced motility of T. b. brucei and T. congolense after 60 min exposure time [39]. Methanol extract of Cassytha filiformis (Lauraceae) stem (4 mg/mL) stopped T. b. brucei mobility after less than half an hour incubation time [48]. T. b. brucei (100%) was completely immobilized in vitro at MIC of 20 mg/mL of aqueous ethanol extract of Anthocleista vogelii (Loganiaceae) root bark [86,93,94,95].

In North-Eastern Nigeria, leaves and fruits of Strychnos spinosa (Loganiaceae) are consumed by lactating Fulani women to stimulate breast milk production [96]. Dichloromethane leaf extract of the plant yielded: saringosterol (16), 24-hydroperoxy-24-vinylcholesterol (17), akagerine (18) and 10-hydroxyakagerine (19). Out of the four compounds, only 16 and 17 exhibited significant antitrypanosomal in vitro activity with IC50 values of 7.8 ± 1.2 and 3.2 ± 1.2 µM respectively [97]. The essential oil from the plant’s leaves was active on T. b. brucei with IC50 13.5 µg/mL. Hoet et al., went further to show that nerolidol (20) and linalool (21), components of this oil, had a higher potency on the trypanosomes with IC50 values of 7.6 and 16.3 µM [98]. Ten and 4 mg/mL methanol extracts of Tamianthus globiferus (Loranthaceae) and Lawsonia inermis (Lythraceae) leaves respectively drastically ceased motility of T. congolense within 5 min [39,99]. A dose of 200 mg/kg of ethyl acetate extract of Punica granatum (Lythraceae) leaf reduced anaemia and promoted weight gain in mice infected with T. b. brucei [100]. The stem barks of Bombax buonopozense (Malvaceae) and Heterotis rotundifolia (Melastomataceae) methanol extracts (200 and 300 mg/kg body weight, respectively) cleared T. b. brucei within seven days of treatment duration, while 67% of T. b. brucei infected mice survived for over 43 days when treated with 400 mg/kg methanol seed extract of Adansonia digitata (Malvaceae) [70,101,102]. Dissotis rotundifolia (Melastomataceae) is a medicinal plant widely used in Nupe (Nigeria) ethno-medicine to treat trypanosomiasis [103,104]. Oral and i.p. administration of 800 mg/kg of ethanol leaf extract of the plant to rat infected with T. b. brucei significantly reduced parasitemia by 66.7% and 78.4%, respectively. The parasite was killed within 45 s of in vitro exposure to the same concentration of the extract. Hyptis spicigera (Lamiaceae) is locally known as “Bunsuru fadama” in Hausa language, Northern Nigeria. When the plant is crushed and applied to the head, it relieves headache [59]. Findings by Ladan et al., demonstrated that 0.5 µg/mL of the volatile oil from H. spicigera leaves, killed T. b. brucei in vitro within 6 min of administration. GC-MS was used to analyze the chemical composition of the essential oil and about 30 constituents were identified [105,106,107,108].

5. Meliaceae, Moraceae, Moringaceae, Myrtaceae, Ochnaceae, Phyllanthaceae, Poaceae, Polygalaceae, Rubiaceae, Rutaceae, Solanaceae, Ulmaceae, Verbenaceace, Vitaceae, Zingiberaceae

A summary of the plants and derived products of the above listed families are in Table 4 and Figure 4 respectively. Various extracts of leaf, bark, stem and seed of Khaya senegalensis (Meliaceae) were reported to treat several human diseases [109]. Rats infected with T. b. brucei showed a significant decrease in blood parasite burden within six days when treated with aqueous stem bark extract of K. senegalensis at 60–100 mg·kg−1 bw i.p. [110]. Umar et al. reported that the stem bark of K. sengalensis possessed the highest in vitro activity among the six extracts tested as it eliminated the parasites within 5 min of incubation time at 1 mg/mL [111]. The following triterpenoids: gedunin (22); methyl-angolensate (23); methyl-6-hydroxyangolensate (24), isolated from K. senegalensis stem bark, though not tested, could be responsible for its trypanocidal activity [112]. aqueous stem bark extract of Securidaca longepedunculata (Polygalaceae) and methanol stem bark extract of Pseudocedrella kotschi (Meliaceae), methanol stem bark extract of Ficus sycomorus (Moraceae), chloroform stem bark extract of Canarium schweinfurthii (Poaceae) and chloroform stem bark extract of Syzygium guineense (Myrtaceae) (4 mg/mL) ceased T. b. brucei motility in vitro within the incubation time of less than one hour [39,113].

Table 4

Plants from Nigeria with activity against African trypanosomes.

Family	Species	Traditional Uses	Plant Part	Ref.
Meliaceae	Khaya senegalensis	to treat malaria	L, AS	[109]
	Pseudocedrella kotschi	to treat diarrhoea, dysentery, epilepsy	SB	[39]
Moraceae	Ficus sycomorus	to treat snakebite, jaundice, dysentery	SB	[39]
Moringaceae	Moringa oleifera	to treat diabetes and intestinal worms	L, S, SB, R	[116]
Myrtaceae	Psidium guajava	to treat diarrhoea, hypertension	L	[118]
	Syzygium guineense	used to bath ill person	SB	[113]
	Eucalyptus camaldulensis	used to treat malaria and typhoid fevers	L	[120]
Ochnaceae	Lophira lanceolata	dermatosis, toothache, muscular tiredness	L, SB	[87]
	Ximenia americana	treatment of fever, jaundice, impotence, sleeping sickness	SB	[123]
Plantaginaceae	Picrorhiza kurroa	to treat asthma, bronchitis, chronic dysentery, malaria	WP	[124]
Poaceae	Canarium schweinfurthii	burnt for fumigation	SB	[39]
Polygalaceae	Securidaca longepedunculata	to treat cough, chest pain, toothache and diabetes	R	[113]
Rubiaceae	Gardenia erubescens	used as dye	L	[92]
	Nauclea latifolia	to treat fever, dental caries, sceptic mouth, malaria	B	[125]
	Nauclea pobeguinii	to treat fever, dental caries, sceptic mouth, malaria	B	[43]
	Keetia leucantha	to treat malaria	L	[127]
	Mitracarpus scaber	to treat headache, toothache, venereal disease, amenorrhoea, dyspepsia, leprosy	L	[128]
	Morinda lucida	used to treat malaria	L	[115]
Rutaceae	Zanthoxylum zanthoxyloides	stomach disorder, worm infection	SB	[70]
Solanaceae	Withania somnifera	to treat external tumors, tubercular glands and ulcer	WP	[43]
Ulmaceae	Trema orientalis	to treat cough, sore throats, asthma, bronchitis, gonorrhea, yellow fever, toothaches	L	[70]
Verbenaceace	Vitex doniana	anemia, gonorrhea, dysentery and to improve fertility	L	[70]
	Vitex simplicifolia	to treat malaria	L	[131]
Vitaceae	Cissus multistriata	for the management of protein deficiency	L	[130]
Zingiberaceae	Zingiber officinale	gastrointestinal diseases, dyspnea, nausea, emesis, diarrhoea and muscular pain	R	[83]

R = root, RB = root bark, SB = stem bark, L = leaves, S = stem, WP = whole plant, Re = rhizome, B = bark, SD = seed, AS = axial stem.

Figure 4

Chemical structures of compounds 22–54.

Morinda lucida (Rubiaceae) root together with Mangifera indica, Carica papya and Cassia podocarba leaves are used in Nigeria to treat malaria [114]. Adewunmi and Adesogan isolated some anthraquinones such as damnacanthol (25) and morindin (26) from M. lucida. Intraperitoneal administration of methanol leaf extract of M. lucida caused a significant reduction in parasitemia in T. b. brucei infested rats and mice [115,116]. An in vitro assay revealed that the petroleum ether extract of the root bark, chloroform extract of the stem bark, methanol extract of the stem and the aqueous extract of all parts of Moringa oleifera (Moringaceae) were active at 4 and 2 mg/mL doses against T. b. brucei [117]. The anti-trypanosomal screening of Psidium guajava (Myrtaceae) leaves revealed that it inhibited growth of T. b. brucei (IC50 of 6.3 μg/mL and 48.9 μg/mL) for 80% and 20% ethanol preparations respectively [118,119,120]. Eucalyptus camaldulensis (Myrtaceae) methanol leaf extract at a dose of 150 mg/kg body weight/day extended the lifespan of T. b. brucei infected mice by six days [120]. GC-MS analysis of E. camaldulensis fraction yielded 9-octadecenamide (27), 1-nonadecene (28), (Z)-9-eicosene (29), hexadecanol (30), 1-pentadecanol (31) for fraction A; and methyl hexadecanoate (32), methyl cis-9-octadecenoate (33), and 1-heptadecanol (34) for fraction B [121,122]. Aqueous extract Lophira lanceolata (Ochnaceae) Leaf and ethanol extract of Gardenia erubescens (Rubiaceae) stem had 100% activity in vitro against T. b. brucei and T. congolense at 20 mg/mL [92,123]. Peter et al. showed that 1000 µg/mL methanol extract of Picrorhiza kurroa (Plantaginaceae) rhizome completely killed T. evansi in vitro after an incubation time of 8 h [124].

Among the three compounds 35, 36 and 37 isolated from the ethanol bark extracts of two plants from the Rubiaceae family (Nauclea pobeguinii and Nauclea latifolia), only 37 had activity in vitro (MIC = 12.5 μg/mL) against T. b. brucei [43,125,126]. Ten compounds with in vitro anti-trypanosomal properties; ursolic acid (38), oleanolic acid (9), betulinic acid (39), β-lonone (40), α-lonone (41), geranylacetone (42), phytol (43), caryophyllene (44) and oleic acid (45) have been isolated from Keetia leucantha (Rubiaceae). Compounds 38 and 9 demonstrated activities with IC50 of 2.5 and 7.3 μg/mL respectively towards T. b. brucei [127]. Mitracarpus scaber (Rubiaceae) is used in Nigeria to treat headache, toothache, venereal diseases, amenorrhoea, dyspepsia and leprosy [128]. Its methanol fraction yielded azaanthraquinone (46) which caused complete disappearance of T. congolense (in vivo) in mice at 50 mg/kg bw for 5 day without relapse. The authors suggested that the mechanism by which the compound performed its trypanocidal effect was by interfering with the mitochondrial electron transport system of the parasite [129]. Ximenia americana (Ochnaceae) is a plant used in traditional medicine for the treatment of malaria, ulcers, and infectious diseases. Aqueous extract of X. americana stem bark exhibited in vitro trypanocidal effect by immobilizing 90% of the T. congolenseafter 30 min incubation [123]. Zanthoxylum zanthoxyloides (Rutaceae) showed only trypanostatic effects and could not completely clear the T. b. brucei in vivo in infected mice [70].

Hexane whole plant extract of Withania somnifera (Solanaceae) had in vitro activity against T. b. brucei (MIC = 50 μg/mL) while 20 µg/mL of compound 47 isolated from the plant immobilized 78% of T. b. brucei [43]. Methanol leaves extracts of Trema orientalis (Ulmaceae) and Vitex doniana (Verbenaceace) exhibited in vitro activities with IC50 values of 3.50 and 6.58 μg/mL respectively against T. b. rhodesiense in less than one hour [70]. Aqueous methanol leaf extract of Cissus multistriata (Vitaceae) was very active against T. b. brucei. Parasitemia level in infected albino rats disappeared on sixth day of treatment with 400 mg/kg b.w of the plant extract intraperitoneally. Omale and Joseph suggested that the high trypanocidal activity of the plant could be due to its flavonoid content [51,130]. After 9 h of exposing T. evansi, in vitro, at 1000 μg/mL of methanol extract of Zingiber officinale (Zingiberaceae) rhizome, 95.86% of the parasites died [83]. Nwodo et al. reported that 2-(5'-methoxyphenyl)-3,4',5,7,8-trihydroxychroman-4-one (48), 2-(5'-methoxyphenyl)-4',5,7-trihydroxy-3-methoxychromen-4-one (49), penduletin (50), 2-(4'-hydroxy-phenyl)-5-hydroxy-3,7-dimethoxychromen-4-one (51), 2-(4-hydroxyphenyl)-3,5,7-trihydroxy-chromen-4-one (52), artemetin (53) and 2-(3',4'-dimethoxyphenyl)-7-hydroxychromen-4-one (54) from Vitex simplicifolia (Verbenaceace) leaf were active (IC50 values range 4.7–23.7 µM) against T. b. rhodesiense after less than one hour of incubation time. Compound 54 showed the most promising and selective trypanocidal activity (IC50 = 4.7 µM) with a selectivity index of 9.8. The authors observed that trypanocidal activity of the compounds increases with increase in methylation of hydroxyl groups. This is expected because methylation increases lipophilicity which increases permeability of molecule across membranes of the parasite [131,132,133].

6. Conclusions

This review, the first of its kind on antitrypanosomal medicinal plants from the Nigerian flora, represents an overview of the potentials of these plants in combating the disease. It is intended to serve as the scientific baseline information for the use of documented plants as well as a starting point for future studies for the discovery of better trypanocidal molecule(s). Most of the plants were evaluated as crude extracts. Only compounds 1, 3, 4, 9, 11–12, 14–17, 20–21, 37–38, 47–54 were actually tested for antitrypanosomal properties. Mode of action of the compounds has almost never been thoroughly studied; only possible mechanisms have been suggested. This calls for more detailed investigations in this direction. In general, even though not all plants reviewed here are native or unique to Nigeria or West Africa, this survey suggests that Nigerian flora is a potential suitable starting point to discovering new and better trypanocidal drug molecules.