Potential of the natural products against leishmaniasis in Old World - a review of in-vitro studies

ABSTRACT

Leishmaniasis is a vector-borne disease among the 10 most Neglected Tropical Diseases with diverse clinical manifestations caused by protozoan parasites of the Leishmania genus. Around 80% of leishmaniasis cases are found in the Old World affecting populations mainly in low and middle-income countries. Its control relies mostly on chemotherapy which still presents many drawbacks. Natural products may offer an inexhaustible source of chemical diversity with therapeutic potential. Despite the lack of knowledge on traditional products with activity against Leishmania parasites, many reports describe the search for natural extracts and compounds with antileishmanial properties against promastigote and amastigote parasite forms. This review summarizes the research of 74 publications of the last decade (2008–2018) focused on the identification of endemic plant-derived products that are active against Old World Leishmania parasites responsible for cutaneous and visceral leishmaniasis. The present review combines data on antileishmanial activity of 423 plants species, belonging to 94 different families, including a large range of crude extracts which lead to the isolation of 86 active compounds. Most studied plants came from Asia and most promising plant families for antileishmanial activity were Asteraceae and Lamiaceae. From the chemical point of view, terpenoids were the most frequently isolated natural products. These studies suggest that natural products isolated from Old World flora are a rich source of new chemical scaffolds for future leishmaniasis treatment as well as for other Neglected Tropical Diseases warranting further investigation.

Introduction

Leishmaniasis is a vector-borne disease (VBD) caused by protozoan parasites of the genus Leishmania sp. and transmitted by infected phlebotomine sand flies. This neglected tropical disease exerts its largest burden on developing countries and is strongly associated to poverty with 4.4 M disability-adjusted life years (DALYs) [1]. It is estimated that over 1 million new cases occur each year worldwide with up to 65 000 annual deaths [2]. There are 98 countries endemic for leishmaniasis [3]. Depending on the species, leishmaniasis can present different clinical forms: visceral leishmaniasis (VL) affecting the spleen, liver, and bone marrow being fatal if untreated; cutaneous leishmaniasis (CL) which generally results in self-limiting but disfiguring skin lesions; and mucocutaneous leishmaniasis (MCL) which is associated with destruction of mucous membranes of the nose, mouth and throat also with disfiguring lesions. Up to 1 million CL new cases are mostly found in Afghanistan, Algeria, Brazil, Colombia, Iran, Pakistan, Peru, Saudi Arabia, Syria, and Tunisia whereas the mucocutaneous form occurs mostly in Bolivia, Brazil and Peru. Up to 90 000 new cases of VL occur mainly in Bangladesh, Brazil, Ethiopia, India, Nepal, South Sudan, and Sudan [2,3].

In the Old World, etiological agents of VL are L. donovani and L. infantum with anthroponotic and zoonotic transmission, respectively. Concerning CL, the main agents in the Old World are L. major and L. tropica with zoonotic and anthroponotic transmission, respectively [4]. L. infantum can also cause CL infections, though less frequently.

Conventional leishmaniasis chemotherapy

Limited investment from the funding agencies and pharmaceutical companies has been forwarded for the development of new chemotherapeutics associated clinical trials for neglected tropical diseases [5,6]. Due to the lack of vaccine for human leishmaniasis and no quite effective vaccines for canine leishmaniasis, the control of this VBD relies on chemotherapy [7]. Most of the currently available drugs for treatment of human leishmaniasis are associated with high toxicity, adverse side effects, high costs and require long-term periods of parental administration. Moreover, parasite drug resistance has become a major concern, mainly for pentavalent antimonials – sodium stibogluconate (Pentostam®) and meglumine antimoniate (Glucantime®) – which have been in use for more than 60 years [8]. Since the 1980s, amphotericin B deoxycholate (Fungizone®) has been used as second-line drug in several countries. This drug presents high toxicity and requires parenteral administration with close clinical monitoring in hospital settings, which increase treatment costs. The liposomal formulations of amphotericin B (e.g. Ambisome®) provide less toxicity with increased efficacy being nowadays the first-line treatment in many countries [9]. Also, paromomycin, an aminoglicosine antibiotic, has been used mainly for treatment of CL cases in parenteral and topical formulations and as combination therapy with gentamicin [10]. The hexadecylphosphocholine – Miltefosine (e.g. Impavido®) – originally an anti-tumor agent, is the only oral drug for leishmaniasis treatment with proven therapeutic utility, although limited due to its teratogenic effects, namely for pregnant women [11]. After being used for decades in monotherapy, declining efficacy started to be observed [12]. Nowadays, miltefosine is used as second line in combination therapy due to its limited availability and affordability both at global and country levels [11]. Currently, combination therapies are considered the best regimens for treating VL in many parts of the world [9]

The World Health Organization, aligned with the Sustainable Development Goals to accelerate control and elimination of Neglected Tropical Diseases by 2030, established on its road map for leishmaniasis control the development of i) more effective, sensitive and user-friendly treatments and diagnostics for VL and CL, and ii) vaccines to prevent transmission of Leishmania parasite [13]. Thus, novel therapeutic alternatives such as identification of both natural and synthetic molecules have the potential to contribute to the development of better therapeutic approaches for this neglected tropical disease [14].

Leishmania in vitro models

During its life cycle, Leishmania alternates between two complex environments to which the parasite must adapt to the mammalian host and the insect vector. Leishmania promastigote forms found in the phlebotomine vector are widely used for preliminary in vitro screens mainly due to the ease of culture growth in laboratory settings, the requirement of small amounts of extracts, fractions or compounds, and the fact that these experiments do not require sophisticated equipment [15]. Nevertheless, Leishmania intracellular amastigotes (IA) are the relevant clinical parasite forms, which multiply inside macrophages, and have been the reference model for in vitro screening. Although high throughput methods, such as the use of reporter genes and flow cytometry [16,17,20] are available for antileishmanial drug screening, microscopic counting using Giemsa staining continues to be the most common method for evaluation of drug efficacy on IA by determining inhibitory concentration values at 50% (IC₅₀) derived from the percentage of infected cells and number of amastigotes per macrophage.

The quest for natural products in traditional medicine

With a global forest area of 31% [21] and estimated 298 000 plant species [23], Earth has a broad potential for the research and application of natural compounds. For many centuries, plants have been a rich source of natural products and continue to be used in traditional medicine, mainly in low and middle-income countries [25]. One of the oldest and most successful reported cases is quinine, a member of the chinchona alkaloid family originally obtained from the Chinchona tree. This compound was the first effective treatment for malaria which has been used since the seventeenth century and remained as the antimalarial drug of choice until the 1940s [27]. Another example of the successful use of natural products is Artemisia annua L., a plant used for centuries in China’s traditional medicine and which also became one of the main therapeutic approaches for the treatment of malaria since the seventies due to isolation of bioactive compound artemisinin [28]. Despite the absence of deep knowledge on traditional products with activity against leishmaniasis, several reviews have described studies on natural products, including extracts and compounds that have shown antileishmanial activity [14,29–31].

Many studies on bioactive compounds against Leishmania are found in New World regions, where there is a high level of vegetal biodiversity, such as the Amazonian region [32]. Many Old World plants studied for antileishmanial activity are also used in traditional medicine for other parasitic diseases like human African trypanosomiasis and malaria but also skin diseases and rheumatism [33,34–36].

As bioactive compounds derived from a given plant species tend to be tested on the parasite species that occur in the region of the biomass origin, we considered it potentially interesting and relevant to analyze and summarize the in vitro natural products studies performed in the Old World, where CL and VL are highly endemic.

Methods

This review was performed based on the literature published between 2008 and 2018 related to the quest for natural products isolated from Old World terrestrial plants with activity against Old World Leishmania species: L. infantum, L. donovani, L. major, L. tropica and L. aethiopica. The studies were retrieved from English research papers from PubMed, Google Scholar and Scopus/Web of Science. The following keywords were used as search indicators in combination with each of the continents Africa, Asia, and Europe: Leishmaniasis, Leishmania, extracts, natural compounds and in vitro activity. The studies here listed are associated with chemo-biological aspects of bioactive metabolites present in crude extracts or in isolated compounds.

According to Nwaka et al. (2009) [6], hits for leishmaniasis are compounds with IC₅₀ lower than 2 µg/mL and selectivity indexes (SI) higher than 20. By contrast Katsuno et al. (2015) [38] consider IC₅₀ values lower than 10 µM as hits, which does not always correspond to the values indicated by the first author for the same compound. As concentration limits and SI values to consider hit compounds are not consensual, attention should be given when analyzing IC₅₀ displayed by natural products. In this review, authors have pointed out some extracts, fractions, essential oils and compounds displaying IC₅₀ lower than 15 µg/ml, a value which safeguards the limits suggested by other authors not limiting the spectrum of potential available and interesting natural products.

When more than one parasite forms were used to evaluate antileishmanial activity, only results obtained with intracellular amastigotes were presented, as this is the most relevant form in human infection. In order to compare compounds activity, the inhibitory concentrations are shown in both µM and µg/ml according to the published references.

Results and discussion

This review included 74 research papers published between 2008 and 2018. Within these publications, 423 species, belonging to 94 families were screened. The most frequent ones were Asteraceae (48 species), Lamiaceae (43 species), and Fabaceae (26 species) (Tables 1 and S1). The region with most significant number of plants studied was Asia with 258 plant species, followed by Africa with 135 species and Europe with 30 species. A total of 662 extracts/fractions, 11 essential oils (Table S1) and 86 isolated compounds (Table 1) were retrieved from the analyzed references.

Table 1.

Compounds isolated from Old World plants that reveal antileishmanial activity (2008-2018).

Plant			Plant origin		Compound			Leishmania		Reference
Family / Species			Continent	Country	Name	IC50 [µM (µg/mL)]	SI	Species	Form*
Annonaceae
		Uvaria grandiflora	Asia	Malaysia	Zeylenol (75)	110.0 (42.28)	0.24	L. donovani	P	[26]
					Ferrudiol (74)	101.1 (49.39)	0.20
	Ferula narthex		Asia	Pakistan	Conferol (73)	39.92 (11.51)	n.d.	Leishmana sp.	AA	[48]
Asteraceae
	Kleinia odora		Asia	Saudi Arabia	3β, 11α-dihydroxyurs-12-ene (12)	3.20 (2)	>18.75d	L. infantum	IA	[76]
					Ursolic acid (13)	7.40 (3.38)	1.54d
					Brein (urs-12-ene-3β,16β-diol) (14)	9.30 (6.33)	>6.88d
	Vernonia mespilifolia		Africa	South Africa	Cynaropicrin (2)	1.56 (0.54)	0.83a	L. donovani	P	[59]
Apocynaceae
	Chilocarpus costatus		Asia	Malaysia	Pinoresinol (27)	213.4 (76.48)	0.13	L. donovani	P	[26]
	Leuconotis eugenifolius		Asia	Malaysia	β-Amyrin (17)	15.4 (6.57)	13.0	L. donovani	P	[26]
Boraginaceae
	Cordia fragrantissima		Asia	Myanmar	Acetylcordiaquinol C (54)	4.26 (1.4)	14.36b/43.50c	L. major	P	[24]
					Cordiachrome B (51)	10.32 (2.5)	5.84b/28.60c
					Cordiaquinol J (55)	10.29 (2.70)	>3.7b/15.44c
					Cordiachrome A (52)	16.92 (4.10)	5.49b/14.90c
					Cordiaquinol C (56)	18.57 (4.50)	n.d.
					Alliodorin (57)	26.89 (7.0)	4.56b/12.76c
					Cordiachrome C (53)	87.08 (21.10)	n.d.
					1,4-p-dibromobenzoylcordiaquinol I (58)	35.55 (23.50)	>4.26b,c
					Acetylcordiaquinol I (59)	244.82 (80.40)	0.63b/0.81c
					Cordiaquinol I (60)	298.94 (81.40)	0.64b/0.88c
					Cordiaquinol K (61)	>90.47 (>25)	>4 b,c
Chrysobalanaceae
	Parinari excelsa		Africa	West and Eastern Africa	3β-hydroxy-olean-5,12-dien-28-oic acid (15)	7.70 (3.52)	7.80a	L. donovani	P	[47]
					Oleanolic acid (16)	8.20 (3.74)	10.70a
Combretaceae
	Anogeissus leiocarpus		Africa	not specified	Castalagin (46)	0.06 (55.00)	<10–27.3j	L. aethiopica	P	[69]
						0.16 (>150)	<10–27.3 j	L. donovani
					Ellagic acid (47)	0.50 (>150)	<10 j	L. aethiopica
						>0.50 (>150)	<10 j	L. donovani
	Terminalia arjuna		Asia	India	Ursolic acid (13)	7.69 (3.51)	2.1a	L. donovani	P	[73]
	Terminalia avicennoides		Africa	not specified	Flavogallonic acid bislactone (48)	0.18 (85.00)	<10–17.7 j	L. aethiopica	P	[69]
						0.32 (>150)	<10–17.7 j	L. donovani
					Punicalagin (49)	0.08 (85.00)	<10–18.75 j	L. aethiopica
						0.14 (>150)	<10–18.75 j	L. donovani
					Terchebulin (50)	0.14 (>150)	<10 j	L. aethiopica
						0.14 (>150)	<10 j	L. donovani
Dilleniaceae
	Dillenia philippinensis		Asia	Philippines	2a,3b-dihydroxyolean-12-en-28-oic acid (18)	45.00 (21.27)	0.56 k	L. major	P	[43]
					2,3-seco-olean-12-ene-2,3-dioic-28-methyl ester (19)	46.60 (25.13)	0.96 k
					2,3-seco-olean-12-ene-2,3-dioic-28-butyl ester (20)	56.70 (32.96)	0.27 k
					Messagenic acid (21)	67.10 (31.85)	0.72 k
Ebenaceae
	Diospyros canaliculata		Africa	Cameroon	Plumbagin (62)	4.52 (0.85)	0.56a	L. donovani	AA	[42]
					Canaliculatin (63)	7.52 (2.73)	6.92a
					Ismailin (64)	10.98 (5.89)	9.61a
					Betulin (22)	225.87 (100)	0.53 a
Fabaceae
	Abrus precatorius		Africa	South Africa	Abruquinone I (66)	3.40 (1.25)	6.5a	L. donovani	P	[54]
					Abruquinone B (67)	2.90 (1.13)	3.5a
					(3S)-7,8,3′‚5′-tetramethoxyisoflavan-1′,4′-quinone (68)	5.00 (1.81)	0.9a
	Piliostigma thonningii		Africa	Nigeria	methyl-ent-3β-hydroxylabd-8(17)-en-15-oate (11)	7.82 (2.63)	n.d.	L. donovani	IA	[45]
					2β- methoxyclovan-9α-ol (8)	>10 (>2.66)	n.d.
					Alepterolic acid (10)	>10 (>3.20)	n.d.
Lamiaceae
	Ocimum sanctum		Asia	Nepal	Ursolic acid (13)	4.82 (2.20)	n.d.	L. major	P	[75]
					Oleanolic acid (16)	37.44 (17.10)	n.d.
					Stigmasterol (76)	>60.58 (>25)	n.d.
					Tulsinol A (23)	>73.02 (>25)	n.d.
					Tulsinol B (24)	89.85 (43.90)	n.d.
					Tulsinol C (29)	16.61 (9.10)	n.d.
					Tulsinol D (25)	96.40 (47.10)	n.d.
					Tulsinol F (26)	48.61 (23.80)	n.d.
					Tulsinol G (31)	261.98 (89.70)	n.d.
					Ferulaldehyde (32)	5.05 (0.90)	n.d.
					Eugenol (33)	>152.25 (>25)	n.d.
					Bieugenol (30)	41.67 (13.60)	n.d.
					Dehydrodieugenol B (28)	51.78 (16.90)	n.d.
					Luteolin (69)	258.18 (73.90)	n.d.
					Apigenin (70)	1328.26 (358.70)	n.d.
					4’,5-dihydroxy-7,8-dimethoxyflavone (71)	>79.54 (>25)	n.d.
					4’,5-dihydroxy-3’,7,8-trimethoxy-flavone (72)	>72.61 (>25)	n.d.
					Caryophyllene oxide (3)	>113.45 (>25)	n.d.
	Salvia repens		Africa	South Africa	12-methoxycarnosic acid (9)	0.75 (0.26)	23.2	L. donovani	AA	[19]
Moringaceae
	Moringa stemopetala		Africa	Ethiopia	1,3-dilinoleoyl-2-olein (85)	45.41 (26.79)	n.d.	L. aethiopica	IA	[49]
					1,3-dioleoyl-2-linolein (86)	30.32 (40.30)	n.d.
Oleaceae										[41]
	Olea europaea v. koroneiki		Europe	Greece	Oleuropein (38)	203.51 (110.00)	3.24i	L. donovani	IA
						60.87 (32.90)	n.d.	L. infantum	P
						260.86 (141.00)	n.d.	L.major	P
					Hydroxytyrosol (37)	251.03 (38.70)	4.65i	L. donovani	IA
						940.55 (145.00)	n.d.	L. infantum	P
						324.98 (50.10)	n.d.	L. major	P
Plumbaginaceae
	Plumbago zeylanica		Asia	India	2-methyl-5-(3’-methyl-but-2’-enyloxy) [1,4,] naphthoquinone (65)	4.06 (1.05)	5.87g	L. donovani	IA	[18]
Solanaceae
	Withania coagulans		Asia	Pakistan	Withanolide J (77)	5.74 (2.70)	n.d.	L. major	P	[40]
					Withanolide G (78)	10.34 (4.70)	n.d.
					Withanolide K (79)	19.97 (5.10)	n.d.
					Withacoagulide C (80)	10.85 (9.40)	n.d.
					Withacoagulin D (81)	22.02 (10.70)	n.d.
					Withacoagulide A (82)	32.72 (15.90)	n.d.
					Ajugin B (83)	42.29 (19.90)	n.d.
	Withania somnifera		Asia	India	Withaferin A (84)	20.19 (9.50)	n.d.	L. donovani	IA	[68]
Valerianaceae
	Valeriana jatamansi		Asia	India	(1S)-4-(Acetoxymethyl)-7-(isovaleryloxymethyl)-1,6,7,7aα-tetrahydrocyclopenta[c]pyran-1α,6α,7beta-triol 1,6-diisovalerate (39)	1.70 (0.72)	0.59 i	L. major	P	[53]
					6α-(Acetyloxy)-4-[(isovaleryloxy)methyl]-7-[[2-(isovaleryloxy)-3-methylbutanoyloxy] methyl]-1alpha-(3-methylpentanoyloxy)-1,4aalpha,5,6,7,7aalpha-hexahydrocyclopenta[c]pyran-7beta-ol (40)	0.87 (1.90)	0.42 i
					Valechlorine (41)	5.01(2.30)	0.83 i
					Bornyl 3-hydroxycinnamate (34)	74.32 (12.20)	0.70 i
					Bornyl isoferulate (35)	86.00 (16.70)	2.08 i
					Bornyl caffeate (36)	48.80 (15.44)	0.05 i
					β-bisabolol (4)	234.74 (52.20)	0.71 i
					Valeranone (5)	273.42 (60.80)	0.70 i
Zingiberaceae
	Curcuma aeruginosa		Asia	Malaysia	Furanodienone (7)	39.5 (9.09)	>1	L. donovani	P	[26]
	Curcuma longa		Africa / Asia	not specified	Curcumin (42)	103.15 (38.00)	n.d.	L. major	P	[22]
					Tris(curcuminato)indium (III) (44)	16.43 (26.00)	n.d.
					Curcumin bis-acetate (43)	114.93 (52.00)	n.d.
					Tris(curcuminato)gallium (III) (45)	17.07 (32.00)	n.d.
	Zingiber zerumbet		Asia	India	Zerumbone (6)	9.36 (2.04)	n.d.	L. donovani	P	[74]
Zygophyllaceae
	Peganum harmala		Asia	India	Peganine hydrochloride (1)	182.48 (41.00)	>5.26i	L. donovani	IA	[39]

Compounds are organized by plants family.

IC₅₀: half maximal inhibitory concentration; SI: selectivity index; some of the SI were calculated based on the anti-Leishmania activity and cytotoxicity values presented in the respective papers. L: leaves; SB: stem barks; F: fruits; FL: flowers; S: stems; BL: bulb; SD: seeds; T: tuber; R: roots; RZ: rhizomes; RB: root bark; RS: resin; W: whole plant; n.d.: not refereed in the reference; ACE: acetone; BuOH: butanol; CF: chloroform; EtAC: ethyl acetate; H₂O: water; HEX: hexane; MeOH: methanol; PET: petroleum ether; ^a – L6; ^b - COS-7 cells; ^c – HuH-7 cells; ^d – MRC-5 cells; ^e – K562 cells; ^f – Vero cells; ^g - RAW264.7; ^h – THP-1 human monocyte cells; ⁱ – J774 cells; ^j – newborn mouse heart fibroblast; ^k - A549 cells; AA: axenic amastigotes; IA: intracellular amastigotes; P: promastigotes; * IA were tested by microscopic observation on Giemsa stained slides, P and AA were tested by colorimetric methods; L.: Leishmania. Classification of the families/species was gauged by the systematic Flora Europeae [37]/Flora Iberica [65].

The results of these studies may contain on its whole, responses that can be used as scaffolds for the development of innovative therapeutic approaches to fight against this neglected disease.

Plant extracts and fractions

Crude extracts can be obtained by using different solvents and solvent mixtures such as water (H₂O), methanol (MeOH), ethanol (EtOH), hexane (HEX), dichloromethane (DCM), butanol (BuOH), ethyl acetate (EtOAc), chloroform (CF) and petroleum ether (PET). MeOH and DCM, pure or in mixture, were the most used solvents in the reviewed studies and the ones displaying higher bioactivities (Table S1).

Most of the studies that report the evaluation of extracts for antileishmanial activity were obtained from Asiatic endemic plants [46]. Abdel-Sattar et al. (2010) performed an extensive study on 52 Saudi Arabian plants for their antiprotozoal activity, in which several sub-fractions of MeOH extracts from different species presented interesting antileishmanial activities. Additionally, medicinal plants from Pakistan are a widely source of extracts and metabolites with anti- Leishmania properties [35]. Solanum villosum L. and Withania somnifera L. (Solanaceae) as well as Verbesina encelioides (Cav.) A. Gray (Asteraceae) shown to be active against L. infantum IA. In addition, antileishmanial effects against L. major IA, one of the causative agents of CL in Middle east, were also observed in extracts from endemic plants from Israel and India: Nuphar lutea L. (Nymphaeaceae) and Valeriana jatamansi DC (Valerianaceae) with low IC₅₀ values: 0.7 µg/mL and 0.8 µg/mL, respectively [50,52,53,55], performed a recent review on the pharmacological activities of Juniperus excelsa M.Bieb in which, among many other properties, extracts and some of its metabolites revealed activity against L. donovani and L. major promastigotes.

Concerning the African continent Et-Touys et al. (2017) [56] made an extensive data collection in African medicinal plants with anti-Leishmania activity, some of them here reported. Extracts and fractions where tested mainly against L. donovani and L. infantum and mostly using the promastigote model. Some of the relevant antileishmanial activities were found in fractions from South African Asteraceae plants. Fractions and essential oils from this family, in which Artemisia species are included, were tested for their parasitic toxicity revealing IC₅₀ ranging from 0.01 to 14.4 µg/ml [57–59,60]. The alkaloid-rich fractions of Pavetta crassipes K. Schum (Rubiaceae) leaves revealed promising results against L. infantum IA (2.0 µg/mL), being also a potential source of active compounds [61].

Essential oils

Essential oils are volatile hydrophobic liquids produced by plants as secondary metabolites and can be obtained from the different parts of a plant. These oils are widely used in different therapeutic applications including antiseptic, anti-inflammatory, spasmolytic, sedative and analgesic [30,62]. Each essential oil is a complex mixture of molecules (mainly terpenes, aliphatic and aromatic compounds) with low molecular weight and at different concentrations [62]. Essential oils are known for their cytotoxicity as they can easily transpose barrier membranes and access the cells and to further induce cell death by apoptosis and/or necrosis, cell cycle arrest, and loss of key organelles function [30,62]. Additionally, essential oils from several plant species have shown antileishmanial activity (Table S1), namely in those obtained from Lamiaceae (Satureja punctata R.Br. and Thymus hirtus Willd) and Rutaceae (Ruta chalepensis L.) [60,63]. Moreover, Artemisia abyssinica Sch.Bip. ex A.Rich (Asteraceae) was tested on both L. aethiopica and L. donovani axenic amastigotes (AA) revealing a stronger growth inhibitory effect and selectivity against the first parasite species [60]. Different compounds were identified by gas chromatography-mass spectrometry on these essential oils, most of them oxygenated monoterpenes and sesquiterpenes, several of those previously described as antileishmanial agents [reviewed by [60]].

Isolated natural products

Plant extracts and their derivatives are well known as sources of compounds that could achieve high biological activities, including against Leishmania parasites [64]. In this review 86 isolated compounds exhibiting antileishmanial activity against one or both parasite life forms of different parasite species were listed (Table 1). This review shows that identified metabolites are chemically diverse, comprising different chemical families. These include alkaloids such a quinazoline derivative (1), terpenoids such as sesquiterpenes (2–8), diterpenes (9–11) and triterpenes (12–22). Lignoids have been described as lignans (23–27) and neolignans (28–30). Minor phenylpropanoids (31–36) and a phenylethanoid (37) were identified. Other aromatic compounds have also been described, as iridoids (38–41), a diarylheptanoid or curcumin (42) and its derivatives (43–45), hydrolyzable tannins (46–50). From the chemical family of quinones, several compounds were studied as benzoquinones (51–53), hydroquinones and derivatives (54–61), naphthoquinones (62–65) and isoflavanquinones (66–68). Flavonoids known as flavones (69–72), a coumarin (73), benzoic acid derivatives (74, 75), steroids (76–84) and triacylglycerols (85, 86) have also been described.

Triterpenoids are known to be the most representative group of phytochemicals broad range of pharmacological effects [66]. In this review triterpenes compounds were the most frequent chemical class. Although not fully elucidated, it is known that triterpenoids, such as ursolic acid, induce apoptosis or autophagic cell death in Leishmania sp. [70]. The sesquiterpene (-)-α-bisabolol has shown to be effective against different Leishmania species, inducing ROS-associated and mitochondrial-dependent apoptosis with depolarization of the mitochondrial membrane [71,72]. Triterpenoid ursolic acid (13) isolated from aerial parts of Terminalia arjuna (Roxb. ex DC.) Wight & Arn. (Combretaceae), Ocimum sanctum L. (Lamiaceae) and Kleinia odora (Forssk.) DC. (Asteraceae) shown to be active against L. donovani [IC₅₀ 7.69 µM; SI 2.1 [73]]; and L. major promastigotes [IC₅₀ 4.82 µM; [75]] and also against L. infantum IA [IC₅₀ of 7.40 µg/ml; SI 1.5; [76]], and at lower concentrations than the conventional drugs used as control. Nevertheless, these compounds presented low selectivity. Ursane triterpenes were also found to be active against other protozoan such as Plasmodium falciparum, Trypanosoma brucei and Trypanosoma cruzi [76]. The sesquiterpene parthenin can block specific targets of parasite responsible for glutathinonylspermidin, trypanothione of cysteine and glutathione synthesis precursors in Leishmania species. Naphthoquinones have been suggested as DNA topoisomerase inhibitors in L. donovani. Also, flavonoids, are known to affect the transport mechanisms in Leishmania and affect the energy level by disrupting the electrons chain transport (reviewed by [56]).

In Leishmania, it was observed that the unsaturated fatty acids displayed more toxicity than their corresponding saturated analogues [78,79], tried to elucidate the possible mechanism of toxicity underneath the acetylenic fatty acids suggesting that they may be interacting directly with the Leishmania DNA topoisomerase enzyme and that there is an alkyl chain dependence, but more studies are needed to elucidate these mechanisms.

Despite recommendations on the higher reliability of results by using IA, we found that drug susceptibility studies using Leishmania promastigotes and/or AA were frequently used since they are an easier and more affordable model for primary drug screening [66,80]. Therefore, a concerted effort in the standardization of the methodologies employed and compounds screening on the IA parasite stage should be prioritized. This will allow us to exclude compounds that lack the capacity to transpose the host cell barrier and to deal with host defense mechanisms. Moreover, with the opportunities given by the Leishmania genomes decoding, mechanisms of action of such compounds can now be explored and novel drug targets be validated by using gene editing high throughput technology [81].

Conclusions

This review covers the literature produced in the last 10 years (2008–2018) concerning extracts and isolated compounds obtained from plant species found in Old World countries and evaluated against Old World Leishmania species. This survey gathers useful information for designing further studies for struggling against Old World leishmaniasis.

Although a high number of molecules did not meet the hit activity criteria suggested by Nwaka et al. (2009) and Katsuno et al. (2015) [6,38], according to the criteria defined in the present review there were some compounds with promising IC₅₀ values and with high selectivity indexes. Moreover, it is of paramount relevance to validate interlaboratory standardized assays and methods to be able to compare and select good hits among active compounds [82]. Complementary approaches to modify the scaffold structures of some of the different compiled active compounds can lead to the improvement of the activity and selectivity of the identified molecules against the Old World Leishmania parasites. The interesting compounds here reviewed, mirror the potential of plant species of this part of the world for the identification of natural products as they remain an extremely rich source of new molecules. The possibility of finding bioactive compounds from plants that may lead to antileishmanial drugs is a growing field as more species, extracts and novel molecules are screened. In 2009, the Drugs for Neglected Diseases initiative (DNDi) launched a training manual under the Pan-Asian Screening Network as a practical and user-friendly guide of assays available to screen natural products against pathogens responsible for some of the neglected diseases [83]. Remarks and recommendations were included in different aspects of screening against kinetoplastids. This data encourages further investigation on natural resources as potential origins for novel therapies for leishmaniasis and even for other neglected tropical diseases. The authors urge an effort from the scientific community to establish further networks and partnerships between groups working with natural compounds, to use high throughput methodologies for screening in a larger range of parasites under identical conditions as well as the use of computational methodologies. All the above-mentioned will increase the possibilities of hit compounds identification for neglected tropical diseases that can seed the downstream drug development pipeline.

Funding Statement

This work was supported by the Fundação para a Ciência e a Tecnologia [UID/Multi/04413/2013 (GHTM)]; Fundação para a Ciência e a Tecnologia [SFRH/BD/78062/2011]; Fundação para a Ciência e a Tecnologia [IF/00743/2015]; CAPES [001]; CNPq [301357/2019-7]; FAPESP [18/07585-1].

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.