The potential of anti-malarial compounds derived from African medicinal plants: a review of pharmacological evaluations from 2013 to 2019

Background African Traditional Medicine (ATM) is used for the healthcare of about 80% of the rural populations of the continent of Africa. The practices of ATM make use of plant-products, which are known to contain plant-based secondary metabolites or natural products (NPs), likely to play key roles in drug discovery, particularly as lead compounds. For various reasons, including resistance of strains of Plasmodium to known anti-malarial drugs, local African populations often resort to plant-based treatments and/or a combination of this and standard anti-malarial regimens. Emphasis has been laid in this review to present the anti-malarial virtue of the most recently published phytochemicals or natural products, which have been tested by in vitro and in vivo assays. Methods The data was based on the current version of the African Compound Libraries, which are constantly being updated based on inputs from journal articles and student theses (M.Sc/Ph.D) from African University libraries. Emphasis was laid on data published after 2012. In order to carry out the original data collection, currently being included in the African Compounds Database, individual journal websites were queried using the country names in Africa as search terms. Over 40,000 articles “hits” were originally retrieved, then reduced to about 9000 articles. The retained articles/theses was further queried with the search terms “malaria”, “malarial”, “plasmodium”, “plasmodial” and a combination of them, resulting in over 500 articles. Those including compounds with anti-malarial activities for which the measured activities fell within the established cut off values numbered 55, which were all cited in the review as relevant references. Results and discussion Pure compounds derived from African medicinal plants with demonstrated anti-malarial/antiplasmodial properties with activities ranging from “very active” to “weakly active” have been discussed. The majority of the 187 natural products were terpenoids (30%), followed by flavonoids (22%), alkaloids (19%) and quinones (15%), with each of the other compound classes being less than 5% of the entire compound collection. It was also observed that most of the plant species from which the compounds were identified were of the families Rubiaceae, Meliaceae and Asphodelaceae. The review is intended to continue laying the groundwork for an African-based anti-malarial drug discovery project.


Background
Malaria is an endemic disease in most tropical countries (Africa, Asia, and Latin America), with about half of the world's population at risk of infection according to the World Health Organization (WHO) [1]. According to the latest World Malaria Report, released in December 2019, there were 228 million cases of malaria in 2018, and the estimated number of malaria deaths stood at 405,000. The causative agents for malaria infections are Plasmodium protozoans (i.e. Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax), although most severe infections are caused by P. falciparum [2][3][4]. Most deaths are recorded among African children below the age of 5 years [1][2][3][4]. This calls for an urgent need for new anti-malarial therapies for any one of the following reasons: • The development of resistance against insecticides (e.g. dichlorodiphenyltrichloroethane, DDT) by the disease vectors (female anopheline mosquitoes) [5][6][7]. • The inefficacy of chemoprophylaxis, which has often resulted in poor results [1,[8][9][10]. • The development of resistance by Plasmodium protozoans against most of the drugs currently used to treat malaria (e.g. chloroquine, artemisinin and its derivatives) [11][12][13].
Plants are known to be a rich reservoir of bioactive secondary metabolites (or natural products, NPs), for example, the anti-malarial drugs quinine and artemisinin (AT) are both of plant origin [14]. The benefits of plants containing bioactive anti-malarial compounds, particularly the bitter principles (alkaloids and terpenoids), include their use in the preparation of traditional remedies against malaria, fever, and inflammation [15]. In fact, more than 80% of the local populations of most tropical countries, including African populations, are dependent on medicinal plants for the treatment of most diseases, including malaria, despite the current wide availability of standard malaria treatments for populations in the rural areas, as well as those in cities [16,17]. It has become of interest to summarize the major findings regarding the most promising secondary metabolites with proven in vitro and in vivo potencies, so as to pave the way for further development with compounds from African sources as leads for anti-malarial drug discovery. Recent reviews have either emphasized plants used in specific countries or regions for the treatment of malaria [18][19][20][21], secondary metabolites from selected plant species [22] or families of species [23], plants used as repellents against the mosquito vectors [24], or reports on the analysis of components of improved traditional preparations against malaria [25,26].
Previous reviews have described the in vitro and in vivo potencies of compounds that have been isolated from African floral matter published data before 2013 [27,28]. These reviews had previously described over 500 NPs, within the major NP classes, including alkaloids, terpenoids, flavonoids, coumarins, phenolics, polyacetylenes, xanthones, quinones, steroids, and lignans. These compounds were described in the literature as exhibiting from weak to very good in vitro anti-malarial activities, based on well-established cut-off values [29][30][31]. Besides, a cheminformatic analysis of the aforementioned dataset, with a focus on molecular descriptors related to "drug-likeness", drug metabolism and pharmacokinetics (DMPK), and some rules of thumb such as the Lipinski "Rule of Five" [32], showed that over 50% of the antimalarial compounds had physicochemical properties that fell within the range of "drug-like" molecules [33].
The present review focuses on compounds with tested activities against various malaria parasites derived from a literature survey from 2013 to 2019 [29][30][31]. A total of 187 NPs belonging to diverse classes, including alkaloids, flavonoids, phenolics, flavonoids, steroids, and terpenoids are described. These compounds have been identified from 45 plant species belonging to 23 families. It is hoped that the results summarized will help for lead compound identification and for further anti-malarial drug discovery. The review describes the NPs with potential anti-malarial properties from African medicinal plants, arranged alphabetically according to the main NP compound classes.
laid on data published after 2012. The original data collection, now being included in the African Compounds Database (http://www.afric an-compo unds.org), was conducted from querying individual journal websites using the country names in Africa and search terms. The list of journals visited have been included in Additional file 1. The "hit" articles were retrieved, i.e. those for which plant materials were collected from Africa were then hand-picked by reading through the "Materials and methods" section to ensure that the plant materials were from Africa. Student theses were also randomly collected as made available from university libraries, constituting a small portion of the data. The folder containing the retained articles/theses was further queried with the search terms "malaria", "malarial", "plasmodium", "plasmodial" and a combination of them. Those for which compounds further showed anti-malarial activities published between 2013 and 2019 and for which the measured activities fell within the established cut-off values were selected and included as relevant references. The  [38][39][40] Plasmodium vinckei petteri Not reported Not reported

Data analysis
The collected data was arranged into spreadsheets according to plant sources, compound classes, activity cut-offs and plasmodial strains tested. All activity data was converted to IC 50 values in μM.
Throughout the text, the term antiplasmodial is referred to as that which counters the growth of parasites of the genus Plasmodium, while anti-malarial is referred to as an agent which prevents or counteracts the progress of the disease caused by the parasite or that which treats the disease (i.e. by killing the parasites in the host). Very often the two terms are used interchangeably in the literature surveyed.

Test methodologies
From the literature collected, a broad range plasmodial strains were tested, including those summarized in Table 1. Aporphines The aporphine alkaloids lysicamine (1), trivalvone (2) (Fig. 1) were identified from the leaves of Annickia kummeriae (Annonaceae) from Tanzania, along with four other (protoberberine) alkaloids. Plants from the genus Annickia (formerly Enantia) are popularly known in West and Central Africa for use in the treatment of malaria [70][71][72]. The study by Maleba et al. [58] showed that compounds 1 and 2 showed respective activities of 8.23 and 2.90 μM against the CQ-resistant K1 strain of P. falciparum.

Promising anti-malarial compounds derived from the African flora Alkaloids
Furoquinolines Four furoquinoline alkaloids (3 to 6) ( Fig. 2) were isolated from the fruits and leaves of Teclea nobilis (Rutaceae) and tested on the chloroquine (CQ)resistant FcB1/Colombia strain of P. falciparum by Lacroix et al. [59]. This species from Uganda has been used to treat a range of ailments from pain and fever to malaria [73]. The isolated compounds, including the novel acetylmontrifoline (3), and the known montrifoline (4), maculine (5), and skimmianine (6), were less potent than the reference drug CQ, showing inhibition against the tested parasite strain at < 300 μM [59].  IC 50 = 35 μM, the other compounds tested being inactive [60].
Strychnogucine B (15) (Fig. 4), which was previously isolated from the roots of the same species by Frédérich et al. [63] was further investigated by Beaufay et al. [62] and the compound now displayed further inhibition against the CQ-sensitive FCA 20/Ghana and CQ-resistant W2/Indochina strains, with IC 50 values of 0.617 and 0.085 μM, respectively.
Protoberberines Maleba et al. [58] showed that against the CQ-resistant K1 strain of P. falciparum protoberberine alkaloids are a subclass of promising anti-malarials. The in vitro testing of compounds 30 to 33 ( Fig. 9) showed that compound 30 (palmatine) was the most active, with an IC 50 value of 0.23 μM. Jatrorrhizine (31) exhibited an IC 50 of 0.71 μM, whereas a mixture of compound 31 and columbamine (32) inhibited the plasmodial strain with an IC 50 value of 0.14 μg/mL, and a mixture of compound 26 and tetrahydro-palmatine (33) inhibited the parasite strain with IC 50 = 0.098 μg/mL, probably explaining the synergistic activity of this plant extract. This justifies its use in African Traditional Medicine for the treatment of malaria [58]. Extracts of Polyalthia longifolium (Annonaceae), used in orally consumed preparations in traditional medicine in Ghana, was investigated in order to identify antimalarial compounds [68]. The protoberberine l-stepholidine (34, Fig. 9) was identified from the stem of species among the isolated compounds [68], but this compound had only a weak antiplasmodial activity against the K1 strain of P. falciparum.
Pyridinones Gbedema et al. also isolated darienine (35, Fig. 10), a known alkaloid with anti-malarial activity [68]. This compound exhibited varying degrees of antiplasmodial activity against the K1 strain of P. falciparum with an IC 50 value of 81.28 μM.
Other alkaloids Gardenine (36, Fig. 10), obtained from the investigation of crude extract of the aerial parts of Canthium multiflorum (Rubiaceae), harvested from Cam-eroon, exhibited antiplasmodial activity against the K1 strain of P. falciparum, with an IC 50 value of 32.12 μM and weak cytotoxicity against L6 cell lines [69].
Compounds 37 to 43 were derived from leaves of Senecio roseiflorus and have shown good to moderate antiplasmodial activities against D6 and W2 strains. The activities in terms of IC 50 values ranged from 11.25 to 56.31 µM for the D6 strain, while for the W2 strain, this ranged from 15.47 to 87.50 µM [76]. Compounds 44 to 46, were derived from roots of Tephrosia villosa and exhibited anti-malarial activities against both the D6 and W2 strains with respective IC 50 values from 11.30 to 14.00 µM for the D6 strain and from 13.10 to 20.40 µM for the W2 strain [77].   [26]. The authors identified two flavonoid glycosides; stachannin (52) and pectolinarigenin-7-O-glucoside (53) [26]. The flavone glycoside morreloflavone-7-O-glucoside (54) [82]. These compounds showed marginal activities (70 to 90% inhibition at 40 μM) against the 3D7 and Dd2 strains of P. falciparum.

Phenolics and quinones
Summaries of the phenolics and quinones with most promising anti-malarial properties have been shown in Table 4 (according to their subclasses), with chemical structures shown in Figs. 15, 16, 17, 18, 19, 20 and 21. Ellagic acid derivatives The plant Terminalia brownii (Combretaceae) is used as a remedy for malaria in Eastern and Central Africa, although the detailed mode of preparation is not fully described in the literature [83]. (78) (Fig. 15), obtained from the stem bark of this plant harvested in Kenya was found to be active against chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of P. falciparum [77]. According to Machumi et al. [84], the IC 50 value obtained against both strains was equal to 8.01 µM.
Anthrones The plant Aloe percrassa (Asphodelaceae), is an indigenous species used in Ethiopian folk medicine to treat malaria, wounds and gastric problems [91]. Aloin A (98) Aloin B (99) microdontin A (100) microdontin B (101) (Fig. 19), are four anthrones derived from the leaf latex of Aloe percrassa by Geremedhin et al. [91]. The anti-malarial activities of the mixtures of Aloin A/B and microdontin A/B were lower than the latex. The mixtures were shown to have suppressed parasitaemia from 36.8 to 66.8% at doses of 100 to 400 mg/kg/day. This suggested that the compounds within the two mixtures may have acted synergistically.
Naphthohydroquinones The plant species Pentas bussei (Rubiaceae) is frequently used in traditional medicine to treat malaria in Kenya, particularly the boiling of the roots and stems for oral consumption [92].
The leaves of Otostegia integrifolia (Lamiaceae) are used in Ethiopian folk medicine for the treatment of several diseases including malaria [97]. The known labdane diterpenoid, otostegindiol (124) (Fig. 26) was isolated from the methanol leaf extract of the species by Endale et al. [97]. The isolated compound 125 displayed a significant (p < 0.001) anti-malarial activity at doses of 25, 50 and 100 mg/kg with chemosuppression values of 50.13, 65.58 and 73.16%, respectively. The previously reported norcassane furanoditerpene, norcaesalpin D (125), was isolated from the roots of Caesalpinia bonducella (Caesalpiniaceae) from Tanzania by Nondo et al. [98]. This compound was active with an IC 50 value of 2.20 and 4.16 µM against the 3D7 and Dd2 strains, respectively [98].
Nyongbela et al. [100] isolated the new sesquiterpene sclerienone C (131) from the rhizomes of Scleria striatonux (Cyperaceae), harvested from Cameroon. According to the authors, this compound exhibited antimicrobial and antiplasmodial activities with IC 50 values against the NF54 and K1 strains of 15.69 and 13.54 μM, respectively [100].
(Ganodermataceae) collected from Egypt [102]. This compound exhibited good anti-malarial activity against the D6 strain of P. falciparum with an IC 50 value of 257.8 nM with no cytotoxicity up to the concentration of 9 μM. The compound also tested positive against the W2 strain with an IC 50 value of 2000 nM [102]. The known limonoid, proceranolide (152) (Fig. 32) was found in the leaves of Ekebergia capensis (Meliaceae) by Irungu et al. [80]. The isolated compound was then evaluated in vitro against the D6 and W2 strains of P. falciparum. This compound exhibited weak antiplasmodial activity against the D6 and W2 strains with IC 50 values of 84.7 and 150.2 µM, respectively [80].

Novel compounds identified and principal compound classes
It was observed that 53 out of the 187 compounds (about 28%) were described in the literature for the very first time. Besides, from Fig. 38, the majority of the NPs were terpenoids (30%), followed by flavonoids (22%), alkaloids (19%) and quinones (15%), the rest of the compound classes, each representing only less than 5% of the entire compound collection. It was also observed that most of the plant species from which the compounds were identified were of the families Rubiaceae, Meliaceae, and Asphodelaceae (Fig. 39).

Compound distribution by plant families
A classification of the compounds by class into the plant families showed that most of the plant families represented their typical (chemotaxonomic) compound classes, often seen in the literature for species harvested from the African continent [34,[111][112][113][114]. As an example, for the collected data (Fig. 40), all the 26 compounds from the Leguminoceae-Fabaceae were flavonoids, while 23 out of the 25 anti-malarial NPs from the Asphodelaceae were quinones. It was also noted that 27 out of the 34 compounds from the Meliacious species were terpenoids, just like the Euphorbiaceous species that included 12 terpenoids out of 13 compounds identified within the family. Meanwhile, all the 12 compounds from the Ancistrocladaceae were alkaloids, just like the Loganiaceae and Annonaceae for which all 8 compounds and 9 out of the 12 identified compounds were, respectively, alkaloids. On the contrary, the compounds from the Rubiaceous species were distributed among different classes, the majority being phenolics and quinones.

The most active compounds
Raw data retrieved from the literature showed activities reported in diverse units. A classification of the compounds by potencies (after all measured IC 50 values were converted to μM), and taking a cut off of 10 μM for the most promising secondary metabolites most likely to be lead compounds. The most active compounds within this range for at least one plasmodial strain, i.e. 25 out of 66 NPs were alkaloids ~ (38%), while 23 of them were terpenoids ~ (35%) and 11 were quinones ~ (17%). Taking a cut off IC 50 value of at most 1 μM left us with 19 compounds, 14 of them being alkaloids. Besides, the majority of the 187 NPs were terpenoids (30%), followed by flavonoids (22%), alkaloids (19%) and quinones (15%), the rest of the compound classes only represent a negligible part of the current collection.

Conclusions
In this review, an attempt has been made to document the anti-malarial/antiplasmodial activities of NPs derived from African medicinal plants in their various compound classes and source species, published between 2013 and 2019. A description of the in vitro and available in vivo activities for 187 compounds is shown, as well as their classification into the various known NP compound classes and plant families of origin. From the collected data, the most active compounds belong to the same compound classes as the malarial drugs of natural origin, e.g. the alkaloid class for quinine and the terpenoid class for artemisinin. A previous report from Titanji et al. [115] had shown that plant-derived alkaloids from African medicinal plants have a great potential for anti-malarial drug development.
Although recently published reviews have described the activities of anti-malarial secondary metabolites of terrestrial and marine origins, input data from African sources has not been the focal point. Tajuddeen and van Heerden recently published a review of 1524 natural compounds from around the world, which have been assayed against at least one strain of Plasmodium, out of which 39% were described as new NPs, with 29% having IC 50 values ≤ 3.0 µM against at least one of the tested plasmodial strains [116]. However, the study was limited to the period between 2010 and 2017 and did not include data from 2018 to 2019. Although the ability of NPs to block the transmission of malaria is still in the early stage, the current review, along with the previous studies that covers data for antiplasmodial compounds from African flora [27,28], could serve as the baseline data for the discovery of new anti-malarial compounds from Africa.