Antimalarial activities of plants with medicinal potential: a systematic review of the literature

Objective: This is a qualitative study, whose objective was to investigate the scientific literature on plant species potentially active against Plasmodium sp. Method: This is a systematic literature review, which aimed to analyze the most recent articles published between the years 2005-2020 in the languages: English and Portuguese. The studies were chosen in an integrative way from the following databases: PubMed (National Library of Medicine), LILACS (Latin American and Caribbean Health Sciences Literature), Science Direct (Explore, scientific and medical) and SciELO (Scientific Electronic Library Online). Results and discussion: 115 species distributed in 50 botanical families were found in antiplasmodial inhibition studies, of which 66 different types of extracts showed action in eliminating these parasites, while 59 of these extracts were inactive. Of this total number, the most studied species belong to the Asteraceae and Fabaceae families. In addition, another 141 botanical species were cited in ethnobotanical surveys in different regions of the world. Aponynaceae and Lamiaceae were the most representative plant families among the studies focused on this topic. The data also allowed us to understand how popular knowledge can help to establish scientific discoveries about plants with antimalarial potential. In addition, environmental conditions were identified as determining factors for the production of chemical constituents in these plants. Conclusion: Efforts to identify plants with active potential in combating the parasite have increased significantly in recent years; however, it is important to emphasize that the preservation of biodiversity needs to be an important aspect of ethnobotanical research in order to guarantee the sustainable use of available resources.

On the other hand, medicinal plants have historically been a means through which studies seek for antimalarial targets; for example, the discovery of quinine extracted from the bark of Cinchona pubescecens, artemisinin which was evaluated from extracts of Artemisina annua L., a traditional Chinese plant used to reduce fever (Bero et al., 2009;Tu, 2011). In this scenario, the search for compounds applicable towards the development of new drugs as alternatives to current antimalarial agents which are facing cases of resistance, remains important (Ménard et al., 2016;Leang et al., 2015;Pluijm et al., 2021).
From this perspective, the discovery of new antimalarials with action against different morphological stages of the parasite and with different mechanisms of action, becomes essential in the control and elimination of this disease (Tse et al., 2019). Thus, considering the lack of access to rapid treatment in some communities, over the decades, plants with medicinal potential have been used as an alternative means of treatment in these locations (Moraes et al., 2020;Martinez et al., 2020).
Considering the importance of adequate and effective treatments as a tool to contain episodes of malaria caused by Plasmodium spp, including the severe cases described in the study by Kotepui et al. (2020), the present study aimed to investigate in the scientific literature active and inactive medicinal plants against malaria parasites, emphasizing changes between different ecosystems.

Materials and Methods
The literature was reviewed in search of scientific articles reporting antispasmodics activities (IC50 or µg/mL) of plants with medicinal potential used around the world for the treatment of malaria.

Search strategy and selection criteria
The study is a systematic and integrative review of the literature carried out in accordance with the study published by Tajbakhsh et al. (2021). A review of studies was carried out in PubMed (National Library of Medicine), LILACS (Latin American and Caribbean Health Sciences Literature), Science Direct (Explore, scientific and medical) databases and in the SciELO virtual library (Scientific Electronic Library Online).
Articles in both English and Portuguese were selected that had the descriptors MESCH/DECS and the following terms combined with the Boolean operators "AND" and "OR": "plant medicine malaria", "herbal malaria", "antimalarials vivax" and "ethnobotany Plasmodium".
Studies carried out between 2005 and 2020, the search was limited to studies published in English and Portuguese bthat were available in full and addressed this theme of medicinal plants, antimalarials and ethnobotanics, were selected.
Project documents, reports, grey literature, papers presented at conferences, articles that overlap the theme, studies on in vivo tests with animals and those repeated in the databases were excluded. The titles and abstracts were subsequently examined by two reviewers, independently (parallel method) to identify articles reporting the antiplasmodial activity of potential medicinal plants.
The following data were extracted from the selected articles by the reviewers: plant species, plant family, place of collection of plant, parts of the plant used, solvent used, isolated compounds. The entire selection process is presented in (Figure   1).
Data collection and analysis were performed by reading the titles and abstracts, choosing the complete texts based on the eligibility criteria, and extracting the data in a standardized Microsoft Office Excel® 2019 spreadsheet, tabulated in GraphPad Prisma Software (version 6).

Results and Discussion
In carrying out this study, it was possible to identify relevant aspects of empirical knowledge and common sense in relation to the use of medicinal plants as antimalarials by populations in different regions of the world, after using the keywords.
In the PubMed data platform, 987 articles were found, but only 110 were selected according to the exclusion criteria.
Similarly, 22 articles were selected from a total of 105 found in LILACS. In the SciELO database, 67 articles were found, but after applying the exclusion criteria, only 3 were in accordance with the objectives of this theme, and of the 2.047 articles found in the Science Direct database, 112 articles were selected. Ultimately, a total of 3.206 articles were found and only 247 were retained after analysis ( Figure 1).
In this research, the evaluation of individual plant species was considered as an independent study, so it is common for an article to have more than one study depending on the number of plant species evaluated (Tajbakhsh et al., 2021).
In this study, 115 plant species were cited within studies aimed at inhibiting P. falciparum, which in turn are distributed among 50 botanical families (Table 1). When analyzing the literature, the most expressive families in relation to species were: Asteraceae, Fabaceae, Euphorbiaceae, Annonaceae, Lamiaceae, Papaveraceae, Cucurbitaceae, Rubiaceae, Rutaceae, and Celastraceae ( Figure 2). As for the results obtained from the ethnobotanical surveys, 141 botanical species used for the treatment of malaria were found, distributed among 59 botanical families (Table 2), namely: Apocynaceae, Lamiaceae, Rutaceae, Solanaceae, Arecaceae, Asteraceae, Euphorbiaceae, Leguminosae, Meliaceae, and Anacardiaceae ( Figure 3). The parts of the plants most used for the extraction of chemical constituents with a possible action on malaria parasites were: leaf, aerial part, root, stem and stem bark, of which 66 botanical extracts, extracted using a variety of methodologies, were active in inhibiting P. falciparum, and 59 extracts were inactive (Table 1). The most cited plant parts in ethnobotanical surveys for therapeutic preparations were leaves, roots, stem bark, aerial part and stem. (Table 2).
Finally, the countries that contain the most plants utilized in studies on P. falciparum inhibition belong to the African continent, especially to the countries of Cameroon and Kenya. The countries with the highest number of botanical species cited were Brazil and Kenya.
It is important to highlight that all the studies came from in vitro assays with P. falciparum. No studies related to P. vivax were found in the literature, since this species presents limitations when maintained in culture (Bermúdez et al., 2018).        In the present study, the most frequently cited species were selected during the review of the literature, which describes in detail which part of the plant was used, which method was used for the extraction of chemical compounds and the result of inhibition of P. falciparum for each species studied (only for the studies that report these details). In addition, scientific knowledge is also linked to common sense, in order to clarify the real effects of these plants with medicinal potential.
In this regard, the botanical species Plectranthus barbatus Andrews, mentioned in the ethnobotanical survey by Mukungu et al. (2016), was tested for its ability to inhibit P. falciparum in vitro. After extraction of the possible active component by Owuor et al. (2012), it was observed that the leaf extract of this plant was inactive when evaluated against strain D6 (sensitive to chloroquine) and strain W2 (resistant to chloroquine).
The species Momordica foetida Schumach., belonging to the Cucurbitaceae family, showed low inhibitory concentration against strains D10 (sensitive to chloroquine) and K1 (resistant to chloroquine) (Waako et al., 2005). However, the study by Adia et al. (2016) using P. falciparum strains NF54 (sensitive to chloroquine) and FCR3 (resistant to chloroquine), showed high inhibition (≤ 10 µg/mL) with the ethyl acetate and water extraction method. The extraction method, plant species and part of the plant used were the same for both studies, so it is not clear why the results diverged. One of the factors that possibly influenced this divergence of results may have been the differences in the extraction solvent, therefore, in the extraction yield and in the extracted metabolite. For example, with dichloromethane, mainly non-polar metabolites are extracted. In contrast, with methanol, polar to nonpolar metabolites are extracted (Tajbakhsh et al., 2021).
Similar to the species M. foetida, another result of inactivity was also observed for the plant Carica papaya L. when the ethanolic extracts of the leaves were tested against two strains P. falciparum, one chloroquine-sensitive and the other chloroquine-resistant (Kovendan et al., 2012). However, the study by Julianti et al. (2013), revealed a high inhibition of 4.8 µg/mL against the P. falciparum K1 strain using a methanolic extract of the leaves. Upon analysis of the two studies, it is evident that the extraction method directly influenced the antiplasmodial action of this ethnospecies.
Unlike the species M. foetida e C. papaya, the results of two studies for the ethnospecies Albizia gummifera (J.F.Gmel.) C.A.Sm. corroborated the inhibition of P. falciparum in vitro at concentrations below 5.0 µg/mL (Orulla et al., 1996;Ofulla et al., 1995). These data reveal high antiplasmodial inhibition, a promising result for this plant.
Similar to that observed for A. gummifera, two studies on the botanical species Flueggea virosa (Roxb. ex Willd.) Royle's inhibition of P. falciparum in vitro showed promising results. Inhibitory activity against chloroquine-sensitive (D6 and 3D7) and chloroquine-resistant (W2 and K1) strains was obtained with concentrations below 25 µg/mL Singh et al., 2017).
Although other plants with results of proven activity were not mentioned in ethnobotanical surveys in this study, they also showed promising results of in vitro inhibition against P. falciparum (Table 1), however, other studies must be carried out in order to confirm this inhibition in models (in vivo) and characterization of secondary metabolites since few studies describe this part of the photochemistry.
The results observed for these ethnospecies validate how important it is to have scientific proof of their true therapeutic effects for medicinal use, not only for the treatment of malaria, but of all the pathologies for which the population makes use of these plant extracts (Martinez et al., 2020).
In this study, in addition to correlating scientific knowledge with popular knowledge, we also sought to carry out a survey of the main environmental conditions that can affect the production of secondary metabolites in a plant, and, consequently, make its possible principle active component ineffective.
Due to challenges along the way, some factors have affected the quality and quantity of active compounds in plants; thus, throughout evolution, plant species have adapted and developed mechanisms that allowed for their survival in different ecosystems of the world. That is, the same plant species is found in different countries with different climates. However, it should be noted that the metabolites depend on conditions such as: temperature, hydric stress, age (period), altitude, seasonality,  Source: Authors.
In addition, it is important to consider some issues regarding the period in which a plant was collected, since the quality and nature of the active constituents are not constant throughout the year (Gobbo-Neto et al., 2007). In this sense, variations in their chemical compounds may occur at different times of the year. Studies report that there are seasonal variations in the secondary metabolites of essential oils (Pitarević et al., 1984;Schwob et al., 2013), phenolic acids (Grace, Logan e Adams, 1998;Zidorn e Stuppner, 2001), flavonoids (Brooks et al., 2004;Jalal et al., 1982), saponins (Kim et al., 1981;Ndamba et al., 1993), alkaloids (Elgorashi et al., 2002;Roca-Pérez et al., 2004), and tannins (Feeny et al., 1968;Salminen et al., 2001).
In the spring, Digitalis obscura leaves present very low concentrations of cardenolides and lanatosides. However, it is possible to observe a rapid accumulation of these substances in the summer, and during the autumn they decrease again (Roca-Pérez et al., 2004). This same variation also occurs with Hypericum perforatum, popularly known as São João's herb. These substances increase from 100 ppm (parts per million) in the winter to 3000 ppm in the summer (Southwell et al., 2001).
The age and development of the plant, as well as the different types of plant organs, can also contribute to the total amount of metabolites produced (Bowers et al., 1993). The sesquiterpene lactones produced by Arnica montana are used as antiinflammatory agents. This plant species in the young phase accumulates helenalin derivative. This substance is reduced to almost zero around six weeks after leaf formation. However, unlike helenalin, the levels of dihydrohelenalin increase greatly and remain constant for a long period of time (Schmidt et al., 1998). Gentiana lutea leaves have a high concentration of C-glycosides in the flowering stage; O-glycosides and isoorientin are found in large amounts before their floral development (Menković et al., 2000).
In addition to age and seasons of the year, the adaptation of each plant species to different biomes has allowed plants to develop in a considerable temperature range, from tropical climates to arid environments and temperatures below 0. However, variations in temperature, as well as hydric variation, directly affect the production of secondary metabolites (Evans, 1996).

Studies by Zobayed, Afreen e Kozai (2005) evaluated the alteration of secondary metabolites under temperature stress in
Hypericum perforatum. In this study, it was possible to observe that temperatures above 35ºC and below 15ºC reduced the photosynthetic efficiency of the leaves, resulting in a low assimilation of CO2, compromising the production of secondary metabolites.
Along with high temperatures, low temperatures also significantly influence the quantity of secondary metabolites.
Artemisia annua, for example, after suffering metabolic stress showed a 60% increase in its levels of artemisinin, an active substance against P. falciparum. On the other hand, it was possible to observe a rapid decrease in dihydroartemisinic acid, which had been converted to artemisinin (Wallaart et al., 2000).
Issues related to seasonality and amount of rainfall can influence the production of secondary metabolites. In Hypericum perforatum, it is possible to observe an increase in the production of flavonoids, hypericins and chlorogenic acid in flowers under hydric stress, while the concentration levels of hyperforins drop drastically (Waterman e Mole, 1989).
Few studies report the relationship between changes in active compounds in high altitude regions. Of the few studies documented, a decrease was observed in deterpene alkaloids in Aconitum napellus and piperidines in Lobelia inflatas at high altitudes (Evans, 1996).

Conclusions
In view of the results obtained in this study, it is possible to observe the growth in in vitro studies with plants with medicinal potential for treating malaria, in addition, it is worth mentioning that many important findings have already been reported and implemented by the pharmaceutical industries. However, it is still necessary to invest in studies and technologies to detect new chemical targets with antimalarial potential.
In this study, in addition to conducting a systematic literature review, we also sought to confirm whether the plants mentioned in ethnobotanical surveys and in vitro studies had effects on malaria parasites. Of the 8 plants cited in botanical surveys, 5 ethnospecies were also being studied for their therapeutic potential for malaria in vitro. This correlation demonstrated the importance of combining empirical and scientific knowledge in the search for strategies for new prototypes of natural origin for various diseases, as well as the geo-environmental conditions of the site of this plant, since these factors can alter its chemical components.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.