Development of Phytomedicines as Novel Antimalarial Lead Molecules: Progress towards Successful Antimalarial Drug Discovery

Abstract

Among numerous life-threatening infectious diseases (HIV/AIDS, TB, NTDs and EIDs), malaria continues to be the deadliest parasitic disease caused by Plasmodium protozoa transmitted by an infective female Anopheles mosquito. Plasmodium falciparum, the potentially fatal malaria parasite, is believed to be responsible for most of the morbidities and mortalities associated with malaria infections. Artemisinin-based Combination Therapies (ACTs) are currently considered to be the frontline therapy against malaria caused by P. falciparum. Despite significant progresses in antimalarial drug discovery, the control and prevention of malaria is still a challenging task. It is primarily because of the reduced clinical efficacy of existing antimalarial therapies including ACTs due to the widespread emergence of drug-resistant strains of malaria parasites, especially P. falciparum. It is, therefore, necessary to discover and develop novel drug candidates and/or alternative therapies for the treatment as well as prevention of resistant malaria. In this chapter, the potential of phytomedicines as natural sources of novel antimalarial lead molecules/ drugs with recent advances in phytomedicine-based antimalarial drug discovery has been reviewed.

1. Introduction

Malaria is a potentially life-threatening parasitic disease caused by Plasmodium protozoa transmitted by an infective female Anopheles mosquito. Along with human immunodeficiency virus/ acquired immunodeficiency syndrome (HIV/AIDS), tuberculosis (TB), neglected tropical diseases (NTDs) and viral hepatitis (hepatitis B), malaria affects billions of people, and causes more than 4 million deaths every year globally [1]. Apart from these infectious diseases, emerging infectious diseases (EIDs) are serious public health threats in the twenty-first century. Some deadly EIDs include severe acute respiratory syndrome (SARS), Ebola virus disease (EVD), Zika virus disease (ZVD), swine flu (H1N1 influenza), bird flu (avian influenza), chikungunya (CHIKV), dengue fever (DENV), hanta pulmonary syndrome (HPS, hanta virus), antibiotic-resistant infections (superbugs) and coronavirus disease (COVID-19, SARS-CoV-2) [2, 3].

According to the latest report by World Health Organization (WHO), about 229 million clinical cases of malaria with a death toll of 409,000 have been documented for the year 2019. In the same year, 94% of all malaria cases and deaths were found in the WHO African region. In the Southeast Asian region of WHO, there were an estimated 7.9 million cases of malaria in 2018. Children under 5 years of age are considerably at higher risk of malaria. They have been accounted for 67% of all malaria deaths worldwide in 2019 [4, 5, 6]. However, Plasmodium falciparum, the deadliest malaria parasite, is attributed to be responsible for most of the morbidity and mortality associated with malaria [7, 8]. Artemisinin-based Combination Therapies (ACTs) are currently considered as the frontline therapy against malaria caused by P. falciparum [9, 10]. Due to the widespread emergence and spread of drug resistant strains of P. falciparum, the clinical utility of existing antimalarial therapies including ACTs has been drastically declined [11, 12]. It has, therefore, become a serious health concern, which urgently necessities the discovery and development of novel drug candidates and/or alternative therapies for the treatment as well as prevention of resistant malaria. In this chapter, the potential of phytomedicines as natural sources of novel antimalarial lead molecules/ drugs with recent advances in phytomedicine-based antimalarial drug discovery has been briefly summarized.

2. Phytomedicines and antimalarial drugs

The discovery of antimalarial drugs from plant sources was started in 200 years back when quinine (QN), a cinchona alkaloid, was isolated from Cinchona bark in the year 1820. Earlier, the extract of Cinchona bark (also known as Peruvian Bark) was traditionally used for the treatment of fever by native Peruvian Indians in 1600s [13]. QN was the only known antimalarial drug for more than three centuries, and until the 1930s was the only effective therapeutic agent for the treatment of malaria. Later, the structure of quinine served as a template for the development of several synthetic congeners as potent antimalarial agents [13, 14]. The introduction of CQ, a 4-aminoquinoline derivative of QN, in the mid-twentieth century (1940) ceased the wide spread use of QN. Soon after its introduction, CQ became the mainstay of malaria chemotherapy, since it was clinically effective, less toxic and cheaper drug [15]. Another synthetic antimalarial, primaquine (PQ, 1950) was also developed thereafter based on the structure of lead QN molecule. PQ is a 8-aminoquinoline analogue of QN. Mefloquine (MQ), a synthetic quinoline methanol derivative of QN, was developed (1975) after CQ to treat resistant cases of malaria. Malaria parasites resistance to CQ and MQ began to appear within a few decades of introduction [16]. Later, several quinoline derivatives related to CQ (amodiaquine, AQ and isoquine, IQ) and MQ [halofantrine (HL), lumefantrine (LUM) and pyronaridine (PYN)] were developed and found effective (in combination with ART-based drugs such as dihydroartemisinin, artemether and artesunate) against CQ-resistant and/or multi-drug resistant (MDR) P. falciparum infections. Hepatotoxicity and cardiotoxicity are some serious toxic effects associated with these drugs [17]. Moreover, rapid development of resistance has severely limited the use of QN-based drugs alone, and therefore, they are used in combination with other drugs in the treatment of resistant malaria. The increasing prevalence of MDR strains of malaria parasites, particularly P. falciparum in most malaria endemic areas (Southeast Asia including Myanmar, Thailand, Vietnam and India, African continent and Eastern Mediterranean region) has significantly reduced the efficacy of CQ and other potent QN-based antimalarials in the treatment of malaria [8]. Figure 1 depicts structures of some QN-based antimalarial drugs.

Figure 1.

Some QN-based antimalarial drugs.

QN-based antimalarials are used widely in the treatment and prophylaxis of malaria. QN still remains an important antimalarial drug due to the emergence of CQ-resistant and MDR strains of malaria parasites, especially P. falciparum. Due to its undesirable side effects, it is now only used as an intravenous injection (as sulphate salt) to treat severe malaria. CQ (as phosphate salt) still remains the first-line drug in the treatment of uncomplicated sensitive P. falciparum malaria, despite its increasing resistance to parasites, due to its easy availability, low cost and good tolerability. In CQ-resistant malaria, the next drug of choice is MQ, followed by QN in combination with tetracycline, doxycycline or sulphadoxine-pyrimethamine (SP). MQ and AQ are widely available and are used to treat cases of uncomplicated malaria in areas where CQ resistance is prevalent [18, 19].

QN-based drugs are blood stage schizonticidal. CQ/MQ is selectively active against the intra-erythrocytic mature forms (trophozoites) and also younger ring forms of malaria parasites, without any activity against gametocytes. QN-based drugs inhibit the heme polymerase enzyme resulting in specific toxicity during the developmental stage of the parasite. CQ accumulates by a weak base mechanism in the acidic food vacuole of trophozoite-infected cells and act by forming a complex with heme in the parasite food vacuole, which prevents heme polymerization and consequently, hemozoin formation. Simply, they these drugs block the polymerisation of heme to haemozoin (malaria pigment). As a result, the heme which is released during haemoglobin degradation builds up toxic accumulation of heme (haematin), thereby kills the parasite with its own toxic effects. The mode of action of QN is similar to CQ. QN binds strongly to heme protein and forms complexes that are toxic to the malaria parasite, as already delineated above. MQ also acts by inhibiting the heme polymerase, similar to CQ [8, 18, 19, 20].

ART, an active constituent of Artemisia annua L. (Sweet wormwood) and related compounds (semi-synthetic derivatives) showed promising antimalarial efficacy in clinical trials in 1970s (1972) and till date they are considered as the most effective and potent antimalarial agents [21]. Since ART is not soluble in water or oil, it has several limitations such as poor aqueous solubility, oral absorption and bioavailability. Reduction of ART (sesquiterpene lactones or cyclic endoperoxide) produced dihydroartemisinin (DHA), a sesquiterpene lactol, which served later as a template for the synthesis of a series of semi-synthetic analogues such as artemether (AM), arteether (AE) and artesunate (AS). They are collectively termed as the first-generation derivatives of ART [22, 23]. First-generation ART derivatives can be further grouped into oil soluble C(10) β-alkyl ethers (AM and AE) and water soluble C(10) β-(substituted) esters (sodium artesunate and sodium artelinate). These drugs possess better oil/water solubility, and therefore, have superior pharmacokinetics properties with increased antimalarial efficacies over the parent compound, ART [8]. Figure 2 represents the structures of ART and some ART-based antimalarial drugs.

Figure 2.

ART and some ART-based antimalarials.

Because of having excellent antiparasitic efficacy against resistant parasites, ART-based drugs mostly replaced the use of QN- and antifolate-based drugs. ART derivatives are fast-acting antimalarials effective against MDR strains of P. falciparum and are used for the treatment of severe and complicated malaria. ART-based drugs showed very rapid clearance of parasites and faster resolution of fever as compared to QN. In some areas of Southeast Asia, combinations of ART-based drug and MQ offer the only reliable treatment for uncomplicated MDR P. falciparum malaria [24, 25].

ART and its analogues are active against all blood stages, particularly against younger ring forms and gametocytes. They have no activity on hepatic stages of parasites. They reduce parasitemia very rapidly and are well tolerated in both adults and children. ART and related compounds are concentrated in parasite-infected erythrocytes and exert their parasiticidal activity subsequent to reductive activation by heme in an irreversible redox reaction, which produces toxic carbon-centred free radicals. Toxic free radicals may lead to alkylation of heme or bring about oxidative damage to parasite proteins/lipids. The endoperoxide group, therefore, appears to be crucial for the antimalarial activity. The antimalarial activity of ART may also result from the inhibition of a parasitic calcium ATPase enzyme [8, 24, 25, 26].

The development of atovaquone (ATO), a 2-hydroxy-1,4-napthoquinone antimalarial, began more than 50 years ago when the outbreak of World War II caused substantial shortages in the supply of QN. ATO is an analogue of lapachol (a prenylnaphthoquinone isolated from Tabebuia species, Lapacho tree, 1892). Lapachol was used as an antimalarial lead molecule for the development of ATO. It is effective against CQ-resistant P. falciparum, but because, when used alone, resistance develops rapidly, ATO is often given in combination with proguanil (PG). A new fixed-dose antimalarial combination of ATO and PG (Malarone, 1998) is available in the market worldwide. Malarone shows good tolerability with minimal side effects in children and adults with uncomplicated malaria. It is used as chemoprophylaxis for the prevention of malaria in travellers. ATO represents a novel class of expensive antimalarial drug. ATO is used in combination with PG (a selective inhibitor of dihydrofolate reductase, DHFR) or tetracycline for the prevention as well as treatment of CQ-resistant malaria, including cerebral malaria caused by P. falciparum. It is as effective as MQ or doxycycline. ATO acts through the inhibition of electron transport at the Plasmodium mitochondrial cytochrome bc1 complex and depolarizes the membranes of Plasmodial mitochondria[15, 16]. The structure of ATO is given in Figure 3.

Figure 3.

Structure of ATO.

3. Approaches to antimalarial drug discovery

The objective of antimalarial drug discovery is to find out new and potent drug candidates based on the knowledge of existing and/or novel drug targets. It is necessary to develop affordable and safe drugs that would be reasonably cheaper, non-toxic to host tissues, and clinically effective against resistant malaria parasites. Suitable in vitro and in vivo experimental methods are, therefore, used for the evaluation of efficacy as well as toxicity of newer antimalarial agents. However, there are several traditional and modern approaches to antimalarial drug discovery programme, which include traditional evaluation of bioactive natural products/phytomedicines, molecular modifications of existing lead molecules, reverse pharmacological or drug repurposing approach and drug discovery based on CADD/SBDD approach [8]. Brief explanations of these approaches are given here under (Figure 4).

Figure 4.

Approaches to antimalarial drug discovery.

3.1 Ethnomedicinal evaluation based approach

The investigation of medicinal plants having traditional/ folkloric uses as antimalarial medicine may be potential sources of novel bioactive compounds that can be further developed into potent antimalarial drugs and/or lead molecules. Several tribes and aboriginals of Asian, African and South American continents still rely on plant-based ethnomedicines for the management of fever and malaria-like illness. QN and ART were discovered from the ethnomedicinal use of Cinchona and Artemisia plants, respectively. They served as lead structures in the development of many more potent antimalarial drugs of current use. Considering the above fact, thousands of medicinal plants and traditional formulations have been screened (in vitro and in vivo) to aid bioactive fraction guided discovery of antimalarial lead molecules [27].

3.2 Random high-throughput screening

Random high-throughput screening of plant extracts is one of the common approaches for antimalarial drug discovery. Scientists and researchers perform random screening of plant extracts against Plasmodium strains by various in vitro methods in search for novel antimalarial compounds. Depending upon preliminary antimalarial efficacy (IC₅₀) and cytotoxicity profile (CC₅₀) obtained in vitro, plant extracts and/or isolated pure compounds can be further subjected to in vivo experimental (ED₅₀ and Pharmacokinetics) studies [28].

3.3 Plasmodium life cycle targeted drug discovery

This is believed to be the most potential approach in antimalarial drug discovery programme. Specific proteins or enzymes that are essential biological components in the life cycle of Plasmodium parasite may provide novel targets for the discovery of drug molecules. For instance, falcipains (FP), plasmepsins (PM), dihydrooroate dehydrogenase (DHOH), phosphatidylisositol-4-kinase (PI4K), cytochrome bc1 (Cyt bc1) and Na⁺-ATPase 4 are some novel drug targets discovered from the biology of P. falciparum [8].

3.4 Indigenous phytomedicine-based reverse pharmacological approach

Reverse Pharmacology deals with the precisely designed preclinical and clinical research of age old herbal medicine used in well documented indigenous system of medicine (Ayurvedic medicine, Chinese medicine etc.) with a view of better understanding of the mechanism of action (even at molecular level) followed by the isolation of bioactive molecule(s) and finally the development of lead molecule(s). In fact, the discovery and development of ART (from A. annua) and its derivatives are the result of reverse pharmacological approach. Another interesting example is the discovery of antiplasmodial protoberberine type alkaloids allocryptopine and protopine from A. mexicana. This approach is considered to be quite reliable and faster technique due to the availability of prior information about therapeutic and toxic properties of the plant species under investigation. However, the discovery of potent lead molecule(s) with desired pharmacological/toxicity profile may sometimes be difficult because herbal medicine/ plant extracts possess therapeutic efficacy due to the synergistic activity of multiple ingredients in the crude mixture [26, 29].

3.5 Drug repurposing approach

Repurposing of existing drugs with new therapeutic indications is also considered as one of the effective alternatives for the discovery of antimalarial drugs. The notable advantage of this approach is that the mechanism of action and toxicity of drugs have already been established in clinical trials for other diseases. Folate antagonists (sulphonamides, sulphones, biguanides, pyrimethamine, triazines, etc.) and several antibacterials/ antibiotics (tetracycline, doxycycline, clindamycin etc.) have been reported to exhibit promising antiplasmodial efficacy against malaria parasites. In recent days, drug repurposing involves the combined efforts of in silico and in vitro methods to identify new therapeutic uses of existing drug molecules on a rational basis. Using the same strategy, researchers have been working on existing drugs in search for new antimalarial drug candidates. Repurposing of azithromycin, auranofin, loperamide hydrochloride, amlodipine besylate, cyclosporin A, esomeprazole magnesium, omeprazole etc. with antimalarial activity have been reported in literature [30, 31].

3.6 Semi-synthetic modifications or designing of analogues

Novel antimalarial drugs can be developed from the semi-synthetic modification of naturally derived lead molecules and/or by designing of newer synthetic analogues/ derivatives of existing drugs based on the structure-activity relationship (SAR) approach. This approach mainly emphasizes on reducing the toxicity with retaining and/or enhancing the therapeutic efficacy of the basic template structure/ lead molecule. Synthetic quinolines like CQ, AQ, IQ (4-aminoquinolines), PQ (8-aminoquinolines) MQ, HL, LUM (quinoline amino alcohols), piperaquine (PIP, bisquinoline analogue) and PYN (benzonaphthyridine derivative) were developed based upon the structural template of QN. Several chemical strategies were involved in structural modification of QN or other lead molecules in order to improve the therapeutic efficacy as well as toxicity of the parent molecule. Tebuquine (4-aminoquinoline derivative, a CQ analogue) and tafenoquine (8-aminoquinoline derivative, a derivative of PQ), are two newer drugs developed recently. Ferroquine (4-aminoquinoline derivative, a CQ analogue, Phase II terminated), AQ-13 (4-aminoquinoline analogue, Phase II) are presently under development. Following similar approach, DHA, AM and AS were also developed from ART. Some newer drugs (belonging to different classes) that are under development include DSM265 [Pf dihydrooroate dehydrogenase (DHOH) inhibitor, a triazolopyrimidine-based drug, Phase II], MMV390048 [Pf phosphatidylisositol-4-kinase (PI4K) inhibitor, Phase I] and KAE609 or cipargamin (Na+-ATPase 4 inhibitor) [8, 32, 33, 34, 35, 36, 37, 38].

3.7 Combination therapy approach

The concept of combination therapy (CT) is based on the synergistic or additive activity of two or more drugs, which improves therapeutic efficacy and also delays the development of resistance to the individual drugs of the combination. In antimalarial combination therapy, two or more drugs are used together that act with independent mode of action probably at different biochemical targets in the life cycle of Plasmodium parasite. WHO recommended combining the rapid schizonticidal ART derivative (DHA, AM or AS) with one or more partner drugs (from different class of antimalarials having longer biological half-lives) for the treatment of resistant P. falciparum malaria. Such combined antimalarial drug regimens (for examples, AM + LUM (Co-Artem, fixed dose, AL), AS + MQ (AM), AS + CQ, AS + SP, AS + DOX, AS + DOX + CQ etc.) are known as ACTs. Some ACTs which are in pipeline include AS + PYN, DHA + PIP (Artekin), DHA-PIP- Trimethoprim and DHA + PIP + MQ [8, 25].

3.8 Drug discovery by CADD/SBDD approach

Traditionally, drugs are discovered by testing naturally derived or synthetically obtained compounds in time-consuming multi-step processes against a battery of in vitro and in vivo screening methods. Compounds having promising therapeutic potential are further investigated for their development as drug candidates after pharmacokinetic, metabolism and toxicity studies. Today’s modern drug discovery process involves rational design and development of novel drug molecules based on a particular disease target using modern tools and techniques of virtual and experimental screening techniques. In virtual screening, computational methods screen large chemical libraries targeted towards a specific biological receptor, using advanced high performance computing environments, data management software and internet. It delivers new drug candidates quickly and at lower costs. Virtual screening is an approach of structure-based drug design (SBDD) that uses computer-based (in silico) methods to discover and develop new drug molecules on the basis of biological structures of particular disease of interest. SBDD methods mainly focus on the design of molecules for a disease target with known three dimensional structures followed by the determination of their binding affinity for the target by molecular docking along with other in silico screening methods (ADMET and toxicity screening) for optimization of molecules during development. The process of SBDD proceeds through design and development of a series of consecutive steps from hit identification to lead optimization followed by preclinical and clinical development of drug candidates [38, 39]. Antimalarial drug discovery based on SBDD approach involves the application of modern tools of molecular modelling and other in silico techniques in the development of novel antimalarial drug candidates (Figure 5).

Figure 5.

Antimalarial drug discovery based on SBDD approach.

4. Phytomedicines and antimalarial lead molecules: recent developments

Phytomedicines (i.e., plant-based/ herbal traditional medicine systems) served as potential sources of lead molecules for the development of several clinically useful antimalarial drug candidates. For example, QN isolated from Cinchona bark was used as a template for the development of CQ and MQ. ART isolated from Artemisia annua has been utilized for the successful development of various semi-synthetic derivatives (DHA, AM and AS) which are currently used in the treatment of CQ-resistant P. falciparum malaria [40, 41]. Apart from QN and ART, some examples of antimalarial natural products that were developed from plants include yingzhausu A, febrifugine, sergeolide, chaparrin, glaucarubin, tehranholide and brusatol [42].

During the last few decades, a large number of plant species have been identified to be effective as antimalarial agents. Pure phytochemicals isolated from these plants have been reported to exhibit antimalarial effectiveness, particularly, against CQ-sensitive and CQ-resistant strains of P. falciparum. It is, therefore, imperative that antimalarial phytochemicals reported with promising in vitro and in vivo activities can be further subjected to preclinical and clinical confirmation for their development as novel antimalarial lead molecules and/ drug candidates. Plant-derived antimalarial compounds belong to several phytochemical classes of natural products such as alkaloids, terpenoids, quassinoids, limonoids, Polyphenols and flavonoids, coumarins, steroids, anthraquinones, naphthoquinones etc.

Terhanolide (artediffusin), a sequiterpene lactone isolated from A. diffusa exhibited antimalarial efficacy against P. falciparum (in vitro) and P. berghei (in vivo) [43]. Halofuginone, an analogue of febrifugine (an alkaloid originally isolated from the plant Dichroa febrifuga) exhibited antiplasmodial effects against CQ-sensitive and CQ-resistant P. falciparum (in vitro) with curative effects in P. berghei-infected mice [44]. Sergeolide, a quassinoid from Picrolemma pseudocoffa showed antimalarial activities in vitro against P. falciparum and in vivo against P. berghei in mice [45]. Further, the antimalarial property licochalcone A (oxygenated chalcone) obtained from Chinese licorice has been reported to exhibit antimalarial activity against CQ-sensitive and CQ-resistant Plasmodium strain. Lichochalcone-A was the first natural derivative of chalcones with antimalarial effectiveness against CQ-resistant strain of P. falciparum [43]. Figure 6 displays structures of some recently developed plant-derived antimalarial compounds.

Figure 6.

Structures of some recently developed plant-derived antimalarial compounds.

Herein, phytomedicine-derived antimalarial compounds are categorized into two broad groups, viz. alkaloids and non-alkaloids [46]. Different alkaloids such as indoles, bisindols, isoquinolines (naphthyl and benzyl), piperidines, pyrroles, quinolones, steroidal alkaloids have been reported to possess antimalarial effectiveness. Polyphenolic compounds and bioflavonoids including dietary flavonoids such as kaempferol, myricetin, quercetin and isoquercitrin possess in vitro antimalarial activities. Different terpenenoids (farnesol, nerolidol, limonene, and linalool), quassinoids, coumarins and limonoids also exhibited antiplasmodial activity when tested in vitro against P. falciparum strains [47, 48]. Semi-synthetic triterpenes such as balsaminoside B, karavilagenin C, S-farnesylthiosalicylic acid, and karavoates B and D have been reported to exhibit in vitro and in vivo antimalarial activity [49, 50, 51]. Table 1 describes phytomedicines as potential sources of novel antimalarial compounds.

Name of specific phytochemical(s)	Type of compound(s)	Plant source (Family)	Antimalarial/ Antiplasmodial activity
Alkaloids
Strychnogucine B; Strychnobaillonine	Bisindole alkaloid	Strychnos icaja Baill.; (Loganiaceae)	In vivo antimalarial activity (30 mg/kg/d dose) against P. berghei in murine model; Potent in vitro antimalarial activity against; CQ-sensitive 3D7 strain of P. falciparum with; IC50 value of 1.1 μM
Lycorine	Indolizidine alkaloid	Plants from Amaryllidaceae family	In vitro antimalarial activity with IC50 value of 0.029 μg/mL against FCR-3 African strain of P. falciparum
Caesalminines A & B	Tetracyclic cassane-type diterpenoids alkaloids	Caesalpinia minax Hance (fabaceae)	Antiplasmodial activity with IC50 values between 0.42 and 0.79 μM
8α-Polyeolinone, polyalthenol, N-acetyl-8α-polyeolinone and N-acetyl-polyveoine	Indolosesquiterpene alkaloid	Polyalthia oliveri Pellegr. (Annonaceae) Polyalthia suaveolens Engl. & Diels. (Annonaceae) (syn.Greenwayodendron suaveolens Engl. & Diels. Verdc. (Annonaceae)	Antiplasmodial activity against NF54 strain of P. falciparum with IC50 of 2.8 μM
Strychnochrysine	Bisindolomonoterpenoid alkaloid	Strychnos nux-vomica Linn. (Loganiaceae)	Antiplasmodial activity against CQ-sensitive and CQ-resistant strains of P. falcipaum
Conessine	Steroidal alkaloid	Holarrhena antidysenterica (L.) Wall. Ex A. DC. (Apocynaceae)	Antimalarial activity against CQ-sensitive P. berghei NK65 strain in BALB/c mice
Conessine	Steroidal alkaloid	Holarrhena antidysenterica (L.) Wall. Ex A. DC. (Apocynaceae)	IC50 values of 1.9 μg/ml and 1.3 μg/ml in the schizont maturation and pLDH assays,
Mokluangin D irehline and mokluangin A	Pregnene-type alkaloid	Holarrhena pubescens (Buch.-Ham.) Wall. Ex G. Don. (Apocynaceae)	Antimalarial activity against MDR P. falciparum K1 strain with IC50 values between 1.2 and 2.0 μM
N-3-benzoyldihydrocyclomicrophylline F	Steroidal alkaloid	Buxus cochinchinensis Pierre ex Gagnep. (Buxaceae)	Antimalarial activity against DR Dd2 strain of P. falciparum with IC50 value of 2.07 μM
Alstonisine	Indole alkaloid	Alstonia macrophylla Wall.ex G.Don (Apocynaceae)	Antiplasmodial activity against P. falciparum with IC50 value of 7.6 μM
20-Epi-dasycarpidone	Indole alkaloid	Aspidosperma ulei Markgr. (Apocynaceae).	Active against MDR K1 strain of P. falciparum with IC50 value of 16.7 μM
16-demethoxycarbonylvoacamine	Sarpagine-type indole alkaloids	Tabernaemontana macrocarpa Jack. (Apocynaceae)	Antiplasmodial activity against 3D7 strain of P. falciparum
Dehydrotylophorine, dehydroantofine and tylophoridicine D	Phenanthroindolizine alkaloids	Ficus septica Burm.f. (Moraceae)	Antimalarial activity against 3D7 strain of P. falciparum with IC50 values of 0.42, 0.028, 0.058 μM
10-Demethylxylopinine	Isoquinoline alkaloids	Actinodaphne macrophylla (Blume)Nees (Lauraceae)	In vitro antiplasmodial activity against 3D7 strain of P. falciparum
(+)-N-methylisococlaurine, atherosperminine, 2-hydroxyathersperminine	Isoquinoline	Cryptocarya nigra R.Br. (Lauraceae)	Antiplasmodial activity against CQ-resistant strain of P. falciparum (K1 strain) with IC50 values of 5.40, 5.80, and 0.75 μM
Dihydronitidine	dihydronitidine	Zanthoxylum heitzii (Aubrey. & Pellegr.) P.G. Waterm. (Rutaceae)	Potent against P. falciparum with IC50 value of 25 nM
(−)-Pseudocurine	Bisbenzylisoquinoline	Stephania abyssinica Oliv. (Menispermaceae)	Antiplasmodial activity against both CQ-susceptible D6 and CQ-resistant W2 strains of P. falciparum (IC50 = 0.29±0.00 and 0.31±0.01 μg/ml, respectively)
(+)-laurotetanine, (+)-norstephasubine	Bisbenzylisoquinoline	Alseodaphne corneri Kosterm. (Lauraceae)	In vitro antiplasmodial efficacy with IC50 values of 0.189 and 0.116 μM
Dioncophylline F	Naphthylisoquinoline alkaloid	Ancistrocladus ileboensis Heubl, Mudogo & G. Bringmann. (Ancistrocladaceae)	Highly effective and specifically active against P. falciparum
Pseudopalmatine; Obtusipetadione; Anonaine; Tavoyanine A, roemerine, Laurolitsine and boldine	Aporphine alkaloid	Stephania rotunda Lour. (Menispermaceae); Dasymaschalon obtusipetalum Jing Wang & R.M.K. Saunders. (Annonaceae); Xylopia sericea A.St.-Hil. (Annonaceae); Phoebe tavoyana (Meissn.) Hook f. (Lauraceae)	Effective against W2 strain of P. falciparum with IC50 value of 2.8 μM; In vitro antiplasmodial activity against MDR P. falciparum strains (TM4 and K1) with IC50 values of 2.46 ± 0.12 and 1.38 ± 0.99 μg/mL; Antiplasmodial activity against CQ-resistant W2 strain of P. falciparum; With IC50 value of 23.2 ± 2.7 μg/ml; Potent inhibitory activity against 3D7 strain of P. falciparum 3D7 with IC50 values of 0.89, 1.49 and 1.65 μg/ml
Sebiferine	Morphinandienone type alkaloid	Phoebe tavoyana (Meissn.) Hook f. (Lauraceae)	Potent inhibitoryactivity against the growth of P. falciparum 3D7 clone, with IC50 values of 2.76 μg/ml
Simplicifolianine	Protoberberine	Meconopsis simplicifolia (D. Don) Walpers (Papaveraceae)	Antiplasmodial activity against P. falciparum strains, TM4/8.2 (CQ-antifolate-sensitive strain) and K1CB1 (MDR) with IC50 values of 0.78 μg/mL and 1.29 μg/mL, respectively
Coptisine	Protoberberine-type alkaloid	Coptis chinesis Franch. (Ranunculaceae)	Potent inhibitory activity against P. falciparum dihydroorotate dehydrogenase (Pf DHODH) with IC50 value of 1.83 ± 0.08 μM
Miliusacunines A	Oxoprotoberberine	Miliusa cuneata (Graib). (Annonaceae)	In vitro antimalarial activity against TM4 strain of P. falciparum with IC50 value of 19.3 ± 3.4 μM
Hymenocardine N-oxide	Cyclopeptide alkaloids	Hymenocardia acida Tul. (Phyllanthaceae)	Antiplasmodial activity against P. falciparum with IC50 value of 12.2 ± 6.6 μM
Microthecaline A	Quinoline alkaloid	Eremophila microtheca F.Muell. (Scrophulariaceae)	Moderate antimalarial activity against P. falciparum (3D7 strain) with IC50 value of 7.7 μM
Sauristolactam	Pyridocoumarin alkaloid	Goniothalamus australis Jessup. (Annonaceae)	Potent antimalarila activity against CQ-sensitive P. falciparum (3D7 strain) with IC50 value of 9.0 μM
Normelicopidine	Acridone Alkaloid	Zanthoxylum simullans Hance (Rutaceae)	Active against drug resistant Dd2 strain of P. falciparum with IC50 value of 18.9 ug/mL
Carpaine	Macrocyclic dilactone	Carica papaya L. (Caricaeae)	Potent antimalarial activity activity against 3D7 (sensitive) and Dd2 (resistant) strains of P. falciparum with IC50 values of 4.21 μM and 4.57 μM, respectively
Palmitine and jatrorrhizine	Indole alkaloid	Penianthus longifolius Miers. (Menispermaceae)	In vitro antimalarial activity against P. falciparum with IC50 values ranging from 0.28 to 0.35 μg mL^−1
Liriodenine	Indole alkaloid	Glossocalyx brevipes Benth. (Siparunaceae)	Antimalarial activity against drug sensitive D-6 strain and NF54 strains of P. falciparum with IC50 values of 2.37 μM and 1.32 μM, respectively
Fagaronine	Indole alkaloid	Fagara zanthoxyloides (Lam). (Rutaceae)	Antimalarial activity in vitro against P. falciparum with IC50 value of 0.018 μg mL^−1
Strychnopentamine chrysopentamine	Indole alkaloid	Strychnos usambarensis Glig ex Engl. (Loganiaceae)	Antimalarial activity against CQ-sensitive (FCA 20) ( IC50 = 117 to 579 nM), moderately CQ-resistant (FCB1-R) (IC50 = 107–550 nM) and CQ-resistant (W2) ( IC50 = 145–507 nM) strains of P. falciparum
Ancistrobrevine; Ancistrobertsonine A, Ancistrobertsonine B, Ancistrobertsonine C, Ancistrobertsonine D	Naphthoisoquinolines	Ancistrocladus robertsoniorum J. Leonard. (Ancistrocladace)	Moderate antimalarial activity against K-1 and NF54 strains of P. falciparum (IC50 values ranges from 2.0 to 15.9 μM)
Habropetaline A, 5′-Odemethyl-dioncohylline A	Naphthoisoquinolines	Triphyophyllum peltatum (Hutch. & Dalz.) Airy Shaw (Dioncophyllaceae)	Antiplasmodial activities against K1 (CQ and pyrimethamine resistant) and NF54 (sensitive to all known drugs) strains of P. falciparum with IC50 values of 5.0 and 2.3 ng mL^−1, respectively
Nitidine	Furoquinolines alkaloid	Toddalia asiatica (L.) Lam. (Rutaceae)	In vitro antiplasmodial activity against K39 strain of P. falciparum with IC50 value of 0.045 μg mL^−1
β-hydroxydihydrochalcone Deguelin, obovatin	Flavonoids	Tephrosia elata Deflers . (Fabaceae)	Antiplasmodial activity against D6 and W2 strains of P. falciparum with IC50 values of 8.2 ± 0.8 and 16.3 ± 0.9 μM, respectively Antimalarial activity against D6 and W2 strains of P. falciparum with IC50 values ranging from 12.4 to 27.6 μM
Chrobisiamone A	Bischromone	Cassia siamea (Lam). (Fabaceae)	In vitro antiplasmodial activity against 3D7 strain of P. falciparum 3D7 (IC50 = 5.6 μM)
Series of twelve biflavonoids (amentoflavone and hinokiflavone derivatives)	Flavonoids	Selaginella bryopteris L. (Selaginellaceae)	Antiplasmodial activity against P. falciparum strains with IC50 value between 0.30 and 0.26 μM
Citflavanone lonchocarpol A 8-prenyldaidzein	Flavonoids	Erythrina fusca Lour. (Fabaceae)	In vitro antiplasmodial activity against P. falciparum at 12.5 μg/mL
Butyraxanthones A-D	Xanthone	Pentadesma butyracea Sabine (Clusiaceae)	Antiplasmodial activity against P. falciparum with IC50 values ranging from 4.4 to 8.0 μM
Kaempferol	Flavonols	Onions, kale, broccoli, apples, cherries, fennel, sorrel, berries, tea	In vitro antiplasmodial activity against P. falciparum with IC50 values 33 ± 7 μM(3D7 strain) and 25 ± 2 μM (7G8 strain)
Myricetin	Flavonols	Onions, kale, broccoli, apples, cherries, fennel, sorrel, berries, tea	In vitro antiplasmodial activity against P. falciparum with IC50 values 40 ± 10 μM (3D7) and 76 ± 2 μM (7G8)
Quercetin	Flavonols	Onions, kale, broccoli, apples, cherries, fennel, sorrel, berries, tea	In vitro antiplasmodial activity against P. falciparum with IC50 values 15± 5μM μM (3D7) and 14 ± 1 μM (7G8)
Isoquercitrin	Flavonols	Onions, kale, broccoli, apples, cherries, fennel, sorrel, berries, tea	In vitro antiplasmodial activity against P. falciparum with IC50 values 66± 10μM (3D7) and 66± 10μM (7G8)
Luteolin	Flavones	Parsley, thyme, celery, sweet red pepper	In vitro antiplasmodial activity against P. falciparum with IC50 values 11± 1μM (3D7) and 12 ± 1 μM (7G8)
Chrysin	Flavones	Parsley, thyme, celery, sweet red pepper	In vitro antiplasmodial activity against P. falciparum with IC50 values 18± 3μM (3D7) and 22 ± 4 μM (7G8)
Okundoperoxide	bicyclofarnesyl sesquiterpene endoperoxide	Scleria striatinux de Wild (syn. S. striatonux) (Cyperaceae)	Antiplasmodial activity against CQ-sensitive (D6) and CQ-resistant (W2) strains of P. falciparum with IC50 values ranging from 176 to 180 μM
Fagraldehyde	Secoiridoid aglycone	Fagraea fragrans (Roxb.)DC. (Gentianaceae)	Effective in vitro against P. falciparum, exhibiting an IC50 value of 116.6 ± 9.4 μM (W2 strain)
6α,7β-Diacetoxyvouacapane	Diterpene	Bowdichia nitida Benth. (Fabaceae)	In vitro antiplasmodial activity against 3D7 strain of P. falciparum (IC50 = 1 μM)
Geraniol	Monoterpene	Pure isolated compound	Antiplasmodial activities against CQ-resistant FcM29-Cameroon strain of P. falciparum (IC50 = 52 μM)
Limonene	Monoterpene	Pure isolated compound	IC50 = 66 μ Mantiplasmodial activities against the chloroquine-resistant FcM29-Cameroon strain of P. falciparum
Ineupatorolide A	Sesquiterpene lactone	Carpesium rosulatum (Asteraceae)	In vitro antiplasmodial activity against CQ-resistant D10 strain of P. falciparum (IC50 = 0.019 μM) In vivo antimalarial activity against P. berghei in mice at doses of 2, 5 and 10 mg.kg^−1·day^−1

Table 1.

Phytomedicines as potential sources of antimalarial compounds [41, 47, 48, 52, 53, 54, 55, 56, 57, 58].

5. Challenges in antimalarial drug discovery

There are several challenges that exist in the domain of antimalarial drug discovery from plant sources. Some major challenges are low natural abundance of phytoconstituents, difficulty in isolation of the specific active compound in pure form, safety/toxicity and ADMET/pharmacokinetics issues, and high cost of production. Due to synergistic nature of crude plant extracts, it is also difficult to select the specific phytochemical responsible for the antimalarial action for isolation. Other issues include limited oral bioavailability and target specificity of natural molecules isolated from plants [59, 60]. Natural products with high degree of structural complexity and chemical instability are the other notable hindrances in the drug discovery pipeline of antimalarial drugs from plants. In vitro screening using parasitic cell cultures is a tedious work protocol which requires an expensive experimental set up and skilled laboratory personnel for the successful evaluation of antiparasitic activity. Similarly, the in vitro toxicity evaluation on normal cell lines requires extensive efforts, skills and labours. Compounds having high in vitro efficacy (IC₅₀ ≤ 1μM) and sufficient oral bioavailability can be considered for further in vivo testing. Compounds with ED₉₀ values of less than 10 mg/kg per os in in vivo murine model is essential for further development [12, 17]. An important challenge is the lacking of efficacy in preclinical trials after the successful in vitro and in vivo studies. Further, development of semi-synthetic derivatives from the natural lead(s) is a challenging task in context of designing scheme of synthesis, synthetic modification, purification of compounds and finally chemical characterization of pure compounds. High-throughput experimental assays eliminate potent antimalarial compounds due to toxicity issues and lack of pharmacokinetic properties [42]. Another challenge is the geochemical and climatic variation of plants. One more important challenge is that since no molecular mechanism and target specificity is known, it is very difficult to choose the in vitro or in vivo models for preliminary screening, and final confirmation of antimalarial efficacy with the exploration of mode(s) of action [59, 60]. Recently, in silico techniques based discovery of antimalarial drugs could reduce the chances of failure in the discovery pipeline. However, newer assays and target based approaches are required to be developed for discovery of newer congeners/ derivatives of naturally occurring potent molecules with desired antimalarial potency and less toxicity.

6. Conclusion

Re-emergence of resistance of existing drugs against P. falciparum, toxicity and unsatisfactory pharmacokinetics and less cost-effectiveness and poor patient compliance, particularly in South-east Asian and African regions are some major concerns in the malaria control and prevention programme worldwide. Although QN- and ART-based existing drugs/ therapies are considered as gold standards in malaria chemotherapy, the clinical utility of these drugs is challenging. Potent antimalarial compounds derived from phytomedicines could serve as potential sources of future antimalarial leads/ agents after a plethora of drug development (pre-clinical and clinical studies) processes. Target-based discovery of bioactive phytochemical entities is required for their successful development as effective and safe antimalarial drug molecules.

Conflict of interest

Authors declare that there is no conflict of interest.