Alkaloids from Plants with Antimalarial Activity: A Review of Recent Studies

Malaria is one of the major health problems in developing countries. The disease kills a large number of people every year and also affects financial status of many countries. Resistance of the plasmodium parasite, the causative agent, to the existing drugs, including chloroquine, mefloquine, and artemisinin based combination therapy (ACT), is a serious global issue in malaria treatment and control. This warrants an urgent quest for novel compounds, particularly from natural sources such as medicinal plants. Alkaloids have over the years been recognized as important phytoconstituents with interesting biological properties. In fact, the first successful antimalarial drug was quinine, an alkaloid, which was extracted from Cinchona tree. In the present review work, the alkaloids isolated and reported recently (2013 till 2019) to possess antimalarial activity are presented. Several classes of alkaloids, including terpenoidal, indole, bisindole, quinolone, and isoquinoline alkaloids, were identified with a promising antimalarial activity. It is hoped that the reports of the review work will spur further research into the structural modification and/or development of the interesting compounds as novel antimalarial drugs.


Introduction
Malaria is an extremely dangerous parasitic disease with ravaging effects in several parts of the world. e World Health Organization (WHO) estimate shows that approximately 3.3 billion people are living at risk places of malaria. Nearly 80% of cases and 90% of deaths are reported from sub-Saharan Africa and children under the age of 5 years and pregnant women are severely affected [1,2]. In 2016, it was estimated that there were 216 million cases of malaria globally and 445,000 deaths due to malaria [3]. Five protozoan species of the genus Plasmodium (Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, Plasmodium vivax, and P. knowlesi) are responsible for human malaria, although the majority of malarial infections are caused by P. falciparum and P. vivax. While P. vivax is less dangerous but more widespread, P. falciparum is fatal and is predominant in Africa [1,4].
Malaria has been treated with quinine, chloroquine, amodiaquine, mefloquine, and artemisinin derivatives (Figure 1), among other drugs. e alkaloidal drug, quinine, is the first antimalarial drug isolated from Cinchona bark. e drug is still quite useful in the treatment of multidrugresistant malaria. Chloroquine, a 4-aminoquinoline, was developed in the 1940s as a synthetic derivative from quinine. It was effective, cheap, and less toxic and was the drug of choice for malarial treatment for decades; however, its use has been restricted in modern malaria therapy due to parasite resistance to the drug [5,6]. Mefloquine is structurally related to quinine and has been introduced to treat chloroquine-resistant malaria, though its use is limited because of resistance and neuropsychiatric side effects [7]. Artemisinin is a natural endoperoxide isolated from sweet warm wood plant Artemisia annua. Artemisinin and its semisynthetic analogs artemether, artether, and artesunate are potent antimalarial agents especially used in the regions where the resistance has developed to other antimalarial agents. e WHO recommends the use of artemisinin analogs in combination with other drugs (ACT) for the treatment of malaria in order to control resistance. Unfortunately, there have been reports of parasite resistance to the ACT [8].
Given the development of resistance of the malarial parasites against many of the current treatment regimens, there has been urgent quest to identify new antimalarial chemotherapeutic agents from natural sources, particularly medicinal plants, in order to possibly avoid problems related to drug resistance [9][10][11][12][13][14]. is is due to the widespread use of plant materials in the treatment of malaria in many traditional medical practices together with the fact that plants were the sources of the two prominent antimalarial lead compounds, quinine and artemisinin.
Several classes of phytoconstituents are responsible for the antimalarial activity of plants including alkaloids, terpenes, steroids, and flavonoids. Alkaloids are considered as an important group exhibiting diverse biological activities, particularly antimalarial activity. ey constitute an important class of structurally diversified compounds that are having the nitrogen atom in the heterocyclic ring and are derived from the amino acids [15]. Large numbers of alkaloids have been isolated from different plant sources and reported for their potent antimalarial activity, some of which have been previously reviewed up till the year 2012 [1,13,[16][17][18][19][20]. However, more updates on the current research on alkaloids as potential antimalarial agents are needed. In the present review work, alkaloids from medicinal plants with antimalarial property which are reported recently from 2013 to 2019 are summarized. ey are discussed in subclasses of alkaloids and the chemical structures of the newly reported compounds and those with antiplasmodial activity are shown in the figures according to their subclasses.
Alkaloids having steroidal nucleus have also been isolated and shown to possess antimalarial activity. Dua et al. [25] investigated the in vitro antimalarial and cytotoxic effects of the known compound, conessine (8), which was isolated from the plant Holarrhena antidysenterica. e four-day in vivo test was also used to test for the antimalarial activity against a chloroquine-sensitive P. berghei NK65 strain in BALB/c mice. Compound (8) shows in vitro antiplasmodial activity with its IC 50 values of 1.9 μg/ml and 1.3 μg/ml in the schizont maturation and pLDH assays, respectively, and cytotoxicity IC 50 of 14 μg/ml. e compound also significantly reduces parasitaemia (88.95% parasite inhibition) in P. berghei-infected mice. In addition, the phytochemical investigation of an alkaloidal extract of the related plant Holarrhena pubescens roots led to the isolation of a new pregnene-type alkaloid, mokluangin D (9), together with nine known steroidal alkaloids. Two of the known compounds, irehline (10)   2 Evidence-Based Complementary and Alternative Medicine relationships in these compounds (illustrated with compound 11) indicates that the C-3 amino group, the nature of the E ring, and the carbonyl group at C-18 are important for the activity against the P. falciparum K1 strain [26]. Furthermore, Pan et al. [27] reported the isolation of a known buxus alkaloid, N-3-benzoyldihydrocyclomicrophylline F (12), together with other compounds from Buxus cochinchinensis Pierre ex Gagnep. (Buxaceae). e buxus alkaloid (12) was found to show significant in vitro antimalarial activity against the drug-resistant Dd2 strain of P. falciparum with IC 50 value of 2.07 μM (cytotoxicity against HT-29 human carcinoma is 1.9 μM and against NF-KB is >20).
In a related study on the root bark of the same plant, Ancistrocladus species from the Democratic Republic of Congo, a new dimeric naphthylisoquinoline alkaloid, jozimine A2(48), was isolated. Compound 48 is one of the as yet very rare naphthylisoquinoline dimers whose central biaryl axis is rotationally hindered. Moreover, it is the first natural dimer of a dioncophyllaceae-type alkaloid that is lacking oxygen functions at C6 and bearing R configurations at C3 in its two isoquinoline portions. e new dimer (48) exhibits excellent, and specific, antiplasmodial activity [41].
ey incorporate a tetrahydroisoquinoline substructure and belong to the isoquinoline class of alkaloids. Several aporphine alkaloids possessing antimalarial activity have been isolated recently (Figure 7).

Protoberberine Alkaloids.
is class of compounds is mainly found in the plants of the Papaveraceae family. e alkaloids are characterized by a tetracyclic ring system that "hides" a substituted phenethylamine or vanillylamine, depending on the viewing "angle" (Figure 8).
Wangchuk et al. [47] carried out phytochemical studies of the aerial parts of Meconopsis simplicifolia (D. Don) Walpers (Papaveraceae), resulting in the isolation of one   (62), an alkaloid from Coptidis rhizoma (Ranunculaceae), was found to be a novel and potent inhibitor of Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) with an IC 50 value of 1.83 ± 0.08 μM [48]. Besides, Promchai et al. [49] isolated five new oxoprotoberberine alkaloids, miliusacunines A-E (63-67), along with nine known compounds from the leaves and twigs of Miliusa cuneata (Annonaceae). All isolated compounds were evaluated for their cytotoxicity against the KB and Vero cell lines and for antimalarial activities against the P. falciparum strains TM4 and K1 (sensitive and multidrug-resistant strains, respectively). Compound 63 exhibits in vitro antimalarial activity against the TM4 strain, with an IC 50 value of 19.3 ± 3.4 μM, while compound 64 demonstrates significant activity against the K1 strain, with an IC 50 value of 10.8 ± 4.1 μM. Both compounds show no discernible cytotoxicity to the Vero cell line at the concentration levels evaluated. In another study, a bioassay-guided fractionation of the tubers of Stephania venosa (Blum) Spreng (Menispermaceae) led to the isolation of four aporphine and one tetrahydroprotoberberine alkaloids including stephanine (50), which was observed to be the most active antiplasmodial compound but is also the most cytotoxic with the lowest selectivity index [50]. Compound 50 was previously reported in Stephania rotunda with significant antiplasmodial activity [42].  Evidence-Based Complementary and Alternative Medicine
Crinane alkaloids were also isolated from the Amaryllidaceae family. A chemical investigation of Amaryllis belladonna Steud. (Amaryllidaceae) bulbs resulted in the isolation of the new crinane alkaloid 1,4-dihydroxy-3-methoxy powellan (78), along with the three known crinane alkaloids, distichamine (79), 11-O-acetylambelline (80), and ambelline (81), as well as the two lycorane alkaloids, acetylcaranine (82) and hippadine (75). Compund 82 shows the most potent inhibitory activity, with an IC 50 value of 3.3 ± 0.3 μM, but compound 75 is inactive despite its structural similarity. Slight structural differences of conjugation and substituents in the tetrahydrophenanthridine moiety seem to greatly affect the inhibitory activity. e crinane-type alkaloid (80) exhibits weak inhibitory activity (IC 50 , 35 ± 1 μM), whereas 81 shows stronger activity with an IC 50 value of 7.3 ± 0.3 μM. e difference in the antiplasmodial activity of these two compounds could be explained by the placement of oxy-substitution at C-11, since acetylation of the oxygenated C-11 of the ethanol bridge slightly decreased the inhibitory activity. e new crinane-type alkaloid 78 showed weak inhibitory activity (IC 50 , 37 ± 3 μM). Compound 79 has little inhibitory activity, despite its similarity to 80 and 81. Only compound 82 shows inhibitory, though very weak, effect against A2780 ovarian cells with an IC 50 value of 56 ± 1 μM [53]. In a recent study, lycorine (83), along with 14 other alkaloids, was isolated from Worsleya procera roots. e compound (83) exhibits antiplasmodial activity against both sensitive (3D7) and resistant (K1) P. falciparum parasite strains with IC 50 values of 2.5 and 3.1 μM, respectively, and a low cytotoxic profile against human hepatocarcinoma cells (HepG2), with a selectivity index greater than 100 [54].
e ring system consists of a hydroxystyrylamine moiety, an amino acid, and a β-hydroxy amino acid; attached to the ring is a side chain, comprised of one or two more amino acid moieties [55]. e structures of the cyclopeptide alkaloids from plants that possess antimalarial activity are shown in Figure 10. Evidence-Based Complementary and Alternative Medicine  (2) Cassane-type diterpene alkaloids
1.11. Pyridocoumarin Alkaloids. Chemical investigation of the aerial parts of Goniothalamus australis resulted in the isolation of two pyridocoumarin alkaloids, goniothalines A (90) and B (91), as well as eight known natural products, including sauristolactam (92), an aristolactam alkaloid, as well as (-)-anonaine (55). Compounds 92 and 55 are active against a chloroquine-sensitive P. falciparum line (3D7) with IC 50 values of 9.0 and 7.0 μM, respectively. e novel natural products (90 and 91) display no in vitro antiparasitic activity at 50 μM. Data from the study suggested that 3-methoxy substituent moderately reduces biological function. Also methylation of the nitrogen and demethoxylation at C-8 in the aristolactam skeleton (compound 92) are important for P. falciparum growth inhibition [60].

Acridone Alkaloids.
e root bark of Zanthoxylum simullans Hance afforded five known acridone alkaloids, together with other compounds. eir antimalarial activity was tested against two different strains of the parasite P. falciparum, 3D7 and Dd2. Normelicopidine (93) is the most active against Dd2 with IC 50 value of 18.9 ug/mL [61].

Macrocyclic Alkaloids.
e macrocyclic dilactone alkaloid, carpaine (94), was isolated from Carica papaya L. leaf. e compound (94) was screened against Plasmodium falciparum 3D7 and Dd2 strains. e cytotoxicity was evaluated against NL20 cells. e compound exhibits good activity against both strains of P. falciparum, 3D7 and Dd2, with IC 50 values of 4.21 μM and 4.57 μM, respectively, and high selectivity for the parasite and was nontoxic to healthy uninfected human red blood cells [62]. e effects of 94 may be related to its macrocyclic dilactone structure, a possible cation chelating structure. e chemical structures of the quinoline, pyridocoumarin, aristolactam, acridone, and macrocytic  [62] alkaloids are shown in Figure 11. Table 1 presents the summary of the alkaloids and their antimalarial activity.

Conclusion
In the present review, attempt has been made to document the alkaloidal compounds isolated from medicinal plants and/or investigated recently for their antimalarial property. ey belong to a great structural diversity including terpenoide, steroid, indole, phenanthroindolizine, isoquinoline, benzylisoquinoline, hasubanane, naphthoisoquinoline, aporphine, morphinandienone, protoberberine, Amaryllidaceae, cyclopeptide, quinoline, pyridocoumarin, acridone, and macrocyclic alkaloids. Many of the compounds with interesting antimalarial property were reported for the first time within this period and they present an array of potential lead compounds towards development of novel antimalarial drugs. eir antiplasmodial and cytotoxicity data were presented. Structure activity relationships were discussed where possible.
Particularly, the most interesting of the newly isolated compounds include the cassane-type diterpene alkaloids, caesalminines A (1) and caesalminines B (2), and the bisindole alkaloid, strychnobaillonine (19). Some other known compounds, including the isoquinolines, cycleanine (25), and 10-demethylxylopinine (26), were also reported to show strong antimalarial property with low IC 50 values. However, some of the compounds have relatively high cytotoxicity profile, while some have not been investigated for their toxicity profile. Another observation is that majority of the study of the antimalarial activity has been based on in vitro studies, while in vivo studies are quite rare.
Given the above, it is suggested that further structural modifications and structure-activity relationship study should be done on the promising compounds. e identified alkaloids with promising antimalarial activity should serve as lead compounds for drug design to obtain compounds with improved efficacy and lower toxicity. ey should also be subjected to toxicity and in vivo efficacy studies. Additionally, the mode of action of the promising compounds should be explored. It is hoped that the outcome of the present review will spur further research into the structural modification and/or development of the promising compounds as novel antimalarial drugs.

Conflicts of Interest
e author declares no conflicts of interest.