The Potential of Secondary Metabolites from Plants as Drugs or Leads against Protozoan Neglected Diseases—Part III: In-Silico Molecular Docking Investigations

Malaria, leishmaniasis, Chagas disease, and human African trypanosomiasis continue to cause considerable suffering and death in developing countries. Current treatment options for these parasitic protozoal diseases generally have severe side effects, may be ineffective or unavailable, and resistance is emerging. There is a constant need to discover new chemotherapeutic agents for these parasitic infections, and natural products continue to serve as a potential source. This review presents molecular docking studies of potential phytochemicals that target key protein targets in Leishmania spp., Trypanosoma spp., and Plasmodium spp.


Introduction
Parasitic protozoal diseases continue to be a cause of considerable morbidity and mortality, particularly in underdeveloped countries around the world. These diseases include malaria [1], Chagas disease [2], human African trypanosomiasis [3], and leishmaniasis [4]. Current chemotherapeutic options for these neglected diseases can have severe side effects, may be ineffective, or even non-existent, and in cases where drug treatment is available, resistance is emerging [5][6][7][8]. Thus, there is a need to discover and develop of new chemotherapeutic agents for these parasitic infections. Natural products have been served as important leads for drug development and databases of natural products provide a convenient source for virtual screening against drug targets [9], including parasitic protozoal diseases [10][11][12]. Natural products offer important complementary opportunities in drug discovery: (a) They occupy different regions of biologically relevant chemical space [13], including abundant oxygen-containing functionalities (rarely nitrogen) and high degrees of chirality and complexity [14]; (b) although outside the "rule-of-five", numerous natural products have proven to be efficacious drugs [15]; (c) they have been optimized for activity, including active transport, by evolution [16]; and (d) natural products serve as lead structures for semisynthetic modification to improve activity, selectivity, or bioavailability [17]. In this review, we present in-silico efforts at natural product drug discovery for neglected parasitic protozoal diseases; molecular docking of phytochemical ligands with potential parasitic protein targets (Table 1).

Molecular Docking Studies
Molecular docking has become one of the most important modeling tools in modern drug discovery. It is a very convenient and cheap means to study protein-ligand interactions. It can be used to rank compounds for prioritization in lead discovery and development. It can also be used to identify potential inhibitors, substrates, activators or binding partners from compound libraries that contains few hundreds to millions of compounds. Several recent reviews on molecular docking have appeared [357][358][359][360], so molecular docking principles and approaches will not be covered here.
Molecular docking has become a well-accepted complement to X-ray crystallography and NMR spectroscopy in studying drug-drug target interactions. It also gives the medicinal chemist a means to access certain ligand binding poses that even X-ray crystallography and NMR spectroscopy may not inform the most experienced structural biologist thereby aiding the medicinal chemist in the creative enterprise of structure-based drug design. In some cases, it has become a replacement or a complement to high throughput compound screening.
Despite current and potential applications, as well as the successes of molecular docking in drug discovery, there remain limitations and caveat in the interpretation of results from molecular docking studies. These limitations stem, mostly, from the inability of the scoring functions in molecular docking algorithms to account for local and global macromolecular dynamics, in addition to inability to accurately predict covalent interactions and solvent accessibilities: (1) in most cases the protein is modeled as a rigid structure without flexibility; (2) solvation of the active/binding site and of the ligand is usually excluded; (3) free-energy estimation of protein-ligand complexes is largely ignored [357,361,362]. Molecular docking methodology, cavity definition and search algorithms, and thermodynamic scoring functions continue to improve, however [363].
Molecular docking has increasing found use in drug discovery programs focused on tropical diseases. The applications include target-based screening of natural products libraries or databases [364,365]. Popular natural product databases include the Dictionary of Natural Products [366], Napralert [367], and the ZINC natural products database [368]. Table 2 lists popular molecular docking software recently used for virtual screening of natural product libraries. The list is not meant as an endorsement, but does reflect the current availability of molecular docking software. In addition, several other commercial and freeware molecular docking packages are available. There are, however, additional effects to be considered in docking studies with natural products: (1) many natural products may have poor bioavailability due to limited solubility, membrane permeability, hydrolysis, or other metabolic transformations; (2) the ligands may also target homologous isozymes in humans and cause serious side effects; (3) the docking studies do not account for possible synergism with the bioactive antiparasitic compounds.
In spite of the above limitations, molecular docking studies of phytochemical ligands with identified protein targets provide the possibilities to identify natural compounds that may themselves function as efficacious drugs, may serve as lead structures for chemical modification and optimization, or provide structural templates for de novo drug synthesis. Some published reports have focused on natural products that are biologically active against one or more protozoan organism or any of their validated drug targets [374][375][376][377][378] while other works have focused on natural products or phytochemicals that were isolated from plants with historical ethnomedicinal therapeutic use [379,380]. Molecular docking has been used to identify, in silico, the selectivity of some compounds or classes of compounds for specific protozoan drug targets. In the reports by us about the selectivity of antiparasitic isoprenoid derivatives for drug targets from Leishmania spp., for example, antiparasitic monoterpenoids were found to selectively dock to L. infantum nicotinamidase, L. major uridine diphosphate-glucose pyrophosphorylase, and methionyl t-RNA synthetase, while germacranolide sesquiterpenoids were selective for L. major methionyl t-RNA synthetase, and dihydroorotate dehydrogenase [375]. It was also shown in that work that diterpenoids generally favored docking to L. mexicana glycerol-3-phosphate dehydrogenase. In addition, the tetranortriterpene limonoids showed some selectivity for L. mexicana glycerol-3-phosphate dehydrogenase and L. major dihydroorotate dehydrogenase while withanolides docked more selectively with L. major uridine diphosphate-glucose pyrophosphorylase.
Also, although not surprising, were the strong docking preference of several steroids and triterpenoids for L. infantum sterol 14α-demethylase (LinfCYP51). Of particular note is the strong docking preference of the hydroperoxy sterol 24-hydroperoxy-24, 25-vinylcholesterol and 24,25-epoxywithanolide D ( Figure 1) to LinfCYP51 ( Figure 2). In vitro evaluation of these compounds as possible inhibitors of LinfCYP51 remains to be tested, but in vitro antileishmanial screening and in silico docking with LinfCYP51 of oleanolic acid ( Figure 1) corroborate these findings [381].  Some published reports have focused on natural products that are biologically active against one or more protozoan organism or any of their validated drug targets [374][375][376][377][378] while other works have focused on natural products or phytochemicals that were isolated from plants with historical ethnomedicinal therapeutic use [379,380]. Molecular docking has been used to identify, in silico, the selectivity of some compounds or classes of compounds for specific protozoan drug targets. In the reports by us about the selectivity of antiparasitic isoprenoid derivatives for drug targets from Leishmania spp., for example, antiparasitic monoterpenoids were found to selectively dock to L. infantum nicotinamidase, L. major uridine diphosphate-glucose pyrophosphorylase, and methionyl t-RNA synthetase, while germacranolide sesquiterpenoids were selective for L. major methionyl t-RNA synthetase, and dihydroorotate dehydrogenase [375]. It was also shown in that work that diterpenoids generally favored docking to L. mexicana glycerol-3-phosphate dehydrogenase. In addition, the tetranortriterpene limonoids showed some selectivity for L. mexicana glycerol-3phosphate dehydrogenase and L. major dihydroorotate dehydrogenase while withanolides docked more selectively with L. major uridine diphosphate-glucose pyrophosphorylase.
Also, although not surprising, were the strong docking preference of several steroids and triterpenoids for L. infantum sterol 14α-demethylase (LinfCYP51). Of particular note is the strong docking preference of the hydroperoxy sterol 24-hydroperoxy-24, 25-vinylcholesterol and 24,25epoxywithanolide D ( Figure 1) to LinfCYP51 ( Figure 2). In vitro evaluation of these compounds as possible inhibitors of LinfCYP51 remains to be tested, but in vitro antileishmanial screening and in silico docking with LinfCYP51 of oleanolic acid ( Figure 1) corroborate these findings [381].

Leishmania and Trypanosoma Targets
The flavonoids (+)-catechin and (−)-epicatechin ( Figure 3) have been shown to be effective inhibitors of Leishmania amazonensis arginase with IC50 values of 0.77 and 1.8 μM, respectively [382]. Molecular docking (MolDock) of these compounds has revealed differences in their interactions with the amino acid residues of the active site of arginase. (+)-Catechin docks in the active site with primary hydrogen-bonding interactions to Ala192, Thr257, and Asp141. In contrast, the primary hydrogenbonding contacts for (−)-epicatechin were with Ser150, His154, Asp245, Asn 152, Thr257, and Asn243.

Leishmania and Trypanosoma Targets
The flavonoids (+)-catechin and (−)-epicatechin ( Figure 3) have been shown to be effective inhibitors of Leishmania amazonensis arginase with IC 50 values of 0.77 and 1.8 µM, respectively [382]. Molecular docking (MolDock) of these compounds has revealed differences in their interactions with the amino acid residues of the active site of arginase. (+)-Catechin docks in the active site with primary hydrogen-bonding interactions to Ala192, Thr257, and Asp141. In contrast, the primary hydrogen-bonding contacts for (−)-epicatechin were with Ser150, His154, Asp245, Asn 152, Thr257, and Asn243.
Herrmann and co-workers have carried out an in-silico screening of a natural products library of 700 structures against T. brucei glyceraldehyde-3-phosphate dehydrogenase (TbGAPDH) [399]. These investigators were able to identify 13 "hits" based on the molecular docking and of these, five compounds (three geranylated benzophenones, flavaspidic acid AB, and a bis-resorcinyl tetradecene derivative, Figure 11) showed significant in vitro inhibitory activity against recombinant TbGAPDH as well as T. brucei rhodesiense.  Figure 11. Phytochemical ligands with encouraging docking properties with Trypanosoma brucei glyceraldehyde 3-phosphate dehydrogenase.
An in-silico analysis of a dataset of 683 flavonoids for molecular docking to L. mexicana pyruvate kinase found that 3-glycosylated flavonoids (seven compounds), 6,8-diglycosyl flavonoids (one compound), and biflavonoids (four compounds) were the most promising ligands [400]. Promastigote surface antigen has been identified as a common protein drug target for L. braziliensis, L. major, and L. infantum. N-Acetylglucosamine was identified as a potential lead target molecule based on docking studies (ArgusDock) [401].
Inhibition of Trypanosoma cruzi silent-information regulator 2 proteins (sirtuins) is known to cause arrested growth of the parasite [402]. Sacconnay and co-workers assembled two conformational states of TcSir2rp1 using homology modeling and carried out molecular docking of a library of antitrypanosomal phytochemicals [403]. Four compounds were found to have particularly promising docking characteristics, (15:2)-anacardic acid, 3,18-diacetoxy-1-octadecene-4,6-diyne-8-ol, aculeatin D, and vismione D ( Figure 12). Leishmania lack the ability to synthesize purines de novo and therefore salvage purines. Adenosine kinase is one of the enzymes in the purine salvage pathway, and Leishmania adenosine kinase is crucial for parasite survival [404]. Molecular docking (Glide, FlexX, GOLD) of a library of An in-silico analysis of a dataset of 683 flavonoids for molecular docking to L. mexicana pyruvate kinase found that 3-glycosylated flavonoids (seven compounds), 6,8-diglycosyl flavonoids (one compound), and biflavonoids (four compounds) were the most promising ligands [400]. Promastigote surface antigen has been identified as a common protein drug target for L. braziliensis, L. major, and L. infantum. N-Acetylglucosamine was identified as a potential lead target molecule based on docking studies (ArgusDock) [401].
Inhibition of Trypanosoma cruzi silent-information regulator 2 proteins (sirtuins) is known to cause arrested growth of the parasite [402]. Sacconnay and co-workers assembled two conformational states of TcSir2rp1 using homology modeling and carried out molecular docking of a library of antitrypanosomal phytochemicals [403]. Four compounds were found to have particularly promising docking characteristics, (15:2)-anacardic acid, 3,18-diacetoxy-1-octadecene-4,6-diyne-8-ol, aculeatin D, and vismione D ( Figure 12). Herrmann and co-workers have carried out an in-silico screening of a natural products library of 700 structures against T. brucei glyceraldehyde-3-phosphate dehydrogenase (TbGAPDH) [399]. These investigators were able to identify 13 "hits" based on the molecular docking and of these, five compounds (three geranylated benzophenones, flavaspidic acid AB, and a bis-resorcinyl tetradecene derivative, Figure 11) showed significant in vitro inhibitory activity against recombinant TbGAPDH as well as T. brucei rhodesiense.  Figure 11. Phytochemical ligands with encouraging docking properties with Trypanosoma brucei glyceraldehyde 3-phosphate dehydrogenase.
An in-silico analysis of a dataset of 683 flavonoids for molecular docking to L. mexicana pyruvate kinase found that 3-glycosylated flavonoids (seven compounds), 6,8-diglycosyl flavonoids (one compound), and biflavonoids (four compounds) were the most promising ligands [400]. Promastigote surface antigen has been identified as a common protein drug target for L. braziliensis, L. major, and L. infantum. N-Acetylglucosamine was identified as a potential lead target molecule based on docking studies (ArgusDock) [401].
Inhibition of Trypanosoma cruzi silent-information regulator 2 proteins (sirtuins) is known to cause arrested growth of the parasite [402]. Sacconnay and co-workers assembled two conformational states of TcSir2rp1 using homology modeling and carried out molecular docking of a library of antitrypanosomal phytochemicals [403]. Four compounds were found to have particularly promising docking characteristics, (15:2)-anacardic acid, 3,18-diacetoxy-1-octadecene-4,6-diyne-8-ol, aculeatin D, and vismione D ( Figure 12). Leishmania lack the ability to synthesize purines de novo and therefore salvage purines. Adenosine kinase is one of the enzymes in the purine salvage pathway, and Leishmania adenosine kinase is crucial for parasite survival [404]. Molecular docking (Glide, FlexX, GOLD) of a library of Leishmania lack the ability to synthesize purines de novo and therefore salvage purines. Adenosine kinase is one of the enzymes in the purine salvage pathway, and Leishmania adenosine kinase is crucial for parasite survival [404]. Molecular docking (Glide, FlexX, GOLD) of a library of natural products with a homology-modeled structure of L. donovani adenosine kinase has revealed 1,6-digalloylglucose and lawsone ( Figure 13) as top hit phytochemical ligands [405].

Plasmodium Targets
Curcumin (Figure 8) has shown antimalarial activity (IC50 5 μM) against P. falciparum, and experimental evidence has suggested disruption of parasite microtubules to be responsible for the antiplasmodial activity [406]. Molecular docking studies (AutoDock) have revealed that curcumin interacts with homology-modeled P. falciparum tubulin dimer at the colchicine binding site of tubulin rather than the paclitaxel or vinblastine binding sites [406].
The sarco/endoplasmic reticulum Ca 2+ -ATPase orthologue of P. falciparum (PfATP6) has been suggested to be a viable drug target for antimalarial chemotherapy [407]. Homology modeling has allowed construction of the three-dimensional structure of PfATP6 and allowed molecular docking/in-silico screening of potential antimalarial drugs, including artemisinin [408] and curcumin [409,410]. Curcumin has also shown selectively strong docking (MolDock) to L. major methionyl tRNA synthetase [377]. Bousejra-El Garah and co-workers, however, found no correlation between in silico docking energies to PfATP6 and antimalarial activities of several structurally diverse antimalarial compounds [411].

Plasmodium Targets
Curcumin ( Figure 8) has shown antimalarial activity (IC 50 5 µM) against P. falciparum, and experimental evidence has suggested disruption of parasite microtubules to be responsible for the antiplasmodial activity [406]. Molecular docking studies (AutoDock) have revealed that curcumin interacts with homology-modeled P. falciparum tubulin dimer at the colchicine binding site of tubulin rather than the paclitaxel or vinblastine binding sites [406].
The sarco/endoplasmic reticulum Ca 2+ -ATPase orthologue of P. falciparum (PfATP6) has been suggested to be a viable drug target for antimalarial chemotherapy [407]. Homology modeling has allowed construction of the three-dimensional structure of PfATP6 and allowed molecular docking/ in-silico screening of potential antimalarial drugs, including artemisinin [408] and curcumin [409,410]. Curcumin has also shown selectively strong docking (MolDock) to L. major methionyl tRNA synthetase [377]. Bousejra-El Garah and co-workers, however, found no correlation between in silico docking energies to PfATP6 and antimalarial activities of several structurally diverse antimalarial compounds [411].

Plasmodium Targets
Curcumin ( Figure 8) has shown antimalarial activity (IC50 5 μM) against P. falciparum, and experimental evidence has suggested disruption of parasite microtubules to be responsible for the antiplasmodial activity [406]. Molecular docking studies (AutoDock) have revealed that curcumin interacts with homology-modeled P. falciparum tubulin dimer at the colchicine binding site of tubulin rather than the paclitaxel or vinblastine binding sites [406].
The sarco/endoplasmic reticulum Ca 2+ -ATPase orthologue of P. falciparum (PfATP6) has been suggested to be a viable drug target for antimalarial chemotherapy [407]. Homology modeling has allowed construction of the three-dimensional structure of PfATP6 and allowed molecular docking/in-silico screening of potential antimalarial drugs, including artemisinin [408] and curcumin [409,410]. Curcumin has also shown selectively strong docking (MolDock) to L. major methionyl tRNA synthetase [377]. Bousejra-El Garah and co-workers, however, found no correlation between in silico docking energies to PfATP6 and antimalarial activities of several structurally diverse antimalarial compounds [411].
Kumar and co-workers have carried out a molecular docking examination (AutoDock) of several bioactive natural products with P. falciparum dihydrofolate reductase [376]. These workers found ochrolifuanine A, chrobisiamone A, ailanthinone, korupensamine A, butyraxanthone B, ancistrolikokine A, calothwaitesixanthone, 7-deacetylkhivorin, 5-prenylbutein, methyl 6-hydroxyangolensate, and aulacocarpin A (Figure 14), to show notable docking energies (i.e., lower than the co-crystallized inhibitor WR99210). Plasmodium lactate dehydrogenase has been identified as a potential drug target for antimalarials due to parasite dependence on glycolysis for ATP production [412]. Molecular docking of the tea flavonoid gallocatechin ( Figure 15) to P. falciparum lactate dehydrogenase revealed strong docking, more strongly than either chloroquine or mefloquine, to the NADH binding site of the enzyme [413]. Glycyrrhetic acid (Figure 4) has exhibited notable (IC50 1.69 μg/mL) in vitro antiplasmodial activity against P. falciparum, and docking studies (Discovery Studio) have also shown glycyrretic acid to dock moderately well to P. falciparum lactate dehydrogenase [414]. (+)-Usnic acid, a secondary metabolite from lichen, was identified as an active and selective inhibitor of the liver stage form of Plasmodium berghei by Lauinger and co-workers [415]. In the molecular docking study to identify the binding affinities and binding sites of (+)-usnic acid and three other lichen secondary metabolites (evernic acid, vulpic acid, and psoromic acid, Figure 16) with Plasmodium type II fatty acid biosynthesis pathway (FAS-II) enzymes, these workers found that the mechanism of action of lichen acids on FAS-II may be different from those of previously described FAS-II enzymes inhibitors. The modeling study they carried out indicated that those compounds appear to inhibit FAS-II enzymes indirectly via binding to allosteric sites on the protein surface and not to the active sites of FAS-II enzymes. This indirect binding is speculated to possibly affect the enzyme conformations and subsequently interfere with the catalytic activities [415].
Enoyl-acyl carrier protein reductase is a critical enzyme in type II fatty acid biosynthesis in the hepatocyte-stage of Plasmodium falciparum. Tallorin and co-workers, based on molecular docking and subsequent in vitro screening, have identified celestrol as a potent PfENR inhibitor [416]. Using molecular docking (AutoDock) coupled with three-dimensional quantitative structure activity relationships (3D-QSAR), Wadhwa and co-workers identified five phytochemicals (3α,20-lupanediol, ergosterol peroxide, 24-methylenecycloartan-3-ol, 2′-epicycloisobrachycoumarinone epoxide, and atalaphyllidine, Figure 16) as potential PfENR inhibitors [417]. Plasmodium lactate dehydrogenase has been identified as a potential drug target for antimalarials due to parasite dependence on glycolysis for ATP production [412]. Molecular docking of the tea flavonoid gallocatechin ( Figure 15) to P. falciparum lactate dehydrogenase revealed strong docking, more strongly than either chloroquine or mefloquine, to the NADH binding site of the enzyme [413]. Glycyrrhetic acid (Figure 4) has exhibited notable (IC 50 1.69 µg/mL) in vitro antiplasmodial activity against P. falciparum, and docking studies (Discovery Studio) have also shown glycyrretic acid to dock moderately well to P. falciparum lactate dehydrogenase [414]. Plasmodium lactate dehydrogenase has been identified as a potential drug target for antimalarials due to parasite dependence on glycolysis for ATP production [412]. Molecular docking of the tea flavonoid gallocatechin ( Figure 15) to P. falciparum lactate dehydrogenase revealed strong docking, more strongly than either chloroquine or mefloquine, to the NADH binding site of the enzyme [413]. Glycyrrhetic acid (Figure 4) has exhibited notable (IC50 1.69 μg/mL) in vitro antiplasmodial activity against P. falciparum, and docking studies (Discovery Studio) have also shown glycyrretic acid to dock moderately well to P. falciparum lactate dehydrogenase [414]. (+)-Usnic acid, a secondary metabolite from lichen, was identified as an active and selective inhibitor of the liver stage form of Plasmodium berghei by Lauinger and co-workers [415]. In the molecular docking study to identify the binding affinities and binding sites of (+)-usnic acid and three other lichen secondary metabolites (evernic acid, vulpic acid, and psoromic acid, Figure 16) with Plasmodium type II fatty acid biosynthesis pathway (FAS-II) enzymes, these workers found that the mechanism of action of lichen acids on FAS-II may be different from those of previously described FAS-II enzymes inhibitors. The modeling study they carried out indicated that those compounds appear to inhibit FAS-II enzymes indirectly via binding to allosteric sites on the protein surface and not to the active sites of FAS-II enzymes. This indirect binding is speculated to possibly affect the enzyme conformations and subsequently interfere with the catalytic activities [415].
Enoyl-acyl carrier protein reductase is a critical enzyme in type II fatty acid biosynthesis in the hepatocyte-stage of Plasmodium falciparum. Tallorin and co-workers, based on molecular docking and subsequent in vitro screening, have identified celestrol as a potent PfENR inhibitor [416]. Using molecular docking (AutoDock) coupled with three-dimensional quantitative structure activity relationships (3D-QSAR), Wadhwa and co-workers identified five phytochemicals (3α,20-lupanediol, ergosterol peroxide, 24-methylenecycloartan-3-ol, 2′-epicycloisobrachycoumarinone epoxide, and atalaphyllidine, Figure 16) as potential PfENR inhibitors [417]. (+)-Usnic acid, a secondary metabolite from lichen, was identified as an active and selective inhibitor of the liver stage form of Plasmodium berghei by Lauinger and co-workers [415]. In the molecular docking study to identify the binding affinities and binding sites of (+)-usnic acid and three other lichen secondary metabolites (evernic acid, vulpic acid, and psoromic acid, Figure 16) with Plasmodium type II fatty acid biosynthesis pathway (FAS-II) enzymes, these workers found that the mechanism of action of lichen acids on FAS-II may be different from those of previously described FAS-II enzymes inhibitors. The modeling study they carried out indicated that those compounds appear to inhibit FAS-II enzymes indirectly via binding to allosteric sites on the protein surface and not to the active sites of FAS-II enzymes. This indirect binding is speculated to possibly affect the enzyme conformations and subsequently interfere with the catalytic activities [415].
Enoyl-acyl carrier protein reductase is a critical enzyme in type II fatty acid biosynthesis in the hepatocyte-stage of Plasmodium falciparum. Tallorin and co-workers, based on molecular docking and subsequent in vitro screening, have identified celestrol as a potent PfENR inhibitor [416]. Using molecular docking (AutoDock) coupled with three-dimensional quantitative structure activity relationships (3D-QSAR), Wadhwa and co-workers identified five phytochemicals (3α,20-lupanediol, ergosterol peroxide, 24-methylenecycloartan-3-ol, 2 -epicycloisobrachycoumarinone epoxide, and atalaphyllidine, Figure 16) as potential PfENR inhibitors [417]. Recently, Gupta and co-workers have used homology modeling to construct parasitic mitogenactivated protein kinases (MAPKs) for Leishmania mexicana, Plasmodium falciparum, and Trypanosoma brucei [418]. These workers carried out a molecular docking study on a small library of 10 antiparasitic phytochemicals. Of these, aspidocarpine showed excellent docking to both LmxMPK4 and TbMAPK5, and cubebin and eupomatenoid 5 ( Figure 17) both docked well with PfMAK2.

Conclusions
This review has catalogued the numerous druggable parasitic protein targets, with more being identified and three-dimensional structures determined, allowing many potential sites for identification and development of new and selective inhibitors. The theoretical predictions need to be experimentally validated, and the results can be used to guide an effective development of selective and targeted natural products analogues. A perusal of the structures in this review reveals several of the phytochemical ligands with promising docking properties are not likely to have suitable druglike properties. Therefore, pharmacokinetic and pharmacodymanic studies as well as structure-based design and optimization studies are needed to resolve issues of bioavailability and selectivity. It is advisable to carry out additional filtering for "drug-likeness" [387,419,420], ADME [421], and toxicity prediction [422].  Recently, Gupta and co-workers have used homology modeling to construct parasitic mitogen-activated protein kinases (MAPKs) for Leishmania mexicana, Plasmodium falciparum, and Trypanosoma brucei [418]. These workers carried out a molecular docking study on a small library of 10 antiparasitic phytochemicals. Of these, aspidocarpine showed excellent docking to both LmxMPK4 and TbMAPK5, and cubebin and eupomatenoid 5 ( Figure 17) both docked well with PfMAK2.  Recently, Gupta and co-workers have used homology modeling to construct parasitic mitogenactivated protein kinases (MAPKs) for Leishmania mexicana, Plasmodium falciparum, and Trypanosoma brucei [418]. These workers carried out a molecular docking study on a small library of 10 antiparasitic phytochemicals. Of these, aspidocarpine showed excellent docking to both LmxMPK4 and TbMAPK5, and cubebin and eupomatenoid 5 ( Figure 17) both docked well with PfMAK2.

Conclusions
This review has catalogued the numerous druggable parasitic protein targets, with more being identified and three-dimensional structures determined, allowing many potential sites for identification and development of new and selective inhibitors. The theoretical predictions need to be experimentally validated, and the results can be used to guide an effective development of selective and targeted natural products analogues. A perusal of the structures in this review reveals several of the phytochemical ligands with promising docking properties are not likely to have suitable druglike properties. Therefore, pharmacokinetic and pharmacodymanic studies as well as structure-based design and optimization studies are needed to resolve issues of bioavailability and selectivity. It is advisable to carry out additional filtering for "drug-likeness" [387,419,420], ADME [421], and toxicity prediction [422].

Conclusions
This review has catalogued the numerous druggable parasitic protein targets, with more being identified and three-dimensional structures determined, allowing many potential sites for identification and development of new and selective inhibitors. The theoretical predictions need to be experimentally validated, and the results can be used to guide an effective development of selective and targeted natural products analogues. A perusal of the structures in this review reveals several of the phytochemical ligands with promising docking properties are not likely to have suitable drug-like properties. Therefore, pharmacokinetic and pharmacodymanic studies as well as structure-based design and optimization studies are needed to resolve issues of bioavailability and selectivity. It is advisable to carry out additional filtering for "drug-likeness" [387,419,420], ADME [421], and toxicity prediction [422].