Marine organisms as potential sources of natural products for the prevention and treatment of malaria

Vector-borne diseases (VBDs) are a worldwide critical concern accounting for 17% of the estimated global burden of all infectious diseases in 2020. Despite the various medicines available for the management, the deadliest VBD malaria, caused by Plasmodium sp., has resulted in hundreds of thousands of deaths in sub-Saharan Africa only. This finding may be explained by the progressive loss of antimalarial medication efficacy, inherent toxicity, the rise of drug resistance, or a lack of treatment adherence. As a result, new drug discoveries from uncommon sources are desperately needed, especially against multi-drug resistant strains. Marine organisms have been investigated, including sponges, soft corals, algae, and cyanobacteria. They have been shown to produce many bioactive compounds that potentially affect the causative organism at different stages of its life cycle, including the chloroquine (CQ)-resistant strains of P. falciparum. These compounds also showed diverse chemical structures belonging to various phytochemical classes, including alkaloids, terpenoids, polyketides, macrolides, and others. The current article presents a comprehensive review of marine-derived natural products with antimalarial activity as potential candidates for targeting different stages and species of Plasmodium in both in vitro and in vivo and in comparison with the commercially available and terrestrial plant-derived products, i.e., quinine and artemisinin.


Introduction
Vector-borne diseases (VBDs) are infectious diseases caused by parasites, bacteria, and viruses transmitted via vectors. About 700 000 deaths are reported officially by the World Health Organization (WHO) from these diseases per year worldwide, including malaria, dengue, schistosomiasis, human African trypanosomiasis, leishmaniasis, Chagas disease, chikungunya fever, Zika virus fever, yellow fever, West Nile fever, Japanese encephalitis, and onchocerciasis, Fig. 1. Based on the WHO reports released in 2020, VBDs are a worldwide concern that accounts for 17% of the estimated global burden of all infectious diseases. 1 VBDs are commonly associated with weather and climate, where the incidence of these diseases is mainly in the tropics and subtropical regions. The low hygiene, sanitation, waste management, and housing in these urban areas help also spread such diseases between the world's poorest people, communities, and countries. 2 Various native to these regions as arthropods, including mosquitoes, ticks, sand ies, triatomine bugs, cockroaches, lice, eas, and aquatic snails, are involved as mediators transmitting VBDs. 3,4 Particularly, malaria is the most challenging VBD that leads to health problems worldwide, especially in developing countries. It is a mosquito-borne infectious disease that affects humans and other animals. An estimated 405 000 malaria deaths worldwide were registered, along with 228 million cases in 2018, compared to 229 million cases and 409 000 deaths with more than 400 000 deaths in 2019, based on the WHO report. 5,6 In other words, malaria accounts for more than 50% of VBDs deaths.
Malaria is transmitted through the bite of an infected Anopheles female mosquito. The infected mosquitoes carry one of several protozoans belonging to the genus Plasmodium (P. falciparum, P. ovale, P. vivax, P. knowlesi, and P. malariae). 7,8 The parasite is then released into the bloodstream causing severe anemia and other signs and symptoms, including chills, fever, profuse sweating, headache, nausea, vomiting, abdominal pain, diarrhea, muscle pain, convulsions, coma, bloody stools. 6,9,10 Serious complications or even death can occur in case of improper diagnosis or treatment. [11][12][13] The life cycle of the malaria parasite of Plasmodium sp. is illustrated in Fig. 2. This gure is of great importance for helping drug discovery respectively, and have been used for hundreds of years and before the mosquito cycle was explored. Even today, in the ght against malaria, both quinine and artemisinin are still of prime importance. 17 In contrast to terrestrial plants, marine organisms do not have a remarkable history of use in traditional medicine. However, recent advances in marine biology and engineering have helped investigation and scientic exploration of the marine environment to identify and isolate novel compounds, which have proven their potential bioactivities against lifethreatening diseases, including tumor and viral infections. [18][19][20] More than 30 000 compounds have been identied from about 240 000 known species of marine organisms. 21 Few of them have been approved by the Food and Drug Administration (FDA), including ziconotide (Prialt®) as a potent analgesic, trabectedin (Yondelis®), and cytarabine or ara-C (Cytosar-U®) as anti-tumor agents, vidarabine or ara-A (Vira-A ® ) and iota-carrageenan (Carragelose ® ) as an antiviral, and omega-3-acid ethyl ester (Lovaza ® ) for treating hypertriglyceridemia (Table 1). [22][23][24] Recently, Nweze, et al. published a review article highlighting the potential of marine-derived natural products for the treatment of some examples of diseases for neglected communities, including malaria, leishmaniasis, and trypanosomiasis. 25 Although some of the previous studies could not identify the chemical structure of bioactive components that acted signicantly against malaria, 26 the current article focuses on malaria. It reviews the different chemical classes, i.e., alkaloids, terpenoids, endoperoxides, phosphotriesters, peptides and depsipeptides, and macrolides, derived from marine organisms, including sponges, cyanobacteria, actinomycete bacteria, so corals, and algae. These bioactive have been conrmed to be potential candidates for managing malaria compared to commercially available products by targeting various stages in Plasmodium sp. life cycle. Moreover, the half maximum cytotoxic (CC 50 ) and inhibitory concentration (IC 50 ) against the different stages of the malaria parasite shall be highlighted, in addition to the possible mechanism of action and structureactivity relationships (SAR) in previous reports investigated the antiplasmodium activity. Hence, the current review may           Netamine I (62) open new frontiers for discovery and approval of novel potent drugs for this life-threatening disease.

Alkaloids
Various classes of marine-derived alkaloids have shown potent antimalarial activity. b-Carboline, indole, imidazole, and pyrrole alkaloids are mostly found. They showed bioactivities against different stages of the Plasmodium parasite with a unique mechanism of action. Fieen classes represented by 67 compounds were reviewed. Among them are manzamine alkaloids which showed inhibitory activity against glycogen synthase 3 (GSK-3) topoisomerase. In addition, salinosporamide showed a potent protease inhibitory effect. Numerous marine-derived alkaloids shall be discussed in detail in the following sub-sections and Table 2, including their sources, IC 50 , chemical structures, SAR, and mechanism of action.

Manzamines
Manzamines are polycyclic (7-8 rings) alkaloids containing a bcarboline moiety. Manzamine A (1) was rst reported from an Okinawan sponge belonging to the Haliclona species (family Chalinidae). 27 They are one of the essential antimalarial alkaloids. In addition to the lack of in vivo toxicity, the manzamines demonstrated greater effectiveness as antimalarial agents than the commonly used drugs artemisinin and CQ. 28 The mechanism of manzamine alkaloids is not completely understood. Still, authors described b-carboline alkaloids as micromolar inhibitors of glycogen GSK-3 by malaria parasites and inhibitors of topoisomerase DNA through intercalation in DNA-base pairs. Hence, a complete SAR investigation of manzamine alkaloids is necessary to understand the importance of each moiety (b-carboline and pentacyclic ring) and the inuence of different substituents on antimalarial activity. Manzamines SAR is summarized into two objectives. The rst is the effect of various substitutions on the b-carboline nucleus, and the other is the effect of substitutions on the pentacyclic ring. 29 The b-carboline moiety of manzamine alkaloids is responsible for antimalarial activity. 9-N alkylation of the b-carboline ring decreases antimalarial activity, indicating that 9-NH is necessary for their antimalarial activity. Hydroxyl-group substitution at position 8 of the b-carboline skeleton does not signicantly affect its antimalarial activity. Hence, 8-hydroxymanzamine (2) has the same effect as manzamine A, while manzamine F (3), a related derivative of manzamine A, is inactive. The eight-membered rings differ between the inactive manzamine F and the active manzamine A. The double bond reduction and/or the incorporation of a ketone group into the adjacent carbon is harmful to antimalarial activity. Likewise, hydroxyl group attachment at position 6 instead of position 8 has a negative effect on antimalarial activity, as shown by the lower potency of 6-hydroxy-manzamine A (4). 27 In vitro and in vivo studies, manzamines A and its 8hydroxy derivative inhibited P. falciparum growth. 30 Several total syntheses of manzamines have been accomplished. 29,31

Neo-kauluamine
Neo-kauluamine (5) is a manzamine dimer, with a highly complex molecule composed of two units of manzamine fragmented by ether bonds between the eight-membered rings isolated from an unspecied genus of Indo-Pacic sponge (Petrosiidae, order Haplosclerida). Despite its structural complexity, neo-kauluamine displayed a strong efficacy in vivo and is considered an up-and-coming agent in malaria. 27 Although this structure, like manzamine F (3), lacks the double bond in the eight-membered ring, it showed the same activity as manzamine A. The lack of antimalarial activity for 12,34-oxamanzamine A (6) suggested that the C-12 hydroxyl, the C-34 methine, or the 8-ring conformation are of great importance for the antimalarial activity. 28

Homofascaplysin A
Homofascaplysin A (11) is also b-carboline alkaloid. It was extracted from the Hyrtios erecta sponge (Thorectidae). 36 This alkaloid presented potent activity against CQ-resistant P. falciparum strains (IC 50 = 0.07 mM) with approximately 10-fold less cytotoxicity. 37 This potent antiplasmodial activity of this compound demonstrated its potential as a lead structure among antimalarial agents and became a synthesis target for the production of other similar analogues. 38

Terpenoids
All terpenes or terpenoids have fundamental repeating vecarbon isoprene units. Terpenes are classied as hemiterpenes (C 5 ), monoterpenes (C 10 ), sesquiterpenes (C 15 ), diterpenes (C 20 ), sesterterpenes (C 25 ), triterpenes (C 30 ), and tetraterpenes/carotenoids (C 40 ). 63 Marine-derived terpenoids have attracted potential interest similar to the terrestrial analogues represented by the sesquiterpene lactone artemisinin and isonitriles-containing terpenes. 7,64,65 More than 30 compounds were isolated and showed antimalarial activity from marine organisms. Unique mechanisms were demonstrated, including the inhibitory activity against heme detoxication by isonitrile derivatives. They are discussed in the following subsections, and their chemical structures are shown in Table 3.

Diterpene isonitriles
A phytochemical investigation of the Cymbastela hooperi sponge provided 15 diterpenes containing isonitriles isocyanate, isothiocyanate, and isonitrile functionalities, which showed higher antimalarial effect and moderate toxicity. 81 The activity of analogues with isocyanate and isothiocyanate functionality was up to ten times lower, demonstrating that the isonitrile group improved activity. An analogue containing only the formamide functional group but no isonitrile group was also ineffective against P. falciparum, implying that the formamide group isn't necessary for antiplasmodial efficacy. 82 Monamphilectine A (56) was a diterpenoid b-lactam alkaloid separated from a Hymeniacidon sp. sponge (Halichondriidae). Monamphilectine A displayed a potent antimalarial activity,. 60 While (−)-8,15diisocyano-11(20)-amphilectene (86), re-isolated from the Svenzea ava sponge (Scopalinidae), was used as a precursor to synthesize ve new products, all of which were tested against laboratory colonies of P. falciparum and Mycobacterium tuberculosis H 37 Rv. 75 In addition, 8a,11-dihydroxypachydictoyl A (87), and 4,18dihydroxydictyolactone (88) were diterpenoids isolated from Dictoyta sp. of the brown alga, which displayed antimalarial activity (IC 50 = 10.0 mM) against K1 strain of P. falciparum. 83 Moreover, so corals and echinoderms living in Vietnamese seas provided several diterpenes. Among them is laevigatol A (89), which had a moderately antiplasmodial activity with an IC 50 < 5.0 mM. 84,85 Also, a series of diterpene glycosides was obtained from the Caribbean so coral Pseudopterogorgia elisabethea (Gorgoniidae). Among them pseudopterosin V (90) exhibited an antimalarial activity (IC 50 = 2.2 mM) against CQresistant P. falciparum colonies. 86

Other terpenoids
Screening of marine sponges extracts from Spongia, and Ircinia genera revealed thr presence of broad-spectrum antiprotozoal meroterpenes, linear triterpenoid, and squalene, with inhibitory effects on P. falciparum and Trypanosoma. The dorisenone D (91), a dimeric C 21 meroterpenoid obtained from Dysidea arenaria sponge (Dysideidae), may become a promising antiplasmodial compound. 87

Endoperoxide-containing compounds
One of the most fundamental advances in malaria chemotherapy was the discovery and manufacturing of endoperoxidecontaining drugs. Undoubtedly, the artemisinin discovery was the beginning of research in this area. Artemisinin is a cadinane sesquiterpene lactone establishing a 1,2,4-trioxane moiety isolated from sweet wormwood Artemisia annua L. leaves (Asteraceae). Artemisinin showed nanomolar potency against CQresistant Plasmodium strains. The endoperoxide linkage is essential for antimalarial activity. One of the artemisinin derivatives lacking the endoperoxide bridge showed no antimalarial activity. 88 These drugs containing endoperoxide were purported to interact via endoperoxide bond with the iron(II) center of the heme unit released in the food vacuole during the digestion of hemoglobin and lead to peroxide bridge cleavage the consequent formation of oxygen-centered radicals. Because of an intramolecular rearrangement, these reactive species were converted into free C-centered radicals, toxic to the parasite through the alkylation of sensitive macromolecular targets. A sarco-endoplasmic reticulum Ca 2+ dependent ATPase of P. falciparum has been proposed as a possible target for these active species. 89,90 Although, most likely, artemisinin activity is not mediated by interaction with a single enzyme. It had been proposed that the Fe 2+ -containing species interacting with the endoperoxide bond is not heme. 91 The marine antimalarial drug-containing endoperoxide was divided according to their structural feature into peroxyketal and non-peroxyketal, as demonstrated below, and the chemical structures are listed in Table 4.

Peroxyketal
Peroxyplakoric acids are the parent compound in the class of peroxyketals/3-alkoxy-1,2-dioxane derivatives, and its methyl esters; peroxyplakoric acids A 3 (92) and B 3 (93). It was extracted from the Okinawan sponge of Plakortis sp. (Plakinidae). 92,93 Peroxyketals derivatives showed potent activity in vitro (IC 50 = 150 and 120 nM against P. falciparum FCR3) and a good selective toxicity index. The long alkyl side chain in these derivatives is important for antimalarial activity as the synthetic analog containing methyl group instead of the nonadienyl group was completely nonactive. It has been observed that transforming the ester group into an amide group increases in vivo antimalarial potency. 94

Non-peroxyketal
Plakortin (94) is a simple 1,2-dioxane metabolite, and was isolated from Plakortis halichondroides. 95 Plakortin and its analogues, named dihydroplakortin (95), 3-epiplakortin (96), plakortide Q (97), plakortide E (98), were re-isolated from the Caribbean sponge Plakortis simplex. 96 All these compounds, except plakortide E, displayed good antimalarial activity against (D10) CQ-sensitive and (W2) CQ-resistant strains of P. falciparum, with no cytotoxicity and a more potent activity on the (W2) strain (IC 50 in D6 = 1.37 nM, and W2 = 1.11 nM). Plakortide E was found to be inactive. It could be ascribed to a vemembered ring presence instead of a six-membered ring and/ or the crowded substituents at carbons anking the endoperoxide linkage. 97,98 Currently, plakortin is among pre-clinically investigated antiplasmodial candidates. 99 On the other side, plakortide L (99), was extracted from a Jamaican sponge Plakortis sp. 97,98 Plakortide O (100) and plakortide P (101), were isolated from plakortis halichondrioides, presented mild antimalarial activity in vitro (IC 50 > 0.023 mM). Despite of their similarities with the plakortin, the congurational changes around the dioxane ring and/or the differences in the alkyl side chains are responsible for the observed decrease of activity. 100 It was established that the role of the endoperoxide system in the antimalarial activity of plakortin derivatives is pivotal, as the diol derivative (102) with an open peroxide ring exhibited no antimalarial activity, when the ester group substitution to the corresponding hydroxyl (103), methoxy (104), or acetoxy (105) derivatives affected the antimalarial activity or the selectivity of these derivatives. 101

Peptides and depsipeptides
Peptides are short cyclic or acyclic chains between two and y amino acids, linked by amide covalent bonds (peptidic bonds). At the same time, depsipeptides are cyclic or acyclic compounds of a-amino and a-hydroxycarboxylic acids linked to each other by esters and amides units. 121 Several peptides and depsipeptides from marine sources were reported for antiplasmodial activity are, summarized in Table 7.
Even though a good number of peptides and depsipeptides have presented good antimalarial activity, their mechanism of action is not well understood. Some of them displayed a strong inhibitory effect on some key enzymes present in the malaria parasite; moreover, the relation between the inhibition and their antimalarial activity remains unestablished.  (147), an acyclic peptide extracted from cyanobacterium Symploca species 126,127 exhibited strong activity against P. falciparum 3D7 colonies, with IC 50 = 74 nM. In addition, four acyclic lipopeptides, dragonamides A (148), B (149), dragomabin (150), and carmabin A (151) have been isolated from the cyanobacterium Moorea producens (Cyanobacteriaceae) (formerly Lyngbya majuscula). dragomabin, carmabin A and dragonamide A displayed good antimalarial activity (IC 50 = 6.0, 4.3 and 7.7 mM, respectively). 128,129 Morover, malyngamide X (152) is the rst (7R)-lyngbic acid connected to a new tripeptide backbone. It was obtained from Bursatella leachii (Aplysiidae), a Thai sea hare, presented a moderate antimalarial activity with a half effective dose (ED 50 ) = 5.44 mM against P. falciparum (K1) multidrug-resistant strain. 130

Phosphotriesters
A new class of antimalarials with long-chain bicyclic phosphotriesters, salinipostins A-K (167-177), Table 8, were obtained from Salinospora sp. bacteria (Micromonosporaceae). SAR fndings indicated that an increase in alkyl chain length attached to the phosphoester oxygen and vinyl carbon led to increased activity while branching of the alkyl causes a slight reduction in activity. The most active compound salinipostin A, did not afect parasite schizonts, indicating that it acts by disrupting the processes required for the establishment or growth of intracellular parasites. Salinispostin A did not inhibit haemozoin formation but cause cellular disorganization and disintegration of internal structure. 146 These compounds showed different activity against P. falciparum W2 strain. Salinipostins A and D displayed the most potent activity (IC 50 = 50 and 82 nM) followed by salinipostins I, B, F, C, G, E and H (IC 50 = 0.126, 0.139, 0.266, 0.1415, 1.52, 3.22, 8.70 mM), respectively. Only, salinipostins K and J displayed weak activity. 146

Polyethers
A polyether ionophore isolated from Streptomyces cinnamonensis (Streptomycetaceae) named Monensin (178), Table 8. Monensin has been displayed a strong antimalarial activity against P. falciparum. 147,148 In a recent study, a polyether metabolite was extracted from Streptomyces sp. strain H668. This polyether displayed in vitro antimalarial activity against both D6 and W2 strains of P. falciparum with IC 50 values from 0.15 to 0.3 nM. 149

Conclusion
Malaria is among the crucial VBDs affecting the global health, based on the WHO official reports. It is easily progressed to impairment of important human body organs, including the liver, and death in case improper diagnosis and treatment. This emergency has acquired a special interest among health care providers and researchers to nd more effective and safer medicaments, especially against the multi-resistant strain of Plasmodium sp. for the people of the developing countries. Particularly, natural-derived treatment of infectious diseases, including malaria is still the most convenient, safe, effective, and diverse. Marine organisms have attracted great potential in the last few decades as a promising and non-traditional source of bioactive compounds. Moreover, recent technological advances have led to isolate and identify thousands of marinederived compounds belonging to various chemical classes. A total of 181 compounds derived from different marine sources, including sponges, cyanobacteria, marine algae, and actinomycetes, were reviewed in the current research possessing potential antimalarial activities with unique SAR and targeting different growth stages, including ring and trophozoite stage. More than half of the compounds belong to three major chemical classes comprising alkaloids, terpenoids, and polyketides. Such chemical diversity, potency, and less cytotoxicity are recognized as great start point for further SAR and clinical investigations of antimalarial candidates. The current article assumed that marine-derived natural products can also open up novel resources of bioactive compounds for novel candidates for management of other infectious diseases, exploring the oceans and seas treasures. Three compounds, including bromophycolides, plakortin, and homogentisic acid, are investigated as antimalarial drugs in pre-clinical trials and may be approved and marketed soon.

Conflicts of interest
Authors declare that there are no known conicts of interest associated with this publication and there has been no signicant nancial support for this work that could have inuenced its outcome.

CQ
Chloroquine EC 50 Half maximal effective concentration ED 50 Half maximal effective dose FB Iron-protoporphyrin IX FDA Food and Drug Administration GSK-3P Glycogen synthase 3 IC 50 Half-maximal inhibitory concentration MIC minimum inhibitory concentration SAR Structure-activity relationships VBDs Vector-borne diseases P. falciparum D6 A West African clone P. falciparum W2 The Indochina clone W2 WHO World Health Organization