A systematic review of pharmacological activities and safety of Moringa oleifera

Introduction Multi-purpose medicinal plants (MMPs) have continued to elicit research interest and commercial attention globally as natural resources considered rich in nutritional and pharmacological properties to treat and cure several diseases (1,2). Several medicinal products as multitherapeutic agents have been derived from such herbs and plants and considered safe for consumption (3,4). Reports indicated that less than 30% of sub-Sahara Africa’s (SSA) population has access to modern health care and pharmaceuticals (5). The other 480 million people in the region rely on traditional medicines, mainly from MMPs (5,6). The medicinal importance and efficacy of the MMPs have been variously associated with the presence of antioxidants and polyphenolic compounds (1,7,8). In recent years, scientific and commercial efforts are being intensified to advance the sustainable utilization of natural antioxidants from plants in the food and nutrition industry as well as preventive health care (9-12). Over a decade ago, the global market worth of medicinal herbs was estimated to the tune of US$65 billion and with increased usage of up to US$5 trillion by 2025 (5). Thus, the relevance of MMPs as a reliable source of nutritive food, medicine and pharmacology will continue to elicit research interest and commercial attention to producing effective pharmaceuticals to combat diseases and http://www.herbmedpharmacol.com doi: 10.34172/jhp.2020.24


Introduction
Multi-purpose medicinal plants (MMPs) have continued to elicit research interest and commercial attention globally as natural resources considered rich in nutritional and pharmacological properties to treat and cure several diseases (1,2). Several medicinal products as multitherapeutic agents have been derived from such herbs and plants and considered safe for consumption (3,4). Reports indicated that less than 30% of sub-Sahara Africa's (SSA) population has access to modern health care and pharmaceuticals (5). The other 480 million people in the region rely on traditional medicines, mainly from MMPs (5,6). The medicinal importance and efficacy of the MMPs have been variously associated with the presence of antioxidants and polyphenolic compounds (1,7,8). In recent years, scientific and commercial efforts are being intensified to advance the sustainable utilization of natural antioxidants from plants in the food and nutrition industry as well as preventive health care (9)(10)(11)(12). Over a decade ago, the global market worth of medicinal herbs was estimated to the tune of US$65 billion and with increased usage of up to US$5 trillion by 2025 (5). Thus, the relevance of MMPs as a reliable source of nutritive food, medicine and pharmacology will continue to elicit research interest and commercial attention to producing effective pharmaceuticals to combat diseases and infections, particularly in the SSA. Ethnopharmacology has proven to be an influential and reliable tool in the search for indigenous plant species with medicinal and pharmacological properties that can be screened for pharmaceuticals toward successful drug discovery and development (13)(14)(15). The adoption of herbal medicine is generally hampered by inadequate knowledge of chemical constituents, pharmacokinetics and pharmacodynamics (16). Other limiting factors include poor quality control, scientific and clinical evidence of their efficacy and effectiveness (17,18). In many African traditional herbal remedies, lack of standardization is one of the major constraints to their safety and adoption (19).
In this systematic review, we present M. oleifera as one of the valuable MMPs globally recognized as a food and medicinal plant providing natural supplements for healthy living (1,20,21). It is a plant with numerous local utilizations with huge pharmaceutical potentials that can be exploited for human health benefits (22,23). The multipurpose medicinal activities and ethnopharmacological use pattern of M. oleifera have been reported from many cultures around the world to treat and cure many illnesses like high fever, diabetes, hypertension, sickle cell anemia and cancer, among others (24)(25)(26). Several authors have also reported various bioactivity and pharmacological mechanisms of M. oleifera parts including leaf, root, seed, and barks (1,(27)(28)(29)(30). The hypocholesterolemic activity has been demonstrated in Wistar rats and rabbits (31,32). A detailed review of hyperglycemia and dyslipidemia activities of the plant has also been published by Mbikay (33). Similarly, various parts are widely used in the treatment of neurodegenerative illnesses (23,25,34,35). Precisely, an avalanche of evidence on pharmacological activities has been highlighted in the literature and commercial products attributing such rich activities to the abundant presence of bioactive molecules including antioxidants, flavonoids, quercetin, Moringine among others (36)(37)(38)(39).
Globally, there is increased scientific research, commercialization, utilization and demand for M. oleifera based pharmaceutics and supplements to challenge various illnesses and support healthier living. A review of existing literature also indicates commensurate studies on animal models to support and confirm the pharmacological properties and safety of M. oleifera. However, safety reports on human studies with no adverse effects are few (40). Herbal extracts and products are limited in use and adoption probably due to the uncertainty surrounding their toxicity level and safety (41). In addition, several reports have shown that toxicity studies are necessary to validate the safety limits of many herbal preparations (42,43). From our observation and market surveys, Moringa-based products and supplements are becoming increasingly available in the open market, as well as online portals without commensurate toxicity assessment tests to establish their safe consumption. Also, different degrees of toxicity have been reported on various seed extracts of M. oleifera and thus requiring more scientific reviews and experimental trials (42,44). Hence, this systematic review is aimed to update existing information on the overall value of the pharmacological activities, toxicity and safety potentials of M. oleifera as an MMP.

Search strategy
We conducted a systematic review on pharmacological, toxicity and safety of M. oleifera over a period of 29 years (1990 to 2019). Five major databases including PubMed, ScienceDirect, Web of Science, Scopus and Mendeley were engaged in the course of searching for relevant information and data. The search terms were "M. oleifera" and ("pharmacological activities", "safety") without narrowing or limiting search items. Table 1 shows the search terms and combinations. For our search strategy, the systematic procedure reported by Page and Moher (45) was adopted.
Study selection Article and study selection was based on originality of research works, keywords and search terms in the title, abstracts and studies involving pharmacological, toxicological and safety assessment of extracts of M. oleifera. Inclusion criteria were papers that evaluated the medicinal and pharmacological properties of M. oleifera parts (leaves, seeds, and barks) in the treatment of several ailments. We excluded review articles, abstracts, case reports and conference proceedings with no link with our inclusion criteria. The articles were reviewed for titles and abstracts by two reviewers (JOP, OSA) while duplicated articles were eliminated. Data extraction One author (JOP) extracted data using standardized forms. Publications with available abstracts were reviewed. Toxicity and safety studies Toxicity and safety studies involving animal models and human clinical trials were considered based on histopathology tests, relative organ weights, and toxicity biomarkers.

Results
After careful searches of databases, we identified and selected 165 articles that clearly matched our selection criteria out of a total of 618 articles searched. The breakdown of the database searches was as follows: 35 articles out of 52 from Scopus, 55 articles out of 456 from ScienceDirect, 50 articles out of 85 from PubMed while 13 from Web of Science and 12 from Mendeley directly related to the purpose of this review were selected. From the 165 articles selected, 50 articles duplicated in two or more databases were excluded leaving us with 115 articles. After preliminary selection of articles based on inclusion benchmarks, a total of 63 published articles were carefully chosen while others were excluded. The data in Figure 1 shows the flowchart of our search strategy. Most of the studies on pharmacological activities, toxicity, and safety on M. oleifera were carried out by experts from Indian, Nigeria, South Africa, Italy, Bangladesh, Egypt, Cameroun, Malaysia, Iran among others. Source of articles identified and selected for this review, articles per database searched, year and number of publications as well as geographical location and number of publications were illustrated in Figure 2.  others were recorded ( Figure 3). Moringa isothiocyanate was variously mentioned and associated with the pharmacological activities of M. oleifera. Information in Table 2 shows the pharmacological properties included in this review, authorship, countries of studies, specific investigation and outcomes.
Toxicity and safety of Moringa oleifera Studies on the toxicity and safety of M. oleifera included in this review are shown in Table 3. Generally, studies on the toxic effects and safety have reported of M. oleifera to be safe in various models on mice, rats, rabbits, in vitro assays and some clinical trials (42,(58)(59)(60). In retrospect, very few toxicity and safety evaluations were encountered in investigations involving humans or clinical trials.
Sixteen studies were evaluated on quality assessment of pharmacological studies of M. oleifera using experimental animals. Among the articles retained, above 75% of studies reported and mentioned the random distribution of animals during experimental procedures. Ten studies clearly mentioned sample size while six did not ( Figure  4). From the articles retained, doses tested on the experimental subjects showed no adverse effects.

Discussion
Database searches and global relevance of M. oleifera The outcomes of this systematic review confirm that M. oleifera is an important plant species with a plethora of pharmacological and therapeutic activities. Our data indicate that Indian researchers were the highest

South Africa
Male Wistar rats were induced with STZ and consequently exposed to methanolic leaf extract of M. oleifera to evaluate its anti-oxidant, inflammatory and hypoglycemic activities.
A significant reduction in plasma glucose level reported in Moringa treated diabetic rats. The hypolipidemic activity was demonstrated in diabetic rats. Antioxidant activity was reported to be high.
Moringa leaf extract showed a depressing impact on biomarkers-TNF-α and IL-6. Diabetic nephropathy was significantly reduced.

Cheraghi et al (50) Iran
Cardioprotective property of M. oleifera leaves using isolated N,α-Lrhamnopyranose VR compound was evaluated in rats induced with doxorubicin cardiac toxicity VR reduced MDA levels in heart tissues. Consequent upon administration, reduced SOD and GSH levels were increased. VR administration inhibited the appearance of markers of heart failure. Cardioprotective

Giacoppo et al (51) Italy
The study evaluated the antiproliferative properties of M. oleifera leaves using isolated isothiocyanate (moringin) complexes on human neuroblastoma cells.
The growth of SH-SY5Y cancer cells was suppressed. MAC treated cells displayed a dosage reliant downregulation of phosphatidylinositol 3-kinase (P13K)/ Akt (protein kinase B)/ mammalian target of rapamycin pathway. MAC treatment also induced apoptotic cell death pathway in the tumor cells. Anti-proliferative effects on PC-3 cells were reported while the safety of the Glucomoringinisothiocyanate rich soluble extracts was confirmed.

Muhammad et al (54)
Malaysia Wound healing abilities of bioactive M. oleifera aqueous fraction on STZ and NAD -induced diabetic Wister rats were evaluated.
Antibacterial susceptibility testing revealed that Moringa oleifera aqueous fraction repressed bacteria growth on chronic wounds of diabetes (S. aureus, P. aeruginosa, and E. coli). The bioactive fraction of Moringa enhanced wound healing in diabetic rats by downregulating proinflammatory cytokines and promoting angiogenesis.
Wound healing

Republic of Korea
In vitro neuroprotective potential of M. oleifera ethanolic leaf extract evaluated.
Leaf extract dose-dependently increased number, length, and branching of neuritis. Moringa leaf extract protected neurons against naturally occurring cell injury by significantly increasing neuronal viability, enhanced neuronal differentiation and synaptogenesis.  No adverse effect on studied organ and organism. The extracts were reported to be safe for consumption in tested organisms.

2.
Asare et al (20) Ghana Rats and human The study evaluated the toxicity and safety of an aqueous leaf extract of M. oleifera in different rats and human blood cells. The rats were fed with high doses of the extracts at 1000 and 3000 mg/kg of and observed for two weeks. Outer blood mononuclear cells in humans were also subjected to graded doses of the extract.
No toxicity was recorded at low and standard doses but at a very high dose of 3000 mg/kg b.wt, the extracts were genotoxic. The dose of 3000 mg/kg was too high which exceeded the recommended doses. Thus, at recommended/standard doses, no traces of genotoxicity were found both in rat's human cells examined.

Jaafaru et al (60) Malaysia Rats and human
Toxicity and antiproliferative actions of M. oleifera glucomoringin-isothiocyanate on cellular multiplication and apoptosis in human prostate cancer cells (PC-3) was investigated The glucomoringin-isothiocyanate was found to be non-toxic, induced apoptosis and inhibited human cancer cell proliferation.

4
Kim et al (42) USA Rats Toxicity evaluation to determine the safety of Moringa isothiocyanate-1 -enriched hydroalcoholic MSE was investigated in rats. Toxicity was determined in various internal organs and cells: gastrointestinal and stomach lesion and discoloration among others.
No mortality was recorded at normal, mid-high and mid-low dose groups. However, mortality was observed in group fed with very high doses of the extracts. Tested doses were not lethal on experimental rats. Genotoxicity parameter were not statistically significant.  Awodele et al (58). Nigeria Mice The toxic effect of M. oleifera aqueous leaf extracts on mice was evaluated using oral administration of 6400 mg/kg extracts and intraperitoneal dose of 1500 mg/kg. In another trial, the mice were orally administered different doses for 60 days.
No toxicity level recorded on examined organs and cells. The study concluded that leaf extracts of M. oleifera were safe in the experimental animals used. 8 Oyagbemi et al (63).

Nigeria Wista Rats
The study investigated the toxicity of M. oleifera methanolic extracts at differing dosages in 30 Wistar rats for 2 months. No adverse effect reported

Anti-inflammatory and analgesic activities
Over 75% of the articles retained highlighted the antiinflammatory and pain relieving activities of M. oleifera which also have been associated with the plant's high antioxidants content. We noticed increased investigation on the anti-inflammatory, antitumor and anticancer abilities of M. oleifera. Inflammation is a multifarious biological reaction of vascular cells to detrimental stimuli, for instance, pathogenic infections, injured cells or irritations (44,73). It is a protecting effort/mechanism by which organisms tend to get rid of harmful impetuses and initiate the healing process (44). Pain, heat, redness, swelling, and loss of functions are some of the usual symptoms of acute inflammation (74). Adedapo    Is the hypothesis/aim/objective of the study clearly described?
Are the main outcomes to be measured clearly described?
Are the main findings of the study clearly described?
Were sample size calculations reported?
Was the allocation adequately reported?
Were the investigators blinded from knowledge with treatment used?
Were animals selected at random for outcome assessment?
Was the outcome assessor blinded?
Has any statistical treatment been applied?
Have the experimental protocols been approved by an ethics committee?

Yes
No Unclear methanolic leaf and root extracts on induced arthritis in rats also established that the extracts considerably reduced all the mechanical symptoms of pain and inflammation among the studied rats (57). A depressing outcome on the inflammatory biomarkers-tumor necrosis factor (TNF-α) and IL-6 diabetic nephropathy in streptozotocin-induced diabetic male Wistar rats were also observed (76). Thus, the analgesic and anti-inflammatory capabilities of M. oleifera parts are not in doubt and could be carefully explored as a cheap and effective drug source to relieve pain and treat inflammation-related disorders. In all the articles retained, isothiocyanate complexes were highlighted to be responsible for its antimicrobial activities. M. oleifera leaf extracts were also found to be potent against infectious diseases caused by multidrug-resistant gram-negative bacteria (81). The investigation of Moura et al (47) clearly established the antimicrobial capabilities of the seed lectins from M. oleifera found to suppress the formation of biofilm by S. marcescens and inhibited bacterial growth. The lectin also suppressed the growth of Bacillus sp. With respect to the antiasthmatic activity of the species, a clinical evaluation of seed kernels on twenty asthmatic patients indicated its efficacy and safety to treat asthma in human clinical trials (64). According to the study, alkaloid moringine relaxes bronchioles which closely bear a resemblance to ephedrine in action and valuable to treat asthma and other related ailments. Based on the noticeable reduction in asthma symptoms and improved breathing functions of the patients, M. oleifera is a very potent herbal source that can be used in asthma prevention and treatment. Infectious diseases are the main cause of death in teenagers (39,81). It is common in developing nations with high mortality being recorded every year (81). Consequently, bacteria are increasingly becoming resistant to antibiotics as a result of a change in response to the use of antibiotics, overuse, and misuse (81)(82)(83). Antibiotics resistance is currently a leading threat in the health sector. The WHO at a recent press conference organized to commemorate the 2018 world antibiotics and diabetes awareness week in Nigeria cautioned the public against the misuse of antibiotics and urged healthy feeding as well as physical exercise to combat diabetes (Read more at https://www.vanguardngr.com/2018/11/who-cautionsagainst-misuse-of-antibiotics).

Cytotoxicity, neuroprotective and anti-cancer activities
The various dietary and behavioural risks causing cancer include indulgence in tobacco and alcohol use, inadequate consumption of fruits and vegetables, lack of physical activities and obesity. Conventional chemotherapy is often characterized by known and unknown sideeffects. Consequently, plant-based alternative drugs and remedies are vigorously being explored as possible therapeutic agents. Findings from this review show that M. oleifera contains rich bioactive compounds which can be harnessed in the treatment and management of chronic diseases including cancer, HIV/AIDS amongst others (84). The antitumor actions of M. oleifera have been connected to the two-well characterized phytochemicals moringinine and quercetin which reverses tumourigenesis (85,86). The recent study of Jaafaru et al (60) provided more insight into the neuroprotection activities of M. oleifera isothiocyanate against hydrogen peroxideinduced oxidative stress. The isolated compounds, 4-(α-Lrhamnopyranosyloxy) benzyl ITC (moringin) complexed with alpha-cyclodextrin (moringin + α-CD; MAC), were shown to possess antiproliferative capabilities on SH-SY5Y human neuroblastoma cells (53,60). The nontoxic M. oleifera complexes induced apoptosis and inhibited the metastasis of cancer cells in man (60). In similar studies, isothiocyanate from M. oleifera seeds was demonstrated to mitigate hydrogen peroxide-induced cytotoxicity and maintained human neuronal cell morphology (53). Previous studies have also shown the antioxidants and anticancer capabilities of M. oleifera crude and aqueous extract with the ability to induce reactive oxygen species (ROS) (87,88). In addition, the antiproliferative activities on two cancer cells HepG2 (liver carcinoma cell line) and MCF7 (breast cancer cell line) confirmed M. oleifera as a probable source of cancer therapy (52,89,90). In another study, the beneficial effect of root extract of M. oleifera was evaluated against beryllium-induced oxidative stress in rats (91). The study confirmed that M. oleifera with piperine therapy was potent, while the mixture of M. oleifera and curcumin was more active and showed better therapeutic outcomes. The potential anticancer efficacy of the plant using the leaf ethanolic extracts has also been demonstrated in human cancer cells such as breast cancer cell type MDA-MB-231, HCT-8 (colorectal) cells and A549 lung cancer cells (89,92). Phytonanoparticles of M. oleifera and its potential antiproliferative agents against cancer are consistent with the safety reports and clinical responses on M. oleifera utilization to treat and cure cancer, neuroblastoma and other related diseases (64,93,94). Suppression of tumor growth and induction of apoptosis on Ehrlich's solid tumor implanted mice also indicated a reliable anti-tumor/anti-cancer capabilities of M. oleifera leaf extracts (95). With the preponderance of scientific reports on the cytotoxicity, neuroprotective and anticancer capabilities of M. oleifera parts, it is hoped that pharmaceutical companies around the globe will step in and develop Moringa-based drugs to cure cancer and other degenerative diseases.

Anti-diabetic and anti-obesity activities
People around the world have exploited herbal medicine to solve their health challenges and ultimately for a healthier living (96). Diabetes, being a protracted metabolic malady of the endocrine system, is characterized by abnormalities in carbohydrates, protein, and fat metabolism (97,98). The incidence of diabetes is increasing at an alarming rate with the attendant high cost of management/treatment and its related complications (96,(99)(100)(101). In 2012 alone, diabetes caused 1.5 million deaths with an increased projection estimated in recent years (99,101). Stroke, heart attack, overweight, kidney failure, obesity, leg amputation, vision loss, and nerve damage are some of the complications of diabetes. Obesity in Africa is also on the increase, being associated with various medical conditions that may lead to death. An imbalance in the intestinal flora is one of the main factors related to obesity and metabolic disorders. The human gut microbiome undertakes a variety of metabolic functions such as the breakdown of complex organic substances into easily digestible sources of energy. M. oleifera possesses antidiabetic, hypo-and hyperglycemia effects (1,102). Consequently, the antidiabetic therapeutic effect of M. oleifera has been attributed to its polyphenols content to reduce blood glucose and lipids concentration after ingestion (96). According to the investigation of Abd Eldaim et al (103), M. oleifera leaves aqueous extract significantly ameliorated hepatotoxicity and reduced hyperglycemia in alloxan-induced diabetic rats. In another study designed to demonstrate the efficacy of M. oleifera ethanolic extract in rats induced with high-fat diet obesity, body weight was significantly reduced in rats treated with the extracts (104). Potential hypoglycemic activity of phenolic glycosides from M. oleifera seed also showed significant therapeutic properties in reducing blood pressure, blood sugar and enhancing the immune system in humans (105). M. oleifera leaves extract was thus confirmed to exhibit hypolipidemic and antiobesity potentials with the capacity to protect the body against negative effects of high fat diet-induced obesity. Numerous non-diabetic and diabetic animal studies have demonstrated the antidiabetic effect of M. oleifera leaf extracts to reduce plasma glucose levels and increase glucose tolerance (29,106,107). A significant decrease in the level of plasma glucose was reported in rats having diabetes using M. oleifera leaves extract (76). The study of Jaiswal et al (107) (62,(110)(111)(112). Overall, M. oleifera leaves were confirmed to lower postprandial blood pressure (50).

Cardioprotective and anti-hypertension activity
Articles retained on anti-hypertension activities of M. oleifera suggest a positive correlation between consumption of raw leaves/extracts of M. oleifera and reduction in blood pressure of tested animals. Globally, hypertension, heart failure, and their associated risk factors have been a major cause of death without restriction to age (113) while almost half the losses in dialysis patients are ascribed to heart disease (114). High blood pressure is a typical symptom of hypertension with severe complications and increases the risk of heart disease, stroke, and death (115,116). Findings of different health interventions by the University College Hospital (UCH), Ibadan, Nigeria, indicated that hypertension is the commonest disease after the age of 40 (117). Management and cure for cardiac arrest and hypertension are cumbersome using synthetic drugs and usually with side effects. Thus, natural remedies that reverse hypertension and lowers high blood pressure are being sought after as plausible solutions to cardiovascular ailments. In this regard, our review outcome indicates that M. oleifera is a good candidate useful to treat hypertension. Its parts show cardiac protective effects in spontaneously hypertensive rats (115). Several studies have also provided scientific proof in support of ethnopharmacological uses of M. oleifera as the antihypertensive plant with strong cardiac protective effects (118,119). Cheraghi et al (50) investigated the cardioprotective property of M. oleifera isolated magnetic hydrogel nanocomposite loaded N,α-Lrhamnopyranosyl vincosamide (VR) on rats induced with doxorubicin cardiac toxicity. It was shown that VR levels of malondialdehyde (MDA) and superoxide dismutase (SOD) in heart tissues were reduced and heart failure biomarkers were inhibited. The studies of Nandave et al (120) also highlighted the cardioprotective capabilities of M. oleifera in isoproterenol (ISP)-induced model of myocardial infarction. Their findings indicated that the effects of ISP-induced hemodynamic perturbations and a rise in lipid peroxidation were significantly repressed when tested organisms treated with M. oleifera extracts. The cardioprotective/antihypertension capacities of the plant might not be unconnected to the high content of antioxidant, antiperoxidative, and myocardial preservative activities (130). Previous studies and personal experiences of one of us (JOP) also validate the consumption of M. oleifera raw leaves or extracts to lower the blood pressure, enhance sleep and prevent cardiac arrest (2,24).

Wound healing and anti-ulcer activities
Wound healing and anti-ulcerative pharmacological capabilities of M. oleifera have been severally reported and associated with rich phytochemical contents of alkaloids and antioxidants (59,77,121,122). Ulcer is one of the neglected tropical diseases affecting millions of people in many countries of the world (123). It is caused by infection with Mycobacterium ulcerans. Valuable anti-ulcer, anti-secretory and cytoprotective activities of M. oleifera were shown in albino Wistar rats using M. oleifera ethanolic root-bark extract, which aligns with the preponderance ethnobotanical information on the plant (24,122). Studies of Ruckmani et al (124) revalidated the anti-ulcerative properties of M. oleifera root and leaves in alkali preparation with healing effects. The evaluation of polyherbal concoction of M. oleifera, Raphanus sativus and Amaranthus tricolor leaf extracts in an experimental model of gastric ulcer in male albino Wistar rats also confirmed the use of M. oleifera as anti-ulcer therapy (56). Their findings concluded that polyherbal concoction exhibited significant anti-ulcerative activity, efficacious in preventing stomach sores caused by reperfusion and ischemia, ethanol and indomethacin. In another study, M. oleifera extract was compared with a commonly used drug for the ulcer, famotidine, and found out that the extract and famotidine significantly reduced the free acidity and total acidity of gastric juice (125). Thus, M. oleifera preparations, particularly in alkali form, can treat and cure ulcers without any adverse effect. The presence of both tannins and flavonoids has been implicated for the antiulcer activity of M. oleifera (39,125,126).
Wound healing has been described as a complex process of tissue repair in response to injury. It is characterized by hemostasis, inflammation, fibroplasia, and maturation (121,127). An aqueous fraction from M. oleifera was reported to heal wounds on diabetic Wister rats induced with STZ and nicotinamide (NAD) (54). The study established that M. oleifera aqueous fraction inhibited the growth of bacteria associated with chronic wounds of diabetes. The bioactive compound Vicenin-2 was implicated to enhance wound healing in diabetic rats by downregulating proinflammatory cytokines and promoting angiogenesis (54). Alcoholic preparation of M. oleifera leaves was also confirmed as an effective therapeutic agent in healing skin wound and improved fibroblast proliferation and spread thereby increasing wound closure rate (121).

Anti-malaria activities
Malaria is a leading and probably the most upsetting infectious disease with an increased incidence of mortality in sub-Saharan Africa (128,129). The mosquito vector, Plasmodium falciparum, is the cause of malaria infection. Though one study indicated that there is a decline in malaria in many parts of Africa, the estimations are not reliable with many deaths recorded recently (130,131). Malaria challenge is compounded by widespread resistance to artemisinin combination therapies by malaria parasites (132). However, as the threat of antimalarial drug resistance increases, persistent efforts are also being expended in developing alternative therapies coupled with the sustainability of existing treatments and control methods. Over the centuries, the use of herbal combinations or preparations as active antimalarial drugs have been seen as an effective approach to treat recurrent malaria infections (133,134). M. oleifera in combination with other plants has been actively used as antimalarial in many parts of Africa and Asia (2,24). For instance, M. oleifera and Gynostemma pentaphyllum leaf extracts were combined with Artesunate in mice infected with Plasmodium berghei and adjudged effective in the treatment of malaria (135). The infected mice were subjected to artesunate treatment (6 mg/kg) in combination with M. oleifera and G. pentaphyllum using standard doses. Their findings showed a significant curative effect and high percentage suppression in a dosedependent manner. Several other studies have established the efficacy of polyherbal preparations containing M. oleifera in combination with other plants, which also inhibited the growth of P. falciparum. This holds enormous potential to serve as an excellent source of herbal drug to combat malaria (112,135,136). The crude preparation of the plant leaves in aqueous form also demonstrated antimalarial activities by suppressing P. berghei ANKA infection in mice (137). Very recently, Olawoye et al (138) studied the actions of M. oleifera powdered leaves on amodiaquine (AQ) pharmacokinetics in healthy twenty human volunteers and established a positive interaction between AQ and M. oleifera suggesting a possible combinatorial therapy for malaria. The methanolic seed extract of M. oleifera also exhibited significant insecticidal activities, which could be exploited for integrated pest management to control the Anopheles mosquito (139).

Other activities
The aphrodisiac effect of M. oleifera is gaining scientific attention as a supplement to enhance sexual performance, although the precise underlying mechanisms are yet to be understood (140). The leaf extracts of M. oleifera inhibited 6-β-hydroxylation of testosterone with monoamine modulation effects, which could help to overcome stressinduced male sexual dysfunction (140,141). A significant correlation between increased sexual urges, erectile function, and consumption of M. oleifera methanolic leaves extract in rats has been reported and attributed to its soluble epoxide hydrolase inhibitory actions (142). In many local communities in Africa and elsewhere, M. oleifera parts have been used in the fight against HIV/AIDs and other viral infections due to its immunostimulatory effects (24,143,144). A study on the impact of leaf powder of M. oleifera as a supplement administered to patients with HIV and on antiretroviral therapy significantly enhanced their nutritional intake and boosted their immune system (145). Thus, the antioxidants in M. oleifera can be carefully utilized to fight free radicals and boost immunity in HIV and other related patients.

Toxicity, human studies, and safety of M. oleifera
The study of Reddy et al (146) clearly demonstrated the non-toxic nature of M. oleifera bark extracts on rats without records of lethality. Adedapo et al (44) also reported no adverse effect on studied organs and organisms suggesting that M. oleifera could be safe on the experimental animals. Likewise, the different tested doses on rats showed no lethality and significant differences in genotoxicity parameters, thus the study confirmed the suitability of M. oleifera as a safe herbal drug (62). Asare et al (20) reported that half-maximal lethal dose (LD 50 ) of M. oleifera aqueous leaf extract was not toxic and observed no changes in the behaviour of the studied rats. In other related studies involving the administration of high dosages of methanolic extract of the plant in male Wistar rats, no toxic reaction or mortality was observed among the rats (147). The toxicity of M. oleifera aqueous extract was investigated in both acute and subchronic studies where mice were orally given an extract of 6400 mg/kg, 250, 500, and 1500 mg/kg, respectively, and intraperitoneally supplied a dose of 1500 mg/kg and found to be safe (58). In another toxicity evaluation involving different standard doses of M. oleifera methanolic extracts for a period of 2 months, there was no negative toxicity reaction on the liver and kidneys of the tested animals (63). According to the findings of the study, all experimental rats fed with M. oleifera leaf extract did not show any symptoms of genotoxicity; however, there was an increase in body weight in a dose-dependent manner. Studies of Asiedu-Gyekye (148) also established the safety of a single dose of 5000 mg/kg and an oral dose of 1000 mg/kg of M. oleifera in tested rats with no overt adverse effects. Recently, the safety of Moringa isothiocyanate-1 enriched hydroalcoholic Moringa seeds extract was demonstrated in mid-low rats groups without toxic reactions on the organs (42). Wang et al (105) reported that phenolic glycosides compound 1, 4 and 5 may be developed as new safe hypoglycemic drugs from M. oleifera, thus validating its safety as a safe therapeutic agent. The therapeutic effect of M. oleifera consumption on diabetic rats also indicated its possible use and safety as an antidiabetic preparation (40,149). Other studies have also reported and confirmed the safety of M. oleifera as an herbal drug to treat many ailments (59,61,64,148).
Several trials demonstrating potential benefits for the treatment of diabetes, ulcer, and asthma and enhancing the immune system of HIV/AIDS patients have been reported (56,150). However, very few clinical trials involving the safety of different preparations of M. oleifera parts have been published. Kushwaha et al (150) studied 30 women at post-menopause supplemented daily with 7 g leaf powder of M. oleifera for 12 weeks and compared with another 30 post-menopause women as a control group. There was a corresponding increase in biomarkers of antioxidant properties showing significant anti-hyperglycemic and anti-dyslipidemia activities without toxic effects. Moringa leaf powder could have served as an appetite stimulant in HIV/AIDS patients, thereby improving their immune systems (60). Agrawal and Mehta (64) also investigated clinical trial and safety of the seed kernel of M. oleifera to treat bronchial asthma and demonstrated that the extracts were safe with no adverse effects recorded. Maurya et al (151) conducted a clinical trial of M. oleifera stem back on 30 patients with urinary tract infection and reported its safety in humans with no adverse effects reported on the patients. Although genotoxicity at the high dose of the supplement was reported in human peripheral blood mononuclear cells, such doses greatly exceeded commonly used doses as demonstrated by the studies of Asare et al (20). However, at recommended doses administered on rats, M. oleifera was found to be safe without genotoxicity observed in the organs tested. Shreds of evidence from the literature and articles retained for this review imply that M. oleifera (leaves, stem and bark or whole leaves) could be safe for consumption as a vegetable, as well as a therapeutic agent to fight diseases and possibly enhance healthy living. Combining the rich pharmacological activities with the safety of M. oleifera, this systematic review concludes that M. oleifera could be further explored in the search for cheaper and safer therapeutic agents against diseases especially in Africa and other resource poor nations.

Conclusion
This review highlighted the multifaceted pharmacological potentials and safety of M. oleifera as a multi-purpose medicinal plant which can be exploited by man in the search for more potent drugs to fight diseases. The increased usage and research efforts on the pharmacological capabilities of the plant (in vivo, in vitro and clinical trials), clearly lend credence to the safety reported in many works of literature and cultures. Though scientific breakthrough has been recorded on many fronts including, identification, isolation, and elucidation of the structure of pharmaceutical principles responsible for many therapeutic actions of M. oleifera, more research efforts are needed to ascertain its safety in humans, as well as the safety of current and future potential herbal formulations of Moringa origin. Many Moringa products are already in the market, which requires quality assessment for their safety and consumption. The importance of M. oleifera cannot be overemphasized in the search for new bioactives that could be harnessed for therapeutic potentials, and for exploring future research options to be engaged in pharmaceutical and allied companies.