The phytochemical constituents and therapeutic uses of genus Aloe: A review

Aloe, the largest genus in the Asphodelaceae family, comprises 548 species, with A. vera, A. arborescens and A. ferox being among the most widely studied species. Aloe species originated in arid climates and cover various habitats, from sea level up to 2700 m, and from desert to closed-canopy forests. For human health, Aloe species are the richest natural sources. The biological activity of Aloe sp. constituents covers a wide spectrum. Most of the indications come from traditional, folkloric use and several have been verified by in vitro or in vivo studies. Emodin, the main phenolic component, has showed anti-neoplastic, anti-inflammatory, antiangiogenic and toxicological potential for use in pharmacology. Polysaccharides, with acemannan being the most important, are present in high abundance in Aloe gels. Acemannan has been reported to have applications in oral, metabolic and cardiovascular diseases, oncology, dentistry and wound healing. The effectiveness of Aloe sp. constituents on colon, liver, duodenum, skin, pancreas, intestine, lungs and kidneys cancers was highly studied with remarkable findings. Regarding the metabolic syndrome, Aloe sp. can be used as an antidiabetic and reduces cholesterol and total body fat. Constituents of Aloe sp. are nontoxic in experimental acute oral studies and are widely used in cosmetology and as bitter agents or consistence modifiers in food and beverages. Traditional Aloe remedies cover most human diseases; however, in order to gain legitimacy, the Aloe-derived drugs must have a well-established composition, with thoroughly investigated adverse effects and conventional drug interactions.


Introduction
Aloe, the largest genus in the Asphodelaceae family, bears its name from the Arabic word "Alloeh," meaning shining bitter substances . The genus Aloe L. comprises 548 accepted species, with at least one-third having some commercial importance (Grace et al., 2009). A. vera, A. arborescens and A. ferox are among the most widely studied Aloe species. The Egyptians called Aloe the "Plant of Immortality" because they can live and even bloom without soil (Mukesh et al., 2010). The plant was widely used by the Aloe ferox Mill. (= A. candelabrum A. Berger), commonly known as the bitter Aloe or Cape aloe, is a polymorphic species indigenous to the Western Cape region of South Africa. It has a single, tree-like stem with succulent leaves protected by reddish spines; hence, the name ferrox (Latin for fierce). Six to twelve branches are present at A. candelabrum, and the flowers have their inner petals tipped with white. The flowers are carried in a large candelabra-like flower-head. There are usually between five and eight branches, each carrying a spikelike head of many flowers (Schmid et al., 1998).

Phytochemical Constituents of Aloe Plants
Aloe leaves, the most commonly used medicinal parts, can be divided into the following structural components: outer green epidermis, consisting of a thick cuticle and under a zone of chlorenchyma (1); outer pulp region, under the epidermis, containing vascular bundles with bitter sap (latex) that exudes from the leaves when they are cut (2); inner leaf pulp, containing large thin-walled parenchyma cells filled with the colorless mucilaginous gel (containing the aloe gel) (3) (Grindlay and Reynolds, 1986;Salehi et al., 2018). Description of the inner central part of the aloe leaf may sometimes be confusing, due to the different terms that are used interchangeably such as inner pulp, mucilage tissue, mucilaginous gel, mucilaginous jelly, inner gel and leaf parenchyma tissue. Technically, the term 'pulp' or 'parenchyma tissue' refers to the intact fleshy inner part of the leaf including the cell walls and organelles, while 'gel' or 'mucilage' refers to the viscous clear liquid within the parenchyma cells (Hamman, 2008). It is important to differentiate between the two medicinal components of A. vera leaves: gel and exudates.
The main classes of bioactive compounds differ among the three components. Thus, the outer green epidermis contains mostly anthraquinones, pre-anthraquinones and corresponding glycosides, while the outer pulp region consists of phenolic compounds (anthraquinones, pre-anthraquinones, flavonoids, chromones, anthrones, coumarins, and pyrones). The pulp is rich in acemannan and phenolic compounds. The Aloe gel from the inner leaf pulp also contains proteins, vitamins, minerals and enzymes.
Flowers of A. vera are a by-product with valuable bioactive compounds whose health benefits are only partially assessed. The flower can be considered as have three maturity stages: immature (1); mature (2); mature, with flowers buds opened (3)(Martínez- . Immature flowers present the highest content of phenolic and antioxidant capacity. As the flower develops the content of these compounds decreases, and the content of fatty acids increases. The last maturity stage has the lowest fatty acid content. These compounds have applications in the cosmetic, nutraceutical, pharmaceutical and food industries. The harvesting period may be chosen depending on the compound of interest and, by removing the flower, the energy consumption of flowers from the plant will be lower, thus favouring plant development . Zapata et al. (2013) noted that the leaf characteristics and gel chemical composition of eight Aloe species studied in freshly harvested leaves in three different seasons within the Mediterranean climate have differences depending of species and harvest seasons. The biological activities of the components are the result of a combined and synergistic action rather than the added effects of single substances (Dagne et al., 2005). The anthraquinones contained by Aloe species are aloesaponarin, helminthosporin, aloechrysone, chrysophanol, aloesaponol, asphodelin and bianthracene (Salehi et al., 2018).
The anthrone class is represented by aloin (synonym -barbaloin), homonataloin and nataloin (Dagne et al., 2005). Aloin comprises two diastereomers: aloin A and aloin B. It is a C-glycoside that can be hydrolyzed in the gut to form aloe-emodin anthrone, which auto-oxidizes to quinone aloe-emodin. Emodin has numerous pharmacological effects. Both in vitro and in vivo studies have demonstrated anti-neoplastic, antiinflammatory, anti-angiogenic and toxicological potential for use in pharmacology (Hsu and Chung, 2012). 5 Table 1. Aloe structural components with class, compounds, source and pharmacological activities (S. Choi and Chung, 2003;Dagne et al., 2005)  The chromones, an abundant phenolic class in leaves, comprise aloesin, aloeresin A and isomeric forms, from aloeresin C to aloeresin F (Cock, 2015).
Feralolide and dihydroisocoumarin glycoside are the coumarins contained in A. species leafs. Aloenin, aloenin aglycone, aloenin acetal and aloenin B are the pyrones identified in several Aloe species leaf exudates. The most common Aloe alkaloids are N-methyltyramine and O,N-dimethyltyramine, while γ-coniceine is only present in a few species (Cock, 2015). Protocatechuic acid, methyl-p-coumarate and pluridone are benzene derivatives frequently identified in Aloes (Salehi et al., 2018).
Naringenin, apigenin, isovitexin and dihydro-isorhamnetin are the major flavonoids detected (Salehi et al., 2018). Phytosterols are represented by cholesterol, β-sitosterol, campesterol and lupeol together with their glucosides. Polysaccharides, the non-phenolic components, with acemannnan being the most important, are present in high abundance in Aloe gels. Acemannan, the main bioactive polysaccharide of A. vera, is a β-(1,4)acetylated soluble polymannose (Liu et al., 2019). It is a storage polysaccharide in the protoplasts of parenchyma cells.
Aloe acemannan content depends greatly on the species and cultivation conditions. Irrigation influenced the amount of polysaccharides. The mannose content decreased with 41% in the case of water deficit. When the aloe was irrigated with seawater, 42% seawater stress treatment only reduced the polysaccharide concentration in the base leaves, without lowering the polysaccharide concentration in the upper and middle parts (Jiang et al., 2014). Considering the age of the plant, the acemannan level reached a peak in three-year-old A. vera plants and then decreased. In addition, increased light intensity resulted in higher acemannan concentrations in A. vera and A. arborescens (Ray and Aswatha, 2013).
Acemannan has been reported to have many pharmacological and biological applications in the medical field, such as oral, metabolic and cardiovascular diseases, oncology, dentistry and wound healing (Liu et al., 2019). Acemannan, when administered orally to mammals, inhibits cholesterol absorption and induces hypocholesterolemia. Parenterally, it induces macrophage activation and interleukin-1 release, stimulates bone marrow activity, promotes wound healing, and inhibits viral replication and tumour growth. This wide range of activities promotes the mannans to potential therapeutic agents and biological response modifiers (Tizard et al., 1989).

Medicinal Use of Aloe Plants
Gastrointestinal disorders, hepatoprotective action and beneficial effects against skin problems such as wounds, injuries, and infectious diseases are among the most frequently mentioned indications in traditional medicine in connection with Aloe species (Akaberi et al., 2016). Aloe sp. dried juice is used traditionally in small doses as carminative and tonic and in larger doses, as a laxative and emmenagogue (Moein et al., 2017). The biological activity of many Aloe species covers a wide spectrum. Most of them come from traditional, folkloric use and some have been verified by in vitro or in vivo studies (Dehdari et al., 2018). The level of experimental or clinical confirmation is very variable, going from anecdotal mentioning to prospective, doubleblind clinical studies.
Antimicrobial and antifungal activities The antimicrobial activity includes bacteria, fungi and viruses. "Smart" biohybrids containing A. vera with triiodide have excellent antifungal and promising antimicrobial activities, are cost-effective, ecofriendly and can be used against surgical site infections (SSI) and as disinfecting agents (Edis and Bloukh, 2020).

7
In vitro activity assessment of Aloe barberae demonstrated antimicrobial effects on gram-positive (Bacillus subtilis and Staphylococcus aureus) and gram-negative bacteria (Escherichia coli and Klebsiella pneumoniae). Aloe sap extract is more effective than leaf extract (Ndhlala et al., 2009). Another study showed that A. vera juice has antimicrobial activity against M. smegmatis, K. pneumoniae, E. faecalis, M. luteus, C. albicans and B. sphericus, but has no effect on Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Salmonella typhimurium (Alemdar and Agaoglu, 2009).
A. vera has better therapeutic, antibacterial and anti-inflammatory effects against staphylococcal pyoderma in dogs than gentamicin (Kamr et al., 2020).
A clinical study performed by Prueksrisakul et al. (2015) on healthy volunteers receiving 250 mL of A. vera gel extract daily demonstrated a significant decrease in the number of oral pathogenic bacteria.
Experimental studies have demonstrated that an aqueous suspension of Aloe polysaccharides can be used to control angular leaf spot disease (Xanthomonas fragariae), which acts both by its antimicrobial activity and by activating latent defence mechanisms in strawberry (Luiz et al., 2017).
In vitro antifungal effects of Aloe species extract have been demonstrated also in Candida albicans (Ndhlala et al., 2009). The purified Aloe protein fraction from the A. vera leaf gel had potent antifungal activity against Candida paraprilosis, Candida krusei and Candida albicans (Das et al., 2011).
A film containing Aloe vera used for coating sliced fruit prolonged the shelf-life of the merchandise by controlling fungal contamination (Benítez et al., 2015;. A vera, A. ferox, A. mitriformis and A. saponaria have high antifungal activity against B. cinerea, P. digitatum, Penicillium expansum and P. italicum. Measured as a percentage of infected leaves, this antifungal activity was positively correlated with aloin content (Zapata et al., 2013).
A study performed by Nidiry et al. (2011) demonstrated that aloin and aloe-emodin are the active principles against two phytopathogenic fungi Colletotrichum gloeosporioides and Cladosporium cucumerinum. Sydiskis et al. (1991) showed that aloe emodin inactivated herpes simplex type 1 and type 2, varicellazoster, pseudorabies and influenza but was not effective against adenovirus and rhinovirus. Electron microscopy demonstrated that the virucidal mechanism consisted of envelope disruption.
A. arborescens has been used for the treatment of upper respiratory tract infections in Central and Eastern European countries for many decades. In vitro study with a mixture containing A. arborescens extract showed a clear dose-dependent antiviral activity against human rhinovirus 14 and Coxsackievirus (both nonenveloped RNA viruses). Respiratory syncytial virus and parainfluenza virus (Paramyxoviridae) were poorly blocked by the test substance, while an adenovirus was not affected by the mixture (Glatthaar-Saalmüller et al., 2015).

Wound healing effect
Skin healing and tissue regeneration are among the most frequently used features in both traditional and modern medicine. Empirical observations were confirmed by in vitro studies, followed by experimental and clinical trials. Liang et al. (2020) suggested that adding gel to the wound dressing could be a simple and standardized way to use A. vera. Inflammation is a normal component of healing, yet, the anti-inflammatory effects of Aloe sp. seem to boost tissue healing (Park et al., 2009). The in vitro study demonstrated anti-inflammatory effects of Aloe sp., indirectly confirmed by the effect of aloe-emodin, comparable to that of kaempferol and quercetin (Ndhlala et al., 2009). The effect of G1G1M1DI2, a glycoprotein fraction isolated from A. vera, on cell migration was confirmed by the accelerated healing of a monolayer of human keratinocytes. It promoted the formation of epidermal tissue in raft culture. Thus, this glycoprotein fraction promotes wound healing by both cell proliferation and migration (Choi et al., 2001). Other healing effects include increased cell phagocytic effect, more rapid wound area contraction rate and collagen synthesis, all due to the mannose contained in A. vera. The polysaccharides present in A. vera induce fibroblast proliferation, hyaluronic acid and 8 hydroxyproline production, which play an important role in extracellular matrix remodeling (Salehi et al., 2018). A. vera green-synthesized silver nanoparticles photobiomodulated by irradiation with laser led to an increase in cell migration in normal wounded and diabetic wounded fibroblast cells ). An experimental study in rats with eight Aloe species (A. arborescens, A. brevifolia (Figure 3), A. eru, A. ferox, A. grandidentata, A. perfoliata, A. saponaria, and A. vera) demonstrated a significantly accelerated healing in the topical application of leaf methanol extracts on diabetic wounds (El Sayed et al., 2016). Oryan et al. (2016) showed that A. vera modulated inflammation, increased wound contraction and epithelialization, decreased scar tissue size, and increased alignment and organization of the regenerated scar tissue. The lesions also demonstrated improved modulus of elasticity, maximum load and ultimate strength. Oral administration of A. vera promotes healing by increasing collagen content and improving angiogenesis and chemotaxis in rats (Ali et al., 2020). Patients with thermal burns dressed with A. vera gel showed advantages compared to those dressed with sulfadiazine regarding early wound epithelialization, earlier pain relief and cost-effectiveness (Shahzad and Ahmed, 2013). Aloe sp. are also efficient in preventing and improving hypertrophic scars (Duansak et al., 2003;Surakunprapha et al., 2020). A. vera may have an anti-inflammatory effect in burn injuries due to the reduction in leukocyte adhesion and pro-inflammatory cytokines. A meta-analysis performed by Guo et al. (2016) concluded that A. vera may be used both as an alternative and integrative way to reduce symptom severity in the wound healing process at the mucocutaneous level. The favourable effects on the digestive system begin with the traditional use as a laxative and cover almost every aspect or organ. Significant antiulcer and gastroprotective activities were observed after the administration of Aloe-containing preparations (Akaberi et al., 2016;Eamlamnam et al., 2006). An interesting direction was provided by a study that noted that the A. vera gel has antibacterial properties against both susceptible and resistant Helicobacter pylori strains. Thus, a combination of A. vera gel with antibiotics may improve the results of H. pylori eradication (Cellini et al., 2014). A study on rats that deal with the effect of A. 9 vera on gastric microcirculatory changes, cytokine levels and gastric ulcer healing showed that A. vera treatment reduced leukocyte adherence and TNF-alpha levels, elevated IL-10 levels and promoted gastric ulcer healing (Eamlamnam et al., 2006).
A study with a A. ferox extract on constipated rats led to improved intestinal motility, increased fecal volume and normalized body weight, confirming the empiric use as a laxative in South Africa (Wintola et al., 2010). A similar effect is targeted in sheep diets containing A. vera extract to reduce enteric methane emission and boost productivity (Akanmu et al., 2020). A recent study has confirmed that acemannan has the advantage of inducing intestinal growth in bacteria such as Bifidobacterium and Lactobacillus (Quezada et al., 2017). A. vera gel had a dose-dependent inhibitory effect on reactive oxygen metabolite production in incubated colorectal mucosal biopsies, which indicated a therapeutic effect in inflammatory bowel disease (Langmead et al., 2004). Aloe extract and a number of its compounds have been shown to ameliorate inflammation and improve clinical and histopathological colitis symptoms in animal models (Akaberi et al., 2016). Any prolonged treatment for chronic inflammatory bowel disease should maintain a high awareness of cancer risk (Harris et al., 2020).
Prospective, randomized, double-blind, placebo-controlled trials in proctology consisted of the application of A. vera cream on the surgical site. The results were similar and demonstrated that the treatment is effective in reducing postoperative pain both at rest and during defecation, healing time, and analgesic requirements (Gaj and Crispino, 2009;Eshghi et al., 2010).
The bitter latex of Aloe ferox is used as a laxative in Africa and Europe and is considered to have tonic, antioxidant, anti-inflammatory, antimicrobial and anticancer properties (Chen et al., 2012).
Hepatoprotective effects A. vera and A. arborescens have hepatoprotective activities (Singab et al., 2015). Thus, polysaccharides exert a protective effect against chronic alcohol-induced liver injury (Cui et al., 2014), toxic solvents such as carbon tetrachloride (Chandan et al., 2007), and aflatoxins (Cui et al., 2017). The hepatoprotective effect appears to be associated with antioxidant capacity and the ability to accelerate lipolysis and inhibit inflammatory response, improve excretory capacity and stimulate bile flow secretion. However, its use in gallbladder conditions that are at risk for carcinogenesis should be discouraged (Puia and Puia, 2013).
Aloe may be used for intestinal drug absorption enhancement in drugs with low bioavailability due to extensive efflux (Josias et al., 2013).
Anticancer activity A review performed by Singab et al. (2015) mentioned several studies with findings about the effectiveness of Aloe sp. on various cancers affecting several organs, including the colon, liver, duodenum, skin, pancreas, intestine, lungs and kidneys. Aloe is thought to be a potential agent for the treatment of gastrointestinal cancers. Qin et al. (2006) noted that the inhibitory effect of aloe-emodin on the proliferation and migration of gastric tumour cell lines is dose-dependent. Pan et al. (2013) showed that aloin inhibits tumour angiogenesis and growth by blocking STAT3 activation. Aloe-emodin and emodin demonstrated anticancer activities in the human gastric cancer MKN45 cell line (Chihara et al., 2015). Emodin also induces apoptosis and cell death in human lung squamous carcinoma cells in vitro . Aloin can be used to radio-sensitize HeLaS3 human cervical carcinoma cells, thus reducing the necessary doses for radiotherapy from 3.4 to 2 Gy (Nićiforović et al., 2007). Paraneoplastic venous thrombosis affects many patients. Fan et al. (2018) demonstrated in vitro that protein and phenolic extracts of four Aloe species have good thrombolytic and fibrinolytic activities. Adding this clot lytic quality to the known anticancer effects may open a new direction in the use of Aloe sp. in oncological treatment (Fan et al., 2018).

Antioxidant activity
In vitro studies have demonstrated that A. vera extracts from both leaves and flowers are good natural antioxidant sources (López et al., 2013). A study performed on several Aloe sp. concluded that the most active antioxidant may be found in A. pillansii along with A. broomii and A. spinosissima, comparable to the better known A. arborescens and A. vera (Sazhina et al., 2016). A study on healthy volunteers receiving 250 mL of A. vera gel extract daily demonstrated a significant increase in the plasma total antioxidant capacity (TAC) (Prueksrisakul et al., 2015).
In a study performed in India on plants harvested from various regions, A. vera extracts from colder climatic regions showed good antiplasmodial activity. There was a significant correlation between the quantities of aloin and aloe-emodin and the antiplasmodial effect . Homonataloin, belonging to the anthrone group, seems to be the most efficient component against chloroquineresistant Plasmodium falciparum strains (van Zyl et al., 2002). Maphosa et al. (2010) provided evidence that A. ferox extract has in-vitro anthelminthic activity, thus encouraging use in the treatment of GI helminthosis.

Antidiabetic effects
The empirical use of Aloe sp. as an antidiabetic has been supported by several studies (Grindlay and Reynolds, 1986). A study by Froldi et al. (2019) demonstrated that both the methanolic and the hydroalcoholic A. arborescens extracts led to the inhibition of glycation and free-radical persistence, without any cytotoxic activity, thus supporting the traditional use of A. arborescens leaf extracts against hyperglycemic conditions. Five phytosterols evaluated for their anti-hyperglycemic effects in type 2 diabetic mice led to a decrease in fasting blood glucose levels between 28% and 64% compared to the control levels (Tanaka et al., 2006). A double-blind randomized controlled trial on 72 patients with pre-diabetes symptoms demonstrated that fasting blood glucose and HbA1C levels improved after 8 weeks (Alinejad-Mofrad et al., 2015). In a randomized double-blind placebo-controlled clinical trial, Huseini et al. (2012) demonstrated that A. vera gel lowered fasting blood glucose and HbA1c levels significantly without affecting any liver/kidney function tests.

Antihyperlipidic effects
In an experimental study performed by Dana et al. (2012) significant differences were observed between cholesterol levels in rats fed a high-cholesterol diet combined with A. vera and a high-cholesterol diet alone.
The formation of fatty streaks in the aorta was also significantly lower in the same animals under the influence of diet with A. vera.
A clinical study showed the effectiveness of A. vera in improving total cholesterol, LDL-C, HDL-C and triglycerides after 4-8 weeks of intake (Alinejad-Mofrad et al., 2015). Huseini et al. (2012) noted that Aloe gel has a favourable effect on total cholesterol and LDL levels and no adverse effects, thus promoting it as a safe anti-hypercholesterolemic agent for hyperlipidemic patients.
The research of Misawa et al. (2012) have shown that A. vera gel powder combined with a high-fat diet induces in rats only a modest decrease of body weight but, much more important, reduces significantly subcutaneous, visceral and total body fat. In an experimental study meant to decipher the anti-obesity mechanism of A. vera gel extract Tada et al. (2020) showed that brown adipose tissue activation contributes to weight loss.

Other favourable effects
Placebo-controlled studies have shown that the consumption of mannans improves cognitive performance in middle-aged patients with mental fatigue. Improvements in memory performance following mannan intake were independent of changes in blood glucose levels (Best et al., 2015).
The ameliorating effect of aqueous extract of A. vera leaves against cartap and malathion toxicity could be used to protect non-target animals from the adverse effects of pesticides (Gupta et al., 2020).

11
In cosmetology, Aloe sp. are used in toothpaste, creams, shampoos and soap production. Industrial applications as bitter agents or consistence modifiers include beverages, ice cream or food supplements.
A panel of experts established that Aloe is not toxic in experimental acute oral studies but can cause significant sperm damage, be abortifacient or produce skeletal abnormalities. Aloin had no carcinogenic effects on mice. Case reports in humans included acute eczema, contact urticaria, and dermatitis, but no phototoxicity in topic use (Andersen, 2007).
A major obstacle in introducing A. spp derived products on a large scale in medicine is the lack of standardization regarding the components and their concentration (Moein et al., 2017).

Conclusions
Many species of the Aloe genus have been in use for a long time in folk medicine and, more recently, as components of food and beverages. Its adaptability led to a worldwide spontaneous or cultivated growth that made Aloe available at a reasonable cost. Traditional Aloe remedies cover most human diseases; however, in order to gain legitimacy, the derived drugs must have a well-established composition, with thoroughly investigated adverse effects and conventional drug interactions.