Citrus maxima (Brum.) Merr. (Rutaceae): Bioactive Chemical Constituents and Pharmacological Activities

Citrus maxima (Burm). Merr. (family Rutaceae), commonly known as Pomelo, is an ethnomedicinally, pharmacologically, and phytochemically valued species. Various ethnomedicinal reports have revealed the use of C. maxima for cough, fever, asthma, diarrhea, ulcer, and diabetes and as a sedative. Numerous phytochemicals have been reported from C. maxima such as polyphenols, terpenoids, sterols, carotenoids, vitamins, and amino acids. The plant possesses significant bioactivities like antioxidant, antimicrobial, anti-inflammatory, analgesic, anticancer, antidiabetic, anti-Alzheimer's disease, insecticidal, anxiolytic, hepatoprotective, antimalarial, and antiobesity. Extensive research is necessary to explore the detailed mechanism of action of extracts and compounds to design effective medicines, herbal products, and functional foods.


Introduction
Citrus maxima (Burm). Merr. (syn. Citrus grandis (L.) Osbeck) (Figure 1) belongs to the family Rutaceae. It is a perennial tree commonly known as Pomelo, Bhogate, Shaddock, Papanus, Pummelo, etc. in various parts of the world, as shown in Table 1. e plant is indigenous to Asia and is commercially grown in China, Nepal, ailand, Malaysia, India, Vietnam, Indonesia, Philippines, Japan, and many other Asian countries. Lately, it has been introduced to many tropical nations [1][2][3]. It grows widely in temperatures 25-32°C and rainfall 1,500-2,500 mm within a 3-4 months dry season. It raises well in rough sand to heavy clay but favors fertile soils [2,3]. Figure 1 shows various plant parts of C. maxima, which include the whole plant, whole fruit, albedo, and pulp. It has big round-shaped edible fruits with pink or white flesh. It is traditionally used for ulcers, febrifuge, dyspepsia, lumbago, fever, cardiotonic, gastrointestinal disorders, diabetes, and cardiovascular disease [4][5][6][7][8][9]. Various chemical constituents are reported from many parts of the C. maxima plant. e extracts or pure compounds from this plant have also been evaluated for a wide range of biological activities. e aim of this review article is to provide comprehensive outline of phytochemistry and pharmacological aspects of the plant and to attract scientific communities for further studies on possible utilization of C. maxima in the field of pharmaceutical, nutraceutical, and cosmeceutical industry.

Methodology
Scientific information about ethnomedicinal uses, phytoconstituents, and in vivo and in vitro biological activities of different parts of C. maxima was collected from published articles retrieved through several relevant databases including Google Scholar, PubMed, Chemical Abstract, Scifinder, Web of Science, and Scopus. e database was searched with the keywords such as Citrus maxima, pummelo, and Citrus grandis along with pharmacological activity, phytochemicals, ethnomedicinal uses, toxicity, etc.

Traditional Uses
It is well documented for its ethnomedicinal values in many countries [4,5,10,11]. Fruits are used as stomach tonic, appetizer, cardiac stimulants and for the treatment of inflammation, cough, asthma, obesity, leprosy, mental aberration, epilepsy, headache, diarrhea [12][13][14][15], antipyretic, and antiemetics agents [16]. Pulp has been used traditionally for cosmetic purpose. e seeds are used against lumbago, dyspepsia, and coughs. Leaves are used for the treatment of epilepsy, cholera, and convulsive cough while decoction is useful on swellings and ulcers [17][18][19]. e details of traditional use of the plant are given in Table 2.

Flavonoids.
Flavonoids are one of the most reported chemical classes from this plant (  [62][63][64][65]67]. Structures of some of the main flavonoids are shown in Figure 5.

Miscellaneous
Compounds. In addition to the compounds mentioned above, a few compounds like L-ascorbic acid, citric acid, decyl acetate, fumaric acid, hexanal, malonic acid, succinic acid, α-tocopherol, pentadecanoic acid, hexadecanoic acid, tetradecanoic acid have been isolated from fruit juice, peel, and leaves of C. maxima [28,79]. Headache, flu, fever, sore throats, breathing disorders, and dyspepsia ailand [24] Fruits peel A decoction of peel has been used to improve coughs, swellings, ulcers, and epilepsy Kenya [25] Fruits peel Obesity and hypertension China [26] Leaves Leaves are chewed to expel the intestinal worms Nepal [22] Leaves and flowers As sedative in nervous affections, convulsive cough, cholera, epilepsy, haemorrhagic diseases, and a lotion of boiled leaves used in painful swellings India [27] Leaves, flowers, fruits, and seed As decoctions to treat coughs, fevers, and gastric disorders e Philippines and southeast Asia [28] Evidence-Based Complementary and Alternative Medicine

Pharmacological Activities
Various studies have been performed regarding the pharmacological effects of C. maxima extracts and their isolated compounds. Modern pharmacological studies confirm the traditional efficacy of this plant as an antiepileptic, antidepressant, and anti-inflammatory agent. e plant is highly potent for treating anxiety, depression, Alzheimer's disease (AD), and other neurological diseases. e plant also exhibits additional antioxidant, analgesic, hepatoprotective, antimicrobial, and anticancer activities. In this review, we collected the available information and described major pharmacological properties like antioxidant, antidepressant, anxiolytic, anti-Alzheimer's disease, antitumor, insecticidal, antidiabetic, antimicrobial, hepatoprotective, anti-obesity, anti-inflammatory, and analgesic activities.  [81]. e in vivo antioxidant activity of methanolic leaf extract (200 and 400 mg/kg, b.w.) was evaluated against paracetamol-induced hepatotoxicity in Wistar albino rats. Leaf extract at 400 mg/kg· b.w. showed reduced lipid peroxidation in paracetamol-treated rat liver as compared to that of saline control. It was also able to restore the depleted catalase and reduce glutathione levels in the paracetamol-intoxicated rat liver to the normal levels, indicating the in vivo antioxidant potential of extracts in paracetamol challenged rats [82]. e freeze-dried fruit extract of C. maxima exhibited 6609 μ·mol Fe 2+ /L antioxidant power through the ferric-reducing antioxidant powder (FRAP) assay which is very similar to the standard drug ascorbic acid [83]. e presence of major phytochemicals might be the reason for showing significant antioxidant activity by C. maxima extracts [14,84,85].

Antidepressant Activity.
e aqueous leaf extracts (100,200, and 300 mg/kg) of C. maxima were evaluated in mice for their antidepressant potential using different models. Fluoxetine (20 mg/kg, i.p.) and imipramine (30 mg/kg, i.p.) were used as standard drugs. e aqueous leaf extracts reduced the immobility time in both the tail suspension test (TST) and the forced swimming test (FST). e exact mechanism for exhibiting antidepressants was not reported, but it might be due to enhancement of norepinephrine neurotransmission in mice [20]. Similarly, the per-oral administration of ethanolic extracts (200 and 400 mg/kg) of C. maxima in mice increased the number of rearing in both the TST and FST models while imipramine (1 mg/kg) noticeably reduced the immobility time [86].
Hesperidin and naringin were evaluated against antidepressant activity using the FST and TST models. Both compounds exhibited significant antidepressant activity [87,88]. e antidepressant effects of plant extracts might be due to the interaction with the serotonergic 5-HT1A and κ-opioid receptors [89,90]. It was concluded that C. maxima extract was useful in its motor-stimulating effects.

Anti-Alzheimer's Disease Activity.
Alzheimer's disease is a neurodegenerative progressive disease that occurs in the elderly population. During the experiments performed using Ellman's colorimetric and scopolamine-induced Alzheimer's methods, ethanolic, hexane, ethyl acetate, and aqueous extracts of C. maxima fruit peel exhibited potent anti-Alzheimer's activity. Similarly, it was found that the brain acetylcholinesterase level was decreased by leaf extract and showed anti-Alzheimer's activity [14,90].
Naringin (40 and 80 mg/kg, p.o.) showed anti-Alzheimer's activity in colchicine tempted cognitive impaired rats through the elevated plus maze and Morris water maze methods. Colchicine (15 µg/5 mL) was given intracerebroventricularly which causes poor memory retention and reduces acetylcholinesterase activity in both the models [88]. e anti-Alzheimer's activity might be due to the development in the cognitive act and diminished oxidative stress by lowering malondialdehyde and nitrite levels. Also, it might be due to the renewal of superoxide dismutase, catalase, and glutathione S-transferase, and a reduction in glutathione as well as the acetylcholinesterase level in tested mice [91].

Anticancer and Antitumor Activity.
e leaf extract of C. maxima tested against Ehrlich ascites carcinoma (EAC) models in swiss albino rats decreased the white blood cell

Evidence-Based Complementary and Alternative Medicine
(WBC) count and increased the lifespan. e biochemical parameters were also in the normal level as compared to the control group [92]. e methanolic extract of the leaves and its fractions in n-hexane, n-butanol, chloroform, ethyl acetate, and water were tested in normal cells and different cancerous cells through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium (MTT) assay. Importantly, the chloroform fraction of leaf extract reduced the survival of HeLa cells [93]. Naringin exhibited potent anticancer activity in various experiments. Naringin (10, 25, and 35 mg/kg i.p.), when treated on rats bearing Walker 256 carcinosarcoma (W256) reduced tumor growth by 75% and TNF-α and IL-6 levels decreased in comparison with the control [94,95]. Naringenin also exhibited cell proliferation and cell migration in B16F10 murine and SK-MEL-28 human melanoma cells. Hesperidin exhibited chemopreventive effects against an azoxymethane (AOM) induced carcinogenesis in the mouse colon. It was found to have significant reducing power for the multiplicities of AOM-induced aberrant crypt foci (ACF) and tumor incidence. It also decreased the proliferative marker proliferating cell nuclear antigen (PCNA) against AOM-induced colon carcinogenesis [96,97]. e presence of flavonoids, limonoids, alkaloids, tannins, saponin, and bioflavonoids plays a prominent role in cancer prevention [59,92,98]. e anticancer activity of naringenin loaded liquid crystalline nanoparticles (LCNs) was evaluated against human lung epithelial carcinoma (A549) and airway epithelium derived basal cells (BCi-NS1.1). Mainly antiproliferative, antimigratory, and anticolony formation activity were studied in which naringenin LCNs showed its significant anticancer properties by inhibiting the migratory and proliferation properties of cells [99].

Antidiabetic Activity.
e in vitro enzyme inhibitory activity of C. maxima fruit juice was examined against α-glucosidase and α-amylase. e percentage inhibition by fruit juice for α-amylase was 75.55%-79.75% and, for α-glucosidase, it was 70.68%-72.83% [100]. e hypoglycaemic property of fruit juice was examined in the streptozotocin (STZ)-induced diabetes mellitus model. e glucose level was lowered in experimental rats than in control rats which is due to the peripheral utilization of glucose or inhibition of gluconeogenic enzymes [23]. e antidiabetic activity of the leaf extracts (200 and 400 mg/kg, b.w.) was evaluated in STZ (65 mg/kg) induced diabetic rats using glibenclamide (0.5 mg/kg, p.o) as the reference standard. e blood glucose level and serum biochemical parameters were measured and found to be normalized in experimental rats than in the control group [101]. e antidiabetic effect of neohesperidin on α-amylase and α-glucosidase improved postprandial hyperglycemic conditions [102]. e antioxidant activity of plants may lead to their defensive effects against chronic metabolic disorders [103]. e antidiabetic activity of methanolic and ethanolic leaf extracts (100 and 200 mg/kg of each extract) of C. maxima was also evaluated against the alloxan (90 mg/kg b.w.) induced diabetes model in mice while glibenclamide (5 mg/kg, p.o.) was used as the standard. e plasma glucose level and parameters of serum lipid profile, serum glutamic pyruvic transaminase (SGPT), serum glutamic oxaloacetic transaminase (SGOT), and C-reactive protein (CRP) were measured and found to be inhibited by the leaf extract in experimental mice as compared to control mice. is finding suggests that both extracts have significant hypoglycaemic effects and can ameliorate the altered lipid profile in diabetic mice. Moreover, the results suggested that the extracts of C. maxima leaf can restore altered levels of liver function enzymes and CRP in diabetic mice, highlighting the hepatoprotective and cardioprotective potentiality of this plant [104].

Antimicrobial Activity.
e antibacterial activity of C. maxima has been widely studied. e ethanolic leaf extract exhibited antibacterial activity against Pseudomonas aeruginosa and Escherichia coli [17]. e ethanolic pulp and seed extracts also exhibited antibacterial activity against Bacillus subtilis, Staphylococcus aureus, and Escherichia coli in the disc diffusion method [105]. In another study, the methanolic extracts of the leaves, seeds, fruits peel, and barks were tested against Escherichia coli, Klebsiella pneumonia, and Staphylococcus aureus. Pulp extract showed the highest zone of inhibition (ZOI) of 26 mm in Klebsiella pneumonia, while none of the other extracts showed significant ZOI. e aqueous extract of the pulp also showed highest antibacterial activity (ZOI of 27 mm) against Staphylococcus aureus [106]. e presence of naringenin and hesperidin might be responsible for its antibacterial activity. e antibacterial activity of hesperidin against Gram-positive and Gramnegative bacteria has already been established [107]. e essential oils from C. maxima also demonstrated antibacterial activity against Escherichia coli, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus licheniformis, and Bacillus altitudinis in the broth dilution method [108].  [110].

5.7.
Hepatoprotective Activity. C. maxima leaf and peel extracts revealed liver protective effect in carbon tetrachloride-induced hepatotoxicity in Wistar rats. Significant reduction of aspartate aminotransferase (AST), alanine transaminase (ALT), and alkaline phosphatase (ALP) levels in experimental rats proved its hepatoprotective activity [111,112]. In another study, hepatoprotective effects of C. maxima methanolic leaf extract (200 mg/kg, b.w.) was examined in paracetamol-induced hepatotoxicity in rats. In this study, leaf extracts were administered for 7 days, paracetamol (2 g/kg) was administered at 5 th day, and silymarin (100 mg/kg, b.w.) was used as the standard drug. Liver was extracted and liver function markers, total bilirubin, total protein in blood serums, and hepatic antioxidants in liver homogenate were evaluated and found normal as compared to the control group [113]. Leaf extracts having antioxidant property might be responsible to decrease the distortion of hepatocytes by elevating the hepatic antioxidant enzymes levels [31].

Anti-obesity Activity.
e anti-obesity activity of ethanolic leaf extract (200 and 400 mg/kg) against olanzapineinduced obesity and cafeteria diet-induced obesity in rats. Body weight, body temperature, and serum parameters were evaluated and found significantly decreased in their values as compared to the obese control group [114]. Ding et al. fed the C. maxima ethanolic peel extract to the mice along with Chow diet for 8 weeks. e diet lowered the weight, decreased fasting blood glucose levels, and also reduced liver lipid and serum insulin levels [115]. Hesperidin also regulates the lipid and glucose metabolism and indirectly facilitates NF-κB signalling way to control inflammation which helps in controlling obesity [116,117].

Analgesic and Anti-Inflammatory Activity.
Various parts of C. maxima have shown analgesic and anti-inflammatory properties. e analgesic property of the methanolic extract of its peel was examined by formalin-induced licking and biting model and acetic acid-induced writhing model. e extract at a higher dose (500 mg/kg) showed satisfactory analgesic activity (73.34%) in the acetic acid-induced pain model as compared to 87.13% activity shown by standard drug diclofenac sodium at 10 mg/kg dose [118]. In another experiment, the analgesic activity of leaf, stem, and fruit was compared by using the tail-flick method in rats, acetic acidinduced writhing, and the hot plate method in mice. Results showed that the leaf extract at 300 mg/kg showed significant analgesic activity in all the models used [119]. Similarly, Kundusen et al. also showed its anti-inflammatory activity in rats when evaluated using formalin, carrageenan, and dextran-induced acute rat paw edema models. Many studies suggested that the mechanism responsible for analgesic and anti-inflammatory activity is due to inhibition of prostaglandin synthesis. Also the presence of flavonoids and their respective aglycones like hesperetin and naringenin might be the reason for the potent anti-inflammatory and analgesic activity [10,[118][119][120].

5.10.
Other Uses. C. maxima fruits are known for their characteristic flavor, making them suitable for breakfast. e peel oil is used as a flavoring agent in food, pharmaceutical products, cosmetics, and perfumery items [25]. Due to refreshing and good-smelling properties, its essential oils are also added in toiletry and insecticidal products [121]. e pectin in rind is used in making jellies and candies, and wood can be used for making suitable tool handles [122].

Conclusion and Future Prospects
C. maxima offers a wide range of medicinal and nutritional uses. Almost all parts of the plant, including whole fruit, fruit pulp, fruit rind, fruit peel, juice, flower, leaf, seed, and essential oils, are traditionally used for the treatment of various diseases. A phytochemical profile showed the presence of many bioactive chemical constituents under several chemical classes including alkaloids, benzenoids, coumarins, carotenoids, phenols, flavonoids, tannin, terpenoids, saponins, amino acids, and carbohydrates. Extracts of various plant parts showed numerous pharmacological properties like antioxidant, antimicrobial, analgesic, anticancer, antidiabetics, antiinflammatory, anti-Alzheimer's, insecticidal, anxiolytic, hepatoprotective, antimalarial, and anti-obesity activities. Isolated compounds like hesperidin, limonene, naringenin, naringin, and neohesperidin have been reported to possess bioactivities like antioxidant, antidepressant, antitumor, anticancer, antimicrobial, hepatoprotective, anti-obesity, insecticidal, analgesic activity, anxiolytic, anti-Alzheimer, antiulcer, and antidiabetic activities. e essential oils from fruits and leaves have enhanced their use in the perfumery and cosmeceutical industry.
Despite the tremendous ethnomedicinal reports, preliminary studies, and promising results, correlations between traditional uses and pharmacological activities are still needed to be established. Bioassay-guided fractionation and isolation of compounds is needed to find more potent and novel compounds for the discovery of lead compounds and to demonstrate their molecular mechanisms to design effective herbal products and functional foods. Extensive in vivo pharmacological tests, pharmacokinetic studies, clinical trials, and toxicity studies are needed. e information regarding the therapeutic dose, dosage form, and safety of the plant products is still an area to be explored. Since the plants can easily grow in south-east Asia including Nepal and India, local farmers can be promoted for the mass cultivation of this plant and small-scale herbal pharmaceutical and juice Evidence-Based Complementary and Alternative Medicine industries can be established. us, viable products and food supplements of this plant species can be designed and marketed at an international level which will ultimately uplift the economic status of the local producer.

Data Availability
No new experimental data were generated during the preparation of this review article.

Conflicts of Interest
e authors declare that there are no conflicts of interest.

Authors' Contributions
BS and PP conceived the idea, collected the literature information, and drafted the manuscript. HPD and PP revised the manuscript. All authors read and approved the final version of the manuscript before submission.