GENUS MALLOTUS (EUPHORBIACEAE): A REVIEW ON TRADITIONAL MEDICINAL USE, PHYTOCHEMISTRY, AND BIOLOGICAL ACTIVITIES

Ethnopharmacological relevance: The genus Mallotus (Euphorbiaceae family) includes 124 accepted names and is distributed in tropical and subtropical regions. Some Mallotus species are used in traditional medicines for the treatment of chronic hepatitis, enteritis, mental disorders, cholelithiasis, anti-cancer, gastric, and duodenal ulcers. Aim of the study: According to the Scifinder database, this is the first review study that focuses on the phytochemistry and pharmacology of Mallotus genus (17 Mallotus species) to understand the link between the traditional medicinal uses, phytochemistry, and bioactivities. Thus, they provide a scientific foundation for further research in the phytochemical and pharmacological activities of their species. Materials and methods: Information about the Mallotus genus was collected using various databases, such as Web of Science, SciFinder, PubMed, Sci-hub, Google Scholar, Wiley, Elsevier, ACS publications, and SpringerLink between 1978 and 2021. Plant names were validated by “The Plant List” (www.theplantlist.org). Results: Up to now, total 325 compounds were reported from Mallotus species, including phloroglucinols, steroids, coumarins, benzopyrans, flavonoids, chalcones, gallic acid and bergenin derivatives. The extracts and phytochemical constituents of the Mallotus genus are a rich source of biological activities, including anti-cancer, anti-inflammatory, immunoregulatory, antioxidant, antibacterial, antifungal, anti-virus, and other activities. Conclusions: The review indicated that the Mallotus genus is a promising source of biological compounds, special anti-cancer chromanes. The results of this review confirm the great potential of Mallotus species. Thus, it will be helpful for further research in the phytochemistry and pharmacology of the Mallotus genus. Nguyen Xuan Nhiem, et al. 142


INTRODUCTION
Nowadays, researchers from the worldwide are interested in new drugs from plant sources due to their low cost and may offer safer medicine than synthetic drugs, which are more costly and have many adverse side effects. Thus, the interest is increasing in the structure-activity relationships of secondary metabolites with potential pharmacological activity.
Mallotus (Euphorbiaceae) is a large genus of trees and shrubs distributed throughout tropical and subtropical regions. Some species of the genus have been used in traditional medicine. Phytochemical studies of the genus have indicated the presence of a large number of phloroglucinols, coumarins, benzopyrans, along with some chalcones, steroids, gallic acid and bergenin derivatives. Furthermore, there are many biological activities of the plants in the genus, such as cytotoxic, anti-inflammatory, immunoregulatory, antioxidant, antibacterial, antifungal, and anti-virus. Herein, we will summarize the literature data concerning the phytochemistry and the biological activities of the Mallotus genus with the aim of providing a comprehensive survey on the traditional medicinal uses of the Mallotus genus in line with their structural and pharmacological data.

MATERIAL AND METHODS
The review was carried out with the help of databases of scientific publications, such as Web of Science, SciFinder, PubMed, Sci-hub, Google Scholar, Wiley, Elsevier, ACS publications, and SpringerLink by using keywords "Mallotus". The cited articles were collected from 1978 to 2021. The Plant List (www.theplantlist.org) was used for confirming species names.

Ethnopharmacology properties
Many Mallotus species have been used as medicinal plants in traditional medicine in Viet Nam and Southeast Asian countries to treat various ailments ranging from minor infections such as gastrointestinal disorders to dysentery, hepatic and cutaneous diseases, fever, and malaria, and a series of other indications. The parts of the Mallotus species that have been studied include barks, stem barks, heartwoods, seeds, leaves, roots, aerial parts, and whole plants. The barks of M. barbatus have been used in Vietnamese oriental medicine to treat stomach ache and duodenal ulcer [1,2]. In India, the decoctions of M. peltatus leaves and stem barks are widely used to treat stomachache [3], intestinal ailments, and skin infections [4]. The ethanol extract of leaves is reported to be helpful in the treatment of trematodic infection [5]. M. repandus has been used in an herbal formula to relieve muscle pain in Thailand [6]. In Taiwan, the leaves of M. repandus have been used as anti-inflammatory drugs [7]. M. apelta has been used in traditional medicine for the treatment of chronic hepatitis. M. furetianus, a kind of tropical plant, is a herb indigenous to Hainan Island of China. Its leaves have been used as a popular aromatic beverage for indigestion. It is also used as a folk medicine for the treatment of cholecystitis disease [8]. M. paxii has played an important role in folk medicine for hundreds of years. Its stems have been used for the treatment of viral infections [9]. The leaves of M. japonicus are used as food wrap while the bark is used to treat various diseases such as gastric ulcer, duodenal ulcer, and gastric hyperacidity [10]. M.roxburghianus has long been traditionally used by the tribal people of Mizoram as a dietary supplement and therapeutic agent for diabetes, ulcer, hypertension, and liver disorders. The leaves of M.roxburghianus are eaten as vegetable (used as greens for salad or cooked with meat or fish) or consumed as a decoction (50 mL thrice daily) for six months to 1 year based on the clinical complications [11].

Phytochemistry
To date, chemical constituents of the Mallotus genus have been widely studied but with more focus on the following seventeen species  Tables 1-15). The parts of Mallotus genus have been studied, including leaves, branches, stem barks, roots, and whole plants The leaves and barks have been found to contain triterpenoids and phenolics. The fruits yielded chalcones and phloroglucinols, while the other parts are characterized by the appearance of flavonoids, steroids, and terpenoids, etc.

Benzopyrans and coumarins
There are 40 benzopyrans and 11 coumarins reported from the Mallotus genus. Benzopyrans

Biological activities
The traditional uses and bioactive compounds from the Mallotus genus have led researcher's interest to study pharmacological activities and to verify the potential uses of the genus. The following discussion provides biological effects of the extracts and isolated compounds from the Mallotus genus. The species of Mallotus genus possesses a wide range of biological effects such as anti-cancer, anti-inflammatory, antioxidant, antibacterial, antifungal, immunoregulatory effects, and anti-virus effects. Below is a summary of the important biological activities of the Mallotus species.  [46,47]. Then, butyrylmallotochromene (108) and isobutyrylmallotochromene (109) were also isolated from the cytotoxic fraction of M. Japonicus in 1988. These two compounds exhibited significant cytotoxic activity against KB cell line with ED 50 values of 3.3 and 0.4 µg/mL, respectively [50]. Butyrylmallotojaponin (128) and isobutyrylmallotojaponin (129) were isolated from the leaves of M. Japonicus exhibited cytotoxic activity against KB cell line with ED 50 values of 0.72 and 0.89 µg/mL, respectively [27]. Isomallotolerin (120) from the fruits of M. japonicus inhibited cytotoxic activity on KB cell line with IC 50 value of 0.84 µg/mL [53]. In 1990, Arisawa and co-workers reported 18 phloroglucinols from the fruits of M. japonicus. All compounds exhibited the cytotoxic activity on Hela cells with IC 50 values ranging from 0.28 µg/mL to 49.10 µg/mL [49]. In 2011, Nam and co-workers isolated two steroids, macrostachyoside A (298) and macrostachyoside B (301) from the leaves of M. macrostachyus. Compounds 298 and 301 showed significant cytotoxic effects against KB and LU-1 cell lines, with the IC 50 values ranging from 4.31±0.09 to 7.12±0.07 µg/mL [77]. Paxiione A (16) was obtained from the stems of M. paxii also showed significant cytotoxic activity on KB cell line, with IC 50 value of 8.62±1.31 µg/mL [9]. In 2014, a coumarine derivative, 7-hydroxy-2hydroxymethyl-8-methoxy-4-oxo-4H-chromene-6-carboxylic acid (20), was found from the twigs of M. apelta by Lu and co-wokers. This compound demonstrated moderate cytotoxic activity against KB and HeLa cells with IC 50 values of 9.50 and 9.23 µg/mL, respectively [20]. In 2005, Prof. Phan Van Kiem and co-wokers reported two benzopyrans, 6-[l′-oxo-3′(R)hydroxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-l-benzopyran (7) and 6-[l′-oxo-3′(R)-methoxybutyl]-5,7-dimethoxy-2,2-dimethyl-2H-l-benzopyran (8), from the leaves of M. apelta. These compounds were evaluated for cytotoxic effects against Hep-2 and RD cell lines, compound 7 exhibited significant cytotoxic effect against two tested human cancer cell lines with IC 50 values of 0.49 µg/mL (Hep-2) and 0.54 µg/mL (RD), while compound 8 showed moderate activity on Hep-2 cell line with IC 50 of 4.22 µg/mL [14]. In the research of Prof. Chau Van Minh and coauthors, 22 compounds from M. apelta were evaluated for their cytotoxic effects against KB, FL, and Hep-2 cancer cell lines. Among them, malloapelta B (26) showed a strong cytotoxic effect against the three cancer cell lines, while the other compounds did not show inhibitory activities with IC 50 values over 50 µM [99].

Cytotoxic and antitumor activities
In addition, ten new chromene derivatives, malloapeltas C-H (12-16,31-32) and one known compound, malloapelta B (26) were isolated from the leaves of M. apelta. All compounds were evaluated for cytotoxic activity using cell counting kit-8 (CCK-8) assay against ovarian cancer cell line (TOV-21G). Compounds 12-16,26 and 31-32 exhibited significant growth and viability inhibitory effects with GI 50 values ranging from 0.06 to 10.39 µM and IC 50 values ranging from 1.62 to 10.42 µM on ovarian cancer cell line, TOV-21G. The most cytotoxic compounds 14,26 and 31/32 were chosen for studying apoptosis mechanisms. These compounds-induced apoptosis was evidenced by activated caspase 8, caspase 9, and PARP, increased Bak and Bax, and decreased Bcl-xL and survivin. Moreover, those compounds significantly inhibited the NF-κB signaling pathway [23]. Compounds 18 and 19 significantly reduced both cell viability of PC-3 cells and ANO1 channel activity. An electrophysiological study revealed that compound 19 is a potent and selective ANO1 inhibitor with an IC 50

Anti-inflammatory and immunoregulatory activities
Besides anti-cancer effects, the anti-inflammatory properties of the extracts and compounds from the Mallotus genus have been reported, suggesting their potential to be developed as antiinflammatory drugs or drug-lead compounds. In 2001, compounds 107, 112-114, 117, 124 (107), mallotochromanol (113), isobutyrylmallotochromanol (114), and isomallotochromanol (124). Among these phloroglucinol derivatives, 124 exhibited strong inhibitory activity on NO production with IC 50 value of 10.7 µM. The phloroglucinol derivatives significantly reduced both the induction of inducible nitric oxide synthase (iNOS) protein and iNOS mRNA expression. NO production by macrophages preactivated with LPS and IFN-gamma for 16 h was also inhibited by the phloroglucinol derivatives [48]. Isomallotochromanol (124) and isomallotochromene (123) were the most potent in inhibiting cytokine production. The phloroglucinol derivatives significantly reduced these cytokine mRNA expressions [100].
Analgesic and anti-inflammatory activities of M. repandus were evaluated using acetic acid induced writhing test, xylene induced ear edema, cotton pellet induced granuloma, and tail immersion methods at doses of 500, 1000, and 2000 mg/kg body weight. The extract of M. repandus exhibited considerable antinociceptive and anti-inflammatory activities against four classical models of pain. In acetic acid induced writhing, xylene induced ear edema, and cotton pellet granuloma models, the extract revealed dose dependent activity. These findings suggest that this plant can be used as a potential source of new antinociceptive and anti-inflammatory candidates. [101]. In 2016, Gangwar et al. evaluated the anti-inflammatory, analgesic, and hypnotic activity of fruit extract M. philippinensis in different rat experimental models. The study revealed that the extract of M. Philippinensis was effective in reducing acute and subacute inflammation and showed effective and similar analgesic activity [102]. In 2019, two new diterpenoids, malloconspur A (258) and malloconspur B (259), and sixteen known terpenoids were isolated from the ethanol extract of M. conspurcatus. Malloconspur B (259) and 17hydroxycleistantha-12,15-dien-3-one (250) substantially inhibited NO production with IC 50 values of 10.47 µM and 9.32 µM, respectively. Compounds 258, 259, and 260 markedly reduced the secretion of PGE2 and TNF-α, LPS-induced in RAW264.7 cells. Compounds 249 and 250 significantly decreased iNOS, NF-κB/p65, and COX-2 protein expressions [79].

CONCLUSION
All the above-mentioned researches have shown that Mallotus is an important genus in the Euphorbiaceae family, but chemical and biological studies are still limited, representing an opportunity to find new bioactive substances. The chemical constituent features of benzopyrans, coumarins, flavonoids, and phloroglucinols have led them to be considered markers of the genus. However, the pre-clinical pharmacological studies found in this review exhibited low methodological quality, which hinders the unambiguous interpretation of the results. It is still noteworthy that several gaps need to be addressed for better applying of the genus Mallotus. This review provides the medicinal potential and basic understanding of the genus Mallotus for further research on the application of medicinal plants. The review is a compilation of information about the Mallotus genus that should be investigated by cytotoxic assays to develop new drugs for the treatment of cancer diseases and symptoms. Declaration of competing interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.