Phytochemical and pharmacological studies on Solanum lyratum: a review

Solanum lyratum is one of the temperate plants, broadly distributed in Korea, China, Japan, India, and South-East Asia and well-documented in those oriental ethnic medicine systems for curing cancers, jaundice, edema, gonorrhea, cholecystitis, phlogosis, rheumatoid arthritis, etc. This review systematically summarized the research progress on S. lyratum respecting the botany, traditional uses, phytochemistry, pharmacology, and toxicology to increase people's in-depth understanding of this plant, by data retrieval in a series of online or off-line electronic databases as far as we can reach. Steroidal saponins and alkaloids, terpenoids, nitrogenous compounds, and flavonoid compounds are the main chemical constituents in S. lyratum. Among them, steroidal alkaloids and saponins are the major active ingredients ever found in S. lyratum, exerting activities of anti-cancer, anti-inflammation, anti-microbial, anti-allergy, and anti-oxidation in vivo or in vitro. As a result, S. lyratum has been frequently prescribed for the abovementioned therapeutic purposes, and there are substantial traditional and modern shreds of evidence of its use. Graphical Abstract


Introduction
Solanum lyratum Thunb. is a herbaceous vine of the family Solanaceae, with white villous hairs on violinshaped leaves and stems, distributed throughout China, Japan, Korea, etc. [1]. S. lyratum prefers a warm and humid environment and is distributed widely in valley grass, roadside and field. S. lyratum is commonly known as "Bai-Mao-Teng" in traditional Chinese medicine and "Back-Mo-Deung" in traditional Korean medicine [2]. In traditional Chinese medicine (TCM), S. lyratum has the functions of clearing heat and removing toxicity ("Qingre Jiedu" in Chinese), dispelling wind and eliminating dampness ("Qufeng Lishi" in Chinese). Therefore, S. lyratum has been traditionally prescribed mainly for healing jaundice, edema, gonorrhea, cholecystitis, phlogosis, and rheumatoid arthritis [3]. Modern phytochemistry and pharmacological studies revealed that S. lyratum consists of a variety of active ingredients, including steroidal saponins, steroidal alkaloids, terpenoids, lignans, and flavonoids [4]. And steroidal saponins and steroidal alkaloids have been used in the modern clinic to treat various cancers, especially lung cancer, cervical cancer, and liver cancer [3].
In the last ten years, dozens of reviews on the research progress of Solanum plants have been published, occasionally referring few phytochemistry and pharmacological reports on S. lyratum [5][6][7][8] (Fig. 1). However, there is no specialized and systematic research review on the S. lyratum species, especially on its phytochemistry and pharmacological aspects. Thus, this review intends to provide an updated and comprehensive summary on the botanical characterization, phytochemistry, and pharmacological and toxicity studies of S. lyratum to fill a gap in the research review of this plant and provides for a better exploration and application of S. lyratum. The literature for this manuscript was obtained from reports published from 1981 to Mar 2022. [2]. The botanical characteristics of S. lyratum were recorded in many classics of TCM, including "Tang Xinxiu Bencao" in the Tang Dynasty [9], "Zhenglei Bencao" in the Song Dynasty [10], and "Compendium of Materia Medica" in Ming Dynasty [11].
S. lyratum is commonly known as "Bai-Mao-Teng" in TCM. It should be noted that there are several adulterants of S. lyratum, including Aristolochia mollissima, Paederia scandens (Lour.) Merr. and Solanumrr  Products and Bioprospecting (2022) 12:39 dulcamara L., all of which are so called as "Bai-Mao-Teng" that it may easily cause an event of medication confusion [12]. For example, a misuse of A. mollissima instead of S. lyratum, has ever led to a renal failure event in patients of Hong Kong [13]. Those adulterants closely resemble S. lyratum in botanical morphology, it is very important to seek advice from a professional or pharmacist before use. The plant morphological characteristics of S. lyratum are as follows: Root is slender and cylindrical. Leaves are mostly violin-shaped, with 3.5-5.5 cm long and 2.5-4.8 cm wide, and the base is 3-5 cm deep-lobed. Lateral lobes are smaller near the base. Middle lobes are usually larger oval and tend to apex acuminate. Both sides of the leaves were covered with white shiny villous hairs, and the levels own midvein and lateral veins. Flowers are sparsely terminal inflorescence or extra-axillary inflorescence, and the pedicel is approximately 2-2.5 cm long. Corollas are blue-purple or white and corollas are about 1.1 cm in diameter. Fruits are spherical and about 8 cm in diameter, which become reddish-black when it matures. Seeds are nearly disc-shaped and about 1.5 mm in diameter. The flowering period of S. lyratum is between May and June, while the fruiting period is between August and October. Significantly, the suitable harvest time has been recommended to be between October and December ( Fig. 2) [14,17].

Traditional uses
In TCM, S. lyratum has been considered as one of the "Top-grade" herbs in "Shennong Bencao Jing" (100 BC-200 AD, Han Dynasties) [18]. For centuries, it has been used for the treatments of cold and fever, malaria, jaundice, nephritis, edema, cholecystitis, rheumatoid arthritis, vaginitis, uterine erosion, and several types of cancer including lung cancer, cervical cancer, and gastric cancer [19,21]. External applications of S. lyratum [22] have been recorded to treat carbuncle, furuncle and swollen poison, etc. In the "Compendium of Materia Medica", S. lyratum is documented [11] to have the effects of clearing heat, detoxification, and expelling rheumatism, for the treatment of rubella, erysipelas, malaria, cancer, etc. The traditional uses of S. lyratum in Korea, Japan, and the Indochina Peninsula focused mainly on the treatments of several types of cancers, warts, herpes, pyretic syndrome, diarrhea, etc., as summarized in Table 1.

NMR characteristics of steroidal alkaloids
Representative NMR data of the common steroidal alkaloids and saponins from S. lyratum were summarized in Tables 3, 4, 5 and 6. 13 C NMR spectra of the spirosolanetype, spirostanol-type, and furostanol-type steroidal alkaloids or saponins, with intrinsic twenty-seven steroid skeleton, normally exhibit characteristic carbon signals for C-22 around δ C 98.0 [35], 109.0 [22], and 112.0 [40] ppm, respectively. In addition, in the high field of the 1 H NMR spectra of steroidal alkaloids and saponins, the resonance signals of methylene and methine protons are generally around δ H 1.1-3.0, while the four methyl groups show proton resonances at δ H 0.6-1.4, among which there are two singlets for the methyl groups at C-18 and C-19 [44], and two doublets for those methyls at C-21 and C-27 [45].

Other compounds
In addition to the abovementioned constituents, other compounds including anthraquinones and fatty acids have also been isolated from S. lyratum (Table 12, Fig. 17).

Pharmacology
Water decoction of the whole herb of S. lyratum was commonly used to treat various diseases, and the fresh whole herb was mashed to remedy herpes and warts for external use. Modern pharmacological evaluations revealed that extracts, fractions or compounds isolated from S. lyratum possessed various therapeutic potentials.
Recently, the plant has been most extensively studied for its anti-cancer pharmacological properties. Meanwhile, other pharmacological effects such as anti-inflammatory, anti-oxidant, anti-microbial, anti-allergy, and hepatoprotective activities of S. lyratum have also been assessed, as summarized in Table 13.

Extracts and fractions
The heat-clearing and detoxicating property of S. lyratum is favorable in the treatment of cancer [17,19,21]. It has been reported that S. lyratum treats various cancers by inhibiting the tumor growth [74,75], enhancing immunity [76,78] and inducing apoptosis via activating both extrinsic and intrinsic apoptotic pathways [27,34,79]. In S 180 tumor-bearing mice, both ethanol and aqueous extracts of S. lyratum could improve immune function and exhibited anti-cancer potential with certain tumor inhibitory effect by improving the activities of natural killer (NK) and cluster of differentiation 4 (CD4) cells, and elevating the contents of serum Interleukin-2 (IL-2) and tumor necrosis factor-α (TNF-α) [77,78,80]. Total alkaloids from S. lyratum (SLTA, 24 mL/kg) could inhibit the tumor growth in mice with Lewis lung cancer, and when combined with cisplatin, a synergistic effect had been shown to down-regulate the mRNA expression of Notch1, Notch3 and Jagged1 in Notch signaling pathway [81]. In addition, the hexane fraction of the methanol extract (50 mg/kg) of S. lyratum showed similar inhibitory activity on tumor growth in mice with Lewis lung carcinoma tumor, potentially acting through upregulating Fas, caspase-8, caspase-3, and p53, and downregulating FasL and B-cell lymphoma-2 (Bcl-2) in the mitochondrial pathway [27,79].
Further studies revealed that the 70% ethanol extract of S. lyratum (SLE) could suppress tumor angiogenesis in vitro by repressing migration, invasion, and tube formation of tumor-derived vascular endothelial cells (Td-ECs). The mechanism of the anti-angiogenic effect of SLE may be related to the inhibitory activity of vascular endothelial growth factor (VEGF) via reducing the number of lipid rafts in the cell membrane [43] and interfering with the lipid rafts by agglutinating cell membrane cholesterol [75]. These changes led to the inhibition of VEGFR2 phosphorylation and activation of its downstream signaling molecules, thereby inhibiting tumor angiogenesis [43]. In addition, SLTA could induce apoptosis of lung carcinoma A549 cells by inhibiting the nuclear factor-kappa B (NF-κB) signaling pathway [82], while glycoalkaloids of S. lyratum (SLGS) significantly inhibited the activity of A549-derived exosomes with IC 50 = 99.59 μg/mL [43].

Compounds
The cytotoxic tests involved in most in vitro studies of S. lyratum have shown that the compounds isolated  from S. lyratum possess a good cytotoxic potential for several cancer cells.

Extracts and fractions
Inflammation is closely related to cancer disease [84]. The detoxication and detumescence effect of S. lyratum can be used as a supplement to modern anti-inflammatory agents.

Antioxidant activity 5.3.1 Extracts
Modern pharmacological studies have revealed that cancer or other diseases are primarily associated with the production and accumulation of excessive free radicals [87], which are commonly produced by the continuous contact between our body and the outside world. Thus, antioxidants can effectively relieve the harmful effects of free radicals. It has been confirmed that S. lyratum extracts and compounds possess significant antioxidant activities. 50% Ethanol extract of S. lyratum (10 μg/mL) could protect against oxidized low-density lipoprotein (Ox-LDL)-induced injury in cultured human umbilical vein endothelial cells (HUVECs) by direct antioxidative action [88]. In the DPPH radical-scavenging tests in vitro using the spectrophotometric method with vitamin C as the positive control, the ethanol and ethyl acetate extracts from S. lyratum showed antioxidative potential with IC 50 of 0.230 mg/mL and 1.010 mg/mL, respectively [89]. Five flavones 236, 248, 249, 251, and 258 from the ethanol extracts of S. lyratum possessed the capability of scavenging DPPH free radicals with IC 50 values of 2.56-21.33 μg/mL [70]. It seems that the glycosidation of the flavone C-3 and C-5 is essential for the scavenging of DPPH free radicals.

Extracts and fractions
There were reports verified the antibacterial potential of S. lyratum extracts. For example, the water-soluble polysaccharide of S. lyratum exerted a significant antibacterial activity against Staphylococcus aureus, Salmonella, Pasteurella, Escherichia coli, Candida albicans, and Pseudomonas aeruginosa. The inhibition zone diameters were > 13 mm at a concentration of 120 mg/mL [90].

Other activities
The extracts of S. lyratum showed a therapeutic potential on the tetrachloride-induced liver damage in rats, by decreasing significantly the activity of transaminase in rat serum [91,92]. Further, there was an in vivo report revealed that the aqueous extract of S. lyratum possessed strong antiallergy activity [93] by inhibiting dose-dependently the histamine release from the rat peritoneal mast cells and decreasing the mRNA expression of L-histamine decarboxylase [94]. Lastly, the ethanol extract of S. lyratum possesses molluscicidal activities with IC 50 values of 30-50 mg/mL [95]. Notably, several Chinese patent prescriptions with S. lyratum as the major component herb, showed significant anticancer efficacies in reported clinic trials. For example, 'Baiyingtang' (composed of S. lyratum, Herba Patriniae, Houttuynia cordata, Lilium brownii var., Aspa raguscochinchinensis(Lour.)Merr.) retention enema could reduce plasma transforming growth factor-β1 (TGF-β1), interleukin-6 (IL-6) levels and increase plasma IL-4 levels in patients with pelvic tumors receiving radiotherapy [96]. 'Baiying decoction' (composed of S. lyratum, Ophiocordyceps sinensis, Houttuynia cordata, Lilium brownii var., Asparaguscochinchinensis(Lour.)Merr.) treatment could ameliorate the marrow suppression and the quality of life in patients with advance non-small cell lung cancer, with high safety [97].

Toxicology
Up to now, the toxicity studies of the isolated compounds and extracts of S. lyratum may have been overlooked by researchers, while few studies have found the toxic potential of Solanum glycoalkaloid [125]. The toxic properties of Glycoalkaloids including solamargine (8) have been reviewed by Sinani AI S.S.S. et al. are due to (1) their ability to disrupt cell-membrane function by complexation with membrane 3β-hydroxysterols to form aggregates and damage the membrane integrity [126], (2) their anti-acetylcholinesterase activity on the central nervous system [126,127], and (3) changes caused by them in active transport of ions through membranes, resulting in disorders in general body metabolism [126]. Additionally, in the acute toxicity experiment with rats, neither mortality nor clinical alterations were shown, except for the mild transient diarrhea with 70% ethanol extract of S. lyratum at 5000 mg/kg [31]. In future, more pharmacological evidences should be sought on the possible adverse effects and the toxicities of S. lyratum extracts and their bioactive constituents when

Conclusion
This review summarized the latest advancements of S. lyratum in botany, traditional uses, phytochemistry, pharmacology, and toxicology. Phytochemical and pharmacological studies have validated many modern usages of this plant. A total of 270 chemical constituents have been isolated from S. lyratum, including steroidal alkaloids, steroidal saponins, terpenoids, nitrogenous compounds, phenylpropanoids, flavonoids, etc. It has been popular in traditional practices due to its potential efficacy on cancer and inflammation, and showed important biological properties in scientific investigations. In the phytochemical analysis of S. lyratum extracts, aqueous and ethanol extracts were commonly acquired from S. lyratum, whose main components included total alkaloids and total saponins. In modern pharmacological studies, compounds and extracts from S. lyratum were evaluated in vivo and in vitro, and their anticancer and cytotoxic, antiinflammatory, antioxidant, antimicrobial, anti-allergy, and hepatoprotective activities have been demonstrated. However, many aspects of S. lyratum have not been studied yet and some relative studies on S. lyratum should be further explored in the following aspects in the future. Firstly, the pharmacological activities are mostly proven from the aqueous and ethanol extracts from S. lyratum, while insufficient pharmacological studies have been conducted on pure compounds. In addition, some activities are lacking comparisons to standards or positive and negative controls. Other studies especially on anticancer and anti-inflammatory activities have shown that the IC 50 values of the test extracts/compounds of S. lyratum are above 200 μg/mL, which can be considered that such extracts/compounds are actually poorly active.
Secondly, pharmacological studies were mostly performed in cell models and animals while investigations in humans have been seldomly performed. Hence, the future investigation should be focused on the bioactivity of S. lyratum in various clinical studies with humans. In addition, the DPPH radical scavenging test and antimicrobial activities also should be guaranteed in vivo, instead of solely relying on method models in vitro.
Thirdly, global quality control standards of S. lyratum are needed urgently and should be improved. Simultaneous qualitative and quantitative measures are recommended to be used for those major active constituents of S. lyratum.      Finally, in toxicological studies of S. lyratum, no unequivocal proof of the toxicological activities in human exists. Further, relationship studies between systematic toxicity and safety evaluation are still needed to assure safety for clinical application in the future. Pharmacological effects of S. lyratum have been demonstrated by ethanol and aqueous extracts of high doses, the effectiveness of high doses extracts in treating diseases provides the possibility of finding active compounds. Therefore, it is important to study the therapeutic window (the range between the doses that produce the desired therapeutic effect and doses that produce toxicity) and the long-term in vivo toxicity for further research on S. lyratum.
In summary, S. lyratum can be considered as an important and valuable resources for human's health. Further research is needed in terms of quality control, toxicity and pharmacological mechanism to provide a theoretical basis for exploitation of the medicinal functions of S. lyratum.