Polygonum capitatum, the Hmong Medicinal Flora: A Comprehensive Review of Its Phytochemical, Pharmacological and Pharmacokinetic Characteristics

Polygonum capitatum, known as “Tou Hua Liao” (Chinese name), is a crucial source of Hmong medicinal plants that has benefited human health for a long time. This folk-medicinal plant is widely distributed in the south-west of China for the treatment of various urologic disorders including urinary tract infections, pyelonephritis, and urinary calculus. The purpose of this paper was to provide a systematic and comprehensive overview of the traditional usages, botany, phytochemistry, pharmacology, pharmacokinetics and clinical applications of this flora. Up until the end of 2022, at least 91 compounds had been reported from P. capitatum, mainly covering the classes of flavonoids, lignanoids, phenols and other components. The compounds and extracts isolated from P. capitatum exhibit a wide range of pharmacological activities, such as anti-inflammatory, antioxidant, antimicrobial, anticancer, analgesic, hypothermic, diuretic and other pharmacological effects. Qualitative and quantitative chemical analyses were also covered. Furthermore, the possible development trends and perspectives for future research on this medicinal plant were also discussed.


Introduction
Polygonum capitatum is a well-known and large-scale Miao medicinal plant with a long history of economic and medicinal value. Amongst Chinese people, especially in Guizhou province, P. capitatum is commonly named TouHuaLiao [1]. It is often named Gastrochilus panduratum RIDL., Kaempferia pandurata Roxb., Curcuma rotunda L., and Boesenbergia rotunda Linn [2]. At present, P. capitatum shows a variety of pharmacological activities, including anti-inflammatory, antioxidant, antimicrobial, anticancer, analgesic, hypothermic, diuretic and other pharmacological effects [2][3][4][5][6][7]. These pharmacological activities are attributed to the chemical constituents and extracts of P. capitatum. So far, more than 90 compounds have been isolated from P. capitatum. Phenolic acids and flavonoids are believed to be responsible for the bioactivities of P. capitatum.
With recently increasing levels of research into P. capitatum, it is especially valuable to review its current status in order to provide reference for a deep exploration of its ethnic medicinal potential. This review summarized progress in the chemical study of P. capitatum, mainly covering the classes of flavonoids, lignanoids, phenols, and other components. Moreover, we systematically organized the development of the medicinal flora into traditional usages, botany, and pharmacology. Qualitative and quantitative chemical analyses 1.2. Botanical Description P. capitatum, is derived from the dried herbs of Polygonaceae species (Polygonaceae family), and is a perennial herb, 10-15 cm tall, with stolons, rooting at its nodes, annual branches ascending upward, and a red surface. Leaves are alternately oval-, base-, and wedge-shaped, sometimes with "V"-shaped markings on the leaf surface, inflorescent, terminal, perianth reddish, five-parted, and flowering from June to October each year. It is worth noting that the stem of the transverse section is composed of one to two rows of epidermis cells. The outermost part has glandular hairs where the cortex is narrow and composed of many rows of tightly arranged parenchyma cells that contain more clusters of calcium oxalate crystals. In addition, the leaf of P. capitatum is a heterofacial leaf. That is to say, the upper epidermis of the leaf of the main vein protrudes slightly upward, semicircularly, and the parenchyma cells of the protruding part are small and dense with glandular hairs often growing in the lower epidermis. Furthermore, the vascular bundles in the middle of the main veins are arranged in a ring; they are externally tough vascular bundles. Parenchyma cells are round, scattered calcium oxalate clusters. The palisade tissue is generally composed of two to three rows of long oval cells, which are closely arranged with scattered clusters of calcium oxalate crystals [13,14].

Geographic Distribution
P. capitatum is generally distributed in the southwest of China, mainly in Jiangxi, Guizhou, Hunan, Hubei, Sichuan, Guangxi, Yunnan, and Tibet provinces. It is also found in other Asian countries, including India, Nepal, Bhutan, Myanmar and Vietnam. The plants of P. capitatum are fond of growing in cool and moist places, they are suitable for growing in the sandy loam of sunny valleys with good air permeability and mild acidic soil without water shortages [13,14].

Phytochemistry
To date, there are a total of 91 compounds  with the phytochemical composition of P. capitatum. They can be classified into four classes: 30 flavonoids, 10 lignanoids, 25 phenols, and 26 other compounds. Each phytochemical is numbered (1-91) and their names, formulas, molecular weights, and the parts of plant used in these compounds, are cited in the SI (Supportting Information) (Table S1).

Lignanoids
Ten lignanoids were isolated and identified from P. capitatum. The structures of these compounds are shown in Figure 1. Moreover, it belongs to isolariciresinol (31)

Biological Activities and Medicinal Potential
As a folk medicine, the whole of the P. capitatum plant has been used to treat urinary tract infections, dysentery, eczema, urolithiasis and pyelonephritis by the Hmong residents from China. It has long been conceived that gallic acid is the only composition underwriting the pharmacological effects of P. capitatum. However, the anti-inflammatory effect of P. capitatum extract has been ascribed to gallic acid-free fractions abounding in flavonoids. Thus, the phenolics and flavonoids are both considered as crucial bioactive constituents of P. capitatum.
Plenty of investigations have been reported on the pharmacological activities of P. capitatum extracts and its major compounds. In the past two decades, pharmacological studies on P. capitatum have indicated diverse biological activities, including anti-inflammatory, antioxidant, anti-hepatocellular carcinoma, antibacterial, antitumor, analgesic, hypothermic, and diuretic activity. This research is summarized here with special focus on flavonoids and phenolic acids with medicinal potential ( Figure 3, Table 1).
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Biological Activities and Medicinal Potential
As a folk medicine, the whole of the P. capitatum plant has been used to treat urinary tract infections, dysentery, eczema, urolithiasis and pyelonephritis by the Hmong residents from China. It has long been conceived that gallic acid is the only composition underwriting the pharmacological effects of P. capitatum. However, the anti-inflammatory effect of P. capitatum extract has been ascribed to gallic acid-free fractions abounding in flavonoids. Thus, the phenolics and flavonoids are both considered as crucial bioactive constituents of P. capitatum.
Plenty of investigations have been reported on the pharmacological activities of P. capitatum extracts and its major compounds. In the past two decades, pharmacological studies on P. capitatum have indicated diverse biological activities, including anti-inflammatory, antioxidant, anti-hepatocellular carcinoma, antibacterial, antitumor, analgesic, hypothermic, and diuretic activity. This research is summarized here with special focus on flavonoids and phenolic acids with medicinal potential (Figure 3, Table 1).

Anti-Inflammatory Activities
The pharmacological effects of P. capitatum on anti-inflammatory activity have been fully summarized. The aqueous and ethanol extract of P. capitatum exhibits anti-inflammatory effects by inhibiting the levels of inflammatory cytokines NO and TNF-α in RAW 264.7 macrophages [31]. The largest study was reported by Liao, the total flavonoid fractions were tested on Kunming mice (18-22 g), administrated orally through gavage in a

Anti-Inflammatory Activities
The pharmacological effects of P. capitatum on anti-inflammatory activity have been fully summarized. The aqueous and ethanol extract of P. capitatum exhibits anti-inflammatory effects by inhibiting the levels of inflammatory cytokines NO and TNF-α in RAW 264.7 macrophages [31]. The largest study was reported by Liao, the total flavonoid fractions were tested on Kunming mice (18-22 g), administrated orally through gavage in a single dose of 0.6 g/kg, 0.3 g/kg, and 0.15 g/kg per day for seven consecutive days. The results showed significant anti-inflammatory activity with inhibition rates of 86.15 % at 0.6 g/kg [32]. Furthermore, treatment with flavonoid-rich extract of P. capitatum (the major constituents were luteolin-7-O-glucoside, rutin, and quercitrin) at 90 and 180 mg/kg body weight in rats for 6 weeks remarkably decreased serum TNF-α, and interleukin-6 (IL-6) levels, which mechanism implied that total flavonoids suppressed the development of atherosclerosis, possibly by inhibiting inflammatory response [7]. Later, the anti-inflammation effects of total flavonoids of both wild and cultivated P. capitatum were also observed in mouse abdominal cavity capillary permeability, the xylene-induced ear swelling model and carrageenaninduced mouse pedal swelling test, and the results showed an inhibitory effect in the same dose [33]. To screen effective anti-inflammatory extracts from P. capitatum, they reported that the aqueous extract and the protein-free water extract of P. capitatum could significantly inhibit the release of NO, TNF-α and IL-6 in LPS-induced RAW264.7 cells. In particular, the protein-free water extract of P. capitatum had the best effect on NO, TNF-α and IL-6 inhibition and was the main effective anti-inflammatory ingredient [34]. Recently, quercetin, one flavonoid, was isolated from P. capitatum, and regulated the balance of gastric cell proliferation and apoptosis to protect against gastritis. Its mechanism was that quercetin protects against gastric inflammation and apoptosis associated with Helicobacter pylori infection by affecting the levels of p38MAPK, BCL-2 and BAX genes [35]. At the same time, flavonoid glycosides of P. capitatum protect against inflammation associated with Helicobacter pylori infection, and the results suggested that flavonoid glycoside has repairing functions for gastric injuries [36].
In addition, the P. capitatum extract powder (1.58 g/kg body weight, DW) in CMC-Na solution, was orally administered for SD rats once daily for 14 consecutive days. The results proved P. capitatum could inhibit the activation of the AKT/PI3K pathway by upregulating PTEN expression; thus, gastric mucosal inflammation induced by H. pylori can be improved [37]. P. capitatum has a significant therapeutic effect on allergic contact dermatitis, which may be related to suppression of levels of IL-4 and TNF-α [38]. In particular, Relinqing granules (14.4, 7.2 g/kg DW) promisingly inhibited dimethylbenzeneinduced auricle tumefaction of mice. Relinqing granules (3.6, 7.2 g/kg DW) significantly inhibited granuloma with cotton ball in rats. Relinqing granules (7.2 g/kg DW) significantly decreased the number of white blood cells in rat urine with chronic urinary tract infections, and improved kidney function and pathological changes [39].
The search for a better model system to explore the effective constituents and the mechanism of action of anti-inflammatory P. capitatum was studied through the method of network pharmacology. The results showed a total of 6 active compounds, and 41 potential targets and 76 signal pathways were screened and obtained [40].
P. capitatum extract has demonstrated obvious anti-oxidant activity in vitro. Experimental studies have shown that an 80% methanol extract of leaves and stems from P. capitatum demonstrate strong antioxidant activities against ABTS + /OH − (23.08%) and Fe 2+ chelating capacity activities (17.3% EDTA/g DW) [41]. Some flavonoids isolated from P. capitatum, quercitrin, protocatechuic acid, quercetin and kaempferol possessed strong scavenging free radical capacity against H 2 O 2 , with an IC 50 of 0.044 µM, 0.276 µM, 0.098 µM and 0.029 µM, respectively [42]. For in vitro experiments, the ethanol extract revealed stronger anti-oxidant activities than the aqueous extracts of P. capitatum; its IC 50 values were 1.71 mg/mL and 0.15 mg/mL, respectively [43]. The same result was shown in another study; the methanol extract of P. capitatum showed higher scavenging activity against DPPH radical and ABTS radical Particularly, the methanol extract exhibited more significant antioxidant activity than that of positive drug BHT [44]. In addition, the EtOAc extract of P. capitatum exhibited remarkable scavenging activity against DPPH radical and ABTS radical. The results further elucidate that EtOAc extract could be used as an important part of antioxidant substances, and that polyphends were the major active ingredients of antioxidant activity for P. capitatum [58]. However, it is of great importance to note that only a small part of the research conducted into anti-oxidant activity has employed in vitro based methods and that further in vivo verifications should be encouraged.

Antimicrobial Activities
Plenty of investigations have been reported on the antimicrobial activities of P. capitatum extracts, and the major compounds P. capitatum possesses and their promising antibacterial activities (Figure 4). Previous studies have reported that crude extracts of P. capitatum significantly inhibit the growth of the bacteria Listeria monocytogenes and Salmonella anatum, at the minimum inhibitory concentration (MIC) of 6.25 mg/mL [41]. Liu et al. reported that the 60% ethanol extract (250 µg/disc) displayed a better antibacterial activity against the multidrug-resistant Staphylococcus aureus [46]. Moreover, in another study, plant extracts and fractions of P. capitatum demonstrated antimicrobial properties against bacterial strains, and through the determination of the MIC and the minimum bactericidal concentration (MBC), the results showed that the crude extracts or fractions FV (flavonoid-enriched fraction) and TN (tannin-enriched fraction) have antibacterial and bactericidal properties [32]. Additionally, in an in vitro antibacterial test, 40 µg/mL or higher concentrations of extracts (flavonoid glycosides) of P. capitatum inhibited the growth of H. pylori; the resistance of MIC was regarded as >40.0 µg/mL, while the resistance of MIC of amoxicillin was regarded as >1.0 µg/mL [36]. REVIEW  11 of 16 glucose, 1, 3, 6-trigalacyl glucose and Davidiin [48]. Moreover, the different polar of seven fractions in the 70% ethanol extract of P. capitatum had high antibacterial activity against EScherichia coli, the MIC was 0.20 mg/mL, and the MBC was 0.78 mg/mL [49]. These findings show that antimicrobial activity is an essential property of P. capitatum and that this flora should be a fundamental source of preservatives for the pharmaceutical industry.

Anti-Tumor Activities
Some pharmacological studies have shown that different extract and compound prescriptions derived from P. capitatum have significant antineoplastic effects against diseases. In 2013, Wang et al. showed that emodin at doses of 10-120 mΜ could effectively inhibit production with a dose-dependent manner of HCC cell lines. The possible mechanism of action inhibited the expression of the proteasome-dependence of EZH2 [29]. It was also found that intraperitoneal administration (single dose of 10 mg/kg/day, sp) significantly inhibits tumor progression in hepatoma xenograft mice [30]. It is well known that davidiin displays extensive antitumor activity. Davidiin, a natural product isolated from P. capitatum, has an antitumor mechanism of changing the metabolism of sphingolipids. When HepG2 cells were treated with 50 µM davidian for 72 h, the levels of several types of sphingolipids significantly changed, including Cer, LacCer and So; they decreased markedly to 26.2%, 27.8% and 19.7%, respectively [50].

Other Biological Activities
Apart from anti-inflammatory, anti-oxidant, antimicrobial and anti-tumor activities, P. capitatum has a remarkable effect on anti-atherosclerosis, a hypoglycemic effect, and defervescence and analgesic action. Wang et al. reported that luteolin-7-O-glucoside, rutin and quercitrin total flavonoids, separated from P. capitatum, exerted an anti-atherosclerosis effect in hyperlipidemia rats through regulating blood lipid metabolism, and modulating a proinflammatory profile [7]. At the same time, the lignans (isoidulinol, 5'-methoxy-isolaridosin-9-O-β-D-xylopyranoside) isolated from P. capitatum showed significant hypoglycemic activity against type two diabetes [51]. Later, it was reported that aqueous extract of P. capitatum at a dose of 450 mg/kg DW significantly reduced the body temperature of rabbits with a fever induced by an intravenous injection of typhoid fever and paratyphoid bacillus [52]. Furthermore, the alcohol and water extracts of P. capitatum exhibited a prominent analgesic effect on the writhing response induced by acetic acid in mice [33,53]. In addition, P. capitatum extracts (5 g/kg, 10 g/kg, 20 g/kg DW, 4 weeks) demon- Simultaneously, P. capitatum inhibits H. pylori growth via interfering with and inhibiting the expression of Helicobacter pylori protein [47]. Moreover, four effective parts of the alcohol extract of P. capitatum were found to have outstanding potential antimicrobial activities; the main antibacterial components could be 6-galacyl glucose, 3, 6-digalacyl glucose, 1, 3, 6-trigalacyl glucose and Davidiin [48]. Moreover, the different polar of seven fractions in the 70% ethanol extract of P. capitatum had high antibacterial activity against EScherichia coli, the MIC was 0.20 mg/mL, and the MBC was 0.78 mg/mL [49]. These findings show that antimicrobial activity is an essential property of P. capitatum and that this flora should be a fundamental source of preservatives for the pharmaceutical industry.

Anti-Tumor Activities
Some pharmacological studies have shown that different extract and compound prescriptions derived from P. capitatum have significant antineoplastic effects against diseases. In 2013, Wang et al. showed that emodin at doses of 10-120 mM could effectively inhibit production with a dose-dependent manner of HCC cell lines. The possible mechanism of action inhibited the expression of the proteasome-dependence of EZH2 [29]. It was also found that intraperitoneal administration (single dose of 10 mg/kg/day, sp) significantly inhibits tumor progression in hepatoma xenograft mice [30]. It is well known that davidiin displays extensive antitumor activity. Davidiin, a natural product isolated from P. capitatum, has an antitumor mechanism of changing the metabolism of sphingolipids. When HepG2 cells were treated with 50 µM davidian for 72 h, the levels of several types of sphingolipids significantly changed, including Cer, LacCer and So; they decreased markedly to 26.2%, 27.8% and 19.7%, respectively [50].

Other Biological Activities
Apart from anti-inflammatory, anti-oxidant, antimicrobial and anti-tumor activities, P. capitatum has a remarkable effect on anti-atherosclerosis, a hypoglycemic effect, and defervescence and analgesic action. Wang et al. reported that luteolin-7-O-glucoside, rutin and quercitrin total flavonoids, separated from P. capitatum, exerted an anti-atherosclerosis effect in hyperlipidemia rats through regulating blood lipid metabolism, and modulating a proinflammatory profile [7]. At the same time, the lignans (isoidulinol, 5'-methoxyisolaridosin-9-O-β-D-xylopyranoside) isolated from P. capitatum showed significant hypoglycemic activity against type two diabetes [51]. Later, it was reported that aqueous extract of P. capitatum at a dose of 450 mg/kg DW significantly reduced the body temperature of rabbits with a fever induced by an intravenous injection of typhoid fever and Paratyphoid bacillus [52]. Furthermore, the alcohol and water extracts of P. capitatum exhibited a prominent analgesic effect on the writhing response induced by acetic acid in mice [33,53]. In addition, P. capitatum extracts (5 g/kg, 10 g/kg, 20 g/kg DW, 4 weeks) demonstrated a hypoglycemic effect. This mechanism may be related to the expression of AMPK and GLUT4 genes up-regulated in the liver to further promote the uptake of glucose by the liver tissue [54].

Quality Control
LC/MS or HPLC are currently the most powerful techniques for global chemical analysis of TCM. They have been extensively used for the analysis of chemical constituents of P. capitatum. The previous literature has reported flavonoids and phenolic acids were considered to be the vital active constituents of P. capitatum. In the 2003 edition of the "Quality standards of Chinese medicinal materials and ethnic medicinal materials in Guizhou Province", only gallic acid (the content > 0.05%) was included as a standard for the evaluation of P. capitatum quality [45]. Zhang et al. reported an HPLC method to analyze the herbs of P. capitatum; the average content of gallic acid was 0.2% [59]. Over the past few years, the use of reversed-phase HPLC has been developed for the analysis of flavonoids; quercitrin, derived from P. capitatum, was linear and ranged from 0.082-0.408 µg [60]. In 2010, a scientist established a simple HPLC method for the characterization of quercetin from three parts (flower, stem and leaf) of P. capitatum. The results showed that the quercetins ranged from 0.25% to 0.62%, and the highest content of quercetin was found in leaves [61]. Recently, the Beijing Institute of Materia Medica, Chinese Academy of Medical Sciences, has completed the quality standard of P. capitatum. The content of gallic acid and quercetin should not be less than 0.015 g/100 g DW and 0.1 g/100 g DW, respectively [62].

Pharmacokinetic and Metabolite Analysis
A comparative pharmacokinetic study of crude herb from P. capitatum was carried out. Several research groups have studied the metabolism of gallic acid (GA) and protocatechuic acid (PCA) in the aqueous extract of P. capitatum. Administration of aqueous extract of P. capitatum was at oral doses of 60 mg/kg (equivalent to 12 mg/kg DW of GA and 0.9 mg/kg DW of PCA) to rats; after 1 h, the concentration of GA and PCA in kidney tissue, respectively, reached 1218.62 ng/g and 43.98 ng/g, indicating that extensive metabolism of GA and PCA occurred after ingestion [3]. He et al. studied the material metabolism of the bioactive extracts of P. capitatum. The results showed that the metabolic pathways of intestinal flora in P. capitatum were hydrolysis, reduction and oxidation [55]. After that, the metabolic characteristics of FR429 were evaluated in male Wistar rats (260-280 g), a total of eight metabolites were detected from bile and urine. It was deduced that the main metabolic pathway of FR429 in rats was methylation and subsequent glucuronidation [56]. Recently, the extract of P. capitatum 700 mg/kg DW (equivalent to gallic acid 21.35 mg/kg DW, quercetin 2.17 mg/kg DW, quercetin content of 0.392 mg/kg DW, respectively,) was orally administered to rats. As a result, gallic acid and quercitrin were detected in plasma, but quercetin was not detected [2]. Similarly, ultra-high performance liquid chromatographytandem mass spectrometry (UPLC-MS/MS) was used to determine the plasma levels of P. capitatum extracts. Compared with the normal group, the absorption of GA, PCA and quercetin (QR) in pyelonephritis rats was increased, and excretion was decreased [57] ( Figure 5).

Conclusions
P. capitatum is a traditional medicinal plant of the Miao people and has been used to treat a variety of urological disorders in China over a long history, such as dysentery, pyelonephritis, cystitis, urolithiasis, pelvic inflammation and rheumatic pain. In this work, we reviewed the available information concerning the traditional uses, phytochemistry, pharmacology and quality control of P. capitatum. In total, 91 compounds from P. capitatum were isolated, including 30 flavonoids, 10 lignanoids, 25 phenols, and 26 other constituents. Furthermore, P. capitatum has clear pharmacological properties such as antibacterial, anti-inflammatory, antioxidant, antitumor, antipyretic and analgesic effects, and has potential hypoglycemic development prospects. These research results could provide a referential merit for the processing, quality control and clinical medication guidance of P. capitatum. In addition, some drugs have been derived from P. capitatum and are presently used in clinic such as Relinqing granule and Milins capsules, but the development of its related medical products is still very limited. However, it is also necessary to further study the drug-forming properties and pharmacokinetics of the active constituents of P. capitatum, as well as to establish quality control standards for different areas of P. capitatum, to investigate their safety evaluation, adverse reactions and toxicity, and to carry out research at the cellular and molecular levels. We hope that this review highlights the important value of P. capitatum and promotes its all-round development.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: The compounds isolated from P. capitatum.

Conclusions
P. capitatum is a traditional medicinal plant of the Miao people and has been used to treat a variety of urological disorders in China over a long history, such as dysentery, pyelonephritis, cystitis, urolithiasis, pelvic inflammation and rheumatic pain. In this work, we reviewed the available information concerning the traditional uses, phytochemistry, pharmacology and quality control of P. capitatum. In total, 91 compounds from P. capitatum were isolated, including 30 flavonoids, 10 lignanoids, 25 phenols, and 26 other constituents. Furthermore, P. capitatum has clear pharmacological properties such as antibacterial, antiinflammatory, antioxidant, antitumor, antipyretic and analgesic effects, and has potential hypoglycemic development prospects. These research results could provide a referential merit for the processing, quality control and clinical medication guidance of P. capitatum. In addition, some drugs have been derived from P. capitatum and are presently used in clinic such as Relinqing granule and Milins capsules, but the development of its related medical products is still very limited. However, it is also necessary to further study the drug-forming properties and pharmacokinetics of the active constituents of P. capitatum, as well as to establish quality control standards for different areas of P. capitatum, to investigate their safety evaluation, adverse reactions and toxicity, and to carry out research at the cellular and molecular levels. We hope that this review highlights the important value of P. capitatum and promotes its all-round development.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27196407/s1, Table S1: The compounds isolated from P. capitatum.  Institutional Review Board Statement: The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Ethics.