Phellodendri Cortex: A Phytochemical, Pharmacological, and Pharmacokinetic Review

Background Phellodendri Cortex (PC) or Huang Bai. According to the scientific database of China Plant Species and Chinese pharmacopeia 2015 edition, PC has two main species which are Phellodendron amurense Rupr (PAR) or “Guan Huang bai” in Chinese and Phellodendron chinense Schneid (PCS) or “Chuan Huang bai” in Chinese. The crude drugs of PAR and PCS are also called Phellodendri amurensis cortex (PAC) and Phellodendri chinense cortex (PCC), respectively. The medicinal part of the plant is the dried trunk bark. PC has comprehensive therapeutic effects which include anti-inflammatory, antimicrobial, anticancer, hypotensive, antiarrhythmic, antioxidant, and antipyretic agents. The exact ingredients in PC and its species are not fully summarised. Aim of the Study This study was designed to review and evaluate the pharmacological actions of compounds and to explore the pharmacokinetic knowledge of PC and its species and to also identify the chemical compound(s) with a potential therapeutic effect on atopic dermatitis. Methods “Huang Bai” and its English, botanical, and pharmaceutical names were used as keywords to perform database search in Encyclopaedia of traditional Chinese Medicines, PubMed, EMBASE, MEDLINE, Science Direct, Scopus, Web of Science, and China Network Knowledge Infrastructure. The data selection criteria included all the studies that were related to the phytochemical, pharmacological, and pharmacokinetic perspectives of PC and its species or their active constituents. More importantly, the voucher number has been provided to ensure the genuine bark of PC used as the medicinal part in the studies. Results 140 compounds were summarized from PC and its species: specifically, 18 compounds from PCC, 44 compounds from PCS, 34 compounds from PAC, and 84 compounds from PAR. Obacunone and obaculactone are probably responsible for antiatopic dermatitis effect. PC and its species possess a broad spectrum of pharmacological actions including anti-inflammatory effect, antibacterial effect, antiviral effect, antitumor effect, antigout effect, antiulcer effect, neuroprotective effect, and antiatopic dermatitis effect. PC could widely distribute in plasma, liver, spleen, kidney, and brain. Berberine may be responsible for the toxic effect on the susceptible users with hemolytic disease or in the peripartum and neonatal period. Conclusions The compounds of the crude bark of PC and its subspecies have showcased a wide range of pharmacological effects. Pharmacological efficacies of PC are supported by its diverse class of alkaloid, limonoid, phenolic acid, quinic acid, lignan, and flavonoid. Obacunone and obaculactone could be the bioactive compounds for atopic dermatitis management. PC and its subspecies are generally safe to use but extra care is required for certain conditions and group of people.


Introduction
Phellodendri Cortex (PC) is also known as "Huang bai" in Chinese and "Obaku" in Japanese. PC is a plant grown in China, Korea, Japan, Vietnam, and Far East of Russia. The earliest record of this plant was on "Shennong's Classic of Materia Medica" [1]. According to the scientific database of China Plant Species and Chinese pharmacopeia 2015 edition, PC has two main species which are Phellodendron amurense Rupr (PAR) or "Guan Huang bai" in Chinese and Phellodendron chinense Schneid (PCS) or "Chuan Huang bai" in Chinese. The crude drugs of PAR and PCS are called Phellodendri amurensis cortex (PAC) and Phellodendri chinense cortex (PCC), respectively. PAR and PCS are naturally grown in the Northeast and Southwest part of China, respectively [2]. According to the latest information from "information system of Chinese rare and endangered plants," PAR is categorized as one of the second degrees of endangered plants. PAR and PCS could be interchangeably used in clinical application because both species contain

Bioactive Compounds
The crude barks of PC and its species contain alkaloids, isoquinoline alkaloid, limonoids, phenolic acid, quinic acid, lignans, and flavonoid, and so on. A summary of the bioactive compounds of the PC and its species reported in the included studies is presented in Table 1. Molecular formula, molecular weight, and chemical structure of the major constituted alkaloids of PC, including berberine, palmatine, and jatrorrhizine, are listed in Tables 2, 3, 4, and 5. They could exert a broad spectrum of pharmacological influence which contains antimicrobial, anti-inflammation, antitumor, antidepressant, and antiulcer effects [47]. Limonoids including limonin and obakunone play an important role in the anti-inflammatory effects of PC [11]. Phenolic acid or phenol carboxylic acids belong to aromatic acid compound substances which are characterized by a phenolic ring and an organic carboxylic acid function [61]. According to the description on Buchler quinine plant in Braunschweig, Germany, quinic acid is a natural sugary compound which can be found in multiple plants such as well-known coffee beans and tobacco leaves. Based on the description on PubChem, lignans affiliate a class of dibenzylbutane derivatives which exists in plants and in body fluids such as bile, serum, urine, etc. These compounds have anticancer potential. Quercetin belongs to flavonoids which can reduce coronary heart disease according to PubChem data. Two species of PC, PCS and PAR, can be differentiated based on the contents of the chemical compounds. Specifically, the 2005 edition of Chinese Medicinal encyclopedia described the PCS's minimal content of berberine hydrochloride and phellodendron hydrochloride to be 3.0% and 0.34%, respectively; the PAR's minimal content of berberine hydrochloride and palmatine hydrochloride is 0.60% and 0.30%, respectively [62].

Pharmacology
. . Anti-Inflammatory Effects. The PAR extract could efficiently adjust lipopolysaccharide (LPS)-induced release of nitric oxide (NO) and inducible nitric oxide sythase (iNOS) production in microglia of both BV2 cells and mice. Besides, PAR extract could also attenuate the LPS-stimulated release of tumor necrosis factor-(TNF-) and interleukin 1 (IL-1 ) from microglia. More importantly, the latter mechanism is more significant than the previous one in terms of IL-1 release. The inhibition of NO suggested that the extract of PAR probably could affect NO-induced neuronal cell death [28]. Another study had also vindicated the antiinflammatory effect of PC extract on ear swelling model of mice. Magnoflorine and phellodenrine belong to alkaloids isolated from PC that are the effective compounds against oxazolone-induced contact-delayed type hypersensitivity (DTH) reaction induced by picryl chloride. Another mechanism in this study is that PC could reduce myeloperoxidase (MPO) activity to its utmost by restraining leukocyte mobility and/or a secretory activity by phellodendron. In contrast, PC extract exerts no effect on phospholipase A2 (PLA 2 ) activity and less effect in arachidonic acid (AA) induced swelling [29]. PC showcased the inhibitory effect on the build-up of NO in LPS-stimulated macrophage Raw 264.7 cells and inhibited iNOS expression. In contrast, PC has no effect on cyclooxygenase-2 (COX-2) expression in LPSinduced RAW 264.7 cells [30]. PCC and PAC could not only shrink the size of edema, but also reduce the activity of MPO and the content of reactive oxygen species (ROS) caused by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). They can also restrain the levels of TNF-, Il-1 , IL-6, and COX-2 in mice treated by TPA. Remarkably, there are a number of chemical compounds in these two species of PC including berberine, palmatine, and phellodendrine. And they are viewed as the anti-inflammatory active candidates. In addition, they may jointly take effect in this regard [3]. The nonalkaloid PAR extract suppressed NO production; besides, limonin and obakunone significantly downregulated NO production and iNOS gene expression via an nuclear factor-B (NF-B)-mediated pathway [11]. PC could reverse the airway inflammation by reducing the infiltration of inflammatory cells and releasing of inflammatory mediators into the affected lung and airways. This could vindicate its applications on the infectious pulmonary diseases [31].

g/mol
Another study had proved that PAR has a broad spectrum of functions against virus-like VSV-GFP, PR8-GFP, NDV-GFP, HSV-GSP, H3-GFP, and EV-71 in vitro and also has effects on different strains of influenza A such as H1N1, H5N2, H7N3, and H9N2 in vivo mice model [41].
. . Antitumor Effects. There are 3 compounds in PCS that have been found to resist three types of tumours which included leukemic cell lines K562 and HEL, breast cancer cell line MDA, and prostate cancer cell line PC3. The compound [( S, R) epoxy--hydroxy-, -diethoxy] tirucalla-en--one has a relatively strong effect as Adriamycin against four tumors with the measurement of IC 50 ; toonaciliatin K and piscidinol A have a comparably moderate effect in this regard [42]. There are 9 most effective compounds of PC for prostate cancer, namely, magnoflorine-O-glucuronide, (phy-droxybenzyl)-, -dihydroxy-N-methyl tetrahydro isoquinoline--O-p-D-glucopyranosid, magnoflorine, menisperine-O-glucuronide, menisperine, berberine, Jatrorrhizine, obaculactone, and obacunone [43]. Polysaccharides from an aqueous extract of PCS act on cell-mediated stimulation and humoral immunity instead of tumor cell inhibition to exert tumoricidal activity. Specifically, some polysaccharides stimulate macrophages and NK cells via -glucan-binding lectin site of complement receptor type 3 [44].
. . Antiulcer Effects. Peptic ulcers are associated with psychological stress and mental illness. The middle dosage of PC extract could significantly reduce the levels of serotonin in the brain and noradrenaline in the adrenal gland. Both serotonin and noradrenaline take effect in the mental depression. Besides, for the molecule in PC, berberine has the ability to inhibit monoamine oxidase-A and modulate the brain noradrenaline, serotonin, and dopamine levels [47]. Another study revealed that PC could protect the gastric mucosa by reinforcing the gastric mucosal barrier through endogenous sulfhydryl compounds and diethyldithiocarbamate-sensitive compounds [48].
. . Antioxidant Effects. The antioxidant activity of PAR is proportional to its extract's concentration. In another aspect, the ethanol extract exhibited a better antioxidant effect because of its high concentration of phenolics and flavonoids than aqueous extract [33]. Phellodendrine from PC could play an antioxidant role by modulating the AKT/NF-B pathway in the zebrafish embryo. Besides, phellodendrine could undo the expression of AKT and NF-B, IKK, and COX-2 [49].
. . Sun Screening Effects. The sun screening effects of PC have been demonstrated through an experiment which was designed for sun screening effect of 50% alcohol extracts of 100 Chinese herbal medicines. The study showed PC could absorb 91.8% of ultraviolet-C, 79.1% of ultraviolet-B, and 50.7% of ultraviolet-A. It is regarded as highly effective for sun screening if the absorption rate of ultraviolet is higher than 90%. Therefore, PC could be a strong candidate for sunproof of ultraviolet-C [50]. Also, PC could also improve skin oxidative lesion induced by ultraviolet radiation via decreasing lipid peroxidation and increasing antioxidant enzymes activities [51].
. . Other Effects. PC stimulates longitudinal bone growth and chondrocyte proliferation via upregulating bone morphogenetic protein-2 (BMP-2) and insulin-like growth factor (IGF-1) expression in the growth cartilage [52]. Besides, PC could activate the fibrinogen system to take the hemostatic effect. This validated charcoaled PC could stop bleeding [53]. The extract of PC also shows neuroprotective effect through adjusting the PC-12 cell apoptosis which was induced by 1-methyl-4-phenylpyridinium (MPP + ) and hindered the release of cytochrome C into the cytosol [54]. PAR could ease the symptoms of atopic dermatitis by decreasing the numbers of mast cells, serum levels of TNF-, and INFand the expression levels of cytokines [56]. PAR could delay or even prevent the progression of diabetic nephropathy by correcting the high blood sugar state, antioxidant enzyme system, and kidney malfunction and reversing histopathological changes inflicted by diabetes on kidney [57]. To further elaborate its mechanism on the compound level, berberine could attenuate the renal malfunction by inhibiting renal aldose reductase and decreasing oxidative stress [58]. Magnoflorine and phellodendrine could inhibit the immune response by suppressing local graft versus host reaction and induction phase of picryl chloride-induced delayedtype hypersensitivity [59]. To counter asthma attack, the extract of PCS inhibits tracheal smooth muscle contraction induced by high K + . Meanwhile, it could also block tracheal smooth muscle concentration induced by Nifedipine [6]. The pharmacological activities of PC and its related derivatives have been listed in Table 6.

Pharmacokinetics
According to Chinese Pharmacopeia (Edition 2015), phellodendrine has been used as one of the evaluating indexes of PC. Phellodendrine could be rapidly absorbed in tissues such as plasma, liver, spleen, kidney, and brain. Besides, kidney is the major distribution tissue and the target organ of phellodendrine. Furthermore, the experimental study on animal suggested that this constituent has no long-term build-up effect on the tissues. Intriguingly, the extract of phellodendrine could be found in the animal brain tissue indicating that this constituent may penetrate the common medication's biggest hurdle: blood-brain barrier. The maximum concentration time is at 5 minutes after intravenous administration. The elimination half-life is no longer than 2 hours [63]. Another constituent from PC is magnoflorine which shows low bioavailability and high absorption and elimination rates after oral and intravenous administration of this constituent in pure compound form. While its bioactivity can be dramatically increased, absorption and elimination rates can be significantly decreased after oral administration of the PC decoction. Similarly, with oral administration of mixture, magnoflorine (40 mg/kg) and berberine (696.4 mg/kg, the equivalent dosage in PC decoction), the bioavailability and absorption and elimination rates have a similar trend. It suggested berberine plays an important role in the drug-drug interaction with magnoflorine in the PC decoction. On the other hand, these findings also warn us that the mingling use of berberine and magnoflorine possibly increases the risk of toxicity which possibly gives some support to the theory that herbal medicine may achieve better therapeutic effects and fewer side effects [64]. When it comes to the different type of processed PC, they have different kinds of effects. For the raw PC, it could downregulate CYP1A2 and activate CYP3A4. As for rice-wine and salt-water processed PC, they can alter the activities of cytochrome P450. Also, rice-wine processed PC alone can counter the inhibitory effect of CYP1A2 and promote the induction of CYP3A4 [65].
. . Toxicity and Contraindication. Several studies have been conducted regarding the toxicology of PC applications. So far, no conclusive result has been reached due to the controversial results from different studies. The common allegation for PC application is neonatal jaundice and kernicterus. Due to this concern, it even causes the complete ban on the use of related herbs including PC in Singapore since 1978. Besides, according to the latest Singaporean official regulations for health supplements guideline, PC is still on the list of prohibited or restricted ingredients. However, according to a cohort study, under the guidance of Chinese medicine practitioners, the application of PC's berberine is clinically safe even in patients who have hematological diseases with profound cytopenia and multiple comorbidities. Despite these, some precautionary measures such as bilirubin and hemoglobin monitoring are still required for the patients who have underlining hemolytic disease. On the other hand, the restriction for PC is necessary for the users in their peripartum and neonatal period due to the concern of its aggravation risk for neonatal jaundice and kernicterus [66].
. . Processing, Differentiation, and Authentication. Traditional Chinese medicine (TCM) processing or preparation or "Pao Zhi" in Chinese is a unique technique and process in TCM. Pao Zhi is a technique to turn the raw herbs into decoction pieces. This technique must be performed under the guidance of TCM's theory to satisfy the different requirements of medicinal materials and special production processes. The quality of Pao Zhi directly affects the therapeutic effects of herbal medicine [67]. It has a time-honored history because as early as 5 B.C. during the Jin dynasty, "Leigong Treatise on the Preparation" was composed as a book for Pao Zhi. It systematically summarized the herbal processing techniques until the Jin dynasty. Intriguingly, it stated that the right way of using the bark of PC is to remove the coarse bark [68]. In the Song dynasty, Pao Zhi had regulated a mandatory process for Chinese medicinal product [67].
In terms of PC's processing, TCM doctors in history emphasized the importance of PC's processing and recorded 16 kinds of methods for PC's processing in the books. Nowadays, the most common types of processing are raw PC, PC fried with salt, PC fried with wine, PC fried with honey, and fried-to-charcoal PC [69]. However, it still lacks official standard when it comes to quantity and quality of the processed PC and its subspecies; some of the existing pioneer studies could explain this ambiguous and abstract concept in a scientific way. Besides, for PC's quality control, the most practical approach is to use thin layer chromatography (TLC) for qualitative and high-performance liquid chromatography (HPLC) for quantitative measurement [70]. According to the results of thin layer chromatography, water percentage in the raw PC is less than 10.0%, PC fried with salt is less than 8.0%, PC fried with wine is less than 7.0%, and PC fried with honey is less than 8.0%. Total ash content in the raw PC is less than 8.0%; acid insoluble ash content is less than 0.8%; PC fried with salt is less than 8.0%; acid insoluble ash content is less than 0.6%; PC fried with wine is less than 8.0%; acid insoluble ash content is less than 0.8%; PC fried with honey is less than 8.0%; acid insoluble ash content is less than 0.4%. For alcohol-soluble extract, the raw PC is more than 16.0%, PC fried with salt is more than 20.0%, PC fried with wine is more than 16.0%, and PC fried with honey is more than 22.0%. For percentage of phellodendrine, the raw PC is more than 0.41%, and berberine is more than 3.92%; PC fried with salt is more than 0.36%, and berberine is more than 3.89%; PC fried with    Anti-asthmatic effect n-butyl alcohol extract of PCS with voucher specimens In vivo BALB/c mice asthmatic model induced by saline solution The IC50 of n-butyl alcohol extract of PCS was 12.2 ± 1.3 g/mL intranasally [6] 42 Evidence-Based Complementary and Alternative Medicine honey is more than 0.37%, and berberine is more than 3.90% [69]. Previously, the differentiation of PCS and PAR is based on the experiences of the herbal professionals to distinguish the minor differences from their appearances in naked eyes and microscope. Due to the increasing mixing applications and counterfeits, more efficient and accurate approaches are required. It is noticeable that the mass fractions of obaculactone and obacunone have a plunging order among raw PC, PC fried with wine, and PC fried with salt. Therefore, it is reasonable to deduce limonin and obacunone have been undergoing a series of chemical reactions during herbal processing. For differentiation of PCS and PAR, the mass fractions of limonin and obacunone have a significant difference between PCS and PAR. The mass fraction of obaculactone and obacunone in PAR is 10 times higher than in PCS. As a result, limonin and obacunone could be utilized as the differential targeted chemical constituents for PC's differentiation [71]. HPLC method demonstrates that PC and charcoaled PC have a significant disparity in terms of characteristic chromatograms and chemical constituents. The peak numbers and proportions of characteristic chromatograms are reducing with the increasing temperature of charcoaled PC. On the other hand, due to the heating effect, berberine hydrochloride will transfer into berberrubine by losing one methyl. As a result, berberrubine is vindicated to be another targeted chemical constituent for charred PC identification [72]. For authentication of PC and its subspecies, berberine hydrochloride could be used as another targeted chemical constituent except for limonin and obacunone as mentioned above [73].

Discussion
This review work has illustrated the diverse bioactive properties of PC and its species associated with active pharmacological actions in vitro and in vivo. Its clinical applications which are demonstrated in these experimental studies indicated the potency of the bioactive compounds and its pharmacological effects.
The diverse derivatives are the backbone for the pharmaceutical efficacies of PC and its species. The alkaloids play a very significant role in this regard not only because they account for a great proportion of constituents in the whole herb, but also because these constituents are relatively well-studied compared to other constituents in other derivatives. Berberine, magnoflorine, palmatine, and phellodendrine are viewed as the anti-inflammatory active candidates in experimental studies. Besides, palmatine and berberine could exert antimicrobial effects. Berberine also has the ability to inhibit monoamine oxidase-A and modulate the brain noradrenaline, serotonin, and dopamine for antiulcer efficacy. The nonalkaloid in PAR extract suppressed NO production; besides, limonin and obakunone significantly downregulated NO production and iNOS gene expression via an NF-B-mediated pathway. [(21S, 23R) Epoxy-24-hydroxy-21 , 25-diethoxy] tirucalla-7-en-3-one, toonaciliatin K, and piscidinol have tumor-shrinking effects on four types of tumors. Polysaccharides from an aqueous extract of PCS act on cell-mediated stimulation and humoral immunity to exert tumoricidal activity.
Phellodendrine and magnoflorine have well absorption rate in the animal organs which could indicate their safety for human trials. However, berberine could interact with magnoflorine or other constituents which could further increase the tissue absorption rates. This should be noticed by other clinical trials or advanced studies to avoid adverse effects. It is also noticeable that different processing techniques or "Pao Zhi" could render the herb with different kind of therapeutic effects. For the raw PC, it could downregulate CYP1A2 and activate CYP3 A4. As for rice-wine and salt-water processed PC, they can alter the activities of cytochrome P450. Also, for rice-wine processed PC alone, it can counter the inhibitory effect of CYP1A2 and promote the induction of CYP3A4. For toxicology, there is no affirmative conclusion in terms of PC and its species' absolute clinical safety. Therefore, safety precautionary measures are still required for vulnerable groups of people. For the processed PC and its species, berberrubine, limonin, and obacunone became targeted chemical constituents for authentication of charred PC, PC, and its subspecies. Furthermore, obacunone and obaculactone are probably responsible for antiatopic dermatitis effect [56].

Conclusions
In summary, the compounds of the crude bark of PC and its species have showcased a wide range of pharmacological effects. Pharmacological efficacies of PC are supported by its diverse class of alkaloids, limonoids, phenolic acid, quinic acid, lignans, and flavonoid. Through this review, in total, 140 chemical compounds had been summarized. Among these compounds are 18 compounds from PCC, 44 compounds from PCS, 34 compounds from PAC, and 84 compounds from PAR. However, more studies are still needed to demonstrate more knowledge to allow a better understanding of this herb and its species.