Psoralen and isopsoralen are secondary plant metabolites found in many fruits (such as Ficus carica
L.), vegetables (such as Celery
), and medicinal herbs (such as Psoralea corylifolia
L.). In addition, they are primarily used clinically to treat various skin diseases, such as psoriasis and vitiligo [1
], since the US Food and Drug Administration approved the use of oral psoralens plus ultraviolet light A radiation (PUVA) to treat psoriasis in 1982. Modern pharmacological studies have further found that they are potential antimicrobial, antitumor, tyrosinase-activating, antidepressant, and estrogenic agents [2
However, the ingestion of psoralens-containing plants, including the medicinal use of these compounds, can cause human health hazards. Bai-shi Wan and Zhuang-gu-guan-jie Wan are both proprietary Chinese medicines with Psoralea corylifolia
(bu-gu-zhi) being the main constituent. There are 11 cases of liver injury associated with the use of Bai-shi Wan for the treatment of vitiligo in the Chinese literature from 1978 to 2005, and 47 cases of liver injury associated with the use of Zhuang-gu-guan-jie Wan for the treatment of degenerative osteoarthrosis in the Chinese literature from 2001 to 2008 [6
]. Additionally, the Roussel Uclaf Causality Assessment Method (RUCAM) has been widely used to evaluate the causality of drug-induced liver injury (DILI) formation, and it can evaluate the liver damage caused by herbs and drugs [8
]. Some causality assessments have been established to show a probable causality for extracts from psoralea leaves, which indicated the possibility that psoralen and isopsoralen reduce hapatotoxicity [10
Since the 1980s, psoralen has been associated with hepatitis due to PUVA in humans [11
]. Recently, animal studies have shown that psoralen and isopsoralen exhibit hepatotoxicity in mice, giving further evidence for psoralen and isopsoralen being hepatoxic agents in Bu-gu-zhi [12
]. As ingredients that have widespread use in vegetables and Chinese medicines, it is important to understand the potential mechanism of psoralen- and isopsoralen-induced hepatotoxicity, and the consequential risks and benefits.
It is generally accepted that many chemicals require metabolic activation before they exert their toxic effects, e.g., methimazole, ticlopidine, and tienilic acid. Furan ring structure is one of the active functional groups that may produce reactive metabolites via CYP450s. The oxidation of the double bond of the furan ring to give the reactive furanoepoxide or γ-ketoenal intermediate, resulting in the modification of CYP450s, has been well-documented during the irreversible inactivation (also called mechanism-based inhibition) of CYP450s by the psoralen analogues 8-Methoxypsoralen (8-MOP), 5-Methoxypsoralen (5-MOP), and bergamottin [14
]. Recently, psoralen and isopsoralen have also been found to give the irreversible inactivation of CYP1A2 in human and rat liver microsomes [18
], and the formation of reactive metabolites has been reported [19
]. Besides, some reactive furanoepoxide has been reported to be associated with hepatotoxicity or even carcinogenicity, such as the reactive epoxide of furan neoclerodane diterpenoids teucrin A, which can react with proteins leading to a mitochondrial permeability transition in rat and mouse hepatocytes [21
], and aflatoxin B1-2,3-epoxide, which may covalently bind to DNA resulting in tumors [22
]. Therefore, it is reasonable to presume that the formation of reactive metabolites is a plausible cause for psoralen and isopsoralen hepatotoxicity.
The molecular docking simulation approach has been widely employed towards understanding and predicting the possibilities of ligand–CYP450s interactions, especially in the discovery of candidate compounds, the exploration of potential targets, and the research of interaction mechanisms [23
], but few papers have focused on studying and modeling the binding mode of mechanism-based inhibition with molecular docking so far.
A large number of studies have shown that the activity center of CYP450s is iron porphyrin binding with a sulfur amino acid residue of half oboro glycine within a distance of 2.2 Å as a fifth axial coordination ligand [27
]. Besides, chemical bonding and the shapes of molecular structures is a rather recent issue, and is explained by the concepts of hybrid orbitals, valence bond theory, Valence Shell Electron Pair Repulsion (VSEPR), resonance structures, and the outlet rule [29
]. Meanwhile, there exists two spin states of Fe(II) influenced by the occupied ligand [32
]. In the molecular mechanism of the CYP450s enzyme catalytic reaction (Scheme 1
is more likely to be low-spin under the action of the ligand field, and the redox potential value of (Fe2+
) is a minus, which contributes to Fe(II) being oxidized to Fe(III) (Scheme 1
To investigate this presumption, a mechanism-based inhibition (MBI) evaluation and in vitro liver microsomal incubation experiments were designed to determine and identify the reactive metabolites of psoralen and isopsoralen. A molecular docking simulation was applied to confirm the binding interaction of these unstable reactive metabolites on CYP3A4. The glutathione (GSH)-depleted agent L-buthi-onine-S, R-sulfoxinine (BSO) and a non-specific inhibitor of CYP450s, 1-Aminobenzotriazole (ABT), were administered to mice to elucidate the factors involved in psoralen- and isopsoralen-induced hepatotoxicity. Based on the results of the above studies, the mechanisms of psoralen- and isopsoralen-induced hepatotoxicity via reactive metabolites have been raised in the present study.
In the present study, five metabolites of psoralen and isopsoralen were found in HLM and MLM: (1) hydroxylation metabolites; (2) hydrolysis metabolites; and (3) furan ring oxidation metabolites (Figure 2
), suggesting the similar metabolic pathway of 8-MOP [41
]. As the peak area reflected (Table 1
), both M4 and M5 (M′4 and M′5) are the main metabolites of psoralen (isopsoralen), suggesting that the formation of reactive metabolites was the main pathway for psoralen/isopsoralen metabolism in HLM and MLM. Subsequently, the trapping of furanoepoxide and γ-ketoenal intermediate by GSH provides further evidence for the existence of reactive metabolites, and their irreversible inactivation of CYP3A4 (Figure 1
). Furthermore, as the IC50
shift folds reflected (Table 1
), psoralen causes a more significant IC50
shift than isopsoralen, suggesting that the reactive metabolite of psoralen has a more potent electrophilicity to covalently bind to proteins.
Due to the low electron density, reactive metabolites can react with cellular components, such as proteins, DNA, and membranes (e.g., mitochondrial membrane), resulting in cell stress, which is involved in the occurrence of liver toxicities [42
]. In a study of 207 of the most commonly prescribed oral medications in the U.S., 62–69% of compounds with reactive metabolite formation were associated with drug-induced liver injury [44
]. GSH-depleted animal models are a standard, commonly used model to investigate the underlying mechanisms of hepatotoxicity caused by reactive metabolites [40
]. In the present study, BSO significantly reinforced the psoralen (300 mg/kg) hepatotoxicity in mice by increasing the plasma ALT activities (6 h: from 42.17 to 59.90 IU/L; 24 h: from 128.42 to 326.07 IU/L). Treatment with ABT, a non-specific inhibitor of CYP450, resulted in a remarkable protection against the hepatotoxicity of psoralen or isopsoralen alone or in combination with BSO (Figure 5
and Figure 6
). Overall, these results suggest that the reactive metabolites of psoralen and isopsoralen are responsible for their hepatotoxicity. Furthermore, the hepatotoxicity of isopsoralen is weaker than that of psoralen, which can be explained by the fact that isopsoralen metabolism results in reactive metabolites with less electrophilicity than that of psoralen (Table 1
). Through a molecular docking analysis, the -Cdocker energy score value of psoralen γ-ketoenal intermediates was found to be slightly higher than that of isopsoralen, which is consistent with the IC50 shift assay
result that psoralen metabolism produces reactive metabolites with higher mechanism-based inhibitory effects than that of isopsoralen (Table 3
In the present study, psoralen and isopsoralen were oxidized by CYP450s, causing the mechanism-based inhibition (MBI) of CYP3A4. Although the mechanism of MBI is unknown for now, we can find some clues from the molecular docking study. On one hand, the psoralen and isopsoralen γ-ketoenal intermediates bound to human CYP3A4 more tightly (21.4426 kcal/mol, 19.635 kcal/mol, respectively) than psoralen/isopsoralen (19.4697 kcal/mol, 18.5824 kcal/mol, respectively) and the furanoepoxide intermediates (−21.3421 kcal/mol, −23.4627 kcal/mol). On the other hand, according to the bond valence model [45
] and the empirical data of published bond lengths from the Inorganic Crystal Structure Database (ICSD) and the Cambridge Structure Database (CSD) [45
], the O atom of the γ-ketoenal intermediate of psoralen/isopsoralen, which can provide one lone electron pair, is close to the iron capable of accepting a lone electron pair on the sixth axial coordination position with a distance of 2.31 Å and 2.226 Å, suggesting that the γ-ketoenal intermediates of psoralen/isopsoralen have the possibility to coordinate to bond to CYP3A4 (Figure 8
a). In this occasion, electrons will be concentrated around the central ion, which is not in accordance with the electric neutrality principle [49
]. Therefore, we have reason to believe that there exists a π backbinding [50
], in which electrons are transferred from the dxz
atomic orbital of central ion Fe2+
to the 2pz
π* anti-bonding molecular orbital on the O=C group of the γ-ketoenal intermediates’ ligand. The correlation of this coordination binding can be described as Figure 8
b. These findings may account for the mechanism of coordination binding as a vital role in mechanism-based inhibition between the γ-ketoenal intermediates and CYP3A4. Consistently, high-dose 5-MOP (560 mg/kg daily for 12 days) has been shown to cause hepatocyte changes in animals [52
]. In humans, transient elevations of liver enzymes were found in 5% of patients (4 of 80) within 3.5–20 months of commencement of PUVA 5-MOP [53
]. However, there was no liver injury report associated with 8-MOP till now. In the present and previous study, psoralen, isopsoralen, and 5-MOP were found to form furanoepoxide or γ-ketoenal intermediate via CYP450s, while 8-MOP was only found to form furanoepoxide [17
]. It is then presumed that γ-ketoenal intermediate may be the more plausible cause for the hepatotoxicity of psoralen, isopsoralen, and 5-MOP; however, further studies need to confirm this.
Apiaceous vegetables (which includes carrots, celery, parsnips, parsley, dill, cilantro, etc.) are rich in furanocoumarins psoralen, 5-MOP, and 8-MOP [54
], but there has been no hepatotoxicity report associated with these vegetables to present. In contrast, Bu-gu-zhi (the recommended dose is 6–15 g/day, with psoralen and isopsoralen as the main ingredients) has four case reports of liver injury, and one case of acute cholestatic hepatitis associated with the use of Bu-gu-zhi over 10 times the usual dosage [52
]. To present, there are no data on the therapeutic or toxic dosage of psoralen in humans. In animal studies of oral psoralen or isopsoralen 40 mg daily for 28 days in mice, liver injury by psoralen or isopsoralen was associated with elevated serum ALT, AST, and/or ALP levels [55
]. As a mechanism exploration study, a psoralen and isopsoralen dose of 300 mg/kg was set at approximately one-half the oral LD50 value (615 mg/kg in mice), and liver injury was observed as mild pathology damage (granular degeneration) after a single oral psoralen and isopsoralen dose. Together, all the studies suggest that both psoralen and isopsoralen are modest hepatotoxic agents, which can be partly explained by the fact that furanoepoxide or the γ-ketoenal intermediate can be deactivated with H2
O and GSH in liver in the present study. This also provides support for the safe ingestion of apiaceous vegetables, while other factors (such as psoralens content in vegetables) should also be considered. In addition, the co-administration of GSH could be considered while taking psoralens or psoralen-containing plants in order to avoid or relive liver injury in a clinical setting. The present study identified the reactive furanoepoxide or γ-ketoenal intermediate produced by the CYP450-mediated metabolism of psoralen and isopsoralen, and then how it binds to CYPs or other macromolecules. Thus, we believe that psoralen and isopsoralen may cause idiosyncratic drug-induced liver injury, which is responsible for hepatotoxicity in mice.
The ethanol extracts of the seeds of Bu-gu-zhi have been proposed as a food additive for the preservation of some processed foods in Japan. Takizawa et al found that long-term (90 days) ingestion of the extracts of the seeds of Bu-gu-zhi showed testicular toxicity in rats [56
]. Interestingly, CYP2D4/18 mRNAs are expressed in rat testis [57
], while the rat and human CYP2D isoforms share a high sequence identity (>70%) [58
]. Then, it is presumed that the reactive metabolites are maybe a plausible cause for Bu-gu-zhi rat testicular toxicity, while the related toxicity in humans and its underlying mechanism need to be further considered.
According to clinical experience, the chemical composition and pharmacological activity of traditional Chinese medicine is very complicated, and it is very important to avoid long term treatment and inappropriate medication to prevent liver injury from Traditional Chinese Medicine (TCM), but there are few precise and effective ways to solve this problem. Thus, we believe RUCAM can play a bigger role in providing prediction for safe and rational drug use [8
4. Materials and Methods
4.1. Inhibitory Effects of Psoralen and Isopsoralen on CYP3A4
Psoralen, isopsoralen, phenacetin, acetaminophen, and midazolam maleate were obtained from the National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). β-nicotinamide adenine dinucleotide phosphate (NADPH), dextromethorphan, dextrorphan, and 1-hydroxymidazolam were purchased from Sigma Chemical Co. (St. Louis, MO, USA). l -buthi-onine-S,R-sulfoxinine (BSO) was obtained from Acros Organics (New Jersey, NJ, USA), and 1-Aminobenzotriazole (ABT) was purchased from Tokyo Chemical Industry Co., LTD (Tokyo, Japan). All other reagents were of analytical or high-performance liquid chromatography grade. Pooled human liver microsomes (HLM), which were purchased from the Research Institute for Liver Diseases Co., Ltd. (Shanghai, China), were prepared from human liver tissues under Chinese organ donation regulations with the full consent of the patients. Mice liver microsomes (MLM) were purchased from the Research Institute for Liver Diseases Co., Ltd. (Shanghai, China). Liver microsomal samples were stored at −80 °C until further use.
4.2. MBI Evaluation of Psoralen and Isopsoralen
IC50 shift assay. In the present study, an IC50 shift assay was performed using a two-step procedure. First, psoralen and isopsoralen at seven concentrations (from 2.5 to 100 µM, except for a psoralen test of 200 µM on CYP3A4) were pre-incubated with HLM for 30 min at 37 °C with/without (+/−) NADPH. After pre-incubation, a 0.02 mL aliquot of the mixture was added to a mixture containing a standard CYP substrate (final concentrations at the Km) in 0.18 mL of potassium phosphate buffer to measure CYP activities. After incubation for 15 min at 37 °C, the reactions were ceased by adding four volumes of pre-cooled methanol. The supernatants were centrifuged at 14,000 rpm for 15 min and then stored at −20 °C until the LC/MS/MS analysis of metabolites.
4.3. Effects of Scavenging Agents (GSH) on Inhibition Ability of Psoralen and Isopsralen
In the present study, GSH was applied to interfere with the inhibition ability of psoralen and isopsralen (at IC50 concentration as detected in IC50 shift assay) on CYP3A4, and the incubation procedure was the same as for the “IC50 shift assay (with NADPH)” experiment, except for the addition of GSH (5 mM at final incubations).
Analysis of samples. The LC/MS/MS method was conducted as described in [59
4.4. Metabolism of Psoralen and Isopsoralen in Liver Microsome
Phase I Metabolism. Psoralen or isopsoralen (50 μM) was suspended in 150 μL of buffer containing 0.1 M potassium phosphate (pH 7.4) and HLM (or MLM) (1.0 mg/mL) and was pre-incubated at 37 °C for 5 min. Subsequently, 150 μL of NADPH solution (final concentration of 1 mM) was added to each reaction mixture. After incubation at 37 °C for 90 min in a shaking water bath, the reaction was terminated by adding 600 μL of ice-cold methanol. The contents were vortexed for 30 s and then centrifuged at 4000 rpm for 15 min. The metabolites of psoralen and isopsoralen in the supernatant were identified by UPLC-MS/MS.
GSH capture of reactive metabolites. The incubation method was the same as for Phase I Metabolism, execpt for the addition of GSH (final concentration of 5 mM).
Metabolite analysis by UPLC-MS/MS. The UPLC/MS/MS method was carried out using a Waters Acquity UPLC Sample Manager and a Waters Acquity UPLC Binary Solvent Manager connected to a Waters Quattro Premier XE triple-quadruple mass spectrometer equipped with a combined ESI probe and Mass Lynx 1.4 software (Waters, Milford, MA, USA). An Acquity UPLC BEH C18 column (1.7 µm, 2.1 mm × 100 mm) was used. The instrument settings were as follows: ESI+; source temperature, 120 °C; dissolution temperature, 350 °C; capillary voltage, 3.2 kV; dissolution N2, 600 L/h; cone N2, 50 L/h. Acquisition was carried out from m/z 50 Da to 1000 Da in the total ion scan mode. MS/MS spectra were acquired from m/z 50 Da to 600 Da in the daughter ion scan mode. The solvent system consisted of solvent A (0.1% CH3COOH) and solvent B (100% ACN). The flow rate was 0.2 mL/min. The gradient elution method was 5 to 80% B over 12 min, followed by 5% B from 12 to 15 min.
Quantitation analysis of psoralen and isopsoralen metabolites by UPLC-MS/MS. The UPLC/MS/MS MRM method was developed to permit the detection and quantitation of psoralen (or isopsoralen) metabolites based on structurally specific fragmentation obtained from collision-induced dissociation: M1, M2, M′1, or M′2 m/z 203.3→147.1; M3 m/z 219.4→157.5; M′3 205→177.1; M4, M5, M′4, or M′5 m/z 221→174.9; GSH-reactive metabolite adducts m/z 510→363.1. The HPLC method and electrospray ionization were performed with the same conditions as those for metabolite analysis by UPLC-MS/MS in liver microsomes, except for the cone (30 V) and collision energy (20 V).
4.5. Animals, Treatment, and Sampling
For the mechanism exploration study, a psoralen and isopsoralen dose of 300 mg/kg was set at approximately one-half the oral LD50 value (615 mg/kg in mice) in the present study. Male 6-week-old KunMing (KM) mice were purchased from the Animal Institute of the Tianjin University of Traditional Chinese Medicine (Tianjin, China), and were used after a 1-week acclimatization period. The animals were housed in mice cages in a unidirectional airflow room under controlled conditions (20–24 °C, 12:12-h light:dark cycles, and free access to water). We assured that all animals received humane care according to the criteria outlined in the “Guide for the Care and Use of Laboratory Animals” prepared by the National Academy of Sciences and published by the National Institutes of Health ((NIH) publication 86–23 revised 1985).
In the hepatotoxicity study, the mice were randomly assigned to 22 groups (n = 10/group). The mice were intraperitoneally injected with BSO (650 mg/kg) in saline (0.2 mL/20 g). The intraperitoneal dosing of ABT (100 mg/kg) in saline (0.2 mL/20 g) was simultaneous with the BSO dosing. One hour later, the mice were orally administered psoralen and isopsoralen (300 mg/kg) in 0.5% carboxymethylcellulose sodium (0.2 mL/20 g). Food was removed 14–16 h before BSO dosing, and was supplied again 6 h after administration of the test compounds. At 6 and 24 h, blood was collected by eyeball removal, and then the liver was removed. Plasma was separated from these blood samples, and alanine aminotransferase (ALT) was determined with a commercial kit, the GPT-UV test (Nanjing jiancheng Bioengineering Institute). A piece of each liver sample was fixed in 10% formalin, embedded in paraffin wax, sectioned, and stained with hematoxylin and eosin for histopathological evaluation. For the groups treated with psoralen/isopsoralen in the absence/presence of ABT, or groups treated with BSO alone, the hepatic GSH has been determined.
4.6. Molecular Docking Simulation
Ligand structure preparation. The structures of psoralen (ID: 6199) and isopsoralen (ID: 10,658) were downloaded from the PubChem database. Combining a recent report and our above study [20
], the structure of furanoepoxide and γ-ketoenal intermediates were edited from ChemDraw Ultra 16.0
). All compounds were prepared and optimized with a CharMm force field by using the Ligand Preparation module of Discovery Studio software 4.5
Protein Structure Preparation. Here, the X-ray crystallographic structures of the human CYP3A4 (Protein Data Bank code: 1W0F) were obtained from the Protein Data Bank. For the docking study, a protein preparation module was utilized to optimize the structure of CYP3A4 by protonating, removing water, building missing side chains, and then adding a forcefield CharMm.
Molecular Docking simulation. This process was carried out by Cdocker module, a grid-based molecular docking method that employs CharMm [60
]. With Cdocker, the initial ligand conformations were sampled via high temperature molecular dynamics and were also allowed to flex during the refinement (via simulated annealing Moleculer Dynamic). Moreover, Cdocker also provided a physics-based scoring function via the CharMm energy of the docked complex. Cdocker has been shown to give highly accurate docked poses [61
The conformations of small molecules were flexible, while protein atoms were fixed to their positions of Discovery Studio 4.5. Every ligand was put into an active site. To ensure the docked poses were diverse, we set the Pose Cluster Radius to 0.5 Å, and the rest of the parameters were set to their defaults. After varying the model, a 1000-step algorithm minimization was carried out to obtain a low-energy conformation without steric clashes in protein structure. The result was evaluated by an empirical function −Cdocker energy and iron–ligand distance was recorded.
4.7. Data Analysis
IC50 value calculation. The IC50 value was calculated using GraphPad Prism (version 4.0) software (GraphPad Software, San Diego, CA, USA).
Statistical analysis. The results from the serum ALT assay were analyzed by Kruskal–Wallis non-parametric analysis of variance followed by Mann–Whitney’s U test. For the data of the GSH assay and CYP450s relative activity, comparisons between the two groups were made by independent-samples t test. SPASS 11.5 was used for these statistical analyses. Differences were considered significant when p < 0.05 or p < 0.01.