Inhibitory Effect of Selaginellins from Selaginella tamariscina (Beauv.) Spring against Cytochrome P450 and Uridine 5′-Diphosphoglucuronosyltransferase Isoforms on Human Liver Microsomes

Selaginella tamariscina (Beauv.) has been used for traditional herbal medicine for treatment of cancer, hepatitis, and diabetes in the Orient. Numerous bioactive compounds including alkaloids, flavonoids, lignans, and selaginellins have been identified in this medicinal plant. Among them, selaginellins having a quinone methide unit and an alkylphenol moiety have been known to possess anticancer, antidiabetic, and neuroprotective activity. Although there have been studies on the biological activities of selaginellins, their modulatory potential of cytochrome P450 (P450) and uridine 5′-diphosphoglucuronosyltransferase (UGT) activities have not been previously evaluated. In this study, we investigated the drug interaction potential of two selaginellins on ten P450 isoforms (CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 2J2 and 3A) and six UGT isoforms (UGT1A1, 1A3, 1A4, 1A6, 1A9 and 2B7) using human liver microsomes and liquid chromatography-tandem mass spectrometry. Selaginellin and selaginellin M had high inhibitory potential for CYP2C8-mediated amodiaquine O-demethylation with IC50 values of 0.5 and 0.9 μM, respectively. Selaginellin and selaginellin M also showed medium inhibitory potential against CYP2C9, CYP2J2, UGT1A1, and UGT1A3 (1 μM < IC50 < 5 μM). These two selaginellins had low inhibitory potential against CYP1A2, CYP2A6, CYP2E1, and UGT1A6 (IC50 > 25 μM). This information might be helpful to predict possible drug interaction potential of between selaginellins and co-administered drugs.

Use of botanical drugs to prevent common disease is on the rise among the global population [15]. Since botanical drugs share the same drug metabolizing enzymes with commonly used commercial drugs, the potential for herb-drug interaction is substantial [16]. Several medicinal herbs and foods, including St. John's wort [17] and grapefruit juice [18] as well as their active constituents (hyperforin [19] and bergamottin [20]) have been reported to cause severe drug interactions. Undoubtedly, the early evaluation of herb-drug interactions is necessary to prevent potential dangerous clinical outcomes.
Modulation of drug-metabolizing enzymes is one of the important causes of drug-drug or herbdrug interaction. Among the numerous drug-metabolizing enzymes, cytochrome P450s (P450s) and uridine 5′-diphosphoglucuronosyltransferases (UGTs), which are responsible for the metabolic clearance of 90% of commercial drugs, have been shown to a play key roles in drug metabolism and drug interactions [21]. For example, bergamottin is reported to increase the blood concentration of drugs through inhibition of hepatic CYP3A activity, thereby enhancing the toxicity of drugs such as atorvastatin, felodipine, and verapamil [22]. Pre-treatment with psoralidin, which has inhibitory potential against UGT1A1-mediated SN-38 glucuronidation (Ki = 5.8 μM), was shown to increase the toxicity of irinotecan [23]. Accordingly, P450-and UGT-mediated drug interactions are even more critical.
Therefore, modulation of selaginellins on P450 and UGT activities may result in potential increase of the systemic exposures of co-administered drugs. To the best of our knowledge, however, no previous study has reported the modulatory effects of selaginellins against human P450 and UGT activities. Here, we investigated the inhibitory potential of two selaginellins ( Figure 1) on ten P450and six UGT-isoform activities in human liver microsomes (HLMs) using cocktails of P450 or UGT probe substrates to evaluate the possibility of drug interactions of selaginellins.

Results and Discussion
In the present study, we investigated the inhibitory effect of two selaginellins against ten cytochrome P450 isoforms and six UGT isoforms using human liver microsomes ( Figure 2). The results showed that selaginellin inhibited most of the P450 and UGT isoforms tested in a concentration-dependent manner. The inhibitory potential of selaginellins is categorized into high (IC50 < 1 μM), medium (1 μM < IC50 < 10 μM), and low (IC50 > 10 μM) classes based on Krippendorff's criteria [24].

Results and Discussion
In the present study, we investigated the inhibitory effect of two selaginellins against ten cytochrome P450 isoforms and six UGT isoforms using human liver microsomes ( Figure 2). The results showed that selaginellin inhibited most of the P450 and UGT isoforms tested in a concentration-dependent manner. The inhibitory potential of selaginellins is categorized into high (IC 50 < 1 µM), medium (1 µM < IC 50 < 10 µM), and low (IC 50 > 10 µM) classes based on Krippendorff's criteria [24].
The effects on CYP1A1, CYP2A6, CYP2E1, and UGT1A6 activities were assumed to be a negligible (IC 50 > 25 µM) ( Table 1). These findings suggest that clinical interactions between these compounds and CYP1A1, CYP2A6, CYP2E1, or UGT1A6 would not occur.
Selaginella tamariscina (Beauv.) Spring has been used for centuries as a Traditional Chinese Medicine to treat various human diseases, including inflammation, human cancer, and hyperglycemia [33]. Therefore, it might be used with anticancer or antidiabetic drugs which are metabolized by CYP2C8 (paclitaxel), CYP2C9 (tolbutamide), or UGT1A1 (irinotecan) [32]. Selaginellins should be used carefully with these drugs to avoid drug interactions in cancer and diabetic patients.

Inhibitory Effects of Selaginellins on UGT Activity
The inhibitory effects of two selaginellins on the metabolism of six UGT probe substrates were evaluated using previously reported method with minor modification [34]. In brief, HLMs (0.25 mg/mL) were activated by incubation in the presence of alamethicin (25 µg/mL) for 15 min on ice. After the addition of UGT isoform-selective substrates and selaginellins (0, 0.5, 2, 5, 20 and 50 µM) the final concentrations of organic solvent (methanol) for the cocktail incubation conditions were 1.0% (v/v). The incubation reaction mixtures were pre-incubated for 5 min. The reaction was initiated by the addition of 5 mM UDPGA and further incubated for 60 min. All reactions were terminated by adding ice-cold acetonitrile containing estrone glucuronide (IS). After mixing and centrifugation, aliquots were injected into a LC-MS/MS as described previously [34] (Table 2). All microsomal incubations were performed in triplicate.

Data Analysis
The IC 50 values (concentration of the inhibitor causing 50% inhibition of the original enzyme activity) were calculated using WinNonlin software (Pharsight, Mountain View, CA, USA): percentage of control activity = 100 × [1 − (I/(I + IC 50 ))], where I is the inhibitor concentration, and IC 50 is the inflection point on the curve [34].