UPLC-MS/MS analysis of dextromethorphan-O-demethylation kinetics in rat brain microsomes

A High-Performance Liquid Chromatography with Photodiode Array Detection Method for Simultaneous Determination of Three Compounds Isolated from Wikstroemia ganpi: Assessment of the Effects on Cytochrome P450-Mediated Metabolism In Vitro and In Vivo

In natural products, the content and quality of the marker components differ depending on the part, production area, collection period, and extraction method; therefore, a standardized analysis method is required to obtain consistent results. This study developed a simultaneous analysis method for three marker components (7-methoxylutolin-5-O-glucoseide, pilloin 5-O-β-d-glucopyranoside, rutarensin) isolated and purified from Wikstroemia ganpi (W. ganpi). Simultaneous analysis was performed using high-performance liquid chromatography with photodiode array detection (HPLC-PDA) method that was validated according to the International Council for Harmonisation (ICH) guidelines. The developed analytical method exhibited linearity (r2 > 0.999), detection limits (0.72-3.34 μg/mL), and quantification limits (2.19-10.22 μg/mL). The relative standard deviation (RSD) value of intra- and inter-day precisions was less than 1.68%, and analyte recoveries (93.42-117.55%; RSD < 1.86%) were validated according to the analytical procedures, and all parameters were within the allowable range. Quantitative analysis of the three marker components from W. ganpi MeOH extract (WGM) showed 7-methoxylutolin-5-O-glucoseide with the highest content (51.81 mg/g). The inhibitory effects of WGM on cytochrome P450 (CYP) substrate drugs were further investigated. The in vitro study revealed that WGM inhibited the CYP3A-mediated metabolism of buspirone and that 7-methoxylutolin-5-O-glucoseide and pilloin 5-O-β-d-glucopyranoside inhibited the metabolism of buspirone with IC50 values of 2.73 and 18.7 μM, respectively. However, a single oral dose of WGM did not have significant effects on the pharmacokinetics of buspirone in rats, suggesting that WGM cannot function as an inhibitor of CYP3A-mediated metabolism in vivo.

Differential expression and activities of cytochrome P450 3A in the rat brain microsomes and mitochondria

Midazolam (MDZ), a benzodiazepine derivative, is metabolized to 1'- and 4-hydroxylated metabolites (1'-OH-MDZ and 4-OH-MDZ, respectively) by cytochrome P450 3A (CYP3A). The purpose of this study was to investigate the CYP3A-mediated hydroxylation of MDZ in the rat brain mitochondria (MT). Brain microsomes (MC) and MT fractions were prepared from rats (n = 8) using differential and density gradient centrifugations, and the purity of the fractions was evaluated using VDAC1 and calreticulin as markers of MT and MC, respectively. The formation rates of 1'-OH-MDZ and 4-OH-MDZ in the rat brain MC and MT samples were determined using an LC-MS/MS method after validation. Subsequently, Michaelis-Menten kinetics of 1'- and 4-hydroxylation of MDZ were estimated. Western blot (WB) analysis was used to determine the protein expression of CYP3A in the rat brain MC and MT. The MC fractions had 5.93% ± 3.01% mitochondrial impurity, and the MT fractions had 19.3% ± 7.8% microsomal impurity (mean ± SD). The maximum velocity (V_max ) values of the formation of the hydroxylated metabolites in the brain MT were 2.4-9-fold higher than those in MC. Further, the V_max values of 4-OH-MDZ in both MC and MT fractions were substantially higher than those of 1'-OH-MDZ. The WB analysis showed that the intensity of the CYP3A immunoreactive band in MT was more than twofold higher than that in MC. It is concluded that compared with MC, rat brain MT contains substantial CYP3A, which may affect the pharmacology or toxicology of centrally acting xenobiotic and endogenous substrates of this enzyme.

A Comparison of Calcium Aggregation and Ultracentrifugation Methods for the Preparation of Rat Brain Microsomes for Drug Metabolism Studies

Preparation of brain microsomes by the calcium chloride aggregation method has been suggested as an alternative to the ultracentrifugation method. However, the effects of the calcium chloride concentration on the quality of the microsomal fractions are not known. Brain microsomes were prepared from the adult rat brains using the high-speed ultracentrifugation and low-speed calcium chloride (10-100 mM) aggregation methods (n = 5-6 per group). The microsomal protein yield (spectrometry), the cytochrome P450 reductase (CPR) activity (spectrometry), and the monooxygenase activities (UPLC-MS/MS) of CYP2D and CYP2E1 were determined in the obtained fractions. Increasing the concentrations of calcium chloride progressively increased the protein yield of the low-speed microsomal fractions. However, the increased yield was associated with a significant decrease in the activities of CPR, CYP2D, and CYP2E1. Additionally, the CYP2D and CYP2E1 activities were significantly correlated with the CPR activities of the fractions. In conclusion, when an ultracentrifuge is available, preparation of brain microsomes by the ultracentrifugation method might be preferable. However, the calcium aggregation method at a calcium chloride concentration of 10 mM is an acceptable alternative to the ultracentrifuge method.

In vivo and in vitro metabolism study of traditional Chinese medicine formula Dingkun Dan in rats by using ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry

Dingkun Dan (DKD), a reputable traditional Chinese medicine formula, has been used to treat gynecological diseases and showed significant clinical effects since ancient times. However, the application and development of DKD are seriously hampered by the unclear active substances. Structural characterization of compounds absorbed in vivo and their corresponding metabolites is significant for clarifying the pharmacodynamic material basis. In this study, an integrated strategy using ultra-performance liquid chromatography, coupled with quadrupole time-of-flight mass spectrometry and UNIFI™ software, was used to identify prototypes and metabolites after oral administration of DKD in rats. As a result, a total of 261 compounds, including 140 prototypes and 121 metabolites, were tentatively characterized in rat plasma, urine, and feces. The metabolic pathways of prototypes have been studied to clarify their possible transformation process in vivo. Moreover, an in vitro metabolism study was applied for verifying the metabolites under simulating the metabolic environment in vivo. This first systematic metabolic study of DKD is important for elucidating the metabolites and metabolic pathways and could provide a scientific basis for explaining the integrative mechanism in further pharmacology study.

UPLC-MS/MS analysis of the Michaelis-Menten kinetics of CYP3A-mediated midazolam 1'- and 4-hydroxylation in rat brain microsomes

Midazolam (MDZ) is a short-acting benzodiazepine with rapid onset of action, which is metabolized by CYP3A isoenzymes to two hydroxylated metabolites, 1'-hydroxymidazolam and 4-hydroxymidazolam. The drug is also commonly used as a marker of CYP3A activity in the liver microsomes. However, the kinetics of CYP3A-mediated hydroxylation of MDZ in the brain, which contains much lower CYP content than the liver, have not been reported. In this study, UPLC-MS/MS and metabolic incubation methods were developed and validated for simultaneous measurement of low concentrations of both hydroxylated metabolites of MDZ in brain microsomes. Different concentrations of MDZ (1-500 µM) were incubated with rat brain microsomes (6.25 µg) and NADPH over a period of 10 min. After precipitation of the microsomal proteins with acetonitrile, which contained individual isotope-labeled internal standards for each metabolite, the analytes were separated on a C₁₈ UPLC column and detected by a tandem mass spectrometer. Accurate quantitation of MDZ metabolism in the brain microsomes presented several challenges unique to this tissue, which were resolved. The optimized method showed validation results in accordance with the FDA acceptance criteria, with a linearity ranging from 1 to 100 nM and a lower limit of quantitation of 0.4 pg on the column for each of the two metabolites. The method was successfully used to determine the Michaelis-Menten (MM) kinetics of MDZ 1'- and 4-hydroxylase activities in rat brain microsomes (n = 5) for the first time. The 4-hydroxylated metabolite had 2.4 fold higher maximum velocity (p < 0.01) and 1.9 fold higher (p < 0.05) MM constant values than the 1'-hydroxylated metabolite. However, intrinsic clearance values of the two metabolites were similar. The optimized analytical and metabolic incubation methods reported here may be used to study the effects of various pathophysiological and pharmacological factors on the CYP3A-mediated metabolism of MDZ in the brain.

Using Estrogenic Activity and Nontargeted Chemical Analysis to Identify Contaminants in Sewage Sludge

Diverse organic compounds, many derived from consumer products, are found in sewage sludge worldwide. Understanding which of these poses the most significant environmental threat following land application can be investigated through a variety of predictive and cell-based toxicological techniques. Nontargeted analysis using high-resolution mass spectrometry with predictive estrogenic activity modeling was performed on sewage sludge samples from 12 wastewater treatment plants in California. Diisobutyl phthalate and dextrorphan were predicted to exhibit estrogenic activity and identified in >75% of sludge samples, signifying their universal presence and persistence. Additionally, the application of an estrogen-responsive cell bioassay revealed reductions in agonistic activity during mesophilic and thermophilic treatment but significant increases in antagonism during thermophilic treatment, which warrants further research. Ten nontarget features were identified (metoprolol, fenofibric acid, erythrohydrobupropion, oleic acid, mestranol, 4'-chlorobiphenyl-2,3-diol, medrysone, scillarenin, sudan I, and N,O-didesmethyltramadol) in treatment set samples and are considered to have influenced the in vitro estrogenic activity observed. The combination of predictive and in vitro estrogenicity with nontargeted analysis has led to confirmation of 12 estrogen-active contaminants in California sewage sludge and has highlighted the importance of evaluating both agonistic and antagonistic responses when evaluating the bioactivity of complex samples.

Kinetics of dextromethorphan-O-demethylase activity and distribution of CYP2D in four commonly-used subcellular fractions of rat brain

The purpose of this study was to compare the enzymatic kinetics and distribution of cytochrome P450 2D (CYP2D) among different rat brain subcellular fractions. Rat brains were used to prepare total membrane, crude mitochondrial, purified mitochondrial, and microsomal fractions, in addition to total homogenate. Michaelis-Menten kinetics of the brain CYP2D activity was estimated based on the conversion of dextromethorphan (DXM) to dextrorphan using UPLC-MS/MS. Protein levels of CYP2D and subcellular markers were determined by Western blot. Microsomal CYP2D exhibited high affinity and low capacity, compared with the mitochondrial CYP2D that had a much lower (∼50-fold) affinity but a higher (∼six-fold) capacity. The apparent CYP2D affinity and capacity of the crude mitochondria were in between those of the microsomes and purified mitochondria. Additionally, the CYP2D activity in the whole homogenate was much higher than that in the total membranes at higher DXM concentrations. A CYP2D immune-reactive band in the brain mitochondria appeared at a lower MW but had a much higher intensity than that in the microsomes. Mitochondrial brain CYP2D has a much higher capacity than its microsomal counterpart. Additionally, brain homogenate is more representative of the overall CYP2D activity than the widely-used total membrane fraction.