Simultaneous Determination of Kaempferide , Kaempferol and Isorhamnetin in Rat Plasma by Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry and its Application to a Pharmacokinetic Study

A selective, rapid and sensitive method using ultra-high performance liquid chromatography was developed and used for the simultaneous determination of kaempferide, kaempferol, and isorhamnetin in rat plasma after oral administration of Sedum sarmentosum Bunge extract. A 6430 triple-quadrupole tandem mass spectrometer in multiple reaction monitoring mode, with a negative electrospray ionization source, was used for the detection. A Waters Symmetry C18 column with isocratic elution (methanol:5 mM ammonium acetate at 75:25, v/v) was used for separation at a flow rate of 0.3 mL min. The method was linear for all analytes over the investigated concentration range (r > 0.9900). The intraand inter-day precisions ranged between 3.88 and 7.74% and the accuracies ranged between 88.63 and 98.74%. The mean recoveries and matrix effect were higher than 85.66 and 92.36%, respectively. All the three compounds tested were stable during storage and analyses. The method was successfully applied to a pharmacokinetic study.


Introduction
Traditional Chinese medicine (TCM) has been used for thousands of years in China and other Asian countries.2][3][4][5] It was also officially listed in the 2015 edition of the Chinese Pharmacopoeia. 6Previous pharmacological studies revealed that SSB can inhibit the production of inflammatory exudates and has significant anti-inflammation, anti-tumor and anti-viral activity. 7,8he aqueous extract exhibits anti-proliferative activity against HepG2 cells by down-regulating the expression of Bcl-2, vascular endothelial growth factor (VEGF), and phosphorylated signal transducer and activator of transcription 3 (pSTAT3) and by inducing apoptosis. 9,10dditionally, in vivo and in vitro renal anti-fibrotic effects suggested that the extract may have therapeutic potential for fibrotic kidney diseases. 11,12hytochemical investigations have shown that this plant contains various components including alkaloids, sarmentosin, flavonoids and triterpenoid among others. 13,14ome investigations have shown that sarmentosin is the active constituent in hepatitis therapy, but its chemical properties were unstable. 10,15Recently, researchers isolated several bioactive flavonoids such as isorhamnetin, kaempferide, kaempferol and quercetin that exhibit pharmacological activities.Isorhamnetin is an O-methylated flavonol, a chemical compound derived from several plants that exhibits anti-inflammatory, anti-oxidant and anti-cancer properties. 16,17][20] Although some methods including high-performance liquid chromatography (HPLC) or liquid chromatographymass spectrometry (LC-MS) have been reported for the detection of kaempferide, kaempferol or isorhamnetin in rat plasma, [21][22][23][24] simultaneous determination of these three constituents in plasma samples after oral administration of Sedum sarmentosum Bunge extract (SSBE) has not been reported.Thus, it was important to develop a method to investigate the pharmacokinetic parameters of SSBE.In this study, a fast, sensitive and simple ultrahigh-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was validated for the determination of the three active compounds in plasma and applied to a pharmacokinetic study using rat plasma.

Reagents and materials
The reference standards of isorhamnetin (purity > 98.0%), kaempferide (purity > 98.0%), and kaempferol (purity > 98.0%) were purchased from Chengdu Must Bio-Technology Co., Ltd.(Chengdu, Sichuan, China).Tinidazole (purity > 99.9%) was used as an internal standard (IS) and purchased from the National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China).SSB was obtained from Suzhou Tianling Chinese Herbal Medicine Co. Ltd. (Jiangsu, China) and identified by Professor Jianwei Chen in Nanjing University of Chinese Medicine.β-Glucuronidase and sulfatase were supplied by Sigma-Aldrich (St. Louis, MO, USA).Methanol and acetonitrile were of HPLC grade (Merck, USA).All other reagents were of AR (analytical reagent) grade.Ultrapure water used for the UHPLC-MS/MS was from Milli-Q system (Millipore, Billerica, MA, USA).

UHPLC-MS/MS instrumentation and chromatographic conditions
The Agilent UHPLC 1290 system (Agilent, USA) consisting of a quaternary pump, an auto-sampler, and an online degasser was used to perform the analysis.The chromatographic separation was performed on a Waters Symmetry C18 reversed phase analytical column (50 × 2.1 mm, 3.5 μm).Methanol:water (75:25; v/v) containing 5 mM ammonium acetate solvent system was used as the mobile phase.The flow rate was 0.3 mL min -1 and the sample injection volume was 2 μL.The temperature of the column and auto-sampler was maintained at 35 and 4 °C, respectively.The total elution time was 2 min.
The UHPLC-MS/MS system consisted of G6430 tandem quadrupole mass spectrometer, with turbo ion spray ionization as the electro spray ionization (ESI) source, operated in a negative ionization mode, using multiple reaction monitoring (MRM) to monitor precursor to product ion transition.The ion MS of isorhamnetin, kaempferide, kaempferol and IS were given in Figure 1.All analytical data were processed using MassHunter software, version B.05.00 (Agilent).

Procedure for sample extraction
All samples were thawed at room temperature before use.50 μL of plasma samples and 10 μL IS solution (100 ng mL -1 ) were transferred to a 1.5 mL centrifuge tube.Next, 50 μL β-glucuronidase (activity 1500 U mL -1 ) hydrolase and 50 μL sulfatase (200 U mL -1 ) hydrolase were added to each plasma sample.After being subjected to a heat shock at 37 °C for 60 min, 800 μL of ethyl acetate/ acetone solvent (1:1) was added to the mixture, which was vortexed for 3 min and centrifuged for 5 min at 12,000 rpm.Subsequently, the supernatant was dried under 40 °C nitrogen, and re-dissolved in 100 μL 50% methanol.Finally, 2 μL supernatant was injected into the UHPLC-MS/MS system for analysis.

Method validation
The method was verified adequately for specificity, selectivity, carryover, linearity (r), extraction recovery, matrix effect (ME), precision, accuracy and stability.

Specificity and selectivity
Specificity was determined by comparing the retention times of six blank plasma samples and blank plasma spiked with kaempferide, kaempferol, isorhamnetin, and IS from the respective chromatograms.Additionally, 2 μL working solution was diluted with mobile phase and injected to check for interference.

Carryover
Carryover was evaluated by three injections of an upper limits of quantification (ULOQ) sample of the calibration curve, immediately followed by three injections of a blank plasma sample.Carryover was considered acceptable if the mean peak area counts of analytes and ISs were not more than 20% for analytes, and 5% for IS, compared to the area counts in the lower limits of quantification (LLOQ) sample.

Linearity and LLOQ
In order to assess the linearity for each analyte, seven nonzero concentrations of calibration samples were determined to establish the calibration curves.Blank plasma samples were analyzed to discard the presence of interferences.The linearity was confirmed through the comparison of the ratio of the peak area of the analytes to that of the IS solution with the analyte concentrations through least squares linear regression analysis, which is described in the form of Y = aX + b (weighting factor = 1/x 2 ).
The LLOQ defined as the amount detected with a signal-to-noise ratio of 10, was determined in five replicates with a precision of less than 20% using the relative standard deviation (RSD) and an accuracy between 80 and 120% of the spiked concentration.

ME and extraction recovery
The ME was measured by comparing the peak area of an analyte added into post-extracted blank plasma samples with the average peak area obtained from corresponding standard solution of the analyte freshly prepared in the reconstitution solution.Meanwhile, the extraction recovery for kaempferide, kaempferol, and isorhamnetin was determined by comparing the peak area of the analyte in extracted samples with the average peak area of the samples spiked with the analyte after extraction.The experiments were performed at three QC levels in six different batches.

Precision and accuracy
The intra-batch precision and accuracy for kaempferide, kaempferol, and isorhamnetin were evaluated using five replicates at three QC levels in the same analytical run.Inter-batch precision and accuracy were evaluated through three different analytical runs by repeated analysis.The concentrations were determined from the standard calibration curve.The accuracy was expressed as percentage relative error (RE, %), and the precision was defined as relative standard deviation (RSD, %).Accuracy is required to be within ± 15% and precision should not exceed 15% for QC samples.

Stability
Kaempferide, kaempferol, and isorhamnetin stability in rat plasma was determined by assessing three replicates in different storage conditions as follows: (i) stability during sample preparation was assessed by keeping the samples at room temperature for 6 h; (ii) freeze-thaw stability for the plasma samples was analyzed over three freeze (-20 °C) and thaw (room temperature) cycles; (iii) stability of the treated plasma samples in the auto-sampler was evaluated by preparing and storing QC samples at the auto-sampler temperature (4 °C) for 24 h, and then injected for analysis; (iv) the long-term stability was determined by assaying the plasma at -20 °C for at least one month.

Pharmacokinetic and data analysis
Eight male Sprague-Dawley (SD) rats were used in the pre-clinical pharmacokinetic study of SSBE.The experiments were carried out under the supervision of the Animal Ethics Committee of Nanjing University of TCM, 14008.All animals were pathogen-free and kept in an environmentally controlled room (24 ± 1 °C and 12/12 h light/dark cycle) for at least one week, with free access to standard chow and water.SD rats were given a single oral dose of 0.2 g kg -1 of SSBE.While water intake was permitted, the rats were subjected to a 12 h fast prior to the experiment.Plasma samples were collected at 0, 0.083, 0.167, 0.25, 0.5, 0.75, 1, 1.5, 2, 4, 8, 12, 24 h after oral administration of SSBE.Blood samples (150 μL) were collected into heparinized tubes through the fossa orbitalis vein and immediately centrifuged at 4000 rpm for 10 min.The separated plasma samples were collected into labeled plastic vials and stored at -20 °C until analysis.
To calculate the pharmacokinetic parameters including compartment and statistic parameters, the data were analyzed by Drug and Statistic (DAS) 3.0 pharmacokinetic software (Chinese Pharmacological Association, Anhui, China).Data were determined as the mean ± standard deviation (SD) with parallel determination of three copies.

Results and Discussion
Optimization of UHPLC-MS/MS conditions MS conditions were optimized to obtain more stable and stronger signal intensities for kaempferide, kaempferol, isorhamnetin, and IS.We chose the positive and negative scan modes to evaluate ion signal, which showed that the negative ion mode was superior to the positive ion mode and the product ions at m/z 299.0 → 284.0 for kaempferide, m/z 285.0 → 93.0 for kaempferol, m/z 315.0 → 300.1 for isorhamnetin, and m/z 245.9 → 125.9 for IS.
The source parameters of the mass spectrometer were set as follows: capillary voltage 4.0 kV, gas temperature 400 °C and gas flow 10 L min -1 .The dependent parameters of the compounds, such as fragmentor and collision energy, were respectively optimized as follows: 120 and 15 V for kaempferide, 140 and 10 V for kaempferol, 140 and 15 V for isorhamnetin, 110 and 20 V for IS.Dwell time was set as 200 ms for all analytes and IS.
Chromatography conditions were investigated to obtain a higher response, sensitivity, and a short run-time.Several mobile phase conditions such as acetonitrile-water and methanol-water, and different buffer solutions such as ammonium formate (2 and 5 mM) and formic acid (0.1 and 0.2%) were evaluated.Isocratic elution using a mobile phase consisting of methanol/5 mM ammonium formate (75:25, v/v), in 2 min using a Symmetry C18 column (1.8 μm, 2.1 × 50 mm) at a flow rate of 0.3 mL min -1 gave the best separation.

Sample preparation
To obtain minimum interference and high recoveries for kaempferide, kaempferol, isorhamnetin, and IS, protein precipitation using methanol and acetonitrile, ethyl acetate, acetone, chloroform or methylene chloride was investigated by liquid-liquid extraction (LLE).LLE using ethyl acetate/acetone (1:1) exhibited a four-fold higher extraction rate than that of other solvents, allowing the effective extraction of the analytes from the plasma.Due to the high recovery rates and low interference, LLE extraction using ethyl acetate/acetone (1:1) provides a simple and efficient method for sample preparation.
Figures 2a-2c show the blank rat plasma chromatogram; blank plasma spiked with the three standard compounds and IS; and rat plasma samples 30 min after oral administration of SSBE, respectively.
The retention times were about 1.19, 1.21, 1.62 and 1.15 min for kaempferide, kaempferol, isorhamnetin, and IS, respectively.No endogenous peak was detected in the plasma samples within the retention times of the three standard compounds and IS.Therefore, it can be concluded that the method can be used to determine the levels of the three components in rat plasma.

Carryover
No peak was observed at the retention times of analytes or IS in the chromatogram of a blank sample analyzed after the injection of ULOQ sample, indicating the absence of carryover.

ME and recovery
The recovery and ME were estimated by analyzing the three levels of QC samples.As seen in the data summarized in Table 1, the QC sample recoveries were within 85.66-92.21%and ME was between 92.36-101.32%.These results indicate that the extraction method for the analytes is robust and reliable.

Accuracy and precision
The precision and accuracy for kaempferide, kaempferol, and isorhamnetin in rat plasma are summarized in Table 2.The accuracy data were within 88.63-98.74%,and the intra-and inter-day precision RSD% between 3.88-7.23and 4.44-7.74,respectively.The results demonstrated that the accuracy and precision values were within the acceptable range.

Stability
Stability results (n = 3) are summarized in Table 3, which show that all analytes were stable in rat plasma for 6 h at room temperature, three freeze-thaw cycles carried out at room temperature for 4 h, and after being stored at -20 °C for at least one month.There was no significant degradation when the extracted plasma samples were stored at 4 °C for 24 h in an auto sampler.The deviation was within 9.75%, which indicated that the method was reliable for routine analysis.

Application to the pharmacokinetic study
The above method was successfully used to study the pharmacokinetics of SSBE in rats with a single-dose, oral administration of 0.2 g kg -1 in eight rats.Pharmacokinetic parameters such as AUC 0-24 and AUC 0-∞ (area under the concentration time curve truncated), t 1/2 (half-life of drug elimination during the terminal phase), C max (maximum concentration observed) and T max (time to reach C max ) , for kaempferide, kaempferol and isorhamnetin were determined (Table 4).The mean plasma concentrationtime curves after oral administration of SSBE are shown in Figure 3.The data conformed to a two-compartment, first-order pharmacokinetic model.
The difference in the level of the three components in the SSBE could be responsible for the variable results.The T max was between 0.50 ± 0.00 h and 1.50 ± 0.58 h, which indicates the difference in metabolism of the three compounds in vivo.The AUC 0-24 was between 195.88 ± 9.55 and 683.64 ± 98.54 ng h mL -1 .These results provide a basis for further studies on SSB to determine the effectiveness of this TCM in clinical treatment.

Conclusions
The primary aim of this study was to present a novel and validated UHPLC-MS/MS method, which could be used for the simultaneous analysis of kaempferide, kaempferol, and isorhamnetin in rat plasma after oral administration of SSBE.The advantages of the method presented in this paper are simple sample preparation and simultaneous determination of three analytes within a short run time (2 min), providing a high throughput approach for sample analysis.The small starting volume of plasma (50 μL) is especially useful because of the low blood volume obtained The results of our study will be useful for future studies on the mechanism of SSB, and could provide valuable information for the clinical application of TCM.

Figure 2 .
Figure 2. Compound chromatograms.(a) blank rat plasma sample; (b) blank plasma spiked with three standard compounds and IS; (c) rat plasma sample after an oral administration of SSBE at 30 min intervals.

Table 2 .
Intra-and inter-day accuracy and precision for kaempferide, kaempferol, and isorhamnetin in rat plasma

Table 3 .
Analyte stability in rat plasma under different storage conditions (mean ± standard deviation, n = 3) from a single rat for routine pharmacokinetic studies.Additionally, this UHPLC-MS/MS with a simple liquid extraction can ultimately be used in other pre-clinical and clinical samples (plasma, urine, tissue) for future pharmacokinetic studies.