2.1 Chemicals and reagents
Methanol, acetonitrile, and formic acid (Fisher Scientific, Loughborough, UK) were used HPLC-grade. Ultrapure water was prepared by the Milli-Q plus ultrapure water system (Millipore, Bedford, USA). Alanine was obtained from National Institutes for Food and Drug Control (Beijing, China). Glutamine, glutamate, and α-ketoglutaric acid were obtained from Shanghai Aladdin Bio-Chem Technology Co., LTD (Shanghai, China). Fumarate, succinate, citrate, cis-aconitate, and malate were obtained from Harveybio (Beijing) Gene Technology Co., LTd (Beijing, China). L-Glutamic Acid-d5 (Glu-d5) was purchased from Toronto Research Chemicals (Toronto, Canada). Phosphate buffer saline (PBS) was acquired from BasalMedia (Shanghai, China). Fetal bovine serum (FBS), DMEM culture medium, penicillin, and streptomycin were obtained from Gibco (Gibco, USA).
2.2 Cell culture
PC-9 acquired gefitinib‐resistant (PC-9/GR) cells were purchased from Hunan Fenghui Biotechnology Co., Ltd. (Hunan, China), which were cultured in DMEM medium (Gibco, USA),which contains 800 ng/mL gefitinib with the 10% (v/v) FBS. In addition, 100 U/mL penicillin and 100 g/mL streptomycin (Beijing Dingguo Co., Ltd., Beijing, China) are also necessary, and then placed at a temperature of 37°C in a humidified atmosphere of 5% CO2.
2.3 Cell viability assay
The inhibitory effects of gefitinib on cell proliferation were evaluated by CCK-8 (Dojindo, Kumamoto, Japan). PC-9/GR were cultured in 96-well plates at a density of 1 × 104 cells per well for 24h and then treated with gefitinib and/or SJZ at the indicated concentrations. After incubating for 48 h, the cells were washed twice with PBS and incubated with CCK-8 working solution for 1 h at 37°C, according to the manufacturer’s protocol. Through iMark™ Microplate Absorbance Reader (Molecular Devices, Sunnyvale) measuring absorbance at 450 nm.
2.4 Standard stock solutions, calibration standards, and quality control samples
Glutamate, glutamine, alanine, cis-aconitate, α-ketoglutaric acid, fumarate, succinate, citrate, malate, and glu-d5 were dissolved in 0.1% formic acid in water to obtain a concentration of 5 mg/mL standard stock solutions. Putting stock solutions of all analytes and IS at -20°C for further study. The working solutions of four analytes such as glutamate, glutamine, alanine, and cis-aconitate were determined in ESI+ mode, and the other working solution of five analytes such as α-ketoglutaric acid, fumarate, succinate, citrate, and malate were measured in ESI- mode. Stock solutions of the analytes were prepared by step wisely diluting with cell extraction solvent to generate the calibration standards at 6-12 different concentrations. For the investigation of precision and stability, quality control (QC) samples were prepared by using separate aliquot of the stock solution in accordance with the same way but independently from the calibrators. The levels of lower limits of quantification (LLOQ), and QCs were shown in Table 2. All standard and QC solutions contain and all measured samples contain internal standard (IS) (173.2 ng/mL). All of them were consisted of methanol-water (80:20, v/v).
2.5 Sample collection and preparation
PC-9/GR cells were seeded in a six-well culture plate (Corning) at a density of 5 × 105 cells/well. Harvesting the cells when the cells are 80-90% confluence, discarding the medium, and washing twice with cold PBS, then using liquid nitrogen quenched. Cells were lysed with cold extraction solvent and by using a cell scraper to harvest. Transfer the extract to a 1.5 mL test tube (from Eppendorf) and place it in a refrigerator at -80°C for later use. Centrifuge the mixture at 14,000 rpm (4°C, 15 minutes) to remove cell debris and collect the supernatant. Transfer the supernatant to a vial for analysis and aliquot and store it at -80°C. The content of each analyte is quantified by the protein concentration of the cell pellet.
2.6 Chromatography and mass spectrometry conditions
A Triple Quad™ 4500 Triple Quadrupole Tandem Mass Spectrometer (SCIEX, Japan) with an electrospray ionization source (ESI). Performed on the The specific detection of analytes is achieved through multiple reaction monitoring (MRM). MS/MS method development is accomplished by injecting a standard solution (1 µg/mL) directly into the mass spectrometer. The tuning parameters of the ion spray voltage are 5500 V (ESI+), -4500 V (ESI-); other tuning parameters are the same as the positive and negative ion modes, and the parameters are as follows: temperature, curtain gas, ion source gas 1 (atomizer) and 2 (Turbo ion spray) were set to 550°C,25 psi, 55 psi༌respectivly. The specific precursor/product ion pairs, collision energy (CE), and declustering potential (DP) of the analyte are shown in Table 1. Liquid chromatography was carried out on a liquid chromatography system (SCIEX, Japan). Chromatographic retention was achieved on an ACQUITY HSS T3 column (2.1 × 100 mm, 1.8 µm) with a column temperature at 40°C. The flow rate was 0.30 mL/min. Solvent A in the mobile phase is composed of 0.1% formic acid in water, and solvent B is composed of 0.1% formic acid in acetonitrile. The total elution time of positive ionization mode was 11 min and the gradient was as follows: 0-0.5 min, 0% B; 0.5-5 min, 0% - 10% B; 5-5.5 min, 10% - 95% B; 5.5-6.5 min, 95% B; 6.5-6.9 min, 95% - 0% B; 6.9-11 min, 0% B. The total elution time of negative ionization mode was 12 min and the gradient was as follows: 0-0.5 min, 0% B; 0.5-5 min, 0% - 10% B; 5-6 min, 10% - 95% B; 6-8 min 95% B; 8-8.5 min, 95% - 0% B; 8.5-12 min, 0% B. Each sample injection was 3 µL. All the above data acquisition and analysis are done by Analyst® software version 1.6.3 (SCIEX, Japna) and MultiQuant™ MD software version 3.0.3 (SCIEX, Japan).
2.7 Therapeutic drug treatments
After culturing the PC-9/GR cells for 24 hours, they were cultured in a medium containing Sijunzi tang, Gefitinib, Sijunzi tang/gefitinib and complete medium according to the aforementioned conditions for 48 hours, respectively. Then the cells are processed according to the previously described steps.
2.8 Method validation
The calibration curve of the analyte is obtained by plotting the relationship between the peak area ratio of each analyte to IS and the concentration of the analyte. Linearity is measured by the correlation coefficient (r). Using MultiQuant™ software (version 3.0.3) to integrate the peak area. Theoretically, the requirement of limit of detection (LOD) is 3. LOQ is defined as a S/N ratio of 10. LLOQ is the lowest concentration of an analyte that can be accurately quantified in a sample. Since the 9 analytes are endogenous, cell lysates are used as a biological matrix. The accuracy of QC samples is expressed as relative standard deviation (RSD). The stability is tested by evaluating the concentration change of analytes in the QC sample after preparation. The prepared QC sample was tested five times at room temperature for 12 hours, and then analyzed.
2.9 Statistical analysis
The experimental results were analyzed using Graphpad Primer 8.0 statistical software. One-way analysis of variance (one-way ANOVA) was used for the comparison of measurement data among multiple groups, and the t-test was used for comparison between the two groups. When P<0.05 indicates a significant difference, and P<0.01 indicates a very significant difference.