Stability
The stability of the four major diterpenoids in biological matrices was evaluated under different conditions. The results showed that AP1, AP3, AP4 and AP6 were stable in plasma and urine during five cycles of freeze–thaw storage. The analytes were also found to be stable after 3 months of “long-term” storage at − 80 °C. The extracted samples kept in the autosampler (at 4 °C) were stable for at least 24 h. No significant levels of degradation products were detected during the sample preparation process. The samples were stable for up to 8 h at room temperature as determined by quantitative analysis of all four analytes. The stability results in plasma and urine are summarized in Table Stability of andrographolide (AP1); 14-deoxy-11, 12-didehydroandrographolide (AP3); neoandrographolide (AP4); and 14-deoxyandrographolide (AP6) in (a) human plasma and (b) urine under different storage conditions.Short term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at -80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerShort term stability8 h at room temperatureFreeze–thaw stability5 cyclesLong term stability3 months at − 80 °CPost preparative stability24 h at autosamplerData expressed as mean ± SD (n = 5).Low quality control concentration (LQC concentration), medium quality control concentration (MQC concentration), high quality control concentration (HQC concentration).