Bioanalysis of propylparaben and p-hydroxybenzoic acid, and their sulfate conjugates in rat plasma by liquid chromatography–tandem mass spectrometry

•New bioanalytical assays for propylparaben and its three major metabolites were reported.•It was discovered that the esterase inhibitor (PMSF) reacted with analyte of interest.•Citric acid was used to inhibit esterase activity to stabilize propylparaben in rat plasma.•Strategies were reported to overcome challenges for analyte stability and interferences.•The two bioanalytical assays were validated and applied to a toxicological study in rats.

Two rugged liquid chromatography–tandem mass spectrometry (LC–MS/MS) methods for the determination of propylparaben, its major metabolite, p-hydroxybenzoic acid (pHBA), and their sulfate conjugates have been developed and validated in citric acid-treated rat plasma. To prevent propylparaben being hydrolyzed to pHBA ex vivo, rat plasma was first treated with citric acid; then collected and processed at a reduced temperature (ice bath). Stable isotope labeled internal standards, d4-propylparaben, 13C6-pHBA, and the d4-labeled internal standards of their sulfate conjugates were used in the methods. The analytes were extracted from the matrix using protein precipitation, followed by chromatographic separation on a Waters ACQUITY UPLC HSS T3 column. Quantification using negative ion electrospray was performed on a Sciex API 4000 mass spectrometer. The analytical ranges were established from 2.00 to 200 ng/mL for propylparaben, 50.0–5000 ng/mL for pHBA, 50.0–10,000 ng/mL for the sulfate conjugate of propylparaben (SPP) and 200–40,000 ng/mL for the sulfate conjugate of pHBA (SHBA). Inter- and intra-run precision for the quality control samples were less than 5.3% and 4.4% for all analytes; and the overall accuracy was within ±5.7% of the nominal values. The validated bioanalytical methods demonstrated excellent sensitivity, specificity, accuracy and precision and were successfully applied to a rat toxicology study under the regulations of Good Laboratory Practices (GLP). Strategies have been developed and applied toward overcoming the challenges related to analyte stability, and environmental and endogenous background.