Quantification of HSK21542 in rat plasma and tissues by LC-MS/MS: an application to pharmacokinetics and tissue distribution study

BACKGROUND: HSK21542 is a novel, peripherally selective kappa opioid receptor (KOR) agonist under development for the treatment of pain and pruritus. This study aimed to develop a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantifying HSK21542 in rat plasma and tissue homogenate. RESEARCH DESIGN AND METHODS: -HSK21542 as the internal standard was developed and validated in rat plasma and tissue homogenates. Sprague Dawley (SD) rats received single intravenous doses of HSK21542 at 0.1, 0.3, or 1.0 mg/kg, or repeated intravenous doses of 0.3 mg/kg twice daily for 7 days. Tissue distribution was evaluated after a single intravenous dose of 0.3 mg/kg. RESULTS: The method was fully validated and demonstrated good linearity for rat plasma (0.500 - 500 ng/mL) and rat liver homogenate (2.50 - 2500 ng/g). HSK21542 exhibited high clearance (mean CL 36.0 - 44.7 mL/min/kg), a short terminal half-life (2-3 h), multi-exponential pharmacokinetic characteristics, and dose-proportional exposure, with no pharmacologically significant accumulation after multiple doses. Tissue distribution studies indicated rapid and wide distribution, with the highest concentrations found in the kidney, bladder, and liver. CONCLUSION: The validated LC-MS/MS method was suitable for preclinical HSK21542 analysis. These pharmacokinetic and tissue distribution profiles provided essential data supporting its further clinical development.