A Validated Eco-Friendly High-Performance Liquid Chromatography Assay for Therapeutic Drug Monitoring of GS-441524 in Cats with Feline Infectious Peritonitis.

Feline infectious peritonitis (FIP) is responsive to treatment with the adenosine analogue GS-441524 (GS-44) and its prodrug, remdesivir (REM); both are now available on veterinary prescription in many countries. Therapeutic drug monitoring (TDM) of GS-44 has the potential to support dose selection for individual cats; however, TDM assays are currently only offered by one UK-based laboratory. This study describes a simple, cost-effective, and environmentally conscious high-performance liquid chromatography (HPLC) method for the quantification of GS-44 in feline plasma or serum. The method was validated in accordance with the International Council for Harmonisation M10 guidelines for bioanalytical methods. Calibration standards demonstrated linearity across a range of 2.59 to 5048.28 ng/mL, R2 = 0.9880, with a lower limit of quantification of 2.59 ng/mL and upper limit of quantification of 5048 ng/mL; this is equivalent to an assay range of 0.06 to 208 µM. Precision, accuracy and spike recovery were within ±15% for 14 of 17 standard concentrations (±20% at the lowest three). Carry-over, dilution integrity, and analyte stability under common storage conditions all met the method's requirements. This is a simple, robust and accurate eco-friendly method suitable for adoption by diagnostic laboratories, enabling routine TDM for cats undergoing treatment for FIP with GS-44 and/or REM.