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 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 15 to 5100 ng/mL, R² = 0.9978, with a lower limit of quantification 8.0 ng/mL and upper limit of quantification 5048 ng/mL; equivalent to an assay range of 0.33 to 208 µM. Precision, accuracy and spike recovery were within ±5% for most standard concentrations (±10% at the lower two). Carry-over, dilution integrity, and analyte stability under common storage conditions all met the guideline criteria. This is a simple, robust and accurate eco-friendly method suitable for being adopted by diagnostic laboratories, enabling routine TDM for cats undergoing treatment for FIP with GS-44 and/or REM.