Quantification of GS-441524 concentration in feline plasma using high performance liquid chromatography with fluorescence detection

The adenosine analogue GS-441524 has demonstrated efficacy in treatment of feline infectious peritonitis (FIP). With no commercially registered formulations of GS-441524 available, global focus shifted to its pro-drug remdesivir, as it became more accessible throughout the COVID-19 pandemic. This study developed and validated a simple liquid chromatography equipped with a fluorescence detector to quantify plasma concentrations of GS-441524 applicable for routine therapeutic monitoring of remdesivir or GS-441524 therapy for FIP infected cats. A Waters X-Bridge C18, 5 µm, 150 × 4.6 mm, column was used and mixtures of 20 mM ammonium acetate (pH 4.5) with acetonitrile of 5% and 70% were prepared for gradient mobile phase. With a simple protein precipitation using methanol to clean plasma sample, GS-441524 was monitored at excitation and emission wavelengths of 250 nm and 475 nm, respectively. Using an external standard, the lowest and highest limits of quantification were 19.5 ng/mL to 10,000 ng/mL, respectively. The intra- and inter day trueness of the quality controls (QCs) were within 10% of their nominal concentrations and intra- and inter day precision of the QCs (expressed as the coefficient of variation) ranged from 1.7 to 5.7%, This assay was able to quantify plasma trough levels of GS-441524 (23.7–190.1 ng/mL) after the administration of remdesivir (9.9–15.0 mg/kg BW, IV or SC) in FIP cats (n = 12). Accordingly, this study generated an alternative and cost-effective way to quantify GS-441524 in feline biological fluids at least up to 24 hr after administrations of remdesivir.