{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,1]],"date-time":"2026-09-01T13:22:07Z","timestamp":1788268927556,"version":"build-2803163510"},"reference-count":41,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T00:00:00Z","timestamp":1788134400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Microsampling Laboratory Ltd."},{"id":[{"id":"https:\/\/ror.org\/05vmx1h48","id-type":"ROR","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Pathogens"],"abstract":"<jats:p>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 \u00b5M. Precision, accuracy and spike recovery were within \u00b115% for 14 of 17 standard concentrations (\u00b120% at the lowest three). Carry-over, dilution integrity, and analyte stability under common storage conditions all met the method\u2019s 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.<\/jats:p>","DOI":"10.3390\/pathogens15090913","type":"journal-article","created":{"date-parts":[[2026,9,1]],"date-time":"2026-09-01T12:41:18Z","timestamp":1788266478000},"page":"913","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["A Validated Eco-Friendly High-Performance Liquid Chromatography Assay for Therapeutic Drug Monitoring of GS-441524 in Cats with Feline Infectious Peritonitis"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2913-6991","authenticated-orcid":false,"given":"Stephen W.","family":"Cooke","sequence":"first","affiliation":[{"name":"The Microsampling Laboratory, 9 Plough Close, Shillingford, Wallingford OX10 7EX, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-3214-8391","authenticated-orcid":false,"given":"Rachael","family":"Hammond","sequence":"additional","affiliation":[{"name":"The Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Edinburgh EH25 9RG, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2088-503X","authenticated-orcid":false,"given":"Dani\u00e8lle A.","family":"Gunn-Moore","sequence":"additional","affiliation":[{"name":"The Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Edinburgh EH25 9RG, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,31]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Tasker, S., Addie, D.D., Egberink, H., Hofmann-Lehmann, R., Hosie, M.J., Truyen, U., Bel\u00e1k, S., Boucraut-Baralon, C., Frymus, T., and Lloret, A. (2023). Feline Infectious Peritonitis: European Advisory Board on Cat Diseases Guidelines. Viruses, 15.","DOI":"10.3390\/v15091847"},{"key":"ref_2","unstructured":"(2025, September 01). Feline Infectious Peritonitis (FIP). Available online: https:\/\/icatcare.org\/articles\/feline-infectious-peritonitis-fip."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"226","DOI":"10.1016\/j.vetmic.2018.04.026","article-title":"The Nucleoside Analog GS-441524 Strongly Inhibits Feline Infectious Peritonitis (FIP) Virus in Tissue Culture and Experimental Cat Infection Studies","volume":"219","author":"Murphy","year":"2018","journal-title":"Vet. Microbiol."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1080\/01652176.2023.2246553","article-title":"Quantification of GS-441524 Concentration in Feline Plasma Using High Performance Liquid Chromatography with Fluorescence Detection","volume":"43","author":"Kimble","year":"2023","journal-title":"Vet. Q."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Cook, S., Wittenburg, L., Yan, V.C., Theil, J.H., Castillo, D., Reagan, K.L., Williams, S., Pham, C.-D., Li, C., and Muller, F.L. (2022). An Optimized Bioassay for Screening Combined Anticoronaviral Compounds for Efficacy against Feline Infectious Peritonitis Virus with Pharmacokinetic Analyses of GS-441524, Remdesivir, and Molnupiravir in Cats. Viruses, 14, Correction in Viruses\u00a02024, 16, 397. https:\/\/doi.org\/10.3390\/v16030397.","DOI":"10.3390\/v16030397"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"109781","DOI":"10.1016\/j.vetmic.2023.109781","article-title":"Better Therapeutic Effect of Oral Administration of GS441524 Compared with GC376","volume":"283","author":"Yan","year":"2023","journal-title":"Vet. Microbiol."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"1772","DOI":"10.1111\/jvim.16803","article-title":"Outcomes of Treatment of Cats with Feline Infectious Peritonitis Using Parenterally Administered Remdesivir, with or without Transition to Orally Administered GS-441524","volume":"37","author":"Coggins","year":"2023","journal-title":"Vet. Intern. Med."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"495","DOI":"10.1111\/jsap.13849","article-title":"Pharmacokinetics of GS-441524 Following Intravenous Remdesivir in Six Cats and Results of Therapeutic Drug Monitoring during Treatment of Feline Infectious Peritonitis: 22 Cases (2021\u20132024)","volume":"66","author":"Coggins","year":"2025","journal-title":"J. Small Anim. Pract."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1098612X231194460","DOI":"10.1177\/1098612X231194460","article-title":"Retrospective Study and Outcome of 307 Cats with Feline Infectious Peritonitis Treated with Legally Sourced Veterinary Compounded Preparations of Remdesivir and GS-441524 (2020\u20132022)","volume":"25","author":"Taylor","year":"2023","journal-title":"J. Feline Med. Surg."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"271","DOI":"10.1177\/1098612X19825701","article-title":"Efficacy and Safety of the Nucleoside Analog GS-441524 for Treatment of Cats with Naturally Occurring Feline Infectious Peritonitis","volume":"21","author":"Pedersen","year":"2019","journal-title":"J. Feline Med. Surg."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1587","DOI":"10.1111\/jvim.15780","article-title":"Antiviral Treatment Using the Adenosine Nucleoside Analogue GS-441524 in Cats with Clinically Diagnosed Neurological Feline Infectious Peritonitis","volume":"34","author":"Dickinson","year":"2020","journal-title":"J. Vet. Intern. Med."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1784","DOI":"10.1111\/jvim.16804","article-title":"Thirty-two Cats with Effusive or Non-effusive Feline Infectious Peritonitis Treated with a Combination of Remdesivir and GS-441524","volume":"37","author":"Green","year":"2023","journal-title":"Vet. Intern. Med."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"1098612X231183250","DOI":"10.1177\/1098612X231183250","article-title":"Long-Term Follow-up of Cats in Complete Remission after Treatment of Feline Infectious Peritonitis with Oral GS-441524","volume":"25","author":"Zwicklbauer","year":"2023","journal-title":"J. Feline Med. Surg."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1098612X251335189","DOI":"10.1177\/1098612X251335189","article-title":"Efficacy of Oral Remdesivir in Treating Feline Infectious Peritonitis: A Prospective Observational Study of 29 Cats","volume":"27","author":"Renner","year":"2025","journal-title":"J. Feline Med. Surg."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"617","DOI":"10.1111\/jsap.13869","article-title":"High-Dose Induction Therapy and Treatment Termination Criteria for Feline Infectious Peritonitis with Remdesivir, GS-441524 and Adjunctive Mefloquine: 46 Cases (2023)","volume":"66","author":"Kamiyoshi","year":"2025","journal-title":"J. Small Anim. Pract."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"e17249","DOI":"10.1111\/jvim.17249","article-title":"Predictive Factors Associated with Short-term Mortality in Cats with Feline Infectious Peritonitis Treated with Remdesivir or GS-441524 or Both","volume":"39","author":"Goto","year":"2024","journal-title":"J. Vet. Intern. Med."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Zuzzi-Krebitz, A.-M., Buchta, K., Bergmann, M., Krentz, D., Zwicklbauer, K., Dorsch, R., Wess, G., Fischer, A., Matiasek, K., and H\u00f6nl, A. (2024). Short Treatment of 42 Days with Oral GS-441524 Results in Equal Efficacy as the Recommended 84-Day Treatment in Cats Suffering from Feline Infectious Peritonitis with Effusion\u2014A Prospective Randomized Controlled Study. Viruses, 16.","DOI":"10.3390\/v16071144"},{"key":"ref_18","first-page":"20551169251360625","article-title":"Navigating Neurological Re-Emergence in Feline Infectious Peritonitis: Challenges and Insights from GS-441524 and Remdesivir Treatment","volume":"11","author":"Rodary","year":"2025","journal-title":"JFMS Open Rep."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Cosaro, E., Pires, J., Castillo, D., Murphy, B.G., and Reagan, K.L. (2023). Efficacy of Oral Remdesivir Compared to GS-441524 for Treatment of Cats with Naturally Occurring Effusive Feline Infectious Peritonitis: A Blinded, Non-Inferiority Study. Viruses, 15.","DOI":"10.3390\/v15081680"},{"key":"ref_20","first-page":"259","article-title":"Outcomes of Treatment of Cats with Effusive Feline Infectious Peritonitis Using Parenterally Administered Remdesivir with Two Different Maintenance Dose Concentrations","volume":"31","author":"Anwer","year":"2025","journal-title":"Kafkas Univ. Vet. Fak. Derg."},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Jones, S., Novicoff, W., Nadeau, J., and Evans, S. (2021). Unlicensed GS-441524-Like Antiviral Therapy Can Be Effective for at-Home Treatment of Feline Infectious Peritonitis. Animals, 11.","DOI":"10.3390\/ani11082257"},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Negash, R., Li, E., Jacque, N., Novicoff, W., and Evans, S.J.M. (2024). Owner Experience and Veterinary Involvement with Unlicensed GS-441524 Treatment of Feline Infectious Peritonitis: A Prospective Cohort Study. Front. Vet. Sci., 11.","DOI":"10.3389\/fvets.2024.1377207"},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Larson, K., Hart, E., Negash, R., Novicoff, W., Jacque, N., and Evans, S. (2025). Prospective Analysis of Clinicopathologic Correlates of At-Home Feline Infectious Peritonitis Treatment Using GS-441524. Pathogens, 14.","DOI":"10.3390\/pathogens14050507"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"489","DOI":"10.2460\/javma.23.08.0466","article-title":"Unlicensed Antiviral Products Used for the At-Home Treatment of Feline Infectious Peritonitis Contain GS-441524 at Significantly Different Amounts than Advertised","volume":"262","author":"Kent","year":"2024","journal-title":"J. Am. Vet. Med. Assoc."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1","DOI":"10.2460\/ajvr.23.10.0221","article-title":"Quality Assessment and Characterization of Unregulated Antiviral Drugs for Feline Infectious Peritonitis: Implications for Treatment, Safety, and Efficacy","volume":"85","author":"Mulligan","year":"2024","journal-title":"Am. J. Vet. Res."},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Jelliffe, R.W., and Neely, M. (2017). Individualized Drug Therapy for Patients, Academic Press.","DOI":"10.1016\/B978-0-12-803348-7.00011-3"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"1772","DOI":"10.1093\/jac\/dkaa152","article-title":"Development and Validation of a UHPLC-MS\/MS Method for Quantification of the Prodrug Remdesivir and Its Metabolite GS-441524: A Tool for Clinical Pharmacokinetics of SARS-CoV-2\/COVID-19 and Ebola Virus Disease","volume":"75","author":"Avataneo","year":"2020","journal-title":"J. Antimicrob. Chemother."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"1461","DOI":"10.1515\/cclm-2020-0612","article-title":"Quantification of Plasma Remdesivir and Its Metabolite GS-441524 Using Liquid Chromatography Coupled to Tandem Mass Spectrometry. Application to a COVID-19 Treated Patient","volume":"58","author":"Alvarez","year":"2020","journal-title":"Clin. Chem. Lab. Med. (CCLM)"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"5811","DOI":"10.1007\/s00216-021-03561-8","article-title":"Quantitative HPLC-MS\/MS Determination of Nuc, the Active Metabolite of Remdesivir, and Its Pharmacokinetics in Rat","volume":"413","author":"Du","year":"2021","journal-title":"Anal. Bioanal. Chem."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"113935","DOI":"10.1016\/j.jpba.2021.113935","article-title":"Simultaneous Quantification of Seven Repurposed COVID-19 Drugs Remdesivir (plus Metabolite GS-441524), Chloroquine, Hydroxychloroquine, Lopinavir, Ritonavir, Favipiravir and Azithromycin by a Two-Dimensional Isotope Dilution LC\u2013MS\/MS Method in Human Serum","volume":"196","author":"Habler","year":"2021","journal-title":"J. Pharm. Biomed. Anal."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Wang, A.Q., Hagen, N.R., Padilha, E.C., Yang, M., Shah, P., Chen, C.Z., Huang, W., Terse, P., Sanderson, P., and Zheng, W. (2022). Preclinical Pharmacokinetics and In Vitro Properties of GS-441524, a Potential Oral Drug Candidate for COVID-19 Treatment. Front. Pharmacol., 13.","DOI":"10.3389\/fphar.2022.918083"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"782","DOI":"10.1203\/00006450-197609000-00004","article-title":"Spectrophotometric Characteristics of Bilirubin","volume":"10","author":"Lee","year":"1976","journal-title":"Pediatr. Res."},{"key":"ref_33","first-page":"157","article-title":"Proteins and Triglycerides Measurement in Blood under UV\/VIS Spectroscopy (190-1100 Nm)","volume":"55","author":"Janjua","year":"2023","journal-title":"Lasers Eng."},{"key":"ref_34","unstructured":"(2022). Bioanalytical Method Validation and Study Sample Analysis. ICH Harmonised Guideline M10. Final version (Standard No. GS-441524). Available online: https:\/\/go.drugbank.com\/drugs\/DB15686."},{"key":"ref_35","unstructured":"(2025, October 21). Available online: https:\/\/database.ich.org\/sites\/default\/files\/M10_Guideline_Step4_2022_0524.pdf."},{"key":"ref_36","unstructured":"(2024, November 29). When Should an Internal Standard Be Used?. Available online: https:\/\/www.chromatographyonline.com\/view\/when-should-internal-standard-be-used-0."},{"key":"ref_37","unstructured":"Jelliffe, R., and Neely, M. (2016). Individualized Drug Therapy for Patients: Basic Foundations, Relevant Software and Clinical Applications, Academic Press."},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Cooke, S.W., Hammond, R., and Gunn-Moore, D.A. (2026). Therapeutic Drug Monitoring of GS-441524 in Cats with Feline Infectious Peritonitis: Pharmacokinetic Variability and Implications for Dose Optimization. Pathogens, 15.","DOI":"10.20944\/preprints202601.0788.v1"},{"key":"ref_39","doi-asserted-by":"crossref","unstructured":"N\u00fa\u00f1ez, O., Sentellas, S., Granados, M., and Saurina, J. (2023). Green Strategies toward Eco-Friendly HPLC Methods in Pharma Analysis. High Performance Liquid Chromatography-Recent Advances and Applications, IntechOpen.","DOI":"10.5772\/intechopen.105269"},{"key":"ref_40","first-page":"488","article-title":"Detoxification of Acetonitrile-Water Wastes from Liquid Chromatography","volume":"41","author":"Gilomen","year":"1995","journal-title":"Chromatographia"},{"key":"ref_41","doi-asserted-by":"crossref","unstructured":"Reckers, A., Wu, A.H.B., Ong, C.M., Gandhi, M., Metcalfe, J., and Gerona, R. (2021). A Combined Assay for Quantifying Remdesivir and Its Metabolite, along with Dexamethasone, in Serum. J. Antimicrob. Chemother., dkab094.","DOI":"10.1093\/jac\/dkab094"}],"container-title":["Pathogens"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2076-0817\/15\/9\/913\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,1]],"date-time":"2026-09-01T12:58:11Z","timestamp":1788267491000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2076-0817\/15\/9\/913"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,8,31]]},"references-count":41,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2026,9]]}},"alternative-id":["pathogens15090913"],"URL":"https:\/\/doi.org\/10.3390\/pathogens15090913","relation":{},"ISSN":["2076-0817"],"issn-type":[{"value":"2076-0817","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,8,31]]}}}