TDM

Therapeutic Drug Monitoring (TDM)

GS-441524 and Remdesivir (GS-5734) have antiviral activity against FCoV and FIPV3. Both drugs are now legally formulated (as “Specials”) for animal treatment in the UK, much of Europe, Australia and more recently the USA, and can be used to treat cats with FIP. Currently, GS-441524 can be purchased on prescription from BOVA Veterinary Specials, UK and Summit Veterinary Pharmaceuticals Ltd. in Europe (and BOVA Veterinary Specials in Australia and NZ) and Stokes Pharmacy in the USA. GS-441524 is available in tablet, powder for suspension, and liquid suspension, whilst Remdesivir is given intravenously (or subcutaneously which is not recommend as it cause some degree of pain in 50% of cats).

TDM can be performed alongside serum AGP measurement (£25 +VAT), and, together, may further indicate how well the cat is responding to treatment. As the treatment of FIP with these antivirals is constantly evolving we have a dedicated flow chart to aid in monitoring for both 6 and 12-week courses of treatment which can requested. Further advice can be given by contacting the FIP Clinical Research Team (FIPstudy@ed.ac.uk) if required. 

Please note: we can only assess blood from cats if they are being treated with legal drugs, not black-market forms of GS-441524 or Remdesivir (as the content of antiviral is not regulated as it is for legally formulated preparations). All cats will remain under the care of their primary veterinary surgeon.


A minimum of 0.5ml of sample (whole blood EDTA, serum or lithium heparin plasma) is needed to determine the effectiveness of drug dosing in individual patients. TDM measures the concentration of the drug at a particular time after its administration and can determine whether that concentration is within or outside the effective therapeutic range (for more information about this, please contact SWC@microsampling-laboratory.co.uk).

TDM samples should be taken after 3-5 days of dosing (i.e., when the drug is at a steady state) and be collected at 3-5 hours after the last dose was administered (for the peak sample) or at 9-11 hours post-dose (for the trough sample). Submitting both peak and trough samples taken on one day will provide the best information as we can generate a pharmacokinetic (PK) graph indicating how your patient is metabolising the medication and determine if any dose modifications are necessary. We realise that this may not always be convenient for the owner/patient; however, the information gathered may allow for timely dose modifications and a better patient outcome, including being able to consider shorter treatment periods (i.e., 6 weeks rather than 12 weeks). 

GS-441524 kidney and/or bladder stones have been occasionally seen in cats (seen in ~10 cases where higher doses of black-market drugs were administered). If there is concern about this possibility, urine samples can be assessed for the presence of GS-441524 crystals (£25+ VAT).

Please note: we can only assess blood from cats if they are being treated with legal drugs, not black-market forms of GS-441524 or Remdesivir (as the content of antiviral is not regulated as it is for legally formulated preparations). All cats will remain under the care of their primary veterinary surgeon.

 


Sample should be sent together with a submission form ideally next-day or 1st class delivery to minimise delay. Please note this is a Monday-Friday service.

To allow us to interpret the results, please submit the following information on our submission form, as without this information we cannot provide you with interpretations to help your patient.

  • Body weight
  • Dose given mg/kg per dose, frequency of dosing, formulation (liquid or tablet) and supplier)
  • Time last dose given
  • Time sample/s taken

Samples should be addressed to:

Easter Bush Pathology

The Royal (Dick) School of Veterinary Studies 

University of Edinburgh

Easter Bush Campus

Midlothian

Roslin EH25 9RG


Turnaround time for TDM is up to 7 days.


TDM: £25 +VAT (per sample/time-point)


The results will be reported as “this cat’s dose of GS-441524 is likely to be (either) adequate or inadequate to ensure effective FIPV viral inhibition”. Recommendations may also include switching formulations, increasing or decreasing the dose being administered and/or possibility of reducing the duration of treatment. This assay may aid responsible decision-making; however, results are to be used at the discretion of the primary care veterinary surgeon, they do not override the responsibility or judgement of the practitioner to do what is best for the animal in their care. 


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