4 Jul 2023
Treatment for FIP is expensive – although recently in the UK the price has been reduced – and it is important to update systems to acknowledge this and consider reducing standard mark-ups, as the antiviral drugs are efficacious and the best current options.
For some owners, this means treating according to current guidelines will not be possible. In these scenarios, veterinarians can consider the following options:
Giving only oral GS-441524 for the full course of treatment, instead of initially starting with remdesivir then transitioning to GS-441524.
Starting on remdesivir/GS-441524 for as long as possible, before swapping to mefloquine tablets dosed as previously mentioned.
If an increase in remdesivir or GS-441524 dosage becomes necessary (because of the development of neurological signs, or failure of response or relapse), but this increase cannot be afforded, mefloquine treatment can be added as an adjunctive treatment, although further studies are needed to judge the efficacy of this combination.
Occasional cases have been successfully treated with mefloquine plus IFNω or PI.
Many products are available on the black market claiming to be GS‑analogues (for example, GS-441524, molnupiravir; see later); however, the purity and safety of these products is unknown, and contamination has been reported.
As of January 2022, the VMD stated that any veterinarian obtaining these drugs for the treatment of FIP in the UK will be acting illegally, as will any owners who administer these drugs to their cat.
In a previous study, which looked at almost 400 cats treated with illegal GS-analogues, only 8.7 per cent of owners reported receiving significant help from their veterinarian as to how to safely administer these drugs, nor how to monitor response to treatment or care for their critically sick cats (Jones et al, 2021).
The authors therefore encourage all owners to pursue legal options so they can be guided and supported through the process by their veterinarian, and the purity and strength of drug administered is known.
The 3C-like protease inhibitor GC376 has shown a promise in the treatment of FIP in client-owned cats at dose of 15mg/kg SC every 12 hours; however, relapses no longer responsive to treatment occurred in that study (Pedersen et al, 2018). The effect on cats with neurological form of FIP was not evaluated and several side effects arose, such as transient stinging on injection, occasional foci of SC fibrosis and hair loss, and retarded development and abnormal eruption of permanent teeth in cats treated younger than 16 to 18 weeks of age (Pedersen et al, 2018).
Molnupiravir is a newer therapeutic option (EIDD-2801). It is available legally in some countries that are not ensuring it is for human use only, and is available illegally from the internet everywhere. Currently, it is for human use only in the UK, but this situation is likely to change in the future.
It is being used in some countries as a first‑line therapy for suspect FIP, and as a rescue therapy to treat cats that have persistent or relapsed clinical signs of FIP after GS-441524 and/or GC376 therapy.
In a recent study, molnupiravir was used as a rescue therapy with an average starting dosage of 12.8mg/kg orally twice daily, and an average ending dosage of 14.7mg/kg orally twice daily, and was given for a median of 12 weeks. In this study, 24 of 26 cats were still living disease‑free at the time of writing. Few adverse effects were reported – folded ears, broken whiskers, and severe leukopenia – but all of these were seen at dosages higher than 23mg/kg twice daily.
However, a new study has shown the EIDD‑2801 dose of 10mg/kg achieves plasma levels greater than the established corresponding EC50 values, which are sustained over 24 hours (Cook et al, 2022).
The use of molnupiravir may be a good option for patients that have previously relapsed on another antiviral therapy, but more studies are needed (Roy et al, 2022).
