FIP Treatment Protocols

Although GS-441524 is currently the most commonly used antiviral treatment for FIP, several other antiviral options are also available. Choosing the appropriate antiviral and treatment modality may depend on the patient's form of FIP and degree of stability, the owner's ability to administer the various forms, availability and veterinary regulations in your country or state.

For more information on monitoring treatment, dealing with poor response to treatment and evaluating readiness to end treatment, please visit the Supporting FIP Treatment section.

GS-441524

This is the most commonly used treatment for all forms FIP, as well as the most documented.

GS-441524 is available in many countries in compounded oral formulations. Injectable and oral formulations are also found on the black market and are often used by pet owners.

Oral GS-441524 dosages are roughly double the SC equivalent.

The typical protocol is 12 weeks (84 days) of continuous treatment with the following oral dosages:

15 mg/kg/day for cases without ocular or neurological involvement
15-20 mg/kg/day for ocular FIP cases
10 mg/kg BID (minimum) for neurological FIP.

For more information, please check the ISFM guidelines.

For sub-cutaneous administration, dosages are as follows:

6-7.5 mg/kg for cases without ocular or neurological involvement
8-10 mg/kg (minimum) for ocular FIP cases
10 mg/kg (minimum) for neurological FIP.

Wherever available, regulated products should be preferred to black market options.

Ocular and neurological cases in particular may require higher doses as there is variation between cats in the amount of GS-441524 that crosses the blood/brain barrier -- ranging from about 7% to 21%.

Please note that many black market suppliers label their products by an estimated bioavailable dose, a dose equivalent, or other means. Click here for more info.

A recent study has shown that a shorter course of therapy may be sufficient for some cats. A larger study is currently in progress to validate this for a larger cohort of cats.

Molnupiravir

This is a newer option for treating FIP. A study has been published showing it to be an effective rescue treatment for cats that have failed treatment with other antivirals for FIP. Studies using it for first-line treatment of FIP have also shown it to be as effective as GS. (

Current dosing recommendations:

10-15 mg/kg PO BID for non-ocular/neurological FIP

15 mg/kg PO BID for ocular FIP

15-20 mg/kg PO BID for neurological FIP

Due to concerns about long term usage and mutagenic properties, including accelerating emergence of viral variants of concern, it is recommended that the lowest effective dosage be used. Unlike GS-441524, Remdesivir and GC376, it is teratogenic and should not be used in pregnant or breeding queens.

Reference studies:

Paxlovid (nirmatrelvir + ritonavir)

Paxlovid is an antiviral treatment which is FDA approved to treat COVID19 in humans. It consists of nirmatrelvir, a 3CLPro inhibitor paired with ritonavir, a protease inhibitor and CYP3A inhibitor commonly used to boost the potency of other antivirals. It is not currently used as a first line of treatment due to lack of data. It has however found use as a rescue treatment paired with GS-441524 or molnupiravir as part of combination therapy to combat viral resistance to GS-441524 or other cases where antiviral therapy has failed.

As a rescue treatment, Paxlovid is combined with GS-441524 or molnupiravir at a dose of 75 mg nirmatrelvir plus 25 mg ritonavir. (When using the human formulation packs, this corresponds to a half tablet of nirmatrelvir, and quarter tablet of ritonavir.)

Remdesivir

Remdesivir is most often used only for the start of treatment and then combined with GS-441524. This treatment is primarily in use in Australia and the UK where Remdesivir is legal and available to veterinarians for prescription.

The treatment protocol is 12 weeks of treatment, however frequently Remdesivir is used only for the initial few weeks, and cats switch to GS-441524 oral tablets for the remainder of treatment.

Dosing is as follows:

10-12 mg/kg IV or SC for non-ocular/neurologcal FIP

15 mg/kg IV or SC for ocular FIP

20 mg/kg IV or SC for neurological FIP

Note that IV administration does have approximately 10% greater chance of worsening existing pleural effusion, or causing new occurrence of pleural effusion so close monitoring is warranted.

For more information on treating with Remdesivir we recommend this excellent pdf put together by vets and researchers in the UK and Australia.

GC376

GC376 is a 3CLPro inhibitor and was the first antiviral documented to successfully cure FIP. It is typically less commonly used as a first line of treatment as it has comparatively low penetration of the blood/brain barrier, making it less effective for neurological FIP cases. It has however found use paired with GS-441524 as part of combination therapy to combat viral resistance to GS-441524.

Due to poor oral absorption, GC376 is only administered as a sub-cutaneous injection.

For non-neurological FIP, GC376 is dosed at 15-20 mg/kg SC BID.

For cats with viral resistance to GS-441524, GC376 is added to the current GS-441524 dosage at a dosage of 20 mg/kg SC BID regardless of form of FIP. This is sufficient for most cats, including many neuro FIP cats, but some will need higher dosages. If this fails to achieve remission of clinical signs or bloodwork is concerning, the GC376 dosage is increased by 10 mg/kg increments to as high as 50 mg/kg SC BID.

FIP Treatment Protocols

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