Pharmacologic Approaches to Feline Infectious Peritonitis

In the past, a diagnosis of feline infectious peritonitis (FIP) carried a grave prognosis, and most cats died or were euthanized within days to months of diagnosis. However, in recent years, considerable advancements have been made in the treatment of FIP, particularly nucleotide prodrugs such as remdesivir and molnupiravir, which are metabolized to their nucleoside analogues GS-441524 and β-d-N⁴-hydroxycytidine (NHC; EIDD-1931), respectively. Also showing promise for adjunctive treatment of refractory FIP are viral protease inhibitors (GC376 and nirmatrelvir).^1-16

In many countries, including the United States, veterinarians can prescribe some or all of the following compounded formulations of remdesivir, GS-441524, molnupiravir, and NHC. Additionally, off-label prescription of registered human drugs are additional legal avenues open to veterinarians.

Cats may still require initial supportive therapy specific to their FIP presentation and disease severity; however, for most patients with FIP, clinicians can anticipate rapid clinical improvement within days of commencing effective antiviral therapy. It is rare that veterinary medicine has taken a disease from an untreatable death sentence to an 85% long-term survival rate with essentially a monotherapy approach.^9,11

This article focuses on the mechanisms of action of these antivirals, the currently understood pharmacokinetics in cats, and the recommended dosing protocols for the treatment of FIP.

Nucleotide and Nucleoside Analogues

Nucleotide and nucleoside analogues are a class of drug with diverse treatment utility, although they are primarily used as antiviral and chemotherapeutic agents.¹⁷ A nucleotide (e.g., remdesivir, molnupiravir) is the prodrug form that incorporates 1 or more phosphate groups into the nucleoside structure, and a nucleoside (e.g., GS-441524, NHC) consists of a nucleobase linked to a sugar moiety.¹⁸ In vivo, nucleotide analogues are typically metabolized to release their core nucleoside, which is subsequently converted into its active form. The active metabolite disrupts virus replication by mimicking native RNA nucleosides and interfering with viral RNA synthesis. How RNA synthesis is ultimately disrupted varies.

Remdesivir and GS-441524

Remdesivir is a nucleotide prodrug that was variably approved around the world for parenteral administration to humans for treatment of SARS-CoV-2 during the COVID-19 pandemic.^19,20 Remdesivir was approved by the U.S. FDA and is available for off-label veterinary use in the United States (although sourcing from a human hospital pharmacy can be challenging); compounded parenteral formulations are also available in Australia, New Zealand, and the United Kingdom.

Remdesivir undergoes several hydrolysis steps to form the primary metabolite GS-441524 and then the pharmacologically active triphosphate GS-443902.^21-25 GS-443902 acts as an adenosine analogue and disrupts virus replication by incorporating into viral RNA via RNA-dependent RNA polymerase, causing delayed chain termination, inhibiting viral synthesis (FIGURE 1).^21,22,24,26 In cats, the in vivo conversion from remdesivir to GS-441524 is likely facilitated by blood esterases (rather than hepatic metabolism).²⁷ Regardless of whether a clinician administers remdesivir (the prodrug) or GS-441524 (the core nucleoside), the active triphosphate GS-443902 ultimately disrupts virus replication (FIGURE 1).

The pharmacokinetics of GS-441524 in specific pathogen–free cats after subcutaneous (n = 2) or intravenous (n = 2) administration of 5 mg/kg have been described.⁴ The intracellular concentration of the active triphosphate in peripheral blood mononuclear cells remains 8 to 20 times above the EC₅₀ (half maximal effective concentration) for 72 hours, supporting the effectiveness of once-daily subcutaneous or intravenous dosing of 5 mg/kg GS-441524.⁴ The in vivo pharmacokinetics of GS-441524 after intravenous administration of remdesivir to 3 clinically normal specific pathogen–free cats and cats with FIP have also been described.²⁸ In this study, 10 mg/kg PO molnupiravir, 25 mg/kg PO GS-441524 and remdesivir, and 7 mg/kg IV remdesivir resulted in plasma levels greater than the established corresponding EC₅₀ values, which are sustained over 24 hours for GS-441524 and remdesivir.²⁸

The pharmacokinetics of GS-441524 after intravenous remdesivir administration in 6 cats clinically affected with FIP has also been described; the mean peak plasma concentration (C_max) of GS-441524 after a single 15 mg/kg IV dose of remdesivir was 2632 ng/mL (standard deviation [SD] 862), time to reach C_max was 1 hour (SD 0), and elimination half-life was 5.14 hours (SD 0.81).²⁹

Although direct pharmacokinetic studies of GS-441524 administered orally to cats remain limited, the outcome data strongly support the efficacy of current oral dosing protocols. Across the literature, treatment with remdesivir and/or GS-441524 demonstrated remission and long-term survival rates of 77% to 96% after subcutaneous and oral administration (TABLE 1).^2,7-12

Molnupiravir and NHC

Molnupiravir (EIDD-2801), an isopropyl ester prodrug of the nucleoside analogue NHC (EIDD-1931), interferes with replication of various viruses, including SARS-CoV-2.^30,31 Because of its ability to tautomerize, molnupiravir can substitute for either cytidine or uridine and subsequently pair with either guanosine or adenosine in the RNA. In this way, molnupiravir acts as a mutagenizing agent that causes increased mutation frequencies above a tolerable threshold (i.e., a mutation rate that surpasses the level needed to maintain virus viability). This process is called “error catastrophe” and results in production of replication-incompetent genomes in subsequent rounds of RNA synthesis, which ultimately leads to virus death. A study showing pharmacokinetic data for molnupiravir showed promise for its use and safety in cats.²⁸ After oral administration at 10 mg/kg, molnupiravir was detected in the serum of all 3 study cats at low levels in the first 12 hours after administration, and its NHC metabolite was detected at much higher levels 12 to 24 hours after administration. NHC levels varied markedly between individual cats; molnupiravir and NHC levels peaked at 1.5 to 9 hours, and the level of NHC was 10- to 100-fold greater than that of molnupiravir. NHC was more consistently and quantifiably detected over the 24-hour period, except in 1 cat, for which NHC was no longer detected (based on limit of detection) 12 hours after administration. The study reported no evidence of acute organ toxicity in any of the cats according to pre- and post-treatment CBCs and serum biochemistry panels; however, all 3 molnupiravir-treated cats demonstrated variable signs of nausea, including hypersalivation and/or vomiting.

One study that examined 18 cats with FIP that received molnupiravir showed that molnupiravir might be an effective and safe treatment for domestic cats with FIP at a dosage of 10 to 20 mg/kg q12h.¹⁴ In this study, increased serum alanine aminotransferase (ALT) activity was found in 3 of the 18 cats at days 7 to 9, but all cats recovered without any medications or need to stop the treatment.¹⁴ Uncontrolled data from client surveys suggest effectiveness of molnupiravir as a rescue treatment after failure of GS-441524 therapy as well as a first-line treatment option for cats with FIP.¹³

Among cats with effusive FIP, an open-label longitudinal, noninferiority trial of 10 cats treated with molnupiravir (10 to 15 mg/kg PO q12h) for 84 days showed that molnupiravir is an effective antiviral treatment for effusive FIP.¹⁶ A prospective clinical trial with 62 cats with FIP that received molnupiravir showed treatment success for 77% of cats and relapse for 11%, but all responded to a higher dose of molnupiravir.³² A study of EIDD-1931 given PO q12h at a median dose of 18.75 mg/kg (range, 12 to 23.8) showed complete response for 9 cats (6 with effusive FIP and 3 with noneffusive FIP); however, this study reported several adverse effects such as neutropenia (3), increased ALT levels (transient in 3, permanent in 1), broken whiskers (1), and hyporexia (6). One cat that experienced relapse responded to an increased dosage (25 mg/kg q12h).³³ A recent study comparing treatment with GS-441524 and molnupiravir showed similar effects and safety for cats with FIP.¹⁵

By targeting mitochondrial RNA, molnupiravir/EIDD-1931 is intrinsically mutagenic for human RNA and DNA.³⁴ Therefore, gloves should be worn when handling the drugs, and the drugs should not be handled by pregnant women. Molnupiravir is not prescribed for humans under 18 years of age, and safety in kittens has yet to be assessed.

Despite preliminary data suggesting that survival outcomes after first-line treatment with molnupiravir and EIDD-1931 may be similar to those after remdesivir and GS-441524 treatment, the increased adverse effects and mutagenic potential of molnupiravir lead the authors to recommend using remdesivir and GS-441524 as first-line treatment for cats with newly diagnosed FIP when possible (TABLE 2).

Protease Inhibitors

After coronavirus entry into host cells, virus RNA is released and translated, resulting in the production and subsequent processing of 2 key enzymes that are vital for virus replication, 3-chymotrypsin-like protease (3CL^pro) and papain-like protease.^35-38 The enzymes are highly conserved across coronavirus species; therefore, drugs that inhibit them offer promise as broad-spectrum coronavirus antivirals.³⁵ The 2 main protease inhibitors of interest for FIP treatment are GC376 (Anivive Lifesciences, anivive.com) and nirmatrelvir (present in Paxlovid).

GC376

GC376 is a 3CL^pro protease inhibitor that stops the cleavage and activation of functional viral proteins required for replication and transcription in host cells. In vitro assessment of GC376 demonstrated potent antiviral effects against FIP virus in various cell cultures.³⁹ Among cats with experimentally induced FIP, 6 of 8 experienced reversal after treatment with GC376,³⁹ but among client-owned cats with naturally acquired FIP, the survival rate was lower (35%) than that achieved with nucleoside analogues.¹ A new study of combination therapy of 46 cats with FIP (36 effusive and 10 noneffusive) that received GC376 and GS-441524 for 4 weeks in different dosage regimens reported that 97.8% of cats responded well to the 4-week therapy although 2 cats required treatment extension of 7 to 12 weeks.⁴⁰ The study demonstrated that GS-441524 combined with GC376 can be safely and effectively used to treat FIP for a shortened treatment period. The absence of a GS-441524 monotherapy control group, however, makes it unclear whether similar outcomes could have been achieved with GS-441524 alone, particularly given the reported success of 6-week GS-441524 monotherapy.⁷ Further studies are needed to evaluate the effectiveness of combination therapy. At the time of this writing, there is no legal access to GC376; however, the drug is undergoing FDA approval for its use in cats with FIP.

Nirmatrelvir

Paxlovid, Pfizer’s antiviral combination for SARS-CoV-2, variably approved around the world, is available to veterinarians via off-label prescription in some countries, including the United States and Australia. Two tablets are contained within Paxlovid: nirmatrelvir (another 3CL^pro protease inhibitor but with a very short half-life) and ritonavir (a pharmacokinetic booster, which inhibits cytochrome P450 to delay nirmatrelvir metabolism). Nirmatrelvir inhibits in vitro replication of FIP virus serotypes I and II, and efficacy is increased when used in combination with other antivirals.²⁸

Paxlovid is being actively investigated for FIP treatment, although anecdotally it has been used as a last resort in combination with a nucleoside analogue for a handful of highly refractory cases. The lead author is aware of unpublished and anecdotal data describing 13 cases in which Paxlovid led to subsequent remission. Its use at this time therefore remains preliminary and experimental; however, for nonresponding cats with confirmed FIP, a dose of nirmatrelvir at 75 mg per cat and ritonavir at 25 mg per cat PO q12h could be considered in combination with continued nucleoside analogue treatment. Because ritonavir is a cytochrome P450 inhibitor, it may delay the metabolism of other medications processed by this pathway.

Conclusions

Judicious use of antivirals by the veterinary community is needed to mitigate risk for development of antiviral resistance (antivirals are antimicrobials too). According to the literature, despite massive use in humans within an 18-month period of the COVID-19 pandemic, very low levels of remdesivir resistance were observed worldwide.⁴¹ In contrast, analysis of SARS-CoV-2 genomes suggests that molnupiravir-associated mutational signatures now exist globally after introduction of molnupiravir for COVID treatment.⁴² In human medicine, introduction of molnupiravir has created new mutations in SARS-CoV-2. Neither of these antivirals should be used in healthy cats to eliminate shedding of feline coronavirus due to the risk for resistance as well as the high likelihood of reinfection among cats in multicat households.

Summary

Treatment for FIP has undergone a revolutionary transformation; antiviral therapies now offer improved survival rates among cats with a condition once considered a death sentence. Continued research and refinement of these therapies promise an even brighter future for affected cats and their caregivers.