Abstract
Objectives
Current antiviral therapy for feline infectious peritonitis (FIP) has limited availability and can be cost-prohibitive. The nucleoside analogue EIDD-1931 is an effective inhibitor against FIP virus serotype I and II in vitro. Our objective was to determine whether EIDD-1931 is an effective treatment option for cats with naturally occurring FIP.
Methods
A prospective clinical trial was conducted with nine client-owned cats diagnosed with effusive or non-effusive FIP including neurological involvement. Cats were administered EIDD-1931 orally twice daily for 12 weeks. A complete response was defined as resolution of all abnormalities associated with FIP. Clinical variables, haematology, biochemistry and imaging findings were monitored during treatment and after discontinuing treatment.
Results
Six cats with effusive FIP and three cats with non-effusive FIP (median age 1.0 year, range 0.5–7.9) were treated with EIDD-1931 (median dose 16.2 mg/kg PO q12h, range 12.0–20.5) for 12 weeks in eight cats and 14 weeks in one cat. All cats showed a complete response to treatment. Adverse effects included transient neutropenia (three cats), elevated alanine transaminase (transient in three cats, persistent in one cat), broken whiskers (one cat) and suspected treatment-induced relative hyporexia (six cats). Adverse effects were not dose dependent. Follow-up was available for more than 12 months after treatment discontinuation. One cat relapsed on two occasions, responding to repeat treatment at escalating dosages before subsequent development of treatment-resistant lymphoma, resulting in euthanasia 545 days after study enrolment. No other relapse events or diseases were observed.
Conclusions and relevance
EIDD-1931 administered at 15–20 mg/kg PO q12h for 12 weeks is a feasible treatment option for naturally occurring FIP. Adverse effects may be more common than treatment with GS-441524.
Introduction
Feline infectious peritonitis (FIP) was previously an almost universally fatal multi-organ disease caused by the FIP virus (FIPV), the second biotype of the single-strand RNA virus feline coronavirus (FCoV). 1 Recent advances in antiviral therapy have resulted in a potential cure for FIP, with the nucleoside analogue GS-441524 and its parent drug remdesivir becoming standard-of-care treatment in countries with access to these antivirals.2 –4 These drugs, however, are not widely available globally, as well as being cost-prohibitive, resulting in many owners electing humane euthanasia or turning to unlicensed therapies.5,6
A novel antiviral, EIDD-1931, and its parent drug molnupiravir, are effective in vitro and in naturally occurring FIP, respectively.7 –9 These compounds are less expensive in Australia and many other countries; therefore, they have the potential to be an accessible treatment to a much wider population. Licensed molnupiravir is available via private prescription in Australia. However, it does not come in an appropriate form to provide accurate dosage to cats and thus needs to be compounded.
Limited information is currently available regarding the pharmacokinetics of molnupiravir in cats. Pharmacokinetics of orally administered molnupiravir (10 mg/kg single dose) has been published in a small group of healthy cats, with measurement of both molnupiravir and its metabolite EIDD-1931. 7 In this study, EIDD-1931 reached a maximum concentration at a mean time of 4.3 h, with serum exposure levels greater than the half-maximal effective concentration (EC50) determined for FIPV early within the 24-h period, but falling below the EC50 by the 12-h time point in one cat, which supports the currently accepted twice daily dose protocol. 7 There are no current data on the pharmacokinetics of the compound EIDD-1931 in cats.
The aim of this pilot study was to determine whether the compound EIDD-1931 can cure naturally occurring FIP and therefore be a viable alternative for cats with FIP. Our hypothesis was that EIDD-1931 would be efficacious in the treatment of FIP.
Materials and methods
Study design
A single-institution, prospective, open-label, clinical trial was performed in cats with a confirmed or presumptive diagnosis of FIP between May 2023 and October 2023. The standard care recommended was oral, once daily treatment with the legally available nucleoside analogue GS-441524 produced locally by BOVA Compounding for a period of 84 days. Cats with a confirmed or presumptive diagnosis of FIP were offered enrolment if standard treatment was declined because of financial constraints, and the owners would otherwise elect for humane euthanasia, or if standard treatment had been unsuccessful in cats with a confirmed diagnosis of FIP.
Written client-informed consent for this pilot study was explicitly obtained. Ethics approval was obtained from NSW Government Animal Care and Ethics Committee (project number RVF 23/2698).
Inclusion criteria
A diagnosis of FIP was made by the lead investigator (AK, resident in internal medicine) under supervision of a board-certified internist (JW) following the Advisory Board on Cat Disease and American Association of Feline Practitioners (now Feline Veterinary Medical Association) guidelines.1,10 A presumptive diagnosis of FIP was made on a combination of signalment, clinical signs, haematology and biochemical findings, and cytological examination of effusions or lymph node or organ fine-needle aspirates. A confirmed diagnosis via positive direct immunofluorescence (IFA) or PCR on effusions or aspirates was desirable but not required for cats receiving EIDD-1931 for first-line treatment. A confirmed diagnosis was required for cats receiving EIDD-1931 as second-line treatment.
Treatment protocol
Cats were treated with EIDD-1931 at a dose as close as practical to 15 mg/kg PO q12h (range 12–20), taking into account the practicality of tablet size, for 84 days. The treatment was provided by BOVA Compounding in the form of scored 60 mg tablets that could be quartered. EIDD-1931 was produced by BOVA Compounding as a commercially available quality-assured compounded product. 11 The dose was estimated based on the reported dosage for the prodrug molnupiravir of 10–15 mg/kg PO q12h (range 10–20) at the time of study design. 5
Cats were assessed by the lead investigator at enrolment (day 0) and at weeks 2, 6 and 11, with a physical examination, haematology and serum biochemistry performed at each visit and repeat imaging performed dependent on each cat’s form of disease. Cats with effusive disease had repeat point-of-care ultrasound (POCUS) performed at week 2 and then weekly until effusion resolved. Cats with dry disease had repeat imaging performed at week 11. Cats with neurological disease did not have repeat imaging performed. Cats were then followed for the next 12 months with recommended reassessment at 1, 3, 6 and 12 months after the treatment course. Body condition score (BCS) was recorded and compared at each visit, with the medication dose adjusted in cats with significant weight gain. 12
Disease remission, defined as resolution of clinical signs, haematological and biochemical abnormalities, and resolution of effusion or other imaging abnormalities, was assessed at week 11. Ongoing remission at follow-up assessments was defined by resolution of clinical signs and stable body weight, with repeat haematology and biochemistry performed where possible.
Supportive care was provided throughout the study at the discretion of the attending veterinarian.
Efficacy of EIDD-1931 for the treatment of FIP was assessed as follows: (1) by survival of patients throughout the trial period; (2) by resolution of clinical signs and resolution to normal physical examination parameters; and (3) by resolution of organ dysfunction and with resolution of laboratory abnormalities to values within the reference interval (RI).
Statistical analysis
Descriptive statistics were performed.
Results
Study population
Nine cats were enrolled in the trial. Eight cats were enrolled in the trial after a diagnosis (3/8 confirmed, 5/8 presumptive) of FIP and declining standard treatment due to financial constraints. One cat was enrolled in the trial after a confirmed diagnosis of abdominal effusive FIP and failing to respond to escalating treatment with standard treatment. There were seven male castrated cats and two female spayed cats with a median age of 1 year (range 0.5–7.9). On initial examination, 5/9 cats had a fever and their median BCS was 3.5/9 (Table 1). Baseline haematological and biochemical parameters were determined (Table 2).
Descriptive features of cats
ALT = alanine transaminase; DMH = domestic mediumhair; DSH = domestic shorthair; FS = female spayed; ICCJ = ileocaecalcolic junction; IFA = immunofluorescence; LN = lymph node; MC = male castrated; N/A = not available
Haematological and biochemical changes and body condition score (BCS) throughout initial treatment course
Data are median (range)
A:G = albumin:globulin; ALT = alanine transaminase; BCS = body condition score; RI = reference interval
Five cats had peritoneal effusion, one cat had pleural effusion, two cats had intestinal mural granulomas at the ileocaecalcolic junction (ICCJ) with palpable mass effect, and one cat had abdominal lymphadenopathy and concurrent T3–L3 myelopathy. Cats with effusive disease underwent POCUS imaging, while cats with dry disease had a full abdominal ultrasound performed by a board-certified radiologist. All cats had cytology of effusion or fine-needle aspirate samples reviewed by a board-certified pathologist, with no evidence of bacterial or atypical infection, neoplasia or other concurrent disease process noted in cases with effusion. Cats with fine-needle aspirate samples of lymph nodes or organs (n = 3) had results consistent with pyogranulomatous inflammation without evidence of micro-organisms.
Direct IFA was the preferred viral testing performed, given the high specificity (100%) for IFA in Australia. 13 Two cats with effusive disease had direct IFA performed, which was positive in both cases (including the cat receiving EIDD-1931 as a second-line treatment), while one cat with effusive disease had PCR performed, which was positive for FIPV biotype. Direct IFA was performed on both cats with ICCJ granuloma and was positive in one cat. Direct IFA was also performed on lymph node aspirates in cat 4 with neurological signs and was positive.
The median starting dose was 16.2 mg/kg PO q12h (range 12.0–20.5). This was based on the practicalities of tablet size and weight of each cat at the time of enrolment. Cat 4, with neurological signs, had progression in the first 48 h of treatment and subsequently had his dose increased from 16 to 20 mg/kg and concurrently received a 6-day course of prednisolone (1.25 mg/kg PO q24h).
Survival analysis
All nine cats responded to initial therapy, with disease remission documented at 11 weeks for 8/9 cats and at 14 weeks for one cat (cat 1), which had persistent hyperglobulinaemia at 11 weeks. The cat (cat 1) with persistent hyperglobulinaemia received 14 weeks of antiviral therapy, while all other cats received 12 weeks of therapy.
The cat (cat 2) that received EIDD-1931 as second-line therapy after failing GS-441524 therapy initially entered treatment remission; however, he was confirmed to have relapsed 10 weeks after discontinuing antiviral therapy, when he re-presented for weight loss and was found to have recurrent abdominal effusion, marked lymphadenomegaly, non-regenerative anaemia, hyperglobulinaemia and hypoalbuminaemia. Extensive diagnostics did not reveal any new or concurrent disease process, and lymph node biopsy was consistent with reactive hyperplasia. T- and B-cell immunohistochemistry on the lymph node was not consistent with neoplasia. FIP immunohistochemistry on the lymph node, as well as direct IFA on abdominal effusion, were negative. This cat was restarted on EIDD-1931 at an increased dose of 22 mg/kg PO q12h (initially treated with 12 mg/kg PO q12h), and remission was achieved after a further 12 weeks of therapy. Cat 2 was subsequently maintained on mefloquine (62.5 mg PO twice weekly) (outside the scope of this trial) until it was discontinued after 4 months because of the development of vomiting. Two weeks later, 411 days after originally entering the trial, this cat represented for weight loss, with persistent peripheral lymphadenopathy, moderate abdominal effusion and recurrent non-regenerative anaemia, hypoalbuminaemia and hyperglobulinaemia. Treatment was restarted with EIDD-1931 at a dose of 25 mg/kg PO q12h with concurrent nirmatrelvir/ritonavir (Paxlovid) (outside the scope of this trial). Despite achieving remission for FIP at day 64, the cat had progressive weight loss, and repeat imaging revealed a hepatic mass with cytology consistent with large cell lymphoma. This cat entered combined antiviral and chemotherapy treatment but developed progressive clinical signs of malaise and was humanely euthanased. Necropsy revealed large cell lymphoma (hepatic mass, optic nerve), with no evidence of persistent vasculitis, peritonitis or pyogranulomatous disease in any organ.
All other cats remain well more than 12 months after completing antiviral therapy. The median survival time was therefore not reached for this group of cats.
Adverse events and comorbidities
The most common adverse event was mild biochemical increase in ALT, which was observed in 4/9 cats. This increase resolved after discontinuation of antiviral therapy in three cats and persisted in one (cat 1). Diagnostics to evaluate the persistent ALT elevation in one cat were recommended but declined by the owner on financial grounds, as the cat was clinically well.
Three cats (cats 3, 8 and 9) had neutropenia at week 2, which was severe in two cats (0.7 × 109/l and 0.1 × 109/l, respectively; RI 1.8–9.6]) and mild in one cat (1.4 × 109/l). All cats were normothermic and subclinical. Antiviral therapy was not changed; however, the cat (cat 8) with a neutrophil count of 0.1 was prescribed prophylactic antibiotics (potentiated amoxicillin 20 mg/kg PO q12h), and all cats were monitored with serial haematology. Neutropenia resolved at weeks 3 (cat 9), 4 (cat 3) and 6 (cat 8).
Cat 9 was diagnosed with concurrent bacterial pyelonephritis on day 28 of treatment after presenting for acute hyporexia and vomiting, with neutrophilia, mild progression in creatinine, positive urine culture with Escherichia coli and sonographic changes supportive of pyelonephritis. Treatment-associated neutropenia had resolved in this cat 1 week before the development of clinical signs consistent with pyelonephritis. Treatment with marbofloxacin (5 mg/kg PO q24h for 14 days) was associated with resolution of clinical signs, neutrophilia and azotaemia. Urine culture was negative after antimicrobial therapy.
Complete shedding of whiskers occurred in one cat (cat 9), noted at week 11 of treatment (Figure 1), which was then noted as regrowing 3 months after discontinuation of therapy (Figure 2).
Missing whiskers at week 11 of treatment
Whisker regrowth, 3 months after discontinuing treatment
Secondary outcomes
Time for resolution of fever was not documented as cats were treated as outpatients. A general improvement in demeanour and appetite was observed in 2–5 days in all cats. Effusion was documented as resolved at a median of 17.8 days (range 14–24). Neurological deficits were resolved at 14 days. Hyperbilirubinaemia was documented as resolved in 14 days. Decreased albumin:globulin ratios normalised by a median of 52.6 days (range 14–85). Granulomatous lesions were still palpable on physical examination at week 6 of treatment and were resolved based on repeat examination and repeat imaging at 84 days. The median BCS at the end of the treatment period was 5.1/9 (range 4–6), with a median BCS of 6.2/9 (range 5–7) 12 weeks after completing treatment.
Discussion
EIDD-1931 was an effective treatment for naturally occurring FIP. Clinical remission occurred in all cats treated with EIDD-1931 as a first-line treatment, with no evidence of relapse over the subsequent 12 months.
Survival rates in this group were similar to those published for GS-441524 and molnupiravir as a first-line therapy.2 –4,8,9 This is useful information as this pilot group included cats with dry and neurological involvement in addition to effusive cases.
Cats with both dry and effusive disease were treated with the same dose, with a goal starting dose of 15 mg/kg. It is possible that cats with effusive disease may be able to be treated with a lower dose; however, the cat with effusive disease (cat 2) that received the 12 mg/kg dose relapsed. The relapse could be due to features of this cat’s specific virus or its host response given that this is also the cat that did not respond to GS therapy; however, this could indicate that a dose below 15 mg/kg may be ineffective.
The cat with neurological signs (cat 4) had his dose increased from 16 mg/kg to 20 mg/kg in the first week of treatment because of progressive neurological signs in the first 48 h. It is unknown whether this dose increase was truly needed or if subsequent improvement was due to concurrent administration of anti-inflammatory steroid therapy. Given that the dose of 20 mg/kg was well tolerated, this remains a proposed dose of EIDD-1931 for cats with neurological involvement.
The major limitations of this pilot study are the lack of pharmacokinetic data of EIDD-1931, the current lack of knowledge over optimal dosing of EIDD-1931 and the small number of cats in this study, making dosage recommendations limited. EIDD-1931 serum measurement was not performed within the scope of this study because of the lack of availability of this assay in Australia. Residual blood samples have been stored with the aim of being able to determine pharmacokinetic data in the future. Doses described in this study showed efficacy in treating FIP; however, the optimal dosing is not known. When considering the molecular weights of the prodrug molnupiravir (329.31 g/mol) and the core nucleoside EIDD-1931 (259.23 g/mol), 15 mg of molnupiravir would be expected to yield 11.8 mg of EIDD-1931.14,15 Based on this, the initial intention was to treat cats with a dose closer to 10 mg/kg of EIDD-1931, to correlate more closely with accepted molnupiravir dosing. However, the first cat (cat 1) in the study was initially treated at a dose rate of 15.7 mg/kg because of the practicality of tablet size and was observed to have a slower response than typically seen with GS-441524 treatment, with persistent significant abdominal effusion at day 15 that did not resolve until day 24 of treatment. It was this initial experience that resulted in aiming for as close as practical to 15 mg/kg; therefore, these cats were administered a slightly higher dose compared with the described dose for molnupiravir. How both these compounds are converted to the active triphosphate in vivo may differ, however, and there may be significant inter-individual variations in metabolism given the variation in molnupiravir metabolism. 7 The relationship between serum drug concentration and treatment outcomes has not been established for any antiviral used for treating FIP but could influence doses for all antivirals in the future.
Effusion was potentially slower to resolve (median 17.8 days) compared with a reported median of 9.5 days with remdesivir/GS-441524 therapy. A similar palpable mass effect remained present at 6 weeks in dry cases compared with reported resolution at 6 weeks with remdesivir/GS-441524 therapy in a prospective study. 4 This suggests that EIDD-1931 may not induce disease remission as quickly as remdesivir/GS-441524 therapy, and a 12-week treatment recommendation remains based on these data, in comparison with a recent study suggesting that a 6-week treatment course could be equally efficacious in cats with effusive disease receiving GS-441524 therapy. 16 However, given the small study size, only limited conclusions can be drawn, and randomised controlled studies would be required to determine the true differences in treatment response.
Subjectively, these cats gained weight more slowly than in the authors’ clinical experience with GS-441524 therapy, with a median BCS of 5.1/9 at week 11 of treatment, with some cats remaining under conditioned. All cats gained further weight and owners reported improved appetite after discontinuing therapy, consistent with EIDD-1931-induced nausea or decreased appetite. One benefit of the lack of significant weight gain during treatment for adult cats was that no dosage adjustment was required during the treatment period.
No adverse effect required treatment discontinuation; however, a greater proportion of cats experienced some type of adverse effect compared with treatment studies with alternative antivirals.2,8,9 This may be due to the small number of cats in this study or may indicate a less favourable side effect profile, which is plausible given the increased cytotoxicity of EIDD-1931 in vitro. 17 Shedding of whiskers has been reported once before with the use of unlicensed molnupiravir and raises concern about increased cytotoxicity of EIDD-1931. 5 Dose-related myelosuppression has been reported with doses of molnupiravir greater then 23 mg/kg q12h. 5 Neutropenia was observed in three cats that were receiving 18–19 mg/kg, but this adverse effect was not clearly dose-dependent, with two cats receiving more than 20 mg/kg (due to neurological signs in one cat and relapse in the second) not experiencing any haematological adverse effects.
One cat (cat 2) developed lymphoma during the observation period and was subsequently euthanased. This cat failed to respond to GS-441524 therapy and was treated for multiple relapse events. It is not known if chronic antigenic stimulation from persistent FIP infection or cytotoxicity from antiviral therapy could have increased this cat’s risk for developing neoplasia or if this development was completely unrelated given the cat was 7 years of age and negative for feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) on point-of-care testing (Anigen Rapid FIV antibody/FeLV antigen). No other diseases were noted in the 12-month observation period in the remaining eight cats.
A limitation of this study was that cats enrolled were typically well enough to be treated as an outpatient; therefore, efficacy in severely ill patients has not been evaluated. However, this was not a specific exclusion criterion for enrolment. At this stage, there is no evidence that one antiviral is more favourable than others in inducing treatment remission in the acute clinical setting.
Another limitation of this study was the lack of a confirmed diagnosis in most cases; however, clinical remission occurred in all cats in the first treatment course, confirming the presumptive diagnosis. This approach reflects a common scenario encountered in clinical practice.1,10
Conclusions
EIDD-1931 administered at 15–20 mg/kg PO q12h for 12 weeks is a feasible treatment option for naturally occurring FIP. Adverse effects may be more common than in treatment with GS-441524. Further research is required to determine optimal dosage and true differences in efficacy and adverse effects between antiviral protocols.
Footnotes
Author note
This article was presented as an abstract at the American College of Veterinary Internal Medicine Forum 2024.
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
Support was provided in kind from BOVA Compounding.
Ethical approval
The work described in this manuscript involved the use of non-experimental (owned or unowned) animals and procedures that differed from established internationally recognised high standards (‘best practice’) of veterinary clinical care for the individual patient. The study therefore had prior ethical approval from an established (or ad hoc) committee as stated in the manuscript.
Informed consent
Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers, tissues and samples) for all procedure(s) undertaken (prospective or retrospective studies). For any animals or people individually identifiable within this publication, informed consent (verbal or written) for their use in the publication was obtained from the people involved.
