Evaluation of the course of improvement with molnupiravir treatment for feline infectious peritonitis

Abstract

Objective

To clarify the clinical course during molnupiravir treatment for feline infectious peritonitis (FIP).

Animals and procedure

Cats diagnosed with FIP and treated with molnupiravir at Hokkaido University Veterinary Teaching Hospital (Sapporo, Hokkaido, Japan) were retrospectively reviewed.

Results

Eleven cats were eligible for inclusion. Six cats had effusive FIP and 5 had non-effusive FIP. In noneffusive cases, 2 cats had neurological abnormalities at diagnosis, whereas 1 additional cat developed neurological signs during treatment. The median initial dosage of molnupiravir was 13.0 mg/kg (range: 10.0 to 15.0 mg/kg), PO, q12h. One cat died after 11 d and the remaining 10 cats completed an 84-day course of treatment. All neurological cases were given dosage increases, extended treatment duration, or both. The median final dosage of molnupiravir in non-neuro-FIP cases was 13.1 mg/kg (range: 10.0 to 15.0 mg/kg), PO, q12h, whereas dosages in neuro-FIP cases were 15.0, 15.2, and 17.2 mg/kg, PO, q12h in the 3 affected cats, respectively. In non-neurological cases, dysrexia, lethargy, and high serum amyloid A were resolved within 15 d. Total follow-up duration ranged from 175 to 362 d. No relapses were observed.

Conclusion and clinical relevance

Monitoring responses to molnupiravir treatment requires observing clinical signs and conducting clinicopathological evaluations, including acute-phase protein evaluation.

INTRODUCTION

Feline infectious peritonitis (FIP) is caused by feline coronavirus, one of the positive-stranded ribonucleic acid (RNA) viruses that belongs to the family Coronaviridae. There are various FIP phenotypes, including effusive or non-effusive; both can have neurological or ocular involvement, or both. If left untreated, FIP is a lethal disease for affected cats. The best-documented effective treatment is the adenosine nucleoside analog GS-441524 and its prodrug, remdesivir. Reported survival rates are from 84 to 86%, with recurrence rates ranging from 0 to 31%, depending on treatment protocol (1–3).

Molnupiravir (EIDD-2801) is another ribonucleoside analog, the oral prodrug of beta-D-N4-hydroxycytidine (NHC) (EIDD-1931). This compound has proven broadspectrum antiviral activities against RNA viral diseases (4,5), including SARS-CoV-2, which also belongs to the family Coronaviridae and is the causative agent of coronavirus disease 2019 (COVID-19) (6). During viral replication by viral RNA-dependent RNA polymerase, NHC replaces cytidine triphosphate or uridine triphosphate as a substrate and prevents viral replication through induction of RNA mutation (7). In December 2021, molnupiravir received emergency-use authorization from the Food and Drug Administration and Japan’s Ministry of Health, Labor and Welfare for treatment of COVID-19, but it is unlicensed for the treatment of FIP in cats.

Information on the use of molnupiravir for cats with FIP is currently limited. In a previous study, molnupiravir was primarily used as a rescue treatment for relapsed cases with GS-441524, especially for neurological cases (8). However, that study was based on an online survey, relying on caregivers’ responses about diagnosis and remission. Another study monitored its effectiveness against FIP with standardized dosage and duration, but the treatment course was not detailed, except for hematocrit, serum α1-acid protein (AGP), and albumin:globulin ratio (9,10).

Therefore, the objective of this study was to characterize responses to molnupiravir treatment for FIP, including neurological cases.

MATERIALS AND METHODS

From January 2022 to January 2023, a retrospective study was conducted at Hokkaido University Veterinary Teaching Hospital (Sapporo, Hokkaido, Japan) on cats diagnosed with FIP. All cats received treatment with molnupiravir, the only nucleoside analog used for FIP treatment at Hokkaido University during this interval. This work involved only privately owned animals. Written consent was obtained from the caregivers of all animals, in accordance with institutional guidelines for client-owned animals. Given the retrospective nature of this study, approval of the Institutional Animal Care and Use Committee was not required.

The diagnosis of FIP was established via either histopathologic or quantitative reverse transcription polymerase chain reaction (RT-qPCR) (FIP Virus RealPCR Test; IDEXX Laboratories, Westbrook, Maine, USA) evaluation of cavital effusion, lymph nodes, or spleen in cases without neurological signs at diagnosis. Cases with neurological signs proceeded to magnetic resonance imaging (MRI) (Vantage Galan 3T; Canon Medical Systems, Tochigi, Japan) and were diagnosed as presumptive FIP based on MRI findings (11). We collected data on the following: signalment, clinical signs, physical examination findings, complete blood (cell) count (ProCyte Dx Veterinary CBC Hematology Analyzer; IDEXX Laboratories), serum biochemistry (DRI-CHEM NX500 or NX700; Fujifilm, Tokyo, Japan), serum amyloid A (SAA) concentration (DRI-CHEM IMMUNO AU10V; Fujifilm), molnupiravir dosage, treatment duration, and possible adverse reactions. However, AGP was not measured. Molnupiravir was marketed for use in humans (LAGEVRIO Capsules 200 mg; Merck Sharp & Dohme, Rahway, New Jersey, USA) and used with caregivers’ consent. Lymph nodes were considered enlarged if the width was > 6 mm and unremarkable if thinner than this cutoff (12). The molnupiravir starting dosage and duration were set between 10 and 15 mg/kg, PO, q12h for 84 d, with dosage and treatment duration modified at the clinician’s discretion. For starting dosages, there were no planned differences between phenotypes (effusive versus noneffusive and with versus without neurological involvement). Follow-up plans and recheck appointments were determined by each attending veterinarian. Regarding analysis of the interval to normalization for clinical, clinicopathological, and imaging abnormalities, cases were excluded if follow-up information within 30 d of the start of treatment was unavailable. Because a relapse was reported within 84 d after the course of treatment in a previous GS-441524 study (1), follow-up phone calls were made at least 84 d after the end of the treatment. Caregivers were asked about appetite, activity levels, and the presence or absence of any clinical signs suggestive of relapse.

RESULTS

Patient data

Eleven cats were included in this study. The median age was 18 mo (range: 3 to 85 mo). Four castrated males, 6 spayed females, and 1 intact female were included. Breeds included were as follows: domestic shorthair (n = 8), munchkin (n = 1), Persian (n = 1), and minuet (n = 1). Patient information is summarized in Table 1.

TABLE 1.

Information regarding 11 cats with feline infectious peritonitis (FIP) that were treated with molnupiravir.

Case	Age (mo)	Breed	Effusion	Visceral lesions	Method of diagnosis	Initial dose (mg/kg, q12h)	Final dose (mg/kg, q12h)	Other treatment	Outcome	Follow-up (d)
1	20	Persian	—	NA	MRI, CSF inaccessible	15.0	15.0	PSL	Remission	190
2	14	DSH	—	−	MRI, CSF inaccessible	12.0	17.2	None	Remission	362
3	18	Minuet	—	+	PCR (spleen FNA)	11.1	15.2	None	Remission	176
4	9	DSH	—	+	Histopathology	14.5	13.1	Surgery	Remission	180
5	12	Munchkin	—	+	PCR (LN FNA)	10.6	11.7	PSL	Remission	246
6	75	DSH	Ascites	+	PCR (pleural effusion, at rDVM)	10.0	10.0	PSL	Remission	289
7	24	DSH	Ascites	−	PCR (ascites)	11.9	11.4	PSL, mirtazapine	Remission	204
8	47	DSH	Pleural effusion	−	PCR (pleural effusion)	14.3	15.0	PSL, mosapride, mirtazapine	Remission	235
9	11	DSH	Ascites	−	PCR (ascites)	13.0	13.4	PSL, mirtazapine	Remission	189
10	85	DSH	Pleural effusion	NA	PCR (pleural effusion)	14.3	13.6	None	Remission	175
11	3	DSH	Ascites	−	PCR (ascites)	14.0	—	None	Deceased	—

CSF — Cerebrospinal fluid; DSH — Domestic shorthair; FNA — Fine-needle aspiration; LN — Lymph node; MRI — Magnetic resonance imaging; NA — Not available; PCR — Polymerase chain reaction; PSL — Prednisolone; rDVM — Referring veterinarian.

Based on the examination findings, 6 cases were classified as effusive FIP and the diagnosis was established based on RT-qPCR using pleural or abdominal effusion. The remaining 5 cases were classified as non-effusive FIP. In non-effusive cases without neurological signs at the first appointment, the diagnosis was established based on histopathology (n = 1) or RT-qPCR of lymph nodes (n = 1), and spleen (n = 1). Two cats (Cat 1 and Cat 2) had ataxia at diagnosis and proceeded to MRI. The MRI findings in both cases included a failure to suppress the cerebrospinal fluid (CSF) signal in T2-weighted fluid-attenuated inversion recovery images, enlarged lateral ventricles, meningeal contrast enhancement in the cervical spinal cord, and herniation of the caudoventral aspect of the cerebellum. Cat 1 also had ependymal contrast enhancement of the lateral ventricles. In both cats, CSF could not be obtained due to suspicion of high intracranial pressure. Another cat (Cat 3) developed hind-limb paresis during treatment, and 3 cats were finally considered presumptive neuro-FIP.

Dysrexia was detected in 91% (10/11) and lethargy in 82% (9/11) of cases. Furthermore, 45% (5/11) of cats were pyrexic at diagnosis. Ascites, mesenteric lymph node enlargement, and pleural effusion occurred in 40% (4/10), 27% (3/11), and 20% (2/10), respectively; with lymph nodes that were 6.1, 10.0, and 20.0 mm wide, respectively. Vomiting, diarrhea, labored breathing, and ataxia were detected in 2 cases each. Regarding clinicopathological abnormalities, all cats had a low albumin:globulin ratio (median: 0.42, range: 0.32 to 0.58), whereas 82% (9/11) of cats had high SAA and 73% (8/11) had hyperproteinemia (Table 2). A complete ophthalmic examination was not done in all cases.

TABLE 2.

Clinical and clinicopathological abnormalities in 11 cats at diagnosis of feline infectious peritonitis (FIP).

Clinical abnormality	%	Proportion	Clinicopathological abnormality	Reference interval	%	Proportion
Dysrexia	91	(10/11)	Low albumin/globulin	≥ 0.6	100	(11/11)
Lethargy	82	(9/11)	High SAA	< 5.5 μg/mL	82	(9/11)
Pyrexia (> 39.2°C)	45	(5/11)	Hyperproteinemia	57 to 78 g/L	73	(8/11)
Ascites	40	(4/10)	Hyperglobulinemia	27 to 52 g/L	55	(6/11)
Mesenteric LN enlargement	27	(3/11)	Anemia	24.0 to 45.0%	36	(4/11)
Pleural effusion	20	(2/10)	Neutrophilia	1.5 to 10.0 × 109/L	36	(4/11)
Vomiting	18	(2/11)	Hyperbilirubinemia	1.7 to 6.8 μmol/L	29	(2/8)
Diarrhea	18	(2/11)	Hypoalbuminemia	23 to 35 g/L	18	(2/11)
Labored breathing	18	(2/11)
Ataxia	18	(2/11)

LN — Lymph node; SAA — Serum amyloid A.

Treatment responses, dosages, and concurrent drugs

The summary of treatment courses is presented in Figure 1. The median molnupiravir starting dosage was 13 mg/kg (range: 10 to 15 mg/kg), PO, q12h, with no obvious differences between cats with effusive or non-effusive FIP and cats with or without neurological involvement. One cat with effusive FIP died 11 d after starting treatment; this was suspected to be due to progression of FIP with neurological involvement and worsening of bloodwork parameters, but was not confirmed by necropsy. The remaining 10 cats successfully finished the treatment course. Cat 10 stopped receiving treatment in the middle of the course due to a communication error. The cat was doing clinically well during cessation of treatment and the reason for restarting was unavailable in the medical record. The total treatment duration for Cat 10 was 65 d.

FIGURE 1.

Time scale of treatments and clinical outcomes for the 11 cats included in this study.

FIP — Feline infectious peritonitis; MOV — Molnupiravir.

All cats with neuro-FIP received dosage increases, prolongation of treatment, or both. Two cats were given dosage increases. One cat (Cat 2) had a resolution of ataxia and was doing clinically well on Day 57. However, because it developed high SAA (12.1 μg/mL), molnupiravir dosage was increased to 15.8 mg/kg, PO, q12h, on the same day. The cat maintained a clinically normal state and SAA was < 3.8 μg/mL on Day 85. Still, the attending clinician decided to treat the cat with an increased dosage of 17.2 mg/kg, PO, q12h, for an additional week (total of 92 d). Another cat (Cat 3), initially treated with 11.1 mg/kg, q12h, developed hind-limb paresis on Day 21, and the dose was increased to 15.7 mg/kg on Day 22. The remaining neuro-FIP case (Cat 1) was treated with 15 mg/kg, q12h, throughout the course. In a first follow-up MRI on Day 50, there was remaining contrast enhancement in the cervical spinal cord. The cat was treated until the second follow-up MRI on Day 95. As a result, all neuro-FIP cases (Cats 1 to 3) were ultimately treated with a dosage of at least 15 mg/kg, q12h (15.0, 17.2, and 15.2 mg/kg, q12h, respectively). Non-neuro-FIP cases (Cats 4 to 10) were finally treated with a median dosage of 13.1 mg/kg (range: 10.0 to 15.0 mg/kg), PO, q12h. The median final molnupiravir dosage of cats overall was 13.6 mg/kg (range: 10 to 17.2 mg/kg), PO, q12h.

At the start of treatment, 6 cats were concurrently prescribed prednisolone, with dosages from 0.7 to 1.0 mg/kg, PO, q24h, and a median duration of 9 d (range: 1 to 36 d). Two cats were prescribed mirtazapine topical ointment (q24h) and 1 cat was prescribed mosapride (0.7 mg/kg, PO, q24h for 4 d).

Normalization for clinical, clinicopathological, and imaging abnormalities

The interval to normalization of abnormal findings after the diagnosis or a dosage increase is shown in Figure 2. Two cats without blood examinations within 30 d were excluded from analysis of blood examination results. Dysrexia and lethargy resolved in a median of 11 d (range: 4 to 31 d). In 5 out of 6 non-neuro-FIP cases with follow-up within 30 d, caregivers noticed improvement in as early as 2 to 6 d. A cat that took 31 d to fully recover appetite and energy levels was a neuro-FIP case (Cat 1). In neuro-FIP cases, ataxia or hind-limb paresis resolved in 15, 67, and 77 d, respectively. The median resolution duration of pleural or abdominal effusion was 14 d (range: 12 to 18 d). Cats with mesenteric lymph node enlargement were serially assessed with repeated ultrasonography, and complete resolution was observed on Days 13, 19, and 44. The low albumin:globulin ratio, hyperproteinemia, and hyperglobulinemia were not resolved in 4/7, 3/6, and 2/6 cases (out of the cases with an abnormality at diagnosis and follow-up within 30 d). High SAA was resolved within 15 d in non-neuro-FIP cases.

FIGURE 2.

Days to normalize abnormal findings. The median is shown as a line within the box, whereas the 1st and 3rd quartiles correspond with the lower and upper hinges. Upper and lower whiskers represent the largest and smallest values no further than 1.5 times from the hinge. Dots represent outliers. Cases were excluded if follow-up information within 30 d was unavailable or abnormalities did not resolve during observation.

LN — Lymph node; SAA — Serum amyloid A.

In non-neuro-FIP cases, SAA was high at diagnosis and decreased rapidly after the start of molnupiravir treatment (Figure 3). Cat 11 died though SAA showed a downward trend. In contrast, in presumptive neuro-FIP cases, SAA was within the reference interval at the time of emergence of neurological signs and was mildly elevated during treatment. Cat 2 (Figure 3, light green line) became clinically normal on Day 15. Serum amyloid A started to elevate at Day 57; the dosage was increased to 15.8 mg/kg, q12h; and SAA normalized 28 d after the dosage increase. Cat 3 (Figure 3, dark blue line) developed hind-limb paresis despite SAA normalization. The dosage of molnupiravir was increased, and all clinical signs had resolved on Day 89. Treatment was therefore not extended, though SAA was mildly elevated (13.4 μg/mL, reference: < 5.5 μg/mL) on Day 89.

FIGURE 3.

Change in serum amyloid A (SAA) concentration during observation, with the Y-axis in a logarithmic scale. The dotted horizontal line is the reference range (< 5.5 μg/mL) (Fuji DRI-CHEM IMMUNO AU10V; Fujifilm, Tokyo, Japan).

FIP — Feline infectious peritonitis.

Possible adverse reaction

One cat with neuro-FIP (Cat 1) developed acute vomiting on Day 91, and molnupiravir was stopped on Day 95. The cat was prescribed maropitant (2 mg/kg, PO, q24h for 3 d), sucralfate (1000 mg, PO, q24h), and famotidine (0.7 mg/kg, PO, q24h for 7 d) for gastrointestinal signs, and vomiting resolved in several days. In another cat with non-neuro-FIP (Cat 8), intermittent diarrhea was observed from Day 12 but had resolved at the time of a follow-up phone call. Alanine aminotransferase (ALT) was not measured after diagnosis in all cats, and these follow-up data were unavailable. Leukopenia, folded ear tips, pruritis, hair loss, and obvious development of jaundice were not observed during follow-up.

Follow-up results

Follow-up information was available for all 10 cats that completed the treatment. The duration of follow-up after the end of molnupiravir treatment ranged from 85 to 271 d (median: 113 d). Overall, the duration of follow-up after the diagnosis was between 175 and 362 d (median: 197 d). According to caregivers, all cats maintained good appetite and energy levels, and no signs of relapse were observed.

DISCUSSION

This report describes the course of improvement during molnupiravir treatment and monitoring of clinical signs, clinicopathological findings, and imaging abnormalities. This information is necessary to aid future clinical decision-making regarding dosage increases during molnupiravir treatment or possible switch to GS-441524 treatment. In non-neuro-FIP cases, clinical signs of dysrexia and lethargy resolved within 2 wk, pleural or abdominal effusion resolved within 18 d, and mesenteric lymph node enlargement resolved in up to 44 d. Although low albumin:globulin ratio and hyperglobulinemia were not resolved in some cases, these animals also maintained remission during the observation period. These persistent abnormalities were also observed in the previous report on molnupiravir (9). Whether these findings can predict long-term consequences after treatment is still unknown and further follow-up information should be obtained.

In non-neuro-FIP cases, SAA was high at diagnosis but normalized within 15 d. Serum amyloid A is a major acutephase protein in cats, produced in the liver in response to inflammatory cytokines (13). It is known to be elevated in FIP cases (14) but can also increase in other diseases (15,16). In a report, AGP had better sensitivity and specificity than SAA in diagnosing effusive FIP cases (17). However, AGP normalization was reported to take 6 to 7 wk to monitor the treatment response of molnupiravir (10). During recovery from surgery-induced inflammation, SAA decreased more rapidly than AGP (15). Although our study lacked a direct comparison of the duration of normalization between SAA and AGP, SAA seemed useful as an early clinicopathological indicator of recovery in nonneurological cases.

No cats with neuro-FIP had high SAA when they had neurological signs, and 2 of 3 cats developed mild elevation of SAA during treatment. Perhaps neuro-FIP, without other organ involvement, produces less inflammatory cytokines and SAA. In support of this hypothesis, a previous report indicated that 32% of neuro-FIP cases had normal hematology and biochemistry (11). In another study, brain tissue from neuro-FIP cases without systemic spread had lower concentrations of cytokines including interleukin-1, −6, and −18, and tumor necrosis factor-α, than tissue from generalized FIP cases (18). Further investigation is necessary to clarify the relationship between neuro-FIP and SAA and determine whether mild SAA elevation should be treated with higher dosages or extended intervals.

Some other characteristics were observed in neuro-FIP cases. First, only cats with neuro-FIP had dosages > 15 mg/kg and treatment durations > 84 d. A similar trend was observed with GS-441524 treatment. Neuro-FIP cases seemed to require higher dosages of GS-441524 (5 to 10 mg/kg, SC, q24h) than non-neuro-FIP cases (2 to 4 mg/kg, SC, q24h) (1,19), consistent with a pharmacokinetic study in cats (20). For molnupiravir and NHC, there is no information available on transport across the bloodbrain barrier in cats. It would be desirable to have results from pharmacokinetic studies of cats to support these substances’ effectiveness in neuro-FIP cases. Moreover, in cases of neuro-FIP, resolution of clinical signs took a maximum of 77 d. In rats, molnupiravir had a lower concentration than NHC in the CSF than in serum (21), perhaps increasing the difficulty to reach an effective inhibitory concentration against viral infection in the central nervous system (CNS). In human medicine, the intracellular tropism of the virus is considered a factor that causes persistent viral infection in the CNS (22), and this also applies to FIP (23). Therefore, drug pharmacokinetics in the CNS and the intracellular tropism of the viral disease may contribute to the difficulty involved or the prolonged interval needed to eliminate FIP infection from the CNS.

Two cats in this study had possible adverse reactions. Vomiting occurred on Day 91 in 1 cat (Cat 1) that was treated at a higher dosage of 15.0 mg/kg. Vomiting resolved with the end of molnupiravir treatment and supportive care. Another cat (Cat 8) had intermittent diarrhea that had resolved at follow-up after the end of the treatment. In a previous experimental study in which 3 cats received 10 mg/kg of molnupiravir, all cats had nausea, including vomiting (24). Also, in humans given molnupiravir, diarrhea, vomiting, and nausea occurred in 2.5, 0.3, and 0.2% of cases, respectively (25). Therefore, vomiting and diarrhea observed during molnupiravir treatment might be considered adverse reactions. These adverse reactions resolved after the end of molnupiravir treatment and no long-term adverse reactions were observed. Other adverse reactions reported with molnupiravir treatment in cats, such as leukopenia, folded ear tips (8,10), and development of jaundice (9), were not observed in this study. An increase in ALT was reported in previous studies (9,10), but ALT assessment was not included in the follow-up blood examination in this study.

This study had some limitations. First, because of the retrospective nature and the limited number of cases that included various types of FIP (effusive or non-effusive, with or without neurological or ocular involvement), this study had a low level of data sets. Second, neuro-FIP was diagnosed presumptively because of the unavailability of CSF. Other important differential diagnoses in feline neurological cases were protozoal, fungal, or bacterial meningoencephalitis, and neoplasia (26,27). Combining the signalment, absence of clinical signs suggestive of any nidus outside the CNS, clinicopathological test results, MRI findings consistent with FIP, and improvement with molnupiravir treatment, these cases were still included as presumptive neuro-FIP cases. Moreover, the treatment protocol and follow-up plans were not strictly standardized. If the starting dosage was higher in neurological cases, as indicated in a previous study (9), then the time for remission would change. Also, the time it took for each abnormality to return to normal may have been overestimated. This means that the actual treatment response time could have been shorter than the recheck dates. In addition, 6 cats were prescribed prednisolone, which could have affected clinicopathological test results.

In conclusion, 10 of 11 cats, including neuro-FIP cases receiving higher dosages, completed the course of treatment, and no relapse was observed during the study period. Also, if the initial treatment regimen was successful, the resolution of dysrexia, lethargy, and high SAA typically occurred within 15 d. Therefore, the treatment response with molnupiravir should be monitored with clinical signs and clinicopathological findings, considering that each abnormality has its own time frame for improvement.