Feline infectious peritonitis (FIP) is a common and highly lethal coronavirus disease of domestic cats. Recent studies of diseases caused by several RNA viruses in people and other species indicate that antiviral therapy may be eﬀective against FIP in cats. The small molecule nucleoside analog GS-441524 is a molecular precursor to a pharmacologically active nucleoside triphosphate molecule. These analogs act as an alternative substrate and RNA-chain terminator of viral RNA dependent RNA polymerase. We determined that GS-441524 was non-toxic in feline cells at concentrations as high as 100 uM and eﬀectively inhibited FIPV replication in cultured CRFK cells and in naturally infected feline peritoneal macrophages at concentrations as low as 1 uM. We determined the pharmacokinetics of GS-441524 in cats in vivo and established a dosage that would sustain eﬀective blood levels for 24 h. In an experimental FIPV infection of cats, GS-441524 treatment caused a rapid reversal of disease signs and return to normality with as little as two weeks of treatment in 10/10 cats and with no apparent toxicity.
GS-441524 has been successfully used to treat feline infectious peritonitis (FIP) in cats. However, the use of its prodrug, remdesivir, in combination with a PO GS-441524 containing product for the treatment of FIP has not yet been described.
Describe treatment protocols, response to treatment and outcomes in cats with FIP treated with a combination of PO GS-441524 and injectable remdesivir.
Thirty-two client-owned cats diagnosed with effusive or non-effusive FIP including those with ocular and neurological involvement.
Cats diagnosed with FIP at a single university hospital between August 2021 and July 2022 were included. Variables were recorded from time of diagnosis, and subsequent follow-up information was obtained from the records of referring veterinarians. All surviving cats were observed for the entire 12-week treatment period.
Cats received treatment with different combinations of IV remdesivir, SC remdesivir, and PO GS-441524 at a median (range) dosage of 15 (10-20) mg/kg. Clinical response to treatment was observed in 28 of 32 cats (87.5%) in a median (range) of 2 (1-5) days. Twenty-six of 32 cats (81.3%) were alive and in clinical and biochemical remission at the end of the 12-week treatment period. Six of 32 cats (18.8%) died or were euthanized during treatment with 4 of the 6 cats (66%) dying within 3 days of starting treatment.
We describe the effective use of injectable remdesivir and PO GS-441524 for the treatment of FIP in cats. Success occurred using different treatment protocols and with different presentations of FIP including cats with ocular and neurological involvement.
Feline infectious peritonitis (FIP) is caused by a mutant biotype of the feline enteric coro- navirus. The resulting FIP virus (FIPV) commonly causes central nervous system (CNS) and ocular pathology in cases of noneffusive disease. Over 95% of cats with FIP will suc- cumb to disease in days to months after diagnosis despite a variety of historically used treatments. Recently developed antiviral drugs have shown promise in treatment of non- neurological FIP, but data from neurological FIP cases are limited. Four cases of naturally occurring FIP with CNS involvement were treated with the antiviral nucleoside analogue GS-441524 (5-10 mg/kg) for at least 12 weeks. Cats were monitored serially with physi- cal, neurologic, and ophthalmic examinations. One cat had serial magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) analysis (including feline coronavirus [FCoV]) titers and FCoV reverse transcriptase [RT]-PCR) and serial ocular imaging using Fourier- domain optical coherence tomography (FD-OCT) and in vivo confocal microscopy (IVCM). All cats had a positive response to treatment. Three cats are alive off treatment (528, 516, and 354 days after treatment initiation) with normal physical and neurologic examinations. One cat was euthanized 216 days after treatment initiation following relapses after primary and secondary treatment. In 1 case, resolution of disease was defined based on normalization of MRI and CSF findings and resolution of cranial and caudal segment disease with ocular imaging. Treatment with GS-441524 shows clinical efficacy and may result in clearance and long-term resolution of neurological FIP. Dosages required for CNS disease may be higher than those used for nonneurological FIP.
By Niels C Pedersen DVM, PhD Centre for Companion Animal Health, School of Veterinary Medicine, University of California, Davis, USA
Feline infectious peritonitis (FIP) has been my fascination for the past 50 years. Although caused by a virus, FIP most closely resembles mycobacterial infections that cause human and feline tuberculosis and leprosy. The disease has yielded its secrets grudgingly and each new discovery has led to yet more questions. As the famous poem by Robert Frost proclaims: ‘We dance round the ring and suppose, but the secret sits in the middle and knows.’ I am fortunate to have reached a final landmark in my career having identified a safe and effective treatment for FIP. This stage was made possible by a lot of work and great collaborations with teams of people in the US at places like Kansas State and Wichita State Universities, and Gilead Sciences.
We now know that small molecules targeting specific proteins involved in RNA virus replication are capable of curing various forms of FIP with a high degree of safety. These small molecules include a protease inhibitor GC376 and a nucleoside analog GS-441524. Both are based on drugs currently used to treat common human diseases such as hepatitis C and HIV/AIDS and undergoing testing for exotic infections with names like MERS (Middle East respiratory syndrome), SARS (severe acute respiratory syndrome) and Ebola. It is important to state that small field trials of the types we have completed and published, the most recent in this issue of JFMS (Figure 1),1 are primarily for proof-of-concept and not rapidly translated into approved and commercially available products. Some researched drugs may be pre-empted or delayed by human applications and all will require a lengthy process to obtain final approval, even for animals. Ultimately, private veterinary pharmaceutical companies will have responsibility for marketing them. Is the demand for such drugs and the willingness of owners to bear the cost a sufficient incentive for these companies?
Unfortunately, initial reports of successful treatment have only stimulated efforts by desperate owners to access these drugs immediately and have created a growing black market. Therefore, I suspect that the next couple of years will test both our patience and ethics. None of this should take away from the fact that over 50 years of research has gotten us to this eventful point. However, much more is left to discover. How does virus replication in macrophages lead to immunity in many cats and disease in an unfortunate few? Can this knowledge finally lead to effective vaccines? What is the best way to care for kittens in catteries, shelters and rescues to minimize FIP losses? Are there even better drugs awaiting discovery? Can small molecule inhibitors synergize with each other and with entirely different treatment modalities?
I will leave these questions, and more, to my fellow FIP researchers.
Reference 1 Pedersen NC, Perron M, Bannasch M, et al. Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious perito- nitis. J Feline Med Surg 2019; 21: 271–281.
There is currently no effective treatment that is legally available for cats with confirmed FIP. Until new treatments can be approved and marketed, treatment remains largely symptomatic. A low to moderate dosage of prednisolone or prednisone (starting at 2 mg/kg, orally, once a day for two weeks and then 0.5-1 mg/kg indefinitely), coupled with a diet high in animal protein (e.g., 1/2 cooked chicken, turkey or rabbit and 1/2 a favorite commercial cat food) and a lot of personal care, is the simplest and possibly most effective symptomatic treatment. Symptomatic treatment ultimately depends on the cat’s immune system to cure the infection. Some cats may have mild or subclinical disease isolated to a single intestinal lymph node, which may be detected as an abdominal mass upon routine physical examination or during a spay operation. Cats with more severe clinical signs will often go into a more chronic and less severe stage of disease after several weeks. As we gain more experience with treating rather than euthanizing cats when FIP is diagnosed, we begin to appreciate that a proportion of cats may survive for many weeks, months, and rarely a year or more. However, it is still fair to say that FIP is ultimately fatal to most cats if left to run its natural course.
There are misconceptions on the value of removing fluid effusions. Cats with chest involvement and breathing difficulties can benefit greatly by removal of pleural fluid. Chest fluid also tends to be slowly replaced, especially when cats are treated with prednisolone. Removal of abdominal fluid should be discouraged unless it is so massive that it interferes with breathing. Abdominal effusions tend to be rapidly replaced at the expense of body fluids and proteins. Owners can be encouraged to maintain symptomatic and palliative treatment for as long as weight and activity are maintained. This can be days, weeks, sometimes months, and rarely a year or more.
However, owners should be apprised of the extremely high morality that occurs among cats with clinically active FIP.
There is some debate on whether certain non-steroidal anti-inflammatory drugs (e.g., TNF-alpha blockers such as pentoxifylline, thalidomide), specific immunomodulators (e.g., feline interferon omega, human recombinant alpha or beta interferon), and non-specific immunostimulants (e.g., several plant- or microbial-based biologics) have any efficacy against FIP. Although an initial study with feline interferon omega indicated efficacy, a subsequent large double blind and placebo-controlled trial showed it to be without efficacy. Human alpha and beta interferon are also of doubtless benefit and are immunogenic to cats and will be ultimately destroyed by the resulting antibodies. A similar large-scale trial with pentoxifylline also showed it to be non- effective against FIP.
Another product in the US is called Peritan FP and it can be found at – http://www.greenpetdepot.com/products/peritan. There are several others as well. Interestingly, advertised statements about the efficacy of Peritan for FIP conclude with the following statement
– “These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.” People should also be aware that some of the anecdotes attesting to the efficacy of a certain product for FIP are deliberately posted. Furthermore, many of the cats described may never have had FIP in the first place.
The use of non-specific immunostimulants has been popular in veterinary medicine for decades, usually for treating specific signs of feline leukemia and feline immunodeficiency virus infection such as anemia or low lymphocyte counts. There are occasional anecdotal reports of cats with “FIP” being cured or their lives prolonged by such treatments. These types of immunostimulants include substances such as Staphylococcal A protein, ImmunoRegulin (Propriobacterium acnes), Acemannan (mucopolysaccharide extract of Aloe Vera leaves) and Imulan (lymphocyte T-cell immunomodulator). There is no evidence that these biologics have any beneficial effect on actual cases of FIP.
The most popular non-specific immunostimulant being used to treat cats with dry FIP is an extract of tree leaves. Polyprenyl immunostimulant (PI) is classed as a biologic by the USDA and is manufactured by Sass and Sass in Tennessee (http://www.sassandsass.com/). PI has been given a conditional license by the USDA as therapeutic for “symptoms” of feline herpes virus infection. According to the Sass and Sass website they are conducting research on its use for FIP and other diseases of companion animals. Although not currently approved for the treatment of FIP by the USDA, it is being widely used off label for prolonging the life of cats with milder forms of FIP. Polyprenyl immunostimulant as a potential treatment for cats with FIP has an interesting history that extends from Russia to the United States. The research and chemical structure of PI is based on a biologic “plant extract” called Phosprenyl, which is used in Russia to treat a wide range of viral infections in many animal species (http://www.2ndchance.info/fip-gamavite.pdf). PI may also have a Russian origin. The following web posts were published inRussian, but with the help of Google translator it is possible to access much of this material in English – http://translate.google.com/translate?hl=en&sl=ru&u=http://pesikot.org/forum/index.php%3Fsho wtopic%3D219269&prev=/search%
Eight of the 60 cats survived over 200 days, and 4 of 60 survived over 300 days No historical control cat that suffered solely from dry FIP and not treated with PI lived longer than 200 days. The conclusion was that PI treatment prolonged the life of cats with the dry form of FIP.
The published results with PI are difficult to interpret, but it can be safely concluded from the researchers words that cats with wet FIP are not responsive to treatment and that there may be a small increase in survival time for cats with the milder dry form of FIP. It is also important to note that the USDA has not added FIP to its list of approved diseases for PI treatment and that the makers of PI do not promote its use for FIP. However, it has gained a vocal following on the web that generates a considerable market for treating FIP. PI may cost over $400 a month if used on an average size cat and dosed accordingly and this expense can be magnified by associated veterinary expenses.
Non-specific Anti-viral Drugs
Common drugs such as cyclosporine, cholesterol inhibitors such as itraconazole, various antibiotics, and several herbal extracts inhibit FIP virus in cell-culture, but the anti-viral effect is weak and potential toxicity great. These substances have their biologic effects by inhibiting normal metabolic processes of cells and some of these processes are usurped by viruses to aid their own replication. The amount of drug required to achieve the needed level of virus inhibition would be toxic or damaging to the cells and thus to the host cat.
Targeted Anti-viral Drug Therapy
The current hope for treatment of FIP rests with several of the same types of specific anti-viral drugs that have been used to treat viral infections of humans such as hepatitis C and HIV/AIDS. These are small molecules that are readily absorbed into cells and specifically target viral proteins that are essential for virus replication. Their toxicity for non-viral processes (i.e., cellular functions) is extremely low, making them both safe and efficacious. We have described our laboratory and field experiences with GC376, a viral protease inhibitor, in an article in the Journal of Feline Medicine and Surgery. An abstract of this article can be accessed at the PubMed website (https://www.ncbi.nlm.nih.gov/pubmed/28901812). The rights for GC376 have been obtained by Anivive and they are starting the lengthy process of obtaining FDA approval for treating cats with FIP and eventual marketing – https://www.k- state.edu/media/newsreleases/2018-09/fipantiviral92018.html.
We have also published our initial research studies on a second compound (nucleoside analog GS-441524- Gilead Sciences, Inc.) and these results can be found at the Veterinary Microbiology journal open access article website (https://www.sciencedirect.com/science/article/pii/S0378113518301603). We are in the process of publishing field trial results with this very promising viral RNA inhibitor in the Journal of Feline Medicine and Surgery. Similar reports will be forthcoming as other drugs go through experimental and field testing. We are convinced based on our research that anti-viral drugs of the type currently used for HIV/AIDS and hepatitis C virus (HCV) infection, and in test phase for Ebola, Marburg, MERS, SARS, and bat coronavirus infections will provide the best chance for curing this terrible disease of cats. These drugs include protease inhibitors, nucleoside analogs, RNA polymerase inhibitors, as well as other classes of anti-viral drugs that might target specific aspects of RNA virus replication.
Unfortunately, the research phase for these drugs has ended and no more field trials are contemplated at this time and no drug is legally available for veterinarians or owners. The processes involved in getting these new drugs FDA approved and commercialized is long and it may be 2-5 years before these or similar drugs find their way to veterinarians for use in the treatment of cats with FIP. This delay has created a vigorous and growing black-market for drugs like GC376 and GS-441524. GC376 is being illegally produced in China and sold through subsidiaries in Europe and US. GS-441524 is also being produced illegally in China but has not yet appeared on the market. Manufacturers and secondary suppliers state that these drugs are to be used for research purposes only and not for use veterinary or human applications but are well- aware of their great demand and willingness of many cat owners to pay an extremely high price for them. We have no idea of the purity or biological activity of these black-market compounds and veterinarians have no experience with preparing them for treatment or using them to treat cats with FIP. The treatment period for naturally occurring FIP is a minimum of 12 weeks, not the two weeks based on treating cats with experimental FIP. Many owners have spent thousands of dollars on black-market drugs and have had to stop treatment before this time, even though their cats are responding, due to the cost. The FIP will relapse if treatment is stopped too soon. Owners and veterinarians using should be also aware of possible legal and ethical consequences arising from the use of black-market drugs.
The initial field testing of GS-441524 for FIP treatment involved subcutaneous injection. This route of administration was based on prior pharmacokinetic (PK) studies done on laboratory cats. Intravenous and subcutaneous routes of injection yielded similar high blood levels that were sustained at virus inhibitory concentrations for over 24 hours. Oral administration was also found to provide blood levels, but somewhat delayed and only at about 40% peak levels of subcutaneous and intravenous routes (Pedersen NC, unpublished data, 2018). However, dogs which have a longer intestinal tract evolved for omnivorous diets, can absorb up to 85% of GS441524 by the oral route [1, 5]. Dogs have often been used as surrogates for humans in oral absorption studies, so oral absorption in humans is also likely to be higher than in cats. Therefore, the subcutaneous route was chosen for field testing in cats based on ease of administration and resulting blood levels.
Current brands of capsules/tablets are sold as supplements and their labels list several common innocuous chemical compounds and medicinal herbs and do not list GS-441524 as one of the ingredients. This is probably done to avoid scrutiny by customs. Regardless of the list of ingredients, the active component in all oral products is GS-441524. The exact concentration of GS-441524 in the various oral products is kept secret by the sellers, but it is obviously several times higher than would be needed if the drug were given by the subcutaneous route.
We were initially critical of the oral route for two reasons. First, oral forms were more wasteful of what was initially a rare and expensive resource. Second, published research on oral absorption of nucleosides (GS-441524 is a nucleoside) document a concentration limit or ceiling for oral absorption [2-5]. This limitation would make it theoretically difficult to achieve the extremely high blood concentrations required to treat certain forms of FIP (e.g., neurological) and/or to overcome the problem of acquired drug resistance. Newer information obtained from field use of the oral forms of GS-441524, indicate that this problem may not be as serious as first believed as most forms of FIP respond equally well whether given pills or injections.
It appears that more and more owners and veterinarians are embracing oral GS-441524 for part or all the treatment. The cost of oral GS-441524 preparations has steadily declined over the last two years and quality increased. The problem of injection site reactions, coupled with more effective oral preparations of GS-441524, have encouraged the oral treatment. Steadily increasing numbers of cats are being treated with oral drug either for part or all of the treatment.
Formulation and Dosing
Suppliers of oral GS-441524 do not list the amount of active drug in their tablets or capsules. Some suppliers also provide pills with a higher concentration of GS-441524 for use in cats with ocular and neurological FIP to limit pills that must be given at one time. In addition, one supplier has tablets labeled for administration every 12 hour (h) and yet another for every 24h. The 1 tablet/kg q12 h tablet contains one half as much GS-441524 as a 1 tablet/kg q24h tablet the rationale being that the q12h dosing would prevent a fall-off in the blood concentration prior to 24h. This belief is inconsistent with the original pharmacokinetic data, which shows blood levels to be sustained at effective levels for at least 24h. Regardless, both the q12h and q24h pills seem equally effective when given according to instructions, although most owners prefer dosing once a day.
FIP Doctor Oral tablet dosage recommendation
Form of FIP
Kittab 10 q24h (10mg GS-441524)
Kittab 25 q24h (25mg GS-441524)
Kittab 50 q24h (50mg GS-441524)
All oral brands have similar instructions for administering capsules or tablets. Fasting for half an hour before and after giving the medication is generally recommended. A small amount of treat may encourage cats to take them, and many cats will consume them when put on a plate with a coating treat.
The price of oral GS has significantly decreased in the last year. Nevertheless, the relative cost of oral GS-441524 is 20-40% higher (depending on the supplier) than their injectable version. Cost calculator may help cat owners to estimate the daily or monthly cost.
Factors affecting oral vs. injection
Cats currently experiencing vomiting/regurgitation and diarrhea are generally considered poor candidates for oral GS-441524. Therefore, cats with serious gastro-intestinal disease are often started on injections, at least until the problems are resolved. Most people, especially in the past, have started with injectable GS-441524. The injection form is cheaper, and the dosage is more accurately managed. Absorption of GS-441524 is also more reliable by the subcutaneous than oral route, which is often a critical factor in the initial treatment of cats that are severely ill and unstable at the onset. Whether or not a cat continues injectable GS-441524 is often conditioned on the ability of the owner to do injections in the most effective manner, the willingness of the cat to adapt to the injection pain, and the occurrence of injection site sores. Oral medication is often a welcome respite for owner and feline patient in such situations.
Comparison of treatment success between injectable and oral GS-441524
Assuming that dosages are accurately calculated, and dosing properly done, the success rate with oral GS-441524 currently mirrors that of injectable formulations. Nevertheless, differences in responses between oral and injectable GS-441524 have been reported. A small number of cats have not responded well to oral GS-441524 as initial treatment or have led to relapses when replacing injections. Alternatively, switching cats to oral GS-441524 at an equivalent dosge has resolved disease that was not responding well to injections. It is difficult to assign these dramatic differences in response to the drug form, as GS-441524 given by subcutaneous or oral routes ends up in the bloodstream and ultimately in the tissues. It is more likely that the brands of injectable or oral GS-441524 used prior to such switching were not good. Indeed, there have been many cases when switching to a different oral or injectable brand immediately improved the response.
It was assumed that only the injectable form of GS-441524 could achieve the extremely high blood and cerebrospinal fluid levels necessary to effectively treat neurological disease, especially in situations where the virus has evolved variable degrees of drug resistance. However, oral brands such as Aura/Lucky have been quite effective on cats with neurological FIP. This has also included some cats who were failing to respond to an extremely high dosage of injectable GS441524. More and more cats with neurological FIP are being cured with entirely oral treatment. This is either due to more experience with oral treatment in difficult cases of FIP, or equally likely, to the increased quality of oral formulations.
Summary of currently available brands of oral GS-441524
Information on oral forms of GS-441524 is sparse regarding treatment outcomes but there are a growing number of brands that are available, attesting to the popularity of this form of treatment. Information on these brands is updated at the FIP Warrior CZ/SK website . This website also contains excellent information on FIP and GS-441524 treatment.
The recommended dosages vary from brand to brand and do not always correspond to the equivalent dosage for injectable GS-441524. GS-441424 is absorbed from the intestine with about 50% efficiency as subcutaneous or intravenous administration. There is also a theoretical upward limit to absorption through the intestine, which would also limit the blood levels that can be obtained. Given the absorption limitations of oral GS-441524, one would expect the oral dosage to be around twice that of injections. However, most oral brands are recommended at an equivalent dosage to injections. This suggests that the actual concentration of GS-441524 in oral preparations may be higher than for injectable GS-441524 as listed in the tables below and as provided by the FIP Warrior CZ/SK website.
Referenced studies on GI absorption of nucleosides related to GS-441524 and GS-441524
1. Thomas L. A precursor to remdesivir shows therapeutic potential for COVID-19.
2. Painter GR, Bowen RA, Bluemling GR, et al. The prophylactic and therapeutic activity of a broadly active ribonucleoside analog in a murine model of intranasal venezuelan equine encephalitis virus infection. Antiviral Res. 2019; 171:104597. doi: 10.1016/j.antiviral.2019.104597
4. de Miranda, P., Krasny, H.C., Page, D.A., Elion, G.B., 1981. The disposition of acyclovir indifferent species. J. Pharmacol. Exp. Ther. 219 (2), 309–315
5. Vijayalakshmi, D., Belt, J.A., 1988. Sodium-dependent nucleoside transport in mouse intestinal epithelial cells. Two transport systems with differing substrate specificities. Biol. Chem. 263 (36), 19419–19423.
6. Yan VC, Khadka S, Arthur K, Ackroyd JJ, Georgiou DK, Muller FL. Pharmacokinetics of Orally Administered GS-441524 in Dogs. bioRxiv, doi: https://doi.org/10.1101/2021.02.04.429674
Feline infectious peritonitis (FIP) caused by feline coronavirus (FCoV) is a common dis-ease in cats, fatal if untreated, and no effective treatment is currently legally available. The aim of this study was to evaluate efficacy and toxicity of the multi-component drug Xraphconn® in vitro and as oral treatment in cats with spontaneous FIP by examining survival rate, development of clinical and laboratory parameters, viral loads, anti-FCoV antibodies, and adverse effects. Mass spectrometry and nuclear magnetic resonance identified GS-441524 as an active component of Xraphconn®. Eighteen cats with FIP were prospectively followed up while being treated orally for 84 days. Values of key parameters on each examination day were compared to values before treatment initiation using linear mixed-effect models. Xraphconn® displayed high virucidal activity in cell culture. All cats recovered with dramatic improvement of clinical and laboratory parameters and massive reduction in viral loads within the first few days of treatment without serious adverse effects. Oral treatment with Xraphconn® containing GS-441524 was highly effective for FIP without causing serious adverse effects. This drug is an excellent option for the oral treatment of FIP and should be trialed as potential effective treatment option for other severe coronavirus-associated diseases across species.