FIP treatment with oral formulations of GS-441524 

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, peaking at 2 hours, but reaching only about 40% peak levels of  subcutaneous and intravenous routes (Pedersen NC, unpublished data, 2018).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.  

Chinese suppliers of GS-441524 copied the diluent, drug concentration, subcutaneous route of  administration that were used in the initial published field trial. The first company to offer  GS441524 on the unapproved market was Mutian. Mutian was also the first to research and offer  an oral form of the drug. Mutian researchers found that effective blood levels of GS-441524  could be achieved by merely increasing the concentration of the drug in their oral preparations.  Other companies (e.g., Aura, Lucky) subsequently offered their own versions of orally  administered GS-441524. However, ss of September 2021, Mutian no longer lists oral GS  preparations (of any form) on their website. Currently, Aura, Lucky, and Capella are the most  used oral forms of GS441524 in the US. 

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 higher  (1.5 – 2 times?) 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]. Results with EIDD-1931, a related nucleoside showed bioavailability  drop from 56 to 36% as the oral dosage increased [6]. 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. Oral  bioavailability can also be significantly decreased by certain substances in the diet, and cat  owners are notorious for using a large range of dietary supplements, some of which could negatively affect the treatment. 

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 and increasing  numbers of cats are being cured with oral drug either for part or all their treatment.  

Formulation and Labeling 

Most established oral preparations are smaller tablets, which are easier to administer than larger  capsules. Newer preparations such as Sweeper offer a soluble film form of GS-441524 to avoid  difficulties in “pilling” some cats.  

The actual amount of GS-441524 in a tablet/capsule and seller recommended dosage for oral  medications varies greatly by form of FIP, the seller, and experience of owner and FIP treatment  groups. Therefore, the actual amount (mg) of GS-441524 in a pill or capsule is not usually given.  In lieu of an actual amount of GS-441524 in a pill or capsule, seller dosages are often based on the number of pills needed per kg of weight, e.g., 1 pill/kg per os (P0) every (q)24 hour (h) for  cats with wet or dry FIP and no ocular or neurological involvement. The amount of GS-41524 in  such one such pill given PO q24h is equivalent to the 4-6 mg/kg SC q24h dosage, but the actual  amount of GS in the one pill may be twice the amount as in 1 ml of injectable GS to compensate  for decreased bioavailability by the PO route.  

GS-441524 For Cat FIPV

In addition, one supplier (Aura/Spark) has tablets labeled for administration q12h and another for  q24h dosing. The 1 tablet/kg PO q12 h tablet contains half as much GS-441524 (presumably 4-6  mg) as a 1 tablet/kg PO q24h tablet (presumably 10 mg) – the rationale being that the q12h  dosing will prevent a fall-off in the blood concentration prior to 24h. However, effective blood  levels after a single PO or SC dose are both sustained for 24h or more. At dosages equivalent to 10-15 mg/kg SC q24h there may be another advantage in q8h or q12h dosing over q24h, as it  may help to circumvent the absorption ceiling. Therefore, dividing PO doses into q8h or q12h increments is often used for cats with dosages equivalent to 10-15 mg/kg SC q24h or higher. 

Dosing 

GS-441524 for cats wet or dry FIP and no neurological or ocular signs the recommended starting  dosage is 4-6 mg/kg SC q24h. The injection dosage for cats with ocular disease is 8 mg/kg SC  q24h and cats with neurological disease 10 mg/kg SC q24h. If a cat starts treatment for wet FIP  and then develops ocular disease, the dosage is immediately increased to 8 mg/kg SC q24h, and  if neurological signs develop it is increased to 10 mg/kg SC q24h. Failure to cure FIP at dosages  higher than 15 mg/kg SC q24h are indicative of drug resistance. The PO dosages equivalent to 4- 6, 8 and 10 mg/kg SC q24h, are 10, 16, and 20 mg/kg PO q24h. (Note: some oral preparations  are labeled as SC equivalents but actually contain up to twice the labelled mg of GS) The  recommended treatment is 12 weeks, with dosage increases when deemed necessary. However, it  is recognized that some cats can be cured in 6 weeks with either form of GS-441524, more in 8- 10 weeks, and almost all in 12 weeks. Young cats with abdominal wet FIP tend to respond the  fastest, cats with dry FIP slower, and cats with neurological FIP the slowest. Therefore, the  “universal” recommendation is to treat every cat with FIP, regardless of form, for a minimum of  12 weeks. The daily PO dosage can be divided q12h, which may be advantageous when treating at the higher dosages in order to avoid the absorption ceiling. SC and PO treatments may alternate q12h in order to avoid large injection volumes. 

Oral GS dosing is less exact than with injections. Tablets are difficult to divide, as they tend to  crumble, so cutting in half is often the best that can be done. When a calculated PO dosage falls  between the labeled tablet dosage it is recommended to always round up to the nearest half  tablet.  

Administration  

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 (e.g., Churu).  

Cost  

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.  

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. Some substances given  PO may interfere with GS-441524 absorption. Therefore, the inclusion of other drugs and dietary  supplements should be avoided if not essential to the FIP treatment.  

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 dosage 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 it is due to the  brands of injectable or oral GS-441524 used prior to such switching were not good or that there were issues with absorption or administration. 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 many 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.