This article is also available in Japanese 🇯🇵 (see download link at the end of the article). 本記事は日本語版もご用意しています🇯🇵（記事末尾のダウンロードリンクをご覧ください）

Felycin®-CA1, sirolimus delayed release tablets) received conditional approval from the FDA-Center for Veterinary Medicine in March of 2025. Also known as rapamycin, this is a drug you may have heard of from the human health span/longevity movement that has shown some anti-aging properties.

According to Dr. Bonagura, it should be appreciated that the CVM has recently offered conditional approval to (veterinary) drugs for diseases with no proven therapy.

This conditional approval does not demonstrate efficacy, only potential, but allows for ongoing evaluation of the drug in clinical patients.

Presently there is an ongoing (FDA approved) pivotal clinical trial (HALT) underway. It might take years to learn the results. It should be noted that in humans drugs with conditional or accelerated approval obtain full approval about 50 to 63% of the time.

A recent survey of ACVIM Cardiologists presented at the North American Veterinary Cardiology Summit in October indicated that >90% of veterinary cardiologists surveyed are not yet prescribing Felycin®-CA1 to feline patients with hypertrophic cardiomyopathy. (This is based on a survey conducted by the Cardiac Education Group.org, by Drs. T. DeFrancesco & W. Church).

He indicated that while he hopes the therapy proves effective, the conditional FDA approval has confused many cardiologists (as clearly suggested by the survey results and the low adoption rate for its use among veterinary cardiologists).

Some of the concerns that have been raised about the conditional approval include:

➡️ the RAPACAT trial, which was the basis for the conditional approval (JAm Vet Med Assoc, July 2024) involved only 43 cats with about 2/3 taking the drug at two doses

➡️ the study was quite short (6 months)

➡️ different segments of the left ventricular (LV) walls were apparently used for measurements on follow-up examinations

➡️ incremental changes in wall thicknesses were within fractions of millimeters and well within day-to-day measurement variation (as shown by numerous feline studies)

➡️ a dose-effect was not evident (the lower dose showed no change in wall thickness; the upper dose progressed)

➡️ and a number of the HCM cats had normal NT-proBNP test results based on review of graphical data.

All of these have led to criticism that the FDA was premature in offering conditional approval based on the available data.

That perspective seemingly fits with a relatively widespread opinion among cardiologists that we need more information and additional studies on this drug before routinely prescribing it to cats with preclinical HCM.

That said, John mentioned that if an owner comes in asking for Felycin®-CA1, he has a conversation with them about what we do and don’t know about the drug’s efficacy, and he also indicates that it appears to be well-tolerated.

✅ If a client still wants to try it, he’ll prescribe it.

It should be noted that because the FDA permits direct consumer and veterinary advertising of conditionally approved drugs, and there has been widespread marketing of this treatment by the manufacturer/distributor/study sponsors, you are very likely to be presented with this situation in your own practice.

He reminded us not to condemn every cat with a murmur. He fears we are over-diagnosing HCM and reminded us that other diseases aside from HCM can cause a murmur.

He indicated that while cardiologists don’t agree on the specific criteria for the diagnosis of HCM,  when the left atrium is dilated the diagnosis of feline heart disease is likely and that this finding is also the strongest prognostic indicator of future events like heart failure or thromboembolism.

I also learned that transient myocardial thickening (likely from myocarditis), can resolve in a few weeks and very much resembles feline HCM. It most often presents as acute congestive heart failure and is more common in younger cats (Journal of Veterinary Internal Medicine, 32(1), 48–56.)

Felycin®-CA1 is an interesting drug to watch. We need additional research to better evaluate outcomes in these patients. Many cardiologists aren’t yet convinced that it’s a miracle worker, but if a client wants to try it, at least in a sample size of 43 cats, there were not any serious side effects.

My perspective as an anesthesiologist: I haven’t noticed anything unusual about HCM cats prescribed Felycin undergoing general anesthesia. Interestingly, it appears rapamycin is neuroprotective when given before anesthesia, potentially decreases neuropathic pain risk, and helps prevent post-operative cognitive dysfunction in RODENT models that undergo general anesthesia.

This ENTIRE article is free to download as a PDF here ⬇️ in English and Japanese 🇯🇵.