Hope for Treatment of Feline Hypertrophic Cardiomyopathy

New Use for Older Drug Shows Promise

Lauri Henry

There is big news in veterinary cardiology! A drug treatment that targets heart muscle enlargement in subclinical feline hypertrophic cardiomyopathy (HCM) (*see notes) will be available by prescription for cats any day now. Here is what you need to know.

What is the Drug?

Illustration of a (human) heart with HCM, showing the thickened heart wall muscle.

The new drug is actually an old drug called rapamycin. Rapamycin was isolated from soil bacteria collected during an expedition to Easter Island in 1964. The indigenous name for Easter Island is Rapa Nui, and rapamycin takes its name from the island. Rapamycin was later given the generic drug name “sirolimus” by the USAN.(1) Sirolimus and rapamycin are the same drug, and the names are used interchangeably.

From the beginning, rapamycin was determined to have diverse uses, including antifungal and anticancer, and contained certain immunosuppressive properties. There have also been studies related to its potential for anti-aging benefits in people.(2) In multiple studies, rapamycin prevented and reversed cardiac wall thickness in rodents with laboratory-induced cardiac hypertrophy.(3,4) HCM is a condition where the heart muscle is enlarged and no longer functions well. If heart muscle thickening can be addressed, then a solution for HCM might be imminent. It is too soon to say if rapamycin will be the solution for HCM in cats, but that is the hope.

The RAPACAT Trial

Clinical trials of rapamycin for cats with HCM are being sponsored by the drug company TriviumVet. TriviumVet is producing rapamycin in an extended-release capsule called felycin®-CA1, a name which combines abbreviations of feline and rapamycin. The release of rapamycin for subclinical feline HCM is based on the results of one small study called the RAPACAT trial.(5) However, the results of that trial were promising enough and safe enough that the FDA gave the drug “conditional approval” for use in cats with subclinical HCM on March 14, 2025.(6)

The results of the RAPACAT trial were initially published in July of 2023.(5) They were recently presented at a feline cardiology symposium for veterinary professionals and cat fanciers at NC State College of Veterinary Medicine in April 2025.(7) The RAPACAT trial studied 41 cats over 180 days. The study design was “double-blinded, multicentered, randomized, and placebo-controlled.”(5) Feline participants receiving the drug were further divided into high-dose and low-dose groups. Neither the investigators nor the pet owners knew which cats were receiving high dose, low dose, or placebo. The RAPACAT trial was conducted simultaneously at NC State CVM’s Cardiology Services and at UC Davis School of Veterinary Medicine’s Cardiology Services.

At the end of the study, the RAPACAT trial concluded that left ventricular “myocardial wall thickness, at any location, was significantly lower in the low-dose DR rapamycin group compared to placebo.” The study also noted that the drug appeared to be safe since there were “no significant differences in adverse events” between the groups studied. This is an early study, and it is important to point out that early studies are typically focused on safety rather than efficacy. It is remarkable that this study appeared to show actual benefit in addition to safety.  One small study is not enough to conclude that the treatment for feline HCM has been discovered. Regardless, it is significant as the first treatment aimed at the primary issue in feline HCM: increased muscle wall thickness. Other drugs have been focused on treating the complications following HCM, such as blood clots, but not at the disease itself. If it works consistently in the broader clinical trial ahead, felycin®-CA1 will be a historic drug.

What is “Conditional FDA Approval”?

“Conditional FDA approval” means that the drug will be made available by veterinary prescription while it is undergoing further study. In order to receive conditional FDA approval, the drug company must prove “the animal drug is safe when used according to the label.”(8)  It must also “show that the drug has a ‘reasonable expectation of effectiveness.’”(8) Drugs receiving conditional approval must have “CA” and a number in their name. “CA” stands for conditional approval. The number after CA is the application number. Drugs can apply for up to five conditional approvals while still undergoing trials for full approval. The first rapamycin drug will be felycin®-CA1. The clinical approval of each application expires after one year.(8)

What’s Next: the HALT Study

TriviumVet is also sponsoring the next trial of rapamycin for cats with subclinical HCM called the “HALT study.” The HALT study is similar in design to the RAPACAT trial except that it is enrolling 300 cats, it will be a very long study, and it will be conducted at many different locations across the United States.

How Does My Cat Get This Drug?

The HALT study is already in progress and already halfway enrolled. The website can be found at: https://www.hcmincats.com/halt. If you and your veterinarian believe the study is a good fit for your cat, your veterinarian can enroll your cat. There will be $500 compensation at the end of the study, and diagnostics and treatment will be free. The disadvantages will be that you will not know if your cat is receiving the drug or the placebo. Also, it will be a very long-time commitment: “The study will consist of up to a 14-day screening period, followed by a 12-month once-weekly dosing period, with five post-enrollment study visits.”(9)

Alternatively, TriviumVet expects to release felycin®-CA1 “in the summer of 2025,” which is any day now. When it is released, your veterinarian will be able to prescribe it for your cat.(10) For updates on the release of felycin®-CA1, you can sign up on the manufacturer’s website.  https://www.triviumvet.com/felycin-ca1. It would probably be sufficient to ask your veterinarian to let you know when it becomes available.

For completeness, it is worth noting that rapamycin is a very old drug and is currently available by prescription in United States pharmacies. There is at least one veterinarian who is prescribing rapamycin to cats for HCM on the internet at present. However, it is not clear exactly what dose or formulation of rapamycin is being prescribed. Release of the medication and dose intended for cats is imminent, and it would be prudent to wait for the release of this information.

There will be a lot more to say about rapamycin in the next few years. For now, for your HCM cats, please be aware that this drug is about to become available. And, as with all things of veterinary importance, please discuss it with your veterinarian.

TriviumVet is also enrolling cats in a clinical trial of rapamycin for a completely unrelated feline disease, feline chronic kidney disease (CKD). For more information, see the REVERSE Feline CKD Study: https://www.ckdincats.com/home

 

*Notes

“Subclinical HCM refers to cats with left ventricular (LV) hypertrophy (LV wall thickness of ≥6 mm at end diastole by 2D or M-mode assessment) in the absence of systemic hypertension, other causes of compensatory myocardial hypertrophy, current or historic symptoms of congestive heart failure, arterial thromboembolism, and severe LV outflow tract obstruction”. (11)

References

  1. Sehgal SN. Sirolimus: its discovery, biological properties, and mechanism of action. Transplant Proc. 2003 May;35(3 Suppl):7S-14S. doi: 10.1016/s0041-1345(03)00211-2. PMID: 12742462.
  2. Kellogg, Doug. The origin story of rapamycin: systemic bias in biomedical research and cold war politics. ASCB: Molecular Biology of the Cell, Vol. 33, No. 13. Published online 13 Oct 2022.
  3. Gu, Jun; Hu, Wei; Song, Zhi-Ping; Chen, Yue-Guang; Zhang, Da-Dong; Wang, Chang-Qian. Rapamycin Inhibits Cardiac Hypertrophy by Promoting Autophagy via the MED/ERK/Beclin-1Pathway. Front. Physiol., Sec. Systems Biology Archive. Volume 7-2016.
  4. McMullen, PhD, Julie R; Sherwood, MBBS, FRACP, Megan C.; Tarnavski, MD, Oleg; Zhang, MS, Li; Dorfman, MD, Adam L.; Shioi, MD, PhD, Tetsuo; and Izumo, MD, Seigo. Inhibition of mTOR Signaling With Rapamycin Regresses Established Cardiac Hypertrophy Induced by Pressure Overload. Circulation. Volume 109, Number 24. Originally published 7 June 2004.
  5. Kaplan JL, Rivas VN, Walker AL, Grubb L, Farrell A, Fitzgerald S, Kennedy S, Jauregui CE, Crofton AE, McLaughlin C, Van Zile R, DeFrancesco TC, Meurs KM, Stern JA. Delayed-release rapamycin halts progression of left ventricular hypertrophy in subclinical feline hypertrophic cardiomyopathy: results of the RAPACAT trial. J Am Vet Med Assoc. 2023 Jul 26;261(11):1628-1637. doi: 10.2460/javma.23.04.0187. PMID: 37495229; PMCID: PMC10979416.
  6. U.S. Food & Drug Administration. “FDA Conditionally Approves Drug for Management of Ventricular Hypertrophy in Cats”. Content current as of: 03/14/2025
    https://www.fda.gov/animal-veterinary/cvm-updates/fda-conditionally-approves-drug-management-ventricular-hypertrophy-cats
  7. Stern, DVM, PhD, DACVIM (Cardiology), Joshua A. “Feline Hypertrophic Cardiomyopathy: Hope is on the Horizon!”. Lecture presented on April 26, 2025 at the Symposium by NC State CVM Continuing education “Health Breakthroughs for Every Cat: Focusing on Future Therapies for Feline Heart Disease and Beyond”.  Zoom and live lecture.
  8. U.S. Food & Drug Administration. “Conditional Approval Explained: A Resource for Veterinarians”. Content current as of: 09/17/2020. https://www.fda.gov/animal-veterinary/resources-you/conditional-approval-explained-resource-veterinarians#CA1
  9. The HALT Study recruitment webpage. Enrollment information. hcmincats.com.
  10. TriviumVet product link. “felycin-CA1 (sirolimus delayed release tablets). https://www.triviumvet.com/felycin-ca1
  11. Freedom of Information Summary (FOI) Application for Conditional Approval. Application number 141-604 felycin-CA1 (sirolimus delayed-release tablets) Cats. https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/16672