08 May Felicyn and Feline HCM: What We Know So Far About Rapamycin for Cats with Heart Disease
Hypertrophic cardiomyopathy (HCM) is the most common heart disease in a cat. This condition causes thickening of the heart muscle, making it harder for the heart to fill with blood and pump efficiently. Some cats live for years with mild forms of the disease, while others progress quickly and develop congestive heart failure, blood clots, or sudden death. Despite decades of research, there have been limited advancements in treatments that actually modify the course of this disease-until recently.
Enter Felicyn: A New Hope for a Cat with Heart Disease?
Felicyn (Felycin-CA1) is a newly conditionally approved, delayed-release oral formulation of sirolimus (also known as rapamycin), granted FDA approval in March 2025 for the management of ventricular hypertrophy in cats with subclinical (pre-symptomatic) HCM. This is the first FDA-approved drug for use in feline HCM for any indication. The approval is based on early evidence suggesting that sirolimus may help slow or reverse some of the harmful changes that occur in the heart muscle during the early stages of the disease.
Although conditionally approved, Felicyn is not yet commercially available. It is expected to launch sometime in the summer of 2025, but an exact release date has not been announced.
Rapamycin is not entirely new to the medical world. It has been used in human medicine for years, primarily as an immunosuppressive drug for transplant recipients, and more recently studied for its anti-aging and anti-fibrotic effects. Its use in cardiology is based on its ability to inhibit mTOR (mammalian target of rapamycin) signaling a key pathway involved in abnormal cardiac muscle growth and remodeling.
What Does the Research Show?
The most significant study to date is the RAPACAT trial, a six-month clinical trial that evaluated the effects of rapamycin in cats with early-stage HCM. The study showed a small but statistically significant reduction-about 1 mm-in left ventricular wall thickness in cats receiving the drug, compared to those on placebo. While this may sound modest, small changes like this can be clinically relevant in feline cardiology. Just as importantly, the medication appeared well-tolerated in the short term, with no evidence of immunosuppression at the approved dosage.
However, the study did not demonstrate improvements in survival, time to congestive heart failure, or overall quality of life. In other words, we don’t yet know whether Felicyn helps cats live longer or feel better, only that it may slow the structural progression of the disease. Additional long-term studies are needed to evaluate those outcomes.
Is It Right for My Cat with Heart Disease?
Felicyn is not appropriate for every cat with heart disease. It is currently approved only for use in cats with subclinical HCM (also referred to as ACVIM Stage B)-meaning there are echocardiographic signs of ventricular hypertrophy, but the cat has no symptoms such as difficulty breathing, lethargy, or fainting.
It should not be used in cats with:
- Congestive heart failure (Stage C or D)
- Diabetes mellitus
- Pre-existing liver disease
- Certain drug interactions, which your veterinarian will review
Before starting Felicyn, cats should be screened for liver health. A study evaluating rabies vaccine response showed no impairment to immune function, and the once-weekly oral dosing (0.3 mg/kg) was not found to be immunosuppressive.
What Should Pet Owners Do Now?
If your cat has been diagnosed with HCM, especially in its early stages, it’s worth having a discussion with your veterinary cardiologist about Felicyn. While the medication shows promise, it is still under conditional approval, which means more data is being collected to confirm long-term safety and effectiveness. It is not yet considered a standard of care, and it is not yet available for veterinarians to prescribe.
For now, treatment recommendations remain individualized based on disease stage, rate of progression, clinical signs, and overall quality of life. Felicyn offers a new, proactive option for managing subclinical disease in appropriately selected patients, but it is not a cure, and it’s not the right fit for every cat.
Stay Tuned!
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As additional studies are completed and more real-world experience accumulates, we’ll continue to evaluate the role of Felicyn in feline cardiology. New treatments take time to find their proper place in practice, but for the first time, we may be entering an era where early intervention could slow this devastating disease.
If you have questions about HCM or want to learn whether your cat may benefit from newer treatments like Felicyn, talk with your veterinarian or schedule a consultation with a veterinary cardiologist. We’re here to help guide you through every step of the journey.
Images used under creative commons license – commercial use (05/08/2025) Image by Daga_Roszkowska from Pixabay