Changing what’s possible in feline heart care: Felycin-CA1 offers hope for cats with hypertrophic cardiomyopathy

This content is sponsored by PRN Pharmacal.

Last June, my colleagues at PRN Pharmacal launched sirolimus delayed-release tablets (Felycin-CA1) a new therapy for the management of ventricular hypertrophy in cats with subclinical hypertrophic cardiomyopathy (HCM).^1,2 As the first product conditionally approved by the FDA for use in cats with HCM, Felycin-CA1 marked an industrywide turning point for feline HCM care, offering hope for cats—and owners—affected by this life-threatening disease.^1,3

Feline HCM—deadly and silent

Feline HCM is a progressive, often irreversible condition that causes the heart’s muscular walls to thicken, impairing its ability to pump blood effectively.^1,3 It can lead to congestive heart failure, arterial thromboembolism, or even sudden death, among other complications.¹ Althoughthe causes of HCM aren’t entirely clear, some research has shown higher prevalence in certain breeds—such as Maine Coon, Ragdoll, Persian, and British Shorthair breeds—as well as identified cardiac gene mutations in those most commonly affected, suggesting genetic implications.²

Despite being the most common cardiovascular disease in cats—affecting 1 in 7 domestic cats—and the No. 1 cause of mortality in adult cats (>9 years of age), feline HCM can be extremely difficult to detect, let alone manage.^3,4,5 In fact, it’s often referred to as a silent disease, since cats with HCM are typically subclinical, progressing without showing any outward signs or symptoms, and if they do show signs, it’s sometimes not until later stages of the disease.^4,5

The FDA’s conditional approval of Felycin-CA1

The active ingredient in Felycin-CA1, sirolimus (also known as rapamycin), was first FDA approved in 1999 as a human immunosuppressant for organ transplants.^2,3,6 Now formulated as a delayed-release tablet specifically designed for cats, Felycin-CA1 has been proven to help slow the progression of subclinical HCM, via results from the RAPACAT trial, a well-controlled pilot field study that led to its conditional approval, in March 2025.^1,2

To receive conditional approval from the FDA, a drug must meet several requirements. Most notably, it must address a serious or life-threatening disease, or an unmet animal or human health need; meet certain manufacturing, safety, and quality standards; and demonstrate a reasonable expectation of effectiveness.¹ Through the RAPACAT trial, in addition to the data already established from the use of sirolimus in other species, Felycin-CA1 met the FDA’s conditional approval requirements.^1-3 Although the FDA’s efficacy requirements differ between full approval and conditional approval, the safety standards are the same.⁷

Addressing the unmet need in feline HCM care

Prior to Felycin-CA1, we’ve had very few options to help manage HCM, particularly beyond palliative care.^2,3 In fact, the therapies that were available weren’t managing HCM at all; they were managing the symptoms.^2,3

For example, if a cat was diagnosed with HCM and experienced congestive heart failure as a result, a diuretic might be used to help clear fluid from the lungs. Or, in more severe cases, if a blood clot had entered the bloodstream, an antithrombotic might be administered. But ultimately, we had no other choice but to explain to these cat owners that the best they could do was manage the symptoms as the disease progressed.

Hence why Felycin-CA1 is considered so groundbreaking.² Previous available therapies in this space had failed to slow disease progression, improve quality of life, or show survival benefit in the subclinical setting.² The RAPACAT trial demonstrated that the left ventricular wall had reduced in those cats with subclinical HCM receiving prolonged administration of Felycin-CA1, suggesting that it may help slow or delay disease progression.³ Consequently, with Felycin-CA1, veterinarians finally have an option that may help manage the pathology associated with subclinical HCM—vs merely what happens to the patient when the heart muscle is failing.^1-3

Convenience of Felycin-CA1

Felycin-CA1 is given to cats at a target dose of 0.3 mg/kg orally once weekly, in conjunction with a meal.⁸ The tablets are available in 3 strengths—0.4 mg, 1.2 mg, and 2.4 mg—and are quite small, about the size of a Tic Tac, making Felycin-CA1 both convenient and easy to administer across a range of patient sizes.⁸ But most importantly, owners of cats with subclinical HCM can breathe a little easier knowing that we veterinarians now have something to start their pet on, instead of waiting for the cat to progress and then supporting them as their heart fails.

As a reminder, Felycin-CA1 is only available by prescription from a licensed veterinarian. Cats should be screened for preexisting liver disease before starting Felycin-CA1, and it should not be used in cats with preexisting liver disease or diabetes mellitus.

Screening for subclinical HCM

Now that we have the first-ever approved disease-modifying agent for subclinical HCM, how do we determine whether a patient is an appropriate candidate? For a cat to be eligible for Felycin-CA1, they must be diagnosed with subclinical HCM.⁷ Although no single, standardized screening protocol exists for feline HCM—subclinical or not—there are recommendations and best practices.^4,5,8,9

As work continues to find an ideal diagnostic pathway for screening cats for subclinical HCM, echocardiography is currently considered the gold standard.^8,9 Additionally, a comprehensive physical examination, along with blood pressure measurement, should always be performed.⁹ This should involve auscultation, as heart murmurs or gallop sounds have been linked to the condition.⁴ It is also recommended that other tests and imaging, like bloodwork and radiographs, be done to help confirm the diagnosis and rule out other causes, such as dehydration and hyperthyroidism.⁸ Biomarker and/or genetic testing may be used as well—not in isolation, however, but as a supplement to the entire diagnostic workup, and with caution, as results have been found to be inaccurate.^4,9

Practitioners—listen up!

On a more fundamental level, most of this relies on cat owners bringing their pets into the clinic for examination in the first place (which has its own separate challenges). That’s why it’s critical for practitioners not only to encourage cat owners to bring their pet in but also, when they do come in, to have conversations about the importance of screening and early intervention for conditions like HCM, especially with those who have predisposed breeds.

A couple of small tips that can make a big difference:

1. That practitioners kindly (and firmly) express to cat owners that it is in their cat’s best interest that they be (and stay) quiet during auscultation, so that practitioners can listen more closely and discern a heart murmur. For some reason, in my experience, I’ve always found that pet owners become most talkative as soon as the stethoscope goes on.
2. To extend feline HCM care beyond the clinic by showing cat owners how to monitor their cat’s sleeping respiratory rate at home. While their cat’s asleep, they can simply watch their chest rise and fall to count the number of breaths per minute. It should be fewer than 30 breaths, but if it’s more, advise cat owners to schedule a veterinarian appointment immediately, as increased respiratory rate has been linked to the progression of feline HCM.^4,9

The HALT Study and the path to full approval

Right now, the HALT Study is underway to provide the substantial evidence of effectiveness necessary for the full FDA approval of Felycin-CA1 (referred to as TRIV202 in HALT) for the management of ventricular hypertrophy in cats with subclinical HCM.^10,11

Launched in June 2025 by TriviumVet, in partnership with Pegasus Laboratories, the HALT Study is a multisite, blinded, randomized, placebo-controlled clinical field study conducted across various clinical sites experienced in treating cats with HCM.¹⁰ It will be both considerably larger than the RAPACAT trial, aiming to enroll 300 cats (vs 43 cats enrolled at baseline in RAPACAT), and longer, examining cats with subclinical HCM over a 12-month dosing period (vs the 6-month period in the RAPACAT trial), with 5 postenrollment study visits.^3,10,11

The size and duration of the HALT Study highlight our hope for greater insights into the efficacy as well as safety of Felycin-CA1—and our commitment to better understanding its long-term implications.

For more information on the HALT Study, including current study sites and enrollment criteria, or to register your cat or a patient, visit www.hcmincats.com.

More hope on the horizon for feline HCM care

Felycin-CA1 represents a big step forward in feline HCM care—and we’re not stopping there. Through the RAPACAT trial, ongoing patient cases, and now the HALT Study, we will continue to learn more about the effects of Felycin-CA1 on managing this complicated condition and push the boundaries of what’s possible in feline heart care.

For full prescribing information, please visit felycin-ca1.com.

References

1. FDA conditionally approves drug for management of ventricular hypertrophy in cats. FDA. March 14, 2025. Accessed October 9, 2025. https://www.fda.gov/animal-veterinary/cvm-updates/fda-conditionally-approves-drug-management-ventricular-hypertrophy-cats
2. Crossley KC. Novel cardiology therapy for cats is launched. dvm360. June 18, 2025. Accessed October 9, 2025. https://www.dvm360.com/view/novel-cardiology-therapy-for-cats-is-launched
3. Kaplan JL, Rivas VN, Walker AL, et al. 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;261(11):1628-1637. Published 2023 Jul 26. doi:10.2460/javma.23.04.0187
4. Kittleson M, Côté E. The feline cardiomyopathies: 2. hypertrophic cardiomyopathy. J Feline Med Surg. 2021;23(11):1028‑1051. doi:10.1177/1098612X211020162
5. McCafferty C. FDA conditionally approves first drug for feline subclinical hypertrophic cardiomyopathy. dvm360. March 14, 2025. Accessed October 9, 2025. https://www.dvm360.com/view/fda-conditionally-approves-first-drug-for-feline-subclinical-hypertrophic-cardiomyopathy
6. Beck L. First drug OK’d for most common heart condition in cats. VIN News Service. April 24, 2025. Accessed October 9, 2025. https://news.vin.com/vinnews.aspx?articleId=86835
7. Conditional approval explained: a resource for veterinarians. FDA. Updated September 17, 2020. Accessed November 3, 2025. https://www.fda.gov/animal-veterinary/resources-you/conditional-approval-explained-resource-veterinarians
8. Felycin-CA1. Prescribing information. PRN Pharmacal; 2025. Accessed October 23, 2025. https://www.triviumvet.com/hubfs/Felycin-PI-Detailer_PRN_02JUN2025.pdf
9. Häggström J, Luis Fuentes V, Wess G. Screening for hypertrophic cardiomyopathy in cats. J Vet Cardiol. 2015;17(suppl 1):S134-S149. doi:10.1016/j.jvc.2015.07.003
10. The HALT HCM Study. HCMinCats.com. Accessed October 23, 2025. https://www.hcmincats.com/halt
11. TriviumVet launch the HALT HCM Study recruiting 300 cats with diagnosed subclinical hypertrophic cardiomyopathy (HCM). TriviumVet. June 25, 2024. Accessed October 23, 2025. https://www.triviumvet.com/blog/triviumvet-launch-the-halt-hcm-study-recruiting-300-cats-with-diagnosed-subclinical-hypertrophic-cardiomyopathy-hcm