FIV. Thoracotomy. Enucleation. Kidney Failure. HCM. This Cat Is Surviving All of It.

Harry wasn't supposed to be a cardiology case.

He was a thoracotomy case. An enucleation case. A cryptococcal infection case. An acute kidney injury case. He was, by any reasonable clinical measure, already more than enough to manage. But on the fourth day of his hospitalization for acute kidney failure, while his mom Kristyn Allen, who, by the way, is a Senior IT Project Specialist at VCA Animal Hospitals, sat in the waiting room, someone listened to his heart and heard something new.

A grade 1 to 2 murmur. First time ever detected.

That moment is the reason Harry is alive and thriving today. Not because the murmur itself was an emergency. Not because hypertrophic cardiomyopathy (HCM) had already done significant damage. But because it was found early enough to mean something, at exactly the moment in veterinary cardiology when finding it early actually matters.

Subclinical HCM and the Intervention Window

HCM is the most common cardiac disease in cats, and for most of its clinical history, the conversation around it has been a conversation about management. You monitored. You waited. You treated symptoms as they emerged and hoped the trajectory was slow. Early detection existed as a concept, but it didn't come with a meaningful intervention on the other end.

Felycin®-CA1 (sirolimus delayed-release tablets) changed that.

The only conditionally FDA-approved management of ventricular hypertrophy in cats with subclinical HCM, Felycin®-CA1 works precisely in the window that the profession has historically had the least to offer: before symptoms appear, before the left atrium enlarges, before the thickening of the left ventricular wall has progressed beyond the point of meaningful intervention. Studies show that cats receiving Felycin-CA1 experienced measurable improvement in heart wall measurements compared to cats that didn't receive treatment. The drug doesn't just slow the process. It may help prevent and delay it.

That makes early detection not just clinically interesting but clinically urgent in a way it has never been before.

What Harry's Case Teaches Us About the ProBNP

Kristyn is not a clinician. She is, by her own description, Harry's mom, a veterinary professional working at VCA with 14 years of field experience and an encyclopedic knowledge of her cat's medical history. But the clinical instinct she developed through Harry's journey is one worth borrowing.

When the cardiologist appointment was booked out nine weeks, her primary vet recommended a proBNP blood test in the meantime. The result came back normal. It bought them a window of relative relief while they waited and gave them a baseline to work from once they got into cardiology. Harry was ultimately diagnosed with mild HCM, thankfully so mild he wasn't even given a grade, with slight left ventricular thickening and no left atrial enlargement. Exactly the subclinical profile Felycin®-CA1 was built for.

Kristyn now requests proBNP testing for all of her cats at every exam, alongside their full bloodwork. Not because she expects the worst. Because she knows what it means to find something early.

For veterinary professionals, the clinical takeaway is straightforward: proBNP testing is an accessible, affordable way to build cardiac surveillance into routine wellness care for feline patients. It is not a replacement for echocardiography. But it is a flag, a baseline, and in the right moment, the thing that starts the conversation that saves a life.

The Accidental Diagnosis That Was Actually Right on Time

Harry's HCM was found the way a lot of the most important findings are found in veterinary medicine: sideways, while everyone was focused on something else. His mom wasn't looking for heart disease the day after Christmas while her cat was crashing from an E. coli-driven acute kidney injury. She was just trying to get him home.

But the murmur was there. And it was heard. And nine weeks later it had a name and a diagnosis and eventually a treatment plan that included the first drug ever conditionally FDA-approved for this specific condition in cats.

Kristyn started Harry on Felycin®-CA1 in September 2025. Within the first two weeks she noticed more energy, more playfulness, improved appetite, and full sprinting around the house. As of 26 weeks on the drug, he has maintained that baseline entirely. His kidney values have shown slight improvement. He regularly bosses his three brothers around and has a great deal to say about it.

He is, against considerable odds, thriving.

The Window Is Open. The Question Is Whether You're Looking Through It.

The profession now has a drug that makes subclinical HCM detection worth acting on. What it needs next is a culture of detection that matches the intervention available.

That means listening at every visit. It means running the proBNP. It means understanding that the mild murmur you heard on day four of a complicated hospitalization might be the most important thing you found all week.

That's where every HCM success story starts. Talk to a veterinary cardiologist about which of your feline patients may be candidates.

Felycin-CA1 is conditionally approved by FDA pending a full demonstration of effectiveness under application number 141-604. See prescribing information for complete details regarding adverse events, warnings, and precautions. For more information on which of your feline patients may be candidates, consult a veterinary cardiologist.

Prescribing Information

Felycin-CA1 (sirolimus delayed-release tablets)

Cardiac drug for oral use in cats only.

Conditionally approved by FDA pending a full demonstration of effectiveness under application number 141-604. It is a violation of federal law to use this product other than as directed in the labeling. For complete prescribing information, see full package insert.

Caution: Federal law restricts this drug to use by or on the order of a licensed veterinarian.

Indication

For the management of ventricular hypertrophy in cats with subclinical hypertrophic cardiomyopathy (HCM).

Dosage and Administration

Administer Felycin-CA1 at a target dose of 0.3 mg/kg orally once weekly. (please see full dosing and administration guidelines in the prescribing information).

Tablets should be swallowed whole and not chewed. Do not split or crush tablets. Felycin-CA1 should be administered with a meal. Cats weighing less than 2.5 kg cannot be accurately dosed.

Contraindications

Felycin-CA1 should not be used in cats with diabetes mellitus. Discontinue immediately if a cat receiving Felycin-CA1 is diagnosed with diabetes mellitus. The administration of Felycin-CA1 to a cat that developed diabetes mellitus was associated with the development of diabetic ketoacidosis and death. Do not administer in cats with pre-existing liver disease.

User Safety Warnings

Not for human use. Keep out of reach of children. Contact a physician in case of accidental ingestion by humans. Pregnant and breastfeeding women should avoid contact with Felycin-CA1. People with known hypersensitivity to sirolimus should administer Felycin-CA1 with caution.

Animal Safety Warnings

Administration of Felycin-CA1 with drugs that inhibit cytochrome P-450 3A4 or P-glycoprotein, such as calcium channel blockers, amiodarone, azoles, or cyclosporine, may increase risk for toxicity. Use caution when administering in cats with the MDR1 mutation or when administering concomitantly with another P-gp substrate. Treatment with Felycin-CA1 could impact the cat’s ability to mount an adequate immune response to vaccinations. Concurrent administration of Felycin-CA1 did not impact the cat’s ability to mount an adequate immune response to a killed rabies vaccine. The impact of Felycin-CA1 for FHV-1, FCV, FPV, and FeLV has not been evaluated. Keep Felcyin-CA1 in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose.

Precautions

For use only in otherwise healthy cats with subclinical HCM in the absence of other causes of compensatory myocardial hypertrophy (e.g. systemic hypertension), current or historic symptoms of congestive heart failure, arterial thromboembolism, and sever LV outflow tract obstruction. Treatment with Felycin-CA1 has been associated with the elevation of the transaminase enzymes, which include alanine aminotransferase (ALT) and aspartate aminotransferase (AST). The use of Felycin-CA1 in cats with viral disease like feline viral rhinotracheitis has not been evaluated. The safety and effectiveness of Felycin-CA1 has not been evaluated in cats with other cardiomyopathy phenotypes, in cats receiving beta blockers or corticosteroids, in cats with kidney disease, hyperthyroidism, or other significant systemic disease. The effectiveness of Felycin-CA1 has not been evaluated in sexually intact cats, therefore, should not be used in animals intended for breeding.

Adverse Reactions

In a well-controlled pilot field study, 43 cats with subclinical HCM were administered either the label dose of Felycin-CA1 (0.3 mg/kg once weekly), twice the label dose (0.6 mg/kg once weekly), or a placebo control tablet and followed for 180 days or until removal from the study. The most frequently observed adverse reactions were cardiovascular in nature, relating to the progression of HCM, and included arrhythmia, congestive heart failure, syncope, and pericardial effusion. Other adverse reactions were lethargy, vomiting, diarrhea, and inappetence.

Clinical Pharmacology

Sirolimus is an immunosuppressant that targets and inhibits the mammalian target of rapamycin C1 (mTORC1) protein complex, a central regulator of cell growth and nutrient response. Studies in rodent models suggest mTOR inhibition by sirolimus attenuates cardiac hypertrophy by promoting autophagy, attenuating oxidative stress and blocking proinflammatory responses, thereby resulting in an improvement in cardiac function in rodents.

Reasonable Expectation of Effectiveness

This product is conditionally approved by FDA pending a full demonstration of effectiveness.

To obtain full product information for Felycin-CA1 please call 1-800-874-9764.

Manufactured for: PRN Pharmacal, Pensacola, FL 32514

PRN is a trademark of Pegasus Laboratories, Inc.

Dosage and Administration

For use in cats only. The total recommended dosage for Felycin-CA1 is 0.3 mg/kg once per week, dosed in conjunction with a meal.

Storage

Store at 20-25°C (68-77°F).

WARNINGS

NOT FOR USE IN HUMANS. KEEP THIS AND ALL DRUGS OUT OF REACH OF CHILDREN.

4-2025

BRAIN CHECK

Harry the cat was already managing FIV, kidney failure, and more when a grade 1 to 2 heart murmur was detected for the first time. What blood test gave his care team a baseline while they waited 9 weeks for a cardiology appointment?

A. Troponin I

B. proBNP

C. SDMA

D. Total T4

CORRECT ANSWER: The answer is proBNP. It came back normal and gave Harry's team a baseline before cardiology even got involved. He was ultimately diagnosed with mild subclinical HCM and started on Felycin-CA1, the only conditionally FDA-approved treatment for ventricular hypertrophy in cats with subclinical HCM. Within two weeks he was back to full sprinting around the house.