Felycin-CA1 for Cat HCM: 43 Cats, 6 Months

About one in seven apparently healthy cats already has hypertrophic cardiomyopathy. In the CatScan study, 115 of 780 cats screened by echocardiography met the criteria — 14.7%. Until 2025 nothing was approved to slow that thickening down. On 14 March 2025 the FDA conditionally approved Felycin-CA1 (sirolimus delayed-release tablets), and the 43-cat trial behind it found the treated group's thickest ventricular wall had thinned by 0.17 mm after six months while placebo cats thickened by 0.94 mm.

What was actually approved, and on what terms

The application is NADA 141-604, sponsored by TriviumVet (Waterford, Ireland). It is the first product approved for cats with HCM for any purpose. The word that matters is conditional: the FDA accepted a reasonable expectation of effectiveness rather than a demonstrated one, because enrolling enough cats with a disease that needs advanced imaging is slow and costly. The approval runs one year at a time and can be renewed for up to four additional years while the sponsor completes the HALT HCM trial, a pivotal study running at more than 20 US sites. Product reached clinics on 30 July 2025.

The label is narrow: the 6 mm line

Felycin-CA1 is indicated for ventricular hypertrophy in cats with subclinical HCM, defined as a left ventricular wall of 6 mm or more at end-diastole on 2D or M-mode echo, with no systemic hypertension, no other cause of compensatory thickening such as hyperthyroidism, no current or past congestive heart failure, no arterial thromboembolism and no severe outflow tract obstruction. That is a stage B2 cat: structurally affected, clinically silent. A murmur alone does not qualify a cat. Cats with heart failure, clot history or severe obstruction were excluded from the trial, so there is no evidence for them.

What RAPACAT measured in 43 cats

It was double-blinded, placebo-controlled and run at two referral cardiology centres. Forty-three client-owned cats were randomised 1:1:1 to placebo, 0.3 or 0.6 mg/kg weekly for 180 days; 36 finished. The primary endpoint was maximum left ventricular wall thickness at any location, measured by echo at days 60 and 180. At day 180 the low dose was significantly better than placebo (P = .01), and the two doses pooled were also different (P = .05). The high dose missed significance, which the authors attribute partly to only 10 cats remaining in that arm. Higher baseline NT-proBNP predicted faster thickening (r² = 0.26, P = .001).

Two-tenths of a millimetre of regression over six months is not a cure; it is the difference between a wall that keeps growing and one that holds still. The trial measured anatomy, not lifespan.

Side effects: 69 events and one label change

Sixty-nine adverse events were recorded — 21 with placebo, 23 with the low dose, 25 with the high dose (P = .82). Only two were judged probably related and 14 possibly related. Five were serious, four of them in treated cats: two developed congestive heart failure, one died suddenly, and one went into diabetic ketoacidosis. All were either disease progression or a known mTOR effect. The DKA cat was obese and on a weight-loss diet before enrolment, which is why the label now states plainly: do not use in cats with diabetes mellitus, and stop immediately if diabetes develops. A second cat, later found to have pre-existing liver disease, declined after several months and was euthanased; cats are now screened for liver disease first, and the label calls for stopping if ALT or AST exceeds twice the upper limit of normal.

Dose, tablet maths and yearly cost

The target dose is 0.3 mg/kg once weekly, swallowed whole, with or without food. Three strengths are combined by weight band; cats under 2.5 kg cannot be dosed accurately. Prices below are manufacturer minimum advertised prices for a 12-tablet carton, which is a 12-week supply for cats on one tablet a week.

Cat's weight	Weekly tablets	Sirolimus/week	12-week carton	Cost per week
2.5–3.2 kg	Two 0.4 mg	0.8 mg	$53.39	$8.90
3.3–4.8 kg	One 1.2 mg	1.2 mg	$150.74	$12.56
4.9–6.4 kg	0.4 mg + 1.2 mg	1.6 mg	$204.13	$17.01
6.5–9.6 kg	One 2.4 mg	2.4 mg	$251.26	$20.94

That is roughly $460 to $1,090 a year for the drug alone, before the diagnostics it requires: a feline echocardiogram typically runs $400–900, a cardiology consultation $150–300, plus bloodwork at one to two months and then every 6–12 months.

Interactions, handling and the compounding trap

Sirolimus is cleared through CYP3A4 and P-glycoprotein. Drugs that inhibit either pathway — azole antifungals such as ketoconazole, amiodarone, some calcium channel blockers, cyclosporine — can push exposure up, so every add-on medication needs checking. Cats with the MDR1 mutation and cats on other P-gp substrates need caution. Tablets are enteric-coated and delayed-release: never split or crush them, because that destroys the release profile the dosing was built on. Pregnant and breastfeeding women should not handle the tablets. The label also notes treatment could blunt vaccine responses; the sponsor ran a rabies vaccine study to check.

Do not substitute human sirolimus (Rapamune) or a compounded rapamycin capsule. Those are different formulations at different doses, and both the FDA and the manufacturer have warned that substituting them for Felycin-CA1 is unapproved and potentially dangerous.

What it does not do

The REVEAL study followed cats with preclinical HCM and found that 30.5% developed congestive heart failure, arterial thromboembolism or both — 24.2% heart failure, 11.6% clots, 2.2% sudden death — against 0.97% of healthy cats. Median survival with subclinical disease was 10.9 years, and the risk of cardiac death climbed from 6.7% at one year to 22.8% at five and 28.3% at ten. CatScan II saw events in 7 of 34 cats over a median 3.6 years. Crucially, REVEAL found no survival benefit from the cardiovascular drugs cats were already taking at the preclinical stage. Wall thickness is a surrogate marker; whether Felycin-CA1 buys years rather than millimetres is exactly what HALT is meant to answer.

A five-step path if your cat has a murmur

Get an echo from someone who scans cats routinely, and ask for wall measurements in millimetres.
At the same visit: blood pressure, total T4, CBC and chemistry, urinalysis. NT-proBNP is optional ($60–120) and does not replace imaging.
A wall of 6 mm or more with no hypertension, thyroid disease, heart failure, clot history or severe obstruction means the label covers your cat; any one of those rules it out.
Start 0.3 mg/kg weekly, recheck liver enzymes and glucose at one to two months, then every 6–12 months, and repeat the echo at three to six months.
Count resting breaths at home. More than 30 a minute, new hiding or a dropping appetite means call the clinic now.

This article does not replace veterinary diagnosis. Felycin-CA1 is prescription-only and conditionally approved; whether it suits an individual cat is a decision for that cat's veterinarian, made on that cat's echocardiogram and bloodwork.

Terms mentioned in this article

HCM MM T4

Frequently asked

Is Felycin-CA1 a cure for HCM?

No. Over six months it held wall thickness steady or slightly reduced it (0.17 mm mean regression versus a 0.94 mm increase on placebo) in cats with subclinical disease, but no trial has shown that it prevents heart failure, clots or death. Approval is conditional, and the HALT HCM trial is still collecting the outcome data needed for full approval.

Can I give my cat human sirolimus or a compounded rapamycin instead?

No. Rapamune and compounded rapamycin are different formulations at different doses, with no feline safety or efficacy data for HCM. The FDA and the manufacturer warn that substituting them is unapproved and can be dangerous, and the delayed-release coating on Felycin-CA1 is part of why once-weekly dosing is tolerated.

My cat has a heart murmur. Can we start the drug now?

Not without an echocardiogram. The label requires a left ventricular wall of 6 mm or more at end-diastole plus normal blood pressure, no hyperthyroidism, no heart failure or clot history and no severe outflow obstruction. Murmurs are common and often functional, so a murmur on its own is not a diagnosis.