Sirolimus (Felycin-CA1) for Cats: Dosage, Safety, and Side‑Effect Profile

Sirolimus, also known as rapamycin and going by the trade name felycin-CA-1, is a newer, very promising medication to treat hypertrophic cardiomyopathy (HCM) in cats and significantly slow down or prevent progression of the disease. In this article, you’ll learn how this medication works to treat HCM, criteria for its use, and information about ongoing research studies.

Sirolimus For Cats Overview

Medication Type:

mTOR inhibitor

Form:

Delayed release tablets

Prescription Required?:

Yes

Brand Names:

Felycin-CA-1 (only brand approved for use in cats), Rapamune

Common Names:

Sirolimus, rapamycin

Available Dosages:

As sirolimus delayed-release tablets (felycin-CA-1): 0.4mg, 1.2mg, 2.4mg

About Sirolimus for Cats

Sirolimus is a newer medication with conditional approval by the FDA for treating hypertrophic cardiomyopathy (HCM) in cats. HCM is the most common heart disease in cats, affecting 1 in 7 of all adult cats. In cats over 9 years of age, this risk increases to 1 out of 3 cats.

Many cats with HCM have no visible or obvious signs of disease until a sudden, devastating event occurs, which is typically either congestive heart failure (CHF) or aortic thromboembolism (FATE).

Although the first apparent signs of HCM often appear suddenly, the condition itself develops slowly over many months or even years with progressive heart muscle thickening. This early development of HCM can only be diagnosed by looking for it with specific testing.

In cats who have these early changes prior to developing congestive heart failure, preliminary studies have shown significant evidence that early heart muscle thickening caused by HCM can be slowed down and even reversed in cats taking sirolimus.

Rapamycin is a natural compound discovered in the 1970s. Researchers found antimicrobial, immunosuppressant, and anti-tumor effects of it with applications for human medicine. Its effects on the heart in cats were discovered much more recently.

Sirolimus and rapamycin are technically the same medication and the names may be used interchangeably. Rapamycin is the original name given to the compound when it was first discovered. Sirolimus is the generic name used for FDA-approved medications. The brand felycin-CA-1, manufactured by TriviumVet is specifically the targeted form of the medication that has acquired conditional FDA approval for cats.

Sirolimus is an inhibitor of mTOR (mechanistic target of rapamycin). mTOR is an enzyme called  a kinase which is involved with regulation of various proteins. When activated, it helps form proteins called mTOR complex 1 and 2 (mTORC1 and mTORC2).

Regulation of these complexes is crucial for cardiac health. mTORC1 is activated in heart muscle cells undergoing chronic hemodynamic (blood flow) stress. This chronic stress with adequate blood flow to heart muscles is part of what leads to remodeling of heart muscle and the disease-causing thickening (hypertrophy) seen with HCM in cats.

A study conducted by Dr. Joshua Stern, a veterinary cardiologist and associate dean at NC State’s College of Veterinary Medicine showed that sirolimus worked so well to address HCM-related heart changes in cats, that it was given conditional approval in March 2025, after the study was completed in 2023.

The RAPACAT Trial

The research project that led to rapamycin/sirolimus’ conditional approval by the FDA and conducted at NC State, was known as the RAPACAT trial.

This study involved 43 client-owned cats that had confirmed subclinical HCM. For HCM, subclinical means these cats had evidence of heart muscle thickening and enlargement but had not yet developed complications of congestive heart failure.

The study was conducted over a 6 month period. All cats had retesting and follow-up throughout that period. At the 6 month mark, there was a significant reduction in left ventricular wall muscle thickness compared to the placebo. Adverse effects appeared to be minimal with the low-dose trial of rapamycin and were similar between the medication and placebo groups.

Two dose levels of rapamycin were evaluated. Interestingly, the low dose trial was noticeably more successful compared to the high dose, which is what led to the recommended 0.3mg/kg once weekly dose.

The reason for this is believed to be the balance between the impact on mTORC1 and mTORC2. Rapamycin impacts mTORC1 far more, and mTORC1 is connected more directly with heart muscle regulation. mTORC2 contributes more to other body system processes, like glucose regulation and insulin resistance in the liver.

Partially inhibiting mTORC1 without impacting mTORC2 through low level, intermittent dosing is believed to be vital to proper dosing of this medication for addressing HCM.

Ongoing Research: The HALT Study

Results of the RAPACAT trial were strong enough to gain conditional FDA approval of sirolimus to treat cats with HCM. However, researchers recognize the need for a far larger study involving many more cats to better evaluate efficacy and safety of sirolimus.

The HALT study will involve 300 or more cats evaluated over 12 months. It began in April 2025 and will continue through December of 2026. Any cat with subclinical/early HCM may be eligible. For more information about the study and enrollment, see the main website for the HALT HCM study.

What Does Conditional Approval Mean?

The results of the RAPACAT trial were so promising that the FDA granted conditional approval for this medication as Felycin CA-1 in early 2025. Criteria for conditional approval include:

• A medication must address a serious or life-threatening health condition
• The medication addresses an unmet need in human or animal health
• Efficacy must be demonstrated through a preliminary study

Conditional approval by the FDA also comes with restrictions on how the medication can be used, which are discussed further below. In general, early HCM must be supported through appropriate diagnostic testing. The medication cannot be prescribed simply for cats believed to be at risk.

Heart murmurs in cats can be highly variable and are often present in cats without true heart disease if they are under stress (such as during a veterinary visit). A cat who simply has a heart murmur without proven heart disease is also not an appropriate candidate for sirolimus without confirmatory testing to support HCM.

What is Needed for My Cat to Start Sirolimus?

For sirolimus to be prescribed by your vet, two specific criteria must be met. This is not only for appropriate medical care, but is required to align with the currently only conditional approval granted by the FDA for use of sirolimus in cats:

1. Cats must be “otherwise healthy”, subclinical (for HCM) and without systemic hypertension, other causes of compensatory myocardial hypertrophy (heart muscle thickening), current or historical symptoms of congestive heart failure, arterial thromboembolism, and severe left ventricular outflow obstruction (which can only be seen with an ultrasound of the heart)
2. Echocardiographic measurement of left ventricular wall thickness to diagnose subclinical hypertrophic cardiomyopathy (defined as greater than or equal to 6 millimeters at end diastole by 2D or M-mode assessment (with heart ultrasound))

This means, to be certain sirolimus is appropriate, needed testing includes:

• Feline NT-proBNP test (either being “abnormal” with the in-clinic SNAP test or more ideally a quantitative value). This test is only supportive of heart disease but not confirmatory for HCM.

• Heart ultrasound (echocardiographic) measurements of the left ventricular wall. Any veterinarian can learn to acquire these images and measurements, but it does require training, experience, and the right equipment for the measurements to be accurate. Veterinarians may send heart ultrasound images for review by a veterinary cardiologist for the most accurate results.
• A full echocardiogram is most ideal to acquire a full picture of heart disease and best support an HCM diagnosis. Ruling out a left ventricular outflow tract obstruction (which can be caused by the mitral valve obstructing flow into the aorta during atrial contraction/systole or can be similarly caused by severe thickening of ventricular heart muscle also blocking blood flow to the aorta) is important and may require a veterinary cardiologist to perform an echo or having a cardiologist review another vet’s echo images to screen for these abnormalities.

• Blood pressure. Cats in earlier stages of HCM are often not hypertensive (high blood pressure). Systemic hypertension is one contraindication for starting sirolimus.
• Bloodwork. Other causes of heart muscle thickening, such as hyperthyroidism, must be ruled out. Contraindications also include liver disease with elevations on bloodwork, and diabetes mellitus. It’s very important to screen for these conditions before starting sirolimus.

This testing may not be easily available in all areas or affordable to everyone. Petspan is a telemedicine company that has developed a veterinarian-approved protocol for assisting with ensuring that rapamycin/sirolimus is prescribed appropriately for HCM. Learn more about their specific HCM protocol for prescribing rapamycin/sirolimus.

Dosing of Sirolimus for Cats

The FDA conditionally-approved dose for sirolimus in cats that meet the proper criteria, is 0.3mg/kg given once a week.

Tablets must be given whole and cannot be crushed or split.

Only the individual tablet currently being used should be removed from the packaging at a time.

Given the conditionally-approved nature of the medication currently, any deviation from this protocol or off-label use of the medication is prohibited by law.

Given the tablet dosing sizes, the following chart can be used (dosing is based on kilograms, the pound ranges are approximations). More than one number in a column represents the need to combine two different tablet sizes for the proper dosage.

Body Weight 2.5-3.2kg (5.5-7lb) 3.3-4.8kg (7.1-10.5lb) 4.9-6.4kg

(10.6-14lb)

6.5-9.6kg

(14.1-21lb)

Over 9.6kg

(over 21lb)

Number of 0.4mg tablets weekly 2 N/A 1 N/A N/A Number of 1.2mg tablets weekly N/A 1 1 N/A 1 Number of 2.4mg tablets weekly N/A N/A N/A 1 1

How to Administer Sirolimus to Cats

Sirolimus only comes in a tablet form which cannot be split or crushed. It is given once daily by mouth. The whole tablet may be hidden in food or a treat, like a Pill Pocket.

For tips on giving your cat a whole pill, see links to our articles below:

• The 7 Best Cat Foods to Hide Pills In
• How to Give Your Cat a Pill (With 7 Proven Tips!

Compounding into a liquid form may be an available option in cases where administering a whole pill is not practical or feasible.

As sirolimus/rapamycin can have immunosuppressant qualities (though the dose is thought to be too low to have this effect) human caregivers are advised to take certain precautions:

• When administering sirolimus, it is important to wash your hands after handling or administering the whole pill.
• If handling vomit or saliva for a cat recently dosed with sirolimus/rapamycin, it’s important to wear gloves when cleaning. The same is true if there is a need to handle broken pills or pill fragments.

Side Effects of Sirolimus for Cats.

The most common side effects noted during the RAPACAT study included digestive upset

• Vomiting
• Diarrhea
• Reduced appetite

Lethargy has also been reported.

Elevation of liver values (though not resulting in illness) was seen in some cats after starting sirolimus.

Although rapamycin has immunosuppressant qualities, the low and infrequent dose used for HCM in cats is not expected to be immunosuppressive. It has been used in conjunction with rabies vaccination and adequate immune response to vaccination was achieved.

Overall, of 69 adverse events reported during the RAPACAT trial, 53 of them were deemed as either unrelated to the medication or a connection to the medication was otherwise not apparent.

Contraindications of Sirolimus for Cats

Sirolimus should not be started in the following situations:

• Cats with diabetes mellitus
• Cats with medically significant liver disease
• Cats with heart disease that is not hypertrophic cardiomyopathy (i.e. dilated cardiomyopathy, cardiomyopathy associated with hyperthyroidism, etc.)
• Cats with systemic hypertension
• Cats with medically significant left ventricular outflow tract obstruction (VOTO)

Sirolimus should be used cautiously in the following situations:

• Cats with other, unrelated illnesses (such as kidney disease; sirolimus has not been fully evaluated in cats with other health conditions)
• Cats weighing less than 2.5kg (5.5lb) (the tablets cannot be accurately dosed)
• Use together with vaccinations other than feline rabies vaccine (sirolimus has not been evaluated alongside other vaccines like FVRCP/HCP, FeLV, etc.)
• Cats without observed liver disease/illness but with liver value elevations on bloodwork

Overdose and Emergencies

In the RAPACAT study, there was no significant difference in adverse events between the low dose group (0.3mg/kg and the conditionally approved dosage) and the high dose group (0.6mg/kg).

Signs of illness were also not observed in cats receiving oral doses up to 7.5 times the labeled dose.

Given the tablet sizes and very low frequency of dosing once a week, the risk of an overdose is hopefully very low.

However, given that this is a very new medication, if an overdose is suspected to have occurred, it’s best to contact one of the following, even before contacting your own veterinarian:

• ASPCA Animal Poison Control Center (1-888-426-4435)
• Pet Poison Helpline (1-855-764-7661)

Potential Drug Interactions With Sirolimus

The following list of medication interactions is either known, based on knowledge of human medicine with rapamycin, or theoretical. Unless noted specifically, use together may not be contraindicated but caution should be considered and details discussed with your veterinarian.

• Azole antifungals (itraconazole): may increase sirolimus concentration. In humans, use together is advised to be avoided).
• Cannabidiol (CBD): may increase sirolimus concentration
• Cyclosporine: may increase sirolimus concentration and may enhance immunosuppressive effects, increasing the risk for secondary infection
• CYP3A inducers (phenobarbital): may increase sirolimus concentration
• Immunosuppressants (glucocorticoids like prednisolone used at an immunosuppressive dose, JAK inhibitors like oclacitinib/Apoquel): may enhance immunosuppressive effects, though when dosed properly, sirolimus is not expected to be immunosuppressive
• P-Glycoprotein substrates (selamectin/Revolution, maropitant/Cerenia): may increase sirolimus concentration
• Verapamil: may increase concentration of both sirolimus and verapamil

Monitoring Cats on Sirolimus

Bloodwork is advised while on sirolimus, especially for monitoring liver values and blood sugar/blood glucose. Your vet may wish to check bloodwork before starting, shortly after starting, and at periodic intervals every few months.

Bloodwork should also be checked if any changes to health are noted. This is especially true for any cats that develop signs of increased drinking and urinating.

Because this medication is designed for cats with subclinical HCM who don’t have visible signs of heart disease, sirolimus’ impact on heart muscle thickness and health may only be evaluated through recheck testing. This may involve rechecking a quantitative feline NT-proBNP to see if the value decreases, and/or through recheck measurements of ventricular wall thickness via heart ultrasound.

How to Store Sirolimus

Sirolimus delayed-release tablets in the original packaging may be stored at room temperature from 68-77 degrees F (20-25 degrees C). Brief or temporary excursions are okay between 59-86 degrees F (15-30 degrees C)

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