Rapamycin (Sirolimus) for Feline Hypertrophic Cardiomyopathy: RAPACAT Trial and Evidence

Bottom line

• In the RAPACAT trial — a double-blind, multicenter, randomized, placebo-controlled clinical trial in 43 cats with subclinical non-obstructive HCM — maximum left ventricular myocardial wall thickness at Day 180 was significantly lower in the low-dose delayed-release rapamycin group than in the placebo group (P = 0.01), establishing the first randomized evidence of disease-modifying effect in feline HCM.[1]
• Delayed-release rapamycin was well tolerated in the RAPACAT trial with no significant differences in adverse events between treatment and placebo groups throughout the 6-month study.[1]
• A parallel pilot study using a feline research colony with naturally occurring hereditary HCM found transcriptomic and proteomic evidence of dose-responsive suppression of myocardial hypertrophy, anti-coagulant and anti-thrombotic protein changes, and stimulatory effects on autophagy following once-weekly oral rapamycin.[2]
• Rapamycin (sirolimus) is not FDA-approved for feline use; its use in cats is investigational. Additional studies are required before this can be recommended as standard of care.

Drug facts

• Class: mTOR inhibitor (macrolide, immunosuppressant)
• Mechanism: Binds intracellular FK-binding protein 12 (FKBP12) → forms complex → inhibits mTORC1 (mammalian target of rapamycin complex 1) → reduces mTOR-driven transcription of hypertrophic gene programs; distinct from calcineurin inhibitors
• Route/interval: Oral; delayed-release (DR) formulation used in RAPACAT; weekly administration in research context
• Indication: Not FDA-approved for veterinary use; FDA-approved for human organ transplant rejection (Rapamune). Use in cats is investigational.
• Approval: No veterinary approval. Human FDA-approved product exists. Use in cats requires off-label extrapolation and owner informed consent.
• Label contraindications: No feline label. Caution anticipated in immunocompromised cats or cats with active infection.
• Label common AEs: No feline label. RAPACAT trial reported no significant difference in adverse events vs. placebo at the doses studied.

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What the evidence shows

Background: HCM in cats and the therapeutic gap

Feline hypertrophic cardiomyopathy (HCM) is the most common cardiomyopathy in cats, with a prevalence as high as 15% in some populations. Despite decades of recognition, standard pharmacologic interventions — atenolol, diltiazem, clopidogrel — remain symptomatic or directed at complications (arrhythmia, thromboembolism) rather than the underlying myocardial hypertrophy. No drug had demonstrated disease-modifying effects on the hypertrophic process itself in cats prior to the RAPACAT trial.

The mechanistic rationale for rapamycin in HCM originates from rodent models, in which mTOR pathway inhibition prevents and reverses cardiac hypertrophy, and from human transplant medicine, where sirolimus-based immunosuppression was associated with reduced cardiac wall thickness in renal allograft recipients — suggesting a transferable effect in mammalian myocardium.

RAPACAT trial: randomized evidence in client-owned cats

Kaplan et al. (2023, J Am Vet Med Assoc (https://pubmed.ncbi.nlm.nih.gov/37495229/)) conducted the RAPACAT trial, a double-blinded, multicenter, NIH-funded, randomized, placebo-controlled clinical trial in 43 client-owned cats with subclinical, non-obstructive HCM. Cats were allocated to low-dose DR rapamycin, high-dose DR rapamycin, or placebo and followed with echocardiography, hematology, biochemistry, urinalysis, N-terminal pro-B-type natriuretic peptide (NT-proBNP), and cardiac troponin I at baseline and Days 60, 120, and 180.

The primary outcome variable was maximum LV myocardial wall thickness at any location. At Day 180, this was significantly lower in the low-dose DR rapamycin group compared to placebo (P = 0.01). The high-dose group did not show a statistically significant effect, a finding the authors noted warrants further investigation. Crucially, baseline echocardiographic and clinicopathologic values were not significantly different between groups, confirming successful randomization. Adverse event profiles did not differ significantly between groups, supporting the tolerability of the drug at the doses evaluated.

The clinical relevance statement from the trial: delayed-release rapamycin may prevent or delay progressive LV hypertrophy in cats with subclinical HCM, with the low-dose regimen showing the more consistent signal.

Pilot study: mechanistic underpinnings in hereditary HCM colony

Rivas et al. (2023, Animals (https://pubmed.ncbi.nlm.nih.gov/37893908/)) used a feline research colony with naturally occurring hereditary HCM (n = 9) to investigate the tissue-, urine-, and plasma-level mechanistic effects of once-weekly delayed-release oral rapamycin at low and high doses for 8 weeks. Transcriptomic and proteomic analyses provided mechanistic depth not captured by echocardiography alone.

Key findings: transcriptomic differences between low- and high-dose groups supported dose-responsive suppressive effects on the myocardial hypertrophy gene program and stimulatory effects on autophagy — consistent with the known biology of mTORC1 inhibition. The myocardial proteome showed differences consistent with potential anti-coagulant and anti-thrombotic effects, cellular remodeling, and metabolic changes. The authors noted these findings closely recapitulate what is seen in the human rapamycin literature, reinforcing the translational rationale.

Importantly, rapamycin was safe and well tolerated in this colony at both doses across the 8-week study period. The pilot study's nine-cat design limits statistical power but provides hypothesis-generating mechanistic context for the clinical trial results.

What the RAPACAT results do and do not establish

The RAPACAT trial provides the first randomized clinical evidence that a drug can reduce the progression of LV wall thickening in cats with subclinical HCM. Several important caveats attend the current evidence: the study enrolled only 43 cats, the high-dose arm did not achieve statistical significance (raising questions about the dose-response relationship), the study used a proprietary delayed-release formulation, and follow-up was 6 months — long enough to show echocardiographic change but too short to address outcomes such as congestive heart failure, atrial fibrillation, or survival. Confirmation trials with larger samples and longer follow-up are required.

Ongoing safety and monitoring considerations

The mTOR pathway regulates immune function, wound healing, and cell proliferation. In the RAPACAT context, no significant adverse events emerged; however, the immunosuppressive potential of rapamycin means that monitoring for infectious complications, hematologic changes, and metabolic effects (the drug affects lipid and glucose metabolism at higher exposures) would be prudent in any clinical use. Cats with concurrent systemic disease, active infection, or planned surgery represent higher-risk candidates.

How this fits clinical practice

At present, rapamycin for feline HCM is best described as an evidence-supported investigational option for subclinical, non-obstructive disease. The RAPACAT trial result is a meaningful scientific advance, but the study's size and duration, the specific formulation used (delayed-release), and the absence of outcome data mean that clinicians considering off-label use should do so within the context of informed owner consent, baseline and periodic echocardiographic and clinicopathologic monitoring, and ideally enrollment in or proximity to ongoing research.

For cats with overt clinical HCM (symptomatic CHF, dynamic LVOTO, atrial thrombus), standard-of-care management remains the appropriate first approach; rapamycin's benefit in symptomatic disease has not been tested.

The translational signal from human transplant medicine, the rodent model data, and the now-consistent results across the RAPACAT trial and the pilot colony study together constitute a body of evidence sufficient to inform discussion with owners of cats with subclinical HCM who ask about emerging options.

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References

1. 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. https://pubmed.ncbi.nlm.nih.gov/37495229/
2. Rivas VN, Kaplan JL, Kennedy SA, et al. Multi-Omic, Histopathologic, and Clinicopathologic Effects of Once-Weekly Oral Rapamycin in a Naturally Occurring Feline Model of Hypertrophic Cardiomyopathy: A Pilot Study. Animals (Basel). 2023;13(20):3184. https://pubmed.ncbi.nlm.nih.gov/37893908/

Changelog

• 2026-06-08: First published.

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