One year after the first feline HCM drug: felycin-CA1 and just 39 sirolimus reports
On 2025-03-14, the U.S. FDA granted Conditional Approval to felycin-CA1 (sirolimus delayed-release tablets) — the first product approved for use in cats with hypertrophic cardiomyopathy (HCM) for any indication. A year on, the safety signal from the marketplace is striking not for what it shows, but for how small it is: as of the most recent adverse-event pull, sirolimus carries just 39 total reports, and 38 of them are feline. This article walks through the approval, the trial that supported it, and why a low report count is its own headline.
A first of its kind
felycin-CA1 is built on sirolimus, the same molecule (also known as rapamycin) first approved for humans in 1999 as an organ-transplant immunosuppressant (Rapamune). For cats, the veterinary use was conditionally approved on 2025-03-14 under NADA 141-604, sponsored by TriviumVet (Waterford, Ireland), with the underlying research led by Dr. Joshua Stern at NC State College of Veterinary Medicine.
The indication is narrow but meaningful: management of ventricular hypertrophy in subclinical HCM — cats with a left ventricular (LV) wall thickness of ≥6 mm at end diastole, confirmed by echocardiography, but without clinical signs of congestive heart failure. Sirolimus is a macrocyclic lactone produced by Streptomyces hygroscopicus; in the heart it works by inhibiting the mTOR pathway, which blocks pathological cardiomyocyte hypertrophy. Notably for a drug in the immunosuppressant family, the feline dose is reported not to produce immunosuppression (a finding supported by a rabies-vaccine-response study).
Why "Conditional" — and why it can renew
Conditional approval is the statutory route for an animal drug that "addresses a serious or life-threatening disease, or addresses an unmet animal or human health need, for which demonstrating effectiveness would require a complex or particularly difficult study." Subclinical HCM progressing toward clinical HCM (congestive heart failure, death, or aortic thromboembolism) fits that definition.
The practical upshot: felycin-CA1's Conditional Approval is valid 1 year, renewable up to 4 times (annually). The sponsor must keep gathering the effectiveness evidence that a full approval would otherwise require. One administrative detail worth noting — because it is not a designated new animal drug, the product carries no exclusive marketing rights (§573(c) does not apply). It is Rx only, requiring a licensed veterinarian's prescription. Commercial strengths are 0.4 mg / 1.2 mg / 2.4 mg tablets, dosed at 0.3 mg/kg orally once weekly, and the tablet must be swallowed whole — not split or mixed into food.
What the clinical data showed
The RAPACAT trial was a well-controlled pilot field study: 43 cats enrolled, of which 36 were evaluable at Day 180, comparing felycin-CA1 at 0.3 mg/kg and 0.6 mg/kg against placebo. The primary endpoint was left ventricular maximal wall thickness (MWT):
| Group | Mean change vs baseline | n |
|---|---|---|
| felycin-CA1 0.3 mg/kg (1X) | ↓0.17 mm | 14 |
| felycin-CA1 0.6 mg/kg (2X) | ↑0.50 mm | 10 |
| Placebo | ↑0.94 mm | 12 |
The difference between the felycin-CA1 group and the control group reached statistical significance at both Day 60 and Day 180, analyzed with a two-sided alpha = 0.10 (the FOI Summary reports alpha = 0.10; exact P values are not disclosed). A separate 43 client-owned-cat extension study (n=15 low-dose, n=15 high-dose, n=13 placebo) saw the low-dose group's MWT decrease at Day 180, while in the high-dose group 3 of 15 cats progressed to congestive heart failure during the study.
Before prescribing, the label requires echocardiographic confirmation of HCM (within 9 months), a blood-pressure examination, and liver-function tests (before medication, after 1 month, then every 6–12 months). Contraindications include diabetic cats, severe heart failure, cats with liver disease, and combination with other immunosuppressants.
The quiet signal: 39 reports, 38 of them feline
Here is the number that frames this whole story. Pulling the FDA Center for Veterinary Medicine's adverse-event database (openFDA animalandveterinary/event.json) on 2026-07-22, sirolimus shows:
- Total reports: 39
- Cat: 38
- Dog: 1
For a drug whose only approved veterinary use is in cats, the species split is almost a tautology — and that is the point. One year after launch, the entire reported experience for the first feline HCM therapy is 39 reports, and 38/39 are feline. A single canine report is the lone outlier.
A required interpretive disclaimer
The adverse-event counts above are cumulative as of the openFDA pull on 2026-07-22 and reflect everything reported to FDA CVM through that snapshot. They are report-level: one report may list multiple reactions and multiple animals, so the figures are not additive and do not imply causation. Reported ≠ causal — a report in this database is a signal for follow-up, not proof that the drug caused the event. Reaction terms are coded using VEDDRA veterinary terminology; methodologically, lack-of-efficacy entries are not in themselves adverse reactions, so this count measures reported events, not treatment failures.
What the reaction terms show
The top reported reactions (VEDDRA terms) among the 39 sirolimus reports are:
- Vomiting — 12
- Weight loss — 9
- Congestive heart failure — 6
- Elevated alanine aminotransferase (ALT) — 6
- Lethargy — 6
- Abnormal radiograph finding — 5
- Abnormal ultrasound finding — 5
- Administration error NOS — 5
- Decreased appetite — 5
- Hypertension — 5
Several of these are notable precisely because the population is cats with heart disease: "congestive heart failure" and "elevated ALT" appear as reported events, while "administration error NOS" and the abnormal imaging findings point to the ordinary messiness of real-world dosing and monitoring rather than to a clean pharmacological signal. None of this establishes causation — it is the raw material for surveillance, not a verdict.
Where felycin-CA1 sits in the 2024–2026 pet-drug landscape
The FDA Green Book's recent-animal-drug-approvals pet segment spans 2024-11-06 to 2026-06-29, with 55 pet-related rows out of 93 total. Within that window, felycin-CA1 (NADA 141-604, 2025-03-14) is called out as the first novel animal drug for subclinical feline HCM — the "CA1" suffix itself marks its conditionally approved status. Its sponsor, TriviumVet, is profiled in that slice as a specialist in rare-disease novel animal drugs, pairing felycin-CA1 (sirolimus, feline HCM) with LIAVIUM-CA1 (pregabalin, canine Chiari-like malformation). In a cohort dominated by ectoparasite, anti-inflammatory, and generic entries, a conditionally approved therapy for a previously unaddressed feline cardiac condition stands apart.
Why a low count is the story
A year in, the headline for felycin-CA1 is not a safety scare — it is the absence of one. 39 reports, 38 feline, is a thin trail for the first drug ever approved for feline HCM, and thinness here is reassuring rather than suspicious: it means the post-market experience so far is small, watchable, and unremarkable at the aggregate level. The conditional-approval pathway — 1 year, renewable up to 4 times — exists precisely so that a drug for an unmet need can reach cats now while the effectiveness evidence keeps building. The low report volume is the first real-world vote of confidence in that pathway working as intended.
That said, "quiet" is not "cleared." The counts are a snapshot as of 2026-07-22, they are reported rather than confirmed, and a population of cardiac cats will always generate congestive-heart-failure reports that must be disentangled from the drug's effect. The right read is steady watching, not celebration — and the 39-report baseline is exactly the kind of honest, low-noise starting point that makes watching possible.
Sources
- fda_nada_141_604_felycin_ca1_sirolimus_cat_hcm — FDA NADA 141-604 felycin-CA1 (Sirolimus Delayed-Release Tablets): Conditional Approval 2025-03-14, indication (LV wall thickness ≥6 mm at end diastole), RAPACAT trial data (43 enrolled / 36 evaluable; MWT ↓0.17 mm at 0.3 mg/kg vs ↑0.94 mm placebo; two-sided alpha = 0.10 at Day 60 & Day 180), 43 client-owned-cat extension (3/15 high-dose progressed to CHF), strengths 0.4/1.2/2.4 mg, dosing 0.3 mg/kg weekly, renewable up to 4 times, no exclusive marketing rights. Caliber: official FDA FOI Summary PDF + FDA CVM Update announcement (primary sources), substring-verified.
- ade_sirolimus_adverse_events — FDA ADE reports for sirolimus: total 39 (Cat 38, Dog 1); top VEDDRA reactions (Vomiting 12, Weight loss 9, Congestive heart failure 6, Elevated ALT 6, Lethargy 6, Abnormal radiograph 5, Abnormal ultrasound 5, Administration error NOS 5, Decreased appetite 5, Hypertension 5). Caliber: openFDA
animalandveterinary/event.json, pulled 2026-07-22, report-level counts, substring-verified. - fda_green_book_recent_approvals_pet_drug_2024_2026 — FDA Green Book Recent Animal Drug Approvals, pet segment 2024-11-06 to 2026-06-29 (55 pet rows of 93); felycin-CA1 (NADA 141-604, 2025-03-14) listed as first novel animal drug for subclinical feline HCM; TriviumVet profiled as rare-disease novel-animal-drug specialist. Caliber: official FDA Green Book recent-approvals page extract, substring-verified.