EIDD-1931 for FIP: the active metabolite behind molnupiravir

Short answer: EIDD-1931 (β-D-N4-hydroxycytidine, "NHC") is the active antiviral metabolite of molnupiravir / EIDD-2801. When a cat takes molnupiravir, the body converts it into EIDD-1931, which is what actually attacks the FIP virus. In practice you dose the prodrug (molnupiravir); EIDD-1931 is the reason it works.

What it is and how it works

EIDD-1931 is a ribonucleoside analog. The FIP virus mistakes it for a normal building block and incorporates it while copying its RNA. But EIDD-1931 is a shape-shifter — once inside the viral genome it pairs incorrectly, forcing the virus to write error after error into its own code. After enough copying cycles the viral population collapses under its own mutations. Virologists call this lethal mutagenesis (or "error catastrophe"), and it's a fundamentally different attack from the replication-stalling of GS-441524.

Why you give molnupiravir, not EIDD-1931 directly

EIDD-1931 on its own is poorly suited to being swallowed and absorbed reliably. Molnupiravir (EIDD-2801) is the prodrug engineered around that problem: it's absorbed well by mouth, then converted in the body into EIDD-1931. So the drug you actually administer for FIP is molnupiravir — EIDD-1931 is the active form it becomes. This is the same prodrug/active relationship as remdesivir → GS-441524.

That's why the FIP evidence, dosing, and cautions all live on the molnupiravir page — including the head-to-head data showing it performs comparably to GS-441524, and the note that its supply, like the rest of the FIP market, is largely unregulated.

Not veterinary advice. FIP treatment is a decision for a veterinarian who has examined your cat.

References

1. GS-441524 and molnupiravir are similarly effective for the treatment of cats with FIP. Frontiers in Veterinary Science, 2024. Read on Frontiers
2. Cook S.E. et al. Antiviral activity of nucleoside analogs including EIDD-1931 / molnupiravir against feline coronavirus. Find on PubMed