Rapamune
Meaning of Rapamune: Named for the island where the soil bacterium was dug up, and the enzyme it blocks was later named after the drug.
Definition of Rapamune
Rapamune is a brand name for sirolimus, a medicine that damps down the immune system and is used to stop the body rejecting a transplanted kidney. It joins a partner protein inside the cell, and that pair jams a master controller of cell growth known as mTOR. Immune cells that would otherwise multiply against a foreign organ stall instead.
How does it differ from the older transplant drugs?
| Group | Point of action | Effect on the kidney |
|---|---|---|
| Calcineurin inhibitors | Block the signal that produces interleukin 2 | Toxic to the kidney over years |
| Sirolimus | Blocks the cell's response to interleukin 2 | Not directly toxic to the kidney |
| Steroids | Damp immune gene activity broadly | Not directly toxic |
| Antimetabolites | Starve dividing cells of DNA building blocks | Not directly toxic |
That difference is the reason it is chosen when a transplanted kidney is being spared further harm. Its own drawbacks lie elsewhere: wounds heal slowly, cholesterol and triglycerides climb, mouth ulcers are common, protein can leak into the urine, and an uncommon inflammation of the lungs is recognized. Healing problems are also why it is kept away from the earliest weeks after certain transplants, where a fresh join in an artery or an airway has to knit.
Why is it called an antibiotic?
Only by chemistry and origin. It was extracted from a soil bacterium collected on Rapa Nui, the island also called Easter Island, and it belongs to the macrolide group that includes several genuine antibiotic drugs. It was first noticed for activity against fungi. It is never used to treat an infection, and by suppressing immunity it raises the risk of one. Its target, mTOR, is a kinase enzyme, though the drug does not plug the active site the way most kinase blockers do; it binds a small partner protein first and the resulting pair clamps the enzyme from the side.
What else is it used for?
The same growth control it interrupts is overactive in several unrelated conditions, which explains a scattered list of uses. It treats a rare lung disease of women in which abnormal muscle-like cells spread through the lungs, and it is used in some vascular malformations and in prevention of graft-versus-host disease after bone marrow transplantation. A version of the molecule coats coronary stents, releasing slowly to stop scar tissue narrowing the vessel again. The compound was originally called rapamycin, and the enzyme it acts on was named the target of rapamycin in its honor.
Also known as
Used in a sentence
Rapamune was substituted for the calcineurin inhibitor to protect kidney function.
Good to know
Sirolimus is a macrolide by structure but is not used to treat infection, and it blocks mTOR indirectly, binding a partner protein first rather than occupying the enzyme's active site as most kinase inhibitors do.