Sirolimus (Rapamune), Blood

Sirolimus is often referred to by the brand name Rapamune.

Sirolimus is an immunosuppressant drug used in the prophylaxis of organ rejection in patients receiving transplants.

Sirolimus can be used in combination with ciclosporin, tacrolimus or mycophenolate.

Sirolimus levels are measured in order to establish the correct dose, maintain therapeutic levels and ensure that toxic levels are avoided.

This test measures the amount of sirolimus in the blood. Sirolimus is a newer immunosuppressive drug that is given to patients who have had an organ transplant. Normally, the immune system recognizes a transplanted organ as 'foreign' and begins to attack it, leading to rejection. Sirolimus limits this response and helps prevent organ rejection by inhibiting immune cell activation and prliferation, and antibody production.

Siroliums is typically taken by mouth (orally) and, after ingestion, is absorbed from the digestive tract. The concentration peaks in the blood within a couple of hours and then gradually declines. Only a small percentage of the dose taken is present in a form that the body can use. Sirolimus has a half-life of about 60 hours. It is metabolised by the liver into several new compounds, and is removed from the body in the stool (more than 90%) and urine (less than 2%).

Sirolimus is currently approved for use in adults who have kidney transplants. It appears to be less toxic to the kidneys than other drugs and can be given in conjunction with the other immunosuppressive drugs ciclosporin and tacrolimus. Studies to evaluate its use in other age groups and types of organ transplantation, such as lung, are in progress. Like other immunosuppressive agents, sirolimus may cause side effects and adverse reactions, and is associated with an increased risk of infection and the development of lymphoma.

Sirolimus levels in the blood must be maintained within a narrow concentration range. If the concentration is too low, organ rejection may occur; if it is too high, then the patient may have symptoms associated with toxicity. The sirolimus blood level is usually measured as 'trough' level, with blood collection just prior to the next dose - when the drug concentration in the blood is at its lowest.  

Typically, sirolimus is initially given with ciclosporin and corticosteroids (siroliumus given 4 hours after ciclosporin). Dosages must be tailored to the individual, and sirolimus and ciclosporin must both be monitored. Often, patients will begin with a high dose of sirolimus at the beginning of treatment which is then tapered down to a lower dose. Ciclosporin is normally withdrawn after 2-3 months.

Sirolimus is usually taken once a day at set intervals. Food may affect absorption, and the drug should be taken at the same time each day with respect to food.