At a glance

Also known as

Rapamune (brand name in Australia), rapamycin

Why get tested?

To determine the concentration of sirolimus in the blood in order to establish a dosing regimen, maintain therapeutic levels, and detect toxic levels

When to get tested?

Early in the course of sirolimus therapy and whenever the dose is changed; frequently at first, then at regular intervals to monitor concentrations over time; whenever excess or deficient levels are suspected

Sample required?

A blood sample drawn from a vein in your arm

What is being tested?

This test measures the amount of sirolimus in the blood. Sirolimus is a relatively new immunosuppressive drug that is given orally to patients who have had an organ transplant. Sirolimus is a compound produced by the bacterium Streptomyces hygroscopicus. It was originally discovered in a soil sample from Rapa Nui (Easter Island), about 3,200 kilometres east of Chile in the South Pacific. When ingested, sirolimus is absorbed from the gastrointestinal tract. Concentrations peak in the blood within a couple of hours and then gradually decline. Only a small percentage of the dose taken is bioavailable - 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 metabolites and is excreted from the body in the stool (more than 90%) and urine (less than 2%).

Sirolimus is currently under the Commonwealth/State Highly Specialised Drugs Program and approved for use in kidney transplant rejection. It appears to be less toxic to the kidneys than some other options and can be given in conjunction with the other immunosuppressive drugs, cyclosporin and tacrolimus. There are insufficient data regarding its use in children and the elderly, and there are cautions against its use in pregnant and breastfeeding women. 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.

Normally, a person's immune system recognises a transplanted kidney as foreign and begins to attack it. Sirolimus limits this response and helps to prevent organ rejection by inhibiting T-lymphocyte activation and proliferation and antibody production.

Sirolimus levels in the blood must be maintained within a narrow therapeutic 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.

Typically, sirolimus is given with cyclosporin and corticosteroids. Dosages must be tailored to the individual, and sirolimus and cyclosporin must be monitored. Often, patients will begin with a loading dose of sirolimus which is followed by lower doses, which are adjusted based on the results of the sirolimus blood tests. In patients considered at low risk for complications, cyclosporin may be weaned after 2-4 months and sirolimus concentrations increased.

Sirolimus is usually taken once a day at set intervals, consistently with or without food. If cyclosporin is also being taken, then the dosages of both drugs should be separated by several hours. The sirolimus blood test is usually measured as a 'trough' level, with blood collection just prior to the next dose - at the drug's lowest concentration in the blood.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

 

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

The sirolimus test is ordered to measure the amount of drug in the blood to determine whether concentrations have reached therapeutic levels and are below toxic levels. If someone is taking both sirolimus and cyclosporin (or tacrolimus), then both drug concentrations will be measured. Other tests may also be periodically ordered to evaluate organ function and the body's blood cell counts because sirolimus may decrease a person's platelet count, increase their blood lipids, and, when taken long-term with other immunosuppressants, affect kidney function.

When is it requested?

Sirolimus is ordered frequently at the start of therapy. Because of its long half-life, sirolimus concentrations may take some time to reach equilibrium in the blood. Therefore patients should not be tested for a week or two after a dosage change. Once dosages have been established and shown to be well tolerated, then the frequency of sirolimus testing may decrease. The test is performed whenever there is a change in dose, a change in other medications or whenever patients have symptoms that suggest side effects, toxicity, or organ rejection. Monitoring is necessary as long as the patient is taking sirolimus.

What does the test result mean?

Concentrations that are higher than the established therapeutic range may cause symptoms associated with toxicity. Concentrations that are too low may lead to rejection of the transplanted organ.

If the person is also taking other immunosuppressants or medications, then patients should work closely with their doctor and transplant team to ensure that there is no potential interaction between the medications.

Is there anything else I should know?

Drugs that inhibit or accelerate the metabolism of sirolimus may affect blood levels. Drugs that can increase sirolimus blood levels include antifungal medications, clarithromycin, erythromycin, isoniazid, protease inhibitors, and quinidine. Those that can decrease sirolimus levels include carbamazepine, phenobarbitol, phenytoin, rifampin and nafcillin. Grapefruit juice and herbal supplements such as St. John's Wort must be avoided while taking sirolimus.

Patients should not alter their dose or the time that they take their dose without consulting their doctor. Sirolimus should be taken consistently with respect to meals. Women should not become pregnant while taking sirolimus. Those considering pregnancy should talk to their doctor about associated risks.

Patients taking sirolimus should also talk to their doctor before getting any vaccines.

Patients may have side effects and experience complications such as infections and slow healing even when taking sirolimus properly. Side effects may include:

 

Common Questions

How long will I need to be on sirolimus?

Transplant patients must take sirolimus or other immunosuppressants throughout their life. Talk to your doctor about the best choice for your condition. This choice may change over time.

Who orders sirolimus tests?

Sirolimus will usually be monitored by the doctor and transplant team that performed your surgery or by a doctor who has specific knowledge of transplantation and your condition.

Where are sirolimus tests performed?

Sirolimus tests may be performed in a local hospital laboratory but may also be sent to a reference laboratory. Because different laboratories use different methods, each laboratory will have different reference ranges. Your doctor will usually send your tests consistently to one laboratory for testing so that your levels can be followed over time by the same testing method.

Should I tell my other health care providers that I am taking sirolimus?

Yes, this is important information for them, partially because of drug interactions and partially because of the immunosuppressive action of sirolimus. Sirolimus can affect your ability to heal, to get vaccinations, and may increase the risk of developing certain cancers (such as skin cancers and lymphoma).

Last Review Date: September 27, 2015