What is being tested?
Sirolimus is an immunosuppressive drug that is given orally to patients who have had kidney transplants. 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.
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 drugs and can be given in conjunction with the other immunosuppressants, cyclosporin and tacrolimus. There is 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.
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 high dose of sirolimus which is tapered down to 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.
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 (sufficient to prevent rejection) and are below toxic levels. Monitoring is necessary because the range in which the drug is effective and not toxic is very narrow. 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 the platelet count, increase 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 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.
To help reduce the chances of developing skin cancer it is important to protect the skin from the effects of ultra-violet light while taking sirolimus. Patients should try reducing the time skin is exposed to sunlight or ultraviolet light by using a high protection sun screen and by wearing appropriate clothing.
Sirolimus should not be used with certain drugs that weaken the immune system and increase risk of infection. These include voriconazole, natalizumab, and rituximab.
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:
Some of the more dangerous signs and symptoms of sirolimus toxicity that warrant an immediate call to the health practitioner include:
Common questions
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.
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.
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.
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).
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