Managing the Body's Response to Organ Transplants

Organ transplants save the lives of nearly 20,000 people every year—and that's just in the U.S. While that number may make transplants seem commonplace and routine, the outcome is never a sure thing. That's because most organs come from a donor who is genetically different from the recipient and the body's natural reaction is to attack and reject a transplanted kidney, liver, heart or other organ.

For decades, transplant surgeons have been battling host rejection with drugs that suppress the immune system. This approach has enabled 90% to 95% of transplant recipients to retain their new organ for at least a year. The rejection rate, however, rises sharply with time. Even with continued use of immunosuppressant drugs—patients must take them for life—by 10 years after the transplant, the immune systems of roughly 50% of patients have rejected the organ.

Managing the body's natural rejection response can increase the chances of a successful outcome, but suppressing the immune system requires a delicate balance. The patient's immune system needs to retain some ability to fight viruses and infections while being restrained from attacking the new organ. Finding the right dosage is crucial and this is usually done by carefully monitoring the blood concentrations of immunosuppressant drugs in the recipient's body.

Monitoring at lower levels

The type of organ transplanted affects the choice of medication, and the U.S. Food and Drug Administration generally limits the use of immunosuppressant drugs to specific organs. For example, the drugs sirolimus and tacrolimus are approved for use only in kidney transplant patients. This combination of drugs can be used at lower dosages than previous-generation immunosuppressants but has a narrow effective range and a potential for toxicity, which makes monitoring essential.

Monitoring of sirolimus and tacrolimus is typically done with biological or immunochemical assays. However, these methods may not have sufficient precision, accuracy or sensitivity to measure combined immunosuppressant therapies at lower doses. Successful monitoring of these therapies requires accurate analysis at the low detection limits of <1 µg/L.

Recently, researchers at the Clinical Laboratories of the University of Wisconsin Hospital and Clinics developed an improved and more-sensitive method for the monitoring of sirolimus and tacrolimus. Their work is described in an Agilent application note entitled "Simultaneous Measurement of Sirolimus and Tacrolimus Concentrations in Blood by Semi-Automated Extraction and Liquid Chromatography-Electrospray Ionization Mass Spectrometry." This method relies on the automation of extraction and analysis to increase sample throughput and the productivity of multiple analysts.

Assessing the improved method

The process begins with the isolation of sirolimus and tacrolimus from whole-blood specimens using a solid-phase sorbent. Next, the compounds are extracted and then analyzed by liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS). Both drugs are measured using the same extraction and LC/MS conditions.

With this approach, sirolimus and tacrolimus can be measured simultaneously, with improved speed and sensitivity. For both drugs, response is linear over the range 0.5 to 120 µg/L and between-day CVs are about 16% at 1.5 µg/L and 3% at 48 µg/L.

Agilent 1100 series LC/MS system

The method is also cost effective. Rather than the more costly LC/MS triple quadrupole, single-analyzer LC/MS instrumentation can be used to implement this method. This provides the additional advantage of minimal maintenance due to the cleanliness of the sample extracts produced by the LC/MS. What's more, low reagent costs further reduce the expense of monitoring.

After six months of use, the Clinical Laboratory team concluded that this method proved its utility for daily therapeutic monitoring of sirolimus and tacrolimus.

For more information

The outcome of organ transplants may never be a sure thing, but improved, sensitive monitoring methods can help enhance the quality of life for transplant recipients. To learn more about Agilent's LC/MS solutions, please see the LC and LC/MS Products section of our Web site. For additional information about Agilent's full range of products and resources, please go to the Life Sciences/Chemical Analysis main page.