Therapeutic drug monitoring of immunosuppressant drugs by high-performance liquid chromatography-mass spectrometry.

The currently expanding range of immunosuppressant agents has placed new challenges on therapeutic drug-monitoring (TDM) services. Many of these drugs require the measurement of concentrations with subsequent dosage adjustment to maximize efficacy while minimizing toxicity. HPLC-mass spectrometry (HPLC-MS) is a relatively new technique for drug quantification and thus TDM of immunosuppressant drugs. Although mass spectrometry relies on producing, differentiating, and detecting ions in the gas phase, the development of the atmospheric pressure ionization interface (electrospray and chemical ionization) has enabled the direct coupling of solution introduction of compounds, via HPLC, to the mass analyzer. The impetus for using HPLC-MS for immunosuppressant measurement has come from the highly potent low-dose immunosuppressant drugs tacrolimus and sirolimus, which have low nanogram per milliliter circulating concentrations. A number of strategies have been reported for sample preparation and ways to automate these processes with solid-phase extraction and 2-dimensional chromatography. The disadvantages of HPLC-MS are initial cost of equipment and availability of suitably skilled scientific staff. The advantages of HPLC-mass spectrometry are high sensitivity, specificity, small sample requirements, minimal sample preparation, rapid throughput, and simultaneous measurement. Further, scientists have the ability to develop methods to measure new immunosuppressant drugs by HPLC-MS before commercial assays become available. With potential applications increasing in immunosuppressive drug monitoring, it can be envisaged that HPLC-MS may become standard equipment in TDM laboratories of the future.