Measurement of free drug and clinical end-point by high-performance liquid chromatography–mass spectrometry: Applications and implications for pharmacokinetic and pharmacodynamic studies Review Article

Free drug measurement and pharmacodymanic markers provide the opportunity for a better understanding of drug efficacy and toxicity. High-performance liquid chromatography (HPLC)–mass spectrometry (MS) is a powerful analytical technique that could facilitate the measurement of free drug and these markers. Currently, there are very few published methods for the determination of free drug concentrations by HPLC–MS. The development of atmospheric pressure ionisation sources, together with on-line microdialysis or on-line equilibrium dialysis and column switching techniques have reduced sample run times and increased assay efficiency. The availability of such methods will aid in drug development and the clinical use of certain drugs, including anti-convulsants, anti-arrhythmics, immunosuppressants, local anaesthetics, anti-fungals and protease inhibitors. The history of free drug measurement and an overview of the current HPLC–MS applications for these drugs are discussed. Immunosuppressant drugs are used as an example for the application of HPLC–MS in the measurement of drug pharmacodynamics. Potential biomarkers of immunosuppression that could be measured by HPLC–MS include purine nucleoside/nucleotides, drug–protein complexes and phosphorylated peptides. At the proteomic level, two-dimensional gel electrophoresis combined with matrix-assisted laser desorption/ionisation time-of-flight (TOF) MS is a powerful tool for identifying proteins involved in the response to inflammatory mediators.