Advances in therapeutic drug monitoring with a focus on phenytoin analysis

With the growing demand for a therapeutic drug monitoring (TDM) in clinical laboratories the development of appropriate and improved analytical methods to monitor drug concentrations and optimize patient care has become imperative. This dissertation contributes in this regard by developing sensitive, specific, and reliable GC-MS and LC-MS/MS methods to measure drug concentrations and validating them according to the FDA Guidance for Industry. The methods are performed based on the example of the critical dose drug phenytoin (PHT), which has characteristics – such as a small therapeutic range, nonlinear pharmacokinetics, interactions with other drugs, inter-individual genetic differences in the metabolism, and long-term therapy – that render it a highly suitable model substance and a reliable standard for the development and application of analytical methods to measure concentrations also of other medications.
Both methods allow the measurement of low drug concentrations in different body compartments (i.e., blood, saliva, and samples from brain tissue microdialysis with relatively small sample volumes) and therefore could be used to measure drug concentrations in the compartment of interest for various other drugs. By systematically comparing the two methods, the dissertation concludes that although GC-MS is a sensitive and suitable method (e.g., for the measurement of volatile and thermally stable substitutes in a sample), the LC-MS/MS method offers significant advantages (in terms of its sensitivity, LOD/LOQ, needed sample volume, sample preparation, analysis time, and costs), which make it highly suitable for larger sample numbers as in pharmacokinetic/pharmacodynamic studies as well as for bedside and routine analyses. While these findings are based on the model substance PHT, other promising drugs for TDM are, for example, Midazolam, Ceftriaxon, Morphine, and Levatiracitam, which are often administered concomitantly in neurosurgery. In the future, the routinely used matrices (i.e., blood or urine) for TDM will be expanded with other sample sources like tissue biopsies, dried blood spots, and oral fluids. LC-MS/MS methods provide the advantage of drug quantification in such matrices which are easier to handle. Thus, the dissertation concludes that LC-MS/MS methods are the current gold standard for TDM.
Furthermore, in this dissertation also a theoretical estimation method to calculate free drug concentrations has been tested, which could be used in the absence of corresponding lab methods or data – not every hospital or institution is able to afford a LC-MS/MS or a GC-MS instrument. The dissertation tested the usefulness of the Sheiner-Tozer algorithm for the correct estimation of the free PHT concentrations in hypoalbuminemic patients and compared the estimated with the measured free PHT levels. There were no statistically significant differences between the two methods and the estimated values highly correlated with the measured free PHT values. The results support the usefulness of the Sheiner-Tozer formula to calculate free PHT concentrations from the total PHT and the serum albumin value of a patient. These findings are of interest for all drugs with a high albumin binding such as immunosuppressants, antibiotics, or anticancer drugs.