An Investigation into the Use of Dried Blood Spot Analysis in Pharmacokinetic Studies

The ethical and practical issues of obtaining a blood sample pose a significant challenge to performing pharmacokinetic studies in children, infants and neonates. Dried blood spot analysis, based on the collection of a micro blood sample has potential to overcome these difficulties. There are at present a limited number of reports on the utility of dried blood spot analysis in clinical pharmacokinetic studies. The studies described in this thesis were undertaken to investigate the accuracy and precision of dried blood spot sampling coupled with mass spectrometry detection for drug quantification, and clinically validate the robustness and feasibility of this technique for pharmacokinetic studies in preterm neonates. Dried blood spot methods were developed for application to pharmacokinetic studies of test drugs dexamethasone and caffeine. Investigations were focused on the blood collection system, analyte recovery and optimisation of the detection system. In-vitro validation results indicated developed methods were precise, accurate and selective in accordance with the Food and Drug Administration regulatory guidelines on the assessment of bioanalytical methods. Results were not significantly affected by small variations in the blood volume spotted or the presence of petroleum jelly, which is often used on the sampling site during capillary blood collection in neonates. Variability in haematocrit was determined to be the single most important factor affecting assay accuracy. Stability assessments by comparison with freshly prepared samples verified the suitability of sample drying, storage and post sample extraction conditions. An investigation of method transferability between different analytical instruments was undertaken with caffeine to provide an assessment of the robustness of dried blood spot analysis. Results generated from a single and triple quadrupole mass spectrometer were comparable with an expected lower limit of quantification with the latter technique most likely due to a greater ionisation and detection efficiency. Intravenous dexamethasone pharmacokinetics was determined in 5 preterm neonates receiving treatment for chronic lung disease. Individual pharmacokinetic analyses were performed using a one compartment model to estimate primary pharmacokinetic parameters, clearance (mean, 0.18 l/h/kg) and volume of distribution (mean, 1.33 l/kg). The whole blood derived mean estimates were similar to previous plasma clearance and volume estimates of 0.14 l/h/kg and 1.91 l/kg, respectively reported in neonates (n=7). This highlights the potential for dried blood spot analysis as an alternative to conventional plasma based methods for dexamethasone dose optimisation studies in neonates. The population pharmacokinetics of oral / intravenous caffeine was determined in 67 preterm neonates. A one compartment model was used to describe the blood concentration-time data. Model evaluation using a bootstrapping technique confirmed the robustness and stability of the developed model. Pharmacokinetic parameters derived from dried blood spot drug measurements were estimated with precision (relative standard error < 10%) and were comparable to estimates of plasma clearance (mean, 7.3 vs. 7.0 ml/h/kg) and volume of distribution (mean, 593 vs. 851 ml/kg) from a previous population study in neonates (n=110). Weight and postnatal age were the most influential covariates in the clearance model which is in agreement with previous population studies. These results demonstrate that dried blood spot analysis is a practical technique, with significant potential as a robust method for use in clinical pharmacokinetic studies in vulnerable populations such as preterms. Haematocrit related effects on paper will need to be accounted for if this potential is to be realised. Further investigations to determine the reproducibility of capillary blood sampling in neonates and the impact of using blood drug measurements on pharmacokinetic parameter estimation will be necessary before widespread use of the technique is possible