Liquid chromatography-high resolution mass spectrometry applied to therapeutic drug monitoring using DBS sampling

Introduction: Quantitative dried blood spot (DBS) analysis was used to investigate if the accurate mass capability (±2ppm m/z) of high resolution mass spectrometry (HRMS) would provide compound specificity comparable to the multiple reaction monitoring (mrm) approach used with a triple quadrupole mass spectrometer. Some initial clinical data from DBS for selected drugs is reported. Methods: Known concentrations of atenolol, caffeine, captopril and dexamethasone in whole blood were prepared as DBS samples. The target drugs were solvent extracted from punched DBS disks and analysed in positive electrospray ionisation mode to prepare calibration graphs based on TOF detection. Individual LC conditions were developed and applied to dried blood spots obtained from neonate patient samples to obtain pharmacokinetic data and from adults to assess adherence to therapy. Results: The extraction efficiency for the recovery of all target drugs from spiked dried blood spots was >90% and the drugs were stable in DBS for at least 12 weeks at room temperature. Use of the stabiliser 1,4 dithiothreitol (DTT) in the paper and extraction solvent was essential for captopril analyses. Validation using spiked blood calibration standards showed good accuracy, precision and linearity.

MH+ m/z LoQ (S/N >10) Injection volume
Atenolol 276.1703 50.0ng/ml 5l Caffeine 195.0883 100.0ng/ml 5l Captopril 218.0845 20.0ng/ml 10l Dexamethasone 393.2034 15.0ng/ml 20l

Conclusion: Comparison of MS/MS and HRMS data demonstrated good selectivity for the two approaches. It was also possible to re-interrogate the TOF recorded data for other ions without the need to re-run the sample.