Quantification of ceftazidime/avibactam in human plasma and dried blood spots: Implications on stability and sample...

Ceftazidime is an established third-generation cephalosporin antibiotic frequently administered to intensive care patients. To overcome drug resistance of pathogens, it is combined with the newly developed non-ß-lactam ß-lactamase inhibitor avibactam under the brand name Zavicefta®. To facilitate therapeutic drug monitoring (TDM), we developed a method for the simultaneous quantification of these substances by LC-MS/MS. A problem for TDM is the low stability of the analytes in plasma, requiring transport times of less than 6 h at 23 °C. Thus, we evaluated dried blood spots (DBS) as matrix for better stability. For both analytes, stable isotope labelled internal standards were applied. Plasma samples were prepared by protein precipitation, DBS by liquid extraction. The chromatographic separation took place on a polar-modified C18 column, and detection was achieved by tandem mass spectrometry with ESI ionization in positive mode for ceftazidime and negative mode for avibactam. Calibration was linear in the ranges of 5 - 100 µg/mL for ceftazidime and 1.25 - 25 µg/mL for avibactam in plasma and 2.5 - 50 µg/mL and 0.625 - 12.5 µg/mL in DBS, respectively. Precision was better than 7 % and accuracy better than 10% for plasma as well as for DBS. The stability of ceftazidime and avibactam was better in DBS than in plasma or full blood, extending maximal transport times at 23 °C from 6 h in plasma or full blood to 24 h for DBS samples. However, robust estimation of plasma concentrations from DBS measurements requires validation by future clinical studies.