Quantitation of 10 antibiotics in plasma: Sulfosalicylic acid combined with 2D-LC-MS/MS is a robust assay for beta-lactam therapeutic drug monitoring.

Therapeutic drug monitoring (TDM) of antibiotics is particularly important in populations with high pharmacokinetic variabilities, such as critically ill patients, leading to unpredictable plasma concentrations and clinical outcomes. Here, we i) describe an original method for the simultaneous quantification of ten antibiotics (cefepime, ceftazidime, ampicillin, piperacillin/tazobactam, cefotaxime, amoxicillin, cloxacillin, oxacillin, linezolid) using 5-sulfosalicylic acid dihydrate (SSA) solution for protein precipitation together with 2D-LC-MS/MS, and ii) evaluate its impact in a one-year retrospective study. The method involved simple dilution with an aqueous mix of deuterated internal standards and plasma protein precipitation with SSA. Twenty microliters of the supernatant was injected into a C8 SPE online cartridge (30 × 2.1 mm) without any evaporation step and back-flushed onto a C18 UHPLC (100 × 2.1 mm) analytical column. Mass spectrometry detection (Xevo TQD) was performed in positive electrospray, in scheduled MRM mode. Overall analytical runtime was 7 min. Due to analytical constraints and the physicochemical properties of the antibiotics, protein precipitation using organic solvents could not be applied. As an alternative, SSA used with 2D-LC offered various advantages: i) lack of dilution resulting in better assay sensitivity, and ii) good chromatography of hydrophilic compounds. Ten microliters of 30% SSA in water eliminated>90% of plasma proteins, including the most abundant high molecular weight proteins at 55 and 72 kDa. The assay was successfully validated according to FDA and EMA guidelines for all the antibiotics, and the coefficients of variation of the quality control (QC) run during sample analysis over one year were below 10%, whatever the QC levels or the antibiotics. The use of 2D-LC combined with SSA precipitation allowed development of a robust, sensitive and rapid quantification assay. Feedback to clinicians was reduced to 24 h, thus allowing rapid dosage adjustment. During one year, 3,304 determinations were performed in our laboratory: 41% were not in the therapeutic range, 58% of which were sub-therapeutic, underlining the importance of early TDM of antibiotics to limit therapeutic failures and the emergence of bacterial resistance.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.