Quantification of Fosfomycin in Plasma by LC-MS/MS: Development, Validation and Application to Pharmacological Monitoring

Fosfomycin is an antibiotic of growing interest for the treatment of infections caused by multidrug-resistant bacteria. Its marked pharmacokinetic variability, especially in critically ill patients, makes therapeutic drug monitoring (TDM) useful to ensure adequate exposure and to limit the risk of treatment failure, resistance development, and toxicity. Given the high polarity of fosfomycin, a hydrophilic interaction liquid chromatography– tandem mass spectrometry (HILIC-MS/MS) method for its quantification in human plasma was developed and validated, using a Phenomenex Luna Polar Pesticides column, a Shimadzu LCMS-8050 mass spectrometer operated in negative electrospray ionisation mode and fosfomycin-d5 as internal standard. Sample preparation relied on a single-step protein precipitation of 40 µL of plasma, with a retention time of 2.8 min and a total run time of seven min. Validation according to the International Council for Harmonisation (ICH) M10 guideline demonstrated linearity over 1.56–200 mg/L (lower limit of quantification of 1.56 mg/L; limit of detection 0.19 mg/L), intra- and inter-day precision and accuracy within acceptance criteria acceptable recovery and matrix effect in six independent plasma lots, and no relevant endogenous interference. The method was applied to 44 samples from six patients receiving intravenous fosfomycin, and incurred sample reanalysis met ICH M10 acceptance criteria. This method provides a sensitive, selective, and robust tool for fosfomycin TDM pharmacokinetic studies.