A new validated HPLC-UV method for therapeutic monitoring of daptomycin in comparison with reference LC-MS/MS

Daptomycin, a cyclic lipopeptide antibiotic with broad spectrum of activity against Grampositive bacteria is also active against multi-resistant bacterial strains, as well as methicillinresistant S. aureus A or penicillin-resistant S. pneumoniae [1,2]. For these reasons, it is a viable alternative for treatment of persisting infections. However, the therapeutic drug monitoring of daptomycin is recommended because the known variability in drug disposition and the severe clinical conditions of patients [3]. Therefore, we developed a simple and fast UV-HPLC method according to FDA guidelines to monitor plasma concentrations of the drug and compared with LC-MS/MS gold standard method. After a liquid-liquid extraction, plasma calibration samples, quality controls and patients’ samples were injected in a HPLC instrument daptomycin and gentamicin (internal standard) that were resolved by a C18 250 × 4.6 mm, 5 µm stationary phase and peaks were monitored at UV = 262 nm. Mobile phase (isocratic flow of 1 mL/min) consisted of acetonitrile-buffer (KH2 PO4 20 mM pH = 3.2) 46:54, vol/vol. Under these conditions, IS and daptomycin peaked at 4.1 and 5.8 min after injection. Values of limits of detection and quantitation accounted for 1.65 and 5.00 (µg/ml), respectively. Values of method linearity (r2) in range 5−100 mg/L were 0.9975 and 0.9956 plasma samples and solvent standard, respectively. Inter- and intra-day variability coefficients were lower than 15 %. Method was applied to 122 patient plasma samples (r2=0.9474) and the output obtained (chromatographic peaks) has been processed with an algorithm (patent protected®) which allows to resolve any interfering peaks with analytes and obtain concentrations not significantly different from LC–MS/MS. These interfering peaks were processed also with other methods (“split peak” and “valley-valley”) commercially available but results were significantly different from those obtained with LC-MS/MS. In conclusion, the present method is demonstrated to be reliable and suitable for daptomycin TDM in clinical routine.