Development and validation of an UPLC-PDA method to quantify daptomycin in human plasma and in dried plasma spots

Quantification of daptomycin in plasma samples may be useful in optimizing therapy especially in special patients' population. Nevertheless, therapeutic drug monitoring of daptomycin is still limited probably for the low number of laboratories which perform this analysis and for high shipment costs. We developed and validated a new UPLC-PDA method to quantify daptomycin in plasma and in dried plasma spots (DPS) collected on dried sample spots devices (DSSD). Daptomycin and quinoxaline, used as internal standard, were monitored at 262nm and 253nm, respectively. Daptomycin was extracted from plasma using acetonitrile and from DPS using an extraction solution (ethyl acetate-acetic acid-acetone-water; 50:20:20:10, v/v/v/v). Both assays were linear over the calibration range of 0.781 to 200μg/ml. Considering the method of extraction from plasma, mean intra and inter-day accuracy was -1.18% and -2.79%, respectively. Mean intra and inter-day precision was 7.91% and 9.22%, respectively. Regarding the extraction method from DPS, mean intra and inter-day accuracy was 2.21% and 2.41%, respectively. Mean intra and inter-day precision was 8.01% and 9.19%, respectively. Daptomycin in DPS was found to be stable for 7 days at room temperature (20-25°C) and for at least 30 days at 4°C. A statistically significant (p<0.001) linear correlation was found between daptomycin extracted from plasma and from DPS (r(2)=0.919). DPS represents a safe and cheap strategy to store and ship plasma samples. Thus, it is suited for pharmacokinetic studies and therapeutic drug monitoring of daptomycin in hospitals without a therapeutic drug monitoring laboratory.