Development and validation of a new UPLC-PDA method to quantify linezolid in plasma and in dried plasma spots.

Linezolid is an oxazolidinone antibiotic used for the treatment of pneumonia and uncomplicated and complicated skin and soft tissues infections caused by Gram positive bacteria. It is also used as second line agent in multi-drug resistant tuberculosis. Therapeutic drug monitoring (TDM) of linezolid represents a valid tool in clinical practice to optimize therapy, especially in critically ill patients. Spreading of TDM is mainly limited by high costs shipment and lack of laboratories that offer a TDM service. To overcome these problems, the use of dried plasma spots or dried blood spots is increasing. The aim of this work was to develop and validate a new chromatographic method to analyze linezolid in plasma and in dried plasma spots and to evaluate the correlation between the two extraction methods. Linezolid extraction from plasma and from dried plasma spots was obtained using acetonitrile. Quinoxaline was used as internal standard. Analysis was performed by an ultra performance liquid chromatography (UPLC) system coupled with photo diode array (PDA) detector, at 254nm. Both analytical methods were linear (r(2)>0.999) over the calibration range of 30-0.117mg/L. Limit of quantification and limit of detection were 0.117mg/L and 0.058mg/L, respectively. Intra and inter-day precision (R.S.D.%) and accuracy (%) were <15%. Long term stability of linezolid in dried plasma spots showed absence of degradation at room temperature (20-25°C) and at 4°C, for at least one month. Linear regression analysis confirmed that the two methods of extraction have good correlation. Thus they are suited for TDM of linezolid and for pharmacokinetic studies.