Determination of venlafaxine and O-desmethylvenlafaxine in dried blood spots for TDM purposes, using LC-MS/MS.

Dried blood spot (DBS) sampling and quantitative analyses of many current therapeutic drug monitoring (TDM)-guided drugs are advantageous because of the minimal invasive sampling strategy. Here, a fast and robust LC-MS/MS method was developed and analytically validated for simultaneous determination of venlafaxine (VEN) and O-desmethylvenlafaxine (ODV) in DBS. Six-millimeter circles were punched out from DBS collected on Whatman DMPK-C paper, and the DBS was extracted with acetonitrile/methanol at 1:3. The total run time was 4.8 min. The assay was linear in the range of 20-1,000 µg/L for both VEN and ODV. Assay accuracy and precision was well within limits of acceptance (LLOQ = 20 µg/L). Normal hematocrit concentrations (0.30-0.50) did not influence the results neither did a normal spot volume (40-80 µL). Punch position at the perimeter instead of the center of the blood spot gave a bias ranging from 2.4 to 10.4%. Correlation between plasma and spiked DBS samples was high. The concentrations found in spiked DBS samples were higher than those in plasma, indicating that a conversion factor for translation of DBS to plasma values is needed. This analytically validated method is suitable for determination of VEN and ODV in DBS and applicable for TDM. The method will be used for TDM of VEN in the Dutch CYSCE multicenter trial (NCT01778907).