Determination of free and encapsulated cytarabine and daunorubicin in rat plasma after intravenous administration of liposomal formulation using ultra-high performance liquid chromatography tandem mass spectrometry.

Liposome encapsulating cytarabine (CYT) and daunorubicin (DNR) is applied for treating Acute Myeloid Leukemia (AML) patients. To evaluate and compare relationship between the pharmacokinetics of free drug (drug which is not entrapped in liposomes) and liposome-encapsulated drug and the toxicity/efficacy, it is crucial to have trustworthy methods for separating the free and the encapsulated of the drug. In this study, methods were developed and validated to isolate and measure the free DNR/CYT (F-DNR/CYT), the encapsulated DNR/CYT (E-DNR/CYT) and the total DNR/CYT (T-DNR/CYT) in rat plasma. The methods involved solid-phase extraction (SPE) using reverse adsorbents for separating the F-DNR and E-DNR, SPE using cation exchange adsorbents for separating the E-CYT, ultrafiltration for isolating the F-CYT and protein precipitation (PPT) for releasing the T-DNR and T-CYT totally from the liposomal forms. The analytes were subsequently quantified on ultra-high performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) individually with multiple reaction monitoring (MRM) mode using positive electrospray ionization (ESI). The calibration curves showed good linear relationships over the concentration range of 0.22-44 μg/mL for E-DNR and T-DNR, 2-1000 ng/mL for F-DNR, 0.5-100 μg/mL for E-CYT and T-CYT, 4-2000 ng/mL for F-CYT respectively. For all the analytes, the within-and between-run precisions were less than13.6% and the accuracies (in terms of RE%) were within -12.5%. Besides, extraction recovery, matrix effect, dilution integrity and stability were also assessed. The methods were successfully applied to investigate the pharmacokinetics in Sprague-Dawley rats following i.v. administration liposomal formulation.