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Liposomal formulation of doxorubicin has been widely applied in clinic for treatment of various cancers. The separation and measurement of free drug (drug which is not entrapped in liposomes) and liposomal drug in the plasma after injection of liposomal doxorubicin is of prime importance due to toxicity and activity concerns. In this study, a rapid and convenient method was developed to isolate and determine the non-liposomal and liposomal drugs in plasma. Plasma samples were prepared by solid phase extraction (SPE) using Oasis HLB cartridges. Liposomal doxorubicin (L-DOX) was collected in the aqueous eluate with its internal standard (IS), metformin; and non-liposomal doxorubicin (NL-DOX) and its isotope labelling IS were eluted from the cartridge by methanol containing 0.5% formic acid. After SPE separation, L-DOX and NL-DOX were subsequently quantified by a validated sensitive LC-MS/MS method individually. The calibration curves were found to be linear for L-DOX in the range of 0.156-40.0 mu g/mL and for NL-DOX in the range of 3.13-200 ng/mL. The extraction recovery was about 97% for L-DOX and about 65% for NL-DOX. This method was further applied to investigate the pharmacokinetics of doxorubicin in Beagle dogs after an intravenous dose of 1.0 mg/kg Doxil. After injection of Doxil (R), L-DOX was the predominant component circulating in plasma, whose amount was about 1000 fold higher than that of NL-DOX. The analytical method might be helpful in pharmacokinetics and toxicity assessment of liposomal formulation.