A method for the simultaneous measurement of liposome-encapsulated and released drugs in mouse plasma by on-line solid phase extraction (SPE)-SPE-HPLC with direct plasma injection was developed using a doxorubicin (DXR)-containing liposome formulation as the model drug. During SPE, the released DXR was extracted on the 1st restricted-access media (RAM) SPE column, whereas the liposomes were eluted. The eluted liposomes were collapsed on-line, and the released DXR was delivered to the 2nd RAM SPE column for extraction. The retained DXR on the SPE columns was analyzed via HPLC-fluorescent detector by switching the valves. The method was validated and applied to the pharmacokinetic study of DXR in mice after intravenous injection of DXR-containing liposomes.
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