Separation of liposome-associated doxorubicin from non-liposome-associated doxorubicin in human plasma: implications for pharmacokinetic studies.

To characterize the pharmacokinetics of liposome-associated drugs, the fraction of drug circulating in liposome-associated form and the absolute plasma drug levels must be determined. In this report, we describe our methodological approach to quantitate plasma liposome-associated doxorubicin separately from protein-bound and free doxorubicin. The method is based on the affinity of a cation-exchange resin for doxorubicin and the repulsion by the same resin of negatively-charged liposomes. The methodology is technically simple and reproducible, and lends itself to the analysis of multiple plasma samples as required in pharmacokinetic studies. The validity of this approach was confirmed by separation of liposome-associated from non-liposome-associated drug using gel exclusion chromatography.