Factors influencing the uptake and release of doxorubicin by liposomes

The use of liposomes exhibiting a transmembrane pH gradient (inside acidic) to accumulate doxorubicin into the interior aqueous compartment has been shown to achieve drug trapping efficiencies in excess of 98% in a manner which is independent of lipid composition. Doxorubicin entrapment appears to be most efficient at 60°C with 100% drug accumulation occurring after 5 minutes. An increase in internal buffering capacity and trap volume of the vesicles significantly enhances doxorubicin sequestration. Initial drug to lipid ratios as high as 2:1 (wt:wt) have been used, although trapping efficiencies fall below 95% at drug to lipid ratios in excess of 1:2 (wt:wt). As vesicle size is decreased the initial drug to lipid ratio must be reduced to 1:10 to maintain high trapping efficiencies. In addition to effecting efficient doxorubicin entrapment, the transmembrane pH gradient also reduces the rate of doxorubicin leakage. For example, in liposomes exhibiting a pH gradient greater than 2 units, release is less than 5% of the encapsulated doxorubicin over 24 hours at 37°C whereas release rates are significantly higher in the absence of a pH gradient. Finally, a procedure for a rapid colorimetric test for determining the amount of unencapsulated doxorubicin is described. The test is based on a spectral shift of doxorubicin peak absorption from 480nm to approximately 600nm upon addition of alkali to the liposomal doxorubicin. The resulting color change of untrapped drug from orange to purple can be quantitated spectrophotometrically or visually.Medicine, Faculty ofBiochemistry and Molecular Biology, Department ofGraduat