CONDUCTIVITY MEASUREMENT AS A CONVENIENT TECHNIQUE FOR DETERMINATION OF LIPOSOME CAPTURE VOLUME

The reduction in conductivity seen between a buffer solution and a lip osome preparation in that buffer was evaluated as a means of measuring liposome capture volume. Using DOPC and DOPG lipid to form negatively charged liposomes, conductivity measurements showed that conductivity of the liposome dispersion decreased as lipid concentrations of lipos ome preparations increased. Independent measurement of capture volumes by gel filtration chromatography showed that conductivity changes cor related with a liposome concentration dependent increase in capture vo lume. It is proposed that ions from the hydrating/suspending buffer no rmally contributing to conductivity were trapped within liposomes upon vesicle formation. These internalized and therefore shielded ions wer e not able to effectively contribute to conductivity of the liposome d ispersion. For multilamellar vesicles (MLVs), capture volume was deter mined by reduction in conductivity over a large lipid concentration ra nge and a broad buffer ionic strength range. Capture volume could also be determined for small unilamellar vesicles (SUVs). However, the gre ater number of exposed phospholipid head groups in high surface area S UVs contributed to conductivity of the liposome dispersion thereby lim iting range of utility. A much higher ionic strength buffer (relative to MLVs) was required before conductivity of phospholipid no longer in fluenced conductivity of the dispersion. To expand this study, multila mellar vesicles having either neutral (DOPC) or positive (DOPC/stearyl amine) charge were evaluated. Similar correlations were found between reduction in conductivity and mannitol entrapment (capture volume). Th ese studies have confirmed that measurement of reduction in conductivi ty provides an easy and convenient method for determining liposome cap ture volume.