Usefulness of liposomes carrying losefamate for CT opacification of liver and spleen.

Losefamate, a hepatobiliary contrast agent, was encapsulated into liposomes to increase its ability to opacify the liver and spleen on computed tomographic (CT) images. Multilamellar lipid vesicles (lecithin, cholesterol, and stearylamine, in 4:1:1 molar ratio) containing iosefamate in their aqueous phase were prepared. Seven dogs received intravenous injections of 100-300 mg I/kg in one of three forms: encapsulated, unencapsulated, or a mixture of the two in equal parts. Animals that received the opaque vesicles had marked opacification of their livers, bile ducts, gallbladders, spleens (maximum 106 H enhancement), and gastrointestinal tracts. Spleen CT values (an indicator of encapsulated material uptake) were always higher in these dogs than in the animals receiving equivalent amounts of unencapsulated iosefamate alone. At the high-dose level, liver uptake of the encapsulated materials was also greater. Liposome-encapsulated hepatobiliary contrast agents are effective liver and spleen opacifiers for CT imaging in the dog.

Liposome dialysis for improved size distributions.

A technique is described which allows reproducible preparation of liposomes with improved size-frequency distributions. The recent procedure of extrusion of crude liposome dispersions through controlled-pore polycarbonate membranes is used to control the upper limit of liposome diameter. Subsequent dialysis, using the same type of membrane, can remove the majority of liposomes smaller than a predetermined size. The pattern of dialysis of a liposome preparation is a function of the size-frequency distribution (as well as the membrane pore size) and can be used to approximate the distribution and/or used to monitor the reproducibility of liposome preparations.

Liposome disposition in vivo II: Dose dependency.

The dose-dependent disposition of extruded multilamellar (diameter approximately 1 micrometer) negatively charged liposomes containing entrapped [14C]inulin was studied in mice. Mice received 1500, 300, or 15 mumoles of liposomal lipid/kg iv. Carbon 14 levels were measured in the blood, liver, spleen, and carcass for 72 hr. A pronounced saturation effect, consistent with the known dose behavior of other colloids, was seen at early times; it was manifested by higher dose values in the blood and spleen but by lower liver values as the dose increased. This dose effect was attenuated in the liver but was maintained in the spleen at later times, and percent dose values approached plateau values in all tissues for all doses at later times. [14C]Inulin was used at the liposomal marker because of its inability to enter cells (or, presumably, leave them if delivered there by liposomes) in its free form. An early decline in carbon 14 levels (over the first 48 hr) was seen in the liver for the low and medium doses. Because of the known ability of blood factors to cause liposomes to leak their contents, this decline was interpreted as being a loss of [14C]inulin from extracellularly bound liposomes during this period. Moreover, the plateau carbon 14 levels at later times were interpreted as approximating the true level of intracellular inulin delivery by the liposomes.

Liposome disposition in vivo: effects of pre-dosing with lipsomes.

The effect of a high, intravenous dose of extruded multilamellar liposomes (1.1 g lipid/kg body weight) upon the subsequent ability of mouse tissues to take-up or bind a second intravenous dose of similar liposomes encapsulating 14C-inulin has been studied in vivo. The first and second doses were separated by either 1,5 or 24 hours. All tissue levels were measured one hour after the second dose. Controls received only the second dose. When the two doses were separated by one hours, 14C-levels in liver were depressed 6-fold and blood levels rose 29-fold relative to controls. However spleen uptake of lipsomes increased to three times control levels. When the two doses were separated by 24 hrs, the first dose had only a minimal effect on the disposition of the second dose. The results are consistent with a reversible blockade of hepatic, but not spleenic uptake and/or binding sites, by the first dose and indicate that adjusting a liposome dose (i.e. number of lipsomes) or use of a drug-free liposome pre-dose may be a useful technique for reducing hepatic uptake, increasing the circulation life time and/or modifying the tissue disposition properteries of therapeutic liposomes without changing liposome composition or size.