Liposomes as carriers for paramagnetic gadolinium chelates as organ specific contrast agents for magnetic resonance imaging (mri)

Abstract

Small unilamellar liposomes were used as carriers for chelates of gadolinium as organ specific magnetic resonance imaging (MRI) contrast agents. The pharmacokinetic and imaging properties of the lipophilic liposome membrane associated chelate diethylenetriaminepentaacetate-stearylamide (DTPA-SA) were investigated. Gadolinium-DTPA-SA liposomes accumulated in the liver of rats at a peak concentration of 60% of the injected dose 4 hours after application. The elimination half-life from the liver was 61 h. Tl-weighted MR images of this liposomal Gd-chelate in rats and dogs gave a strong signal enhancement of the abdominal organs, liver and spleen. High blood concentrations of the Gd-DTPASA liposomes, reaching 60% of the injected dose after 30 min., decreasing to 40% after 2 hours, suggest their potential as a contrast agent for the blood pool. The gadolinium chelate benzoyloxypropionictetraacetate (Gd-BOPTA) was entrapped in liposomes of different lipid composition. Pharmacokinetic studies of liposome preparations containing a poly(ethylene)glycol (PEG) modified lipid showed that high levels of 80 - 60 % of the injected dose remained in the blood, 15 to 60 minutes after application. Peak blood concentrations of liposomes without PEG reached only 30%, with a correspondingly higher uptake in the liver and the spleen. Thus, both the lipophilic chelate Gd-DTPA-SA, as well as Gd-BOPTA entrapped within the aqueous volume of liposomes possess not only a potential as a liver and spleen specific contrast agent, but also for the imaging of the vascular system.     

Monosialoganglioside liposome-entrapped enzyme uptake by hepatic cells.

Monosialoganglioside liposomes are rapidly taken up by the liver as compared to dicetylphosphate, phosphatidic acid or neutral liposomes. Asialoganglioside GM 1 liposomes are taken up with the same avidity as ganglioside GM 1 liposomes. Competition experiments with asialofetuin suggest that this uptake is mediated by specific recognition of the terminal galactose residues of the glyco-lipid liposomes by the receptor present on the plasma membrane of the parenchymal cells of liver. Thus liposomes containing glycolipids with terminal beta-galactosyl residues should provide an approach for specifically directing biologically active molecules to liver parenchymal cells.     

A comparison of negatively and positively charged liposomes containing entrapped polynosinic · polycytidylic acid for interferon induction in mice

Intravenous injection of negatively charged liposomes containing entrapped poly(I) · poly(C) induced a vigorous interferon response in mice with serum titers of interferon reaching twenty times those observed with comparable dosages of free poly(I) · poly(C). The response did not persist over an extended time period as was observed earlier for enhanced interferon production stimulated by positively charged liposomes containing the inducer. Both negatively and positively charged liposomes containing [¹⁴C]poly(I) · poly(C) were taken up chiefly by the liver when given intravenously. Negatively charged particles were concentrated somewhat preferentially by the spleen (7–9% of the dose compared to 4–6%). Less radioactivity was found in liver and spleen when negatively charged particles were given intraperitoneally than was the case when positively charged particles were injected by this route. Free [¹⁴C]poly(I) · poly(C) was extensively metabolized to low molecular weight materials within four hours of injection, while encapsulation of the polymer provided protection against in vivo degradation. When both preferential localization and protection were considered, from three to five times as much high molecular weight [¹⁴C]poly(I) · poly(C) was recovered from liver at four hours after intravenous injection when the compound was given in encapsulated form compared to the free polymer. Similarly, for spleen, seven times and three times as much polymeric [¹⁴C]poly(I) · poly(C) was recovered following injection of negatively charged liposomes and positively charged liposomes respectively compared to free [¹⁴C]poly(I) · poly(C). At 48 h after an intravenous injection of positively charged liposomes, as much as four percent of the dose remained in high molecular weight form in the liver and one percent in the spleen. Following intraperitoneal injections, polymeric [¹⁴C]poly(I) · poly(C) recovered from the liver never exceeded 4.3% of the dose, showing that most of the radioactivity in the liver consisted of metabolites. These results suggest that elevated and prolonged production of interferon in animals treated with encapsulated inducer results from a combination of factors including preferential tissue location and protection of the inducer from hydrolytic cleavage.     

Enzyme replacement with liposomes containing beta-galactosidase from Charonia lumpas in murine globoid cell leukodystrophy (twitcher)

Enzyme replacement with liposomes containing beta-galactosidase obtained from charonia lumpas was carried out in murine globoid cell leukodystrophy (GLD). Charonia lumpas beta-galactosidase was able to hydrolyze galactocerebroside trapped into liposomes prepared from lecithin, cholesterol and sulfatide (molar ratio; 7:2:1). Liposomes containing charonia lumpas beta-galactosidase were successfully incorporated into the mouse tissues. 3H-galactocerebroside labeled liposomes were also incorporated into mouse liver, spleen and other tissues. The accumulation rate of 3H-galactocerebroside into twithcer mice liver and spleen was almost 40 to 100 times higher than those of controls and degraded to 70 to 80% of accumulated radioactivity of 3H-galactocerebroside by single injection of liposomes containing charonia lumpas beta-galactosidase. Results suggest that exogeneous enzyme trapped in liposomes can be useful for the correction of accumulated compound.     

Monosialoganglioside liposome-entrapped enzyme uptake by hepatic cells

Monosialogangliosde liposomes are rapidly taken up by the liver as compared to dicetylphosphate, phosphatidic acid or neutral liposomes. Asialoganglioside G_{M 1} liposomes are taken up with the same avidity as ganglioside G_{M 1} liposomes. Competition experiments with asialofetuin suggest that this uptake is mediated by specific recognition of the terminal galactose residues of the glycolipid liposomes by the receptor present on the plasma membrane of the parenchymal cells of liver. Thus liposomes containing glycolipids with terminal β-galactosyl residues should provide an approach for specifically directing biologically active molecules to liver parenchymal cells.     

REVERSAL BY ADENINE OF THE ETHIONINE-INDUCED LIPID ACCUMULATION IN THE ENDOPLASMIC RETICULUM OF THE RAT LIVER : A Preliminary Report

Within 3.5 to 4 hours after thionine administration, numerous small osmiophilic bodies, liposomes, appear in the endoplasmic reticulum of the liver cells. By fusion, the liposomes lead to the formation of larger collections of fat, giant liposomes. Adenine administration to ethionine-treated rats removes the liposomes from the hepatocytes and causes the transitory appearance of osmiophilic droplets in the sinusoidal space of Disse. The characteristic disaggregation of hepatic polysomes seen in the liver after ethionine administration is corrected by the injection of adenine.     

Differential uptake of liposomes varying in size and lipid composition by parenchymal and kupffer cells of mouse liver

Using liposomes differing in size and lipid composition, we have studied the uptake characteristics of the liver parenchymal and Kupffer cells. Desferal labeled with iron-59 was chosen as a radiomarker for the liposomal content, because Desferal in its free form does not cross cellular membranes. At various time intervals after an intravenous injection of liposomes into mice, the liver was perfused with collagenase, and the cells were separated in a Percoll gradient. It was found that large multilamellar liposomes (diameter of about 0.5 μm) were mainly taken up by the Kupffer cells. For these large liposomes, the rate of uptake by Kupffer cells was rapid, with maximum uptake at around 2 hours after liposome injection. Unexpectedly, small unilamellar liposomes (diameter of about 0.08 μm) were less effectively taken up by Kupffer cells, and the rate of uptake was slow, with a maximum uptake at about 10 hours after liposome injection. In contrast, parenchymal cells were more effective in taking up small liposomes and the uptake of large liposomes was negligible. In addition, liposomes made with a galactolipid as part of the lipid constituents appeared to have higher affinity to parenchymal cells than liposomes made without the galactolipid. These findings should be of importance in designing suitable liposomes for drug targeting.     

Large liposomes containing ganglioside GM1 accumulate effectively in spleen

Large liposomes, with a composition of egg phosphatidylcholine, cholesterol and ganglioside GM1, prepared by an extrusion method, were injected intravenously into mice. After 24 h, up to 50% of injected dose was accumulated in spleen compared with about 15% in spleen for liposomes containing no GM1. The effect of GM1 on spleen accumulation of liposomes was liposome size dependent. Only relatively large liposomes (d greater than 300 nm) showed high accumulation; smaller liposomes were progressively less accumulated. The spleen accumulation increased with increasing injection dose of the liposomes. It was noted that the enhanced uptake by spleen was accompanied by a decrease in the liver uptake, but the total uptake of liposomes by liver and spleen was not dependent on the diameter of liposome or the presence of the ganglioside GM1. Autoradiographs of fixed and sectioned spleen using 125I-labeled tyraminylinulin as a content marker for the liposomes, showed that liposomes localized at the reticular meshwork of the red pulp. These results suggest that larger liposomes containing GM1 are filtered by the spleen during the circulation in blood. The smaller ones with a mean diameter of less than 100 nm are not retained by the filter. The function of GM1 is to prevent liposomes from a rapid uptake by the liver so that liposomes may circulate through the spleen and be filtered. These results, together with the observation that the liposome-entrapped proteins were degraded by the spleen, suggest the potential use of these liposomes for specific drug delivery to the spleen.     

Interaction of cells from murine organs with liposomes of various composition

The distribution of liposomes prepared from total mouse liver lipids and containing (3H)-labelled platelet activation factor in mouse organs was studied. It was shown that the majority of intraperitoneally injected liposomes prepared from total mouse liver lipids were transported to mouse liver and spleen. The interaction of liposomes with spleen cells in vitro revealed that the affinity of liposomes prepared from total spleen macrophage or total spleen lymphocyte lipids for mouse spleen cells was much higher than that of liposomes prepared from a model lipid mixture. The liposome binding to isolated spleen macrophages or lymphocytes was much higher than the liposome uptake by these cells in the total population of mouse spleen cells.     

Tissue distribution of EDTA encapsulated within liposomes containing glycolipids or brain phospholipids.

Multilameller liposomes were prepared with various asialoglycolipids, gangliosides, sialic acid, or brain phospholipids in the liposome membrane and with ethylenediaminetetraacetic acid (EDTA) encapsulated in the aqueous compartments. The liposomes containing glycolipids or sialic acid were prepared from a mixture of phosphatidylcholine, cholesterol, and one of the following test substances: galactocerebroside, glucocerebroside, galactocerebroside sulfate, mixed gangliosides, monosialoganglioside GM1, monosialoganglioside GM2, monosialoganglioside GM3, disialoganglioside GD1a, or sialic acid. The liposomes containing brain phospholipids were mixtures of either sphingomyelin and cholesterol or a brain total phospholipid extract and cholesterol. Distributions of 14C-labeled EDTA were determined in mouse tissues from 15 min to 6 h or 12 h after a single injection of liposome preparation. Liver uptake up encapsulated EDTA was lowest from all liposome preparations containing sialic acid or sialogangliosides, regardless of the amount of sialic acid moiety present or the identity of the particular ganglioside; highest uptake of encapsulated EDTA by liver was from liposomes containing galactocerebroside or brain phospholipids. Lungs and brain took up the largest amounts of EDTA from liposomes containing sphingomyelin and lesser amounts from liposomes containing GD1a. Use of mouse brain phospholipid extract to prepare liposomes did not increase uptake of encapsulated EDTA by the brain. EDTA in liposomes containing monosialogangliosides, brain phospholipids, galactocerebroside, or sialic acid was taken up well by spleen and marrow. Highest thymus uptake of encapsulated EDTA was from liposomes containing GD1a. These results demonstrate that inclusion of sialogangliosides in liposome membranes decreases uptake of liposomes by liver, thus making direction of encapsulated drugs to other organs more feasible. Liposomes containing glycolipids also have potential uses as probes of cell surface receptors.     

Differentiation in hepatic and splenic phagocytic activity during reticuloendothelial blockade with cholesterol-free and cholesterol-rich liposomes

We have shown earlier that liver and spleen reticuloendothelial cells have low affinity to phagocyte liposomes containing cholesterol. In the present study, we predosed mice with cholesterol-rich (identical to = 46.6 mol% cholesterol content) and cholesterol-free (identical to 0 mol%) liposomes to saturate the reticuloendothelial cells and examined the tissue distribution of the second dose of the test liposomes containing an aqueous marker, 125I-labelled poly(vinylpyrrolidone). The result shows that both preparations of the predosed liposomes caused suppression in hepatic uptake and delay in the blood clearance of the test liposomes, but the cholesterol-free liposomes were more effective in producing these effects than the cholesterol-rich liposomes. The suppression in hepatic phagocytic function, in accordance with the 'spillover' phenomenon [16, 17], caused an enhancement in spleen and lung uptake. The increase in lung uptake was proportionally related to the degree of suppression in the hepatic uptake, but the results of the splenic uptake showed some discrepancy. The predosed cholesterol-free liposomes which caused the maximum spillover of the test liposomes from the liver did not achieve maximum enhancement in the splenic uptake. Instead, the maximum enhancement was recorded with the predosed cholesterol-rich liposomes. This discrepancy in splenic uptake suggests that the predosed liposomes caused saturation of not only liver also the spleen reticuloendothelial system. However, instead of suppression in the splenic uptake due to the saturation, enhancement in uptake of the test liposomes was observed. We suggest the cause of this apparent increase the splenic phagocytic activity may be due to stimulation, by some unknown mechanism of splenic macrophages endothelial cells and/or lymphocytes, to phagocyte the excess of the test liposomes spillover from the liver with impaired phagocytic function.     

Tissue distribution of EDTA encapsulated within liposomes containing glycolipids or brain phospholipids

Multilamellar liposomes were prepared with various asialoglycolipids, gangliosides, sialic acid, or brain phospholipids in the liposome membrane and with ethylenediaminetetraacetic acid (EDTA) encapsulated in the aqueous compartments. The liposomes containing glycolipids or sialic acid were prepared from a mixture of phosphatidylcholine, cholesterol, and one of the following test substances: galactocerebroside, glucocerebroside, galactocerebroside sulfate, mixed gangliosides, monosialoganglioside G_M1, monosialoganglioside G_M2, monosialoganglioside G_M3, disialoganglioside G_D1a, or sialic acid. The liposomes containing brain phospholipids were mixtures of either sphingomyelin and cholesterol or a brain total phospholipid extract and cholesterol. Distribution of ¹⁴C-labeled EDTA were determined in mouse tissues from 15 min to 6 h or 12 h after a single injection of liposome prepartion. Liver uptake of encapsulated EDTA was lowest from all liposome preparations containing sialic acid or sialogangliosides regardless of the amount of sialic acid moiety present or the identity of the particular ganglioside; highest uptake of encapsulated EDTA by liver was from the liposomes containing galactocerebroside or brain phospholipids. Lungs and brain took up the largest amounts of EDTA from liposomes containing sphingomyelin and lesser amounts from liposomes containing G_D1a. Use of mouse brain phospholipid extract to prepare liposomes did not increase uptake of encapsulated EDTA by the brain. EDTA in liposomes containing monosialogangliosides, brain phospholipids, galactocerebroside, or sialic acid was taken up well by spleen and marrow. Highest thymus uptake of encapsulated EDTA was from liposomes containing G_D1a. These results demonstrate that inclusion of sialogangliosides in liposome membranes decreases uptake of liposomes by liver, thus making direction of encapsulated drugs to other organs more feasible. Liposomes containing glycolipids also have potential uses as probes of cell surface receptors.     

Uptake of liposomes containing phosphatidylserine by liver cells in vivo and by sinusoidal liver cells in primary culture: in vivo-in vitro differences

The interaction with liver cells of liposomes containing different mol fractions of phosphatidylserine was investigated in vivo and in vitro. Increasing the amount of liposomal phosphatidylserine from 10 to 30 mol% leads to a faster blood disappearance of the liposomes. Within the liver, which is mainly responsible for this elimination, these liposomes are only taken up by the hepatocytes and Kupffer cells. By contrast, sinusoidal endothelial cells, in vitro, do bind and internalize liposomes containing >/=30% phosphatidylserine at least as actively as Kupffer cells. The uptake by endothelial and Kupffer cells is inhibited by poly(inosinic acid) and other anionic macromolecules, suggesting the involvement of scavenger receptors. The lack of liposome uptake by endothelial cells under in vivo conditions can be attributed to plasma effects since addition of various sera caused severe reduction of in vitro uptake of liposomes. In vivo the phosphatidylserine head groups may be masked by plasma proteins adsorbed to the liposomal surface, thus preventing recognition by receptors, which are intrinsically able to recognize phosphatidylserine.     

Inhibition of liver metastases in murine colon adenocarcinoma by liposomes containing human C-reactive protein or crude lymphokine

Previous studies from our laboratory have shown that liposomes containing LYNK or CRP inhibit lung metastases in mice bearing the malignant fibrosarcoma, T241. We have now extended these observations to the murine colon adenocarcinoma (MCA-38), which metastasizes to the liver. MCA-38 tumor cells (1 X 10(6)) were implanted in the wall of the cecum by orthotopic transplantation. Three-hundred twenty-six mice bearing such tumors were divided into four treatment groups as follows: (1) no treatment (2) liposomes containing control medium (3) liposomes containing LYNK, and (4) liposomes containing CRP. Treatment was started from day 2, day 18, or 34 after tumor implantation and it was administered on 3 days per week. Each treatment, given parenterally, consisted of 4 mumol liposomes (PS-PC, 1:1) containing the appropriate agents. Mice receiving liposomes containing LYNK or CRP had significantly fewer and smaller liver metastases (25%-28%), than those in the two control groups (53%-54%). Also, a significantly better survival was noted in the two treated groups than in the two control groups. The most likely mechanism of tumor inhibition appears to be through macrophage activation. In the Winn tumor neutralization test, peritoneal macrophages harvested from normal mice 24 h after a single injection of liposomes (2.5 mumol) containing LYNK or CRP markedly inhibited tumor growth when a mixture of effector-target cells (40 : 1) was injected in the footpad. These studies further confirm the suggested role of CRP as an 'immunomodulator' or 'biological response modifier' in yet another tumor system.     

Hepatic uptake and antihepatotoxic properties of vitamin E and liposomes in the mouse

Intravenous administration of soybean phosphatidylcholine liposomes containing different amounts of tocopherol acetate leads to a dose and time dependent increase of mouse liver tocopherol content, which was not observed when the preparation was given orally. When benzo[a]pyrene pretreated mice intoxicated with 400 mg/kg AAP were pretreated 2 h before with 1 g/kg phosphatidylcholine liposomes containing 4 mg/kg vitamin E acetate, these animals were protected against liver damage. Vitamin E alone or liposomes lacking vitamin E showed no protection. In an inflammatory liver disease model, i.e. fulminant hepatitis induced by intraperitoneal administration of 700 mg/kg galactosamine and 1 microgram/kg lipopolysaccharide phosphatidylcholine liposomes protected at a dose of 1 g/kg i.v. In this case, however, the protection was not due to the presence of vitamin E. These findings demonstrate the usefulness of phosphatidylcholine for liver protection and show that the protective spectrum is improved when they contain vitamin E. The data suggest that phosphatidylcholine is an excellent carrier for delivery of vitamin E to the liver.     

Grafting of different glycosides on the surface of liposomes and its effect on the tissue distribution of 125I-labelled γ-globulin encapsulated in liposomes

Different glycosides were grafted on the surface of liposomes containing ¹²⁵I-labelled γ-globulin by two ways: (1) by using glycolipid and (2) by covalent coupling of $\text{p-}\text{aminophenyl-}\text{d}\text{-glycosides}$ to phosphatidylethanolamine liposomes using glutaraldehyde. The distribution of ¹²⁵I-labelled γ-globulin was determined in mouse tissues from 5–60 min after a single injection of these liposomes. The liver uptake of encapsulated ¹²⁵I-labelled γ-globulin was highest from liposomes having galactose and mannose on the surface. Competition experiments and cross-inhibition studies indicate that this uptake are mediated by specific recognition of the surface galactose and mannose residues of liposomes by the receptors present on the plasma membrane of liver cells. Stearylamine-containing liposomes were found to be more efficient in mediating the uptake of ¹²⁵I-labelled γ-globulin by the lung, whereas in the case of spleen, phosphatidylethanolamine liposomes were more efficient. The extent of uptake of ¹²⁵I-labelled γ-globulin from all types of liposome decreases as the amount of given liposomes increases. The uptake of ¹²⁵I-labelled γ-globulin from liposomes containing asialogangliosides depends upon the phospholipid/ glycolipid ratio. These experiments clearly demonstrate that enhanced liposome uptake by liver cells could be achieved by grafting galactose and mannose on the liposomal surface.     

Inhibition of the hyperglycemic effect of cAMP by intravenous injection to mice of carbonic anhydrase entrapped in liposomes.

Mice treated with acetazolamide or cAMP received intravenously liposomes containing carbonic anhydrase. The hyperglycemic effect of both these substances was suppressed by carbonic anhydrase entrapped in liposomes. The later also abolished the glycogenolytic action of cAMP on liver.     

Tissue distribution and cellular distribution of liposomes encapsulating muramyltripeptide phosphatidyl ethanolamide

In previous studies it was shown that administration of liposome-encapsulated MTPPE (LE-MTPPE) led to resistance againstKlebsiella pneumoniae infection. To get more insight in the cell types that are involved in this by LE-MTPPE induced antibacterial resistance, the tissue distribution of liposomes encapsulating MTPPE and the distribution over the cells in the main target organs were investigated. After intravenous injection of the liposomes in mice a substantial amount was recovered from liver and spleen and a smaller amount from the lung. In the liver 83% of the liposomes was taken up by the macrophages. In the spleen also most liposomes were taken up by macrophages of the red and white pulp as well as by dendrocytes. The liver and spleen were also the organs in which, after intravenous inoculation,K. pneumoniae was trapped. It was observed that cells containing LE-MTPPE often had not taken up bacteria. Most bacteria, about 73%, were found in cells not containing liposomes. The capacity of the liposome-containing cells to take up bacteria did not change with time. This suggests that the by LE-MTPPE immunostimulating effect is due to the production of cytokines by the cells that take up LE-MTPPE. These cytokines might stimulate other cells to the killing of bacteria.     

Liposomes containing chelating agents. Cellular penetration and a possible mechanism of metal removal

Electron microscope studies were done on mouse liver, from 5 min to 8 wk after an intravenous injection of liposomes containing ethylenediaminetetraacetic acid (EDTA). Livers of mice receiving an injection of liposomes containing KCL instead of EDTA or an injection of a solution of EDTA were also examined. Liposomes were shown to be phagocytized by hepatocytes as well as by Kupffer cells within minutes after the injection. Initially, there was a close contact between the liposomal membrane and the cellular membrane, followed by an invagination of the latter and the formation of a distinct vesicle surrounding a single liposome or a cluster of several liposomes. No fusion between the liposomal membrane and the cell membrane was observed. Between 15 min and 6 h after liposome injection, the Kupffer cells were found to have an increased number of lysosomes and autophagic vacuoles. Within the latter, morphologically intact liposomes or remnants of liposomes could be seen. At 12 h after injection, a striking increase in macrophages was observed in the liver sinusoids of EDTA-liposome-injected mice, but not in those of KCl- liposome-injected mice. Within the macrophages, remnants of liposomes occasionally could be observed. However, the origin and the physiological role of these cells are unknown. In the hepatocytes, morphological changes were first observed 24 h after injection; there were large numbers of autophagic vacuoles, and some cells showed extensive areas of focal cytoplasmic degeneration. The morphology of the liver cells returned to normal about 7 days after injection. No morphological changes were observed in livers of mice receiving EDTA solution without liposomes. A possible mechanism by which the liposome- encapsulated chelating agents can successfully remove intracellular toxic metals is discussed. The use of liposomes as carriers seems to be a useful tool for intracellular delivery of chelating agents or drugs in general.     

Effect of bile anionic polypeptidic fraction on the fate of cholesterol carried by liposomes in the rat

[14C]Cholesterol associated with liposomes with or without anionic polypeptidic fraction was administered intravenously to the rat. The cholesterol originated from liposomes including anionic polypeptidic fraction is secreted in bile much later, is stored in liver in higher quantity, and is metabolized into bile salts in lesser quantity during the 4 hr of experimentation than the cholesterol issued from liposomes exempt of anionic polypeptidic fraction. From these results it can be postulated that the cholesterol associated with liposomes containing anionic polypeptidic fraction might be directed in a particular liver pathway.