Liposomes loaded with nonionic contrast media. Hepatosplenic computed tomographic enhancement

Bio-inspired nano tools for neuroscience

Research and treatment in the nervous system is challenged by many physiological barriers posing a major hurdle for neurologists. The CNS is protected by a formidable blood brain barrier (BBB) which limits surgical, therapeutic and diagnostic interventions. The hostile environment created by reactive astrocytes in the CNS along with the limited regeneration capacity of the PNS makes functional recovery after tissue damage difficult and inefficient. Nanomaterials have the unique ability to interface with neural tissue in the nano-scale and are capable of influencing the function of a single neuron. The ability of nanoparticles to transcend the BBB through surface modifications has been exploited in various neuro-imaging techniques and for targeted drug delivery. The tunable topography of nanofibers provides accurate spatio-temporal guidance to regenerating axons. This review is an attempt to comprehend the progress in understanding the obstacles posed by the complex physiology of the nervous system and the innovations in design and fabrication of advanced nanomaterials drawing inspiration from natural phenomenon. We also discuss the development of nanomaterials for use in Neuro-diagnostics, Neuro-therapy and the fabrication of advanced nano-devices for use in opto-electronic and ultrasensitive electrophysiological applications. The energy efficient and parallel computing ability of the human brain has inspired the design of advanced nanotechnology based computational systems. However, extensive use of nanomaterials in neuroscience also raises serious toxicity issues as well as ethical concerns regarding nano implants in the brain. In conclusion we summarize these challenges and provide an insight into the huge potential of nanotechnology platforms in neuroscience.

Distribution of liposome-encapsulated iodixanol in rat liver cells

Distribution of liposome-encapsulated [(125)I]iodixanol in different types of liver cells following intravenous injection was studied in rats. The data showed that liposome-encapsulated [(125)I]iodixanol was rapidly taken up by the liver; after 15 min, radioactivity corresponding to nearly 25% of the injected radioactivity could be recovered therein. After 4 hr, approximately 60% of the injected radioactivity was in the liver. One week after injection, nearly 30% of the encapsulated radioactivity could still be recovered in the liver. Liposome-encapsulated [(125)I]iodixanol was taken up both by hepatocytes and the Kupffer cells. On a per cell basis, the uptake of liposome-encapsulated [(125)I]iodixanol in Kupffer cells was more than 10-fold greater than that in hepatocytes, while the contribution of liver endothelial cells to uptake was negligible. Osmotic protection studies showed that iodixanol does not readily diffuse across lysosomal membranes, indicating that loss of iodixanol from the liver probably occurred by recycling rather than by diffusion across phagolysosomal and plasma membranes.

Evaluation of liposomal contrast agents for liver CT in healthy rabbits

RATIONALE AND OBJECTIVES

To evaluate the efficiency of two new liposomal contrast agents aimed at the reticuloendothelial system for liver CT in comparison to an extracellular contrast agent.

METHODS

Two liposomal contrast agents (BR2 and BR21, respectively), containing free as well as encapsulated iomeprol at different concentrations, and commercially available free extracellular iomeprol were studied. In 60 experiments, the three contrast agents were tested at five different doses in rabbits. Intravenous injection was followed by dynamic CT studies over a period of 0 to 120 minutes (0 to 6 hours in 3 animals). A quantitative analysis of the enhancement in aorta and liver was performed.

RESULTS

In healthy rabbits, the two liposomal contrast agents induced a significantly higher and more persistent increase in liver density at doses of > or = 1.5 mL/kg compared with the extracellular agent. The density enhancement induced by the two liposomal agents was dose-dependent and reached a maximum of +102 Hounsfield units (HU), compared with +87 HU for the extracellular contrast agent, at 2.0 mL/kg, without any appreciable increases at higher dosages. An adequate liver enhancement of at least +40 HU persisted for up to 90 minutes after injection of the liposomal contrast agents, compared with < 5 minutes after the extracellular agent. BR2 tended to provide a higher and more persistent enhancement than BR21.

CONCLUSIONS

Liposomal contrast agents induce a more pronounced and much more persistent increase in liver density than conventional extracellular agents. Liposomes thus have the potential for optimizing CT examinations of the liver by providing a larger imaging window.

CT blood pool enhancement in primates with lopromide-carrying liposomes containing soy phosphatidyl glycerol

RATIONALE AND OBJECTIVES

The purpose of this study was to determine the feasibility of using iodinated liposomes as blood pool agents for computed tomography (CT) in nonhuman primates.

MATERIALS AND METHODS

Five normal adult baboons (15-21 kg) were anesthetized and intravenously injected with iopromide containing soy phosphatidyl glycerol liposomes with a diameter of 195 nm. Each animal received a dose of 300 mg total iodine per kilogram (46% encapsulation).

RESULTS

The animals tolerated the injections well, experiencing no measurable electrocardiographic changes, and recovered uneventfully from anesthesia. Sequential helical CT scans of the baboons from the base of the skull to the symphysis pubis acquired up to 40 minutes after injection showed persistent blood pool enhancement. Maximum mean enhancement of major vascular structures was 106 HU at 1 minute after contrast medium injection. Mean blood pool enhancement was 76, 72, and 67 HU at 10, 20, and 40 minutes after injection, respectively. Liver and spleen were enhanced by 40 and 41 HU, respectively, 40 minutes after injection. No significant enhancement was measured in the brain and pancreas.

CONCLUSION

Soy phosphatidyl glycerol with iopromide liposomes produces prolonged vascular enhancement and has potential as a blood pool CT contrast agent in primates.

Blood pool and liver enhancement in CT with liposomal lodixanol: comparison with lohexol

RATIONALE AND OBJECTIVES

The authors compared the time course and blood pool and hepatic enhancement of three different doses of liposomal iodixanol with those of iohexol.

MATERIALS AND METHODS

A liposomal iodixanol formulation was prepared with 200 mg of iodine per milliliter total and 80 mg of iodine per milliliter encapsulated. Twelve normal New Zealand white rabbits divided into four groups received 75-, 100-, or 150-mg encapsulated iodine per kilogram doses of liposomal iodixanol or 2 mL/kg iohexol with 300 mg of iodine per milliliter. A liver section was scanned with serial computed tomography (CT) before the injection, immediately afterward, and at 1-minute intervals for 10 minutes. Region-of-interest measurements of the aorta and liver were plotted at each time point, and contrast enhancement was plotted as a function of time and iodine dose.

RESULTS

All liposomal iodixanol doses produced greater liver enhancement than iohexol. Results were significant (P < .05) for 100 mg and 150 mg iodine per kilogram dose groups at time points beyond 2 minutes. Peak hepatic enhancement (change in attenuation) was 54.9 HU +/- 7.6 with iohexol, compared with 59.6 HU +/- 6.1, 73.3 HU +/- 3.6, and 104.1 HU +/- 8.8 for 75, 100, and 150 mg encapsulated iodine per kilogram doses, respectively. Hepatic enhancement increased rapidly after injection of liposomal iodixanol, plateauing 2-3 minutes later. Blood pool enhancement decreased rapidly. Steady-state liver enhancement with liposomal iodixanol increased linearly with dose. Aortic enhancement was greater with iohexol.

CONCLUSION

Liposomal iodixanol yielded greater hepatic enhancement at lower total iodine doses than iohexol. Although liver enhancement occurred rapidly after injection, blood pool enhancement was brief.

Liposomes as delivery agents for medical imaging

Medical imaging requires an appropriate intensity of signal from the area of interest in order to differentiate certain structures from surrounding tissues, regardless of the modality used. In the majority of cases, contrast agents specific for each imaging modality are necessary to achieve a sufficiently intense signal. To facilitate the accumulation of contrast in the required zone, various microparticulates have been suggested as carriers for contrast agents. Among these carriers, liposomes-microscopic artificial phospholipid vesicles-draw special attention because of their easily controlled properties and useful pharmacological characteristics. This review will discuss how the advantages of liposomes have been used so far in the rapidly growing field of diagnostic medical imaging.

CT and MR imaging of the liver using liver-specific contrast media. A comparative study in a tumour model

PURPOSE

A new type of liposomal liver-specific contrast medium (CM) in CT was studied, and the results were compared with those obtained with Mn-DPDP, a paramagnetic hepatobiliary CM, in MR imaging. The contrasts of normal liver tissue to tumorous tissue and the importance of the CM for tumour detection in the 2 modalities were studied in a rabbit tumour model. CT and T1-weighted pre- and postcontrast and T2-weighted MR images precontrast were obtained.

MATERIAL, METHODS AND RESULTS

Compared to precontrast images, significantly higher contrasts of normal liver tissue to tumorous tissue were obtained after CM administration in both CT and MR examinations. At radiologic evaluation, significantly more tumours were detected after CM administration in CT and in T1-weighted MR images than in precontrast images in CT and T1-weighted MR. There were no significant differences in tumour detection frequency in MR studies including a T2-weighted pulse sequence, postcontrast CT, or postcontrast T1-weighted MR imaging.

CONCLUSION

The use of liver-specific CM improves visualization of liver tumours in CT and T1-weighted MR imaging.

A new liposomal contrast medium for CT of the liver. An imaging study in a rabbit tumour model

A new type of liposomal liver-specific contrast medium was studied in an experimental tumour model. Rabbits were inoculated with VX2-carcinoma directly in the liver of laparotomy. CT studies were carried out 14 days after inoculation. The liver-specific contrast medium consisted of a suspension of liposomes in a 100 mg I/ml iodixanol solution, with equal amounts of encapsulated and nonencapsulated iodixanol. It was administered at a dose of 200 mg I/kg. The contrast of normal liver tissue to tumorous tissue was significantly increased by contrast medium administration, the increase being largest 10 min after injection.

Liver contrast enhancement in primates using iopromide liposomes

RATIONALE AND OBJECTIVES

We studied the feasibility of using iodinated liposomes as computed tomography (CT) liver contrast agents in nonhuman primates.

METHODS

Iopromide-containing liposomes were investigated as reticuloendothelial (RES) contrast agents for CT scanning of the liver in normal adult baboons. For intravenous (i.v.) injection, liposomes were resuspended in mannitol solution, filtered under sterile conditions, and injected i.v. at doses of 200 and 400 mg l/kg to each of five anesthetized adult baboons.

RESULTS

Animals tolerated the injections without measurable electrocardiographic changes and recovered uneventfully from anesthesia. Sequential CT scans of the baboons' upper abdomen acquired up to 60 min postinjection showed persistent enhancement of the liver 10-60 min after injection. Maximum enhancement levels were 36 and 61 delta Hounsfield units (delta H) after the 200- and 400-mg/kg doses, respectively. The mean time to plateau enhancement was 20 min with the 200-mg/kg dose and 10 min with the 400-mg/kg dose. The greatest splenic enhancements were 181 and 301 delta H after the 200- and 400-mg/kg doses, respectively.

CONCLUSION

Iopromide liposomes are effective as RES contrast agents in primates.

Enhancement of computed tomography liver contrast using iomeprol-containing liposomes and detection of small liver tumors in rats

RATIONALE AND OBJECTIVES

We evaluated iomeprol-containing liposomes (Lipiom), a new contrast medium for computed tomography (CT) liver scanning, in an animal model of chemically induced hepatocellular carcinomas and other liver tumors in rats.

METHODS

Liver tumors were induced by administration of carcinogens to rats, either 0.55% (w/w) 1'-hydroxysafrole in the diet or induction by 3'-methyl-4-diethylaminoazobenzene followed by promotion with carbon tetrachloride. CT scanning was performed 1-3 hr after intravenous injection of iomeprol-containing liposomes.

RESULTS

After injection of iomeprol-containing liposomes at a dose of 70 mg of liposome-entrapped iodine per kilogram of body weight, the normal liver parenchyma showed a contrast enhancement, in Hounsfield units, of more than 60% over the control value before bolus. Liver tumors with no or few Kupffer cells were not enhanced and appeared as dark areas within the normal parenchyma. Tumors and pretumoral lesions devoid of Kupffer cells, as small as 3 mm in diameter, could be distinguished using this non-invasive method.

CONCLUSION

CT liver scanning after injection of iomeprol-containing liposomes appears to be promising method for detecting liver tumors and focal liver lesions.

The determination of liposome captured volume

Manipulating the process by which lipids assemble to form bilayer membranes has produced a myriad of protocol-dependent liposome types. For each of these systems the arrangement of bilayers is characteristic and can be described by parameters such as aqueous entrapment per mole lipid or captured volume, vesicle size distribution, the average number of lamellae per vesicle and shape. For specific applications as model systems or drug delivery systems specific characteristics are desired. Consequently over the years many techniques have evolved to better quantitate these parameters. Here we focus on and detail several methods to quantitate liposome captured volume. We also briefly describe the available methods to measure the other aforementioned physical properties and discuss their interdependency with captured volume.