The distribution and fate of 131I-labelled liposomes

Optical and nuclear imaging of glioblastoma with phosphatidylserine-targeted nanovesicles

Multimodal tumor imaging with targeted nanoparticles potentially offers both enhanced specificity and sensitivity, leading to more precise cancer diagnosis and monitoring. We describe the synthesis and characterization of phenol-substituted, lipophilic orange and far-red fluorescent dyes and a simple radioiodination procedure to generate a dual (optical and nuclear) imaging probe. MALDI-ToF analyses revealed high iodination efficiency of the lipophilic reporters, achieved by electrophilic aromatic substitution using the chloramide 1,3,4,6-tetrachloro-3α,6α-diphenyl glycoluril (Iodogen) as the oxidizing agent in an organic/aqueous co-solvent mixture. Upon conjugation of iodine-127 or iodine-124-labeled reporters to tumor-targeting SapC-DOPS nanovesicles, optical (fluorescent) and PET imaging was performed in mice bearing intracranial glioblastomas. In addition, tumor vs non-tumor (normal brain) uptake was compared using iodine-125. These data provide proof-of-principle for the potential value of SapC-DOPS for multimodal imaging of glioblastoma, the most aggressive primary brain tumor.

Techniques for Loading Technetium-99m and Rhenium-186/188 Radionuclides into Preformed Liposomes for Diagnostic Imaging and Radionuclide Therapy

Liposomes can serve as carriers of radionuclides for diagnostic imaging and therapeutic applications. Herein, procedures are outlined for radiolabeling liposomes with the gamma-emitting radionuclide, technetium-99m (^99mTc), for noninvasive detection of disease and for monitoring the pharmacokinetics and biodistribution of liposomal drugs, and/or with therapeutic beta-emitting radionuclides, rhenium-186/188 (^186/188Re), for radionuclide therapy. These efficient and practical liposome radiolabeling methods use a post-labeling mechanism to load ^99mTc or ^186/188Re into preformed liposomes prepared in advance of the labeling procedure. For all liposome radiolabeling methods described, a lipophilic chelator is used to transport ^99mTc or ^186/188Re across the lipid bilayer of the preformed liposomes. Once within the liposome interior, the pre-encapsulated glutathione or ammonium sulfate (pH) gradient provides for stable entrapment of the ^99mTc and ^186/188Re within the liposomes. In the first method, ^99mTc is transported across the lipid bilayer by the lipophilic chelator, hexamethylpropyleneamine oxime (HMPAO) and ^99mTc-HMPAO becomes trapped by interaction with the pre-encapsulated glutathione within the liposomes. In the second method, ^99mTc or ^186/188Re is transported across the lipid bilayer by the lipophilic chelator, N,N-bis(2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA), and ^99mTc-BMEDA or ^186/188Re-BMEDA becomes trapped by interaction with pre-encapsulated glutathione within the liposomes. In the third method, an ammonium sulfate (pH) gradient loading technique is employed using liposomes with an extraliposomal pH of 7.4 and an interior pH of 5.1. BMEDA, which is lipophilic at pH 7.4, serves as a lipophilic chelator for ^99mTc or ^186/188Re to transport the radionuclides across the lipid bilayer. Once within the more acidic liposome interior, ^99mTc/^186/188Re-BMEDA complex becomes protonated and more hydrophilic, which results in stable entrapment of the ^99mTc/^186/188Re-BMEDA complex within the liposomes. Since many commercially available liposomal drugs use an ammonium sulfate (pH) gradient for drug loading, these liposomal drugs can be directly radiolabeled with ^99mTc-BMEDA for noninvasive monitoring of tissue distribution during treatment or with ^186/188Re-BMEDA for combination chemo-radionuclide therapy.

Liposome imaging agents in personalized medicine

In recent years the importance of molecular and diagnostic imaging has increased dramatically in the treatment planning of many diseases and in particular in cancer therapy. Within nanomedicine there are particularly interesting possibilities for combining imaging and therapy. Engineered liposomes that selectively localize in tumor tissue can transport both drugs and imaging agents, which allows for a theranostic approach with great potential in personalized medicine. Radiolabeling of liposomes have for many years been used in preclinical studies for evaluating liposome in vivo performance and has been an important tool in the development of liposomal drugs. However, advanced imaging systems now provide new possibilities for non-invasive monitoring of liposome biodistribution in humans. Thus, advances in imaging and developments in liposome radiolabeling techniques allow us to enter a new arena where we start to consider how to use imaging for patient selection and treatment monitoring in connection to nanocarrier based medicines. Nanocarrier imaging agents could furthermore have interesting properties for disease diagnostics and staging. Here, we review the major advances in the development of radiolabeled liposomes for imaging as a tool in personalized medicine.

Techniques for loading technetium-99m and rhenium-186/188 radionuclides into pre-formed liposomes for diagnostic imaging and radionuclide therapy

Liposomes can serve as carriers of radionuclides for diagnostic imaging and therapeutic applications. Herein, procedures are outlined for radiolabeling liposomes with the gamma-emitting radionuclide, technetium-99m ((99m)Tc), for non-invasive detection of disease and for monitoring the pharmacokinetics and biodistribution of liposomal drugs, and/or with therapeutic beta-emitting radionuclides, rhenium-186/188 ((186/188)Re), for radionuclide therapy. These efficient and practical liposome radiolabeling methods use a post-labeling mechanism to load (99m)Tc or (186/188)Re into pre-formed liposomes prepared in advance of the labeling procedure. For all liposome radiolabeling methods described, a lipophilic chelator is used to transport (99m)Tc or (186/188)Re across the lipid bilayer of the pre-formed liposomes. Once within the liposome interior, the pre-encapsulated glutathione or ammonium sulfate (pH) gradient provides for stable entrapment of the (99m)Tc and (186/188)Re within the liposomes. In the first method, (99m)Tc is transported across the lipid bilayer by the lipophilic chelator, hexamethylpropyleneamine oxime (HMPAO) and (99m)Tc-HMPAO becomes trapped by interaction with the pre-encapsulated glutathione within the liposomes. In the second method, (99m)Tc or (186/188)Re is transported across the lipid bilayer by the lipophilic chelator, N,N-bis(2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA), and (99m)Tc-BMEDA or (186/188)Re-BMEDA becomes trapped by interaction with pre-encapsulated glutathione within the liposomes. In the third method, an ammonium sulfate (pH) gradient loading technique is employed using liposomes with an extraliposomal pH of 7.4 and an interior pH of 5.1. BMEDA, which is lipophilic at pH 7.4, serves as a lipophilic chelator for (99m)Tc or (186/188)Re to transport the radionuclides across the lipid bilayer. Once within the more acidic liposome interior, (99m)Tc/(186/188)Re-BMEDA complex becomes protonated and more hydrophilic, which results in stable entrapment of the (99m)Tc/(186/188)Re-BMEDA complex within the liposomes. Since many commercially available liposomal drugs use an ammonium sulfate (pH) gradient for drug loading, these liposomal drugs can be directly radiolabeled with (99m)Tc-BMEDA for non-invasive monitoring of tissue distribution during treatment or with (186/188)Re-BMEDA for combination chemo-radionuclide therapy.

High-Activity Radio-Iodine Labeling of Conventional and Stealth Liposomes

A new method to label preformed liposomes with high activities of radiohalogenated compounds has been developed. It uses activated esters of simple synthetic molecules that may be readily halogenated, such as Bolton-Hunter reagent (BH), and arginine-containing liposomes. BH, in the form of an activated ester, crosses the liposome membrane to react with arginine inside the liposomes, as demonstrated by thin-layer chromatography and by the fact that saline-containing liposomes, or hydrolyzed BH or the water soluble sulfo-BH afforded only marginal encapsulation yields. Under optimized conditions, between 37 and 55 degrees C, 62 +/- 4% (mean +/- SD) of radiolabeled BH were consistently encapsulated in the liposomes within 30 min. In molar amounts, this corresponds to a mean of 56 nmol of BH per micromol of lipids. Based on achievable specific activity, up to 2.8 GBq of iodine-131 could be entrapped per micromol of lipids. Leakage of radioactivity was very low, with less than 5% of the encapsulated activity released within 6 days at 4 degrees C in phosphate-buffered saline and less than 50% within 24 h in human serum at 37 degrees C. The labeling stability, and the fact that both conventional and PEGylated liposomes can be readily labeled with high doses of radioactivity, will make this technique useful for in vivo targeting applications, such as tumor detection (using iodine-123 or iodine-124) or therapy (with iodine-131 or astatine-211).

Effect of macrophage activation on plasma disposition of niosomal 3H-methotrexate in sarcoma-180 bearing mice

Methotrexate niosomes were formed by reverse phase evaporation technique using sorbitan monostearate (Span 60) surfactant. The pharmacokinetic parameters of methotrexate (MTX) after macrophage activation via muramyl dipeptide gelatin conjugate were studied and the results compared with those obtained after free MTX and MTX niosomes without macrophage activation. In mice MTX concentrations and tumour regression were higher after macrophage activation.

The organ distribution and circulation time of intravenously injected colloidal carriers sterically stabilized with a block copolymer--poloxamine 908

Colloidal carriers injected intravenously are normally removed rapidly and efficiently by the liver and this represents a major barrier to drug targeting. By coating model microspheres and emulsions with a block co-polymer (poloxamine) it has been possible to keep the carrier circulating in the vascular compartment with little or no uptake by the reticuloendothelial system.

The effect of non-ionic surfactant vesicle (niosome) entrapment on the absorption and distribution of methotrexate in mice

Non-ionic surfactant vesicles (niosomes) prepared from a non-ionic surfactant, cholesterol and dicetyl phosphate and containing methotrexate (MTX) have been administered to mice. Given intravenously the niosomes prolong the levels of MTX in the blood, large amounts of the drug being taken up by the liver. There was also an increased uptake of MTX into the brain, perhaps due to an effect of the niosome components on the permeability of the blood brain barrier. Absorption of the drug from the gastrointestinal tract following oral ingestion, appeared to be increased at some doses; most of the entrapped MTX was taken up by the liver, but uptake of MTX into the brain was also increased. The metabolic profile of the drug is altered by the niosomes which appear to prevent the rapid formation of 7-hydroxy methotrexate.

Interaction of 99mTc-labeled liposomes with Walker tumour cells in vitro. Liposome-mediated introduction of thaliblastine into resistant Walker tumour cells

A method for labeling phospholipid vesicles (liposomes) with 99mTc-pertechnetate is described. The labeling methodology involves the chemical reduction of sodium 99mTc-pertechnetate by stannous chloride, which produces a 99mTc-SnCl2 complex with high binding capacity to phospholipids. Binding studies in vitro showed that Walker tumour cells can incorporate large amounts of 99mTc-labeled liposomes without cytotoxic effects. Fusion of liposomes with the cell plasma membrane may represent an important pathway for lipid incorporation. A Walker cell line resistant to thaliblastine, on treatment with the antitumour alkaloid thaliblastine entrapped in liposomes, showed a marked growth inhibition at concentrations that had little effect when used as free drug. The usefulness of 99mTc-labeled liposomes for kinetic binding studies with tumour cells in cultures and their possible use as carriers of cytotoxic agents in selective cancer chemotherapy, are discussed.