Radiolabelling liposomes

External stimuli have also been used to further target liposomes. In one such study magnetite particles were incorporated in radiolabeled liposomes and a magnet positioned over the right kidney of a test animal. The liposomes were selectively targeted to that kidney in concentrations that were viewed as significantly high for relevant clinical applications [66],... 

One method for the labeling of liposomes with chelator, hexamethylpropyleneamine oxime (HMPAO) was developed by Phillips et al. (16). Lipophilic HMPAO enters the liposome where it interacts with glutathione and becomes converted to the hydrophilic form, which is trapped in the liposome. A detailed protocol for radiolabeling liposomes using Tc-HMPAO has been reported (3). In a typical experiment, 10 to 15mCi (370-555 MBq) of Tc in the form of sodium pertechnetate... 

Goins B, Phillips WT. Radiolabelled liposomes for imaging and biodistribution studies. In Torchilin VP, Weissig V, eds. Liposomes A Practical Approach. Oxford, U.K. Oxford University Press, 2003 319. 

Awasthi VD, Goins B, Klipper R, Phillips WT. Dual radiolabeled liposomes biodistribution studies and localization of focal sites of infection in rats. Nucl Med Biol 1998 25 155. 

Love WG, Amos N, Williams BD, Kellaway IW. Effect of liposome surface charge on the stability of technetium ( Tc) radiolabelled liposomes. J Micro-encapsul 1989 6 105. 

Ogihara-Umeda, I., Sasaki,T.,Kojima,S., and Nishigori, H. (1996), Optimal radiolabelled liposomes for tumour imaging, J. Nucl. Med., 37,326-332. 

Incubate approx 65 x 106 erythrocytes with doubly radiolabeled liposomes (100-500 nmol of lipid P) at 37°C for 30 min. 

Begent, R. H. J., Green, A. J., Bagshawe, K. D., Jones, B. E., Keep, P. A., Searle, F., Jewkes, R. F., Barrat, G. M., and Ryman, B. E. (1982). Liposomally entrapped second antibody improves tumor imaging with radiolabelled (first) antitumor anti-body. Lancet, 2, 739-742. 

Materials. Egg phosphatidylcholine (PC), bovine brain phosphatidylserine (PS) were obtained from Avanti Polar Lipids Inc. (Birmingham, AL) and cholesterol was from Sigma (St. Louis, MO). Ganglioside GMj, bovine, was obtained from Calbiochem (San Diego, CA). Diethylenetriamine pentaacetic acid distearylamide complex (DPTA-SA) was synthesized according to ref. 17 and nlIn-DTPA-SA was prepared as described (7). This lipophilic radiolabel is not transferred to the serum components from liposomes (unpublished data), nor is it rapidly metabolized in vivo (7). The synthesis of N-(glutaryl)phosphatidylethanolamine(NGPE) has been described (18). Dipalmitoyl deoxyfluorouridine(dpFUdR) was synthesized as described (24). 

Dubey, A. K., Eryomin, Y. A., Taraschi, T. F. and Janes, N. (1996). Alcohol binding to liposomes by 2H NMR and radiolabel binding assays does partitioning describe binding Biophys. J., 70, 2307-2315. 

Rudolph AS, Klipper RW, Goins B, et al. In vivo biodistribution of a radiolabeled blood substitute Tc-labeled liposome-encapsulated hemoglobin in an anesthetized rabbit. Proc Natl Acad Sci USA 1991 88 10976. 

There are two major ways in which liposomes have been radiolabeled by stably trapping the radionuclide in the liposome interior (i) use of second molecule encapsulated in liposome and (ii) chemical gradient with pH or ammonium sulfate. In the first method, a radionuclide is incubated with a lipophilic chelator and then mixed with an aliquot of liposomes encapsulating a second molecule. Once the lipophilic chelator carries the radionuclide across the lipid bilayer, the second molecule interacts with the radionuclide-chelator causing the radionuclide to become trapped within the interior of the liposome. This interaction may be due to the second molecule having a higher affinity for the radionuclide than the original lipophilic chelator. An... 

Recently, this method was adapted to label two commercially available liposomal formulations doxorubicin encapsulated in polyethylene glycol (PEG)-coated liposomes (Caelyx /Doxil ) (14) and daunorubicin encapsulated in small distearoyl-phosphatidyl-choline/cholesterol liposomes (Daunoxome ) (15). Although no DTPA was encapsulated in these liposomes, the labeling efficiency was typically between 70% and 80% and the radiolabeled preparations were stable in vivo during the time course of the experiment (four hours). Most likely, the lipophilic In-oxine avidly associates with the lipid bilayer and encapsulation of DTPA might not be necessary when the experimental observation period does not exceed four to six hours. 

A new method for radiolabeling glutathione-containing liposomes with using a different chelator based on SNS/S pattern complexes has been recently reported (21). One particular chelator, A,A-bis(2-mercaptoethyl)-A, A diethyl-ethylenediamine (BMEDA), was shown to efficiently label the liposomes. 

Harrington KJ, Mohammadtaghi S, Uster PS, et al. Effective targeting of solid tumors in patients with locally advanced cancers by radiolabeled pegylated liposomes. Clin Cancer Res 7, 243, 2001. 

Laverman P, Boerman OC, Storm G. Radiolabeling of liposomes for scintigraphic imaging. Methods Enzymol 2003 373 234—248. 

Bao A, Goins B, Klipper R, Negrete G, Mahindaratne M, Phillips WT. A novel liposome radiolabeling method using Tc-SNS/S complexes in vitro and in vivo evaluation. J Pharm Sci 2003 92 1893. 

DNA and/or protein vaccine entrapment in DRV liposomes is monitored by measuring the vaccine in the suspended pellet and combined supernatants. The most convenient way to monitor DNA entrapment is by using radio-labelled or DNA. For protein entrapment, the use of I-labelled protein tracer is recommended. If a radiolabel is not available or cannot be used, appropriate quantitative techniques should be employed. To determine DNA or protein by such techniques, a sample of the liposome suspension is mixed with Triton X-100 (up to 5% final concentration) or, preferably, with isopropanol (1 1 volume ratio) so as to liberate the entrapped materials. However, if Triton X-100 or the solubilized liposomal lipids interfere with the assay of the materials, liposomal lipids or the DNA must be extracted using appropriate techniques (6). Entrapment values for protein and DNA, whether alone or coentrapped, range between about 20% to 80% (protein) and 30%i to 100%i (DNA) of the initial material depending on the DNA or protein used and, in the case of DNA, the presence or absence of cationic charge. Values are highest for DNA when it is entrapped into cationic DRV (typical values in Table 1). 

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