Liposomes In Tumor-bearing Mice Research Articles

Enhanced circulation time and antitumor activity of doxorubicin by comblike polymer-incorporated liposomes

Polymer incorporation on liposomal membranes has been extensively studied as a method of enhancing the circulation time of liposomes in the bloodstream. In this study, we investigated the in vitro and in vivo characteristics of liposomes whose surface was modified using a comblike polymer comprised of a poly(methyl methacrylate) (PMMA) backbone and short poly(ethylene oxide) (PEO) side chains. Doxorubicin (DOX)-loaded liposomes incorporating with the comblike polymer were prepared and their circulation time, biodistribution and antitumor activity were evaluated in B16F10 melanoma tumor-bearing mice. The circulation half-life time in the bloodstream of the comblike polymer-incorporated liposomes (CPILs) was approximately 14- or 2-fold higher than those of the conventional or polyethyleneglycol-fixed liposomes (PEG-liposomes), respectively. Additionally, in the biodistribution assay, the accumulation of the CPILs in the tumor was higher than those of the other liposomes. Based on this result, the antitumor activities of the CPILs were higher than those of conventional liposome formulation of DOX or free DOX due to the higher passive targeting efficiency of the long-circulating CPILs to tumor. This study suggests that the incorporation of the comblike polymer on the liposomal membrane is a promising tool to further improve circulation time of liposomes in tumor-bearing mice.

Biodistribution and tumor-accumulation of gadolinium (Gd) encapsulated in long-circulating liposomes in tumor-bearing mice for potential neutron capture therapy

To deliver and maintain a sufficient amount of Gd into tumors is required for a successful Gd neutron capture therapy (Gd-NCT), but it has been proven to be rather challenging to achieve. Previously, we have reported a Gd-encapsulated liposome formulation that has the potential to overcome this challenge. In the present study, we sought to systemically evaluate the biodistribution and the tumor-accumulation of the Gd in model tumor-bearing mice. The Gd-encapsulated liposomes were injected into mice pre-grafted with two different model tumors. The Gd content in the tumors and other organs were determined at various time after the injection. A sufficient amount of Gd was readily delivered into those two different model tumors. Increasing the dose of Gd by injecting the Gd-encapsulated liposomes multiple times tended to increase the uptake of the Gd by the tumors. Finally, the uptake of Gd by tumors was inversely correlated with the size of the tumors. The Gd-encapsulated liposomes hold great potentials as a Gd delivery system for NCT of small- and medium-size tumors. Alternative strategies may have to be adopted in order to use NCT to treat large, advanced solid tumors, although for which, Gd-NCT might be advantageous over boron-NCT.

Preparation, characterization, and biodistribution study of technetium-99m -labeled leuprolide acetate-loaded liposomes in Ehrlich ascites tumor-bearing mice.

The purpose of this study was to prepare conventional and sterically stabilized liposomes containing leuprolide acetate in an attempt to prolong the biological half life of the drug, to reduce the uptake by reticuloendothelial system (RES), and to reduce the injection frequency of intravenously administered peptide drugs. The conventional and sterically stabilized liposomes containing leuprolide acetate were prepared by reverse phase evaporation method and characterized for entrapment efficiency and particle size. Radiolabeling of leuprolide acetate and its liposomes was performed by direct labeling with reduced technetium-99m. Its biodistribution and imaging characteristics were studied in ehrlich ascites tumor (EAT)-bearing mice after labeling with technetium-99m. The systemic pharmacokinetic studies were performed in rabbits. A high uptake by tumor was observed by sterically stabilized liposome containing leuprolide acetate compared with free drug and conventional liposomes. The liver/tumor uptake ratio of free drug, conventional (LL), and sterically stabilized liposomes (SLL5000 and SLL2000) was found to be 20, 7.99, 1.63, and 1.23, respectively, which showed the increased accumulation of sterically stabilized liposomes in tumor compared with the free drug and conventional liposomes at 24 hours postinjection. Liver uptake of sterically stabilized liposomes was still 7-fold less than the conventional liposomes. The marked accumulation of liposomes in the tumor-bearing mice was also documented by gamma scintigraphic studies. The findings demonstrate the distribution of these liposomes within solid tumor and prove that the sterically stabilized liposomes experience increased tumor uptake and prolonged circulation half life. Hence these findings will be relevant for the optimal design of long circulating liposomes for the peptide drugs and for targeting of liposomes toward tumor.

Liposomes as drug delivery vehicles for boron agents.

The successful treatment of cancer by boron neutron capture therapy (BNCT) requires the selective concentration of boron-10 within malignant tumors. The potential of liposomes to deliver boron-rich compounds to tumors has been assessed by examination of the biodistribution of boron delivered by liposomes in tumor-bearing mice. Small unilamellar vesicles have been found to stably encapsulate high concentrations of water-soluble ionic boron compounds. Alternatively, lipophilic boron-containing species have been embedded within the phospholipid bilayer of liposomes, and both hydrophilic and lipophilic boron compounds have been incorporated within the same liposome formulation. The biodistribution of boron was determined at several time points over 48 hr after i.v. injection of liposomal suspensions in BALB/c mice bearing EMT6 tumors. The tumor-selective delivery of boron by the liposomes was demonstrated as tumor-boron concentrations increased for several hours post-injection. Even at the low injected doses employed (6-18 mg boron/kg body weight) therapeutic tumor boron concentrations were observed (> 30 micrograms boron/g tissue) and high tumor/blood ratios were achieved (> 5). The most favorable results were obtained with the polyhedral borane Na3[a2-B20H1-NH2CH2CH2NH2]. Liposomes encapsulating this species produced a tumor boron concentration of 45 micrograms/g tissue at 30 hr post-injection, at which time the tumor/blood boron ratio was 9.3.

The effect of cytokine binding liposomes in tumor bearing mice

The effect of cytokine binding liposomes in tumor bearing mice

Tumor imaging with technetium-99m-DTPA encapsulated in RES-avoiding liposomes

For passive targeting of liposomes to tumor tissues, we earlier developed reticuloendothelial system (RES)-avoiding liposomes modified with a uronic acid derivative, palmityl- d-glucuronide (PGlcUA). In the present study, we encapsulated technetium-99m ( 99mTc)-diethylenetriaminepentaacetic acid (DTPA) in PGlcUA-liposomes (dipalmitoylphosphatidylcholine:cholesterol:PGlcUA = 40:40:20 as a molar ratio) and studied the biodistribution of the liposomes in tumor-bearing mice. 99mTc-DTPA encapsulated in liposomes effectively accumulated in tumor tissues after intravenous administration. Corresponding to these results, tumor was strongly imaged by a γ-camera when 99mTc-DTPA-encapsulated PGlcUA-liposomes were used.

Model studies directed toward the boron neutron-capture therapy of cancer: boron delivery to murine tumors with liposomes.

The successful treatment of cancer by boron neutron-capture therapy (BNCT) requires the selective concentration of boron-10 within malignant tumors. The potential of liposomes to deliver boron-rich compounds to tumors has been assessed by the examination of the biodistribution of boron delivered by liposomes in tumor-bearing mice. Small unilamellar vesicles with mean diameters of 70 nm or less, composed of a pure synthetic phospholipid (distearoyl phosphatidylcholine) and cholesterol, have been found to stably encapsulate high concentrations of water-soluble ionic boron compounds. The hydrolytically stable borane anions B10H10(2-), B12H11SH2-, B20H17OH4-, B20H19(3-), and the normal form and photoisomer of B20H18(2-) were encapsulated in liposomes as their soluble sodium salts. The tissue concentration of boron in tumor-bearing mice was measured at several time points over 48 h after i.v. injection of emulsions of liposomes containing the borane anions. Although the boron compounds used do not exhibit an affinity for tumors and are normally rapidly cleared from the body, liposomes were observed to selectively deliver the borane anions to tumors. The highest tumor concentrations achieved reached the therapeutic range (greater than 15 micrograms of boron per g of tumor) while maintaining high tumor-boron/blood-boron ratios (greater than 3). The most favorable results were obtained with the two isomers of B20H18(2-). These boron compounds have the capability to react with intracellular components after they have been deposited within tumor cells by the liposome, thereby preventing the borane ion from being released into blood.

Tumor accumulation of novel RES-avoiding liposomes

For passive targeting of liposomes to tumor tissues, we earlier developed reticuloendotheelial system (RES)-avoiding liposomes modified with a uronic acid derivative, palmityl- d-glucuronide (PGlcUA) (Namba, Y., Sakakibara, T., Masada, M., Ito, F. and Oku, N. (1990) Chem. Pharm. Bull. 38, 1663–1666). In this present study, we examined the blood clearance and biodistribution of PGlcUA-liposomes (dipalmitoylphoshatidylcholine/cholesterol/PGlcUA = 40: 40: 20 as a molar ratio ) in normal and tumor-bearing mice. Liposomes containing dipalmitoylphophatidylglycerol (DPPG) instead of PGlcUA was also examined as a control. When [ 3H]inulin-encapsulated PGlcUA-liposomes and DPPG-liposomes were intravenously injected into normal mice, approx. 50% of the 3H radioactivity was recovered from the liver, the bulk of RES, at 12 h after administration of DPPG-liposomes, while only approx. 20% of it was found there when PGlcUA-liposomes were administered. Radioactivity remaining in the plasma at 12 h after injection was 5-fold higher when PGlcUA-liposomes were injected than when DPPG-liposomes were used. Biodistribution of liposomes in tumor-bearing mice was also examined. Mice were inoculated with 10 7 S180 cells into the hind leg. After 1 week, liposomes were injected. Radioactivity of [ 3H]inulin originally encapsulated in the PGlcUA-liposomes accumulated in the tumor to an extent 3–4-fold higher than that of the marker in DPPG-liposomes. Liver/tumor ration of the radioactivity was 12 for DPPG-liposomes and only 2 for PGlcUA-liposomes. This latter value is the lowest of various liposome formulations ever reported.