Improvement of biodistribution and therapeutic index via increase of polyethylene glycol on drug-carrying liposomes in an HT-29/luc xenografted mouse model.

Liposomes modified with a high concentration of polyethylene glycol (PEG) could significantly prolong the retention time of the carried drug in the circulation, thus improving the drug accumulation in the tumor. In this study, 6 mol% rather than 0.9 mol% PEGylated liposomes (100 nm in diameter) encapsulated with indium-111 were used in a human colorectal carcinoma HT-29/luc tumor-bearing mouse model for comparing the PEGylation effect. Pharmacokinetics, biodistribution, passive-targeted assay, bioluminescence imaging (BLI) and tumor growth measurements were used for the spatial and temporal distribution, tumor localization and therapeutic evaluation of the drug. Pharmacokinetic studies indicated that the terminal half-life (T((1/2))lambdaz) and C(max) of 6 mol% PEG (111)In liposomes were similar to those of 0.9 mol% PEG (111)In liposomes. In the blood, the total body clearance (Cl) of 6 mol% PEG (111)In liposomes was about 1.7-fold lower and the area under the curve (AUC) was 1.7-fold higher than those of 0.9 mol% PEG (111)In liposomes. These results showed that the long-term circulation and localization of 6 mol% PEGylated liposomes was more appropriate for use in the tumor-bearing animal model. In addition, the biodistribution of 6 mol% PEG (111)In liposomes showed significantly lower uptake in the liver, spleen, kidneys, small intestine and bone marrow than those of 0.9 mol% PEG (111)In liposomes. The clearance rate of both drugs from the blood decreased with time, with the maximum at 24 h post intravenous (i.v.) injection. Prominent tumor uptake and the highest tumor/muscle ratios were found at 48 h post injection. Both AUC and relative ratio of the AUCs (RR-AUC) also showed that 6 mol% PEGylated liposomes significantly reduced the uptake of drugs in the reticuloendothelial system (RES), yet enhanced the uptake in the tumor. Gamma scintigraphy at 48 h post injection also demonstrated more distinct tumor uptake with 6 mol% PEG (111)In liposomes as compared to that of 0.9 mol% PEGylated liposomes (p<0.01). BLI and in vivo tumor growth tracing showed that growth in tumor volume could largely be inhibited by 6 mol% PEG (111)In liposomes. The results suggest that 6 mol% PEGylated liposomes might be a more suitable liposomal carrier for drug delivery than 0.9 mol% PEGylated liposomes, not only by reducing the drug accumulation in the RES or its related organs, but by prolonging drug circulation and eventually enhancing the targeting efficiency in the tumor to reach a better therapeutic index.