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 λz) 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. Liposomes are polymeric nanoparticles which consist of one or more concentric phospholipid bilayers and are widely used as carriers for diagnostic and therapeutic agents (1-5). Liposomes comprise a promising drug delivery system owing to their slow drug release, and spatial and temporal distribution of drugs for targeted therapies. Liposomal carriers could be used to effectively encapsulate chemotherapeutic drugs for cancer therapy, antisense oligonucleotides for gene therapy, peptides for the treatment of infectious diseases, antigens to stimulate immune response, as well as radiopharmaceuticals for targeted imaging and radiotherapy (6-8). These encapsulated agents showed some improvement in pharmacokinetic stability, better biodistribution i.e. higher accumulation in target organs and lower normal tissue