Search

Injectable drug delivery systems (DDS) such as particulate carriers and water-soluble polymers are being used and developed for a wide variety of therapeutic applications. However, a number of immunological risks with serious clinical implications are associated with administration of DDS. These immunological events can compromise the efficacy and safety of these systems by changing the pharmacokinetics, biodistribution and targeting capability of DDS, and by inducing hypersensitivity reactions. Antibodies induced by administration of DDS can be directed against the carrier material, the drug and/or targeting ligands associated with the DDS. Complement activation and opsonization of DDS, which may or may not be associated with antibody formation, may lead to accelerated clearance, hypersensitivity reactions and formation of membrane attack complexes resulting in premature release of the drug. Also platelets have been reported to play a role in DDS immunogenicity. Despite our curtailed understanding of the relationships between physicochemical characteristics and immunogenicity of DDS, several risk factors have been identified. Insight into these factors should be employed in the development of novel DDS with low immunological risk.

The chemical and enzymatic degradation of monodisperse oligo(e‐caprolactone) (OCL) and its amphiphilic block oligomer with methoxy poly(ethylene glycol) (mPEG) were investigated in order to obtain insight into the degradation of mPEG‐ b ‐OCL micelles. Hydrolytic degradation was studied as function of pH and dielectric constant of the medium, and enzymatic degradation was investigated at different enzyme and substrate concentrations. The degradation was monitored by HPLC and MS, and the micelle destabilization with DLS. It was found that the hydrolytic degradation followed pseudo first order kinetics, and that the rate depended on the pH and dielectric constant. Degradation essentially occurred via a random scission process, and induced micelle destabilization after approximately 1.5 degradation half‐lives. At physiological pH and temperature, OCLs are very stable, reflected by an estimated degradation half‐life of mPEG‐ b ‐OCL micelles of several years. However, the presence of lipase resulted in an accelerated degradation with half‐lives of a few days to hours. The enzymatic degradation of mPEG‐ b ‐OCL followed Michaelis–Menten kinetics. The results indicate that mPEG‐ b ‐OCL micelles are very stable in vitro , but their susceptibility to enzymes such as lipase makes these systems suitable for the hydrolysis controlled release of drugs in vivo . © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:506–518, 2008

A promising strategy to improve the immunogenic potential of DNA vaccines is the formulation of plasmid DNA (pDNA) with cationic liposomes. In this respect, particle size may be of crucial importance. This study aimed at the evaluation of high-pressure extrusion as a method for sizing cationic liposomes after entrapment of pDNA. This is a well-known sizing method for liposomes, but so far, it has not been applied for liposomes that are already loaded with pDNA. Liposomes composed of egg PC, DOTAP, and DOPE with entrapped pDNA were prepared by the dehydration-rehydration method and subjected to various extrusion cycles, comparing different membrane pore sizes and extrusion frequencies. At optimized extrusion conditions, liposome diameter (Zave) and polydispersity index (PDI) were reduced from 560 nm and 0.56 to 150 nm and 0.14 respectively, and 35% of the pDNA was retained. Importantly, gel electrophoresis and transfection experiments with pDNA extracted from these extruded liposomes demonstrated the preservation of the structural and functional integrity of the pDNA. The reduction in size resulted in enhanced transfection of HeLa cells, as detected by functional expression of the fluorescent protein, eGFP. In addition, these liposomes were able to stimulate Toll-like receptor 9, indicating efficient endosomal uptake and release of the included pDNA. In conclusion, high-pressure extrusion is a suitable technique to size cationic liposomes with entrapped pDNA and allows preparation of well-defined nanosized pDNA-liposomes, with preserved pDNA integrity. Their improved transfection efficiency and ability to activate an important pattern-recognition receptor are favorable properties for DNA vaccine delivery vehicles.

The formulation of plasmid DNA (pDNA) in cationic liposomes is a promising strategy to improve the potency of DNA vaccines. In this respect, physicochemical parameters such as liposome size may be important for their efficacy. The aim of the current study was to investigate the effect of vesicle size on the in vivo performance of liposomal pDNA vaccines after subcutaneous vaccination in mice. The tissue distribution of cationic liposomes of two sizes, 500 nm (PDI 0.6) and 140 nm (PDI 0.15), composed of egg PC, DOPE and DOTAP, with encapsulated OVA-encoding pDNA, was studied by using dual radiolabeled pDNA-liposomes. Their potency to elicit cellular and humoral immune responses was investigated upon application in a homologous and heterologous vaccination schedule with 3 week intervals. It was shown that encapsulation of pDNA into cationic lipsomes resulted in deposition at the site of injection, and strongest retention was observed at large vesicle size. The vaccination studies demonstrated a more robust induction of OVA-specific, functional CD8(+) T-cells and higher antibody levels upon vaccination with small monodisperse pDNA-liposomes, as compared to large heterodisperse liposomes or naked pDNA. The introduction of a PEG-coating on the small cationic liposomes resulted in enhanced lymphatic drainage, but immune responses were not improved when compared to non-PEGylated liposomes. In conclusion, it was shown that the physicochemical properties of the liposomes are of crucial importance for their performance as pDNA vaccine carrier, and cationic charge and small size are favorable properties for subcutaneous DNA vaccination. (C) 2011 Elsevier Ltd. All rights reserved.

Intravenous injection of an endotoxin-free solution of poloxamine-908 to rats can enhance the phagocytic clearance capacity of tissue macrophages, particularly those of the liver and the spleen. Such stimulated cells were able to clear a significant portion of intravenously injected methoxypoly(ethyleneglycol)2000 liposomes (mean size of 87 nm), labelled with technetium-99m via the N-hydroxysuccinimidyl hydrazine nicotinate hydrochloride derivative of distearoyl phosphatidylethanolamine, within 4 h post administration. These liposomes, otherwise, exhibit long circulatory behaviour in control animals, with poor localization to the liver and spleen. We suggest that such technetium-99m-labelled engineered vesicles may be of aid for detection of the liver and spleen macrophages with enhanced phagocytic clearance capacity by gamma scintigraphy. Alterations in the phagocytic activity of liver and spleen macrophages is known to occur during cancer. Therefore, such diagnostic procedures may prove useful for patient selection or for monitoring the progress of treatment with long circulating nanoparticles carrying anti-cancer agents, thus minimizing damage to this important line of body’s defence cells, and are discussed.