Clearance properties of liposomes involving conjugated proteins for targeting

Liposomal doxorubicin for active targeting: surface modification of the nanocarrier evaluated in vitro and in vivo: challenges and prospects

Due to the inability of classical chemotherapeutic agents to exclusively target tumor cells, these treatments are associated with severe toxicity profiles. Thus, long-circulating liposomes have been developed in the past to enhance accumulation in tumor tissue by passive targeting. Accordingly, commercially available liposomal formulations of sterically stabilized liposomal doxorubicin (Caelyx^®, Doxil^®, Lipodox^®) are associated with improved off-target profiles. However, these preparations are still not capable to selectively bind to target cells. Thus, in an attempt to further optimize existing treatment schemes immunoliposomes have been established to enable active targeting of tumor tissues. Recently, we have provided evidence for therapeutic efficacy of anti-IGF1R-targeted, surface modified doxorubicin loaded liposomes. Our approach involved a technique, which allows specific post-modifications of the liposomal surface by primed antibody-anchor conjugates thereby facilitating personalized approaches of commercially available liposomal drugs. In the current study, post-modification of sterically stabilized liposomal Dox was thoroughly investigated including the influence of different modification techniques (PIT, SPIT, SPIT60), lipid composition (SPC/Chol, HSPC/Chol), and buffers (HBS, SH). As earlier in vivo experiments did not take into account the presence of non-integrated ab-anchor conjugates this was included in the present study. Our experiments provide evidence that post-modification of commercially available liposomal preparations for active targeting is possible. Moreover, lyophilisation represents an applicable method to obtain a storable precursor of surface modifying antibody-anchor conjugates. Thus, these findings open up new approaches in patient individualized targeting of chemotherapeutic therapies.

The Role of Cell-Penetrating Peptide and Transferrin on Enhanced Delivery of Drug to Brain

The challenge of effectively delivering therapeutic agents to brain has led to an entire field of active research devoted to overcome the blood brain barrier (BBB) and efficiently deliver drugs to brain. This review focusses on exploring the facets of a novel platform designed for the delivery of drugs to brain. The platform was constructed based on the hypothesis that a combination of receptor-targeting agent, like transferrin protein, and a cell-penetrating peptide (CPP) will enhance the delivery of associated therapeutic cargo across the BBB. The combination of these two agents in a delivery vehicle has shown significantly improved (p < 0.05) translocation of small molecules and genes into brain as compared to the vehicle with only receptor-targeting agents. The comprehensive details of the uptake mechanisms and properties of various CPPs are illustrated here. The application of this technology, in conjunction with nanotechnology, can potentially open new horizons for the treatment of central nervous system disorders.

Engineering anti-GD2 monoclonal antibodies for cancer immunotherapy

Ganglioside GD2 is highly expressed on neuroectoderm-derived tumors and sarcomas, including neuroblastoma, retinoblastoma, melanoma, small cell lung cancer, brain tumors, osteosarcoma, rhabdomyosarcoma, Ewing's sarcoma in children and adolescents, as well as liposarcoma, fibrosarcoma, leiomyosarcoma and other soft tissue sarcomas in adults. Since GD2 expression in normal tissues is restricted to the brain, which is inaccessible to circulating antibodies, and in selected peripheral nerves and melanocytes, it was deemed a suitable target for systemic tumor immunotherapy. Anti-GD2 antibodies have been actively tested in clinical trials for neuroblastoma for over the past two decades, with proven safety and efficacy. The main limitations have been acute pain toxicity associated with GD2 expression on peripheral nerve fibers and the inability of antibodies to treat bulky tumor. Several strategies have been developed to reduce pain toxicity, including bypassing complement activation, using blocking antibodies, or targeting of O-acetyl-GD2 derivative that is not expressed on peripheral nerves. To enhance anti-tumor efficacy, anti-GD2 monoclonal antibodies and fragments have been engineered into immunocytokines, immunotoxins, antibody drug conjugates, radiolabeled antibodies, targeted nanoparticles, T-cell engaging bispecific antibodies, and chimeric antigen receptors. The challenges of these approaches will be reviewed to build a perspective for next generation anti-GD2 therapeutics in cancer therapy.

Nanocarrier-mediated targeting of tumor and tumor vascular cells improves uptake and penetration of drugs into neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in children, accounting for about 8% of childhood cancers. Despite aggressive treatment, patients suffering from high-risk NB have very poor 5-year overall survival rate, due to relapsed and/or treatment-resistant tumors. A further increase in therapeutic dose intensity is not feasible, because it will lead to prohibitive short-term and long-term toxicities. New approaches with targeted therapies may improve efficacy and decrease toxicity. The use of drug delivery systems allows site specific delivery of higher payload of active agents associated with lower systemic toxicity compared to the use of conventional (“free”) drugs. The possibility of imparting selectivity to the carriers to the cancer foci through the use of a targeting moiety (e.g., a peptide or an antibody) further enhances drug efficacy and safety. We have recently developed two strategies for increasing local concentration of anti-cancer agents, such as CpG-containing oligonucleotides, small interfering RNAs, and chemotherapeutics in NB. For doing that, we have used the monoclonal antibody anti-disialoganglioside (GD₂), able to specifically recognize the NB tumor and the peptides containing NGR and CPRECES motifs, that selectively bind to the aminopeptidase N-expressing endothelial and the aminopeptidase A-expressing perivascular tumor cells, respectively. The review will focus on the use of tumor- and tumor vasculature-targeted nanocarriers to improve tumor targeting, uptake, and penetration of drugs in preclinical models of human NB.

Cell penetrating peptide tethered bi-ligand liposomes for delivery to brain in vivo: Biodistribution and transfection

Targeted nano-particulate systems hold extraordinary potential for delivery of therapeutics across blood brain barrier (BBB). In this work, we investigated the potential of novel bi-ligand (transferrin-poly-l-arginine) liposomal vector for delivery of desired gene to brain, in vivo. The in vivo evaluation of the delivery vectors is essential for clinical translation. We followed an innovative approach of combining transferrin receptor targeting with enhanced cell penetration to design liposomal vectors for improving the transport of molecules into brain. The biodistribution profile of 1, 1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine iodide(DiR)-labeled liposomes was evaluated in adult rats after single intravenous injection at dose of 15.2μmoles of phospholipids/kg body weight. We demonstrated that bi-ligand liposomes accumulated in rat brain at significantly (p<0.05) higher concentrations as compared to the single-ligand (transferrin) or plain liposomes. In addition, the bi-ligand liposomes resulted in increased expression of β-galactosidase(β-gal) plasmid in rat brain tissue in comparison to the single-ligand liposomes. Histological examination of the transfected tissues did not show any signs of tissue necrosis or inflammation. Hemolysis assay further authenticated the biocompatibility of bi-ligand liposomes in blood up to 600 nmoles of phospholipids/1.4×10(7) erythrocytes. The findings of this study provide important and detailed information regarding the distribution of bi-ligand liposomes in vivo and accentuate their ability to demonstrate improved brain penetration and transfection potential over single-ligand liposomes.

Liposomal doxorubicin-based treatment in a preclinical model of adrenocortical carcinoma

Adrenocortical carcinoma (ACC) is a rare endocrine tumor entity with poor prognosis. Medical treatment is limited to common cytotoxic agents, which are associated with low treatment responses. Thus, lack of therapeutic efficacy demands innovative treatment options for patients with advanced ACC. Recently, we have developed and characterized anti-IGF1 receptor (IGF1-R) immunoliposomes (SSLD-1H7) for the treatment of neuroendocrine tumors of the gastroenteropancreatic system. As previous results indicated putative applicability also for other IGF1-R-overexpressing tumor entities, we initiated testing of liposomal preparations in in vitro and in vivo models of ACC. Adrenocortical NCIh295 cells were used for in vitro association studies with different liposomal formulations. Thereby, flow cytometry revealed high cellular association and internalization of anti-IGF1-R immunoliposomes (soy phosphatidylcholine (SPC)/cholesterol (Chol)-polyethyleneglycol (PEG)-1H7, 50.1±2.2%). Moreover, internalization of pegylated liposomes (SPC/Chol-PEG, 57.1±2.4%) and an even higher uptake of plain liposomes (84.6±0.8%; P<0.0001) were detectable in adrenocortical tumor cells. In vivo, liposomal treatments were investigated on NCIh295 tumor xenografts in pharmacokinetic and therapeutic experiments. A significant reduction in tumor size was detectable in NCIh295 tumor-bearing mice after a single treatment with SSLD-1H7 (0.89±0.15 cm; P=0.006) and a diminished efficacy for SSLD-PEG+ (1.01±0.19 cm; P=0.04) in comparison with untreated controls (1.5±0.0 cm). Thus, anti-IGF1-R immunoliposomes have been successfully tested in vitro and in vivo in a preclinical model for ACCs and could, therefore, represent a promising therapeutic approach for this tumor entity. Moreover, a combination of mitotane plus liposomally encapsulated cytostatic agents instead of free drugs could also be an interesting novel treatment option for ACC in the future.

PEGylated liposomal doxorubicin targeted to α5β1-expressing MDA-MB-231 breast cancer cells

Targeting drugs selectively to cancer cells can potentially benefit cancer patients by avoiding side effects generally associated with several cancer therapies. One of the attractive approaches to direct the drug cargo to specific sites is to incorporate ligands at the surface of the delivery systems. Integrin α(5)β(1) is overexpressed in tumor vasculature and cancer cells, thus making it an attractive target for use in drug delivery. Our group has developed a fibronectin-mimetic peptide, PR_b, which has been shown to bind specifically to integrin α(5)β(1), thereby providing a tool to target α(5)β(1)-expressing cancer cells in vitro as well as in vivo. Our current work focuses on designing modified stealth liposomes (liposomes functionalized with polyethylene glycol, PEG) for combining the benefits associated with PEGylation, as well as imparting specific targeting properties to the liposomes. We have designed PEGylated liposomes that incorporate in their bilayer the fibronectin-mimetic peptide-amphiphile PR_b that can target several cancer cells that overexpress α(5)β(1), including the MDA-MB-231 breast cancer cells used in this study. We have encapsulated doxorubicin inside the liposomes to enhance its therapeutic potential via PEGylation as well as active targeting to the cancer cells. Our results show that PR_b-functionalized stealth liposomes were able to specifically bind to MDA-MB-231 cells, and the binding could be controlled by varying the peptide concentration. The intracellular trafficking of the doxorubicin liposomes was examined, and within minutes after delivery the majority of them were found to be in the early endosomes, whereas after a longer period of time they had accumulated in the late endosomes and lysosomes. The functionalized liposomes were found to be equally cytotoxic as the free doxorubicin, especially at higher doxorubicin concentrations, and provided higher cytotoxicity than the nontargeted and GRGDSP-functionalized stealth liposomes. Thus, the PR_b-functionalized PEGylated nanoparticles examined in this study offer a promising strategy to deliver their therapeutic payload directly to the breast cancer cells, in an efficient and specific manner.

Aptamer-conjugated nanobubbles for targeted ultrasound molecular imaging

Targeted ultrasound contrast agents can be prepared by some specific bioconjugation techniques. The biotin-avidin complex is an extremely useful noncovalent binding system, but the system might induce immunogenic side effects in human bodies. Previous proposed covalently conjugated systems suffered from low conjugation efficiency and complex procedures. In this study, we propose a covalently conjugated nanobubble coupling with nucleic acid ligands, aptamers, for providing a higher specific affinity for ultrasound targeting studies. The sgc8c aptamer was linked with nanobubbles through thiol-maleimide coupling chemistry for specific targeting to CCRF-CEM cells. Further improvements to reduce the required time and avoid the degradation of nanobubbles during conjugation procedures were also made. Several investigations were used to discuss the performance and consistency of the prepared nanobubbles, such as size distribution, conjugation efficiency analysis, and flow cytometry assay. Further, we applied our conjugated nanobubbles to ex vivo ultrasound targeted imaging and compared the resulting images with optical images. The results indicated the availability of aptamer-conjugated nanobubbles in targeted ultrasound imaging and the practicability of using a highly sensitive ultrasound system in noninvasive biological research.

Nanomaterials: applications in cancer imaging and therapy

The application of nanomaterials (NMs) in biomedicine is increasing rapidly and offers excellent prospects for the development of new non-invasive strategies for the diagnosis and treatment of cancer. In this review, we provide a brief description of cancer pathology and the characteristics that are important for tumor-targeted NM design, followed by an overview of the different types of NMs explored to date, covering synthetic aspects and approaches explored for their application in unimodal and multimodal imaging, diagnosis and therapy. Significant synthetic advances now allow for the preparation of NMs with highly controlled geometry, surface charge, physicochemical properties, and the decoration of their surfaces with polymers and bioactive molecules in order to improve biocompatibility and to achieve active targeting. This is stimulating the development of a diverse range of nanometer-sized objects that can recognize cancer tissue, enabling visualization of tumors, delivery of anti-cancer drugs and/or the destruction of tumors by different therapeutic techniques.

A maleimide-based in-vitro model for ultrasound targeted imaging

Intricate variations and poor visual access result in the difficulty in studying the property of adherent targeted bubbles using an in vivo model. Here, we propose a simple in-vitro model based on the natural adhesion of maleimide bubbles to gelatin. We validated the maleimide-mediated bubble adhesion using flat gelatin phantoms. Treating the gelatin surfaces with reducing agent yielded abundant cysteine molecules for attaching maleimide bubbles. An optical microscope and a homemade ultrasound imaging system equipped with a 40-MHz transducer were adopted to observe the acoustic responses of adherent bubbles. Comparing the results of optical observations from experimental and control tests support the bubble adhesion indeed relying on maleimide-cysteine tethering. The intensity of the echoes from a bubble-bound gelatin surface increased with the bubble adhesion density compared with that from a clear gelatin surface. The echo enhancement reached a plateau at 40-42 dB as the bubble adhesion densities were higher than 1.47 × 10(5) bubbles/mm(2). The adherent bubbles would be disrupted rapidly under the exposure of 300-kPa ultrasound pulses. However, increasing the adhesion density to 3.62 × 10(5) bubbles/mm(2) resulted in the echo enhancement being maintained at a duration of 40 min. The advantages of this in-vitro model over previously proposed ones include better stability, less expense, and fewer preparation procedures.

In vivo evaluation of pH-sensitive polymer-based immunoliposomes targeting the CD33 antigen

The purpose of this study was to evaluate in vivo a targeted pH-sensitive liposomal formulation tailored to promote the efficient intracellular delivery of 1-beta-d-arabinofuranosylcytosine (ara-C) to human myeloid leukemia cells. Specifically, pH-sensitive immunoliposomes were obtained by anchoring a copolymer of dioctadecyl, N-isopropylacrylamide and methacrylic acid in bilayers of PEGylated liposomes (LP) and by coupling the whole anti-CD33 monoclonal antibody (mAb) or its Fab' fragments. Their pharmacokinetic and biodistribution profiles were assessed in Balb/c and leukemic HL60-bearing immunodepressed (SCID) mice. In naive mice, nontargeted and pH-sensitive Fab'-LP had longer circulation times than LP with whole mAb. In SCID/HL60 (CD33(+)) mice, the pharmacokinetic and biodistribution profiles of LP and encapsulated ara-C were comparable between nontargeted and pH-sensitive Fab'-LP. In leukemic mice, only pH-insensitive, ara-C-loaded Fab' induced prolonged survival times. The apparent absence of pH-sensitive Fab'-LP effect could be related to lower exposure to ara-C in SCID mice.

Enhanced antitumor effect of novel dual-targeted paclitaxel liposomes

A novel dual-targeted peptide containing an alpha V integrins specific ligand and a neuropilin-1 specific motif was developed which showed an increased specific targeting affinity to tumors. Active dual-targeted liposomes were then produced with this peptide and exhibited greater binding activity than single-targeted liposomes in vitro. Paclitaxel entrapped in this formulation greatly increased the uptake of paclitaxel in the targeting cells and significantly suppressed the growth of HUVEC and A549 cells compared with general paclitaxel injections (Taxol) and single-targeted paclitaxel liposomes. The treatment of tumor xenograft models with dual-targeted paclitaxel liposomes also resulted in better tumor growth inhibition than any other treatment groups. Therefore, the dual-targeted paclitaxel liposomes prepared in the present study might be a more promising drug for cancer treatment. Furthermore, the dual-targeting approach may produce synergistic effects that can be applied in the development of new targeted drug delivery systems.

Curvature-tuned preparation of nanoliposomes

Numerous methods have been reported for the preparation of liposomes, many of which, in addition to requiring time-consuming preparative steps and the use of organic solvents, result in heterogeneous liposome populations of incontrollable size. Taking into consideration the phenomenon of spontaneous vesiculation and the theory of curvature, here we present an extremely rapid and simple, solvent-free method for the preparation of monodisperse solutions of highly stable small unilamellar vesicles using both charged and zwitterionic lipids mixed with lyso-palmitoylphosphatidylcholine, exploiting a combination of a rapid pH change followed by a defined period of equilibration. Various experimental parameters and their interactions were evaluated in terms of their effect on resulting liposome size and shape, as well as on liposome stability and size distribution, with transmission electron microscope imaging being used to visualize the formed liposomes, and photon correlation spectroscopy to obtain statistical data on mean diameter and monodispersity of the liposome population. zeta potential measurements also provided information about the interpretation of vesiculation kinetics and liposome stability. The time interval of pH jump, operation temperature, equilibration time, and lipid type were shown to be the determining factors controlling the size, shape, and monodispersity of the liposomes. Buffer type was also found to be important for the long-term storage of the liposomes. Ongoing work is looking at the application of the developed method for encapsulation of bioactive molecules, such as drugs, genetic materials, and enzymes.

pH-sensitive immunoliposomes specific to the CD33 cell surface antigen of leukemic cells

A promising avenue in cancer therapy using liposomal formulations is the combination of site-specific delivery with triggered drug release. The use of trigger mechanisms in liposomes could be relevant for drugs susceptible to lysosomal hydrolytic/enzymatic degradation. Here, we propose a polymeric pH-sensitive liposome system that is designed to release its content inside the endosomes through a polymer structural change following receptor-mediated internalization. Specifically, pH-sensitive immunoliposomes (ILs) were obtained by including a terminally alkylated copolymer of N-isopropylacrylamide (NIPAM) in the liposome bilayer and by coupling the anti-CD33 monoclonal antibody to target leukemic cells. In vitro release of encapsulated fluorescent probes and cytosine arabinoside (ara-C) revealed that pH-sensitivity of the vector was retained in the presence of the antibody upon incubation in plasma. Flow cytometry and confocal microscopy analyses demonstrated that the pH-sensitive ILs were efficiently internalized by various CD33+ leukemic cell lines while limited interaction was found for liposomes decorated with an isotype-matched control antibody. Finally, the pH-sensitive ILs-CD33 formulation exhibited the highest cytotoxicity against HL60 cells, confirming the role of the NIPAM copolymer in promoting the escape of intact ara-C in the endosomes. These results suggest that this pH-sensitive liposomal formulation could be beneficial in the treatment of acute myeloid leukemia.

Testis specific lactate dehydrogenase as target for immunoliposomes

PROBLEM

Is it possible to deliver therapeutic agents to testis through specific targeting?

METHOD OF STUDY

Immunoliposomes are designed by incorporating antibodies to lactate dehydrogenase-C4 (LDH-C(4)), which is the product of a testis specific gene. Their targeting and delivering ability is investigated in vitro and in vivo.

RESULTS

It is observed that LDHC(4)-immunoliposomes are able to discriminate and recognize antigens on spermatozoa and testes both in vitro and in vivo.

CONCLUSION

Specific targeting through LDH-C(4) appears to be a feasible strategy for delivering therapeutic as well as anti-spermatogenic agents to testis.

Nanoengineering artificial lipid envelopes around adenovirus by self-assembly

We have developed a novel, reproducible, and facile methodology for the construction of artificial lipid envelopes for adenoviruses (Ad) by self-assembly of lipid molecules around the viral capsid. No alteration of the viral genome or conjugation surface chemistry at the virus capsid was necessary, therefore difficulties in production and purification associated with generating most surface-modified viruses can be eliminated. Different lipid bilayer compositions produced artificially enveloped Ad with physicochemical and biological characteristics determined by the type of lipid used. Physicochemical characteristics such as vector size, degree of aggregation, stability, and surface charge of the artificially enveloped Ad were correlated to their biological (gene transfer) function. In monolayer cell cultures, binding to the coxsackie and adenovirus receptor (CAR) was blocked using a zwitterionic envelope, whereas enhanced binding to the cell membrane was achieved using a cationic envelope. Envelopment of Ad by both zwitterionic and cationic lipid bilayers led to almost complete ablation of gene expression in cell monolayers, due to blockage of virion endosomal escape. Alternatively, artificial Ad envelopes built from lipid bilayers at the fluid phase in physiological conditions led to enhanced penetration of the vectors inside a three-dimensional tumor spheroid cell culture model and delayed gene expression in the tumor spheroid compared to nonenveloped adenovirus. These results indicate that construction of artificial envelopes for nonenveloped viruses by lipid bilayer wrapping of the viral capsids constitutes a general strategy to rationally engineer viruses at the nanoscale with control over their biological properties.

Clostridium neurotoxin fragments as potential targeting moieties for liposomal gene delivery to the CNS

Targeted transfection of the CNS with synthetic, nonviral vectors represents a huge technical challenge. The approach explored here attempts to combine self-assembly ABCD nanoparticles (Kostarelos and Miller, Chem. Soc. Rev. 2005, 34, 970), with the potential of Clostridium neurotoxin fragments to effect receptor-mediated transfection of neuronal cells. Cationic liposome-plasmid DNA complexes were first modified with a PEG stealth layer, before the addition of C-terminal fragments of tetanus toxin (TH(C)), botulinum toxin (BH(C)) or the truncated C-terminal domain of TH(C) as biological "targeting" ligands. First-generation nanoparticles were identified for the transfection of two neuronal cell lines (human SH-5YSY and rat/mouse hybrid N18-RE105); control studies were also performed with HeLa cells. ABCD nanoparticle transfections of the neuronal cell lines were up to 30-fold higher than corresponding control transfections with nanoparticles that lacked the protein ligand. We also demonstrate apparent receptor-mediated uptake by means of competition-binding and real-time confocal experiments. By contrast, nanoparticle transfection of HeLa cells appeared to involve alternative nonspecific enhanced cellular uptake mechanism(s). Receptor-mediated and nonspecific mechanisms appear to be in competition, potentially harming the capacity of receptor-mediated delivery to effect proper targeted delivery of nucleic acids to cells ex vivo and in vivo.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Design of targeted lipid nanocapsules by conjugation of whole antibodies and antibody Fab’ fragments

Immunonanocapsules were synthesized by conjugation to lipid nanocapsules (LNC) of whole OX26 monoclonal antibodies (OX26 MAb) directed against the transferrin receptor (TfR). The TfR is overexpressed on the cerebral endothelium and mediates the transcytosis mechanism. Fab' fragments, known for their reduced interaction with the reticuloendothelial system, were also conjugated to LNC. This coupling was facilitated by the incorporation of lipid PEG(2000) functionalized with reactive-sulfhydryl maleimide groups (DSPE-PEG(2000)-maleimide) into LNC shells by a post-insertion procedure, developed initially for liposome pegylation. An interfacial model using the dynamic rising drop technique helped determine the parameters influencing the DSPE-PEG(2000)-maleimide insertion and the quality of the anchorage. Heat was essential to promote both an important and stable adsorption of DSPE-PEG(2000)-maleimide onto LNC. OX26 MAb were thiolated to react with maleimide functions whereas thiol residues on Fab' fragments were used directly. The number of ligands per nanocapsule was adjusted according to their initial quantity in the coupling reaction mixture, with densities from 16 to183 whole antibodies and between 42 and 173 Fab' fragments per LNC. The specific association of immunonanocapsules to cells overexpressing TfR was thus demonstrated, suggesting their ability to deliver drugs to the brain.

Active targeting of brain tumors using nanocarriers

The delivery of drugs to brain tumors is limited by the presence of the blood-brain barrier (BBB) separating the blood from the cerebral parenchyma. An understanding of the specific mechanisms of the brain capillary endothelium has led to the development of various strategies to enhance the penetration of drugs into the brain tissue. Active targeting is a non-invasive approach, which consists in transporting drugs to target organs using site-specific ligands. Drug-loaded nanocarriers capable of recognizing brain capillary endothelial cells and cerebral tumoral cells have shown promising potential in oncology. Endogenous and chimeric ligands binding to carriers or receptors of the BBB have been directly or indirectly conjugated to nanocarriers. This review indexes the main targeted colloidal systems used for drug delivery to the brain. Their pharmacological behavior and their therapeutic effect are discussed.

The use of immunoliposome for nutrient target regulation (a review)

Although research on the role of genetically engineered antibodies and liposomes in the immunology or the nutrition field is extensive, there is no case for immunoliposome to nutrient target regulation. It is known that liposomes are spherical particles that encapsulate a fraction of the solvent, in which they freely diffuse (float) into their interior. Therefore, identification of immunoliposomes in hypothalamic site or intestinal epithelial cells that are differentially regulated by liposomes encapsulating nutrients or drugs will be an important step toward understanding the role of immunoliposomes in nutrition regulation progression and ingredient stability. Consequently, a useful model (immunoliposomal nutrient delivery system, ILNDS) of nutrient target regulation via immunoliposomes is designed to regulate the endocrine system effectively. This review focuses on antibody libraries' construction, display and selection, a brief introduction of immunoliposome, and how to use ILNDS for nutrient target regulation.

Development of ligand-targeted liposomes for cancer therapy

The continued evolution of targeted liposomal therapeutics has resulted in new agents with remarkable antitumour efficacy and relatively mild toxicity profiles. A careful selection of the ligand is necessary to reduce immunogenicity, retain extended circulation lifetimes, target tumour-specific cell surface epitopes, and induce internalisation and subsequent release of the therapeutic substance from the liposome. Methods for assembling targeted liposomes, including a novel micellar insertion technology, for incorporation of targeting molecules that efficiently transforms a non-targeted liposomal therapeutic to a targeted one, greatly assist the translation of targeted liposome technology into the clinic. Targeting strategies with liposomes directed at solid tumours and vascular targets are discussed. The authors believe the development of ligand-targeted liposomes is now in the advanced stage and offers unique and important advantages among other targeted therapies. Anti-HER2 immunoliposomal doxorubicin is awaiting Phase I clinical trials, the results of which should provide new insights into the promise of ligand-targeted liposomal therapies.

Current methods for attaching targeting ligands to liposomes and nanoparticles

Liposomes and nanoparticles have emerged as versatile carrier systems for delivering active molecules in the organism. These colloidal particles have demonstrated enhanced efficacy compared to conventional drugs. However, the design of liposomes and nanoparticles with a prolonged circulation time and ability to deliver active compounds specifically to target sites remains an ongoing research goal. One interesting way to achieve active targeting is to attach ligands, such as monoclonal antibodies or peptides, to the carrier. These surface-bound ligands recognize and bind specifically to target cells. To this end, various techniques have been described, including covalent and noncovalent approaches. Both in vitro and in vivo studies have proved the efficacy of the concept of active targeting. The present review summarizes the most common coupling techniques developed for binding homing moieties to the surface of liposomes and nanoparticles. Various coupling methods, covalent and noncovalent, will be reviewed, with emphasis on the major differences between the coupling reactions, on their advantages and drawbacks, on the coupling efficiency obtained, and on the importance of combining active targeting with long-circulating particles.

Biodistribution Study of Doxorubicin Encapsulated in Liposomes: Influence of Peptide Coating and Lipid Composition

In this paper we describe the biodistribution of doxorubicin (DXR) encapsulated in three different types of liposomes. Common composition was hydrogenated phosphatidylcholine (HPC)/phosphatidylglycerol (PG) cholesterol (Chol)/X, X being either 10% N-glutaryl phosphatidylethanolamine (NGPE), 10% NGPE + 6% distearoyl-phosphatidylethanolamine-polyethyleneglycol 2000 (DSPE-PEG), or 10% NGPE + 6% DSPE-PEG-COOH. These series of vesicles were coated with an active or an inactive sequence of laminin (laminin receptors, integrins, are overexpressed in tumor cells). Single doses of these preparations were injected, i.v., into healthy mice. For biodistribution experiments, mice were sacrificed at three different time-points post-treatment. Doxorubicin and doxorubicinol (DXOH) levels were determined in plasma, heart, lung, kidney, spleen, and liver using HPLC with daunorubicin (DNR) as internal standard. The results obtained indicate that compositions containing DSPE-PEG have the longest half-lives in plasma, as was to be expected according to the data in the literature. However, the presence of the peptides on the surface of liposomes reduces concentration values in this tissue. Distribution in other organs reveals high differences, among the liposomal samples studied, depending mainly on the presence of active or inactive peptide on the surface of vesicles. Liposomes coated with the laminin active sequence show lower accumulation in studied tissues than free DXR. This indicates that heart toxicity, associated to DXR treatments, could be diminished, and open promising perspectives for its future study in tumor-bearing animals.

Targeting of immunoliposomes to endothelial cells using a single-chain Fv fragment directed against human endoglin (CD105)

We generated immunoliposomes targeting proliferating endothelial cells by chemically coupling a single-chain Fv fragment (scFv A5) directed against human endoglin to the liposomal surface. For this purpose, we introduced an additional cysteine residue at the C-terminus of the scFv fragment. This scFv' fragment was expressed in soluble form in bacteria and allowed for a site-directed coupling to sulfhydryl-reactive lipids incorporated into the lipid bilayer. The immunoliposomes (ILA5) showed rapid and strong binding to human endoglin-expressing endothelial cells (HUVEC, HDMEC), while no binding was observed with various endoglin-negative cell lines and blood lymphocytes. In vitro, ILA5 were stable for several hours in serum- or plasma-containing medium. Incubation of endothelial cells with ILA5 at 37 degrees C led to increased binding and internalisation of the liposomes as evidenced by a perinuclear accumulation. In vitro, doxorubicin-loaded ILA5 showed an increased cytotoxicity towards endothelial cells compared to untargeted liposomes and free doxorubicin. Since the vasculature of tumours is easily accessible to drug carrier systems, the described endothelial cell-specific immunoliposomes may be useful for the development of efficacious and safe vascular targeting agents in cancer therapy.

Imparting bone affinity to glycoproteins through the conjugation of bisphosphonates

PURPOSE

To develop a novel means of conjugating bisphosphonates onto the carbohydrate moieties of glycoproteins to enhance protein affinity to bone.

METHODS

1-Amino-1,1-diphosphonate methane (aminoBP) was conjugated onto the carbohydrate moietites of oxidized fetuin by using 4-(maleimidomethyl)cyclohexane-1-carboxyl-hydrazide (MMCCH). Bone affinity of the resulting conjugates was compared to proteins obtained from another means of conjugation, whereby aminoBP was conjugated onto fetuin's lysine moieties by using succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC).

RESULTS

The use of the MMCCH resulted in the conjugation of up to seven aminoBPs per molecule of fetuin. These conjugates gave a 2.6-, 2.0-, 30.5-. and 1.84-fold increased affinity for untreated, ashed, demineralized bone and hydroxyapatite, respectively, as compared to conjugates from the SMCC reaction. Both conjugates exhibited a pH-independent, equally slow degradation in adult bovine serum-containing media.

CONCLUSION

The use of the MMCCH chemistry to conjugate aminoBP onto fetuin was feasible. Furthermore, the described processes of conjugation resulted in amino-BP-dependent increase in the glycoprotein's affinity to various bone matrices in a manner that exceeds the affinity produced by the previously established method, which used SMCC.

Targeting Stealth liposomes in a murine model of human small cell lung cancer

Tumor accumulation and therapeutic activity of Stealth liposomes loaded with doxorubicin (DXR) were examined in Balb/c nude mice xenografts inoculated subcutaneously with the human small cell lung cancer (SCLC) cell line, H69. Mice were treated with non-targeted liposomes (SL) or liposomes targeted with antagonist G coupled to the liposome surface (SLG). SLG showed 30-44-fold higher binding to H69 cells harvested from H69 xenografts than SL. At 48 and 72 h post injection, tumor accumulation of [(125)I]tyraminylinulin-containing liposomes was shown to be dependent on liposome size but independent of the presence of the targeting ligand. Maximum tumor uptake of either SLG or SL ranged from 2 to 4% of injected dose/g of tissue. In therapeutic studies, mice received three weekly injections of 3 or 6 mg free DXR/kg or 3 or 10 mg liposomal DXR/kg at initial tumor volumes of either 7 or 33 mm(3). The therapeutic efficacy of DXR-containing SL or SLG was significantly improved over free DXR, but SLG did not improve anti-tumor efficacy relative to SL. Stealth liposomes containing DXR have potential as a therapy against human SCLC tumors.

Characterization, stability and in-vivo distribution of asialofetuin glycopeptide incorporating DSPC/CHOL liposomes prepared by mild cholate incubation

In this study, a small triantennary asialoglycopeptide of fetuin (A-F2) was used as a ligand to direct liposomes to hepatocytes. A-F2 was cleaved from asialofetuin, purified, conjugated with fatty acids and incorporated into pre-formed sonicated DSPC/Chol (2:1) liposomes. A mild cholate incubation method for incorporating the A-F2 ligand on pre-formed vesicles was used. In preliminary in vivo experiments 111In3+ encapsulated in A-F2/palmityl liposomes was seen to accumulate in the liver of mice significantly faster than when encapsulated in non-ligand bearing liposomes of the same lipid composition (studied before), justifying further investigation of this system. The presence of the A-F2/fatty acid conjugate in a functional form on the vesicle surface was confirmed by their reversible agglutination in the presence of Ricinus communis agglutinin (RCA120). Effects of ligand incorporation on the vesicle size distribution, z-potential, membrane integrity and stability were monitored. The results demonstrate that highest ligand incorporation was achieved when liposomes and ligand were co-incubated in the presence of 1 mM sodium cholate. Incorporation increased with the length of the fatty acid used for A-F2 conjugation. Ligand-bearing liposomes were demonstrated to be smaller in diameter (about 30%) with a more positive z-potential in comparison to control vesicles while ligand incorporation did not influence the liposome membrane integrity. The size of the ligand-incorporating vesicles was maintained after 24 hours of incubation in isotonic buffer, proving that the vesicles do not aggregate. Although the preliminary biodistribution results may suggest that ligand bearing liposomes are accumulating in the liver, further cell culture, in vivo distribution and especially liver fractionation studies are required in order to clarify the intrahepatic localization of these liposomes and the ability to target liver hepatocytes in vivo.

Immunoliposomes for the targeted delivery of antitumor drugs

This review presents an overview of the field of immunoliposome-mediated targeting of anticancer agents. First, problems that are encountered when immunoliposomes are used for systemic anticancer drug delivery and potential solutions are discussed. Second, an update is given of the in vivo results obtained with immunoliposomes in tumor models. Finally, new developments on the utilization of immunoliposomes for the treatment of cancer are highlighted.