Current methods for attaching targeting ligands to liposomes and nanoparticles

Nanoencapsulation II. Biomedical applications and current status of peptide and protein nanoparticulate delivery systems

The concept of polymeric nanoparticles for the design of new drug delivery systems emerged a few years ago, and recent rapid advances in nanotechnology have offered a wealth of new opportunities for diagnosis and therapy of various diseases. Recent progress has made possible the engineering of nanoparticles to allow the site-specific delivery of drugs and to improve the pharmacokinetic profile of numerous compounds with biomedical applications such as peptide and protein drugs. Biologically active peptides and their analogues are becoming an increasingly important class of drugs. Their use for human and animal treatment is problematic, however, because some of these drugs are generally ineffective when taken orally and thus have been administered chiefly by the parenteral route. This review covers some of the historical and recent advances of nanotechnology and concludes that polymeric nanoparticles show great promise as a tool for the development of peptide drug delivery systems.

Targeted Liposomes: Convenient Coupling of Ligands to Preformed Vesicles Using “Click Chemistry”

An efficient and convenient chemoselective conjugation method based on "click chemistry" was developed for coupling ligands to the surface of preformed liposomes. It can be performed under mild conditions in aqueous buffers; the use of a water soluble Cu(I) chelator, such as bathophenanthrolinedisulfonate, was essential to obtain good yields in reasonable reaction times. A model reaction was achieved in which, in a single step, an unprotected alpha-D-mannosyl derivative carrying a spacer arm functionalized with an azide group was conjugated to the surface of vesicles presenting a synthetic lipid carrying a terminal alkyne function. When liposomes composed of saturated phospholipids were used, the reaction conditions developed in the present work did not damage the membranes as measured by the absence of leakage of entrapped 5,6-carboxyfluorescein. Moreover, as assessed by agglutination experiments using concanavalin A, the mannose residues were perfectly accessible on the surface of the targeted vesicles.

Generation of immunoliposomes using recombinant single-chain Fv fragments bound to Ni-NTA-liposomes

Recombinant single-chain Fv antibody fragments (scFv) can be combined with liposomes to generate immunoliposomes for targeted drug delivery. Recent studies have shown that scFv molecules modified to express a C-terminal cysteine residue can be used for site-directed chemical conjugation. Here, we present a new method by immobilizing scFv fragments via their C-terminal hexahistidyl-tag on liposomes containing Ni-NTA-lipids (Ni-NTA-DOGS) in their lipid bilayer without the need to introduce additional reactive groups in the protein. Using an anti-endoglin scFv as a model antibody, we could show that scFv molecules are efficiently immobilized on the liposome surface and that these immunoliposomes bind specifically and strongly to endoglin-expressing endothelial cells. This approach allows for a rapid and flexible generation of target cell-specific immunoliposomes.

Epidermal Growth Factor-Conjugated Poly(ethylene glycol)-block- Poly(δ-valerolactone) Copolymer Micelles for Targeted Delivery of Chemotherapeutics

Epidermal growth factor (EGF)-conjugated copolymer micelles were prepared from a mixture of diblock copolymers of methoxy poly(ethylene glycol)-block-poly(delta-valerolactone) (MePEG-b-PVL) and EGF-PEG-b-PVL for targeted delivery to EGF receptor (EGFR)-overexpressing cancers. The block copolymers and functionalized block copolymers were synthesized using PEG as the macroinitiator and HCl-diethyl ether as the catalyst. The MePEG-b-PVL and the carboxyl-terminated PEG-b-PVL (HOOC-PEG-b-PVL) copolymers were found to have molecular weights of 5940 and 5900, respectively, as determined by gel permeation chromatography (GPC) analyses. The HOOC-PEG-b-PVL copolymers were then activated by N-hydroxysuccinimide and subsequently reacted with EGF to form the EGF-PEG-b-PVL copolymers. The efficiency for the conjugation of EGF to the copolymer was found to be 60.9%. A hydrophobic fluorescent probe, CM-DiI, was loaded into both the nontargeted MePEG-b-PVL micelles and the targeted EGF-conjugated PEG-b-PVL micelles. The effective mean diameters of the CMDiI-loaded nontargeted and the CMDiI-loaded targeted micelles were found to be 32 +/- 1 nm and 45 +/- 2 nm, respectively, as determined by dynamic light scattering (DLS). The zeta potentials for the nontargeted micelles (no CM-DiI-loaded), CM-DiI-loaded nontargeted micelles, and CM-DiI-loaded targeted micelles were found to be -6.5, -8.7, and - 13.5 mV, respectively. Evaluation of the in vitro release of CM-DiI from the MePEG-b-PVL micelles in phosphate buffer saline (0.01 M, pH = 7.4) containing 10% (v/v) fetal bovine serum at 37 degrees C revealed that approximately 20% of the probe was released within the first 2 h. Confocal laser scanning microscopy (CLSM) analysis revealed that the targeted micelles containing CM-DiI accumulated intracellularly in EGFR-overexpressing MDA-MB-468 breast cancer cells following a 2 h incubation period, while no detectable cell uptake was observed for the nontargeted micelles. Results obtained from the confocal images were confirmed in an independent study by measuring the intracellular CM-DiI fluorescence in cell lysate. In addition, the presence of free EGF was found to decrease the extent of uptake of the targeted micelles. Nuclear staining of the cells with Hoechst 33258 indicated that the targeted micelles mainly localized in the perinuclear region and some of the micelles were localized in the nucleus. These results demonstrate that the EGF-conjugated copolymer micelles developed in this study have potential as vehicles for targeting hydrophobic drugs to EGFR-overexpressing cancers.

Shell click-crosslinked (SCC) nanoparticles: a new methodology for synthesis and orthogonal functionalization

A new methodology for the preparation of well-defined core-shell nanoparticles was developed, based upon the employment of a multifunctional crosslinker to coincidently stabilize supramolecular polymer assemblies and imbed into the shell unique chemical functionalities. Amphiphilic diblock copolymers of poly(acrylic acid)(80)-b-poly(styrene)(90) that had been assembled into micelles and partially functionalized throughout the corona with alkynyl groups were utilized as Click-readied nanoscaffolds for the formation of shell Click-crosslinked nanoparticles (SCCs). Divergently grown dendrimers of the zero, first, second, and third generations having increasing numbers of azide terminating groups ((N(3))(2)-[G-0], (N(3))(4)-[G-1], (N(3))(8)-[G-2], and (N(3))(16)-[G-3], respectively) were investigated as crosslinkers via Click reactions with the alkynyl groups to form covalent linkages throughout the block copolymer micelle corona, thus forming a crosslinked shell. The crosslinking reactions were characterized by (1)H NMR and IR spectroscopies, differential scanning calorimetry (DSC), and dynamic light scattering (DLS) measurements. Only the first generation dendrimer ((N(3))(4)-[G-1]) possessed a sufficient balance of polyvalency and water solubility to achieve crosslinking and establish a robust nanostructure. The resulting SCC was further characterized with atomic force microscopy (AFM), transmission electron microscopy (TEM), and analytical ultracentrifugation (AU). The dendritic crosslinker is important as it also allows for the incorporation of excess functionality that can undergo complementary reactions. Within the shell of this SCC the remaining azide termini of the dendrimer crosslinker were then consumed in a secondary Click reaction with an alkynyl-functionalized fluorescein to yield a fluorescently labeled SCC that was characterized with DLS, AFM, TEM, AU, UV-vis, and fluorescent measurements as a function of pH.

The design and evaluation of a novel targeted drug delivery system using cationic emulsion-antibody conjugates

In an attempt to design a targeted drug delivery system to tumors' over-expressing H-ferritin specifically recognized by a monoclonal antibody, AMB8LK, a cationic emulsion - AMB8LK conjugate was prepared. A novel cross-linker molecule bearing maleimide group was synthesized and added to cationic emulsion formulation for AMB8LK Fab' fragment covalent coupling. NMR spectroscopy confirmed the cross-linker synthesis and the preservation of the active maleimide function. SDS gel-electrophoresis results corroborated the formation of the Fab' fragment. Different densities of Fab' fragments (10-200 Fab'/oil droplet) were conjugated to emulsion droplet interface and no changes in the physico-chemical properties were observed ( approximately 120 nm size and zeta potential of approximately +30 mV). The coupling efficiency ranged from 55% to 70% and was visualized by TEM showing gold particles attached to the droplet interface. Cell culture studies demonstrated specific binding to cells as confirmed by the occurrence of the marked reduction in binding when free AMB8LK Mab was incubated before adding the AMB8LK-emulsion conjugate to the cells. The coupling of AMB8LK Fab' fragment to the cationic emulsion increased the cells uptake by 50% as compared to non-conjugated respective cationic emulsion. Appropriate conditions were, thus, identified for coupling AMB8LK Fab' fragment to cationic emulsion without altering the specificity and affinity of the Mab fragment to the tumor antigen.

Role of nanotechnology in targeted drug delivery and imaging: a concise review

The use of nanotechnology in drug delivery and imaging in vivo is a rapidly expanding field. The emphases of this review are on biophysical attributes of the drug delivery and imaging platforms as well as the biological aspects that enable targeting of these platforms to injured and diseased tissues and cells. The principles of passive and active targeting of nanosized carriers to inflamed and cancerous tissues with increased vascular leakiness, overexpression of specific epitopes, and cellular uptake of these nanoscale systems are discussed. Preparation methods-properties of nanoscale systems including liposomes, micelles, emulsions, nanoparticulates, and dendrimer nanocomposites, and clinical indications are outlined separately for drug delivery and imaging in vivo. Taken together, these relatively new and exciting data indicate that the future of nanomedicine is very promising, and that additional preclinical and clinical studies in relevant animal models and disease states, as well as long-term toxicity studies, should be conducted beyond the "proof-of-concept" stage. Large-scale manufacturing and costs of nanomedicines are also important issues to be addressed during development for clinical indications.

Architectural and structural optimization of the protective polymer layer for enhanced targeting

Using Monte Carlo simulations we study the influence of ligand architecture (valence, branching length) and structure (polydispersity) of a flat protective polymer layer on the accessibility of its functional groups and efficiency of receptor targeting. Two types of receptor surfaces were considered: the surface homogeneously covered with receptors and the surface containing a finite number of receptor sites. We found that multivalent ligands provide a larger density of targeting groups on the periphery of the layer compared to monovalent ligands for the same overall number of targeting groups per polymer layer. Because of their cooperativity in binding, multivalent ligands were also considerably more efficient in binding to both types of receptor surfaces. With an increase of ligand valence the number of functional groups attached to receptors noticeably increases. Short-branched divalent ligands show an especially high cooperativity in binding to closely packed receptors. However, in the case of immobile receptors separated by a finite distance from each other, the average distance between the functional groups belonging to the same short divalent ligand is too small to reach different receptors simultaneously and the receptor binding is less efficient than in the monovalent ligand case. Using a bidisperse protective polymer layer formed by short nonfunctional polymers and long functionalized polymers considerably increases the fraction of functional groups on the periphery of the layer. Simulations of receptor binding confirm the high efficiency of receptor targeting by bidisperse polymer layers, which is achieved by means of larger compressibility and higher capability of the ligands to reach out compared to the corresponding monodisperse layers. The concepts of multivalent ligands and a bidisperse protective polymer layer each have their own advantages which can be combined for an enhanced targeting effect.

Regional Radiochemotherapy Using In Situ Hydrogel

PURPOSE

To evaluate the feasibility of regional radiochemotherapy of mammary tumors using in situ hydrogel loaded with cisplatin (CDDP) and rhenium-188 ((188)Re).

METHODS

Sodium alginate (SA) and calcium chloride were used to create a hydrogel for delivery of CDDP and (188)Re. In vitro studies were performed to evaluate cytotoxic effects of (188)Re-hydrogel and sustained-release ability of the CDDP-hydrogel. Tumor-bearing rats were injected with (188)Re-hydrogel (0.5-1 mCi/rat), (188)Re-perrhenate (0.5-1 mCi/rat, intratumoral, I.T.), CDDP-hydrogel (3 mg/kg), and (188)Re-hydrogel loaded with CDDP (3 mg/kg body weight, 0.5-1 mCi/rat), respectively, and groups receiving (188)Re were imaged at 24 and 48 h postinjection. Tumor volume, body weight, imaging, and kidney function were assessed as required for each group.

RESULTS

Successful formation of the hydrogel was demonstrated by cytotoxic effects of (188)Re-hydrogel and slow release of CDDP-hydrogel in vitro. Tumor volume measurements showed significant delay in tumor growth in treated vs. control groups with minimal variation in normal kidney function for the CDDP-hydrogel group. Scintigraphic images indicated localization of (188)Re-hydrogel in the tumor site up to 48 h postinjection.

CONCLUSIONS

Our data demonstrate the feasibility of using hydrogel for delivery of chemotherapeutics and radiation locally. This technique may have applications involving other contrast modalities as well as treatment in cases where tumors are inoperable.