The characterization of a novel dendritic system for gene delivery by isothermal titration calorimetry

riDOM, a cell penetrating peptide. Interaction with phospholipid bilayers

Melittin is an amphipathic peptide which has received much attention as a model peptide for peptide-membrane interactions. It is however not suited as a transfection agent due to its cytolytic and toxicological effects. Retro-inverso-melittin, when covalently linked to the lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (riDOM), eliminates these shortcomings. The interaction of riDOM with phospholipid membranes was investigated with circular dichroism (CD) spectroscopy, dynamic light scattering, ζ-potential measurements, and high-sensitivity isothermal titration calorimetry. riDOM forms cationic nanoparticles with a diameter of ~13nm which are well soluble in water and bind with high affinity to DNA and lipid membranes. When dissolved in bilayer membranes, riDOM nanoparticles dissociate and form transient pores. riDOM-induced membrane leakiness is however much reduced compared to that of authentic melittin. The secondary structure of the ri-melittin is not changed when riDOM is transferred from water to the membrane and displays a large fraction of β-structure. The (31)P NMR spectrum of the nanoparticle is however transformed into a typical bilayer spectrum. The Gibbs free energy of riDOM binding to bilayer membranes is -8.0 to -10.0kcal/mol which corresponds to the partition energy of just one fatty acyl chain. Half of the hydrophobic surface of the riDOM lipid extension with its 2 oleic acyl chains is therefore involved in a lipid-peptide interaction. This packing arrangement guarantees a good solubility of riDOM both in the aqueous and in the membrane phase. The membrane binding enthalpy is small and riDOM binding is thus entropy-driven.

Targeted delivery of doxorubicin-utilizing chitosan nanoparticles surface-functionalized with anti-Her2 trastuzumab

Background

Targeting drugs to their sites of action to overcome the systemic side effects associated with most antineoplastic agents is still a major challenge in pharmaceutical research. In this study, the monoclonal antibody, trastuzumab, was used as a targeting agent in nanoparticles carrying the antitumor drug, doxorubicin, specifically to its site of action.

Methods

Chitosan-doxorubicin conjugation was carried out using succinic anhydride as a crosslinker. Trastuzumab was conjugated to self-assembled chitosan-doxorubin conjugate (CS-DOX) nanoparticles (particle size, 200 nm) via thiolation of lysine residues and subsequent linking of the resulted thiols to chitosan. Conjugation was confirmed by gel permeation chromatography, differential scanning calorimetry, Fourier transform infrared spectroscopy, and ¹H nuclear magnetic resonance spectroscopy studies. Dynamic light scattering, transmission electron microscopy, and zeta potential determination were used to characterize the nanoparticles.

Results

CS-DOX conjugated nanoparticles had a spherical shape and smooth surface with a narrow size distribution and core-shell structure. Increasing the ratio of doxorubicin to chitosan in the conjugation reaction gave rise to a higher doxorubicin content but lower conjugation efficiency. Trastuzumab-decorated nanoparticles (CS-DOX-mAb) contained 47 μg/mg doxorubicin and 33.5 μg/mg trastuzumab. Binding of trastuzumab to the nanoparticles was further probed thermodynamically by isothermal titration calorimetry. Fluorescence microscopy demonstrated enhanced and selective uptake of CS-DOX-mAb by Her2⁺ cancer cells compared with nontargeted CS-DOX nanoparticles and free drug.

Conclusion

Antibody-conjugated nanoparticles were shown to discriminate between Her2⁺ and Her2⁻ cells, and thus have the potential to be used in active targeted drug delivery, with reduction of drug side effects in Her2⁺ breast and ovarian cancers.

Elucidating the molecular mechanism of PAMAM-siRNA dendriplex self-assembly: effect of dendrimer charge density

Dendrimers are attractive vehicles for nucleic acid delivery due to monodispersity and ease of chemical design. The purpose of this study was to elucidate the self-assembly process between small interfering RNA (siRNA) and different generation poly(amidoamine) dendrimers and to characterize the resulting structures. The generation 4 (G4) and G7 displayed equal efficiencies for dendriplex aggregate formation, whereas G1 lacked this ability. Nanoparticle tracking analysis and dynamic light scattering showed reduced average size and increased polydispersity at higher dendrimer concentration. The nanoparticle tracking analysis indicated that electrostatic complexation results in an equilibrium between differently sized complex aggregates, where the centre of mass depends on the siRNA:dendrimer ratio. Isothermal titration calorimetric data suggested a simple binding for G1, whereas a biphasic binding was evident for G4 and G7 with an initial exothermic binding and a secondary endothermic formation of larger dendriplex aggregates, followed by agglomeration. The initial binding became increasingly exothermic as the generation increased, and the values were closely predicted by molecular dynamics simulations, which also demonstrated a generation dependent differences in the entropy of binding. The flexible G1 displayed the highest entropic penalty followed by the rigid G7, making the intermediate G4 the most suitable for dendriplex formation, showing favorable charge density for siRNA binding.

How to study dendriplexes II: Transfection and cytotoxicity

This paper reviews different techniques for analyzing the transfection efficiencies and cytotoxicities of dendriplexes-complexes of nucleic acids with dendrimers. Analysis shows that three plasmids are mainly used in transfection experiments: plasmid DNA encoding luciferase from the firefly Photinus pyralis, beta-galactosidase, or green fluorescent protein. The effective charge ratio of transfection does not directly correlate with the charge ratio obtained from gel electrophoresis, zeta-potential or ethidium bromide intercalation data. The most popular cells for transfection studies are human embryonic kidney cells (HEK293), mouse embryonic cells (NIH/3T3), SV40 transformed monkey kidney fibroblasts (COS-7) and human epithelioid cervical carcinoma cells (HeLa). Cellular uptake is estimated using fluorescently-labeled dendrimers or nucleic acids. Transfection efficiency is measured by the luciferase reporter assay for luciferase, X-Gal staining or beta-galactosidase assay for beta-galactosidase, and confocal microscopy for green fluorescent protein. Cytotoxicity is determined by the MTT test and lactate dehydrogenase assays. On the basis of the papers reviewed, a standard essential set of techniques for characterizing dendriplexes was constructed: (1) analysis of size and shape of dendriplexes in dried/frozen state by electron or atomic force microscopy; (2) analysis of charge/molar ratio of complexes by gel electrophoresis or ethidium bromide intercalation assay or zeta-potential measurement; (3) analysis of hydrodynamic diameter of dendriplexes in solution by dynamic light scattering. For the evaluation of transfection efficiency the essential techniques are (4) luciferase reporter assay, beta-galactosidase assay or green fluorescent protein microscopy, and (5) cytotoxicity by the MTT test. All these tests allow the transfection efficiencies and cytotoxicities of different kinds of dendrimers to be compared.