Drug Delivery Systems from Self-Assembly of Dendron-Polymer Conjugates †

This review highlights the utilization of dendron-polymer conjugates as building blocks for the fabrication of nanosized drug delivery vehicles. The examples given provide an overview of the evolution of these delivery platforms, from simple micellar containers to smart stimuli- responsive drug delivery systems through their design at the macromolecular level. Variations in chemical composition and connectivity of the dendritic and polymeric segments provide a variety of self-assembled micellar nanostructures that embody desirable attributes of viable drug delivery systems

Trastuzumab targeted micellar delivery of docetaxel using dendron-polymer conjugates

Incorporation of a therapeutic antibody into nanosized drug delivery systems can improve their target specificity. This work reports an antibody-conjugated targeted delivery system composed of polymer-dendron conjugates. Trastuzumab is chosen as the targeting moiety, since it is clinically used against tumor cells expressing HER2 receptors. A micellar delivery system was generated using amphiphilic polymer-dendron conjugates containing a fourth-generation polyester dendron as the hydrophobic block and a linear poly(ethylene glycol) (PEG) chain as the hydrophilic block. After preparation of docetaxel loaded (ca. 10% wt) micelles, trastuzumab was conjugated onto the micellar shell using an amidation reaction. Micelles remained stable after conjugation of the antibody, with a slight increase in size from 179 nm to 185 nm upon functionalization. Docetaxel release was determined to be responsive to acidic pH, and over the course of 30 h, 54% drug release was measured in acidic media, whereas it was around 30% under neutral conditions. Cytotoxicity experiments on MCF-7 and SK-OV-3 cell lines displayed improved toxicity levels for targeted micelles in comparison with the non-targeted counterparts, whereas pulse-chase experiments indicated effectiveness of micellar formulations and the presence of targeting groups. Cellular internalization experiments using fluorescence microscopy and flow cytometry further demonstrated the enhanced cellular uptake of antibody conjugated targeted micelles

Combretastatin A‑4 Conjugated Antiangiogenic Micellar Drug Delivery Systems Using Dendron–Polymer Conjugates

Employment of polymeric nanomaterials in cancer therapeutics is actively pursued since they often enable drug administration with increased efficacy along with reduced toxic side effects. In this study, drug conjugated micellar constructs are fabricated using triblock dendron–linear polymer conjugates where a hydrophilic linear polyethylene glycol (PEG) chain is flanked by well-defined hydrophobic biodegradable polyester dendrons bearing an antiangiogenic drug, combretastatin-A4 (CA4). Variation in dendron generation is utilized to obtain a library of micellar constructs with varying sizes and drug loadings. In particular, a family of drug appended dendron–polymer conjugates based on polyester dendrons of generations ranging from G1 to G3 and 10 kDa linear PEG were obtained using [3 + 2] Huisgen type “click” chemistry. The final constructs benefit from PEG’s hydrophilicity and antibiofouling character, as well as biodegradable nature of the hydrophobic polyester dendrons. The hydrophobic–hydrophilic–hydrophobic character of these constructs leads to the formation of flower-like micelles in aqueous media. In addition to generation-dependent subnanomolar range critical micelle concentrations, the resulting micelles possess hydrodynamic diameters suitable for passive tumor targeting through enhanced permeability and retention (EPR) effect; thereby they are suitable candidates as controlled drug delivery agents. For all constructs, <i>in vitro</i> cytotoxicities were investigated and inhibitory effect of Comb-G3-PEG on tube formation was shown on human umbilical vein endothelial cells (HUVECs)

Designing Dendron–Polymer Conjugate Based Targeted Drug Delivery Platforms with a “Mix-and-Match” Modularity

Polymeric micellar systems are emerging as a very important class of nanopharmaceuticals due to their ability to improve pharmacokinetics and biodistribution of chemotherapy drugs, as well as to reduce related systemic toxicities. While these nanosized delivery systems inherently benefit from passive targeting through the enhanced permeation and retention effect leading to increased accumulation in the tumor, additional active targeting can be achieved through surface modification of micelles with targeting groups specific for overexpressed receptors of tumor cells. In this project, nontoxic, biodegradable, and modularly tunable micellar delivery systems were generated using two types of dendron–polymer conjugates. Either an AB type dendron–polymer construct with 2K PEG or an ABA type dendron–polymer–dendron conjugate with 6K PEG based middle block was used as primary construct; along with an AB type dendron–polymer containing a cRGDfK targeting group to actively target cancer cells overexpressing α_υβ₃/α_υβ₅ integrins. A set of micelles encapsulating docetaxel, a widely employed chemotherapy drug, were prepared with varying feed ratios of primary construct and targeting group containing secondary construct. Critical micelle concentrations of all micellar systems were in the range of 10^–6 M. DLS measurements indicated hydrodynamic size distributions varying between 170 to 200 nm. An increase in docetaxel release at acidic pH was observed for all micelles. Enhanced cellular internalization of Nile red doped micelles by MDA-MB-231 human breast cancer cells suggested that the most efficient uptake was observed with targeted micelles. <i>In vitro</i> cytotoxicity experiments on MDA-MB-231 breast cancer and A549 lung carcinoma cell lines showed improved toxicity for RGD containing micelles. For A549 cell line EC₅₀ values of drug loaded micellar sets were in the range of 10^–9 M whereas EC₅₀ value of free docetaxel was around 10^–10 M. For MDA-MB-231 cell line EC₅₀ value of free docetaxel was 6 × 10^–8 M similar to EC₅₀ of nontargeted AB type docetaxel doped micellar constructs whereas the EC₅₀ value of its targeted counterpart decreased to 5.5 × 10^–9 M. Overall, in this comparative study, the targeting group containing micellar construct fabricated with a 2 kDa PEG based diblock dendron–polymer conjugate emerges as an attractive drug delivery vehicle due to the ease of synthesis, high stability of the micelles, and efficient targeting

Diels–Alder “Clickable” Biodegradable Nanofibers: Benign Tailoring of Scaffolds for Biomolecular Immobilization and Cell Growth

Biodegradable polymeric nanofibers have emerged as promising candidates for several biomedical applications such as tissue engineering and regenerative medicine. Many of these applications require modification of these nanofibers with small ligands or biomolecules such as peptides and other growth factors, which necessitates functionalization of these materials in mild and benign fashion. This study reports the design, synthesis, and functionalization of such nanofibers and evaluates their application as a cell culture scaffold. Polylactide based copolymers containing furan groups and triethylene glycol (TEG) units as side chains were synthesized using organocatalyzed ring opening polymerization. The furan moiety, an electron rich diene, provides “clickable” handles required for modification of nanofibers since they undergo facile cycloaddition reactions with maleimide-containing small molecules and ligands. The TEG units provide these fibers with hydrophilicity, enhanced biodegradability, and antibiofouling characteristics to minimize nonspecific adsorption. A series of copolymers with varying amounts of TEG units in their side chains were evaluated for fiber formation and antibiofouling characteristics to reveal that an incorporation of 7.5 mol % TEG-based monomer was optimal for nanofibers containing 20 mol % furan units. Facile functionalization of these nanofibers in a selective manner was demonstrated through attachment of a dienophile containing fluorophore, namely, fluorescein maleimide. To show efficient ligand-mediated bioconjugation, nanofibers were functionalized with a maleimide appended biotin, which enabled efficient attachment of the protein, Streptavidin. Importantly, the crucial role played by the TEG-based side chains was evident due to lack of any nonspecific attachment of protein to these nanofibers in the absence of biotin ligand. Furthermore, these nanofibers were conjugated with a cell adhesive cyclic peptide, cRGDfK-maleimide, at room temperature without the need of any additional catalyst. Importantly, comparison of the cell attachment onto nanofibers with and without the peptide demonstrated that fibers appended with the peptides promoted cells to spread nicely and protrude actin filaments for enhanced attachment to the support, whereas the cells on nonfunctionalized nanofibers showed a rounded up morphology with limited cellular spreading

Dendrons and Multiarm Polymers with Thiol-Exchangeable Cores: A Reversible Conjugation Platform for Delivery

Disulfide exchange reaction has emerged as a powerful tool for reversible conjugation of proteins, peptides and thiol containing molecules to polymeric supports. In particular, the pyridyl disulfide group provides an efficient handle for the site-specific conjugation of therapeutic peptides and proteins bearing cysteine moieties. In this study, novel biodegradable dendritic platforms containing a pyridyl disulfide unit at their focal point were designed. Presence of hydroxyl groups at the periphery of these dendrons allows their elaboration to multivalent initiators that yield poly­(ethylene glycol) based multiarm star polymers via controlled radical polymerization. The pyridyl disulfide unit at the core of these star polymers undergoes efficient reaction with thiol functional group containing molecules such as a hydrophobic dye, namely, Bodipy-SH, glutathione, and KLAK sequence containing peptide. While conjugation of the hydrophobic fluorescent dye to the PEG-based multiarm polymer renders it water-soluble, it can be cleaved off the construct through thiol–disulfide exchange in the presence of an external thiol such as dithiothreitol. The multiarm polymer was conjugated with a thiol group containing apoptotic peptide to increase its solubility and cellular transport. In vitro cytotoxicity and apoptosis assays demonstrated that the resultant peptide–polymer conjugate had almost five times more apoptotic potential primarily through triggering apoptosis by disrupting mitochondrial membranes of human breast cancer cell line (MDA-MB-231) compared to naked peptide. The novel dendritic platform disclosed here offers an attractive template that can be modified to multiarm polymeric constructs bearing a “tag and release” characteristic