Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
188.8.131.52 Biocompatibility of Dendrimers
Dendrimers [5098-5105, 6015] are tree-shaped synthetic macromolecules with a regular highly-branched structure emanating outward from a core. Dendrimers are formed almost nanometer by nanometer, with the number of synthetic layers or “generations” dictating the exact size of the particles. Each molecule is typically a few nanometers wide but some have been constructed up to 30 nanometers wide, incorporating more than 100,000 atoms. The peripheral layer of the dendrimer molecule can be made to form a dense layer of molecular groups that serve as hooks for attaching other useful molecules, such as DNA, in the outermost branches . Dendrimers offer many exciting near-term opportunities in nanomedicine for the design of novel drug-carriers [5107, 5108], gene delivery systems [5109-5127], imaging or contrast agents [5128-5138], cell labeling agents , biosensors [5140-5145], artificial catalytic sites [5146-5149], catalytic antibodies , and DNA/protein microarrays [5151-5154]. Dendrimers also hold great promise in tissue targeting applications and controlled drug release , affording relatively easy passage across biological barriers by transcytosis [5156-5159] due to their controllable nanoscopic architecture and flexibility for tailored functionalization [2397, 5160-5162].
In 1998, James R. Baker Jr. co-founded the Center for Biologic Nanotechnology at the University of Michigan to bring together doctors, medical researchers, chemists and engineers to pursue the use of dendrimers as a safer and more effective medical therapy agent . For Baker, these nanostructures are attractive because they can sneak DNA and other materials into cells while avoiding triggering an immune response, unlike the viral vectors commonly employed today for transfection. The dendrimer molecule is decorated with specific snippets of DNA, then injected into biological tissue. Upon encountering a living cell, dendrimers of a certain size trigger endocytosis, in which a vesicle encloses the dendrimer and admits the particle into the cell’s interior. Once inside, the DNA is released and migrates to the nucleus where it becomes part of the cell’s genome. The technique was first tested on a variety of mammalian cell types , and in 2001 Baker began animal trials of dendrimer gene therapy. Baker and Donald Tomalia, another co-founder of the Center for Biologic Nanotechnology, report using glycodendrimer “nanodecoys” to trap and deactivate some influenza virus subtype strains . Here the glycodendrimers present a surface that mimics the sialic acid groups normally found in the mammalian cell membrane. This causes virus particles to adhere to the outer branches of the decoys instead of the natural cells.
The biocompatibility of dendrimers is determined by the nature and conformational mobility of their exterior. One of the earliest studies  of dendrimer biocompatibility looked at Starburst dendrimers. These are spherical macromolecules composed of repeating polyamidoamino (PAMAM ) units that can be produced in successive generations, each with a defined size, molecular weight, and number of terminal amino groups. Roberts et al  investigated Generation 3 (G3; MW = 5,147; 24 terminal amines), Generation 5 (G5; MW = 21,563; 96 amines), and Generation 7 (G7; MW = 87,227; 384 amines) PAMAMs in V79 cells or in Swiss-Webster mice for a number of biological properties, including in-vitro toxicity, in-vivo toxicity, immunogenicity, and biodistribution. Potential biological complications were observed only with G7, and there was no evidence of immunogenicity. Dendrimer G3 showed the highest accumulation in kidney tissue, whereas G5 and G7 preferentially localized in the pancreas. G7 showed extremely high urinary excretion.
A more comprehensive study of dendrimer biocompatibility by Malik et al  looked at polyamidoamine (PAMAM, Starburst), poly(propyleneimine) with either diaminobutane or diaminoethane as core, and poly(ethylene oxide) (PEO) grafted carbosilane (CSi-PEO) dendrimers to study systematically the effect of dendrimer generation and surface functionality on biological properties in vitro. Dendrimers with -NH2 termini displayed concentration- and (in PAMAM) generation-dependent hemolysis. Changes in red cell morphology were observed after 1 hour even at low concentrations (10 µg/ml). At concentrations below 1 mg/ml CSi-PEO dendrimers and dendrimers with carboxylate (COONa) terminal groups were neither hemolytic nor cytotoxic towards a panel of cell lines in vitro, but cationic dendrimers were cytotoxic with IC50 values of 50-300 µg/ml depending on dendrimer type, cell type, and generation . Polyether dendrimers with carboxylate and malonate surfaces were not hemolytic at 1 hour, but were lytic after 24 hours, unlike anionic PAMAM dendrimers . Cationic 125I-labelled PAMAM G3 and G4 dendrimers administered intravenously to Wistar rats at ~10 µg/ml were cleared rapidly from the circulation, with <2% recovered dose in blood at 1 hour. Anionic PAMAM dendrimers (G2.5, G3.5 and G5.5) showed longer circulation times, with 20-40% recovered dose in blood at 1 hour and generation-dependent clearance rates (lower generations circulated longer) . PAMAM dendrimers injected intraperitoneally appeared in the bloodstream within 1 hour and their subsequent biodistribution mirrored that seen following intravenous injection . Malik et al  concluded that inherent toxicity probably ruled out using higher generation cationic dendrimers for parenteral administration, especially if they needed to be used at a high dose, and that dendrimer structure would have to be carefully tailored to avoid rapid hepatic uptake if targeting elsewhere (e.g., tumor targeting) was a primary objective . Other related studies have examined dendrimer interactions with human arterial endothelial cells , muscle cells , proteins , and nuclear pores ; complement activation by DNA-dendrimer complexes ; microvascular extravasation profiles of dendrimers ; whole-body biodistribution of dendrimer-based agents ; modification of cell adhesion to surfaces ; the synthesis of dendrimer-fullerene films ; and the morphology of DNA-dendrimer complexes as a function of ionic strength . Thus, toxicity relates mostly to charge and surface functionality.
In applications, dendrimeric macromolecules have been investigated as delivery vehicles for antisense [2396, 5177-5180] or therapeutic [5181-5184] oligonucleotides, antiangiogenic agents , antiapoptosis agents , selectin antagonists , plasmid-based gene delivery vectors , photodynamic therapy  and radioimmunotherapy [5190-5193] agents, adjuvants , vaccines , bacterial toxin inhibitors , various anti-cancer drugs [2398, 5107, 5160, 5197-5202] including 5-fluorouracil , and even as potentially useful objects for DNA-based bottom-up nanoassembly . Dendrimeric peptides selective for microbial surfaces have been developed which have broad antimicrobial activity while achieving low hemolytic activity to human erythrocytes , and other antimicrobial [2399, 5205], antiviral [5206-5210], and antiprion  dendrimeric agents have been investigated. E. Pinkhassik suggests that solid particles coated with the same residues as the dendrimers might exhibit identical solubility and biocompatibility.
Last updated on 30 April 2004