Polymer-Nucleic Acid Complexes

In view of the potential importance of the polymer nueleie aeid eomplexes in gene therapies and in the advantages that responsive polymers offer in this area, considered in depth here are the mechanisms by which the synthetic and biological macromolecules interact and how they must function in order to deliver and express a therapeutic transgene. 

For nonviral systems to be effective vectors, the polymer must not only condense and deliver DNA intact to the target site but must also enable nucleic acid to be transported through the cell membrane, and be translocated from the cytoplasm to the nucleus. Many researchers have now investigated DNA complexed by electrostatic interactions to cationic polymers as a method by which the therapeutic gene can be transported. In most cases these cationic polymers form condensed complexes with DNA that both contract the nucleic acid to facilitate cellular uptake, and which protect it from serum and cytosolic nuclease degradation. Mechanisms of DNA condensation, cellular uptake and transport to the nucleus, as well as strategies to improve toxicity, transfection potential and nuclear targeting of polycation mediated dehvery systems are discussed below. 

Many polymers are able to condense DNA to these toroidal and contracted nano/micro-sphere structures in vitro and the importance of efficient condensation for transfection has been demonstrated. There are a number of factors that influence DNA condensation and the size of polymer-DNA complexes. For example, low molecular weight linear PEI forms much larger complexes with DNA than higher molecular weight and branched PEI, and these complexes possess lower transfection abilities [57, 58]. 

4- FalUniJff driven by H pump ictivates hyrolytic enzymes that digest material within lysosome 

The most common uptake pathway into the cell for nontargeted polymers occurs via passive endocytosis, and indeed has been shown to be the pathway for both PEI and PEL [59, 60]. It is also likely that transfection occurring via endocytosis takes place for many polymers through calcium mediated cell anchorage to the extracellular matrix. The normal 

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Polyplexes are formed by mixing a poly(nucleic acid) salt (typically the sodium salt) and a cationic polymer. Electrostatic attraction and counterion release leads to a hydrophobic collapse of the polymer-nucleic acid complex into a nanoparticle called a polyplex. 

Ccmmercially available as well as synthetically prepared samples of ATA consist mostly of a heterogeneous collection of polymers as revealed by fractionation schemes utilizing both dialysis and ultra-filtration, axid by molecular weight measurements. Carbon-13 NNR studies suggest that the polymeric material is of the phenol-formaldehyde type, and inhibitory assays that depend on the formation of a protein-nucleic acid complex reveal that potency varies directly with the molecular weight of the polymer. ATA fractions of molecular weight 400 are essentially inactive. 

It is also interesting that rod assemblies can be apphcable to biomimetic or bioconjugate systems, hi Sect. 2.7, artificial nucleic acid complexes consisting of amphiphihc rods and linear nucleic acid polymers showed single twisted right-handed hehcal fibers. In addition, carbohydrate-coated supramolecular objects self-assembled by rod-coils or wedge-coils revealed outstanding fig-... 

At one level, life can be regarded as a collection of hugely complex reactions taking place between organic compounds in oddly shaped containers. Many of these organic compounds are polymers, including the cellulose of wood, natural fibers such as cotton and silk, the proteins and carbohydrates in our food, and the nucleic acids of our genes. 

Only two nucleic acids exist. They are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). The structural complexity of nucleic acids falls far short of that of proteins. Like proteins, however, nucleic acids are polymers, with nucleotides being the monomer units. 

Now that we have the monomers, which are pretty complex in this case, it remains to define how they are joined together to create the polymer. The amino acids in proteins are linked by peptide bonds. The nucleotides in nucleic acids are linked by phosphodiester bonds, as is shown in figure 12.2. These DNA phosphodiester bonds are very stable. Indeed, samples of largely intact DNA can be recovered from organisms that have been extinct for thousands of years. This remarkable stability should not come as a surprise the central importance of DNA in all forms of life requires that it be stable to various sorts of insults. 

Relatively few simple pyrimidine monomers have been incorporated into heterocyclic polymers (70MI11100). By far the greatest efforts with this ring system have involved polymerizable derivatives of nucleic acid bases and related compounds (81MH1102). The majority of this work has involved the synthesis and free radical polymerization of suitable vinyl- and acrylic-functional monomers, e.g. (134)-(136), although epoxy and other derivatives have also been studied. A number of the polymers exhibit base-paired complex formation with natural nucleic acid polymers or synthetic analogues, have found use in... 

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