Hybrid Polymer Therapeutics Incorporating Bioresponsive, Coiled Coil Peptide Linkers

Nanopharmaceuticals or nanomedicines are defined as nanometer sized (1 to 1000 nm) complex systems, consisting of at least two components, one of which being the biologically active moiety such as a drug, peptide, protein or nucleotide. Several nano-sized hybrid therapeutics (e.g. polymer-protein conjugates) and drug delivery systems (liposomes and nanoparticles) have been approved for routine use in clinics. The term "polymer therapeutics" describes biologically active polymeric drugs, polymer-drug conjugates, polymer-protein conjugates, polymeric micelles to which a drug is covalently bound and multi-component polyplexes (containing covalent linkers) being developed as non-viral vectors for gene and protein delivery. Over the last decade, anticancer polymer therapeutics consisting of a drug covalently attached to a biocompatible polymeric carrier have been investigated intensively, and are currently in Phase I and II clinical trials. Polymers that have been most widely used to prepare these therapeutics are poly(N-(2-hydroxypropyl)methacrylamide), poly(ethylene glycol), polyglutamate and dextran, to which drug molecules (paclitaxel, campothecin or doxorubicin) are attached. These drugs are attached to the polymer backbone via a covalent linker that is stable in the circulation, but which releases the drug on arrival within the tumor cells. Linkers that have been used so far include peptides that are enzymatically cleaved by lysosomal enzymes as well as pH-sensitive cis-aconityl, hydrazone and acetal linkers that are hydrolyzed in the endosomal and lysosomal vesicles because of the local, acidic pH (4 - 6.5). Compared to low-molecular weight anticancer drugs, polymer therapeutics can offer a number of distinct advantages such as enhanced drug solubility and stability, increased plasma half-life as well as possibilities for passive targeting based on the enhanced permeability and retention (EPR) effect, which is based on the tendency of macromolecules of sufficient high molecular weight larger than 40 kDa to preferentially accumulate in tumor tissue. Keeping in mind the increasing clinical demand for innovative therapeutics, the aim of this Thesis was to prepare novel polymer therapeutics containing (more) effective non-covalent linkers to promote intracellular delivery of a bioactive cargo. In our conceptually novel design of polymer therapeutics, the bioactive cargo is attached to a polymer backbone via a non-covalent, biologically-inspired coiled coil linker, which is formed by heterodimerization of two complementary peptide sequences that are linked to the polymer carrier, respectively, the cargo. This new class of polymer therapeutics was designed to have similar properties and to follow the same endocytic cell uptake pathway as their covalent analogues. Delivery of the therapeutic cargo proceeds through a drop in pH in the endosomal compartments, which would disrupt the coiled coil complex. The motivation for using a heterodimeric coiled-co