Synthesis And Characterization Of Surface Functionalized Poly(Amidoamine) Dendrimers As Nanovectors For Drug Delivery

Dendrimers belong to a special family of macromolecules, and are distinct from the traditional polymers by virtue of their unique three-dimensional architecture, well-defined structures and a large number of surface terminals located within a nanosized volume. They are an ideal platform for drug delivery in nanobiotechnology and nanomedicine thanks to their nanosize dimension and high drug payload to offer potentially high local concentration for more sensitive defection and better therapeutic effects. In my PhD thesis, I will briefly present the unique properties of dendrimers and discuss their promising potentials in biomedical applications.My host laboratory is a leading group in the design and synthesis of functional dendrimers as safe and efficacious nanovectors for nucleic acid delivery in gene therapy. Earlier work from this group demonstrated that high generations of structurally flexible poly(amido)amine (PAMAM) dendrimers bearing a triethanolamine (TEA) core are excellent nanovectors to deliver RNAi therapeutics in different disease models in vitro and in vivo. The dendrimer of generation5has been scheduled for clinical trials in USA and UK. However, the large-scale synthesis of this high generation dendrimer is technically demanding, making it difficult to meet the "good manufacturing practice"(GMP) grade required for clinical trials. Therefore, developing lower generation dendrimers of this family through structural modification to improve delivery activity constitutes a worthwhile goal. My Ph.D project mainly focused on the surface modifications of these dendrimers to improve their nucleic acid delivery efficiency and explore the possible effective nanovectors of low generation dendrimer for gene therapy.It is known that arginine-rich motifs have the properties to promote cell penetration. We hence designed and synthesized TEA-core PAMAM dendrimers bearing arginines on the dendrimer surface in order to enhance the cell membrane penetration and foster delivery efficiency. Indeed, the arginine-terminated TEA-core PAMAM dendrimer of generation4(G4Arg) proved to be effective at delivering siRNAs. Further investigation confirmed that this dendrimer was bestowed with a superior capacity to form stable dendriplexes with siRNA and enhance cell uptake, resulting in significantly magnified gene silencing compared to its non-arginine-terminating dendrimer counterpart G4. Moreover, G4Arg displayed no discernible toxicity and was able to deliver siRNA in prostate cancer models, producing significant gene silencing and potent anticancer activity both in vitro and in vivo. All these findings demonstrate that1) decoration of the dendrimer surface with arginine residues is an effective strategy to improve the delivery ability of dendrimers; and2) our G4Arg is a highly promising nanovector for efficacious siRNA delivery and holds great potential for further therapeutic applications.Meanwhile, in our continuous efforts to fight against life-threatening cancers, radio-pharmaceutics constitutes an important means for both cancer diagnostics and clinic treatment. However, due to the non-specific systemic delivery, radioactive diagnostic reagents are often randomly dispersed in the body, causing radiodamage in normal cells/tissues and at the same time lowering detection sensitivity and reduce therapeutic effects. On the basis of the multivalent property of dendrimers, dendrimer-based delivery of radio-pharmaceutics offers a promising solution to overcome these limitations. In the second part of my thesis, I described the design, synthesis and characterization of an amphiphilic dendrimer harbouring a chelating agent on the surface with the aim of carrying radioactive isotope as theranostic in cancer therapy. We also designed and synthesized an amphiphile containing the cancer targeting motif RGDC, with which we expect to increase the cancer targeting ability, and hence enhance the sensitivity for cancer diagnostics and increase the anticancer effect while reduce the non-specific toxicity for cancer treatment.In the last part of my thesis, I focused on the design and synthesis of deuterium (D) labeled dendrimers in order to further study the mechanism of dendrimer-mediated drug delivery using the non-destructive label-free Raman spectroscopy. The synthesis of these D-labelled dendrimer proved straightforward following the well-established protocols developed in this thesis. All analytical data including NMR, MS and Raman demonstrated that the deuterium atoms have been successfully incorporated into the dendrimers, which offer an unique means for further Raman study on the dendrimer-based drug delivery mechanism.In conclusion, we have successfully designed and synthesized a series of surface functionalized dendrimers, including arginine-terminated, deuterium labeled and radio-pharmaceutics chelating agent carrying dendrimers. Among them, arginine-terminated dendirmers could promote efficient cell uptake, improve effectively siRNA delivery for gene silencing, leading to potent anticancer effect both in vitro and in vivo, thus it holds great potential for further clinical applications. The radioactive isotope chelating agent bearing dendrimers developed in this thesis are aimed to provide tumor tissue targeted delivery to improve tumor detection sensitivity, increase anticancer activity while reduce non-specific toxicity and radiodamage. Finally, the D-labeled dendrimers will be used as the label-free probes for the study on the drug delivery mechanism using Raman spectroscopy. We are actively pursuing our research in these directions.