PAMAM Dendrimer as Quantized Building Blocks for Biomedical Applications.

Abstract

Heterogeneity, whether from polydispersity within the polymeric scaffold or due to a broad distribution of covalently conjugated products, is a remaining challenge in theranostics. Chapter 1 of this thesis presents the challenges conjugate heterogeneity introduces to synthesis, characterization, and application of multivalent conjugates. A brief review of recent literature approaches to address sample heterogeneity is also provided. Chapter 2 highlights the heterogeneity of the scaffold of interest, poly(amidoamine) dendrimer. It was found that the mass range of commercial polymer was ~1.4-115 kDa, and that skeletal defects in the dendrimer structure occur at a rate of 8-15%. Methods to isolate dendrimer monomers with decreased polydispersity are introduced. Chapter 3 of this thesis presents the first application of these monomers as soft superatoms for the assembly of new, precise nanostructures. New chromatography strategies to obtain ligand-dendrimer conjugates with precise ratios are introduced, and these conjugates are employed as building blocks for novel megamers. In Chapter 4, dendrimers with precise numbers of ring-strain promoted click ligands are clicked to folic acid-azide derivatives to synthesize dendrimer-folic acid conjugates with narrow ligand-to-dendrimer ratio distributions. These materials are evaluated for binding to a folate binding protein, and the results used to interpret the mechanism of increased binding of these conjugates compared to free folic acid. Mathematical models were developed to differentiate between three proposed mechanisms, and it was determined that the observed increase in avidity is due to nonspecific interactions between the polymer scaffold and protein, which are initiated by a specific interaction between folic acid and the protein. Chapter 5 examines the impact of scaffold size, ligand-to-scaffold linking systems, and conjugate valency on the activity of dendrimer-methotrexate conjugates. The larger mass and diameter of dimer impurities contributed significantly to observed activity of these and previous conjugates. The weaker methotrexate-folate binding protein interaction was not strong enough to key the nonspecific interactions until higher valencies in larger conjugates. The final chapter provides conclusions as to how this thesis impacts the current field of multivalent conjugates for targeted drug delivery, and gives some insight to where the field must continue to grow.