Doctor of Philosophy
Dendrimers are a class of polymers with a highly branched, three-dimensional architecture comprised of an initiator core, several interior layers of repeating units, and multiple active surface terminal groups. Dendrimers have been recognized as the most versatile compositionally and structurally controlled nanoscale building blocks for drug and gene delivery. Polyamidoamine (PAMAM) dendrimers have been most investigated because of their unique structures and properties. Polycationic PAMAM dendrimers form compacted polyplexes with nucleic acids at physiological pH, holding great potential for gene delivery.
Folate receptor (FRα) is expressed at very low levels in normal tissues but expressed at high levels in cancers in order to meet the folate demand of rapidly dividing cells under low folate conditions. Our primary aim was to investigate folic acid (FA)-conjugated PAMAM dendrimer generation 4 (G4) conjugates (G4-FA) for targeted gene delivery. The in vitro cellular uptake and transfection efficiency of G4-FA conjugates and G4-FA/DNA polyplexes were investigated in Chapter 4. It was found the cellular uptake of G4-FA conjugates and G4-FA/DNA polyplexes was in a FR-dependent manner. Free FA competitively inhibited the cellular uptake of G4-FA conjugates and G4-FA/DNA polyplexes. G4-FA/DNA polyplexes were preferentially taken up by FR-positive HN12 cells but not FR-negative U87 cells. In contrast, the cellular uptake of G4 dendrimers and G4/DNA polyplexes was non-selective via absorptive endocytosis. G4-FA conjugates significantly enhanced cytocompatibility and transfection efficiency compared to G4 dendrimers. This work demonstrates that G4-FA conjugates allow FR-targeted gene delivery, reduce cytotoxicity, and enhance gene transfection efficiency.
The in vivo biodistribution of G4-FA conjugates and anticancer efficacy of G4-FA/siRNA polyplexes were investigated in Chapter 5. Vascular endothelial growth factor A (VEGFA) is one of the major regulators of angiogenesis, essential for the tumor development. It was found G4-FA/siVEGFA polyplexes significantly knocked down VEGFA mRNA expression and protein release in HN12 cells. In the HN12 tumor-bearing nude mice, G4-FA conjugates were preferentially taken up by the tumor and retained in the tumor for at least 21 days following intratumoral (i.t.) administration. Two-dose i.t. administration of G4-FA/siVEGFA polyplexes significantly inhibited tumor growth by lowering tumor angiogenesis. In contrast, two-dose i.t. administration of G4/siVEGFA polyplexes caused severe skin lesion, presumably as a result of local toxicity. Taken together, this work shows great potential for the use of G4-FA conjugates in targeted gene delivery and cancer gene therapy.
We also explored polyanionic PAMAM dendrimer G4.5 as the underlying carrier to carry camptothecin (CPT) for glioblastoma multiforme therapyin Chapter 6. "Click" chemistry was applied to improve polymer-drug coupling reaction efficiency. The CPT-conjugate displayed a dose-dependent toxicity with an IC50 of 5 μM, a 185-fold increase relative to free CPT, presumably as a result of slow release. The conjugated CPT resulted in G2/M arrest and cell death while the dendrimer itself had little to no toxicity. This work indicates highly efficient "click" chemistry allows for the synthesis of multifunctional dendrimers for sustained drug delivery.
Immobilizing PAMAM dendrimers to the cell surface may represent an innovative method of enhancing cell surface loading capacity to deliver therapeutic and imaging agents. In Chapter 7, macrophage RAW264.7 (RAW) was hybridized with PAMAM dendrimer G4.0 (DEN) on the basis of bioorthogonal chemistry. Efficient and selective cell surface immobilization of dendrimers was confirmed by confocal microscopy. It was found the viability and motility of RAW-DEN hybrids remained the same as untreated RAW cells. Furthermore, azido sugar and dendrimer treatment showed no effect on intracellular AKT, p38, and NFκB (p65) signaling, indicating that the hybridization process neither induced cell stress response nor altered normal signaling. This work shows the feasibility of applying bioorthogonal chemistry to create cell-nanoparticle hybrids and demonstrates the noninvasiveness of this cell surface engineering approach.
In summary, these studies indicate surface-modification of PAMAM dendrimer G4 with FA can effectively target at FR-positive cells and subsequently enhance in vitro transfection efficiency and in vivo gene delivery. G4-FA conjugates may serve as a versatile targeted gene delivery carrier potentially for cancer gene therapy. PAMAM dendrimers G4.5 may serve as a drug delivery carrier for the controlled release of chemotherapeutics. The immune cell-dendrimer hybrids via bioorthogonal chemistry may serve as an innovative drug and gene delivery carrier potentially for cancer chemotherapy. Taken together, engineering of PAMAM dendrimers may advance anticancer drug and gene delivery.
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