Impact of Dendritic Polymer Architecture on Self-assembly, Cellular Interactions, and Protein Adsorption

Nanocarriers have demonstrated their great potential to revolutionize the diagnosis, treatment, and prevention of a variety of diseases, especially cancer. In this dissertation research, we systematically explored the role of dendritic polymer architecture on the self-assembly and cellular interactions of nanocarriers. A set of novel PEGylated dendron-based copolymers (PDC) were synthesized and characterized along with self-assembled dendron micelles (DM) with various surface functional groups and hydrophilic-lipophilic balances (HLB). The critical micelle concentration (CMC) of PDCs varied from 6.50 × 10-8 to 9.3 × 10-7 M and compared to synthesized linear-block copolymer (LBC) counterparts, the CMCs of PDCs were up to 100-times lower at similar HLBs, demonstrating their superior thermodynamic stability. Molecular dynamics (MD) simulations revealed that the hydrophobic core of the DM was more completely covered by dense poly(ethylene glycol) (PEG) compared to the linear micelle (LM). In vitro analyses of the DMs revealed that both the formation of non-specific and specific cellular interactions were controllable through modulation of the PEG corona length. Using folic acid (FA) as the model targeting agent, variation of the PEG corona length and PDC-FA content resulted in targeted DMs that achieved modular cellular interactions, ranging from minimal to 27-fold enhancements, compared to non-targeted DMs. The targeting efficiency of FA-targeted DMs was then compared to FA-targeted LMs at physiologically relevant (high serum) culture conditions. DMs and LMs exhibited similar targeting efficiencies in non-serum containing media; however, the use of serum containing media substantially reduced the targeting ability of LM, while DMs remained unaffected. Further studies provided evidence for the positive role of dendritic polymer architectures for overcoming the negative effect of protein corona formation on targeted cellular interactions. The controlled, high stability self-assembled structures produced by PDCs as well as the ability to engineer the cellular interactions of DMs through simple manipulation of physicochemical properties such as PEG corona length provide compelling evidence for the future development of DMs as targeted drug delivery platforms

Impact of Light Intensity on Control in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization

Organic photoredox catalysts have been shown to operate organocatalyzed atom transfer radical polymerizations (O-ATRP) using visible light as the driving force. In this work, the effect of light intensity from white LEDs was evaluated as an influential factor in control over the polymerization and the production of well-defined polymers. We posit the irradiation conditions control the concentrations of various catalyst states necessary to mediate a controlled radical polymerization. Systematic dimming of white LEDs allowed for consideration of the role of light intensity on the polymerization performance. The general effects of decreased irradiation intensity in photoinduced O-ATRP were investigated through comparing two different organic photoredox catalysts: perylene and an 3,7-di­(4-biphenyl) 1-naphthalene-10-phenoxazine. Previous computational efforts have investigated catalyst photophysical and electrochemical characteristics, but the broad and complex effects of varied irradiation intensity as an experimental variable on the mechanism of O-ATRP have not been explored. This work revealed that perylene requires more stringent irradiation conditions to achieve controlled polymer molecular weight growth and produce polymers with dispersities <1.50. In contrast, the 3,7-di­(4-biphenyl) 1-naphthalene-10-phenoxazine is more robust, achieving linear polymer molecular weight growth under relative irradiation intensity as low as 25%, to produce polymers with dispersities <1.50. This finding is significant, as the discovery of highly robust catalysts is necessary to allow for the adoption of successful O-ATRP in a wide scope of conditions, including those which necessitate low light intensity irradiation

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization Using Continuous Flow

Organocatalyzed atom transfer radical polymerization (O-ATRP) has emerged as a metal-free variant of historically transition-metal reliant atom transfer radical polymerization. Strongly reducing organic photoredox catalysts have proven capable of mediating O-ATRP. To date, operation of photoinduced O-ATRP has been demonstrated in batch reactions. However, continuous flow approaches can provide efficient irradiation reaction conditions and thus enable increased polymerization performance. Herein, the adaptation of O-ATRP to a continuous flow approach has been performed with multiple visible-light absorbing photoredox catalysts. Using continuous flow conditions, improved polymerization results were achieved, consisting of narrow molecular weight distributions as low as 1.05 and quantitative initiator efficiencies. This system demonstrated success with 0.01% photocatalyst loadings and a diverse methacrylate monomer scope. Additionally, successful chain-extension polymerizations using 0.01 mol % photocatalyst loadings reveal continuous flow O-ATRP to be a robust and versatile method of polymerization

Dendritic nanoparticles: the next generation of nanocarriers?

Dendritic polymers have attracted a great deal of scientific interests due to their well-defined unique structure and capability to be multifunctionalized. Here we present a comprehensive overview of various dendrimer-based nanoparticles that are currently being investigated for drug delivery and diagnostics applications. Through a critical review of the old and new dendritic designs, we highlight the advantages and disadvantages of these systems and their structure- biological property relationships. This paper also focuses on the major challenges facing the clinical translation of these nanodevices, and how these challenges are being (or should be) addressed, which will greatly benefit the overall progress of dendrimer-based technologies for theranostics

Size and Surface Charge of Engineered Poly(amidoamine) Dendrimers Modulate Tumor Accumulation and Penetration: A Model Study Using Multicellular Tumor Spheroids

An enormous effort has been put into designing nanoparticles (NPs) with controlled biodistributions, prolonged plasma circulation times, and/or enhanced tissue targeting. However, little is known about how to design NPs with precise distributions in the target tissues. In particular, understanding NP tumor penetration and accumulation characteristics is crucial to maximizing the therapeutic potential of drug molecules carried by the NPs. In this study, we employed poly­(amidoamine) (PAMAM) dendrimers, given their well-controlled size (<10 nm) and surface charge, to understand how the physical properties of NPs govern their tumor accumulation and penetration behaviors. We demonstrate for the first time that the size and surface charge of PAMAM dendrimers control their distributions in both a 3D multicellular tumor spheroid (MCTS) model and a separate extracellular matrix (ECM) model, which mimics the tumor microenvironment. Smaller PAMAM dendrimers not only diffused more rapidly in the ECM model but also efficiently penetrated to the MCTS core compared to their larger counterparts. Furthermore, cationic, amine-terminated PAMAM dendrimers exhibited the greatest accumulation in MCTS compared to either charge-neutral or anionic dendrimers. Our findings indicate that the size and surface charge of PAMAM dendrimers may tailor their tumor accumulation and penetration behaviors. These results suggest that controlled tumor accumulation and distinct intratumoral distributions can be achieved by simply controlling the size and surface charge of dendrimers, which may also be applicable for other similarly sized NPs

Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (Φ_ISC) in a highly reducing (<i>E</i>⁰* = −1.7 V vs SCE), visible-light-absorbing phenoxazine-based PC. Simple substitution of <i>N</i>-phenyl for <i>N</i>-naphthyl is shown to dramatically increase Φ_ISC from 0.11 to 0.91 without altering catalytically important properties, such as <i>E</i>⁰*

Organocatalyzed Atom Transfer Radical Polymerization Using <i>N</i>‑Aryl Phenoxazines as Photoredox Catalysts

<i>N</i>-Aryl phenoxazines have been synthesized and introduced as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization for the synthesis of well-defined polymers. Experiments confirmed quantum chemical predictions that, like their dihydrophenazine analogs, the photoexcited states of phenoxazine photoredox catalysts are strongly reducing and achieve superior performance when they possess charge transfer character. We compare phenoxazines to previously reported dihydrophenazines and phenothiazines as photoredox catalysts to gain insight into the performance of these catalysts and establish principles for catalyst design. A key finding reveals that maintenance of a planar conformation of the phenoxazine catalyst during the catalytic cycle encourages the synthesis of well-defined macromolecules. Using these principles, we realized a core substituted phenoxazine as a visible light photoredox catalyst that performed superior to UV-absorbing phenoxazines as well as previously reported organic photocatalysts in organocatalyzed atom transfer radical polymerization. Using this catalyst and irradiating with white LEDs resulted in the production of polymers with targeted molecular weights through achieving quantitative initiator efficiencies, which possess dispersities ranging from 1.13 to 1.31

Poly(ethylene glycol) Corona Chain Length Controls End-Group-Dependent Cell Interactions of Dendron Micelles

To systematically investigate the relationship among surface charge, PEG chain length, and nano–bio interactions of dendron-based micelles (DMs), a series of PEGylated DMs with various end groups (−NH₂, −Ac, and −COOH) and PEG chain lengths (600 and 2000 g/mol) are prepared and tested <i>in vitro</i>. The DMs with longer PEG chains (DM_2K) do not interact with cells despite their positively charged surfaces. In sharp contrast, the DMs with shorter PEG chains (DM₆₀₀) exhibit charge-dependent cellular interactions, as observed in both <i>in vitro</i> and molecular dynamics (MD) simulation results. Furthermore, all DMs with different charges display enhanced stability for hydrophobic dye encapsulation compared to conventional linear-block copolymer-based micelles, by allowing only a minimal leakage of the dye <i>in vitro</i>. Our results demonstrate the critical roles of the PEG chain length and polymeric architecture on the terminal charge effect and the stability of micelles, which provides an important design cue for polymeric micelles

Positively Charged Dendron Micelles Display Negligible Cellular Interactions

PEGylated dendron-based copolymers (PDCs) with different end-group functionalities (−NH₂, −COOH, and −Ac) were synthesized and self-assembled into dendron micelles to investigate the effect of terminal surface charges on size, morphology, and cellular interactions of the micelles. All of the dendron micelles exhibited similar sizes (20–60 nm) and spherical morphologies, as measured using dynamic light scattering and transmission electron microscopy, respectively. The cellular interactions of dendron micelles were evaluated using confocal microscopy and flow cytometry. Surprisingly, although amine-terminated dendrimers are known to strongly interact with cells nonspecifically, all of the surface-modified dendron micelles exhibited charge-independent low levels of cellular interactions. The unexpected results, particularly from the amine-terminated dendron micelles, could be attributed to: (i) minimal end-group effects, as each PDC has an approximately 10-fold lower charge-number-to-molecular-weight ratio compared to the dendrimer, and (ii) intra- and intermolecular hydrogen bonding between positively charged terminal groups with poly­(ethylene glycol) (PEG) backbones, which leads to the sequestration of the charges, as demonstrated by atomistic molecular dynamics simulations. With the narrow size distribution, uniform morphologies, and low levels of nonspecific cellular interactions, the dendron micelles offer a promising drug delivery platform

Structure–Property Relationships for Tailoring Phenoxazines as Reducing Photoredox Catalysts

Through the study of structure–property relationships using a combination of experimental and computational analyses, a number of phenoxazine derivatives have been developed as visible light absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-DFT) computational modeling of the photoexcitation of <i>N</i>-aryl and core modified phenoxazines guided the design of PCs with absorption profiles in the visible regime. In accordance with our previous work with <i>N</i>,<i>N</i>-diaryl dihydrophenazines, characterization of noncore modified <i>N</i>-aryl phenoxazines in the excited state demonstrated that the nature of the <i>N</i>-aryl substituent dictates the ability of the PC to access a charge transfer excited state. However, our current analysis of core modified phenoxazines revealed that these molecules can access a different type of CT excited state which we posit involves a core substituent as the electron acceptor. Modification of the core of phenoxazine derivatives with electron-donating and electron-withdrawing substituents was used to alter triplet energies, excited state reduction potentials, and oxidation potentials of the phenoxazine derivatives. The catalytic activity of these molecules was explored using organocatalyzed atom transfer radical polymerization (O-ATRP) for the synthesis of poly­(methyl methacrylate) (PMMA) using white light irradiation. All of the derivatives were determined to be suitable PCs for O-ATRP as indicated by a linear growth of polymer molecular weight as a function of monomer conversion and the ability to synthesize PMMA with moderate to low dispersity (dispersity less than or equal to 1.5) and initiator efficiencies typically greater than 70% at high conversions. However, only PCs that exhibit strong absorption of visible light and strong triplet excited state reduction potentials maintain control over the polymerization during the entire course of the reaction. The structure–property relationships established here will enable the application of these organic PCs for O-ATRP and other photoredox-catalyzed small molecule and polymer syntheses