Guorong Sun

Polymeric Nanostructures for Imaging and Therapy.

Medical diagnosis and therapy are essential for providing patients with proper care, although inefficient diagnosis and therapy are usually associated with either improper detection of the diseases, unsatisfactory therapeutic outcomes and/or serious adverse reactions. Advances in the design of various diagnostic and therapeutic agents, and the recent trend of utilizing molecules for both therapeutic and diagnostic applications (i.e. theranostics), still have not achieved the maximum benefits of controlling the navigation and biodistribution of these molecules within the biological system. The key challenges towards the use of these agents, from small molecules to macromolecular drugs (e.g. natural, proteins and nucleic acids-based drugs, or synthetic, polymer-based conjugates, carriers or other systems), are, for example, the loss of activity via rapid clearance or degradation, inefficient delivery to the target sites, and inappropriate probing of the disease states, dependent on the particular disease and its location in the body. The concept of nanotechnology has been initiated early in 1959 by Richard Feynman in his famous historical talk at Caltech “There’s Plenty of Room at the Bottom”, with introduction of the possibility of manipulating materials at the atomic and molecular levels.1 In 1974, Norio Taniguchi, at Tokyo University, first utilized the term “nanotechnology” referring to the design of materials on the nanoscale.2 In the early 1990’s and until now, the use of nanomaterials of different nature (organic and inorganic), and for various applications (multiple disciplines) has been greatly expanded, in particular, over the last couple of decades.3-4 In the medical field, nanotechnology has emerged to include non-invasive systems for probing of disease and also capable of carrying cargo for localized high concentration delivery, known as “nanomedicine”, with reduction of off-target effects. The use of nanomaterials, in particular polymeric nanostructures, has demonstrated efficiency in improving delivery of diagnostic and therapeutic agents to the target sites, and the feasibility of incorporating several therapeutic/diagnostic/targeting moieties within specific compartments of the nanoparticles, with control of their navigation in the body and to the target sites. Further understanding of the nature and microenvironments of biological systems (e.g. different pH, temperature, permeability, drainage, or overexpressing proteins, enzymes or receptors), and the barriers towards the delivery of various moieties to their destinations, which could be either intra- or extracellular, has aided the design of nanomaterials that could evade the various physiological barriers. Selective delivery to the site of the disease can increase the therapeutic efficacy, imaging contrast and accuracy, reduce adverse reactions, and reduce the dose and cost of medications. Initially, platform technologies were the target for nanostructure designs, but with the complications of biological systems, it has been recognized over the past decade that disease- and patient-specific medical treatment is needed for efficacy—this review highlights a few examples developed within the past couple of years, with a focus on in vivo studies together with novel designs and significant advances in syntheses. The advantages of polymeric nanostructures over other types of nanomaterials are based upon the flexibility over which their structures can be modified to yield materials of various compositions, morphologies, sizes, surface properties, with possibility of hierarchical assembly of several nanomaterials of various components into one construct that can be accommodated with a variety of therapeutic, diagnostic and/or targeting moieties, within selective compartments of the nanodevices. High efficiency in diagnosis and treatment of diseases and improving patient quality of life and compliance can be achieved through understanding the molecular events associated with various diseases, and combining the advances in the design of therapeutic and diagnostic agents and nanomaterials, together with the innovative instruments utilized for monitoring these agents. This review will focus on several recent advances in the design of polymeric nanoparticles that have been utilized for delivery of diagnostic and/or therapeutic agents, and the various barriers towards the clinical development of these materials. After a brief overview of the capabilities and challenges with medical imaging and therapy, in general, disease-specific examples of polymer nanoparticles designed specifically to overcome the challenges and address unmet medical needs will be discussed in detail.

Gold nanocages covered with thermally-responsive polymers for controlled release by high-intensity focused ultrasound

This paper describes the use of Au nanocages covered with smart, thermally-responsive polymers for controlled release with high-intensity focused ultrasound (HIFU). HIFU is a highly precise medical procedure that uses focused ultrasound to heat and destroy pathogenic tissue rapidly and locally in a non-invasive or minimally invasive manner. The released dosage could be remotely controlled by manipulating the power of HIFU and/or the duration of exposure. We demonstrated localized release within the focal volume of HIFU by using gelatin phantom samples containing dye-loaded Au nanocages. By placing chicken breast tissues on top of the phantoms, we further demonstrated the feasibility of this system for controlled release at depths up to 30 mm. Because it can penetrate more deeply into soft tissues than near-infrared light, HIFU is a potentially more effective external stimulus for rapid, on-demand drug release.

Copper‐64‐Alloyed Gold Nanoparticles for Cancer Imaging: Improved Radiolabel Stability and Diagnostic Accuracy

Gold nanoparticles, especially positron-emitter- labeled gold nanostructures, have gained steadily increasing attention in biomedical applications. Of the radionuclides used for nanoparticle positron emission tomography imaging, radiometals such as (64) Cu have been widely employed. Currently, radiolabeling through macrocyclic chelators is the most commonly used strategy. However, the radiolabel stability may be a limiting factor for further translational research. We report the integration of (64) Cu into the structures of gold nanoparticles. With this approach, the specific radioactivity of the alloyed gold nanoparticles could be freely and precisely controlled by the addition of the precursor (64) CuCl2 to afford sensitive detection. The direct incorporation of (64) Cu into the lattice of the gold nanoparticle structure ensured the radiolabel stability for accurate localization in vivo. The superior pharmacokinetic and positron emission tomography imaging capabilities demonstrate high passive tumor targeting and contrast ratios in a mouse breast cancer model, as well as the great potential of this unique alloyed nanostructure for preclinical and translational imaging.

Facile, efficient approach to accomplish tunable chemistries and variable biodistributions for shell cross-linked nanoparticles.

The in vivo behavior of shell cross-linked knedel-like (SCK) nanoparticles is shown to be tunable via a straightforward and versatile process that advances SCKs as attractive nanoscale carriers in the field of nanomedicine. Tuning of the pharmacokinetics was accomplished by grafting varied numbers of methoxy-terminated poly(ethylene glycol) (mPEG) chains to the amphiphilic block copolymer precursors, together with chelators for the radioactive tracer and therapeutic agent (64)Cu, followed by self-assembly into block copolymer micelles and chemical cross-linking throughout the shell regions. (64)Cu-radiolabeling was then performed to evaluate the SCKs in vivo by means of biodistribution experiments and positron emission tomography (PET). It was found that the blood retention of PEGylated SCKs could be tuned, depending on the mPEG grafting density and the nanoparticle surface properties. A semiquantitative model of the density of mPEG surface coverage as a function of in vivo behavior was applied to enhance the understanding of this system.

Multicompartment Polymer Nanostructures with Ratiometric Dual-Emission pH-Sensitivity

Pyrazine-labeled multicompartment nanostructures are shown to exhibit enhanced pH-responsive blue-shifted fluorescence emission intensities compared to their simpler core-shell spherical analogs. An amphiphilic linear triblock terpolymer of ethylene oxide, N-acryloxysuccinimide, and styrene, PEO(45)-b-PNAS(105)-b-PS(45), which lacks significant incompatibility for the hydrophobic block segments and undergoes gradual hydrolysis of the NAS units, underwent supramolecular assembly in mixtures of organic solvent and water to afford multicompartment micelles (MCMs) with a narrow size distribution. The assembly process was followed over time and found to evolve from individual polymer nanodroplets containing internally phase segregated domains, of increasing definition, and ultimately to dissociate into discrete micelles. Upon covalent cross-linking of the MCMs with pH-insensitive pyrazine-based diamino cross-linkers, pH-responsive, photonic multicompartment nanostructures (MCNs) were produced. These MCNs exhibited significant enhancement of overall structural stability, in comparison with the MCMs, and internal structural tunability through the cross-linking chemistry. Meanwhile, the complex compartmentalized morphology exerted unique pH-responsive fluorescence dual-emission properties, indicating promise in ratiometric pH-sensing applications.

Orthogonally dual-clickable Janus nanoparticles via a cyclic templating strategy.

Synthetic asymmetrical systems, Janus particles and patchy particles, are capable of undergoing hierarchical assembly processes that mimic those of Nature, to serve as switchable devices, optical probes, phase-transfer catalysts, and multifunctional drug carriers, each of which benefits from opposing surface patterns that behave differently. Production of nanometer-sized Janus particles that are equipped with efficient chemistries remains a challenge. A robust Janus-faced polymer nanoparticle framework that presents two orthogonally click-reactive surface chemistries has been generated by a recyclable strategy that involves reactive functional group transfer by templating against gold nanoparticle substrates. This anisotropic functionalization approach is compatible with a wide range of soft materials, providing Janus nanoparticles for the construction of dual-functionalized devices by accurately controlling chemical functionality at the nanoscopic level.

Nanoscopic Cylindrical Dual Concentric and Lengthwise Block Brush Terpolymers as Covalent Preassembled High-Resolution and High-Sensitivity Negative-Tone Photoresist Materials

We describe a high-resolution, high-sensitivity negative-tone photoresist technique that relies on bottom-up preassembly of differential polymer components within cylindrical polymer brush architectures that are designed to align vertically on a substrate and allow for top-down single-molecule line-width imaging. By applying cylindrical diblock brush terpolymers (DBTs) with a high degree of control over the synthetic chemistry, we achieved large areas of vertical alignment of the polymers within thin films without the need for supramolecular assembly processes, as required for linear block copolymer lithography. The specially designed chemical compositions and tuned concentric and lengthwise dimensions of the DBTs enabled high-sensitivity electron-beam lithography of patterns with widths of only a few DBTs (sub-30 nm line-width resolution). The high sensitivity of the brush polymer resists further facilitated the generation of latent images without postexposure baking, providing a practical approach for controlling acid reaction/diffusion processes in photolithography.

Benzaldehyde-functionalized polymer vesicles.

Polymer vesicles with diameters of ca. 100-600 nm and bearing benzaldehyde functionalities within the vesicular walls were constructed through self-assembly of an amphiphilic block copolymer PEO(45)-b-PVBA(26) in water. The reactivity of the benzaldehyde functionalities was verified by cross-linking the polymersomes and also by a one-pot cross-linking and functionalization approach to further render the vesicles fluorescent, each via reductive amination. In vitro studies found these labeled nanostructures to undergo cell association.

Directing Self-Assembly of Nanoscopic Cylindrical Diblock Brush Terpolymers into Films with Desired Spatial Orientations: Expansion of Chemical Composition Scope

Diblock brush terpolymers (DBTs) with different fluorinated methacrylate-based block segments are synthesized through sequential ring-opening metathesis polymerizations and are used to prepare polymer thin films with predictable film thicknesses. These DBTs exhibit preferable substrate vertical alignments within the films, induced by the relatively lower surface energy of the fluorinated structural components, together with the overall cylindrical morphology of the brush architecture.

A fundamental investigation of cross-linking efficiencies within discrete nanostructures, using the cross-linker as a reporting molecule

Various bi-functional pyrazine-based chromophores were used as cross-linkers to probe directly the efficiencies of their incorporation into the shell of block copolymer micelles. In addition, the block copolymer micelles were made to carry pre-installed reactive functionalities along the central block of an amphiphilic triblock copolymer. Unique photo-physical characteristics were observed, depending upon the type of pyrazine cross-linker, the conditions used for cross-linking and the stoichiometries applied.