Polymeric Nanoparticles for Drug Delivery.

The recent emergence of nanomedicine has revolutionized the therapeutic landscape and necessitated the creation of more sophisticated drug delivery systems. Polymeric nanoparticles sit at the forefront of numerous promising drug delivery designs, due to their unmatched control over physiochemical properties such as size, shape, architecture, charge, and surface functionality. Furthermore, polymeric nanoparticles have the ability to navigate various biological barriers to precisely target specific sites within the body, encapsulate a diverse range of therapeutic cargo and efficiently release this cargo in response to internal and external stimuli. However, despite these remarkable advantages, the presence of polymeric nanoparticles in wider clinical application is minimal. This review will provide a comprehensive understanding of polymeric nanoparticles as drug delivery vehicles. The biological barriers affecting drug delivery will be outlined first, followed by a comprehensive description of the various nanoparticle designs and preparation methods, beginning with the polymers on which they are based. The review will meticulously explore the current performance of polymeric nanoparticles against a myriad of diseases including cancer, viral and bacterial infections, before finally evaluating the advantages and crucial challenges that will determine their wider clinical potential in the decades to come.

Self-Immolative Polymer Nanoparticles with Precise and Controllable pH-Dependent Degradation.

Polymer nanoparticles have generated significant interest as delivery systems for therapeutic cargo. Self-immolative polymers (SIPs) are an interesting category of materials for delivery applications, as the characteristic property of end-to-end depolymerization allows for the disintegration of the delivery system, facilitating a more effective release of the cargo and clearance from the body after use. In this work, nanoparticles based on a pH-responsive polymer poly(ethylene glycol)-b-(2-diisopropyl)amino ethyl methacrylate) and a self-immolative polymer poly[N,N-(diisopropylamino)ethyl glyoxylamide-r-N,N-(dibutylamino)ethyl glyoxylamide] (P(DPAEGAm-r-DBAEGAm)) were developed. Four particles were synthesized based on P(DPAEGAm-r-DBAEGAm) polymers with varied diisopropylamino to dibutylamino ratios of 4:1, 2:1, 2:3, and 0:1, termed 4:1, 2:1, 2:3, and 0:1 PGAm particles. The pH of particle disassembly was tuned from pH 7.0 to pH 5.0 by adjusting the ratio of diisopropylamino to dibutylamino substituents on the pendant tertiary amine. The P(DPAEGAm-r-DBAEGAm) polymers were observed to depolymerize (60-80%) below the particle disassembly pH after ∼2 h, compared to <10% at pH 7.4 and maintained reasonable stability at pH 7.4 (20-50% depolymerization) after 1 week. While all particles exhibited the ability to load a peptide cargo, only the 4:1 PGAm particles had higher endosomal escape efficiency (∼4%) compared to the 2:3 or 0:1 PGAm particles (<1%). The 4:1 PGAm particle is a promising candidate for further optimization as an intracellular drug delivery system with rapid and precisely controlled degradation.

Lewis Base Catalyzed Synthesis of Sulfur Heterocycles via the C1-Pyridinium Enolate.

While the addition of C1-Lewis base enolates to carbonyls and related structures are well established, the related addition to thiocarbonyls compounds are unknown. Herein, we report a reaction cascade in which a C1-pyridinium enolate undergos addition to dithioesters, trithiocarbonates and xanthates. The reaction provides access to a range of dihydrothiophenes and dihydrothiopyrans (28-examples). Mechanistic investigations, including isolation of intermediates, electronic correlation, and kinetic isotope effect studies support the viability of an activated acid intermediate giving rise to the C1-pyridinium enolate which undergoes turnover limiting cyclization. Subsequent formation of a ß-thiolactone regenerates the catalyst with loss of carbon oxysulfide providing the observed products.

Polyoxometalates as chemically and structurally versatile components in self-assembled materials.

Polyoxometalates (POMs) are anionic molecular metal oxides with expansive diversity in terms of their composition, structure, nuclearity and charge. Within this vast collection of compounds are dominant structural motifs (POM platforms), that are amenable to significant chemical tuning with minimal perturbation of the inorganic oxide molecular structure. Consequently, this enables the systematic investigation of these compounds as inorganic additives within materials whereby structure and charge can be tuned independently i.e. [PW12O40]3- vs. [SiW12O40]4- while also investigating the impact of varying the charge balancing cations on self-assembly. The rich surface chemistry of POMs also supports their functionalisation by organic components to yield so-called inorganic-organic hybrids which will be the key focus of this perspective. We will introduce the modifications possible for each POM platform, as well as discussing the range of nanoparticles, microparticles and surfaces that have been developed using both surfactant and polymer building blocks. We will also illustrate important examples of POM-hybrids alongside their potential utility in applications such as imaging, therapeutic delivery and energy storage.

Understanding the Biological Interactions of pH-Swellable Nanoparticles.

pH-responsive nanoparticles have generated significant interest for use as drug delivery systems due to their potential for inducible release at low pH. The pH variation from the bloodstream (pH 7.4) to intracellular compartments of cells called endosomes/lysosomes (pH < 5.0) has been of particular interest. However, one of the limitations with nanoparticle delivery systems is the inability to migrate out of these compartments to the cytosol or other organelles, via a process termed endosomal escape. Previous studies have postulated that pH-responsive nanoparticles can facilitate endosomal escape through a range of mechanisms including membrane interaction, pH-induced swelling, and the proton-sponge effect. In this study, a series of pH-swellable nanoparticles (85-100 nm) are designed and their impact on biological interactions, particularly endosomal escape, are investigated. The particles exhibit tunable pH-induced swelling (from 120% to 200%) and have good buffering capacity. The cellular association is studied using flow cytometry and endosomal escape is determined using a calcein leakage assay. Interestingly, no endosomal escape with all nanoparticle formulations is found, which suggests there are limitations with both the proton-sponge effect and pH-induced swelling mechanism as the primary methods for inducing endosomal escape.

Quantifying the Endosomal Escape of pH-Responsive Nanoparticles Using the Split Luciferase Endosomal Escape Quantification Assay.

All nanoparticles have the potential to revolutionize the delivery of therapeutic cargo such as peptides, proteins, and RNA. However, effective cytosolic delivery of cargo from nanoparticles represents a significant challenge in the design of more efficient drug delivery vehicles. Recently, research has centered on designing nanoparticles with the capacity to escape endosomes by responding to biological stimuli such as changes in pH, which occur when nanoparticles are internalized into the endo-/lysosomal pathway. Current endosomal escape assays rely on indirect measurements and yield little quantitative information, which hinders the design of more efficient drug delivery vehicles. Therefore, we adapted the highly sensitive split luciferase endosomal escape quantification (SLEEQ) assay to better understand nanoparticle-induced endosomal escape. We applied SLEEQ to evaluate the endosomal escape behavior of two pH-responsive nanoparticles: the first with a poly(2-diisopropylamino ethyl methacrylate) (PDPAEMA) core and the second with 1:1 ratio of poly(2-diethylamino ethyl methacrylate) (PDEAEMA) and PDPAEMA. SLEEQ directly measured the cytosolic delivery and showed that engineering the nanoparticle disassembly pH could improve the endosomal escape efficiency by fivefold. SLEEQ is a versatile assay that can be used for a wide range of nanomaterials and will improve the development of drug delivery vehicles in the future.

Acid-Responsive Poly(glyoxylate) Self-Immolative Star Polymers.

Self-immolative polymers have significant potential for applications such as drug or gene delivery. However, to realize this potential, such materials need to be customized to respond to specific variations in biological conditions. In this work, we investigated the design of new star-shaped self-immolative poly(ethyl glyoxylate)s (PEtGs) and their incorporation into responsive nanoparticles. PEtGs are a subclass of stimulus-responsive self-immolative polymers, which can be combined with different stimuli-responsive functionalities. Two different tetrathiol initiators were used for the polymerization in combination with a variety of potential pH-responsive end-caps, yielding a library of star PEtG polymers which were responsive to pH. Characterization of the depolymerization behavior of the polymers showed that the depolymerization rate was controlled by the end caps rather than the architecture of the polymer. A selection of the star polymers were modified with amines to allow introduction of charge-shifting properties. It was shown that pH-responsive nanoparticles could be prepared from these modified polymers and they demonstrated pH-dependent particle disruption. The pH responsiveness of these particles was studied by dynamic light scattering and 1H nuclear magnetic resonance spectroscopy.

Multicompartment Polymeric Nanocarriers for Biomedical Applications.

Multicompartment polymeric nanocarriers which mimic the compartmentalized architecture of living cells have received considerable research attention in the biomedical field. The advancement of synthetic polymeric chemistry has allowed multicompartment polymeric nanocarriers to be tailored for biomedical applications such as drug delivery, encapsulated catalysis, and artificial cellular mimics. In this review, polymer-based multicompartment nanocarriers (multicompartment micelles, multicompartment polymersomes, and capsosomes) have been discussed. This review focuses on multicompartment systems applied to biomedical applications over the last ten years. The synthetic procedures and structural properties that impact the specific application are also highlighted.

pH-Responsive Polymer Nanoparticles for Drug Delivery.

Stimuli-responsive nanoparticles have the potential to improve the delivery of therapeutics to a specific cell or region within the body. There are many stimuli that have shown potential for specific release of cargo, including variation of pH, redox potential, or the presence of enzymes. pH variation has generated significant interest for the synthesis of stimuli-responsive nanoparticles because nanoparticles are internalized into cells via vesicles that are acidified. Additionally, the tumor microenvironment is known to have a lower pH than the surrounding tissue. In this review, different strategies to design pH-responsive nanoparticles are discussed, focusing on the use of charge-shifting polymers, acid labile linkages, and crosslinking.

The Endosomal Escape of Nanoparticles: Toward More Efficient Cellular Delivery.

Many emerging therapies rely on the delivery of biological cargo into the cytosol. Nanoparticle delivery systems hold great potential to deliver these therapeutics but are hindered by entrapment and subsequent degradation in acidic compartments of the endo/lysosomal pathway. Engineering polymeric delivery systems that are able to escape the endosome has significant potential to address this issue. However, the development of safe and effective delivery systems that can reliably deliver cargo to the cytosol is still a challenge. Greater understanding of the properties that govern endosomal escape and how it can be quantified is important for the development of more efficient nanoparticle delivery systems. This Topical Review highlights the current understanding of the mechanisms by which nanoparticles escape the endosome, and the emerging techniques to improve the quantification of endosomal escape.

Controlling endosomal escape using nanoparticle composition: current progress and future perspectives.

Polymer nanoparticles offer significant benefits for improving delivery of biological therapeutics such as DNA and proteins, as they allow the cargo to be protected until it is delivered to a target cell. However, there are still challenges with achieving efficient delivery to the optimal cellular region. One significant roadblock is escape of nanoparticles from within the endosomal/lysosomal compartments into the cytosol. Here, we review the recent advances in understanding endosomal escape of polymer nanoparticles. We also discuss the current progress on investigating how nanoparticle structure can control endosomal escape. It is important to understand the fundamental biological processes that govern endosomal escape in order to design more effective therapeutic delivery systems.

Correction: Engineered hydrogen-bonded polymer multilayers: from assembly to biomedical applications.

Correction for 'Engineered hydrogen-bonded polymer multilayers: from assembly to biomedical applications' by Georgina K. Such et al., Chem. Soc. Rev., 2011, 40, 19-29.

The potential of nanoparticle vaccines as a treatment for cancer.

A complex and multifaceted relationship exists between cancer and the immune system. Advances in our understanding of this relationship have resulted in significant clinical attention in the possibilities of cancer immunotherapy. Harnessing the immune system's potent and selective destructive capability is a major focus of attempts to treat cancer. Despite significant progress in the field, cancer therapy still remains significantly deficient, with cancer being one of the largest contributors to morbidity and mortality in the developed world. It is evident that the design of new treatment regimes is required to exploit cancer immunotherapy. Herein we review the potential for nanotechnology to overcome the challenges that have limited the more widespread implementation of immunotherapy to cancer treatment.

Nanoescapology: progress toward understanding the endosomal escape of polymeric nanoparticles.

Using nanoparticles to deliver drugs to cells has the potential to revolutionize the treatment of many diseases, including HIV, cancer, and diabetes. One of the major challenges facing this field is controlling where the drug is trafficked once the nanoparticle is taken up into the cell. In particular, if drugs remain localized in an endosomal or lysosomal compartment, the therapeutic can be rendered completely ineffective. To ensure the design of more effective delivery systems we must first develop a better understanding of how nanoparticles and their cargo are trafficked inside cells. This needs to be combined with an understanding of what characteristics are required for nanoparticles to achieve endosomal escape, along with methods to detect endosomal escape effectively. This review is focused into three sections: first, an introduction to the mechanisms governing internalization and trafficking in cells, second, a discussion of methods to detect endosomal escape, and finally, recent advances in controlling endosomal escape from polymer- and lipid-based nanoparticles, with a focus on engineering materials to promote endosomal escape. WIREs Nanomed Nanobiotechnol 2017, 9:e1452. doi: 10.1002/wnan.1452 For further resources related to this article, please visit the WIREs website.

pH-Responsive Transferrin-pHlexi Particles Capable of Targeting Cells in Vitro.

Targeting nanoparticles to specific cellular receptors has the potential to deliver therapeutic compounds to target sites while minimizing side effects. To this end, we have conjugated a targeting protein, holo-transferrin (holo-Tf), to pH-responsive polymers, poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) and poly(2-(diethylamino)ethyl methacrylate)-ran-poly(2-(diisopropylamino)ethyl methacrylate (PDEAEMA-r-PDPAEMA). These protein-polymer hybrid materials were observed to self-assemble when the pH is increased above the pKa of the polymer. We demonstrate that their response to pH could be tuned depending on the polymer constituent attached to holo-Tf. Importantly, the targeting behavior of these nanoparticles could be maximized by tuning the density of holo-Tf on the nanoparticle surface by the introduction of a (PDEAEMA-r-PDPAEMA)-b-poly(ethylene glycol) (PEG) copolymer.

Probing Endosomal Escape Using pHlexi Nanoparticles.

The effective escape of nanocarriers from endosomal compartments of the cell remains a major hurdle in nanomedicine. The endosomal escape of pH-responsive, self-assembled, dual component particles based on poly[2-(diethylamino)ethyl methacrylate)(PDEAEMA) and poly(ethylene glycol)-b-poly[2-(diethylamino)ethyl methacrylate) (PEG-b-PDEAEMA) has been recently reported. Herein, we report that polymer molecular weight (Mn ) can be used to tune endosomal escape of nanoparticle delivery systems. PDEAEMA of Mn 7 kDa, 27 kDa, 56 kDa and 106 kDa was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization and co-assembled with PEG-b-PDEAEMA (16 kDa) via nanoprecipitation. All particles had similar size, displayed pH-responsive behaviour, and low toxicity regardless of molecular weight. Ovalbumin was loaded in the particles to demonstrate loading and release capabilities and as a marker to study internalization and endosomal escape. Association and endosomal escape was found to depend on molecular weight, with enhanced escape observed for high Mn PDEAEMA: 42% of cells with particle induced endosomal escape for 106 kDa nanoparticles, compared to minimal escape for 7 kDa particles. The results show that a simple variation in molecular weight can enhance the endosomal escape of polymeric carriers, and thus improve their effectiveness for intracellular delivery of therapeutics.

HD Flow Cytometry: An Improved Way to Quantify Cellular Interactions with Nanoparticles.

Histogram deconvolution flow cytometry enables improved quantification of nanomaterial-cell interactions. The algorithm identifies the positive cells in highly overlapped populations and calculates the fluorescence intensity of the positive population. This technique performs better than commercially available methods with the additional benefit of visualizing the output.

Quantifying Nanoparticle Internalization Using a High Throughput Internalization Assay.

PURPOSE: The internalization of nanoparticles into cells is critical for effective nanoparticle mediated drug delivery. To investigate the kinetics and mechanism of internalization of nanoparticles into cells we have developed a DNA molecular sensor, termed the Specific Hybridization Internalization Probe - SHIP. METHODS: Self-assembling polymeric 'pHlexi' nanoparticles were functionalized with a Fluorescent Internalization Probe (FIP) and the interactions with two different cell lines (3T3 and CEM cells) were studied. The kinetics of internalization were quantified and chemical inhibitors that inhibited energy dependent endocytosis (sodium azide), dynamin dependent endocytosis (Dyngo-4a) and macropinocytosis (5-(N-ethyl-N-isopropyl) amiloride (EIPA)) were used to study the mechanism of internalization. RESULTS: Nanoparticle internalization kinetics were significantly faster in 3T3 cells than CEM cells. We have shown that ~90% of the nanoparticles associated with 3T3 cells were internalized, compared to only 20% of the nanoparticles associated with CEM cells. Nanoparticle uptake was via a dynamin-dependent pathway, and the nanoparticles were trafficked to lysosomal compartments once internalized. CONCLUSION: SHIP is able to distinguish between nanoparticles that are associated on the outer cell membrane from nanoparticles that are internalized. This study demonstrates the assay can be used to probe the kinetics of nanoparticle internalization and the mechanisms by which the nanoparticles are taken up by cells. This information is fundamental for engineering more effective nanoparticle delivery systems. The SHIP assay is a simple and a high-throughput technique that could have wide application in therapeutic delivery research.

Multifunctional Thrombin-Activatable Polymer Capsules for Specific Targeting to Activated Platelets.

Smart poly(2-oxazoline) (POx)-based multifunctional polymer capsules that specifically target glycoprotein (GP) IIb/IIIa on the surface of activated platelets are degraded by the serine protease thrombin and release the urokinase plasminogen activator loaded into the polymer capsules, only in the area of acute thrombosis.

Interfacing materials science and biology for drug carrier design.

Over the last ten years, there has been considerable research interest in the development of polymeric carriers for biomedicine. Such delivery systems have the potential to significantly reduce side effects and increase the bioavailability of poorly soluble therapeutics. The design of carriers has relied on harnessing specific variations in biological conditions, such as pH or redox potential, and more recently, by incorporating specific peptide cleavage sites for enzymatic hydrolysis. Although much progress has been made in this field, the specificity of polymeric carriers is still limited when compared with their biological counterparts. To synthesize the next generation of carriers, it is important to consider the biological rationale for materials design. This requires a detailed understanding of the cellular microenvironments and how these can be harnessed for specific applications. In this review, several important physiological cues in the cellular microenvironments are outlined, with a focus on changes in pH, redox potential, and the types of enzymes present in specific regions. Furthermore, recent studies that use such biologically inspired triggers to design polymeric carriers are highlighted, focusing on applications in the field of therapeutic delivery.