Water-Soluble, Biocompatible Polyphosphazenes with Controllable and pH-Promoted Degradation Behavior

Phosphorus and Silicon-Based Macromolecules as Degradable Biomedical Polymers

Synthetic polymers are indispensable in biomedical applications because they can be fabricated with consistent and reproducible properties, facile scalability, and customizable functionality to perform diverse tasks. However, currently available synthetic polymers have limitations, most notably when timely biodegradation is required. Despite there being, in principle, an entire periodic table to choose from, with the obvious exception of silicones, nearly all known synthetic polymers are combinations of carbon, nitrogen, and oxygen in the main chain. Expanding this to main-group heteroatoms can open the way to novel material properties. Herein the authors report on research to incorporate the chemically versatile and abundant silicon and phosphorus into polymers to induce cleavability into the polymer main chain. Less stable polymers, which degrade in a timely manner in mild biological environments, have considerable potential in biomedical applications. Herein the basic chemistry behind these materials is described and some recent studies into their medical applications are highlighted.

Degradable Bottlebrush Polypeptides and the Impact of their Architecture on Cell Uptake, Pharmacokinetics, and Biodistribution In Vivo

Bottlebrush polymers are highly promising as unimolecular nanomedicines due to their unique control over the critical parameters of size, shape and chemical function. However, since they are prepared from biopersistent carbon backbones, most known bottlebrush polymers are non-degradable and thus unsuitable for systemic therapeutic administration. Herein, we report the design and synthesis of novel poly(organo)phosphazene-g-poly(α-glutamate) (PPz-g-PGA) bottlebrush polymers with exceptional control over their structure and molecular dimensions (Dh ≈ 15-50 nm). These single macromolecules show outstanding aqueous solubility, ultra-high multivalency and biodegradability, making them ideal as nanomedicines. While well-established in polymer therapeutics, it has hitherto not been possible to prepare defined single macromolecules of PGA in these nanosized dimensions. A direct correlation was observed between the macromolecular dimensions of the bottlebrush polymers and their intracellular uptake in CT26 colon cancer cells. Furthermore, the bottlebrush macromolecular structure visibly enhanced the pharmacokinetics by reducing renal clearance and extending plasma half-lives. Real-time analysis of the biodistribution dynamics showed architecture-driven organ distribution and enhanced tumor accumulation. This work, therefore, introduces a robust, controlled synthesis route to bottlebrush polypeptides, overcoming limitations of current polymer-based nanomedicines and, in doing so, offers valuable insights into the influence of architecture on the in vivo performance of nanomedicines.

Characterization Techniques of Polymer Aging: From Beginning to End

Polymers have been widely applied in various fields in the daily routines and the manufacturing. Despite the awareness of the aggressive and inevitable aging for the polymers, it still remains a challenge to choose an appropriate characterization strategy for evaluating the aging behaviors. The difficulties lie in the fact that the polymer features from the different aging stages require different characterization methods. In this review, we present an overview of the characterization strategies preferable for the initial, accelerated, and late stages during polymer aging. The optimum strategies have been discussed to characterize the generation of radicals, variation of functional groups, substantial chain scission, formation of low-molecular products, and deterioration in the polymers' macro-performances. In view of the advantages and the limitations of these characterization techniques, their utilization in a strategic approach is considered. In addition, we highlight the structure-property relationship for the aged polymers and provide available guidance for lifetime prediction. This review could allow the readers to be knowledgeable of the features for the polymers in the different aging stages and provide access to choose the optimum characterization techniques. We believe that this review will attract the communities dedicated to materials science and chemistry.

Cyclo- and Polyphosphazenes for Biomedical Applications

Cyclic and polyphosphazenes are extremely interesting and versatile substrates characterized by the presence of -P=N- repeating units. The chlorine atoms on the P atoms in the starting materials can be easily substituted with a variety of organic substituents, thus giving rise to a huge number of new materials for industrial applications. Their properties can be designed considering the number of repetitive units and the nature of the substituent groups, opening up to a number of peculiar properties, including the ability to give rise to supramolecular arrangements. We focused our attention on the extensive scientific literature concerning their biomedical applications: as antimicrobial agents in drug delivery, as immunoadjuvants in tissue engineering, in innovative anticancer therapies, and treatments for cardiovascular diseases. The promising perspectives for their biomedical use rise from the opportunity to combine the benefits of the inorganic backbone and the wide variety of organic side groups that can lead to the formation of nanoparticles, polymersomes, or scaffolds for cell proliferation. In this review, some aspects of the preparation of phosphazene-based systems and their characterization, together with some of the most relevant chemical strategies to obtain biomaterials, have been described.

Factors Controlling Degradation of Biologically Relevant Synthetic Polymers in Solution and Solid State

The field of biodegradable synthetic polymers, which is central for regenerative engineering and drug delivery applications, encompasses a multitude of hydrolytically sensitive macromolecular structures and diverse processing approaches. The ideal degradation behavior for a specific life science application must comply with a set of requirements, which include a clinically relevant kinetic profile, adequate biocompatibility, benign degradation products, and controlled structural evolution. Although significant advances have been made in tailoring materials characteristics to satisfy these requirements, the impacts of autocatalytic reactions and microenvironments are often overlooked resulting in uncontrollable and unpredictable outcomes. Therefore, roles of surface versus bulk erosion, in situ microenvironment, and autocatalytic mechanisms should be understood to enable rational design of degradable systems. In an attempt to individually evaluate the physical state and form factors influencing autocatalytic hydrolysis of degradable polymers, this Review follows a hierarchical analysis that starts with hydrolytic degradation of water-soluble polymers before building up to 2D-like materials, such as ultrathin coatings and capsules, and then to solid-state degradation. We argue that chemical reactivity largely governs solution degradation while diffusivity and geometry control the degradation of bulk materials, with thin "2D" materials remaining largely unexplored. Following this classification, this Review explores techniques to analyze degradation in vitro and in vivo and summarizes recent advances toward understanding degradation behavior for traditional and innovative polymer systems. Finally, we highlight challenges encountered in analytical methodology and standardization of results and provide perspective on the future trends in the development of biodegradable polymers.

Polyphosphazene-Based Nanocarriers for the Release of Camptothecin and Epirubicin

The design and study of efficient polymer-based drug delivery systems for the controlled release of anticancer drugs is one of the pillars of nanomedicine. The fight against metastatic and invasive cancers demands therapeutic candidates with increased and selective toxicity towards malignant cells, long-term activity and reduced side effects. In this sense, polyphosphazene nanocarriers were synthesized for the sustained release of the anticancer drugs camptothecin (CPT) and epirubicin (EPI). Linear poly(dichloro)phosphazene was modified with lipophilic tocopherol or testosterone glycinate, with antioxidant and antitumor activity, and with hydrophilic Jeffamine M1000 to obtain different polyphosphazene nanocarriers. It allowed us to encapsulate the lipophilic CPT and the more hydrophilic EPI. The encapsulation process was carried out via solvent exchange/precipitation, attaining a 9.2-13.6 wt% of CPT and 0.3-2.4 wt% of EPI. CPT-loaded polyphosphazenes formed 140-200 nm aggregates in simulated body physiological conditions (PBS, pH 7.4), resulting in an 80-100-fold increase of CPT solubility. EPI-loaded polyphosphazenes formed 250 nm aggregates in an aqueous medium. CPT and EPI release (PBS, pH 7.4, 37 °C) was monitored for 202 h, being almost linear during the first 8 h. The slow release of testosterone and tocopherol was also sustained for 150 h in PBS (pH 7.4 and 6.0) at 37 °C. The co-delivery of testosterone or tocopherol and the anticancer drugs from the nanocarriers was expected. Cells of the human breast cancer cell line MCF-7 demonstrated good uptake of anticancer-drug-loaded nanocarriers after 6 h. Similarly, MCF-7 spheroids showed good uptake of the anticancer-drug-loaded aggregates after 72 h. Almost all anticancer-drug-loaded polyphosphazenes exhibited similar or superior toxicity against MCF-7 cells and spheroids when compared to raw anticancer drugs. Additionally, cell-cycle arrest in the G2/M phase was increased in response to the drug-loaded nanocarriers. Almost no toxicity of anticancer-drug-loaded aggregates against primary human lung fibroblasts was observed. Furthermore, the aggregates displayed no hemolytic activity, which is in contrast to the parent anticancer drugs. Consequently, synthesized polyphosphazene-based nanocarriers might be potential nanomedicines for chemotherapy.

Biodegradable AuNP-Based Plasmonic Nanogels as Contrast Agents for Computed Tomography and Photoacoustics

Gold nanoparticles (AuNP) are well-established contrast agents in computed tomography (CT) and photoacoustic imaging (PAI). A wide variety of AuNP sizes, shapes, and coatings have been reported for these applications. However, for clinical translation, AuNP should be excretable to avoid long-term accumulation and possible side effects. Sub-5 nm AuNP have the benefit to be excretable through kidney filtration, therefore their loading in biodegradable nanogels holds promise to result in contrast agents that have long circulation times in the vasculature and subsequent biodegradation for excretion. Polyphosphazenes are intrinsically biodegradable polymers capable of forming nanogels with high payloads, and to release their payloads upon degradation. The significant development in polyphosphazenes that have tailored degradation kinetics, and their formulation with drugs or contrast agents, has shown potential as a biodegradable platform for imaging vasculature and endogenous molecules, by combination of CT and PA modalities. Therefore, we herein present methods for the formulation of AuNP assemblies loaded in nanogels composed of biodegradable polyphosphazenes, with a size range from 50 to 200 nm. We describe protocols for their characterization by UV-vis spectroscopy, Fourier-transform infrared spectroscopy, various microscopy techniques, elemental quantification by induced coupling plasma optical emission spectroscopy and contrast production in both CT and PAI. Finally, we detail the methods to investigate their effect on cells, distribution in cells and imaging properties for detection of endogenous molecules.

Synthesis and Characterization of Macroinitiators Based on Polyorganophosphazenes for the Ring Opening Polymerization of N-Carboxyanhydrides

Among the various biocompatible amphiphilic copolymers, biodegradable ones are the most promising for the preparation of drug delivery systems since they are destroyed under physiological conditions, that, as a rule, reduce toxicity and provide controlled release of the drug. Hybrid graft-copolymers consisting of the main inorganic polyphosphazene chain and polypeptide side chains are of considerable interest for the development of delivery systems with a controlled degradation rate, since the main and side chains will have different degradation mechanisms (chemical and enzymatic hydrolysis, respectively). Variable particle degradation rate, controlled by the adjusting the composition of substituents, will allow selective delivery in vivo and controlled drug release. The present work proposes the preparation of biodegradable macroinitiators based on polyorganophosphazenes for the synthesis of hybrid copolymers. Synthesis of novel biodegradable macroinitiators based on polyorganophosphazenes was performed via macromolecular substitution of a polydichlorophosphazene chain with the sodium alcoholates, amines and amino acids. The composition of copolymers obtained was calculated using NMR. These polyorganophosphazenes bearing primary amino groups can be considered as convenient macroinitiators for the polymerization of NCA of α-amino acids in order to prepare hybrid copolymers polyphosphazene-graft-polypeptide. The developed macroinitiators were amphiphilic and self-assembled in the aqueous media into nanoparticles. Furthermore, the ability to encapsulate and release a model substance was demonstrated. In addition, the in vitro cytotoxicity of synthesized polymers was evaluated using two cell lines.

Facile preparation of pH/redox dual-responsive biodegradable polyphosphazene prodrugs for effective cancer chemotherapy

In order to maximize the therapeutic effect and and minimize the systemtic side effect of the small molecule anticancer drugs, biodegradable drug delivery systems (DDSs) that respond to tumor microenvironment (TME) have attracted significant attention. Herein, a novel redox/pH dual-responsive and biodegradable polyphosphazene (PPZ) nano-prodrugs have been prepared via one-pot crosslinking of vanillin modified DOX (VMD, acid-sensitive) and 4,4'-dihydroxydiphenyl disulfide (HPS, GSH-responsive) with hexachlorocyclotriphosphazene (HCCP). The phenol groups of the as-synthesized VMD and HPS have high nucleophilic substitution activity towards HCCP under base catalyst and afforded PPZ nano-prodrugs, denoted as HCCP-VMD-HPS, with a high drug loading ratio of up to 56.4 %. As expected, the skeleton of the PPZ consisting of imine bonds in VMD and the disulfide bonds in HPS and cyclotriphosphazenes inclined to be decomposed in low pH conditions and high level of GSH environments. The antitumor drug DOX was found to be controlled released in TME conditions (extracellular, pH∼6.8 and endosomes, lysosomes pH∼5.0 with ∼10 mM GSH), rather than neutral physiological conditions (pH 7.4 with ∼20 μM GSH). Moreover, the resulting HCCP-VMD-HPS nano-prodrug have obvious cytotoxicity to cancer cells while a negligible side effect to normal cells. We therefore believe that the prepared redox/pH dual-responsive and biodegradable PPZ DDSs have great potential in various field.

Practical phosphorylation of polymers: an easy access to fully alcohol soluble synthetically and industrially important polymers

A simple method for the phosphorylation of synthetically and industrially important polymers is introduced to the polymer community.

One-pot synthesis of acid-degradable polyphosphazene prodrugs for efficient tumor chemotherapy

In order to improve the therapeutic efficacy and reduce the side effects of anticancer drugs, stimuli-responsive and biodegradable drug-delivery systems have attracted significant attention in the past three decades. Herein, we report acid-responsive and degradable polyphosphazene nano-prodrugs synthesized via a one-pot cross-linking reaction of 4-hydroxybenzhydrazide-modified doxorubicin (BMD) with hexachlorocyclotriphosphazene (HCCP). The phenol groups in the as-synthesized BMD exhibited a high reactivity towards HCCP and in the presence of a basic catalyst the determined drug loading ratio of the nanoparticles, denoted as HCCP-BMD, was up to 85.64%. Interestingly, the hydrazone bonds in BMD and the skeleton of polyphosphazene tended to break down in acidic environments, and the antitumor active drug DOX was found to be released in an acidic tumor microenvironment (pH ∼ 6.8 for extracellular, and pH ∼ 5.0 for endosomes and lysosomes). The resulting HCCP-BMD prodrug exhibited high cytotoxicity to HeLa cells and could effectively suppress tumor growth, with negligible damage to normal tissues. We therefore believe that this acid- degradable polyphosphazene prodrug may offer great potential in various biomedical fields.

Biomedical applications of polyphosphazenes

Ever since the pioneering research efforts on their utility in biomedicine, polyphosphazene polymers have witnessed enormous growth and expansion in several biomedical applications due to their unique properties. The development of this exceptional biodegradable system with extraordinary design flexibility, property tunability and neutral bioactivity could stimulate an unprecedented paradigm in biomaterial design. Thus, polyphosphazenes are, undoubtedly, the next-generation biomaterials. This editorial provides a brief perspective on the promising prospects of polyphosphazene-based biomaterials for medical device technology, focusing mainly on the authors' work on this particular polymeric system.

Hybrid Porous Microparticles Based on a Single Organosilica Cyclophosphazene Precursor

Porous organosilica microparticles consisting of silane-derived cyclophosphazene bridges were synthesized by a surfactant-mediated sol-gel process. Starting from the substitution of hexachlorocyclotriphosphazene with allylamine, two different precursors were obtained by anchoring three or six alkoxysilane units, via a thiol-ene photoaddition reaction. In both cases, spherical, microparticles (size average of ca. 1000 nm) with large pores were obtained, confirmed by both, scanning and transmission electron microscopy. Particles synthesized using the partially functionalized precursor containing free vinyl groups were further functionalized with a thiol-containing molecule. While most other reported mesoporous organosilica particles are essentially hybrids with tetraethyl orthosilicate (TEOS), a unique feature of these particles is that structural control is achieved by exclusively using organosilane precursors. This allows an increase in the proportion of the co-components and could springboard these novel phosphorus-containing organosilica microparticles for different areas of technology.

Acid-Responsive and Biologically Degradable Polyphosphazene Nanodrugs for Efficient Drug Delivery

To enhance the therapeutic effects and reduce the damage to normal tissues in cancer chemotherapy, it is indispensable to develop drug delivery carriers with controllable release and good biocompatibility. In this work, acid-responsive and degradable polyphosphazene (PPZ) nanoparticles were synthesized by the reaction of hexachlorotripolyphosphonitrile (HCCP) with 4-hydroxy-benzoic acid (4-hydroxy-benzylidene)-hydrazide (HBHBH) and anticancer drug doxorubicin (DOX). The controlled release of DOX could be realized based on the acid responsiveness of acylhydrazone in HBHBH. Experimental results showed that polyphosphazene nanoparticles remained stable in the body's normal fluids (pH ∼ 7.4), while they were degraded and controllable release of DOX in an acidic environment such as tumors (pH ∼ 6.8) and lysosome and endosome (∼5.0) in cancer cells In particular, the doxorubicin (DOX)-loading ratio was fair high and could be tuned from 10.6 to 52.6% by changing the dosing ratio of DOX to HBHBH. Meanwhile, the polyphosphazene nanodrugs showed excellent toxicity to tumor cells and reduced the side effect to normal cells both in vitro and in vivo due to their enhanced permeability and retention (EPR) effect and pH-sensitive degradation properties. Therefore, the constructed pH-sensitive drug delivery system has great potential for cancer chemotherapy.

Main-Chain Phosphorus-Containing Polymers for Therapeutic Applications

Polymers in which phosphorus is an integral part of the main chain, including polyphosphazenes and polyphosphoesters, have been widely investigated in recent years for their potential in a number of therapeutic applications. Phosphorus, as the central feature of these polymers, endears the chemical functionalization, and in some cases (bio)degradability, to facilitate their use in such therapeutic formulations. Recent advances in the synthetic polymer chemistry have allowed for controlled synthesis methods in order to prepare the complex macromolecular structures required, alongside the control and reproducibility desired for such medical applications. While the main polymer families described herein, polyphosphazenes and polyphosphoesters and their analogues, as well as phosphorus-based dendrimers, have hitherto predominantly been investigated in isolation from one another, this review aims to highlight and bring together some of this research. In doing so, the focus is placed on the essential, and often mutual, design features and structure-property relationships that allow the preparation of such functional materials. The first part of the review details the relevant features of phosphorus-containing polymers in respect to their use in therapeutic applications, while the second part highlights some recent and innovative applications, offering insights into the most state-of-the-art research on phosphorus-based polymers in a therapeutic context.

Pyrene-BODIPY-substituted novel water-soluble cyclotriphosphazenes: synthesis, characterization, and photophysical properties

In the present work, pyrene-boron-dipyrromethene (BODIPY)-substituted novel water-soluble cyclotriphosphazene derivatives (6 and 7) were synthesized by click reactions between a cyclotriphosphazene derivative with a hydrophilic glycol side group (2) and BODIPYs (4 and 5). All of the new compounds (2, 6, and 7) were characterized by Fourier-transform infrared and nuclear magnetic resonance spectroscopy, as well as mass spectrometry and elemental analysis. The photophysical properties of the BODIPY-substituted cyclotriphosphazenes (6 and 7) were investigated by UV-Vis and fluorescence emission spectroscopy in water and water/solvent mixtures. It was found that the target compounds were soluble in water and could be potential candidates as water-soluble fluorescent dyes for the desired applications.

131 I-Labeled Multifunctional Polyphosphazene Nanospheres for SPECT Imaging-Guided Radiotherapy of Tumors

Design of theranostic nanoplatforms represents a major topic for current nanomedicine. Here, the preparation of multifunctional poly(cyclotriphosphazene-co-polyethylenimine) nanospheres (PNSs) labeled with radionuclide ¹³¹ I for single photon emission computed tomography (SPECT) imaging-guided radiotherapy of tumors is reported. In this work, PNSs are prepared using hexachlorocyclotriphosphazene as a crosslinker to crosslink branched polyethylenimine (PEI) via a nucleophilic substitution reaction, modified with 3-(4'-hydroxyphenyl) propionic acid-OSu (HPAO) for ¹³¹ I labeling, and reacted with 1,3-propane sulfonate (1,3-PS) to render the particles with antifouling property, followed by acetylation of the remaining surface amines and labeling with ¹³¹ I. The acquired PNS.NHAc-HPAO(¹³¹ I)-PS particles are well characterized. It is shown that the multifunctional PNSs with an average size of 184 ± 29.3 nm exhibit favorable antifouling properties, high ¹³¹ I labeling efficiency (76.05 ± 3.75%), and excellent radiostability and colloidal stability. With these properties owned, the developed PNS.NHAc-HPAO(¹³¹ I)-PS spheres enable much more efficient SPECT imaging and radiotherapy of a xenografted tumor model in vivo than the PEI counterpart material (PEI.NHAc-HPAO(¹³¹ I)-PS). The developed PNSs may be used as a versatile platform for further development of different forms of nanomedicine for various biomedical applications.

Novel Water-Soluble Cyclotriphosphazene-Bodipy Conjugates: Synthesis, Characterization and Photophysical Properties

In the present work, novel water-soluble cyclotriphosphazene derivatives (3b and 4b) were synthesized by 'click' reactions between cyclotriphosphazene derivative with hydrophilic glycol side groups (2) and Bodipy's (3a and 4a). All newly synthesized compounds (2, 3b and 4b) were characterized by fourier-transform infrared (FTIR), mass and NMR spectroscopy techniques and elemental analysis (EA). The photophysical properties of Bodipy substituted novel cyclotriphosphazenes (3a and 4a) were examined via UV-Vis absorption and fluorescence emission spectroscopy inside water and many organic solvents such as acetone, tetrahydrofuran, dichloromethane, dimethyl sulfoxide, etc., and the results were compared with the each other. Graphical Abstract.

Stimuli-Responsive Phosphorus-Based Polymers

This microreview details recent developments in stimuli-responsive polymers with phosphorus in the main-chain, in particular polyphosphazenes and polyphosphoesters. The presence of phosphorus in the polymers endows unique properties onto the macromolecules, which can be utilized for the preparation of materials capable of physically responding to specific stimuli. Achieving the desired responsiveness has been much facilitated by recent developments in synthetic polymer chemistry, in particular controlled synthesis and backbone functionalization phosphorus-based polymers, in order to achieve the required properties and hence responsiveness of the materials. The development of phosphorus-based polymers which respond to the most important stimuli are discussed, namely, pH, oxidation, reduction, temperature and biological triggers. The polymers are placed in the context not just of each other but also with reference to state-of-the-art organic polymers.

Polyphosphazene-based nanocarriers for the release of agrochemicals and potential anticancer drugs

The synthesised polyphosphazene-based nanocarriers allowed sustained diosgenin and brassinosteroid release over 4 days, with strong to moderate MCF-7 cytotoxicity and good agrochemical activity at medium and low concentrations.

Coumarin-Caged Polyphosphazenes with a Visible-Light Driven On-Demand Degradation

Polymers that, upon photochemical activation with visible light, undergo rapid degradation to small molecules are described. Through functionalization of a polyphosphazene backbone with pendant coumarin groups sensitive to light, polymers which are stable in the dark could be prepared. Upon irradiation, cleavage of the coumarin moieties exposes carboxylic acid moieties along the polymer backbone. The subsequent macromolecular photoacid is found to catalyze the rapid hydrolytic degradation of the polyphosphazene backbone. Water-soluble and non-water-soluble polymers are reported, which due to their sensitivity toward light in the visible region could be significant as photocleavable materials in biological applications.

Hydrolytically Degradable PEGylated Polyelectrolyte Nanocomplexes for Protein Delivery

Novel oppositely charged polyphosphazene polyelectrolytes containing grafted poly(ethylene glycol) (PEG) chains were synthesized as modular components for the assembly of biodegradable PEGylated protein delivery vehicles. These macromolecular counterparts, which contained either carboxylic acid or tertiary amino groups, were then formulated at near physiological conditions into supramolecular assemblies of nanoscale level, below 100 nm. Nanocomplexes with electroneutral surface charge, as assessed by zeta potential measurements, were stable in aqueous solutions, which suggests their compact polyelectrolyte complex "core"-hydrophilic PEG "shell" structure. Investigation of PEGylated polyphosphazene nanocomplexes as agents for noncovalent PEGylation of the therapeutic protein l-asparaginase (L-ASP) in vitro demonstrated their ability to dramatically reduce protein antigenicity, as measured by antibody binding using enzyme linked immunosorbent assay (ELISA). Encapsulation in nanocomplexes did not affect enzymatic activity of L-ASP, but improved its thermal stability and proteolytic resistance. Gel permeation chromatography (GPC) experiments revealed that all synthesized polyphosphazenes exhibited composition controlled hydrolytic degradability in aqueous solutions at neutral pH and showed greater stability at lower temperatures. Overall, novel hydrolytically degradable polyphosphazene polyelectrolytes capable of spontaneous self-assembly into PEGylated nanoparticulates in aqueous solutions can potentially enable a simple and effective approach to modifying therapeutic proteins without the need for their covalent modification.

Degradable, Dendritic Polyols on a Branched Polyphosphazene Backbone

Herein, we present the design, synthesis, and characterization of fully degradable, hybrid, star-branched dendritic polyols. First multiarmed polyphosphazenes were prepared as a star-branched scaffold which upon functionalization produced globular branched hydroxyl-functionalized polymers with over 1700 peripheral functional end groups. These polyols with unique branched architectures could be prepared with controlled molecular weights and relatively narrow dispersities. Furthermore, the polymers are shown to undergo hydrolytic degradation to low molecular weight degradation products, the rate of which could be controlled through postpolymerization functionalization of the phosphazene backbone.

A pH-sensitive Macromolecular Prodrug as TLR7/8 Targeting Immune Response Modifier

The chemical synthesis and biological activity of novel functionalized imidazoquinoline derivatives (ImQ) to generate Toll-like receptor (TLR) 7/8 specific prodrugs are presented. In vivo activity of ImQs to induce inflammation was confirmed in zebrafish larvae. After covalent ligation to fully biodegradable polyphosphazenes (ImQ-polymer), the macromolecular prodrugs were designed to undergo intracellular pH-sensitive release of ImQs to induce inflammation through binding to endosomal TLR7/8 (danger signal). We showed ImQ dissociation from prodrugs at a pH 5 pointing towards endosomal prodrug degradability. ImQ-polymers strongly activated ovalbumin-specific T cells in murine splenocytes as shown by increased proliferation and expression of the IL-2 receptor (CD25) on CD8+ T cells accompanied by strong IFN-γ release. ImQ prodrugs presented here are suggested to form the basis of novel nanovaccines, for example, for intravenous or intratumoral cancer immunotherapeutic applications to trigger physiological antitumor immune responses.

Synthesis and in vivo anticancer evaluation of poly(organo)phosphazene-based metallodrug conjugates

Within this work we aimed to improve the pharmacodynamics and toxicity profile of organoruthenium and -rhodium complexes which had previously been found to be highly potent in vitro but showed unselective activity in vivo. Different organometallic complexes were attached to a degradable poly(organo)phosphazene macromolecule, prepared via controlled polymerization techniques. The conjugation to hydrophilic polymers was designed to increase the aqueous solubility of the typically poorly soluble metal-based half-sandwich compounds with the aim of a controlled, pH-triggered release of the active metallodrug. The synthesized conjugates and their characteristics have been thoroughly studied by means of ³¹P NMR and UV-Vis spectroscopy, ICP-MS analyses and SEC coupled to ICP-MS. In order to assess their potential as possible anticancer drug candidates, the complexes, as well as their respective macromolecular prodrug formulations were tested against three different cancer cell lines in cell culture. Subsequently, the anticancer activity and organ distribution of the poly(organo)phosphazene drug conjugates were explored in vivo in mice bearing CT-26 colon carcinoma. Our investigations revealed a beneficial influence of this macromolecular prodrug by a significant reduction of adverse effects compared to the free metallodrugs.

Oxidation Responsive Polymers with a Triggered Degradation via Arylboronate Self-Immolative Motifs on a Polyphosphazene Backbone

Oxidation responsive polymers with triggered degradation pathways have been prepared via attachment of self-immolative moieties onto a hydrolytically unstable polyphosphazene backbone. After controlled main-chain growth, postpolymerization functionalization allows the preparation of hydrolytically stable poly(organo)phosphazenes decorated with a phenylboronic ester caging group. In oxidative environments, triggered cleavage of the caging group is followed by self-immolation, exposing the unstable glycine-substituted polyphosphazene which subsequently undergoes to backbone degradation to low-molecular weight molecules. As well as giving mechanistic insights, detailed GPC and 1H and 31P NMR analysis reveal the polymers to be stable in aqueous solutions, but show a selective, fast degradation upon exposure to hydrogen peroxide containing solutions. Since the post-polymerization functionalization route allows simple access to polymer backbones with a broad range of molecular weights, the approach of using the inorganic backbone as a platform significantly expands the toolbox of polymers capable of stimuli-responsive degradation.

Synthesis of polyphosphazenes with different side groups and various tactics for drug delivery

Polyphosphazenes (PPZs) are hybrid polymers comprising a main chain containing nitrogen and phosphorous linked through interchanging single and double bonds, and side chains.

Preparation of polyphosphazenes: a tutorial review

Poly(organo)phosphazenes are a family of inorganic molecular hybrid polymers with very diverse properties due to the vast array of organic substituents possible. This tutorial review aims to introduce the basics of the synthetic chemistry of polyphosphazenes, detailing for readers outside the field the essential knowledge required to design and prepare polyphosphazenes with desired properties. A particular focus is given to some of the recent advances in their chemical synthesis which allows not only the preparation of polyphosphazenes with controlled molecular weights and polydispersities, but also novel branched architectures and block copolymers. We also discuss the preparation of supramolecular structures, bioconjugates and in situ forming gels from this diverse family of functional materials. This tutorial review aims to equip the reader to prepare defined polyphosphazenes with unique property combinations and in doing so we hope to stimulate further research and yet more innovative applications for these highly interesting multifaceted materials.

Macromolecular Pt(IV) Prodrugs from Poly(organo)phosphazenes

The preparation of novel macromolecular prodrugs via the conjugation of two platinum(IV) complexes to suitably functionalized poly(organo)phosphazenes is presented. The inorganic/organic polymers provide carriers with controlled dimensions due to the use of living cationic polymerization and allow the preparation of conjugates with excellent aqueous solubility but long-term hydrolytic degradability. The macromolecular Pt(IV) prodrugs are designed to undergo intracellular reduction and simultaneous release from the macromolecular carrier to present the active Pt(II) drug derivatives. In vitro investigations show a significantly enhanced intracellular uptake of Pt for the macromolecular prodrugs when compared to small molecule Pt complexes, which is also reflected in an increase in cytotoxicity. Interestingly, drug-resistant sublines also show a significantly smaller resistance against the conjugates compared to clinically established platinum drugs, indicating that an alternative uptake route of the Pt(IV) conjugates might also be able to overcome acquired resistance against Pt(II) drugs. In vivo studies of a selected conjugate show improved tumor shrinkage compared to the respective Pt(IV) complex.

Polyphosphazene Based Star-Branched and Dendritic Molecular Brushes

A new synthetic procedure is described for the preparation of poly(organo)phosphazenes with star-branched and star dendritic molecular brush type structures, thus describing the first time it has been possible to prepare controlled, highly branched architectures for this type of polymer. Furthermore, as a result of the extremely high-arm density generated by the phosphazene repeat unit, the second-generation structures represent quite unique architectures for any type of polymer. Using two relativity straight forward iterative syntheses it is possible to prepare globular highly branched polymers with up to 30 000 functional end groups, while keeping relatively narrow polydispersities (1.2-1.6). Phosphine mediated polymerization of chlorophosphoranimine is first used to prepare three-arm star polymers. Subsequent substitution with diphenylphosphine moieties gives poly(organo)phosphazenes to function as multifunctional macroinitiators for the growth of a second generation of polyphosphazene arms. Macrosubstitution with Jeffamine oligomers gives a series of large, water soluble branched macromolecules with high-arm density and hydrodynamic diameters between 10 and 70 nm.

Biodegradable Polyphosphazene Based Peptide-Polymer Hybrids

A novel series of peptide based hybrid polymers designed to undergo enzymatic degradation is presented, via macrosubstitution of a polyphosphazene backbone with the tetrapeptide Gly-Phe-Leu-Gly. Further co-substitution of the hybrid polymers with hydrophilic polyalkylene oxide Jeffamine M-1000 leads to water soluble and biodegradable hybrid polymers. Detailed degradation studies, via ³¹P NMR spectroscopy, dynamic light scattering and field flow fractionation show the polymers degrade via a combination of enzymatic, as well as hydrolytic pathways. The peptide sequence was chosen due to its known property to undergo lysosomal degradation; hence, these degradable, water soluble polymers could be of significant interest for the use as polymer therapeutics. In this context, we investigated conjugation of the immune response modifier imiquimod to the polymers via the tetrapeptide and report the self-assembly behavior of the conjugate, as well as its enzymatically triggered drug release behavior.

Electron-withdrawing ability tunable polyphosphazene frameworks as novel heterogeneous catalysts for efficient biomass upgrading

Novel polyphosphazene frameworks as green heterogeneous catalysts are discovered for efficient production of 5-HMF from fructose, which is due to the unique cyclotriphosphazene unit and the electron-withdrawing nature of the polymer backbone.

Water-triggered self-assembly polycondensation for the one-pot synthesis of cyclomatrix polyphosphazene nanoparticles from amino acid ester

Water-triggered self-assembly polycondensation was proposed for preparation of cyclomatrix polyphosphazene nanoparticles from amino acid ester, and a critical solubility parameter was found to determine whether the nanoparticles were formed.

Recent advances in characterization of nonviral vectors for delivery of nucleic acids: impact on their biological performance

INTRODUCTION

Nucleic acid delivery is a complex process that requires transport across numerous extracellular and intracellular barriers, whose impact is often neglected during optimization studies. As such, the development of nonviral vectors for efficient delivery would benefit from an understanding of how these barriers relate to the physicochemical properties of lipoplexes and polyplexes.

AREAS COVERED

This review focuses on the evaluation of parameters associated with barriers to delivery such as blood and immune cells compatibility which, as a collective, may serve as a useful prescreening tool for the advancement of nonviral vectors in vivo. An outline of the most relevant rationally developed polyplexes and lipoplexes for clinical application is also given.

EXPERT OPINION

The evaluation of scientifically recognized parameters enabled the identification of systemic delivered nonviral vectors' behavior while in blood as one of the key determinants of vectors function and activity both in vitro and in vivo. This multiparametric approach complements the use of in vitro efficacy results alone for prescreening and improves in vitro-in vivo translation by minimizing false negatives. Further, it can aid in the identification of meaningful structure-function-activity relationships, improve the in vitro screening process of nonviral vectors before in vivo use and facilitate the future development of potent and safe nonviral vectors.

Robust and Operationally Simple Synthesis of Poly(bis(2,2,2-trifluoroethoxy) phosphazene) with Controlled Molecular Weight, Low PDI, and High Conversion

Synthetically straightforward conditions have been developed for the preparation of poly(bis 2,2,2-trifluoroethoxy)phosphazene with low PDI (<1.15) at high conversion (75-99%) and on a multigram scale. A combination of ³¹P NMR and GPC analyses demonstrate that molecular weight increases linearly as a function of monomer consumption, exhibiting first order kinetics with respect to monomer concentration up to high monomer conversion. Thus, the molecular weight can be controlled by varying the initiator (H₂O) to monomer ratio.