Hydrolysis pathways for aminophosphazenes

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.

Recent research progress on polyphosphazene-based drug delivery systems

In recent years, synthetic polymer materials have become a research hotspot in the field of drug delivery. Compared with natural polymer materials, synthetic polymer materials have more flexible structural adjustability, and can be designed to obtain clinically required delivery vehicles. Polyphosphazenes are one of the most promising biomedical materials in the future due to their controllable degradation properties and structural flexibility. These materials can be designed by controlling the hydrophilic and hydrophobic balance, introducing functional groups or drugs to form different forms of administration, such as nanoparticles, polyphosphazene-drug conjugates, injectable hydrogels, coatings, etc. In addition, the flexible backbone of polyphosphazenes and the flexibility of substitution enable them to meet researchers' design requirements in terms of stereochemistry, nanostructures, and topologies. At present, researchers have achieved a lot of successful practices in the field of targeted delivery of anticancer drugs/proteins/genes, bone tissue engineering repair, cell imaging tracking, photothermal therapy, and immunologic preparations. This review provides a summary of the progress of the recent 10 years of polyphosphazene-based drug delivery systems in terms of of chemical structure and functions.

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.

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.

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.

Generational Biodegradable and Regenerative Polyphosphazene Polymers and their Blends with Poly (lactic-co-glycolic acid)

New fields such as regenerative engineering have driven the design of advanced biomaterials with a wide range of properties. Regenerative engineering is a multidisciplinary approach that integrates the fields of advanced materials science and engineering, stem cell science, physics, developmental biology, and clinical translation for the regeneration of complex tissues. The complexity and demands of this innovative approach have motivated the synthesis of new polymeric materials that can be customized to meet application-specific needs. Polyphosphazene polymers represent this fundamental change and are gaining renewed interest as biomaterials due to their outstanding synthetic flexibility, neutral bioactivity (buffering degradation products), and tunable properties across the range. Polyphosphazenes are a unique class of polymers composed of an inorganic backbone with alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two substituents, with a wide variety of side groups available for property optimization. Polyphosphazenes have been investigated as potential biomaterials for regenerative engineering. Polyphosphazenes for use in regenerative applications have evolved as a class to include different generations of degradable polymers. The first generation of polyphosphazenes for tissue regeneration entailed the use of hydrolytically active side groups such as imidazole, lactate, glycolate, glucosyl, or glyceryl groups. These side groups were selected based on their ability to sensitize the polymer backbone to hydrolysis, which allowed them to break down into non-toxic small molecules that could be metabolized or excreted. The second generation of degradable polyphosphazenes developed consisted of polymers with amino acid ester side groups. When blended with poly (lactic acid-co-glycolic acid) (PLGA), the feasibility of neutralizing acidic degradation products of PLGA was demonstrated. The blends formed were mostly partially miscible. The desire to improve miscibility led to the design of the third generation of degradable polyphosphazenes by incorporating dipeptide side groups which impart significant hydrogen bonding capability to the polymer for the formation of completely miscible polyphosphazene-PLGA blends. Blend system of the dipeptide-based polyphosphazene and PLGA exhibit a unique degradation behavior that allows the formation of interconnected porous structures upon degradation. These inherent pore-forming properties have distinguished degradable polyphosphazenes as a potentially important class of biomaterials for further study. The design considerations and strategies for the different generations of degradable polyphosphazenes and future directions are discussed.

Conformation-Dependent Affinity of Thermoresponsive Biodegradable Hydrogels for Multifunctional Ligands: A Differential Scanning Calorimetry Approach

We investigated energetics of binding of multifunctional pyranine ligands to hydrogels of the cross-linked poly(methoxyethylaminophosphazene) (PMOEAP) from data on the thermotropic volume phase transition of the gels by means of high-sensitivity differential scanning calorimetry. Dependences of the transition temperature, enthalpy, and width on the concentration of pyranines were obtained, and the excess transition free energy as a function of the pyranine concentration was calculated. We found that the affinity of the gels for the pyranine ligands increased very significantly upon the gel collapse. The intrinsic binding constants and free energies of binding of the ligands to the gels in the collapsed state were estimated from the DSC data. They revealed a significant increase in the hydrogel affinity for pyranines proportional to the number of anionic groups in the ligand structure. The affinity of the PMOEAP hydrogels for the multifunctional ligands was not affected by an increase in the cross-linking density of the gels and only slightly reduced by physiological salt concentrations.

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.

Biodegradable Polyphosphazene-Based Blends for Regenerative Engineering

The occurrence of musculoskeletal tissue injury or disease and the subsequent functional impairment is at an alarming rate. It continues to be one of the most challenging problems in the human health care. Regenerative engineering offers a promising transdisciplinary strategy for tissues regeneration based on the convergence of tissue engineering, advanced materials science, stem cell science, developmental biology and clinical translation. Biomaterials are emerging as extracellular-mimicking matrices designed to provide instructive cues to control cell behavior and ultimately, be applied as therapies to regenerate damaged tissues. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and the ability to be excreted or resorbed by the body. Herein, the focus will be on biodegradable polyphosphazene-based blend systems. The synthetic flexibility of polyphosphazene, combined with the unique inorganic backbone, has provided a springboard for more research and subsequent development of numerous novel materials that are capable of forming miscible blends with poly (lactide-co-glycolide) (PLAGA). Laurencin and co-workers has demonstrated the exploitation of the synthetic flexibility of Polyphosphazene that will allow the design of novel polymers, which can form miscible blends with PLAGA for biomedical applications. These novel blends, due to their well-tuned biodegradability, and mechanical and biological properties coupled with the buffering capacity of the degradation products, constitute ideal materials for regeneration of various musculoskeletal tissues.

LAY SUMMARY

Regenerative engineering aims to regenerate complex tissues to address the clinical challenge of organ damage. Tissue engineering has largely focused on the restoration and repair of individual tissues and organs, but over the past 25 years, scientific, engineering, and medical advances have led to the introduction of this new approach which involves the regeneration of complex tissues and biological systems such as a knee or a whole limb. While a number of excellent advanced biomaterials have been developed, the choice of biomaterials, however, has increased over the past years to include polymers that can be designed with a range of mechanical properties, degradation rates, and chemical functionality. The polyphosphazenes are one good example. Their chemical versatility and hydrogen bonding capability encourages blending with other biologically relevant polymers. The further development of Polyphosphazene-based blends will present a wide spectrum of advanced biomaterials that can be used as scaffolds for regenerative engineering and as well as other biomedical applications.

Biodegradable Poly[(Amino Acid Ester)phosphazenes] for Biomedical Applications

Polyphosphazene derivatives with amino acid ester side groups were prepared by reacting poly(dichlorophosphazene) with ethyl esters of amino acids and were characterized by 1H, 31P-NMR and DSC analyses. The in vitro rate of degradation of these polymers depended on the nature of the amino acids while introducing small amounts of depsipeptide ester co-substituents increased degradation rates. The rate of hydrolytic degradation of the poly[(organo)phosphazene] materials could be controlled by the amino acid ester by the depsipeptide ester side group content and by blending poly[(amino acid ester)phosphazenes] with poly[(amino acid ester)-co-(depsipeptide ester)phosphazenes]. Poly[(glycine ethyl ester)phosphazenes] prepared from poly[(dichloro)phosphazenes] degradation rates were independent of molecular weight. Degradations of polymer blends indicate that an intramolecular catalysis ofthe polymer by the pendent carboxylic acid.

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

The synthesis of a series of novel, water-soluble poly(organophosphazenes) prepared via living cationic polymerization is presented. The degradation profiles of the polyphosphazenes prepared are analyzed by GPC, 31P NMR spectroscopy, and UV-Vis spectroscopy in aqueous media and show tunable degradation rates ranging from days to months, adjusted by subtle changes to the chemical structure of the polyphosphazene. Furthermore, it is observed that these polymers demonstrate a pH-promoted hydrolytic degradation behavior, with a remarkably faster rate of degradation at lower pH values. These degradable, water soluble polymers with controlled molecular weights and structures could be of significant interest for use in aqueous biomedical applications, such as polymer therapeutics, in which biological clearance is a requirement and in this context cell viability tests are described which show the non-toxic nature of the polymers as well as their degradation intermediates and products.

Polyphosphazenes: Multifunctional, Biodegradable Vehicles for Drug and Gene Delivery

Poly[(organo)phosphazenes] are a unique class of extremely versatile polymers with a range of applications including tissue engineering and drug delivery, as hydrogels, shape memory polymers and as stimuli responsive materials. This review aims to divulge the basic principles of designing polyphosphazenes for drug and gene delivery and portray the huge potential of these extremely versatile materials for such applications. Polyphosphazenes offer a number of distinct advantages as carriers for bioconjugates; alongside their completely degradable backbone, to non-toxic degradation products, they possess an inherently and uniquely high functionality and, thanks to recent advances in their polymer chemistry, can be prepared with controlled molecular weights and narrow polydispersities, as well as self-assembled supra-molecular structures. Importantly, the rate of degradation/hydrolysis of the polymers can be carefully tuned to suit the desired application. In this review we detail the recent developments in the chemistry of polyphosphazenes, relevant to drug and gene delivery and describe recent investigations into their application in this field.

Biomedical applications of degradable polyphosphazenes

This article describes the synthesis of biodegradable polyphosphazenes. The rate of degradation can be varied in a controllable manner by the introduction of hydrolysis-sensitive amino acid ester side groups or by blending of polymers. Biodegradable polyphosphazenes can be used for the preparation of drug-containing implants and this is illustrated for devices containing the cytostatic agent mitomycin C. This article reviews data about the degradation characteristics of poly[(amino acid ester)phosphazene] derivatives that have been discussed previously. Some new data about MMC-containing poly[(organo)phosphazene] devices are discussed as well. (c) 1996 John Wiley & Sons, Inc.

Mechanical properties and osteocompatibility of novel biodegradable alanine based polyphosphazenes: Side group effects

The versatility of polymers for tissue regeneration lies in the feasibility to modulate the physical and biological properties by varying the side groups grafted to the polymers. Biodegradable polyphosphazenes are high-molecular-weight polymers with alternating nitrogen and phosphorus atoms in the backbone. This study is the first of its kind to systematically investigate the effect of side group structure on the compressive strength of novel biodegradable polyphosphazene based polymers as potential materials for tissue regeneration. The alanine polyphosphazene based polymers, poly(bis(ethyl alanato) phosphazene) (PNEA), poly((50% ethyl alanato) (50% methyl phenoxy) phosphazene) (PNEA(50)mPh(50)), poly((50% ethyl alanato) (50% phenyl phenoxy) phosphazene) (PNEA(50)PhPh(50)) were investigated to demonstrate their mechanical properties and osteocompatibility. Results of mechanical testing studies demonstrated that the nature and the ratio of the pendent groups attached to the polymer backbone play a significant role in determining the mechanical properties of the resulting polymer. The compressive strength of PNEA(50)PhPh(50) was significantly higher than poly(lactide-co-glycolide) (85:15 PLAGA) (p<0.05). Additional studies evaluated the cellular response and gene expression of primary rat osteoblast cells on PNEA, PNEA(50)mPh(50) and PNEA(50)PhPh(50) films as candidates for bone tissue engineering applications. Results of the in vitro osteocompatibility evaluation demonstrated that cells adhere, proliferate, and maintain their phenotype when seeded directly on the surface of PNEA, PNEA(50)mPh(50), and PNEA(50)PhPh(50). Moreover, cells on the surface of the polymers expressed type I collagen, alkaline phosphatase, osteocalcin, osteopontin, and bone sialoprotein, which are characteristic genes for osteoblast maturation, differentiation, and mineralization.

Novel factor-loaded polyphosphazene matrices: potential for driving angiogenesis

Currently employed bone tissue engineered scaffolds often lack the potential for vascularization, which may be enhanced through the incorporation of and regulated release of angiogenic factors. For this reason, the objective here was to fabricate and characterize protein-loaded amino acid ester polyphosphazene (Pphos)-based scaffolds and evaluate the novel sintering method used for protein incorporation, a method which will ultimately allow for the incorporation of proangiogenic agents. To test the hypothesis, Pphos and their composite microspheres with nanocrystalline hydroxyapatite (Pphos-HAp) were fabricated via the emulsion solvent evaporation method. Next, bovine serum albumin (BSA)-containing microsphere matrices were created using a novel solvent-non-solvent approach for protein loading. The resulting protein (BSA) loaded circular porous microsphere based scaffolds were characterized for morphology, porosity, protein structure, protein distribution and subsequent protein release pattern. Scanning electron microscopy revealed porous microsphere scaffolds with a smooth surface and sufficient level of sintering, illustrated by fusion of adjacent microspheres. The porosity measured for the poly(ethyl phenylalanato:glycinato)phosphazene (PNPhGly) and poly(ethyl phenylalanato:glycinato)phosphazene-hydroxyapatite (PNPhGly-HAp) scaffolds were 23 +/- 0.11% and 18 +/- 4.02%, respectively, and within the range of trabecular bone. Circular dichroism confirmed an intact secondary protein structure for BSA following the solvent sintering method used for loading and confocal microscopy verified that FITC-BSA was successfully entrapped both between adjacent microspheres and within the surface of the microspheres while sintering. For both Pphos and their composite microsphere scaffolds, BSA was released at a steady rate over a 21 day time period, following a zero order release profile. HAp particles in the composite scaffolds served to improve the release profile pattern, underscoring the potential of HAp for growth factor delivery. Moreover, the results of this work suggest that the solvent-non-solvent technique for protein loading is an optimal one that will allow for future development of angiogenic factor-loaded Pphos matrices with the capacity to invoke neovascularization.

Recent advances in synthetic bioelastomers

This article reviews the degradability of chemically synthesized bioelastomers, mainly designed for soft tissue repair. These bioelastomers involve biodegradable polyurethanes, polyphosphazenes, linear and crosslinked poly(ether/ester)s, poly(ε-caprolactone) copolymers, poly(1,3-trimethylene carbonate) and their copolymers, poly(polyol sebacate)s, poly(diol-citrates) and poly(ester amide)s. The in vitro and in vivo degradation mechanisms and impact factors influencing degradation behaviors are discussed. In addition, the molecular designs, synthesis methods, structure properties, mechanical properties, biocompatibility and potential applications of these bioelastomers were also presented.