Partial Hydrolysis of Poly(2-ethyl-2-oxazoline) and Potential Implications for Biomedical Applications?

A Concise Review on Effect of PEGylation on the Properties of Lipid-Based Nanoparticles

Nanoparticle-based drug delivery systems have emerged as promising platforms for enhancing therapeutic efficacy while minimizing off-target effects. Among various strategies employed to optimize these systems, polyethylene glycol (PEG) modification, known as PEGylation-the covalent attachment of PEG to nanoparticles, has gained considerable attention for its ability to impart stealth properties to nanoparticles while also extending circulation time and improving biocompatibility. PEGylation extends to different drug delivery systems, in specific, nanoparticles for targeting cancer cells, where the concentration of drug in the cancer cells is improved by virtue of PEGylation. The primary challenge linked to PEGylation lies in its confirmation. Numerous research findings provide comprehensive insights into selecting PEG for various PEGylation methods. In this review, we have endeavored to consolidate the outcomes concerning the choice of PEG and diverse PEGylation techniques.

Poly(2-isopropenyl-2-oxazoline) as a Versatile Functional Polymer for Biomedical Applications

Functional polymers play an important role in various biomedical applications. From many choices, poly(2-isopropenyl-2-oxazoline) (PIPOx) represents a promising reactive polymer with great potential in various biomedical applications. PIPOx, with pendant reactive 2-oxazoline groups, can be readily prepared in a controllable manner via several controlled/living polymerization methods, such as living anionic polymerization, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) or rare earth metal-mediated group transfer polymerization. The reactivity of pendant 2-oxazoline allows selective reactions with thiol and carboxylic group-containing compounds without the presence of any catalyst. Moreover, PIPOx has been demonstrated to be a non-cytotoxic polymer with immunomodulative properties. Post-polymerization functionalization of PIPOx has been used for the preparation of thermosensitive or cationic polymers, drug conjugates, hydrogels, brush-like materials, and polymer coatings available for drug and gene delivery, tissue engineering, blood-like materials, antimicrobial materials, and many others. This mini-review covers new achievements in PIPOx synthesis, reactivity, and use in biomedical applications.

Hydrolysis of hydrophobic poly(2-oxazoline)s and their subsequent modification via aza-Michael addition

Partially hydrolysed poly(2-oxazoline)s possess unique properties. However, much of the focus in this area has been on water soluble poly(2-oxazoline)s. Where hydrophobic poly(2-oxazoline)s have been used, this is often for selective hydrolysis. However, hydrolysis of very hydrophobic polymers could lead to interesting solution behaviour. Herein, we describe universal conditions for the hydrolysis of poly(2-alkyl-2-oxazoline)s suitable for both hydrophobic and hydrophilic 2-oxazolines. We show that the system utilised gives comparable rates to that of water alone for poly(2-ethyl-2-oxazoline). In addition, poly(2-fatty acid-2-oxazoline) was hydrolysed using the developed system and was found to proceed in a controlled manner allowing the targeting of specific degrees of hydrolysis, albeit much slower than for poly(2-ethyl-2-oxazoline). Finally, we demonstrate the partial functionalisation of poly(2-oxazoline)-poly(ethylene imine) co-polymers via aza-Michael addition.

Synthesis of Double Hydrophilic Block Copolymers Poly(2-isopropyl-2-oxazoline-b-ethylenimine) and their DNA Transfection Efficiency

Gene delivery is now a part of the therapeutic arsenal for vaccination and treatments of inherited or acquired diseases. Polymers represent an opportunity to develop new synthetic vectors for gene transfer, with a prerequisite of improved delivery and reduced toxicity compared to existing polymers. Here, the synthesis in a two-step's procedure of linear poly(ethylenimine-b-2-isopropyl-2-oxazoline) block copolymers with the linear polyethylenimine (lPEI) block of various molar masses is reported; the molar mass of the poly(2-isopropyl-2-oxazoline) (PiPrOx) block has been set to 7 kg mol^-1 . Plasmid DNA condensation is successfully achieved, and in vitro transfection efficiency of the copolymers is at least comparable to that obtained with the lPEI of same molar mass. lPEI-b-PiPrOx block copolymers are however less cytotoxic than their linear counterparts. PiPrOx can be a good alternative to PEG which is often used in drug delivery systems. The grafting of histidine moieties on the lPEI block of lPEI-b-PiPrOx does not provide any real improvement of the transfection efficiency. A weak DNA condensation is observed, due to increased steric hindrance along the lPEI backbone. The low cytotoxicity of lPEI-b-PiPrOx makes this family a good candidate for future gene delivery developments.

Synthesis and Evaluation of Poly(3-hydroxypropyl Ethylene-imine) and Its Blends with Chitosan Forming Novel Elastic Films for Delivery of Haloperidol

This study aimed to develop novel elastic films based on chitosan and poly(3-hydroxypropyl ethyleneimine) or P3HPEI for the rapid delivery of haloperidol. P3HPEI was synthesized using a nucleophilic substitution reaction of linear polyethyleneimine (L-PEI) with 3-bromo-1-propanol. ¹H-NMR and FTIR spectroscopies confirmed the successful conversion of L-PEI to P3HPEI, and the physicochemical properties and cytotoxicity of P3HPEI were investigated. P3HPEI had good solubility in water and was significantly less toxic than the parent L-PEI. It had a low glass transition temperature (T_g = -38.6 °C). Consequently, this new polymer was blended with chitosan to improve mechanical properties, and these materials were used for the rapid delivery of haloperidol. Films were prepared by casting from aqueous solutions and then evaporating the solvent. The miscibility of polymers, mechanical properties of blend films, and drug release profiles from these formulations were investigated. The blends of chitosan and P3HPEI were miscible in the solid state and the inclusion of P3HPEI improved the mechanical properties of the films, producing more elastic materials. A 35:65 (%w/w) blend of chitosan-P3HPEI provided the optimum glass transition temperature for transmucosal drug delivery and so was selected for further investigation with haloperidol, which was chosen as a model hydrophobic drug. Microscopic and X-ray diffractogram (XRD) data indicated that the solubility of the drug in the films was ~1.5%. The inclusion of the hydrophilic polymer P3HPEI allowed rapid drug release within ~30 min, after which films disintegrated, demonstrating that the formulations are suitable for application to mucosal surfaces, such as in buccal drug delivery. Higher release with increasing drug loading allows flexible dosing. Blending P3HPEI with chitosan thus allows the selection of desirable physicochemical and mechanical properties of the films for delivery of haloperidol as a poorly water-soluble drug.

Tunable Drug Release from Fused Deposition Modelling (FDM) 3D-Printed Tablets Fabricated Using a Novel Extrudable Polymer

Three-dimensional (3D) printing is proving to be a pivotal technology for developing personalized dosage forms with bench to bedside feasibility. Fused deposition modelling (FDM) 3D printing has emerged as the most used technique wherein molten drug-loaded polymer filaments are deposited layer-by-layer to fabricate a predefined shape and internal geometry. However, for precise FDM 3D printing, it is imperative for the filaments to have peculiar mechanical/physicochemical properties, which the majority of the FDA/GRAS approved polymers lack. In the current study, a novel water-soluble polymer, Poly(2-ethyl-tetra-oxazoline) [PETOx] has been investigated as an extrudable and printable polymer with two different types of drug molecule—dextromethorphan hydrobromide (DXM) and hydrochlorothiazide (HCTZ). Hot-stage microscopy experiments of drug:polymer (1:1 w/w) and filaments were carried out at 25−275 °C. HCTZ-loaded filament showed higher toughness of 17 ± 3.25 × 106 J/m3 compared with DXM and drug-free filament. Moisture sorption and flexural analysis was performed to understand the correlation of mechanical properties and storage humidity to printability. Varying the number of outer perimeters of each layer (shell number) was observed to affect the drug release pattern from the printlets. The DXM one-shell printlet showed >80%, whereas the DXM five-shell printlet showed >60% of the drug release within 60 min. PETOx could prove to be a high-performance and versatile 3D printable polymer.

Protected amine-functional initiators for the synthesis of α-amine homo- and heterotelechelic poly(2-ethyl-2-oxazoline)s

Screening a series of protected amine cationic ring-opening polymerization initiators revealed the commercially available N-(3-bromopropyl)phthalimide as the most suitable to achieve defined polymers with high degree of amine functionalization.

Stochasticity of poly(2-oxazoline) oligomer hydrolysis determined by tandem mass spectrometry

Understanding modification of synthetic polymer structures is necessary for their accurate synthesis and potential applications. In this contribution, a series of partially hydrolyzed poly(2-oxazoline) species were produced forming poly[(2-polyoxazoline)-co-(ethylenimine)] (P(EtOx-co-EI)) copolymers; EI being the hydrolyzed product of Ox. Bulk mass spectrometry (MS) measurements accurately measured the EI content. Tandem mass spectrometry analysis of the EI content in the copolymer samples determined the distribution of each monomer within the copolymer and corresponded to a theoretically modelled random distribution. The EI distribution across the polymers was shown to be effected by the choice of terminus, with a permanent hydrolysis event observed at an OH terminus. A neighbouring group effect wasn't observed at the polymer length analysed (approximately 25-mer species), suggesting that previously observed neighbouring group effects require a larger polymer chain. Although clearly useful for random polymer distribution this approach may be applied to many systems containing non-specific modifications to determine if they are directed or random locations across peptides, proteins, polymers, and nucleic acids.

Tandem mass spectrometry can be used to better understand modification sites of synthetic polymer structures providing more complete chemical knowledge which is necessary for their accurate synthesis and potential applications.

Poly(2-allylamidopropyl-2-oxazoline)-Based Hydrogels: From Accelerated Gelation Kinetics to In Vivo Compatibility in a Murine Subdermal Implant Model

A rapid photo-curing system based on poly(2-ethyl-2-oxazoline-co-2-allylamidopropyl-2-oxazoline) and its in vivo compatibility are presented. The base polymer was synthesized from the copolymerization of 2-ethyl-2-oxazoline (EtOx) and the methyl ester containing 2-methoxycarboxypropyl-2-oxazoline (C₃MestOx) followed by amidation with allylamine to yield a highly water-soluble macromer. We showed that spherical hydrogels can be obtained by a simple water-in-oil gelation method using thiol-ene coupling and investigated the in vivo biocompatibility of these hydrogel spheres in a 28-day murine subdermal model. For comparison, hydrogel spheres prepared from poly(ethylene glycol) were also implanted. Both materials displayed mild, yet typical foreign body responses with little signs of fibrosis. This is the first report on the foreign body response of a poly(2-oxazoline) hydrogel, which paves the way for future investigations into how this highly tailorable class of materials can be used for implantable hydrogel devices.

Block copolymers comprising degradable poly(2-ethyl-2-oxazoline) analogues via copper-free click chemistry

We present the synthesis development of amphiphilic, degradable poly(2-ethyl-2-oxazoline) (PEtOx) analogue block copolymers in a modular fashion utilizing the strain-promoted azide–alkyne cycloaddition (SPAAC).

Poly(2-ethyl-2-oxazoline-co-ethyleneimine)-block-poly(ε-caprolactone) based micelles: synthesis, characterization, peptide conjugation and cytotoxic activity

Here we present self-assembled polymeric micelles as potential delivery systems for therapeutic agents with highly tunable properties.

Poly(2-ethyl-2-oxazoline-co-N-propylethylene imine)s by controlled partial reduction of poly(2-ethyl-2-oxazoline): synthesis, characterization and cytotoxicity

Cationic polymers obtained via partial reduction of poly(2-ethy-2-oxazoline)s were studied on their cytocompatibility and their buffer capacity in acidic environment.

Selective Partial Hydrolysis of 2-isopropyl-2-oxazoline Copolymers towards Decreasing the Ability to Crystallize

Poly(2-isopropyl-2-oxazoline) (PiPrOx) is readily prone to crystallization both in solid and from solutions. This feature is detrimental for certain applications. Here, we examine whether the presence of unsubstituted ethyleneimine (EI) units, a gradient distributed within a polymer chain composed of 2-isopropyl-2-oxazoline (iPrOx) and 2-methyl-2-oxazoline (MOx) units, decreases the ability to crystallize the copolymer and affects thermal properties compared to the homopolymer of iPrOx. We assumed that the separation of stiff iPrOx units by the more flexible EI will affect the spatial arrangements of the ordered chains, slightly plasticize and, as a result, decrease their ability to crystallize. The selective hydrolysis of gradient iPrOx and 2-methyl-2-oxazoline (MOx) copolymers, carried out under mild conditions, led to iPrOx/MOx/EI copolymers. To the best of our knowledge, the selective hydrolysis of these copolymers has never been carried out before. Their thermal properties and crystallization abilities, both in a solid state and from an aqueous solution, were analyzed. Based on the analysis of polymer charge and cytotoxicity studies, the potential use of the copolymers obtained was indicated in some biological systems.

On-Demand Dissoluble Diselenide-Containing Hydrogel

On-demand dissolution of hydrogels is being increasingly studied for their potential use in burn wound dressing applications. Herein, a dynamic diselenide-containing hydrogel is developed through a very simple one-pot and two-step process starting from the selenol functionalization of a partially hydrolyzed poly(2-ethyl-2-oxazoline) with γ-butyroselenolactone. The hydrogel spontaneously cross-links via an in situ oxidation of the selenol functionalities in air. The gelation process and the final viscoelastic properties of the gel are characterized by rheological experiments. The mechanical properties of those new diselenide-containing hydrogels are easily tuned by varying the concentration of γ-butyroselenolactone. The materials also show good skin adhesion and UV light responsiveness. A unique feature of the hydrogel is its capability to be fully and rapidly dissolved on-demand, via oxidation or reduction of the diselenide cross-links, making them particularly attractive for burn wound dressing applications.

Recent advances and prospects in nano drug delivery systems using lipopolyoxazolines

Facing the growing demand in nano drug delivery systems (nDDS), hybrid excipients based on natural molecules and well-defined synthetic polymers are intensively investigated. Lipopolyoxazolines (LipoPOx) composed of a polyoxazoline block (POx) and a lipid or lipid-like derivative are detailed in this review. The nature of lipids used, the route to synthesize LipoPOx and their advantages for the formulation of drugs are reported. The place of POx family in nanomedicine is discussed compared to PEG, considered as the gold standard of hydrophilic polymers. LipoPOx nanoformulations including liposomes, mixed micelles, lipid nanocapsules are provided alongside discussion of the nDDS for intravenous or topical administration.

Organocatalytic Ring-Opening Polymerization of N-Acylated-1,4-oxazepan-7-ones Toward Well-Defined Poly(ester amide)s: Biodegradable Alternatives to Poly(2-oxazoline)s

We report a series of poly(ester amide)s (PEAs) synthesized by organocatalytic ring-opening polymerization (ROP) of N-acylated-1,4-oxazepan-7-one (OxP) monomers, produced from N-acylated-4-piperidones using the Baeyer-Villiger oxidation reaction. The ROP of OxPs, conducted in CH₂Cl₂ at room temperature with benzyl alcohol as initiator and TBD/TU (1,5,7-triazabicyclo[4.4.0]dec-5-ene/thiourea) as a binary organocatalytic system, revealed a controlled/living character. The thermodynamics of the ROP highly depends on the N-acylated substituent of monomers, with the following reactivity order: OxP_Ph > OxP_Me > OxP_Pr > OxP_Bn. Based on NMR results, it seems that our system follows the hydrogen bonding bifunctional activation mechanism. All intermediates and final products were characterized by NMR, MALDI-TOF MS, SEC, and DSC techniques. All poly(N-acylated-1,4-oxazepan-7-one) (POxP) polymers are amorphous with different glass transition temperatures (T_g), depending on the N-acylated substituent (T_g: -2.90 to 43.75 °C). Among the synthesized polymers, only POxP_Me was water-soluble and it degraded much faster than polycaprolactone in an aqueous phosphate buffer saline solution (pH = 7.4). Therefore, poly(N-acylated-1,4-oxazepan-7-one)s are potential biodegradable alternatives to poly(2-oxazoline)s.

Preparation and biological imaging of fluorescent hydroxyapatite nanoparticles with poly(2-ethyl-2-oxazoline) through surface-initiated cationic ring-opening polymerization

Fluorescent hydroxyapatite (HAp) nanoparticles have received significant attention in biomedical fields due to their outstanding advantages, such as low immunogenicity, excellent biocompatibility and biodegradability. However, fluorescent HAp nanoparticles with well controlled size and morphology are coated with hydrophobic molecules and their biomedical applications are largely restricted by their poor dispersibility in physiological solutions. Therefore, surface modification of these hydrophobic fluorescent HAp nanoparticles to render them water dispersibility is of utmost importance for biomedical applications. In this work, we reported for the first time for preparation of water-dispersible hydrophilic fluorescent Eu³⁺-doped HAp nanoparticles (named as HAp-PEOTx) through the cationic ring-opening polymerization using hydrophilic and biocompatible 2-ethyl-2-oxazoline (EOTx) as the monomer. The characterization techniques, such as nuclear magnetic resonance (NMR) spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize these samples. Results confirmed that we could successfully obtain the hydrophilic fluorescent HAp-PEOTx composites through the strategy described above. These fluorescent HAp-PEOTx composites display great water dispersibility, unique fluorescent properties and excellent biocompatibility, making them promising for in vitro bioimaging applications.

Long circulating liposomes: challenges and opportunities

The poor pharmacokinetic parameters and low solubility of many anticancer therapeutics have warranted the use of drug-delivery systems such as liposomes. Overcoming some drawbacks of the conventional liposomes, targeted liposomal delivery by longer circulation time by addition of poly(ethylene glycol) to the liposomal surface and further adding specific ligands to achieve ligand selective retention and uptake has been introduced. PEGylated liposomes are the only second-generation liposomal formulations in clinical use and are now being challenged with the allergenic response they pose even in the treatment of naive patients. This article will review the challenges and hindrances in the use of long circulating liposomes and explore the opportunities to overcome this issue.

Spermine modified polymeric micelles with pH-sensitive drug release for targeted and enhanced antitumor therapy

Tumor targeting delivery of chemotherapeutic drugs by nanocarriers has been demonstrated to be a promising strategy for cancer therapy with improved therapeutic efficacy. In this work, we reported a novel type of active targeting micelle with pH-responsive drug release by using biodegradable poly(lactide)-poly(2-ethyl-2-oxazoline) di-block copolymers functionalized with spermine (SPM). SPM has been considered as a tumor binding ligand through its specific interaction with the polyamine transport system (PTS), a transmembrane protein overexpressed on various types of cancer cell, while its application in nano-drug delivery systems has rarely been explored. The micelles with spherical shape (∼110 nm) could load hydrophobic paclitaxel (PTX) with high capacity, and release the payload much faster at acidic pH (4.5–6.5) than at pH 7.4. This pH-responsive property assisted the rapid escape of drug from the endo/lysosome after internalization as demonstrated by confocal laser scanning microscopy images using coumarin-6 (Cou-6) as a fluorescent probe. With surface SPM modification, the micelles displayed much higher cellular uptake than SPM lacking micelles in various types of cancer cells, demonstrating tumor targeting ability. The uptake mechanism of SPM modified micelles was explored by flow cytometry, which suggested an energy-consuming sag vesicle-mediated endocytosis pathway. As expected, the micelles displayed significantly enhanced anti-cancer activity. This work demonstrates that SPM modified pH-sensitive micelles may be potential drug delivery vehicles for targeting and effective cancer therapy.

Tumor targeting delivery of SPM functionalized micelles via PTS binding and their endocytosis and pH-triggered endo/lysosome drug release for anti-cancer therapy.

One Size Does Not Fit All: The Effect of Chain Length and Charge Density of Poly(ethylene imine) Based Copolymers on Delivery of pDNA, mRNA, and RepRNA Polyplexes

Nucleic acid delivery systems are commonly translated between different modalities, such as DNA and RNA of varying length and structure, despite physical differences in these molecules that yield disparate delivery efficiency with the same system. Here, we synthesized a library of poly(2-ethyl-2-oxazoline)/poly(ethylene imine) copolymers with varying molar mass and charge densities in order to probe how pDNA, mRNA, and RepRNA polyplex characteristics affect transfection efficiency. The library was utilized in a full factorial design of experiment (DoE) screening, with outputs of luciferase expression, particle size, surface charge, and particle concentration. The optimal copolymer molar mass and charge density was found as 83 kDa/100%, 72 kDa/100%, and 45 kDa/80% for pDNA, RepRNA, and mRNA, respectively. While 10 of the synthesized copolymers enhanced the transfection efficiency of pDNA and mRNA, only 2 copolymers enhanced RepRNA transfection efficiency, indicating a narrow and more stringent design space for RepRNA. These findings suggest that there is not a "one size fits all" polymer for different nucleic acid species.

Full and partial hydrolysis of poly(2-oxazoline)s and the subsequent post-polymerization modification of the resulting polyethylenimine (co)polymers

This review discusses the full and partial hydrolysis of poly(2-oxazoline)s as well as the synthetic methods that have been reported to modify the resulting secondary amine groups.

3rd generation poly(ethylene imine)s for gene delivery

In this study, a series of high molar mass poly(2-oxazoline)-based copolymers was synthesized, introducing 2-ethyl-2-oxazoline, ethylene imine, and primary amine bearing monomer units representing a new generation of PEI.

Multidimensional nanofibrous scaffolds of poly(lactide-co-caprolactone) and poly(ethyl oxazoline) with improved features for cardiac tissue engineering

Aim: The aim of the study is to develop scaffolds that mimic native tissue properties for effective regeneration of the myocardium, which is affected by the gradual thinning of left ventricular tissue after an infarction. Materials & methods: Heterogenous nanofibrous scaffolds made of poly(lactide-co-caprolactone) and poly(ethyl oxazoline) were characterized for physico-chemical properties. The biocompatibility of the scaffolds was evaluated by studying the adhesion, proliferation and differentiation of H9c2 cells. Results: The scaffolds mimic the cardiac extracellular matrix and showed enhanced tensile strength, improved cell compatibility along with the expression of cardiac marker proteins. Conclusion: Our experimental data confirmed the importance of native tissue architecture and mechanical strength for improved cell response in cardiac tissue engineering.

Microwave-assisted cationic ring-opening polymerization of 2-oxazolines

Unlike any other polymer class, the (co-)poly(2-oxazoline)s have tremendously benefited from the introduction of microwave reactors into chemical laboratories. This review focuses on the research activities in the area of (co-)poly(2-oxazoline)s prepared by microwave-assisted syntheses and, correspondingly, summarizes the current-state-of the-art of the microwave-assisted synthesis of 2-oxazoline monomers and the microwave-assisted ring-opening (co-)polymerization of 2-oxazolines as well as prominent examples of post-polymerization modification of (co-)poly(2-oxazoline)s. Special attention is attributed to the kinetic analysis of the microwave-assisted polymerization of 2-oxazolines and the discussion of non-thermal microwave effects.

A Low Protein Binding Cationic Poly(2-oxazoline) as Non-Viral Vector

Developing safe and efficient non-viral gene delivery systems remains a major challenge. We present a new cationic poly(2-oxazoline) (CPOx) block copolymer for gene therapy that was synthesized by sequential polymerization of non-ionic 2-methyl-2-oxazoline and a new 2-oxazoline monomer, 2-(N-methyl, N-Boc-amino)-methyl-2-oxazoline, followed by deprotection of the pendant secondary amine groups. Upon mixing with plasmid DNA (pDNA), CPOx forms small (diameter ≈80 nm) and narrowly dispersed polyplexes (PDI <0.2), which are stable upon dilution in saline and against thermal challenge. These polyplexes exhibited low plasma protein binding and very low cytotoxicity in vitro compared to the polyplexes of pDNA and poly(ethylene glycol)-b-poly(L-lysine) (PEG-b-PLL). CPOx/pDNA polyplexes at N/P = 5 bound considerably less plasma protein compared to polyplexes of PEG-b-PLL at the same N/P ratio. This is a unique aspect of the developed polyplexes emphasizing their potential for systemic delivery in vivo. The transfection efficiency of the polyplexes in B16 murine melanoma cells was low after 4 h, but increased significantly for 10 h exposure time, indicative of slow internalization of polyplexes. Addition of Pluronic P85 boosted the transfection using CPOx/pDNA polyplexes considerably. The low protein binding of CPOx/pDNA polyplexes is particularly interesting for the future development of targeted gene delivery.