Perspective Article
Synthesis and characterization of redox-sensitive polyurethanes based on l-glutathione oxidized and poly(ether ester) triblock copolymers

https://doi.org/10.1016/j.reactfunctpolym.2021.104986Get rights and content

Highlights

  • Novel segmented polyurethanes temperature-dependent synthese

  • Tunable hydrophobic/hydrophilic polymer characte

  • Film casting processing

  • Redox-sensitive release of methotrexate

Abstract

Segmented polyurethanes, based on PCL-PEG-PCL copolymers, 1,4-diisocyanatobutane and l-glutathione oxidized, used as chain extender, were synthetized. Three different reactions conditions were investigated using three different copolymers having ɛ-CL/polyethylene glycol molar ratios equal to 12, 24 and 36 and three different reaction conditions. As investigated by size exclusion chromatography analyses and quantification of l-glutathione, the polymerization and the extension phase's efficiency depended on the ɛ-CL/PEG ratio and the extension phase's operating temperature. Three selected polyurethanes were characterized by spectroscopic, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. Polyurethanes processed as films showed a hydrolysis pattern dependent on the crystallinity of their soft segments, which was not influenced by the incubation in a reductive mimicking environment. Instead, a redox-sensitive release of methotrexate, used as a model drug, was demonstrated.

Introduction

Biodegradable polyurethanes (PUs) represent an exciting class of synthetic polymers used as biomedical devices and for applications in regenerative medicine [[1], [2], [3], [4], [5]]. Essential features of these polymers, such as chemical and mechanical resistance or biological inertia, can be tuned during synthetic steps varying the building blocks employed. Repeated representative sequences can be distinguished, namely hard and soft segments. Hard segments (HS) are represented by polyurethane from isocyanate chemistry, while soft segments (SS) generally derive from polyols. In particular, poly(ester urethane) (PUs) can be prepared via the reaction of a diisocyanate with PCL and a small diol used as a chain extender [4,6], while linear poly(ester urethane urea) PUU can be obtained using a diamine molecule, as a chain extender [7]. Small common aliphatic diisocyanates, like 1,4-butane diisocyanate (BDI), and 1,6-hexamethylene diisocyanate (HDI) are often used in degradable polyurethanes to avoid any potential toxicity concerns despite the loss of mechanical properties offered from the use of aromatic diisocyanates [6].

Most polyurethanes assumed biocompatible features when produced by inserting biodegradable soft segments [[1], [2], [3]], such as polyesters (PLA, PLGA, or PCL) and their copolymers. The introduction of enzymatically [4,5] or hydrolytically cleavable bonds minimizes biocompatibility issues of the material. As soft segments, the use of PCL improved tensile stress resistance, hydrolytic and enzymatic degradation rate. In contrast, polyethylene glycol -containing PUs resulted in softer materials with higher water affinity and prolonged degradation rate [6].

With the aim to impart multifunctional properties, stimuli-responsive moieties have been inserted on biodegradable PU using appropriately functionalized diisocyanate and chain extenders [[7], [8], [9], [10]].

Hydrolytically labile PUU elastomers, incorporating peptide sequences sensitive for ECM's specific enzymatic activities, were designed and proposed for appropriate tissue engineering applications [4]. Some copolymers, such as PEG-PLA-PEG, provided thermo-sensitivity to PUs when included in their synthesis. Such products exploited these features when employed as self-assembling materials for tissue engineering and nanomedical purposes [[11], [12], [13]]. pH-responsive [14] and reductive sensitive [15] groups were used as target-responsive moieties demonstrating their preferable reactivity in the tumour microenvironment. Among the stimulus sensitive groups, disulfide bridges could be cleaved and regenerated under thermodynamic control through the reversible thiol-disulfide exchange reactions, which can be triggered by redox stimuli [16], photoirradiation [17], mechanical or thermal stresses. Disulfide bonds induced a dynamic reorganization in polymers (disulfide metathesis) which consisted of structural changes of the material favouring its biodegradation in biological systems [[18], [19], [20]]. Many polythiourethanes were produced and reported in the literature where the disulfide bridges were exploited as sensitive moieties for redox-stimulated doxorubicin delivery systems, especially for cancer therapy [[21], [22], [23], [24]]. For example, Wang et al. introduced cystine dimethyl ester as a chain extender, obtaining a GSH responsive material proposed for tumour targetable polymeric systems [25]. In fact, in the light of the critical role that GSH/GSSG redox couple plays on several tissues responses [26], GSH or GSSG were used to achieve several types of stimulus-sensitive polymeric derivatives in different biomedical therapeutic strategies [19,20]. The GSH/GSSG couple contains both donor and acceptor groups that can establish a supramolecular network, imparting redox properties to the polymeric material [28]. The cell adhesive properties of GSH (due to the sequence of glutamic acid, cysteine and glycine) were exploited to produce cell-adhesive polysaccharide derivatives [27]. Here, a series of biodegradable poly(ether ester urethane) ureas (PUUs), were synthetized by employing three triblock copolymers PCL-PEG-PCL as soft segments, which ends formed hard segments with 1,4-diisocyanatobutane (BDI) and the tetramethyl ester of oxidized l-glutathione (GSSG-OMe4) as a chain extender. Polyethyleneglicole (PEG) is water soluble polymer with a known profile of biocompatibility; polyethylene glycol conjugation is a known strategy to improve bioavaiability of biomaterials. Poly(ε-caprolactone) (PCL) is a biocompatible and biodegradable hydrophobic polyesters. Amphiphilic copolymers composed of PEG and polycaprolactone have good biocompatibility and adjustable biodegradability profiles [[28], [29], [30]]. BDI was selected as one of the most biocompatible diisocyanate considering that the hydrolyzed product, the putrescine, naturally occurs in the body [31]. The disulfide bond was introduced into the polymers to offer reactive moieties in reducing conditions that could be exploited to trigger the release of loaded drugs or improve the degradation rate.

Reaction conditions for the synthesis of these new PUU derivatives (labelled as PolyCEGS) were standardized, and derivatives were physically characterized. Sensitivity to simulated reductive environments of selected derivatives processed as films was assayed by evaluating mass loss and Ssingle bondS reduction via colorimetric measurements. Finally, methotrexate (MTX) was selected to test the redox-sensitive drug delivery potential of selected PolyCEGSs. MTX is an antagonist of the synthesis of folic acid used for several anticancer therapies and autoimmune diseases like rheumatoid arthritis (RA). The study highlighted the interactions of the drug with the polyurethanes, correlating any changes in films when immersed in simulated redox environments, as the main trigger for drug release, thus exploring the potential application of PolyCEGS to produce redox controlled drug delivery systems (DDS).

Section snippets

Materials

Poly (ethylene glycol) (PEG Mw 600), ε-caprolactone (ε-CL), oxidized l-glutathione (GSSG) and reduced l-glutathione (GSH), 5,5-dithiobis-2-nitrobenzoic acid (DTNB), EDTA, LiBr, hexane, diethylamine (DEA), NaN3, polystyrene standards were acquired from Sigma Aldrich srl, Italy. Polyethylene glycol was dried by azeotropic distillation in toluene using a 1:4 volume ratio, while ε-caprolactone was purified by vacuum distillation using CaH2 as reported in the literature [4,6,32]. CaH2, stannous

Synthesis of the triblock copolymers

PolyCE copolymers were synthesized with a yield above 95%. The 1H NMR spectra of PolyCE12, reported as an example (Fig. 1), showed the characteristic peaks of methylene protons of PCL block, labelled as a, b, c and d at 1.5, 1.9, 2.5 and 4.3 ppm respectively, while the ethylene oxide protons of polyethylene glycol (e) were displayed at 3.7 ppm. Two small peaks at 4.4 and 4.5 ppm are attributable to methylene groups (referred to protons d in Fig. 1) of the terminal ω methylene protons of the

Conclusions

Novel biodegradable segmented polyurethanes were synthesized using three different starting triblock copolymers PCL-PEG-PCL with increasing PCL/PEG ratio. The BDI was used as reactive for the generation of hard segment following the chemistry of isocyanates and the tetramethyl ester of glutathione as a redox-sensitive chain extender. Three synthetic protocols were used to obtain the most suitable polymer for drug delivery purposes, where the main parameter which governed the extension phase was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could affect the work reported in this paper.

Acknowledgments

This work was financially supported by MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca) grants.

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