The production of hydrogel microspheres (MS) for cell immobilization, maintaining the favorable properties of alginate gels but presenting enhanced performance in terms of in vivo durability and physical properties, is desirable to extend the therapeutic potential of cell transplantation. A novel type of hydrogel MS was produced by straightforward functionalization of sodium alginate (Na-alg) with heterotelechelic poly(ethylene glycol) (PEG) derivatives equipped with either end thiol or 1,2-dithiolane moieties. Activation of the hydroxyl moieties of the alginate backbone in the form of imidazolide intermediate allowed for fast conjugation to PEG oligomers through a covalent carbamate linkage. Evaluation of the modified alginates for the preparation of MS combining fast ionic gelation ability of the alginate carboxylate groups and slow covalent cross-linking provided by the PEG-end functionalities highlighted the influence of the chemical composition of the PEG-grafting units on the physical characteristics of the MS. The mechanical properties of the MS (resistance and shape recovery) and durability of PEG-grafted alginates in physiological environment can be adjusted by varying the nature of the end functionalities and the length of the PEG chains. In vitro cell microencapsulation studies and preliminary in vivo assessment suggested the potential of these hydrogels for cell transplantation applications. ■ INTRODUCTIONProgress in therapies relying on the allo-or xenotransplantation of immobilized cells and tissue strongly depends on the quality of the immobilizing material. Currently, the translation of related therapies to the clinics, for example to treat end-stage organ failure and end-stage diseases such as cancer, diabetes mellitus, and acute liver failure, 1,2 is hindered by the lack of materials having the perfect properties. Hydrogels prepared from the biopolymer sodium alginate (Na-alg) or derivatives of it have been reported in abundant papers as particularly advantageous because they fulfill general requirements such as the spontaneous formations of hydrogels in the presence of divalent cations (e.g., Ca 2+ , Ba 2+ ) under mild conditions of temperature and pH, and high biocompatibility. 3−6 Maintaining these advantages but overcoming several drawbacks, including the lack of in vivo mechanical resistance and stability, and defects in permselectivity, 7,8 are the focus of current research. There are additional limitations caused by the dimension of the targeted final application. This calls in particular for therapies which intend the transplantation of allo-or xenogeneic cells immobilized in microspheres (MS) in order to allow for miniinvasive surgery. 9,10 The stabilization of alginate gel beads by coating the MS with polycations such as (poly(L-lysine), poly(L-ornithine), and poly(L-guanidine) was investigated, but impaired cell graft function in vivo. 11 Another strategy to improve the performance of alginate MS relies on the combination with other polymers such as poly(ethylene glycol) (P...
The controlled release of small molecular modulators of the immune response from hydrogel microspheres (MS) used for cell immobilization is an attractive approach to reduce pericapsular fibrotic overgrowth (PFO) after transplantation. Ketoprofen is a well-known nonsteroidal anti-inflammatory drug involved in the early stage inflammation cascade. PEGylated derivatives of ketoprofen, presenting either ester or amide linkage to the drug, were synthesized and conjugated to the hydroxyl groups of sodium alginate (Na-alg). Functionalized cell-free and MIN6 cells containing MS were produced from the resulting modified alginates. In vitro quantification of ketoprofen release indicated regular and sustained drug delivery over 14 days, resulting from the hydrolytic cleavage of the ester bond. The release kinetics was enhanced over the initial 7 days by the presence of MIN6 cells, probably as a result of cell esterase activity. In the presence of amide bond, traces of ketoprofen were released over 14 days due to a much slower hydrolysis kinetics. Cell-free and MIN6 cells containing MS were transplanted in immune-competent mice, either in the peritoneal cavity or under the kidney capsule, with a follow-up period of 30 days. Comparison with nonmodified Ca-alg MS transplanted in the same conditions demonstrated a clear reduction in the severity of PFO for MS functionalized with ketoprofen. Quantification of collagen deposition on MIN6 cells containing MS transplanted under the kidney capsule revealed the significant effect of ketoprofen release to decrease fibrotic tissue formation. The impact was more pronounced when the drug was covalently conjugated by an ester linkage, allowing higher concentration of the anti-inflammatory compound to be delivered at the transplantation site. The functionality of microencapsulated MIN6 cells 30 days after transplantation was confirmed by detection of insulin positive cell content.
Two novel types of hydrogel microspheres (MS) are presented. First, one-component microspheres (1-comMS) were produced from sodium alginate (Na-alg) equipped with thiolfunctionalized hydroxyl groups. The functionalization pathway included the conversion of Na-alg into tetrabutylammonium alginate, insertion of new carboxyl groups, grafting of α-amine-ωthiol poly(ethylene glycol), and restoration of the sodium salt. This modification conserves all original carboxyl groups of Na-alg and allows for covalent disulfide bond formation in addition to ionic cross-linking. Second, two-component microspheres (2-comMS) were obtained from a mixture of Na-alg and Na-alg functionalized with cysteamine. This functionalization was achieved by grafting cystamine dihydrochloride on some carboxyl groups followed by the reduction to cysteamine. Using the one-step MS formation process developed for both MS types, very fast ionic gelation with calcium ions conserves the spherical shape of the polymer solution droplets upon extrusion into the gelation bath, while simultaneously occurring slow covalent cross-linking reinforces the hydrogels. The physical properties of both MS types are adjustable by varying the polymer concentration, the degree of grafting, and the mixing ratio. In vitro cell microencapsulation studies confirmed the cytocompatibility of 1-comMS and 2-comMS.
Cell microencapsulation and subsequent transplantation of the microencapsulated cells require multidisciplinary approaches. Physical, chemical, biological, engineering, and medical expertise has to be combined. Several natural and synthetic polymeric materials and different technologies have been reported for the preparation of hydrogels, which are suitable to protect cells by microencapsulation. However, owing to the frequent lack of adequate characterization of the hydrogels and their components as well as incomplete description of the technology, many results of in vitro and in vivo studies appear contradictory or cannot reliably be reproduced. This review addresses the state of the art in cell microencapsulation with special focus on microencapsulated cells intended for xenotransplantation cell therapies. The choice of materials, the design and fabrication of the microspheres, as well as the conditions to be met during the cell microencapsulation process, are summarized and discussed prior to presenting research results of in vitro and in vivo studies. Overall, this review will serve to sensitize medically educated specialists for materials and technological aspects of cell microencapsulation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.