Engineering surface adhered poly(vinyl alcohol) physical hydrogels as enzymatic microreactors

Engineering Extracellular Vesicles with the Tools of Enzyme Prodrug Therapy

Extracellular vesicles (EVs) have recently gained significant attention as important mediators of intercellular communication, potential drug carriers, and disease biomarkers. These natural cell-derived nanoparticles are postulated to be biocompatible, stable under physiological conditions, and to show reduced immunogenicity as compared to other synthetic nanoparticles. Although initial clinical trials are ongoing, the use of EVs for therapeutic applications may be limited due to undesired off-target activity and potential "dilution effects" upon systemic administration which may affect their ability to reach their target tissues. To fully exploit their therapeutic potential, EVs are embedded into implantable biomaterials designed to achieve local delivery of therapeutics taking advantage of enzyme prodrug therapy (EPT). In this first application of EVs for an EPT approach, EVs are used as smart carriers for stabilizing enzymes in a hydrogel for local controlled conversion of benign prodrugs to active antiinflammatory compounds. It is shown that the natural EVs' antiinflammatory potential is comparable or superior to synthetic carriers, in particular upon repeated long-term incubations and in different macrophage models of inflammation. Moreover, density-dependent color scanning electron microscopy imaging of EVs in a hydrogel is presented herein, an impactful tool for further understanding EVs in biological settings.

Substrate mediated enzyme prodrug therapy

Substrate mediated enzyme prodrug therapy (SMEPT) is a biomedical platform developed to perform a localized synthesis of drugs mediated by implantable biomaterials. This approach combines the benefits and at the same time offers to overcome the drawbacks for traditional pill-based drug administration and site-specific, implant mediated drug delivery. Specifically, SMEPT offers the flexibility of delivering multiple drugs - individually as monotherapy, in sequence, or as a combination therapy, all of which is also accomplished in a site-specific manner. This technology is also unique for site-specific synthesis of drugs with short half-life, such as nitric oxide. This review presents historical development of SMEPT from early reports to the most recent examples, and also outlines potential avenues for subsequent development of this platform.

Enzyme Prodrug Therapy Engineered into Electrospun Fibers with Embedded Liposomes for Controlled, Localized Synthesis of Therapeutics

Enzyme prodrug therapy (EPT) enables localized conversion of inert prodrugs to active drugs by enzymes. Performance of EPT necessitates that the enzyme remains active throughout the time frame of the envisioned therapeutic application. β-glucuronidase is an enzyme with historically validated performance in EPT, however it retains its activity in biomaterials for an insufficiently long period of time, typically not exceeding 7 d. Herein, the encapsulation of β-glucuronidase in liposomal subcompartments within poly(vinyl alcohol) electrospun fibers is reported, leading to the assembly of biocatalytically active materials with activity of the enzyme sustained over at least seven weeks. It is further shown that liposomes provide the highly beneficial stabilization of the enzyme when incubated in cell culture media. The assembled biocatalytic materials successfully produce antiproliferative drugs (SN-38) using externally administered prodrugs (SN-38-glucuronide) and effectively suppress cell proliferation, with envisioned utility in the design of cardiovascular grafts.

Poly(vinyl alcohol) Physical Hydrogels: Matrix-Mediated Drug Delivery Using Spontaneously Eroding Substrate

Poly(vinyl alcohol) hydrogels have a long and successful history of applications in biomedicine. Historically, these matrices were developed to be nondegradable—limiting their utility to applications as permanent implants. For tissue engineering and drug delivery, herein we develop spontaneously eroding physical hydrogels based on PVA. We characterize in detail a mild, noncryogenic method of producing PVA physical hydrogels using poly(ethylene glycol) as a gelating agent, and investigate PVA molar mass as a means to define the kinetics of erosion of these biomaterials. PVA hydrogels are characterized for associated inflammatory response in adhering macrophages, antiproliferative effects mediated through delivery of cytotoxic drugs to myoblasts, and pro-proliferative activity achieved via presentation of conjugated growth factors to endothelial cells. Together, these data present a multiangle characterization of these novel multifunctional matrices for applications in tissue engineering and drug delivery mediated by implantable biomaterials.

Design of thiol-ene photoclick hydrogels using facile techniques for cell culture applications†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4bm00187gClick here for additional data file

Thiol-ene 'click' chemistries have been widely used in biomaterials applications, including drug delivery, tissue engineering, and controlled cell culture, owing to their rapid, cytocompatible, and often orthogonal reactivity. In particular, hydrogel-based biomaterials formed by photoinitiated thiol-ene reactions afford spatiotemporal control over the biochemical and biomechanical properties of the network for creating synthetic materials that mimic the extracellular matrix or enable controlled drug release. However, the use of charged peptides functionalized with cysteines, which can form disulfides prior to reaction, and vinyl monomers that require multistep syntheses and contain ester bonds, may lead to undesired inhomogeneity or degradation under cell culture conditions. Here, we designed a thiol-ene hydrogel formed by the reaction of allyloxycarbonyl-functionalized peptides and thiol-functionalized poly(ethylene glycol). Hydrogels were polymerized by free radical initiation under cytocompatible doses of long wavelength ultraviolet light in the presence of water-soluble photoinitiators (lithium acylphosphinate, LAP, and 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone, Irgacure 2959). Mechanical properties of these hydrogels were controlled by varying the monomer concentration to mimic a range of soft tissue environments, and hydrogel stability in cell culture medium was observed over weeks. Patterns of biochemical cues were created within the hydrogels post-formation and confirmed through the incorporation of fluorescently-labeled peptides and Ellman's assay to detect free thiols. Human mesenchymal stem cells remained viable after encapsulation and subsequent photopatterning, demonstrating the utility of the monomers and hydrogels for three-dimensional cell culture. This facile approach enables the formation and characterization of hydrogels with well-defined, spatially-specific properties and expands the suite of monomers available for three-dimensional cell culture and other biological applications.

Biocatalytic polymer thin films: optimization of the multilayered architecture towards in situ synthesis of anti-proliferative drugs

We report on the assembly of multi-layered polyelectrolyte thin films containing an immobilized enzyme to perform conversion of externally administered prodrugs and achieve delivery of the resulting therapeutics to adhering cells. Towards this goal, multi-layered coatings were assembled using poly(sodium styrene sulfonate) and poly(allylamine hydrochloride). Activity of the incorporated enzyme was quantified as a function of the assembly conditions, position of the enzyme within the multi-layered architecture, concentration of the enzyme in the adsorption solution, and concentration of the administered prodrug. Biocatalytic coatings exhibited sustained levels of enzymatic activity over at least one week of incubation in physiological buffers without signs of loss of activity of the enzyme. Developed enzyme-containing polymer films afforded zero-order release of the in situ synthesized cargo with kinetics of synthesis (nM per hour) covering at least 3 orders of magnitude. Internalization of the synthesized product by adhering cells was visualized using a fluorogenic enzyme substrate. Therapeutic utility of biocatalytic coatings was demonstrated using a myoblast cell line and a prodrug for the anti-proliferative agent, 5-fluorouridine. Taken together, this work presents a novel approach to delivery of small molecule drugs using multi-layered polymer thin films with utility in surface-mediated drug delivery, assembly of therapeutic implantable devices, and tissue engineering.

Cholesterol modification of (Bio)polymers using UV-Vis traceable chemistry in aqueous solutions

Cholesterol modification of synthetic and biological polymers is achieved using of thiocholesterol (TC) and thiol-disulfide exchange. TC is reacted with Ellman's reagent to produce a mixed disulfide (TC-ER) which is activated towards thiol-disulfide exchange. TC-ER is used to obtain an inclusion complex with methyl-β-cyclodextrin, which is then employed to achieve cholesterol functionalization of a model peptide, synthetic polymers, and physical hydrogels based on poly(vinyl alcohol). It is anticipated that the established techniques will significantly broaden the use of cholesterol in bio- and nanotechnology and specifically biomedicine.

Lipogels: surface-adherent composite hydrogels assembled from poly(vinyl alcohol) and liposomes

Drug-eluting engineered surface coatings are of paramount importance for many biomedical applications from implantable devices to tissue engineering. Herein, we present the assembly of lipogels, composite physical hydrogels assembled from poly(vinyl alcohol) and liposomes using thiol-disulfide exchange between end group modified PVA and thiocholesterol containing liposomes, and the response of adhering cells to these coatings. We demonstrate the controlled loading of liposomes into the polymer matrix and the preserved mechanical properties of the lipogels. Furthermore, the lipogels are successfully rendered cell adhesive by incorporation of poly(l-lysine) into the PVA polymer matrix or by poly(dopamine) coating of the lipogels. The successful lipid uptake from the lipogels by macrophages, hepatocytes, and myoblasts was monitored by flow cytometry. Finally, the delivery of active cargo, paclitaxel, to adherent myoblasts is shown, thus illustrating the potential of the lipogels as a drug eluting interface for biomedical applications.

Microstructured, functional PVA hydrogels through bioconjugation with oligopeptides under physiological conditions

In this work, bioconjugation techniques are developed to achieve peptide functionalization of poly(vinyl alcohol), PVA, as both a polymer in solution and within microstructured physical hydrogels, in both cases under physiological conditions. PVA is unique in that it is one of very few polymers with excellent biocompatibility and safety and has FDA approval for clinical uses in humans. However, decades of development have documented only scant opportunities in bioconjugation with PVA. As such, materials derived thereof fail to answer the call for functional biomaterials for advanced cell culture and tissue engineering applications. To address these limitations, PVA is synthesized with terminal thiol groups and conjugated with thiolated peptides using PVA in solution. Further, microstructured, surface-adhered PVA physical hydrogels are assembled, the available conjugation sites within the hydrogels are quantified, and quantitative kinetic data are collected on peptide conjugation to the hydrogels. The success of bioconjugation in the gel phase is quantified through the use of a cell-adhesive peptide and visualization of cell adhesion on PVA hydrogels as cell culture substrates. Taken together, the presented data establish a novel paradigm in bioconjugation and functionalization of PVA physical hydrogels. Coupled with an excellent safety profile of PVA, these results deliver a superior biomaterial for diverse biomedical applications.

Bioresorbable surface-adhered enzymatic microreactors based on physical hydrogels of poly(vinyl alcohol)

Hydrogel biomaterials based on poly(vinyl alcohol), PVA, have an extensive history of biomedical applications, yet in their current form suffer from significant shortcomings, such as a lack of mechanism of biodegradation and poor opportunities in controlled drug release. We investigate physical hydrogels of PVA as surface-adhered materials and present biodegradable matrices equipped with innovative tools in substrate-mediated drug release. Toward the final goal, PVA chains with narrow polydispersities (1.1-1.2) and molecular weights of 5, 10, and 28 kDa are synthesized via controlled radical polymerization (RAFT). These molecular weights are shown to be suitably high to afford robust hydrogel matrices and at the same time suitably low to allow gradual erosion of the hydrogels with kinetics of degradation controlled via polymer macromolecular characteristics. For opportunities in controlled drug release, hydrogels are equipped with enzymatic cargo to achieve an in situ conversion of externally added prodrug into a final product, thus giving rise to surface-adhered enzymatic microreactors. Hydrogel-mediated enzymatic activity was investigated as a function of polymer molecular weight and concentration of solution taken for assembly of hydrogels. Taken together, we present, to the best of our knowledge, the first example of bioresorbable physical hydrogel based on PVA with engineered opportunities in substrate-mediated enzymatic activity and envisioned utility in surface-mediated drug delivery and tissue engineering.

Substrate mediated enzyme prodrug therapy

In this report, we detail Substrate Mediated Enzyme Prodrug Therapy (SMEPT) as a novel approach in drug delivery which relies on enzyme-functionalized cell culture substrates to achieve a localized conversion of benign prodrug(s) into active therapeutics with subsequent delivery to adhering cells or adjacent tissues. For proof-of-concept SMEPT, we use surface adhered micro-structured physical hydrogels based on poly(vinyl alcohol), β-glucuronidase enzyme and glucuronide prodrugs. We demonstrate enzymatic activity mediated by the assembled hydrogel samples and illustrate arms of control over rate of release of model fluorescent cargo. SMEPT was not impaired by adhering cells and afforded facile time - and dose - dependent uptake of the in situ generated fluorescent cargo by hepatic cells, HepG2. With the use of a glucuronide derivative of an anticancer drug, SN-38, SMEPT afforded a decrease in cell viability to a level similar to that achieved using parent drug. Finally, dose response was achieved using SMEPT and administration of judiciously chosen concentration of SN-38 glucuronide prodrug thus revealing external control over drug delivery using drug eluting surface. We believe that this highly adaptable concept will find use in diverse biomedical applications, specifically surface mediated drug delivery and tissue engineering.