Mathematical model accurately predicts protein release from an affinity-based delivery system

Redesigned chondroitinase ABC degrades inhibitory chondroitin sulfate proteoglycans in vitro and in vivo in the stroke-injured rat brain

Injuries to the central nervous system, such as stroke and traumatic spinal cord injury, result in an aggregate scar that both limits tissue degeneration and inhibits tissue regeneration. The aggregate scar includes chondroitin sulfate proteoglycans (CSPGs), which impede cell migration and axonal outgrowth. Chondroitinase ABC (ChASE) is a potent yet fragile enzyme that degrades CSPGs, and thus may enable tissue regeneration. ChASE37, with 37-point mutations to the native enzyme, has been shown to be more stable than ChASE, but its efficacy has never been tested. To answer this question, we investigated the efficacy of ChASE37 first in vitro using human cell-based assays and then in vivo in a rodent model of stroke. We demonstrated ChASE37 degradation of CSPGs in vitro and the consequent cell adhesion and axonal sprouting now possible using human induced pluripotent stem cell (hiPSC)-derived neurons. To enable prolonged release of ChASE37 to injured tissue, we expressed it as a fusion protein with a Src homology 3 (SH3) domain and modified an injectable, carboxymethylcellulose (CMC) hydrogel with SH3-binding peptides (CMC-bp) using inverse electron-demand Diels-Alder chemistry. We injected this affinity release CMC-bp/SH3-ChASE37 hydrogel epicortically to endothelin-1 stroke-injured rats and confirmed bioactivity via degradation of CSPGs and axonal sprouting in and around the lesion. With CSPG degradation shown both in vitro by greater cell interaction and in vivo with local delivery from a sustained release formulation, we lay the foundation to test the potential of ChASE37 and its delivery by local affinity release for tissue regeneration after stroke.

Acrylic Acid Modified Poly-ethylene Glycol Microparticles for Affinity-Based release of Insulin-Like Growth Factor-1 in Neural Applications

Sustained release of bioactive molecules via affinity-based interactions presents a promising approach for controlled delivery of growth factors. Insulin-like growth factor-1 (IGF-1) has gained increased attention due to its ability to promote axonal growth in the central nervous system. In this work, we aimed to evaluate the effect of IGF-1 delivery from polyethylene-glycol diacrylate (PEG-DA) microparticles using affinity-based sustained release on neurons. We developed PEG-DA-based microparticles with varying levels of acrylic acid (AA) as a comonomer to tune their overall charge. The particles were synthesized via precipitation polymerization under UV light, yielding microparticles (MPs) with a relatively low polydispersity index. IGF-1 was incubated with the PEG-DA particles overnight, and formulations with a higher AA content resulted in higher loading efficiency and slower release rates over 4 weeks, suggesting the presence of binding interactions between the positively charged IGF-1 and negatively charged particles containing AA. The released IGF-1 was tested in dorsal root ganglion (DRG) neurite outgrowth assay and found to retain its biological activity for up to two weeks after encapsulation. Furthermore, the trophic effect of IGF-1 was tested with stem cell-derived V2a interneurons and found to have a synergistic effect when combined with neurotrophin-3 (NT3). To assess the potential of a combinatorial approach, IGF-1-releasing MPs were encapsulated within a hyaluronic acid (HA) hydrogel and showed promise as a dual delivery system. Overall, the PEG-DA MPs developed herein deliver bioactive IGF-1 for a period of weeks and hold potential to enable axonal growth of injured neurons via sustained release.

Colloidal Hydrogel with Staged Sequestration and Release of Molecules Undergoing Competitive Binding

Competitive binding of distinct molecules in the hydrogel interior can facilitate dynamic exchange between the hydrogel and the surrounding environment. The ability to control the rates of sequestration and release of these molecules would enhance the hydrogel's functionality and enable targeting of a specific task. Here, we report the design of a colloidal hydrogel with two distinct pore dimensions to achieve staged, diffusion-controlled scavenging and release dynamics of molecules undergoing competitive binding. The staged scavenging and release strategy was shown for CpG oligodeoxynucleotide (ODN) and human epidermal growth factor (hEGF), two molecules exhibiting different affinities to the quaternary ammonium groups of the hydrogel. Fast ODN scavenging from the ambient environment occurred via diffusion through submicrometer-size hydrogel pores, while delayed hEGF release from the hydrogel was governed by its diffusion through nanometer-size pores. The results of the experiments were in agreement with simulation results. The significance of staged ODN-hEGF exchange was highlighted by the dual anti-inflammation and tissue proliferation hydrogel performance.

Facile synthesis of double-cross-linked alginate-based hydrogel: Characterization and use in a context of circular economy for cationic dye removal

Hydrogels based on natural polysaccharides have received special attention in the last decade due to their interesting features, such as availability, biocompatibility, biodegradability and safety. Such characteristics may make them sustainable and eco-friendly materials for water and wastewater treatment, meeting the concept of circular economy. In this study, a novel double-cross-linked alginate-based hydrogel has been successfully synthesized using epichlorhydrin and sodium trimetaphosphate (STMP) as cross-linker agents and then used for the removal of methylene blue (MB) dye under different operating conditions. The obtained hydrogel was deeply characterized by using various analytical techniques, namely Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Experimental results showed that the synthesized double cross-linked hydrogel with relatively high STMP concentration (0.26 M) has promising structural and textural properties. This material exhibited excellent removal ability towards MB with a maximum adsorption capacity of about 992 mg/g for an initial pH of 10. The kinetic and isotherm modeling study revealed that the pseudo-second-order and Freundlich models fitted well the measured adsorption experimental data. The MB adsorption process onto the synthesized hydrogel is exothermic, feasible and spontaneous. It mainly includes electrostatic interaction and hydrogen bonds. These findings suggest that double-cross-linked alginate-based hydrogel can be considered as an attractive and potential adsorbent for an effective cationic dye removal from aqueous environments. The use of such a green adsorbent for the treatment of organic-pollutants-rich industrial wastewaters promotes sustainability and circular economy concepts.

Affinity-controlled release of rod-derived cone viability factor enhances cone photoreceptor survival

Retinitis pigmentosa (RP) is a group of genetic diseases that results in rod photoreceptor cell degeneration, which subsequently leads to cone photoreceptor cell death, impaired vision and eventual blindness. Rod-derived cone viability factor (RdCVF) is a protein which has two isoforms: a short form (RdCVF) and a long form (RdCVFL) which act on cone photoreceptors in the retina. RdCVFL protects photoreceptors by reducing hyperoxia in the retina; however, sustained delivery of RdCVFL remains challenging. We developed an affinity-controlled release strategy for RdCVFL. An injectable physical blend of hyaluronan and methylcellulose (HAMC) was covalently modified with a peptide binding partner of the Src homology 3 (SH3) domain. This domain was expressed as a fusion protein with RdCVFL, thereby enabling its controlled release from HAMC-binding peptide. Sustained release of RdCVFL was demonstrated for the first time as RdCVFL-SH3 from HAMC-binding peptide for 7 d in vitro. To assess bioactivity, chick retinal dissociates were harvested and treated with the affinity-released recombinant protein from the HAMC-binding peptide vehicle. After 6 d in culture, cone cell viability was greater when cultured with released RdCVFL-SH3 relative to controls. We utilized computational fluid dynamics to model release of RdCVFL-SH3 from our delivery vehicle in the vitreous of the human eye. We demonstrate that our delivery vehicle can prolong the bioavailability of RdCVFL-SH3 in the retina, potentially enhancing its therapeutic effects. Our affinity-based system constitutes a versatile delivery platform for ultimate intraocular injection in the treatment of retinal degenerative diseases. STATEMENT OF SIGNIFICANCE: Retinitis pigmentosa (RP) is the leading cause of inherited blindness in the world. Rod-derived cone viability factor (RdCVF), a novel protein paracrine factor, is effective in preclinical models of RP. To extend its therapeutic effects, we developed an affinity-controlled release strategy for the long form of RdCVF, RdCVFL. We expressed RdCVFL as a fusion protein with an Src homology 3 domain (SH3). We then utilized a hydrogel composed of hyaluronan and methylcellulose (HAMC) and modified it with SH3 binding peptides to investigate its release in vitro. Furthermore, we designed a mathematical model of the human eye to investigate delivery of the protein from the delivery vehicle. This work paves the way for future investigation of controlled release RdCVF.

Osmotic release of drugs via deswelling dynamics of microgels: modeling of collaborative flow and diffusions

Hydrogel colloids, i.e., micro- or nano-gels, are increasingly engineered as promising vehicles for polymer-based drug delivery systems. We report a continuum theory of deswelling dynamics of nanocomposite microgels driven by external osmotic shocks and further develop a universal framework, by introducing a buffer release domain, to quantitatively characterize a continuous drug release from deswollen microgels towards surroundings. The drug release is shown to proceed accompanied by an active outward solvent flow created by the elastically shrunken gel network. We further find that a declining trend in the cumulative release plateau with the drug size is followed by an apparent increase again as the drug size increases above a threshold. These findings highlight a nontrivial behavior that the resulting hydrodynamic interactions coexist collaboratively with the passive diffusions to facilitate a desired drug release. We show that deswelling of a stiffer microgel (the mesh size reduces slowly) or loading the larger drugs could bring a control-like release type, otherwise a burst-like release type emerges. Compared with a uniform microgel, the fuzzy-corona-like microgel enables a more productive drug release before reaching deswelling equilibrium. Our model not only predicts well the existing experiments, but also serves as a versatile paradigm to help understand the reciprocal roles of the solvent flow, the gel dynamics, and the diffusions in the polymer-based drug delivery systems.

Affibody-mediated controlled release of fibroblast growth factor 2

Protein therapeutics possess high target affinity and specificity, yet short residence times, which limit their broad utility. To overcome this challenge, we used affinity interactions to modulate protein release from a hydrogel delivery vehicle thereby prolonging therapeutic availability. Specifically, we designed an affibody-modified hyaluronan (HA)-based hydrogel as a delivery platform for fibroblast growth factor 2 (FGF2), a neuroprotective and neuroregenerative factor in the central nervous system (CNS). We identified a highly specific affibody binding partner with moderate affinity for FGF2 using yeast surface display and flow cytometry-based screening. Importantly, we demonstrated controlled release of bioactive FGF2 from the hydrogel by varying the ratio of affibody to protein and showed increased thermal stability of FGF2 in the presence of affibody. This versatile delivery platform will allow the distinct, simultaneous release of multiple proteins based on specific affinity interactions.

Drug repurposing: An emerging strategy in alleviating skin cancer

Skin cancer is one of the most common forms of cancer. Several million people are estimated to have affected with this condition worldwide. Skin cancer generally includes melanoma and non-melanoma with the former being the most dangerous. Chemotherapy has been one of the key therapeutic strategies employed in the treatment of skin cancer, especially in advanced stages of the disease. It could be also used as an adjuvant with other treatment modalities depending on the type of skin cancer. However, there are several shortfalls associated with the use of chemotherapy such as non-selectivity, tumour resistance, life-threatening toxicities, and the exorbitant cost of medicines. Furthermore, new drug discovery is a lengthy and costly process with minimal likelihood of success. Thus, drug repurposing (DR) has emerged as a new avenue where the drug approved formerly for the treatment of one disease can be used for the treatment of another disease like cancer. This approach is greatly beneficial over the de novo approach in terms of time and cost. Moreover, there is minimal risk of failure of repurposed therapeutics in clinical trials. There are a considerable number of studies that have reported on drugs repurposed for the treatment of skin cancer. Thus, the present manuscript offers a comprehensive overview of drugs that have been investigated as repurposing candidates for the efficient treatment of skin cancers mainly melanoma and its oncogenic subtypes, and non-melanoma. The prospects of repurposing phytochemicals against skin cancer are also discussed. Furthermore, repurposed drug delivery via topical route and repurposed drugs in clinical trials are briefed. Based on the findings from the reported studies discussed in this manuscript, drug repurposing emerges to be a promising approach and thus is expected to offer efficient treatment at a reasonable cost in devitalizing skin cancer.

Protein release from nanocellulose and alginate hydrogels: The study of adsorption and desorption kinetics

This work presents a study of the lysozyme release from crosslinked TEMPO nanocellulose (TOCNF) and alginate (ALG) hydrogels in a medium with different ionic strength and temperature. The main objective is to develop a mathematical model for a detailed study of the concurrent action of diffusion mechanism and adsorption/desorption kinetics. Model fit parameters provide important information about the initial (maximum) adsorption rate and its deceleration with increasing ionic strength of the release medium. Similarly, the initial (minimum) desorption rate and its acceleration with increasing salt concentration can be determined. The model leads us to the conclusion that the initial adsorption rate is higher in the case of TOCNF, but due to fewer electrostatic interactions and morphology as well as topography of the surface, it decreases to a negligible value much faster than in the case of ALG, where the diffusion process becomes dominant.

Quantifying How Drug-Polymer Interaction and Volume Phase Transition Modulate the Drug Release Kinetics from Core-Shell Microgels

This paper presents a simple experimental-informed theory describing the drug release process from a temperature-responsive core-shell microgel. In stark contrast to the commonly employed power-law models, we couple electric, hydrophobic, and steric factors to characterize the impact of drug-polymer pair interaction on the release kinetics. To this end, we also propose a characteristic time, depicting the drug release process as an interplay between kinetics and thermodynamics. In some instances, the negative correlation between the diffusivity and the (thermodynamics) drug-polymer interaction renders the drug release time non-trivial. In conclusion, our theory establishes a mechanistic understanding of the drug release process, exploring the effect of (hydrophobic adhesion) attractive and (steric exclusion) repulsive pair interactions between the drugs and the microgel in the presence of temperature-induced volume phase transition.

Peptide hydrogels for affinity-controlled release of therapeutic cargo: Current and potential strategies

The development of devices for the precise and controlled delivery of therapeutics has grown rapidly over the last few decades. Drug delivery materials must provide a depot with delivery profiles that satisfy pharmacodynamic and pharmacokinetic requirements resulting in clinical benefit. Therapeutic efficacy can be limited due to short half-life and poor stability. Thus, to compensate for this, frequent administration and high doses are often required to achieve therapeutic effect, which in turn increases potential side effects and systemic toxicity. This can potentially be mitigated by using materials that can deliver drugs at controlled rates, and material design principles that allow this are continuously evolving. Affinity-based release strategies incorporate a myriad of reversible interactions into a gel network, which have affinities for the therapeutic of interest. Reversible binding to the gel network impacts the release profile of the drug. Such affinity-based interactions can be modulated to control the release profile to meet pharmacokinetic benchmarks. Much work has been done developing affinity-based control in the context of polymer-based materials. However, this strategy has not been widely implemented in peptide-based hydrogels. Herein, we present recent advances in the use of affinity-controlled peptide gel release systems and their associated mechanisms for applications in drug delivery.

Computational pharmaceutics - A new paradigm of drug delivery

In recent decades pharmaceutics and drug delivery have become increasingly critical in the pharmaceutical industry due to longer time, higher cost, and less productivity of new molecular entities (NMEs). However, current formulation development still relies on traditional trial-and-error experiments, which are time-consuming, costly, and unpredictable. With the exponential growth of computing capability and algorithms, in recent ten years, a new discipline named "computational pharmaceutics" integrates with big data, artificial intelligence, and multi-scale modeling techniques into pharmaceutics, which offered great potential to shift the paradigm of drug delivery. Computational pharmaceutics can provide multi-scale lenses to pharmaceutical scientists, revealing physical, chemical, mathematical, and data-driven details ranging across pre-formulation studies, formulation screening, in vivo prediction in the human body, and precision medicine in the clinic. The present paper provides a comprehensive and detailed review in all areas of computational pharmaceutics and "Pharma 4.0", including artificial intelligence and machine learning algorithms, molecular modeling, mathematical modeling, process simulation, and physiologically based pharmacokinetic (PBPK) modeling. We not only summarized the theories and progress of these technologies but also discussed the regulatory requirements, current challenges, and future perspectives in the area, such as talent training and a culture change in the future pharmaceutical industry.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Not only in silico drug discovery: Molecular modeling towards in silico drug delivery formulations

The use of methods at molecular scale for the discovery of new potential active ligands, as well as previously unknown binding sites for target proteins, is now an established reality. Literature offers many successful stories of active compounds developed starting from insights obtained in silico and approved by Food and Drug Administration (FDA). One of the most famous examples is raltegravir, a HIV integrase inhibitor, which was developed after the discovery of a previously unknown transient binding area thanks to molecular dynamics simulations. Molecular simulations have the potential to also improve the design and engineering of drug delivery devices, which are still largely based on fundamental conservation equations. Although they can highlight the dominant release mechanism and quantitatively link the release rate to design parameters (size, drug loading, et cetera), their spatial resolution does not allow to fully capture how phenomena at molecular scale influence system behavior. In this scenario, the "computational microscope" offered by simulations at atomic scale can shed light on the impact of molecular interactions on crucial parameters such as release rate and the response of the drug delivery device to external stimuli, providing insights that are difficult or impossible to obtain experimentally. Moreover, the new paradigm brought by nanomedicine further underlined the importance of such computational microscope to study the interactions between nanoparticles and biological components with an unprecedented level of detail. Such knowledge is a fundamental pillar to perform device engineering and to achieve efficient and safe formulations. After a brief theoretical background, this review aims at discussing the potential of molecular simulations for the rational design of drug delivery systems.

A Polymeric Delivery System Enables Controlled Release of Genipin for Spatially-Confined In Situ Crosslinking of Injured Connective Tissues

An emerging approach toward repair of connective tissues applies exogenous crosslinkers to mechanically augment injured structures in vivo. One crosslinker that has been explored for this purpose is the plant-derived small molecule genipin. However, genipin's high reactivity to primary amines in proteins, small size, and high diffusion coefficient necessitate localizing and controlling its delivery to avoid off-target or adverse effects. In this study, genipin-loaded polymers were evaluated for sustained local administration. Insoluble polymers comprising subunits of α-, β-, or γ-cyclodextrin, cyclic oligosaccharides possessing increasing cavity sizes, were compared to polymers comprising subunits of the non-cyclic polysaccharide dextran. Polymers made from β-cyclodextrin showed prolonged genipin release for over ten times longer than polymers made from α- or γ-cyclodextrins or dextran, indicating that genipin possesses molecular affinity for the β-cyclodextrin cavity. Modeling of complexation between genipin and cyclodextrin hosts supported this finding. Genipin released from all polymers was confirmed to be functional by exogenous collagen crosslinking through fluorometric and mechanical readouts. Co-incubation of genipin-loaded polymers with bovine tendon explants showed genipin crosslink-mediated coloration that was confined to the sites of exposure. Altogether, results indicate that host-guest interactions within a polymeric delivery vehicle can help to control and confine genipin release.

Zero-order drug delivery: State of the art and future prospects

Pharmaceutical drugs are an important part of the global healthcare system, with some estimates suggesting over 50% of the world's population takes at least one medication per day. Most drugs are delivered as immediate-release formulations that lead to a rapid increase in systemic drug concentration. Although these formulations have historically played an important role, they can be limited by poor patient compliance, adverse side effects, low bioavailability, or undesirable pharmacokinetics. Drug delivery systems featuring first-order release kinetics have been able to improve pharmacokinetics but are not ideal for drugs with short biological half-lives or small therapeutic windows. Zero-order drug delivery systems have the potential to overcome the issues facing immediate-release and first-order systems by releasing drug at a constant rate, thereby maintaining drug concentrations within the therapeutic window for an extended period of time. This release profile can be used to limit adverse side effects, reduce dosing frequency, and potentially improve patient compliance. This review covers strategies being employed to attain zero-order release or alter traditionally first-order release kinetics to achieve more consistent release before discussing opportunities for improving device performance based on emerging materials and fabrication methods.

Elucidating the Structure-Function Relationship of Solvent and Cross-Linker on Affinity-Based Release from Cyclodextrin Hydrogels

Minocycline (MNC) is a tetracycline antibiotic capable of associating with cyclodextrin (CD), and it is a frontline drug for many instances of implant infection. Due to its broad-spectrum activity and long half-life, MNC represents an ideal drug for localized delivery; however, classic polymer formulations, particularly hydrogels, result in biphasic release less suitable for sustained anti-microbial action. A polymer delivery system capable of sustained, steady drug delivery rates poses an attractive target to maximize the antimicrobial activity of MNC. Here, we formed insoluble hydrogels of polymerized CD (pCD) with a range of crosslinking densities, and then assessed loading, release, and antimicrobial activity of MNC. MNC loads between 5-12 wt % and releases from pCD hydrogels for >14 days. pCD loaded with MNC shows extended antimicrobial activity against S. aureus for >40 days and E. coli for >70 days. We evaluated a range of water/ethanol blends to test our hypothesis that solvent polarity will impact drug-CD association as a function of hydrogel swelling and crosslinking. Increased polymer crosslinking and decreased solvent polarity both reduced MNC loading, but solvent polarity showed a dramatic reduction independent of hydrogel swelling. Due to its high solubility and excellent delivery profile, MNC represents a unique drug to probe the structure-function relationship between drug, affinity group, and polymer crosslinking ratio.

Heparin-mediated delivery of bone morphogenetic protein-2 improves spatial localization of bone regeneration

Supraphysiologic doses of bone morphogenetic protein-2 (BMP-2) are used clinically to promote bone formation in fracture nonunions, large bone defects, and spinal fusion. However, abnormal bone formation (i.e., heterotopic ossification) caused by rapid BMP-2 release from conventional collagen sponge scaffolds is a serious complication. We leveraged the strong affinity interactions between heparin microparticles (HMPs) and BMP-2 to improve protein delivery to bone defects. We first developed a computational model to investigate BMP-2-HMP interactions and demonstrated improved in vivo BMP-2 retention using HMPs. We then evaluated BMP-2-loaded HMPs as a treatment strategy for healing critically sized femoral defects in a rat model that displays heterotopic ossification with clinical BMP-2 doses (0.12 mg/kg body weight). HMPs increased BMP-2 retention in vivo, improving spatial localization of bone formation in large bone defects and reducing heterotopic ossification. Thus, HMPs provide a promising opportunity to improve the safety profile of scaffold-based BMP-2 delivery.

Vascularization strategies for skin tissue engineering

Lack of proper vascularization after skin trauma causes delayed wound healing. This has sparked the development of various tissue engineering strategies to improve vascularization.

Modulated Protein Delivery to Engineer Tissue Repair

IMPACT STATEMENT

Achieving targeted protein delivery to injured tissues is a core focus of the field of tissue engineering and has enormous clinical potential. This article highlights significant advances made in biomaterial-based protein delivery strategies over the last 25 years and how they will influence research in the next 25 years. These advances will enable protein release rates to be tuned with increased flexibility to deliberately address the challenges of the dynamic injury environment and ultimately lead to better solutions for patients.

From Microscale to Macroscale: Nine Orders of Magnitude for a Comprehensive Modeling of Hydrogels for Controlled Drug Delivery

Because of their inherent biocompatibility and tailorable network design, hydrogels meet an increasing interest as biomaterials for the fabrication of controlled drug delivery devices. In this regard, mathematical modeling can highlight release mechanisms and governing phenomena, thus gaining a key role as complementary tool for experimental activity. Starting from the seminal contribution given by Flory-Rehner equation back in 1943 for the determination of matrix structural properties, over more than 70 years, hydrogel modeling has not only taken advantage of new theories and the increasing computational power, but also of the methods offered by computational chemistry, which provide details at the fundamental molecular level. Simulation techniques such as molecular dynamics act as a "computational microscope" and allow for obtaining a new and deeper understanding of the specific interactions between the solute and the polymer, opening new exciting possibilities for an in silico network design at the molecular scale. Moreover, system modeling constitutes an essential step within the "safety by design" paradigm that is becoming one of the new regulatory standard requirements also in the field-controlled release devices. This review aims at providing a summary of the most frequently used modeling approaches (molecular dynamics, coarse-grained models, Brownian dynamics, dissipative particle dynamics, Monte Carlo simulations, and mass conservation equations), which are here classified according to the characteristic length scale. The outcomes and the opportunities of each approach are compared and discussed with selected examples from literature.

A hyaluronan/methylcellulose-based hydrogel for local cell and biomolecule delivery to the central nervous system

Regenerative medicine strategies rely on exogenous cell transplantation and/or endogenous cell stimulation. Biomaterials can help to increase the regenerative potential of cells and biomolecules by controlling transplanted cell fate and provide a local, sustained release of biomolecules. In this review, we describe the use of a hyaluronan/methylcellulose (HAMC)-based hydrogel as a delivery vehicle to the brain, spinal cord, and retina to promote cellular survival and tissue repair. We discuss various controlled release strategies to prolong the delivery of factors for neuroprotection. The versatility of this hydrogel for a diversity of applications highlights its potential to enhance cell- and biomolecule-based treatment strategies.

Injectable liquid polymers extend the delivery of corticosteroids for the treatment of osteoarthritis

Drug delivery strategies generally use inert materials, such as high molecular weight polymers, to encapsulate and control the release rate of therapeutic drugs. Diffusion governs release and depends on the ease of permeation of the polymer alongside the device thickness. Yet in applications such as osteoarthritis, the physiological constraints and limited intra-articular joint space prevent the use of large, solid drug delivery implants. Other investigators have explored the use of micro- and nanoparticle drug delivery systems. However, the small size of the systems limits the total drug that may be encapsulated and its short diffusion distance causes rapid release. Ordinarily, the extremely low diffusivity of a polymer fluid would make this an unsuitable delivery system. Our technology takes advantage of specific molecular interactions between drug and polymer, which can control the rate of release beyond diffusion. With this "affinity-based drug delivery", we have shown that delivery rates from solid polymer can be prolonged from hours and days, to weeks and months. In this paper, we demonstrate that this affinity-based mechanism also applies to low diffusivity fluid-phase polymers. They show release rates that are substantially slower than chemically similar polymers incapable of forming those inclusion complexes. The similarity of this study's liquid polymers to the viscoelastic fluids used in current clinical practice makes it an ample delivery system for osteoarthritic application. We confirmed the capacity of anti-inflammatory delivery of corticosteroids: hydrocortisone, triamcinolone, and dexamethasone; from both solid implants and polymer fluids. Further, we demonstrated that viscoelastic properties are widely tunable, and within the range of native synovial fluid. Lastly, we determined these polymer fluids have no impact on the differentiation of mesenchymal stem cells to cartilage and are not cytotoxic to a common cell line.

Endogenous IgG-based affinity-controlled release of TRAIL exerts superior antitumor effects

The inefficiency of recombinant tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-based clinical regimens has been dominantly attributed to the short half-life of TRAIL. Affinity-controlled release using endogenous long-acting proteins, such as IgG and albumin, as carriers is extremely attractive for improving the pharmacokinetics of TRAIL. Up to now, it is unclear whether IgG-binding is efficient for affinity-controlled release of TRAIL.

Methods: An IgG-binding affibody, IgBD, was genetically fused to the N-terminus of TRAIL to produce IgBD-TRAIL.The IgG-binding ability, cytotoxicity, serum half-life, and in vivo antitumor effect of IgBD-TRAIL were compared with that of TRAIL. In addition, an albumin-binding affibody, ABD, was fused to TRAIL to produce ABD-TRAIL. The cytototoxicity, serum half-life, and antitumor effect of IgBD-TRAIL and ABD-TRAIL were compared.

Results: IgBD fusion endowed TRAIL with high affinity (nM) for IgG without interference with its cytotoxicity. The serum half-life of IgBD-TRAIL is 50-60 times longer than that of TRAIL and the tumor uptake of IgBD-TRAIL at 8-24 h post-injection was 4-7-fold that of TRAIL. In vivo antitumor effect of IgBD-TRAIL was at least 10 times greater than that of TRAIL. Owing to the high affinity (nM) for albumin, the serum half-life of ABD-TRAIL was 80-90 times greater than that of TRAIL. However, after binding to albumin, the cytotoxicity of ABD-TRAIL was reduced more than 10 times. In contrast, binding to IgG had little impact on the cytotoxicity of IgBD-TRAIL. Consequently, intravenously injected IgBD-TRAIL showed antitumor effects superior to those of ABD-TRAIL.

Conclusions: Endogenous long-acting proteins, particularly IgG-based affinity-controlled release, prolonged the serum half-life as well as significantly enhanced the antitumor effect of TRAIL. IgBD-mediated endogenous IgG binding might be a novel approach for the affinity-controlled release of other protein drugs.

Combined delivery of chondroitinase ABC and human induced pluripotent stem cell-derived neuroepithelial cells promote tissue repair in an animal model of spinal cord injury

The lack of tissue regeneration after traumatic spinal cord injury in animal models is largely attributed to the local inhibitory microenvironment. To overcome this inhibitory environment while promoting tissue regeneration, we investigated the combined delivery of chondroitinase ABC (chABC) with human induced pluripotent stem cell-derived neuroepithelial stem cells (NESCs). ChABC was delivered to the injured spinal cord at the site of injury by affinity release from a crosslinked methylcellulose (MC) hydrogel by injection into the intrathecal space. NESCs were distributed in a hydrogel comprised of hyaluronan and MC and injected into the spinal cord tissue both rostral and caudal to the site of injury. Cell transplantation led to reduced cavity formation, but did not improve motor function. While few surviving cells were found 2 weeks post injury, the majority of live cells were neurons, with only few astrocytes, oligodendrocytes, and progenitor cells. At 9 weeks post injury, there were more progenitor cells and a more even distribution of cell types compared to those at 2 weeks post injury, suggesting preferential survival and differentiation. Interestingly, animals that received cells and chABC had more neurons than animals that received cells alone, suggesting that chABC influenced the injury environment such that neuronal differentiation or survival was favoured.

Enhanced in vivo retention of low dose BMP-2 via heparin microparticle delivery does not accelerate bone healing in a critically sized femoral defect

Bone morphogenetic protein-2 (BMP-2) is an osteoinductive growth factor used clinically to induce bone regeneration and fusion. Some complications associated with BMP-2 treatment have been attributed to rapid release of BMP-2 from conventional collagen scaffolds, motivating the development of tunable sustained-release strategies. We incorporated BMP-2-binding heparin microparticles (HMPs) into a hydrogel scaffold to improve spatiotemporal control of BMP-2 delivery to large bone defects. HMPs pre-loaded with BMP-2 were mixed into alginate hydrogels and compared to hydrogels containing BMP-2 alone. BMP-2 release from scaffolds in vitro, BMP-2 retention within injury sites in vivo, and bone regeneration in a critically sized femoral defect were evaluated. Compared to hydrogel delivery alone, BMP-2-loaded HMPs reduced BMP-2 release in vitro and increased early BMP-2 retention in the bone defect. BMP-2-loaded HMPs induced bone formation at both ectopic and orthotopic sites; however, the volume of induced bone was lower for defects treated with BMP-2-loaded HMPs compared to hydrogel delivery. To better understand the effect of HMPs on BMP-2 release kinetics, a computational model was developed to predict BMP-2 release from constructs in vivo. The model suggested that HMPs limited BMP-2 release into surrounding tissues, and that changing the HMP density could modulate BMP-2 release. Taken together, these experimental and computational results suggest the importance of achieving a balance of BMP-2 retention within the bone defect and BMP-2 release into surrounding soft tissues. HMP delivery of BMP-2 may provide a method of tuning BMP-2 release in vivo that can be further investigated to improve current methods of bone regeneration.

STATEMENT OF SIGNIFICANCE

The development of effective biomaterials for sustained protein delivery is a crucial component of tissue engineering strategies. However, in most applications, including bone repair, the optimal balance between protein presentation in the injury site and protein release into the surrounding tissues is unknown. Herein, we introduced heparin microparticles (HMPs) into a tissue engineered construct to increase in vivo retention of bone morphogenetic protein-2 (BMP-2) and enhance healing in femoral defects. Although HMPs induced bone regeneration, no increase in bone volume was observed, leading to further experimental and computational analysis of the effect of HMP-BMP-2 interactions on protein retention and release. Ultimately, this work provides insight into designing tunable protein-material interactions and their implications for controlling BMP-2 delivery.

Designing Peptide and Protein Modified Hydrogels: Selecting the Optimal Conjugation Strategy

Hydrogels are used in a wide variety of biomedical applications including tissue engineering, biomolecule delivery, cell delivery, and cell culture. These hydrogels are often designed with a specific biological function in mind, requiring the chemical incorporation of bioactive factors to either mimic extracellular matrix or to deliver a payload to diseased tissue. Appropriate synthetic techniques to ligate bioactive factors, such as peptides and proteins, onto hydrogels are critical in designing materials with biological function. Here, we outline strategies for peptide and protein immobilization. We specifically focus on click chemistry, enzymatic ligation, and affinity binding for transient immobilization. Protein modification strategies have shifted toward site-specific modification using unnatural amino acids and engineered site-selective amino acid sequences to preserve both activity and structure. The selection of appropriate protein immobilization strategies is vital to engineering functional hydrogels. We provide insight into chemistry that balances the need for facile reactions while maintaining protein bioactivity or desired release.

Nanoparticle-based drug delivery systems: What can they really do in vivo?

In the past few decades, there has been explosive growth in the construction of nanoparticle-based drug delivery systems (NDDSs), namely nanomedicines, owing to their unique properties compared with traditional drug formulations. However, because of a variety of challenges, few nanomedicines are on sale in the market or undergoing clinical trial at present. Thus, it is essential to look back and re-evaluate what these NDDSs can really do in vivo, why nanomedicines are regarded as potential candidates for next-generation drugs, and what the future of nanomedicine is. Here, we focus mainly on the properties of NDDSs that extend blood circulation, enhance penetration into deep tumor tissue, enable controllable release of the payload into the cytoplasm, and overcome multi-drug resistance. We further discuss how to promote the translation of nanomedicines into reality. This review may help to identify the functions of NDDSs that are really necessary before they are designed and to reduce the gap between basic research and clinical application.

Combinatorial Therapies After Spinal Cord Injury: How Can Biomaterials Help?

Traumatic spinal cord injury (SCI) results in an immediate loss of motor and sensory function below the injury site and is associated with a poor prognosis. The inhibitory environment that develops in response to the injury is mainly due to local expression of inhibitory factors, scarring and the formation of cystic cavitations, all of which limit the regenerative capacity of endogenous or transplanted cells. Strategies that demonstrate promising results induce a change in the microenvironment at- and around the lesion site to promote endogenous cell repair, including axonal regeneration or the integration of transplanted cells. To date, many of these strategies target only a single aspect of SCI; however, the multifaceted nature of SCI suggests that combinatorial strategies will likely be more effective. Biomaterials are a key component of combinatorial strategies, as they have the potential to deliver drugs locally over a prolonged period of time and aid in cell survival, integration and differentiation. Here we summarize the advantages and limitations of widely used strategies to promote recovery after injury and highlight recent research where biomaterials aided combinatorial strategies to overcome some of the barriers of spinal cord regeneration.

Local Affinity Release

The use of hydrogels for therapeutic delivery is a burgeoning area of investigation. These water-swollen polymer matrices are ideal platforms for localized drug delivery that can be further combined with specific ligands or nanotechnologies to advance the controlled release of small-molecule drugs and proteins. Due to the advantage of hydrophobic, electrostatic, or specific extracellular matrix interactions, affinity-based strategies can overcome burst release and challenges associated with encapsulation. Future studies will provide innovative binding tools, truly stimuli-responsive systems, and original combinations of emerging technologies to control the release of therapeutics spatially and temporally. Local drug delivery can be achieved by directly injecting a therapeutic to its site of action and is advantageous because off-target effects associated with systemic delivery can be minimized. For prolonged benefit, a vehicle that provides sustained drug release is required. Hydrogels are versatile platforms for localized drug release, owing to the large library of biocompatible building blocks from which they can be formed. Injectable hydrogel formulations that gel quickly in situ and provide sustained release of therapeutics are particularly advantageous to minimize invasiveness. The incorporation of polymers, ligands or nanoparticles that have an affinity for the therapeutic of interest improve control over the release of small-molecule drugs and proteins from hydrogels, enabling spatial and temporal control over the delivery. Such affinity-based strategies can overcome drug burst release and challenges associated with protein instability, allowing more effective therapeutic molecule delivery for a range of applications from therapeutic contact lenses to ischemic tissue regeneration.

Encapsulation-free controlled release: Electrostatic adsorption eliminates the need for protein encapsulation in PLGA nanoparticles

Encapsulation of therapeutic molecules within polymer particles is a well-established method for achieving controlled release, yet challenges such as low loading, poor encapsulation efficiency, and loss of protein activity limit clinical translation. Despite this, the paradigm for the use of polymer particles in drug delivery has remained essentially unchanged for several decades. By taking advantage of the adsorption of protein therapeutics to poly(lactic-co-glycolic acid) (PLGA) nanoparticles, we demonstrate controlled release without encapsulation. In fact, we obtain identical, burst-free, extended-release profiles for three different protein therapeutics with and without encapsulation in PLGA nanoparticles embedded within a hydrogel. Using both positively and negatively charged proteins, we show that short-range electrostatic interactions between the proteins and the PLGA nanoparticles are the underlying mechanism for controlled release. Moreover, we demonstrate tunable release by modifying nanoparticle concentration, nanoparticle size, or environmental pH. These new insights obviate the need for encapsulation and offer promising, translatable strategies for a more effective delivery of therapeutic biomolecules.

Local release from affinity-based polymers increases urethral concentration of the stem cell chemokine CCL7 in rats

The protein chemokine (C-C motif) ligand 7 (CCL7) is significantly over-expressed in urethral and vaginal tissues immediately following vaginal distention in a rat model of stress urinary incontinence. Further evidence, in this scenario and other clinical scenarios, indicates CCL7 stimulates stem cell homing for regenerative repair. This CCL7 gradient is likely absent or compromised in the natural repair process of women who continue to suffer from SUI into advanced age. We evaluated the feasibility of locally providing this missing CCL7 gradient by means of an affinity-based implantable polymer. To engineer these polymers we screened the affinity of different proteoglycans, to use them as CCL7-binding hosts. We found heparin to be the strongest binding host for CCL7 with a 0.323 nM dissociation constant. Our experimental approach indicates conjugation of heparin to a polymer backbone (using either bovine serum albumin or poly (ethylene glycol) as the base polymer) can be used as a delivery system capable of providing sustained concentrations of CCL7 in a therapeutically useful range up to a month in vitro. With this approach we are able to detect, after polymer implantation, significant increase in CCL7 in the urethral tissue directly surrounding the polymer implants with only trace amounts of human CCL7 present in the blood of the animals. Whole animal serial sectioning shows evidence of retention of locally injected human mesenchymal stem cells (hMSCs) only in animals with sustained CCL7 delivery, 2 weeks after affinity-polymers were implanted.

Designer protein delivery: From natural to engineered affinity-controlled release systems

Exploiting binding affinities between molecules is an established practice in many fields, including biochemical separations, diagnostics, and drug development; however, using these affinities to control biomolecule release is a more recent strategy. Affinity-controlled release takes advantage of the reversible nature of noncovalent interactions between a therapeutic protein and a binding partner to slow the diffusive release of the protein from a vehicle. This process, in contrast to degradation-controlled sustained-release formulations such as poly(lactic-co-glycolic acid) microspheres, is controlled through the strength of the binding interaction, the binding kinetics, and the concentration of binding partners. In the context of affinity-controlled release--and specifically the discovery or design of binding partners--we review advances in in vitro selection and directed evolution of proteins, peptides, and oligonucleotides (aptamers), aided by computational design.

Local retention of antibodies in vivo with an injectable film embedded with a fluorogen-activating protein

Herein we report an injectable film by which antibodies can be localized in vivo. The system builds upon a bifunctional polypeptide consisting of a fluorogen-activating protein (FAP) and a β-fibrillizing peptide (βFP). The FAP domain generates fluorescence that reflects IgG binding sites conferred by Protein A/G (pAG) conjugated with the fluorogen malachite green (MG). A film is generated by mixing these proteins with molar excess of EAK16-II, a βFP that forms β-sheet fibrils at high salt concentrations. The IgG-binding, fluorogenic film can be injected in vivo through conventional needled syringes. Confocal microscopic images and dose-response titration experiments showed that loading of IgG into the film was mediated by pAG(MG) bound to the FAP. Release of IgG in vitro was significantly delayed by the bioaffinity mechanism; 26% of the IgG were released from films embedded with pAG(MG) after five days, compared to close to 90% in films without pAG(MG). Computational simulations indicated that the release rate of IgG is governed by positive cooperativity due to pAG(MG). When injected into the subcutaneous space of mouse footpads, film-embedded IgG were retained locally, with distribution through the lymphatics impeded. The ability to track IgG binding sites and distribution simultaneously will aid the optimization of local antibody delivery systems.

Affinity-based release of polymer-binding peptides from hydrogels with the target segments of peptides

Peptides with affinities for the target segments of polymer hydrogels were identified by phage display methods and exhibited affinity-based release capability from the hydrogels. The sustained anticancer effects of the drug-conjugated peptides were also demonstrated by their release from the hydrogels.

Sustained Small Molecule Delivery from Injectable Hyaluronic Acid Hydrogels through Host-Guest Mediated Retention

Self-assembled and injectable hydrogels have many beneficial properties for the local delivery of therapeutics; however, challenges still exist in the sustained release of small molecules from these highly hydrated networks. Host-guest chemistry between cyclodextrin and adamantane has been used to create supramolecular hydrogels from modified polymers. Beyond assembly, this chemistry may also provide increased drug retention and sustained release through the formation of inclusion complexes between drugs and cyclodextrin. Here, we engineered a two-component system from adamantane-modified and β-cyclodextrin (CD)-modified hyaluronic acid (HA), a natural component of the extracellular matrix, to produce hydrogels that are both injectable and able to sustain the release of small molecules. The conjugation of cyclodextrin to HA dramatically altered its affinity for hydrophobic small molecules, such as tryptophan. This interaction led to lower molecule diffusivity and the release of small molecules for up to 21 days with release profiles dependent on CD concentration and drug-CD affinity. There was significant attenuation of release from the supramolecular hydrogels (~20% release in 24h) when compared to hydrogels without CD (~90% release in 24h). The loading of small molecules also had no effect on hydrogel mechanics or self-assembly properties. Finally, to illustrate this controlled delivery approach with clinically used small molecule pharmaceuticals, we sustained the release of two widely used drugs (i.e., doxycycline and doxorubicin) from these hydrogels.