Drug delivery from gelatin-based systems.

Review

Drug delivery from gelatin-based systems Maytal Foox & Meital Zilberman† †

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Introduction

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Gelatin sources and crosslinking

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Gelatin as a drug delivery

Expert Opin. Drug Deliv. Downloaded from informahealthcare.com by Nyu Medical Center on 05/20/15 For personal use only.

carrier 4.

Gelatin-based particles

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Gelatin-based fibers

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Gelatin-based hydrogels and bioadhesives

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Conclusion

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Expert opinion

Tel-Aviv University, Department of Biomedical Engineering, Faculty of Engineering, Tel-Aviv, Israel

Introduction: Carriers for controlled drug release offer many advantages compared with conventional dosage forms. Gelatin has been investigated extensively as a drug delivery carrier, due to its properties and history of safe use in a wide range of medical applications. Areas covered: Gelatin was shown to be versatile due to its intrinsic features that enable the design of different carrier systems, such as microparticles and nanoparticles, fibers and even hydrogels. Gelatin microparticles can serve as vehicles for cell amplification and for delivery of large bioactive molecules, whereas gelatin nanoparticles are better suited for intravenous delivery or for drug delivery to the brain. Gelatin fibers contain a high surface area-to-volume ratio, whereas gelatin hydrogels can trap molecules between the polymer’s crosslink gaps, allowing these molecules to diffuse into the blood stream. Another interesting area is the combination of tissue bioadhesivebased gelatin with controlled drug release for pain management and wound healing. Expert opinion: The modification of gelatin and its combinations with other biomaterials have demonstrated the flexibility of these systems and can be employed for meeting the challenges of finding ideal carrier systems that enable specific, targeted and controlled release in response to demands in the body. Keywords: fibers, gelatin, hydrogels, microparticles, nanoparticles, tissue bioadhesive Expert Opin. Drug Deliv. [Early Online]

1.

Introduction

Gelatin is a natural, biocompatible, biodegradable and multifunctional biopolymer. It is widely used in food, pharmaceutical, cosmetic and medical applications due to its unique mechanical and technological properties. In the medical and pharmaceutical fields, gelatin is currently used as a matrix for implants, device coatings and as a stabilizer in vaccines against measles, mumps, rubella, Japanese encephalitis, rabies, diphtheria and tetanus toxin [1]. It is also used in intravenous infusions, hard and soft capsules, plasma expanders, wound dressings, tissue bioadhesives, hemostats, sealants and in drug delivery systems [2-5]. Gelatin was first produced commercially in 1685 in Holland [6] and has been used for intravenous infusions since World War I [5]. The demand for gelatin for many applications has increased over the years and has reached an annual global output of nearly 326,000 tons [7]. Gelatin’s many qualities, along with its ability to produce a thermoreversible gel, have turned it into a good candidate for targeted drug delivery carriers. The collagen source, the type of hydrolytic treatment used, the extraction method, the amount of thermal denaturation employed and the crosslinking degree of gelatin influence the mechanical properties, swelling behavior, thermal properties and other physiochemical properties [5,8-10]. The versatility in gelatin properties enables choosing the most suitable conditions for desired drug-release profiles. The aim of this article is to review the different sources and production methods of gelatin as well as its 10.1517/17425247.2015.1037272 © 2015 Informa UK, Ltd. ISSN 1742-5247, e-ISSN 1744-7593 All rights reserved: reproduction in whole or in part not permitted

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M. Foox & M. Zilberman

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A great demand exists for the development of controlled release of bioactive molecules. The unique properties of gelatin and its extensive investigation as a biomaterial for controlled release of bioactive molecules are discussed. Gelatin was shown to be versatile due to its intrinsic features that enable the design of different carrier systems such as microparticles and nanoparticles, fibers and even hydrogels. Gelatin microparticles can serve as vehicles for cell amplification and delivery of large bioactive molecules, whereas gelatin nanoparticles have higher intracellular uptake and are better suited for intravenous delivery or for drug delivery to the brain. Gelatin fibers contain a high surface area-to-volume ratio and high porosity, whereas gelatin hydrogels can trap molecules within the gaps between the polymer crosslinks, allowing the drugs to diffuse into the blood stream. Another interesting area is the combination of tissue bioadhesive-based gelatin with controlled drug release for pain management and wound healing. Gelatin-based systems exhibit a high ability to absorb water. However, this can be monitored by changing the gelatin source, its molecular weight and the degree of the crosslinking. The challenge is to find a good crosslinking technique that will not change the unique and desired properties of the gelatin or lower its biocompatibility. It is concluded that work is continuing to allow the release of a wider variety of biomolecules for a broad range of applications by the modification of gelatin and its combination with other biomaterials.

This box summarizes key points contained in the article.

modification to produce different types of carries with or without synthetic or natural polymers in order to optimize and specify the drug-release profile for a broad range of applications in regenerative medicine. 2.

Gelatin sources and crosslinking

Gelatin sources Gelatin is a water-soluble polypeptide that can be obtained from acid, alkaline or enzymatic hydrolysis of collagen, the main protein component of the skin, bones and connective tissue of animals, including fish and even insects. Gelatin derived from an acid treatment, such as hydrochloric acid or sulfuric acid, is known as type A, whereas gelatin derived from an alkaline treatment is known as type B. After both kinds of treatments, the solutions are filtered, deionized and concentrated by membrane filtration and/or vacuum evaporation. Minerals, fats and albuminoids found in bones or skin are then removed by chemical and physical treatment in order to obtain purified gelatin [6]. Porcine skin-derived gelatin is the most popular source (46%), followed by bovine skin (29.4%), bone (23.1%) and 2.1

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other sources (1.5%) [7]. However, these sources suffer from major constrains and concerns among consumers due to religious issues (porcine-derived products are forbidden in both Judaism and Islam, and Hindus do not consume bovine-derived products) as well as the increase in vegetarianism throughout the world. Furthermore, there is a health concern of transmitting pathogenic vectors such as prions [11]. Other sources of gelatin, including poultry, fish, vertebrates and even recombinant gelatin, have therefore been introduced during the past decade. Due to the fact that the commercial production of gelatin from poultry skin is currently limited by low yields, it is not very popular as a source of gelatin [12]. Gelatin from fish (especially warm-water fish) possesses similar characteristics to porcine gelatin and may thus be an alternative for it. Production of gelatin from fish may also serve as a means for utilizing some of the by-products of the fishing industry. However, the primary amino acid sequence level of fish gelatin is different from that of porcine and bovine skin gelatin. Fish gelatin (especially from cold-water fish) has a lower melting temperature, which can decrease its thermal stability and effectiveness at body temperature. Fish gelatin also raises a concern for possible allergic reactions [13,14]. Recombinant gelatins were developed in order to overcome the disadvantages associated with animal tissuederived material. This technology allows the production of gelatins with specific properties to match a specific application [15]. Crosslinking of gelatin Gelatin should be crosslinked before use in order to increase its mechanical properties and lower its solubility and degradation rate in aqueous solutions [16]. Physical crosslinking by microwave energy [17], dehydrothermal treatment and ultraviolet irradiation [18], biological crosslinking by enzymes or chemical crosslinking by agents such as formaldehyde, [19], glutaraldehyde [20], glyceraldehyde [21], genipin [22] and carbodiimide [23] have been reported. Physical crosslinking can be achieved using environmental triggers (such as pH, temperature or ionic strength) or physicochemical interactions (such as hydrophobic interactions, charge condensation, hydrogen bonding, stereocomplexation or supramolecular chemistry). When physical crosslinking is employed, there is no need for chemical modification or the addition of crosslinking agents that might be toxic [24]. However, it is difficult to control the crosslinking density and the crosslinking procedure is often less efficient [25]. There are two types of chemical crosslinkers: non-zerolength and zero-length [25]. Non-zero-length crosslinkers are built into the biomaterial by bridging free amine groups of lysine and hydroxylysine or free carboxylic acid residues of glutamic and aspartic acid of the protein molecules. Aldehydes (formaldehyde, glutaraldehyde, glyceraldehydes), polyepoxides and isocyanates are often used as crosslinkers, with glutaraldehyde being the most commonly used. However, during the biomaterial’s degradation, reactive and toxic 2.2

Expert Opin. Drug Deliv. (2015) 12(8)


Drug delivery from gelatin-based systems

compounds might leach into the body and cause damage [26]. In contradistinction, zero-length crosslinkers activate carboxylic acid residues to react with free amine residues, resulting in the formation of an amide bond without incorporation of foreign structures into the network. Acyl azide and water-soluble carbodiimide crosslinkers are representatives of this group of crosslinkers. The acyl azide crosslinking is a four-step reaction which requires the use of the toxic reagent hydrazine [4]. However, a simpler and less toxic variation of this reaction is achieved when using diphenylphosphoryl azide instead of hydrazine [27,28]. Carbodiimides are reactive organic compounds with a R-N=C=N-R structure. The R is usually alkyl, aryl, acyl, aroyl, imidoyl or sulfonyl, but can also be nitrogen, silicon, phosphorous and metal [29]. Nobel laureate Sheehan used water-soluble carbodiimides in the synthesis of penicillin and was the first to use carbodiimides to crosslink gelatin. The workhorse in this application is the water-soluble N-ethyl-N-(3-dimethylaminopropyl) carbodiimide (EDC). The carbodiimide crosslinking can be performed in one step and involves the use of less harmful reagents [30,31]. However, when using chemical crosslinkers, unreacted crosslinker molecules might be left inside the matrix and there is a risk of the creation of toxic products by the reaction between the substrate and the crosslinking agent during in vivo biodegradation. Means to lower the concentrations of chemical crosslinkers and finding naturally derived crosslinking agents with low cytotoxicity are therefore being examined. One example is the use of genipin, a natural crosslinking reagent obtained from geniposide, an iridoid glucoside isolated from the Genipa americana and Gardenia jasminoides Ellis fruits [32]. Studies have shown that crosslinking with genipin resulted in lower cytotoxicity and higher biocompatibility compared to glutaraldehyde, formaldehyde and epoxy compounds. Additionally, in vivo studies showed that lacerations treated by a genipin-crosslinked bioadhesive induced significantly fewer inflammatory responses and recovered sooner than lacerations treated by aldehyde-crosslinked bioadhesives [22,33-35]. Gelatin dissolves rapidly in aqueous environments. Its use as a carrier for long-term delivery systems is thus ineffective. Crosslinking of gelatin can improve its thermal and mechanical stability and its hydration potential under physiological conditions, as well as lower its degradation in vivo, and allows it to maintain its long-term release function as a carrier [36,37]. The increase in the resistance against degradation probably results from the crosslinking which modifies cleavage sites of gelatin molecules, resulting in an inhibition of the enzymesubstrate interaction [38]. The crosslinking density of the gelatin carrier can be altered by either prolonging the crosslinking reaction period or increasing its concentration. As a consequence, the period of bioactive molecule release can be regulated [38,39].

3.

Gelatin as a drug delivery carrier

Significant efforts have been made to improve formulations for better stabilization of drugs over a sufficient storage time. In the body, the therapeutic effect of a drug is most effective when its concentration in the blood is above the minimum effective level and below the toxic level. However, every drug has its own biological half-life and can therefore be maintained at an effective concentration for only a specific and short time. One solution for this problem is to increase the dose of the drug. However, it is important not to reach the toxic response region. Another solution, which is less convenient for the patient, is to take the drug several times during a period of time. Extensive efforts are consequently being made to develop dosage forms that prolong the biological activity of a protein in the body or assist in targeting it to a specific tissue. One possible way to achieve these goals is to incorporate the drug into an appropriate matrix. Drug controlled-release carriers offer many advantages compared to conventional dosage forms, including improved efficacy, maintenance of the desired drug concentration in the blood for a long period of time without reaching a toxic level or dropping below the effective level, reduced toxicity and improvement in patient compliance and convenience. In spite of these advantages, the patient can be harmed if the controlled drug delivery system is not properly designed. The ideal drug delivery system must therefore be biocompatible without toxic degradable products, mechanically strong with the ability to load large amounts of drug with no concern of accidental release, simple to fabricate and sterilize, easy to place and remove and comfortable for the patient [29]. There are several mechanisms by which the release of active agents from drug delivery systems to the surrounding can be controlled: i) diffusion; ii) swelling followed by diffusion (mostly in hydrogels); iii) diffusion and degradation (erodible systems); iv) hydrolysis of the covalent bond in case the drug is covalently bound to the biodegradable polymer (pendant chain systems); v) osmotic pressure; and vi) externally or self-regulated systems. Polymeric carriers, which physically entrap molecules of interest, need to be shielded to avoid fast recognition by the immune system followed by rapid clearance from the body. The physicochemical properties of drug carriers, such as size, hydrophilicity and zeta potential have enormous effects on their identification and phagocytosis by the macrophage system. The suppression of nonspecific interactions with the body, including blood components (opsonization) and activation of the complement system, would reduce blood clearance of drug carriers. Drug delivery carriers can be coated with a hydrophilic polymer, such as gelatin, to allow both inhibition of opsonization and improvement of water solubility [40]. In addition, using a biodegradable polymer as a drug carrier would protect the drug against corrosion by gastric acid and


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disassembly by enzymes, thus preserving the efficient activity of the drug in the body [41]. Gelatin has been extensively investigated as a drug delivery carrier for many classes of drugs due to its properties as a natural biomaterial and history of safe use in a wide range of medical and pharmaceutical applications. Inflammatory drugs, antineoplastic compounds, antibacterial agents and recently nucleic acid and hydrophobic materials were reported in the literature [42-50]. Gelatin properties can be adjusted to maximize drug-loading efficiency [51]. Gelatin’s isoelectric point can be adjusted to the electrostatic properties of the chosen drug molecule by using an alkaline or an acidic treatment [52]. Furthermore, the hydrophilic nature of gelatin facilitates the penetration of body fluids into the particles and thereby increases the diffusion-mediated release of bioactive molecules [53]. Drug-release profiles from gelatin can also be optimized by changing the gelatin source, molecular weight and the degree of crosslinking [39]. Furthermore, modification of gelatin to different types of carriers and addition of synthetic or natural polymers can be optimized and the drug profile release can be specified to a broad range of tissue engineering [54,55], cancer therapy (46 -- 47), and therapeutic angiogenesis [56,57] applications. As a drug delivery carrier, gelatin has been shown to be versatile due to its intrinsic features that enable the design of different carrier systems. The most widely used gelatin-based systems are described in the sections below. 4.

Gelatin-based particles

Gelatin microparticles and nanoparticles have been extensively studied as carrier systems for many applications. Microparticles have the advantage of a large surface area which enables sufficient exchange of nutrients and metabolic wastes and allows rapid cell development [58]. They can therefore serve as vehicles for cell amplification and can simplify the delivery of these expanded cells or other large bioactive molecules to the desired site [59]. The size of nanoparticles offers other advantages, including a relatively higher intracellular uptake by various areas of the body [60]. For example, nanoparticles are better suited for intravenous delivery [61] or for delivery of drugs to the brain, due to their ability to better accumulate in macrophage-rich organs which can easily cross the blood--brain barrier [62]. Due to their unique design, liposomes have the ability to incorporate both hydrophilic and hydrophobic drugs, protect them from degradation, target them to the desired site and reduce the toxicity or side effects of those molecules [63-67]. Unfortunately, liposomes have low encapsulation efficiency, poor storage stability and watersoluble drugs can rapidly leak in the presence of body fluids [68]. However, some researchers showed that embedding liposomes into a gelatin-based system resulted in an improvement in their stability, viscosity and the half-life of the loaded drug and the liposome [69]. 4

Nanoparticles Nanoparticles are defined as particles with a size in the range of 1 -- 1000 nm [42]. They have good in vivo stability, can be easily sterilized, scaled-up and their manufacture can be free of contamination with pyrogens. These advantages can be used in the fields of biomaterials and medicine [70]. Nanoparticles can be used to deliver hydrophilic and hydrophobic drugs, proteins, vaccines and other types of bioactive molecules to various areas in the body, such as the lymphatic system, brain, arterial walls, lungs, liver, spleen or even for long-term systemic circulation [61]. A successful nanoparticle system should have high loading capacity to reduce the amount of carrier needed for administration. Depending on the drug and carrier properties, the drug can be dissolved, entrapped, encapsulated or attached to the nanoparticle. The loading of the drug can be performed at the time of the nanoparticle production, during the polymerization process or after the formation of the nanoparticles by incubating them with the drug solution [68]. The amount of bound drug and the type of interaction between the drug and the nanoparticles depend on the chemical structure of the drug and the polymer and the conditions of the drug-loading procedure [71]. Gelatin nanoparticles have been widely used for encapsulating many bioactive molecules [72]. Li et al. encapsulated bovine serum albumin (BSA) as a model protein drug, using gelatin-based nanoparticles. BSA release was followed by a diffusion-controlled mechanism and was enhanced by the nanoparticles’ water uptake capacity of 51 -- 72% [73]. Nanoparticles can better accumulate in macrophage-rich organs which can easily cross the blood--brain barrier and reach the brain, compared to other carrier systems [62]. The oral administration of didanosine, a widely used antiHIV drug, is associated with poor gastrointestinal tolerability, narrow therapeutic index, low plasma protein binding, short plasma half-life (30 min to 4 h) and dose-dependent side effects with the need for frequent dosing [74]. Therefore, gelatin nanoparticles with a mannan coating to further enhance the macrophage uptake and the distribution in organs that act as major HIV reservoirs were loaded with didanosine. The ex vivo study indicated that the rate of didanosine uptake by the macrophages was fivefold higher from mannan-coated gelatin nanoparticles than from free didanosine in PBS solution [70]. In order to reduce the toxic side effects of chloroquine phosphate, a well-known antimalarial drug, it was encapsulated in gelatin nanoparticles [75]. The results showed that chloroquine phosphate release was controlled via diffusion, increased with increasing temperature, but decreased in the physiological fluids and was found to be optimal near the physiological pH (7.4). The addition of the crosslinker glutaraldehyde to the nanoparticles decreased the drug release rate. The type of gelatin also affected the release profile. Gelatin type B nanoparticles showed higher drug release than gelatin type A [75]. 4.1




The release of growth factors in vivo is known to be very challenging, since a special and time-specific controlled delivery of multiple factors must be obtained in order to achieve maximum efficiency. Since degradation rates of subpopulations of specific blocks in colloidal gels can be controlled, they can be suitable for independent release of multiple biomaterials [54]. For this reason, colloidal gels made of oppositely charged gelatin nanospheres were loaded with both osteogenic bone morphogenetic protein-2 (BMP-2) and angiogenic basic fibroblast growth factor (bFGF), using a twin syringe in order to improve control over their dual delivery. In vitro results showed that the crosslinking degree of gelatin has a greater effect on the delivery rates of both BMP-2 and bFGF than the gelatin type. However, the combined delivery of both BMP-2 and bFGF resulted in an inhibitory effect on osteogenesis under the current experimental conditions. More experiments are needed in order to maximize the ability of these nanospheres [54]. In another study, gelatin nanoparticles prepared by nanoprecipitation using water and ethanol were loaded with three different drugs (tizanidine hydrochloride, gatifloxacin and fluconazole). The addition of glutaraldehyde as a crosslinker enhanced the tizanidine hydrochloride-loading efficiency, but lowered the gatifloxacin-loading efficiency due to competition of activated groups [42]. No loading was achieved in the case of fluconazole. Ibuprofen sodium, an anti-inflammatory drug, was loaded into PEGylated gelatin nanoparticles. The results demonstrated an improved plasma half-life of ibuprofen sodium when encapsulated within the nanoparticles. These results might allow lowering the rate of ibuprofen sodium needed for treatment [45]. Targeting nano- and micro-vesicles to enhance drug uptake only in tumor cells, while sparing healthy cells, is a unique strategy which is currently being investigated. It is known that the extracellular pH of tumor tissue is lower than that of normal tissue. The pH-responsive carriers would therefore have an advantage in accelerating local drug release in tumor tissue [76]. Doxorubicin, an anticancer drug, was encapsulated in an amphiphilic gelatin--iron oxide core/calcium phosphate shell nanoparticle. The addition of a calcium phosphate shell acts as a drug reservoir and turned the nanoparticles into highly pH-responsive drug-release carriers [76]. These nanoparticles were found to be biocompatible and were taken up by HeLa cells [47]. In another study, paclitaxel-loaded gelatin nanoparticles were shown to be effective against human bladder transitional cancer cells with a very rapid release rate of 90% at 37 C after 2 h in PBS and urine solutions [53]. Surface-modified gelatin nanoparticles were also tested as carriers for double-stranded DNA and RNA oligonucleotides via ionic interactions. The results showed that the loading capacity of the oligonucleotides was dependent on the particle’s zeta potential and on the incubation medium [77]. A new tumor-targeted small interfering RNA (siRNA) delivery system using a gelatin nanocarrier was developed. The

primary amines of gelatin and the 5¢-end of the siRNA were thiol-modified in order to increase the interactions between them. The encapsulation of siRNA molecules in the selfassembled thiolated gelatin nanoparticles using chemical crosslinking protected them from enzymatic degradation. The nanoparticles showed a great potential as a systemic siRNA delivery system for cancer therapy, based on their characteristics of low toxicity, tumor accumulation and effective siRNA delivery [47]. Microparticles Gelatin microparticles have been extensively studied as a carrier system for many applications [78-80]. They can be produced by several methods: emulsion polymerization [59,81,82], solvent evaporation [58], coacervation [83] and spray-drying [84]. However, gelatin microparticles have poor mechanical properties and rapid dissolution rates in aqueous environments which accelerate drug release at body temperature [21]. Gelatin microparticles are typically crosslinked with formaldehyde, glutaraldehyde [85], genipin and/or carbodiimides [38] in order to solve these problems. However, methacrylation of gelatin has been reported to be less cytotoxic and to enable a larger range of crosslinking densities compared to the traditional chemical crosslinking methods, thus offering an alternative method to better control the extent of hydrogel crosslinking [86]. It was shown that fewer methacrylated microparticles had decreased elastic moduli and larger mesh sizes but could be loaded with up to a 10-fold higher relative amount of growth factor compared to glutaraldehyde crosslinked microparticles. Furthermore, a reduced gelatin methacrylate crosslinking density resulted in higher release of BMP-4 and bFGF and accelerated release rate with collagenase treatment [78]. Solorio et al. succeeded in developing a self-assembled system of gelatin microspheres loaded with TGF-b1 as a mean of enhancing neo-cartilage tissue formation in a human mesenchymal stem cells (hMSCs) culture. In addition to the production of hMSC sheets with superior mechanical properties, this system could decrease the culture time necessary before implanting hMSCs and might also avoid the problem of the loss of chondrogenic phenotype in vivo by providing continued local exposure of these hMSCs to growth factor by the microparticles [87]. Additional researches demonstrated the efficacy of gelatin microparticles in releasing VEGF or BMP-2. Both VEGF and BMP-2 release kinetics were dependent on the gelatin crosslinking degree [88,89]. Gelatin microparticles showed several advantages for pulmonary delivery, such as a large distribution to lung epithelial cells and higher availability of bioactive molecules to the infected cells. Rifampicin and isoniazid are part of the therapeutic treatment against tuberculosis. However, these drugs are relatively toxic, can cause several side effects and might cause drug resistance when not followed to completion. The use of microcarriers loaded with these drugs may allow reducing the therapeutic dose, extending the duration of 4.2


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action, ensuring sustained drug delivery to the lungs and as a result improving patient compliance. Therefore, a new polymeric microparticle system based on gelatin covalently bound to isoniazid and containing rifampicin was developed. The results showed that the microparticles demonstrated low cytotoxicity, were able to encapsulate both rifampicin (51 ± 6%) and isoniazid (22 ± 1%) and had a good nebulization efficiency which is important in pulmonary antitubercular drug delivery systems [90]. It is known that most anticancer drugs have pharmacokinetic limitations such as low bioavailability and a high effective dose (ED50) index. A new strategy of targeting microvesicles to enhance specific drug uptake by tumor cells without harming healthy cells is increasingly investigated. Sterically stabilized gelatin micro-assemblies loaded with the anticancer drug noscapine were developed for targeting human NSCLC. In vitro release studies indicated that these micro-assemblies followed first-order release kinetics and exhibited an initial burst followed by slow release of the drug. The in vitro studies showed that the stabilized gelatin micro-assemblies had an approximately threefold lower IC50 compared to free noscapine. The micro-assemblies extended the release of noscapine and increased the plasma half-life of noscapine by ~ 9.57-fold with an accumulation of ~ 48% drug in the lungs [46]. Propolis ethanolic extractive solution (bee glue) is usually used in wound healing, tissue regeneration, burns, neurodermatitis, leg ulcers, psoriasis, herpes simplex and genitalis, rheumatism and sprains, periodontal diseases, candidiasis, cheilitis, stomatitis, influenza and cold [84]. Gelatin microparticles loaded with propolis prepared by the spray-drying method have been shown to be a feasible and inexpensive method. In addition, this propolis dosage form would be without the propolis extraction solution’s strong and unpleasant taste, aromatic odor and high ethanol concentration, which can cause packaging and transport difficulties and patient inconvenience [84]. In another research, novel microparticles combined with sustained and localized delivery of bFGF and gyrus-patterned surface were tested for both delivery systems and scaffolds for proliferation and attachment of cells. The results suggested that this unique system might be a good candidate for rapid cell expansion and has a potential for application in cartilage tissue engineering [59]. Liposomes Liposomes are small spheres of an aqueous core entrapped by one or more phospholipids which form closed bilayered systems [91]. Liposomes are widely used as an advanced technology for delivering bioactive molecules due to their high biocompatibility, ability to incorporate hydrophilic and hydrophobic drugs and their ability to deliver bioactive molecules directly to the desired site [64-67]. Hydrophobic drugs are usually encapsulated in the lipid bilayers of liposomes, whereas hydrophilic drugs may either be encapsulated inside 4.3

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the aqueous cores of liposomes or be located in the external aqueous phase. A liposomal hydrogel system consisting of a PEG--gelatin hydrogel loaded with liposomes containing the antibiotic ciprofloxacin was developed in order to reduce bacterial adhesion to silicone catheter material. Liposomal hydrogel-coated catheters were shown to have an antimicrobial efficacy against Pseudomonas aeruginosa [92]. Another study demonstrated a controlled release of liposomes loaded with calcein fluorescence dye or calcein labeled with rhodamine from gelatin carboxymethyl cellulose films. The release rate of the loaded liposomes depended mainly on the amount of liposomes entrapped inside the films, the swelling degree and the network density of the film, and the glutaraldehyde crosslinking degree [93]. A successive release of sodium periodate (oxidizing reagent) from thermal liposomes entrapped inside a stimuli-responsive gelatinous derivative hydrogel was also demonstrated [94]. Embedding liposomal drug delivery systems into a polymer-based system improves the liposome stability, viscosity, half-life of the loaded drug and the embedded liposome. It also allows a sustained and efficient drug release over prolonged periods of time [69]. 5.

Gelatin-based fibers

Polymer fibers can be used for various biomedical applications such as tissue-engineered matrices and wound-healing patches, due to their morphological similarity to the structure of the native extracellular matrix [95,96]. In the past five decades, an enormous number of studies on gelatin films have been published [61]. Lin and Tsai developed an ultraviolet crosslinked mixture of gelatin and phenyl azide-conjugated poly (acrylic acids) electrospun fibers for tissue engineering applications. They also demonstrated that these fibers can be incorporated with bioactive substance, such as hydroxyapatite nanoparticles for enhancing osteoconductivity, which possess a great potential in bone tissue engineering [97]. The production of polymer fibers can be carried out by several methods. Large diameter fibers are usually produced by melt spinning [98], wet spinning [99,100], gel spinning [100-102] or dry spinning [103], whereas fabrication of nanofibers is usually performed by electrospinning [104,105], centrifugal [105,106] and solution/melt blow spinning [105,107]. The development of polymer-based nanofibers using electrospinning has attracted much attention in biomedical fields during the past years. The fibers produced via electrospinning form a porous network with a very high surface areato-volume ratio, high porosity and controllable pore size. As a drug carrier, a high surface-to-volume ratio would accelerate the solubility of the drug in the aqueous solution and enhance the efficiency of the drug. Many parameters that can be controlled during the procedure such as solution properties, temperature and humidity can influence the morphology of nanofibers, which affects the release rate of the drug [108]. For example, a poly (D, L-lactide-co-glycolide) (PLGA) and




gelatin mixture was fabricated via the electrospinning technique to produce nanofiber scaffolds which were loaded with fenbufen, a NSAID. The results showed that increased gelatin contents enhanced the hydrophilic nature of PLGA/ gelatin scaffolds which increased the fenbufen release rate. As was seen in other researches, the addition of a crosslinker agent (glutaraldehyde vapor) reduced the drug release rate [41]. The main problem with the topical application of antifungal agents is their uncontrolled release rate which requires reapplication to achieve a local therapeutic concentration. Gelatin-based fibers were therefore loaded with different antifungal agents. The in vitro results showed that incorporation of polyene antifungals resulted in the inhibition of the growth of yeasts and filamentous fungal pathogens. It was also found that polyene antifungals had strong interactions with the gelatin matrix which led to a reduction in the hemolytic activity of polyenes, making them noncytotoxic to primary human corneal and sclera fibroblasts. This system may provide new opportunities for management of superficial skin infections [109]. Another research studied the possible potential of genipin crosslinked gelatin fibers loaded with human VEGF as a system for inducing early angiogenesis. The studies demonstrated that the VEGF-loaded gelatin fibers induced cell viability, endothelial differentiation, chemoattractive properties of hMSCs and induced an angiogenic potential in stimulating new vessel formation. These results may suggest a great potential for stimulating and inducing angiogenesis in tissue engineering applications and for improving the clinical outcome of implanted tissue-engineered scaffolds [110]. Clearly, a wide variety of drugs can be delivered using gelatin particles and fibers via several routes (Figure 1). 6.

Gelatin-based hydrogels and bioadhesives

Hydrogels have been of great interest to biomaterial scientists for many years. One of the most interesting properties of these polymers is the distension of polymeric chains after contact with a solvent (polymer swelling) [111]. Hydrogels are considered biocompatible and their structure is similar to the macromolecular-based components in the body. They were therefore studied as scaffolds for various applications in tissue engineering, wound dressing as well as drug delivery carriers [112,113]. Hydrogels Hydrogels are semisolid forms of hydrophilic polymers that can swell in water and hold a large amount of water while maintaining their structure [114]. They are usually crosslinked to allow control over the swelling rate and are used in longterm applications [115,116]. This crosslinking can be carried out by covalent bonds, hydrogen bonding, van der Waals interactions or physical interactions [117]. Hydrogels for tissue engineering scaffolds usually contain large pores to accommodate living cells, or may dissolve or 6.1

degrade while releasing growth factors and as a result create pores into which living cells may penetrate and proliferate [118]. An injectable methacrylated gelatin hydrogel was developed, which is capable of rapid gelation via visible light activated crosslinking in air or in an aqueous solution. The results showed that the hydrogel supported human bone marrow-derived mesenchymal stem cell growth and TGF-b3-induced chondrogenesis. They, thus, offer a promising scaffold for cell-based repair and resurfacing of articular cartilage defects [119]. In another study, a wound dressing hydrogel produced from chitosan, honey and gelatin was developed. The results showed a powerful antibacterial efficacy against Staphylococcus aureus and Escherichia coli and significantly promoted burn healing [120]. Hydrogels can be also used as delivery carriers by trapping molecules within the gaps between the polymer crosslinks. In the body, due to a direct contact with water, they swell and the gaps between the polymer crosslinks increase, thus allowing the drugs to diffuse into the blood stream. Hydrogels can protect the drug from hostile environments and also control drug release by changing the gel structure in response to environmental stimuli [114]. Hydrogels can be made from several synthetic and natural polymers, including PLGA, poly (hydroxyethyl methacrylate), polyvinyl alcohol or natural polymers such as chitosan, gelatin and alginate. Hydrogels consisting of gelatin, varying concentrations of polyvinyl alcohol and the anticancer drug cisplatin were synthesized as a slow-release drug delivery system. In vivo results showed that hydrogels containing a low dose of cisplatin were as effective in inhibiting tumor growth as the conventional treatment of intraperitoneal administration of high doses of free cisplatin. This would allow reducing the unpleasant side effects of the drug and improving the quality life of the patients during the treatment [121]. In another study, gelatin was chemically derivatized to give succinylated gelatin with an anionic charge in order to optimize stromal cell-derived factor-1 (SDF-1) release from the hydrogel. The release profile rate of SDF-1 from the hydrogel could be controlled by changing the water content of the hydrogel which could be modified by changing the hydrogel preparation conditions. When the succinylated gelatin hydrogel loaded with SDF-1 was implanted subcutaneously, it significantly enhanced angiogenesis and the mRNA level of SDF-1 receptor compared to an injection of SDF-1 solution [56]. Saito and Tabata chemically introduced ethylenediamine into the gelatin carboxyl groups in order to obtain a cationized gelatin. The cationized gelatin was mixed with a low-molecular-weight heparin and was dehydrothermally crosslinked to create a gelatin hydrogel-incorporating complex. The complex did not dissolve or release heparin in a PBS solution at 37 C. When collagenase was added, the hydrogel was enzymatically degraded and heparin was released from it. Increasing the time of the crosslinking process resulted in a slower degradation and release


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A.

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E.

Figure 1. Scanning electron microscopic images showing: (A and B) gelatin particles. (A) Gyrus-patterned surface and (B) smooth surface. Magnification: 3000 (A), 2000 (B). (C) Crosslinked gelatin mat with 0.1% w/v genipin, scale bar 1 µm. (D) Electrospun fenbufen-loaded poly (D, L-lactide-co-glycolide)/gelatin (9/1) nanofibers crosslinked by glutaraldehyde vapor for 5 h and (E) corresponding film. A and B: Adapted from [59] with permission. C: Adapted from [110] with permission. D and E: Adapted from Ref. [41] with permission.

rate of the heparin. The gelatin hydrogel-incorporating complex showed an antifibrotic effect when tested in a mouse model [57]. Bioadhesives Novel tissue bioadhesives based on gelatin, with alginate as a polymeric additive and crosslinked by EDC, were developed and studied by us. These formulations were loaded with anesthetic drugs and antibiotic drugs [122,123]. The purpose of this study was to develop a bioadhesive with a therapeutic effect of releasing drugs to the lacerated area for pain management. Gelatin and alginate were selected for this study due to their unique properties. Gelatin polar groups, such as amine and carboxyl groups, enable it to bind to other compounds. Gelatin’s ability to form physically crosslinked hydrogel structures [124], its natural sticky behavior in solution along with its other qualities, such as being biocompatible, biodegradable and non-immunogenic, has made it one of the most extensively investigated materials for tissue bioadhesives. Alginate, a natural polysaccharide, possesses a bioadhesive nature and is classified, with its carboxyl end groups, as an anionic mucoadhesive polymer [125]. Its properties of biodegradability under physiological conditions, a controllable gel porosity and allowing high diffusion of macromolecules, has turned it into 6.2

8

a good candidate for protein delivery. It is also known to induce cytokine production and can thus influence the healing process [126]. The incorporation of bupivacaine in the gelatin-alginate bioadhesive improved the bioadhesive’s bonding strength, probably due to the reinforcing effect of its crystals (Figure 2). Bupivacaine exhibited a burst release of 44 -- 74% after 6 h with ~ 99% release during the first 3 days. It was also showed that the EDC concentration, which controlled the swelling ratio, had a major effect on the burst release, whereas the effects of gelatin and alginate concentrations were less notable. These results suggested that the bupivacaine release profile was controlled mainly by the swelling, allowing water penetration into the hydrogel and hydrophilic group concentration of the bioadhesive. The drug’s hydrophilic nature and the electrical interactions between the polymeric components and the drug also had some effect on the release profile. A model describing the dependence of the drug release profile on the bioadhesive and drug characterization is presented in Figure 3. The obtained release profiles are beneficial for the treatment of wounds because pain usually decreases with the healing process.



B. 100 80 60 40 100 mg/ml gelatin

20

200 mg/ml gelatin

0 5

10

100 80 60 40

20 mg/ml aiginate 40 mg/ml aiginate

20

60 mg/ml aiginate 0

15

0

5

Time [days]

10

15

Time [days]

C. Cumulative bupivacaine release [%]


0

Cumulative bupivacaine release [%]

Cumulative bupivacaine release [%]

A.

D.

10 8 6 4 2

100 µm

20 µm

Figure 2. Drug delivery results of the bioadhesives research showing the effect of the gelatin (A) and alginate (B) concentrations on the release profile of bupivacaine from the basic adhesive formulation (200 mg/ml gelatin, 40 mg/ml alginate, 20 mg/ml EDC) loaded with 3% w/v bupivacaine. (C and D) ESEM fractographs showing the basic formulation loaded with 1% w/v bupivacaine. Adapted from [122] with permission. EDC: N-ethyl-N-(3-dimethylaminopropyl) carbodiimide.

In a continuing research, N-hydroxysuccinimide was also added to the crosslinking reaction of the gelatin-alginate bioadhesive (Figure 4) in order to enable a decrease in the EDC content and thereby the cytotoxic effect, without decreasing the bonding strength. Three antibiotic drugs were incorporated in these formulations: clindamycin, vancomycin and ofloxacin (Table 1). However, only clindamycin was found to be inert toward the crosslinking reaction and did not decrease the bonding strength of the bioadhesive (Figure 5). These results can be explained by the chemical structure of the drugs. Ofloxacin contains carboxyl groups, vancomycin contains both amine and carboxyl groups, whereas clindamycin does not contain either of these groups (Table 1). The drugs’ primary amine and carboxyl groups can be crosslinked with gelatin or alginate by the EDC crosslinking reaction, reducing the amount of EDC molecules available for the crosslinking of gelatin and alginate, and as a result decreasing the crosslinking degree of the bioadhesive and its bonding strength. The results also showed low cytotoxicity of the

gelatin-alginate bioadhesive toward human fibroblasts cells. The release profile of clindamycin was highly effective against two relevant bacterial strains, Staphylococcus albus and S. aureus, which were eradicated within < 48 h (Figure 5). Delivering an antibiotic or anesthetic drug locally to the laceration area using the bioadhesive could reduce the risk of infections or relieve the pain caused by the wound and thereby increase the therapeutic effect of the bioadhesive itself. 7.

Conclusion

Incorporating bioactive molecules into appropriate carriers offers many advantages compared to conventional dosage forms. It can improve patient compliance and convenience by reducing possible toxic side effects of the drug while sustaining the effective drug level and even allowing easy delivery of multiple growth factors which is usually a very challenging and expensive process. Gelatin has been extensively investigated as drug delivery carrier due to its properties and history


9


Cross-linking density of the adhesive network

Major effect

The swelling rate of the bioadhesive

The bioadhesive’s hydrophilic groups concentration

Durg release profile


Hydrop hilicity of the drug

Minor effect

Diffusion of the drug

Durg-polymer electrical interaction

Figure 3. Bioadhesive research: a schematic representation of a qualitative model describing the dependence of the drugrelease profile on the characteristics of the adhesive and of the drug. Adapted from [122] with permission.

R1 NH

AI AI

O C

O

O

C OH Gel

C OH +

R1= O

CH3

R2=

Gel C N

+ NH Cr

CH3 CH3

R2 N-acylurea

O Gel

AI

R1 N

C OH + C + O N C OH R2

H+

AI

R1 O

NH

Gel C O O-Iso-Acylurea

C + NH2 Gel N

Nucleophilic attack

R2

+

EDC

O

H

O

C NH Gel

Gel +

HO N

NHS O O

AI

O

Gel

C

O N

+

O

AI C OH Gel + Urea NH-R’ O C NH–R’ NH2–Gel

NHs activated carboxylic acid group O

Figure 4. Crosslinking reaction of gelatin and alginate with EDC and NHS. Adapted from [123] with permission. Al: Alginate; EDC: N-ethyl-N-(3-dimethylaminopropyl) carbodiimide; Gel: Gelatin; NHS: N-hydroxysuccinimide; NHS: N-hydroxysuccinimide.

of safe use in a wide range of medical applications. Gelatin’s properties can be modified and adjusted to maximize drug loading and efficiency of release for many classes of drugs. The release profile from gelatin carriers was shown to be optimized by changing the gelatin source, its molecular weight 10

and the degree of its crosslinking. The amount of loaded drug and the type of interaction between the drug and the carrier depend on the chemical structure of the drug and the carrier and the conditions of the drug-loading procedure. Gelatin versatility enabled the design of different carrier systems.



Table 1. The chemical structure of the antibiotic drugs used in the bioadhesives research. Antibiotic drug

Chemical structure

Clindamycin H3C

Functional groups that can react with the crosslinking agent EDC (circled in red) -

CH3

H N

CI

N CH3 O HO

O OH SCH3 OH O

• HCI


Ofloxacin

O F

OH

N

N

N

O

Vancomycin H2N CH3 H3C HO O

OH

O O

HN

O HO

N H

O

O

O H N

O O

• HCI CI

O

HO O

Carboxyl and amine groups

HO HO

CI

HO

Carboxyl groups

N H O

H N

NH2

OH O N H O H3C

H N

CH3

CH3

OH OH

Adapted from [123] with permission. EDC: N-ethyl-N-(3-dimethylaminopropyl) carbodiimide.

Gelatin microparticles and nanoparticles have been widely used for encapsulating many bioactive molecules. Microparticles have a relatively large surface area and can therefore serve as vehicles for cell amplification and delivery of expanded cells or large bioactive molecules to the desired site. Nanoparticles have a higher intracellular uptake and are better suited for intravenous or drug delivery in different areas in the body. Due to their unique design, liposomes have the ability to incorporate both hydrophilic and hydrophobic drugs, protect them from degradation, target them to the desired site and reduce the toxicity or side effects of those molecules. Embedding liposomes into a gelatin-based system resulted in an improvement in their stability and viscosity and in the half-life of the loaded drug and the liposome. As a drug carrier, gelatin fibers contain a high surface areato-volume ratio, high porosity and controllable pore size and can therefore accelerate the solubility of the drug in the aqueous solution and enhance the drug’s efficiency. Gelatin hydrogels can trap molecules within the gaps between the polymer crosslinks. In the body, due to a direct contact with water, they swell and the gaps between the

polymer crosslinks increase, allowing the drugs to diffuse into the blood stream. However, work is continually being carried out in order to improve gelatin release technology by modification of gelatin to allow release of a wider variety of biomolecules from gelatin carriers for a broad range of applications. 8.

Expert opinion

There is a growing need for controlled release of bioactive molecules which is becoming larger due to their production on an industrial scale. Significant efforts have therefore been made in order to achieve a sustained, effective and timespecific controlled release of bioactive molecules from different carriers. It is thus important to improve efficacy, maintain the desired drug concentration in the blood for a long period of time without reaching a toxic level or dropping below the effective level, reduce toxicity and improve patient compliance and convenience. The unique properties of gelatin and its extensive investigation as a biomaterial for controlled release of different


11


*

12

10.02

10.09 10 7.16

8

6.62

6 4 2 0

No drug

Vancomycin Antibiotic drug

Ofloxacin

8 7 6 5 4 3 2 1 0

Clindamycin

0

24

48

72

48

72

Time (h)

B.

D. 18

Bonding strength (KPa)


*

Log10 (microorganisms/ml)

C.

14

*

*

16 14 12 10.09 10 8 6

11.59

12.45

12.57

10.02

4 2 0 No clind 1% 3% 5% Clindamycin concentration (%w/v)

7%

Log10 (microorganisms/ml)

Bonding strength (KPa)

A.

8 7 6 5 4 3 2 1 0

0

24 Time (h)

Figure 5. (A and B) Bonding strength of bioadhesives showing the effect of antibiotic incorporation on the bioadhesive’s bonding strength: (A) the effect of drug type on the bonding strength of the basic formulation (200 mg/ml gelatin, 40 mg/ml alginate and 20 mg/ml N-ethyl-N-(3-dimethylaminopropyl) carbodiimide) loaded with %1 w/v drug. (B) The effect of clindamycin content on the bonding strength of the basic formulation is shown. Values are expressed as means ± SD. (C and D) Microbiological results showing the effect of antibiotic-eluting bioadhesives on bacterial inhibition. The remaining number of Staphylococcus aureus (C) and Staphylococcus albus (D) CFU versus time when initial bacterial concentrations of 107 CFU/ml were used is shown. The releasing samples were derived from formulations containing 200 mg/ml gelatin, 40 mg/ml alginate and EDC (mg/ml)--NHS (mg/ml)--clindamycin (%) concentrations of ( ) 20--0--0, ( ) 10--1--0, ( ) 20--0--3, ( ) 20--0--7, and ( ) 10--1--7. Bacteria in the presence of PBS only served as control ( ). Significant differences are marked with (*). Adapted from [123] with permission.

bioactive molecules are described in the current review article. Gelatin was shown to be versatile due to its intrinsic features that enable the design of different carrier systems. The release of inflammatory drugs, antineoplastic compounds, antibacterial agents, growth factors and even nucleic acids and hydrophobic materials were reported in the literature. Furthermore, modification of gelatin to different types of carriers and the addition of synthetic or natural polymers can enable higher flexibility and diversity in terms of system degradation and optimized and specific drug release, while maintaining and improving the properties of the bulk material. These modifications enabled the development of microparticle- or nanoparticle-based gelatin that can be injected subcutaneously to different areas of the body. Another unique property of gelatin-based nanoparticles loaded with drugs is their ability to be taken up by macrophages, pass the 12

blood--brain barrier and release bioactive molecules in the brain for neural regeneration applications. The main issue regarding the use of gelatin as a carrier for drug delivery systems is its high ability to absorb water. This high water-absorption capacity leads to a very fast release profile and does not allow sustained and effective release of water-soluble drugs. The water uptake can be partially controlled by changing the gelatin source, its initial molecular weight and the degree of its crosslinking. Advanced solutions may be based on new methods to crosslink gelatin. Unfortunately, most of the common crosslinking agents are not highly biocompatible. There are several techniques based on modifications of gelatin, such as methacrylation or enzymes as crosslinking agents. However, the main challenge is to develop a good crosslinking technique that would not change the unique and desired properties of the gelatin or decrease its




biocompatibility. The balance is between biocompatibility and effectiveness of the crosslinking reaction. New crosslinking methods may also enable improvement of the gelatin’s mechanical properties and may lower its solubility and degradation rate in aqueous solutions. New methods for preparation of gelatin-natural polymer and gelatin--synthetic polymer mixtures may enable desired water uptake and swelling behavior for various applications. Another direction that should be investigated in order to increase the diversity and efficiency of these systems is the use of different sources of gelatin, rather than the common use of porcine or bovine gelatin. Gelatin from warm water fish possesses similar characteristics to porcine gelatin and may thus be an alternative for it. In contradistinction, coldwater fish gelatin has a lower melting temperature which can be beneficial when low viscosity is needed, especially in applications that require injection through a catheter. These injectable formulations could be administered more readily into the body, thereby facilitating in vivo release and biological activity. An alternative gelatin source that can eliminate the disadvantages associated with animal tissue-derived material is the development of recombinant gelatin which should be studied further. This technology allows the production of gelatin with specific properties to match a specific application. Understanding the system’s behavior, the process parameters and their effects on the mechanical and biological properties will enable effective and diverse advanced gelatin-based systems for many applications. Gelatin can be used for drug-release application only or be used for active implants that in addition to their regular role also release drug molecules in a controlled desired manner. Examples of the latter systems can be wound dressings that also release antibiotic or analgesic drugs to the wound site and scaffolds for various biomedical applications that also release growth factors. Both scaffolds and wound dressings are based on films or fibers, which means that these drugeluting structures must combine desired mechanical and physical properties with a beneficial drug-release profile. Thus, dense and porous novel gelatin-based drug-eluting structures may make a significant contribution to the new emerging field of active implants. Not only gelatin films and fibers are relevant to active implants. Gelatin hydrogels or microparticles/nanoparticles can be also used to fill the spaces in the pores of porous scaffolds designed for tissue engineering. In addition, drug-loaded gelatin and mixtures of gelatin formulations with other natural or synthetic polymers can be used to coat various implants and thus convert them into drug-eluting implants. These new directions will require the development of novel methods for processing drug-loaded gelatin formulations. Another interesting area is the combination of tissue bioadhesives with controlled drug release for pain management and

wound healing. Gelatin’s ability to form physically crosslinked hydrogel structures, its natural sticky behavior in solution, along with its other qualities such as being biocompatible, biodegradable and non-immunogenic, has turned it into one of the most extensively investigated materials for tissue bioadhesives. Novel gelatin-based bioadhesive systems with drug delivery were described in the current article and will be studied by us also in animal models. The demand for gelatin in many applications has increased over the years. We believe that this tendency will continue in both the pharmaceutical and the medical fields. The next level of research in this field should focus on in vivo models which will be followed by clinical trials. The animal studies related to the various drug-loaded gelatin systems will enable elucidation of the performance of gelatin systems in the fields of regenerative medicine, neurological rehabilitation and soft tissue restoration. Overall, the search for an ideal drug delivery carrier based on gelatin continues. For the time being, the modification of gelatin and its combinations with other biomaterials demonstrate the flexibility of these systems and ensure their continued role as a carrier in the field of drug delivery and other areas such as tissue engineering and wound healing. Each type of gelatin carrier should be further optimized to a specific field that it is most suitable. For example, gelatin microparticles have an advantage as vehicles for cell amplification and delivery of large bioactive molecules, whereas gelatin nanoparticles have higher intracellular uptake and are better suited for drug delivery to the brain, which is a very demanded and important field. Gelatin fibers, as well as gelatin hydrogels, contain a high surface area-to-volume ratio and high porosity and can therefore combined drug delivery with tissue engineering or wound healing systems. The future of drug delivery systems based on gelatin raises challenges of finding the ideal carrier systems which enable a specific, sustained, targeted and controlled release in direct response to local microenvironment demands in the body.

Acknowledgments The authors are grateful to the Office of the Chief Scientist in the Israel Ministry of Industry, Trade and Labor, for supporting research, which is briefly described in this article.

Declaration of interest The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents, received or pending, or royalties.


13


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Affiliation

Maytal Foox1 & Meital Zilberman†2 PhD † Author for correspondence 1 Tel-Aviv University, Department of Biomedical Engineering, Faculty of Engineering, Tel-Aviv 69978, Israel 2 Professor, Tel-Aviv University, Department of Biomedical Engineering, Faculty of Engineering, Tel-Aviv 69978, Israel Tel: +972 3 6405842; Fax: +972 3 6407939; E-mail: [email protected]

17

Drug delivery systems 5A. Oral drug delivery.

Drug delivery systems. 6. Transdermal drug delivery.

Implantable drug-delivery systems.

Lipid-Based Drug Delivery Systems.

ATP-Responsive Drug Delivery Systems.

MEMS: Enabled Drug Delivery Systems.

Kidney-targeted drug delivery systems.

pH-responsive drug-delivery systems.

Implantable neurosurgical drug delivery systems.

Smart drug delivery systems: from fundamentals to the clinic.

From smart materials to advanced drug delivery systems.

Ocular drug delivery systems: An overview.

Supramolecular hydrogels as drug delivery systems.

Radiation sterilization of new drug delivery systems.

Marine Origin Polysaccharides in Drug Delivery Systems.

Raman imaging of drug delivery systems.

Drug delivery systems versus dosage forms.

Pulmonary drug delivery systems for tuberculosis treatment.

Biomedical Imaging in Implantable Drug Delivery Systems.

Mucoadhesive systems in oral drug delivery.

Advanced drug delivery systems for therapeutic applications.

Implantable drug delivery systems: practical considerations.

Albumin nanostructures as advanced drug delivery systems.

In situ gelling systems for drug delivery.