Chitosan-based hydrogels for controlled, localized drug delivery

Hydrogels are high-water content materials prepared from cross-linked polymers that are able to provide sustained, local delivery of a variety of therapeutic agents. Use of the natural polymer, chitosan, as the scaffold material in hydrogels has been highly pursued thanks to the polymer's biocompatibility, low toxicity, and biodegradability. The advanced development of chitosan hydrogels has led to new drug delivery systems that release their payloads under varying environmental stimuli. In addition, thermosensitive hydrogel variants have been developed to form a chitosan hydrogel in situ, precluding the need for surgical implantation. The development of these intelligent drug delivery devices requires a foundation in the chemical and physical characteristics of chitosan-based hydrogels, as well as the therapeutics to be delivered. In this review, we investigate the newest developments in chitosan hydrogel preparation and define the design parameters in the development of physically and chemically cross-linked hydrogels.     

Thermosetting gel for the delivery of 5-aminolevulinic acid esters to the cervix

5-Aminolevulinic acid (5-ALA)-mediated photodynamic therapy has been proposed as an alternative, cervix-sparing treatment for cervical intraepithelial neoplasia (CIN). In this context, topical application of 5-ALA to the cervix is beneficial due to the small necessary dose and its minimal side effects. Therefore, lipophilic 5-ALA esters, such as hexylaminolevulinate (HAL), have led to improved local bioavailability and therapeutic efficacy. Hydrogels have shown to be more appropriate for the local delivery of these derivatives, but due to the limited long-term stability of such formulations at 25 degrees C, the development of an extemporaneously prepared hydrogel targeting CIN can be advantageous. Therefore, a poloxamer 407 thermosetting gel, which is liquid at room temperature and becomes a semi-solid when in contact with the female genital tract, has been evaluated in vitro and in vivo. Rheological evaluation has shown that a 17.0% poloxamer 407 hydrogel with a sol-gel transition at 24.8 +/- 0.6 degrees C was the best formulation for easy application and optimal residence time. Furthermore, similarly to other hydrogels previously tested, such a formulation shows a more complete HAL release in vitro than conventional cream vehicles, and tends to increase porphyrin accumulation in nude mice skin. Finally, in vitro release profiles were correlated to the in vivo results.     

Efficacy study of two novel hyaluronic acid-based formulations for viscosupplementation therapy in an early osteoarthrosic rabbit model

Viscosupplementation (VS) is a therapy for osteoarthrosis (OA) consisting of repetitive intra-articular injections of hyaluronic acid (HA). It is known to be clinically effective in relieving pain and increasing joint mobility by restoring joint homeostasis. In this study, the effects of two novel HA-based VS hydrogel formulations were assessed and challenged against a pure HA commercial formulation for the first time and this in a rabbit model of early OA induced by anterior cruciate ligament transection (ACLT). The first formulation tested was a hybrid hydrogel composed of HA and reacetylated chitosan, a biopolymer considered to be chondroprotective, assembled thanks to an ionic shielding. The second formulation consisted of a novel HA polymer grafted with antioxidant molecules (HA-4AR) aiming at decreasing OA oxidative stress and increasing HA retention time in the articulation.ACLT was performed on rabbits in order to cause structural changes comparable to traumatic osteoarthrosis. The protective effects of the different formulations were observed on the early phase of the pathology in a full randomized and blinded manner. The cartilage, synovial membrane, and subchondral bone were evaluated by complementary investigation techniques such as gross morphological scoring, scanning electron microscopy, histological scoring, and micro-computed tomography were used.In this study, ACLT was proven to successfully reproduce early OA articular characteristics found in humans. HA and HA-4AR hydrogels were found to be moderately protective for cartilage as highlighted by μCT. The HA-4AR was the only formulation able to decrease synovial membrane hypertrophy occurring in OA. Finally, the hybrid HA-reacetylated chitosan hydrogel surprisingly led to increased subchondral bone remodeling and cartilage defect formation. This study shows significant effects of two innovative HA modification strategies in an OA rabbit model, which warrant further studies toward more effective viscosupplementation formulations.     

Thermosensitive hydrogels for nasal drug delivery: The formulation and characterisation of systems based on N-trimethyl chitosan chloride

Towards the development of a thermosensitive drug-delivery vehicle for nasal delivery, a systematic series of N-trimethyl chitosan chloride polymers, synthesised from chitosans of three different average molecular weights, have been co-formulated into a hydrogel with poly(ethylene glycol) and glycerophosphate. Rheological evaluations have shown that hydrogels derived from N-trimethyl chitosan with a low degree of quaternisation and high or medium average molecular weight exhibit relatively short sol-gel transition times at physiologically relevant temperatures. Also, the same hydrogels display good water-holding capacity and strong mucoadhesive potential, and their mixtures with mucus exhibit rheological synergy. An aqueous hydrogel formulation, derived from N-trimethyl chitosan of medium average molecular weight and low degree of quaternisation, appears particularly promising in that it exhibits most favourable rheological and mucoadhesive behaviour and a sol-gel transition that occurs at 32.5 °C within 7 min.     

Physical characterization of a chitosan-based hydrogel delivery system

Hydrogel formation from a mixture of biocompatible chitosan, beta-glycerol phosphate, and hydroxyethyl cellulose was studied. The rheological properties of the formed hydrogels were examined using a Bohlin VOR rheometer. The effect of hydrogel composition and temperature on both the gelation rate and the elastic strength of hydrogels was investigated, from which possible hydrogel formation mechanisms were inferred. The formed hydrogels as potential vehicles for delivering pilocarpine were examined. The advantages and disadvantages of chitosan/hydroxyethyl cellulose hydrogel as a delivery system are discussed.     

Hydrogels based on interpenetrating network of chitosan and polyvinyl pyrrolidone for pH-sensitive delivery of repaglinide

The aim of this study was to develop a pH-sensitive chitosan/polyvinyl pyrrolidone (PVP) based controlled drug release system for repaglinide. The hydrogels were synthesised by crosslinking chitosan and PVP blend with glutaraldehyde to form a semi-interpenetrating polymer network (semi-IPN). These semi-IPNs were studied for their content uniformity, swelling index (SI), mucoadhesion, wettability, in vitro release and their release kinetics. The hydrogels showed more than 95% loading of repaglinide. These hydrogels showed high swelling and mucoadhesion under acidic conditions. The swelling was found due to the protonation of a primary amino group on chitosan. In acidic condition chitosan was ionized, and adhesion occurred between the positively charged chitosan and the negatively charged mucus. In the physiological condition less swelling was noticed. In vitro release study revealed that formulation containing chitosan (2% w/v) and PVP (4% w/v) in the ratio of 14:6 w/w showed complete drug release after 12h. Release profile showed that all the formulations followed non-fickian diffusion mechanism (diffusion coupled with swelling). Fourier transform infrared (FTIR) spectroscopic analysis revealed proper crosslinking of polymer and formation of semi-IPN as well as presence of drug in the formulation. Differential scanning calorimetry (DSC) and powder x-ray diffraction (p-XRD) study revealed the presence of repaglinide in crystalline form in the formulations. The surface morphology of semi-IPN was studied before and after dissolution in simulated gastric fluid (SGF, pH 1.2) which indicated generation of open channel-like structure in hydrogel after dissolution. The results of study suggest that semi-IPNs of chitosan/PVP are potent candidates for delivery of repaglinide in acidic environment.     

In situ forming hydrogels based on chitosan for drug delivery and tissue regeneration

In situ forming hydrogels with simple sol–gel transition are more practicable as injectable hydrogels for drug delivery and tissue regeneration. State-of-the-art in situ gelling systems can easily and efficiently be formed by different mechanisms in situ. Chitosan is a kind of natural polysaccharide that is widely exploited for biomedical applications due to its good biocompatibility, low immunogenicity and specific biological activities. Chitosan-based in situ gelling systems have already gained much attention as smart biomaterials in the development of several biomedical applications, such as for drug delivery systems and regeneration medicine. Herein, we review the typical in situ gelling systems based on chitosan and mechanisms involved in hydrogel forming, and report advances of chitosan-based in situ gels for the applications in drug delivery and tissue regeneration. Finally, development prospects of in situ forming hydrogels based on chitosan are also discussed in brief.     

Evaluation of properties microcrystalline chitosan as a drug carrier. Part 1. In vitro release of diclofenac from mictocrystalline chitosan hydrogel

The influence of microcrystalline chitosan hydrogel, alone (MCCh) as well as in combination with methylcellulose (MC) or Carbopol (CP), on the release of diclofenac free acid (DA) and its salt (DS) was studied in vitro. Commercial Olfen gel (Mepha Ltd., Switzerland) was applied as a reference preparation. The influence of hydrophilizing agents (1,2-propylene glycol and glycerol) and methycellulose hydrogel on the rheological properties of the vehicle and on the release of drug from modified MCCh hydrogel was studied. The quantity of the released substance was determined by UV-spectroscopy. The results confirmed that release was dependent on the chemical character of the drug and on the type of vehicle. The process of diclofenac release from MCCh hydrogels as well as from Carbopol hydrogels runs in two phases. The first phase is characterised by rapid release whereas in the second phase the release is much slower. The most suitable basis for diclofenac is microcrystalline chitosan hydrogel with addition glycerol, 1,2-propylene glycol, and methylcellulose hydrogel.     

Controlled releases of FGF-2 and paclitaxel from chitosan hydrogels and their subsequent effects on wound repair, angiogenesis, and tumor growth

A photocrosslinkable chitosan (Az-CH-LA) aqueous solution resulted in an insoluble hydrogel like a soft rubber within 30 sec of ultraviolet light (UV)-irradiation. The photocrosslinked chitosan hydrogel showed strong sealing strength and potential use as a new tissue adhesive in surgical application. Paclitaxel, which is an anti-tumor reagent and a vascularization-inhibitor, retained in the photocrosslinked chitosan hydrogel, and were gradually released from the photocrosslinked chitosan hydrogel in vivo upon the degradation of the hydrogel. The paclitaxel-incorporated photocrosslinked chitosan hydrogels effectively inhibited tumor growth and angiogenesis in mice. On the other hand, the fibroblast growth factor (FGF)-2 molecules also retained in both the photocrosslinked chitosan and an injectable chitosan/IO(4)-heparin hydrogels, and were gradually released from the hydrogels upon their in vivo biodegradations. The activity of FGF-2 in the hydrogels was stable for long time (more than 14 days). The controlled release of biologically active FGF-2 molecules from the hydrogels caused an induction of the angiogenesis and, possibly, collateral circulation occurred in the healing-impaired diabetic (db/db) mice and the ischemic limbs of rats. The purpose of this review is to describe the effectiveness of the chitosan hydrogels (photocrosslinkable chitosan hydrogel and chitosan/IO(4)-heparin hydrogel) as a local drug delivery carrier for FGF-2 and paclitaxel to control wound repair, tumor growth, and angiogenesis. It is thus proposed that the chitosan hydrogels may be a promising new local carrier for drugs such as FGF-2 and paclitaxel.     

The effect of lipid composition and liposome size on the release properties of liposomes-in-hydrogel

To study the release of liposome-associated drugs into hydrogels, we designed and synthesized two pH-sensitive rhodamine derivatives to use as model compounds of different lipophilicities. The dyes were fluorescent when in the free form released from liposomes into the chitosan hydrogel, but not when incorporated within liposomes. The effect of liposomal composition, surface charge and vesicle size on the release of those incorporated dyes was evaluated. The lipophilicity of the rhodamine derivatives affected both the amount and rate of release. While liposome size had only a minor effect on the release of dyes into the hydrogel, the surface charge affected the release to a greater extent. By optimizing the characteristics of liposomes we could develop a liposomes-in-hydrogel system for application in wound therapy. We further characterized liposomes-in-hydrogel for their rheological properties, textures and moisture handling, as well as their potential to achieve a controlled release of the dye. The polymer-dependent changes in the hydrogel properties were observed upon addition of liposomes. The charged liposomes exhibited stronger effects on the textures of the chitosan hydrogels than the neutral ones. In respect to the ability of the system to handle wound exudates, chitosan-based hydrogels were found to be superior to Carbopol-based hydrogels.     

In situ Hydrogels Prepared by Photo-initiated Crosslinking of Acrylated Polymers for Local Delivery of Antitumor Drugs

A triblock copolymer was synthesized by ring opening polymerization of ε‐caprolactone in the presence of poly(ethylene glycol) (PEG). The resulted PCL-PEG-PCL triblock copolymer, PEG and monomethoxy (MPEG) were functionalized by end group acrylation. NMR and FT-IR analyses evidenced the successful synthesis and functionalization of polymers. A series of photo-crosslinked hydrogels composed of acrylated PEG-PCL-Acr and MPEG-Acr or PEG-Acr were prepared by exposure to visible light using lithium phenyl-2,4,6-trimethylbenzoylphosphinate as initiator. The hydrogels present a porous and interconnected structure as shown by SEM. The swelling performance of hydrogels is closely related to the crosslinking density and hydrophilic content. Addition of MPEG or PEG results in increase in water absorption capacity of hydrogels. In vitro degradation of hydrogels was realized in the presence of a lipase from porcine pancreas. Various degradation rates were obtained which mainly depend on the hydrogel composition. MTT assay confirmed the good biocompatibility of hydrogels. Importantly, in situ gelation was achieved by irradiation of a precursor solution injected in the abdomen of mice. Doxorubicin (DOX) was selected as a model antitumor drug to evaluate the potential of hydrogels in cancer therapy. Drug-loaded hydrogels were prepared by in situ encapsulation. In vitro drug release studies showed a sustained release during 28 days with small burst release. DOX-loaded hydrogels exhibit antitumor activity against A529 lung cancer cells comparable to free drug, suggesting that injectable in situ hydrogel with tunable properties could be most promising for local drug delivery in cancer therapy.     

Controlled release of lipase from Candida rugosa loaded into hydrogels of N-isopropylacrylamide and itaconic acid

The series of poly(N-isopropylacrylamide-co-itaconic acid) hydrogels, with lipase from Candida rugosa as a model protein, were synthesized by free radical copolymerization. The composition of hydrogels was varied by monomers ratio, crosslinking agent concentration and amounts of lipase, which was loaded by in situ polymerization. All samples were characterized regarding morphology. The investigation of hydrogel swelling properties revealed their pH and temperature sensitive character. Protein loading efficiency, release profiles and the specific activity yield of the released lipase were also investigated as a function of hydrogel composition, protein content and pH, at the physiological temperature of 37°C. Copolymers of N-isopropylacrylamide and itaconic acid presented high lipase loading efficiency. Another very important feature of these copolymers was that the protein release kinetic strongly depended on the pH value of the medium. The diffusion exponents values around 1 denoted that these hydrogel compositions could be adjusted to follow near zero-order kinetics. Namely, hydrogel formulations released low amounts of lipase at pH 2.20, but much higher released protein quantities were observed at pH 6.80 enabling these copolymers to be attractive candidates as site specific protein oral drug delivery systems.     

Nasal Delivery of Insulin Using Novel Chitosan Based Formulations: A Comparative Study in Two Animal Models Between Simple Chitosan Formulations and Chitosan Nanoparticles

Purpose. To investigate whether the widely accepted advantages associated with the use of chitosan as a nasal drug delivery system, might be further improved by application of chitosan formulated as nanoparticles. Methods. Insulin-chitosan nanoparticles were prepared by the ionotropic gelation of chitosan glutamate and tripolyphosphate pentasodium and by simple complexation of insulin and chitosan. The nasal absorption of insulin after administration in chitosan nanoparticle formulations and in chitosan solution and powder formulations was evaluated in anaesthetised rats and/or in conscious sheep. Results. Insulin-chitosan nanoparticle formulations produced a pharmacological response in the two animal models, although in both cases the response in terms of lowering the blood glucose levels was less (to 52.9 or 59.7% of basal level in the rat, 72.6% in the sheep) than that of the nasal insulin chitosan solution formulation (40.1% in the rat, 53.0% in the sheep). The insulin-chitosan solution formulation was found to be significantly more effective than the complex and nanoparticle formulations. The hypoglycaemic response of the rat to the administration of post-loaded insulin-chitosan nanoparticles and insulin-loaded chitosan nanoparticles was comparable. As shown in the sheep model, the most effective chitosan formulation for nasal insulin absorption was a chitosan powder delivery system with a bioavailability of 17.0% as compared to 1.3% and 3.6% for the chitosan nanoparticles and chitosan solution formulations, respectively. Conclusion. It was shown conclusively that chitosan nanoparticles did not improve the absorption enhancing effect of chitosan in solution or powder form and that chitosan powder was the most effective formulation for nasal delivery of insulin in the sheep model.     

Structure and interactions in chitosan hydrogels formed by complexation or aggregation for biomedical applications.

The aim of this review was to provide a detailed overview of physical chitosan hydrogels and related networks formed by aggregation or complexation, which are intended for biomedical applications. The structural basis of these systems is discussed with particular emphasis on the network-forming interactions, the principles governing their formation and their physicochemical properties. An earlier review discussing crosslinked chitosan hydrogels highlighted the potential negative influence on biocompatibility of covalent crosslinkers and emphasised the need for alternative hydrogel systems. A possible means to avoid the use of covalent crosslinkers is to prepare physical chitosan hydrogels by direct interactions between polymeric chains, i.e. by complexation, e.g. polyelectrolyte complexes (PEC) and chitosan/poly (vinyl alcohol) (PVA) complexes, or by aggregation, e.g. grafted chitosan hydrogels. PEC exhibit a higher swelling sensitivity towards pH changes compared to covalently crosslinked chitosan hydrogels, which extends their potential application. Certain complexed polymers, such as glycosaminoglycans, can exhibit interesting intrinsic properties. Since PEC are formed by non-permanent networks, dissolution can occur. Chitosan/PVA complexes represent an interesting alternative for preparing biocompatible drug delivery systems if pH-controlled release is n/ot required. Grafted chitosan hydrogels are more complex to prepare and do not always improve biocompatibility compared to covalently crosslinked hydrogels, but can enhance certain intrinsic properties of chitosan such as bacteriostatic and wound-healing activity.     

In situ injectable nano-composite hydrogel composed of curcumin, N,O-carboxymethyl chitosan and oxidized alginate for wound healing application

In this paper, an in situ injectable nano-composite hydrogel composed of curcumin, N,O-carboxymethyl chitosan and oxidized alginate as a novel wound dressing was successfully developed for the dermal wound repair application. Nano-curcumin with improved stability and similar antioxidant efficiency compared with that of unmodified curcumin was developed by using methoxy poly(ethylene glycol)-b-poly(?-caprolactone) copolymer (MPEG-PCL) as carrier followed by incorporating into the N,O-carboxymethyl chitosan/oxidized alginate hydrogel (CCS-OA hydrogel). In vitro release study revealed that the encapsulated nano-curcumin was slowly released from CCS-OA hydrogel with the diffusion-controllable manner at initial phase followed by the corrosion manner of hydrogel at terminal phase. In vivo wound healing study was performed by injecting hydrogels on rat dorsal wounds. Histological study revealed that application of nano-curcumin/CCS-OA hydrogel could significantly enhance the re-epithelialization of epidermis and collagen deposition in the wound tissue. DNA, protein and hydroxyproline content in wound tissue from each group were measured on 7th day of post wounding and the results also indicated that combined using nano-curcumin and CCS-OA hydrogel could significantly accelerate the process of wound healing. Therefore, all these results suggested that the developed nano-curcumin/CCS-OA hydrogel as a promising wound dressing might have potential application in the wound healing.     

In vitro skin penetration and pharmacodynamic evaluation of prostaglandin E1 ethyl ester, a vasoactive prodrug of prostaglandin E1, formulated into alcoholic hydrogels

Alcoholic hydrogels containing prostaglandin E1 ethyl ester (PGE1-EE), a prodrug of PGE1 as a therapeutic agent for erectile dysfunction, were formulated. The prodrug was stable against chemical hydrolysis in aqueous solution (pH 7.4), devoid of esterase activities, but was hydrolyzed to the parent drug in rat skin homogenates within 240 min. In the rat skin penetration study for 24 h, the steady-state flux values (microg/cm2/h) of PGE1-EE and PGE, from alcoholic hydrogels having 20% ethanol content were 7.6 and 1.8, respectively. PGE1-EE was superior to PGE, from a skin penetration point of view due to its increased lipophilicity. The fastest skin penetration rate was obtained for PGE1-EE in 20% alcoholic hydrogel together with limonene or cineole. These formulations increased the flux of PGE1-EE up to about 4-fold compared to control hydrogel in the absence of penetration enhancers. In the pharmacodynamic study using a cat, alcoholic hydrogel with limonene or cineole showed a significant effect in terms of increasing intracavernosal pressure compared to control hydrogel. Therefore, the transdermal alcoholic hydrogel formulation of PGE1-EE with limonene or cineole can be a promising transdermal delivery system to overcome inconvenience associated with frequent intracavernosal injections for the treatment of erectile dysfunction.     

Preparation and in vitro evaluation of melatonin-loaded HA/PVA gel formulations

Melatonin-loaded hyaluronic acid (HA) and poly(vinyl alcohol) (PVA) gels were prepared by using freeze–thaw technique and an emulsion method followed by freeze–thaw technique to produce a new synergistic system for topical application. Freeze–thaw hydrogels and emulgels were characterized by means of Fourier transform infrared spectroscopy, rheology and swelling tests. The porous structure of the hydrogels was shown by scanning electron microscopy observations and thermal properties were tested by differential scanning calorimetry measurements. Bioadhesion and in vitro release characterization of formulations were performed by texture profile analysis and dialysis bag method, respectively. The pore size of both formulations was ranging from 900?nm to 30?μm. Melatonin showed a good compatibility with the polymeric matrices as the pores were smaller for the drug-loaded systems. In vitro release studies showed that the release was improved by emulgel formulations. After 24?h, the release percentage was found to be 13.240%?±?1.094 and 15.192%?±?2.270 for hydrogel and emulgel, respectively. Emulgels had better bioadhesion properties than simple freeze–thaw samples. As a conclusion, regarding the in vitro characterization studies HA and PVA hydrogel and emulgel formulations and their lyophilized forms could be promising systems for topical application of melatonin.     

In vivo retention of poloxamer-based in situ hydrogels for vaginal application in mouse and rat models

The purpose of this study is to evaluate the in vivo retention capabilities of poloxamer-based in situ hydrogels for vaginal application with nonoxinol-9 as the model drug. Two in situ hydrogel formulations, which contained 18% poloxamer 407 plus 1% poloxamer 188 (GEL1, relative hydrophobic) or 6% poloxamer 188 (GEL2, relative hydrophilic), were compared with respect to the rheological properties, in vitro hydrogel erosion and drug release. The vaginal retention capabilities of these hydrogel formulations were further determined in two small animal models, including drug quantitation of vaginal rinsing fluid in mice and isotope tracing with ^99mTc in rats. The two formulations exhibited similar phase transition temperatures ranging from 27 to 32 °C. Increasing the content of poloxamer 188 resulted in higher rheological moduli under body temperature, but slightly accelerated hydrogel erosion and drug release. When compared in vivo, GEL1 was eliminated significantly slower in rat vagina than GEL2, while the vaginal retention of these two hydrogel formulations behaved similarly in mice. In conclusion, increases in the hydrophilic content of formulations led to faster hydrogel erosion, drug release and intravaginal elimination. Rats appear to be a better animal model than mice to evaluate the in situ hydrogel for vaginal application.     

PVA-PEG physically cross-linked hydrogel film as a wound dressing: experimental design and optimization

The development of hydrogel films as wound healing dressings is of a great interest owing to their biological tissue-like nature. Polyvinyl alcohol/polyethylene glycol (PVA/PEG) hydrogels loaded with asiaticoside, a standardized rich fraction of Centella asiatica, were successfully developed using the freeze–thaw method. Response surface methodology with Box–Behnken experimental design was employed to optimize the hydrogels. The hydrogels were characterized and optimized by gel fraction, swelling behavior, water vapor transmission rate and mechanical strength. The formulation with 8% PVA, 5% PEG 400 and five consecutive freeze–thaw cycles was selected as the optimized formulation and was further characterized by its drug release, rheological study, morphology, cytotoxicity and microbial studies. The optimized formulation showed more than 90% drug release at 12?hours. The rheological properties exhibited that the formulation has viscoelastic behavior and remains stable upon storage. Cell culture studies confirmed the biocompatible nature of the optimized hydrogel formulation. In the microbial limit tests, the optimized hydrogel showed no microbial growth. The developed optimized PVA/PEG hydrogel using freeze–thaw method was swellable, elastic, safe, and it can be considered as a promising new wound dressing formulation.