Preactivated thiomers: permeation enhancing properties

Design and evaluation of propranolol hydrochloride loaded thiolated Zein/PEO electrospun fibrous matrix for transmucosal drug delivery

Thiolated polymers have garnered wide attention from researchers on mucoadhesive drug delivery. This work explores the thiolation of zein protein using cysteine amino acid via the EDC crosslinker. The optimization of thiolation and purification have been done and confirmed using Ellman's assay and Raman spectra. The thiolated Zein/PEO polymer blend has been appraised for electrospinning to fabricate fibrous matrices. The extent of thiol modification augmented the mechanical properties and adhesion in rabbit intestinal mucosa. In vitro cytotoxicity evaluations such as direct contact assay, MTT assay, and live dead assay performed in RPMI 2650 cells corroborated the non-cytotoxicity of the fabricated matrices with and without propranolol hydrochloride (PL). Detailed drug release studies were conducted in PBS. Drug release in PBS followed the Korsmeyer Peppas model of release. On treating RPMI 2650 cells with the matrix, F-actin and adherens junctional proteins retained integrity, and consequently, drug permeation would proceed through the transcellular transport mechanism. Transepithelial electrical resistance (TEER) measurement of the RPMI 2650 cell monolayer also supported the transcellular transport mechanism. Ex vivo permeation study through porcine buccal mucosa showed 41.26 ± 0.56% PL permeation within 24 h of study. It validated the competence of the electrospun thiolated Zein/PEO matrix for transmucosal drug delivery.

Surface design of nanocarriers: Key to more efficient oral drug delivery systems

As nanocarriers (NCs) can improve the solubility of drugs, prevent their degradation by gastrointestinal (GI) enzymes and promote their transport across the mucus gel layer and absorption membrane, the oral bioavailability of these drugs can be substantially enhanced. All these properties of NCs including self-emulsifying drug delivery systems (SEDDS), solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs), liposomes, polymeric nanoparticles, inorganic nanoparticles and polymeric micelles depend mainly on their surface chemistry. In particular, interaction with food, digestive enzymes, bile salts and electrolytes, diffusion behaviour across the mucus gel layer and fate on the absorption membrane are determined by their surface. Bioinert surfaces limiting interactions with gastrointestinal fluid and content as well as with mucus, adhesive surfaces providing an intimate contact with the GI mucosa and absorption enhancing surfaces can be designed. Furthermore, charge converting surfaces shifting their zeta potential from negative to positive directly at the absorption membrane and surfaces providing a targeted drug release are advantageous. In addition to these passive surfaces, even active surfaces cleaving mucus glycoproteins on their way through the mucus gel layer can be created. Within this review, we provide an overview on these different surfaces and discuss their impact on the performance of NCs in the GI tract.

Thiolated Nanoparticles for Biomedical Applications: Mimicking the Workhorses of Our Body

Advances in nanotechnology have generated a broad range of nanoparticles (NPs) for numerous biomedical applications. Among the various properties of NPs are functionalities being related to thiol substructures. Numerous biological processes that are mediated by cysteine or cystine subunits of proteins representing the workhorses of the bodies can be transferred to NPs. This review focuses on the interface between thiol chemistry and NPs. Pros and cons of different techniques for thiolation of NPs are discussed. Furthermore, the various functionalities gained by thiolation are highlighted. These include overall bio- and mucoadhesive, cellular uptake enhancing, and permeation enhancing properties. Drugs being either covalently attached to thiolated NPs via disulfide bonds or being entrapped in thiolated polymeric NPs that are stabilized via inter- and intrachain crosslinking can be released at the diseased tissue or in target cells under reducing conditions. Moreover, drugs, targeting ligands, biological analytes, and enzymes bearing thiol substructures can be immobilized on noble metal NPs and quantum dots for therapeutic, theranostic, diagnostic, biosensing, and analytical reasons. Within this review a concise summary and analysis of the current knowledge, future directions, and potential clinical use of thiolated NPs are provided.

Nanocomposite systems for precise oral delivery of drugs and biologics

Oral delivery is considered the favoured route of administration for both local and systemic delivery of active molecules. Formulation of drugs in conventional systems and nanoparticles has provided opportunities for targeting the gastrointestinal (GI) tract, increasing drug solubility and bioavailability. Despite the achievements of these delivery approaches, the development of a product with the ability of delivering drug molecules at a specific site and according to patients' needs remains a challenging endeavour. The complexity of the physicochemical properties of colloidal systems, their stability in different regions of the gastrointestinal tract, and interaction with the restrictive biological barriers hampered their success for oral precise medicine. To overcome these issues, nanoparticles have been combined with polymers to create hybrid nanosystems, namely nanocomposites. They offer enormous possibilities of structural and mechanical modifications to both nanoparticles and polymeric matrixes to generate systems with new properties, functions, and applications for oral delivery. In this review, nanocomposites' physicochemical and functional properties intended to target specific regions of the GI tract-oral cavity, stomach, small bowel, and colon-are analysed. In parallel, it is provided an insight in the nanocomposite solutions for oral delivery intended for systemic and local absorption, together with a focus on inflammatory bowel diseases (IBDs). Additional difficulties in managing IBD related to the alteration in the physiology of the intestine are described. Finally, future perspectives and opportunities for advancement in this field are discussed.

Multifunctional coordination polymers based on copper(i) and mercaptonicotinic ligands: synthesis, and structural, optical and electrical characterization

Three new coordination polymers (CPs) named [Cu(6mna)]n (CP1), [CuCl(H6mna)(H2O)0.33]n (CP2), and {[(CuI)2H2dtdn].MeCN}n (CP3), (H6mna = 6-mercaptonicotinic acid, and H2dtdn = 6,6'-dithiodinicotinic acid) have been synthesized and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 3 are 2D-CPs while complex 2 is a 1D-CP. The optical properties of these complexes have been evaluated in the solid state, at room temperature and at 77 K, and compared with those of the starting ligands. The electrical conductivity of CPs 1-3 has been evaluated and their thermal stabilities have been studied. CP2 shows an interesting crystal arrangement, where the connection between the ligand and the copper forms a channel-like structure characterized by an intrinsic disorder. Crystal data collected at low temperatures for this complex revealed minor structural changes in the CuCu distances and Cu-S-Cu angles along the chain, excluding phase transition. In CP1, the N and S atoms are involved in metal coordination bonds giving rise to a 2D coordination polymer. In CP3, the Cu-I bonds compose double ladder-like structures, bridged by H2dtdn ligands. The electrical conductivities of CPs 1-3 suggest their semiconductive behavior.

Synthesis of HA-SS-MP: A Prodrug With High Specificity for Cancer Cells

Thiolated hyaluronic acid was synthesized, and a quantification method was established to determine the content of the thiol group. Three thiol compounds, 2-nitro-5-mercapto-benzoic acid (TNB), 2- mercaptonicotinic acid (2-MNA), and mercaptopyridine (MPD), were tested for their efficiency in forming adducts with hyaluronic acid (HA) through disulfide bonds. Our results showed that MPD exhibited the highest disulfide exchange efficiency when the reaction was conducted at the optimal pH value. The synthesized hyaluronic acid-SS-mercaptopurine (HA-SS-MP) was characterized by1H-nuclear magnetic resonance spectroscopy. The HA-SS-MP micelle showed a particle size of 264.4 nm in aqueous solution. An in vitro release experiment demonstrated that the releasing rate of 6-MP from HA-SS-MP in the release medium containing glutathione (GSH) was much higher than that in the release medium without GSH. Cell uptake experiments demonstrated the CD44 targeting of hyaluronic acid. Cytotoxicity tests in mouse melanoma cells (B16F10) showed that HA-SS-MP can kill melanoma cells effectively and with lower cytotoxicity, compared with the active ingredient, 6-MP.

Per-6-Thiolated Cyclodextrins: A Novel Type of Permeation Enhancing Excipients for BCS Class IV Drugs

The purpose of the study was to develop a per-6-thiolated α-cyclodextrin (α-CD) by substituting all primary hydroxyl groups of α-CD with thiol groups and to assess its solubility-improving and permeation-enhancing properties for a BCS Class IV drug in vitro as well as in vivo. The primary hydroxyl groups of α-CD were replaced by iodine, followed by substitution with -SH groups. The structure of per-6-thiolated α-CD was approved by FT-IR and ¹H NMR spectroscopy. The per-6-thiolated was characterized for thiol content, -SH stability, cytotoxicity, and solubility-improving properties by using the model BCS Class IV drug furosemide (FUR). The mucoadhesive properties of the thiolated oligomer were investigated via viscoelastic measurements with porcine mucus, whereas permeation-enhancing features were evaluated on the Caco-2 cell monolayer and rat gut mucosa. Furthermore, oral bioavailability studies were performed in rats. The per-6-thiolated α-CD oligomer displayed 4244 ± 402 μmol/g thiol groups. These -SH groups were stable at pH ≤ 4, exhibiting a pK_a value of 8.1, but subject to oxidation at higher pH. Per-6-thiolated α-CD was not cytotoxic to Caco-2 cells in 0.5% (m/v) concentration within 24 h. It improved the solubility of FUR in the same manner as unmodified α-CD. The addition of per-6-thiolated α-CD (0.5% m/v) increased the mucus viscosity up to 5.8-fold at 37 °C within 4 h. Because of the incorporation in per-6-thiolated α-CD, the apparent permeability coefficient (P_app) of FUR was 6.87-fold improved on the Caco-2 cell monolayer and 6.55-fold on the intestinal mucosa. Moreover, in vivo studies showed a 4.9-fold improved oral bioavailability of FUR due to the incorporation in per-6-thiolated α-CD. These results indicate that per-6-thiolated α-CD would be a promising auxiliary agent for the mucosal delivery of, in particular, BCS Class IV drugs.

Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature

Thiolated polymers designated "thiomers" are obtained by covalent attachment of thiol functionalities on the polymeric backbone of polymers. In 1998 these polymers were first described as mucoadhesive and in situ gelling compounds forming disulfide bonds with cysteine-rich substructures of mucus glycoproteins and crosslinking through inter- and intrachain disulfide bond formation. In the following, it was shown that thiomers are able to form disulfides with keratins and membrane-associated proteins exhibiting also cysteine-rich substructures. Furthermore, permeation enhancing, enzyme inhibiting and efflux pump inhibiting properties were demonstrated. Because of these capabilities thiomers are promising tools for drug delivery guaranteeing a strongly prolonged residence time as well as sustained release on mucosal membranes. Apart from that, thiomers are used as drugs per se. In particular, for treatment of dry eye syndrome various thiolated polymers are in development and a first product has already reached the market. Within this review an overview about the thiomer-technology and its potential for different applications is provided discussing especially the outcome of studies in non-rodent animal models and that of numerous clinical trials. Moreover, an overview on product developments is given.

Mucoadhesive S-protected thiolated cyclodextrin-iodine complexes: a promising strategy to prolong mucosal residence time of iodine

Aim: The purpose of this study was to develop S-protected thiolated α-cyclodextrin–iodine complexes providing prolonged mucosal residence time and sustained release of the antimicrobial agent. Materials & methods: First, L-cysteine was conjugated with 2-mercaptonicotinic acid to generate cysteine-2-mercaptonicotinic acid (Cys-MNA). Subsequently, α-CD was oxidized with NaIO4 and Cys-MNA was bound to the resulting aldehyde groups via reductive amination. Finally, iodine was incorporated into complex. Result: S-protected thiolated α-CD displayed 3804 μmol/g MNA groups. The inclusion constant (KC) between iodine and S-protected thiolated α-CD was 5.37 × 104 M-1. In vitro release of iodine was around 15% per hour, whereas mucoadhesive properties showed 38-fold improvement in mucoadhesion. The complex did not show cytotoxicity at a concentration of 0.5% (m/v). In addition, complexes exhibited pronounced antimicrobial activity against Staphylococcus aureus and Escherichia coli. Conclusion: According to these results, S-protected thiolated α-CD-iodine complex might be a promising novel formulation for the mucosal use of iodine.

Synthesis and Characterization of Thiolated PVP-Iodine Complexes: Key to Highly Mucoadhesive Antimicrobial Gels

The aim of this study was to synthesize iodine containing polymeric excipients for mucosal treatment of microbial infection exhibiting a prolonged mucosal residence time by forming an adhesive gel on the mucosal surface. In order to achieve this aim, 2-(2 acryloylamino-ethyldisulfanyl)-nicotinic acid (ACENA) was copolymerized with N-vinylpyrrolidone (NVP) to obtain thiolated polyvinylpyrrolidone (PVP) for complexation with iodine. The average molecular mass of different thiolated PVP variants was determined by size exclusion chromatography. The structure of thiolated PVP was confirmed by ¹H NMR. Thiolated PVP variants were characterized for thiol content, cytotoxicity, iodine loading capacity, rheological behavior, and adhesion time on mucosa. The highest achieved degree of thiolation was 610 ± 43 μmol/g, and the maximum recorded iodine loading was 949 ± 31 μmol/g of polymer. Thiolated PVP variants (0.5% m/v) showed no toxicity after incubation on Caco-2 cells for the period of 3 and 24 h, respectively. Thiolated PVP and thiolated PVP-iodine complexes exhibited a 5.4- and 4.4-fold increased dynamic viscosity in porcine mucus in comparison to PVP and PVP-iodine complex, respectively. Compared to PVP and PVP-iodine complex thiol-functionalized PVP and PVP-iodine complexes demonstrated significantly prolonged attachment to mucosal surface over a period of 3 h. Thiol functionalized PVP proved to be a promising novel excipient for complexation with iodine and to exhibit strongly improved mucoadhesive properties.

Mannosylated thiolated polyethylenimine nanoparticles for the enhanced efficacy of antimonial drug against Leishmaniasis

AIM

Our aim was to inhibit trypanothione reductase (TR) and P-gp efflux pump of Leishmania by the use of thiolated polymers. Thus, increasing the intracellular accumulation and therapeutic effectiveness of antimonial compounds.

METHODS

Mannosylated thiolated chitosan and mannosylated thiolated chitosan-polyethyleneimine graft were synthesized and characterized. Meglumine antimoniate-loaded nanoparticles were prepared and evaluated for TR and P-gp efflux pump inhibition, biocompatibility, macrophage uptake and antileishmanial potential.

RESULTS

Thiomers inhibited TR with Ki 2.021. The macrophage uptake was 33.7- and 18.9-fold higher with mannosylated thiolated chitosan-polyethyleneimine graft and mannosylated thiolated chitosan nanoparticles, respectively, as compared with the glucantime. Moreover, the in vitro antileishmanial activity showed 14.41- and 7.4-fold improved IC₅₀ for M-TCS-g-PEI and M-TCS, respectively as compared with glucantime.

CONCLUSION

These results encouraged the concept that TR and P-gp inhibition by the use of thiomers improves the therapeutic efficacy of antimonial drugs.

A Polymer Chemistry Point of View on Mucoadhesion and Mucopenetration

Although oral is the preferred route of administration of pharmaceutical formulations, the long-standing challenge for medically active compounds to efficiently cross the mucus layer barrier limits its wider applicability. Efforts in nanomedicine to overcome this hurdle consider mucoadhesive and mucopenetrating drug carriers by selectively designing (macromolecular) building blocks. This review highlights and critically discusses recent strategies developed in this context including poly(ethylene glycol)-based modifications, cationic and thiolated polymers, as well as particles with high charge density, zeta-potential shifting ability, or mucolytic properties. The latest advances in ex vivo test platforms are also reviewed.

Thiolated polymers as mucoadhesive drug delivery systems

Mucoadhesion is the process of binding a material to the mucosal layer of the body. Utilising both natural and synthetic polymers, mucoadhesive drug delivery is a method of controlled drug release which allows for intimate contact between the polymer and a target tissue. It has the potential to increase bioavailability, decrease potential side effects and offer protection to more sensitive drugs such as proteins and peptide based drugs. The thiolation of polymers has, in the last number of years, come to the fore of mucoadhesive drug delivery, markedly improving mucoadhesion due to the introduction of free thiol groups onto the polymer backbone while also offering a more cohesive polymeric matrix for the slower and more controlled release of drug. This review explores the concept of mucoadhesion and the recent advances in both the polymers and the methods of thiolation used in the synthesis of mucoadhesive drug delivery devices.

Thiomers and their potential applications in drug delivery

INTRODUCTION

Thiomers are the product of the immobilization of sulfhydryl-bearing ligands onto the polymer backbone of a conventional polymer, which results in a significant improvement in mucoadhesion; in situ gelation and efflux inhibition compare with unchanged polymers. Because of thiol groups, thiomers have more reactivity and enhanced protection against oxidation. Since the late 1990s, extensive work has been conducted on these promising polymeric excipients in the pharmaceutical field. Areas covered: This review covers thiomers, their classification and their different properties. Various techniques for the synthesis, purification and characterization of thiomers are described in detail. This review also encompasses their various properties such as mucoadhesion, permeation enhancement, in situ gelation and efflux inhibition, as well as different formulations based on thiomers. In addition to the use of thiomers as multifunctional excipients, this review also encompasses their use as drugs. Expert opinion: The synthesis is realized by linkage of sulfhydryl-bearing ligands but reported methods give low yields. Higher degrees of modification are not necessary and would probably lead to extreme changes in properties. Nevertheless, an accurate characterization of the final product is important. The scale-up procedure for industrial manufacturing has been adapted to produce GMP materials; Lacrimera® eye drops have already entered the European market.

Novel in vitro transport method for screening the reversibility of P-glycoprotein inhibitors

The purpose of this study was to establish a novel in vitro method for screening reversibility of P-glycoprotein (P-gp) inhibitors. Caco-2 cells with 21days of cultivation were used as an in vitro model. Transport of rhodamine 123 in the presence of various inhibitors and after removing of inhibitors was determined. Transport of rhodamine 123 at 4°C and in the secretory direction assured that Caco-2 cells exhibited P-gp function at all time of experiment. The apparent permeability coefficient (Papp) of rhodamine 123 in the presence of verapamil, cyclosporin A, ritonavir, quinidine, N-ethylmaleimide, Cremophor® EL, Tween 80 and poly(acrylic acid)-cysteine-2-mercaptonicotinic acid (PAA-cys-2MNA) was 2.3-, 3.8-, 2.3-, 3.1, 7.5-, 2.1-, 2.9- and 2.5-fold higher than Papp of rhodamine 123 alone. After removing of the inhibitors, Papp decreased to the same range of control except in the case of N-ethylmaleimide which was 2.4-fold higher than the control. These results revealed a reversible inhibition of verapamil, cyclosporin A, ritonavir, quinidine, Cremophor® EL, Tween 80 and PAA-cys-2MNA and an irreversible inhibition of N-ethylmaleimide for P-gp. Thus, this novel established that in vitro method might be an effective tool for screening the reversibility of inhibition of P-gp inhibitors.

Permeation enhancement via thiolation: in vitro and ex vivo evaluation of hyaluronic acid-cysteine ethyl ester

It was the aim of this study to evaluate the permeation-enhancing effect of synthesized thiolated hyaluronic acid (HA). HA, a naturally found polysaccharide, was chemically modified with l-cysteine ethyl ether (C) via amide bond formation. In vitro permeation enhancement was tested on Caco-2 cells with two compounds, sulforhodamine (SR) and fluorescein isothiocyanate-dextran (FD4). Cytotoxicity assays as lactate dehydrogenase and thiazolyl blue tetrazolium bromide (MTT) were performed on colon carcinoma cell line. Transepithelial electrical resistance (TEER) measurements were conducted. Ex vivo evaluation was accomplished on rat intestinal mucosa in order to predict the permeation enhancing effect with SR, sodium fluorescein (SF), and FD4, respectively. The MTT as well as lactate dehydrogenase revealed no toxicity over time periods of 3 and 12 h, respectively. The bioconjugate is biocompatible and safe to use. Furthermore, TEER measurements showed the integrity of tight junctions. The in vitro permeation studies on cell studies exhibit 1.28-fold enhancement for SR and 1.47-fold enhancement for FD4 with hyaluronic acid-cysteine ethyl ester (HAC) in comparison to unmodified one. The ex vivo transport studies exhibit 1.9-fold enhancement for SF, 1.31-fold enhancement for Rhodamine123, and 1.3-fold enhancement for FD4 with HAC in comparison to unmodified one, respectively. Thus, the promising results encourage further investigations and exploitation of this versatile polysaccharide.

Preactivated thiomers: their role in drug delivery

INTRODUCTION

Since thiolated polymers - known as thiomers - have entered the pharmaceutical arena in the late 1990 s, more and more academic and industrial research groups have started to work with these promising polymeric excipients. Meanwhile, various thiomers are the subject of clinical trials and the first product based on thiolated chitosan will reach the market in 2015. Due to the formation of disulfide bonds with mercaptopyridine substructures, thiol groups of thiomers are on the one hand more reactive and on the other hand are protected toward oxidation. These so-called preactivated thiomers representing the second generation of thiomers are subject of this review.

AREAS COVERED

Within this review, preactivated thiomers are classified and their mode of action is described. Furthermore, different synthetic pathways, purification and chemical characterization methods of preactivated thiomers are explained. Their properties including mucoadhesive, permeation-enhancing, efflux pump inhibitory and in situ gelling properties are described. In addition, various formulations based on preactivated thiomers are introduced.

EXPERT OPINION

The first-generation thiomers have already shown great potential resulting in various product developments. Preactivated thiomers - representing the second generation of thiomers - offer the additional advantage of even comparatively more reactive sulfhydryl ligands and of stability toward oxidation. According to this, they are promising novel polymeric excipients for various applications.

Thiomers: A Blessing to Evaluating Era of Pharmaceuticals

Thiomers are the polymers modified for the mucoadhesive properties and other additive properties by incorporating thiol moieties in the backbone of the unmodified polymeric chain by substitution reactions or simple oxidation reactions. Drugs that are less soluble and permeable can be complexed with thiomers for their increased absorption through the mucosal membranes by increase in contact time and prolonged stay in body due to mucoadhesion. Immobilization of thiol group therefore increases the mucoadhesive properties of the modified polymer by 2–140-folds. The prepared thiomers are characterized and made stable by different techniques. Thiomers also give the controlled delivery of the active pharmaceutical ingredients in the body. Different polymers that are modified by thiolation are chitosan, polyacrylic acid, sodium alginate, sodium carboxy methyl cellulose, guar gum, and so forth. Thiomeric formulations are a challenge to deliver drugs with low therapeutic compatibility. Micro- and nanopreparations containing thiomers can be prepared by different techniques such as covalent crosslinking, in situ gelation, radical emulsion polymerization, and emulsification. Nowadays thiomers have wide range of applications as a promising pharmaceutical excipient in the evaluating era of pharmaceutical technology.

Strategies for improving mucosal drug delivery

Within this review we will provide a comprehensive understanding in order to improve existing strategies and to develop new systems to lower the barrier for improving mucosal drug delivery. Mucosal administration of drugs achieves a therapeutical effect as the permeation of significant amounts of a drug is permitted through the absorption membrane. The absorption membrane relies on the mucosal layer and the epithelial tissue. In order to overcome barriers, drug delivery systems have to exhibit various functions and features, such as mucoadhesive and protective activity, solubility improving, permeation and uptake enhancing, and drug release controlling properties. This review also aims to provide an insight of well-distinguished strategies to date, as well as provide a focus on the enhancement of membrane permeability. Furthermore, since the development and functions of drug delivery systems exert a high influence on the ability of drug permeation through membrane, these considerations will also be discussed in this review.

Thiomers--from bench to market

Thiolated polymers or designated thiomers are obtained by immobilization of sulhydryl bearing ligands on the polymeric backbone of well-established polymers such as poly(acrylates) or chitosans. This functionalization leads to significantly improved mucoadhesive properties compared to the corresponding unmodified polymers, as disulfide bonds between thiol groups of thiomers and cysteine-rich glycoproteins of the mucus gel layer are formed. Furthermore, enzyme- and efflux-pump inhibiting as well as improved permeation-enhancing properties are advantages of thiolization. By the covalent attachment of mercaptonicotinamide substructures via disulfide bonds to thiolated polymers these properties are even substantially further improved and stability towards oxidation even in aqueous media can be provided. Meanwhile, more than 50 research groups worldwide are working on thiolated polymers. For certain thiomers the scale up process for industrial production has already been done and GMP material is available. Furthermore, safety of thiolated poly(acrylic acid), thiolated chitosan and thiolated hyaluronic acid could be demonstrated via orientating studies in human volunteers and via various clinical trials. The first product (Lacrimera® eye drops, Croma-Pharma) containing a chitosan-N-acetylcysteine conjugate for treatment of dry eye syndrome will enter the European market this year. It is the only product providing a sustained protective effect on the ocular surface due to its comparatively much more prolonged residence time worldwide. Various further products utilizing, for instance, thiolated hyaluronic acid in ocular surgery are in the pipeline.

Synthesis and characterization of pH tolerant and mucoadhesive (thiol-polyethylene glycol) chitosan graft polymer for drug delivery

The objective of this study was to generate a water-soluble thiolated chitosan to enable the permeation-enhancing effect of chitosan at pH of at least 5.5 without losing the advantages of improved mucoadhesive properties. Therefore, the thiol-bearing polyoxyethylene ligand {O-(3-carboxylpropyl)-O'-[2-[3-mercaptopropionylamino)ethyl]-polyethyleneglycol} was conjugated via amide bond formation to the amino group of chitosan. Resulting novel chitosan derivative (Chito-PEG-SH) exhibited 250 μmol free thiol groups per gram polymer. By the attachment of the thiol-bearing PEG ligand, an improvement of permeation-enhancing effect on rat intestine (2.7-fold improvement) as well as on a Caco-2 monolayer model (1.9-fold improvement) could be found. Cytotoxicity studies on Caco-2 cells revealed no change in biocompatibility. Mucoadhesion was improved 3.1-fold by the formation of disulfide bonds with mucus glycoproteins. The mucoadhesive effect of Chito-PEG-SH turned out to be similar to thiolated chitosan and more pronounced than mucoadhesive properties of unmodified chitosan. The graft polymer is soluble in water and aqueous solutions over a broad pH range. In aqueous media, the novel polymer does not precipitate at pH of 8.6 or less. According to these results, Chito-PEG-SH might show potential as auxiliary agent in oral drug delivery where its solubility even up to pH 8 is likely beneficial.

Approaches for enhancing oral bioavailability of peptides and proteins

Oral delivery of peptide and protein drugs faces immense challenge partially due to the gastrointestinal (GI) environment. In spite of considerable efforts by industrial and academic laboratories, no major breakthrough in the effective oral delivery of polypeptides and proteins has been accomplished. Upon oral administration, gastrointestinal epithelium acts as a physical and biochemical barrier for absorption of proteins resulting in low bioavailability (typically less than 1-2%). An ideal oral drug delivery system should be capable of (a) maintaining the integrity of protein molecules until it reaches the site of absorption, (b) releasing the drug at the target absorption site, where the delivery system appends to that site by virtue of specific interaction, and (c) retaining inside the gastrointestinal tract irrespective of its transitory constraints. Various technologies have been explored to overcome the problems associated with the oral delivery of macromolecules such as insulin, gonadotropin-releasing hormones, calcitonin, human growth factor, vaccines, enkephalins, and interferons, all of which met with limited success. This review article intends to summarize the physiological barriers to oral delivery of peptides and proteins and novel pharmaceutical approaches to circumvent these barriers and enhance oral bioavailability of these macromolecules.