Degradation of proteins by guinea pig intestinal enzymes

Lipid-Based Nanoparticles as Oral Drug Delivery Systems: Overcoming Poor Gastrointestinal Absorption and Enhancing Bioavailability of Peptide and Protein Therapeutics

Delivery and formulation of oral peptide and protein therapeutics have always been a challenge for the pharmaceutical industry. The oral bioavailability of peptide and protein therapeutics mainly relies on their gastrointestinal solubility and permeability which are affected by their poor membrane penetration, high molecular weight and proteolytic (chemical and enzymatic) degradation resulting in limited delivery and therapeutic efficacy. The present review article highlights the challenges and limitations of oral delivery of peptide and protein therapeutics focusing on the application, potential and importance of solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) as lipid-based drug delivery systems (LBDDSs) and their advantages and drawbacks. LBDDSs, due to their lipid-based matrix can encapsulate both lipophilic and hydrophilic drugs, and by reducing the first-pass effect and avoiding proteolytic degradation offer improved drug stability, dissolution rate, absorption, bioavailability and controlled drug release. Furthermore, their small size, high surface area and surface modification increase their mucosal adhesion, tissue-targeted distribution, physiological function and half-life. Properties such as simple preparation, high-scale manufacturing, biodegradability, biocompatibility, prolonged half-life, lower toxicity, lower adverse effects, lipid-based structure, higher drug encapsulation rate and various drug release profile compared to other similar carrier systems makes LBDDSs a promising drug delivery system (DDS). Nevertheless, undesired physicochemical features of peptide and protein drug development and discovery such as plasma stability, membrane permeability and circulation half-life remain a serious challenge which should be addressed in future.

In vitro fermentation potential of gut endogenous protein losses of growing pigs

Fermentation of dietary and endogenous protein in the hindgut is generally considered detrimental to the health of pigs. We investigated the in vitro fermentation potential of porcine endogenous protein in ileal digesta and colonic mucus, using a N-free buffer with an excess of fermentable carbohydrates. Urea, whey protein isolate (WPI, positive control), WPI hydrolysate (WPIH), and combinations of the latter two were used to validate the assay. A new biphasic model, including a linear end simulation, fitted to the gas production data over a 48-h period identified the time point when substrate fermentation ended. A higher degree of hydrolysis of WPI resulted in a higher maximum gas production rate (Rmax, P < 0.01). Differences in Rmax and the time required to reach Rmax were observed among ileal digesta samples, with Rmax increasing with the insoluble protein content, and the highest Rmax occurring with colonic mucus samples (P < 0.05). The endogenous proteins entering the large intestine of pigs can ferment more rapidly compared to highly soluble and digestible protein sources, with Rmax positively correlated with decreasing solubility of endogenous nitrogenous components.

Assessment of hydrophobic-ion paired insulin incorporated SMEDDS for the treatment of diabetes mellitus

To overcome the low oral bioavailability of insulin, we hypothesized that the insulin-hydrophobic ion pairing (HIP) complex incorporated self-microemulsifying drug delivery system (SMEDDS) would be beneficial. In the present study, an oral insulin delivery system was developed and estimated using the HIP technique and SMEDDS. Further insulin-HIP complexes were characterized using various spectroscopical techniques. Additionally, insulin-HIP complexes were subjected to analysis of complexes' conformational stability in the real physiological solution using computational approaches. On the other hand, in vitro, and in vivo studies were carried out to investigate the permeability and hypoglycemic effect. Subsequently, in an in vitro non-everted gut sac study, the apparent permeability coefficient (P_app) was approximately 8-fold higher in the colon than in the jejunum, and the HIP-incorporated SMEDDS showed an approximately 3-fold higher P_app value than the insulin solution. The hypoglycemic effect after in situ colon instillation, the HIP complex between insulin and sodium docusate-incorporated SMEDDS showed a pharmacological availability of 2.52 ± 0.33 % compared to the subcutaneously administered insulin solution. Thus, based on these outcomes, it can be concluded that the selection of appropriate counterions is important in developing HIP-incorporated SMEDDS, wherein this system shows promise as a tool for oral peptide delivery systems.

An Update on Pharmaceutical Strategies for Oral Delivery of Therapeutic Peptides and Proteins in Adults and Pediatrics

While each route of therapeutic drug delivery has its own advantages and limitations, oral delivery is often favored because it offers convenient painless administration, sustained delivery, prolonged shelf life, and often lower manufacturing cost. Its limitations include mucus and epithelial cell barriers in the gastrointestinal (GI) tract that can block access of larger molecules including Therapeutic protein or peptide-based drugs (TPPs), resulting in reduced bioavailability. This review describes these barriers and discusses different strategies used to modify TPPs to enhance their oral bioavailability and/or to increase their absorption. Some seek to stabilize the TTPs to prevent their degradation by proteolytic enzymes in the GI tract by administering them together with protease inhibitors, while others modify TPPs with mucoadhesive polymers like polyethylene glycol (PEG) to allow them to interact with the mucus layer, thereby delaying their clearance. The further barrier provided by the epithelial cell membrane can be overcome by the addition of a cell-penetrating peptide (CPP) and the use of a carrier molecule such as a liposome, microsphere, or nanosphere to transport the TPP-CPP chimera. Enteric coatings have also been used to help TPPs reach the small intestine. Key efficacious TPP formulations that have been approved for clinical use will be discussed.

The role of polysaccharides from natural resources to design oral insulin micro- and nanoparticles intended for the treatment of Diabetes mellitus: A review

Oral administration of insulin (INS) would represent a revolution in the treatment of diabetes, considering that this route mimics the physiological dynamics of endogenous INS. Nano- and microencapsulation exploiting the advantageous polysaccharides properties has been considered an important technological strategy to protect INS against harsh conditions of gastrointestinal tract, in the same time that improve the permeability via transcellular and/or paracellular pathways, safety and in some cases even selectivity for targeting delivery of INS. In fact, some polysaccharides also give to the systems functional properties such as pH-responsiveness, mucoadhesiveness under specific physiological conditions and increased intestinal permeability. In general, all polysaccharides can be functionalized with specific molecules becoming more selective to the cells to which INS is delivered. The present review highlights the advances in the past 10 years on micro- and nanoencapsulation of INS exploiting the unique natural properties of polysaccharides, including chitosan, starch, alginate, pectin, and dextran, among others.

How to design the surface of peptide-loaded nanoparticles for efficient oral bioavailability?

The oral administration of proteins is a current challenge to be faced in the field of therapeutics. There is currently much interest in nanocarriers since they can enhance oral bioavailability. For lack of a clear definition, the key characteristics of nanoparticles have been highlighted. Specific surface area is one of these characteristics and represents a huge source of energy that can be used to control the biological fate of the carrier. The review discusses nanocarrier stability, mucus interaction and absorption through the intestinal epithelium. The protein corona, which has raised interest over the last decade, is also discussed. The universal ideal surface is a myth and over-coated carriers are not a solution either. Besides, common excipients can be useful on several targets. The suitable design should rather take into account the composition, structure and behavior of unmodified nanomaterials.

Oral delivery of insulin for treatment of diabetes: status quo, challenges and opportunities

Objectives

Diabetes mellitus is characterised by progressive β-cell destruction and loss of function, or loss of ability of tissues to respond to insulin. Daily subcutaneous insulin injection is standard management for people with diabetes, although patient compliance is hard to achieve due to the inconvenience of injections, so other forms of delivery are being tested, including oral administration. This review summarises the developments in oral insulin administration.

Methods

The PubMed database was consulted to compile this review comparing conventional subcutaneous injection of insulin to the desired oral delivery.

Key findings

Oral administration of insulin has potential benefits in reducing pain and chances of skin infection, improving the portal levels of insulin and avoiding side effects such as hyperinsulinemia, weight gain and hypoglycaemia. Although oral delivery of insulin is an ideal administration route for patients with diabetes, several physiological barriers have to be overcome. An expected low oral bioavailability can be attributed to its high molecular weight, susceptibility to enzymatic proteolysis and low diffusion rate across the mucin barrier.

Conclusions

Strategies for increasing the bioavailability of oral insulin include the use of enzyme inhibitors, absorption enhancers, mucoadhesive polymers and chemical modification for endogenous receptor-mediated absorption. These may help significantly increase patient compliance and disease management.

Proteins and peptides: The need to improve them as promising therapeutics for ulcerative colitis

The present review briefly describes the nature, type and pathogenesis of ulcerative colitis, and explores the potential use of peptides and proteins in the treatment of inflammatory bowel disease, especially ulcerative colitis. Intestinal absorption and the barrier mechanism of peptide and protein drugs are also discussed, with special emphasis on various strategies which make these drugs better therapeutics having high specificity, potency and molecular targeting ability. However, the limitation of such therapeutics are oral administration, poor pharmacokinetic profile and decreased bioavailability. The recent findings illustrated in this review will be helpful in designing the peptide/protein drugs as a promising treatment of choice for ulcerative colitis.

Nanoparticles for oral delivery: targeted nanoparticles with peptidic ligands for oral protein delivery

As the field of biotechnology has advanced, oral protein delivery has also made significant progress. Oral delivery is the most common method of drug administration with high levels of patient acceptance. Despite the preference of oral delivery, administration of therapeutic proteins has been extremely difficult. Increasing the bioavailability of oral protein drugs to the therapeutically acceptable level is still a challenging goal. Poor membrane permeability, high molecular weight, and enzymatic degradation of protein drugs have remained unsolved issues. Among diverse strategies, nanotechnology has provided a glimpse of hope in oral delivery of protein drugs. Nanoparticles have advantages, such as small size, high surface area, and modification using functional groups for high capacity or selectivity. Nanoparticles with peptidic ligands are especially worthy of notice because they can be used for specific targeting in the gastrointestinal (GI) tract. This article reviews the transport mechanism of the GI tract, barriers to protein absorption, current status and limitations of nanotechnology for oral protein delivery system.

Oral colon delivery of insulin with the aid of functional adjuvants

Oral colon delivery is currently considered of importance not only for the treatment of local pathologies, such as primarily inflammatory bowel disease (IBD), but also as a means of accomplishing systemic therapeutic goals. Although the large bowel fails to be ideally suited for absorption processes, it may indeed offer a number of advantages over the small intestine, including a long transit time, lower levels of peptidases and higher responsiveness to permeation enhancers. Accordingly, it has been under extensive investigation as a possible strategy to improve the oral bioavailability of peptide and protein drugs. Because of a strong underlying rationale, most of these studies have focused on insulin. In the present review, the impact of key anatomical and physiological characteristics of the colon on its viability as a protein release site is discussed. Moreover, the main formulation approaches to oral colon targeting are outlined along with the design features and performance of insulin-based devices.

Oral delivery of therapeutic protein/peptide for diabetes--future perspectives

Diabetes is a metabolic disease and is a major cause of mortality and morbidity in epidemic proportions. A type I diabetic patient is dependent on daily injections of insulin, for survival and also to maintain a normal life, which is uncomfortable, painful and also has deleterious effects. Extensive efforts are being made worldwide for developing noninvasive drug delivery systems, especially via oral route. Oral route is the most widely accepted means of administration. However it is not feasible for direct delivery of peptide and protein drugs. To overcome the gastro-intestinal barriers various types of formulations such as polymeric micro/nanoparticles, liposomes, etc. are investigated. In the recent years lot of advances have taken place in developing and understanding the oral peptide delivery systems. Simultaneously, the development and usage of other peptides having anti-diabetic potentials are also considered for diabetes therapy. In this review we are focusing on the advances reported during the past decade in the field of oral insulin delivery along with the possibility of other peptidic incretin hormones such as GLP-1, exendin-4, for diabetes therapy.

Degradation of teriparatide by gastro-intestinal proteolytic enzymes

Teriparatide, a recombinant parathyroid hormone (1-34) is the first approved agent for the treatment of osteoporosis that stimulates new bone formation. Currently, the drug is administered daily by s.c. injection. Because of the obvious advantages of oral teriparatide administration, the development of such a delivery system would be of great benefit. Besides other barriers, the enzymatic barrier caused by gastro-intestinal (GI) proteolytic enzymes is believed to be responsible for negligible teriparatide oral bioavailability. It was therefore the aim of the study to evaluate the stability of teriparatide towards a variety of GI proteases under physiological conditions. Results indicate that teriparatide is entirely degraded by trypsin, chymotrypsin and pepsin within 5 min. In contrast, even after 3 h of incubation with elastase about 85% of undegraded teriparatide could still be detected. Within an incubation period of 3 h in the presence of rat small intestinal mucosa, approximately half of the teriparatide was degraded. Experiments with isolated aminopeptidase N demonstrated that this membrane bound peptidase is primarily involved in the degradation process. Results gained from and recorded in this study provide a precise characterisation of the enzymatic barrier for oral teriparatide administration and represents a prerequisite for the development of oral teriparatide delivery systems.

Pharmacokinetic modeling of absorption behavior of 9-aminocamptothecin (9-AC) released from colon-specific HPMA copolymer-9-AC conjugate in rats

PURPOSE

To quantitate and predict colon-specific 9-aminocamptothecin (9-AC) release from the N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-9-AC conjugate and its absorption behavior after oral administration in rats.

METHODS

Drug distribution in the gastrointestinal (GI) tract and the plasma concentration-time profile of 9-AC released from the HPMA copolymer conjugate were predicted using the degradation, transit, and absorption rate constants in cecum. The fate of 9-AC in cecum and liver was measured by in-situ cecum absorption and liver perfusion.

RESULTS

Following oral administration of the conjugate, 9-AC was released rapidly in cecum. Based on the pharmacokinetic model, up to 60% of the dose was in the cecum at ~6 h, and 7% of the dose still remained there at 24 h. The predicted plasma concentration curve for released 9-AC after an oral dose of 3 mg/kg of 9-AC equivalent increased gradually and reached a peak of 98 nM at 7 h, then started decreasing slowly to 16 nM at 24 h. The bioavailability value was estimated as 0.31 after the first-pass elimination.

CONCLUSIONS

A pharmacokinetic model delineated the impact of GI transit, drug absorption rate, and first-pass metabolism on drug disposition following oral administration of HPMA copolymer-9-AC conjugate in rats.

Azo compounds in colon-specific drug delivery

Azo compounds have the potential to act as drug carriers that facilitate the selective release of therapeutic agents to the colon, and also to effect the oral administration of those macromolecular drugs that require colon-specific drug delivery. With some further research-driven refinements, these materials may lead to more efficient treatments for local conditions, such as colonic cancer or inflammatory bowel disease. This article provides an overview of the azo-based systems developed to date, identifies the requirements for an ideal carrier, and highlights the directions for further developments in the field of azo group-facilitated colonic delivery.

Design of gelatin nanoparticles as swelling controlled delivery system for chloroquine phosphate

Gelatin nanoparticles were prepared by a single W/O emulsion technique and characterized by infrared (IR) spectra, scanning electron microscopy (SEM) and particle size analysis. The prepared nanoparticles were loaded with chloroquine phosphate (CP), a well known antimalarial drug, and the release dynamics of entrapped drug was investigated as a function of various experimental factors such as percent loading of the drug, chemical architecture of the nanocarriers, and pH, temperature, ionic strength and nature of the release medium. The nanoparticles were also studied for their water sorption capacity by optical microscopic method taking advantage of the aggregation of nanoparticles. The drug release processes was analyzed kinetically using Ficks power law and a correlation was established between the quantity of released drug and swelling of the nanoparticles.

Synthesis and characterization of novel aromatic azo bond-containing pH-sensitive and hydrolytically cleavable IPN hydrogels

Novel interpenetrating network (IPN) hydrogels, composed of pH-sensitive, aromatic azo group containing network as one component (Network A), and a hydrolyzable network as the other (Network B), were prepared by a sequential process. The first network was formed by crosslinking of a reactive polymer precursor (copolymer of N,N-dimethylacrylamide, acrylic acid, N-tert.butylacrylamide, and N-methacryloylglycylglycine p-nitrophenyl ester) with an aromatic azo group containing diamine ((N,N'-epsilon-aminocaproyl)-4,4'-diaminoazobenzene). The second network was formed by radical crosslinking copolymerization of N-(2-hydroxypropyl)methacrylamide with N,O-dimethacryloylhydroxylamine. The composition of the hydrogels was manipulated to determine the influence of hydrogel composition on the equilibrium degree of swelling, modulus of elasticity in compression, and on the rate of degradation of Network B. Modeling of network structure was accomplished using the statistical branching theory. The major advantage of IPN hydrogels, when compared to traditional pH-sensitive networks, is the linear swelling profile following abrupt change of pH from 2 to 7.4. This indicates the suitability of IPN as carriers for oral drug delivery.

Peptidase activity on the surface of the porcine buccal mucosa

Peptide drugs in buccal bioadhesive delivery systems are exposed to the surface of the buccal mucosa at high concentrations over long periods of time. The peptidase activity on the surface of the buccal mucosa has not been evaluated as a barrier to peptide buccal delivery. The in vitro stability of various synthetic substrates on the surface of intact porcine buccal mucosa was determined. No carboxypeptidase or dipeptidyl peptidase IV activity was detected on the buccal mucosa, while aminopeptidase N activity was detected using Leu-p-nitroanilide. No endopeptidase activity was observed towards the peptide substrates. Insulin and insulin B-chain were intact at the 2 h time point at 37 degrees C, while the percent of parent Leu-enkephalin remaining was 18+/-9 (mean+/-S.D., n=9). In the presence of aminopeptidase inhibitors, amastatin, sodium deoxycholate and EDTA, the degradation of Leu-enkephalin was dramatically reduced. This work suggests that the buccal route maybe advantageous for the delivery of peptides that are susceptible to such activities. The inclusion of aminopeptidase inhibitors in buccal bioadhesive delivery systems could improve buccal bioavailability of Leu-enkephalin. We suggest that compared with the existing in vitro metabolism methods, the analysis of peptide or protein metabolism on intact buccal mucosa could better predict the degradation of the drug as it crosses the tissue.

Design of novel bioconjugates for targeted drug delivery

This paper summarizes recent work on the design and development of targeted polymeric bioconjugates based on N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers. Polymerizable antibody Fab' fragment (MA-Fab') has been developed and used in the preparation of targeted HPMA copolymer-mesochlorin e6 conjugates for the treatment of human ovarian carcinomas. The reactivity of the MA-Fab' in copolymerization with HPMA depended on the length of the spacer between the monomer double bond and the antibody Fab' fragment. The biological activity of the antibody Fab' fragment was maintained after incorporation into the HPMA copolymer. Novel aromatic azo spacers were designed and incorporated into HPMA copolymer-drug (cyclosporin A, 9-aminocamptothecin) conjugates for the colon-specific drug delivery and for the treatment of colon diseases. The colon-specific drug release from the conjugates was controlled by the structures of both drug and spacers. Lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), were conjugated to the colon-specific polymer drug conjugates to enhance specific adhesion onto colon tissues.

Biorecognizable HPMA copolymer-drug conjugates for colon-specific delivery of 9-aminocamptothecin

N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates for colon-specific delivery of 9-aminocamptothecin (9-AC) were designed. They hold 9-AC bound via spacers containing amino acid residues and aromatic azo bonds. In vitro release profiles of 9-AC from HPMA copolymer conjugates were evaluated under artificial conditions that simulated large intestinal azoreductase and peptidase activities. The studies indicated that the azo bond was reduced first, followed by the release of unmodified 9-AC from the 9-AC containing fragment by peptidases. Release profiles depended on the chemical structure of the peptide part of the spacer. Conjugates containing leucylalanine showed high colon-specific release of 9-AC when compared to alanine containing conjugates. It appears that the studied conjugates are suitable as colon-specific drug delivery systems.

Thiolated carboxymethylcellulose: in vitro evaluation of its permeation enhancing effect on peptide drugs

The purpose of this study was to evaluate the effect of sodium carboxymethylcellulose (NaCMC) and carboxymethylcellulose-cysteine (CMC-Cys) conjugates on the intestinal permeation of sodium fluorescein (NaFlu) and model peptide drugs, bacitracin and insulin. Cysteine was covalently linked to carbodiimide activated NaCMC. Iodometric titration of the polymer conjugates was used to determine the extent of immobilised cysteine. Permeation studies were performed on guinea pig small intestinal mucosa mounted in Ussing-type chamber. Unmodified NaCMC (1% m/v) significantly improved the transport ratio (R= P(app) polymer/ P(app) control) of NaFlu to 1.3 and 1% (m/v) NaCMC conjugated with cysteine further enhanced the permeation. Cysteine conjugation at 3.6, 5.3 and 7.3% (m/m) resulted in R-values of 1.4, 1.7 and 1.8, respectively. Decreasing the concentration of CMC-Cys, exhibiting 7.3% (m/m) of immobilised cysteine (CMC-Cys7.3) from 1% (m/v) to 0.5% (m/v) decreased the R-value of NaFlu from 1.8 to 1.2. NaCMC at 1% (m/v) in the presence of free cysteine had no significant effect on the R-value of NaFlu compared to NaCMC alone. Formulation of fluorescence labelled bacitracin and insulin in unconjugated NaCMC (1% m/v) did not significantly improve the permeation, however in the presence of 1% (m/v) CMC-Cys7.3 a significantly improved permeation was observed (R= 1.3). Conjugation at NaCMC with cysteine moieties significantly improves the intestinal permeation of the hydrophilic molecule NaFlu and the model peptide drugs bacitracin and insulin in vitro, therefore this conjugated system maybe useful for peroral administration of peptide drugs in the future.

In vitro evaluation of the permeation-enhancing effect of thiolated polycarbophil

The objective of this study was to investigate the permeation-enhancing effect of thiolated polycarbophil (PCP) on peptide drugs. Mediated by a carbodiimide, increasing amounts of cysteine (Cys) were covalently bound to sodium neutralized PCP (NaPCP). The extent of covalently attached Cys was determined by quantifying the share of thiol groups on the resulting polymer-Cys conjugates via iodometric titration. The permeation-enhancing effect of polymer-Cys conjugates was evaluated in Ussing-type chambers using intestinal mucosa from guinea pigs. Whereas the transport enhancement ratio (P(app) polymer/P(app) control) for 0.5% (m/v) NaPCP was 1.14 using sodium fluorescein as model drug, it was 1.63 for 0.5% (m/v) PCP-Cys displaying a share of 2.2% (m/m) Cys on the conjugate (PCP-Cys 2.2%). Moreover, the substitution of sodium fluorescein by bacitracin-fluorescein isothiocyanate (bacitracin-FITC) led to ratios of 1.03 and 1.36 and in the case of insulin-fluorescein isothiocyanate (insulin-FITC) to ratios of 1.07 and 1.33, respectively (means; n = 3). Additional permeation studies with 0.5% (m/v) PCP-Cys conjugates exhibiting a share of 1.8% up to 4.2% of cysteine showed enhancement ratios of 1.22 up to 1.47 for sodium fluorescein within 3 h. In contrast, the permeation-enhancing effect of PCP could not be improved by the addition of free unconjugated Cys. Because of their permeation-enhancing effect for the paracellular route of absorption, PCP-Cys conjugates probably represent a new tool for the peroral administration of peptide drugs.

Oral peptide drug delivery: polymer-inhibitor conjugates protecting insulin from enzymatic degradation in vitro

A drug-carrier matrix has been developed which protects embedded insulin from degradation by the luminally secreted serine-proteases trypsin (EC 3.4.21.4), chymotrypsin (EC 3.4.21.1) and elastase (EC 3.4.21.36) in vitro. Increasing amounts of the Bowman-Birk inhibitor (BBI) and elastatinal, respectively, were thereby covalently bound to the mucoadhesive polymer sodium carboxymethylcellulose (Na-CMC). The inhibitory efficacy of resulting polymers was evaluated. On the one hand, all polymer-BBI conjugates showed a strong inhibitory activity towards trypsin and chymotrypsin whereas it was markedly lower towards elastase. The polymer-elastatinal conjugates, on the other hand, displayed a comparatively higher inhibitory activity towards elastase. In an artificial intestinal fluid containing trypsin, chymotrypsin and elastase in physiological concentrations insulin, being incorporated in unmodified Na-CMC, was rapidly degraded at 37 degrees C. Within 1 h 98.7 +/- 0.4% (mean +/- SD, n = 3) of the peptide drug were thereby metabolized. On the contrary, the incorporation of insulin in a mixture of the two polymer-inhibitor conjugates CMC-BBI (40%; w/w) and CMC-elastatinal conjugate (60%; w/w) led to a peptide degradation of 22.3 +/- 2.5% (mean +/- SD, n = 3) within the same time period. Even after 4 h of incubation, 33.6 +/- 3.2% (mean +/- SD, n = 3) of the therapeutic agent remained stable towards enzymatic attack. Hence, the polymer-inhibitor conjugates described in this study seem to be a useful tool in overcoming the luminal enzymatic barrier in peroral insulin delivery.

Chitosan and its derivatives: potential excipients for peroral peptide delivery systems

In the 1990s chitosan turned out to be a useful excipient in various pharmaceutical formulations. By modifications of the primary amino group at the 2-position of this poly(beta1-->4 D-glucosamine), the features of chitosan can even be optimised according to a given task in drug delivery systems. For peroral peptide delivery these tasks focus on overcoming the absorption (I) and enzymatic barrier (II) of the gut. On the one hand, even unmodified chitosan proved to display a permeation enhancing effect for peptide drugs. On the other hand, a protective effect for polymer embedded peptides towards degradation by intestinal peptidases can be achieved by the immobilisation of enzyme inhibitors on the polymer. Whereas serine proteases are inhibited by the covalent attachment of competitive inhibitors such as the Bowman-Birk inhibitor, metallo-peptidases are inhibited by chitosan derivatives displaying complexing properties such as chitosan-EDTA conjugates. In addition, because of the mucoadhesive properties of chitosan and most of its derivatives, a presystemic metabolism of peptides on the way between the dosage form and the absorption membrane can be strongly reduced. Based on these unique features, the co-administration of chitosan and its derivatives leads to a strongly improved bioavailability of many perorally given peptide drugs such as insulin, calcitonin and buserelin. These polymers are therefore useful excipients for the peroral administration of peptide drugs.

Synthesis of bioadhesive lectin-HPMA copolymer-cyclosporin conjugates

An amino group containing cyclosporin A (CsA) derivative has been synthesized and conjugated to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer via an aromatic azo bond, which can be specifically cleaved by azoreductase activity in colon to release the drug for the treatment of colon diseases. Lectins, peanut (Arachis hypogea) agglutinin (PNA) and wheat germ agglutinin (WGA), have been conjugated to HPMA copolymer-CsA derivative conjugates (PCsA), respectively, to give bioadhesive conjugates. The PNA and WGA are the targeting proteins that can bind to diseased colon tissue and healthy tissue, respectively. There were on average four P(CsA) copolymer chains attached on one WGA molecule with a drug content of 16.0 wt % and five P(CsA) copolymer chains attached on one PNA molecule with a drug content of 11.5 wt %. The incubation of a P(CsA) copolymer with the rat cecal contents resulted in the cleavage of the azo bond and release of the cyclosporin derivative. The biological evaluation of the conjugates is under way.

Effect of calcium concentration, hardening agent and drying condition on release characteristics of oral proteins from calcium pectinate gel beads

Pectin has been investigated for its ability to produce solid calcium pectinate gel (CPG) beads containing bovine serum albumin (BSA). Several factors can influence the properties and release characteristics of the CPG beads. In this study, the effect of calcium concentration, hardening agent and drying condition on the encapsulation and release characteristics of BSA from the matrix gel beads made of calcium pectinate were studied. BSA release studies under conditions mimicking mouth to colon transit have shown that calcium pectinate protects the drug from being released completely in the physiological environment of the upper gastrointestinal tract, and is susceptible to the enzymatic action with consequent drug release. In addition, the release of BSA from CPG beads was strongly affected by calcium concentration and drying condition. However, the release was not particularly affected by the presence of hardening agent at the concentration of 1% or lower. Since the release of BSA as a model protein drug could be controlled by the regulation of the preparation conditions of CPG beads, the CPG beads may be used for a potential oral controlled release system for protein drugs.

The use of inhibitory agents to overcome the enzymatic barrier to perorally administered therapeutic peptides and proteins

The peroral administration of peptide drugs is a major challenge to pharmaceutical science. In order to provide a sufficient bioavailability of these therapeutic agents after oral dosing, several barriers encountered with the gastrointestinal (GI) tract have to be overcome by a suitable galenic. One of these barriers is caused by proteolytic enzymes, leading to a severe presystemic degradation in the GI tract. Besides some other strategies to overcome the so-called enzymatic barrier, the use of inhibitory agents has gained considerable scientific interest, as various in vivo studies could demonstrate a significantly improved bioavailability of therapeutic peptides and proteins, due to the co-administration of such excipients. In vitro techniques to evaluate the actual potential of inhibitory agents incubation with pure proteases, freshly collected gastric or intestinal fluids, mucosal homogenates, brush border vesicles and freshly excised mucosa. In situ techniques are based on single-pass perfusion studies cannulating different intestinal segments and determining the amount of undegraded model drug in perfusion solutions or blood. For in vivo studies, insulin is mostly used as a model drug, offering the advantage of a well-established method to evaluate the biological response after oral dosing by determining the decrease in blood glucose level. Generally, inhibitory agents can be divided into: inhibitors which are not based on amino acids (I), such as p-aminobenzamidine, FK-448 and camostat mesilate; amino acids and modified amino acids (II), such acid derivatives; peptides and modified peptides (III), e.g. bacitracin, antipain, chymostatin and amastatin; and polypeptide protease inhibitors (IV), e.g. aprotinin, Bowman-Birk inhibitor and soybean trypsin inhibitor. Furthermore, complexing agents and some mucoadhesive polymers also display enzyme inhibitory activity. Drawbacks of inhibitory agents, such the risk of toxic side effects or high production costs, might be excluded by the development of advanced drug delivery systems. Initial steps in this direction can be seen in the development of delivery system containing mucoadhesive polymers providing an intimate contact to the mucosa, thereby reducing the drug degradation between delivery system and absorbing membrane, controlled release systems which provide a simultaneous release of drug and inhibitor and in the immobilisation of enzyme inhibitors on delivery systems.

Targeting drugs to the colon: delivery systems for oral administration

The oral delivery of drugs to the colon has applications in a variety of therapeutic areas. This review is concerned with the approaches taken to achieve a universal system for delivery. The design of such a system requires the identification and exploitation of a unique feature of the colonic environment. The use of transit times, pH and bacterial enzymes are critically assessed. In addition, the system must provide protection for the drug during transit to the colon. Upper gastro-intestinal physiology and the transit of pharmaceuticals through these regions are reviewed with reference to their relevance in achieving site specificity.