Enhancement of the intestinal absorption of peptides and nonpeptides

Oral administration of peptides and proteins: nanoparticles and cyclodextrins as biocompatible delivery systems

This review discusses drawbacks to peptide and protein oral formulations related to these drugs’ chemical and physical instability. Means used to overcome such limitations are mentioned and discussed in parallel with manufacturing considerations, metabolism, absorption mechanisms and the efflux systems that peptides and proteins experience as they travel through the gastrointestinal tract. Special focus is given to the use of delivery systems based on nanoparticles and cyclodextrins. Advantages of these systems relate to the protection from degradation, enhancement of absorption, targeting and controlling the release of the drug. Biodistribution and safety issues are discussed once material from the delivery system is expected to be absorbed by the body and thus interact with biological components. Operating parameters regarding nanoparticle manufacture and composition are also overviewed since nanoparticle physicochemical characteristics influence the ability to successfully entrap the intended drug as well as interaction with body.

Novel pH responsive polymethacrylic acid-chitosan-polyethylene glycol nanoparticles for oral peptide delivery

In present study, novel pH sensitive polymethacrylic acid-chitosan-polyethylene glycol (PCP) nanoparticles were prepared under mild aqueous conditions via polyelectrolyte complexation. Free radical polymerization of methacrylic acid (MAA) was carried out in presence of chitosan (CS) and polyethylene glycol (PEG) using a water-soluble initiator and particles were obtained spontaneously during polymerization without using organic solvents or surfactants/steric stabilizers. Dried particles were analyzed by scanning electron microscopy (SEM) and particles dispersed in phosphate buffer (pH 7.0) were visualized under transmission electron microscope (TEM). SEM studies indicated that PCP particles have an aggregated and irregular morphology, however, TEM revealed that these aggregated particles were composed of smaller fragments with size less than 1 micron. Insulin and bovine serum albumin (BSA) as model proteins were incorporated into the nanoparticles by diffusion filling method and their in vitro release characteristics were evaluated at pH 1.2 and 7.4. PCP nanoparticles exhibited good protein encapsulation efficiency and pH responsive release profile was observed under in vitro conditions. Trypsin inhibitory effect of these PCP nanoparticles was studied using casein substrate and these particles displayed lesser inhibitory effect than reference polymer carbopol. Preliminary investigation suggests that these particles can serve as good candidate for oral peptide delivery.

Absorption enhancement, mechanistic and toxicity studies of medium chain fatty acids, cyclodextrins and bile salts as peroral absorption enhancers

The objective of the present investigation was to evaluate an oral 'drug delivery' approach, which involves co-administration of absorption enhancers (AEs). The representative low permeable hydrophilic (biopharmaceutic classification system (BCS) Class III) drugs used in the study comprised of cefotaxime sodium and ceftazidime pentahydrate, whereas low permeable lipophilic (BCS Class IV) drugs include cyclosporin A and lovastatin. AEs from three different chemical classes, namely, medium chain fatty acids (sodium caprylate and caprate), cyclodextrins (beta-cyclodextrin, hydroxypropyl beta-cyclodextrin) and bile salts (sodium cholate and deoxycholate) were evaluated for absorption enhancement efficacy, mechanism of action and toxicity using in vitro everted intestinal sac model. These AEs were found to enhance intestinal permeability of drugs from 2- to 27-fold. Light microscopy studies of intestinal sac incubated with AEs for 120 min revealed morphological changes in absorptive mucosa and rank order of toxicity were cyclodextrins>bile salts congruent with medium chain fatty acids. Fluorescence polarization studies indicated that brush bordered membrane vesicles labeled with lipophilic (DPH, 12AS) and hydrophilic dyes (ANS), when treated with AEs exhibited concentration and time dependent decrease in fluorescence polarization. Total protein released in presence of AEs was more than control but considerably less than EDTA (0.58% w/v), which is known to cause toxic release of proteins from cell. Overall, AEs were found to significantly enhance drug permeability by decreasing lipid membrane fluidity and/or interacting with hydrophilic domains of membrane, and has the potential to improve oral delivery.

Oral Delivery of Peptide Drugs

A wide variety of peptide drugs are now produced on a commercial scale as a result of advances in the biotechnology field. Most of these therapeutic peptides are still administered by the parenteral route because of insufficient absorption from the gastrointestinal tract. Peptide drugs are usually indicated for chronic conditions, and the use of injections on a daily basis during long-term treatment has obvious drawbacks. In contrast to this inconvenient and potentially problematic method of drug administration, the oral route offers the advantages of self-administration with a high degree of patient acceptability and compliance. The main reasons for the low oral bioavailability of peptide drugs are pre-systemic enzymatic degradation and poor penetration of the intestinal mucosa. A considerable amount of research has focused on overcoming the challenges presented by these intestinal absorption barriers to provide effective oral delivery of peptide and protein drugs. Attempts to improve the oral bioavailability of peptide drugs have ranged from changing the physicochemical properties of peptide molecules to the inclusion of functional excipients in specially adapted drug delivery systems. However, the progress in developing an effective peptide delivery system has been hampered by factors such as the inherent toxicities of absorption-enhancing excipients, variation in absorption between individuals, and potentially high manufacturing costs. This review focuses on the intestinal barriers that compromise the systemic absorption of intact peptide and protein molecules and on the advanced technologies that have been developed to overcome the barriers to peptide drug absorption.

Oral Absorption Enhancement of Cromolyn Sodium Through Noncovalent Complexation

PURPOSE

To determine the effect of Sodium N-[8-(2-hydroxybenzoyl)amino]caprylate (SNAC) on the permeation of cromolyn across Caco-2 cell monolayers and explore the molecular basis for the enhanced absorption.

METHODS

Transport studies of cromolyn across Caco-2 cell monolayers were conducted in the presence of various SNAC concentrations. Permeation of cellular transport markers and lactate dehydrogenase (LDH) release were measured to evaluate cell integrity. Molecular interactions betweent the two compounds were investigated using isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR), and Fourier-transfrom infrared (FTIR) spectroscopies and molecular dynamics simulations.

RESULTS

The absorption of cromolyn across Caco-2 monolayers was enhanced markedly by SNAC. SNAC did not cause significant LDH leakage and changes in the permeation of transport markers. ITC, spectroscopies, and molecular dynamic simulations indicated the existence of intermolecular interactions between cromolyn and SNAC that involve the 2-hydroxybenzamide moiety on SNAC and weaken the hydrogen bonding between cromolyn and surrounding water molecules.

CONCLUSIONS

SNAC increases the permeability of Caco-2 monolayers to cromolyn without measurable cell damage. SNAC interacts with cromolyn mainly via ring stacking. One major mode of interaction appears to involve the insertion of the aromatic ring of SNAC between cromolyn's rings. Such interaction appears to reduce the hydration of cromolyn and thus optimize its hydrophobicity for oral absorption.

Improved paracellular uptake by the combination of different types of permeation enhancers

This study had the purpose to improve the paracellular uptake of drugs by combining the thiomer/reduced glutathione (GSH) permeation-enhancing system with a proteolytic enzyme. Due to the covalent binding of 2-iminothiolane to chitosan the thiomer chitosan-TBA (chitosan-4-thiobutylamidine) was obtained. Permeation studies were performed with freshly excised intestinal mucosa of guinea pigs mounted in Ussing-type chambers using on the one hand the low-molecular size marker flurescein (Na-Flu) and on the other hand the high-molecular size marker FITC-dextran. Apparent permeability coefficient (P(app)) as well as enhancement ratios (=P(app) permeation-enhancing system/P(app) control) were calculated. Trypsin, papain and bromelain displayed a permeation-enhancing effect for Na-Flu on the small intestinal mucosa. Enhancement ratios of 1.84, 1.63 and 1.78 were identified for 2% trypsin, 0.5% papain and 2% bromelain solutions, respectively. However, only bromelain could guarantee a significant permeation enhancement of FITC-dextran with a P(app) of 4.45+/-0.44 x 10(-6) cm/s representing an enhancement ratio of 1.57. A similar enhancement of FITC-dextran permeation was reached by the use of the chitosan-TBA (0.5%)/GSH (5%) system. Moreover, an additive permeation-enhancing effect of the chitosan-TBA/GSH system in combination with bromelain (2%) was observed, leading to a maximum P(app) of 5.91+/-0.51 x 10(-6) cm/s, which corresponds to an enhancement ratio of 2.1. According to these results, the combination of the thiomer/GSH system with bromelain might represent a new promising strategy in order to raise the in vivo efficacy of non-invasive administered hydrophilic macromolecular drugs.

Optimisation of a lipid based oral delivery system containing A/Panama influenza haemagglutinin

Vaccine antigens administered by the oral route are often degraded by gastric secretions during gastrointestinal transit. This necessitates larger and more frequent doses of antigen for vaccination. A delivery system, which overcomes this, is a lipid vesicle containing bile salts (bilosome), which prevents antigen degradation and enhances mucosal penetration. The effect of bilosome formulation modification on vaccine transit efficacy across the mucosa was determined. Specific antibody levels were assessed by end-point titre ELISA and the subclasses determined. Significant IgG1 titres were induced when the protein loading was doubled from 15 to 30 microg (P=0.009) and was equivalent to antigen administration by the subcutaneous route. No IgG2a was induced, indicating the generation of a TH2 response. Significant mucosal IgA levels were also observed with this treatment group (P=0.05).

Thiomers: potential excipients for non-invasive peptide delivery systems

In recent years thiolated polymers or so-called thiomers have appeared as a promising alternative in the arena of non-invasive peptide delivery. Thiomers are generated by the immobilisation of thiol-bearing ligands to mucoadhesive polymeric excipients. By formation of disulfide bonds with mucus glycoproteins, the mucoadhesive properties of these polymers are improved up to 130-fold. Due to formation of inter- and intramolecular disulfide bonds within the thiomer itself, dosage forms such as tablets or microparticles display strong cohesive properties resulting in comparatively higher stability, prolonged disintegration times and a more controlled release of the embedded peptide drug. The permeation of peptide drugs through mucosa can be improved by the use of thiolated polymers. Additionally some thiomers exhibit improved inhibitory properties towards peptidases. The efficacy of thiomers in non-invasive peptide delivery could be demonstrated by various in vivo studies. Tablets comprising a thiomer and pegylated insulin, for instance, resulted in a pharmacological efficacy of 7% after oral application to diabetic mice. Furthermore, a pharmacological efficacy of 1.3% was achieved in rats by oral administration of calcitonin tablets comprising a thiomer. Human growth hormone in a thiomer-gel was applied nasally to rats and led to a bioavailability of 2.75%. In all these studies, formulations comprising the corresponding unmodified polymer had only a marginal or no effect. According to these results drug carrier systems based on thiomers seem to be a promising tool for non-invasive peptide drug delivery.

Effects of superporous hydrogels on paracellular drug permeability and cytotoxicity studies in Caco-2 cell monolayers

The aim of this study was to evaluate the effect of superporous hydrogel (SPH) and SPH composite (SPHC) as permeation enhancers for peptide drug delivery on Caco-2 cell monolayers. Moreover, the cytotoxic effects of these polymers were also studied using trypan blue test, MTT assay and propidium iodide staining. Transepithelial electrical resistance (TEER) studies revealed that both SPH and SPHC polymers were able to decrease TEER values to about 40% of initial values, indicating the ability of these polymers to open tight junctions. Recovery studies of TEER showed that the effects of polymers on Caco-2 cell monolayers were reversible, indicating viability of the cells after incubation with polymers. Both polymers were able to enhance the transport of the hydrophilic marker 14C-mannitol up to 2.7 and 3.8-fold in comparison to the control group. The cumulative transport of fluorescein isothiocyanate labelled dextrans with a molecular weight of 4400 Da (FD4) and 19600 Da (FD20) was enhanced by SPH and SPHC polymers by opening of tight junctions; however, this enhancement was inversely proportional to the molecular weight of marker compounds. Cytotoxicity studies confirmed that the transport enhancing properties of SPH and SPHC polymers were not caused by damage of the Caco-2 cell monolayers. The cells were able to exclude trypan blue as well as propidium iodide after incubation with SPH and SPHC polymers. MTT assay showed that the number of viable cells was higher than 95% after incubation with SPH and SPHC polymers. This indicates that the mitochondrial metabolic activities of the cells were preserved after application of the polymers.

Evaluation of superporous hydrogel (SPH) and SPH composite in porcine intestine ex-vivo: assessment of drug transport, morphology effect, and mechanical fixation to intestinal wall

The objective of this study was to investigate the potential of superporous hydrogel (SPH) and SPH composite (SPHC) polymers to enhance the transport of N-alpha-benzoyl-L-arginine ethylester (BAEE) and fluorescein isothiocyanate-dextran 4400 (FD4) across porcine intestinal epithelium ex-vivo, and to study any possible morphological damage to the epithelium by applying these polymers. In addition, the ability of these polymers to attach to the gut wall by mechanical pressure was examined by using a specifically designed centrifuge model. The transport of BAEE and FD4 across the intestinal mucosa was enhanced 2- to 3-fold by applying SPHC polymer in comparison to negative control. No significant morphological damage was observed by applying these polymers inside the intestinal lumen. Moreover, the SPH and SPHC polymers were able to attach mechanically to the intestinal wall by swelling and did not move in the intestinal lumen even when a horizontal force of 13 gms(-2) was applied. In conclusion, these polymers are appropriate vehicles for enhancing the intestinal absorption of peptide and protein drugs.

Oral immunisation with peptide and protein antigens by formulation in lipid vesicles incorporating bile salts (bilosomes)

The ability of non-ionic surfactant vesicles to induce systemic immune responses in mice following oral immunisation was studied using a standard antigen (bovine serum albumin), a synthetic measles peptide and an influenza sub-unit vaccine. The effectiveness of this formulation was significantly increased by incorporating bile salts (in particular deoxycholate) into the formulation. We have named the resulting vesicles bilosomes. We found that the most effective immunisation protocol was to give two doses of vaccine three days apart and then repeat this protocol two weeks later. Following this method, preparation of measles peptide in bilosomes produced a specific cell mediated response, as measured by splenocyte proliferation and IL-2 production. Of particular significance, these studies demonstrate that oral administration of bilosomes incorporating the influenza sub-unit vaccine could induce as potent an antibody response as the parenterally administered vaccine containing the same quantity of antigen. In addition, the Th1/Th2 balance, as measured by antibody subclasses, was similar whether animals were immunised by the oral or the parenteral vaccine route. As bilosomes are prepared from naturally occurring lipids and have no apparent toxicity associated with their use, they represent a useful modification of conventional lipid vesicle based systems for the oral delivery of proteins and peptides.

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.

Evaluation of absorption enhancement for a potent cyclopeptidic alpha(nu)beta(3)-antagonist in a human intestinal cell line (Caco-2)

Different absorption enhancing principles for a potent cyclopeptidic alpha(nu)beta(3)-antagonist (EMD 121974) were investigated in monolayers of a human intestinal cell line (Caco-2). Transepithelial transport was quantitated by reversed-phase high-performance liquid chromatography. Cytotoxic effects were characterized by determination of transepithelial electrical resistances (TEERs), propidium iodide (PI)-influx, FITC-phalloidin staining and the release of cytosolic lactate dehydrogenase (LDH). Medium chain fatty acids (MCFAs, NaC10, NaC12) and taurocholate (NaTC) were the most efficient enhancers of cyclopeptide and FITC-dextran 4400 permeability coefficients, displaying different time profiles of activity. Whereas NaTC (15 mM) showed almost a constant permeation enhancing effect from 20 min up to 120 min (ca. 12-fold), MCFA absorption enhancement was markedly dependent on incubation time (NaC10, 20 min: 1.2-fold, 120 min: 17-fold; NaC12, 20 min: 4.3-fold, 120 min: 13-fold). All cytotoxicity assays demonstrated that MCFAs were significantly more cytotoxic than NaTC. Ion pairing with hydrophobic amino acids and heptane sulfonate distinctly increased octanol-buffer partition coefficients of the cationic cyclopeptide but did not enhance its transepithelial permeability. Nanoparticles as well as beta-cyclodextrin neither affected integrity of the cells nor transport properties of the cyclopeptide. In summary, significant absorption enhancement was only observed with NaTC or MCFAs. Increase in permeability coefficients using NaTC occurred rapidly with acceptable cytotoxicities and merits further investigations.

Intestinal permeation enhancers

This review addresses the field of improving oral bioavailability through the use of excipients that increase intestinal membrane permeability. The critical issues to consider in evaluating these approaches are 1) the extent of bioavailability enhancement achieved, 2) the influence of formulation and physiological variables, 3) toxicity associated with permeation enhancement, and 4) the mechanism of permeation enhancement. The categories of permeation enhancers discussed are surfactants, fatty acids, medium chain glycerides, steroidal detergents, acyl carnitine and alkanoylcholines, N-acetylated alpha-amino acids and N-acetylated non-alpha-amino acids, and chitosans and other mucoadhesive polymers. Some of these approaches have been developed to the stage of initial clinical trials. Several seem to have potential to improve oral bioavailabilities of poorly absorbed compounds without causing significant intestinal damage. In addition, the possible use of excipients that inhibit secretory transport is reviewed.

Effect of degree of quaternization of N-trimethyl chitosan chloride for enhanced transport of hydrophilic compounds across intestinal caco-2 cell monolayers

N-Trimethyl chitosan chloride (TMC) is a permanently quaternized chitosan derivative with improved aqueous solubility compared to native chitosan. TMC is able to open the tight junctions of intestinal epithelia at physiological pH values, where chitosan is insoluble and therefore ineffective. TMCs with degrees of substitution of 40 and 60% were synthesized according to a novel synthesis procedure and their effect on the permeability of the tight junctions of the intestinal Caco-2 monolayers was studied, measuring the transepithelial electrical resistance and the transport of a mainly paracellularly transported compound, [14C]-mannitol. Toxicity studies using nucleic stains were done to establish the transport as a cause of opening of the tight junctions and not of possible cytotoxicity. TMC60 showed higher transport enhancement ratios than TMC40 in all concentrations tested (0.05-1. 0%, w/v). Both derivatives did not affect the viability of the Caco-2 cell monolayers. These results suggest that high charge density is necessary for TMC to substantially improve the paracellular permeability of intestinal epithelia. It is expected that TMC40 and TMC60 will enhance the intestinal permeation of hydrophilic macromolecular drugs such as peptides and proteins.

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.

The effect of a drug-delivery system consisting of soybean phosphatidyl choline and medium-chain monoacylglycerol on the intestinal permeability of hexarelin in the rat

The aim of this study was to investigate if the effective in-situ permeability (Peff) of a new growth hormone-releasing peptide, hexarelin, along rat intestine was enhanced by a lipid matrix drug-delivery system comprising a mixture of soybean phosphatidyl choline and medium-chain monoacylglycerol (PC-MG). The study was performed with and without a protease inhibitor, Pefabloc SC. To enable better understanding of the mechanism of action of this delivery system we also studied the uptake of a small hydrophilic molecule, atenolol. PC-MG at a concentration of 15 mmol L(-1) increased the jejunal Peff of hexarelin approximately 20-fold, both in the presence and absence of Pefabloc SC, whereas Peff was not increased in the ileum and colon. PC-MG had no effect on the jejunal, ileal and colonic Peff of atenolol. Complete recovery of the non-absorbable molecule PEG 4000 showed that functional intestinal viability was maintained in all experiments. Although the results obtained in this study are promising, pharmacokinetic and toxicological studies are required to investigate if this delivery system is a suitable and safe candidate for improving the oral bioavailability of hexarelin.