Absorption enhancement in intestinal epithelial Caco-2 monolayers by sodium caprate: assessment of molecular weight dependence and demonstration of transport routes

Recent Progress in the Oral Delivery of Therapeutic Peptides and Proteins: Overview of Pharmaceutical Strategies to Overcome Absorption Hurdles

Purpose

Proteins and peptides have secured a place as excellent therapeutic moieties on account of their high selectivity and efficacy. However due to oral absorption limitations, current formulations are mostly delivered parenterally. Oral delivery of peptides and proteins (PPs) can be considered the need of the hour due to the immense benefits of this route. This review aims to critically examine and summarize the innovations and mechanisms involved in oral delivery of peptide and protein drugs.

Methods

Comprehensive literature search was undertaken, spanning the early development to the current state of the art, using online search tools (PubMed, Google Scholar, ScienceDirect and Scopus).

Results

Research in oral delivery of proteins and peptides has a rich history and the development of biologics has encouraged additional research effort in recent decades. Enzyme hydrolysis and inadequate permeation into intestinal mucosa are the major causes that result in limited oral absorption of biologics. Pharmaceutical and technological strategies including use of absorption enhancers, enzyme inhibition, chemical modification (PEGylation, pro-drug approach, peptidomimetics, glycosylation), particulate delivery (polymeric nanoparticles, liposomes, micelles, microspheres), site-specific delivery in the gastrointestinal tract (GIT), membrane transporters, novel approaches (self-nanoemulsifying drug delivery systems, Eligen technology, Peptelligence, self-assembling bubble carrier approach, luminal unfolding microneedle injector, microneedles) and lymphatic targeting, are discussed. Limitations of these strategies and possible innovations for improving oral bioavailability of protein and peptide drugs are discussed.

Conclusion

This review underlines the application of oral route for peptide and protein delivery, which can direct the formulation scientist for better exploitation of this route.

Gastrointestinal Permeation Enhancers for the Development of Oral Peptide Pharmaceuticals

Recently, two oral-administered peptide pharmaceuticals, semaglutide and octreotide, have been developed and are considered as a breakthrough in peptide and protein drug delivery system development. In 2019, the Food and Drug Administration (FDA) approved an oral dosage form of semaglutide developed by Novo Nordisk (Rybelsus^®) for the treatment of type 2 diabetes. Subsequently, the octreotide capsule (Mycapssa^®), developed through Chiasma's Transient Permeation Enhancer (TPE) technology, also received FDA approval in 2020 for the treatment of acromegaly. These two oral peptide products have been a significant success; however, a major obstacle to their oral delivery remains the poor permeability of peptides through the intestinal epithelium. Therefore, gastrointestinal permeation enhancers are of great relevance for the development of subsequent oral peptide products. Sodium salcaprozate (SNAC) and sodium caprylate (C8) have been used as gastrointestinal permeation enhancers for semaglutide and octreotide, respectively. Herein, we briefly review two approved products, Rybelsus^® and Mycapssa^®, and discuss the permeation properties of SNAC and medium chain fatty acids, sodium caprate (C10) and C8, focusing on Eligen technology using SNAC, TPE technology using C8, and gastrointestinal permeation enhancement technology (GIPET) using C10.

Regulation of the intestinal barrier by nutrients: The role of tight junctions

Tight junctions (TJs) play an important role in intestinal barrier function. TJs in intestinal epithelial cells are composed of different junctional molecules, such as claudin and occludin, and regulate the paracellular permeability of water, ions, and macromolecules in adjacent cells. One of the most important roles of the TJ structure is to provide a physical barrier to luminal inflammatory molecules. Impaired integrity and structure of the TJ barrier result in a forcible activation of immune cells and chronic inflammation in different tissues. According to recent studies, the intestinal TJ barrier could be regulated, as a potential target, by dietary factors to prevent and reduce different inflammatory disorders, although the precise mechanisms underlying the dietary regulation remain unclear. This review summarizes currently available information on the regulation of the intestinal TJ barrier by food components.

Exploring the Impact of Surfactant Type and Digestion: Highly Digestible Surfactants Improve Oral Bioavailability of Nilotinib

The scientific rationale for selection of the surfactant type during oral formulation development requires an in-depth understanding of the interplay between surfactant characteristics and biopharmaceutical factors. Currently, however, there is a lack of comprehensive knowledge of how surfactant properties, such as hydrophilic-lipophilic balance (HLB), digestibility, and fatty acid (FA) chain length, translate into in vivo performance. In the present study, the relationship between surfactant properties, in vitro characteristics, and in vivo bioavailability was systematically evaluated. An in vitro lipolysis model was used to study the digestibility of a variety of nonionic surfactants. Eight surfactants and one surfactant mixture were selected for further analysis using the model poorly water-soluble drug nilotinib. In vitro lipolysis of all nilotinib formulations was performed, followed by an in vivo pharmacokinetic evaluation in rats. The in vitro lipolysis studies showed that medium-chain FA-based surfactants were more readily digested compared to long-chain surfactants. The in vivo study demonstrated that a Tween 20 formulation significantly enhanced the absolute bioavailability of nilotinib up to 5.2-fold relative to an aqueous suspension. In general, surfactants that were highly digestible in vitro tended to display higher bioavailability of nilotinib in vivo. The bioavailability may additionally be related to the FA chain length of digestible surfactants with an improved exposure in the case of medium-chain FA-based surfactants. There was no apparent relationship between the HLB value of surfactants and the in vivo bioavailability of nilotinib. The impact of this study's findings suggests that when designing surfactant-based formulations to enhance oral bioavailability of the poorly water-soluble drug nilotinib, highly digestible, medium chain-based surfactants are preferred. Additionally, for low-permeability drugs such as nilotinib, which is subject to efflux by intestinal P-glycoprotein, the biopharmaceutical effects of surfactants merit further consideration.

In Vitro, Ex Vivo and In Vivo Evaluation of Microcontainers for Oral Delivery of Insulin

Enhancing the oral bioavailability of peptides has received a lot of attention for decades but remains challenging, partly due to low intestinal membrane permeability. Combining a permeation enhancer (PE) with unidirectionally releasing microcontainers (MCs) has previously been shown to increase insulin permeation across Caco-2 cell monolayers. In the present work, this setup was further employed to compare three common PEs—sodium caprate (C₁₀), sodium dodecyl sulfate (SDS), and lauroyl carnitine. The concept was also studied using porcine intestinal tissue with the inclusion of 70 kDa fluorescein isothiocyanate-dextran (FD70) as a pathogen marker. Moreover, a combined proteolysis and Caco-2 cell permeation setup was developed to investigate the effect of soybean trypsin inhibitor (STI) in the MCs. Lastly, in vivo performance of the MCs was tested in an oral gavage study in rats by monitoring blood glucose and insulin absorption. SDS proved to be the most potent PE without increasing the ex vivo uptake of FD70, while the implementation of STI further improved insulin permeation in the combined proteolysis Caco-2 cell setup. However, no insulin absorption in rats was observed upon oral gavage of MCs loaded with insulin, PE and STI. Post-mortem microscopic examination of their gastrointestinal tract indicated lack of intestinal retention and optimal orientation by the MCs, possibly precluding the potential advantage of unidirectional release.

Food-Derived Bioactive Peptides in Human Health: Challenges and Opportunities

Recent scientific evidence suggests that food proteins not only serve as nutrients, but can also modulate the body's physiological functions. These physiological functions are primarily regulated by some peptides that are encrypted in the native protein sequences. These bioactive peptides can exert health beneficial properties and thus are considered as a lead compound for the development of nutraceuticals or functional foods. In the past few decades, a wide range of food-derived bioactive peptide sequences have been identified, with multiple health beneficial activities. However, the commercial application of these bioactive peptides has been delayed because of the absence of appropriate and scalable production methods, proper exploration of the mechanisms of action, high gastro-intestinal digestibility, variable absorption rate, and the lack of well-designed clinical trials to provide the substantial evidence for potential health claims. This review article discusses the current techniques, challenges of the current bioactive peptide production techniques, the oral use and gastrointestinal bioavailability of these food-derived bioactive peptides, and the overall regulatory environment.

Rhamnolipids Enhance in Vivo Oral Bioavailability of Poorly Absorbed Molecules

PURPOSE

This report describes the effect of rhamnolipids (RLs) on the tight junctions (TJ) of the intestinal epithelium using the rat in-situ closed loop model.

METHODS

We investigated the transport of 5 (6)-carboxyfluorescein (CF) and fluorescein isothiocyanate-labeled dextrans with average molecular weights of 4.4 and 10 kDa (FD-4 and FD-10) when co-administered with different concentrations of RLs. Lactate dehydrogenase (LDH) leakage assay and histopathological examination of treated intestinal loops were used to assess potential toxicity of RLs. Further, the effect of kaempferol on accelerating the resealing of the tight junctions in vivo was also investigated RESULTS: Data shows that administration of different RLs concentrations (1.0-5.0% v/v) increased CF absorption through rat intestine by 2.84- and 15.82-folds with RLs concentrations of 1.0% and 5.0% v/v, respectively. RLs exhibited size-dependent increase on FD-4 and FD-10 absorption. Dosing RLs at 1.0% v/v didn't cause a significant LDH leakage or histopathological changes to intestinal mucosa compared to higher concentrations, which showed a progressive damaging effect. Using kaempferol, a natural flavonoid that stimulates the assembly of the TJs, proved to enhance the recovery of barrier properties of the intestinal mucosa treated with high concentrations of RLs (2.5% and 5% v/v).

CONCLUSIONS

These results collectively illustrate the ability of RLs to enhance oral bioavailability of different molecules across the intestinal epithelial membrane in a concentration- and time-dependent fashion.

Induction of rhythmic transient depolarizations associated with waxing and waning of slow wave activity in intestinal smooth muscle

Cannon described in 1902 the segmentation motor activity of the small intestine (Canon WB. J Med Res 7: 72-75, 1902). This motor pattern can arise when low-frequency transient depolarizations are evoked in the interstitial cells of Cajal associated with the deep muscular plexus (ICC-DMP) network, which then affect the omnipresent slow wave activity: changing its regular amplitude into a waxing and waning pattern. The objective of the present study was to investigate physiological stimuli that could induce the low-frequency component. Intracellular recordings were obtained from circular muscle with or without attached mucosa. Decanoic acid (1 mM) and butyric acid (10 mM) both evoked low-frequency transient depolarizations but through different mechanisms. Decanoic acid-induced waxing and waning was initiated by purely myogenic means when perfused onto exposed circular muscle. Butyric acid required the intact mucosa and uninhibited neural activity to elicit the low-frequency response. Evidence is provided that the transient rhythmic depolarizations occur in the absence of interstitial cells of Cajal associated with the myenteric plexus (ICC-MP). Onset of the slow transient depolarizations was stimulated by addition of N(ω)-nitro-l-arginine (l-NNA; 100 μM); thus the low-frequency component seems to be under chronic inhibition by nitric oxide. Excitatory tachykinergic stimulation induced the low-frequency component since substance P (0.5 μM) evoked it in the presence of neural blockade. In summary, interplay between two networks of myogenic pacemakers, neural activity, and nutrient factors such as fatty acids plays a role in the generation of the rhythmic low-frequency component that is essential for the development of the checkered segmentation motor pattern.

Nonvascular drug-eluting stent coated with sodium caprate-incorporated polyurethane for the efficient penetration of paclitaxel into tumor tissue

To increase the therapeutic potency of nonvascular drug-eluting stents, sodium caprate was employed as a drug-penetration enhancer. A polytetrafluoroethylene-covered drug-eluting stent was coated with a mixture containing sodium caprate, paclitaxel, and polyurethane via the rolling coating technique. The coated stent has a smooth membrane surface with a 40-µm membrane thickness. Paclitaxel was released from the coated stent for two months. In the multilayered cell sheet model, sodium caprate in the polyurethane membrane (PUSC10) showed the possibility of enhancing the paclitaxel tissue penetration. The amount of penetrated paclitaxel for the sodium caprate-containing polyurethane membrane (PUSC10) was two times higher than that of sodium caprate-free polyurethane membrane. Additionally, the potential of sodium caprate was confirmed by a tumor-bearing small animal model. PUSC10 incorporated with Nile red (as a model fluorescence dye for visualization of drug penetration; PUSC10–Nile red) or PUSC10 incorporated with paclitaxel (PUSC10–paclitaxel) membrane was implanted at tumor sites in Balb/c mice. In the case of PUSC10–Nile red, the tissue penetration depth of Nile red was significantly increased from 30 µm (without sodium caprate) to 1060 µm (with sodium caprate). After seven days, an almost four times higher therapeutic area of PUSC10–paclitaxel was observed compared to that of polyurethane–paclitaxel (without sodium caprate) by a terminal deoxynucleotidyl transferase dUTP nick end labeling assay. The results indicate that sodium caprate improves the penetration and therapeutic efficiencies of drugs in drug-eluting stents, and thus, it has potential for local stent therapy.

Efficacious intestinal permeation enhancement induced by the sodium salt of 10-undecylenic acid, a medium chain fatty acid derivative

10-undecylenic acid (UA) is an OTC antifungal therapy and a nutritional supplement. It is an unsaturated medium chain fatty acid (MCFA) derivative, so our hypothesis was that its 11-mer sodium salt, uC11, would improve intestinal permeation similar to the established enhancer, sodium caprate (C10), but without the toxicity of the parent saturated MCFA, decylenic acid (C11). MTT assay and high-content screening (HCS) confirmed a cytotoxicity ranking in Caco-2 cells: C11 > C10 = uC11. Five to ten millimolars of the three agents reduced TEER and increased the Papp of [(14)C]-mannitol across Caco-2 monolayers and rat intestinal mucosae, a concentration that matched increases in plasma membrane permeability seen in HCS. Although C11 was the most efficacious enhancer in vitro, it damaged monolayers and tissue mucosae more than the other two agents at similar concentrations and exposure times and was therefore not pursued further. Rat jejunal and colonic in situ intestinal instillations of 100 mM C10 or uC11 with FITC-dextran 4000 (FD4) solutions yielded comparable regional enhancement ratios of ~10 and 30%, respectively, for each agent with acceptable tissue histology. Mini-tablets of uC11 and FD4 however delivered more FD4 compared to C10-FD-4 mini-tablets in both regions, as reflected by a statistically higher AUC, and with no evidence of membrane perturbation. The unsaturated bond in uC11 therefore confers a reduction in lipophilicity and cytotoxicity compared to C11, and the resulting permeation enhancement is on a par with or superior to that of C10, a key component of formulations in current phase II oral peptide clinical trials.

Zanamivir oral delivery: enhanced plasma and lung bioavailability in rats

The objective of this study was to enhance the oral bioavailability (BA) of zanamivir (ZMR) by increasing its intestinal permeability using permeation enhancers (PE). Four different classes of PEs (Labrasol^®, sodium cholate, sodium caprate, hydroxypropyl β-cyclodextrin) were investigated for their ability to enhance the permeation of ZMR across Caco-2 cell monolayers. The flux and P_app of ZMR in the presence of sodium caprate (SC) was significantly higher than other PEs in comparison to control, and was selected for further investigation. All concentrations of SC (10-200 mM) demonstrated enhanced flux of ZMR in comparison to control. The highest flux (13 folds higher than control) was achieved for the formulation with highest SC concentration (200 mM). The relative BA of ZMR formulation containing SC (PO-SC) in plasma at a dose of 10 mg/kg following oral administration in rats was 317.65% in comparison to control formulation (PO-C). Besides, the AUC_{0-24 h} of ZMR in the lungs following oral administration of PO-SC was 125.22 ± 27.25 ng hr ml^-1 with a C_max of 156.00 ± 24.00 ng/ml reached at 0.50±0.00 h. But, there was no ZMR detected in the lungs following administration of control formulation (PO-C). The findings of this study indicated that the oral formulation PO-SC containing ZMR and SC was able to enhance the BA of ZMR in plasma to an appropriate amount that would make ZMR available in lungs at a concentration higher (>10 ng/ml) than the IC₅₀ concentration of influenza virus (0.64-7.9 ng/ml) to exert its therapeutic effect.

Sodium caprate as an enhancer of macromolecule permeation across tricellular tight junctions of intestinal cells

Sodium caprate is a promising candidate for inducing drug absorption enhancement. The mechanism of that uptake-enhancing effect is not fully understood so far. We investigated how caprate acts in an established human intestinal cell line, HT-29/B6, on the transient opening of transcellular (across the cell membranes) and paracellular (across the tight junction) pathways. Sodium caprate (10 mm) caused a rapid and reversible decrease of transepithelial resistance which is based, as measured by two-path impedance spectroscopy, exclusively on resistance changes of the paracellular pathway. Measurements of paracellular marker fluxes revealed an increased permeability for fluorescein (330 Da) and FITC-dextran (4 and 10 kDa), indicating an opening of the paracellular barrier. Confocal microscopy revealed a marked reduction of tricellulin in tricellular tight junctions and of claudin-5 in bicellular tight junctions. This was not due to altered protein expression, as occludin, claudins or tricellulin were not significantly changed in Western blots. Visualization of the translocation site of the cell membrane-impermeable marker molecule sulpho-NHS-SS-biotin (607 Da) indicated the tricellular tight junction to be the predominant pathway. We suggest that caprate's known enhancing effect on intestinal drug uptake is based on increased permeability in tricellular cell contacts, mediated by reversible removal of tricellulin from the tricellular tight junction.

Cell-based in vitro models for predicting drug permeability

INTRODUCTION

In vitro cell models have been used to predict drug permeation in early stages of drug development, since they represent an easy and reproducible method, allowing the tracking of drug absorption rate and mechanism, with an advantageous cost-benefit ratio. Such cell-based models are mainly composed of immortalized cells with an intrinsic ability to grow in a monolayer when seeded in permeable supports, maintaining their physiologic characteristics regarding epithelium cell physiology and functionality.

AREAS COVERED

This review summarizes the most important intestinal, pulmonary, nasal, vaginal, rectal, ocular and skin cell-based in vitro models for predicting the permeability of drugs. Moreover, the similitude between in vitro cell models and in vivo conditions are discussed, providing evidence that each model may provisionally resemble different drug absorption route.

EXPERT OPINION

Despite the widespread use of in vitro cell models for drug permeability and absorption evaluation purposes, a detailed study on the properties of these models and their in vitro-in vivo correlation compared with human data are required to further use in order to consider a future drug discovery optimization and clinical development.

Improvement of adenoviral vector-mediated gene transfer to airway epithelia by folate-modified anionic liposomes

Despite remarkable progress in the development of both viral and nonviral gene delivery vectors for airway disease treatment, poor gene transfer efficiency to the airway epithelium is a major obstacle in clinical application. To take advantage of the unique features of viral and nonviral vectors, we have developed complexes of adenovirus vector and anionic liposomes (AL-Ad5) by the calcium-induced phase change method. In the current study, based on the fact that there are overexpressed folate receptors on the surface of airway epithelia, we further modified the AL-Ad5 complexes with folate (F-AL-Ad5) to improve the transduction ability of Ad5 in airway epithelia. The transduction efficiencies of the obtained F-AL-Ad5 and AL-Ad5 complexes were assessed in primary-cultured airway epithelia in vitro. Our results indicated that compared with naked adenovirus vector, both AL-Ad5 and F-AL-Ad5 could significantly enhance the gene transduction efficiency of adenovirus vector in primary-cultured airway epithelial cells. Moreover, the enhancement mediated by F-AL-Ad5 was more dramatic than that by AL-Ad5. These results suggested that F-AL-Ad5 may be a useful strategy to deliver therapeutic genes to the airway epithelia and is promising in clinical application.

Thiolated polycarbophil/glutathione: defining its potential as a permeation enhancer for oral drug administration in comparison to sodium caprate

Thiolated polyacrylates were shown to be permeation enhancers with notable potential. The aim of this study was to evaluate the permeation enhancing properties of a thiolated polycarbophil/glutathione (PCP-Cys/GSH) system for oral drug application in comparison to a well-established permeation enhancer, namely sodium caprate. In vitro permeation studies were conducted in Ussing-type chambers with sodium fluoresceine (NaFlu) and fluoresceine isothiocyanate labeled dextran (molecular mass 4 kDa; FD4) as model compounds. Bioavailability studies were carried out in Sprague Dawley rats with various formulations. Moreover, cytotoxic effects of both permeation enhancers were compared. Permeation enhancement ratios of 1% sodium caprate were found to be 3.0 (FD4) and 2.3 (NaFlu), whereas 1% PCP-Cys/0.5% GSH displayed enhancement ratios of 2.4 and 2.2. Both excipients performed at a similar level in vivo. Sodium caprate solutions increased oral bioavailability 2.2-fold (FD4) and 2.3-fold (NaFlu), while PCP-Cys hydrogels led to a 3.2-fold and 2.2-fold enhancement. Cell viability experiments revealed a significantly higher tolerance of Caco-2 cells towards 0.5% PCP-Cys (81% survival) compared to 0.5% sodium caprate (5%). As PCP-Cys is not absorbed from mucosal membranes due to its comparatively high molecular mass, systemic side-effects can be excluded. In conclusion, both systems displayed a similar potency for permeation enhancement of hydrophilic compounds. However, PCP-Cys seems to be less harmful to cultured cells.

Safety and efficacy of sodium caprate in promoting oral drug absorption: from in vitro to the clinic

A major challenge in oral drug delivery is the development of novel dosage forms to promote absorption of poorly permeable drugs across the intestinal epithelium. To date, no absorption promoter has been approved in a formulation specifically designed for oral delivery of Class III molecules. Promoters that are designated safe for human consumption have been licensed for use in a recently approved buccal insulin spray delivery system and also for many years as part of an ampicillin rectal suppository. Unlike buccal and rectal delivery, oral formulations containing absorption promoters have the additional technical hurdle whereby the promoter and payload must be co-released in high concentrations at the small intestinal epithelium in order to generate significant but rapidly reversible increases in permeability. An advanced promoter in the clinic is the medium chain fatty acid (MCFA), sodium caprate (C(10)), a compound already approved as a food additive. We discuss how it has evolved to a matrix tablet format suitable for administration to humans under the headings of mechanism of action at the cellular and tissue level as well as in vitro and in vivo efficacy and safety studies. In specific clinical examples, we review how C(10)-based formulations are being tested for oral delivery of bisphosphonates using Gastro Intestinal Permeation Enhancement Technology, GIPET (Merrion Pharmaceuticals, Ireland) and in a related solid dose format for antisense oligonucleotides (ISIS Pharmaceuticals, USA).

Fatty acids increase paracellular absorption of aluminium across Caco-2 cell monolayers

Passive paracellular absorption, regulated by tight junctions (TJs), is the main route for absorption of poorly absorbed hydrophilic substances. Surface active substances, such as fatty acids, may enhance absorption of these substances by affecting the integrity of TJ and increasing the permeability. It has been suggested that aluminium (Al) absorption occurs mainly by the paracellular route. Herein, we investigated if physiologically relevant exposures of fully differentiated Caco-2 cell monolayers to oleic acid and docosahexaenoic acid (DHA), which are fatty acids common in food, increase absorption of Al and the paracellular marker mannitol. In an Al toxicity test, mannitol and Al absorption through Caco-2 cell monolayers were similarly modulated by Al concentrations between 1 and 30mM, suggesting that absorption of the two compounds occurred via the same pathways. Exposure of Caco-2 cell monolayers to non-toxic concentrations of Al (2mM) and (14)C-mannitol in fatty acid emulsions (15 and 30mM oleic acid, 5 and 10mM DHA) caused a decreased transepithelial electrical resistance (TEER). Concomitantly, fractional absorption of Al and mannitol, expressed as percentage of apical Al and mannitol retrieved at the basolateral side, increased with increasing dose of fatty acids. Transmission electron microscopy was applied to assess the effect of oleic acid on the morphology of TJ. It was shown that oleic acid caused a less structured morphology of TJ in Caco-2 cell monolayers. Taken together our findings indicate that fatty acids common in food increase the paracellular intestinal absorption of Al. These findings may influence future risk assessment of human Al exposure.

Effect of sodium caprate on the oral absorptions of danshensu and salvianolic acid B

The current study aims to investigate the effect of sodium caprate on the intestinal absorption and bioavailabilities of danshensu and salvianolic acid B, the major active components in Salvia miltiorrhiza Bge (Danshen). Biopharmaceutics and pharmacokinetics properties of the two compounds have been characterized by in vitro, in situ models as well as in vivo in rats. Based on the identified biopharmaceutics characteristics of the two compounds, effect of sodium carparate as absorption enhancer on the intestinal absorption and pharmacokinetics of danshensu and salvianolic acid B in pure compound form as well as extract form were investigated both in vitro and in vivo. Both danshensu and salvianolic acid B demonstrated very limited intestinal permeabilities, leading to oral bioavailabilities of only 11.09% and 3.90% in rats, respectively. Results from both in vitro and in vivo studies consistently indicated that sodium caprate could significantly enhance intestinal permeabilities as well as the in vivo bioavailabilities of both danshensu and salvianolic acid B. The current findings not only identified the usefulness of sodium caprate for the improved delivery of Danshen product but also demonstrated the importance of biopharmaceutics characterization in the dosage form development of traditional Chinese medicine.

Regulation of tight junction permeability by sodium caprate in human keratinocytes and reconstructed epidermis

Tight junctions (TJs) restrict paracellular flux of water and solutes in epithelia and endothelia. In epidermis, the physiological role of TJs is not fully understood. In this study, sodium caprate (C10), which dilates intestinal TJs, was applied to cultured human epidermal keratinocytes and reconstructed human epidermis to investigate the effects of C10 on epidermal TJs. C10 treatment decreased transepithelial electrical resistance and increased paracellular permeability, although Western blots showed that the expression of TJ-related transmembrane proteins was not decreased. The effects of C10 were reversible. Immunofluorescence microscopy and immuno-replica electron microscopy showed that the localization of TJ strands were disintegrated, concomitant with the dispersion and/or disappearance of TJ-related molecules from the cell surface. These findings suggest that C10 impairs barrier function by physically disrupting TJ conformation in the epidermis. Furthermore, these results also show that proper localization of the molecules on the cellular membrane is important for TJ barrier function.

Melittin as a permeability enhancer II: in vitro investigations in human mucus secreting intestinal monolayers and rat colonic mucosae

PURPOSE

Melittin has shown potential as a non-cytotoxic absorption enhancer in Caco-2 monolayers. Our objectives were to assess in vitro efficacy and cytotoxicity of melittin in two intestinal permeability models and investigate the potential mechanism by which melittin might enhance gastrointestinal absorption.

MATERIALS AND METHODS

The effects of melittin were examined in the mucus-secreting intestinal cell monolayers, HT29-MTX-E12 (E12), using transepithelial electrical resistance (TER), transmission electron microscopy (TEM) and the MTT viability assay. The effects of melittin on TER, permeability and short circuit current (Isc) were also investigated in rat colon mucosae mounted in Ussing chambers. Ion transporting capacity of tissue was measured in response to secretagogues as surrogate markers of cytotoxicity. Melittin stability was examined by a means of a hemolytic assay. The mechanism by which melittin decreases TER across the rat mucosa was examined with a range of enzymatic inhibitors.

RESULTS

Apical addition of melittin resulted in a reversible non-cytotoxic concentration-dependent decrease in TER across E12 monolayers, which was independent of the presence of mucus. Apical addition of melittin reduced TER and increased the permeability of [(14)C]-mannitol across rat colonic mucosae. The melittin-induced drop in TER in rat colon was significantly attenuated by W7 suggesting partial mediation by calmodulin.

CONCLUSIONS

The rapid and reversible nature of melittin's permeation enhancing properties and its limited cytotoxicity in polarized intestinal epithelia, suggests a potential drug delivery role for the peptide in oral formulations of poorly absorbed drugs.

Melittin as an epithelial permeability enhancer I: investigation of its mechanism of action in Caco-2 monolayers

PURPOSE

Melittin is an amphipathic antimicrobial peptide which has been shown to enhance the permeability of mannitol and reduce transepithelial electrical resistance (TER) across Caco-2 monolayers. The aim of this work was to further examine the potential of melittin as a paracellular permeability enhancer and to investigate the mechanism of interaction with tight junction proteins in Caco-2.

MATERIALS AND METHODS

The permeability of a range of fluorescent markers of differing molecular weights across monolayers was examined and immunofluorescence and western blotting analysis of tight junction proteins were also carried out. The mechanism of TER reduction was also examined using cell signalling inhibitors.

RESULTS

Apical but not basolateral addition of melittin increased the permeability of a range FITC-dextrans (4-70 kDa) across monolayers. Melittin effects were reversible and no cytotoxicity was evident in polarized Caco-2 epithelia at the concentrations used. Altered expression of ZO-1, E-cadherin and F-actin was also detected. The phospholipase A2 inhibitors, aristolochic acid and indomethacin and the cyclooxygenase inhibitor, piroxicam, partially attenuated melittin-induced TER reduction, suggesting that part of the mechanism by which melittin opens tight junctions involves prostaglandin signalling.

CONCLUSIONS

Apically-added melittin opens tight junctions, causing dramatic TER reductions with significant increases in flux of dextrans. These effects appear mediated in part via PLA2 and involve alterations in specific tight junction proteins.

Disruption of epithelial tight junctions by yeast enhances the paracellular delivery of a model protein

PURPOSE

The aim of this study was to investigate the effect of heat-killed yeast cells on the integrity of epithelial tight junctions in vitro.

METHODS

Changes in barrier potential of Caco-2 cell monolayers were assessed by transepithelial electrical resistance (TEER) measurements and by an increasing permeability to a marker protein, horse-radish peroxidase (HRP). Visualisation of tight junction disruption was carried out directly through electron microscopy and indirectly through fluorescence confocal microscopy and immunoblotting of the tight junction-associated proteins zonula occludens ZO-1, occludin and actin.

RESULTS

Yeast cells opened tight junctions in a reversible dose- and time-dependent manner, as shown by a decrease in TEER and an increase in HRP permeability. These changes to barrier potential were shown not to be due to cytotoxic effects but due to modulation of the tight junctions. ZO-1, actin and occludin proteins were demonstrated to be involved in yeast-induced tight junction opening through the use of confocal microscopy and western blotting. Electron microscopy confirmed a direct opening of tight junctions after application of yeast.

CONCLUSION

Yeast modulated epithelial tight junctions in a reversible manner by contraction of the actin cytoskeleton and shift of ZO-1 and occludin tight junction proteins from the membrane to cytoskeletal areas of the cell.

Oral delivery of low-molecular-weight heparin using sodium caprate as absorption enhancer reaches therapeutic levels

The primary objective of this study was to evaluate sodium caprate as an oral penetration enhancer for low molecular weight heparin (LMWH), ardeparin. In vitro studies using Caco-2 cell monolayer indicated that 0.0625% of sodium caprate gave approximately 2-fold enhancement of ardeparin compared to negative control with almost 100% cell survival as evaluated by MTT cytotoxicity assay. In vivo studies in rats with ardeparin (1,200 IU/kg) and sodium caprate (100 mg/kg) led to a relative bioavailability of 27% with plasma anti-factor Xa levels within the therapeutic range (>0.2 IU/ml). Moreover, under these conditions, histological examination provided evidence that there was no damage to the gastrointestinal wall. Regional permeability studies using rat intestine indicated the colon as the region of maximum permeation. These results suggest that, at the dose administered, sodium caprate acts as a relatively safe and efficient absorption enhancer in the quest for alternatives for the oral delivery of LMWH.

Evaluation of the Oral Bioavailability of Low Molecular Weight Heparin Formulated With Glycyrrhetinic Acid as Permeation Enhancer

Low molecular weight heparin (LMWH) is the agent of choice for anticoagulant therapy and prophylaxis of thrombosis and coronary syndromes. However, its therapeutic use is limited due to poor oral bioavailability. The aim of this study was to investigate the oral delivery of LMWH, ardeparin formulated with 18-beta glycyrrhetinic acid (GA), as an alternative to currently used subcutaneous (sc) delivery. Drug transport through Caco-2 cell monolayers was monitored in the presence and absence of GA by scintillation counting and transepithelial electrical resistance. Regional permeability studies using rat intestine were performed using a modified Ussing chamber. Cell viability in the presence of various concentrations of enhancer was determined by MTT assay. The absorption of ardeparin after oral administration in rats was measured by an anti-factor Xa assay. Furthermore, the eventual mucosal epithelial damage was histologically evaluated. Higher ardeparin permeability (~7-fold) compared to control was observed in the presence of 0.02 % GA. Regional permeability studies indicated predominant absorption in the duodenal segment. Cell viability studies showed no significant cytotoxicity below 0.01 % GA. Ardeparin oral bioavailability was significantly increased (F(relative)/(S.C). = 13.3%) without causing any damage to the intestinal tissues. GA enhanced the oral absorption of ardeparin both in vitro and in vivo. The oral formulation of ardeparin with GA could be absorbed in the intestine. These results suggest that GA may be used as an absorption enhancer for the oral delivery of LMWH.

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.

Relationship between lipophilicity of BCS class III and IV drugs and the functional activity of peroral absorption enhancers

Absorption enhancers (AEs) have been shown to be specific in permeation enhancement capabilities because of which they increase absorption of some drug molecules more than others. Present study was designed to investigate the relationship between lipophilicity of drug molecules and the absorption enhancement potential of AEs. Four drug molecules of different lipophilicity were selected as model compounds, namely, cefotaxime sodium, ceftazidime pentahydrate, lovastatin and cyclosporin A. Their apparent permeability coefficients in the absence and presence of three classes of AEs (fatty acids, cyclodextrins, and bile salts) were determined using in vitro everted rat intestinal sac absorption model. Significant relationship was observed between log P of drug and absorption enhancement ratios by AEs. This relationship was found to be functionally directly or indirectly proportional depending upon nature of AE and explain the mechanism of permeation enhancement.

In situ and in vivo efficacy of peroral absorption enhancers in rats and correlation to in vitro mechanistic studies

The present investigation attempts to increase intestinal permeability and hence absorption of biopharmaceutic classification system (BCS) Class III (cefotaxime sodium (CX)) and Class IV (cyclosporin A (CSA)) drugs by employing certain absorption enhancers. Drugs were co-perfused with sodium caprate (SC, 0.25% w/v), piperine (P, 0.004% w/v) and sodium deoxycholate (SD, 1.0% w/v) separately in rat in situ single pass intestinal perfusion model. These additives increased intestinal permeability (P(app)) and absorption rate constant (K(a)) up to two and fourfold, respectively. SC exhibited substantial absorption enhancement of both CX and CSA, while SD and P enhanced absorption of CX and CSA, respectively. Co-administration of SC significantly enhanced peroral bioavailability of CX (from 29.4 +/- 1.7 to 69.6 +/- 3.2) and CSA (from 18.4 +/- 15.6 to 49.6 +/- 25.1) in rats, while P increased bioavailability of CSA (from 18.4 +/- 15.6 to 33.1 +/- 17.7). Transmission electron microscopy of intestinal mucosa revealed that SC and SD act on lipid and protein domains of absorptive membrane. P showed no effect on intestinal P(app) and oral bioavailability of CX but has a profound effect on CSA, a known P-glycoprotein (P-gp) substrate. These results indicated that P enhances intestinal absorption of CSA by modulating P-gp mediated efflux transport. Release of lactate dehydrogenase in situ from intestinal mucosa in the presence of absorption enhancer was taken as index of its local toxicity. All the absorption enhancers showed significantly less release of LDH compared to positive control, sodium dodecyl sulfate (60% w/v). Overall, the data indicate that the features of these commonly used food ingredients or endogenous bile salts can effectively improve bioavailability of various BCS Class III and Class IV drugs.

Strategies to improve oral drug bioavailability

Efforts to improve oral drug bioavailability have grown in parallel with the pharmaceutical industry. As the number and chemical diversity of drugs has increased, new strategies have been required to develop orally active therapeutics. The past two decades have been characterised by an increased understanding of the causes of low bioavailability and a great deal of innovation in oral drug delivery technologies, marked by an unprecedented growth of the drug delivery industry. The advent of biotechnology and consequent proliferation of biopharmaceuticals have brought new challenges to the drug delivery field. In spite of the difficulties associated with developing oral forms of this type of therapeutics, significant progress has been made in the past few years, with some oral proteins, peptides and other macromolecules currently advancing through clinical trials. This article reviews the approaches that have been successfully applied to improve oral drug bioavailability, primarily, prodrug strategies, lead optimisation through medicinal chemistry and formulation design. Specific strategies to improve the oral bioavailability of biopharmaceuticals are also discussed.

Assessment and modulation of acamprosate intestinal absorption: comparative studies using in situ, in vitro (CACO-2 cell monolayers) and in vivo models

The purpose of this study was to explore the intestinal absorption mechanism of acamprosate and to attempt to improve the bioavailability (BA) of the drug through modulation of its intestinal absorption using two enhancers (polysorbate 80 and sodium caprate) based on in situ, in vitro and in vivo models and comparing the results obtained. Intestinal transport of the drug, in the absence and in presence of polysorbate 80 (0.06, 0.28 and 9.6 mM) or sodium caprate (13 and 16 mM) was measured by using an in situ rat gut technique and Caco-2 cell monolayers. Additionally, the effect of sodium caprate on drug oral bioavailability, measured as urinary recovery, was quantified by performing in vivo experiments with the rat as animal model. Only sodium caprate was able to increase the absorption rate constant (ka) of acamprosate in the mid-intestine of the rats from 0.29 +/- 0.07 h-1 in the absence of the promoter to 0.51 +/- 0.19 h-1 in the presence of C10 16 mM, along with the apparent permeability (Papp) obtained in Caco-2 cells (around two-fold). However, the drug bioavailability in rats (around 20%) did not improve in the presence of any of the concentrations tested (13, 16 and 50 mM). It is concluded that acamprosate absorption likely occurs via paracellular pathway and can be enhanced by sodium caprate in situ and in vitro but not in vivo-thus suggesting that although in situ and in vitro studies could be useful in early screening to select a potential promoter, in vivo studies in animal models are necessary to confirm the utility of the enhancer and to determine the influence of physiological variables.

Transport Of Surface-Modified Nanoparticles Through Cell Monolayers

We synthesized three peptides, a D-polyarginyl peptide (r8(FITC)), a Tat peptide (Tat(FITC)), and a control peptide (Cp(FITC)) and attached each to amino-CLIO, a nanoparticle 30 nm in diameter. We then examined the effective permeability, Peff, of all six materials through CaCo-2 monolayers. The transport of peptide-nanoparticles was characterized by a lag phase (0-8 h) and a steady-state phase (9-27 h). The steady-state Peff values for peptides were in the order r8(FITC)>Tat(FITC)=Cp(FITC). When r8(FITC) and Tat(FITC) peptides were attached to the nanoparticle, they conferred their propensity to traverse cell monolayers onto the nanoparticle, whereas Cp(FITC) did not. Thus, when the r8(FITC) peptide was attached to the amino-CLIO nanoparticle, the resulting peptide-nanoparticle had a Peff similar to that of this poly-D-arginyl peptide alone. The Peff of r8(FITC)-CLIO (MW approximately 1000 kDa) was similar to that of mannitol (MW=182 Da), a poorly transported reference substance, with a far lower molecular weight. These results are the first to indicate that the modification of nanoparticles by attachment of membrane-translocating sequence-based peptides can alter nanoparticle transport through monolayers. This suggests that the surface modification of nanoparticles might be a general strategy for enhancing the permeability of drugs and that high-permeability nanoparticle-based therapeutics can be useful in selected pharmaceutical applications.

PAR2 activation alters colonic paracellular permeability in mice via IFN-gamma-dependent and -independent pathways

Activation of colonic proteinase-activated receptor-2 (PAR(2)) caused inflammation and increased mucosal permeability in mouse colon. The present study was aimed at characterizing the possible links between these two phenomena. We evaluated the effects of intracolonic infusion of PAR(2)-activating peptide, SLIGRL, on colonic paracellular permeability and inflammation at two different doses, 5 and 100 microg per mouse, in an attempt to discriminate between both PAR(2)-mediated effects. We further investigated the possible involvement of interferon gamma (IFN-gamma) and calmodulin-dependent activation of myosin light chain kinase (MLCK), and alterations of zonula occludens-1 (ZO-1) localization in PAR(2)-induced responses. Thus, at the lower dose, SLIGRL increased colonic permeability without causing inflammation. Western blotting showed phosphorylation of mucosal myosin light chain (MLC) expression after both doses of SLIGRL. Moreover, while the MLCK inhibitor, ML-7, abolished the permeability effects of the low dose of SLIGRL, it only partially inhibited that of the high dose. In IFN-gamma-deficient mice (B6 ifng(-/-)), the increases in permeability were similar for both doses of SLIGRL and prevented by ML-7. In addition, MLCK immunoprecipitation revealed an increase of calmodulin binding to MLCK in the mucosa of mice treated with either dose of SLIGRL. Finally, we have shown that direct activation of PAR(2) on enterocytes is responsible for increased permeability and ZO-1 disruption. Moreover, SLIGRL at a dose that does not produce inflammation increases permeability via calmodulin activation, which binds and activates MLCK. The resulting tight junction opening does not depend upon IFN-gamma secretion, while the increased permeability in response to the high dose of PAR(2) agonist involves IFN-gamma secretion.

Absorption enhancers for nasal drug delivery

This paper describes the basic concepts for the transmucosal delivery of drugs, and in particular the use of the nasal route for delivery of challenging drugs such as polar low-molecular-weight drugs and peptides and proteins. Strategies for the exploitation of absorption enhancers for the improvement of nasal delivery are discussed, including consideration of mechanisms of action and the correlation between toxic effect and absorption enhancement. Selected enhancer systems, such as cyclodextrins, phospholipids, bioadhesive powder systems and chitosan, are discussed in detail. Examples of the use of these enhancers in preclinical and clinical studies are given. Methods for assessing irritancy and damage to the nasal membrane from the use of absorption enhancers are also described. Finally, the mucosal use of absorption enhancers (chitosan) for the improved nasal delivery of vaccines is reported with reference to recent phase I/II clinical studies.

Prediction of the oral absorption of low-permeability drugs using small intestine-like 2/4/A1 cell monolayers

PURPOSE

To characterize the paracellular route of 2/4/A1 monolayers and to compare the permeabilities of incompletely absorbed oral drugs in 2/4/A1 with those in Caco-2 monolayers.

METHODS

The cells were cultivated on permeable supports. The 2/4/ A1 expression of genes associated with tight junctions was compared with that in the small intestine using RT-PCR. The aqueous pore radii were determined using paracellular marker molecules. The permeabilities of a series of incompletely absorbed drugs (defined as having a fraction absorbed 0 to 80%) after oral administration to humans were studied.

RESULTS

Occludin and claudin 1 and 3 were expressed in 2/4/A1. The pore radius of 2/4/A1 was 9.0 +/- 0.2 A. which is similar to that in the human small intestine, although the pore radius was smaller (3.7 +/- 0.1 A) in Caco-2. The relationship between permeability and fraction absorbed of 13 drugs was stronger in 2/4/A1 than in Caco-2. The relationships were used to predict the intestinal absorption of another seven drugs. The prediction was more accurate in 2/4/A1 (RMSE = 15.6%) than in Caco-2 (RMSE = 21.1%). Further, Spearman's rank coefficient between FA and permeability was higher in 2/4/A1.

CONCLUSION

The improved 2/4/A1 cell culture model has a more in vivo-like permeability and predicted the oral absorption of incompletely absorbed drugs better than Caco-2 cells.

Regional intestinal absorption of FITC-dextran 4,400 with nanoparticles based on beta-sitosterol beta-D-glucoside in rats

Nanoparticles (NP) are potential carriers for drug delivery to the targeted intestine. NP based on beta-sitosterol beta-D-glucoside (Sit-G) enhanced the colon-specific absorption of FITC-dextran 4,400 (FD-4), because the concentration-dependent increase of bioavailability appeared in only the colon. In a permeation study, the absorption enhancement in the colon was suppressed in the following conditions: (1) the addition of Sit-G NP to serosa; (2) a permeation study at 4 degrees C; (3) the addition of endocytosis inhibitor, cytochalasin B. NP based on sitosterol, the aglycon of Sit-G, did not increase the FD-4 colonic permeation. The addition of Sit-G NP to the mucosal side induced a decrease of transepithelial resistance (TEER), but this phenomenon was suppressed by an inhibitor of Na(+)-dependent specific glucose transporter, phrolidzin, which did not affect FD-4 permeation. These findings suggested that absorption enhancement by Sit-G NP may not be due to opening of a tight junction, but might be related to endocytosis via glucose residue of Sit-G.

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.