Effects of surfactant-based permeation enhancers on mannitol permeability, histology, and electrogenic ion transport responses in excised rat colonic mucosae

LabrafacTM MC60 is an efficacious intestinal permeation enhancer for macromolecules: Comparisons with Labrasol® ALF in ex vivo and in vivo rat studies

Labrafac™ MC60 (glycerol monocaprylocaprate) is a lipid-based excipient used in oral formulations as a solubiliser. Due to the high proportions of established permeability enhancers, caprylate (C8) and caprate (C10), in Labrafac™ MC60, we hypothesised that it might behave as an intestinal permeation enhancer. We therefore evaluated this using two paracellular markers (ex vivo) and insulin (in vivo) as model molecules. Ex vivo studies were conducted in isolated muscle-stripped rat colonic mucosae mounted in Ussing chambers. Apical addition of Labrafac™ MC60 (8, 12, and 16 mg/ml) enhanced the apparent permeability coefficients (Papp) of [14C] mannitol and FITC-dextran 4 kDa (FD4) across colonic mucosae. Similar effects were observed in isolated jejunal mucosae, but at higher concentrations (40 mg/ml). The enhancing capacity of Labrafac™ MC60 was transient due to reversibility of reductions in transepithelial electrical resistance (TEER) upon wash-out and effects on fluxes were molecular weight-dependent (MW) as suggested by fluxes of a set of high MW FITC-dextrans. The permeability enhancing effects of Labrafac™ MC60 ex vivo were maintained in the presence of simulated intestinal fluids, FaSSIF and FaSSCoF, in both jejunal and colonic mucosae, respectively. Following intra-intestinal regional instillations to rats, the relative bioavailability of 50 IU/kg insulin ad-mixed with Labrafac™ MC60 was 5 % in jejunum (40 mg/ml) and 6 % in colon (8 mg/ml). When Labrafac™ MC60 was combined with PEG-60 hydrogenated castor oil (1 % v/v), this further increased the bioavailability of insulin to 8 % in jejunum. Absorption enhancement was also maintained in the presence of FaSSIF in jejunal instillations. Histology after 120 min exposure to Labrafac™ MC60 in vivo for both jejunum and colon was similar to untreated control. Labrafac™ MC60 therefore acts as a non-damaging intestinal permeation enhancer for macromolecules and can be considered as another excipient in screening programmes to develop orally administered macromolecules.

Head-to-Head Comparison of Caco-2 Transwell and Gut-on-a-Chip Models for Assessing Oral Peptide Formulations

Oral delivery of potent peptide drugs provides key formulation challenges in the pharmaceutical industry: stability, solubility, and permeability. Intestinal permeation enhancers (PEs) can overcome the low oral bioavailability by improving the drug permeability. Conventional in vitro and ex vivo models for assessing PEs fail to predict efficacy in vivo. Here, we compared Caco-2 cells cultured in the conventional static Transwell model to a commercially available continuous flow microfluidic Gut-on-a-Chip model. We determined baseline permeability of FITC-Dextan 3 kDa (FD3) in Transwell (5.3 ± 0.8 × 10-8 cm/s) vs Chip (3.2 ± 1.8 × 10-7 cm/s). We screened the concentration impact of two established PEs sodium caprate and sucrose monolaurate and indicated a requirement for higher enhancer concentration in the Chip model to elicit equivalent efficacy e.g., 10 mM sodium caprate in Transwells vs 25 mM in Chips. Fasted and fed state simulated intestinal fluids (FaSSIF/FeSSIF) were introduced into the Chip and increased basal FD3 permeability by 3-fold and 20-fold, respectively, compared to 4-fold and 4000-fold in Transwells. We assessed the utility of this model to peptides (Insulin and Octreotide) with PEs and observed much more modest permeability enhancement in the Chip model in line with observations in ex vivo and in vivo preclinical models. These data indicate that microfluidic Chip models are well suited to bridge the gap between conventional in vitro and in vivo models.

Gastrointestinal Permeation Enhancers Beyond Sodium Caprate and SNAC - What is Coming Next?

Oral peptide delivery is trending again. Among the possible reasons are the recent approvals of two oral peptide formulations, which represent a huge stride in the field. For the first time, gastrointestinal (GI) permeation enhancers (PEs) are leveraged to overcome the main limitation of oral peptide delivery-low permeability through the intestinal epithelium. Despite some success, the application of current PEs, such as salcaprozate sodium (SNAC), sodium caprylate (C8), and sodium caprate (C10), is generally resulting in relatively low oral bioavailabilities (BAs)-even for carefully selected therapeutics. With several hundred peptide-based drugs presently in the pipeline, there is a huge unmet need for more effective PEs. Aiming to provide useful insights for the development of novel PEs, this review summarizes the biological hurdles to oral peptide delivery with special emphasis on the epithelial barrier. It describes the concepts and action modes of PEs and mentions possible new targets. It further states the benchmark that is set by current PEs, while critically assessing and evaluating emerging PEs regarding translatability, safety, and efficacy. Additionally, examples of novel PEs under preclinical and clinical evaluation and future directions are discussed.

The Current and Promising Oral Delivery Methods for Protein- and Peptide-Based Drugs

Drugs based on peptides and proteins (PPs) have been widely used in medicine, beginning with insulin therapy in patients with diabetes mellitus over a century ago. Although the oral route of drug administration is the preferred one by the vast majority of patients and improves compliance, medications of this kind due to their specific chemical structure are typically delivered parenterally, which ensures optimal bioavailability. In order to overcome issues connected with oral absorption of PPs such as their instability depending on digestive enzymes and pH changes in the gastrointestinal (GI) system on the one hand, but also their limited permeability across physiological barriers (mucus and epithelium) on the other hand, scientists have been strenuously searching for novel delivery methods enabling peptide and protein drugs (PPDs) to be administered enterally. These include utilization of different nanoparticles, transport channels, substances enhancing permeation, chemical modifications, hydrogels, microneedles, microemulsion, proteolytic enzyme inhibitors, and cell-penetrating peptides, all of which are extensively discussed in this review. Furthermore, this article highlights oral PP therapeutics both previously used in therapy and currently available on the medical market.

Oral Absorption of Middle-to-Large Molecules and Its Improvement, with a Focus on New Modality Drugs

To meet unmet medical needs, middle-to-large molecules, including peptides and oligonucleotides, have emerged as new therapeutic modalities. Owing to their middle-to-large molecular sizes, middle-to-large molecules are not suitable for oral absorption, but there are high expectations around orally bioavailable macromolecular drugs, since oral administration is the most convenient dosing route. Therefore, extensive efforts have been made to create bioavailable middle-to-large molecules or develop absorption enhancement technology, from which some successes have recently been reported. For example, Rybelsus® tablets and Mycapssa® capsules, both of which contain absorption enhancers, were approved as oral medications for type 2 diabetes and acromegaly, respectively. The oral administration of Rybelsus and Mycapssa exposes their pharmacologically active peptides with molecular weights greater than 1000, namely, semaglutide and octreotide, respectively, into systemic circulation. Although these two medications represent major achievements in the development of orally absorbable peptide formulations, the oral bioavailability of peptides after taking Rybelsus and Mycapssa is still only around 1%. In this article, we review the approaches and recent advances of orally bioavailable middle-to-large molecules and discuss challenges for improving their oral absorption.

Formulation and Evaluation of Insulin-Loaded Sodium-Alginate Microparticles for Oral Administration

The development of oral insulin drug delivery systems is still an ongoing challenge for pharmaceutical technology researchers, as the formulation process has to overcome a number of obstacles due to the adverse characteristics of peptides. The aim of this study was to formulate different sodium-alginate microparticles as a possible method for oral insulin administration. In our previous studies, the method has been successfully optimized using a small model peptide. The incorporation of insulin into alginate carriers containing nonionic surfactants has not been described yet. In order to enhance the absorption of insulin through biological barriers, Labrasol ALF and Labrafil M 2125 CS were selected as permeation-enhancing excipients. They were applied at a concentration of 0.10% (v/v%), along with various combinations of the two, to increase oral bioavailability. Encapsulation efficiency showed sufficient drug incorporation, as it resulted in over 80% in each composition. In vitro dissolution and enzymatic stability test results proved that, as a pH-responsive polymer, alginate bead swelling and drug release occur at higher pH, thus protecting insulin against the harsh environment of the gastrointestinal tract. The remaining insulin content was 66% due to SIF degradation after 120 min. Permeability experiments revealed the impact of permeation enhancers and natural polymers on drug absorption, as they enhanced drug transport significantly through Caco-2 cells in the case of alginate microparticle formulations, as opposed to the control insulin solution. These results suggest that these formulations are able to improve the oral bioavailability of insulin.

Safety of surfactant excipients in oral drug formulations

Surfactants are a diverse group of compounds that share the capacity to adsorb at the boundary between distinct phases of matter. They are used as pharmaceutical excipients, food additives, emulsifiers in cosmetics, and as household/industrial detergents. This review outlines the interaction of surfactant-type excipients present in oral pharmaceutical dosage forms with the intestinal epithelium of the gastrointestinal (GI) tract. Many surfactants permitted for human consumption in oral products reduce intestinal epithelial cell viability in vitro and alter barrier integrity in epithelial cell monolayers, isolated GI tissue mucosae, and in animal models. This suggests a degree of mis-match for predicting safety issues in humans from such models. Recent controversial preclinical research also infers that some widely used emulsifiers used in oral products may be linked to ulcerative colitis, some metabolic disorders, and cancers. We review a wide range of surfactant excipients in oral dosage forms regarding their interactions with the GI tract. Safety data is reviewed across in vitro, ex vivo, pre-clinical animal, and human studies. The factors that may mitigate against some of the potentially abrasive effects of surfactants on GI epithelia observed in pre-clinical studies are summarised. We conclude with a perspective on the overall safety of surfactants in oral pharmaceutical dosage forms, which has relevance for delivery system development.

Sugar-Based Monoester Surfactants: Synthetic Methodologies, Properties, and Biological Activities

Glycolipids are biocompatible and biodegradable amphiphilic compounds characterized by a great scientific interest for their potential applications in various technological areas, including pharmaceuticals, cosmetics, agriculture, and food production. This report summarizes the available synthetic methodologies, physicochemical properties, and biological activity of sugar fatty acid ester surfactants, with a particular focus on 6-O-glucose, 6-O-mannose, 6-O-sucrose, and 6'-O-lactose ones. In detail, the synthetic approaches to this class of compounds, such as enzymatic lipase-catalyzed and traditional chemical (e.g., acyl chloride, Steglich, Mitsunobu) esterifications, are reported. Moreover, aspects related to the surface activity of these amphiphiles, such as their ability to decrease surface tension, critical micelle concentration, and emulsifying and foaming ability, are described. Biological applications with a focus on the permeability-enhancing effect across the skin or mucosa, antimicrobial and antifungal activities, as well as antibiofilm properties, are also presented. The information reported here on sugar-based ester surfactants is helpful to broaden the interest and the possible innovative applications of this class of amphiphiles in different technological fields in the future.

Intestinal Absorption Study: Challenges and Absorption Enhancement Strategies in Improving Oral Drug Delivery

The oral route is the most common and practical means of drug administration, particularly from a patient’s perspective. However, the pharmacokinetic profile of oral drugs depends on the rate of drug absorption through the intestinal wall before entering the systemic circulation. However, the enteric epithelium represents one of the major limiting steps for drug absorption, due to the presence of efflux transporters on the intestinal membrane, mucous layer, enzymatic degradation, and the existence of tight junctions along the intestinal linings. These challenges are more noticeable for hydrophilic drugs, high molecular weight drugs, and drugs that are substrates of the efflux transporters. Another challenge faced by oral drug delivery is the presence of first-pass hepatic metabolism that can result in reduced drug bioavailability. Over the years, a wide range of compounds have been investigated for their permeation-enhancing effect in order to circumvent these challenges. There is also a growing interest in developing nanocarrier-based formulation strategies to enhance the drug absorption. Therefore, this review aims to provide an overview of the challenges faced by oral drug delivery and selected strategies to enhance the oral drug absorption, including the application of absorption enhancers and nanocarrier-based formulations based on in vitro, in vivo, and in situ studies.

Impact of Intestinal Concentration and Colloidal Structure on the Permeation-Enhancing Efficiency of Sodium Caprate in the Rat

In this work, we set out to better understand how the permeation enhancer sodium caprate (C10) influences the intestinal absorption of macromolecules. FITC-dextran 4000 (FD4) was selected as a model compound and formulated with 50–300 mM C10. Absorption was studied after bolus instillation of liquid formulation to the duodenum of anesthetized rats and intravenously as a reference, whereafter plasma samples were taken and analyzed for FD4 content. It was found that the AUC and C_max of FD4 increased with increasing C10 concentration. Higher C10 concentrations were associated with an increased and extended absorption but also increased epithelial damage. Depending on the C10 concentration, the intestinal epithelium showed significant recovery already at 60–120 min after administration. At the highest studied C10 concentrations (100 and 300 mM), the absorption of FD4 was not affected by the colloidal structures of C10, with similar absorption obtained when C10 was administered as micelles (pH 8.5) and as vesicles (pH 6.5). In contrast, the FD4 absorption was lower when C10 was administered at 50 mM formulated as micelles as compared to vesicles. Intestinal dilution of C10 and FD4 revealed a trend of decreasing FD4 absorption with increasing intestinal dilution. However, the effect was smaller than that of altering the total administered C10 dose. Absorption was similar when the formulations were prepared in simulated intestinal fluids containing mixed micelles of bile salts and phospholipids and in simple buffer solution. The findings in this study suggest that in order to optimally enhance the absorption of macromolecules, high (≥100 mM) initial intestinal C10 concentrations are likely needed and that both the concentration and total dose of C10 are important parameters.

In Silico-Based Experiments on Mechanistic Interactions between Several Intestinal Permeation Enhancers with a Lipid Bilayer Model

Oral administration of drugs is generally considered convenient and patient-friendly. However, oral administration of biological drugs exhibits low oral bioavailability (BA) due to enzymatic degradation and low intestinal absorption. A possible approach to circumvent the low BA of oral peptide drugs is to coformulate the drugs with permeation enhancers (PEs). PEs have been studied since the 1960s and are molecules that enhance the absorption of hydrophilic molecules with low permeability over the gastrointestinal epithelium. In this study, we investigated the impact of six PEs on the structural properties of a model membrane using molecular dynamics (MD) simulations. The PEs included were the sodium salts of the medium chain fatty acids laurate, caprate, and caprylate and the caprylate derivative SNAC─all with a negative charge─and neutral caprate and neutral sucrose monolaurate. Our results indicated that the PEs, once incorporated into the membrane, could induce membrane leakiness in a concentration-dependent manner. Our simulations suggest that a PE concentration of at least 70-100 mM is needed to strongly affect transcellular permeability. The increased aggregation propensity seen for neutral PEs might provide a molecular-level mechanism for the membrane disruptions seen at higher concentrations in vivo. The ability for neutral PEs to flip-flop across the lipid bilayer is also suggestive of possible intracellular modes of action other than increasing membrane fluidity. Taken together, our results indicate that MD simulations are useful for gaining insights relevant to the design of oral dosage forms based around permeability enhancer molecules.

Safety and Toxicity Issues of Therapeutically Used Nanoparticles from the Oral Route

The emerging science of nanotechnology sparked a research attention in its potential benefits in comparison to the conventional materials used. Oral products prepared via nanoparticles (NPs) have garnered great interest worldwide. They are used commonly to incorporate nutrients and provide antimicrobial activity. Formulation into NPs can offer opportunities for targeted drug delivery, improve drug stability in the harsh environment of the gastrointestinal (GI) tract, increase drug solubility and bioavailability, and provide sustained release in the GI tract. However, some issues like the management of toxicity and safe handling of NPs are still debated and should be well concerned before their application in oral preparations. This article will help the reader to understand safety issues of NPs in oral drug delivery and provides some recommendations to the use of NPs in the drug industry.

Formulation strategies to improve the efficacy of intestinal permeation enhancers. Adv Drug Deliv Rev 2021;177:113925. [PMID: 34418495 DOI: 10.1016/j.addr.2021.113925]

The use of chemical permeation enhancers (PEs) is the most widely tested approach to improve oral absorption of low permeability active agents, as represented by peptides. Several hundred PEs increase intestinal permeability in preclinical bioassays, yet few have progressed to clinical testing and, of those, only incremental increases in oral bioavailability (BA) have been observed. Still, average BA values of ~1% were sufficient for two recent FDA approvals of semaglutide and octreotide oral formulations. PEs are typically screened in static in vitro and ex-vivo models where co-presentation of active agent and PE in high concentrations allows the PE to alter barrier integrity with sufficient contact time to promote flux across the intestinal epithelium. The capacity to maintain high concentrations of co-presented agents at the epithelium is not reached by standard oral dosage forms in the upper GI tract in vivo due to dilution, interference from luminal components, fast intestinal transit, and possible absorption of the PE per se. The PE-based formulations that have been assessed in clinical trials in either immediate-release or enteric-coated solid dosage forms produce low and variable oral BA due to these uncontrollable physiological factors. For PEs to appreciably increase intestinal permeability from oral dosage forms in vivo, strategies must facilitate co-presentation of PE and active agent at the epithelium for a sustained period at the required concentrations. Focusing on peptides as examples of a macromolecule class, we review physiological impediments to optimal luminal presentation, discuss the efficacy of current PE-based oral dosage forms, and suggest strategies that might be used to improve them.

A deep and permeable nanofibrous oval-shaped microwell array for the stable formation of viable and functional spheroids

Despite the potential of a nanofibrous (NF) microwell array as a permeable microwell array to improve the viability and functions of spheroids, thanks to the superior permeability to both gases and solutes, there have still been difficulties regarding the stable formation of spheroids in the NF microwell array due to the low aspect ratio (AR) and the large interspacing between microwells. This study proposes a nanofibrous oval-shaped microwell array, named the NOVA microwell array, with both a high AR and a high well density, enabling us to not only collect cells in the microwell with a high cell seeding efficiency, but also to generate multiple viable and functional spheroids in a uniform and stable manner. To realize a deep NOVA microwell array with a high aspect ratio (AR = 0.9) and a high well density (494 wells cm^-2), we developed a matched-mold thermoforming process for the fabrication of both size- and AR-controllable NOVA microwell arrays with various interspacing between microwells while maintaining the porous nature of the NF membrane. The human hepatocellular carcinoma (HepG2) cell spheroids cultured on the deep NOVA microwell array not only had uniform size and shape, with a spheroid circularity of 0.80 ± 0.03 at a cell seeding efficiency of 94.29 ± 9.55%, but also exhibited enhanced viability with a small fraction of dead cells and promoted functionality with increased albumin secretion, compared with the conventional impermeable microwell array. The superior characteristics of the deep NOVA microwell array, i.e. a high AR, a high well density, and a high permeability, pave the way to the production of various viable and functional spheroids and even organoids in a scalable manner.

Permeability-enhancing effects of three laurate-disaccharide monoesters across isolated rat intestinal mucosae

Laurate (C₁₂)-sucrose esters are established intestinal epithelial permeation enhancers (PEs) with potential for use in oral delivery. Most studies have examined blends of ester rather than specific monoesters, with little variation on the sugar moiety. To investigate the influence of varying the sugar moiety on monoester performance, we compared three monoesters: C₁₂-sucrose, C₁₂-lactose, and C₁₂-trehalose. The assays were: critical micellar concentration (CMC) in Krebs-Henseleit buffer, MTS and lactate dehydrogenase assays in Caco-2 cells, transepithelial electrical resistance (TEER) and apparent permeability coefficient (P_app) of [¹⁴C] mannitol across isolated rat intestinal mucosae, and tissue histology. For CMC, the rank order was C₁₂-trehalose (0.21 mM) < C₁₂-sucrose (0.34 mM) < C₁₂-lactose (0.43 mM). Exposure to Caco-2 cells for 120 min produced TC₅₀ values in the MTS assay from 0.1 to 0.4 mM. Each ester produced a concentration-dependent decrease in TEER across rat mucosae with 80% reduction seen with 8 mM in 5 min, but C₁₂-trehalose was less potent. C₁₂-sucrose and C₁₂-lactose increased the P_app of [¹⁴C] mannitol across mucosae with similar potency and efficacy, whereas C₁₂-trehalose was not as potent or efficacious, even though it still increased flux. In the presence of the three esters, gross intestinal histology was unaffected except at 8 mM for C₁₂-sucrose and C₁₂-lactose. In conclusion, the three esters enhanced permeability likely via tight junction modulation in rat intestinal tissue. C₁₂-trehalose was not quite as efficacious, but neither did it damage tissue to the same extent. All three can be considered as potential PEs to be included in oral formulations.

Enhancing Intestinal Absorption of a Model Macromolecule via the Paracellular Pathway using E-Cadherin Peptides

Membrane permeation enhancers have received significant attention in recent years for enabling the oral absorption of poorly permeable drug molecules. In this study, we investigated the ability of His-Ala-Val (HAV) and Ala-Asp-Thr (ADT) peptides derived from the extracellular-1 (EC1) domain of E-cadherin proteins to increase the paracellular permeation and intestinal bioavailability of the poorly permeable model macromolecule, fluorescein-isothiocyanate dextran with average molecular weight 4000 (FD4). The in vitro enzymatic stability of linear and cyclic E-cadherin peptides was characterized under simulated gastric and intestinal conditions, and the cyclic E-cadherin peptides, HAVN1 and ADTC5, which demonstrated excellent stability in vitro, were advanced to in vivo intestinal instillation studies and compared against the established surfactant membrane permeation enhancer, sodium caprate (C₁₀). Cyclic HAVN1 and ADTC5 peptides increased FD4 bioavailability by 7.2- and 4.4-fold compared to control, respectively (not statistically significant). In contrast, C₁₀ provided a statistically significant 10.7-fold relative bioavailability enhancement for FD4. Importantly, this study represents the first report of cyclic E-cadherin peptides as intestinal membrane permeation enhancers. The findings described herein demonstrate the potential of enzymatically stabilized cyclic E-cadherin peptides for increasing poorly permeable drug absorption via the oral route.

Comparison of the effects of the intestinal permeation enhancers, SNAC and sodium caprate (C10): Isolated rat intestinal mucosae and sacs

SNAC and C₁₀ are intestinal permeation enhancers (PEs) used in formulations of peptides for oral delivery in clinical trials. Our aims were to compare their: (i) mechanism of action in isolated rat intestinal mucosae mounted in Ussing chambers and in non-everted gut sacs, (ii) effects on mucosa integrity in those models and also in in situ intra-jejunal instillations and (iii) interactions with intestinal mucus. SNAC increased the apparent permeability coefficient (P_app) of the paracellular marker, FITC-dextran 4000 (FD4), across isolated rat gastric mucosae in concentration-dependent fashion, whereas C₁₀ did not, while both reduced the transepithelial electrical resistance (TEER). In isolated jejunal and colonic mucosae, both agents increased the P_app of [¹⁴C]-mannitol and FD4 whereas C₁₀ but not SNAC reduced TEER. 20 mM SNAC was required to achieve the efficacy of 10 mM C₁₀ in jejunal and colonic mucosae. In isolated non-everted jejunal and colonics sacs, FD4 flux increases were observed in the presence of both PEs. Histology of mucosae revealed that both PEs induced minor epithelial damage to the mucosa at concentrations that increased fluxes. Jejunal tissue withstood epithelial damage in the following order: intra jejunal in situ instillations > jejunal sacs > isolated jejunal mucosae. Both PEs modulated viscoelastic properties of porcine jejunal mucus without altering rheological properties. In conclusion, SNAC and C₁₀ are reasonably efficacious PEs in rat intestinal tissue with common overall mechanistic features. Their potency and toxic potential are low, in agreement with clinical trial data.

Nano and microscale delivery platforms for enhanced oral peptide/protein bioavailability

In recent years, peptide/protein drugs have attracted considerable attention owing to their superior targeting and therapeutic effect and fewer side effects compared with chemical drugs. Oral administration modality with enhanced patient compliance is increasingly being recognized as an ideal route for peptide/protein delivery. However, the limited permeation efficiency and low oral bioavailability of peptide/protein drugs significantly hinder therapeutic advances. To address these problems, various nano and microscale delivery platforms have been developed, which offer significant advantages in oral peptide/protein delivery. In this review, we briefly introduce the transport mechanisms of oral peptide/protein delivery and the primary barriers to this delivery process. We also highlight the recent advances in various nano and microscale delivery platforms designed for oral peptide/protein delivery. We then summarize the existing strategies used in these delivery platforms to improve the oral bioavailability and permeation efficiency of peptide/protein therapeutics. Finally, we discuss the major challenges faced when nano and microscale systems are used for oral peptide/protein delivery. This review is expected to provide critical insight into the design and development of oral peptide/protein delivery systems with significant therapeutic advances.

Intestinal permeation enhancers to improve oral bioavailability of macromolecules: reasons for low efficacy in humans

INTRODUCTION

Intestinal permeation enhancers (PEs) are substances that transiently alter the intestinal epithelial barrier to facilitate permeation of macromolecules with low oral bioavailability (BA). While a number of PEs have progressed to clinical testing in conventional formulations with macromolecules, there has been only low single digit increases in oral BA, irrespective of whether the drug met primary or secondary clinical endpoints.

AREAS COVERED

This article considers the causes of sub-optimal BA of macromolecules from PE dosage forms and suggests approaches that may improve performance in humans.

EXPERT OPINION

Permeation enhancement is most effective when the PE is co-localized with the macromolecule at the epithelial surface. Conditions in the GI tract impede optimal co-localization. Novel delivery systems that limit dilution and spreading of the PE and macromolecule in the small intestine have attempted to replicate promising enhancement efficacy observed in static drug delivery models.

Advanced delivery strategies facilitating oral absorption of heparins

Heparins show great anticoagulant effect with few side effects, and are administered by subcutaneous or intravenous route in clinics. To improve patient compliance, oral administration is an alternative route. Nonetheless, oral administration of heparins still faces enormous challenges due to the multiple obstacles. This review briefly analyzes a series of barriers ranging from poorly physicochemical properties of heparins, to harsh biological barriers including gastrointestinal degradation and pre-systemic metabolism. Moreover, several approaches have been developed to overcome these obstacles, such as improving stability of heparins in the gastrointestinal tract, enhancing the intestinal epithelia permeability and facilitating lymphatic delivery of heparins. Overall, this review aims to provide insights concerning advanced delivery strategies facilitating oral absorption of heparins.

Solid lipid nanocarriers diffuse effectively through mucus and enter intestinal cells - but where is my peptide?

Peptides are therapeutic molecules with high potential to treat a wide variety of diseases. They are large hydrophilic compounds for which absorption is limited by the intestinal epithelial border covered by mucus. This study aimed to evaluate the potential of Hydrophobic Ion Pairing combined with Solid Lipid Nanoparticles (SLN) and Nanostructured Lipid Carriers (NLC) to improve peptide transport across the intestinal border using Caco-2 cell monolayers (enterocyte-like model) and Caco-2/HT29-MTX co-cultured monolayers (mucin-secreting model). A Hydrophobic Ion Pair (HIP) was formed between Leuprolide (LEU), a model peptide, and sodium docusate. The marked increase in peptide lipophilicity enabled high encapsulation efficiencies in both NLC (84%) and SLN (85%). After co-incubation with the nanoparticles, confocal microscopy images of the cell monolayers demonstrated particles internalization and ability to cross mucus. Flow cytometry measurements confirmed that 82% of incubated SLN and 99% of NLC were internalized by Caco-2 cells. However, LEU transport across cell monolayers was not improved by the nanocarriers. Indeed, combination of particles platelet-shape and HIP low stability in the transport medium led to LEU burst release in this environment. Improvement of peptide lipidization should maintain encapsulation and enable benefit from nanocarriers enhanced intestinal transport.

Salcaprozate sodium (SNAC) enhances permeability of octreotide across isolated rat and human intestinal epithelial mucosae in Ussing chambers

Octreotide is approved as a one-month injectable for treatment of acromegaly and neuroendocrine tumours. Oral delivery of the octapeptide is a challenge due mainly to low intestinal epithelial permeability. The intestinal permeation enhancer (PE) salcaprozate sodium (SNAC) has Generally Regarded As Safe (GRAS) status and is a component of an approved oral peptide formulation. The purpose of the study was to examine the capacity of salcaprozate sodium (SNAC), to increase its permeability across isolated rat intestinal mucosae from five regions and across human colonic mucosae mounted in Ussing chambers. Apical-side buffers were Kreb's-Henseleit (KH), fasted simulated intestinal fluid (FaSSIF-V2), rat simulated intestinal fluid (rSIF), and colonic simulated intestinal fluid (FaSSCoF). The basal apparent permeability coefficient (P_app) of [³H]-octreotide was equally low across rat intestinal regional mucosae in KH, rSIF, and FaSSIF-V2. Apical addition of 20 mM SNAC increased the P_app across rat tissue in KH: colon (by 3.2-fold) > ileum (3.4-fold) > upper jejunum (2.3-fold) > duodenum (1.4-fold) > stomach (1.4-fold). 20 mM and 40 mM SNAC also increased the P_app by 1.5-fold and 2.1-fold respectively across human colonic mucosae in KH. Transepithelial electrical resistance (TEER) values were reduced in the presence in SNAC especially in colonic regions. LC-MS/MS analysis of permeated unlabelled octreotide across human colonic mucosae in the presence of SNAC indicated that [³H]-octreotide remained intact. No gross damage was caused to rat or human mucosae by SNAC. Attenuation of the effects of SNAC was seen in rat jejunal mucosae incubated with FaSSIF-V2 and rSIF, and also to some extent in human colonic mucosae using FaSSCoF, suggesting interaction between SNAC with buffer components. In conclusion, SNAC showed potential as an intestinal permeation enhancer for octreotide, but in vivo efficacy may be attenuated by interactions with GI luminal fluid contents.

The In Vivo Effect of Transcellular Permeation Enhancers on the Intestinal Permeability of Two Peptide Drugs Enalaprilat and Hexarelin

Permeation enhancers like sodium dodecyl sulfate (SDS) and caprate increase the intestinal permeability of small model peptide compounds, such as enalaprilat (349 Da). However, their effects remain to be investigated for larger low-permeability peptide drugs, such as hexarelin (887 Da). The objective of this single-pass perfusion study in rat was to investigate the effect of SDS at 5 mg/mL and of caprate administered at different luminal concentrations (5, 10, and 20 mg/mL) and pH (6.5 and 7.4). The small intestinal permeability of enalaprilat increased by 8- and 9-fold with SDS at 5 mg/mL and with caprate at 10 and 20 mg/mL but only at pH 7.4, where the free dissolved caprate concentration is higher than at pH 6.5 (5 vs. 2 mg/mL). Neither SDS nor caprate at any of the investigated luminal concentrations enhanced absorption of the larger peptide hexarelin. These results show that caprate requires doses above its saturation concentration (a reservoir suspension) to enhance absorption, most likely because dissolved caprate itself is rapidly absorbed. The absent effect on hexarelin may partly explain why the use of permeation enhancers for enabling oral peptide delivery has largely failed to evolve from in vitro evaluations into approved oral products. It is obvious that more innovative and effective drug delivery strategies are needed for this class of drugs.

Evaluation of Sucrose Laurate as an Intestinal Permeation Enhancer for Macromolecules: Ex Vivo and In Vivo Studies

Oral delivery of macromolecules requires permeation enhancers (PEs) adaptable to formulation. Sucrose laurate (SL) (D1216), a food grade surfactant, was assessed in Caco-2 monolayers, isolated rat intestinal tissue mucosae, and rat intestinal instillations. Accordingly, 1 mM SL increased the apparent permeability coefficient (Papp) of [14C]-mannitol and reduced transepithelial electrical resistance (TEER) across monolayers. It altered expression of the tight junction protein, ZO-1, increased plasma membrane potential, and decreased mitochondrial membrane potential in Caco-2 cells. The concentrations that increased flux were of the same order as those that induced cytotoxicity. In rat colonic tissue mucosae, the same patterns emerged in respect to the concentration-dependent increases in paracellular marker fluxes and TEER reductions with 5 mM being the key concentration. While the histology revealed some perturbation, ion transport capacity was retained. In rat jejunal and colonic instillations, 50 and 100 mM SL co-administered with insulin induced blood glucose reductions and achieved relative bioavailability values of 2.4% and 8.9%, respectively, on a par with the gold standard PE, sodium caprate (C10). The histology of the intestinal loops revealed little damage. In conclusion, SL is a candidate PE with high potential for emulsion-based systems. The primary action is plasma membrane perturbation, leading to tight junction openings and a predominant paracellular flux.

Labrasol® is an efficacious intestinal permeation enhancer across rat intestine: Ex vivo and in vivo rat studies

Labrasol® ALF (Labrasol®), is a non-ionic surfactant excipient primarily used as a solubilising agent. It was investigated here as an intestinal permeation enhancer in isolated rat colonic mucosae in Ussing chamber and in rat in situ intestinal instillations. Labrasol® comprises mono-, di- and triglycerides and mono- and di- fatty acid esters of polyethylene glycol (PEG)-8 and free PEG-8, with caprylic (C₈)- and capric acid (C₁₀) as the main fatty acids. Source components of Labrasol® as well as Labrasol® modified with either C₈ or C₁₀ as the sole fatty acid components were also tested to determine which element of Labrasol® was responsible for its permeability-enhancing properties. Labrasol® (4, 8 mg/mL) enhanced the transport of the paracellular markers, [¹⁴C] mannitol, FITC-dextran 4000, and FITC-insulin across colonic mucosae. The enhancement was non-damaging, transient, and molecular weight-dependent. The PEG ester fraction of Labrasol® also had enhancing properties. When insulin was administered with Labrasol® in instillations, it had a relative bioavailability of 7% in jejunum and 12% in colon. C₈- and C₁₀ versions of Labrasol® and the PEG ester fraction also induced similar bioavailability values in jejunal instillations: 6, 5 and 7% respectively. Inhibition of lipases in instillations did not reduce the efficacy of Labrasol®, suggesting that its mechanism as a PE is not simply due to liberated medium chain fatty acids. Labrasol® acts as an efficacious intestinal permeation enhancer and has potential for use in oral formulations of macromolecules and BCS Class III molecules.

Intestinal Permeation Enhancers for Oral Delivery of Macromolecules: A Comparison between Salcaprozate Sodium (SNAC) and Sodium Caprate (C10)

Salcaprozate sodium (SNAC) and sodium caprate (C10) are two of the most advanced intestinal permeation enhancers (PEs) that have been tested in clinical trials for oral delivery of macromolecules. Their effects on intestinal epithelia were studied for over 30 years, yet there is still debate over their mechanisms of action. C10 acts via openings of epithelial tight junctions and/or membrane perturbation, while for decades SNAC was thought to increase passive transcellular permeation across small intestinal epithelia based on increased lipophilicity arising from non-covalent macromolecule complexation. More recently, an additional mechanism for SNAC associated with a pH-elevating, monomer-inducing, and pepsin-inhibiting effect in the stomach for oral delivery of semaglutide was advocated. Comparing the two surfactants, we found equivocal evidence for discrete mechanisms at the level of epithelial interactions in the small intestine, especially at the high doses used in vivo. Evidence that one agent is more efficacious compared to the other is not convincing, with tablets containing these PEs inducing single-digit highly variable increases in oral bioavailability of payloads in human trials, although this may be adequate for potent macromolecules. Regarding safety, SNAC has generally regarded as safe (GRAS) status and is Food and Drug Administration (FDA)-approved as a medical food (Eligen®-Vitamin B12, Emisphere, Roseland, NJ, USA), whereas C10 has a long history of use in man, and has food additive status. Evidence for co-absorption of microorganisms in the presence of either SNAC or C10 has not emerged from clinical trials to date, and long-term effects from repeat dosing beyond six months have yet to be assessed. Since there are no obvious scientific reasons to prefer SNAC over C10 in orally delivering a poorly permeable macromolecule, then formulation, manufacturing, and commercial considerations are the key drivers in decision-making.

Application of Permeation Enhancers in Oral Delivery of Macromolecules: An Update

The application of permeation enhancers (PEs) to improve transport of poorly absorbed active pharmaceutical ingredients across the intestinal epithelium is a widely tested approach. Several hundred compounds have been shown to alter the epithelial barrier, and although the research emphasis has broadened to encompass a role for nanoparticle approaches, PEs represent a key constituent of conventional oral formulations that have progressed to clinical testing. In this review, we highlight promising PEs in early development, summarize the current state of the art, and highlight challenges to the translation of PE-based delivery systems into safe and effective oral dosage forms for patients.

Synthesis, Structure⁻Activity Relationships and In Vitro Toxicity Profile of Lactose-Based Fatty Acid Monoesters as Possible Drug Permeability Enhancers

Permeability enhancers are receiving increased attention arising from their ability to increase transepithelial permeability and thus, bioavailability of orally or pulmonary administered biopharmaceutics. Here we present the synthesis and the in vitro assaying of a series of lactose-based non-ionic surfactants, highlighting the relationship between their structure and biological effect. Using tensiometric measurements the critical micelle concentrations (CMCs) of the surfactants were determined and demonstrate that increasing hydrophobic chain length reduces surfactant CMC. In vitro testing on Caco-2 intestinal and Calu-3 airway epithelia revealed that cytotoxicity, assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) release assays, is presented for most of the surfactants at concentrations greater than their CMCs. Further biological study demonstrates that application of cytotoxic concentrations of the surfactants is associated with depolarizing mitochondrial membrane potential, increasing nuclear membrane permeability and activation of effector caspases. It is, therefore, proposed that when applied at cytotoxic levels, the surfactants are inducing apoptosis in both cell lines tested. Importantly, through the culture of epithelial monolayers on Transwell^® supports, the surfactants demonstrate the ability to reversibly modulate transepithelial electrical resistance (TEER), and thus open tight junctions, at non-toxic concentrations, emphasizing their potential application as safe permeability enhancers in vivo.

Labrasol® and Salts of Medium-Chain Fatty Acids Can Be Combined in Low Concentrations to Increase the Permeability of a Macromolecule Marker Across Isolated Rat Intestinal Mucosae

In addition to their solubilizing properties, excipients used in lipid-based formulations can improve intestinal permeability of macromolecules. We determined whether admixing of medium-chain fatty acid (MCFA) permeation enhancers with a lipoidal excipient (Labrasol^®) could potentiate transepithelial flux of a poorly permeable macromolecule (fluorescein isothiocyanate dextran 4 kDa [FD4]) across rat intestinal mucosae mounted in Ussing chambers. Low concentrations of sodium caprate (C₁₀), sodium undecylenate (C_11:1), or sodium laurate (C₁₂) combined with Labrasol^® increased the apparent permeability coefficient (P_app) of FD4 to values typically seen with higher concentrations of MCFAs or Labrasol^® alone. For example, combination of C_11:1 (0.5 mg/mL) with Labrasol^® (1 mg/mL) increased the P_app of FD4 by 10- and 11-fold over the respective individual agents at the same concentrations where no enhancement was evident. The increased enhancement ratios seen with the combinations were associated with some perturbation in intestinal histology and with attenuation of an epithelial functional measure, carbachol-stimulated inward short-circuit current. In conclusion, combining three MCFAs separately with Labrasol^® increased the P_app of FD4 to values greater than those seen for MCFAs or Labrasol^® alone. Ultimately, this may permit lower concentrations of MCFA to be used in combination with other excipients in oral formulations of poorly permeable molecules.