Influence of Bile Composition on Membrane Incorporation of Transient Permeability Enhancers

The stomach, small intestine, and colon-specific gastrointestinal tract delivery systems for bioactive nutrients

Oral administration is a convenient way to deliver bioactive nutrients. However, the complex and dynamic environment of the gastrointestinal (GI) tract poses distinct challenges. These include the acidic environment of the stomach, limited transport across the GI mucosa, and the risk of enzymatic degradation, all of which can compromise the nutritional effectiveness of orally delivered nutrients. In response to these challenges, various GI tract delivery systems have been developed to target specific regions, such as the stomach, small intestine, or colon, to precisely control the release of bioactive nutrients and enhance their health-promoting benefits. This review critically examines the principles underlying stomach-, small intestine-, and colon-targeted delivery systems, highlighting the selection of appropriate wall materials and the interactions between delivery systems and the mucosal epithelial barrier. Moreover, we describe relevant biological models and quantitative analyses to measure these interactions. In particular, we emphasize the significant advantages offered by colon-targeted delivery systems in maintaining a healthy colonic microenvironment. This review aims to inspire novel concepts and stimulate further research into GI tract delivery systems, offering promising avenues for maximizing the therapeutic effects of bioactive nutrients in practical applications.

The Impact of Permeation Enhancers on Transcellular Permeation of Small Molecule Drugs

Passive permeation through an epithelial membrane may be enhanced by using a class of amphiphilic molecules known as permeation enhancers (PEs). PEs have been studied in clinical trials and used in coformulations with peptides and small molecule drugs, and yet, an understanding of the permeant-PE interactions leaves much to be desired. This manuscript uses all-atom molecular dynamics (MD) simulations to showcase the effects of sodium caprate (C10) and salcaprozate sodium (SNAC), two commonly applied PEs, on membrane properties and the free energy profiles of five small molecule drugs (mannitol, atenolol, ketoprofen, decanedecaol, mucic acid). Our results show that both C10 and SNAC make the lipid molecules pack more densely, but C10 increases the lipid lateral diffusivity while SNAC decreases it. The change in the lipid order parameter also shows both PEs increasing the order near the lipid heads, possibly due to the dense packing in the membrane. A decrease in the central barrier of the permeation free energy was observed by embedding PEs into a lipid bilayer and SNAC is more efficient in doing so than C10. Neither SNAC nor C10 has a large impact on the diffusion coefficient of the small molecules. The analysis of the MD simulations revealed that PEs make the membrane tail region more hydrophilic by forming hydrogen bonds with small molecule drugs, i.e., decreasing the central barrier of the permeation free energy. While this study was only limited to small molecule drugs, this lays the groundwork for future studies to which the effects of the PEs in the permeation of macromolecules and peptides may be observed.

Molecular Investigation of SNAC as an Oral Peptide Permeation Enhancer in Lipid Membranes via Solid-State NMR

Oral peptide therapeutics are increasingly favored in the pharmaceutical industry for their ease of use and better patient adherence. However, they face challenges with poor oral bioavailability due to their high molecular weight and surface polarity. Permeation enhancers (PEs) like salcaprozate sodium (SNAC) have shown promise in clinical trials, achieving about 1% bioavailability. One proposed mechanism for enhancing permeation is membrane perturbation or fluidization, though direct experimental proof and quantitative analysis of these effects are still needed. This study employs solid-state NMR (ssNMR) to investigate how SNAC interacts with hydrated DMPC liposomes, measuring enhancements in membrane fluidity across interfacial and transmembrane regions. The methodology involves analyzing phosphate lipid headgroups and acyl chains using static 31P chemical shift anisotropy and 2H quadrupolar coupling measurements alongside 1H and 13C magic angle spinning NMR for motional averaging of 1H-1H and 1H-13C dipolar couplings. Our findings indicate an overall increase in the uniaxial motion of phospholipids with SNAC in a PE concentration-dependent manner. It boosts lipid headgroup dynamics and enhancement plateaus at 25% between 24 and 72 mM concentrations. SNAC effectively enhances the fluidity of the hydrophobic center by 43% at 72 mM PE concentration, more significantly than the interfacial region. It is worth noting that the extent of liposome dissolution and conversion to micelles increases as SNAC concentration rises. Including a model peptide drug, octreotide, introduces a competitive equilibrium in this complex PE-lipid-peptide system, further influencing membrane dynamics for peptide permeation. Interestingly, the membrane enhancement does not show the expected plateau, and a less significant lipid mobility increase is observed in the presence of octreotide, suggesting a less substantial impact compared to peptide-free systems, which is likely due to peptide-PE interactions that consume monomeric SNAC, reducing its interaction with the lipid membrane. This study provides the first quantitative and site-specific ssNMR measurements of membrane mobility influenced by one representative PE as a snapshot of PE lipid interaction in a liposome model, demonstrating how peptide drugs modulate competitive equilibria and PE-induced lipid dynamics.

Development of 3D-Printed Two-Compartment Capsular Devices for Pulsatile Release of Peptide and Permeation Enhancer

OBJECTIVE

The oral absorption of a peptide is driven by a high local concentration of a permeation enhancer (PE) in the gastrointestinal tract. We hypothesized that a controlled release of both PE and peptide from a solid formulation, capable of maintaining an effective co-localized concentration of PE and peptide could enhance oral peptide absorption. In this study, we aimed to develop a 3D-printed two-compartment capsular device with controlled pulsatile release of peptide and sodium caprate (C10).

METHODS

3D-printed two-compartment capsular device was fabricated using a fused deposition modeling method. This device was then filled with LY peptide and C10. The release profile was modulated by changing the thickness and polymer type of the capsular device. USP apparatus II dissolution test was used to evaluate the impacts of device thickness and polymer selection on release profile in vitro. An optimal device was then enteric coated with HPMCAS.

RESULTS

A strong linear relationship between the thickness of capsular devices and the delay in the release onset time was observed. An increase in the device thickness or the use of PLA decreased the release rate. The capsular device with compartment 1, compartment 2 and fence thickness of 0.4; 0.95 and 0.5 mm, respectively, and the use of PVA achieved desired pulsatile release profiles of both peptide and C10. Furthermore, enteric-coated capsular devices with HPMCAS had similar pulsatile release profiles compared to non-enteric coated devices.

CONCLUSION

These findings suggest potential application of 3D-printing techniques in the formulation development for complex modified drug release products.

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.

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.

Drug solubilization in dog intestinal fluids with and without administration of lipid-based formulations

The use of animal experiments can be minimized with computational models capable of reflecting the simulated environments. One such environment is intestinal fluid and the colloids formed in it. In this study we used molecular dynamics simulations to investigate solubilization patterns for three model drugs (carvedilol, felodipine and probucol) in dog intestinal fluid, a lipid-based formulation, and a mixture of both. We observed morphological transformations that lipids undergo due to the digestion process in the intestinal environment. Further, we evaluated the effect of bile salt concentration and observed the importance of interindividual variability. We applied two methods of estimating solubility enhancement based on the simulated data, of which one was in good qualitative agreement with the experimentally observed solubility enhancement. In addition to the computational simulations, we also measured solubility in i) aspirated dog intestinal fluid samples and ii) simulated canine intestinal fluid in the fasted state, and found there was no statistical difference between the two. Hence, a simplified dissolution medium suitable for in vitro studies provided physiologically relevant data for the systems explored. The computational protocol used in this study, coupled with in vitro studies using simulated intestinal fluids, can serve as a useful prescreening tool in the process of drug delivery strategies development.

Interactions of oral permeation enhancers with lipid membranes in simulated intestinal environments

The oral bioavailability of therapeutic peptides is generally low. To increase peptide transport across the gastrointestinal barrier, permeation enhancers are often used. Despite their widespread use, mechanistic knowledge of permeation enhancers is limited. To address this, we here investigate the interactions of six commonly used permeation enhancers with lipid membranes in simulated intestinal environments. Specifically, we study the interactions of the permeation enhancers sodium caprate, dodecyl maltoside, sodium cholate, sodium dodecyl sulfate, melittin, and penetratin with epithelial cell-like model membranes. To mimic the molecular composition of the real intestinal environment, the experiments are performed with two peptide drugs, salmon calcitonin and desB30 insulin, in fasted-state simulated intestinal fluid. Besides providing a comparison of the membrane interactions of the studied permeation enhancers, our results demonstrate that peptide drugs as well as intestinal-fluid components may substantially change the membrane activity of permeation enhancers. This highlights the importance of testing permeation enhancement in realistic physiological environments and carefully choosing a permeation enhancer for each individual peptide drug.

Interaction between Permeation Enhancers and Lipid Bilayers

Permeation enhancers (PEs) are a class of molecules that interact with the epithelial membrane and transiently increase its transcellular permeability. Although there have been few clinical trials of PE coformulated drugs, the mechanism of action of PEs remains elusive. In this paper, the interaction between two archetypes of PEs [salcaprozate sodium (SNAC) and sodium caprate (C10)] and membranes is investigated with extensive all-atom molecular dynamics simulations. The simulations show that (1) the association between the neutral PEs and membranes is favored in free energy, (2) the propensity of neutral PE aggregation is larger in aqueous solution than in lipid bilayers, (3) the equilibrium distribution of neutral PEs in membranes is fast, e.g., accessible with unbiased MD simulations, and (4) the micelle of neutral PEs formed in aqueous solution does not rupture the membranes (e.g., not forming pores or breaking up the membrane) under simulation conditions. All results combined, this study indicates that PEs insert into the membranes in an equilibrium or near equilibrium process. This study lays the foundation for future investigations of how PEs impact the free energy of permeation for small molecules.

Medium-Chain Triacylglycerols (MCTs) and Their Fractions in Drug Delivery Systems : A Systematic Review

Medium-chain triacylglycerol (MCT) is a type of triacylglycerol that has six or seven to twelve carbon chains. It consists of three molecules of fatty acids attached to one molecule of glycerol. Drug delivery system (DDS) is defined as a formulation to distribute drugs into the human body. The unique properties of MCTs have garnered interest in using them as excipients in DDS. Even though there are many significant effects attributed to the use of MCTs, especially in modulating the rate of drug delivery in various DDS, they are all limited and intermittent. This warrants a detailed summary of the previous studies on the use of MCTs in various DDS. Therefore, this review focuses on presenting a systematic review of previous studies on the use of MCTs in the last six years and explores the types and effects of MCTs on DDS that employ various types of delivery routes. A systematic search through PubMed, Science Direct and Scopus was performed. Keywords like "medium-chain triglycerides", "medium-chain fatty acids", "medium-chain triglycerides and their fractions", "medium-chain fatty acids and their fractions", "MCTs", "MCFA", "in drug delivery", "in drug delivery system" and their combinations were used. The synonyms of the words were also used to extend the search. A total of 17 articles that met the inclusion criteria were identified. Findings from this review have identified the several MCTs and their fractions used in DDS that employed the oral/enteral, topical, transdermal, parenteral, and pulmonary routes of drug delivery. The review also highlights that the usage of MCTs in DDS results in a better transportation of drugs into the human body.

Improved Therapeutic Efficacy of MT102, a New Anti-Inflammatory Agent, via a Self-Microemulsifying Drug Delivery System, in Ulcerative Colitis Mice

MT-102 is a new anti-inflammatory agent derived from Juglans mandshurica and Isatis indigotica. Its therapeutic potential is hindered by low aqueous solubility, impacting its in vivo efficacy. Therefore, this study aimed to develop a self-microemulsifying drug delivery system (SMEDDS) for MT-102 to enhance its oral efficacy in treating ulcerative colitis. Solubility assessment in different oils, surfactants, and cosurfactants led to a SMEDDS formulation of MT-102 using Capmul MCM, Tween 80, and propylene glycol. Based on a pseudoternary phase diagram, the optimal SMEDDS composition was selected, which consisted of 15% Capmul MCM, 42.5% Tween 80, and 42.5% propylene glycol. The resulting optimized SMEDDS (SMEDDS-F1) exhibited a narrow size distribution (177.5 ± 2.80 nm) and high indirubin content (275 ± 5.58 µg/g, a biomarker). Across an acidic to neutral pH range, SMEDDS-F1 showed rapid and extensive indirubin release, with dissolution rates approximately 15-fold higher than pure MT-102. Furthermore, oral administration of SMEDDS-F1 effectively mitigated inflammatory progression and symptoms in a mouse model of ulcerative colitis, whereas pure MT-102 was ineffective. SMEDDS-F1 minimized body weight loss (less than 5%) without any significant change in colon length and the morphology of colonic tissues, compared to those of the healthy control group. In addition, oral administration of SMEDDS-F1 significantly inhibited the secretion of pro-inflammatory cytokines such as IL-6 and TNF-α. In conclusion, the SMEDDS-F1 formulation employing Capmul MCM, Tween 80, and propylene glycol (15:42.5:42.5, w/w) enhances the solubility and therapeutic efficacy of MT-102.

Capric Acid and Myristic Acid Permeability Enhancers in Curved Liposome Membranes

Liposomes are considered as advanced drug delivery systems for cancer treatment. A generation of pH-sensitive liposomes is being developed that use fatty acids (FAs) as a trigger for drug release in tumor tissues. However, FAs are also known to enhance permeability, and it is unclear whether FAs in liposomes may cause drug leakage or premature drug release. The passive permeability of the drug through the membrane of the liposome is thus a crucial factor for timely drug delivery. To investigate how the curvature and lipid composition of liposomes affect their passive permeability, coarse-grained molecular dynamics were performed. The permeability was determined with a counting method. Flat bilayers and three liposomes with varying diameters were studied, which had varying lipid compositions of dipalmitoylphosphatidylcholine, cholesterol, and deprotonated or neutral saturated FAs. The investigated permeants were water and two other small permeants, which have different free energy profiles (solubility) across the membrane. First, for the curvature effect, our results showed that curvature increases the water permeability by reducing the membrane thickness. The permeability increase for water is about a factor of 1.7 for the most curved membranes. However, a high curvature decreases permeability for permeants with free energy profiles that are a mix of wells and barriers in the headgroup region of the membrane. Importantly, the type of experimental setup is expected to play a dominant role in the permeability value, i.e., whether permeants are escaping or entering the liposomes. Second, for the composition effect, FAs decrease both the area per lipid (APL) and the membrane thickness, resulting in permeability increases of up to 55%. Cholesterol has a similar effect on the APL but has the opposite impact on membrane thickness and permeability. Therefore, FAs and cholesterol have opposing effects on permeability, with cholesterol's effect being slightly stronger in our simulated bilayers. As all permeability values were well within a factor of 2, and with liposomes usually being larger and less curved in experimental applications, it can be concluded that the passive drug release from a pH-sensitive liposome does not seem to be significantly affected by the presence of FAs.

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.

SNAC for Enhanced Oral Bioavailability: An Updated Review

The delivery of proteins and peptides via an oral route poses numerous challenges to improve the oral bioavailability and patient compliance. To overcome these challenges, as well as to improve the permeation of proteins and peptides via intestinal mucosa, several chemicals have been studied such as surfactants, fatty acids, bile salts, pH modifiers, and chelating agents, amongst these medium chain fatty acid like C10 (sodium caprate) and Sodium N-[8-(2-hydroxybenzoyl) amino] caprylate (SNAC) and its derivatives that have been well studied from a clinical perspective. This current review enumerates the challenges involved in protein and peptide delivery via the oral route, i.e., non-invasive routes of protein and peptide administration. This review also covers the chemistry behind SNAC and toxicity as well as mechanisms to enhance the oral delivery of clinically proven molecules like simaglutide and other small molecules under clinical development, as well as other permeation enhancers for efficient delivery of proteins and peptides.

In Vivo Mechanism of Action of Sodium Caprate for Improving the Intestinal Absorption of a GLP1/GIP Coagonist Peptide

Sodium caprate (C10) has been widely evaluated as an intestinal permeation enhancer for the oral delivery of macromolecules. However, the effect of C10 on the intestinal absorption of peptides with different physicochemical properties and its permeation-enhancing effect in vivo remains to be understood. Here, we evaluated the effects of C10 on intestinal absorption in rats with a glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GIP-GLP1) dual agonist peptide (LY) and semaglutide with different enzymatic stabilities and self-association behaviors as well as the oral exposure of the LY peptide in minipigs. Furthermore, we investigated the mechanism of action (MoA) of C10 for improving the intestinal absorption of the LY peptide in vivo via live imaging of the rat intestinal epithelium and tissue distribution of the LY peptide in minipigs. The LY peptide showed higher proteolytic stability in pancreatin and was a monomer in solution compared to that in semaglutide. C10 increased in vitro permeability in the minipig intestinal organoid monolayer to a greater extent for the LY peptide than for semaglutide. In the rat jejunal closed-loop model, C10 increased the absorption of LY peptide better than that of semaglutide, which might be attributed to higher in vitro proteolytic stability and permeability of the LY peptide. Using confocal live imaging, we observed that C10 enabled the rapid oral absorption of a model macromolecule (FD4) in the rat intestine. In the duodenum tissues of minipigs, C10 was found to qualitatively reduce the tight junction protein level and allow peptide uptake to the intestinal cells. C10 decreased the transition temperature of the artificial lipid membrane, indicating an increase in membrane fluidity, which is consistent with the above in vivo imaging results. These data indicated that the LY's favorable physicochemical properties combined with the effects of C10 on the intestinal mucosa resulted in an ∼2% relative bioavailability in minipigs.

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.

Considerations in the developability of peptides for oral administration when formulated together with transient permeation enhancers

This paper reviews many of the properties of a peptide that need to be considered prior to development as an oral dosage form when co-formulated with a permeation enhancer to improve oral bioavailability, including the importance and implications of peptide half-life on variability in pharmacokinetic profiles. Clinical considerations in terms of food and drug-drug interactions are also discussed. The paper further gives a brief overview how permeation enhancers overcome barriers that limit oral absorption of peptides and thereby improve their oral bioavailability, albeit bioavailabilities are still low single digit and variability is high.

Characterization of the physicochemical interactions between exenatide and two intestinal permeation enhancers: sodium caprate (C10) and salcaprozate sodium (SNAC)

A common approach to tackle the poor intestinal membrane permeability of peptides after oral administration is to formulate them with a permeation enhancer (PE). Increased oral bioavailability for oral peptide candidates has been reported from clinical trials when either salcaprozate sodium (SNAC) or sodium caprate (C₁₀) is incorporated in the formulation. However, little is known about how they physically interact with peptides in solution. Our objective was to compare the biophysical interactions between the GLP-1 analogue exenatide (Byetta®, Lilly), and C₁₀ or SNAC using a variety of advanced analytical techniques. First, critical micelle concentration was measured in different buffers for both PEs. Dynamic light scattering (DLS) measurements revealed specific supramolecular structures arising from exenatide-PE association. Surface plasmon resonance (SPR) indicated the formation of exenatide-PE complexes with a high contribution from non-specific interactions and rapid binding kinetics, resulting in overall low affinities. DLS and isothermal titration calorimetry (ITC) were used to examine the supramolecular organization of the PEs, and revealed thermodynamic signatures characterized by unfavourable enthalpic contributions compensated by favourable entropic ones, but with low-affinity estimates in water (K_D in the 10-100 µM range). With affinity capillary electrophoresis (ACE), weak interactions between exenatide and SNAC or C₁₀ were confirmed in saline, with a dissociation constant around 10 µM and 30 µM respectively. In biorelevant intestinal media, the bile salts in FaSSIF and FeSSIF further reduced the binding of both agents to exenatide (K_D ≈ 100 µM), indicating that the interaction between the PEs and exenatide might be inhibited by bile salts in the GI lumen. This study suggests that the interactions of both PEs with exenatide follow a similar non-covalent mechanism and are of low affinity.

Micellar Composition Affects Lipid Accretion Kinetics in Molecular Dynamics Simulations: Support for Lipid Network Reproduction

Mixed lipid micelles were proposed to facilitate life through their documented growth dynamics and catalytic properties. Our previous research predicted that micellar self-reproduction involves catalyzed accretion of lipid molecules by the residing lipids, leading to compositional homeostasis. Here, we employ atomistic Molecular Dynamics simulations, beginning with 54 lipid monomers, tracking an entire course of micellar accretion. This was done to examine the self-assembly of variegated lipid clusters, allowing us to measure entry and exit rates of monomeric lipids into pre-micelles with different compositions and sizes. We observe considerable rate-modifications that depend on the assembly composition and scrutinize the underlying mechanisms as well as the energy contributions. Lastly, we describe the measured potential for compositional homeostasis in our simulated mixed micelles. This affirms the basis for micellar self-reproduction, with implications for the study of the origin of life.

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.

Explicit-pH Coarse-Grained Molecular Dynamics Simulations Enable Insights into Restructuring of Intestinal Colloidal Aggregates with Permeation Enhancers

Permeation enhancers (PEs) can increase the bioavailability of drugs. The mechanisms of action of these PEs are complex, but, typically, when used for oral administration, they can transiently induce the alteration of trans- and paracellular pathways, including increased solubilization and membrane fluidity, or the opening of the tight junctions. To elucidate these mechanistic details, it is important to understand the aggregation behavior of not only the PEs themselves but also other molecules already present in the intestine. Aggregation processes depend critically on, among other factors, the charge state of ionizable chemical groups, which is affected by the pH of the system. In this study, we used explicit-pH coarse-grained molecular dynamics simulations to investigate the aggregation behavior and pH dependence of two commonly used PEs—caprate and SNAC—together with other components of fasted- and fed-state simulated intestinal fluids. We also present and validate a coarse-grained molecular topology for the bile salt taurocholate suitable for the Martini3 force-field. Our results indicate an increase in the number of free molecules as a function of the system pH and for each combination of FaSSIF/FeSSIF and PEs. In addition, there are differences between caprate and SNAC, which are rationalized based on their different molecular structures and critical micelle concentrations.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

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.

Investigation of Self-Emulsifying Drug-Delivery System Interaction with a Biomimetic Membrane under Conditions Relevant to the Small Intestine

Self-emulsifying drug-delivery systems (SEDDS) have been extensively shown to increase oral absorption of solvation-limited compounds. However, there has been little clinical and commercial use of these formulations, in large part because the demonstrated advantages of SEDDS have been outweighed by our inability to precisely predict drug absorption from SEDDS using current in vitro assays. To overcome this limitation and increase the biological relevancy of in vitro assays, an absorption function can be incorporated using biomimetic membranes. However, the effects that SEDDS have on the integrity of a biomimetic membrane are not known. In this study, a quartz crystal microbalance with dissipation monitoring and total internal reflection fluorescence microscopy were employed as complementary methods to in vitro lipolysis-permeation assays to characterize the interaction of various actively digested SEDDS with a liquescent artificial membrane comprising lecithin in dodecane (LiDo). Observations from surface analysis showed that interactions between the digesting SEDDS and LiDo membrane coincided with inflection points in the digestion profiles. Importantly, no indications of membrane damage could be observed, which was supported by flux profiles of the lipophilic model drug felodipine (FEL) and impermeable marker Lucifer yellow on the basal side of the membrane. There was a correlation between the digestion kinetics of the SEDDS and the flux of FEL, but no clear correlation between solubilization and absorption profiles. Membrane interactions were dependent on the composition of lipids within each SEDDS, with the more digestible lipids leading to more pronounced interactions, but in all cases, the integrity of the membrane was maintained. These insights demonstrate that LiDo membranes are compatible with in vitro lipolysis assays for improving predictions of drug absorption from lipid-based formulations.

Transient Permeation Enhancer® (TPE®) technology for oral delivery of octreotide: a technological evaluation

INTRODUCTION

The FDA approval of oral semaglutide for type 2 diabetes (2019) and oral octreotide for acromegaly (2020) is evidence that selected niche peptides can be administered orally if formulated with selected intestinal permeation enhancers.

AREAS COVERED

We evaluated the oral octreotide formulation, MYCAPSSA® (Chiasma Pharmaceuticals, Needham, MA, USA). An outline of the current standard of care in acromegaly and the benefits of oral octreotide versus depot injections is provided. We discuss the Transient Permeation Enhancer (TPE®) technology used and detail the safety and efficacy data from animal models and clinical trials.

EXPERT OPINION

TPE® is an oily suspension of octreotide that includes a number of excipients that can transiently alter epithelial barrier integrity by opening of intestinal epithelial tight junctions arising from transcellular perturbation. Phase I studies using 20 mg octreotide capsules yielded a relative oral bioavailability of ~0.7% and primary endpoints were achieved in two Phase III studies. The oral octreotide dose required to achieve these endpoints was over 200 times that of the 0.1 mg immediate-release subcutaneous injection, a reminder of the difficulty in achieving oral absorption of macromolecules. Many acromegaly patients will prefer a convenient twice-daily oral formulation of octreotide compared to monthly depot injections.

Understanding the Synergistic Correlation between the Spatial Distribution of Drug-Loaded Mixed Micellar Systems and In Vitro Behavior via Experimental and Computational Approaches

To better promote the application of polymeric mixed micelles (PMMs), a coarse-grained molecular dynamics simulation (CGMD) has been employed to investigate the factors controlling the spatial distribution within the PMMs and predict their drug-loading properties, meanwhile, combined with experimental methods to validate and examine it. In this study, the snapshots obtained from CGMD and the results of proton nuclear magnetic resonance (¹H NMR) and transmission electron microscopy (TEM) provide new insights into the distribution principle that the spatial distribution depends on the hydrophobic compatibility of drugs with the regions within PMMs. Docetaxel (DTX) is located within the interior or near the core-corona interface of the HS15 hydrophobic core inside FS/PMMs (PMMs fabricated from a nonionic triblock copolymer (F127)) and a nonionic surfactant (HS15), and therefore, the system with a high HS15 ratio, such as system I, is more suitable for loading DTX. In contrast, the more water-soluble puerarin (PUE) is more likely to be solubilized in the "secondary hydrophobic area," mainly formed by the hydrophobic part of F127 within FS/PMMs. However, when the initial feeding concentration of the drug is increased or the FS mixing ratios are changed, an inappropriate distribution would occur and hence influence the drug-loading stability. Also, this impact was further elucidated by the calculated parameters (solvent-accessible surface area (SASA), the radius of gyration (R_g), and energy landscape), and the analysis of the drug leakage, concluding that inappropriate distribution of the drug would lower the stability of the drug in the PMMs. These results combined together provide new insights into the distribution principle that the spatial distribution of drugs within PMMs depends on the hydrophobic compatibility of drugs with the regions formed by micellar materials. Additionally, in vitro drug release yielded a consistent picture with the above conclusions and provides evidence that both the location of the drug within the systems and the stability of the drug-loading system have a great influence on the drug release behavior. Accordingly, this work demonstrates that we can tune the drug-loading stability and drug release behavior via the drug-PMM interaction and drug location study, and CGMD technology would be a step forward in the search for suitable drug-delivery PMMs.

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.

Synergistic Computational Modeling Approaches as Team Players in the Game of Solubility Predictions

Several approaches to predict and model drug solubility have been used in the drug discovery and development processes during the last decades. Each of these approaches have their own benefits and place, and are typically used as standalone approaches rather than in concert. The synergistic effects of these are often overlooked, partly due to the need of computational experts to perform the modeling and simulations as well as analyzing the data obtained. Here we provide our views on how these different approaches can be used to retrieve more information on drug solubility, ranging from multivariate data analysis over thermodynamic cycle modeling to molecular dynamics simulations. We are discussing aqueous solubility as well as solubility in more complex mixed solvents and media with colloidal structures present. We conclude that the field of computational pharmaceutics is in its early days but with a bright future ahead. However, education of computational formulators with broad knowledge of modeling and simulation approaches is imperative if computational pharmaceutics is to reach its full potential.