Human intestinal permeability

ADMET profiling of geographically diverse phytochemical using chemoinformatic tools

Aim: Phytocompounds are important due to their uniqueness, however, only few reach the development phase due to their poor pharmacokinetics. Therefore, preassessing the absorption, distribution, metabolism, excretion and toxicity (ADMET) properties is essential in drug discovery. Methodology: Biologically diverse databases (Phytochemica, SerpentinaDB, SANCDB and NuBBE_DB) covering the region of India, Brazil and South Africa were considered to predict the ADMET using chemoinformatic tools (Qikprop, pkCSM and DataWarrior). Results: Screening through each of pharmacokinetic criteria resulted in identification of 24 compounds that adhere to all the ADMET properties. Furthermore, assessment revealed that five have potent anticancer biological activity against cancer cell lines. Conclusion: We have established an open-access database (ADMET-BIS) to enable identification of promising molecules that follow ADMET properties and can be considered for drug development.

Cyclodextrins in Drug Delivery Systems and Their Effects on Biological Barriers

Cyclodextrins are widely used excipients, composed of glucopyranose units with a cyclic structure. One of their most important properties, is that their inner cavity is hydrophobic, while their surface is hydrophilic. This enables them for the complex formation with lipophilic molecules. They have several applications in the pharmaceutical field like solubility enhancers or the building blocks of larger drug delivery systems. On the other hand, they have numerous effects on cells or biological barriers. In this review the most important properties of cyclodextrins and cyclodextrin-based drug delivery systems are summarized with special focus on their biological activity.

"Development of Fixed Dose Combination Products" Workshop Report: Considerations of Gastrointestinal Physiology and Overall Development Strategy

The gastrointestinal (GI) tract is one of the most popular and used routes of drug product administration due to the convenience for better patient compliance and reduced costs to the patient compared to other routes. However, its complex nature poses a great challenge for formulation scientists when developing more complex dosage forms such as those combining two or more drugs. Fixed dose combination (FDC) products are two or more single active ingredients combined in a single dosage form. This formulation strategy represents a novel formulation which is as safe and effective compared to every mono-product separately. A complex drug product, to be dosed through a complex route, requires judicious considerations for formulation development. Additionally, it represents a challenge from a regulatory perspective at the time of demonstrating bioequivalence (BE) for generic versions of such drug products. This report gives the reader a summary of a 2-day short course that took place on the third and fourth of November at the Annual Association of Pharmaceutical Scientists (AAPS) meeting in 2018 at Washington, D.C. This manuscript will offer a comprehensive view of the most influential aspects of the GI physiology on the absorption of drugs and current techniques to help understand the fate of orally ingested drug products in the complex environment represented by the GI tract. Through case studies on FDC product development and regulatory issues, this manuscript will provide a great opportunity for readers to explore avenues for successfully developing FDC products and their generic versions.

Analytical method for the determination of polyethylenglycole 400 as marker in porcine plasma

Polyethylenglycole (PEG) is a widespread linear polymer which can be utilized as a solute digestive and intestinal permeability marker in nutritional physiology studies depending on chain length/molecular mass. PEG 400 is proposed to be an ideal permeability marker. Due to its molecular mass (238-590 g/mol) and characteristics, PEG 400 is suggested to be used as a surrogate for studying the paracellular permeability of small hydrophilic molecules. For this purpose, a liquid chromatographic-tandem mass spectrometric method has been developed for the determination of the major oligomers of PEG 400 in porcine plasma. The analysis included a simple and rapid clean-up step where proteins were precipitated. The most intense ions corresponding to seven PEG 400 oligomers were separated within 7 min. Validation of the optimized method was performed in the range of 500-18,000 ng/mL. Mean recoveries between 93 and 105% were achieved using spiked plasma samples in three different concentration levels. The limit of quantification ranged between 11 and 244 ng/mL. The applicability of the method was demonstrated by the analysis of porcine plasma samples obtained from an animal experiment with barrows. The kinetic course of administrated PEG 400 was shown based on the dataset of two barrows selected from the control group, and it was figured out that relative proportion of each PEG oligomer in portal plasma decreased with increasing molecular mass.

Drug Transport across Porcine Intestine Using an Ussing Chamber System: Regional Differences and the Effect of P-Glycoprotein and CYP3A4 Activity on Drug Absorption

Drug absorption across viable porcine intestines was investigated using an Ussing chamber system. The apparent permeability coefficients, P_app,pig, were compared to the permeability coefficients determined in humans in vivo, P_eff,human. Eleven drugs from the different Biopharmaceutical Classification System (BCS) categories absorbed by passive diffusion with published P_eff,human values were used to test the system. The initial experiments measured P_app,pig for each drug after application in a Krebs–Bicarbonate Ringer (KBR) buffer and in biorelevant media FaSSIF V2 and FeSSIF V2, mimicking fasted and fed states. Strong sigmoidal correlations were observed between P_eff,human and P_app,pig. Differences in the segmental P_app,pig of antipyrine, cimetidine and metoprolol confirmed the discrimination between drug uptake in the duodenum, jejunum and ileum (and colon); the results were in good agreement with human data in vivo. The presence of the P-gp inhibitor verapamil significantly increased P_app,pig across the ileum of the P-gp substrates cimetidine and ranitidine (p < 0.05). Clotrimazole, a potent CYP3A4 inhibitor, significantly increased P_app,pig of the CYP3A4 substrates midazolam, verapamil and tamoxifen and significantly decreased the formation of their main metabolites. In conclusion, the results showed that this is a robust technique to predict passive drug permeability under fasted and fed states, to identify regional differences in drug permeability and to demonstrate the activity of P-gp and CYP3A4.

Challenges and Recent Progress in Oral Drug Delivery Systems for Biopharmaceuticals

Routes of drug administration and the corresponding physicochemical characteristics of a given route play significant roles in therapeutic efficacy and short term/long term biological effects. Each delivery method has favorable aspects and limitations, each requiring a specific delivery vehicles design. Among various routes, oral delivery has been recognized as the most attractive method, mainly due to its potential for solid formulations with long shelf life, sustained delivery, ease of administration and intensified immune response. At the same time, a few challenges exist in oral delivery, which have been the main research focus in the field in the past few years. The present work concisely reviews different administration routes as well as the advantages and disadvantages of each method, highlighting why oral delivery is currently the most promising approach. Subsequently, the present work discusses the main obstacles for oral systems and explains the most recent solutions proposed to deal with each issue.

Deconstructing the physical processes of digestion: reductionist approaches may provide greater understanding

I provide a broad overview of the physical factors that govern intestinal digestion i.e. the admixture of food particles in digesta with secreted enzymes and the subsequent mass transfer of liberated nutrients from the surfaces of particles to the gut wall, with a view to outlining the quantitative work that is required to determine the relative importance of these factors in the digestion of particular foods. I first discuss what is known of the mechanical forces generated by contraction of the walls of the various segments of the gut and the level of diffusive, and advective mixing that it generates within the lumen. I then discuss the particular physical effects that may limit the digestion of solid, physically and/or chemically homogenous and heterogeneous food particles, notably capillarity, porosity, poro-elastic flow and compaction and their likely effects on diffusive and convective mass transfer at particulate surfaces. Similarly, I discuss mucins and morphology on mass transfer of nutrients to the gut wall i.e. the mucosa.

Intestinal Villi Model with Blood Capillaries Fabricated Using Collagen-Based Bioink and Dual-Cell-Printing Process

The human intestine, a vital organ in our digestive system, shows an anatomically complex architecture. The fabrication of three-dimensional (3D) intestinal models containing villus structures has been an important topic for intestine regeneration or organ-on-a-chip, because a 3D model can provide broad surface area and help absorption and transportation of digested nutrients. In this study, we developed a 3D intestinal villi model containing an epithelium layer and a blood capillary structure, using an innovative cell-printing process. The epithelium and capillary network of the 3D model were fabricated using two collagen-based bioinks laden with Caco-2 cells and human umbilical vein endothelial cells (HUVECs). The fabricating conditions were optimized to obtain a unique 3D villus structure, with capillary in the core and high cell viability. A fabricated single villus was 183 ± 12 μm in diameter and 770 ± 42 μm in height, which means the aspect ratio of the structure was 4.2 ± 0.3. The results indicate that the cell-laden intestinal villi successfully mimicked the 3D geometry of human intestinal villi. In vitro cellular activity of the 3D villi model containing epithelium and capillary demonstrated significantly higher cell growth and expression of enzymes and MUC17, compared to those of 2D models and a 3D villi model without the capillary network. The suggested 3D intestinal villi also exhibited the enhancement of the barrier function as compared to those of the others, and even demonstrated an increase of the permeability coefficient of FITC-dextran and glucose uptake ability (FITC, fluorescein isothiocyanate). These results indicate that a 3D intestinal villi model would be a highly promising for mimicking the human intestine.

Fifty-Eight Years and Counting: High-Impact Publishing in Computational Pharmaceutical Sciences and Mechanism-Based Modeling

With this issue of the Journal of Pharmaceutical Sciences, we celebrate the nearly 6 decades of contributions to mechanistic-based modeling and computational pharmaceutical sciences. Along with its predecessor, The Journal of the American Pharmaceutical Association: Scientific Edition first published in 1911, JPharmSci has been a leader in the advancement of pharmaceutical sciences beginning with its inaugural edition in 1961. As one of the first scientific journals focusing on pharmaceutical sciences, JPharmSci has established a reputation for publishing high-quality research articles using computational methods and mechanism-based modeling. The journal's publication record is remarkable. With over 15,000 articles, 3000 notes, and more than 650 reviews from industry, academia, and regulatory agencies around the world, JPharmSci has truly been the leader in advancing pharmaceutical sciences.

Integrating theoretical and experimental permeability estimations for provisional biopharmaceutical classification: Application to the WHO essential medicines

The accuracy of the provisional estimation of the Biopharmaceutics Classification System (BCS) is heavily influenced by the permeability measurement. In this study, several theoretical and experimental models currently employed for BCS permeability classification have been analysed. The experimental models included the in situ rat intestinal perfusion, the ex vivo rat intestinal tissue in an Ussing chamber, the MDCK and Caco-2 cell monolayers, and the parallel artificial membrane (PAMPA). The theoretical models included the octanol-water partition coefficient and the QSPeR (Quantitative Structure-Permeability Relationship) model recently developed. For model validation, a dataset of 43 compounds has been recompiled and analysed for the suitability for BCS permeability classification in comparison with the use of human intestinal absorption and oral bioavailability values. The application of the final model, based on a majority voting system showed a 95.3% accuracy for predicting human permeability. Finally, the present approach was applied to the 186 orally administered drugs in immediate-release dosage forms of the WHO Model List of Essential Medicines. The percentages of the drugs that were provisionally classified as BCS Class I and Class III was 62.4%, suggesting that in vivo bioequivalence (BE) may potentially be assured with a less expensive and more easily implemented in vitro dissolution test, ensuring the efficiency and quality of pharmaceutical products. The results of the current study improve the accuracy of provisional BCS classification by combining different permeability models.

Segmental-Dependent Intestinal Drug Permeability: Development and Model Validation of In Silico Predictions Guided by In Vivo Permeability Values

The goal of this work was to develop an in silico model that allows predicting segmental-dependent permeability throughout the small intestine (SI). In vivo permeability of 11 model drugs in 3 SI segments (jejunum, mid-SI, ileum) was studied in rats, creating a data set that reflects the conditions throughout the SI. Then, a predictive model was developed, combining physicochemical drug properties influencing the underlying mechanism of passive permeability: Log p, polar surface area, M_W, H-bond count, and Log f_u, with microenvironmental SI conditions. Excellent correlation was evident between the predicted and experimental data (R² = 0.914), with similar predictability in each SI segment. Log p and Log f_u were identified as the major determinants of permeability, with similar contribution. Total H-bond count was also a significant determinant, followed by polar surface area and M_W. Leaving out any of the model parameters decreased its predictability. The model was validated against 5 external drugs, with excellent predictability. Notably, the model was able to predict the segmental-dependent permeability of all drugs showing this trend experimentally. Model predictability was better in the high-permeability versus low-permeability range. Overall, our approach of constructing a straightforward in silico model allowed reliable predictions of segmental-dependent intestinal permeability, providing new insights into relative effects of drug-related factors and gastrointestinal environment on permeability.

Preclinical models for colonic absorption, application to controlled release formulation development

Oral controlled release (CR) formulations have many benefits and have become a valuable resource for the local and systemic administration of drugs. The most important characteristic of these pharmaceutical products is that drug absorption occurs mainly in the colon. Therefore, this review analyses the physiological and physicochemical features that may affect an orally administered CR product, as well as the different strategies to develop a CR dosage form and the methods used to evaluate the formulation efficacy. The models available to study the intestinal permeability and their applicability to colonic permeability determinations are also discussed.

Multivariate assessment of azo dyes' biological activity parameters

Lipophilicity as key molecular descriptor of potential biological activity for selected derivatives of azo dyes was determined mathematically, by using relevant software packages and by reversed-phase thin-layer chromatography (RPTLC) on C18 and cyano modified carriers in mixtures of water/n-propanol and water/acetone. The obtained chromatographic parameters, R_M⁰ and m, of the examined azo dyes were correlated with the standard measure of lipophilicity, log P, important pharmacokinetic predictors and selected toxicity parameters applying linear regression analysis. Thereby, good correlations for each applied system were obtained (average correlation coefficient, r, 0.944, 0.885 and 0.919). Also, the correlations between the studied parameters of azo dyes were examined applying two multivariate methods (Cluster Analysis and Principal Component Analysis). It was shown that the polarity of the substituent, and to a lesser extent its electronic effects has the greatest influence on the studied parameters of the azo dyes derivatives. Multivariate methods pointed out the similarity of the chromatographic retention constant, R_M⁰, with the parameters of lipophilicity, unlike the chromatographic parameter m, which exhibits better agreement with the toxicity parameters.

Drug permeability profiling using cell-free permeation tools: Overview and applications

Cell-free permeation systems are gaining interest in drug discovery and development as tools to obtain a reliable prediction of passive intestinal absorption without the disadvantages associated with cell- or tissue-based permeability profiling. Depending on the composition of the barrier, cell-free permeation systems are classified into two classes including (i) biomimetic barriers which are constructed from (phospho)lipids and (ii) non-biomimetic barriers containing dialysis membranes. This review provides an overview of the currently available cell-free permeation systems including Parallel Artificial Membrane Permeability Assay (PAMPA), Phospholipid Vesicle-based Permeation Assay (PVPA), Permeapad®, and artificial membrane based systems (e.g. the artificial membrane insert system (AMI-system)) in terms of their barrier composition as well as their predictive capacity in relation to well-characterized intestinal permeation systems. Given the potential loss of integrity of cell-based permeation barriers in the presence of food components or pharmaceutical excipients, the superior robustness of cell-free barriers makes them suitable for the combined dissolution/permeation evaluation of formulations. While cell-free permeation systems are mostly applied for exploring intestinal absorption, they can also be used to evaluate non-oral drug delivery by adjusting the composition of the membrane.

Intestinal absorption and activation of decitabine amino acid ester prodrugs mediated by peptide transporter PEPT1 and enterocyte enzymes

Decitabine (DAC), a potent DNA methyltransferase (DNMT) inhibitor, has a limited oral bioavailability. Its 5'-amino acid ester prodrugs could improve its oral delivery but the specific absorption mechanism is not yet fully understood. The aim of this present study was to investigate the in vivo absorption and activation mechanism of these prodrugs using in situ intestinal perfusion and pharmacokinetics studies in rats. Although PEPT1 transporter is pH dependent, there appeared to be no proton cotransport in the perfusion experiment with a preferable transport at pH 7.4 rather than pH 6.5. This suggested that the transport was mostly dependent on the dissociated state of the prodrugs and the proton gradient might play only a limited role. In pH 7.4 HEPES buffer, an increase in P_eff was observed for L-val-DAC, D-val-DAC, L-phe-DAC and L-trp-DAC (2.89-fold, 1.2-fold, 2.73-fold, and 1.90-fold, respectively), compared with the parent drug. When co-perfusing the prodrug with Glysar, a known substrate of PEPT1, the permeabilities of the prodrugs were significantly inhibited compared with the control. To further investigate the absorption of the prodrugs, L-val-DAC was selected and found to be concentration-dependent and saturable, suggesting a carrier-mediated process (intrinsic K_m: 7.80 ± 2.61 mM) along with passive transport. Determination of drug in intestinal homogenate after perfusion further confirmed that the metabolic activation mainly involved an intestinal first-pass effect. In a pharmacokinetic evaluation, the oral bioavailability of L-val-DAC, L-phe-DAC and L-trp-DAC were nearly 1.74-fold, 1.69-fold and 1.49-fold greater than that of DAC. The differences in membrane permeability and oral bioavailability might be due to the different stability in the intestinal lumen and the distinct PEPT1 affinity which is mainly caused by the stereochemistry, hydrophobicity and steric hindrance of the side chains. In summary, the detailed investigation of the absorption mechanism by in vivo intestinal perfusion and pharmacokinetic studies showed that the prodrugs of DAC exhibited excellent permeability and oral bioavailability, which might be attributed to a hybrid (partly PEPT1-mediated and partly passive) transport mode and a rapid activation process in enterocytes.

Pharmaceutical cocrystallization: an effective approach to modulate the physicochemical properties of solid-state drugs

This highlight presents an update on applications of cocrystallization to modify properties relevant to efficacy, safety, and manufacturability of drugs.

Closed-Loop Doluisio (Colon, Small Intestine) and Single-Pass Intestinal Perfusion (Colon, Jejunum) in Rat-Biophysical Model and Predictions Based on Caco-2

PURPOSE

The effective rat intestinal permeability (P _eff ) was deconvolved using a biophysical model based on parameterized paracellular, aqueous boundary layer, transcellular permeabilities, and the villus-fold surface area expansion factor.

METHODS

Four types of rat intestinal perfusion data were considered: single-pass intestinal perfusion (SPIP) in the jejunum (n = 40), and colon (n = 15), closed-loop (Doluisio type) in the small intestine (n = 78), and colon (n = 74). Moreover, in vitro Caco-2 permeability values were used to predict rat in vivo values in the rat data studied.

RESULTS

Comparable number of molecules permeate via paracellular water channels as by the lipoidal transcellular route in the SPIP method, although in the closed-loop method, the paracellular route appears dominant in the colon. The aqueous boundary layer thickness in the small intestine is comparable to that found in unstirred in vitro monolayer assays; it is thinner in the colon. The mucosal surface area in anaesthetized rats is 0.96-1.4 times the smooth cylinder calculated value in the colon, and it is 3.1-3.6 times in the small intestine. The paracellular permeability of the intestine appeared to be greater in rat than human, with the colon showing more leakiness (higher P _para ) than the small intestine.

CONCLUSION

Based on log intrinsic permeability values, the correlations between the in vitro and in vivo models ranged from r² 0.82 to 0.92. The SPIP-Doluisio method comparison indicated identical log permeability selectivity trend with negligible bias.

Importance of the Unstirred Water Layer and Hepatocyte Membrane Integrity In Vitro for Quantification of Intrinsic Metabolic Clearance

Prediction of clearance-a vital component of drug discovery-remains in need of improvement and, in particular, requires more incisive assessment of mechanistic methodology in vitro, according to a number of recent reports. Although isolated hepatocytes have become an irreplaceable standard system for the measurement of intrinsic hepatic clearance mediated by active uptake transport and metabolism, the lack of prediction reliability appears to reflect a lack of methodological validation, especially for highly cleared drugs, as we have previously shown. Here, novel approaches were employed to explore fundamental experimental processes and associated potential limitations of in vitro predictions of clearance. Rat hepatocytes deemed nonviable by trypan blue staining showed undiminished metabolic activity for probe cytochrome P450 (P450) substrates midazolam and propranolol; supplementation with NADPH enhanced these activities. Extensive permeabilization of the plasma membrane using saponin showed either full or minimal P450 activity, depending on the presence or absence of 1 mM NADPH, respectively. The shaking of incubations facilitated P450 metabolic rates up to 5-fold greater than static incubation, depending on intrinsic clearance, indicating the critical influence of the unstirred water layer (UWL). Permeabilization allowed static incubation metabolic rates to approach those of shaking for intact cells, indicating an artificially induced breakdown of the UWL. Permeabilization combined with shaking allowed an increased metabolic rate for saquinavir, resolving the membrane permeability limitation for this drug. These findings advance the interpretation of the rate-limiting processes involved in intrinsic clearance measurements and could be critical for successful in vitro prediction.

Revisiting atenolol as a low passive permeability marker

BACKGROUND

Atenolol, a hydrophilic beta blocker, has been used as a model drug for studying passive permeability of biological membranes such as the blood-brain barrier (BBB) and the intestinal epithelium. However, the extent of S-atenolol (the active enantiomer) distribution in brain has never been evaluated, at equilibrium, to confirm that no transporters are involved in its transport at the BBB.

METHODS

To assess whether S-atenolol, in fact, depicts the characteristics of a low passive permeable drug at the BBB, a microdialysis study was performed in rats to monitor the unbound concentrations of S-atenolol in brain extracellular fluid (ECF) and plasma during and after intravenous infusion. A pharmacokinetic model was developed, based on the microdialysis data, to estimate the permeability clearance of S-atenolol into and out of brain. In addition, the nonspecific binding of S-atenolol in brain homogenate was evaluated using equilibrium dialysis.

RESULTS

The steady-state ratio of unbound S-atenolol concentrations in brain ECF to that in plasma (i.e., K_p,uu,brain) was 3.5% ± 0.4%, a value much less than unity. The unbound volume of distribution in brain (V_{u, brain}) of S-atenolol was also calculated as 0.69 ± 0.10 mL/g brain, indicating that S-atenolol is evenly distributed within brain parenchyma. Lastly, equilibrium dialysis showed limited nonspecific binding of S-atenolol in brain homogenate with an unbound fraction (f_u,brain) of 0.88 ± 0.07.

CONCLUSIONS

It is concluded, based on K_p,uu,brain being much smaller than unity, that S-atenolol is actively effluxed at the BBB, indicating the need to re-consider S-atenolol as a model drug for passive permeability studies of BBB transport or intestinal absorption.

Low colonic absorption drugs: risks and opportunities in the development of oral extended release products

INTRODUCTION

Currently numerous drugs have been observed with lower colonic absorption than small intestine absorption, which can significantly impact in vivo performance of their oral extended release (ER) products.

AREAS COVERED

We reviewed over 300 publications, patents, book chapters, and commercial reports of drug products from regulatory agencies for low colonic absorption (LCA) drugs and critical findings are discussed. The focuses of this article are (1) current findings on the causes of low colonic absorption to support early assessment of LCA candidates, and (2) current knowledge on successful ER strategies and technical platforms used for LCA drugs in commercial drug products to facilitate oral ER product development.

EXPERT OPINION

Colonic drug absorption is one of the critical considerations in successful development of oral ER products. The root causes of low colonic absorption in many LCA drugs are still unclear. It is recommended to evaluate colonic drug absorption of drug candidate at early stage of oral ER product development. After evaluation, the selection of a formulation platform to develop an oral ER product needs to be carefully considered for LCA drugs. Based on the current commercial oral ER formulation platforms for LCA drugs, compounds are first divided into five types (I-V) and different ER formulation approaches with higher success rate are recommended for each type.

Mass Spectrometry Imaging proves differential absorption profiles of well-characterised permeability markers along the crypt-villus axis

Knowledge about the region-specific absorption profiles from the gastrointestinal tract of orally administered drugs is a critical factor guiding dosage form selection in drug development. We have used a novel approach to study three well-characterized permeability and absorption marker drugs in the intestine. Propranolol and metoprolol (highly permeable compounds) and atenolol (low-moderate permeability compound) were orally co-administered to rats. The site of drug absorption was revealed by high spatial resolution matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) and complemented by quantitative measurement of drug concentration in tissue homogenates. MALDI-MSI identified endogenous molecular markers that illustrated the villi structures and confirmed the different absorption sites assigned to histological landmarks for the three drugs. Propranolol and metoprolol showed a rapid absorption and shorter transit distance in contrast to atenolol, which was absorbed more slowly from more distal sites. This study provides novel insights into site specific absorption for each of the compounds along the crypt-villus axis, as well as confirming a proximal-distal absorption gradient along the intestine. The combined analytical approach allowed the quantification and spatial resolution of drug distribution in the intestine and provided experimental evidence for the suggested absorption behaviour of low and highly permeable compounds.

Drug-transporter mediated interactions between anthelminthic and antiretroviral drugs across the Caco-2 cell monolayers

BACKGROUND

Drug interactions between antiretroviral drugs (ARVs) and anthelminthic drugs, ivermectin (IVM) and praziquantel (PZQ) were assessed by investigating their permeation through the Caco-2 cell monolayers in a transwell. The impact of anthelminthics on the transport of ARVs was determined by assessing the apical to basolateral (AP → BL) [passive] and basolateral to apical (BL → AP) [efflux] directions alone, and in presence of an anthelminthic. The reverse was conducted for the assessment of the influence of ARVs on anthelminthics.

METHODS

Samples from the AP and BL compartments were taken at 60, 120, 180 and 240 min and quantified either by HPLC or radiolabeled assay using a liquid scintillating counter for the respective drugs. Transepithelial resistance (TEER) was used to assess the integrity of the monolayers. The amount of compound transported per second (apparent permeability, Papp) was calculated for both AP to BL (PappAtoB), and BL to AP (PappBtoA) movements. Samples collected after 60 min were used to determine the efflux ratio (ER), quotient of secretory permeability and absorptive permeability (PappBL-AP/PappAP-BL). The reverse, (PappAP-BL/PappBL-AP) constituted the uptake ratio. The impact of SQV, EFV and NVP on the transport of both IVM and PZQ were investigated. The effect of LPV on the transport of IVM was also determined. The influence of IVM on the transport of SQV, NVP, LPV and EFV; as well as the effect PZQ on the transport of SQV of was also investigated, and a two-tailed p value of <0.05 was considered significant.

RESULTS

IVM significantly inhibited the efflux transport (BL → AP movement) of LPV (ER; 6.7 vs. 0.8, p = 0.0038) and SQV (ER; 3.1 vs. 1.2 p = 0.00328); and increased the efflux transport of EFV (ER; 0.7 vs. 0.9, p = 0.031) suggesting the possibility of drug transporter mediated interactions between the two drugs. NVP increased the efflux transport of IVM (ER; 0.8 vs. 1.8, p = 0.0094).

CONCLUSIONS

The study provides in vitro evidence of potential interactions between IVM, an anthelminthic drug with antiretroviral drugs; LPV, SQV, NVP and EFV. Further investigations should be conducted to investigate the possibility of in vivo interactions.

Dopamine enhances duodenal epithelial permeability via the dopamine D5 receptor in rodent

AIM

The intestinal barrier is made up of epithelial cells and intercellular junctional complexes to regulate epithelial ion transport and permeability. Dopamine (DA) is able to promote duodenal epithelial ion transport through D1-like receptors, which includes subtypes of D₁ (D₁ R) and D₅ (D₅ R), but whether D1-like receptors influence the duodenal permeability is unclear.

METHODS

FITC-dextran permeability, short-circuit current (I_SC ), Western blot, immunohistochemistry and ELISA were used in human D₅ R transgenic mice and hyperendogenous enteric DA (HEnD) rats in this study.

RESULTS

Dopamine induced a downward deflection in I_SC and an increase in FITC-dextran permeability of control rat duodenum, which were inhibited by the D1-like receptor antagonist, SCH-23390. However, DA decreased duodenal transepithelial resistance (TER), an effect also reversed by SCH-23390. A strong immunofluorescence signal for D₅ R, but not D₁ R, was observed in the duodenum of control rat. In human D₅ R knock-in transgenic mice, duodenal mucosa displayed an increased basal I_SC with high FITC-dextran permeability and decreased TER with a lowered expression of tight junction proteins, suggesting attenuated duodenal barrier function in these transgenic mice. D₅ R knock-down transgenic mice manifested a decreased basal I_SC with lowered FITC-dextran permeability. Moreover, an increased FITC-dextran permeability combined with decreased TER and tight junction protein expression in duodenal mucosa were also observed in HEnD rats.

CONCLUSION

This study demonstrates, for the first time, that DA enhances duodenal permeability of control rat via D₅ R, which provides new experimental and theoretical evidence for the influence of DA on duodenal epithelial barrier function.

Pokharkar V. Understanding peroral absorption: regulatory aspects and contemporary approaches to tackling solubility and permeability hurdles

Oral drug absorption is a process influenced by the physicochemical and biopharmaceutical properties of the drug and its inter-relationship with the gastrointestinal tract. Drug solubility, dissolution and permeability across intestinal barrier are the key parameters controlling absorption. This review provides an overview of the factors that affect drug absorption and the classification of a drug on the basis of solubility and permeability. The biopharmaceutical classification system (BCS) was introduced in early 90׳s and is a regulatory tool used to predict bioavailability problems associated with a new entity, thereby helping in the development of a drug product. Strategies to combat solubility and permeability issues are also discussed.

Biowaiver Monographs for Immediate-Release Solid Oral Dosage Forms: Enalapril

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence testing for the marketing authorization of immediate-release, solid oral dosage forms containing enalapril maleate are reviewed. Enalapril, a prodrug, is hydrolyzed by carboxylesterases to the active angiotensin-converting enzyme inhibitor enalaprilat. Enalapril as the maleate salt is shown to be highly soluble, but only 60%-70% of an orally administered dose of enalapril is absorbed from the gastrointestinal tract into the enterocytes. Consequently, enalapril maleate is a Biopharmaceutics Classification System class III substance. Because in situ conversion of the maleate salt to the sodium salt is sometimes used in production of the finished drug product, not every enalapril maleate-labeled finished product actually contains the maleate salt. Enalapril is not considered to have a narrow therapeutic index. With this background, a biowaiver-based approval procedure for new generic products or after major revisions to existing products is deemed acceptable, provided the in vitro dissolution of both test and reference preparation is very rapid (at least 85% within 15 min at pH 1.2, 4.5, and 6.8). Additionally, the test and reference product must contain the identical active drug ingredient.

Kinetics of lipid bilayer permeation of a series of ionisable drugs and their correlation with human transporter-independent intestinal permeability

For low molecular weight drugs, lipid bilayer permeation is considered the major route for in vivo cell barrier passage. We recently introduced a fluorescence assay with liposomes to determine permeation kinetics of ionisable compounds across the lipid bilayer by monitoring drug-induced pH changes inside the liposomes. Here, we determined the permeability coefficients (P_FLipP, FLipP for "Fluorescence Liposomal Permeability") across 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers of 35 ionisable drugs at pH6.0 and compared them to available in vivo human jejunal permeability (P_eff) data. P_FLipP values were furthermore compared with published Caco-2 cell permeability coefficients (P_Caco-2), permeability coefficients determined with the parallel artificial membrane permeability assay (PAMPA) and with log D (pH6.0). The log P_FLipP, corrected for predicted para-cellular diffusion, and log P_Caco-2 correlated best with log P_eff, with similar adjusted R² (0.75 and 0.74, n=12). Our results suggest that transporter-independent intestinal drug absorption is predictable from liposomal permeability.

Dissolution and dissolution/permeation experiments for predicting systemic exposure following oral administration of the BCS class II drug clarithromycin

In order to save time and resources in early drug development, in vitro methods that correctly predict the formulation effect on oral drug absorption are necessary. The aim of this study was to 1) evaluate various BCS class II drug formulations with in vitro methods and in vivo in order to 2) determine which in vitro method best correlates with the in vivo results. Clarithromycin served as model compound in formulations with different particle sizes and content of excipients. The performed in vitro experiments were dissolution and dissolution/permeation experiments across two types of membrane, Caco-2 cells and excised rat intestinal sheets. The in vivo study was performed in rats. The oral absorption was enhanced by downsizing drug particles and by increasing the excipient concentration. This correlated strongly with the flux across Caco-2 cells but not with the other in vitro experiments. The insufficient correlation with the dissolution experiments can be partly explained by excipient caused problems during the filtration step. The very poor correlation of the in vivo data with the flux across excised rat intestinal sheets might be due to an artificially enlarged mucus layer ex vivo. In conclusion, downsizing BCS class II drug particles and the addition of surfactants enhanced the in vivo absorption, which was best depicted by dissolution/permeation experiments across Caco-2 cells. This setup is proposed as best model to predict the in vivo formulation effect. Also, this is the first study to evaluate the impact of the nature of the permeation membrane in dissolution/permeation experiments.

Hydrotropic Solubilization of Lipophilic Drugs for Oral Delivery: The Effects of Urea and Nicotinamide on Carbamazepine Solubility-Permeability Interplay

Hydrotropy refers to increasing the water solubility of otherwise poorly soluble compound by the presence of small organic molecules. While it can certainly increase the apparent solubility of a lipophilic drug, the effect of hydrotropy on the drugs’ permeation through the intestinal membrane has not been studied. The purpose of this work was to investigate the solubility–permeability interplay when using hydrotropic drug solubilization. The concentration-dependent effects of the commonly used hydrotropes urea and nicotinamide, on the solubility and the permeability of the lipophilic antiepileptic drug carbamazepine were studied. Then, the solubility–permeability interplay was mathematically modeled, and was compared to the experimental data. Both hydrotropes allowed significant concentration-dependent carbamazepine solubility increase (up to ∼30-fold). A concomitant permeability decrease was evident both in vitro and in vivo (∼17-fold for nicotinamide and ∼9-fold for urea), revealing a solubility–permeability tradeoff when using hydrotropic drug solubilization. A relatively simplified simulation approach based on proportional opposite correlation between the solubility increase and the permeability decrease at a given hydrotrope concentration allowed excellent prediction of the overall solubility–permeability tradeoff. In conclusion, when using hydrotropic drug solubilization it is prudent to not focus solely on solubility, but to account for the permeability as well; achieving optimal solubility–permeability balance may promote the overall goal of the formulation to maximize oral drug exposure.

Quantitative Targeted Absolute Proteomics of Transporters and Pharmacoproteomics-Based Reconstruction of P-Glycoprotein Function in Mouse Small Intestine

The purpose of this study was to investigate whether a pharmacokinetic model integrating in vitro mdr1a efflux activity (which we previously reported) with in vitro/in vivo differences in protein expression level can reconstruct intestinal mdr1a function. In situ intestinal permeability-surface area product ratio between wild-type and mdr1a/1b (-/-) mice is one of the parameters used to describe intestinal mdr1a function. The reconstructed ratios of six mdr1a substrates (dexamethasone, digoxin, loperamide, quinidine, verapamil, vinblastine) and one nonsubstrate (diazepam) were consistent with the observed values reported by Adachi et al. within 2.1-fold difference. Thus, intestinal mdr1a function can be reconstructed by our pharmacoproteomic modeling approach. Furthermore, we evaluated regional differences in protein expression levels of mouse intestinal transporters. Sixteen (mdr1a, mrp4, bcrp, abcg5, abcg8, glut1, 4f2hc, sglt1, lat2, pept1, mct1, slc22a18, ostβ, villin1, Na(+)/K(+)-ATPase, γ-gtp) out of 46 target molecules were detected by employing our established quantitative targeted absolute proteomics technique. The protein expression amounts of mdr1a and bcrp increased progressively from duodenum to ileum. Sglt1, lat2, and 4f2hc were highly expressed in jejunum and ileum. Mct1 and ostβ were highly expressed in ileum. The quantitative expression profiles established here should be helpful to understand and predict intestinal transporter functions.

Model-based dietary optimization for late-stage, levodopa-treated, Parkinson's disease patients

Levodopa has been the gold standard for Parkinson’s disease treatment for more than 40 years. Its bioavailability is hindered by dietary amino acids, leading to fluctuations in the motor response particularly in late-stage (stage 3 and 4 on Hoehn and Yahr scale) patients. The routine dietary intervention consists of low-protein (<0.8 g/kg) diets or the redistribution of daily protein allowance to the last meal. Computational modeling was used to examine the fluctuation of gastrointestinal levodopa absorption under consideration of the diet by (i) identifying the group of patients that could benefit from dietary interventions, (ii) comparing existing diet recommendations for their impact on levodopa bioavailability, and (iii) suggesting a mechanism-based dietary intervention. We developed a multiscale computational model consisting of an ordinary differential equations-based advanced compartmentalized absorption and transit (ACAT) gut model and metabolic genome-scale small intestine epithelial cell model. We used this model to investigate complex spatiotemporal relationship between dietary amino acids and levodopa absorption. Our model predicted an improvement in bioavailability, as reflected by blood concentrations of levodopa with protein redistribution diet by 34% compared with a low-protein diet and by 11% compared with the ante cibum (a.c.) administration. These results are consistent with the reported better outcome in late-stage patients. A systematic analysis of the effect of different amino acids in the diet suggested that a serine-rich diet could improve the bioavailability by 22% compared with the a.c. administration. In addition, the slower gastric emptying rate in PD patients exacerbates the loss of levodopa due to competition. Optimizing dietary recommendations in quantity, composition, and intake time holds the promise to improve levodopa efficiency and patient’s quality of life based on highly detailed, mechanistic models of gut physiology endowed with improved extrapolative properties, thus paving the way for precision medical treatment.

Computational prediction of formulation strategies for beyond-rule-of-5 compounds

The physicochemical properties of some contemporary drug candidates are moving towards higher molecular weight, and coincidentally also higher lipophilicity in the quest for biological selectivity and specificity. These physicochemical properties move the compounds towards beyond rule-of-5 (B-r-o-5) chemical space and often result in lower water solubility. For such B-r-o-5 compounds non-traditional delivery strategies (i.e. those other than conventional tablet and capsule formulations) typically are required to achieve adequate exposure after oral administration. In this review, we present the current status of computational tools for prediction of intestinal drug absorption, models for prediction of the most suitable formulation strategies for B-r-o-5 compounds and models to obtain an enhanced understanding of the interplay between drug, formulation and physiological environment. In silico models are able to identify the likely molecular basis for low solubility in physiologically relevant fluids such as gastric and intestinal fluids. With this baseline information, a formulation scientist can, at an early stage, evaluate different orally administered, enabling formulation strategies. Recent computational models have emerged that predict glass-forming ability and crystallisation tendency and therefore the potential utility of amorphous solid dispersion formulations. Further, computational models of loading capacity in lipids, and therefore the potential for formulation as a lipid-based formulation, are now available. Whilst such tools are useful for rapid identification of suitable formulation strategies, they do not reveal drug localisation and molecular interaction patterns between drug and excipients. For the latter, Molecular Dynamics simulations provide an insight into the interplay between drug, formulation and intestinal fluid. These different computational approaches are reviewed. Additionally, we analyse the molecular requirements of different targets, since these can provide an early signal that enabling formulation strategies will be required. Based on the analysis we conclude that computational biopharmaceutical profiling can be used to identify where non-conventional gateways, such as prediction of 'formulate-ability' during lead optimisation and early development stages, are important and may ultimately increase the number of orally tractable contemporary targets.

The solubility-permeability interplay and oral drug formulation design: Two heads are better than one

Poor aqueous solubility is a major challenge in today's biopharmaceutics. While solubility-enabling formulations can significantly increase the apparent solubility of the drug, the concomitant effect on the drug's apparent permeability has been largely overlooked. The mathematical equation to describe the membrane permeability of a drug comprises the membrane/aqueous partition coefficient, which in turn is dependent on the drug's apparent solubility in the GI milieu, suggesting that the solubility and the permeability are closely related, exhibit a certain interplay between them, and treating the one irrespectively of the other may be insufficient. In this article, an overview of this solubility-permeability interplay is provided, and the available data is analyzed in the context of the effort to maximize the overall drug exposure. Overall, depending on the type of solubility-permeability interplay, the permeability may decrease, remain unchanged, and even increase, in a way that may critically affect the formulation capability to improve the overall absorption. Therefore, an intelligent design of solubility-enabling formulation needs to consider both the solubility afforded by the formulation and the permeability in the new luminal environment resulting from the formulation.