In vitro models to evaluate the permeability of poorly soluble drug entities: Challenges and perspectives

The role of phospholipids in drug delivery formulations - Recent advances presented at the Researcher's Day 2023 Conference of the Phospholipid Research Center Heidelberg

This Focus on Meetings contribution summarizes recent advances in the research on phospholipids and their applications for drug delivery and analytical purposes that have been presented at the hybrid Researcher's Day 2023 Conference of the Phospholipid Research Center (PRC), held on July 3-5, 2023, in Bad Dürkheim, Germany. The PRC is a non-profit organization focused on expanding and sharing scientific and technological knowledge of phospholipids in pharmaceutical and other applications. This is accomplished by, e.g., funding doctoral and postdoctoral research projects. The progress made with these projects is presented at the Researcher's Day Conference every two years. Four main topics were presented and discussed in various lectures: (1) formulation of phospholipid-based nanocarriers, (2) therapeutic applications of phospholipids and phospholipid-based nanocarriers, (3) phospholipids as excipients in oral, dermal, and parenteral dosage forms, and (4) interactions of phospholipids and phospholipid-based vesicles in biological environment and their use as analytical platforms.

The development of a novel simultaneous in vitro dissolution - in situ perfusion system as a potential tool for studying the absorption of solid oral formulation in rat

The aim of this work is to develop a novel simultaneous in vitro dissolution - in situ perfusion system (SDPS) as a potential tool to evaluate the in vivo performance of solid oral formulation in rat. The innovative nitrendipine (NTD) tablet of Bayotensin mite® made in Germany was used as reference listed drug (RLD), and five generic products from Chinese market were compared with RLD using the in vitro dissolution test method specified by the orange book and the SDPS method developed in this study. Four self-prepared NTD tablets with different proportions of microcrystalline cellulose/starch were employed to investigate the discriminatory ability of the SDPS for formulation. In addition, the predictivity of the SDPS in relation to data from in vivo pharmaceutics studies was evaluated. The 45-min dissolution test and multiple-pH dissolution profiles of generic product 1 and 2 have no difference compared with the RLD, but their dissolution profiles from the SDPS showed statistically significant differences. A biexponential formula successfully described the concentration profiles of self-prepared formulations in SDPS experiments. The kdis (0.08 ± 0.01 ∼ 0.2 ± 0.03 min-1) and ka (about 2.30 × 10-3 min-1) values calculated by the formulas of F1-F3 suggested that the used excipients had no effect on the intestinal absorption of NTD, and it might be the property of active pharmaceutical ingredient that led to the difference among the generics. Furthermore, the in vivo rat pharmacokinetics study results of F1-F3 showed a good correlation (R2 = 0.99) with the SDPS data. In summary, the SDPS is a promising tool to detect the unexpected quality changes of pharmaceutical products in weakly regulated markets, facilitate formulation screening, and potentially reduce animal testing for estimating the in vivo absorption behavior of solid oral formulations. The absorption performance of generic drugs in vivo should be further investigated.

In vitro dissolution/permeation tools for amorphous solid dispersions bioavailability forecasting I: Experimental design for PermeaLoop™

Along with the increasing demand for candidate-enabling formulations comes the need for appropriate in vitro bioavailability forecasting. Dissolution/permeation (D/P) systems employing cell-free permeation barriers are increasingly gaining interest, due to their low cost and easy application as passive diffusion bio-predictive profiling in drug product development, as this accounts for nearly 75% of new chemical entities (NCEs) absorption mechanism. To this end, this study comprises theoretical considerations on the design and experimental work towards the establishment and optimization of a PermeaLoop™ based dissolution/permeation assay to simultaneously evaluate the drug release and permeation using Itraconazole (ITZ)-based amorphous solid dispersions (ASD) formulations, with different drug loads, based on a solvent-shift approach. Alternative method conditions were tested such as: donor medium, acceptor medium and permeation barrier were screened using both PermeaPad® and PermeaPlain® 96-well plates. A range of solubilizers, namely Sodium Dodecyl Sulfate, Vitamin E-TPGS and hydroxypropyl-β-cyclodextrin, were screened as possible solubilizing additives to the acceptor medium, while donor medium was varied between blank FaSSIF (phosphate buffer) and FaSSIF. The method optimization also included the ITZ dose selection, being the ITZ single dose (100 mg) considered the most adequate to be used in further experiments to allow the comparison with in vivo studies. In the end, a standardized approach that may be applied to predict the bioavailability of weakly basic poorly soluble drug-based formulations is described, contributing to strengthening the analytical portfolio of in vitro pre-clinical drug product development.

In vitro dissolution/permeation tools for amorphous solid dispersions bioavailability forecasting II: Comparison and mechanistic insights

Along with the increasing demand for complex formulations comes the need for appropriate in vitro methodologies capable of predicting their corresponding in vivo performance and the mechanisms controlling the drug release which can impact on in vivo drug absorption. In vitro dissolution-permeation (D/P) methodologies that can account for the effects of enabling formulations on the permeability of drugs are increasingly being used in performance ranking during early development stages. This work comprised the application of two different cell-free in vitro D/P setups: BioFLUX™ and PermeaLoop™ to evaluate the dissolution-permeation interplay upon drug release from itraconazole (ITZ)- HPMCAS amorphous solid dispersions (ASDs) of different drug loads. A solvent-shift approach was employed, from a simulated gastric environment to a simulated intestinal environment in the donor compartment. PermeaLoop™ was then combined with microdialysis sampling to separate the dissolved (free) drug from other species present in solution, like micelle-bound drug and drug-rich colloids, in real time. This setup was applied to clarify the mechanisms for drug release and permeation from these ASDs. In parallel, a pharmacokinetic study (dog model) was conducted to assess the drug absorption from these ASDs and to compare the in vivo results with the data obtained from each in vitro D/P setup, allowing to infer which would be the most adequate setup for ASD ranking. Even though both D/P systems resulted in the same qualitative ranking, BioFLUX™ overpredicted the difference between the in vivo AUC of two ASDs, whereas PermeaLoop™ permeation flux resulted in a good correlation with the AUC observed in pharmacokinetic studies (dog model) (R2 ≈ 0.98). Also, PermeaLoop™ combined with a microdialysis sampling probe clarified the mechanisms for drug release and permeation from these ASDs. It demonstrated that the free drug was the only driving force for permeation, while the drug-rich colloids kept permeation active for longer periods by acting as drug reservoirs and maintaining constant high levels of free drug in solution, which are then immediately able to permeate. Hence, the data obtained points BioFLUX™ and PermeaLoop™ applications to different momentums in the drug product development pipeline: while BioFLUX™, an automated standardized method, poses as a valuable tool for initial ASD ranking during the early development stages, PermeaLoop™ combined with microdialysis sampling allows to gain mechanistic understanding of the dissolution-permeation interplay, being crucial to fine tune and identify leading ASD candidates prior to in vivo testing.

Methods of Analysis and Identification of Betulin and Its Derivatives

This scientific work presents practical and theoretical material on the methods of analysis and identification of betulin and its key derivatives. The properties of betulin and its derivatives, which are determined by the structural features of this class of compounds and their tendency to form dimers, polymorphism and isomerization, are considered. This article outlines ways to improve not only the bioavailability but also the solubility of triterpenoids, as well as any hydrophobic drug substances, through chemical transformations by introducing various functional groups, such as carboxyl, hydroxyl, amino, phosphate/phosphonate and carbonyl. The authors of this article summarized the physicochemical characteristics of betulin and its compounds, systematized the literature data on IR and NMR spectroscopy and gave the melting temperatures of key acids and aldehydes based on betulin.

Experimental and computational models to investigate intestinal drug permeability and metabolism

Oral administration is the preferred route for drug administration that leads to better therapy compliance. The intestine plays a key role in the absorption and metabolism of oral drugs, therefore, new intestinal models are being continuously proposed, which contribute to the study of intestinal physiology, drug screening, drug side effects, and drug-drug interactions.Advances in pharmaceutical processes have produced more drug formulations, causing challenges for intestinal models. To adapt to the rapid evolution of pharmaceuticals, more intestinal models have been created. However, because of the complexity of the intestine, few models can take all aspects of the intestine into account, and some functions must be sacrificed to investigate other areas. Therefore, investigators need to choose appropriate models according to the experimental stage and other requirements to obtain the desired results.To help researchers achieve this goal, this review summarised the advantages and disadvantages of current commonly used intestinal models and discusses possible future directions, providing a better understanding of intestinal models.

Pharmaceutical Methods for Enhancing the Dissolution of Poorly Water-Soluble Drugs

Low water solubility is the main hindrance in the growth of pharmaceutical industry. Approximately 90% of newer molecules under investigation for drugs and 40% of novel drugs have been reported to have low water solubility. The key and thought-provoking task for the formulation scientists is the development of novel techniques to overcome the solubility-related issues of these drugs. The main intention of present review is to depict the conventional and novel strategies to overcome the solubility-related problems of Biopharmaceutical Classification System Class-II drugs. More than 100 articles published in the last 5 years were reviewed to have a look at the strategies used for solubility enhancement. pH modification, salt forms, amorphous forms, surfactant solubilization, cosolvency, solid dispersions, inclusion complexation, polymeric micelles, crystals, size reduction, nanonization, proliposomes, liposomes, solid lipid nanoparticles, microemulsions, and self-emulsifying drug delivery systems are the various techniques to yield better bioavailability of poorly soluble drugs. The selection of solubility enhancement technique is based on the dosage form and physiochemical characteristics of drug molecules.

Rivaroxaban lyospheres prepared by a dimethyl sulfoxide-based spray-freeze-drying process

Spray-freeze-drying (SFD) processes are usually using aqueous solvent systems, which however, exclude the use of SFD for poorly water-soluble drugs. Here, we evaluated dimethyl sulfoxide for its suitability in formulating SFD particles (lyospheres®). Rivaroxaban was spray-freeze-dried from DMSO solutions containing polyvinyl pyrrolidone (PVP; Kollidon® 25), vinylpyrrolidone-vinyl acetate copolymer (PVP-VA; Kollidon® VA64) or polyvinyl alcohol 4-88 (PVA) forming porous lyospheres® (median particle size 250 to 350 µm). Rivaroxaban was amorphous with all three polymers, which in combination with their high porosity resulted in rapid dissolution in vitro within 10 minutes. Consequently, this translated in lower T_max (0.5-1.0 hour) after oral administration of lyospheres® to rats (compared with T_max of 4 hours with coarse rivaroxaban). Lyosphere formulations achieved a distinct bioavailability increase (AUC_(0-inf) = 1487±657 ng*h/ml with PVP; 4426±1553 ng*h/ml with PVP-VA; 9569±3868 ng*h/ml with PVA lyospheres®; whereas 385±145 ng*h/ml with coarse rivaroxaban). These in vitro and in vivo results underlined the benefit of using DMSO in SFD that can broaden the applicability of the SFD process to much a larger repertoire of poorly water-soluble drugs.

Permeability of Olmesartan Medoxomil from Lipid Based and Suspension Formulations using an Optimized HDM-PAMPA Model

Hexadecane membrane-parallel artificial membrane permeability assay (HDM-PAMPA) is based on an artificial hexadecane membrane that separates the two compartments (donor and acceptor compartment). This model is used to predict the permeability of drugs in gastrointestinal tract and to simulate the passive absorption. In vivo behaviour of the drugs can be estimated with these systems in drug development studies. In our study we optimized HDM-PAMPA model to determine permeability of olmesartan medoxomil (OM) lipid based drug delivery system (OM-LBDDS). In order to prove that LBDDS formulation facilitates the weak permeability of OM, permeation rates were compared with the OM suspension formula (containing 0.25% v/w CMC). The experiment was performed on a 96-well MultiScreen® PAMPA filter plate (MAIPN4510). The permeability of olmesartan formulations from the donor to acceptor compartment separated by a HDM membrane were determined by the previous validated HPLC method. We created positive control series without coating hexadecane membrane to present the LBDDS and suspension formulation permeability from uncoated plates. The effective permeability constant (P_e) was calculated by the formula and improvement of permeability of OM-LBDDS formulation from hexadecane membrane was confirmed. On the contrary there was no permeation of OM-Suspension in the hexadecane coated plates. As a result, the intestinal permeability of OM-LBDDS was calculated to be at least 100 times more than the suspension. OM-Suspension permeation was only observed in the hexadecane uncoated positive control plates. This was also manifestation of HDM-PAMPA mimicking permeability of intestines because of its lipidic construction.

PAMAM Dendrimers: A Review of Methodologies Employed in Biopharmaceutical Classification

The oral route is the preferred way of drug administration for most drugs, whose treatment success is directly related to the compound intestinal absorption. This absorption process, in its turn, is influenced by several factors impacting the drug bioavailability, which is extremely dependent on the maximum solubility and permeability. However, optimizing these last two factors, without chemical structural modification, is challenging. Although poly(amidoamine) dendrimers (PAMAM) are an innovative and promising strategy as drug delivery compounds, there are few studies that determine the permeability and solubility of PAMAM-drugs derivatives. Considering this scenario, this paper aimed to carry out a literature review of the last five years concerning biopharmaceutical characterizations of dendrimer delivery systems. In vitro methodologies, such as the Parallel artificial membrane permeability assay (PAMPA) (non-cellular based model) and Caco-2 cells (cellular based model), used for the permeability evaluation in the early stages of drug discovery proved to be the most promising methodologies. As a result, we discussed, for instance, that through the usage of PAMPA it was possible to evaluate the higher capacity for transdermal delivery of DNA of TAT-conjugated PAMAM, when in comparison with unmodified PAMAM dendrimer with a P<0.05. We also presented the importance of choosing the best methods of biopharmaceutical characterization, which will be essential to guarantee the efficacy and safety of the drug candidate.

Nanocarrier Drug Delivery Systems: Characterization, Limitations, Future Perspectives and Implementation of Artificial Intelligence

There has been an increasing demand for the development of nanocarriers targeting multiple diseases with a broad range of properties. Due to their tiny size, giant surface area and feasible targetability, nanocarriers have optimized efficacy, decreased side effects and improved stability over conventional drug dosage forms. There are diverse types of nanocarriers that have been synthesized for drug delivery, including dendrimers, liposomes, solid lipid nanoparticles, polymersomes, polymer-drug conjugates, polymeric nanoparticles, peptide nanoparticles, micelles, nanoemulsions, nanospheres, nanocapsules, nanoshells, carbon nanotubes and gold nanoparticles, etc. Several characterization techniques have been proposed and used over the past few decades to control and predict the behavior of nanocarriers both in vitro and in vivo. In this review, we describe some fundamental in vitro, ex vivo, in situ and in vivo characterization methods for most nanocarriers, emphasizing their advantages and limitations, as well as the safety, regulatory and manufacturing aspects that hinder the transfer of nanocarriers from the laboratory to the clinic. Moreover, integration of artificial intelligence with nanotechnology, as well as the advantages and problems of artificial intelligence in the development and optimization of nanocarriers, are also discussed, along with future perspectives.

Evaluation of the permeability and potential toxicity of polycyclic aromatic hydrocarbons to pulmonary surfactant membrane by the parallel artificial membrane permeability assay model

Polycyclic aromatic hydrocarbons (PAHs) can penetrate and accumulate in the pulmonary surfactant (PS) membranes, leading to abnormalities of biological macromolecules and the destruction of membrane structure and properties. In the present study, the bioavailability, apparent permeability, effective permeability and residual coefficient of 10 PAHs on PS membrane was assessed by the parallel artificial membrane permeability assay (PAMPA). The influence of various forces on permeability is obtained by analyzing the correlation between parameters and physicochemical properties. Research shows that octanol-water partition coefficient (K_ow) cannot directly predict permeability, and permeability has no significant relationship with polarity. Dispersion, induction, coupling/polarization promote permeation, while hydrogen bonded acid and n-n electron pair inhibit permeation. Further surface pressure-area (π-A) isotherms test and Brewster angle microscope observation manifested that there are huge differences in the transmembrane ability and effects on the membrane of PAHs with different structures. This work has considerable significance that will help to evaluate the bioavailability and human health risk of PAHs.

A Stability Indicating RP-HPLC Method for Determination of the COVID-19 Drug Molnupiravir Applied Using Nanoformulations in Permeability Studies

Antiviral drugs have gained much more attention in recent years due to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection and many drug candidates are currently under investigation in order to end pandemic. Molnupiravir, a prodrug of the synthetic nucleoside derivative N4-hydroxycytidine, is one of the promising candidates for SARS-CoV-2 treatment. In this study, a RP-HPLC method was developed for the determination of Molnupiravir and applied for in vitro permeability studies of self-emulsifying drug delivery system (SEDDS) formulations using Caco-2 cell line. Discovery® HS C18 Column (75 ×4.6 mm, 3 µm) was used at 30 °C. Isocratic elution was performed with ACN:water (20:80 v/v) mixture. The flow rate was 0.5 mL/min and UV detection was at 240 nm. Molnupiravir eluted within 5 min. Molnupiravir was exposed to thermal, photolytic, hydrolytic, and oxidative stress conditions. Peak homogeneity data of Molnupiravir in the stressed samples peak obtained using photodiode array detector, in the stressed sample chromatograms, demonstrated the specificity of the method for their estimation in presence of degradants. The developed method was validated according to the International Council for Harmonisation (ICH) guidelines and found to be linear within the range 0.1–60.0 μg/mL. The method was simple, rapid, selective, sensitive, accurate, precise, robust and rugged. Thus, it was applied successfully for permeability quantitation of Molnupiravir in nanoformulations. The apparent permeability of Molnupiravir in SEDDS formulations, which have droplet size under 350 nm, was calculated as 3.20 ± 0.44 × 10⁻⁶ cm/s.

Hexafluoroisopropyl carbamates as selective MAGL and dual MAGL/FAAH inhibitors: biochemical and physicochemical properties

A series of hexafluoroisopropyl carbamates with indolylalkyl‐ and azaindolylalkyl‐substituents at the carbamate nitrogen was synthesized and evaluated for inhibition of the endocannabinoid degrading enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). The synthesized derivatives with butyl to heptyl spacers between the heteroaryl and the carbamate moiety were inhibitors of both enzymes. For investigated compounds in which the alkyl chain was partially incorporated into a piperidine ring, different results were obtained. Compounds with a methylene spacer between the piperidine ring and the heteroaromatic system were found to be selective MAGL inhibitors, while an extension of the alkyl spacer to two to four atoms resulted in dual inhibition of FAAH/MAGL. The only small change in enzyme inhibitory activity with variation of the heteroaromatic system indicates that the reactive hexafluoroisopropyl carbamate group is mainly responsible for the strength of the inhibitory effect of the compounds. Selected derivatives were also tested for hydrolytic stability in aqueous solution, liver homogenate and blood plasma as well as for aqueous solubility and for permeability in a Caco‐2 cell model. Some compounds showed a slightly higher MAGL inhibitory effect than the known selective MAGL inhibitor ABX‐1431 and also partly surpassed this substance with regard to certain physicochemical and biochemical properties such as water solubility and cell permeability.

Cell Interplay Model to Assess the Impact of Glioma Cells on Blood-Brain Barrier Permeability

The blood-brain barrier (BBB) is the most selective protecting layer of the central nervous system (CNS) with unique neurovascular features. The BBB is known to undergo a process of molecular alterations during disease state, such as in the case of glioma. This results in a non-uniform permeability along the BBB layer, which retains intact regions but develops focal sites of higher leakiness, especially in the surrounds of the tumor core. Although essential to guarantee brain homeostasis, the BBB has been the Achilles heel of drug delivery to the brain since the early times of the first classification as "barrier," more than a century ago. Due to the presence of the BBB, the transport of drug molecules from the bloodstream to the brain parenchyma is highly restricted, and, therefore, clinically relevant therapeutic concentrations cannot be achieved. Research efforts have focused on the development of novel tools to ameliorate drug permeability across the BBB, including drug formulation into non-invasive delivery systems with brain targeting properties and techniques that allow a temporary disruption of the BBB. To strengthen the advancement of potential drug candidates, in vitro models that recapitulate the main in vivo features of BBB are required to perform a preliminary screening of permeability, both in health and disease conditions. Herein, a protocol to assemble a BBB in vitro model to screen drug permeability in a glioma disease state is detailed. The model consists of a BBB and glioma cell co-culture and aims at exploiting the effect of the interplay between the cell constituents on the permeability of drug molecules. Although simple and straightforward, the herein in vitro model presents a high reproducibility, cost-effectiveness, and a favorable time-benefit balance.

Combined Methodologies for Determining In Vitro Bioavailability of Drugs and Prediction of In Vivo Bioequivalence From Pharmaceutical Oral Formulations

With the aim of developing an in vitro model for the bioavailability (BA) prediction of drugs, we focused on the study of levonorgestrel (LVN) released by 1.5 mg generic and brand-name tablets. The developed method consisted in combining a standard dissolution test with an optimized parallel artificial membrane permeability assay (PAMPA) to gain insights into both drug release and gastrointestinal absorption. Interestingly, the obtained results revealed that the tablet standard dissolution test, combined with an optimized PAMPA, highlighted a significant decrease in the release (15 ± 0.01 μg min⁻¹ vs 30 ± 0.01 μg min⁻¹) and absorption (19 ± 7 × 10^–6 ± 7 cm/s Pe vs 41 ± 15 × 10^–6 cm/s Pe) profiles of a generic LVN tablet when compared to the brand-name formulation, explaining unbalanced in vivo bioequivalence (BE). By using this new approach, we could determine the actual LVN drug concentration dissolved in the medium, which theoretically can permeate the gastrointestinal (GI) barrier. In fact, insoluble LVN/excipient aggregates were found in the dissolution media giving rise to non-superimposable dissolution profiles between generic and brand-name LVN tablets. Hence, the results obtained by combining the dissolution test and PAMPA method provided important insights confirming that the combined methods can be useful in revealing crucial issues in the prediction of in vivo BE of drugs.

Versatile membrane-based microfluidic platform for in vitro drug diffusion testing mimicking in vivo environments

Mimicking the diffusion that drugs suffer through different body tissues before reaching their target is a challenge. In this work, a versatile membrane-based microfluidic platform was developed to allow for the identification of drugs that would keep their cytotoxic properties after diffusing through such a barrier. As an application case, this paper reports on a microfluidic device capable of mimicking the diffusion that free or encapsulated anticancer drugs would suffer in the intestine before reaching the bloodstream. It not only presents the successful fabrication results for the platform but also demonstrates the significant effect that the analyzed drugs have over the viability of osteosarcoma cells. This intestine-like microfluidic platform works as a tool to allow for the identification of drugs whose cytotoxic performance remains effective enough once they enter the bloodstream. Therefore, it allows for the prediction of the best treatment available for each patient in the battle against cancer.

Comparative permeability of three saikosaponins and corresponding saikogenins in Caco-2 model by a validated UHPLC-MS/MS method

Saikosaponins (SSs) are the main active components extracted from Bupleuri Radix (BR) which has been used as an important herbal drug in Asian countries for thousands of years. It has been reported that the intestinal bacteria plays an important role in the in vivo disposal of oral SSs. Although the deglycosylated derivatives (saikogenins, SGs) of SSs metabolized by the intestinal bacteria are speculated to be the main components absorbed into the blood after oral administration of SSs, no studies have been reported on the characteristics of SGs for their intestinal absorption, and those for SSs are also limited. Therefore, a rapid UHPLC-MS/MS method was developed to investigate and compare the apparent permeability of three common SSs (SSa, SSd, SSb₂) and their corresponding SGs (SGF, SGG, SGD) through a bidirectional transport experiment on Caco-2 cell monolayer model. The method was validated according to the latest FDA guidelines and applied to quantify the six analytes in transport medium samples extracted via liquid-liquid extraction (LLE). The apparent permeability coefficient (P _app ) determined in this study indicated that the permeability of SGs improved to the moderate class compared to the corresponding parent compounds, predicting a higher in vivo absorption. Moreover, the efflux ratio (ER) value demonstrated an active uptake of SSd and the three SGs, while a passive diffusion of SSa and SSb₂.

An overview of in vitro, ex vivo and in vivo models for studying the transport of drugs across intestinal barriers

Oral administration is the most commonly used route for drug delivery owing to its cost-effectiveness, ease of administration, and high patient compliance. However, the absorption of orally delivered compounds is a complex process that greatly depends on the interplay between the characteristics of the drug/formulation and the gastrointestinal tract. In this contribution, we review the different preclinical models (in vitro, ex vivo and in vivo) from their development to application for studying the transport of drugs across intestinal barriers. This review also discusses the advantages and disadvantages of each model. Furthermore, the authors have reviewed the selection and validation of these models and how the limitations of the models can be addressed in future investigations. The correlation and predictability of the intestinal transport data from the preclinical models and human data are also explored. With the increasing popularity and prevalence of orally delivered drugs/formulations, sophisticated preclinical models with higher predictive capacity for absorption of oral formulations used in clinical studies will be needed.

Clofazimine functionalized polymeric nanoparticles for brain delivery in the tuberculosis treatment

Central nervous system tuberculosis (CNS-TB) is the most severe form of the disease especially due to the inability of therapeutics to cross the blood-brain barrier (BBB). Clofazimine (CFZ) stands out for presenting high in vitro activity against multi-drug resistant strains of Mycobacterium tuberculosis, however, CFZ physicochemical and pharmacokinetics properties limit drug penetration into the CNS and, consequently, its clinical use. The aim of this work was to develop polymeric nanoparticles (NPs) of poly(lactic-co-glycolic acid) (PLGA) and polyethylene glycol (PEG) loaded with CFZ and functionalized with a transferrin receptor (TfR)-binding peptide, aiming brain drug delivery for CNS-TB treatment by the intravenous route. The poor water solubility and high lipophilicity of CFZ was overcome through its entrapment into PLGA-PEG NPs manufactured by both conventional and microfluidic techniques using the nanoprecipitation principle. In vitro studies in brain endothelial hCMEC/D3 cells demonstrated that CFZ incorporation into the NPs was advantageous to reduce drug cytotoxicity. The TfR-binding peptide-functionalized NPs showed superior cell interaction and higher CFZ permeability across hCMEC/D3 cell monolayers compared to the non-functionalized NP control, thus indicating the efficacy of the functionalization strategy on providing CFZ transport through the BBB in vitro. The functionalized NPs demonstrate suitability for CFZ biological administration, suggested with low plasma protein binding, off-target biodistribution and precise delivery of CFZ towards the brain parenchyma.

Estimating the Oral Absorption from Self-Nanoemulsifying Drug Delivery Systems Using an In Vitro Lipolysis-Permeation Method

The aim of this study was to design an in vitro lipolysis-permeation method to estimate drug absorption following the oral administration of self-nanoemulsifying drug delivery systems (SNEDDSs). The method was evaluated by testing five oral formulations containing cinnarizine (four SNEDDSs and one aqueous suspension) from a previously published pharmacokinetic study in rats. In that study, the pharmacokinetic profiles of the five formulations did not correlate with the drug solubilization profiles obtained during in vitro intestinal lipolysis. Using the designed lipolysis-permeation method, in vitro lipolysis of the five formulations was followed by in vitro drug permeation in Franz diffusion cells equipped with PermeaPad^® barriers. A linear in vivo–in vitro correlation was obtained when comparing the area under the in vitro drug permeation–time curve (AUC_0–3h), to the AUC_0–3h of the plasma concentration–time profile obtained from the in vivo study. Based on these results, the evaluated lipolysis-permeation method was found to be a promising tool for estimating the in vivo performance of SNEDDSs, but more studies are needed to evaluate the method further.

Current challenges and future perspectives in oral absorption research: An opinion of the UNGAP network

Although oral drug delivery is the preferred administration route and has been used for centuries, modern drug discovery and development pipelines challenge conventional formulation approaches and highlight the insufficient mechanistic understanding of processes critical to oral drug absorption. This review presents the opinion of UNGAP scientists on four key themes across the oral absorption landscape: (1) specific patient populations, (2) regional differences in the gastrointestinal tract, (3) advanced formulations and (4) food-drug interactions. The differences of oral absorption in pediatric and geriatric populations, the specific issues in colonic absorption, the formulation approaches for poorly water-soluble (small molecules) and poorly permeable (peptides, RNA etc.) drugs, as well as the vast realm of food effects, are some of the topics discussed in detail. The identified controversies and gaps in the current understanding of gastrointestinal absorption-related processes are used to create a roadmap for the future of oral drug absorption research.

Development, synthesis, and biological evaluation of sulfonyl-α-l-amino acids as potential anti-Helicobacter pylori and IMPDH inhibitors

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes a crucial step in the biosynthesis of DNA and RNA, and it has been exploited as a promising target for antimicrobial therapy. The present study discusses the development and synthesis of a series of sulfonyl-α-l-amino acids coupled with the anisamide scaffold and evaluates their activities as anti-Helicobacter pylori and IMPDH inhibitors. Twenty derivatives were synthesized and their structures were established by high-resolution mass spectrometry and ¹ H and ¹³ C nuclear magnetic resonance measurements. Four compounds (6, 10, 11, and 21) were found to be the most potent and selective molecules in the series with minimum inhibitory concentration (MIC) values <17 µM, which were selected to test their inhibitory activities against HpIMPDH and human (h)IMPDH2 enzymes. In all tests, amoxicillin and clarithromycin were used as reference drugs. Compounds 6 and 10 were found to have a promising activity against the HpIMPDH enzyme, with IC₅₀  = 2.42 and 2.56 µM, respectively. Moreover, the four compounds were found to be less active and safer against hIMPDH2 than the reference drugs, with IC₅₀  > 17.17 µM, which makes sure that their selectivity is toward HpIMPDH and reverse to that of amoxicillin and clarithromycin. Also, the synergistic antibacterial activity of compounds 6, 10, amoxicillin, and clarithromycin was investigated in vitro. The combination of amoxicillin/compound 6 (2:1 by weight) exhibited a significant antibacterial activity against H. pylori, with MIC = 0.12 µg/ml. The molecular docking study and ADMET analysis of the most active compounds were used to elucidate the mode-of-action mechanism.

The progress on sulfhydryl modified polymers with regard to synthesis, characterization and mucoadhesion

The concepts of mucoadhesion and mucoadhesive polymers were introduced in the 20th century, leading to several advantages. These included enhanced drug absorption and extended residence at specific site of action. Polymeric excipients underwent chemical modification with sulfhydryl groups on the polymeric backbone so as to improve mucoadhesive features as well as potential. This modification resulted in compounds mimicking the nature of secreted mucus glycoproteins. Thus, these thiol group-bearing excipients presented the ability to attach covalently to the mucosa by the disulfide bonding. Nevertheless, the first generation of these thiol-modified polymers, named thiomers, presented disadvantages such as low stability in aqueous media and/or the high susceptibility towards oxidation along with the drawback of low sufficient reactive functional moieties on the polymeric backbone at lower pH. Therefore, in the 21st century, a second generation of preactivated or S-protected polymers with protected thiol moieties were developed, as well as a third generation of thiomers, solving some of the previously described problems. This review article aimed to highlight the progess on a potent sulfhydryl modification during the last decades and the posterior characterization and in vitro/ex vivo/in vivo mucoadhesiveness.

Triple Strategies to Improve Oral Bioavailability by Fabricating Coamorphous Forms of Ursolic Acid with Piperine: Enhancing Water-Solubility, Permeability, and Inhibiting Cytochrome P450 Isozymes

As a BCS IV drug, ursolic acid (UA) has low oral bioavailability mainly because of its poor aqueous solubility/dissolution, poor permeability, and metabolism by cytochrome P450 (CYP) isozymes, such as CYP3A4. Most UA preparations demonstrated a much higher dissolution than that of its crystalline form yet a low drug concentration in plasma due to their lower consideration or evaluation for the permeability and metabolism issues. In the current study, a supramolecular coamorphous system of UA with piperine (PIP) was prepared and characterized by powder X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. In comparison to crystalline UA and UA in physical mixture, such coamorphous system enhanced solubility (5.3-7-fold in the physiological solution) and dissolution (7-8-fold in the physiological solution within 2 h) of UA and exhibited excellent physical stability under 90-day storage conditions. More importantly, the pharmacokinetic study of coamorphous UA in rats exhibited 5.8-fold and 2.47-fold improvement in AUC0-∞ value, respectively, compared with its free and mixed crystalline counterparts. In order to further explore the mechanism of such improvement, the molecular interactions of a coamorphous system in the solid state were investigated. Fourier transform infrared spectroscopy, solid-state 13C nuclear magnetic resonance spectroscopy, and density functional theory modeling suggested that intermolecular hydrogen bonds with strong interactions newly formed between UA and PIP after coamorphization. The in vitro permeability studies across Caco-2 cell monolayer and metabolism studies by rat hepatic microsomes indicated that free PIP significantly increased the permeability of UA and inhibited the enzymatic metabolism of UA by CYP3A4. However, PIP in the coamorphous combination exhibited a much lower level in the bioenhancing than its free form arising from the synchronized dissolution characteristic of the preparation (only 60% of PIP released in comparison to its free counterpart in 2 h). The in situ loop study in rats proposed that the acid-sensitive dissolution in the stomach of the coamorphous preparation helped to improve the effective free drug concentration, thereby facilitating PIP to play its role in bioenhancing. The current study offers an exploratory strategy to overcome poor solubility/dissolution, poor permeability, and metabolism by cytochrome P450 isozymes of the BCS IV drug to improve its oral bioavailability.

A novel hydrogel sheet prevents postoperative pancreatic fistula in a rat model

Aim

To evaluate the efficacy of a novel hydrogel sheet in preventing postoperative pancreatic fistula (POPF).

Background

Postoperative pancreatic fistula is a life‐threatening complication. As no study has reported the use of hydrogel sheets in preventing POPF, their effectiveness for that purpose remains unclear.

Methods

A novel hydrogel sheet made of polyvinyl alcohol (PVA) was prepared by the freeze‐thaw method. The pancreatic ducts and surrounding pancreatic parenchyma of rats were transected to induce a pancreatic fistula. Next, the sheet was attached to the transection site. Ascitic fluid amylase and lipase concentrations were measured. Neoveil^®, a nonwoven polyglycolic acid (PGA) felt, is already clinically used as an absorbable reinforcing material at pancreatic transection sites. Neoveil^® was used for comparison, as was VIEWGEL^®, which is marketed as a wound dressing.

Results

The hydrogel sheet remained in place 48 hours postoperatively. The ascitic amylase concentrations in the control, VIEWGEL^®‐treated, Neoveil^®‐treated, and hydrogel‐treated rats, respectively, were 4992.4 ± 5355.7, 1068.4 ± 269.1, 730.2 ± 425.2, and 303.1 ± 240.1 IU/L; the ascitic lipase concentrations were 2279.8 ± 3395.2, 169.5 ± 100.6, 90.4 ± 71.0, and 86.8 ± 59.8 IU/L. The ascitic amylase and lipase levels were significantly lower in the hydrogel group than in the other groups (P < .05).

Conclusions

This novel hydrogel sheet effectively prevents pancreatic fistulas and has promising clinical application potential.

Biodegradable Nanoparticles: A Recent Approach and Applications

Biodegradable nanoparticles (NPs) are the novel carriers for the administration of drug molecules. Biodegradable nanoparticles have become popular recently because of their special features such as targeted delivery of drugs, improved bioavailability, and better therapeutic effectiveness to administer the drug at a constant rate. Polymeric NPs are very small-sized polymeric colloidal elements in which a drug of interest may be encapsulated or incorporated in their polymeric network or conjugated or adsorbed on the layer. Various polymers are employed in the manufacturing of nanoparticles, some of the frequently employed polymers are agents, chitosan, cellulose, gelatin, gliadin, polylactic acid, polylactic-co-glycolic acid, and pullulan. Nanoparticles have been progressively explored for the delivery of targeted ARVs to cells of HIV-infected and have performed the prolonged kinetic release. Drug embedded in this system can give better effectiveness, diminished resistance of drugs, reduction in systemic toxicity and symptoms, and also enhanced patient compliance. The present review highlights the frequently employed manufacturing methods for biodegradable nanoparticles, various polymers used, and its application in anti-retroviral therapy. Also, common evaluation parameters to check the purity of nanoparticles, ongoing and recently concluded clinical trials and patents filled by the various researchers, and the future implication of biodegradable NPs in an innovative drug delivery system are described. The biodegradable NPs are promising systems for the administration of a broad variety of drugs including anti-retroviral drugs, and hence biodegradable nanoparticles can be employed in the future for the treatment of several diseases and disorders.

Characterization of the Partition Rate of 2-Naphthol (NAP) and Ritonavir (RTV) Across the Water-Octanol Interface and the Influence of Common Pharmaceutical Excipients

The partition rate of 2 model compounds, 2-naphthol (NAP) and ritonavir (RTV), across the water-octanol (W/O) interface was determined from aqueous solutions with and without the presence of a variety of common pharmaceutical excipients. The NAP molecule was present as either a unionized species in the phosphate buffer solution or as an anion in 0.01 M NaOH solution while RTV was present as dications in 0.1 M HCl. Excipients examined include different type of polymers (e.g., PVP-VA, HPMC, HPMCAS, PVP, PEG 8000), small molecules (e.g., glucose, lactose, maltoheptaose), and surfactants (e.g., Tween 80 and SDS). A noticeable to significant reduction of the partition rate of both NAP and RTV across the W/O interface was observed with aqueous media containing 0.01-0.1 μg/mL polymers including PVP-VA, HPMC, and HPMCAS. Reduction of NAP and RTV partition rate associated with such extremely low concentration of polymers in the aqueous media affirms the presence of a surface excess of adsorbed excipients at the W/O interface. Pi values of NAP and RTV across the W/O interface are found to be sensitive to (1) the molecule surface area, molecular weight and the degree of the polymerization of these pharmaceutical excipients, and (2) the ionization state of the model compound.

Design, Synthesis of Novel Tetrandrine-14-l-Amino Acid and Tetrandrine-14-l-Amino Acid-Urea Derivatives as Potential Anti-Cancer Agents

Tetrandrine, a dibenzyltetrahydroisoquinoline alkaloid isolated from the root of the traditional Chinese medicinal plant Stephania tetrandra S. Moore, a member of the Menispermaceae, showed anti-cancer activity by inhibiting cell proliferation, preventing cell cycle progress and induction of cell death and autophagy. In this study, twelve tetrandrine-l-amino acid derivatives and twelve tetrandrine-14-l-amino acid-urea derivatives were designed and synthesized, using C14-aminotetrandrine as raw material. Then the preliminary in vitro anti-cancer activities of these derivatives against human breast cancer cell line MDA-MB-231, human leukemia cell lines HEL and K562 were evaluated. The in vitro cytotoxicity results showed that these derivatives exhibited potent inhibitory effects on cancer cell growth, and the primary structure-activity relationships were evaluated. Notably, compound 3f exhibited satisfactory anticancer activity against all three cancer cell lines, especially the HEL cell line, with the IC₅₀ value of 0.23 µM. Further research showed that 3f could induce G1/S cycle arrest and apoptosis in a dose- and time- dependent manner on the leukemia cell line HEL. The results suggested that 3f may be used as a potential anti-cancer agent for human leukemia.

Nanoformulation strategies for improving intestinal permeability of drugs: A more precise look at permeability assessment methods and pharmacokinetic properties changes

The therapeutic efficacy of orally administered drugs is often restricted by their inherent limited oral bioavailability. Low water solubility, limited permeability through the intestinal barrier, instability in harsh environment of the gastrointestinal (GI) tract and being substrate of the efflux pumps and the cytochrome P450 (CYP) can impair oral drug bioavailability resulting in erratic and variable plasma drug profile. As more drugs with low membrane permeability are developed, new interest is growing to enhance their intestinal permeability and bioavailability. A wide variety of nanosystems have been developed to improve drug transport and absorption. Sufficient evidence exists to suggest that nanoparticles are able to increase the transepithelial transport of drug molecules. However, key questions remained unanswered. What types of nanoparticles are more efficient? What are preclinical (or clinical) achievements of each type of nanoformulation in terms of pharmacokinetic (PK) parameters? Addressing this issue in this paper, we have reviewed the current literature regarding permeability enhancement, permeability assessment methods and changes in PK parameters following administration of various nanoformulations. Although permeability enhancement by various nanoformulations holds great promise for oral drug delivery, many challenges still need to be addressed before development of more clinically successful nanoproducts.

Simultaneous Evaluation of Dissolution and Permeation of Oral Drug Solid Formulations for Predicting Absorption Rate-Limiting Factors and In Vitro-In Vivo Correlations: Case Study Using a Poorly Soluble Weakly Basic Drug

Combined dissolution and permeation systems are designed to simultaneously assess the dissolution of a pharmaceutical dosage form and the permeation of dissolved drugs therefrom. However, there were still some limitations on predicting the possible absorption rate-limiting steps and improving the in vitro-in vivo correlation (IVIVC) of a complete dosage form. In this study, the modified biorelevant media with some solubilizers and pH modifiers were integrated into the drug dissolution/absorption simulating system (DDASS). Indapamide, a poorly soluble compound (pKa = 8.8), was selected to validate the applicability of the modified biorelevant media. The elution and permeation dynamics of indapamide were investigated by using appropriate solubilizing agents in the DDASS. The absorption behaviors were analyzed after oral administration of indapamide in beagle dogs. The absorption rate-limiting steps and IVIVCs were predicted from the dissolution-permeation-absorption dynamic parameters. As a result, the absorption fraction of indapamide in the FaSSIF_mod of DDASS was estimated to be approximately 100%, in accordance with its high permeability. The ratios of permeation rate to elution rate were 2.55 and 3.34 for the immediate- and sustained-release tablets of indapamide, respectively, suggesting a dissolution rate-limiting absorption for indapamine. In addition, point-to-point correlations were established between in vitro elution and in vivo absorption by the nonlinear and linear regression analysis ways (r > 0.85). The findings indicate that DDASS is a promising technique to develop improved IVIVCs of a complete dosage form, and the FaSSIF_mod is suitable to predict the possible absorption rate-limiting steps of poorly soluble drugs in DDASS.

Intestinal Fish Cell Barrier Model to Assess Transfer of Organic Chemicals in Vitro: An Experimental and Computational Study

We studied the role of the fish intestine as a barrier for organic chemicals using the epithelial barrier model built on the rainbow trout (Oncorhynchus mykiss) intestinal cell line, RTgutGC and the newly developed exposure chamber, TransFEr, specifically designed to work with hydrophobic and volatile chemicals. Testing 11 chemicals with a range of physicochemical properties (logK_OW: 2.2 to 6.3, logHLC: 6.1 to 2.3) and combining the data with a mechanistic kinetic model enabled the determination of dominant processes underlying the transfer experiments and the derivation of robust transfer rates. Against the current assumption in chemical uptake modeling, chemical transfer did not strictly depend on the logK_OW but resulted from chemical-specific intracellular accumulation and biotransformation combined with paracellular and active transport. Modeling also identified that conducting elaborate measurements of the plastic parts, including the polystyrene insert and the PET filter, is unnecessary and that stirring in the TransFEr chamber reduced the stagnant water layers compared to theoretical predictions. Aside from providing insights into chemical uptake via the intestinal epithelium, this system can easily be transferred to other cell-based barrier systems, such as the fish gill or mammalian intestinal models and may improve in vitro-in vivo extrapolation and prediction of chemical bioaccumulation into organisms.

Spray Congealing: An Emerging Technology to Prepare Solid Dispersions with Enhanced Oral Bioavailability of Poorly Water Soluble Drugs

The low and variable oral bioavailability of poorly water soluble drugs remains a major concern for the pharmaceutical industry. Spray congealing is an emerging technology for the production of solid dispersion to enhance the bioavailability of poorly soluble drugs by using low-melting hydrophilic excipients. The main advantages are the absence of solvents and the possibility to obtain spherical free-flowing microparticles (MPs) by a relatively inexpensive, simple, and one-step process. This review aims to fully describe the composition, structure, physico-chemical properties, and characterization techniques of spray congealed-formulations. Moreover, the influence of these properties on the MPs performance in terms of solubility and dissolution enhancement are examined. Following, an overview of the different spray congealed systems developed to increase the oral drug bioavailability is provided, with a focus on the mechanisms underpinning the bioavailability enhancement. Finally, this work gives specific insights on the main factors to be considered for the rational formulation, manufacturing, and characterization of spray congealed solid dispersions.

Successful oral delivery of poorly water-soluble drugs both depends on the intraluminal behavior of drugs and of appropriate advanced drug delivery systems

Poorly water-soluble drugs continue to be a problematic, yet important class of pharmaceutical compounds for treatment of a wide range of diseases. Their prevalence in discovery is still high, and their development is usually limited by our lack of a complete understanding of how the complex chemical, physiological and biochemical processes that occur between administration and absorption individually and together impact on bioavailability. This review defines the challenge presented by these drugs, outlines contemporary strategies to solve this challenge, and consequent in silico and in vitro evaluation of the delivery technologies for poorly water-soluble drugs. The next steps and unmet needs are proposed to present a roadmap for future studies for the field to consider enabling progress in delivery of poorly water-soluble compounds.

Lipopolysaccharide Polyelectrolyte Complex for Oral Delivery of an Anti-tubercular Drug

Anti-tuberculosis drug delivery has remained a challenge due to inconsistent bioavailability and inadequate sustained-release properties leading to treatment failure. To resolve these drawbacks, a lipopolysaccharide polyelectrolyte complex (PEC) encapsulated with rifampicin (RIF) (as the model drug) was fabricated, using the solvent injection technique (SIT), with soy lecithin (SLCT), and low-molecular-weight chitosan (LWCT). The average particle size and surface charge of RIF-loaded PEC particulates was 151.6 nm and + 33.0 nm, respectively, with noted decreased particle size and surface charge following increase in SLCT-LWCT mass ratio. Encapsulation efficiency (%EE) and drug-loading capacity (%LC) was 64.25% and 5.84%, respectively. Increase in SLCT-LWCT mass ratio significantly increased %EE with a marginal reduction in %LC. In vitro release studies showed a sustained-release profile for the PEC particulate tablet over 24 h (11.4% cumulative release) where the dominant release mechanism involved non-Fickian anomalous transport shifting towards super case II release as SLCT ratios increased (6.4% cumulative release). PEC-tablets prepared without SIT presented with rapid Fickian-diffusion-based drug release with up to 90% RIF release within 4 h. Ex vivo permeability studies revealed that lipopolysaccharide PEComplexation significantly increased the permeability of RIF by ~ 2-fold within the 8-h study period. These results suggest successful encapsulation of RIF within a PEC structure while imparting increased amorphic regions, as indicated by x-ray diffraction, for potential benefits in improved drug dissolution, bioavailability, and dosing.

Insight into Amorphous Solid Dispersion Performance by Coupled Dissolution and Membrane Mass Transfer Measurements

The tendency of highly supersaturated solutions of poorly water-soluble drugs to undergo liquid-liquid phase separation (LLPS) into drug-rich and water-rich phases when the concentration exceeds the amorphous solubility, for example, during dissolution of some amorphous solid dispersions, is thought to be advantageous from a bioavailability enhancement perspective. Recently, we have developed a high surface area, flow-through absorptive dissolution testing apparatus that enables fast mass transfer providing more in vivo relevant conditions and time frames for formulation testing. Using this apparatus, the absorption behaviors of solutions with different extents of supersaturation below and above the amorphous solubility were evaluated. In addition, simultaneous dissolution-absorption testing of amorphous solid dispersions (ASDs) with varying drug loadings and polymer types was carried out to study and distinguish the absorption behavior of ASDs that do or do not undergo LLPS. When compared with closed-compartment dissolution testing, a significant influence of the absorptive compartment on the dissolution rate of ASDs, particularly at high drug loadings, was observed. The formation of drug-rich nanodroplets, generated by both solvent-addition and ASD dissolution, resulted in a higher amount of drug transferred across the membrane. Moreover, the mass transfer was further enhanced with increasing concentration above the amorphous solubility, thereby showing correlation with an increase in the number of drug-rich particles. The importance of including an absorptive compartment in dissolution testing is highlighted in this study, enabling coupling of dissolution to membrane transport, and providing a more meaningful comparison between different formulations.

Using Human Plasma as an Assay Medium in Caco-2 Studies Improves Mass Balance for Lipophilic Compounds

PURPOSE

To examine the utility of human plasma as an assay medium in Caco-2 permeability studies to overcome poor mass balance and inadequate sink conditions frequently encountered with lipophilic compounds.

METHODS

Caco-2 permeability was assessed for reference compounds with known transport mechanisms using either pH 7.4 buffer or human plasma as the assay medium in both the apical and basolateral chambers. When using plasma, P_app values were corrected for the unbound fraction in the donor chamber. The utility of the approach was assessed by measuring the permeability of selected antimalarial compounds using the two assay media.

RESULTS

Caco-2 cell monolayer integrity and P-gp transporter function were unaffected by the presence of human plasma in the donor and acceptor chambers. For many of the reference compounds having good mass balance with buffer as the medium, higher P_app values were observed with plasma, likely due to improved acceptor sink conditions. The lipophilic antimalarial compounds exhibited low mass balance with buffer, however the use of plasma markedly improved mass balance allowing the determination of more reliable P_app values.

CONCLUSIONS

The results support the utility of human plasma as an alternate Caco-2 assay medium to improve mass balance and permeability measurements for lipophilic compounds.

Amino Acids in the Development of Prodrugs

Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.

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.