Passive Lipoidal Diffusion and Carrier-Mediated Cell Uptake Are Both Important Mechanisms of Membrane Permeation in Drug Disposition

Host cell environments and antibiotic efficacy in tuberculosis

The aetiologic agent of tuberculosis (TB), Mycobacterium tuberculosis (Mtb), can survive, persist, and proliferate in a variety of heterogeneous subcellular compartments. Therefore, TB chemotherapy requires antibiotics crossing multiple biological membranes to reach distinct subcellular compartments and target these bacterial populations. These compartments are also dynamic, and our understanding of intracellular pharmacokinetics (PK) often represents a challenge for antitubercular drug development. In recent years, the development of high-resolution imaging approaches in the context of host-pathogen interactions has revealed the intracellular distribution of antibiotics at a new level, yielding discoveries with important clinical implications. In this review, we describe the current knowledge regarding cellular PK of antibiotics and the complexity of drug distribution within the context of TB. We also discuss the recent advances in quantitative imaging and highlight their applications for drug development in the context of how intracellular environments and microbial localisation affect TB treatment efficacy.

Quantification of Binding of Small Molecules to Native Proteins Overexpressed in Living Cells

The affinity and selectivity of small molecules for proteins drive drug discovery and development. We report a fluorescent probe cellular binding assay (FPCBA) for determination of these values for native (untagged) proteins overexpressed in living cells. This method uses fluorophores such as Pacific Blue (PB) linked to cell-permeable protein ligands to generate probes that rapidly and reversibly equilibrate with intracellular targets, as established by kinetic assays of cellular uptake and efflux. To analyze binding to untagged proteins, an internal ribosomal entry site (IRES) vector was employed that allows a single mRNA to encode both the protein target and a separate orthogonal fluorescent protein (mVenus). This enabled cellular uptake of the probe to be correlated with protein expression by flow cytometry, allowing measurement of cellular dissociation constants (Kd) of the probe. This approach was validated by studies of the binding of allosteric activators to eight different Protein Kinase C (PKC) isozymes. Full-length PKCs expressed in transiently transfected HEK293T cells were used to measure cellular Kd values of a probe comprising PB linked to the natural product phorbol via a carbamate. These values were further used to determine competitive binding constants (cellular Ki values) of the nonfluorescent phorbol ester PDBu and the anticancer agent bryostatin 1 for each isozyme. For some PKC-small molecule pairs, these cellular Ki values matched known biochemical Ki values, but for others, altered selectivity was observed in cells. This approach can facilitate quantification of interactions of small molecules with physiologically relevant native proteins.

Membrane transporters in cell physiology, cancer metabolism and drug response

By controlling the passage of small molecules across lipid bilayers, membrane transporters influence not only the uptake and efflux of nutrients, but also the metabolic state of the cell. With more than 450 members, the Solute Carriers (SLCs) are the largest transporter super-family, clustering into families with different substrate specificities and regulatory properties. Cells of different types are, therefore, able to tailor their transporter expression signatures depending on their metabolic requirements, and the physiological importance of these proteins is illustrated by their mis-regulation in a number of disease states. In cancer, transporter expression is heterogeneous, and the SLC family has been shown to facilitate the accumulation of biomass, influence redox homeostasis, and also mediate metabolic crosstalk with other cell types within the tumour microenvironment. This Review explores the roles of membrane transporters in physiological and malignant settings, and how these roles can affect drug response, through either indirect modulation of sensitivity or the direct transport of small-molecule therapeutic compounds into cells.

Combining SILCS and Artificial Intelligence for High-Throughput Prediction of the Passive Permeability of Drug Molecules

Membrane permeability of drug molecules plays a significant role in the development of new therapeutic agents. Accordingly, methods to predict the passive permeability of drug candidates during a medicinal chemistry campaign offer the potential to accelerate the drug design process. In this work, we combine the physics-based site identification by ligand competitive saturation (SILCS) method and data-driven artificial intelligence (AI) to create a high-throughput predictive model for the passive permeability of druglike molecules. In this study, we present a comparative analysis of four regression models to predict membrane permeabilities of small druglike molecules; of the tested models, Random Forest was the most predictive yielding an R2 of 0.81 for the independent data set. The input feature vector used to train the developed prediction model includes absolute free energy profiles of ligands through a POPC-cholesterol bilayer based on ligand grid free energy (LGFE) profiles obtained from the SILCS approach. The use of the membrane free energy profiles from SILCS offers information on the physical forces contributing to ligand permeability, while the use of AI yields a more predictive model trained on experimental PAMPA permeability data for a collection of 229 molecules. This combination allows for rapid estimations of ligand permeability at a level of accuracy beyond currently available predictive models while offering insights into the contributions of the functional groups in the ligands to the permeability barrier, thereby offering quantitative information to facilitate rational ligand design.

Visualization of Molecular Permeation into a Multi-compartment Phospholipid Vesicle

Passive permeation of small molecules into vesicles with multiple compartments is a critical event in many chemical and biological processes. We consider the translocation of the peptide NAF-144-67 labeled with a fluorescent fluorescein dye across membranes of rhodamine-labeled 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) into liposomes with internal vesicles. Time-resolved microscopy revealed a sequential absorbance of the peptide in both the outer and inner micrometer vesicles that developed over a time period of minutes to hours, illustrating the spatial and temporal progress of the permeation. There is minimal perturbation of the membrane structure and no evidence for pore formation. On the basis of molecular dynamics simulations of NAF-144-67, we extended a local defect model to migration processes that include multiple compartments. The model captures the long residence time of the peptide within the membrane and the rate of permeation through the liposome and its internal compartments. Imaging experiments confirm the semi-quantitative description of the permeation of the model by activated diffusion and open the way for studies of more complex systems.

Permeation of a Homologous Series of NBD-Labeled Fatty Amines through Lipid Bilayers: A Molecular Dynamics Study

Permeation through biomembranes is ubiquitous for drugs to reach their active sites. Asymmetry of the cell plasma membrane (PM) has been described as having an important role in this process. Here we describe the interaction of a homologous series of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled amphiphiles (NBD-Cn, n = 4 to 16) with lipid bilayers of different compositions (1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC):cholesterol (1:1) and palmitoylated sphingomyelin (SpM):cholesterol (6:4)), including an asymmetric bilayer. Both unrestrained and umbrella sampling (US) simulations (at varying distances to the bilayer center) were carried out. The free energy profile of NBD-Cn at different depths in the membrane was obtained from the US simulations. The behavior of the amphiphiles during the permeation process was described regarding their orientation, chain elongation, and H-bonding to lipid and water molecules. Permeability coefficients were also calculated for the different amphiphiles of the series, using the inhomogeneous solubility-diffusion model (ISDM). Quantitative agreement with values obtained from kinetic modeling of the permeation process could not be obtained. However, for the longer, and more hydrophobic amphiphiles, the variation trend along the homologous series was qualitatively better matched by the ISDM when the equilibrium location of each amphiphile was taken as reference (ΔG = 0), compared to the usual choice of bulk water.

Facilitating flip-flop: Structural tuning of molecule-membrane interactions in living bacteria

The first barrier that a small molecule must overcome before trespassing into a living cell is the lipid bilayer surrounding the intracellular content. It is imperative, therefore, to understand how the structure of a small molecule influences its fate in this region. Through the use of second harmonic generation, we show how the differing degrees of ionic headgroups, conjugated system, and branched hydrocarbon tail disparities of a series of four styryl dye molecules influence the propensity to "flip-flop" or to be further organized in the outer leaflet by the membrane. We show here that initial adsorption experiments match previous studies on model systems; however, more complex dynamics are observed over time. Aside from probe molecule structure, these dynamics also vary between cell species and can deviate from trends reported based on model membranes. Specifically, we show here that the membrane composition is an important factor to consider for headgroup-mediated small-molecule dynamics. Overall, the findings presented here on how structural variability of small molecules impacts their initial adsorption and eventual destinations within membranes in the context of living cells could have practical applications in antibiotic and drug adjuvant design.

Screening for bilayer-active and likely cytotoxic molecules reveals bilayer-mediated regulation of cell function

A perennial problem encountered when using small molecules (drugs) to manipulate cell or protein function is to assess whether observed changes in function result from specific interactions with a desired target or from less specific off-target mechanisms. This is important in laboratory research as well as in drug development, where the goal is to identify molecules that are unlikely to be successful therapeutics early in the process, thereby avoiding costly mistakes. We pursued this challenge from the perspective that many bioactive molecules (drugs) are amphiphiles that alter lipid bilayer elastic properties, which may cause indiscriminate changes in membrane protein (and cell) function and, in turn, cytotoxicity. Such drug-induced changes in bilayer properties can be quantified as changes in the monomer↔dimer equilibrium for bilayer-spanning gramicidin channels. Using this approach, we tested whether molecules in the Pathogen Box (a library of 400 drugs and drug-like molecules with confirmed activity against tropical diseases released by Medicines for Malaria Venture to encourage the development of therapies for neglected tropical diseases) are bilayer modifiers. 32% of the molecules in the Pathogen Box were bilayer modifiers, defined as molecules that at 10 µM shifted the monomer↔dimer equilibrium toward the conducting dimers by at least 50%. Correlation analysis of the molecules' reported HepG2 cell cytotoxicity to bilayer-modifying potency, quantified as the shift in the gramicidin monomer↔dimer equilibrium, revealed that molecules producing <25% change in the equilibrium had significantly lower probability of being cytotoxic than molecules producing >50% change. Neither cytotoxicity nor bilayer-modifying potency (quantified as the shift in the gramicidin monomer↔dimer equilibrium) was well predicted by conventional physico-chemical descriptors (hydrophobicity, polar surface area, etc.). We conclude that drug-induced changes in lipid bilayer properties are robust predictors of the likelihood of membrane-mediated off-target effects, including cytotoxicity.

New Betulin Derivatives with Nitrogen Heterocyclic Moiety-Synthesis and Anticancer Activity In Vitro

As part of the search for new medicinal substances with potential application in oncology, the synthesis of new compounds combining the betulin molecule and the indole system was carried out. The structure of the ester derivatives obtained in the Steglich reaction was confirmed by spectroscopic methods (¹H and ¹³C NMR, HR-MS). The obtained new 3-indolyl betulin derivatives were evaluated for anticancer activity against several human cancer cell lines (melanomas, breast cancers, colorectal adenocarcinomas, lung cancer) as well as normal human fibroblasts. The significant reduction in MCF-7 cells viability for 28-hydroxy-(lup-20(29)-ene)-3-yl 2-(1H-indol-3-yl)acetate was observed at a concentration of 10 µg/mL (17 µM). In addition, cytometric analysis showed that this compound strongly reduces the proliferation rate of breast cancer cells. For this, the derivative showing the promising cytotoxic effect on MCF-7 breast cancer cells, the pharmacokinetic profile prediction was performed using in silico methods. Based on the results obtained in the study, it can be concluded that indole-functionalized triterpene EB367 is a promising starting point for further research in the field of breast cancer therapy or the synthesis of new derivatives.

Conformational Effects on the Passive Membrane Permeability of Synthetic Macrocycles

Macrocyclic compounds (MCs) can have complex conformational properties that affect pharmacologically important behaviors such as membrane permeability. We measured the passive permeability of 3600 diverse nonpeptidic MCs and used machine learning to analyze the results. Incorporating selected properties based on the three-dimensional (3D) conformation gave models that predicted permeability with Q² = 0.81. A biased spatial distribution of polar versus nonpolar regions was particularly important for good permeability, consistent with a mechanism in which the initial insertion of nonpolar portions of a MC helps facilitate the subsequent membrane entry of more polar parts. We also examined effects on permeability of 800 substructural elements by comparing matched molecular pairs. Some substitutions were invariably beneficial or invariably deleterious to permeability, while the influence of others was highly contextual. Overall, the work provides insights into how the permeability of MCs is influenced by their 3D conformational properties and suggests design hypotheses for achieving macrocycles with high membrane permeability.

Hypoglycaemic activity of Oleanonic acid, a 3-oxotriterpenoid isolated from Aidia Genipiflora (DC.) Dandy, involves inhibition of carbohydrate metabolic enzymes and promotion of glucose uptake

The present study evaluated the antidiabetic activities of the 70% ethanol stem bark extract of Aidia genipiflora (AGB) and one of its constituents, oleanonic acid in streptozotocin (40 mg/kg)-induced diabetic rats. In vitro assays of glucose uptake and inhibition of carbohydrate metabolizing enzymes were then used to investigate their mechanism(s) of hypoglycaemic action. In silico evaluation of the pharmacokinetic and toxicity properties of the compound was also carried out. Administration of AGB (100-400 mg/kg) and oleanonic acid (15 - 60 mg/kg) resulted in significant reductions (p < 0.001) in the blood glucose and considerable decrease (p < 0.05) in the elevated lipid parameters of the diabetic animals. AGB activity at 200 and 400 mg/kg; and oleanonic acid at 60 mg/kg were comparable to glibenclamide (5 mg/kg). The extract and its isolate strongly inhibited α-glucosidase and α-amylase activity with IC₅₀ values of (10.48 ± 1.39 µg/mL and 14.51 ± 1.26 µg/mL) and (36.52 ± 1.95 µM and 105.84 ± 1.08 µM) respectively. The glucose uptake assays showed that AGB and oleanonic acid exerted both insulin-dependent and independent promotional effect of glucose transport into the periphery by upregulating the expression of PI3K and PPARγ transcripts with a concomitant increase in GLUT-4 transcripts. Although oleanonic acid was predicted to be teratogenic, it was found to be generally non-lethal with favourable pharmacokinetics properties making it suitable for further studies. The study has shown that the stem bark of A. genipiflora is a source of new hypoglycaemic agents and that oleanonic acid possesses hypoglycaemic and anti-hyperlipidaemic activities.

Interactions between Rhodamine Dyes and Model Membrane Systems—Insights from Molecular Dynamics Simulations

Background: rhodamines are dyes widely used as fluorescent tags in cell imaging, probing of mitochondrial membrane potential, and as P-glycoprotein model substrates. In all these applications, detailed understanding of the interaction between rhodamines and biomembranes is fundamental. Methods: we combined atomistic molecular dynamics (MD) simulations and fluorescence spectroscopy to characterize the interaction between rhodamines 123 and B (Rh123 and RhB, respectively) and POPC bilayers. Results: while the xanthene moiety orients roughly parallel to the membrane plane in unrestrained MD simulations, variations on the relative position of the benzoic ring (below the xanthene for Rh123, above it for RhB) were observed, and related to the structure of the two dyes and their interactions with water and lipids. Subtle distinctions were found among different ionization forms of the probes. Experimentally, RhB displayed a lipid/water partition coefficient more than two orders of magnitude higher than Rh123, in agreement with free energy profiles obtained from umbrella sampling MD. Conclusions: this work provided detailed insights on the similarities and differences in the behavior of bilayer-inserted Rh123 and RhB, related to the structure of the probes. The much higher affinity of RhB for the membranes increases the local concentration and explains its higher apparent affinity for P-glycoprotein reconstituted in model membranes.

Simultaneous Determination of Seven Lipophilic and Hydrophilic Components in Salvia miltiorrhiza Bunge by LC-MS/MS Method and Its Application to a Transport Study in a Blood-Brain-Barrier Cell Model

Salvia miltiorrhiza Bunge (SM) has been extensively used in Alzheimer’s disease treatment, the permeability through the blood-brain barrier (BBB) determining its efficacy. However, the transport mechanism of SM components across the BBB remains to be clarified. A simple, precise, and sensitive method using LC-MS/MS was developed for simultaneous quantification of tanshinone I (TS I), dihydrotanshinone I (DTS I), tanshinone IIA (TS IIA), cryptotanshinone (CTS), protocatechuic aldehyde (PAL), protocatechuic acid (PCTA), and caffeic acid (CFA) in transport samples. The analytes were separated on a C18 column by gradient elution. Multiple reaction monitoring mode via electrospray ionization source was used to quantify the analytes in positive mode for TS I, DTS I, TS IIA, CTS, and negative mode for PAL, PCTA, and CFA. The linearity ranges were 0.1–8 ng/mL for TS I and DTS I, 0.2–8 ng/mL for TS IIA, 1–80 ng/mL for CTS, 20–800 ng/mL for PAL and CFA, and 10–4000 ng/mL for PCTA. The developed method was accurate and precise for the compounds. The relative matrix effect was less than 15%, and the analytes were stable for analysis. The established method was successfully applied for transport experiments on a BBB cell model to evaluate the apparent permeability of the seven components.

Best practices in current models mimicking drug permeability in the gastrointestinal tract - an UNGAP review

The absorption of orally administered drug products is a complex, dynamic process, dependent on a range of biopharmaceutical properties; notably the aqueous solubility of a molecule, stability within the gastrointestinal tract (GIT) and permeability. From a regulatory perspective, the concept of high intestinal permeability is intrinsically linked to the fraction of the oral dose absorbed. The relationship between permeability and the extent of absorption means that experimental models of permeability have regularly been used as a surrogate measure to estimate the fraction absorbed. Accurate assessment of a molecule's intestinal permeability is of critical importance during the pharmaceutical development process of oral drug products, and the current review provides a critique of in vivo, in vitro and ex vivo approaches. The usefulness of in silico models to predict drug permeability is also discussed and an overview of solvent systems used in permeability assessments is provided. Studies of drug absorption in humans are an indirect indicator of intestinal permeability, but in vitro and ex vivo tools provide initial screening approaches are important tools for direct assessment of permeability in drug development. Continued refinement of the accuracy of in silico approaches and their validation with human in vivo data will facilitate more efficient characterisation of permeability earlier in the drug development process and will provide useful inputs for integrated, end-to-end absorption modelling.

Route of intracellular uptake and cytotoxicity of sesamol, sesamin, and sesamolin in human melanoma SK-MEL-2 cells

The intracellular uptake concentration determines drug absorption, drug activity, and toxicity. Sesamol, sesamin, and sesamolin are promising bioactive components from Sesame indicum L. Their respective intracellular uptake pathway and cytotoxicity were evaluated using melanoma and non-cancerous cells. Quantitative structure-activity relationship (QSAR) models were built to identify the molecular features affecting drug uptake in cells. The respective intracellular uptake pathway for sesamol vs. sesamin and sesamolin was carrier-mediated vs. passive transport. Topological polar surface area (PSA) and 2D autocorrections increase the intracellular concentration (C/M ratio) of these compounds. Sesamol has the lowest C/M ratio compared to sesamin and sesamolin, but only sesamol inhibits the cell viability of melanoma and provides an inhibition concentration at 50% (IC₅₀) against melanoma cells. The slightly aqueous solubility of sesamin and sesamolin, therefore, limits testing of their cytotoxicity. In conclusion, sesamol has the potential to inhibit melanoma cell growth, but requires improvement of the C/M ratio to increase its physicochemical properties. Thus, in order to investigate the cytotoxicity of sesamin and sesamolin against melanoma cells a solubility enhancer is needed.

4,5,7-Trisubstituted indeno[1,2-b]indole inhibits CK2 activity in tumor cells equivalent to CX-4945 and shows strong anti-migratory effects

Highly pleiotropic and constitutively active protein kinase CK2 is a key target in cancer therapy, but only one small-molecule inhibitor has reached clinical trials-CX-4945. In this study, we present the indeno[1,2-b]indole derivative 5-isopropyl-4-methoxy-7-methyl-5,6,7,8-tetrahydroindeno[1,2-b]indole-9,10-dione (5a-2) that decreased the intracellular CK2 activity in A431, A549, and LNCaP tumor cell lines analogous to CX-4945 (> 75% inhibition at 20 µm) and similarly blocked CK2-specific Akt phosphorylation in LNCaP cells. Cellular uptake analysis demonstrated higher intracellular concentrations of 5a-2 (408.3 nm) compared with CX-4945 (119.3 nm). This finding clarifies the comparable effects of both compounds on the intracellular CK2 activity despite their different inhibitory potency in vitro [IC₅₀  = 25 nm (5a-2) and 3.7 nm (CX-4945)]. Examination of the effects of both CK2 inhibitors on cancer cells using live-cell imaging revealed notable differences. Whereas CX-4945 showed a stronger pro-apoptotic effect on tumor cells, 5a-2 was more effective in inhibiting tumor cell migration. Our results showed that 49% of intracellular CX-4945 was localized in the nuclear fraction, whereas 71% of 5a-2 was detectable in the cytoplasm. The different subcellular distribution, and thus the site of CK2 inhibition, provides a possible explanation for the different cellular effects. Our study indicates that investigating CK2 inhibition-mediated cellular effects in relation to the subcellular sites of CK2 inhibition may help to improve our understanding of the preferential roles of CK2 within different cancer cell compartments.

Tissue-Specific Accumulation, Biotransformation, and Physiologically Based Toxicokinetic Modeling of Benzotriazole Ultraviolet Stabilizers in Zebrafish (Danio rerio)

Benzotriazole ultraviolet stabilizers (BUVSs) are high-production-volume chemicals with ubiquitous occurrence in the aquatic environment. However, little is known about their bioconcentration and biotransformation, and physiologically based toxicokinetic (PBTK) models for BUVSs are lacking. This study selected six BUVSs for which experiments were performed with zebrafish (Danio rerio) exposed to two different levels (0.5 and 10 μg·L^-1). Higher kinetic bioconcentration factors (BCFs) were observed at the lower exposure level with environmental relevance, with BCF of 3.33 × 10³ L·kg^-1 for 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole (UV-327). This phenomenon was interpreted by a nonlinear adsorption mechanism, where binding with specific protein sites contributes to bioconcentration. Muscle exhibited the lowest accumulation, in which depuration half-life of UV-327 was 19.5 d. In kidney, muscle, ovary, gill, and skin, logBCF increased with increase in log K_OW of the BUVSs until log K_OW was ca. 6.5, above which logBCF decreased. However, the trend was not observed in the liver and intestine. Six biotransformation products were identified and mainly accumulated in the liver and intestine. Considering the nonlinear adsorption mechanism in the PBTK model, the prediction accuracy of the model was improved, highlighting the binding of xenobiotics with specific protein sites in assessing the bioconcentration of chemicals for their risk assessment.

Distribution of ion pairs into a bilayer lipid membrane and its effect on the ionic permeability

This work reports the distribution constant of a target ion and a counter-ion between an aqueous phase and an artificial bilayer lipid membrane (BLM) and its influence to the ionic permeability through a BLM. A theoretical formula for ionic permeability through a BLM based on the distribution of the target ion and the counter-ion is also proposed and validated by analyzing the flux of a fluorescent cation [rhodamine 6G (R6G⁺)] through the BLM in the presence of counter-ions (X^- = Br^-, BF₄^-, and ClO₄^-). The transmembrane flux was evaluated by simultaneous measurement of the transmembrane current density and the transmembrane fluorescence intensity as a function of the membrane potential. The distribution constant of R6G⁺ and X^- between the aqueous and BLM phases was determined by a liposome-extraction method. The measured ionic permeability exhibited non-linear dependent on the aqueous concentration of R6G⁺ or X^-, but proportional to the concentration of R6G⁺ and X^- inside the BLM evaluated from the distribution constant of R6G⁺ and X^-. The proportionality demonstrates that the distribution of cations and anions between the aqueous and BLM phases dominates the flux of ion transport through the BLM. The proposed formula can express the dependence of the transmembrane current on the membrane potential and the concentrations of R6G⁺ and X^- in the aqueous phase.

Understanding the brain uptake and permeability of small molecules through the BBB: A technical overview

The brain is the most important organ in our body requiring its unique microenvironment. By the virtue of its function, the blood-brain barrier poses a significant hurdle in drug delivery for the treatment of neurological diseases. There are also different theories regarding how molecules are typically effluxed from the brain. In this review, we comprehensively discuss how the different pharmacokinetic techniques used for measuring brain uptake/permeability of small molecules have evolved with time. We also discuss the advantages and disadvantages associated with these different techniques as well as the importance to utilize the right method to properly assess CNS exposure to drug molecules. Even though very strong advances have been made we still have a long way to go to ensure a reduction in failures in central nervous system drug development programs.

Effect of Water pH on the Uptake of Acidic (Ibuprofen) and Basic (Propranolol) Drugs in a Fish Gill Cell Culture Model

Water pH is predicted to affect the uptake of ionizable pharmaceuticals in fish. The current study used an in vitro primary fish gill cell culture system to assess the effect of pH values in the range of 4.5-8.75 on the uptake rates of the base propranolol (pKa 9.42) and the acid ibuprofen (pKa 4.59). The rate-limiting step in the uptake was the diffusive supply flux of the unionized form from the water to the apical membrane, with subsequent rapid transfer across the epithelium. Computed uptake rate based on the unionized fraction best described the uptake of propranolol and ibuprofen over the range of pH values 5-8 and 6-8.75, respectively. For ibuprofen, the computed uptake rate overestimated the uptake below pH 6 where the unionized fraction increased from 4% at pH 6 to 55% at pH 4.5. As the unionized fraction increased, the uptake rate plateaued suggesting a saturation of the transport process. For both drugs, large variations in the uptake occur with only small fluctuations in pH values. This occurs between pH values 6 and 8, which is the pH range acceptable in regulatory test guidelines and seen in most of our freshwaters.

Recent Experimental Developments in Studying Passive Membrane Transport of Drug Molecules

The ability to measure the passive membrane permeation of drug-like molecules is of fundamental biological and pharmaceutical importance. Of significance, passive diffusion across the cellular membranes plays an effective role in the delivery of many pharmaceutical agents to intracellular targets. Hence, approaches for quantitative measurement of membrane permeability have been the topics of research for decades, resulting in sophisticated biomimetic systems coupled with advanced techniques. In this review, recent developments in experimental approaches along with theoretical models for quantitative and real-time analysis of membrane transport of drug-like molecules through mimetic and living cell membranes are discussed. The focus is on time-resolved fluorescence-based, surface plasmon resonance, and second-harmonic light scattering approaches. The current understanding of how properties of the membrane and permeant affect the permeation process is discussed.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Myrianthus libericus: Possible mechanisms of hypoglycaemic action and in silico prediction of pharmacokinetics and toxicity profile of its bioactive metabolite, friedelan-3-one

The hypoglycaemic and anti-hyperlipidaemic effects of the 70% ethanol stem bark extract of Myrianthus libericus (MLB), used traditionally in the management of diabetes in Ghana, was evaluated in this study using streptozotocin (45 mg/kg)-induced diabetic rats. In vitro hypoglycaemic activities of the extract and one of its principal compounds, friedelan-3-one were then investigated using α-amylase inhibitory and glucose uptake assay in C2C12 myotubes. In silico analysis of the pharmacokinetic and toxicity properties of the compound was also performed. MLB significantly (p < 0.001) reduced the elevated blood glucose levels and corrected considerably (p < 0.01) the altered serum lipid profiles of the diabetic rats which was comparable to glibenclamide (5 mg/kg). Together with friedelan-3-one, the extract markedly inhibited the activity of α-amylase and promoted glucose uptake in C2C12 cells. Whereas MLB significantly (p < 0.001) up-regulated PI3K and PPARγ transcripts with a corresponding increase in GLUT-4 transcripts within the muscle cells, friedelan-3-one only up-regulated PI3K and GLUT-4 transcripts to promote glucose transport. Friedelan-3-one was shown to be non-carcinogenic, non-hepatotoxic, has decent oral bioavailability and a good compound for optimisation into a drug candidate. The study has demonstrated that MLB possess hypoglycaemic and anti-hyperlipidaemic activities and could be used as a therapeutic agent in the management of diabetes mellitus.

Target Validation Using PROTACs: Applying the Four Pillars Framework

Proteolysis targeting chimeras (PROTACs) are heterobifunctional compounds that recruit the E3 ubiquitin ligase machinery to proteins of interest, resulting in their ubiquitination and subsequent proteasomal degradation. Targeted protein degradation has generated considerable interest in drug discovery because inhibition of one particular function of a protein often does not deliver the therapeutic efficacy that results from whole-protein depletion. However, the physicochemistry and intrinsically complex pharmacology of PROTACs present challenges, particularly for the development of orally bioavailable drugs. Here we describe the application of a translational pharmacology framework (called the four pillars) to expedite PROTAC development by informing pharmacokinetic-pharmacodynamic (PKPD) understanding and helping elucidate structure-activity relationships. Experimental methods are reviewed that help illuminate exposure of the drug or probe at the site of action (pillar 1) and engagement of its target(s) (pillar 2) that drive functional pharmacological effects (pillar 3) resulting in modulation of a relevant phenotype (pillar 4). We hope the guidance will be useful to those developing targeted protein degraders and help establish PROTAC molecules as robust target validation chemical probes.

Physicochemical properties and Mycobacterium tuberculosis transporters: keys to efficacious antitubercular drugs?

Securing novel, safe, and effective medicines to treat Mycobacterium tuberculosis remains an elusive goal, particularly influenced by the largely impervious Mtb envelope that limits exposure and thus efficacy of inhibitors at their cellular and periplasmic targets. The impact of physicochemical properties on pharmacokinetic parameters that govern oral absorption and exposure at sites of infection is considered alongside how these properties influence penetration of the Mtb envelope, with the likely influence of transporter proteins. The findings are discussed to benchmark current drugs and the emerging pipeline, whilst considering tactics for future rational and targeted design strategies, based around emerging data on Mtb transporters and their structures and functions.

Flavanols and triterpenoids from Myrianthus arboreus ameliorate hyperglycaemia in streptozotocin-induced diabetic rats possibly via glucose uptake enhancement and α-amylase inhibition

Myrianthus arboreus is use traditionally as an antidiabetic agent in Ghana. We reported the in vivo antidiabetic activity of its 70 % ethanol stem bark extract (MAB) which we found to be strongly concentrated in its EtOAc fraction using glucose uptake and enzyme inhibitory assays. The present study sought to investigate the in vivo hypoglycaemic and anti-hyperlipidaemic activity of this ethyl acetate fraction of MAB (MAB-EtOAc, 50 and 100 mg/kg) in streptozotocin (STZ)-induced diabetic rats for 21 days, isolate and evaluate the bioactive constituents responsible for the antidiabetic activity. In silico pharmacokinetic and toxicity properties of the most active compound was also determined. MAB-EtOAc significantly (p < 0.001) reduced the blood glucose levels while normalizing considerably the altered serum lipid parameters of the diabetic rats which was comparable to glibenclamide (5 mg/kg). Chemical investigation of MAB-EtOAc led to the isolation of seven known compounds including three flavanols which are reported for the first time in the plant: epicatechin (1), epigallocatechin (2), dulcisflavan (3), euscaphic acid (4), tormentic acid (5), sitosterol-3-O-β-d-glucopyranoside (6) and arjunolic acid (7). The compounds markedly inhibited the action of α-amylase and, except for 4 and 6, which stimulated considerably glucose uptake in C2C12 cells. Compounds 2, 3, 5, 6 and 7 which were further evaluated in STZ-induced diabetic rats demonstrated hypoglycaemic and anti-hyperlipidaemic activities which, however, were not comparable with MAB-EtOAc. Compound 3, the most active compound was predicted to be non-toxic, non-mutagenic, has reasonable oral bioavailability and a decent substrate for further drug development. The findings of this study show that the isolated compounds may contribute to the antidiabetic activity of M. arboreus and could serve as marker compounds for the quality control of herbal medicines that would be made from the plant.

Per Artursson's Major Contributions to the Caco-2 Cell Literature in Pharmaceutical Sciences

This edition of the Journal of Pharmaceutical Sciences is dedicated to the wonderful career of Per Artursson from the Uppsala University. My Commentary focusses on Per's major contributions to the Caco-2 cell literature over the past 30 years. Two especially influential papers have been cited more than 1000 times out of a total citation count of almost 30,000 and a h-index of 93 (Google Scholar), making Per one of the most cited and influential Pharmaceutical scientists of his generation. The Caco-2 field to which Per contributed so many advances has informed the community on key areas including predictive drug fluxes across the intestine, metabolism by intestinal epithelia, the role of transporters during flux, enantiomer-selective flux, excipient interaction with tight junctions, and nanoparticle uptake by enterocytes. In this pioneering work, Per has been careful to emphasise that Caco-2 monolayers have limitations and are a model of the human small intestine where observations must be backed up with in vitro tissue and in vivo work. Throughout, he has paid great attention to detail in methodology, as reflected by co-authorship of two Nature Protocols on Caco-2 assays. The article briefly assesses some of the most important milestones in Per's published Caco-2 research.

Permeation of Biopolymers Across the Cell Membrane: A Computational Comparative Study on Polylactic Acid and Polyhydroxyalkanoate

Polymeric nanoparticles, which by virtue of their size (1-1000 nm) are able to penetrate even into cells, are attracting increasing interest in the emerging field of nanomedicine, as devices for, e.g., drugs or vaccines delivery. Because of the involved dimensional scale in the nanoparticle/cell membrane interactions, modeling approaches at molecular level are the natural choice in order to understand the impact of nanoparticle formulation on cellular uptake mechanisms. In this work, the passive permeation across cell membrane of oligomers made of two employed polymers in the biomedical field [poly-D,L-lactic acid (PDLA) and poly(3-hydroxydecanoate) (P3HD)] is investigated at fundamental atomic scale through molecular dynamics simulations. The free energy profile related to membrane crossing is computed adopting umbrella sampling. Passive permeation is also investigated using a coarse-grained model with MARTINI force field, adopting well-tempered metadynamics. Simulation results showed that P3HD permeation is favored with respect to PDLA by virtue of its higher hydrophobicity. The free energy profiles obtained at full atomistic and coarse-grained scale are in good agreement each for P3HD, while only a qualitative agreement was obtained for PDLA. Results suggest that a reparameterization of non-bonded interactions of the adopted MARTINI beads for the oligomer is needed in order to obtain a better agreement with more accurate simulations at atomic scale.

A bespoke microfluidic pharmacokinetic compartment model for drug absorption using artificial cell membranes

Early prediction of the rate and extent of intestinal absorption is vital for the efficient development of orally administered drugs. Here we show a new type of pharmacokinetic compartment model that shows a threefold improvement in the prediction of molecular absorption in the jejunum than the current state-of-the-art in vitro technique, parallel artificial membrane permeability assays (PAMPA). Our three-stage pharmacokinetic compartment model uses microfluidic droplets and bespoke, biomimetic artificial cells to model the path of a drug proxy from the intestinal space into the blood via an enterocyte. Each droplet models the buffer and salt composition of each pharmacokinetic compartment. The artificial cell membranes are made from the major components of human intestinal cell membranes (l-α-phosphatidylcholine, PC and l-α-phosphatidylethanolamine, PE) and sizes are comparable to human cells (∼0.5 nL). We demonstrate the use of the microfluidic platform to quantify common pharmacokinetic parameters such as half-life, flux and the apparent permeability coefficient (P_app). Our determined P_app more closely resembles that of actual intestinal tissue than PAMPA, which overestimates it by a factor of 20.

Facts, Patterns, and Principles in Drug Discovery: Appraising the Rule of 5 with Measured Physicochemical Data

The rule of 5 was designed to estimate the likelihood of poor absorption or permeation, noting the impact of poor solubility. This Perspective explores the impact of various physicochemical descriptors and contemporary lipophilicity measurements on permeability and solubility, showing that the distribution coefficient log D_7.4 (rather than log P) is the most impactful parameter. Molecular weight, almost invariably the defining characteristic of "beyond the rule of 5" compounds, has little impact on solubility when log D_7.4 measurements and aromaticity are considered. Predicting permeation is more complex, given passive and carrier transport mechanisms; however, notable patterns of behavior are apparent, giving insight even "beyond the rule of 5". Recommended best practices should involve using the facts (measurements) and the patterns they reveal to establish informative principles rather than fastidious rules.

Hypericin photodynamic activity in DPPC liposomes - part II: stability and application in melanoma B16-F10 cancer cells

Hypericin (Hyp) is considered a promising photosensitizer for Photodynamic Therapy (PDT), due to its high hydrophobicity, affinity for cell membranes, low toxicity and high photooxidation activity. In this study, Hyp photophysical properties and photodynamic activity against melanoma B16-F10 cells were optimized using DPPC liposomes (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) as a drug delivery system. This nanoparticle is used as a cell membrane biomimetic model and solubilizes hydrophobic drugs. Hyp oxygen singlet lifetime (τ) in DPPC was approximately two-fold larger than that in P-123 micelles (Pluronic™ surfactants), reflecting a more hydrophobic environment provided by the DPPC liposome. On the other hand, singlet oxygen quantum yield values (Φ_Δ¹O₂) in DPPC and P-123 were similar; Hyp molecules were preserved as monomers. The Hyp/DPPC liposome aqueous dispersion was stable during fluorescence emission and the liposome diameter remained stable for at least five days at 30 °C. However, the liposomes collapsed after the lyophilization/rehydration process, which was resolved by adding the lyoprotectant Trehalose to the liposome dispersion before lyophilization. Cell viability of the Hyp/DPPC formulation was assessed against healthy HaCat cells and high-metastatic melanoma B16-F10 cells. Hyp incorporated into the DPPC carrier presented a higher selectivity index than the Hyp sample previously solubilized in ethanol under the illumination effect. Moreover, the IC₅₀ was lower for Hyp in DPPC than for Hyp pre-solubilized in ethanol. These results indicate the potential of the formulation of Hyp/DPPC for future biomedical applications in PDT treatment.

Polyoxometalates function as indirect activators of a G protein-coupled receptor

A series of multivalent polyoxovanadates were found to activate signaling of a G protein coupled receptor, the luteinizing hormone receptor.

Implication of human drug transporters to toxicokinetics and toxicity of pesticides

Human membrane drug transporters are recognized as major actors of pharmacokinetics. Pesticides also interact with human drug transporters, which may have consequences for pesticide toxicokinetics and toxicity. The present review summarizes key findings about this topic. In vitro assays have demonstrated that some pesticides, belonging to various chemical classes, modulate drug transporter activity, regulate transporter expression and/or are substrates, thus bringing the proof of concept for pesticide-transporter relationships. The expected low human concentration of pesticides in response to environmental exposure constitutes a key-parameter to be kept in mind for judging the in vivo relevance of such pesticide-transporter interactions and their consequences for human health. Existing data about interactions of pesticides with drug transporters remain, however, rather sparse; more extensive and systematic characterization of pesticide-transporter relationships, through the use of high throughput in vitro assays and/or in silico methods, is, therefore, required. In addition, consideration of transporter polymorphisms, pesticide mixture effects and physiological and pathological factors governing drug transporter expression may help to better define the in vivo relevance of pesticide-transporter interactions in terms of toxicokinetics and toxicity for humans. © 2019 Society of Chemical Industry.

Role of L-Type Amino Acid Transporter 1 (LAT1) for the Selective Cytotoxicity of Sesamol in Human Melanoma Cells

Sesamol is effective against melanoma cells with less damage to normal cells. The underlying selective cytotoxicity of sesamol in melanoma vs. non-cancerous cells is undefined. Melanoma cells differ from normal cells by over-expression of the L-type amino acid transporter 1 (LAT1). We sought to clarify the transport mechanism on selective cytotoxicity of sesamol in melanoma cells. A human melanoma cell line (SK-MEL-2) and African monkey epithelial cell line (Vero) were used to study the cellular uptake and cytotoxicity of sesamol. The intracellular concentration of sesamol was quantified by UV-HPLC. The cytotoxicity was determined by neutral red uptake assay. Sesamol showed a higher distribution volume and uptake clearance in SK-MEL-2 than Vero cells. Sesamol was distributed by both carrier-mediated and passive transport by having greater carrier-mediated transport into SK-MEL-2 cells than Vero cells. Higher mRNA expression and function of LAT1 over LAT2 were evident in SK-MEL-2 cells compared to Vero cells. Sesamol uptake and sesamol cytotoxicity were inhibited by the LAT1 inhibitor, suggesting LAT1 had a role in sesamol transport and its bioactivity in melanoma. The LAT1-mediated transport of sesamol is indicative of how it engages cytotoxicity in melanoma cells with promising therapeutic benefits.

5-Fluorouracil-Complete Insight into Its Neutral and Ionised Forms

5-Fluorouracil (5FU), a common anti-cancer drug, occurs in four tautomeric forms and possesses two potential sites of both protonation and deprotonation. Tautomeric and resonance structures of the ionized forms of 5FU create the systems of connected equilibriums. Since there are contradictory reports on the ionized forms of 5FU in the literature, complex theoretical studies on neutral, protonated and deprotonated forms of 5FU, based on the broad spectrum of DFT methods, are presented. These indicate that the O4 oxygen is more willingly protonated than the O2 oxygen and the N1 nitrogen is more willingly deprotonated than the N3 nitrogen in a gas phase. Such preferences are due to advantageous charge delocalization of the respective ions, which is demonstrated by the NBO and ESP analyses. In an aqueous phase, stability differences between respective protonated and deprotonated forms of 5FU are significantly diminished due to the competition between the mesomeric effect and solvation. The calculated pK_a values of the protonated, neutral and singly deprotonated 5FU indicate that 5FU does not exist in the protonated and double-deprotonated forms in the pH range of 0–14. The neutral form dominates below pH 8 and the N1 deprotonated form dominates above pH 8.

Validation of Cell-Based Assay for Quantification of Sesamol Uptake and Its Application for Measuring Target Exposure

The intracellular drug concentration is needed for determination of target exposure at the site of action regarding its pharmacological action and adverse effects. Sesamol is an antiproliferative molecule from Sesamum indicum with promising health benefits. We present a method for measuring the intracellular sesamol content using reverse-phase HPLC with a UV diode array in melanoma cells. Sesamol was completely resolved by isocratic elution (4.152 ± 0.008 min) with methanol/water (70%, v/v) through a 30 °C, 5-µm C-18 column and detection at 297 nm. The present assay offers high sensitivity, fast elution, and an accurate and linear nominal concentration range of 10–1000 ng/mL (R² = 0.9972). The % accuracy of the sesamol quality control sample was −3.36% to 1.50% (bias) with a 0.84% to 5.28% relative standard deviation (RSD), representing high repeatability and high reproducibility. The % recovery was 94.80% to 99.29%, which determined that there was no loss of sesamol content during the sample preparation. The validated method was applied to monitor intracellular sesamol concentration after treatment from 5 min to 24 h. The remaining intracellular sesamol content was correlated with its antiproliferative effect (R² = 0.9483). In conclusion, this assay demonstrated low manipulation, quick elution, and high sensitivity, precision, accuracy, and recovery, and it was successfully applied to the quantification of sesamol in target cells.

Effect of dehydrocholic acid conjugated with a hydrocarbon on a lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine

The effects of bile acids, dehydrocholic acid (DHA) and DHA conjugated with a hydrocarbon (6-aminohexanoate; 6A-DHA) were evaluated using a lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). DOPC formed a homogenous thin membrane in presence or absence of the DHA, while 20 mol% 6A-DHA induced phase separation on the DOPC thin membrane. It was observed formation of a stomatocyte-like liposomes when these membranes were suspended in a basic solvent. Generally, liposome formation can be prevented by some bile acids. It was found that DHA and 6A-DHA did not disrupt liposome formation, while DHA and 6A-DHA perturbed the liposomal membrane, resulting in increased local-fluidity due to the bent structure of DHA and 6A-DHA. DHA and 6A-DHA showed completely different effects on the hydrophobicity of the boundary surface of DOPC liposome membranes. The steroidal backbone of DHA was found to prevent the insertion of water molecules into the liposomal membrane, whereas 6A-DHA did not show the same behavior which was attributed to its conjugated hydrocarbon.

Molecular Adsorption and Transport at Liposome Surfaces Studied by Molecular Dynamics Simulations and Second Harmonic Generation Spectroscopy

A fundamental understanding of the factors that determine the interactions with and transport of small molecules through phospholipid membranes is crucial in developing liposome-based drug delivery systems. Here we combine time-dependent second harmonic generation (SHG) measurements with molecular dynamics simulations to elucidate the events associated with adsorption and transport of the small molecular cation, malachite green isothiocyanate (MGITC), in colloidal liposomes of different compositions. The molecular transport of MGITC through the liposome bilayer is found to be more rapid in 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPG and DOPS, respectively) liposomes, while the molecular transport is slower in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes. Interestingly, MGITC is observed to neither adsorb nor transport in trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (QPADOPE) liposomes due to shielding by the quinone group. The modified Langmuir adsorption isotherm model is used to determine the free energy of adsorption for MGITC, which is found to be less negative in DOPC than in DOPG and DOPS, caused by lower electrostatic interactions between the positively charged dye and the zwitterionic DOPC liposome surface. The results are compared to our previous investigations, which showed that malachite green (MG) adsorbs and transports in DOPG and DOPS liposomes but not in DOPC and QPADOPE liposomes. Molecular dynamics simulations are used to investigate the adsorption and transport properties of MG and MGITC in DOPC and DOPG liposomes using umbrella sampling to determine the free energy profiles and interfacial molecular orientations. Together, these time-resolved SHG studies and corresponding molecular dynamics simulations characterize the complicated chemical interactions at different lipid membranes to provide key molecular-level insights for potential drug delivery applications. The results also point toward understanding the role of chemical functional groups, in this case isothiocyanate, in controlling molecular adsorption at and transport through lipid bilayers.

Dual lysosomal-mitochondrial targeting by antihistamines to eradicate leukaemic cells

Background

Despite great efforts to identify druggable molecular targets for AML, there remains an unmet need for more effective therapies.

Methods

An in silico screening was performed using Connectivity Maps to identify FDA-approved drugs that may revert an early leukaemic transformation gene signature. Hit compounds were validated in AML cell lines. Cytotoxic effects were assessed both in primary AML patient samples and healthy donor blood cells. Xenotransplantation assays were undertaken to determine the effect on engraftment of hit compounds. The mechanism of action responsible for the antileukaemic effect was studied focussing on lysosomes and mitochondria.

Findings

We identified a group of antihistamines (termed ANHAs) with distinct physicochemical properties associated with their cationic-amphiphilic nature, that selectively killed leukaemic cells. ANHAs behaved as antileukaemic agents against primary AML samples ex vivo, sparing healthy cells. Moreover, ANHAs severely impaired the in vivo leukaemia regeneration capacity. ANHAs' cytotoxicity relied on simultaneous mitochondrial and lysosomal disruption and induction of autophagy and apoptosis. The pharmacological effect was exerted based on their physicochemical properties that permitted the passive targeting of both organelles, without the involvement of active molecular recognition.

Interpretation

Dual targeting of lysosomes and mitochondria constitutes a new promising therapeutic approach for leukaemia treatment, supporting the further clinical development.

Fund

This work was funded by the Fundación Mutua Madrileña (RMR), CaixaImpulse (RMR), the Spanish Ministry of Economy (RMR), the Josep Carreras International Leukaemia Foundation (RMR), l'Obra Social “La Caixa” (RMR), and Generalitat de Catalunya (IJC).

Using Physicochemical Measurements to Influence Better Compound Design

During the past decade, the physicochemical quality of molecules under investigation at all stages of the drug discovery process has come under particular scrutiny. The issues associated with excessive lipophilicity and poor solubility in particular are many and varied, ranging from poor outcomes in screening campaigns to promiscuity, limited and/or poorly predictable pharmacokinetic exposure, and, ultimately, greater chances of clinical failure. In this review, contemporary methods to secure key measurements are described along with their relevance to understanding the behavior of molecules in environments pertinent to pharmacological activity. Together, the various measurements contribute to predictive models of both the physicochemical properties themselves and the outcomes they influence.

Intestinal Permeability and Drug Absorption: Predictive Experimental, Computational and In Vivo Approaches

The main objective of this review is to discuss recent advancements in the overall investigation and in vivo prediction of drug absorption. The intestinal permeability of an orally administered drug (given the value P_eff) has been widely used to determine the rate and extent of the drug’s intestinal absorption (F_abs) in humans. Preclinical gastrointestinal (GI) absorption models are currently in demand for the pharmaceutical development of novel dosage forms and new drug products. However, there is a strong need to improve our understanding of the interplay between pharmaceutical, biopharmaceutical, biochemical, and physiological factors when predicting F_abs and bioavailability. Currently, our knowledge of GI secretion, GI motility, and regional intestinal permeability, in both healthy subjects and patients with GI diseases, is limited by the relative inaccessibility of some intestinal segments of the human GI tract. In particular, our understanding of the complex and highly dynamic physiology of the region from the mid-jejunum to the sigmoid colon could be significantly improved. One approach to the assessment of intestinal permeability is to use animal models that allow these intestinal regions to be investigated in detail and then to compare the results with those from simple human permeability models such as cell cultures. Investigation of intestinal drug permeation processes is a crucial biopharmaceutical step in the development of oral pharmaceutical products. The determination of the intestinal P_eff for a specific drug is dependent on the technique, model, and conditions applied, and is influenced by multiple interactions between the drug molecule and the biological membranes.

Effects of absorption-modifying excipients on jejunal drug absorption in simulated fasted and fed luminal conditions

Oral administration of drug products is the preferred administration route. In recent decades there has been an increase in drug candidates with low solubility and/or low permeability. To increase the possibility of oral administration for the poorly permeating drugs, the use of absorption modifying excipients (AMEs) has been proposed. These types of AMEs may also affect the regulatory assessment of a novel drug delivery system if they affect the absorption of a drug from any of the four BCS classes. The effects of AMEs have previously been investigated in various animal models, including the single-pass intestinal perfusion (SPIP) in rats. To further improve the biorelevance and the in vivo predictiveness of the SPIP model, four compounds (atenolol, enalaprilat, ketoprofen, metoprolol) were perfused in fasted or fed state simulated intestinal fluid (FaSSIF or FeSSIF) together with the AMEs N-acetyl-cysteine, caprate, or sodium dodecyl sulfate. For the highly soluble and poorly permeating compounds enalaprilat and atenolol (BCS class III), the flux was increased the most by the addition of SDS in both FaSSIF and FeSSIF. For ketoprofen (BCS class II), the flux decreased in the presence of all AMEs in at least one of the perfusion media. The flux of metoprolol (BCS class I) was not affected by any of the excipients in none of simulated prandial states. The changes in magnitude in the absorption of the compounds were in general smaller in FeSSIF than in FaSSIF. This may be explained by a reduced free concentration AMEs in FeSSIF. Further, the results in FeSSIF were similar to those from intrajejunal bolus administration in rat in a previous study. This suggests that the biorelevance of the SPIP method may be increased when investigating the effects of AMEs, by the addition of intraluminal constituents representative to fasted and/or fed state to the inlet perfusate.

Benzotriazine Di-Oxide Prodrugs for Exploiting Hypoxia and Low Extracellular pH in Tumors

Extracellular acidification is an important feature of tumor microenvironments but has yet to be successfully exploited in cancer therapy. The reversal of the pH gradient across the plasma membrane in cells that regulate intracellular pH (pHi) has potential to drive the selective uptake of weak acids at low extracellular pH (pHe). Here, we investigate the dual targeting of low pHe and hypoxia, another key feature of tumor microenvironments. We prepared eight bioreductive prodrugs based on the benzotriazine di-oxide (BTO) nucleus by appending alkanoic or aminoalkanoic acid sidechains. The BTO acids showed modest selectivity for both low pHe (pH 6.5 versus 7.4, ratios 2 to 5-fold) and anoxia (ratios 2 to 8-fold) in SiHa and FaDu cell cultures. Related neutral BTOs were not selective for acidosis, but had greater cytotoxic potency and hypoxic selectivity than the BTO acids. Investigation of the uptake and metabolism of representative BTO acids confirmed enhanced uptake at low pHe, but lower intracellular concentrations than expected for passive diffusion. Further, the modulation of intracellular reductase activity and competition by the cell-excluded electron acceptor WST-1 suggests that the majority of metabolic reductions of BTO acids occur at the cell surface, compromising the engagement of the resulting free radicals with intracellular targets. Thus, the present study provides support for designing bioreductive prodrugs that exploit pH-dependent partitioning, suggesting, however, that that the approach should be applied to prodrugs with obligate intracellular activation.

Physics-Based Method for Modeling Passive Membrane Permeability and Translocation Pathways of Bioactive Molecules

Assessment of permeability is a critical step in the drug development process for selection of drug candidates with favorable ADME properties. We have developed a novel physics-based method for fast computational modeling of passive permeation of diverse classes of molecules across lipid membranes. The method is based on heterogeneous solubility-diffusion theory and operates with all-atom 3D structures of solutes and the anisotropic solvent model of the lipid bilayer characterized by transbilayer profiles of dielectric and hydrogen bonding capacity parameters. The optimal translocation pathway of a solute is determined by moving an ensemble of representative conformations of the molecule through the dioleoyl-phosphatidylcholine (DOPC) bilayer and optimizing their rotational orientations in every point of the transmembrane trajectory. The method calculates (1) the membrane-bound state of the solute molecule; (2) free energy profile of the solute along the permeation pathway; and (3) the permeability coefficient obtained by integration over the transbilayer energy profile and assuming a constant size-dependent diffusivity along the membrane normal. The accuracy of the predictions was evaluated against experimental permeability coefficients measured in pure lipid membranes (for 78 compounds, R² was 0.88 and rmse was 1.15 log units), PAMPA-DS (for 280 compounds, R² was 0.75 and rmse was 1.59 log units), BBB (for 182 compounds, R² was 0.69 and rmse was 0.87 log units), and Caco-2/MDCK assays (for 165 compounds, R² was 0.52 and rmse was 0.89 log units).

Intrinsic Membrane Permeability to Small Molecules

Spontaneous solute and solvent permeation through membranes is of vital importance to human life, be it gas exchange in red blood cells, metabolite excretion, drug/toxin uptake, or water homeostasis. Knowledge of the underlying molecular mechanisms is the sine qua non of every functional assignment to membrane transporters. The basis of our current solubility diffusion model was laid by Meyer and Overton. It correlates the solubility of a substance in an organic phase with its membrane permeability. Since then, a wide range of studies challenging this rule have appeared. Commonly, the discrepancies have their origin in ill-used measurement approaches, as we demonstrate on the example of membrane CO₂ transport. On the basis of the insight that scanning electrochemical microscopy offered into solute concentration distributions in immediate membrane vicinity of planar membranes, we analyzed the interplay between chemical reactions and diffusion for solvent transport, weak acid permeation, and enzymatic reactions adjacent to membranes. We conclude that buffer reactions must also be considered in spectroscopic investigations of weak acid transport in vesicular suspensions. The evaluation of energetic contributions to membrane translocation of charged species demonstrates the compatibility of the resulting membrane current with the solubility diffusion model. A local partition coefficient that depends on membrane penetration depth governs spontaneous membrane translocation of both charged and uncharged molecules. It is determined not only by the solubility in an organic phase but also by other factors like cholesterol concentration and intrinsic electric membrane potentials.

Challenges in working towards an internal threshold of toxicological concern (iTTC) for use in the safety assessment of cosmetics: Discussions from the Cosmetics Europe iTTC Working Group workshop

The Threshold of Toxicological Concern (TTC) is an important risk assessment tool which establishes acceptable low-level exposure values to be applied to chemicals with limited toxicological data. One of the logical next steps in the continued evolution of TTC is to develop this concept further so that it is representative of internal exposures (TTC based on plasma concentration). An internal TTC (iTTC) would provide threshold values that could be utilized in exposure-based safety assessments. As part of a Cosmetics Europe (CosEu) research program, CosEu has initiated a project that is working towards the development of iTTCs that can be used for the human safety assessment. Knowing that the development of an iTTC is an ambitious and broad-spanning topic, CosEu organized a Working Group comprised a balance of multiple stakeholders (cosmetics and chemical industries, the EPA and JRC and academia) with relevant experience and expertise and workshop to critically evaluate the requirements to establish an iTTC. Outcomes from the workshop included an evaluation on the current state of the science for iTTC, the overall iTTC strategy, selection of chemical databases, capture and curation of chemical information, ADME and repeat dose data, expected challenges, as well as next steps and ongoing work.