Absorption-excipient-pH classification gradient maps: sparingly soluble drugs and the pH partition hypothesis

PAMPA model of gliclazide permeability: The impact of probiotic bacteria and bile acids

Gut microbiota and bile acids possess the ability to modify absorption and pharmacokinetic profile of numerous drugs. Since the variability of gliclazide response in patients cannot be explained only by genetic factors, the influence of gut microbiota and bile acids should be considered. The aim of this study was to determine the effects of probiotic bacteria and bile acids on the gliclazide permeability. The permeability of gliclazide with and without probiotic bacteria and bile acids (cholic acid, CA and deoxycholic acid, DCA) was tested using in vitro PAMPA model, at three different pH values (5.8, 6.5 and 7.4). Concentrations of gliclazide were determined by HPLC analysis. The interactions of gliclazide and bile acids were also investigated by molecular mechanics calculations (MM2). Probiotic bacteria significantly increased the permeability of gliclazide across the PAMPA membrane at all observed pH values while the total amount of gliclazide during incubation with bacteria was significantly reduced at pH 7.4, which could be a consequence of partial metabolism of the drug by enzymes of probiotic bacteria. Bile acids decreased the permeability of gliclazide through PAMPA membrane, with more pronounced effects of DCA, by forming more stable complexes with gliclazide. Given that probiotic bacteria and bile acids are naturally present in the gut and that each individual has a specific bacterial fingerprint, future research should extend the explanation of their effect on the gliclazide bioavailability and therapy individualization in in vivo conditions.

Understanding Cell Penetration of Cyclic Peptides

Approximately 75% of all disease-relevant human proteins, including those involved in intracellular protein-protein interactions (PPIs), are undruggable with the current drug modalities (i.e., small molecules and biologics). Macrocyclic peptides provide a potential solution to these undruggable targets because their larger sizes (relative to conventional small molecules) endow them the capability of binding to flat PPI interfaces with antibody-like affinity and specificity. Powerful combinatorial library technologies have been developed to routinely identify cyclic peptides as potent, specific inhibitors against proteins including PPI targets. However, with the exception of a very small set of sequences, the vast majority of cyclic peptides are impermeable to the cell membrane, preventing their application against intracellular targets. This Review examines common structural features that render most cyclic peptides membrane impermeable, as well as the unique features that allow the minority of sequences to enter the cell interior by passive diffusion, endocytosis/endosomal escape, or other mechanisms. We also present the current state of knowledge about the molecular mechanisms of cell penetration, the various strategies for designing cell-permeable, biologically active cyclic peptides against intracellular targets, and the assay methods available to quantify their cell-permeability.

Gastrointestinal Malabsorption of Thyroxine

Levothyroxine, a largely prescribed drug with a narrow therapeutic index, is often a lifelong treatment. The therapeutic efficacy of T4 may be marred by behavioral, pharmacologic, and pathologic issues acting as interfering factors. Despite a continuous search for an optimal T4 treatment, a significant number of patients fail to show a complete chemical and/or clinical response to this reference dose of T4. Gastrointestinal malabsorption of oral T4 represents an emerging cause of refractory hypothyroidism and may be more frequent than previously reputed. In this review, we examine the pharmacologic features of T4 preparations and their linkage with the intestinal absorption of the hormone. We have stressed the major biochemical and pharmacologic characteristics of T4 and its interaction with the putative transporter at the intestinal level. We have examined the interfering role of nutrients, foods, and drugs on T4 absorption at the gastric and intestinal levels. The impact of gastrointestinal disorders on T4 treatment efficacy has been also analyzed, in keeping with the site of action and the interfering mechanisms. Based on the evidence obtained from the literature, we also propose a schematic diagnostic workup for the most frequent and often hidden gastrointestinal diseases impairing T4 absorption.

Ranking Itraconazole Formulations Based on the Flux through Artificial Lipophilic Membrane

PURPOSE

The goal of the study was to evaluate a miniaturized dissolution-permeation apparatus (μFLUX™ apparatus) for its ability to benchmark several itraconazole (ITZ) formulations for which in vivo PK data was available in the literature.

METHOD

Untreated and micronized powders of ITZ and various enabling formulations of ITZ (commercial Sporanox® solid dispersion, a Soluplus®-based solid dispersion and a nanosuspension) were introduced to the donor compartment of μFLUX™ apparatus. Donor and acceptor chambers were divided from each other by a lipophilic membrane. In addition to the flux evaluations, changes in solid state as a function of time were investigated to gain further insight into the flux changes observed over time for the solid dispersion formulations.

RESULTS

Initial flux values from Sporanox®, the nanosuspension and the micronized ITZ showed ratios of 52/4/1 with a decreasing flux from nanosuspension and both solid dispersions after 2.5-3 h. Although the initial flux from the Soluplus® formulation was 2.2 times lower than the one observed for Sporanox®, the decrease in flux observed was milder and became ~ 2 times higher than Sporanox® after approximately 2.5 h. The total amounts of ITZ in the receiver compartment after 240 min showed the same rank order as the rodent AUCs of these formulations reported in literature.

CONCLUSIONS

It was demonstrated that in vitro flux measurements using lipophilic artificial membranes could correctly reproduce the rank order of PK results for ITZ formulations. The drop in flux over time for solid dispersions could be backed by experimental indications of crystallization.

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.

Analytical technologies for real-time drug dissolution and precipitation testing on a small scale

Objectives

This review focuses on real-time analytics of drug dissolution and precipitation testing on a comparatively small scale.

Key findings

Miniaturisation of test equipment is an important trend in pharmaceutics, and several small-scale experiments have been reported for drug dissolution and precipitation testing. Such tests typically employ analytics in real-time. Fibre optic ultraviolet (UV) analytics has become a well-established method in this field. Novel imaging techniques are emerging that use visible or UV light; also promising is Fourier transform infrared imaging based on attenuated total reflection. More information than just a rate constant is obtained from these methods. The early phase of a dissolution process can be assessed and drug precipitation may eventually be observed. Some real-time techniques are particularly well suited to studying drug precipitation during formulation dispersion; for example, turbidity, focused beam reflectance measurement and Raman spectroscopy.

Summary

Small-scale dissolution tests equipped with real-time analytics have become important to screen drug candidates as well as to study prototype formulations in early development. Future approaches are likely to combine different analytical techniques including imaging. Miniaturisation started with mini-vessels or small vials and future assays of dissolution research will probably more often reach the level of parallel well plates and microfluidic channels.

Sample reduction strategies in discovery bioanalysis

It is a constant challenge to provide timely bioanalytical support for the evaluation of drug-like properties and PK/PD profiles for the ever-increasing numbers of new chemical entities in a cost-effective manner. While technological advancement in various aspects of LC–MS/MS analysis has significantly improved bioanalytical efficiency, a number of simple sample reduction strategies can be employed to reduce the number of samples requiring analysis, and as a result increase the bioanalytical productivity without deploying additional instruments. In this review, advantages and precautions of common sample reduction strategies, such as sample pooling and cassette dosing, are discussed. In addition, other approaches such as reducing calibration standards and eliminating over-the-curve sample reanalysis will also be discussed. Taken together, these approaches can significantly increase the capacity and throughput of discovery bioanalysis without adding instruments, and are viable means to enhance the overall productivity of the bioanalytical laboratory.

pH-gradient PAMPA-based in vitro model assay for drug-induced phospholipidosis in early stage of drug discovery

In the present study we validated a widely used, high-throughput in vitro permeability model (PAMPA) to be used at the early stage of drug discovery for the phospholipidosis (PLD) prediction of drug-like compounds. Regarding the mechanism of action of PLD, our pH-gradient PAMPA system is the first noncell based model to mimic one-way transport of cationic amphiphilic drugs (CADs) from cytosol to the lysosome. Moreover, due to the fact that PLD can mainly occur in lung, liver, brain, kidney and heart, we have used similar commercially available original tissue-derived lipid fractions (heart, liver, brain), and in the case mimicking membrane of kidney and lung tissue we prepared tissue-mimetic artificial lipid mixtures in house. Metabolism of a drug can change the degree of PLD depending on the physicochemical properties of metabolites and the rate of metabolism. Our data from 57 drugs and 4 metabolites of earlier and 2 metabolites of newly recognized outliers (phenacetin and bupropion) using our pH-gradient PAMPA system show a good correlation with in vivo PLD data. Moreover, predictive ability of our best system, the lung specific pH-gradient PAMPA model was significantly better than widely used in silico models and it was also slightly better than that of the known noncell based models on our selection of compounds. Our pH-gradient PAMPA systems therefore offer mechanistically alternative, accurate and cost-effective screening tools for the early prediction of PLD potential of drug-like compounds.

Synthesis and evaluation of 7-substituted 4-aminoquinoline analogues for antimalarial activity

We previously reported that substituted 4-aminoquinolines with a phenyl ether substituent at the 7-position of the quinoline ring and the capability of intramolecular hydrogen bonding between the protonated amine on the side chain and a hydrogen bond acceptor on the amine's alkyl substituents exhibited potent antimalarial activity against the multidrug resistant strain P. falciparum W2. We employed a parallel synthetic method to generate diaryl ether, biaryl, and alkylaryl 4-aminoquinoline analogues in the background of a limited number of side chain variations that had previously afforded potent 4-aminoquinolines. All subsets were evaluated for their antimalarial activity against the chloroquine-sensitive strain 3D7 and the chloroquine-resistant K1 strain as well as for cytotoxicity against mammalian cell lines. While all three arrays showed good antimalarial activity, only the biaryl-containing subset showed consistently good potency against the drug-resistant K1 strain and good selectivity with regard to mammalian cytotoxicity. Overall, our data indicate that the biaryl-containing series contains promising candidates for further study.

Supramolecular encapsulation of benzimidazole-derived drugs by cucurbit[7]uril

UV–vis and NMR spectroscopic techniques were employed to demonstrate the ability of the synthetic macrocyclic host cucurbit[7]uril (CB7) to solubilize and stabilize widely used fungicides and anthelmintic drugs of the benzimidazole family in water, namely, albendazole (ABZ), carbendazim (CBZ), thiabendazole (TBZ), fuberidazole (FBZ), and the parent benzimidazole (BZ). CB7 binds the protonated forms of these guests very strongly (e.g., K = 2.6 × 107 L/mol for ABZ) but their neutral forms significantly more weakly (e.g., K = 6.5 × 104 L/mol for ABZ), which reflects a complexation-induced increase of their pKa values by 2.6 units for ABZ, 2.5 units for CBZ, 4.0 units for TBZ, 3.8 units for FBZ, and 3.5 units for BZ. The absolute drug solubilities increased upon complexation from 0.003 to 0.300 mmol/L for ABZ, from 0.160 to 1.12 mmol/L for CBZ, from 0.110 to 1.11 mmol/L for TBZ, and from 0.25 to 0.75 mmol/L for FBZ (for BZ, the solubility enhancement was found to be insignificant). Complexation by CB7 further improves the photostability of the drugs and alters their photophysical properties.

A new in situ brain perfusion flow correction method for lipophilic drugs based on the pH-dependent Crone-Renkin equation

PURPOSE

To determine the flow-corrected luminal permeability, P(c), of lipophilic drugs measured by the in situ brain perfusion method under circumstances where the traditional Crone-Renkin equation (CRE) method, using diazepam as a flow marker, often fails.

METHODS

The pH-dependent rate of brain penetration of five lipophilic drugs (amitriptyline, atomoxetine, imipramine, indomethacin, maprotiline, sertraline), as well as of atenolol and antipyrine, were measured in Sprague-Dawley rats. A new pH-dependent CRE was derived and applied to remove the hydrodynamic component of effective permeability, P(e), to produce P(c) values.

RESULTS

It was shown by the analysis of the in situ data in the pH 6.5-8.5 interval for the lipophilic bases that the average vascular flow F(pf) = 0.036 mL∙g(-1)∙s(-1), centered in a "flow-limit window" (FLW) bounded by P (e) (min)  = 170 and P (e) (max)  = 776 (10(-6) cm∙s(-1) units). It was shown that the traditional CRE is expected not to work for half of the molecules in the FLW and is expected to underestimate (up to 64-fold) the other half of the molecules.

CONCLUSION

The new pH-CRE flow correction method applied to lipophilic ionizable drugs, based on the pH partition hypothesis, can overcome the limitations of the traditional CRE.

A modified physiological BCS for prediction of intestinal absorption in drug discovery

In this study, the influence of physiologically relevant media on the compound position in a biopharmaceutical classification system (BCS) which resembled the intestinal absorption was investigated. Both solubility and permeability limited compounds (n = 22) were included to analyze the importance of each of these on the final absorption. Solubility was determined in three different dissolution media, phosphate buffer pH 6.5 (PhB 6.5), fasted state simulated intestinal fluid (FaSSIF), and fed state simulated intestinal fluid (FeSSIF) at 37 °C, and permeability values were determined using the 2/4/A1 cell line. The solubility data and membrane permeability values were used for sorting the compounds into a BCS modified to reflect the fasted and fed state. Three of the seven compounds sorted as BCS II in PhB 6.5 (high permeability, low solubility) changed their position to BCS I when dissolved in FaSSIF and/or FeSSIF (high permeability, high solubility). These were low dosed (20 mg or less) lipophilic molecules displaying solvation limited solubility. In contrast, compounds having solid-state limited solubility had a minor increase in solubility when dissolved in FaSSIF and/or FeSSIF. Although further studies are needed to enable general cutoff values, our study indicates that low dosed BCS Class II compounds which have solubility normally restricted by poor solvation may behave as BCS Class I compounds in vivo. The large series of compounds investigated herein reveals the importance of investigating solubility and dissolution under physiologically relevant conditions in all stages of the drug discovery process to push suitable compounds forward, to select proper formulations, and to reduce the risk of food effects.

Coexistence of passive and carrier-mediated processes in drug transport

The permeability of biological membranes is one of the most important determinants of the pharmacokinetic processes of a drug. Although it is often accepted that many drug substances are transported across biological membranes by passive transcellular diffusion, a recent hypothesis speculated that carrier-mediated mechanisms might account for the majority of membrane drug transport processes in biological systems. Based on evidence of the physicochemical characteristics and of in vitro and in vivo findings for marketed drugs, as well as results from real-life discovery and development projects, we present the view that both passive transcellular processes and carrier-mediated processes coexist and contribute to drug transport activities across biological membranes.

How well can the Caco-2/Madin-Darby canine kidney models predict effective human jejunal permeability?

The study aimed to predict effective human jejunal permeability (P(eff)) using a biophysical model based on parametrized paracellular, aqueous boundary layer, and transcellular permeabilities, and the villus-fold surface area expansion factor (k(VF)). Published human jejunal data (119 P(eff), 53 compounds) were analyzed by a regression procedure incorporating a dual-pore size paracellular model. Transcellular permeability, scaled by k(VF), was equated to that of Caco-2 at pH 6.5. The biophysical model predicted human jejunal permeability data within the experimental uncertainty. This investigation revealed several surprising predictions: (i) many molecules permeate predominantly (but not exclusively) by the paracellular route, (ii) the aqueous boundary layer thickness in the intestinal perfusion experiments is larger than expected, (iii) the mucosal surface area in awake humans is apparently nearly entirely accessible to drug absorption, and (iv) the relative "leakiness" of the human jejunum is not so different from that observed in a number of published Caco-2 studies.

Population-based mechanistic prediction of oral drug absorption

The bioavailability of drugs from oral formulations is influenced by many physiological factors including gastrointestinal fluid composition, pH and dynamics, transit and motility, and metabolism and transport, each of which may vary with age, gender, race, food, and disease. Therefore, oral bioavailability, particularly of poorly soluble and/or poorly permeable compounds and those that are extensively metabolized, often exhibits a high degree of inter- and intra-individual variability. While several models and algorithms have been developed to predict bioavailability in an average person, efforts to accommodate intrinsic variability in the component processes are less common. An approach that incorporates such variability for human populations within a mechanistic framework is described together with examples of its application to drug and formulation development.