Methylated β-cyclodextrins: influence of degree and pattern of substitution on the thermodynamics of complexation with tauro- and glyco-conjugated bile salts

Synergetic Effect of β-Cyclodextrin and Its Simple Carbohydrate Substituents on Complexation of Folic Acid and Its Structural Analog Methotrexate

Folic acid (FA) and its structural analog, anticancer medicine methotrexate (MTX), are known to form host/guest complexes with native cyclodextrins, of which the most stable are formed with the medium-sized β-cyclodextrin. Based on our research, proving that simple sugars (D-glucose, D-galactose, and D-mannose) can form adducts with folic acid, we envisioned that combining these two types of molecular receptors (cyclodextrin and simple carbohydrates) into one may be beneficial for the complexation of FA and MTX. We designed and obtained host/guest inclusion complexes of FA and MTX with two monoderivatives of β-cyclodextrin-substituted at position 6 with monosaccharide (glucose, G-β-CD) and disaccharide (maltose, Ma-β-CD). The complexation was proved by experimental (NMR, UV-vis, IR, TG, DSC) and theoretical methods. We proved that derivatization of β-cyclodextrin with glucose and maltose has a significant impact on the complexation with FA and MTX, as the addition of one glucose subunit to the structure of the receptor significantly increases the value of association constant for both FA/G-β-CD and MTX/G-β-CD, while further extending a pendant chain (incorporation of maltose subunit) results in no additional changes.

ITC analysis of polydisperse systems: Unravelling the impact of sample heterogeneity

Binding interactions often involve heterogeneous samples displaying a distribution of binding sites that vary in affinity and binding enthalpy. Examples include biological samples like proteins and chemically produced samples like modified cyclodextrins. Experimental studies often ignore sample heterogeneity and treat the system as an interaction of two homogeneous species, i.e. a chemically well-defined ligand binding to one type of site. The present study explores, by simulations and experiments, the impact of heterogeneity in isothermal titration calorimetry (ITC) setups where one of the binding components is heterogeneous. It is found that the standard single-site model, based on the assumption of two homogeneous binding components, provides excellent fits to simulated ITC data when the binding free energy is normally distributed and all sites have similar binding enthalpies. In such cases, heterogeneity can easily go undetected but leads to underestimated binding constants. Heterogeneity in the binding enthalpy is a bigger problem and may result in enthalpograms of increased complexity that are likely to be misinterpreted as two-site binding or other complex binding models. Finally, it is shown that heterogeneity can account for previously observed experimental anomalies. All simulations are accessible in Google Colab for readers to experiment with the simulation parameters.

Computational study of inclusion complexes of V-type nerve agents (VE, VG, VM, VR and VX) with β-cyclodextrin

The effective detoxification of organophosphate (OP) nerve agents (OPNAs) is a challenging issue for scientists. The host-guest inclusion complexes of five V-type nerve agents (VE, VG, VM, VR and VX) with β-cyclodextrin (β-CD) have been studied by combining quantum mechanical (QM) calculations and molecular dynamics (MD) simulations. The frontier molecular orbital (FMO) and molecular electrostatic potential (MEP) have been analyzed to describe the reactivity parameters and electronic properties. The obtained results clearly reveal that stable complexes were formed in both vacuum and water media, and the complexation process occurred spontaneously. To understand non-covalent interactions, natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) have been used. IR and Raman spectra have been calculated to confirm the formation of complexes and also thermodynamic parameters have been investigated. It was demonstrated that in addition to van der Waals interactions, the presence of intermolecular hydrogen bonds enhances the stability of these complexes. Furthermore, MD simulations were carried out to get a better insight into the inclusion process of the above complexes. From MD simulations, all simulated systems reached full equilibration at 1000 ps and the V-agent molecules consistently remained in the β-CD cavity and only had vibrational motion inside the cavity. More importantly, MD simulations support the findings of QM calculations and indicate that hydrogen bonding can help the leaving groups of V-agents to be released and them to be hydrolyzed. All results have shown that the VR agent formed the most stable complex with β-CD molecule than that of other agents.Communicated by Ramaswamy H. Sarma.

Leveraging the use of in vitro and computational methods to support the development of enabling oral drug products: An InPharma commentary

Due to the strong tendency towards poorly soluble drugs in modern development pipelines, enabling drug formulations such as amorphous solid dispersions, cyclodextrins, co-crystals and lipid-based formulations are frequently applied to solubilize or generate supersaturation in gastrointestinal fluids, thus enhancing oral drug absorption. Although many innovative in vitro and in silico tools have been introduced in recent years to aid development of enabling formulations, significant knowledge gaps still exist with respect to how best to implement them. As a result, the development strategy for enabling formulations varies considerably within the industry and many elements of empiricism remain. The InPharma network aims to advance a mechanistic, animal-free approach to the assessment of drug developability. This commentary focuses current status and next steps that will be taken in InPharma to identify and fully utilize 'best practice' in vitro and in silico tools for use in physiologically based biopharmaceutic models.

Mass Spectrometry of Esterified Cyclodextrins

Cyclodextrins are cyclic oligosaccharides that have received special attention due to their cavity-based structural architecture that imbues them with outstanding properties, primarily related to their capacity to host various guest molecules, from low-molecular-mass compounds to polymers. Cyclodextrin derivatization has been always accompanied by the development of characterization methods, able to unfold complicated structures with increasing precision. One of the important leaps forward is represented by mass spectrometry techniques with soft ionization, mainly matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI). In this context, esterified cyclodextrins (ECDs) benefited also from the formidable input of structural knowledge, thus allowing the understanding of the structural impact of reaction parameters on the obtained products, especially for the ring-opening oligomerization of cyclic esters. The current review envisages the common mass spectrometry approaches such as direct MALDI MS or ESI MS analysis, hyphenated liquid chromatography-mass spectrometry, and tandem mass spectrometry, employed for unraveling the structural features and particular processes associated with ECDs. Thus, the accurate description of complex architectures, advances in the gas phase fragmentation processes, assessment of secondary reactions, and reaction kinetics are discussed in addition to typical molecular mass measurements.

Degree and distribution of substitution of hydroxypropyl-β-cyclodextrin in enantioselective liquid-liquid extraction and countercurrent chromatographic enantioseparation

Nine types of hydroxypropyl-β-cyclodextrin (HP-β-CD) with different degrees and distributions of substitution were synthesised, and nine racemates were selected to investigate the effect of different degrees and distributions of substitution of HP-β-CD on the enantioseparation factor. ¹H NMR and GC/MS were used to characterise the synthesised HP-β-CD. The degree and distribution of substitution had a significant influence on enantioselective liquid-liquid extraction and enantioseparation by countercurrent chromatography. For most of the tested racemates, increasing both the degree of substitution and distribution of substitution at the C-2 position for HP-β-CD would lead to an increasing enantioseparation factor; the optimal enantioseparation factor of 2-phenylbutyric acid, tropic acid, 2,3-diphenylpropionic acid, 2-(4-hydroxylphenyl) propanoic acid, and naproxen was increased to 1.77, 1.53, 1.67, 1.61, and 1.75, respectively. The enantioseparation of racemic naproxen, 2-(4-hydroxylphenyl) propanoic acid, and 2,3-diphenylpropionic acid by countercurrent chromatography was optimised using HP-β-CD with a degree of substitution of 16.5, and peak resolution was significantly improved to 1.03, 1.35, and 1.01, respectively.

Cyclodextrin-Oligocaprolactone Derivatives-Synthesis and Advanced Structural Characterization by MALDI Mass Spectrometry

Cyclodextrins have previously been proven to be active in the catalysis of cyclic ester ring-opening reactions, hypothetically in a similar way to lipase-catalyzed reactions. However, the way they act remains unclear. Here, we focus on β-cyclodextrin's involvement in the synthesis and characterization of β-cyclodextrin-oligocaprolactone (CDCL) products obtained via the organo-catalyzed ring-opening of ε-caprolactone. Previously, bulk or supercritical carbon dioxide polymerizations has led to inhomogeneous products. Our approach consists of solution polymerization (dimethyl sulfoxide and dimethylformamide) to obtain homogeneous CDCL derivatives with four monomer units on average. Oligomerization kinetics, performed by a matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) optimized method in tandem with 1H NMR, revealed that monomer conversion occurs in two stages: first, the monomer is rapidly attached to the secondary OH groups of β-cyclodextrin and, secondly, the monomer conversion is slower with attachment to the primary OH groups. MALDI MS was further employed for the measurement of the ring-opening kinetics to establish the influence of the solvents as well as the effect of organocatalysts (4-dimethylaminopyridine and (-)-sparteine). Additionally, the mass spectrometry structural evaluation was further enhanced by fragmentation studies which confirmed the attachment of oligoesters to the cyclodextrin and the cleavage of dimethylformamide amide bonds during the ring-opening process.

Bile acid sequestrants: a review of mechanism and design

OBJECTIVE

Bile acid sequestrants (BAS) are used extensively in the treatment of hypercholesterolaemia. This brief review aimed to describe the design and evaluation of three types of BAS: amphiphilic copolymers, cyclodextrin/poly-cyclodextrin and molecular imprinted polymers. The mechanisms underlying the action of BAS are also discussed.

KEY FINDINGS

BAS could lower plasma cholesterol, improve glycemic control in patients with type 2 diabetes and regulate balance energy metabolism via receptors or receptor-independent mediated mechanisms. Different types of BAS have different levels of ability to bind to bile acids, different stability and different in-vivo activity.

CONCLUSIONS

A growing amount of evidence suggests that bile acids play important roles not only in lipid metabolism but also in glucose metabolism. The higher selectivity, specificity, stability and in-vivo activity of BAS show considerable potential for lipid-lowering therapy.

Cyclodextrin complexation studies as the first step for repurposing of chlorpromazine

The antipsychotic drug chlorpromazine (CPZ) has potential for the treatment of acute myeloid leukemia, if central nervous system side-effects resulting from its passage through the blood-brain barrier can be prevented. A robust drug delivery system for repurposed CPZ would be drug-in-cyclodextrin-in-liposome that would redirect the drug away from the brain while avoiding premature release in the circulation. As a first step, CPZ complexation with cyclodextrin (CD) has been studied. The stoichiometry, binding constant, enthalpy, and entropy of complex formation between CPZ and a panel of CDs was investigated by isothermal titration calorimetry (ITC). All the tested CDs were able to include CPZ, in the form of 1:1, 1:2 or a mixture of 1:1 and 1:2 complexes. In particular, a substituted γ-CD, sugammadex (the octasodium salt of octakis(6-deoxy-6-S-(2-carboxyethyl)-6-thio)cyclomaltooctaose), formed exclusively 1:2 complexes with an extremely high association constant of 6.37 × 10⁹ M^-2. Complexes were further characterized by heat capacity changes, one- and two-dimensional (ROESY) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics simulations. Finally, protection of CPZ against photodegradation by CDs was assessed. This was accelerated rather than reduced by complexation with CD. Altogether these results provide a molecular basis for the use of CD in delayed release formulations for CPZ.

Complexation Kinetics of Cyclodextrins with Bile Salt Anions: Energy Barriers for the Threading of Ionic Groups

Binding constants for thousands of cyclodextrin complexes have been reported in the literature, but much less is known about the kinetics of these host-guest complexes. In the present study, inclusion complexes of bile salts with β-cyclodextrin, γ-cyclodextrin, and a methylated β-cyclodextrin were studied by nuclear magnetic resonance (NMR) lineshape analysis to explore the structural factors that govern the complexation kinetics. For complexes with β-cyclodextrin, the association rate constants ranged from 2 × 10⁶ to 2 × 10⁷ M^-1 s^-1 while the dissociation rate constants ranged from 12 to 6000 s^-1 at 25 °C. The kinetics were thus significantly slower than for any other β-cyclodextrin complex reported in the literature, due to the large energy barrier for threading the ionic sidechains of the bile salt anions. Bile salts with taurine and glycine sidechains had identical binding affinities, but the kinetics differed by a factor of 10. Introduction of a single hydroxyl group at the binding site of the bile salts reduced the lifetimes and binding constants of the complexes by more than 50 times. The strong temperature dependence of the rate constants revealed that the large activation energies were mainly enthalpic with a small contribution from entropy. The larger γ-cyclodextrin was threaded by the nonionic end of the bile salts, and the kinetics were too fast to be accurately determined. The study demonstrates that ionic groups on guest molecules constitute significant energy barriers for the threading and dethreading of β-cyclodextrin hosts.

Rapid Ion Mobility Separations of Bile Acid Isomers Using Cyclodextrin Adducts and Structures for Lossless Ion Manipulations

Bile acids (BAs) constitute an important class of steroid metabolites often displaying changes associated with disease states and other health conditions. Current analyses for these structurally similar compounds are limited by a lack of sensitivity and long separation times with often poor isomeric resolution. To overcome these challenges and provide rapid analyses for the BA isomers, we utilized cyclodextrin adducts in conjunction with novel ion mobility (IM) separation capabilities provided by structures for lossless ion manipulations (SLIM). Cyclodextrin was found to interact with both the tauro- and glyco-conjugated BA isomers studied, forming rigid noncovalent host-guest inclusion complexes. Without the use of cyclodextrin adducts, the BA isomers were found to be nearly identical in their respective mobilities and thus unable to be baseline resolved. Each separation of the cyclodextrin-bile acid host-guest inclusion complex was performed in less than 1 s, providing a much more rapid alternative to current liquid chromatography-based separations. SLIM provided capabilities for the accumulation of larger ion populations and IM peak compression that resulted in much higher resolution separations and increased signal intensities for the BA isomers studied.

Extracavity Effect in Cyclodextrin/Surfactant Complexation

Cyclodextrin (CD) complexation is a convenient method to sequester surfactants in a controllable way, for example, during membrane-protein reconstitution. Interestingly, the equilibrium stability of CD/surfactant inclusion complexes increases with the length of the nonpolar surfactant chain even beyond the point where all hydrophobic contacts within the canonical CD cavity are saturated. To rationalize this observation, we have dissected the inclusion complexation equilibria of a structurally well-defined CD, that is, heptakis(2,6-di- O-methyl)-β-CD (DIMEB), and a homologous series of surfactants, namely, n-alkyl- N, N-dimethyl-3-ammonio-1-propanesulfonates (SB3- x) with chain lengths ranging from x = 8 to 14. Thermodynamic parameters obtained by isothermal titration calorimetry and structural insights derived from nuclear magnetic resonance spectroscopy and molecular dynamics simulations revealed that, upon inclusion, long-chain surfactants with x = ≥10 extend beyond the canonical CD cavity. This enables the formation of hydrophobic contacts between long surfactant chains and the extracavity parts of DIMEB, which make additional favorable contributions to the stability of the inclusion complex. These results explain the finding that the stability of CD/surfactant inclusion complexes monotonously increases with the surfactant chain length even for long chains that completely fill the canonical CD cavity.

Polymeric bile acid sequestrants: Review of design, in vitro binding activities, and hypocholesterolemic effects

Polymeric bile acid sequestrants (BAS) have recently attracted much attention as lipid-lowering agents. These non-absorbable materials specifically bind bile acids (BAs) in the intestine, preventing bile acid (BA) reabsorption into the blood through enterohepatic circulation. Therefore, it is important to understand the structure-property relationships between the polymer sequestrant and its ability to bind specific BAs molecules. In this review, we describe pleiotropic effects of bile acids, and we focus on BAS with various molecular architectures that result in different mechanisms of BA sequestration. Here, we present 1) amphiphilic polymers based on poly(meth)acrylates, poly(meth)acrylamides, polyalkylamines and polyallylamines containing quaternary ammonium groups, 2) cyclodextrins, and 3) BAS prepared via molecular imprinting methods. The synthetic approaches leading to individual BAS preparation, as well as results of their in vitro BA binding activities and in vivo lipid-lowering activities, are discussed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Design of β-CD-surfactant complex-coated liquid crystal droplets for the detection of cholic acid via competitive host-guest recognition

β-CD-C14TAB complex-coated 5CB droplets are designed by the adsorption of β-CD-C14TAB complexes at the 5CB/aqueous interface. We show that the 5CB droplets can be used as an optical probe for the selective detection of cholic acid in aqueous solution containing uric acid and urea via competitive host-guest recognition.

Direct insight into the nonclassical hydrophobic effect in bile salt:β-cyclodextrin interaction: role of hydrophobicity in governing the prototropism of a biological photosensitizer

The interaction of norharmane with bile salts is reported along with the evidence for nonclassical hydrophobic effect in bile salt:β-cyclodextrin interaction.

Thermodynamic investigation of the interaction between cyclodextrins and preservatives - Application and verification in a mathematical model to determine the needed preservative surplus in aqueous cyclodextrin formulations

Preservatives are inactivated when added to conserve aqueous cyclodextrin (CD) formulations due to complex formation between CDs and the preservative. To maintain the desired conservation effect the preservative needs to be added in apparent surplus to account for this inactivation. The purpose of the present work was to establish a mathematical model, which defines this surplus based upon knowledge of stability constants and the minimal concentration of preservation to inhibit bacterial growth. The stability constants of benzoic acid, methyl- and propyl-paraben with different frequently used βCDs were determined by isothermal titration calorimetry. Based upon this knowledge mathematical models were constructed to account for the equilibrium systems and to calculate the required concentration of the preservations, which was evaluated experimentally based upon the USP/Ph. Eur./JP monograph. The mathematical calculations were able to predict the needed concentration of preservation in the presence of CDs; it clearly demonstrated the usefulness of including all underlying chemical equilibria in a mathematical model, such that the formulation design can be based on quantitative arguments.

Tuning temperature responsive poly(2-alkyl-2-oxazoline)s by supramolecular host-guest interactions

A poly[(2-ethyl-2-oxazoline)-ran-(2-nonyl-2-oxazoline)] random copolymer was synthesized and its thermoresponsive behavior in aqueous solution modulated by the addition of different supramolecular host molecules. The macrocycles formed inclusion complexes with the nonyl aliphatic side-chains present in the copolymer, increasing its cloud point temperature. The extent of this temperature shift was found to depend on the cavitand concentration and on the strength of the host-guest complexation. The cloud point temperature could be tuned in an unprecedented wide range of 30 K by supramolecular interactions. Since the temperature-induced breakage of the inclusion complexes constitutes the driving force for the copolymer phase transition, the shift in cloud point temperature could be utilized to estimate the association constant of the nonyl side chains with the cavitands.

In vitro and in vivo evaluation of novel cross-linked saccharide based polymers as bile acid sequestrants

Bile acid sequestrants (BAS) represent a therapeutic approach for the management of hypercholesterolemia that relies on the cationic polymeric nature of BAS to selectively bind negatively charged bile acids. We hypothesized that the cross-linking of β-cyclodextrin (β-CD) and saccharides such as starch or dextrin with divinyl sulfone (DVS) yields homo- and hetero-polymeric materials with the ability to trap sterols. Our hypothesis was put to test by synthesizing a library of 22 polymers that were screened to evaluate their capability to sequester both cholesterol (CHOL) and cholic and deoxycholic acids (CA and DCA). Three polymers synthesized in high yield were identified as promising. Two were neutral hetero-polymers of β-CD and starch or dextrin and the third was a weakly cationic homo-polymer of starch, highlighting the importance of the cavity effect. They were tested in hypercholesterolemic male Wistar rats and their ability to regulate hypercholesterolemia was similar to that for the reference BAS cholestyramine, but with two additional advantages: (i) they normalized the TG level and (ii) they did not increase the creatinine level. Neither hepatotoxicity nor kidney injury was detected, further supporting them as therapeutical candidates to manage hypercholesterolemia.

Remarkable viscoelasticity in mixtures of cyclodextrins and nonionic surfactants

We report the effect of native cyclodextrins (α, β, and γ) and selected derivatives in modulating the self-assembly of the nonionic surfactant polyoxyethylene cholesteryl ether (ChEO10) and its mixtures with triethylene glycol monododecyl ether (C12EO3), which form wormlike micelles. Cyclodextrins (CDs) generally induce micellar breakup through a host-guest interaction with surfactants; instead, we show that a constructive effect, leading to gel formation, is obtained with specific CDs and that the widely invoked host-guest interaction may not be the only key to the association. When added to wormlike micelles of ChEO10 and C12EO3, native β-CD, 2-hydroxyethyl-β-CD (HEBCD), and a sulfated sodium salt of β-CD (SULFBCD) induce a substantial increase of the viscoelasticity, while methylated CDs rupture the micelles, leading to a loss of the viscosity, and the other CDs studied (native α- and γ- and hydroxypropylated CDs) show a weak interaction. Most remarkably, the addition of HEBCD or SULFBCD to pure ChEO10 solutions (which are low-viscosity, Newtonian fluids of small, ellipsoidal micelles) induces the formation of transparent gels. The combination of small-angle neutron scattering, dynamic light scattering, and cryo-TEM reveals that both CDs drive the elongation of ChEO10 aggregates into an entangled network of wormlike micelles. (1)H NMR and fluorescence spectroscopy demonstrate the formation of inclusion complexes between ChEO10 and methylated CDs, consistent with the demicellization observed. Instead, HEBCD forms a weak complex with ChEO10, while no complex is detected with SULFBCD. This shows that inclusion complex formation is not the determinant event leading to micellar growth. HEBCD:ChEO10 complex, which coexists with the aggregated surfactant, could act as a cosurfactant with a different headgroup area. For SULFBCD, intermolecular interactions via the external surface of the CD may be more relevant.

Tuning the viscoelasticity of nonionic wormlike micelles with β-cyclodextrin derivatives: a highly discriminative process

We report the influence of five β-cyclodextrin (β-CD) derivatives, namely: randomly methylated β-cyclodextrin (MBCD), heptakis (2,6-di-O-methyl)-β-cyclodextrin (DIMEB), heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB), 2-hydroxyethyl-β-cyclodextrin (HEBCD) and 2-hydroxypropyl-β-cyclodextrin (HPBCD), on the self-assembly of mixtures of nonionic surfactants: polyoxyethylene cholesteryl ether (ChEO10) and monocaprylin (MCL). Mixtures of ChEO10/MCL in water form highly viscoelastic wormlike micelle solutions (WLM) over a range of concentrations; herein, the composition was fixed at 10 wt % ChEO10/3 wt % MCL. The addition of methylated β-CDs (MBCD, DIMEB, TRIMEB) induced a substantial disruption of the solid-like viscoelastic behavior, as shown from a loss of the Maxwell behavior, a large reduction in G' and G″ in oscillatory frequency-sweep measurements, and a drop of the viscosity. The disruption increased with the degree of substitution, following: MBCD < DIMEB < TRIMEB. Cryo-TEM images confirmed a loss of the WLM networks, revealing short rods and disc-like aggregates, which were corroborated by small-angle neutron scattering (SANS) measurements. Critical aggregation concentrations (CAC), measured by fluorescence spectroscopy, increased in the presence of DIMEB for both ChEO10 and MCL, suggesting the existence of interactions between methylated β-CDs and both surfactants involved in WLM formation. Instead, hydroxyl-β-CDs had a very different effect on the WLM. HPBCD only slightly reduced the solid-like behavior, without suppressing it. Quite remarkably, the addition of HEBCD reinforced the solid-like characteristics and increased the viscosity 10-fold. Cryo-TEM images confirmed the subsistence of WLM in ChEO10/MCL/HEBCD solutions, while SANS data revealed a slight elongation and thickening of the worms, and an increase of associated water molecules. CAC data showed that HPBCD had little effect on either surfactant, while HEBCD strongly affected the CAC of MCL and only slightly affected the ChEO10. For both DIMEB and HEBCD, time-resolved SANS measurements showed that morphology changes underlying these macroscopic changes occur in less than 100 ms.

Bile salts and their importance for drug absorption

Bile salts are present in the intestines of humans as well as the animals used during the development of pharmaceutical products. This review provides a short introduction into the physical chemical properties of bile salts, a description of the bile concentration and composition of bile in different animal species and an overview of the literature investigating the influence of bile salts on the in vivo performance of different compounds and drug formulations. Generally, there is a positive effect on bioavailability when bile is present in the gastro-intestinal tract, independent of the formulation systems, e.g. suspensions, solutions, cyclodextrin complexes or lipid based formulations, but a few exceptions have also been reported.

Higher order inclusion complexes and secondary interactions studied by global analysis of calorimetric titrations

This paper investigates the use of isothermal titration calorimetry (ITC) as a tool for studying molecular systems in which weaker secondary interactions are present in addition to a dominant primary interaction. Such systems are challenging since the signal pertaining to the stronger primary interaction tends to overshadow the signal from the secondary interaction. The methodology presented here enables a complete and precise thermodynamic characterization of both the primary and the weaker secondary interaction, exemplified by the binding of β-cyclodextrin to the primary and secondary binding sites of the bile salt glycodeoxycholate. Global regression analysis of calorimetric experiments at various concentrations and temperatures provide a precise determination of ΔH, ΔG°, and ΔC(p) for both binding sites in glycodeoxycholate (K1 = 5.67 ± 0.05 × 10(3) M(-1), K2 = 0.31 ± 0.02 × 10(3) M(-1)). The results are validated by a (13)C NMR titration and negative controls with a bile salt with no secondary binding site (glycocholate) (K = 2.96 ± 0.01 × 10(3) M(-1)). The method proved useful for detailed analysis of ITC data and may strengthen its use as a tool for studying molecular systems by advanced binding models.