Multimodal inclusion complexes between barbiturates and 2-hydroxypropyl-beta-cyclodextrin in aqueous solution: isothermal titration microcalorimetry, (13)C NMR spectrometry, and molecular dynamics simulation

Formulation and stability study of an oral paediatric phenobarbital 1% solution containing hydroxypropyl-β-cyclodextrins

OBJECTIVES

Phenobarbital is a barbiturate, used to treat focal and generalised epilepsy. Since the end of marketing of the oral solution KANEURON in 2017, phenobarbital tablets remain the only available dosage form. Development of an oral phenobarbital solution for paediatric use is therefore essential to fulfil clinical needs. A new formulation of phenobarbital with hydroxypropyl-β-cyclodextrins (HPBCD) was developed, and the physicochemical stability of the solution was evaluated.

METHODS

Different excipients have been selected to formulate a solution of phenobarbital. Samples were dosed by High Performance Liquid Chromatography (HPLC) at 216 nm with a LiChroCART C18 endcapped column and mobile phase composed of phosphate buffer pH 3 and methanol (50:50 v/v). Linearity, accuracy, sensibility and specificity of the method were tested, and a forced degradation study was carried out. During stability study, content of phenobarbital, pH, osmolality of the phenobarbital solution and degradation products were followed up for 6 months in line with GERPAC guidelines.

RESULTS

The stability indicating the character of the assay method has been validated. The physicochemical stability study shows that the phenobarbital solution formulated is stable for 6 months, in line with International Conference of Harmonisation (ICH) recommendations Q1A and Q3B (R2) regarding the content of phenobarbital and levels of degradation products (no degradation products >0.01%). Phenobarbital concentration was 101.59±2.6% of initial concentration in refrigerated samples and 101.14±0.5% at 20±5°C. No phenobarbital degradation products (>0.01%) were observed throughout the 6 months. No significant variation of pH or osmolality was observed.

CONCLUSIONS

HPBCD solubilise phenobarbital and create a homogeneous solution. These stability data set the shelf life of this new phenobarbital solution at up to 6 months. A microbiological stability study will be carried out to ensure the possibility of using this solution in children.

Towards Rational Chemosensor Design through Improved Understanding of Experimental Parameter Variation and Tolerance in Cyclodextrin-Promoted Fluorescence Detection

We have previously developed a highly efficient fluorescence-based toxicant-detection method that operates in complex environments to detect aromatic toxicants and toxicant metabolites with high sensitivity and selectivity. This method relies on the ability of γ-cyclodextrin to act as a supramolecular scaffold, and uses a variety of non-covalent interactions between the cyclodextrin, toxicant, and fluorophore to enable efficient detection. Reported herein is an investigation of the effect of various experimental parameters, including host concentration, temperature, pH, salt, and solvent, on the observed energy-transfer efficiencies. These results advance our understanding of γ-cyclodextrin-based association complexes and provide crucial information for the development of fluorescence-based sensors using such complexation and the resultant fluorescence-based detection.

A novel synthetic luteinizing hormone-releasing hormone (LHRH) analogue coupled with modified β-cyclodextrin: insight into its intramolecular interactions

BACKGROUND

Cyclodextrins (CDs) in combination with therapeutic proteins and other bioactive compounds have been proposed as candidates that show enhanced chemical and enzymatic stability, better absorption, slower plasma clearance and improved dose-response curves or immunogenicity. As a result, an important number of therapeutic complexes between cyclodextrins and bioactive compounds capable to control several diseases have been developed.

RESULTS

In this article, the synthesis and the structural study of a conjugate between a luteinizing hormone-releasing hormone (LHRH) analogue, related to the treatment of hormone dependent cancer and fertility, and modified β-cyclodextrin residue are presented. The results show that both the phenyl group of tyrosine (Tyr) as well as the indole group of tryptophan (Trp) can be encapsulated inside the cyclodextrin cavity. Solution NMR experiments provide evidence that these interactions take place intramolecularly and not intermolecularly.

CONCLUSIONS

The study of a LHRH analogue conjugated with modified β-cyclodextrin via high field NMR and MD experiments revealed the existence of intramolecular interactions that could lead to an improved drug delivery.

GENERAL SIGNIFICANCE

NMR in combination with MD simulation is of great value for a successful rational design of peptide-cyclodextrin conjugates showing stability against enzymatic proteolysis and a better pharmacological profile.

Growth, shrinking, and breaking of pluronic micelles in the presence of drugs and/or beta-cyclodextrin, a study by small-angle neutron scattering and fluorescence spectroscopy

The associative structures between F127 Pluronic micelles and four drugs, namely, lidocaine (LD), pentobarbital sodium salt (PB), sodium naproxen (NP), and sodium salicylate (SAL), were studied by small-angle neutron scattering (SANS). Different outcomes for the micellar aggregates are observed, which are dependent on the chemical nature of the drug and the presence of charge or otherwise: the micelles grow with LD, are hardly modified with PB, and decrease in size with both NP and SAL. The partition coefficient, determined by fluorescence spectroscopy, is directly correlated to the amount of charge, following NP approximately SAL < PB < LD. All drugs are found to lie at the interfacial layer, with a slightly deeper localization of LD and more superficial for PB. All drugs can form inclusion complexes with heptakis(2,6-di-O-methyl) beta-cyclodextrin (hep2,6 beta-CD). Hep2,6 beta-CD, as shown in previous studies (Joseph, J.; Dreiss, C. A.; Cosgrove, T. Langmuir, 2008, 24, 10005-10010; Dreiss, C. A.; Nwabunwanne, E.; Liu, R.; Brooks, N. J. Soft Matter, 2009, 5, 1888-1896), is also able to form a complex with F127, resulting in micellar breakup. In the ternary mixtures, a fine balance of forces is involved, which results in drastic micellar changes, as observed from the SANS patterns. Depending on the ratio of drug, polymer, and hep2,6 beta-CD and the nature of the interactions (which is directly linked to the drug chemical structure), the presence of drug either hinders micellar breakup by beta-CD (at high enough concentration of LD or PB) or leads to micellar growth (NP). These effects are mainly attributed to a preferential drug/beta-CD interaction (except for PB), which, at least in the conditions studied here, explains the higher beta-CD concentration needed for micellar breakup to occur.

Computational Modeling of Inclusion Complexes of β-Cyclodextrin with enantiomers of Salsolinol, N-Methyl-Salsolinol, and 1-Benzyl-Tetrahydroisoquinoline

Capillary electrophoresis with β-CD as a chiral selector has successfully separated the two enantiomers of salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline. The migration times of each enantiomer in capillary electrophoresis reflect the stability of their β-CD inclusion complexes. This paper reports a computational modeling study of the inclusion complexes of β-cyclodextrin (β-CD) with salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline by using PM3 (Parametric Method 3) semi-empirical molecular orbital calculations and the ONIOM hybrid method. Two types of the inclusion complexes, cis- and trans-orientations, are considered for each enantiomer of the guest molecules, salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline. In the cis-orientation, the nitrogen in the salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline points toward the secondary hydroxyls of the β-CD, while in the trans-orientation, the nitrogen in salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline points toward the primary hydroxyls of the β-CD. We found that the stabilization energies of these inclusion complexes from these PM3 and ONIOM different methods correlate very well with the migration order deduced from the study of capillary electrophoretic separation.

Advances in physical chemistry and pharmaceutical applications of cyclodextrins

Cyclodextrins (CDs) attract much attention for industrial applications and academic research. A few experimental methods for determination of the binding constant between CD and a guest molecule were reviewed critically. A hydrophile–hydrophobe matching model for host–guest docking was proposed for estimation of the binding constant and the solution structure of the complex. Rather detailed solution structures of CD complexes were determined by proton NMR spectroscopy, aided by calculations of molecular mechanics and surface areas, and were used to analyze the binding constants. The binding constants of CDs with multi-site guests were analyzed on the basis of their solution structures. The working mechanisms and physicochemical predictions in a few pharmaceutical applications of CDs were proposed on the basis of detailed solution structures and accurate binding constants.

NMR analysis of ion pair formation between timolol and sorbic acid in ophthalmic preparations

Ion pair formation between timolol and sorbic acid was investigated using NMR spectroscopy in order to clarify their interactions within ophthalmic preparation. (13)C and (1)H NMR spectra of timolol, sorbic acid, and a mixture of the two were obtained, and the signal changes induced by pairing were observed. The carbon signals of the butylaminopropanol moiety of timolol were markedly shifted in the mixture, as were the carboxyl and conjugated carbons assigned to sorbic acid. The localizations of the changes in each molecule revealed the binding sites. The profiles of butylaminopropanol carbon chemical shifts plotted against a molar ratio of sorbate were synchronized, which suggested a single type of interaction with sorbic acid. The Job plot showed a typical pattern with a single-maximum at a mole function of 0.5, indicating the presence of a 1:1 complex of timolol and sorbic acid. The stability constants (K) of the timolol-sorbate and timolol-maleate pairs were 1.9x10(1) and 2.2x10(2)M(-1), respectively. The higher K value of the timolol-maleate interaction suggested that it was dominant to the timolol-sorbate interaction when maleate and sorbate coexisted within a timolol solution. Here, we demonstrated evidence of an interaction between timolol and sorbic acid using simple NMR measurements, which suggested the existence of ion pair formation derived from charge neutralization. Our analysis using NMR spectroscopy should advance the understanding and optimization of formulations that are based on ion pair.

Sertaconazole/hydroxypropyl-β-cyclodextrin complexation: Isothermal titration calorimetry and solubility approaches

Complexation of sertaconazole (SN) with hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was characterized by phase-solubility diagram measurements and isothermal calorimetry (ITC) in aqueous medium, and by differential scanning calorimetry (DSC), Raman spectroscopy and X-ray diffractometry in solid state. The strongest interaction was observed at pH 1.2, at which two different 1:1 complexes can be formed depending on the hydrophobic ring of the drug involved in the process. At pH 5.8 and 7.4 the likelihood of 1:2 stoichiometry increases as a consequence of the simultaneous complexation of the nonprotonized imidazolyl and the dichlorophenyl groups. In the presence of 20% HP-beta-CD, SN solubility is enhanced by a factor of 116, 107, and 5 at pH 1.2, 5.8, and 7.4, respectively. Complexation enthalpy recorded by ITC showed the same tendency which confirms the practical interest of this technique for fast screening of the potential of CDs as drug solubilizers. Solubility and dissolution rate of the drug from compacts prepared with freeze-dried complexes were significantly greater than those obtained with SN powder or compacts made with physical blends.

Multimodal molecular encapsulation of nicardipine hydrochloride by beta-cyclodextrin, hydroxypropyl-beta-cyclodextrin and triacetyl-beta-cyclodextrin in solution. Structural studies by 1H NMR and ROESY experiments

Proton nuclear magnetic resonance spectroscopy (1H NMR), which has become an important tool for in vitro study of cyclodextrin (CD) complexes, was used to study and structurally characterize the inclusion compounds formed in solution between nicardipine hydrochloride (NC) and beta-cyclodextrin (betaCD), hydroxypropyl-beta-cyclodextrin (HPbetaCD) and triacetyl-beta-cyclodextrin (TAbetaCD). The large variation of chemical shifts from protons located around the interior of the hydrophobic cavity (i.e. H-3, H-5 and H-6) coupled with minimal variation of shifts from protons located on the outer sphere of the betaCD (i.e. H-1, H-2 and H-4) provided clear evidence of inclusion complexation. In the presence of the different CDs, the aromatic protons of NC were the most affected, suggesting a strong involvement of the phenyl groups in the inclusion mechanism. The application of continuous variation method indicated the presence of complexes with a 1:1 host/guest stoichiometry for all the studied CDs. Two-dimensional rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments were carried out to further support the proposed inclusion mode. Inspection of the ROESY spectra allowed the establishment of spatial proximities between several aromatic hydrogens of the guest and the CD protons, indicating that the inclusion occurs by accommodation of the two aromatic groups of NC. All the experimental data were further rationalized to elaborate possible three-dimensional geometric models of inclusion complexes. From the aforementioned observations, we concluded there is no preference for inclusion of a particular aromatic ring. Instead, two types of 1:1 complexes with different inclusion structures may exist simultaneously in solution, being alternatively included through the wider side of the cavity, i.e. the so-called multimodal inclusion occurs in the interaction of NC with the different CDs.