Molecular modelling of the inclusion complexes between beta-cyclodextrin and (R)/(S)-methylphenobarbitone and its application to HPLC

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.

Cyclodextrins as pharmaceutical solubilizers

Cyclodextrins are useful functional excipients that have enjoyed widespread attention and use. The basis for this popularity from a pharmaceutical standpoint, is the ability of these materials to interact with poorly water-soluble drugs and drug candidates resulting in an increase in their apparent water solubility. The mechanism for this solubilization is rooted in the ability of cyclodextrin to form non-covalent dynamic inclusion complexes in solution. Other solubilizing attribute may include the ability to form non-inclusion based complexes, the formation of aggregates and related domains and the ability of cyclodextrins to form and stabilize supersaturated drug solutions. The increase in solubility also can increase dissolution rate and thus improve the oral bioavailability of BCS Class II and IV materials. A number of cyclodextrin-based products have reached the market based on their ability to camouflage undesirable physicochemical properties. This review is intended to give a general background to the use of cyclodextrin as solubilizers as well as highlight kinetic and thermodynamic tools and parameters useful in the study of drug solubilization by cyclodextrins.

Enantioselective chromatography of alkyl derivatives of 5-ethyl-5-phenyl-2-thiobarbituric acid studied by semiempirical AM1 method

Complexation of alkyl derivatives of 5-ethyl-5-phenyl-2-thiobarbituric acid (2-thiophenobarbital) enantiomers by beta-cyclodextrin was investigated by the AM1 method. The inclusion complexes of beta-cyclodextrin with neutral and anionic forms of these enantiomers have been modeled and energetically optimized. The chiral discrimination of enantiomers was analyzed in terms of differences in the interaction energies. The calculated interaction energies between each enantiomer of the investigated 2-thiobarbiturates and beta-cyclodextrin confirm the ability of beta-cyclodextrin to act as a mobile phase additive in reversed-phase HPLC to separate enantiomers by liquid chromatography and rationalize their order of elution.

Atomistic modeling of enantioselection in chromatography

A review of atomistic molecular modeling studies related to chromatographic separations of enantiomers is presented. Only those types of calculations where direct interactions between a selector and a selectand are involved are described in this review; omitted are regression models. An emphasis is placed on comparing methods used for sampling potential energy surfaces implementing different methodologies like quantum and molecular mechanics for energy calculations, and molecular dynamics and Monte Carlo sampling strategies for simulations. Type I-V chiral stationary phases and additives for capillary electrophoresis and ion-pair chromatography are covered in this review.

Monte Carlo simulations of the chiral recognition of fenoprofen enantiomers by cyclomaltoheptaose (beta-cyclodextrin)

Differential complexation of fenoprofen enantiomers by cyclomaltoheptaose (beta-cyclodextrin) was investigated by Monte Carlo docking simulations. The chiral discrimination of (R)- and (S)-fenoprofen by beta-cyclodextrin was discussed in terms of the difference in the interaction energies and the patterns of molecular interactions. The interaction energies between each enantiomer of fenoprofen and beta-cyclodextrin were consistent with the reported experimental results that showed that the S isomer interacted preferentially with beta-cyclodextrin and was retained longer in a separation process than the R isomer. The thermodynamic preference of inclusion complex formation of (S)-fenoprofen could be explained by the orientation of the phenyl group attached to the chiral carbon, which provided closer contact and thus more favorable intermolecular interactions between the host and guest molecule. The results presented here would be very useful for the prediction of chiral recognition ability of beta-cyclodextrin.

Comparison of cyclodextrin-barbiturate noncovalent complexes using electrospray ionization mass spectrometry and capillary electrophoresis

Various noncovalent complexes between native and derivatized cyclodextrins (CDs) and barbiturates were studied using capillary electrophoresis (CE) and electrospray ionization mass spectrometry (ESI-MS). This paper involves the study of four aspects of CD-barbiturate noncovalent inclusion complexes. The first study focused on determining the formation of CD-barbiturate inclusion complexes in ESI-MS. This determination was accomplished by the comparison of migration data from CE with ESI-MS inclusion complex peak abundances, which were found to be complementary. The second study found the possibility of predicting native beta-CD mediated CE elution orders for barbiturates using data from ESI-MS. A third study focused on the formation of barbiturate inclusion complexes with derivatized beta-CD and gamma-CD. As part of this study, the effect of the extent of side chain substitution on native CD complexation behavior was investigated. The results indicated that the number of side chains on the CD does not affect the formation of barbiturate complexes with the hydrophobic CD cavity. Finally, a comparison of the hydroxypropyl-beta-CD-barbiturate and hydroxypropyl-gamma-CD-barbiturate complexes in CE and ESI-MS was made to study the relationship between strength of drug-CD binding and enantioresolution. The results from the above studies indicated that the gas phase and the solution state complexes showed comparable behavior indicating that similar interactions played a role in stabilizing these complexes. While it was possible to use the ESI-MS data to determine drug binding to the CDs, it was not possible to predict whether a separation of the enantiomers of a chiral barbiturate would occur. However, the ESI-MS data could be used to eliminate certain CDs from consideration as chiral selectors.

Enantiomer separation of dihydropyridine calcium antagonists with cyclodextrins as chiral selectors: structural correlation

Native and substituted cyclodextrins (CDs) were used as chiral selectors both in high-performance liquid chromatography and capillary electromigration separations (HPCE and MEKC). Chromatographic data of five dihydropyridine calcium antagonists obtained on three beta-CD chiral stationary phases in reversed-phase mode were compared with those of capillary electrophoresis using beta-CDs in the presence and absence of sodium dodecyl sulfate (SDS). Competition of separated compounds with SDS molecules for penetration into the CD cavity can limit their necessary interaction with the chiral selector and consequently even preclude enantiomer separation. Some insight into this problem can be brought about by comparing the experimental data with computer-aided energy minimization of CD-solute and CD-SDS inclusion complexes.