Host-guest Inclusion Complexes between Mitiglinide and the Naturally Occurring Cyclodextrins α , β , and : A Theoretical Approach

2015 The Authors. This is an Open Access article distributed under the terms of the Creative Commons Attribution (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, as long as the original authors and source are cited. No permission is required from the authors or the publishers. Adv Pharm Bull, 2015, 5(2), 289-291 doi: 10.15171/apb.2015.040 http://apb.tbzmed.ac.ir Advanced Pharmaceutical Bulletin


Introduction
The inclusion complexation with cyclodextrins (CDs) is an attractive and widely used technique for solubility/dissolution enhancement of poorly watersoluble drugs.CDs, are naturally occurring cyclodextrins that commercially available cyclic oligosaccharides containing 6, 7, 8-glucopyranose units and are referred to as α-, βand γ-CDs, respectively. 1The inclusion complexation of drug molecules with CDs usually accompanied in favorable changes in the physicochemical properties of the drug, such as solubility, dissolution rate, stability and bioavailability, thus making them more suitable for oral drug delivery. 2itiglinide (MIT), (2S)-benzyl-4-(cisperhydroisoindol-2-yl) butyric acid (Figure 1).It is a novel insulinotropic agent of the glinide class with rapid onset. 3Analogous to other glinide group members, MIT acts by stimulating the secretion of insulin from pancreatic-beta cells by closing the ATP-sensitive K + [K(ATP)] channels. 4MIT is hydrophobic and has a low aqueous solubility, possibly limiting its range of applications. 5n the current paper, we have investigated the inclusion processes of MIT with the naturally occurring CDs using PM3 method in order to get some insight into the conformation of this complex.Furthermore, to investigate and predict the interaction energies (Ecomp) of the optimal configurations of (1:1) mitiglinide/CDs inclusion complexes by employing molecular simulations using molecular mechanics methods with Autodock to determine the mode of inclusion of MIT within the CD.The obtained structures were further optimized by the semiempirical method PM3 to obtain the binding energies of the studied inclusion complexes.

Computational method
The starting geometries of MIT and the host structures (-CD,-CD, -CD,) were built based on the structures that are generated from the crystallographic parameters provided by the Drug, 6 and the Cambridge Structural Database (CSD). 7,8Each of the starting geometry was separately optimized using the semi empirical method, PM3 available in the Gaussian03 software package. 9The starting geometries of the inclusion complexes were constructed using HyperChem (Version 7.0, Hypercube, Gainesville, FL, USA).The previously optimized structures of MIT and host molecules were allowed to approach each other along the symmetric axis (the Xaxis) passing through the center of the host cavity.For example, in the -CD case, the coordinate system used to define the process of complexation is based on constructing the CD with the seven identical glucose units positioned symmetrically around the Z-axis, such that all the glycosidic oxygens are in the XY plane and their center was defined as the center of the coordination system. 10MIT molecule was docked into the cavity of the CD with the central oxygen atom connecting the two rings coincide with the Z-axis.Multiple starting points were generated by moving the guest molecules along the and +Z-axis from 10 to -10 Å, at 1 Å intervals, and by rotating the guest molecules from 0° -360° at 45° intervals.Different inclusion orientations, i.e the pyrrolidine group facing down, the benzene ring facing down into the cavity of the CDs and the carboxylic group facing up and down horizontally into the cavity of the host, were considered for all cases.Docking of the guest molecule was initially done to maximize the electrostatic interaction and hydrophobic interaction between the host and the guest molecules using the Autodock4.2software. 11The Kollman and Gasteiger charges were assigned on the host while the charges on MIT were added automatically.For the autogrid calculation, the grid map of 40 x 40 x 40 Å 3 was used for α-CD and β-CD while 50 x 50 x 50 Å 3 dimension of grid box was set on γ-CDs.Grid point spacing of 0.375 Å was used and centered on the macromolecule.A total of 21 rigid docking tests in vacuo were constructed using Autodock4.2software and a total of 100 runs adopting the Lamarkian Genetic Algorithm were performed.The results of cluster ranked in the order of increasing in energy and ranked in the order of increasing energy with 2.0 Å tolerance.The configurations obtained were ranked in clusters which list from the lowest to highest binding energy.The results from docking study were then again submitted to further optimization at the PM3 level.The inclusion interactions were simulated in vacuum and the presence of water molecules were ignored to save computational time especially for large molecules.Comparisons were made between the conformation from the semi empirical method and the optimized docking results to determine the lowest energy conformation which is then taken as the final structure.The complexation energy, Ecomp, was calculated for the minimum energy structures by the following equation: where, Ecomp, EG, and EH represent the total energy of the host-guest complex, the free guest molecule and the free host molecule, respectively.The magnitude of the energy change would be an indication of the driving force towards complexation.The more negative the complexation energy change is the more thermodynamically favorable is the inclusion complex.

Results and Discussion
To further understand how molecular recognition takes place at the atomic level, we use two different molecular modeling techniques that are the quantum mechanics calculation using the PM3 semi empirical method as well as the docking calculation to complement the experimental studies.Overall the PM3 semiempirical method proved to be more efficient to locate the minimum energy conformations when compared to docking calculation.The final complexation energies (Ecomp) of the MIT/α-CD, MIT/β-CD, and MIT/γ-CD inclusion complexes were -13.571 kcal/mol, -9.696 kcal/mol and -17.884 kcal/mol, respectively.The optimized geometries for the lowest energy conformation for the inclusion complexes of MIT with the different CDs are shown in Figure 2. In general, the inclusion of MIT into the cavity of -CD results in the most stable conformation when compared to and -CDs and the overall lowest binding energies upon complexation with the different hosts decrease in the order of: -CD > -CD > -CD.In general the results from the lowest rank structure produced via docking calculation needs to be further optimized through the PM3 calculation and the results after closed comparison between the two methods produced an almost similar inclusion site as shown in Figure 2. The dominant driving force for the complexation is evidently H-bondings interaction.Close observation on the structure of the inclusion complexes shows that -CD complexes have more H-bonding interactions compared to the -CD complexes.Various authors have also reported that intermolecular hydrogen bondings between the host-guest interactions resulting from the fitting of small molecules into the cyclodextrin cavities play an important role in the binding energies of a number of cyclodextrin inclusion complexes.In all three hosts, the pyrrolidine group of the MIT was included into the cavity leaving the benzene ring to move freely outside the cyclodextrin cavity.Almost a complete inclusion of the pyrrolidine ring was observed for the MIT/-CD and MIT/-CD.However, the inclusion the pyrrolidine ring in the -CD is shallow and consistently, no H-bondings were observed.A slightly more positive complexation energy displayed for the MIT/-CD inclusion complex, shows that inclusion complex between the two molecules are less favorable from the theoretical point of view.A careful scanning of literature shows that several studies also used semi-empirical PM3 method and HyperChem software as reliable tools for the inclusion complexes as they yield a good estimate. 13,14For example, in our previous work, Khaldun et al., 2010, we used the above mentioned methods to study the enantiomeric inclusion complexes and to rationalize the reasons for the different migration between the ofloxacin and ornidazole enantiomers with sulfated beta cyclodextrin.The results were in full agreements with the experimental part conducted.As a general conclusion, the use of such PM3 and HyperChem methods in theoretical calculations provides sufficient data for a reliable image of the inclusion process occurred.

Conclusion
The inclusion complexation of MIT with the naturally occurring CDs has been investigated theoretically by performing two different molecular modeling techniques that is the quantum mechanics calculation using the PM3 semi empirical method as well as the docking calculation to complement the experimental studies.The complexation is energetically driven by hydrogen bond interactions between the host and guest molecules.The geometry of the most stable complex shows that the pyrrolidine ring is included deeply inside in the hydrophobic cavity of -CD and -CD while it is shallow in the case of -CD.

Figure 1 .
Figure 1.The chemical and 3D structures of mitiglinide (MIT) optimized using the PM3 method.

Figure 2 .
Figure 2. Energy minimized structures obtained from PM3 calculations for the front side and top views of (a): MIT/-CD; (b): MIT/-CD; (c): MIT/-CD and the Autodock calculations for the (d): side and (e): top views of the MIT/-CD inclusion complexes. 12 Host-guest Inclusion Complexes between...