versão impressa ISSN 0327-0793
Lat. Am. appl. res. vol.41 no.2 Bahía Blanca abr. 2011
Study on inclusion interaction of ibuprofen with ?-cyclodextrin derivatives
L.T. Song, X.Y. Jiang*, K.W. Tang and J. B. Miao
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China
Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414000, Hunan, China
Abstract - The inclusion interaction of ibuprofen with hydroxypropyl-β-cyclodextrin (HP-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD) and methyl-β-cyclodextrin (Me-β-CD) was investigated by fluorescence spectroscopy. Experimental conditions affecting the inclusion process, such as host molecule, HP-β-CD concentration and pH, were discussed in detail. The results suggested the formation of inclusion complexes with a stoichiometric ratio of 1:1 and HP-β-CD was more suitable to include neutral ibuprofen molecule. In addition, a phase solubility study was performed by mixing an excess amount of ibuprofen with aqueous solution containing increasing amount of β-CDs using UV-vis. The results indicated that the solubility of ibuprofen was increased by inclusion with β-CD derivatives, and HP-β-CD was most efficient among the three β-CD derivatives. Moreover, stable solid inclusion complexes were established and characterized by DSC.
Keywords - Ibuprofen; β-CD Derivatives; Inclusion Complex; Fluorescence Spectroscopy.
Cyclodextrins(CDs) are macrocyclic compounds with several D-glucopyranoses residues linked by α-1,4-glycosidic bonds. α-, β- and γ-CD are the most common CDs, which have six, seven and eight glucose units, respectively. Their structures are shown in Fig. 1 (Szejt, 1998; Wenz, 1994; Harata and Kawano, 2002; Rekharsky and Inoue, 1998). Because of the 4C1 chair conformation of each glucopyranose unit, the whole molecule has the shape of a hollow truncated cone. The interior of the cavity is composed of hydrogen atoms of C-3, C-5 and oxygen atoms of the glycosidic linkage which make the intracavity hydrophobic, while the exterior of the cavity is hydrophilic due to assembling large numbers of alcoholic hydroxyl groups. CDs can form host-guest inclusion complexes by weak intermolecular interaction with a wide variety of guest (Hapiot et al., 2006; Saenger and Noltemeyer, 1976; Sonoda et al., 2006; Harata, 1998).
Figure 1. Molecular structures of CDs
Upon inclusion or partial inclusion of molecules within their hydrophobic interior, many properties of guest molecules are affected remarkably, such as chemical reactivity (Uekama et al., 1998), volatility (Tong, 2001), absorption spectrum (Aoyagi et al., 1997) and so on (Matsushita et al., 1997; Ueno, 1992). These changes in chemical and physical properties are of both theoretical and practical interest. CDs have been extensively studied in connection with various areas of chemistry, including the sensing of organic molecules, analytical chemistry, pharmaceuticals, food, encapsulation of drugs and other industrial areas (Jiang et al., 2004; Garcia-Rio et al., 1997; Fakayode et al., 2005; Csernak et al., 2006; Pacioni and Veglia, 2003). In particular, CDs were applied to increase the solubility, bioutility (Aigner et al., 1996), and stability (Uekama et al., 1998) of drugs.
Ibuprofen (IBU) is a non steroidal anti-inflammatory drug from the 2-arylpropionic acid family (Fig. 2). Because of its important anti-inflammatory activity, IBU is commercialized in several types of pharmaceutical preparations, such as tablets, capsules, suppositories and oral drops (Matkovic et al., 2005). However, IBU is not soluble easily in water. Therefore, formation of an inclusion complex with CDs can hopefully increase its aqueous solubility. At present, the interaction of CDs and IBU is limited to parent CDs which have poor water solubility and are unsafe due to its nephrotoxicity (Frank et al., 1976; Irie and Uekama, 1997). Therefore, several modified and relatively safe β-CDs have been used, such as HP-β-CD, HE-β-CD and Me-β-CD.
Figure 2. The chemical structure of IBU
In our research, the inclusion behavior of IBU with HP-β-CD, HE-β-CD and Me-β-CD was investigated systematically by fluorescence spectroscopy. In addition, a phase solubility study was performed and stable solid inclusion complexes were established and characterized by DSC.
Ibuprofen was purchased from Zhejiang Juhua Co. Ltd. (Zhejiang. China), HP-β-CD, HE-β-CD and Me-β-CD were bought from Xinda Fine Chemical & Co. Inc. (Shandong. China). Other reagents used were of analytical grade and doubly distilled water was used throughout.
TU-1901 UV-vis spectrofluorimeter (Beijing purkinje general instrument Co. Ltd., Beijing, China). All the fluorescence measurements were performed by a F-4500 spectrofluorimeter (Beijing purkinje general instrument Co. Ltd., Beijing, China) using 1cm quartz cell and both the slits were set at 10nm with the excitation wavelength at 250nm and the emission at 350nm. The pH measurements were made with a model pHS-25 pH meter (Shanghai, China). A Perkin-Elmer DSC instrument (Pyris Diamond TG/DSC, USA) was used in studying of complex formation.
1mL stock solution (5×10-4 mol/L) of IBU was transferred into a 25mL volumetric flask and an appropriate amount of 0.05mol/L HP-β-CD (or HE-β-CD or Me-β-CD) was added. The pH was fixed with 0.1mol/L phosphate buffer solution. The mixed solution was diluted to final volume with distilled water and shaken thoroughly, then was determined after 30min at 20±1oC. The spectra were recorded or fluorescence intensities were measured. In all the measurements of absorption, fluorescence was made against the blank solution treated in the same way without CDs.
D. Phase-solubility study
Solubility measurements were based on the phase-solubility technique established by Higuchi and Connors (1965). For this purpose, aqueous solutions of CDs with different increasing concentrations were prepared. Excess amount of IBU were added to each CDs solution. The solutions were reacted completely by ultrasonic agitation for 30min, equilibrating for 24h, then centrifuged and filtered with syringe through a 0.45µm PTFE filter. Their absorption was measured by UV spectrophotometer at 280 nm. The phase-solubility profile was therefore obtained by plotting the solubility of IBU versus the concentration of CDs. The stability constants, KC, were calculated from the straight-line portion of the phase solubility diagram according to the Higuchi-Connors equation (Eq. (1)):
where S0 is the solubility of IBU in the absence of β-CDs. Slope is the slope of the experimental phase solubility diagram for IBU-β-CDs.
E. Preparation of solid inclusion complex of IBU with HP-β-CD
IBU and HP-β-CD were accurately weighed according to the molar ratio of 1:1, and then HP-β-CD was placed into small amount of distilled water and grinded in a mortar. After that IBU was poured into HP-β-CD solution continuously and grinded into paste. The above mixed paste was dried at 40oC and white powder product was obtained, which is solid inclusion complex of IBU with HP-β-CD. Then four different types of samples were used: ibuprofen alone, HP-β-CD alone, physical mixture and inclusion complex, to identify differences in DSC curves. The scan rate was 20 oC/min between 40oC-300oC under nitrogen environment.
III. RESULTS AND DISCUSSION
A. Theory of inclusion interactions between ibuprofen and β-CDs
The apparent association constant of the inclusion complex can be determined by Benesi-Hildebrand equation (Catena and Bright, 1989):
where [P]0 denotes the initial concentration of IBU and [CD]0 denotes that of CDs. F and F0 are the fluorescence intensities of IBU in the presence and absence of CDs, respectively. K is a formation constant, k is the instrument constant and Q is the fluorescence quantum yield of the inclusion complex.
If the curve of 1/(F-F0) versus 1/[CD]0 exhibits good linearity, it implies that inclusion complexes with a stoichiometry of 1:1 are formed.
B. Effect of the types of CDs
β-CDs have the hydrophilic outer surface and a hydrophobic internal cavity. The inclusion interaction of β-CDs and the guest molecules is affected by the size of the internal cavity and hydrophilic, hydrophobic characters of the host. It is generally believed that dipole-dipole, electrostatic, van der Walls forces, hydrogen bonding, hydrophobic interaction, and the release of distortion energy of CD ring upon guest binding cooperatively govern the stability of an inclusion complex. The effect of HP-β-CD concentration on the fluorescence intensity of IBU is given in Fig. 3. As can be seen from Fig. 3, the fluorescence intensities were enhanced and maximum emission wavelengths had red shifts when CDs were added. The fact of red-shifting suggested the formation of complex. It was also observed in Fig. 3 that the fluorescence intensity of IBU gradually enhanced with the increase of HP-β-CD concentration. It could be explained that the CDs cavity provided an apolar environment for IBU molecule and that thus increased the quantum yield of the fluorescence of IBU. According to Benesi-Hildebrand equation, the double reciprocal plots of 1/(F-F0) versus 1/[CD]0 is shown in Fig. 4. All of them exhibit good linearity, and imply that the inclusion complexes have a stoichiometry of 1:1. The formation constant (K) was obtained from the ratio of the intercept to the slope (Table 1). It can be seen from table 1 that the three CDs showed different inclusion capacity to IBU. The inclusion interaction of IBU with HP-β-CD was stronger than that of IBU with the other two CDs. The inclusion complex interaction, expressed by the formation constant, followed the order HP-β-CD > HE-β-CD > Me-β-CD.
Figure 3. Fluorescence spectra of IBU in the absence and presence of HP-β-CD. The concentration of HP-β-CD: 1-0 mol/L, 2-5×10-3 mol/L, 3-10×10-3 mol/L, 4-15×10-3 mol/L, 5-20×10-3 mol/L, 6-25×10-3 mol/L.
Figure 4. Double reciprocal plots for IBU in the presence of different CDs.
Table 1. The formation constants (K) of IBU with different CDs.
C. Effect of pH
The effect of pH on host-guest inclusion interaction mostly behaves that the conformations of guest are dissimilar at different pH values, namely the polarity of the guest changes. Figure 5 shows the double reciprocal plots of 1/(F-F0) versus 1/[CD]0 for IBU in the presence of HP-β-CD at different values of pH. All the plots exhibited good linearity. This implied that the formation of inclusion complexes with a stoichiometric ratio of 1:1 (HP-β-CD: IBU). It was also noted that the formation constants (Table 2) were very sensitive to the change of pH values. The inclusion interaction of HP-β-CD with IBU was in the order: pH2.0 > pH5.0 > pH6.0.
Figure 5. Double reciprocal plots for IBU in the presence of HP-β-CD at different pH values.
Table 2. The formation constants (K) of IBU/HP-β-CD at different pH values.
One of the major factors affecting the inclusion interaction is the hydrophobic degree of the guest, which is related to the form of IBU. The pKa of IBU is 5.2; so there exists following equilibrium in aqueous solution:
The dissociation constant for Eq. 3 can be described by
where HA and A- are neutral molecule and anion of IBU, respectively. In aqueous solution, HA exists in both states of neutral molecule and anion. If pH < 5.2, the neutral form of IBU is predominant; while pH > 5.2, the anion form of IBU is predominant. The normal HP-β-CD are not charged and the major inclusion interactions are hydrophobic interactions between the cyclodextrin cavity and the guest and hydrogen bonding of the guest to -OH groups or other introduced groups on the CD rings (Zhang et al., 2009). At the condition of pH 2.0, the neutral (uncharged) form of IBU is predominant, which is more hydrophobic than the anion form, so it is more easily to form the inclusion with HP-β-CD.
D. Phase solubility studies
The phase solubility diagram is a widely accepted method for evaluation of the effect of CD complexation on the guest solubility (Davis and Brewster, 2004). The diagram is obtained by measuring the concentration of guest in the presence of increasing concentration of cyclodextrins. Figure 6 shows the phase solubility diagrams of CDs with IBU. The stability constants KC were calculated from the straight-line portion of the phase solubility diagram and summarized in Table 3.
Figure 6. Phase-solubility profile for IBU in CDs.
Table 3. Stability constant (KC) of IBU-CDs complexes.
All three CDs were found to enhance water solubility of IBU and showed typical AL type diagram (Higuchi and Connors, 1965). But the solubilizing efficiency of IBU was different in different modified CD. HP-β-CD showed highest stability constant among the three CDs and was most efficient among the three CDs.
E. DSC spectra studies
Figure 7 represents the DSC results of four different types of samples: HP-β-CD alone, IBU alone, physical mixture and inclusion complex. The ibuprofen DSC scan demonstrated an endothermic peak at 78oC that corresponds to its melting point, which is 75-78oC for pure IBU. Similar value was seen for the physical mixture of IBU and HP-β-CD in Fig. 7 C. The DSC scan of the physical mixture was nearly identical to that of pure IBU, and also showed a strong endothermic peak at approximately 78oC, indicating that no inclusion occurred by this process. The DSC thermogram of HP-β-CD showed an weak endothermic peak at 128oC (Fig. 7 B), possibly due to elimination of the water. As it can be seen in Fig. 7 B and D, the thermograms of the HP-β-CD and of the inclusion complex didn't show any sharp endothermic peak in the temperature range investigated. The disappearance of the endothermic peak assigned to IBU at 78oC (Fig. 7 D) indicated that an inclusion complex between IBU and HP-β-CD was produced, not a simple physical mixture.
Figure 7. DSC thermograms of ibuprofen(A), HP-β-CD(B), physical mixture(C) and inclusion complex(D).
In this paper, fluorescence spectroscopy and phase-solubility give original support for the formation of complexes of IBU/CDs, while DSC analyses unequivocally demonstrate that the inclusion complex is formed. In addition, the following conclusions can be arrived at from the above studies:
(i) IBU forms 1:1 complex with CDs;
(ii) HP-β-CD is more suitable for including IBU than Me-β-CD and HE-β-CD;
(iii) Acidity solution is more suitable for IBU included by HP-β-CD;
(iv) The water solubility of IBU is increased by inclusion with CDs according to the phase-solubility diagram.
This work was supported by the Natural Science Foundation of Hunan (No: 06JJ4097).
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Received: December 9, 2009.
Accepted: March 29, 2010.
Recommended by Subject Editor Ricardo Gómez.