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Solubility enhancement of glibenclamide in choline–tryptophan ionic liquid: Preparation, characterization and mechanism of solubilization
Journal of Molecular Liquids 212 (2015) 629–634
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Solubility enhancement of glibenclamide in choline–tryptophan ionic liquid: Preparation, characterization and mechanism of solubilization Mahmoud A. Alawi a,⁎, Imad I. Hamdan b, ALSayed A. Sallam c, Najiah Abu Heshmeh a a b c
University of Jordan, Department of Chemistry, Amman 11942, Jordan University of Jordan, Faculty of Pharmacy, Amman 11942, Jordan TQ Pharma, Amman, Jordan
a r t i c l e
i n f o
Article history: Received 24 June 2015 Received in revised form 8 September 2015 Accepted 3 October 2015 Available online xxxx Keywords: Choline Tryptophan Ionic liquid Solubility Glibenclamide
a b s t r a c t A new ionic liquid solvent was prepared from choline as a cation and tryptophan as the anion. The produced ionic liquid was characterized by NMR, UV and HPLC. The obtained solvent was shown to form in a ratio of 1:1 (choline: tryptophan). The produced solvent was shown to increase the solubility of the low water solubility drug glibenclamide. The mechanism of the observed solubility enhancing effect of the drug was investigated and it did not seem to be simply due to increase in the pH of the medium. The most likely mechanism of increase in the solubility of glibenclamide (Glib) is the formation of complex hydrogen bonds and π–π interaction of the aromatic rings. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are salts composed of negative and positive ions and present as liquids at room temperature. Although any cation and anion might serve as components of ionic liquids most of the reported studies involved a bulky cation such as alkylpyridinium, tetraalkylammonium or tetraalkyl–phosphonium [1]. The most reported anions for ILs were based on sulfonates, hexafluorophosphates, and tetrafluoroborates [2]. In the last few years ILs were gaining increasing interests because of their peculiar properties and large potential uses. These include their use as solvents in much industrial process such as chemical synthesis, extraction, and chromatography and in chemical batteries [3]. Ionic liquids can exist with various ranges of properties such as viscosity (from low to highly viscous), polarity (low to high polarity), electrical conductivity (generally all have high conductivity), acidity and basicity (low to high) [4]. Although some recent studies have shown the favorable effects of ILs in the pharmaceutical fields, such as solubilizing agents, stabilizing agents and drug delivery modulators [5], their use in medical applications is still limited. The major reason behind that is believed to be concerns regarding their toxicity and biodegradability [6]. However, the toxicity issue might be overcome by choosing the cations and anions
⁎ Corresponding author. E-mail address: [email protected] (M.A. Alawi).
http://dx.doi.org/10.1016/j.molliq.2015.10.006 0167-7322/© 2015 Elsevier B.V. All rights reserved.
of the IL to be known as safe chemical (this is known as Generally Regarded Safe) [7]. A striking increase in the solubility of some drugs (compared to water) has been observed with the use of some ILs. In some cases 60 thousand times increase in solubility has been reported [8]. Drugs whose solubility was significantly improved by the use of ILs included acyclovir, albendazole and danzole [9]. Dissolution of a drug is a key step to its therapeutic effect; hence low water solubility of drugs affects their bioavailability and consequently their therapeutic efficacy [10]. Therefore, ILs might resolve the problem of many low water soluble drugs by working as excellent solvents [8]. In this paper we report on the preparation and characterization of an IL composed of choline as the cation component and the amino acid tryptophan as the anion component. Those ions were chosen so that if successful their use in human would be acceptable as their main components were generally regarded as safe [11]. Glib which is an antidiabetic drug and is known to be of low water solubility, was employed as a model drug in this study to test the solubilizing power of the prepared IL. Glib is a weak acidic drug (pka = 5.5) that is classified as a class II drug (low solubility, high permeability drugs), and has a molecular weight of 490 with a melting point of 173–175 °C [12–17]. Several approaches have been previously reported in attempts to improve the solubility of Glib, including in situ controlled crystallization [12], co-solvent solubilization [13], inclusion complexes with cyclodextrins [18] and raising of pH [19]. To the best of our knowledge there have been no reports on solubilizing Glib with the aid of ionic liquids.
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2. Experimental
water) were added gradually to both of sample and reference cells. After each addition, the mixture was properly mixed (using a micro pipette) and scans recorded, after 3 min, in the range 200–500 nm.
2.1. Chemicals HPLC Grade acetonitrile was obtained from Merck (Germany), choline hydroxide solution (45% w/w) from Acros (New Jersey/USA) and tryptophan (99% purity) were obtained from Fluka (Switzerland). Glib was obtained from Sharon bio-medicine (Mumbai/India). 2.2. Preparation and characterization of choline–tryptophan IL A solution of choline hydroxide (45% w/w) was added to an equivalent molar amount of amino acid (with slight excess of amino acid) in accordance to the equation in Fig. 1, the byproduct, water was evaporated at 40–50 °C using vacuum rotary evaporator. To the residue, 90 ml of acetonitrile and 10 ml of methanol were added to precipitate any excess un-reacted amino acid, and it was stirred vigorously. The mixture was then filtered to remove excess un-reacted amino acid. Filtrate was evaporated using rotary evaporator to remove solvents. Then liquid product was dried under vacuum for 2 days at 80 °C. 2.3. Equipment All UV measurements were made using a spectroscan 80D UV–Vis spectrophotometer, Biotech Engineering Management (UK). Conductivity measurements were carried out using WTW 315I (Germany) while pH measurements were carried out using Hanna/pH 211, (Germany) pH meter. Chromatographic analysis of Glib was carried out using Spectra System P4000 HPLC pump equipped with a Knauer PDA detector. The employed mobile phase consisted of 50% acetonitrile and 50% 25 mM phosphate buffer that was adjusted to pH 4.5 and the detection wavelength was set at 380 nm. The employed column was Ace C18 (100 mm × 4.6 mm × 3 μm). Nuclear magnetic resonance 1 H-NMR spectra was recorded on a Bruker AVANCE III-500 MHz spectrometer with TMS as internal standard. 2.4. Conductimetric, pH, UV measurements, NMR spectra, and titration curve of the IL Solutions containing increasing concentrations of IL (in water) were prepared in the range (9.76 × 10−2 − 50% w/w). Conductivity and pH of each solution were measured along with UV scans in the range 200–400 nm. Samples of IL (100 mg) in 50 ml of distilled water were titrated with 0.1 M HCl that was previously standardized against sodium carbonate. The acid was added in increments of 0.5 ml until no obvious change in pH was observed for five successive additions. Proton NMR spectra were obtained for IL alone (50 mg in 1.0 ml deutorated DMSO), Glib alone (20 mg in 1.0 ml deutorated DMSO), mixture of Glib 20 mg and IL 50 mg (in 1.0 ml deutorated DMSO), and for solutions containing increasing concentrations (0.03–50% w/w) of IL in deutorated D2O. 2.5. UV spectroscopic titration of IL with glib Aliquot (900 μl) of 0.25 M solution of IL in water, was placed in a micro-volume UV quartz cell (both blank and sample cells). Increments of 10 μl of 0.018 M Glib solution (in 0.25 M solution of IL in distilled
2.6. Measurement of solubility of glib in IL The solubility of Glib was measured in solutions containing increasing concentration of IL in water (0.1–50 g %) as well as in phosphate buffers (50 mM) having different pH values in the range 6–12. Excess amount of Glib was added to 1 ml of the relevant phosphate buffer, or IL solutions until no further material could be dissolved. Samples were placed on a shaker water bath (37 °C) for 48 h, centrifuged for 10 min, and 100 μl of the supernatant were transferred to a new sample vial and 900 μl of mobile phase were added in order to bring about ten times dilution of the sample. The diluted sample was injected onto HPLC where the method described by British pharmacopeia (2007) for determination of Glib was adopted. The method employed a mobile phase consisting of 50% of phosphate buffer and 50% acetonitrile at flow rate 1 ml/min and pH 4.5, with UV detection at 380 nm. A calibration curve was constructed using Glib solutions of increasing concentrations in the range (0.0625–0.500 mg/ml) and a linear equation with a correlation coefficient of 0.998 could be obtained. A typical linear equation, that was employed to determine the concentration of Glib in solubility samples, could be given by: A = 1119.6 X + 8.39 where A is the peak area and X is the concentration in mg/ml). 3. Results and discussion A yellowish viscous ionic liquid substance was obtained. The formation of IL between choline and tryptophan was evidenced through different techniques such as NMR, pH measurements UV and conductimetric measurements. NMR spectra of the obtained IL showed disappearance of the acidic proton of tryptophan at about 11.1 ppm (Fig. 2) as also reported previously [20] which indicate the formation of IL through ionic attraction between negative and positive charges of tryptophan and choline respectively. The obtained NMR spectrum for the IL (Fig. 2) showed a distinct singlet at 3.1 ppm that belongs to the 9 methyl protons of choline and five separate signals in the aromatic region 6.8–7.6 ppm (two triplets, one singlet and two doublets) which essentially belong to the five aromatic protons of tryptophan. Since the ratio of the peak integration for the methyl singlet to any individual signal in the aromatic proton region was 9:1; it could be concluded that the stoichiometry of the formed ionic liquid was 1:1. A HPLC method was developed in order to enable measurement of the IL in aqueous phase and consequent determination of partition coefficient. Calibration curve was constructed by plotting peak area against concentration in the range 0.08–20 g %. However, the resulting calibration plot showed two linear phases with the initial phase (at low concentration range; 0.08–1.5 g %) having a slope almost zero. At higher concentration range (2.5–20 g %) the relationship was linear and could be expressed by the equation: A = 164 X + 4.0, where A is the peak area of the IL and X is its concentration (g %). The intersection point between the two phases correspond to ~ 1.7 g %, which suggests that the ionic liquid exits as an ion pair at concentrations higher than 1.7 g % but dissociates to individual ions at concentrations less than that. However, the linear part (in the range 2.5–20 g %) was employed to calculate the concentration of the IL in
Fig. 1. Reaction equation for choline–tryptophan ionic liquid.
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Fig. 4. Plot of conductivity expressed as μS/cm (♦) and pH of solution (■) against concentration of IL in g %.
Fig. 2. 1H NMR of choline–tryptophan ionic liquid (A), choline (B) and tryptophan (C).
the aqueous phase. Accordingly, partition coefficient (Octanol/water) was determined, and found to be − 1.01 (RSD = 2.6%, N = 3), which suggests rather hydrophilic properties. Aqueous solutions of IL exhibited significantly alkaline pH values (8–13) which is attributed to the basic amino group of tryptophan which was not involved in the IL formation. In order to further characterize the obtained IL, different properties (UV absorption, pH and conductivity and NMR spectra) were measured for IL at different concentrations in water. Fig. 3A shows overlaid UV spectra for increasing concentration of IL in water. Within the studied concentration range the absorbance varied, mostly, in the range 300–400 nm. Plot of absorbance at 380 nm against concentration resulted in Fig. 3B. The plot in Fig. 3B is characterized by a linear part with a slope of 0.0918 g %−1 cm−1 which represents the absorptivity coefficient. However, at low concentrations there was almost no change in absorbance (Δ absorbance/Δ concentration) up to about 1 g %. Given the fact that choline by itself has no absorption in that region and tryptophan has only little absorption, it could be that the break in the plot in Fig. 3B reflects the transition from individual species (tryptophan and choline) to the ion pair form. Accordingly at concentrations lower than 1 g %, the IL is not expected to retain its properties as an ion pair including solubilization effect, which accords with the results obtained by HPLC. Fig. 4 represents plots of both pH and conductivity as a function of IL concentration. It is generally accepted that conductivity versus concentration plots for ionic species are linear over a wide range of concentrations unless reactions (association/dissociation) takes place [21]. Therefore, the break (at about 1.8 g %) in the slope of the otherwise
linear conductivity plot (Fig. 4) could be interpreted as a reflection of the dissociation of the IL into two individual species at concentrations lower than ~ 1.8 g %. These observations accord and further support that obtained by UV and HPLC discussed earlier. The plot for pH versus concentration of IL also showed a break in the pH/concentration plot, similar to that seen for the conductivity plot, though at lower concentration ~0.5 g %. Moreover NMR spectra showed shifts in the resonances of some of the protons as the concentration of the IL was changed (Fig. 5). Plot of the obtained resonances of selected protons as a function of IL concentration is shown in Fig. 6. The observed concentration dependency of chemical shifts in Figs. 5 and 6 indicate that associations occur between the molecules of the IL as their concentration is increased in water (D2O) leading to formation of dimmers or potentially other forms of associations or complexes. Although an NMR spectral shift of an exchangeable proton or any proton in different solvents is well known; the phenomenon of concentration dependent NMR chemical shifts, of non exchangeable protons, in the same solvent, has been reported for the first time in 1998 [22]. Since then, similar observations have been reported for some molecules including quinoline derivatives [23], chlorhexidine [24], pyridylalkanols [25], with typical values for the observed shift 0.2–0.4 on δ scale. In all cases the phenomenon was explained in terms of formation of dimmers or higher order structures of the molecules (stacking of molecules) as the concentration is increased. Therefore, it could be concluded that the prepared IL molecules associate through dipole–dipole interaction to form aggregates or complexes as their concentration is increased. 3.1. Solubility of Glib in IL Solubility of Glib was measured in choline–tryptophan ionic liquid (choline–tryptophan) using the HPLC method recommended by the British Pharmacopea [26]. A dramatic increase in solubility was obtained, it was even not possible to determine the exact value of Glib solubility in the neat choline–tryptophan because it exceeded several
Fig. 3. (A) Overlaid UV spectra of increasing concentration (0.1–12.5 g %) of IL in water, (B) plot of absorbance at 380 nm against concentration of IL (g %).
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Fig. 5. NMR spectra for decreasing concentrations of IL in D2O (A = 50%, B = 3.125% and C = 0.0313%).
folds the upper concentration limit of the calibration curve, but it was certainly much higher than 160 mg/ml. The solubility of Glib was studied also in aqueous solutions containing different percentages of choline–tryptophan (0.0195 g %–1.25 g %). Solubility of Glib was found to increase dramatically with increasing the concentration of the IL in water (Fig. 7), but even at the lowest concentration of IL studied the solubility value was 2.7 mg/ml which is around 130–600 times higher than the reported values for the solubility of Glib in water (15–24 μg/ml)[14–16]. At the highest concentration of the IL (6.5 g %) the solubility of Glib was found to be 9.89 mg/ml which is almost 400–2000 times the reported values for aqueous Glib solubility. The break in the solubility of Glib versus concentration of IL is interpreted as a result of dissociation of the IL to individual ions at lower concentration. The concentration that corresponds to the break in the curve (~1 g %) which again accords with its counterparts in the conductivity and absorbance (at 380 nm) versus concentration plots (~ 1.5 and
Fig. 6. Plot of chemical shifts of aromatic proton (tryptophan) and the methyl protons (choline) of the IL as a function of its concentration in D2O.
1.0 g % respectively) which further supports that the IL maintains its ion pair form at high concentrations (N ~ 1.5%) and dissociates into individual ions at lower concentrations. These findings are important because if the IL is to be used as a solubilizer in marketed drugs then questions would be raised regarding its safety. If it is shown that IL dissociates at low concentrations to individual ions, which are known to be safe naturally occurring biomolecules (tryptophan and choline) then that would ensure the safety of the prepared IL. Therefore at concentrations less than 1.5 g % of IL its use can be expected to be safe as it dissociates to choline and tryptophan. At first one might think that the observed dramatic increase in solubility of Glib was simply a result of increase in the pH of the medium and the consequent dissociation of the acidic Glib. The pH values of the aqueous solutions of IL which were used to study the solubility of Glib were all basic with pH values (8–12.5) significantly higher than the reported pKa value for Glib of 5.5 [17]. Thus it is unlikely that the observed increase in solubility is attributed to dissociation of Glib imposed by the increase in pH, since Glib is almost completely ionized over the entire range of the relevant pH values (8.0–12.5). Furthermore, the solubility of Glib was determined in different aqueous buffer solutions (without IL) at different pH values in the range 6–12 i.e. pH solubility profile was obtained (Fig. 8). The results showed that the solubility of Glib increased from 0.028 mg/ml (at pH 6) to 0.8 mg/ml at pH 12. However the solubility of Glib in IL at pH ~12 was much higher (~ 9 mg/ml) and it was about 10 times higher than its value in simple buffer of the same pH-value. Thus the pH alone can't be accepted as the sole mechanism of increasing solubility of Glib when IL is employed. In order to explore the mechanism by which IL exhibited such a dramatic effect on solubility enhancement of Glib, the potential of Glib to form some kind of associations with the IL, was investigated. Therefore, NMR spectra were obtained for a sample of Glib together with IL,
Fig. 7. Solubility of Glibenclamide in aqueous solutions of increasing IL-concentrations.
Fig. 8. Solubility profile of Glib as a function of pH of the medium.
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Table 1 Detailed peak assignment and comparison for Glib NMR spectra in presence and absence of IL. Tryptophane–choline IL
Glibenclamide alone
IL and glibenclamide
Notes
1.4, br sing (2H), trypto, NH2
1.08 and 1.51, m, 10H (CH2 2″ + 3″ + 4″ + 5″ + 6″)
0.98–1.52, m, (10H) Hexane of glib 2.46, s, unknown 2.61, q, 1H, (tryp 9) 2.8, t, 2H, (Glib 10) 3.05, s, 9H, (Chol methyl) 3.13, m, 3H, (trypt NH, +CH 1″)
No obvious change in delta but change in Split pattern
2.46, sing (1H) Chol, OH 2.51, quart(1H) Trypto, 9 2.89, t, 1H (CH2 10) 3.05, sing shar (9H), Chol methyl 3.25, mult over, trypto, NH
3.24, m, 1H (CH 1″)
3.34, trip (2H) chol, 1.
3.34, water 3.49, m, 2H (CH2 9)
3.38, trip over, (2H) chol, 2.
3.75, S, 3H(2-OMe) 3.79, doublet (2H) tryp 8, 4.5, s, br, water 6.3, d, 1H(NH 9′) 6.86, trip (1H) tryp 5. 6.95, trip (1H) tryp 4. 7.1, sing (1H) tryp 1.
3.36, m, 3H, (Chol 1 + Glib CH2 9) 3.4, m, 3H, (Chol 2 + Glib CH2 9) 3.65, overlab, water 3.75, s, 3H, (Glib OMe) 3.79, m, 2H, (Trypt 8) 5.6, s, br, 1H (Glib NH 9’)
7.1, d, 1H (CHar 3) 7.45, m, 3H, (CHar 4 + 3′ + 5′)
7.3, doublet (1H) tryp 6. 7.5, doublet (1H) tryp 3. 7.59, d, 1H (Char 6) 7.78, m, 2H(Char 2′ +6′) 8.22, t, 1H (NH 8) 10.29, S (vbr), 1H (NH 7′)
Increase in freq. in presence of IL Slight decrease in presence of IL No change Decrease in delta ~0.1 Typical value for water No obvious change No obvious change Addition of Glib appeared to bring the signal closer to its typical value (3.33) No significant change No significant change Very much higher than typical value (3.33) Very obvious decrease in frequency together with becoming broader and singlet instead of doublet. No signif change No signif change No significant change
6.89, t, 1H (tryptar 5) 6.99, t, 1H (tryptar 4) 7.08, s + 7.12; s, 2H, (trypt 1 + Glib 3ar) 7.19, d, 1H, (G3′ +5′) 7.44, d, 1H, (Glib 4ar) 7.30, d, 1H, (tryptar 6) 7.52, d, 1H, (trypar 3) 7.63, m, 1H, (Glib 6, 2′, 6′)
Significant decrease in 3′ and 5′ of Glib No significant change No significant change No significant change Significant decrease for the protons 2′ and 6′
8.22, t, 1H, (Glib NH8) 11.10, s, 1H,(Glib NH7′)
No significant change Significant increase in the frequency, and it becomes sharper
dissolved in DMSO, along with spectra for each of IL and Glib alone. The obtained spectra for both Glib and tryptophan were in good agreement with previously published reports [27,20], which confirmed assignments of all protons. The obtained results clearly indicated distinct shifts in the NMR spectrum of Glib in presence of IL as compared to Glib alone. A detailed description of signals and their assignments are presented in Table 1. According to Table 1, the most significantly altered resonances were those of Glib aromatic protons (2′, 6′, 3′, 5′), the sulfonamide proton (N7′) and the amide proton (N9′). Therefore, it could be concluded that Glib associates with the IL at that end of the molecule (aromatic sulfonamide) through the formation of multiple hydrogen bonds between IL and Glib in addition to π–π interaction through the aromatic rings between the two molecules. A proposed structure describing the interaction between the two molecules is presented in Fig. 9. It is noteworthy that the protons of water appeared at significantly higher δ (4.5) than usual in the spectrum of IL alone [28] which suggest
extensive involvement of hydrogen bonding with IL molecules. However, as Glib was added to IL the frequency of water came back to near normal (3.65) suggesting the replacement of water molecules by Glib. Therefore, it could be concluded that Glib slides between the stacked IL molecules through formation of hydrogen bonds and π–π interaction. Further evidence on the interaction (association between Glib and IL) came from UV spectroscopic titration. A sample of IL was titrated with Glib and the obtained overlaid absorption spectra are presented in Fig. 10. As Glib was added to both sample and blank cells, a characteristic shift in the absorption spectrum (general decrease) was observed in the range of 300–350 nm with an isosbestic point at about 338 nm. A plot of absorbance at λmax (380 nm) produced a decreasing linear plot with an equation: absorbance = −614 C + 2.28 with a correlation coefficient = 0.998, where C is the molar concentration of Glib. This trend should not have been observed unless Glib interacts with the IL which further supports NMR results.
Fig. 9. Proposed structure for the association between Glib and IL.
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Fig. 10. Overlaid UV spectra for IL titrated with glibenclamide.
4. Conclusions An IL was prepared from choline as a cation and tryptophan as an anion. The IL was extensively characterized by NMR, and UV techniques. The ability of IL to solubilize the antidiabetic drug Glib was demonstrated were it was increased several folds. Attempts were made to understand the mechanism of solubilization of Glib in IL employing NMR, UV and HPLC. Results indicated the interaction of Glib with IL to form certain types of complexes through hydrogen bonds and π–π interaction of the aromatic rings. The produced IL may be utilized clinically to improve the solubility of the poorly water soluble drug Glib since it exhibited very high solubility enhancing effect, yet water miscible and most likely safe because it dissociates in dilute solutions to the acceptably known as safe compounds: tryptophan and choline. Acknowledgment The authors would like to thank the Deanship for academic research at the University of Jordan (51/2011-2012) for the financial support. References [1] R. Liu, J.F. Liu, Y.G. Yin, X.L. Hu, G.B. Jiang, Ionic liquids in sample preparation, Anal. Bioanal. Chem. 393 (2009) 871–883.
[2] M. Kosmulski, K. Marczewska-Boczkowska, P. Zukowski, J. Subocz, C. Saneluta, Permittivities of 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates, Croat. Chem. Acta 80 (3–4) (2007) 461–466. [3] S. Keskin, D. Kayrak-Talay, U. Akman, O. Hortacsu, A review of ionic liquids towards supercritical fluid applications, J. Supercrit. Fluids 43 (2007) 150–180. [4] S. Gitanjali, A. Kumar, Ionic liquids: physico-chemical, solvent properties and their applications in chemical processes, Indian J. Chem. 47A (2008) 495–503. [5] J. Stoimenovski, D.R. MacFarlane, K. Bica, R.D. Rogers, Crystalline vs. ionic liquid salt forms of active pharmaceutical ingredients: a position paper, Pharm. Res. (2010). [6] D.J.C. Constable, et al., Green Chem. (2007), http://dx.doi.org/10.1039/b703488c. [7] USDA Nutrients database, http://www.nal.usda.gov/fnic/foodcomp/search Accessed Dec 7, 2011. [8] H. Mizuuchi, V. Jaitely, S. Murdan, A.T. Florence, Room temperature ionic liquids and their mixtures: potential pharmaceutical solvents, Eur. J. Pharm. Sci. 33 (2008) 326–331. [9] K.B. Smith, R.H. Bridson, G.A. Leeke, Solubilities of pharmaceutical compounds in ionic liquids, J. Chem. Eng. Data 56 (5) (2011) 2039–2043. [10] A. Chaudhary, U. Upendra Nagaich, N. Gulati, V.K. Sharma, R.L. Khosa, Enhancement of solubilization and bioavailability of poorly soluble drugs by physical and chemical modifications: a recent review, J. Adv. Pharm. Educ. Res. 2 (1) (2012) 32–67. [11] T.K. Chung, H.B. Gelberg, J.L. Dorner, D.H. Baker, Safety of L-tryptophan for pigs, J. Anim. Sci. 69 (1991) 2955–2960. [12] A.A. Elkordy, A. Jatto, E. Essa, In situ controlled crystallization as a tool to improve the dissolution of glibenclamide, Int. J. Pharm. 428 (2012) 118–120. [13] N. Seedher, M. Kanojia, Co-solvent solubilization of some poorly-soluble anti-diabetic drugs, Pharm. Dev. Technol. 14 (2009) 185–192. [14] A.A. Abd Elbary, H.F. Salem, M.E. Maher, In vitro and in vivo evaluation of glibenclamide using surface solid dispersion (SSD) approach, Br. J. Pharmacol. Toxicol. 2 (1) (2011) 51–62. [15] A.A. Elkordy, J. Ayobami, E. Essa, In situ controlled crystallization, as a tool to the dissolution of glibenclamide, Int. J. Pharm. 428 (2012) 118–120. [16] M. Remko, Theoretical study of molecular structure, pK(a), lipophilicity, solubility, absorption, and polar surface area of some hypoglycemic agents, J. Mol. Struct. THEOCHEM 897 (2009) 63–82. [17] A.C. Moffat, J.V. Jackson, M.S. Moss, B. Widdop, Clark's Isolation and Identification of Drugs, The Pharmaceutical Press, London, 1986. [18] R.J. Babu, J.K. Pandit, Enhancement of dissolution rate and hypoglycemic activity of glibenclamide with beta-cyclodextrine, STP Pharma. Sci. 5 (1995) 196–201. [19] P. Lehto, K. Laine, K.T. Kivisto, K.T. Kivisto, P.J. Neuvonen, The effect of pH on the invitro dissolution of three second-generation sulphonylurea preparations: mechanism of antacid–sulphonylurea interaction, J. Pharm. Pharmacol. 48 (1996) 899–901. [20] E. Boyle-Roden, N. Hofer, K.K. Dey, P.J. Grandinetti, M. Caffrey, High resolution 1H NMR of a lipid cubic phase using a solution NMR probe, J. Magn. Reson. 189 (2007) 13–19. [21] G.H. Jeffery, J. Basset, J. Mendham, R.C. Denney, Vogel's Textbook of Quantitative Chemical Analysis, Sixth ed., 1989 (Amazon.com). [22] A. Mitra, P.J. Seaton, T. Williamson, Unprecedented concentration dependent chemical shift variation in 1H-NMR studies: a caveat in the investigations of molecular recognition and structure elucidation, Tetrahedron 54 (1998) 15489–15498. [23] A. Beck, Concentration Dependent 1H-NMR Chemical Shifts of Quinoline Derivatives , (A Master of Science) University of North Carolina, 2007. [24] P. Zeng, G. Zhang, A. Rao, W. Bowle, S. Wiedmann, Concentration dependent aggregation properties of chlorhexidine salts, Int. J. Pharm. 367 (2009) 73–78. [25] J.S. Lomas, A. Adenier, C. Cordier, Proton NMR and IR study of selfassociation in pyridylalkanols: open or cyclic dimers? Higher polymers? J. Phys. Org. Chem. 19 (2006) 295–307. [26] British Pharmacopoeia, Volume 1 and 2. Glibenclamide (and Tablet) Monograph, The Stationary Office, London, 2009. [27] D. Sanz, R.M. Claramunt, I. Alkorta, G. Sánchez-Sanz, J. Elguero, The structure of glibenclamide in the solid stat, Magn. Reson. Chem. 50 (3) (2012) 246–255. [28] H.E. Gottlieb, V. Kotlyar, A. Nudelman, NMR chemical shifts of common laboratory solvents as trace impurities, J. Org. Chem. 62 (1997) 7512–7515.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Solubility enhancement of glibenclamide in choline–tryptophan ionic liquid: Preparation, characterization and mechanism of solubilization Mahmoud A. Alawi a,⁎, Imad I. Hamdan b, ALSayed A. Sallam c, Najiah Abu Heshmeh a a b c
University of Jordan, Department of Chemistry, Amman 11942, Jordan University of Jordan, Faculty of Pharmacy, Amman 11942, Jordan TQ Pharma, Amman, Jordan
a r t i c l e
i n f o
Article history: Received 24 June 2015 Received in revised form 8 September 2015 Accepted 3 October 2015 Available online xxxx Keywords: Choline Tryptophan Ionic liquid Solubility Glibenclamide
a b s t r a c t A new ionic liquid solvent was prepared from choline as a cation and tryptophan as the anion. The produced ionic liquid was characterized by NMR, UV and HPLC. The obtained solvent was shown to form in a ratio of 1:1 (choline: tryptophan). The produced solvent was shown to increase the solubility of the low water solubility drug glibenclamide. The mechanism of the observed solubility enhancing effect of the drug was investigated and it did not seem to be simply due to increase in the pH of the medium. The most likely mechanism of increase in the solubility of glibenclamide (Glib) is the formation of complex hydrogen bonds and π–π interaction of the aromatic rings. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are salts composed of negative and positive ions and present as liquids at room temperature. Although any cation and anion might serve as components of ionic liquids most of the reported studies involved a bulky cation such as alkylpyridinium, tetraalkylammonium or tetraalkyl–phosphonium [1]. The most reported anions for ILs were based on sulfonates, hexafluorophosphates, and tetrafluoroborates [2]. In the last few years ILs were gaining increasing interests because of their peculiar properties and large potential uses. These include their use as solvents in much industrial process such as chemical synthesis, extraction, and chromatography and in chemical batteries [3]. Ionic liquids can exist with various ranges of properties such as viscosity (from low to highly viscous), polarity (low to high polarity), electrical conductivity (generally all have high conductivity), acidity and basicity (low to high) [4]. Although some recent studies have shown the favorable effects of ILs in the pharmaceutical fields, such as solubilizing agents, stabilizing agents and drug delivery modulators [5], their use in medical applications is still limited. The major reason behind that is believed to be concerns regarding their toxicity and biodegradability [6]. However, the toxicity issue might be overcome by choosing the cations and anions
⁎ Corresponding author. E-mail address: [email protected] (M.A. Alawi).
http://dx.doi.org/10.1016/j.molliq.2015.10.006 0167-7322/© 2015 Elsevier B.V. All rights reserved.
of the IL to be known as safe chemical (this is known as Generally Regarded Safe) [7]. A striking increase in the solubility of some drugs (compared to water) has been observed with the use of some ILs. In some cases 60 thousand times increase in solubility has been reported [8]. Drugs whose solubility was significantly improved by the use of ILs included acyclovir, albendazole and danzole [9]. Dissolution of a drug is a key step to its therapeutic effect; hence low water solubility of drugs affects their bioavailability and consequently their therapeutic efficacy [10]. Therefore, ILs might resolve the problem of many low water soluble drugs by working as excellent solvents [8]. In this paper we report on the preparation and characterization of an IL composed of choline as the cation component and the amino acid tryptophan as the anion component. Those ions were chosen so that if successful their use in human would be acceptable as their main components were generally regarded as safe [11]. Glib which is an antidiabetic drug and is known to be of low water solubility, was employed as a model drug in this study to test the solubilizing power of the prepared IL. Glib is a weak acidic drug (pka = 5.5) that is classified as a class II drug (low solubility, high permeability drugs), and has a molecular weight of 490 with a melting point of 173–175 °C [12–17]. Several approaches have been previously reported in attempts to improve the solubility of Glib, including in situ controlled crystallization [12], co-solvent solubilization [13], inclusion complexes with cyclodextrins [18] and raising of pH [19]. To the best of our knowledge there have been no reports on solubilizing Glib with the aid of ionic liquids.
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2. Experimental
water) were added gradually to both of sample and reference cells. After each addition, the mixture was properly mixed (using a micro pipette) and scans recorded, after 3 min, in the range 200–500 nm.
2.1. Chemicals HPLC Grade acetonitrile was obtained from Merck (Germany), choline hydroxide solution (45% w/w) from Acros (New Jersey/USA) and tryptophan (99% purity) were obtained from Fluka (Switzerland). Glib was obtained from Sharon bio-medicine (Mumbai/India). 2.2. Preparation and characterization of choline–tryptophan IL A solution of choline hydroxide (45% w/w) was added to an equivalent molar amount of amino acid (with slight excess of amino acid) in accordance to the equation in Fig. 1, the byproduct, water was evaporated at 40–50 °C using vacuum rotary evaporator. To the residue, 90 ml of acetonitrile and 10 ml of methanol were added to precipitate any excess un-reacted amino acid, and it was stirred vigorously. The mixture was then filtered to remove excess un-reacted amino acid. Filtrate was evaporated using rotary evaporator to remove solvents. Then liquid product was dried under vacuum for 2 days at 80 °C. 2.3. Equipment All UV measurements were made using a spectroscan 80D UV–Vis spectrophotometer, Biotech Engineering Management (UK). Conductivity measurements were carried out using WTW 315I (Germany) while pH measurements were carried out using Hanna/pH 211, (Germany) pH meter. Chromatographic analysis of Glib was carried out using Spectra System P4000 HPLC pump equipped with a Knauer PDA detector. The employed mobile phase consisted of 50% acetonitrile and 50% 25 mM phosphate buffer that was adjusted to pH 4.5 and the detection wavelength was set at 380 nm. The employed column was Ace C18 (100 mm × 4.6 mm × 3 μm). Nuclear magnetic resonance 1 H-NMR spectra was recorded on a Bruker AVANCE III-500 MHz spectrometer with TMS as internal standard. 2.4. Conductimetric, pH, UV measurements, NMR spectra, and titration curve of the IL Solutions containing increasing concentrations of IL (in water) were prepared in the range (9.76 × 10−2 − 50% w/w). Conductivity and pH of each solution were measured along with UV scans in the range 200–400 nm. Samples of IL (100 mg) in 50 ml of distilled water were titrated with 0.1 M HCl that was previously standardized against sodium carbonate. The acid was added in increments of 0.5 ml until no obvious change in pH was observed for five successive additions. Proton NMR spectra were obtained for IL alone (50 mg in 1.0 ml deutorated DMSO), Glib alone (20 mg in 1.0 ml deutorated DMSO), mixture of Glib 20 mg and IL 50 mg (in 1.0 ml deutorated DMSO), and for solutions containing increasing concentrations (0.03–50% w/w) of IL in deutorated D2O. 2.5. UV spectroscopic titration of IL with glib Aliquot (900 μl) of 0.25 M solution of IL in water, was placed in a micro-volume UV quartz cell (both blank and sample cells). Increments of 10 μl of 0.018 M Glib solution (in 0.25 M solution of IL in distilled
2.6. Measurement of solubility of glib in IL The solubility of Glib was measured in solutions containing increasing concentration of IL in water (0.1–50 g %) as well as in phosphate buffers (50 mM) having different pH values in the range 6–12. Excess amount of Glib was added to 1 ml of the relevant phosphate buffer, or IL solutions until no further material could be dissolved. Samples were placed on a shaker water bath (37 °C) for 48 h, centrifuged for 10 min, and 100 μl of the supernatant were transferred to a new sample vial and 900 μl of mobile phase were added in order to bring about ten times dilution of the sample. The diluted sample was injected onto HPLC where the method described by British pharmacopeia (2007) for determination of Glib was adopted. The method employed a mobile phase consisting of 50% of phosphate buffer and 50% acetonitrile at flow rate 1 ml/min and pH 4.5, with UV detection at 380 nm. A calibration curve was constructed using Glib solutions of increasing concentrations in the range (0.0625–0.500 mg/ml) and a linear equation with a correlation coefficient of 0.998 could be obtained. A typical linear equation, that was employed to determine the concentration of Glib in solubility samples, could be given by: A = 1119.6 X + 8.39 where A is the peak area and X is the concentration in mg/ml). 3. Results and discussion A yellowish viscous ionic liquid substance was obtained. The formation of IL between choline and tryptophan was evidenced through different techniques such as NMR, pH measurements UV and conductimetric measurements. NMR spectra of the obtained IL showed disappearance of the acidic proton of tryptophan at about 11.1 ppm (Fig. 2) as also reported previously [20] which indicate the formation of IL through ionic attraction between negative and positive charges of tryptophan and choline respectively. The obtained NMR spectrum for the IL (Fig. 2) showed a distinct singlet at 3.1 ppm that belongs to the 9 methyl protons of choline and five separate signals in the aromatic region 6.8–7.6 ppm (two triplets, one singlet and two doublets) which essentially belong to the five aromatic protons of tryptophan. Since the ratio of the peak integration for the methyl singlet to any individual signal in the aromatic proton region was 9:1; it could be concluded that the stoichiometry of the formed ionic liquid was 1:1. A HPLC method was developed in order to enable measurement of the IL in aqueous phase and consequent determination of partition coefficient. Calibration curve was constructed by plotting peak area against concentration in the range 0.08–20 g %. However, the resulting calibration plot showed two linear phases with the initial phase (at low concentration range; 0.08–1.5 g %) having a slope almost zero. At higher concentration range (2.5–20 g %) the relationship was linear and could be expressed by the equation: A = 164 X + 4.0, where A is the peak area of the IL and X is its concentration (g %). The intersection point between the two phases correspond to ~ 1.7 g %, which suggests that the ionic liquid exits as an ion pair at concentrations higher than 1.7 g % but dissociates to individual ions at concentrations less than that. However, the linear part (in the range 2.5–20 g %) was employed to calculate the concentration of the IL in
Fig. 1. Reaction equation for choline–tryptophan ionic liquid.
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Fig. 4. Plot of conductivity expressed as μS/cm (♦) and pH of solution (■) against concentration of IL in g %.
Fig. 2. 1H NMR of choline–tryptophan ionic liquid (A), choline (B) and tryptophan (C).
the aqueous phase. Accordingly, partition coefficient (Octanol/water) was determined, and found to be − 1.01 (RSD = 2.6%, N = 3), which suggests rather hydrophilic properties. Aqueous solutions of IL exhibited significantly alkaline pH values (8–13) which is attributed to the basic amino group of tryptophan which was not involved in the IL formation. In order to further characterize the obtained IL, different properties (UV absorption, pH and conductivity and NMR spectra) were measured for IL at different concentrations in water. Fig. 3A shows overlaid UV spectra for increasing concentration of IL in water. Within the studied concentration range the absorbance varied, mostly, in the range 300–400 nm. Plot of absorbance at 380 nm against concentration resulted in Fig. 3B. The plot in Fig. 3B is characterized by a linear part with a slope of 0.0918 g %−1 cm−1 which represents the absorptivity coefficient. However, at low concentrations there was almost no change in absorbance (Δ absorbance/Δ concentration) up to about 1 g %. Given the fact that choline by itself has no absorption in that region and tryptophan has only little absorption, it could be that the break in the plot in Fig. 3B reflects the transition from individual species (tryptophan and choline) to the ion pair form. Accordingly at concentrations lower than 1 g %, the IL is not expected to retain its properties as an ion pair including solubilization effect, which accords with the results obtained by HPLC. Fig. 4 represents plots of both pH and conductivity as a function of IL concentration. It is generally accepted that conductivity versus concentration plots for ionic species are linear over a wide range of concentrations unless reactions (association/dissociation) takes place [21]. Therefore, the break (at about 1.8 g %) in the slope of the otherwise
linear conductivity plot (Fig. 4) could be interpreted as a reflection of the dissociation of the IL into two individual species at concentrations lower than ~ 1.8 g %. These observations accord and further support that obtained by UV and HPLC discussed earlier. The plot for pH versus concentration of IL also showed a break in the pH/concentration plot, similar to that seen for the conductivity plot, though at lower concentration ~0.5 g %. Moreover NMR spectra showed shifts in the resonances of some of the protons as the concentration of the IL was changed (Fig. 5). Plot of the obtained resonances of selected protons as a function of IL concentration is shown in Fig. 6. The observed concentration dependency of chemical shifts in Figs. 5 and 6 indicate that associations occur between the molecules of the IL as their concentration is increased in water (D2O) leading to formation of dimmers or potentially other forms of associations or complexes. Although an NMR spectral shift of an exchangeable proton or any proton in different solvents is well known; the phenomenon of concentration dependent NMR chemical shifts, of non exchangeable protons, in the same solvent, has been reported for the first time in 1998 [22]. Since then, similar observations have been reported for some molecules including quinoline derivatives [23], chlorhexidine [24], pyridylalkanols [25], with typical values for the observed shift 0.2–0.4 on δ scale. In all cases the phenomenon was explained in terms of formation of dimmers or higher order structures of the molecules (stacking of molecules) as the concentration is increased. Therefore, it could be concluded that the prepared IL molecules associate through dipole–dipole interaction to form aggregates or complexes as their concentration is increased. 3.1. Solubility of Glib in IL Solubility of Glib was measured in choline–tryptophan ionic liquid (choline–tryptophan) using the HPLC method recommended by the British Pharmacopea [26]. A dramatic increase in solubility was obtained, it was even not possible to determine the exact value of Glib solubility in the neat choline–tryptophan because it exceeded several
Fig. 3. (A) Overlaid UV spectra of increasing concentration (0.1–12.5 g %) of IL in water, (B) plot of absorbance at 380 nm against concentration of IL (g %).
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Fig. 5. NMR spectra for decreasing concentrations of IL in D2O (A = 50%, B = 3.125% and C = 0.0313%).
folds the upper concentration limit of the calibration curve, but it was certainly much higher than 160 mg/ml. The solubility of Glib was studied also in aqueous solutions containing different percentages of choline–tryptophan (0.0195 g %–1.25 g %). Solubility of Glib was found to increase dramatically with increasing the concentration of the IL in water (Fig. 7), but even at the lowest concentration of IL studied the solubility value was 2.7 mg/ml which is around 130–600 times higher than the reported values for the solubility of Glib in water (15–24 μg/ml)[14–16]. At the highest concentration of the IL (6.5 g %) the solubility of Glib was found to be 9.89 mg/ml which is almost 400–2000 times the reported values for aqueous Glib solubility. The break in the solubility of Glib versus concentration of IL is interpreted as a result of dissociation of the IL to individual ions at lower concentration. The concentration that corresponds to the break in the curve (~1 g %) which again accords with its counterparts in the conductivity and absorbance (at 380 nm) versus concentration plots (~ 1.5 and
Fig. 6. Plot of chemical shifts of aromatic proton (tryptophan) and the methyl protons (choline) of the IL as a function of its concentration in D2O.
1.0 g % respectively) which further supports that the IL maintains its ion pair form at high concentrations (N ~ 1.5%) and dissociates into individual ions at lower concentrations. These findings are important because if the IL is to be used as a solubilizer in marketed drugs then questions would be raised regarding its safety. If it is shown that IL dissociates at low concentrations to individual ions, which are known to be safe naturally occurring biomolecules (tryptophan and choline) then that would ensure the safety of the prepared IL. Therefore at concentrations less than 1.5 g % of IL its use can be expected to be safe as it dissociates to choline and tryptophan. At first one might think that the observed dramatic increase in solubility of Glib was simply a result of increase in the pH of the medium and the consequent dissociation of the acidic Glib. The pH values of the aqueous solutions of IL which were used to study the solubility of Glib were all basic with pH values (8–12.5) significantly higher than the reported pKa value for Glib of 5.5 [17]. Thus it is unlikely that the observed increase in solubility is attributed to dissociation of Glib imposed by the increase in pH, since Glib is almost completely ionized over the entire range of the relevant pH values (8.0–12.5). Furthermore, the solubility of Glib was determined in different aqueous buffer solutions (without IL) at different pH values in the range 6–12 i.e. pH solubility profile was obtained (Fig. 8). The results showed that the solubility of Glib increased from 0.028 mg/ml (at pH 6) to 0.8 mg/ml at pH 12. However the solubility of Glib in IL at pH ~12 was much higher (~ 9 mg/ml) and it was about 10 times higher than its value in simple buffer of the same pH-value. Thus the pH alone can't be accepted as the sole mechanism of increasing solubility of Glib when IL is employed. In order to explore the mechanism by which IL exhibited such a dramatic effect on solubility enhancement of Glib, the potential of Glib to form some kind of associations with the IL, was investigated. Therefore, NMR spectra were obtained for a sample of Glib together with IL,
Fig. 7. Solubility of Glibenclamide in aqueous solutions of increasing IL-concentrations.
Fig. 8. Solubility profile of Glib as a function of pH of the medium.
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Table 1 Detailed peak assignment and comparison for Glib NMR spectra in presence and absence of IL. Tryptophane–choline IL
Glibenclamide alone
IL and glibenclamide
Notes
1.4, br sing (2H), trypto, NH2
1.08 and 1.51, m, 10H (CH2 2″ + 3″ + 4″ + 5″ + 6″)
0.98–1.52, m, (10H) Hexane of glib 2.46, s, unknown 2.61, q, 1H, (tryp 9) 2.8, t, 2H, (Glib 10) 3.05, s, 9H, (Chol methyl) 3.13, m, 3H, (trypt NH, +CH 1″)
No obvious change in delta but change in Split pattern
2.46, sing (1H) Chol, OH 2.51, quart(1H) Trypto, 9 2.89, t, 1H (CH2 10) 3.05, sing shar (9H), Chol methyl 3.25, mult over, trypto, NH
3.24, m, 1H (CH 1″)
3.34, trip (2H) chol, 1.
3.34, water 3.49, m, 2H (CH2 9)
3.38, trip over, (2H) chol, 2.
3.75, S, 3H(2-OMe) 3.79, doublet (2H) tryp 8, 4.5, s, br, water 6.3, d, 1H(NH 9′) 6.86, trip (1H) tryp 5. 6.95, trip (1H) tryp 4. 7.1, sing (1H) tryp 1.
3.36, m, 3H, (Chol 1 + Glib CH2 9) 3.4, m, 3H, (Chol 2 + Glib CH2 9) 3.65, overlab, water 3.75, s, 3H, (Glib OMe) 3.79, m, 2H, (Trypt 8) 5.6, s, br, 1H (Glib NH 9’)
7.1, d, 1H (CHar 3) 7.45, m, 3H, (CHar 4 + 3′ + 5′)
7.3, doublet (1H) tryp 6. 7.5, doublet (1H) tryp 3. 7.59, d, 1H (Char 6) 7.78, m, 2H(Char 2′ +6′) 8.22, t, 1H (NH 8) 10.29, S (vbr), 1H (NH 7′)
Increase in freq. in presence of IL Slight decrease in presence of IL No change Decrease in delta ~0.1 Typical value for water No obvious change No obvious change Addition of Glib appeared to bring the signal closer to its typical value (3.33) No significant change No significant change Very much higher than typical value (3.33) Very obvious decrease in frequency together with becoming broader and singlet instead of doublet. No signif change No signif change No significant change
6.89, t, 1H (tryptar 5) 6.99, t, 1H (tryptar 4) 7.08, s + 7.12; s, 2H, (trypt 1 + Glib 3ar) 7.19, d, 1H, (G3′ +5′) 7.44, d, 1H, (Glib 4ar) 7.30, d, 1H, (tryptar 6) 7.52, d, 1H, (trypar 3) 7.63, m, 1H, (Glib 6, 2′, 6′)
Significant decrease in 3′ and 5′ of Glib No significant change No significant change No significant change Significant decrease for the protons 2′ and 6′
8.22, t, 1H, (Glib NH8) 11.10, s, 1H,(Glib NH7′)
No significant change Significant increase in the frequency, and it becomes sharper
dissolved in DMSO, along with spectra for each of IL and Glib alone. The obtained spectra for both Glib and tryptophan were in good agreement with previously published reports [27,20], which confirmed assignments of all protons. The obtained results clearly indicated distinct shifts in the NMR spectrum of Glib in presence of IL as compared to Glib alone. A detailed description of signals and their assignments are presented in Table 1. According to Table 1, the most significantly altered resonances were those of Glib aromatic protons (2′, 6′, 3′, 5′), the sulfonamide proton (N7′) and the amide proton (N9′). Therefore, it could be concluded that Glib associates with the IL at that end of the molecule (aromatic sulfonamide) through the formation of multiple hydrogen bonds between IL and Glib in addition to π–π interaction through the aromatic rings between the two molecules. A proposed structure describing the interaction between the two molecules is presented in Fig. 9. It is noteworthy that the protons of water appeared at significantly higher δ (4.5) than usual in the spectrum of IL alone [28] which suggest
extensive involvement of hydrogen bonding with IL molecules. However, as Glib was added to IL the frequency of water came back to near normal (3.65) suggesting the replacement of water molecules by Glib. Therefore, it could be concluded that Glib slides between the stacked IL molecules through formation of hydrogen bonds and π–π interaction. Further evidence on the interaction (association between Glib and IL) came from UV spectroscopic titration. A sample of IL was titrated with Glib and the obtained overlaid absorption spectra are presented in Fig. 10. As Glib was added to both sample and blank cells, a characteristic shift in the absorption spectrum (general decrease) was observed in the range of 300–350 nm with an isosbestic point at about 338 nm. A plot of absorbance at λmax (380 nm) produced a decreasing linear plot with an equation: absorbance = −614 C + 2.28 with a correlation coefficient = 0.998, where C is the molar concentration of Glib. This trend should not have been observed unless Glib interacts with the IL which further supports NMR results.
Fig. 9. Proposed structure for the association between Glib and IL.
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Fig. 10. Overlaid UV spectra for IL titrated with glibenclamide.
4. Conclusions An IL was prepared from choline as a cation and tryptophan as an anion. The IL was extensively characterized by NMR, and UV techniques. The ability of IL to solubilize the antidiabetic drug Glib was demonstrated were it was increased several folds. Attempts were made to understand the mechanism of solubilization of Glib in IL employing NMR, UV and HPLC. Results indicated the interaction of Glib with IL to form certain types of complexes through hydrogen bonds and π–π interaction of the aromatic rings. The produced IL may be utilized clinically to improve the solubility of the poorly water soluble drug Glib since it exhibited very high solubility enhancing effect, yet water miscible and most likely safe because it dissociates in dilute solutions to the acceptably known as safe compounds: tryptophan and choline. Acknowledgment The authors would like to thank the Deanship for academic research at the University of Jordan (51/2011-2012) for the financial support. References [1] R. Liu, J.F. Liu, Y.G. Yin, X.L. Hu, G.B. Jiang, Ionic liquids in sample preparation, Anal. Bioanal. Chem. 393 (2009) 871–883.
[2] M. Kosmulski, K. Marczewska-Boczkowska, P. Zukowski, J. Subocz, C. Saneluta, Permittivities of 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates, Croat. Chem. Acta 80 (3–4) (2007) 461–466. [3] S. Keskin, D. Kayrak-Talay, U. Akman, O. Hortacsu, A review of ionic liquids towards supercritical fluid applications, J. Supercrit. Fluids 43 (2007) 150–180. [4] S. Gitanjali, A. Kumar, Ionic liquids: physico-chemical, solvent properties and their applications in chemical processes, Indian J. Chem. 47A (2008) 495–503. [5] J. Stoimenovski, D.R. MacFarlane, K. Bica, R.D. Rogers, Crystalline vs. ionic liquid salt forms of active pharmaceutical ingredients: a position paper, Pharm. Res. (2010). [6] D.J.C. Constable, et al., Green Chem. (2007), http://dx.doi.org/10.1039/b703488c. [7] USDA Nutrients database, http://www.nal.usda.gov/fnic/foodcomp/search Accessed Dec 7, 2011. [8] H. Mizuuchi, V. Jaitely, S. Murdan, A.T. Florence, Room temperature ionic liquids and their mixtures: potential pharmaceutical solvents, Eur. J. Pharm. Sci. 33 (2008) 326–331. [9] K.B. Smith, R.H. Bridson, G.A. Leeke, Solubilities of pharmaceutical compounds in ionic liquids, J. Chem. Eng. Data 56 (5) (2011) 2039–2043. [10] A. Chaudhary, U. Upendra Nagaich, N. Gulati, V.K. Sharma, R.L. Khosa, Enhancement of solubilization and bioavailability of poorly soluble drugs by physical and chemical modifications: a recent review, J. Adv. Pharm. Educ. Res. 2 (1) (2012) 32–67. [11] T.K. Chung, H.B. Gelberg, J.L. Dorner, D.H. Baker, Safety of L-tryptophan for pigs, J. Anim. Sci. 69 (1991) 2955–2960. [12] A.A. Elkordy, A. Jatto, E. Essa, In situ controlled crystallization as a tool to improve the dissolution of glibenclamide, Int. J. Pharm. 428 (2012) 118–120. [13] N. Seedher, M. Kanojia, Co-solvent solubilization of some poorly-soluble anti-diabetic drugs, Pharm. Dev. Technol. 14 (2009) 185–192. [14] A.A. Abd Elbary, H.F. Salem, M.E. Maher, In vitro and in vivo evaluation of glibenclamide using surface solid dispersion (SSD) approach, Br. J. Pharmacol. Toxicol. 2 (1) (2011) 51–62. [15] A.A. Elkordy, J. Ayobami, E. Essa, In situ controlled crystallization, as a tool to the dissolution of glibenclamide, Int. J. Pharm. 428 (2012) 118–120. [16] M. Remko, Theoretical study of molecular structure, pK(a), lipophilicity, solubility, absorption, and polar surface area of some hypoglycemic agents, J. Mol. Struct. THEOCHEM 897 (2009) 63–82. [17] A.C. Moffat, J.V. Jackson, M.S. Moss, B. Widdop, Clark's Isolation and Identification of Drugs, The Pharmaceutical Press, London, 1986. [18] R.J. Babu, J.K. Pandit, Enhancement of dissolution rate and hypoglycemic activity of glibenclamide with beta-cyclodextrine, STP Pharma. Sci. 5 (1995) 196–201. [19] P. Lehto, K. Laine, K.T. Kivisto, K.T. Kivisto, P.J. Neuvonen, The effect of pH on the invitro dissolution of three second-generation sulphonylurea preparations: mechanism of antacid–sulphonylurea interaction, J. Pharm. Pharmacol. 48 (1996) 899–901. [20] E. Boyle-Roden, N. Hofer, K.K. Dey, P.J. Grandinetti, M. Caffrey, High resolution 1H NMR of a lipid cubic phase using a solution NMR probe, J. Magn. Reson. 189 (2007) 13–19. [21] G.H. Jeffery, J. Basset, J. Mendham, R.C. Denney, Vogel's Textbook of Quantitative Chemical Analysis, Sixth ed., 1989 (Amazon.com). [22] A. Mitra, P.J. Seaton, T. Williamson, Unprecedented concentration dependent chemical shift variation in 1H-NMR studies: a caveat in the investigations of molecular recognition and structure elucidation, Tetrahedron 54 (1998) 15489–15498. [23] A. Beck, Concentration Dependent 1H-NMR Chemical Shifts of Quinoline Derivatives , (A Master of Science) University of North Carolina, 2007. [24] P. Zeng, G. Zhang, A. Rao, W. Bowle, S. Wiedmann, Concentration dependent aggregation properties of chlorhexidine salts, Int. J. Pharm. 367 (2009) 73–78. [25] J.S. Lomas, A. Adenier, C. Cordier, Proton NMR and IR study of selfassociation in pyridylalkanols: open or cyclic dimers? Higher polymers? J. Phys. Org. Chem. 19 (2006) 295–307. [26] British Pharmacopoeia, Volume 1 and 2. Glibenclamide (and Tablet) Monograph, The Stationary Office, London, 2009. [27] D. Sanz, R.M. Claramunt, I. Alkorta, G. Sánchez-Sanz, J. Elguero, The structure of glibenclamide in the solid stat, Magn. Reson. Chem. 50 (3) (2012) 246–255. [28] H.E. Gottlieb, V. Kotlyar, A. Nudelman, NMR chemical shifts of common laboratory solvents as trace impurities, J. Org. Chem. 62 (1997) 7512–7515.