International Journal of Pharmaceutics 443 (2013) 273–278
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Imidazolium ionic liquids as solvents of pharmaceuticals: Influence on HSA binding and partition coefficient of nimesulide Ana M.O. Azevedo, Diogo M.G. Ribeiro, Paula C.A.G. Pinto ∗ , Marlene Lúcio, Salette Reis, M. Lúcia M.F.S. Saraiva REQUIMTE, Departamento de Química, Faculdade de Farmácia, Universidade do Porto, Rua Jorge Viterbo Ferreira, 4050-313 Porto, Portugal
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Article history: Received 9 October 2012 Received in revised form 14 December 2012 Accepted 20 December 2012 Available online 31 December 2012 Keywords: Ionic liquids Drug solvent/carrier Drug–protein binding Partition coefficient
a b s t r a c t In this work, the influence of imidazolium ionic liquids (ILs) on bio-chemical parameters that influence the in vivo behavior of nimesulide was evaluated. In this context, the binding of nimesulide to human serum albumin (HSA), in IL media, was studied. In parallel, the evaluation of the interaction of drug–IL systems, with micelles of hexadecylphosphocholine (HDPC), enabled the calculation of partition coefficients (Kp ). Both assays were performed in buffered media in the absence and in the presence of emim [BF4 ], emim [Ms] and emim [TfMs] 1%. Even though there was an increase of the dissociation constant (Kd ) in IL media, nimesulide still binds to HSA by means of strong interactions. The thermodynamic analysis indicates that the interaction is spontaneous for all the tested systems. Moreover, the studied systems exhibited properties that are favorable to the interaction of the drug with biological membranes, with Kp values 2.5–3.5 higher than in aqueous environment. The studied nimesulide–IL systems presented promising characteristics regarding the absorption and distribution of the drug in vivo, so that the studied solvents seem to be good options for drug delivery. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are being extensively used in replacement of conventional organic solvents due to some attractive characteristics namely negligible vapor pressure, good chemical and thermal stability, high ionic conductivity and multiple solvation interactions with both organic and inorganic compounds (Olivier-Bourbigou et al., 2010; Wasserscheid and Welton, 2008). Moreover, properties such as viscosity, polarity and density as well as toxicity can be finely tuned by distinct anion/cation combinations aiming the development of tailored compounds for specific purposes. Due to all this, ILs have been applied as solvents or co-solvents in a variety of applications in the field of analytical chemistry, synthesis and biocatalysis, among others (Hallett and Welton, 2011; Pinto et al., 2008a,b; Sun and Armstrong, 2010; van Rantwijk and Sheldon, 2007; Welton, 1999). More recently, the emergence of ILs with biological activity has revolutionized the scientific focus of these compounds and opened interesting perspectives regarding their pharmaceutical application (Hough and Rogers, 2007; Hough et al., 2007). Also in this field, the possibility of engineering the properties of ILs by
∗ Corresponding author. Tel.: +351 220428679; fax: +351 226093483. E-mail address:
[email protected] (P.C.A.G. Pinto). 0378-5173/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijpharm.2012.12.030
manipulating the anion–cation combination, in association with their solvent properties and in some cases water-miscibility, are considered promising characteristics regarding the applicability of ILs as solvents or carriers of pharmaceutical drugs (Moniruzzaman and Goto, 2011; Smith et al., 2011). These possibilities have been explored by several authors in distinct approaches (Mizuuchi et al., 2008; Smith et al., 2011). In this context, ILs have been studied as solvents of poorly soluble active pharmaceutical ingredients like albendazol and danazol (Mizuuchi et al., 2008). This approach can be of the most importance considering that nowadays the majority of drug candidates fail to reach the market due to poor pharmaceutical properties resulting mainly from poor water solubility (Di and Kerns, 2003; Pouton, 2006). Based on the same objectives, ILs have been incorporated on microemulsions aiming the development of drug delivery systems for water insoluble or sparingly soluble drugs (Moniruzzaman et al., 2010a,b,c). Likewise, Jaitely et al. evaluated water-immiscible ILs as drug reservoirs for controlled release of drugs (Jaitely et al., 2008, 2010). As a complement, the toxicity of ILs toward Caco-2 cells was also studied. In 2011, after several proposals in the pharmaceutical field, ILs were outlined as potential good solvents and reagents for pharmaceuticals (Moniruzzaman and Goto, 2011). As can be concluded, the research in this field has been focused mainly on the ability of ILs to solubilize poorly water soluble drugs and on the release profiles of the drugs from IL reservoirs. As far as we know there are no studies regarding the prediction of the
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biological behavior of drugs vehicled by ILs or regarding the influence of ILs on parameters that determine the pharmacokinetics or pharmacodynamics of the drugs. Thus, in this work, we focused on the well known antiinflammatory drug nimesulide and studied, for the first time, its behavior in terms of protein binding affinity and partition coefficient in IL based systems, aiming the prediction of drug’s distribution and membrane interaction. The inclusion of the drug in IL based systems can be justified by its limited solubility in water and can be a good starting point considering the future development of formulations, mainly transdermal ones. The protein binding affinity studies were based on the quenching of human serum albumin (HSA) fluorescence in association with models of drug–protein binding in the equilibrium. HSA is the most abundant protein in human plasma and is the major target for many endogenous and exogenous compounds, interacting reversibly with a broad spectrum of therapeutic agents (Chuang et al., 2009; Kragh-Hansen et al., 2002). Thus, HSA has a strong influence on the pharmacodynamics and pharmacokinetics of the bounded drugs and is nowadays recognized as a model protein for studying drug–protein interaction. Several applications have confirmed fluorescence spectroscopy as an important tool for the routine implementation of this kind of study (Matei and Hillebrand, 2010; Qi et al., 2008; Seedher and Bhatia, 2006). This is related with the characteristics of HSA in terms of intrinsic fluorescence and with the possibility of easily quenching this fluorescence by direct interaction of a drug with the proteic structure. Regarding the lipophilicity issue, it was our aim to evaluate the influence of ILs on the interaction of nimesulide with a membrane model. This will provide an indication of the distribution of the drug between aqueous and lipid phases which is of great interest in predicting drug’s absorption and distribution as well as in assessing its toxic and/or therapeutic effects (Di and Kerns, 2003). In the ILs field, as in others, the determination of partition coefficients has been traditionally performed recurring to the octanol/water system (Ko/w ), not considering the ionic nature of the compounds (Chapeaux et al., 2007; Giaginis and Tsantili-Kakoulidou, 2008; Ropel et al., 2005). Recently, a few authors brought this issue to discussion and proposed alternative pathways to evaluate the lipophilicity of ILs (Lee and Lee, 2009; Ventura et al., 2011). However all these approaches are still based on the octanol/water distribution and are mainly related with the analysis of toxicological data. In this context, it is nowadays accepted and demonstrated that membrane models are more adequate for this purpose since they mimic the hydrophobic core and the charged polar surface of phospholipids present in biomembranes (Giaginis and Tsantili-Kakoulidou, 2008). Indeed, using biomimetic membrane models, the hydrophobic, Hbonding, dipole–dipole and electrostatic interactions between drug and membrane are considered, whereas octanol/water systems can only model nonpolar interactions (Brittes et al., 2010; Ferreira et al., 2003). Even though in this work the main objective was to assess the influence of ILs on the properties of nimesulide, we consider that this matter is of the most importance and that the ionic nature of the ILs under study must be also considered. Thus, we resorted to micelles of hexadecylphosphocholine (HDPC), as membrane model (Magalhaes et al., 2010), and studied the influence of ILs on the partition properties of nimesulide in IL through the calculation of partition coefficients (Kp ), by derivative spectroscopy (Magalhaes et al., 2010). With these studies we intended to forecast the biological behavior of nimesulide in IL based systems and further increase the possibilities of application of these compounds as drug solvents/carriers in the pharmaceutical industry.
2. Materials and methods 2.1. Chemicals All solutions were prepared using chemicals of analytical reagent grade and high purity water (Milli-Q) with a specific conductance
