Solubilization and Interaction Studies of Bile Salts

Appl Biochem Biotechnol DOI 10.1007/s12010-016-1987-x

Solubilization and Interaction Studies of Bile Salts with Surfactants and Drugs: a Review Nisar Ahmad Malik 1

Received: 12 November 2015 / Accepted: 10 January 2016 # Springer Science+Business Media New York 2016

Abstract In this review, bile salt, bile salt–surfactant, and bile salt–drug interactions and their solubilization studies are mainly focused. Usefulness of bile salts in digestion, absorption, and excretion of various compounds and their rare properties in ordering the shape and size of the micelles owing to the presence of hydrophobic and hydrophilic faces are taken into consideration while compiling this review. Bile salts as potential bio-surfactants to solubilize drugs of interest are also highlighted. This review will give an insight into the selection of drugs in different applications as their properties get modified by interaction with bile salts, thus influencing their solution behavior which, in turn, modifies the phase-forming behavior, microemulsion, and clouding phenomenon, besides solubilization. Finally, their future perspectives are taken into consideration to assess their possible uses as bio-surfactants without side effects to human beings. Keywords Bile salts . Surfactants . Drugs . Thermodynamics . Physico-chemical properties . Interactions

Introduction Greenish yellow secretion, bile or gall, is secreted by the liver and stored in the gallbladder where it is concentrated or passed to duodenum part of small intestine. Its main purpose is to emulsify fats and help their absorption in the small intestine. Its main constituents are bile acids and bile salts, cholesterol, phospholipids, water, and pigments. One of the constituents of bile that is bile salts are formed of four different bile acids, namely, cholic, deoxycholic, chenodeoxycholic, and lithocholic. These acids in turn have the capacity to interact and combine with glycine or taurine forming complex acids and salts [1] (Fig. 1).

* Nisar Ahmad Malik [email protected]

1

Department of Chemistry, University of Kashmir, Hazratbal, Srinagar 190006, India

Appl Biochem Biotechnol

Glycine

Taurine

Fig. 1 Structures of bile acids

Duodenum of small intestine is the main portion of intestine where lipid digestion occurs, as duodenum houses various surface active systems such as the enzyme pancreatic lipase and its co-factor colipase, and bile salts. Bile salts help in emulsifying the fatty foods adsorbing on oil–water interfaces and helps in lipase–colipase complex interface for the adsorption. The lipid digestion starts when lipase reaches the oil–water interface which results into the breaking down of the triglycerides from the lipid substrate into mono-glycerides and free fatty acids [2]. After the formation of simpler products due to lipolysis, they are removed from oil–water interface and solubilized via bile salt micelles for consumption by human body. Bile salts are important and help in solubilization and transport of lipophilic substances (nutrients, drugs, etc.) to mucosa of intestine for absorption [3]. Many recent review articles have appeared in literature dealing with the role of bile salts in digestion and drug absorption [3, 4] and their aggregation behavior in solution [5] (Fig. 2). Bile salts present a class of potential bio-surfactants with biological importance [6, 7], for instance as solubilizers to cholesterol and lipids [8], emulsifiers and dispersion agents in cosmetics [9], medicines [10], and chemicals [11]. Mixed micelles are formed when bile salts are added with other amphiphiles [12–16] and also result in the formation of liquid crystalline phases [17–19]. The aggregates of bile acid salts are important owing to their physicochemical properties which are different than conventional amphiphiles. For example, their critical micelle concentration (CMC) is lower than simple ordinary surfactants, and this CMC is characterized by a range rather than an exact value with smaller aggregation number, higher charge density, and higher polydispersity [20–35]. Because of the application in drug delivery

Appl Biochem Biotechnol Fig. 2 Schematic representation of aggregate formation of bile salt in aqueous medium

systems, amphiphile forming aggregates which undergo a transition between vesicles and micelles are of special interest. Electrostatic, van der Waals, hydrophobic, and steric interactions and their delicate balance play an important role in aggregate formation. Thus, to control and understand the self-assembly processes, efforts are made to understand the weak interaction mechanisms [36–39]. Conventional surfactants or amphiphiles, as is well known, are hydrophobic (tail portion) and hydrophilic (head group portion). On the other hand, bile salts are facial amphiphiles formed of a polar face or polar side commonly called α side and a nonpolar face or hydrophobic side also named as β side in a rigid steroid skeleton [40, 41]. In water, bile salts form micelles primarily by hydrophobic interactions over their hydrophobic sides [42, 43]. It is supposed that in hydrophobic phase (liquid), bile salts form micelles (reverse micelles) by hydrogen bond formation [44, 45] (Fig. 3). Various models of aggregate formation of bile salts are proposed. (1) Small’s model: It involves first the formation of primary aggregates followed by secondary aggregates. Hydrophobic interactions between two and nine monomers of steroid nuclei of bile salts form primary aggregates. Primary aggregates further aggregate and are held together by hydrogen bonding between the hydroxyl groups. Globular shape is proposed for primary aggregates and oblate ellipsoidal for secondary aggregates [42]. (2) Oakenfull and

Fig. 3 Schematic representation of hydrophobic (a) and aggregate formation of bile salts (b, c)


Fisher’s Model: This model suggested the formation of dimers. Layer aggregates of bile salts are formed due to the hydrophobic interaction between the dimers. Rod-like structures are formed in secondary aggregation [43]. (3) Kawamura et al. Model: In this model, disc-shaped secondary aggregates were proposed, in which the hydrophobic sides of bile salts face each other, thus rendering the interior of the aggregates as hydrophobic and away from solvent while the outer side hydrophilic side faces toward the solvent molecules. An alternating orientation of bile salt molecules with parallel long axes of the molecules was suggested to form aggregates [44]. (4) Warren et al. Model: A helical-type shape of bile salt aggregates was proposed based on the structure of the crystalline state rather than the liquid state. According to this model, polar interactions of bile salts result in aggregation. From molecular dynamic simulations, helix model has been discounted and thus disc shape of bile salt aggregates is widely recognized [45]. Owing to the unique facial structure, the amphiphilic aggregates of bile salts have a common chemical structure quite different from synthetic surfactants. They have two planes, concave and convex; from the concave side, also called α-plane, few hydroxyl groups pointing outward of the carbon framework and a hydrophobic convex side also called β-plane are present. The methyl groups are present at the C-18, C-19, and C-21 position that are on β-plane and the two or three hydroxyl groups at the C-3, C-7, and C-12 positions as well as a short and flexible aliphatic tail with hydrophilic groups, such as carboxylate, at the end, which is α-plane [46]. In presence of conventional surfactants, these lie between the head groups of the conventional amphiphiles. Steric interactions between the bile salts and amphiphiles due to large steroid skeleton of the bile salt keep the head groups of bile salts apart [47–49]. The average head group area of mixed amphiphiles is increased on the addition of bile salts forming curved aggregates [50–52]. Although bile salts are anionic and shall behave like anionic surfactants, they differ from conventional anionic surfactants in many respects. These are structurally different with more charge density, more microviscosity, and rigid backbone structure having hydroxyl groups on the α-side and methyl groups on the β side rendering them smaller as compared to conventional anionic surfactants [17, 53–56]. It has been observed that the bile salts solubilize many soluble and lipidic substances due to formation of mixed micelles [42]. Attributable to their capacity to solubilize nonpolar substances particularly cholesterol and fatty acids, their importance in solubilizing many biologically important insoluble substances such as phospholipids and monoglycerides is also investigated [42]. Pharmaceutical formulations also use bile salt micelles for solubilization of poorly soluble molecules and in permeation through biological membranes [57–60].

Micellization Behavior of Bile Salts in Aqueous Medium Bile salts due to their amphiphilic nature self-aggregate to form micelles in aqueous solution. Various methods were employed to study the micellar properties of bile salts [20, 61–69]; many review articles were compiled particularly focusing on physico-chemical properties of bile and bile salt self-assembly [5, 70–72]. To understand many important biological processes, such as solubilization of lipids, bilirubin, cholesterol, lecithin, and fat-soluble vitamins in living organisms, their surfactant like properties have been explored [35, 73]. It is difficult to determine precisely the critical micelle concentration (CMC) for bile salts as the CMC lies in a broader range by reason of increasing concentration; the aggregates of bile salts grow in size compared to conventional ones [69]. Multiple aggregation equilibriums at various concentrations of bile salts in aqueous medium are also observed. The minimum concentration where


aggregation has been detected is around 3 mM [74]. Owing to the absence of well-defined hydrophobic tail and hydrophilic head, bile salts have hydroxyl groups generally located on one face and methyl groups on the opposite displaying planar polarity and form smaller two to nine molecules per micelle [75] (Fig. 4). Not only hydrophobic effect but also hydrogen bonding plays an important role in micellization of bile salts. The hydroxyl and acidic groups present in bile salts may be involved in intermolecular hydrogen bonds. Hydrophobic interactions are relatively weak and complex, hydrogen bonding is dependent upon position, and orientation of hydroxyl groups compensating for the micellization in bile salts [76]. Many experimental and theoretical simulation methods are routinely employed to examine the bile

Sodium Cholate

Sodium taurocholate

Sodium glycocholate

Sodium chenodeoxycholate

Sodium glycodeoxycholate

Sodium glycolithocholate

Fig. 4 Structures of some bile salts


salt aggregation in aqueous medium [17, 77–79]. Small et al. [79] proposed the primary and secondary aggregate model which is the widely accepted one. At low concentrations of bile salts, the primary aggregates with hydrophobic binding sites are formed. At higher concentration of bile salts, the primary aggregates cluster together to form hydrogen bonded secondary aggregates of larger size [79]. Various studies [80, 81] confirmed elongated rod-shaped secondary structures with water and ions filling the central hydrophilic core. Several techniques such as surface tension [82], absorption [83], conductivity [84], dynamic light scattering [85], calorimetry [86], freezing point depression [75], fluorescence [87], and capillary electrophoresis [88] were employed to study the micellization and separation behavior of bile salts. It was observed that the micellization is affected by using foreign substances such as pyrene for CMC determination due to low aggregation number values [89]. Schurtenberger and his co-workers using static and dynamic light scattering measurements calculated the mean aggregation number and mean hydrodynamic radius for sodium taurodeoxycholate (NaTDC) as a function of this bile salt concentration. It was observed that the elongated micelles of NaTDC are formed in presence of higher concentration of NaCl above CMC [90]. Matsuoka and Moroi [69] employed the pyrene fluorescence technique to investigate the micelle formation (CMC, the micelle aggregation number, and the degree of counterion binding to micelle) of sodium deoxycholate and sodium ursodeoxycholate. By aqueous solubility change with solution pH, the micellization of NaDC and NaUDC was studied at different temperatures (288.2, 298.2, 308.2, and 318.2 K). From the viewpoint of chemical structure for the growth of micelles, it was observed that the location of the OH group at C-7 and its orientation are the most important factors. For the confirmation of low counterion binding to micelles, the activity measurement for sodium ions was made by a sodium ion selective electrode. Fontell [56] used the X-ray scattering to examine and interpret the X-ray scattering in aqueous bile acid salt solutions. He observed that there is weak diffraction phenomenon in the low-angle range which was dependent upon the type, shape, position, and concentration of bile acid salts, and the observed X-ray scattering phenomenon of bile acid salts in the low-angle region close to the primary beam in aqueous solutions provides the evidence for existence of colloid aggregates. Molecular dynamic simulations were performed on six bile salts, cholate, glycocholate, taurocholate, glycochenodeoxycholate, glycodeoxycholate, and glycolithocholate, to analyze their spontaneous aggregation and micellar structure in aqueous medium. In this study, 8 to 17 molecules of bile salts were observed to form aggregates, and these aggregates varied in shape from oblate, spherical, to prolate. It was also observed that the intermolecular hydrogen bonding plays an important part in micellization [91]. Two physiologically important bile salts, sodium cholate (NaC) and sodium deoxycholate (NaDC), with dipalmitoylphosphatidylcholine (DPPC) below their CMCs (NaC and NaDC, 0.05 to 1 mM, and DPPC vesicles) were investigated to assess their interactions. In both solid gel and liquid crystalline phases of DPPC vesicles at very low (≤1 mM) concentrations of the bile salts, the interactions resulted in significant wetting of the membrane up to the hydrocarbon core regions, which was analyzed by steady state fluorescence and dynamic fluorescence lifetime analysis. It was observed that in the presence of NaDC, the lipid bilayer is wetted more than the NaC [92]. Isothermal titration calorimetry, conductivity measurement, and steady state fluorescence measurements were used in absence and in presence of NaCl (0.15 m) to assess the micellization process of NaC; NaDC; sodium glycocholate, NaG; sodium glycodeoxycholate, NaDG; sodium taurocholate, NaTC; and sodium taurodeoxycholate, NaDTC, in aqueous solution. The micellization process was favored as the increase in the length of the


side chain and an increase in the number of hydroxyl groups suggesting that the hydrophobic effect is the main driving force for micellization, which was also confirmed from the values of ΔmicCp°. It was observed that the added salt that is NaCl diminishes the electrostatic repulsions between head groups while unaffecting the degree of ionization and enthalpy of micellization [93].

Bile Salt–Bile Salt Interactions Cholanology that is the bile acid science plays an essential role in biotechnology and in pharmaceutical industry [94–98]. Bile salt–bile salt interactions are helpful in choosing different drug carriers [99] and as solubilizers of drugs of interest [100], and in stabilizing enzymes [101], these mixed bile salts are helpful in realizing the interaction nature which are prevalent in majority of solution and at interface [18, 102, 103]. Conductivity, surface tension, and density measurements were used to investigate the mixed aggregate formation of sodium dehydrocholate (NaDHC)–sodium deoxycholate (NaDC). It was observed that at higher concentrations of NaDHC, repulsive interactions occurred in mixed aggregates. As the concentration of second bile salt, NaDC, was increased, attractive interactions begin to take place and were attributed to the hydrocarbon backbone structural difference [104]. Tensiometry, conductometry, and microcalorimetry were used to find the CMC of NaC, NaDC, NaTC, NaTDC, and their equimolar binary and ternary mixtures. It was observed that bile salt mixtures showed nonideal behavior evident from CMC, activity coefficients, composition of the bile salt micelles, and intermicellar interaction parameters using Clint [105], Rubingh [106], and Rubingh–Holland [107] models. Micellar electrokinetic chromatography (MEKC) systems with mixed pseudostationary phases of NaC and NaDC have been characterized by means of the solvation parameter model. The importance of characterizing systems with an appropriate set of solutes that embrace a wide range of descriptor values has been proven as they can significantly influence the value of the system constants. The fit of the solvation parameter model to the experimental log k data has been compared for each SC–SDC system when the Abraham descriptors and the Poole optimized descriptors, recently proposed, are used. In both cases, the variation in MEKC surfactant composition results in similar changes in the coefficients of the correlation equations, which in turn leads to similar information on solute–solvent and solute–micelle interactions. It was demonstrated that NaDC is more hydrogen bond acidic and hydrophobic but slightly less polarizable than NaC. Systems with intermediate selectivity are obtained through mixtures of both surfactants [108]. Small-angle neutron scattering (SANS) measurements of D2O solutions (0 × 1 M) of NaC and NaDC were carried out at T = 298 K. Under compositions very much above the critical micelle concentration (CMC), the bile salt micelle size growths were monitored by adopting Hayter–Penfold type analysis of the scattering data. NaC and NaDC solutions show presence of correlation peaks at Q = 0 × 12 and 0 × 1 Å − 1, respectively. Monodisperse ellipsoids of the micelles produce best fits. For NaC and NaDC systems, aggregation number (9 × 0, 16 × 0), fraction of the free counterions per micelle (0 × 79, 0 × 62), semi-minor (8 × 0 Å), and semi-major axes (18 × 4, 31 × 7 Å) values for the micelles were deduced. Extent of micellar growth was studied using ESR correlation time measurements on a suitable probe incorporating NaC and NaDC micelles. The growth parameter (axial ratio) values were found to be 2 × 3 and 4 × 0 for NaC and NaDC systems, respectively and the values were found out to be in close agreement with that of SANS [109].


Bile Salt–Surfactant Interactions The interaction of bile salts with lipids, biocompatible polymers, surfactants, and other bioactive molecules has been extensively studied [103, 110, 111]. By reason of the small aggregation number which results in small size of aggregates, hydrophobic guest molecules such as cholesterol are poorly solubilized by the micelles of bile salts [112]. Bile salts inside human beings have the special capacity to dissolve cholesterol due to the formation of mixed micelles between bile salt and phospholipid that is lecithin [113]. Due to this capability, they solubilize hydrophobic guest molecules in micelles [114]. Different types of interactions occur in bile salts with phospholipid bilayers [115, 116]. The bile salt monomers without disrupting the membranes are inserted into the phospholipid vesicles when their concentration is lower than CMC. Mixed vesicle–mixed micelle systems are formed as the concentration of bile salts reaches above their CMC, and the mixed vesicles become solubilized with more increase in the concentration of bile salts [117] (Table 2). Surface tension, conductivity, light scattering, and viscosity techniques were performed at 298.15 K in aqueous solutions of the mixed micelles of hexadecyl, tetradecyl, and dodecyl-trimethylammonium cholates to find the critical micelle concentrations, conductivities, aggregation numbers, and intrinsic viscosities. Micelles formed were observed to be spherical, small, and slightly charged, and 19–32 aggregation numbers were depicted from the results obtained [14]. Micellization behavior of NaC and NaDC with long-chain alkyltrimethylammonium bromides (CnTABr, n = 12, 14, and 16) in pure and at different compositions were studied using the techniques: conductance, viscosity, fluorescence spectroscopy, dynamic light scattering, zeta potential, and small-angle X-ray scattering in aqueous solutions of these mixed systems. It was observed that the CnTABr–NaC formed clear isotropic phase solutions but CnTABr–NaDC exhibited different phases (micelles, vesicles, and precipitates). CnTABr mixed with NaC and NaDC showed strong synergism calculated with the help of Rubinigh’s model for Regular Solution Theory. It was observed that NaC acted differently than NaDC because of higher hydrophilicity of NaC owing to one more hydroxyl group than NaDC which helps in deeper insertion of NaDC into the mixed micelle [118]. It was also demonstrated that NaDC is more hydrogen bond acidic and hydrophobic but slightly less polarizable than NaC [108]. Mixed micelles of sodium dodecyl sulphate (SDS) and NaC were investigated by nuclear magnetic resonance diffusion and relaxation technique. NaC concentration was varied keeping the concentration of SDS constant. NMR self-diffusion coefficients of SDS and NaC were measured and the deuterium relaxation rates for specifically deuterium labeled SDS molecules. Micellar electrokinetic capillary chromatography (MECC) is a capillary electrophoretic separation technique where ionic surfactants, at concentrations above their CMCs, are added to the electrolyte solution. This MECC is based on the electrophoretic mobilities of the compounds and their partitioning into the micelles. Mixed micelles of SDS-NaC as a pseudostationary phase in MECC and is used in the separation of compounds [119]. Mixed micelles of NaDC and sodium chenodeoxycholate (NaCDC), with nonionic surfactants decanoyl-N-methylglucamide, MEGA-10, and hexa-ethyleneglycol mono n-dodecylether (C12E6), were investigated by the measurement of pKa value of bile acid species. The pKa and pH displayed a marked rise at a particular concentration (CMC) of bile salt, and on increasing the concentration of bile salts above CMC, the pKa and pH reached a constant value. The dissociation state of carboxyl groups in mixed micelles depended on the hydrophilic group structure of nonionic surfactants and was less as compared to the pure bile salts which also depended on the mole fraction of bile salts [120]. Conductivity and surface tension measurements were employed to study the mixed micellization behavior of bile salts


(sodium cholate and sodium deoxycholate) with nonionic surfactants (Tween 20 and Tween 60). Using Clint’s [105], Rubingh’s [106], and Motomura’s [121] models for mixed binary systems, various physicochemical parameters were calculated; synergism between the individual surfactants in the mixed micelles was observed indicated by the negative values of interaction parameter, β. From the results, it was observed that more hydrophilic sodium cholate shows more negative values of interaction parameter, β, confirming stronger synergism with Tween 20 and Tween 60. NaC exhibited more synergism with Tween 20 and Tween 60 than NaDC, owing to more hydrophilicity, as there are more axial hydroxyl groups, that is, more hydrogen bonding between the αaxial hydroxyl groups of NaC and proton acceptor ethoxy groups of the polar head of Tween 20 and Tween 60 [122]. On increasing the number of equatorial OH or oxo groups in the steroid skeleton of the bile acid anion, the absolute value of the interaction parameter, β, increases. Effective formation of hydrogen bonds with the polyoxyethylene chains of the polar head of nonionic surfactant (Tween 40) occurs as the oxygen atoms of these groups are shifted toward the steroid skeleton mean plane [114]. On the mixing of two pure phases (i.e., micelles as separate phases), the Gibbs energy of mixing, ΔmixG, is negative, whereas ΔexcessG is the additional Gibbs energy compared to ΔmixG which refers to the nonideal mixing. The more negative is the ΔexcessG, the more stable is the nonideal mixed micelle compared to the ideal micelle. These parameters are calculated by well-established equations given by Clint [105], Rubingh [106, 107], and Motomura [121] (Table 1). NaC exhibits more synergism with Tween 20 and Tween 60 than NaDC, owing to more hydrophilicity, as there are more axial hydroxyl groups, that is, more hydrogen bonding between the α-axial hydroxyl groups of NaC and proton acceptor ethoxy groups of the polar head of Tweens [122]. In case of cationic surfactants, alkytrimethylammonium bromide (CnTABr, n = 10, 12, 14, 16, and 18), the interaction parameter β (−5.1 to −11.5) becomes more negative as the hydrocarbon chain is increased from 10 to 18 carbon atoms with NaC [122]. Average interaction parameter, β, of NaC with Tx-100 and SDS was reported to be −2.06 and −0.63 [123], respectively, and β = −3.9 with Tween 20 and −5.5 with Tween 60 with NaC [122]. From these studies, the interaction between NaC with cationic > nonionic > anionic surfactants is clearly seen, as expected from hydrogen bonding and electrostatic interactions. Surface tension, conductivity, calorimetry, and fluorescence were performed on the mixed micelles of NaDC and polyoxyethylene tert-Octylphenyl Ether (Triton X-100). Due to the reduction of the repulsion interactions among the anionic species, NaDC, the minimum areas of the mixed surfactants (TX-100 and NaDC) at the air–liquid interface are lower than the pure components. On increasing TX-100 in the mixture, the polarity of the interior of the mixed micelles decreased. Increasing the concentration of NaDC in the mixture resulted in the decrease in the micellar aggregation number. The process of micellization became increasingly endothermic with a fair degree of entropy increase on increasing the mole fraction of NaDC [124]. Binary mixtures of NaDC and polyoxyethylene sorbitan monooleate (Tween 80) were studied by conductance, surface tension, and fluorescence techniques. Various thermodynamic and micellization parameters were calculated using phase separation and mass action models. Using Clint’s, Rubingh’s, and Motomura models, interactions between NaDC and Tween 80 were calculated. An overall attractive force in the mixed micelle state was observed which was evident from the negative values of interaction parameter, β. The values of polarity index and aggregation numbers of mixed micelles decrease with an increasing proportion of NaDC [89].

Appl Biochem Biotechnol Table 1 Physico-chemical parameters of mixed (bile salt–conventional surfactant or amphiphilic drug) surfactant interactions α1 α2 Clint’s equation [105] 1 CM C ideal ¼ CM C 1 þ CM C 2 (Used to calculate the ideal CMC in mixture) C2 xideal ¼ ½ðα1 CM αC12CM Þþðα2 CM C 1 Þ

Motomura

[121]

C ð1−α1 Þ Cα1 X 21 ln XCM ¼ ð1−X 1 Þ2 ln ðCM 1−X 1 ÞCM C 2 1 CM C 1

Rubingh

[106, 107]

f1 = exp[β1(1 − x1) ] f2 = exp[β1x12]

Activity coefficient Activity coefficient

[106, 107] [106, 107]

Interaction parameter

[106, 107]

ΔGex = RT[(X1 ln f1) + (1 − X1)ln f2]

Excess free energy

[106, 107]

ΔHmix = X1(1 − X1)βRT

Enthalpy of mixing

[107, 151]

ΔG0m = (2 − g)RT ln Xcmc

Gibbs free energy (g = degree of couterion dissociation)

[143]

cmc ΔH 0m ¼ −RT 2 ð2−g Þ d lnX dT

Enthalpy (Xcmc = CMC in mole fraction units)

[143]

ΔS 0m ¼ ΔH mT−ΔGm

Entropy

[143]

Free energy of adsorption

[152]

Surface excess

[153]

Amin ¼ N A Γ1 max

Minimum area per molecule

[153]

Πcmc = γ0 − γcmc

Surface pressure (surface tension in pure water surface tension at CMC)

[154]

2

β ¼ ð1−X1

1Þ

2

CM C ln Xα11CM C1

0

ΔG0ads

¼

Γ max ¼

0

cmc ΔG0m −Π Γ max

∂γ 1 − 2RT ∂lnC T;P

α1, α2, CMC1, CMC2, xideal, and β are the mole fractions, experimental critical micelle concentrations, ideal micellar mole fraction, and interaction parameter of surfactant 1 and surfactant 2, respectively. To sense the bile salt–surfactant interactions in ternary aqueous solutions, these parameters are very useful in evaluating the interactions

Bis(quaternary ammonium bromide), a gemini surfactant with NaC, was investigated by surface tension, conductometry, light scattering, light microscopy, and microelectrophoretic techniques in aqueous solution. Nonideal mixing behavior with strong interactions between Gemini surfactant and NaC was observed. The interaction mode and microstructures formed depended upon the electrostatic effects, geometry of molecules, and dissimilar separation of the hydrophobic and hydrophilic moieties in the surfactants. Mixed micelles, vesicles, coacervates, and solid crystalline phases have also been observed [125]. Mixed micelles of Triton X-100 and SDS with cholate, deoxycholate, and 7-oxodeoxycholate were investigated, and their micellization behavior was investigated by surface tension and conductivity techniques. It was observed that TX-100 and SDS show weak synergistic interactions with sodium deoxycholate, while 7oxodeoxycholate shows the strongest attractive interaction with investigated co-surfactants. Increased synergistic interactions were attributed to the hydrophilic groups on bile salts [123]. Dodecylethylammonium bromide (DEAB) and dodecyltrimethylammonium bromide (DTAB) were investigated with NaC and NaDC by freeze-fracture transmission electron microscopy, dynamic light scattering, isothermal titration calorimetry, and absorbance measurements. The effects of electrostatic attractions in catanionic surfactant/bile salt mixed systems were masked by the steric effect of bile salt, thus affecting weak interactions that prevailed in the mixed aggregates [126] (Table 2).

Appl Biochem Biotechnol Table 2 Solubilization and interaction study of bile salts with surfactants Bile salt

Surfactant

Reference

i.Sodium deoxycholate ii.Sodium chenodeoxycholate

i.Decanoyl-N-methylglucamide ii.Hexa-ethyleneglycol mono n-dodecylether

[120]

i.Sodium cholate ii.Sodium deoxycholate

i.Dodecyltrimethylammonium bromides ii.Tetradecyltrimethylammonium bromides iii.Hexadecyltrimethylammonium bromides

[118]

Sodium cholate

Sodium dodecyl sulphate (SDS)

[119]


i.Tween 20 ii.Tween 60

[122]

Sodium deoxycholate Sodium deoxycholate

Polyoxyethylene tert-Octylphenyl ether (Triton X-100) Polyoxyethylene sorbitan monooleate (Tween 80)

[124] [155]

Sodium cholate

Bis(quaternary ammonium bromide) i.Triton X-100 ii.SDS

[125] [123]

i.Dodecyltriethylammonium bromide ii.Dodecyltrimethylammonium bromide

[126]

i.Cholate ii.Deoxycholate iii.7-oxodeoxycholate i.Sodium cholate ii.Sodium deoxycholate

Bile Salt–Drug Interactions Drug-induced liver injury (DILI) is a major and still unresolved scientific problem. DILI can advance on subsequent use of many drugs, both prescription and self-medication, through a variety of mechanisms [127]. Bile salts play a significant part in solubilization of lipids in human beings and other organisms, thus acting as potential carriers in drug delivery [128]. As a potential biological host system, the host–guest interactions of bile salts are reported by many workers using photophysical studies [129, 130]. On the other hand, only a few studies have been reported on bile salt–drug interactions [130–138] (Table 3). The uniqueness of bile salts in having two binding sites makes them capable of delivering hydrophilic and hydrophobic drug molecules [134, 135]. Interactions of phenothiazine tranquilizer drugs (promazine hydrochloride, PMZ, and promethazine hydrochloride, PMT) with bile salts, viz., sodium cholate (NaC) and sodium deoxycholate (NaDC) in aqueous medium, were investigated through conductivity, surface tension, UV-visible, and fluorescence measurements. Mixed micellization of these drugs with bile salts was analyzed by using different theoretical models. Binding of phenothiazine with bile salts as investigated by UV-visible and steady state fluorescence. Benesi–Hildebrand (B–H) equation was used to find the stoichiometric ratios, binding constants, and free energy change for the phenothiazine–bile salt complexes. The CMC values of phenothiazine–bile salt mixed micelles was found to be much lower than their individual components due to which the toxicity of drugs was reduced and their permeability and bioavailability were enhanced. The negative values of ΔG0m and ΔG0ad signified the spontaneity of micellization and adsorption phenomenon, whereas a negative value of ΔG0ex ensured the stability of drug–bile salt mixed micelles [138]. A polyene group based antifungal (natural antibiotic) drug, amphotericin B, has limited solubility at physiological pH in aqueous solution; therefore, its solubility in aqueous medium is enhanced by interacting with biomolecules showing surface activity such as sodium deoxycholate [139–142]. Interaction and aggregation behavior

Appl Biochem Biotechnol Table 3 Solubilization and interaction studies of bile salts with drugs Bile salt

Drug

Reference


i.Iodomethacin ii.Phenylbutazone

[136]

i.Sodium cholate ii.Sodium deoxycholate iii.Sodium taurocholate

Curcumin

[130]


i.Promazine hydrochloride ii.Promethazine hydrochloride

[138]

Sodium deoxycholate

Amphotericin B Imipramine hydrochloride

[139] [143]

Sodium taurocholate

i.Hydrocortisone ii.Triamcinolone iii.Dexamethasone iv.Diazepam v.Betamethasone vi.Pentazocine vii.Griseofulvin viii.Phenytoin ix.Cyclosporine A x.Betamethasone 17-valerate xi.Danazol

[151]


Ellipticine (in the dipalmitoylphosphocholine liposome)

[129]

i.Sodium cholate ii.Sodium deoxycholate iii.Sodium taurocholate iv.Sodium glycocholate i.Sodium cholate ii.Sodium deoxycholate iii.Sodium taurocholate Sodium taurocholate

i.Griseofulvin ii.Hexestrol iii.Glutethimide

[128]

Promethazine hydrochloride (PMT)

[144]

Promazine hydrochloride (PMZ)

[145]


Promazine hydrochloride (PMZ)

[146]


of NaC, NaDC, and NaTC were investigated with antidepressant drug imipramine hydrochloride (IMPH). Various physicochemical parameters (CMC, Gibbs free energy, enthalpy, entropy, and various interaction parameters using different mixed micelle models) were calculated to gain an insight into the interactions prevailing in the system. It was observed that attractive interactions exist between bile salts and IMPH, and the CMC of imipramine hydrochloride reduced in the following order NaDC > NaC > NaTC. The reason for this decrease was that NaDC attaches to positively charged IMP ions strongly reducing head group area more as compared to NaC and NaTC [143]. PMZ and PMT were investigated with NaC, NaDC, and NaTC using conductometry and surface tension; it was observed that the bile salts formed stable mixed aggregates showing synergistic interactions evident from the various interaction parameters such as β, χ1, f1, and f2, etc. calculated using Clint, Rubingh, and Rosen models of mixed amphiphilic systems [144–147] (Fig. 5).


Iodomethacin

Phenylbutazone

Imipramine Hydrochloride

Curcumin

Amphotericin B

Promethazine Hydrochloride

Proazamine Hydrochloride Fig. 5 Structures of some drug molecules

Solubilization Effects of Bile Salts on Drugs To understand the absorption of various poorly soluble drug molecules, the investigation of solubility and the extent of dissolution is the primary step. It has become a well-established research area to increase the productivity and efficiency of the


Hexestrol

Hydrocortisone

Triamcinolone

Dexamethasone

Diazepam

Betamethasone

Fig. 6 Structures of drug molecules

pharmaceutical industry in delivering and manufacturing high-efficiency drugs [10, 148]. Bile salts improve the bioavailability of poorly absorbable drugs by increasing the dissolution rate of the drug or by facilitating the transport rate of the solute across the intestinal wall by lowering the surface tension of gastrointestinal fluid or by solubilization. In general, drugs are composed of two parts—active pharmaceutical ingredient (API) and an inert or inactive part of drug called excipient. Interaction


(R)-Pentazocine

Griseofulvin

Phenytoin

Cyclosporin A

Glutethimide

Betamethasone 17-valerate

Danazol

Fig. 6 (continued)

between API and bile salts still remains a research area to be investigated at molecular level [149]. Mixed micelles of taurocholate-API formation were


investigated. It was observed that due to the amphiphilic properties of both API and taurocholate, solubility of API increased with the taurocholate-API micelle formation [150]. NaDC and NaC were used to enhance the solubility of iodomethacin (a nonsteroidal antiinflammatory drug) and phenylbutazone (a nonsteroidal antiinflammatory drug) in pH 7.3 buffer at 335 K. It was observed that the solubilization of iodomethacin increased due to micellar solubilization and solubilization of phenylbutazone was ascribed to the wetting effect [136]. Miyazaki and his co-workers studied the solubilization and absorption of iodomethacin and phenylbutazone using bile salts in rats; they observed that the bile salts enhanced the absorption of these drugs [137]. Poor water solubility of curcumin (diarylheptanoid) with bile salts NaTC, NaCh, and NaDC was studied using photophysical approach [130]. It was observed that the NaTC is more potent in encapsulating and subduing the degradation of the curcumin than the aggregates of NaC and NaDC. Interaction of acidic group of curcumin with anionic group of the bile salts results in a significant change in the photophysics of curcumin, as curcumin is almost insoluble in water but upon aggregate formation with bile salts results in its increased solubilization which is also evident from the increased absorption intensity [130]. Sodium taurocholate (NaTC) was used as a model bile salt to study the solubilization of triamcinolone, hydrocortisone, dexamethasone, phenytoin, betamethasone, griseofulvin, diazepam, cyclosporine A, betamethasone 17-valerate, pentazocine, and danazol. It was observed that the aqueous solubility (μg/ml) of these drugs followed the order hydrocortisone > triamcinolone > dexamethasone > diazepam > betamethasone > pentazocine > griseofulvin > phenytoin > cyclosporine A > betamethasone 17-valerate > danazol. In presence of NaTC (15 mM), the solubility (μg/ml) followed the order pentazocine > diazepam > griseofulvin > phenytoin > cyclosporin A [150] (Fig. 6).

Conclusions and Future Perspectives Micelles of bile salts are more highly charged with different structure and smaller, as compared to conventional ones. Bile salts solubilize many soluble and lipidic substances owing to formation of mixed micelles. To solubilize nonpolar substances particularly cholesterol and fatty acids, their importance in solubilizing many biologically important insoluble substances such as phospholipids and monoglycerides is a new area of interest. In biotechnology and in pharmaceutical industry, cholanology (bile acid science) plays an important role. The uniqueness of bile salts in having two binding sites makes them capable of delivering hydrophilic and hydrophobic drug molecules. Bile salts inside human beings have the special capacity to dissolve cholesterol due to the formation of mixed micelles between bile salts and phospholipids. Drug-induced liver injury (DILI) is a major and still unresolved scientific problem; thus, these types of interactions can be utilized to overcome this problem. Bile salt–drug interaction studies are not extensively studied; therefore, absorption of various poorly soluble drug molecules with the bile salts can be helpful in further investigation. Thus, being bio-surfactants, their impact on the well-being of humans will be a boon. Acknowledgments Nisar Ahmad Malik would like to thank Head Department of Chemistry, University of Kashmir for providing the necessary facilities and to Dr. Aijaz Ahmad Dar for his valuable suggestions.


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