Journal of Bangladesh Chemical Society, Vol. 27(1 & 2), 49-59, 2014

INTERACTION OF MOXIFLOXACIN HYDROCHLORIDE WITH CETYLDI- METHYLETHYLAMMONIUM BROMIDE SK. MD. ALI AHSAN, MOHAMMED DELWAR HOSSAIN, MD. ANAMUL HOQUE, MOHAMMED ABDULLAH KHAN* Department of Chemistry, JahangirnagarUniversity, Savar, Dhaka- 1342, Bangladesh

Abstract Interaction of a fourth-generation synthetic fluoroquinolone antibacterial agent namely Moxifloxacin hydrochloride (MFH) with a model cationic surfactant namely Cetyldimethylethylammonium Bromide (CDMEAB) has been carried out by

conductance measurements in water and in the presence of salts such as NaCl, Na 2SO4 and Na3PO4 over the temperature range of 298.15-318.15K. Two critical micelle concentrations were obtained for MFH-CDMEAB in all the cases. The change of c* values of CDMEAB due to the addition of MFH is indicative of the interaction between MFH and CDMEAB. For the MFH–CDMEAB system, the values of c* are higher in magnitude in contrast to that of pure CDMEAB in water at a particular temperature. The both c* values for MFH–CDMEAB system in the presence of salts are lower in magnitude compared to those in aqueous medium which indicates added electrolytes 0 favor the micellization of MFH-CDMEAB system. The ∆G m values were negative and the spontaneity of micellization process is found to be increased with rise of temperature. 0 0 The values of ∆H m and ∆S m reveal that the binding interactions between MFH and CDMEAB in water are both electrostatic and hydrophobic in nature while hydrophobic contribution plays the major role at lower temperatures. In some cases enhanced hydrophobic binding between MFH and CDMEAB was observed in presence of salts. 0 0 The presence of linear correlation between ∆H m and ∆S m values was observed in all cases. Introduction Surfactants are extensively used in a wide variety of commercial products such as industrial and household detergents, pharmaceutical formulations to facilitate the preparation, patient tolerability and effective of the dosage form, emulsifiers, adhesives, paints and also be used as diluents, suspending agents, disintegrating agents, and solubilizing agents1-4. In solution surfactants can form supramolecular assembly as well as molecular aggregates known as micelles. Surfactants have multipurpose usage as physical models as simplified model of biomembranes5. Surfactant micelles have been used considerably to enhance the water solubility of many pharmaceutical components that stands for a difficult problem in formulation of an acceptable dosage form 6-10. The interpretation of the interaction of drugs with surfactant micelles can be visualized as estimation for their interactions with biological surfaces. The changes in surfactant structure, the nature of the counter ions, concentration of the added electrolytes, temperature etc.11 can modify the size, flexibility and type of interactions of micelles significantly. Literature survey reveals that a detail study regarding drug-surfactant interactions is still necessary. MFH (Scheme I), a fourth-generation synthetic fluoroquinolone antibiotic drug, has been prescribed for use orally, in parenteral form for intravenous infusion, as an ophthalmic *Author for correspondence: e-mail: [email protected] 2 SK. MD. ALI AHSAN, M. DELWAR HOSSAIN, MD. ANAMUL HOQUE, M. ABDULLAH KHAN solution and for the medication of conjunctivitis. MFH is used to treat multifarious diseases like cellulitis, respiratory tract infections, anthrax, endocarditis, intra-abdominal infections, tuberculosis, meningitis etc. In our previous studies, interaction of some cephalosporin drugs with different ionic surfactants was reported12-14. To the best of our knowledge, the interaction of MFH with CDMEAB has not been studied yet. Keeping this in mind, a study of the interaction of a model quinolone drug like MFH with a model cationic surfactant like CDMEAB has been undertaken using conductometric technique. The values of critical micelle concentration (c*), fraction of counter ion binding (β) and 0 0 0 0 thermodynamic parameters such as ∆G m, ∆H m, ∆S m and ∆mC p associated with the MFH mediated CDMEAB micellization in pure water as well as in different salts solution like

NaCl, Na2SO4 and Na3PO4 have been determined to illustrate the MFH-CDMEAB interactions.

Materials and Method CDMEAB of pureness 99% (Acros Organics, USA), MFH (USP standard sample of pureness 98%, provided by General Pharmaceuticals, Bangladesh), NaCl of pureness

99.5% (BDH, England), Na2SO4 of pureness 99% (Merck, Mumbai) and Na3PO4 (Merck) were used without any further treatment. Distilled-deionized water of specific conductance 0.8-1.5 μS cm-1 was used in all ground works. The specific conductances of the MFH-CDMEAB systems both in water and in aqueous salts solution were measured using a 4510 conductivity meter (Jenway, UK) with a temperature-compensated cell (cell constant provided by manufacture is 0.97cm-1) following the procedure reported in the literatures12-18. The accuracy of the multimeter was within ± 0.5 %. The concentrated CDMEAB (50.0 mM) solution was gradually added to the MFH solution taken in a test tube and then the conductances at each dilution were measured after thorough mixing as well as allowing time for the attainment of temperature equilibrium. The desired constant temperature was maintained using RM6 Lauda circulating water thermostated bath with precision of ±0.1K. To study the effect of salts such as NaCl, Na2SO4, and Na3PO4 on the interaction of MFH with CDMEAB, both MFH and CDMEAB solutions were prepared in presence of same salt. For this purpose both the solution of MFH and CDMEAB were prepared in the same electrolyte solution having the identical ionic strength (I). Results and Discussion The specific conductance value of CDMEAB solutions is changed with the addition of MFH drug in water as well as in presence of salts and a typical plot of specific conductance (k) versus concentration of CDMEAB for MFH-CDMEAB system in water at 303.15 K is shown in Fig. 1. An abrupt change in the κ values with gradual addition of CDMEAB to MFH solution was observed showing a sharp break point and the concentration of surfactant corresponding to the break point is known as the critical micelle concentration12-19. INTERACTION OF MOXIFLOXACIN HYDROCHLORIDE WITH CETYLDIMETHYLETHYLAMMONIUM 3

Fig. 1. Specific conductivity (κ) versus concentration of CDMEAB for MFH-CDMEAB system in water at 298.15K.

Two such break points are observed for MFH-CDMEAB systems both in pure water and in * * * aqueous solutions of salts and are labeled as c 1 and c 2. The c 1 value reveals the concentration of * surfactant at which association between drug and surfactant starts whereas the c 2 value is the * indicative of the drug saturation by surfactants. The observation of more than one c value is also reported for different systems previously13-16. The degree of ionization of micelles (α) was determined from the quotient of the slopes of the two intersecting straight lines corresponding to the upstairs and beneath c*12-16, 20-22. By deducting the value of α from unity, the fraction of counter ion binding, β at c* was determined. Table 1. Values of c* and β for MFH-CDMEAB system in water containing different concentrations of MFH at 303.15K.

* * cdrug / c 1/ c 2 / β1 β2 mM mM mM 0.00a 0.90 3.70 0.79 0.86 0.050 0.98 6.16 0.81 0.90 0.10 1.02 6.28 0.80 0.89 0.50 1.01 7.05 0.85 0.90 1.00 0.94 6.11 0.77 0.85 2.00 0.92 6.09 0.72 0.80 aRef.[13]

* The values of c and β in water containing different concentrations of drugs at 303.15K are shown in Table 1. The values of c* for pure CDMEAB in water was found to be changed with the addition of MFH and the c* values for MFH-CDMEAB system were almost higher in magnitude compared to those of pure CDMEAB in water. There is an alteration in the c* values for MFH-CDMEAB system with the concentrations of drug at 303.15K temperature which indicates the interaction between MFH and CDMEAB. The values of c* and β for MFH-CDMEAB system at 303.15K temperature in the presence of * NaCl, Na2SO4 and Na3PO4 are presented in Table 2. The c values of MFH-CDMEAB system at 303.15K temperature in salts solution were found to be lower in magnitude compared to the reported c* values in water. The c* values were found to increase with increasing of the ionic strength of NaCl, after certain ionic strength of NaCl the c* values were found to decrease gradually with further increase in the ionic strength of the salt. In * case of Na2SO4, the c 1 values were found to decrease gradually with increase of salts * ionic strength whereas the c 2 values were found to increase with increase of the ionic * strength, then after certain ionic strength the c 2 values tend to decrease gradually with * further increase in the ionic strength of salts. In the presence of Na 3PO4 the c values were found to decrease initially, passed through a minimum and then tend to increase with 4 SK. MD. ALI AHSAN, M. DELWAR HOSSAIN, MD. ANAMUL HOQUE, M. ABDULLAH KHAN further increase of salt ionic strength. The c* values of MFH-CDMEAB system at 303.15K at I = 0.50 mM followed the order: cNaCl> cNa3PO4> cNa2SO4 (Table 2). This change of c* values may be due to the presence of ions of different character. Chloride (Cl -) ion is a moderate chaotrope, having large singly charged ion with low charge density, rupture water structures and weaken the stability of hydrophobic aggregates of surfactant molecules. Both Sulfate and phosphate are strong kosmotropes, having small multi charged ion with high charge density, interact with water strongly as water structure maker and stabilize hydrophobic aggregates of CDMEAB molecules. Thus Na2SO4 and Na3PO4 salt out the hydrophobic chains of surfactants from aqueous medium and lowers the c* values of surfactant system significantly compared to that of NaCl. Table 2. Values of c* and β for MFH-CDMEAB system in the presence of salts like

NaCl, Na2SO4 and Na3PO4 at 303.15K.

* * Salts I / c 1/ c 2 / β1 β2 mM mM mM NaCl 0.001 0.90 6.40 0.89 0.93 0.01 0.95 6.42 0.85 0.91 0.05 1.15 6.48 0.82 0.89 0.30 1.03 6.61 0.84 0.89 0.50 0.88 6.35 0.89 0.93 1.00 0.82 6.32 0.84 0.90

Na2SO4 0.30 0.91 4.70 0.80 0.89 0.50 0.61 5.01 0.74 0.85 0.90 0.40 4.31 0.88 0.94 1.50 0.37 4.09 0.90 0.94 3.00 0.34 3.94 0.95 0.96

Na3PO4 0.06 0.99 4.94 0.70 0.80 0.50 0.64 5.69 0.78 0.86 0.60 0.51 5.05 0.70 0.83 1.80 0.47 4.87 0.81 0.86 3.00 0.49 5.36 0.86 0.88 6.00 0.57 6.25 0.87 0.87

The kosmotropic effect of phosphate is found to be comparatively lower than that of sulfate ion. With addition of salt, a decrease of c* values was observed for the micellization of pure ionic surfactants and also for drug-surfactant systems 12-14, 23-26. The entire effects of an electrolyte come into view to verge on the sum of its drifts on the drug and surfactant molecule in connection with the aqueous phase. Hydrophilic groups of the surfactant molecules are associated with the aqueous phase in mutually the monomeric and micellar forms of the surfactant while the hydrophobic groups are associated with the aqueous phase only in the monomeric form. Thus the consequence of the electrolyte on the hydrophilic groups in the monomeric and micellar forms may eliminate each other, INTERACTION OF MOXIFLOXACIN HYDROCHLORIDE WITH CETYLDIMETHYLETHYLAMMONIUM 5 withdrawal the hydrophobic groups in the monomers as the moiety most likely to be conceited by the toting of electrolyte to the aqueous phase. The values of c* and β at different temperatures for MFH–CDMEAB system in pure water and in the presence of salts such as NaCl, Na2SO4 and Na3PO4 are presented in * Tables 3 and 4. The values of c 1 and β at different temperatures in pure water were found * to increase gradually with increasing temperatures whereas the c 2 values initially increased, passing a maximum, then tend to decrease with further increase of temperature. Table 3. Values of c* and β for the pure CDMEAB and MFH–CDMEAB systems in water containing 0.50 mM MFH at different temperatures.

* * System T / c 1 / c 2 / β1 β2

K mM mM

CDMEABa 298.15 0.88 4.10 0.79 0.87 MFH-CDMEAB 298.15 0.92 6.91 0.77 0.83 303.15 1.01 7.05 0.84 0.89 308.15 1.09 6.59 0.75 0.83 313.15 1.13 6.34 0.71 0.86 318.15 0.98 5.83 0.79 0.88 aRef.[13]

Table 4. Values of c* and β for the MFH–CDMEAB system containing 0.50 mM MFH in aqueous solution of salts at different temperatures.

* * Salts I / T / c 1/ c 2/ β1 β2 mM K mM mM

NaCl 0.50 298.15 0.94 7.75 0.84 0.88 303.15 0.88 6.35 0.89 0.93 308.15 0.87 6.75 0.80 0.86 313.15 0.95 6.93 0.79 0.86 318.15 0.99 6.95 0.79 0.86

Na2SO4 0.50 298.15 0.69 4.98 0.81 0.86 303.15 0.61 5.01 0.74 0.85 308.15 0.72 6.27 0.77 0.85 313.15 0.73 5.45 0.74 0.85 318.15 0.75 5.41 0.74 0.86

Na3PO4 0.50 298.15 0.68 5.22 0.80 0.87 6 SK. MD. ALI AHSAN, M. DELWAR HOSSAIN, MD. ANAMUL HOQUE, M. ABDULLAH KHAN

303.15 0.64 5.69 0.78 0.86 308.15 0.69 6.08 0.78 0.86 313.15 0.74 5.35 0.75 0.86 318.15 0.78 5.30 0.76 0.84

In the presence of NaCl both the c* values were found to decrease initially, passed through a minimum, then tend to increase with further increase of temperature. In the * presence of Na2SO4 and Na3PO4 salts, the c 1 values were found to be decrease up to certain temperature, then the values tend to increase with further increase of temperatures * * while the c 2 values followed the opposite trend. The change of c values with temperature can be explained with the change of the mode of hydration surrounding the surfactant monomers as well as the drug mediated CDMEAB micelles. In monomeric form of the surfactant, both the hydrophobic and hydrophilic hydrations are possible whereas only hydrophilic hydration is possible for micellized CDMEAB. Both types of hydrations are expected to decrease with increasing temperature. A decrease in hydrophilic hydration favours the micelle formation while a decrease of hydrophobic dehydration with the increase of temperature disfavours the micelle formation 12, 13. Thus the magnitude of these two factors determine whether the c* values increase or decrease over a particular temperature range.

The thermodynamic parameters are an effective tool to study the mode of interaction in the molecular level. By using the following equations, the thermodynamic parameters of MFH-CDMEAB system containing 1:1 electrolyte surfactant were determined12-16, 27-30:

0 * ∆G m = (1+β) RTln(c ) (1)

0 2 * ∆H m = - (1+β) RT (∂lnc / ∂T) (2)

0 0 0 ∆S m= (∆H m-∆G m) / T (3)

* * Where values of c were in used in mole fraction unit. ln(c 2) vs. T plot (Fig. 2) was found 0 12-15, 25, 29 nonlinear. The plots were used to calculate ∆H m and slopes were drawn at each temperature as equal to ∂ln(c*) ⁄ ∂T 25, 29.

* Fig. 2. ln(c 1) versus T for MFH-CDMEAB system in water.

The thermodynamic parameters for MFH-CDMEAB system in pure water and in the 0 presence of NaCl, Na2SO4 and Na3PO4 salts are presented in Table 5. The ∆G 1, m and 0 ∆G 2, m values for all the systems are found to be negative which indicates that the micellization processes are thermodynamically spontaneous. INTERACTION OF MOXIFLOXACIN HYDROCHLORIDE WITH CETYLDIMETHYLETHYLAMMONIUM 7

0 For MFH–CDMEAB in water, the ∆H 1,m values are found to be initially positive, the 0 sign of ∆H 1,m value changes from positive to negative and the negative values tend to 0 increase with further rise of temperature. The values of ∆S 1,m are found to be positive and the values decrease gradually with increasing temperature. Thus the first aggregation process is found to be entropy controlled at lower temperature and becomes both enthalpy and entropy controlled at higher temperatures. For the MFH–CDMEAB system in water, 0 0 the ∆H 2,m value is positive at 298.15 K, the sign of the ∆H 2,m value changes from 0 positive to negative and then the negative ∆H 2,m value decreases with further rise in temperatures while there is some uncertainty in the magnitude of the value at higher 0 temperature. The values of ∆S 2,m are positive and the positive values decrease with increase of temperature. Thus the MFH mediated CDMEAB micellization process is entropy controlled at temperature 298.15K which becomes both enthalpy and entropy controlled at higher temperatures. The results reveal that the binding interactions between MFH and CDMEAB are both electrostatic and hydrophobic in nature while hydrophobic contribution plays the major role at lower temperatures.

0 In aqueous solution of NaCl salt, the ∆H 1,m values were found to be negative both at 0 lower and higher temperatures whereas the ∆H 1,m values were found to be positive at 0 intermediate temperatures. The ∆H 2,m values were found to be positive both at lower and 0 higher temperatures whereas the ∆H 2,m values were found to be negative at intermediate 0 temperatures. The values of ∆S 1,m are negative both at lower and higher temperatures and 0 0 the values of ∆S 1,m are positive at intermediate temperatures. The ∆S 2,m values were found to be positive, the values decreased up to certain temperature and then tend to increase with temperature. Thus the behaviour of first micellization in presence of NaCl, both at lower and higher temperature was entirely enthalpy controlled while that becomes entropy controlled at the intermediate temperatures. The second micellization process was entropy controlled at lower and higher temperatures and that becomes both entropy and enthalpy controlled at intermediate temperatures. In aqueous solution of Na 2SO4 and Na3PO4 salts, the behavior of the change of thermodynamic parameters indicates that the first micellization process is both entropy and enthalpy controlled both higher and lower temperatures and mostly entropy controlled at the intermediate temperatures. Whereas the second micellization process in presence of Na2SO4 and Na3PO4 salts is similar to MFH- 0 CDMEAB in pure water. The net ∆H m is the sum of the change in enthalpies arising from hydrophobic interactions, electrostatic interactions and hydration of polar head groups. A 0 negative ∆H m may occur when second and third effects become more effective and the 0 0 positive ∆H m may arise when the first effect is dominant. The positive values of ∆S m for MFH mediated surfactant micellization can be explained considering two factors. These are: (1) transfer of hydrophobic chains from hydrated form in aqueous medium to the nonpolar interior of the micelle destroying iceberg structures and (2) intensification of rotational degree of freedom of hydrophobic chains in the micelle interior compared to 31, 32 0 the aqueous environment . The negative values of ∆S m may occur when the formation of iceberg structure surrounding the MFH and CDMEAB is much dominant than the above two effects. 8 SK. MD. ALI AHSAN, M. DELWAR HOSSAIN, MD. ANAMUL HOQUE, M. ABDULLAH KHAN

0 The molar heat capacity changes (∆mC p) for micelle formation, is an important sign of protein structural changes in response to different ligands which is obtained from the 0 33, 34 slope of the plot of ∆H m versus temperature .

0 0 ∆mC p = ((∂H m)/∂T) p (4)

0 For MFH-CDMEAB system in pure water, the values ∆mC 1,p were found to be negative 0 and the ∆mC 2,p values were initially negative and the values change into positive at higher 0 temperatures. In the presence of NaCl, Na2SO4 and Na3PO4 salts, the ∆mC 1,p values are positive at lower temperatures and the negative at higher temperatures whereas the 0 ∆mC 2,p values follow the similar trend that observed in pure water. The change in heat capacity associated with MFH-CDMEAB binding was believed to be associated with motion restriction and is proportional to the funeral of the molecular surface, which generally draws a parallel with a change in the solvent accessible surface area 34. 0 However, the small ΔmC p and the positive binding entropy indicate minor structural rearrangement of CDMEAB micelle during binding with MFH whereas in the case of aggregation the effect was significant at lower temperatures.

0 0 2 The enthalpy-entropy compensation, a linear relationship between ∆H m and ∆S m with R value in the range of 0.994-0.999 was observed in all cases according to the following regression equation 35:

0 0,* 0 ∆H m= ∆H m + Tc ∆S m (5)

Where the slope, Tc the compensation temperature, describes the solvation part of micellization process and assists as the basis of comparison for differing examples of 0,* compensation behavior and the intercept ∆H m, was the intrinsic enthalpy gain. The 0 values of ∆H m and Tc for both systems in pure water and in the presence of salts are shown in Table 6. The Tc values for MFH-CDMEAB system are almost the same both in pure water as well as in the presence of salts. Table 6. Enthalpy-entropy compensation parameters for MFH-CDMEAB system containing 0.50mM in water and in aqueous salts solution.

0,* 0,* Medium I / ∆H 1,m / ∆H 2,m / Tc,1/ Tc,2/ mM kJ.mol-1 kJ.mol-1 K K

H2O 0.00 -49.41 -43.40 302.98 307.25

H2O-NaCl 0.50 -50.32 -43.35 303.44 312.45

H2O- Na2SO4 0.50 -50.77 -43.31 306.47 305.21

H2O- Na3PO4 0.50 -51.02 -42.75 308.27 298.16 INTERACTION OF MOXIFLOXACIN HYDROCHLORIDE WITH CETYLDIMETHYLETHYLAMMONIUM 9

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