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Growth and Shrinkage of Pluronic Micelles by Uptake and Release of Flurbiprofen: Variation of pH Shirin Alexander,† Wiebe M. de Vos,† Thomas C. Castle,‡ Terence Cosgrove,*,† and Stuart W. Prescott*,† †

School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Revolymer Ltd., 1 New Tech Square, Zone 2, Deeside Industrial Park, Deeside, Flintshire CH5 2NT, U.K.

‡

ABSTRACT: The micellization of Pluronic triblock copolymers (P103, P123, and L43) in the presence of flurbiprofen at different pH was studied by small-angle neutron scattering (SANS), pulsed-field gradient stimulated-echo nuclear magnetic resonance (PFGSE-NMR), and surface tension measurements. Addition of flurbiprofen to the Pluronic at low pH leads to an increase in the fraction of micellization, aggregation number, and the core radius of the micelles. However, changing the pH to above the pKa of flurbiprofen in an ethanol/water mixture (∼6.5) reduces the fraction of micellization and results in a weaker interaction between the drug and micelles due to the increased drug solubility in aqueous solution.

1. INTRODUCTION Oral administration is the most common route for drug delivery; however, drugs formulated for this route need to be solubilized in aqueous solvents. This is a challenge for hydrophobic drugs, and different approaches have been used to overcome this problem, such as encapsulation, changing the pH of the solution, and using mixed solvent systems.1,2 In recent years, amphiphilic block copolymers have been studied in this respect, especially for their potential use in drug delivery. In the presence of a selective solvent that is good for one block and a poor solvent for the other, amphiphilic block copolymers may form micelles and the core of the micelle can encapsulate hydrophobic compounds. Many studies in this particular area have focused on releasing drugs from the delivery vehicles using external stimulations, such as ultrasonication, varying pH, and temperature.3−5 One of the most widely studied classes of amphiphilic copolymers in this field is the Pluronic triblock copolymers (nonionic triblock copolymers with a central block of poly(propylene oxide), PPO, and end blocks of poly(ethylene oxide), PEO: [PEO−PPO−PEO]). The aggregation behavior of Pluronics strongly depends on temperature and concentration; multimolecular micelles form above the critical micellization concentration (cmc) and critical micellization temperature (cmt).6,7 Much research has focused on formulating and assessing Pluronic organogels containing hydrophobic, pH-sensitive drugs (such as flurbiprofen) for topical applications.8,9 A drug precipitation assay from dilute Pluronic solutions (below cmc) has also been examined in combination with a pH-sensitive model compound. It was demonstrated that drug release can be inhibited by Pluronic F127 in simulated gastric fluid (pH = 1.2) and released in simulated intestinal fluid (pH = 7.4).10 © 2012 American Chemical Society

Precipitation of drugs in vivo due to pH changes throughout the body has been reported. Gu et al.11 have described the dissolution and precipitation behavior of poorly soluble drugs by simulating the conditions in the gastrointestinal region. It was observed that weak basic drugs when dissolved in the gastric compartment, precipitated in the intestine at higher pH. Similarly, solubilization of the hydrophobic drug hydrochlorothiazide was examined at a range of pH values by Yogesh et al.12 It was shown that the drug solubility in the presence of micelles increased at a low pH at which the drug remains un-ionized. We have previously used PFGSE-NMR and surface tension measurements to study the aggregation behavior of Pluronic micelles (P103, P123, and L43) in the presence of flurbiprofen and ibuprofen under different conditions of concentration of drug and polymer.13,14 It was found that the addition of drug to an aqueous solution of Pluronic copolymers influences properties such as the cmc, size, aggregation number, and the structure of the micelles. These previous studies showed that the drug is in fast exchange between the micelle core and the bulk solvent and, in agreement with other work on Pluronic micelles, that there is a substantial volume fraction of solvent in the core of the micelles.13,14 Flurbiprofen (Figure 1) is a weak acid with a pKa of ∼4.2− 4.515−19 in water. By using a potentiometric titration, we measured a pKa of ∼6.5 in the ethanol/water mixture (10:90) used throughout this work. Oumada et al.20 have also determined the pKa of some water-insoluble nonsteroidal anti-inflammatory drugs (NSAID) in methanol/water mixtures Received: October 31, 2011 Revised: April 3, 2012 Published: April 3, 2012 6539

dx.doi.org/10.1021/la204262w | Langmuir 2012, 28, 6539−6545

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KWS-1 instrument at the JCNS, Germany. LOQ is a fixed-sample detector instrument that uses neutrons with wavelengths between 2.2 and 10.0 Å to provide a Q range of 0.009−1.0 Å−1. All samples were measured in 1 mm path-length rectangular quartz cells, and a D2O/d6ethanol mixture (90:10) was run as a background sample. The measurements were carried out at 298 K. Data reduction was carried out using the Colette program, and data were fitted using the Pedersen model for Pluronic micelles.13,24,25 This model uses a form factor for a spherical core surrounded by noninteracting Gaussian chains (Figure 2).26,27

Figure 1. Structure of flurbiprofen.

and obtained higher pKa compared to pure water. Since ethanol is commonly used in drug formulations, it is important to understand the effect of ethanol on the drug release at various pHs; hence, ethanol/water mixtures were used throughout this study. Flurbiprofen solubility increases by a factor of 4 in water, from pH 5 to 7.21 In 10% ethanol/water mixture this increases approximately by another factor of 2. Changes in the pH, for example in the digestive system, will thus have a profound effect on the aggregation properties of Pluronic−flurbiprofen complexes. We have investigated the structure of Pluronic−drug complexes using small-angle neutron scattering (SANS) for Pluronics P123, P103, and L43; each of these polymers consist of 30% PEO but have different chain lengths, as shown in Table 1. For both P103 and P123 the experimental conditions used

Figure 2. Schematic depiction of a block copolymer micelle based on the Pedersen model, with the core radius (Rc), PEO segment radius of gyration (Rg), and interaction radius (Rint). The model has 20 input parameters that are necessary when examining the complex aggregation behavior of the Pluronics; however, the values of many of parameters can be set to known values in order to obtain consistent data sets. Some of the most important and sensitive parameters used in this model are the aggregation number (Nagg), volume fraction of solvent in the core (ϕsol), radius of gyration of PEO (Rg(PEO)), incoherent background scattering (B/cm−1), the equivalent hard-sphere volume fraction (ϕHS), the interaction radius (Rint/Å) which is taken as Rc + 2Rg(PEO) (this describes the distance from the center of the micelles, over which it is able to interact with other micelles), the (Gaussian) polydispersity of the aggregation number (σNagg), the scattering length density difference of the core (Δρcore/Å−2), the Q resolution, the number of ethylene oxide units (NEO) and propylene oxide units (NPO), concentration (c/w/v), fraction of micellization (f mic), scattering length density of the solvent (ρsol/Å−2), and the core radius (Rc/Å). The aggregation number (Nagg) is directly related to the core radius (Rc) according to24

Table 1. Composition of Pluronics Used ethylene oxide unitsa propylene oxide unitsa Mnb/kg mol−1 Mwb/kg mol−1 polydispersity indexb a

P123

P103

L43

40 70 6.3 7.1 1.13

34 60 5.5 5.6 1.02

13 22 2.0 2.1 1.05

Values supplied by BASF.23 bDetermined using MALDI-TOF MS.

here are well above the cmt and cmc;6,22 for L43 the chosen temperature is below the cmt at the polymer concentration used.14 Our previous studies and the SANS data presented here show similar complexation behavior for P103 and P123 while the formation of L43−drug complexes is unfavorable. Consequently, in the subsequent studies of the dynamics of these systems reported here, we confine our NMR and surface tension experiments to P103−flurbiprofen systems.

Nagg =

4πR c 3(1 − ϕsol) 3NPOVPO

(1)

3

where VPO = 96.3 Å is the propylene oxide volume and is calculated from its mass density (ρPO = 1.01 g/cm3). As was previously shown by Foster et al.,13 the relatively small amount of drug that is located in the Pluronic micelle core does not change the scattering appreciably so including this in the model is unnecessary. Parameters such as the Q resolution, NEO, NPO, c, and ρsol (6.28 × 10−6 Å−2 for d6-ethanol/D2O 10:90) were fixed as they are determined by the experimental conditions. Data fitting was carried out using the absolute intensities measured; no scale factors were required. 2.3. PFGSE-NMR Spectroscopy. PFGSE-NMR measurements were carried out at 298 K on a Bruker DSX-300 MHz spectrometer with a “Diff 30” field gradient probe using a 5 mm 1H/2H coil insert. The gradient pulse duration (δ) was set to 1−2 ms depending on the sample, and the magnetic field gradient (G) was varied from 0.05 to 10 T m−1. The diffusion time (Δ) was set to 150 ms. Calibration of the instrument was carried out using a water/methanol reference sample. The stimulated-echo signals were Fourier-transformed, and the resulting spectra (signal area as a function of gradient strength) were used to calculate the diffusion coefficients (D, m2 s−1) for each chemically distinct species. The attenuation of the echo signal intensity follows eq 2:

2. MATERIALS AND METHODS All compounds were used as received. Pluronics P103, P123, and L43 were provided by BASF. Flurbiprofen was supplied by Sigma-Aldrich. D2O (99.94 at. % D), d6-ethanol (99 at. % D), DCl, 35% in D2O (99.0 at. % D), and NaOD, 40 wt % in D2O (99.5 at. % D), were purchased from Goss Scientific Instruments Ltd. DCl and NaOD were diluted with D2O to produce 0.1 M solutions. 2.1. Sample Preparation. The Pluronics and the solvent (10% d6ethanol in D2O) were weighed into a vial and placed on a roller mixer overnight. The drug was then added to the solutions, and samples were stirred using a magnetic stirrer for 2 h to ensure the complete uptake of the drug by the micelles. They were then left on a roller− mixer for 24 h to reach equilibrium. The pH of each sample was adjusted to the required value by addition of NaOD or DCl and samples were left on the roller−mixer again for 24 h to equilibrate. 2.2. Small-Angle Neutron Scattering. SANS measurements were carried out on the LOQ instrument at ISIS, Didcot, UK, and the 6540


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From Figure 3, it is clear that P123 shows a slightly higher aggregation number than P103 (see Table 2) which is due to the larger chain length of P123.

(2)

where A(0) is the initial peak area, A(G) is the peak area for each gradient step, and γN is the gyromagnetic ratio of the nucleus, which is 2.675 × 108 rad T−1 s−1 for a proton. A plot of ln(A(G)) vs [(GδγN)2(Δ − δ/3)] as shown below gives a straight line with gradient −D. If the plot shows two distinct slopes, then the data are analyzed with two different diffusion coefficients. The hydrodynamic radii (Rh) can then be obtained from the Stokes−Einstein equation (eq 3), assuming that the aggregates are spherical and noninteracting.

Rh =

kBT 6πηD

Table 2. SANS Parameters for Pluronics P123 and P103 at 5% w/v at 298 K in D2O/d6-Ethanol

(3)

where kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the solvent mixture. 2.4. Surface Tension. The surface tension of the Pluronic P103 solutions (10 wt %) in 10% ethanol, 90% H2O, with a drug concentration of 0.6 wt %, at four different pH values (3, 4.5, 6.5, and 12) were determined using a K100 tensiometer at the Krüss Surface Science Centre at the University of Bristol, using the Wilhelmy plate method. All the samples were passed through 0.45 μm Millipore filter to remove any impurities before the measurements. The measurements were carried out at 298 K unless otherwise stated. 2.5. Potentiometric Titration. The pKa of the flurbiprofen in the ethanol/water mixture (90/10) used throughout this study was measured using an Orion 420A pH meter at a flurbiprofen concentration of 0.5% w/v at 298 K.

Pluronics

P123

P103

Nagg ± 2.5 ϕsol ± 0.03 Rg(PEO)/Å ± 0.54 ϕHS ± 0.004 Rint/Å ± 1.5 f mic ± 0.03 Rg(unimer)/Å ± 1.4 Rc ± 2.5/Å

70.4 0.35 17.9 0.13 89.1 0.93 18.7 56.3

41.2 0.37 13.5 0.11 71.4 0.91 14.9 44.8

The aggregation parameters obtained by fitting the data to the Pedersen model are shown in Table 2. These data show that by increasing the hydrophobic part of the Pluronics, the aggregation number, the fraction of micellization, and the volume fraction of the hard spheres increased. This is in line with data from Foster et al.13 and Ganguly et al.,29 who found similar behavior. As can be seen in Figure 3, the SANS of L43 at low Q increases slightly. This implies the presence of large aggregates, and it is believed to be due to the existence of hydrophobic diblock or triblock impurities in the unpurified industrial samples. Similar behavior was observed by Alvarez-Ramirez et al.30 and Kositza et al.,31 who reported an increase in both Rh and light scattering intensity around the CMTs of P103 and L64 that disappeared once the industrial samples were purified and once the micelles are formed. The effect of pH on the aggregation behavior of the three Pluronics without any added drug was examined; all the three Pluronics showed no changes in their aggregation behavior between pH 3 and 12. Hence, any changes in the aggregation behavior of the Pluronics as pH changes in the presence of drug will stem from changes induced by the drug. Addition of 0.5% w/v flurbiprofen leads to significant changes in the SANS data at all pH values, as can be seen in Figure 4. Below the pKa of the drug (∼6.5), the drug promotes micellization; the size of the micelles, the aggregation number, the fraction of polymer micellized increase, and the volume fraction of the solvent in the core decrease. This is because of hydrophobic interactions between the PPO and the drug. However, as the pH is increased to above the pKa, the flurbiprofen is ionized and so becomes more water-soluble; release of the drug from the core of the micelles thus becomes enthapically and entropically favorable. This causes a decrease in drug−micelle hydrophobic interactions and therefore a reduction in the radius, percentage of micellization, and aggregation number and an increase in the solvent in the core (Table 3). The percentage of ionization increases from ∼0.03% at pH 3 to ∼50% at pH 6; this introduces a repulsive interaction that is accounted for by the Pedersen model and gives rise to the observed structure factor in Figure 4 and causes an increase in interaction radius (Rint). L43 data showed different behavior compared to the other two Pluronics (Figure 4c); at high pH the L43 micelles disappeared. At low pH, the scattering data were fitted to the Pedersen model, showing the formation of a small micellar structure with addition of drug. At pH ∼ 6.5 the solution

3. RESULTS AND DISCUSSION 3.1. SANS. Pluronics P123, P103, and L43 were each studied without added drug, and the results are shown in Figure 3. As the L43 experiments were performed well below the

Figure 3. SANS from Pluronics P123, P103, and L43 at 298 K, 5% w/ v in D2O/d6-ethanol (90:10). Solid lines are fits to the Pedersen model for Pluronics P123 and P103 and to the Guinier−Debye function for Pluronic L43.

cmt,14,28 the scattering from L43 was fitted to the Guinier− Debye model for polymers,13 confirming that the polymer was mainly unimeric. The scattering length density difference was fixed at 5.32 × 10−6 Å2, and the volume fraction was fixed at 0.05, which gave fits to radius of gyration of 10.9 ± 1.0 Å. By contrast, it can be seen that both P103 and P123 have the typical scattering expected from a core−shell micelle. 6541


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unambiguously to the Pedersen model. At this temperature and pH, the sample is close to complete phase separation. The data at pH 12 were fitted to the Guinier−Debye function, with Rg = 11.6 ± 1.5 Å, characteristic of unimers. The measured Rg is nearly identical to the polymer in the absence of drug, demonstrating the majority of the drug is released from the micelles, leading to their complete breakup. As can be seen in Figures 4a and 4b, the intensity of the scattering pattern at low Q at pH ∼12 is decreased and the position of side maxima are moved to a higher Q compared to those of the polymers alone. This reflects a smaller aggregation number and a smaller core radius and will be discussed in section 3.2. 3.2. PFGSE-NMR. The change in the size of the micelles was confirmed by PFGSE-NMR. For P103 solutions, on addition of drug, the data showed that nearly all of the flurbiprofen was found to be solubilized within the micelles and the presence of drug promoted the micellization of the polymers. As shown in Figure 5, at low pH, there is a decrease in diffusion coefficients that corresponds to an increase in size of the micelles. As the pH changes to above the pKa of the drug, an increase in the attenuation of the micelle occurs, which means that the micelles diffuse faster. The attenuation plot of P103 alone (Figure 5) has a shallower gradient (hence, larger size) compared to the P103 + drug at pH ∼12; the same behavior was observed by SANS (Figure 4; Tables 2 and 3). We postulate that because at pH ∼ 12 the drug molecules are completely ionized and surface active, they still participate in the micellization process by interacting strongly with the PEO block via ion-dipole interactions,32−34 with the charged drug headgroup introducing electrostatic repulsion. This leads to an increase in the volume fraction of water in the core, a decrease in the aggregation number, and, overall, a shrinking of the micelles. This is consistent with the diffusion data (Table 4), which shows that although the drug releases from the micelles by increasing the pH, nearly 56% of the drug molecules are still interacting with micelles at pH ∼ 12. Similar behavior was observed by Valero35 and Sharma,36 who reported a decrease in the aggregation number and size of the Pluronic micelles upon addition of naproxen salt. As can be seen in Figure 6, below the pKa of drug (∼6.5), the average flurbiprofen diffusion coefficient is identical to the diffusion coefficient of the micelles, meaning that most of the drug molecules must be solubilized within the micelles (Table 4). However, as the pH increases, drug is gradually released from the micelles as is seen by an increase in the diffusion coefficient of the drug. The percentage of flurbiprofen that is in the core of the micelle, ϕM, can be calculated by assuming a fast exchange between environments:14

Figure 4. SANS from (a) Pluronic P103, (b) Pluronic P123, and (c) Pluronic L43, 5% w/v, with 0.5% w/v flurbiprofen, at a range of pH values. Data for P103 and P123 were fitted to the Pedersen model, the results of which can be found in Table 3. L43 data were fitted to either a Guinier−Debye function or the Pedersen model as described.

Dflur = ϕMDmic + (1 − ϕM)Dfree

(4)

where Dflur is the dynamically averaged diffusion coefficient of the flurbiprofen, Dmic is the diffusion coefficient of the micelle obtained from the polymer peak, and Dfree is the diffusion coefficient of free flurbiprofen in solution, taken as 4.35 × 10−10 m2 s−1 obtained independently from a dilute solution. 3.3. Surface Tension. In our previous work, we examined the effect of drug addition on the cmc's of Pluronics and discussed the complex surface tension behavior of the Pluronics in the presence of flurbiprofen, including the minimum in surface tension shown in Figure 7.14 Pluronic copolymers

became opalescent and the SANS intensity increased, indicating the presence of larger aggregates. This could be due to the release of the drug from the micelles and the onset of micelle disintegration; it is postulated that the release of insoluble drug from the micelles leads to the formation of drug precipitates that cause the observed increase in turbidity and a higher SANS intensity. The data at this pH could not be fitted 6542


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Table 3. SANS Parameters for 5% w/v Pluronic P123, P103, and L43 with 0.5% w/v Flurbiprofen at a Range of pH Values, 298 K, in D2O/d6-Ethanol pH 3.0 Nagg ± 2.3 ϕsol ± 0.1 ϕHS ± 0.001 Rint/Å ± 1.1 f mic ± 0.03 Rc/Å ± 2.3

pH 4.5

pH 6.5

pH 12

P123

P103

L43

P123

P103

L43

P123

P103

P123

P103

124 0.17 0.11 85.2 0.95 63

70.5 0.22 0.076 83.6 0.94 50

9.2 0.8 0.18 62.7 0.33 32

120 0.18 0.12 96.6 0.94 62

68.3 0.29 0.12 88.9 0.93 51

10 0.8 0.16 64.2 0.34 33

107 0.27 0.41 148 0.92 62

50.3 0.46 0.26 109 0.91 50

24.3 0.63 0.12 82.3 0.89 48

14.6 0.65 0.11 69.5 0.88 39

Figure 6. Comparison of the diffusion coefficients of P103 micelles with 0.5% w/v flurbiprofen, with the average diffusion coefficient of flurbiprofen solubilized in the micelles, as a function of pH. P103 data were taken from the PEO CH2 group peak at δ ∼ 3.6 ppm. Diffusion coefficients of the drug were calculated from the attenuation of the flurbiprofen aromatic group at δ ∼ 7.5 ppm. Values are ±4%.

Figure 5. Attenuation plots for 5% w/v P103 solutions with 0.5% w/v flurbiprofen, in D2O/d6-ethanol (90:10) at a range of pH values. All data were taken from the PEO CH2 group peak at δ ∼ 3.6 ppm.

Table 4. Aggregation Data Determined Using PFGSE-NMR for 5% w/v Solutions of Pluronic P103 in D2O/d6-Ethanol with Addition of 0.5% w/v Flurbiprofen at 298 K at a Range of pH Values pH

% flurbiprofen in micelle (±4%)

Rh of micelle/Å (±4%)

3.0 4.5 6.5 12.0 6.5 (no drug)

100 99 86 56

94 93 76 64 68

without any added drug showed a single break point in a surface tension vs polymer concentration plot, which represents the concentration at which micelles are formed (cmc). However, the surface tension graph of the micelle−drug complexes showed two break points: the first is the critical aggregation concentration (cac), which is the onset of drug absorption into the micelles, and the second at which the maximum amount of dug is taken up by the micelles and is the onset of the plateau region in Figure 7. It has been shown that the cac/cmc ratio indicates the strength of the interactions between the micelles and the drug.17,37 The smaller the cac/cmc ratio, the stronger the polymer−drug interaction. The surface tension plot of Pluronic P103 alone and with drug as a function of pH is shown in Figure 7. Values of the cmc and cac are summarized in Table 5, and it can be seen that at higher pH, a larger cac/cmc ratio is obtained, which indicates overall weaker flurbiprofen−micelle attractive interactions and the release of drug into the solution. The cmc of P103 at this solvent composition is 0.16 wt %,

Figure 7. Surface tension of P103 in water/ethanol with and without flurbiprofen as a function of concentration at 298 K. Values are average over 10 measurements ±1 SD.

which is higher than that in pure water6 (0.07 wt %). The effect of cosolvent has been reported in the literature previously, and it is observed that polar solvents such as ethanol increase the cmc as they alter the solvent quality for the block copolymers.29,38−40 One should note here the reduction of the surface tension values at pH 12 (Figure 7). This is due to the fact that the drug 6543


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were supported by a beam time allocation from the Science and Technology Facilities Council, and Dr. Ann Terry and Dr. Stephen King are thanked for their support during these experiments. This work is also based on experiments performed at the Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich, Germany.

Table 5. The cmc and cac Values of Pluronic P103, with and without Drug in Water−Ethanol Solutions (90:10) at a Range of pH Values pH

cac/% w/v

cac/cmca

3.0 4.5 6.5 12

0.023 0.035 0.056 0.14

0.14 0.22 0.35 0.87

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(1) Dai, W.-G. In vitro methods to assess drug precipitation. Int. J. Pharm. 2010, 393, 1. (2) Strickley, R. Solubilizing Excipients in Oral and Injectable Formulations. Pharm. Res. 2004, 21, 201. (3) Stevenson-Abouelnasr, D.; Husseini, G. A.; Pitt, W. G. Further investigation of the mechanism of Doxorubicin release from P105 micelles using kinetic models. Colloids Surf., B 2007, 55, 59. (4) Foster, B.; Cosgrove, T.; Espidel, Y. PFGSE-NMR Study of pHTriggered Behavior in Pluronic-Ibuprofen Solutions. Langmuir 2009, 25, 6767. (5) Hyung-Jun, G.; Hyun, K.; Sang-Cheol, C. Release of Flurbiprofen from Poloxamer 407 Gel. Arch. Pharm. Res 1994, 17, 240. (6) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Micellization of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Triblock Copolymers in Aqueous Solutions: Thermodynamics of Copolymer Association. Macromolecules 1994, 27, 2414. (7) Ma, J.-h.; Guo, C.; Tang, Y.-l.; Liu, H.-z. 1H NMR Spectroscopic Investigations on the Micellization and Gelation of PEO−PPO−PEO Block Copolymers in Aqueous Solutions. Langmuir 2007, 23, 9596. (8) Pandey, M. S.; Belgamwar, V. S.; Surana, S. J. Topical delivery of Flurbiprofen from Pluronic Lecithin organogel. Indian J. Pharm. Sci. 2009, 71, 87. (9) El Gendy, A. M.; Jun, H. W.; Kassem, A. A. In Vitro Release Studies of Flurbiprofen from Different Topical Formulations. Drug Dev. Ind. Pharm. 2002, 28, 823. (10) Dai, W.-G.; Dong, L. C.; Li, S.; Deng, Z. Combination of Pluronic/Vitamin E TPGS as a potential inhibitor of drug precipitation. Int. J. Pharm. 2008, 355, 31. (11) Gu, C. H.; Rao, D.; Gandhi, R. B.; Hilden, J.; Raghavan, K. Using a novel multicompartment dissolution system to predict the effect of gastric pH on the oral absorption of weak bases with poor intrinsic solubility. J. Pharm. Sci. 2005, 94, 199. (12) Kadam, Y.; Yerramilli, U.; Bahadur, A.; Bahadur, P. Micelles from PEO-PPO-PEO block copolymers as nanocontainers for solubilization of a poorly water soluble drug hydrochlorothiazide. Colloids Surf., B 2011, 83, 49. (13) Foster, B.; Cosgrove, T.; Hammouda, B. Pluronic Triblock Copolymer Systems and Their Interactions with Ibuprofen. Langmuir 2009, 25, 6760. (14) Alexander, S.; Cosgrove, T.; Prescott, S. W.; Castle, T. C. Flurbiprofen Encapsulation Using Pluronic Triblock Copolymers. Langmuir 2011, 27, 8054. (15) Bones, J.; Thomas, K.; Nesterenko, P. N.; Paull, B. On-line preconcentration of pharmaceutical residues from large volume water samples using short reversed-phase monolithic cartridges coupled to LC-UV-ESI-MS. Talanta 2006, 70, 1117. (16) Domańska, U.; Pobudkowska, A.; Pelczarska, A.; Gierycz, P. pKa and Solubility of Drugs in Water, Ethanol, and 1-Octanol. J. Phys. Chem. B 2009, 113, 8941. (17) Bandyopadhyay, P.; Ghosh, A. K. pH-Controlled “Off−On− Off” Switch Based on Cu2+-Mediated Pyrene Fluorescence in a PAA− SDS Micelle Aggregated Supramolecular System. J. Phys. Chem. B 2009, 113, 13462. (18) Ràfols, C.; Rosés, M.; Bosch, E. A comparison between different approaches to estimate the aqueous pKa values of several non-steroidal anti-inflammatory drugs. Anal. Chim. Acta 1997, 338, 127. (19) Meloun, M.; Bordovská, S.; Galla, L. The thermodynamic dissociation constants of four non-steroidal anti-inflammatory drugs by the least-squares nonlinear regression of multiwavelength spectrophotometric pH-titration data. J. Pharm. Biomed. Anal. 2007, 45, 552.

a

The cmc at pH 6.5 in the absence of drug was measured to be 0.16% w/v.

is surface active at higher pH and can absorb at the air−water interface (the drug molecules at pH ∼ 12 reduce the surface tension of the solvent mixture by about 7 mN/m).

4. CONCLUSIONS SANS, PFGSE-NMR, and surface tension measurements were used to investigate the structural and aggregation behavior of a series of Pluronics in the presence of a hydrophobic drug, flurbiprofen, with varying pH. Variation of pH for the three Pluronics alone revealed that the aggregation behavior of these copolymers is pH-independent; however, this behavior changes on the addition of a weakly acidic drug. At pH values below the pKa of the drug, both SANS and NMR data indicated a higher aggregation number, lower fraction of solvent in the core, and a larger fraction of polymer micellized. As the pH was raised above the pKa of the drug, a substantial amount of drug was released from the micelles into the solutions, which resulted in a higher cmc/cac ratio. This led to a lower scattering at low Q and an increase in both the diffusion coefficient of the drug and of the micelles. The Pluronic L43 showed very different behavior in comparison to P123 and P103. At lower pH values, the drug increases the fraction of polymer micellized to 35%, and at higher pH values the micelles break down and the drug releases completely. One might conclude that L43 would be more appropriate as a drug delivery vehicle than P103 and P123, as the drug can be released fully into solution. However, the small micellar size and the very high sensitivity of the aggregation to temperature and concentration make its use quite difficult from a formulation prospective. For each of these Pluronics, our data indicate that flurbiprofen should remain encapsulated in the stomach (pH ∼ 1−2) but would be released in the gastrointestinal region (pH ∼ 7.5). In a forthcoming paper, we will discuss the effect of temperature on the drug−Pluronic interactions using SANS.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.C.), stuart.prescott@ bris.ac.uk (S.W.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Rob Dyer from Krüss Surface Science Centre for his assistance with the surface tensiometery measurements, Dr. Youssef Espidel for his support using PFGSE-NMR, and Miss Catherine Cooper for assistance with SANS measurements. We are also grateful to the EPSRC and Revolymer Ltd. for funding. Experiments at the ISIS Pulsed Neutron and Muon Source 6544


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