pubs.acs.org/Langmuir © 2009 American Chemical Society
PFGSE-NMR Study of pH-Triggered Behavior in Pluronic-Ibuprofen Solutions Beth Foster, Terence Cosgrove,* and Youssef Espidel School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom Received January 23, 2009. Revised Manuscript Received April 5, 2009 The effect of drug addition and pH variation on Pluronic copolymer solutions has been investigated using pulsed-field gradient (PFG) NMR. Addition of ibuprofen to Pluronic P104 in solution reduced the overall pH from 7.5 to 4.5, as well as promoting micellization; a substantial increase in the hydrodynamic radius of the micelles, from 57.7 to 102.3 A˚ was observed, along with an increase in the fraction of polymer micellized. The aggregation behavior was attributed primarily to the presence of ibuprofen, rather than the reduction in pH observed, since the micellization of P104 alone was not found to be significantly altered by pH changes in the region of interest. Conversely, for the P104 solutions containing ibuprofen, a strong pH-dependence was observed when raising the pH above the pKa of ibuprofen. The data obtained showed that, above pH 4.5, ibuprofen is gradually released from the micelles as a result of its improved solubility, leading to a reduction in the polymer aggregation toward that observed before the addition of ibuprofen.
1. Introduction Amphiphilic polymeric micelles have long been studied for their potential as drug delivery vehicles, in particular with a view to giving controlled or extended release of a drug within the body. The aggregation behavior of these systems is closely linked to the in vivo release profile, blood circulation time, and bioavailability of the encapsulated drug.1,2 Much recent research in this area has been focused on achieving site-specific release by the development of polymers whose micellization is sensitive to external stimuli, such as temperature, pH, or ultrasonic radiation.3,4 One widely used group of sensitive polymers are the Pluronics, which are amphiphilic copolymers consisting of a central poly (propylene oxide) block (PPO) with poly(ethylene oxide) blocks (PEO) at either end. The aggregation of Pluronics is strongly temperature-sensitive, due to dehydration of the PPO block which occurs at around 18 °C and above. A wide range of Pluronics are available, comprising various combinations of block sizes, with each displaying different micellization and phase behavior. Many research groups have investigated the possibility of combining the temperature-responsiveness of Pluronics with the pH-sensitivity of ionizable groups, to create dually responsive delivery systems. For instance, Pluronics have been blended with chitosan, a pH-sensitive polymer, to create ocular treatment formulations, in which gelation occurs with a change in temperature or pH.5 Pluronics have also been end-functionalized with ionizable groups, such as carboxylic acid moieties, which tend to cause retention of normal Pluronic behavior below the pKa of the new polymer but an alteration in the solubilization and micellization behavior in solutions of higher pH.6 Bromberg *Corresponding author. E-mail:
[email protected]. (1) Kabanov, A. V.; Batrakova, E. V.; Alakhov, V. Y. J. Controlled Release 2002, 82, 189–212. (2) Stolnik, S.; Illum, L.; Davis, S. S. Adv. Drug Delivery Rev. 1995, 16, 195–214. (3) Kabanov, A. V.; Batrakova, E. V.; Alakhov, V. Y. J. Controlled Release 2002, 82, 189–212. (4) Stevenson-Abouelnasr, D.; Husseini, G. A.; Pitt, W. G. Colloids Surf., B 2007, 55, 59–66. (5) Gupta, H.; Jain, S.; Mathur, R.; Mishra, P.; Mishra, A. K.; Velpandian, T. Drug Delivery 2007, 14, 507–515. (6) Custers, J. P. A.; van den Broeke, L. J. P.; Keurentjes, J. T. F. Langmuir 2007, 23, 12857–12863.
Langmuir 2009, 25(12), 6767–6771
and co-workers have carried out substantial research into pentablock copolymers, consisting of various Pluronics, with poly (acrylic acid) (PAA) reacted onto the terminal blocks via atomtransfer radical polymerization.7-9 Above pH ∼ 5.0, the PAA blocks are deprotonated, which increases their solubility as well as the magnitude of their interchain repulsions. This causes dispersion/swelling of the micelle or gel, allowing any encapsulated drug to be released.10-12 Our research group has previously used small-angle neutron scattering (SANS) to study the aggregation behavior of Pluronic systems containing different concentrations of ibuprofen, a nonsteroidal anti-inflammatory drug.13 The presence of ibuprofen in the Pluronic systems studied (P103, P104, and P105) induced aggregation of the polymers, causing a reduction of the critical micellization temperature (CMT) and concentration (CMC) at ambient temperatures, as well as influencing various physical properties of the micelle. The micellization observed was temperature-sensitive, with different aggregation characteristics at a range of temperatures, for samples with and without ibuprofen. Since ibuprofen is a weak acid (structure shown in Figure 1), with a pKa within the physiological pH range (approximately 4.4), it is possible that pH changes, for example, in the digestive system, could also play an important role in influencing aggregation behavior of these Pluronic-ibuprofen conjugates. Unfunctionalized Pluronic copolymers are not expected to exhibit strong pH sensitivity, since they do not contain ionizable groups.14 However, several research groups have reported Pluronic gel systems which show pH-dependent release of acidic or basic drugs, including the (7) Tian, Y.; Ravi, P.; Bromberg, L.; Hatton, T. A.; Tam, K. C. Langmuir 2007, 23, 2638–2646. (8) Bromberg, L. Ind. Eng. Chem. Res. 1998, 37, 4267–4274. (9) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2002, 18, 4944–4952. (10) Barreiro-Iglesias, R.; Bromberg, L.; Temchenko, M.; Hatton, T. A.; Alvarez-Lorenzo, C.; Concheiro, A. Eur. J. Pharm. Sci. 2005, 26, 374–385. (11) Alakhov, V.; Pietrzynski, G.; Patel, K.; Kabanov, A.; Bromberg, L.; Hatton, T. J. Pharm. Pharmacol. 2004, 56, 1233–1241. (12) Tian, Y.; Bromberg, L.; Lin, S. N.; Hatton, T. A.; Tam, K. J. Controlled Release 2007, 121, 137–145. (13) Foster, B.; Cosgrove, T.; Hammouda, B. Langmuir 2009; DOI: 10.1021/ la900298m. (14) Park, S.; Lee, Y.; Bae, K.; Ahn, C.; Park, T. Macromol. Rapid Commun. 2007, 28, 1172–1176.
Published on Web 05/11/2009
DOI: 10.1021/la900299v
6767
Article
nonsteroidal drugs ketoprofen, naproxen, and diclofenac.15,16 Tomida et al. proposed that, for the release of a drug into aqueous solution to be pH-dependent, it must be weakly acidic or basic and have low aqueous solubility.17 Ibuprofen satisfies these criteria. Altering the solubility of a drug in a Pluronic solution, by varying the pH of the solvent, has also been found to influence sol-gel transition boundaries.18-20 Relatively few studies have focused on dilute unfunctionalized Pluronic solutions containing pH-sensitive drugs, although Scherlund and co-workers have studied the release of lidocaine and prilocaine from Pluronic F127 solutions close to the gel point, using dialysis at a range of pH values.19,21 They found that the drug release rate was increased with decreasing pH, which was attributed to greater ionization of the drug molecules at low pH enhancing their solubility. Similarly, La et al. reported increased release of indomethacin from PEO-poly(β-benzyl L-aspartate) block copolymers above its pKa due to increased solubility of the drug.22 Both Kwon et al. and Froemming and Ghaly have reported that mildly acidic drugs show stronger interactions with Pluronics below their pKa than above.23,24 Drug release from dilute micellar systems may therefore be expected to show similar behavior to that from gel networks, but with reduced effects from diffusion limitation and possibly a greater risk of instability; Dai et al. have described the pH-dependency of Pluronic solutions containing a model pH-responsive drug, which precipitates out of solution at raised pH.25 This paper aims to investigate whether the presence of ibuprofen could make the aggregation of the Pluronic micelles pH-responsive, and if so what effect this would have on the micellar properties and drug release in vivo.
2. Materials and Methods 2.1. Materials. Pluronics P103, P104, and P105 were provided by BASF; the composition of each of these Pluronics is listed in Table 1. Ibuprofen, 40 grade, was supplied by Albemarle. D2O, 99.94 atom % D, was purchased from Goss Scientific Instruments Ltd. DCl, 35% in D2O (99.0% D), and NaOD, 40 wt % in D2O (99.5% D), were acquired from Sigma-Aldrich and diluted with D2O to produce 1 M solutions. All other compounds were used as received. 2.2. Preparation of Solutions. All samples were prepared by weighing the Pluronic and solvent into a vial, sonicating for approximately 1 h, and leaving the solution on a roller-mixer for 24 h to equilibrate. The ibuprofen was then added to the solutions, and the samples were sonicated for a further 2 h to allow incorporation of ibuprofen into the micelles. Samples were left on a roller-mixer again for at least 24 h to reach equilibrium. The pH of each sample was adjusted to the value required, by addition of small quantities of DCl or NaOD as necessary, and the samples were left for a final 24 h on a roller-mixer to allow the pH to equilibrate. The accurate pH of the solutions was measured immediately after removing samples from the NMR spectrometer.
Foster et al. Table 1. Composition of Pluronics Used P103
6768 DOI: 10.1021/la900299v
P105
34 54 74 ethylene oxide units 60 61 56 propylene oxide unitsa -1 b 4686 5866 6228 Mn (g mol ) 5408 6386 6900 Mw (g mol-1)b 1.15 1.09 1.11 polydispersity indexb a Values supplied by BASF. b Determined using MALDI-TOF MS as described previously.13
2.3. Pulsed-Field Gradient Stimulated-Echo Nuclear Magnetic Resonance (PFGSE-NMR). PFGSE (diffusion) NMR measurements were carried out at 298 K on a Bruker DSX-300 MHz spectrometer using a diff 30 field gradient probe with 5 mm 1H/2H coil insert. A stimulated echo pulse sequence, as described by Tanner26 was used. This produces an echo at time (τL + 2τ), where τ is the time between the first and second radio frequency pulses (fixed at 6 ms for each sample) and τL is the time between the second and third radio frequency pulses. The gradient pulse duration (δ) was set to 1 ms, and the magnetic field gradient (G) was ramped from 0.05 to 10 T m-1. The diffusion time, Δ, was set between 50 and 250 ms, depending on the sample. To maximize the signal-to-noise ratio, 64 scans were run over at least 50 gradient steps. A calibration of the instrument was carried out using a water/methanol reference sample. The stimulated-echo signals were Fourier-transformed, and the resulting decays (signal area as a function of the gradient strength) were fitted to eq 1, which assumes Brownian motion, to find the diffusion coefficients (D) for each chemically distinct species. h i AðτL þ 2τÞ ¼ A0 exp -γ2 G2 δ2 ðΔ -δ=3ÞD
ð1Þ
where A0 is the initial peak area, A is the peak area for each gradient step, and γ is the gyromagnetic ratio of the nuclei, which is 2.675 108 rad T-1 s-1 for a proton. Because the polymer concentration is below the critical overlap concentration c, the diffusion rate at infinite dilution (D0) can be approximated using eq 2 D0 ¼ D= 1 -ðφP þ φI Þ
ð2Þ
where φP and φI are the volume fraction of polymer and ibuprofen, respectively, in the solution. It should be noted that this equation assumes that free and micellized polymer chains have the same effect on the obstruction in the system, which in reality may not be the case. Similarly, the volume fraction of ibuprofen is taken into account due to its effect on the micelle size, without differentiating between the ibuprofen inside the micelle and that outside, so the equation should only be taken as an approximation. The hydrodynamic radius, Rh, can then be calculated from the diffusion coefficient according to the Stokes-Einstein relation, eq 3, assuming the aggregates are approximately spherical and noninteracting. Rh ¼
(15) Chi, S. C.; Jun, H. W. J. Pharm. Sci. 1991, 80, 280–283. (16) Suh, H.; Jun, H. W. Int. J. Pharm. 1996, 129, 13–20. (17) Tomida, H.; Shinohara, M.; Kuwada, N.; Kiryu, S. Acta Pharm. Suec. 1987, 24, 263–272. (18) Oh, S. H.; Kim, J. K.; Song, K. S.; Noh, S. M.; Ghil, S. H.; Yuk, S. H.; Lee, J. H. J. Biomed. Mater. Res., Part A 2005, 72, 306–316. (19) Scherlund, M.; Brodin, A.; Malmsten, M. Int. J. Pharm. 2000, 211, 37–49. (20) Suh, J. M.; Bae, S. J.; Jeong, B. Adv. Mater. 2005, 17, 118–120. (21) Scherlund, M.; Malmsten, M.; Holmqvist, P.; Brodin, A. Int. J. Pharm. 2000, 194, 103–116. (22) La, S.; Okano, T.; Kataoka, K. J. Pharm. Sci. 1996, 85, 85–90. (23) Kwon, S. H.; Kim, S. Y.; WookHa, K.; JooKang, M.; Huh, J. S.; JongIm, T.; WookChoi, Y. Arch. Pharmacal Res. 2007, 30, 1138–1143. (24) Froemming, K. H.; Ghaly, G. M. Pharm. Ind. 1981, 43, 371–375. (25) Dai, W. G.; Dong, L. C.; Li, S.; Deng, Z. Int. J. Pharm. 2007, 355, 31–37.
P104
a
kB T 6πηD0
ð3Þ
where kB is the Boltzmann constant, T is the temperature, and η is the viscosity of the solvent.
3. Results and Discussion The three Pluronics P103, P104, and P105 were each found to diffuse at two different rates in solution, with the faster rate corresponding to diffusion of the unimer and the slower rate to that of the micelle, indicating that over the diffusional time scale (26) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523–2526.
Langmuir 2009, 25(12), 6767–6771
Foster et al.
Article
Table 2. Aggregation Data Determined Using PFGSE-NMR for 5 wt %/vol Solutions of Pluronics at 298 K, and Comparison with Data Determined Using Small-Angle Neutron Scattering, Surface Tension, and Static Light Scattering Measurementsa Pluronic P103 P104 P105
% micellized, (1.1
Rh of unimer, (0.3 (A˚)
52.5 40.6 36.2
14.5 19.4 21.5
Rh of micelle, (0.7 (A˚) 60.2 57.7 52.2
T (K) 298 298 298
Figure 1. Structure of ibuprofen.
70.0 21.4 43.0 (core) 293 P10b 97.6 39 298 P104c a Note that the radii determined using scattering are radii of gyration rather than hydrodynamic radii. b Data determined using SANS.13 c Data determined using surface tension and static light scattering measurements (fraction micellized calculated from CMC).29
observed there is no appreciable exchange process. This is in agreement with the findings of Malmsten and Lindman,27 who observed extremely long residence times for Pluronic F127 in micelles (hours), but contrasts with the fast exchange (
