preparation and evaluation of modified release ibuprofen

Acta Poloniae Pharmaceutica ñ Drug Research, Vol. 63 No. 6 pp. 521ñ534, 2006

ISSN 0001-6837 Polish Pharmaceutical Society

PREPARATION AND EVALUATION OF MODIFIED RELEASE IBUPROFEN MICROSPHERES WITH ACRYLIC POLYMERS (EUDRAGIT®) BY QUASIEMULSION SOLVENT DIFFUSION METHOD: EFFECT OF VARIABLES BURCU DEVRIM AND KANDEMIR CANEFE* Ankara University, Faculty of Pharmacy, Department of Pharmaceutical Technology 06100-Tandoðan ñ Ankara ñ TURKEY Abstract: The aim of this study was to prepare and evaluate microspheres containing ibuprofen. Microspheres were prepared by modified quasi-emulsion solvent diffusion method. The influence of formulation factors (drug-polymer ratio, volumes of solvent, polyvinyl alcohol concentration and type of polymer) on the morphology, particle size distribution, drug loading capacity, micromeritical properties and the in vitro release characteristics of the microspheres were investigated. Physical characterizations of ibuprofen microspheres were also carried out using scanning electron microscopy, X-ray diffractometry and IR spectrophotometry. It was found that the yield of preparation was dependent on the initial temperature gradient between the emulsion phases. When there was an initial difference of temperature between the aqueous phase and dispersed emulsion phases, yield of preparation was increased distinctly. The drug loading capacities were very high for all formulations of the microspheres which were obtained. Mean particle size changed by changing the drug-polymer ratio, volumes of solvent or polyvinyl alcohol concentration. The flow properties were much improved over those of the original crystals. In vitro dissolution results showed that the release rate of ibuprofen was modified in all formulations. Although ibuprofen release rates from Eudragit® RS microspheres were very slow, they were fast from Eudragit RL microspheres. These results observed that if Eudragit RS and Eudragit RL are used in combination, optimum release profiles may be obtained. Keywords: Ibuprofen; EudragitÆ; microspheres; quasi-emulsion solvent diffusion method; modified release

Ibuprofen, α-methyl-4-(2-methylpropyl)-benzene acetic acid is a non-steroidal anti-imflammatory, antipyretic and analgesic drug. This drug is indicated for the relief of mild to moderate pain and inflammation in conditions such as dysmenorrea, migraine, postoperative pain, dental pain, in which disorders an immediate available dose is requested. It is also used in chronic disorders as ankylosing spondylitis, osteoarthritis and rheumatoid arthritis for all of which a sustained release is desirable (1). The usually daily dose by mouth is 1.2 to 1.8 g divided in different administrations. The drug is readily absorbed from the gastrointestinal tract, and peak plasma concentrations occur about 1 to 2 h after ingestion (2). Unfortunately, ibuprofen causes a certain irritation in the gastrointestinal mucous membrane and possesses a bitter taste and aftertaste. The half-life in plasma is about 2 h. The short halflife and the low single administration dose make ibuprofen a very good candidate for the formulation of controlled release multiple-unit dosage forms (36). At the same time, great attention has been devoted on the possibility to prepare ibuprofen micros-

pheres in order to formulate oral controlled release systems, to protect the gastric mucous membrane from drug irritation or to mask its unpleasant taste. Microspheres are one of the microparticulate systems and are prepared to obtain prolonged or controlled drug delivery, to improve bioavailability or stability and to target drug to specific sites. Microspheres can also offer advantages like limiting fluctuation within therapeutic range, reducing side effects, decreasing dosing frequency and improving patient compliance. They spread out more uniformly in the GI tract, thus avoiding exposure of the mucosa to high concentration of drug and ensuring more reproducible drug absorption. The risk of dose dumping also seems to be lower than with a singleunit dosage form (7-9). In this study, ibuprofen microspheres were prepared by a novel technique, called the quasi-emulsion solvent diffusion method, which was proposed by Kawashima et al. (10). Compared with the solvent evaporation method for microsphere preparation, the solidification of the liquid droplet in this process was much faster. Further, the use of a harmful organic solvent could

* Corresponding author: Tel.: +90-312-212 9151, Fax: +90-312-213 1081, E-mail: [email protected].

521

522

BURCU DEVRIM and KANDEMIR CANEFE between the emulsion phases, drug-polymer ratio, volumes of ethanol and stirring speed) to obtain spherical particles. Then the influences of formulation variables on the microsphere properties were examined and the microsphere formulations suitable to achieve our goal were determined.

be avoided, and the reduced pressure and/or the heating to evaporate the solvent would be unnecessary in this method (11). The rare coalescence of a microspheres observed in the process did not require the addition of antiadhesion agents, such as magnesium stearate and talc, which are often used in the evaporation method. Acrylic polymers are widely used as tablet coatings and as retardants of drug release in sustained release formulations (12, 13). Methacrylate copolymers (Eudragits) have recently received increased attention for modified dosage forms because of their inertness, solubility in relatively non-toxic solvents and availability of resins with different properties (14-16). Eudragit® RS and Eudragit® RL polymers are copolymers of poly(ethylacrylate, methyl-methacrylate and chlorotrimethyl-ammonioethyl methacrylate), containing an amount of quaternary ammonium groups between 4.5-6.8% and 8.8-12% for RS and RL, respectlively (17). The copolymer Eudragit RS PM® differs from Eudragit® RS 100 in that it contains 0.5% talc. Eudragit® RS and Eudragit® RL are insoluble in water and digestive juices, but they are permeable and both have pH-independent release profiles. The permeability of Eudragit® RS and RL in aqueous media is due to the presence of quaternary ammonium groups in their structure; Eudragit® RL has a greater proportion of these groups and as such is more permeable than Eudragit® RS (18, 19). In this study, ibuprofen loaded microspheres were prepared by using quasi-emulsion solvent diffusion method. Firstly, we investigated some formulation variables (initial difference of temperature

EXPERIMENTAL Reagents and equipment Reagents: ibuprofen (Eczacýbaþý), Eudragit® RS PM, Eudragit® RS 100, Eudragit® RL 100 (Rˆhm-Pharma), ethanol (Riedel de-HaÎn), polyvinyl alcohol (PVA 72 000), sodium hydroxide, potassium dihydrogen phosphate (Merck). The other chemicals were of analytical grade and distilled water was used for all experiments. Equipment: UV spectrophotometer (Shimadzu UV-1202), pH meter (Meter Lab), laser diffractometer (Sympatec HELOS-H0728), X-ray diffractometer (Rigaku, D-Max/2200), IR-spectrophotometer (Jasco, FT/IR 420), dissolution tester (Aymes), SEM analysis (JEOL JSM-840A). METHODS Preparation of microspheres In order to produce the microspheres, modified quasi-emulsion solvent diffusion method was used (20). Weighed amounts of ibuprofen and acrylic polymer were dissolved in 5 mL of ethanol at 45OC. The formed ethanolic solution was poured into water containing polyvinyl alcohol (0.025-

Table 1. Formulations of the microspheres.

Eudragit® (%)

Formulation code

Ibuprofen (%)

PVA 72.000 (%)

F1

100

-

0.05

5

F2

80

20(RSPM)

0.05

5

F3

75

25(RSPM)

0.05

5

F4

71.43

28.57(RSPM)

0.05

5

F5

66.67

33.33(RSPM)

0.05

5

F6

66.67

33.33(RSPM)

0.05

3

F7

66.67

33.33(RSPM)

0.05

6

F8

66.67

33.33(RSPM)

0.025

5

F9

66.67

33.33(RSPM)

0.1

5

F10

66.67

33.33(RS100)

0.05

5

F11

66.67

33.33(RL100)

0.05

5

*Microspheres were produced with the agitation speed of 450 rpm.

Ethanol (mL)

Preparation and evaluation of modified release ibuprofen microspheres...

523

microspheres of ibuprofen were measured. Before measuring the X-ray powder diffractogram, the microspheres were crushed into powder because they were too big to measure. The data were collected in the continuous scan mode using step size of 0.01O 2θ. The scanned 2 range was 2θ to 35O. Infrared spectroscopy Infrared spectra of the pure drug and of the microspheres were determined using an IR-spectrophotometer (Jasco, FT/IR 420), using the KBr disk technique (about 10 mg sample for 100 mg of dry KBr). The scanning range used was from 4000 to 200 cm-1 at a scan period of 3 min. Particle size analysis The particle size of the microspheres was determined by laser diffractometry using Sympatec HELOS (H0728) particle size analyzer. In this study, filtered and degassed purified water was used as a carrier fluid. About 0.3-0.5 mg of microspheres were dispersed in purified water in the sample unit and were circulated 2000 times per minute. Each determination was carried out in triplicate.

Figure 1. Preparation procedure of microspheres.

0.1% w/v) and stirring continuously with a propeller type agitator (Model RZR-2000; Heidolph Electro). The system was thermally controlled at 20OC. Ethanol solution was finely dispersed in the aqueous phase as discrete droplets. The finely dispersed droplets of the polymer solution of the drug were solidified in the aqueous phase via counter diffusions of ethanol and water out of and into the droplets. After 30 min of stirring, the microspheres were separated by filtration, washed twice with 50 mL of water and then dried in oven at 37OC for 24 h. Dried microspheres were stored in desiccator containing CaCl2. Three independent batches were prepared. In Figure 1, the processing of this technique is illustrated. The representative formulations for the preparation of microspheres are tabulated in Table 1. Microspheres dried at 37OC were then weighed and the yield of microsphere preparation was calculated using the Eq. (1): The amount of microspheres obtained (g) Percent yield = ñññññññññññññññññññññññññññññññññññ× 100 The theoretical amount (g)

(1)

CONTROLS ON MICROSPHERES Powder X-ray diffractometry Powder X-ray diffractometric analyses were performed using an X-ray diffractometer (Rigaku, D-Max/2200). Diffraction of raw crystals and

Determination of the bulk density Determination of the bulk density of the microspheres was obtained by a tapping method and found in g/mL (21, 22). 10 mL of microspheres were weighed and were tapped 20 times. Upon tapping the volume was measured again. Bulk density was determined according to the ratio of the mass and volume of the microspheres. Determination of repose angle Determination of the repose angle was obtained for 10 g microspheres (23). For this study, microspheres were poured into a conical flask which had a 0.9 cm diameter and was placed 10 cm above the surface. Repose angle was calculated according to the tangent of the ratio of the height and diameter of the bulk. Determination of drug loading capacity Drug loading capacity of the microspheres was determined by dissolving accurately weighed portions for each batch in ethanol which dissolves both the active agent and the polymer (24). Filtration of microspheres was done by using Whatman No. 42 filter with a porosity of 2.5 µm. The dissolved drug amount was measured spectrophotometrically at 264 nm. The polymer does not interfere with the assay at this wavelength. The drug loading capacity was calculated according to Eq. (2).

524

BURCU DEVRIM and KANDEMIR CANEFE

Assayed drug content Drug loading capacity = ññññññññññññññññññññññññ× 100 Theoretical drug content

(2)

SEM analysis The shape and surface characteristics of microspheres were observed by a scanning electron microscope (JEOL JSM-840A). The samples were dusted onto double sided tape on an aluminum stub and sputter-coated with gold particles in an argon atmosphere. In vitro dissolution studies The dissolution rate of pure drug and the drug release rate from the microspheres were studied using USP 24 Type 2 (paddle) method (25). The USP paddle method was followed using a standard

1-L dissolution vessel and 7.5 cm diameter paddle, assembled as per the USP 24 Physical Test section on dissolution. The vessel was partially immersed in a suitable water bath that permitted holding the temperature inside the vessel at 37 ± 0.5OC during the test and keeping the bath fluid in constant, smooth motion. The paddle used as the stirring element was formed a blade and a shaft. The shaft was positioned so that its axis was not more than 2 mm at any point from the vertical axis of the vessel, and rotated smoothly without significant wobble. The distance of 25 ± 2 mm between the blade and the inside bottom of the vessel was maintained during the test. The dissolution medium was agitated at 50 rpm for the duration of the test. The samples per batch were

Figure 2. X-ray diffraction patterns of original crystals (A), F5 (B), F10 (C) and F11 (D).


525

Figure 3. IR-spectra of original crystals (A), F5 (B), F10 (C) and F11 (D).

tested in 900 mL of pH 6.8 phosphate buffer as dissolution medium. To improve wetting of the microspheres, a small amount of surfactant (Polysorbate 80, 0.1% w/v) was added to the dissolution media. Accurately weighed samples of ibuprofen or microspheres were added to dissolution medium kept at 37 ± 0.5OC. In order to keep the total surface area of the microsphere samples constant and thus to get comparable results, the release studies all were carried out with 350 µm size fractions of microspheres prepared. An aliquot of the release medium was withdrawn at predetermined time intervals and passed

through a 2.5 µm filter (Whatman No. 42). The initial volume of the release medium was maintained by adding equivalent amount of fresh medium after each sampling. The absorption of the samples was recorded at a wavelength of 264 nm spectrophotometrically. From the absorbance readings, cumulative percentage of ibuprofen dissolved was calculated. Ibuprofen sink conditions were determined in phosphate buffer pH 6.8 (using an amount of drug equivalent to three times of the dose in the pharmaceutical formulation in 900 mL of medium) and were maintained during all measurements.

526


Table 2. Mean particle size of microspheres.

Formulation code

Mean particle size (µm)

Standart deviation

% 95 C.L.

F1

131.85

0.14

0.08

F2

170.73

0.16

0.09

F3

180.84

0.76

0.44

F4

269.99

0.40

0.23

F5

316.65

0.34

0.20

F6

1595.65

0.27

0.16

F7

89.21

0.18

0.10

F8

618.12

0.20

0.12

F9

185.72

0.54

0.31

F10

272.96

0.20

0.12

F11

391.03

0.15

0.09

*C.L.: Confidence limits

Table 3. Bulk density of the microspheres.

Table 4. Repose angle of the microspheres.

Formulation code

Bulk density (g/mL)

Standart deviation

% 95 C.L*.

F1

0.544

0.004

0.003

F2

0.513

0.009

F3

0.524

F4

Formulation code

Repose angle

Standart deviation

% 95 C.L.*

F1

23.68

0.510

0.426

F2

24.52

0.824

0.589

0.006

F3

23.23

0.168

0.120

0.008

0.006

F4

23.71

0.827

0.393

0.515

0.008

0.006

F5

18.06

0.566

0.180

F5

0.532

0.004

0.003

F6

22.45

0.583

0.417

F6

0.427

0.010

0.007

F7

23.27

0.619

0.443

F7

0.501

0.004

0.003

F8

22.21

0.142

0.102

F8

0.514

0.005

0.003

F9

20.58

0.687

0.492

F9

0.515

0.007

0.005

F10

21.26

0.636

0.455

F10

0.507

0.007

0.005

F11

24.61

0.110

0.092

F11

0.450

0.006

0.004

* C.L.: Confidence limits

Figure 4. Mean particle sizes of microspheres (n = 3).

* C.L.: Confidence limits

527


Table 5. Results of the drug loading capacity.

Formulation code

Theoretical drug content (%)

Assay drug content (%)

Drug loading capacity (%)

Standart deviation

% 95 C.L.*

F1

100

97.16

97.16

0.12

0.287

F2

80

79.29

99.11

0.49

0.777

F3

75

73.98

98.64

0.26

0.203

F4

71.43

70.43

98.60

0.25

0.266

F5

66.67

66.53

99.79

0.28

0.180

F6

66.67

65.74

98.61

0.11

0.273

F7

66.67

66.22

99.33

0.26

0.270

F8

66.67

64.13

96.19

0.30

0.192

F9

66.67

63.47

95.20

0.18

0.188

F10

66.67

66.10

99.15

0.15

0.154

F11

66.67

68.64

102.95

0.35

0.368

*C.L.: Confidence limits

Figure 5. Yield of preparation (n = 3).

Calculations of dissolution test results kinetically Dissolution test results obtained from the paddle method in pH 6.8 phosphate buffer were studied by using SPSS for Windows 11.0 according to zero order, first order, Hixson-Crowell and Higuchi kinetics. Statistical analysis The data obtained from the particle size, encapsulation efficiency and release rate determination studies of ibuprofen microspheres were analyzed statistically with ANOVA and t-test by using SPSS for Windows 11.0. DISCUSSION AND CONCLUSION Modified quasi-emulsion solvent diffusion method was used to prepare ibuprofen microspheres. The reasons to choose this method as the

production method were its simplicity, low cost, success with poor aqueous solubility drugs and the production of microspheres of relatively high drug loading, as reported by Pignatello et al. (18). In this process, the drug and polymer were dissolved in a solvent such as ethanol. The final solution was dispersed into an aqueous phase with constant agitation, forming o/w emulsion droplets. The solvent and water counter-diffused out of and into the droplets, respectively. The diffused water within the droplets may decrease the drug and polymer solubilities. Both components co-precipitated and continued ethanol diffusion resulted in further solidification, producing matrix-type microspheres, as shown in Figure 6. First, the effect of initial difference of temperature between the aqueous phase and dispersed emulsion phases on the microsphere formation was investigated. When there was no difference

528


Figure 6. Scanning electron micrographs of microspheres and their surfaces prepared from the formulations given in Table 1 (A) F2, (B) F5, (C) F6, (D) F7, (E) F10 and (F) F11.

of initial temperature gradient between the emulsion phases, as reported by Kawashima et al. (10), microspheres coalesced together and the resultant yield of microspheres was relatively low. Conversely, no coalescence was observed by initial temperature gradient between the continuous and dispersed emulsion phases as in our study. The inhibitory effect on the coalescence of droplets observed with increase in the gradient temperature between the emulsion phases may be explained by influence of this factor on the microsphere harden-

ing sequence. Due to the heat transfer, the rapid solidification of droplets by precipitation of polymer at the droplet surface prevented the coalescence and fusion of droplets. As a consequence, the yield of microspheres varied from 65 to 89.9%. Then, various formulations with different drug-polymer ratios and ethanol volume were tried and stirring speed was changed to obtain spherical particles. When amount of drug and polymer in dispersed phase was too high (drug-polymer ratio was 1:1, w/w) no spherical particles were obtained. These results


529

Figure 6 (cont.). Scanning electron micrographs of microspheres and their surfaces prepared from the formulations given in Table 1 (A) F2, (B) F5, (C) F6, (D) F7, (E) F10 and (F) F11.

show that the amount of solid, thus the viscosity of the inner phase is an important factor for the preparation of microspheres. Keeping the drug amount and the solvent volume constant, spherical particles were obtained as the amount of polymer was decreased to give a drug-polymer ratio of 2:1, 3:1 or 4:1. The ethanol volume of the inner phase also influenced microsphere formation in the various batches. The microsphere yield declined sharply as larger volumes of ethanol or less viscous dispersed phase solutions were used. When a solvent volume

of 10 mL was used, the resulting ethanol phase was very dilute. The addition of the two phases together produced rapid intermixing of the phases, resulting in immediate precipitation of drug and polymer before droplets could form. Hence, no microspheres were produced. With increasing concentration of ibuprofen, the recoveries of spherical matrices increased rapidly. At a concentration of 1.25 g/mL, the loss of microspheres due to adhesion to the propeller and vessel wall became relatively larger than the recovered amount, since the volume of ethanol

530


Figure 6 (cont.). Scanning electron micrographs of microspheres and their surfaces prepared from the formulations given in Table 1 (A) F2, (B) F5, (C) F6, (D) F7, (E) F10 and (F) F11.

solution was small (2 mL). Three different stirring speeds (400, 450 and 500 rpm) were selected and it was observed that particle size of microspheres decreased with the increasing of the stirring speed, but at 500 rpm, because of the turbulence of aqueous phase, the polymer stuck around the paddle of the mixer and a great loss of the polymer was indicated. Therefore, as a stirring speed of 450 rpm for the preparation of microspheres was suitable. According to the particle size analysis, it was found that particle size was dependent on drug-poly-

mer ratio, volume of ethanol, type of polymer and concentration of polyvinyl alcohol (Table 2). It was observed that when polymer amount increased, particle size of the microspheres increased (p < 0.05). F5 formulation produced with 33.33% polymer concentration had bigger particle size than F2 formulation produced with 20% polymer concentration. Increasing the polymer load led to a more viscous solution. When the viscous polymeric solution was poured into the aqueous phase, larger droplets and thus larger microspheres, were formed.


Figure 7. Release profiles of ibuprofen from microspheres prepared with different drug-polymer ratio in pH 6.8 phosphate buffer at 37 ± 0.5OC (n = 3).

531

Figure 9. Release profiles of ibuprofen from microspheres prepared at different PVA 72 000 concentration in pH 6.8 phosphate buffer at 37 ± 0.5OC (n = 3).

Figure 8. Release profiles of ibuprofen from microspheres prepared with different ethanol volume in pH 6.8 phosphate buffer at 37 ± 0.5OC (n = 3).

Figure 10. Release profiles of ibuprofen from microspheres prepared different polymer in pH 6.8 phosphate buffer at 37 ± 0.5OC (n = 3).

When the amount of solvent was decreased, polymer and drug concentration increased. As a result of the increase in the polymer concentration, microspheres particle size increased (p < 0.05) as shown in Table 2. The biggest particle size was obtained from F6 which was produced with 3 mL of ethanol. In this study, polyvinyl alcohol was preferred as an emulsifying agent rather than sucrose fatty acid ester used in the studies of Kawashima et al. (10). The presence of polyvinyl alcohol significantly prevented aggregation of the droplets with solidified outer shells during the process. The dispersion of the ethanolic solutions of the drug and polymer into droplets in the medium depended on the concentration of polyvinyl alcohol in the medium.

Hence, when polyvinyl alcohol concentration increased in disperse phase, particle size of microspheres decreased (p < 0.05). As polyvinyl alcohol concentration was increased from 0.025 to 0.1%, particle size of microspheres decreased from 618 µm to 186 µm (Table 2). Particle size analysis of microspheres prepared by using different types of Eudragit® observed that mean particle size of the microspheres depended on the type of polymer (p < 0.05) in contrast to the earlier report by DortunÁ and Haznedar (19). Smaller particles were obtained with Eudragit RS as shown in Table 2. The X-ray powder diffraction patterns and IR spectra of the microspheres, as well as those of orig-

532


Table 6. Kinetic evaluation of dissolution rate tests of microsphere formulations in pH 6.8 phosphate buffer. Formulation code

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F11

Kinetics Zero

k0

0.937

0.234

0.197 0.140 0.117

0.106

0.121

0.127

0.103

0.1100 0.184

order

r2

0.689

0.760

0.825 0.853 0.895

0.927

0.898

0.913

0.883

0.868

0.809

First

k1

0.050

0.020

0.007 0.004 0.002

0.002

0.002

0.002

0.002

0.002

0.006

order

2

r

0.863

0.969

0.952 0.962 0.965

0.976

0.968

0.946

0.942

0.939

0.940

Hixson-

k4

0.040

0.010

0.007 0.004 0.003

0.002

0.003

0.003

0.002

0.002

0.006

Crowell

r2

0.813

0.949

0.938 0.934 0.946

0.963

0.948

0.946

0.924

0.918

0.924

Q→t1/2

k

10.5

4.99

4.50

2.67

3.07

3.19

2.63

2.82

4.52

r

0.821

0.911

0.976 0.967 0.986

0.995

0.986

0.980

0.978

0.974

0.948

2

3.62

2.99

k0 ñ Zero order dissolution rate constant (mg/minute); k1 ñ First order dissolution rate constant (minute ); k4 ñ Hixson-Crowell rate constant (minute-1); k ñ Higuchi rate constant (minute-1/2) -1

inal drug crystals are shown in Figures 2 and 3, respectively. The drug was dispersed in the polymeric matrices in a microcrystalline form, without polymorph change or transition phenomena into an amorphous form. Comparison of the X-ray diffaction patterns of ibuprofen (pattern A) and microspheres prepared with Eudragit® RS PM (F5), Eudragit® RS 100 (F10) and Eudragit® RL 100 (F11) (patterns B, C and D), showed no significant reduction in the characteristic peak intensities, suggesting that the extent of ibuprofen crystallinity was not reduced by the polymer. The spectral observations indicated that the principal IR absorption peaks observed in the spectra of ibuprofen were close to those in the spectra of the microspheres (Figure 3). IR spectra of the drug and microspheres showed characteristic bands of C=O vibrations of the esterified carboxylic acid groups at 1730 cm-1 and of the carboxylic acid groups at 1705 cm-1. For all the microspheres were indications that there is no strong interaction between the drug and the polymers. According to bulk density experiments, the smallest value was obtained from the microspheres prepared with 3 mL of ethanol (F6) (Table 3). It was seen that as the particle size increased, the bulk density decreased. That was because, the bulk density decreased due to the increase in the particle size and intraparticular space. Repose angle results showed that all formulations had suitable flow properties as shown in Table 4. All repose angles were under 30O. The flow properties were much improved over those of the original crystals. All formulations presented similar flow properties while the particle sizes were different because all formulations were spherical and their surface was smooth. It seems that the presence of

polymer contributes to improved sphericity of the microspheres. According to Figure 5, the yields of preparation were very high for all microspheres prepared (80.1-89.9%) with initial temperature gradient between emulsion phases. Assays generally showed that the drug content of the microspheres in the various batches showed good correlation with the theoretical drug loadings. The drug was uniformly encapsulated into the microspheres irrespective of initial drug concentrations. As seen in Table 5, drug loading capacities were very high for all microspheres and were not affected by the type of polymer (p > 0.05), the drugpolymer ratio (p > 0.05), volumes of ethanol (p > 0.05) and polyvinyl alcohol concentration (p > 0.05). The high content of ibuprofen in microspheres was believed to be due to the poor solubility of drug in poor solvent. Figure 6 shows the surface morphology and cross-section of the resultant microspheres which were investigated by SEM. It was observed that many micropores were formed on the surface of the microspheres. It was assumed that the coprecipitation of drug and polymers firstly occurred on the surface of the quasi-emulsion droplets and formed the film like a shell on the outer surface of droplets. Further diffusion of the ethanol out of the droplets resulted in the cavity and micropores were left in the inside of the microspheres. The surface morphology of microspheres was dependent on the polymer concentration in the ethanol droplet. With a decrease in the polymer concentration, the surface became rough and porous. Conversely, at higher concentrations, a smooth shell-like structure with small pores was formed on the microspheres. While F2 formulation prepared with 4:1 drug-polymer ratio had a very


porous surface, it was obtained smooth shell-like structure with 2:1 drug-polymer ratio at F5 formulation, as shown in Figure 6. Additionally, it was observed that the surface of microspheres prepared with Eudragit RS was smoother than that prepared with Eudragit RL. Comparison of the batches made with acrylic polymer and the batch made without polymer shows that microencapsulation of ibuprofen with polymer resulted in a marked decrease in the drug release rate of the microspheres (Figure 7). The difference was significant at each time point (p < 0.01). This was consistent with the localization of crystals in the core of the microspheres. Eudragit® RS and RL are not biodegradable and the solvent has to diffuse in polymer to dissolve the drug. Thereafter, the solution of ibuprofen can leave the core of the microspheres. Further, Figure 7 clearly illustrates that the rate of drug release from the microspheres depended on the polymer concentration in the system. When the concentration of the polymer in the system increased the release rate of ibuprofen decreased. The difference was also significant (p < 0.01) for 8 h. It is suggested that a reduced diffusion path and increased tortuosity may retarded the drug release rate from the matrix at higher concentrations of polymer. An inverse relationship was observed between polymer content and drug release rate from prepared microspheres. Microspheres containing 20% Eudragit® RS PM released the drug more rapidly, while those with 33.3% Eudragit® RS PM exhibited a relatively slower drug release profile. The effect of ethanol volume on drug release from microspheres is presented in Figure 8. Varying volumes of ethanol were used in the preparation of the dispersed phase. The quantity of ibuprofen and Eudragit® added was constant for all batches, therefore varying the ethanol volume affected drug and polymer concentration in ethanol (Table 1). Figure 8 illustrates that the drug release profiles differed according to the volume of ethanol used during microencapsulation. This difference was significant (p < 0.01) for 8 h. The highest release rate was achieved from F7 prepared with 6 mL of ethanol. The use of increasing volumes of ethanol produced microspheres with increasing drug release, despite the final composition of the batches being the same. The surface morphology of microsphere was dependent on the concentration of drug and polymer in the ethanol droplet as demonstrated in Figure 6. With a decrease in concentration, the surface became rough and porous. These findings suggested that these pores provided a channel for release of drug from the microspheres.

533

As shown in Figure 9, dissolution rate was not affected by polyvinyl alcohol concentration (p > 0.01). The release profiles of ibuprofen from microspheres prepared with different Eudragit® types are illustrated in Figure 10. The effect of retardation on the dissolution rate depended on the type of Eudragit®. As drug release rates were very slow and incomplete from Eudragit® RS microspheres, the same formulation was prepared using Eudragit® RL as polymer and different drug release profile was observed. The release of ibuprofen from F11 prepared with Eudragit® RL was remarkably faster than from F5 and F10 prepared with Eudragit® RS and the difference was significant (p < 0.01) for 8 h. It could be explained considering the chemical structure of Eudragit®. The Eudragit® RL and RS are synthesized from acrylic and methacrylic esters with high and low content of quaternary ammonium groups (8.8-12% and 4.5-6.8%, respectively) and result in microspheres with different water permeability. Due to the content of the quaternary ammonium groups, Eudragit® RS is only slightly permeable; hence drug release is relatively retarded, whereas Eudragit® RL is freely permeable, so that release is less retarded. These results observed that if Eudragit RS and Eudragit RL are used in combination, optimum release profiles may be obtained. In addition, the release mechanism of ibuprofen from these Eudragit® microspheres was evaluated on the basis of theoretical dissolution equations including zero order, first order, Hixson-Crowell and Higuchi kinetic models, since different release kinetics are assumed to reflect different release mechanisms. The results are shown in Table 6. It shows that the release pattern of ibuprofen from Eudragit® microspheres corresponded best to the Higuchi equation, which shows a linear relationship between the dissolved percent of drug and the square root of residence time. It was demonstrated that the microspheres possessed matrix structure, from which the dissolved drug diffused into the dissolution medium. Furthermore, when polymer concentration was increased in the formulations, correspondence was increased to the Higuchi equation. Consequently, ibuprofen microspheres with acrylic polymers (Eudragit® RS and RL) were prepared successfully by the modified quasi-emulsion solvent diffusion method. It was observed that the initial difference of temperature between the aqueous phase and dispersed emulsion phases influenced the microsphere formation. When there was an initial temperature gradient between the continuous and dispersed emulsion phases no coalescence was observed in the formulations. The formulation vari-

534


ables, i.e. drug-polymer ratio, volume of ethanol, concentration of polyvinyl alcohol and type of polymer were found to have an effect on the particle size, micromeritic and in vitro drug release characteristic of the prepared microspheres. The resultant microspheres have the desired micromeritic properties. The dissolution test results shown that the release rate of ibuprofen decreased when polymer concentration was increased in the system. Ibuprofen release rates from Eudragit RS microspheres were very slow whereas release rates from Eudragit RL microspheres were faster. The drug release profile aimed for peroral administration may be obtained by adding Eudragit RS and RL and changing the ratio of these polymers. In vitro dissolution findings showed that drug release appeared to fit the Higuchi matrix model. Acknowledgments The authors would like to thank Eczacýbaþý (Turkey) for providing ibuprofen. REFERENCES 1. Adams S.S., Buckler J.W.: Ibuprofen and Flurbiprofen. in: E.C. Huskisson (Ed.), AntiRheumatic Drugs, p. 244, Praeger, West Port, CT 1983. 2. Sweetman S.C.: Martindale, The Extra Pharmacopeia, 34th ed., Royal Pharmaceutical Society, London 2005. 3. Gallardo A., Eguiburu J.L., Berridi M.J.F., Roman J.S.: J. Controlled Rel. 55, 171 (1998). 4. Jbilou M., Ettabia A., Guyot-Hermann A.-M., Guyot J.-C.: Drug Dev. Ind. Pharm. 25(3), 297 (1999). 5. Leo E., Forni F., Bernabei M.T.: Int. J. Pharm. 196, 1 (2000). 6. Maheshwari M., Ketkar A.R., Chauhan B., Patil V.B., Paradkar A.R.: Int. J. Pharm. 261, 57 (2003). 7. Davis S.S., Illum L.: Biomaterials 9, 111 (1988). 8. Ritschel W.A.: Drug Dev. Ind. Pharm. 15, 1073 (1989).

9. Follonier N., Doelker E.: S.T.P. Pharma Sciences 2, 141 (1992). 10. Kawashima Y., Niwa T., Handa T., Takeuchi H., Iwamoto T., Itoh K.: J. Pharm. Sci. 78, 68 (1989). 11. RÈ M.I., Biscans B.: Powder Technology 101, 120 (1999). 12. Khanfar M., Salem M., Najib N., Pillai G.: Acta Pharm. Hung. 67, 235 (1997) 13. Jovanovic M., Jovicic G., Djuric Z., Agbaba D., Karljikovic-Rajic K., Nikolic L.: Acta Pharm. Hung. 67, 229 (1997). 14. Zaghloul A.A., Vaithiyalingam S.R., Faltinek J., Reddy I.K., Khan M.A.: J. Microencapsul. 18, 651 (2001). 15. Yang M., Cui F., You B., Fan Y., Wang L., Yue P., Yang H.: Int. J. Pharm. 259, 103 (2003). 16. Mateovic-Rojinik T., Frlan R., Bogataj M., Bukovec P., Mrhar A.: Chem. Pharm. Bull. 53, 143 (2005). 17. Eudragit RS and Eudragit RL data sheets, Rˆhm Pharma GmbH, Darmstadt 1991. 18. Pignatello R., Bucolo C., Ferrara P., Maltese A., Puleo A., Puglisi G.: Eur. J. Pharm. Sci. 16, 53 (2002). 19. DortunÁ B., Haznedar S.: Int. J. Pharm. 269, 131 (2004). 20. Devrim B., Canefe K.: Proceedings of the 8th

21. 22.

23. 24. 25.

European Congress of Pharmaceutical Sciences, Brussels, Belgium; Eur. J. Pharm. Sci. 23 (Suppl. 1), S40 (2004). Canefe K., Ateþoðlu-GˆÁmez S.: Boll. Chim. Farmac. 141, 423 (2002). You J., Cui F., Han X., Wang Y., Yang L., Yu Y., Li Q.: Colloids and Surfaces B: Biointerfaces 48, 35 (2006). Khan M.A., Bolton S., Kýslalýoðlu M.S.: Int. J. Pharm. 102, 185 (1994). Kachrimanis K., Ktistis G., Malamataris S.: Int. J. Pharm. 173, 61 (1998). The United States Pharmacopoeia, 24th ed., United States Pharmacopoeial Convention Inc., National Publishing Philadelphia, PA 2000.

Received: 31.05.2006