DESIGN , FORMULATION AND EVALUATION OF CELECOXIB SOLID DISPERSIONS PREPARED BY THE SOLVENT-EVAPORATION TECHNIQUE

The aim of the present study was to improve dissolution of poorly water-soluble Celecoxib by solid dispersion technique using water soluble carriers. Solid dispersions of Celecoxib were prepared with different polymers or carriers such as Hydroxy Propyl Beta Cyclodextrin, polyvinylpyrrolidone (PVP K30) and Urea. Solid dispersions were formulated in drug polymer ratio 1:2.5, 1:5, 1:7.5 and 1:10 using solvent evaporation method. The prepared formulae were assayed for drug content, production yield and micromeritics properties. Dissolution profiles were done in water containing 1% sodium lauryl sulphate and the in-vitro release was ranked according to the % drug released after 60 and 120 minutes. The results indicated that all of the formulated solid dispersions displayed better dissolution profiles as compared to the pure drug. Formula containing drug to HP-β-CD (1:10) was ranked first and gave the best results out of the 12 solid dispersion formulae prepared.


INTRODUCTION
The term 'solid dispersion' has been utilized to describe a family of dosage forms whereby the drug is dispersed in a biologically inert matrix, usually with a view to enhancing oral bioavailability.Another definition was 'molecular mixtures of poorly water soluble drugs in hydrophilic carriers.
First description of solid dispersions was from Sekiguchi and Obi (1961).They noted that the formulation of eutectic mixtures improve the rate of drug release and, consequently, the bioavailability of poorly water soluble drugs (Sekiguchi et al., 1964).In the late sixties a second generation of solid dispersions appeared, containing amorphous carriers instead of crystalline ones.Recently, it has been shown that the dissolution profile can be improved if the carrier has surface activity or self-emulsifying properties, therefore third generation solid dispersions appeared.These contain a surfactant carrier, or a mixture of amorphous polymers and surfactants as carriers.
The most commonly used hydrophilic carriers for solid dispersions include polyvinylpyrrolidone (Paradkar et al., 2004;Ambike et al., 2004), polyethylene glycols (Samy et al., 2010), Urea (Punitha et al., 2009) and cyclodextrins (CDs) that comprises of a series of cyclic oligosaccharides compounds, and several members of this family are used industrially in pharmaceutical, chemical, and food science applications (Maestre et al., 2007).
Solid dispersion can be prepared by four methods.The first one is the solvent evaporation method.Various drugs are formulated as solid dispersions by solvent method, the most recent candidates are: Aceclofenac (Dahiya, 2010), Tacrolimus (Joe et al., 2010), Allopurniol (Samy et al., 2010), and Isradipine (Tran et al., 2010).Another common process is the co-precipitation method, in which a non-solvent is added drop wise to the drug and carrier solution, under constant stirring.In the course of the non-solvent addition, the drug and carrier are co-precipitated to form microparticles (Pouton, 2006).
The second method is the Fusion method which is sometimes referred to as the melt method, which is correct only when the starting materials are crystalline.The matrix consisted of sulfathiazole and urea as solid dispersion which was melted using a physical mixture at the eutectic composition, followed by a cooling step.Some of recent solid dispersions formed by fusion method are: glibenclamide Hot melt extrusion is the third method of preparation.Melt extrusion is essentially the same as the fusion method except that intense mixing of the components is induced by the extruder.Yang et al. (2010) prepared acetaminophen as solid dispersion by hot melt extrusion.
Supercritical fluid methods is a recent method which mostly applied with carbon dioxide (CO2), which is used as either a solvent for drug and matrix or as an anti-solvent.When supercritical CO2 is used as solvent, matrix and drug are dissolved and sprayed through a nozzle, into an expansion vessel with lower pressure and particles are immediately formed.
In spite of almost several years of research on solid dispersions, their commercial application is limited.Only a few products have been marketed so far.Amongst these are: Gris-PEG (Novartis), griseofulvin in PEG; Cesamet (Lily), nabilone in PVP; Sporanox (Janssen Pharmaceutica/J&J), itraconazole in HPMC and PEG 20,000 sprayed on sugar spheres.
NSAIDs are the most widely prescribed medications in the world.All (NSAIDs) including selective COX-2 inhibitors, are antipyretic, analgesics, and anti-inflammatory.The mechanism of action of NSAIDs has been attributed to their ability to inhibit the cyclooxygenase enzyme (COX).Out of the 2 isoforms of cyclooxygenase, COX -1 is responsible for mediating the production of prostaglandin while COX-2 is primarily associated with inflammation, pain, and fever.The traditional NSAIDs are non selective COX inhibitors.COX-2 selective NSAIDs are, therefore , ideal anti-inflammatory drugs with minimum drug related side effects , since they spare COX-1 activity.
Celecoxib was our drug of choice which is the first specific inhibitor of cycloxygenase-2 (COX-2) to be approved by the United States Food and Drug Administration (FDA), in 1998.The aqueous solubility of CXB is low (3 to 7 μg/mL) when determined in vitro at pH 7 and 40°C as its chemical structure shows high non-polar characteristics.The oral bioavailability of CXB is between 22% and 40%.Thus, it is important to enhance the solubility and dissolution rate of CXB to improve its overall oral bioavailability.There are various techniques for enhancement of CXB solubility as using its amorphous form (Chawlaa, 2003) The aim of this paper was to prepare Celecoxib solid dispersions using HP-β-CD, PVP K30 and Urea in order to enhance its solubility, in-vitro release and hence its bioavailability.

METHODOLOGY Preparation of Celecoxib solid dispersions by the Solvent evaporation method
The calculated amount of Celecoxib and the employed polymers (HP-β-CD, PVP K30 and Urea) in different drug-polymer ratios (1:2.5, 1:5, 1:7.5 and 1:10) were weighed and mixed together in a porcelain dish.Twelf different formulae were prepared by the solvent evaporation method.The mixture was dissolved in the least amount of methanol as a common solvent.Then the solvent was evaporated in oven at temperature 45 o C till complete evaporation.The solid dispersions prepared were pulverized in a mortar and sieved.The fraction of the powder that passed through 200 µm and retained on a 125 µm sieve was collected, stored in a desiccator and used for further investigations.

The production yield
The production yields of the prepared Celecoxib solid dispersions were studied, since it measures the actual weight of the prepared solid dispersion (drug + polymer).This value was calculated by dividing the actual yield of the solid dispersion produced (before sieving) over the theoretical yield and multiplied by 100.

Actual yield of solid dispersion X100 Theoretical yield of solid dispersion
The drug content A specific amount of the prepared Celecoxib solid dispersion equivalent to 5 mg drug was dissolved in 50 ml methanol to produce stock solution (100 µg /ml).One ml of the stock solution was withdrawn and completed to 10 ml using methanol.The concentration of this solution was (10 µg /ml).The solution was assayed spectrophotometrically at 252 nm for calculating the Celecoxib content.The polymers did not show interference with the absorbance of the drug at this wavelength.

Micromeritic properties of the prepared solid dispersions
The prepared solid dispersions were evaluated through determination of the following parameters:

Densities of solid dispersions
Both bulk density (D b ) and tapped density (D t ) were determined.A quantity of 1g solid dispersion powder was introduced into a 10 ml measuring cylinder.The initial volume was observed, and then the cylinder was allowed to stroke.The tapping was continued until no further change in volume was noted.D b and D t were calculated using the following formulae:

Hausner's Ratio
It is the ratio between bulk density and tapped density.It gives an idea about the flow characters of powder particles.

Compressibility percent (Carr's Index)
Compressibility is indirectly related to the relative flow rate, cohesiveness, and particle size of a powder.The compressibility percent of a material can be estimated as:

Angle of Repose
It was measured by passing the solid dispersion powder through a funnel which was maintained at a fixed height in all experiments.The height (h) and radius (r) of the cone were determined.The angle of repose was calculated from the following equation:

From Hard gelatin capsules
The dissolution behaviors of the prepared 12 formulae of Celecoxib solid dispersions were compared with the Celecoxib powder.The dissolution studies were performed by USP dissolution tester, apparatus I (basket method).
An accurately weighed amount of prepared solid dispersion equivalent to 100 mg of Celecoxib was placed in Hard gelatin capsule.Each capsule was placed in a basket in 900 ml of water containing 1% sodium lauryl sulphate.The basket was rotated at 100 rpm.The temperature of the in-vitro release medium was maintained at 37 o C± 0.5 o C. Each sample was run in triplicate in which 5 milliliters aliquots were withdrawn at 10 ,20, 30, 45, 60 , 90 and 120 minutes and replaced by 5 ml of fresh pre-warmed water containing 1% SLS.Samples were analyzed spectrophotometrically at the predetermined λ max using water containing 1% SLS as a blank.The cumulative percentage drug released was calculated.
The experiments were conducted in triplicates and the mean ± SD was calculated using Microsoft office excel, 2007 The data of the in-vitro release of pure Celecoxib and Celecoxib solid dispersion was treated by different kinetic orders or systems to explain the release mechanism for each formula.So, the studied formulations of Celecoxib were subjected to zero, first and Higuchi's diffusion model (Higuchi, 1963).The kinetic parameters and correlation coefficient were calculated for the in-vitro release of Celecoxib.

Preparation of Celecoxib by solid dispersion technique using solvent-evaporation method
Celecoxib was formulated as solid dispersions using the solvent-evaporation technique.Three polymers were employed, i.e., HP-β-CD, PVP K30 and Urea in different drug-polymer ratios (1:2.5, 1:5, 1:7.5 and 1:10).Twelf different formulae of Celecoxib solid dispersions were prepared by the above method.Table (1) contains the suggested formulae of Celecoxib.
The formed solid dispersions varied in their physical properties according to the type of polymer used and the proportions of drug to polymer.Formulae containing PVP were more viscous, sticky and more difficult to be sieved than formulae containing HP-β-CD and Urea.
The lower the ratio of the drug to the polymer, the more elastic the solid dispersion was.This was consistent with the results of Tantishaiyakul et al. (1999) which stated that piroxicam: PVP k90 ratios lower than 1:4 were not investigated due to the stickiness of the preparations with the increasing amount of the polymer.

The production yield
The values of the production yield of the 12 formulae of Celecoxib solid dispersion before sieving were ranging from 90 to 99.62 %.Satisfactory reproducibility of results when repeating the preparations was observed.Table (2) shows the production yield of the prepared formulae.
Formula CXB-Urea (1:7.5)gave the best value for the production yield while formula CXB-PVPK30 (1:2.5)gave the worst value.The obtained results were found to be in good agreement with the specifications of the official pharmacopeias (USP 30, 2007; BP, 2009).

The drug content
As shown in table (2), the drug content of different formulae ranged from 88.57% to 102.492%.The obtained results were found to be in good agreement with the specifications of the official pharmacopeias (USP 30, 2007;BP, 2009).
Table (2): Production yield and Drug Content for Celecoxib solid dispersions

Micromeritic properties of Celecoxib solid dispersions Direct method for calculating the flowability Angle of repose (Ө)
The angle of repose has been used in several branches of sciences to characterize the flow properties of solids.Angle of repose is a characteristic related to interparticulate friction or resistance to movement between particles.Angle of repose results were reported to be very dependent upon the method used.Experimental difficulties may arise as a result of segregation of material and consolidation or aeration of a powder as the cone is formed.Despite its difficulties, the method continues to be used in the pharmaceutical industry, and a

Formula Number
Production yield % (PY) The angle of repose is the constant, three dimensional angle (relative to the horizontal base) assumed by a cone-like pile of material formed by any of several different methods.
The The values obtained for the angle of repose of the prepared Celecoxib solid dispersions ranged from 12.38° to 19.5°, as shown in table (3).These values indicate that all formulae have good to excellent flowability.Good flowability was shown in all formulae.This may give indication that CXB solid dispersion is a good method for improving the flowability.

b-The Hausner ratio
The Hausner ratio is a number that is correlated to the flowability of a powder or granular material.The Hausner ratio is measured from the bulk and tapped density of the powder.The accepted scale of flowability of a powder was described in USP 30 (2007).The value of the Hausner ratio was found to give indication about the flow properties of solid dispersion.The values less than 1.25 indicate better flowability than values more than 1.25.
The values obtained for the Hauser ratio of the prepared Celecoxib solid dispersions ranged from1.039CXB-PVP (1:7.5) to 2.219 CXB-HP-β-CD (1:2.5), as shown in table (3).So, the obtained results showed that four CXB solid dispersions have better flowability while the other eight CXB formulae showed passable flowability.

c-Compressibility % (Carr's index)
Compressibility percent is indirectly related to the relative flow rate, a compressible material will be less flowable.The value of the compressibility percent was found to affect the flow properties of solid dispersion.The obtained ranking order is the same as the obtained from the Hausner ratio data.The best Carr's index was 3.84 for CXB-PVPK30 (1:7.5), while the worst was 20 for CXB-HP-β-CD (1:2.5) , as shown in table (3).

Tiwari et al., (2010)
measured the bulk and tapped densities, angle of repose, hausners ratio and compressibility index of CXB-Sorbitol solid dispersions in various proportions and found it had a good flowability.
The rank order of CXB solid dispersions concerning the micromeritics properties (Angel of repose, Hausner Ratio and Carr's Index) was done.Formula CXB-PVPK30 (1:7.5) was found to be the best formulation of CXB solid dispersion followed by formula CXB-PVPK30 (1:10).

In-vitro release of Celecoxib solid dispersions from Hard gelatin capsule
The in-vitro release refers to the rate of dissolving the drug from an intact dosage form.There are two reasons for performing these tests, one of them is to ensure product uniformity from batch to batch, the other reason is to predict the rate of drug absorption and ⁄ or availability of the drug for absorption (Wagner, 1975).
In-vitro release studies were carried out at 37 °C by the USP Dissolution Tester, Apparatus I (Rotating basket), and using 900 ml of water containing 1% SLS as the dissolution medium and at a rotation rate of 100 rpm.The in-vitro release of Celecoxib solid dispersion from the different suggested formulae was evaluated by measuring the percent of drug released from the Hard gelatin capsule.
The data obtained and calculated for this part are illustrated in figures (1-3).Pure Celecoxib yielded the slowest initial dissolution rate with only about 66.20% in 120 minutes.
Figure ( 1) shows the in-vitro release of pure Celecoxib and the prepared Celecoxib solid dispersion formulae from F1 to F4 using PVPK30.The influence of PVPK30 ratio was studied on the prepared formulae on the in-vitro release of Celecoxib using Hard gelatin capsule.
Figure (2) shows the in-vitro release of pure CXB and the prepared CXB solid dispersion formulae from F5 to F8 using Urea.The influence of Urea was studied on the prepared formulae on the in-vitro release of CXB using hard gelatin capsule.
Tantishaiyakul and coworkers (1999) studied the effect of PVP K90 and PVP K17 on the in vitro release of the piroxicam.They found that the Dissolution rates for solid dispersions were greater than those for piroxicam alone.The enhanced dissolution rates of solid dispersions may be due to many factors such as decreased particle size of drug (Ford et al., 1986), specific form of drug (Simonelli, et al., 1976) in these solid dispersions in addition to the increase in drug wettability and preventing the aggregation of drug by the polymers.
A total conclusive rank order for production yield, drug content and the micromeritics properties and the in-vitro release of the Celecoxib solid dispersion is shown in table (5).From this rank order it was concluded that the best formula was CXB-PVPK30 in which the drug to polymer ratio was 1:10.

Figure (2):
The effect of Urea ratio on the in-vitro release of CXB solid dispersions using hard gelatin capsule (pure CXB + formulae 5-8)

Figure (3):
The effect of HP-β -CD ratio on the in-vitro release of CXB solid dispersions using hard gelatin capsule (pure CXB + formulae 9-12) The kinetic treatment of the in-vitro release of Celecoxib is critical and has to be investigated to achieve an optimal system with desired release characteristics.Furthermore, in-vitro release studies are often performed to predict how a delivery system might work in ideal situations, which might give some indication on its in-vivo performance.Tables (6-8) illustrate the kinetic parameters of the in vitro release of Celecoxib using Hard gelatin capsule.In calculating the kinetic parameters for each order or system, the intercept, the slope, the correlation coefficient, the specific rate constant and the half-life were obtained.

CONCLUSION
This present study showed that when Celecoxib was dispersed in a suitable watersoluble carrier such as HP-β-CD, PVPK30 and Urea, its dissolution was enhanced compared with pure drug.The water soluble carrier may operate in the microenvironment (diffusion layer) immediately surrounding the drug particles in the early stage of dissolution, since the carrier completely dissolves in short time, enhancing the solubility and dissolution of drug.The study clearly showed that addition of HP-β-CD to Celecoxib in ratio 10:1 respectively improved the dissolution rate and this formula ranked first in the in-vitro release rate ranking.Mechanisms involved are solubilization and improved wetting of the drug in the water soluble polymers rich microenvironment formed at the surface of drug crystals after dissolution rate.Finally it could be concluded that solid dispersion of Celecoxib using hydrophilic polymers would improved the aqueous solubility, dissolution rate and thereby enhancing its systemic availability.

Muralidhar
et al., (2010) prepared solid dispersion of CXB and PVP K30 in a similar procedure to our thesis, CXB:PVPK-30 in 1:1, 1:2, 1:4, 1:6 weight ratios by means of solvent method; the same work was reported by Punitha et al., (2010) who prepared CXB-Mannitol solid dispersion by solvent evaporation technique; (Punitha et al., 2009) who prepared CXB-Urea by the solvent method; Ahuja et al. (2007) who prepared Refocoxib solid dispersions with PVP K25; Shavi and coworkers (2010) prepared the solid dispersions by dissolving the mixture of Gliclazide and the PVP K 30 at the weight ratios of 1:0.5, 1:0.75 and 1:1 w/w and Samy et al. (2010) used the solvent evaporation method to prepare solid dispersions of Allopurinol with different polymers (PVP K30 and PVP K90)Table (1): Suggested formulae of Celecoxib [solid dispersions] angle of repose was found to affect the flowability of the particles or granules.The values less than 20° exhibit excellent flowability; the values between 20 and 30° show good flowability; the values between 30 and 34° exhibit passable flowability; while the values above 34° show very poor flowability (Bhowmik et al., 2009).USP specifications (USP 30, 2007) for the values of the angle of repose were different from the previous publication and these values are: 25-30 excellent flow, 31-35° indicates good flow, 36-40° the flowability of a powder is fair, 41-45° passable flowability, 46-55° poor, and > 55° indicates very poor flow properties for a powder.
The values between 5 and 12 show excellent flowability; the values between 12 and 16 exhibit good flowability; the values between 18 and 21 show fair passable flowability; the values between 23 and 35 exhibit poor flowability; while the values between 33 and 38 exhibit very poor flowability (Bhowmik et al., 2009).

Table ( 3
): The data collected for the angle of repose, the bulk density, the tapped density, hausner ratio and compressibility % of CXB solid dispersion formulae

Table ( 4
): Rank Order for the in-vitro release of CXB solid dispersions using different polymers and different drug-polymer ratios using the hard gelatin capsule

Table ( 5
): Conclusive rank order for the production yield, drug content, micrometrics properties and in-vitro release of the solid dispersion formulae.

Table ( 6
): Kinetic parameters for the in-vitro release of CXB formulae using hard gelatin capsule according to Zero-order using kinetics

Table ( 8
): Kinetic parameters for the in-vitro release of CXB formulae using hard gelatin capsule according to Higuchi diffusion model

Table ( 9
): The calculated correlation coefficients for the in-vitro release of CXB formulae from hard gelatin capsule employing different kinetic orders or systems