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RESEARCH ARTICLE

Controlled release Microcapsules for Oral Delivery of Aceclofenac: Formulation and Characterization

Rajesh Palva, Sandeep Kumar Singh, Priya Ranjan Prasad Verma, Praveen R*

Department of Pharmaceutical Science and Technology, Birla Institute of Technology, Mesra, Ranchi, India.

*Corresponding Author E-mail: prav.mpharm@gmail.com

ABSTRACT:

The aim of the present research work was to formulate microcapsules for rate controlled oral delivery of aceclofenac, using cellulose acetate phthalate (CAP) and ethyl cellulose (EC) as release retardant, and to evaluate the physicochemical properties. ‘Polymer deposition by emulsion-solvent evaporation’ technique was employed to produce microcapsules with CAP alone and with different proportions of CAP and EC, using acetone to dissolve the polymers. The formulations were characterized in terms of particle size, morphology, drug loading, entrapment efficiency, nature of flow and drug release. Infrared spectra and differential scanning thermograms confirmed the compatibility of drug with excipients and formulation process. Microcapsules were uniform, spherical, discrete and free flowing with the particle size varying from 397.32 to 526.06 µm. Average particle size increased with increase in proportion of polymer. High drug loading (19.12% to 42.84%) and entrapment efficiency (76.48% to 85.68%) were achieved, both the parameters decreasing with increasing fraction of polymer. The results of powder flow parameters like angle of repose, bulk density, tapped density, compressibility index and Hausner ratio confirmed that the powder was free flowing to enable direct capsule filling. The rate of drug release from microcapsules was found to be inversely related to the proportion of polymer. Substitution of a fraction of CAP with EC further slowed down the drug release rate, but the drug release was above 80% after 8 h. The drug release was dependent on concentration of aceclofenac in the core, with super case II transport mechanism. Delayed controlled release microcapsules of aceclofenac were successfully developed using ethyl cellulose and cellulose acetate phthalate, suitable for oral delivery.

KEYWORDS: Microcapsule, Aceclofenac, Controlled release, Oral delivery, Flow property.

Cellulose acetate phthalate (CAP) and ethyl cellulose (EC) are widely used in oral pharmaceutical formulation as a hydrophobic coating agent for tablet and granules. EC coatings are used to modify the release of a drug, to mask an unpleasant taste or to improve the stability of a formulation while CAP is used as an enteric coating polymer [11,12]. The present study focus to formulate and characterize aceclofenac loaded microcapsules using CAP and a mixture of CAP and EC (CAP-EC).

MATERIALS AND METHODS:

Materials

Aceclofenac was a generous gift from Comed Pharmaceuticals Ltd. (Gujrat, India). Cellulose acetate phthalate was purchased from Samar Chemicals Ltd. (Nagpur, India) and ethyl cellulose was purchased from Loba Chemie Pvt. Ltd. (Mumbai, India). All other reagents and solvents used were of analytical grade.

Formulation of microcapsules

Microcapsules were formulated by ‘polymer deposition by solvent evaporation’ method [13]. Aceclofenac was dispersed in liquid paraffin containing polysorbate 80 (1% w/v) by ultra-sonication. The polymer (CAP/CAP-EC) was dissolved in acetone and was added slowly to stirred drug dispersion by means of a burette. The resultant mixture was stirred at room temperature (25°C) for 24 h to evaporate acetone completely. The liquid paraffin was decanted and the microcapsules were collected, washed twice with chloroform dried in air for 12 h. The microcapsules were stored in desiccator until further use. The amount of each components used in the formulation are summarized in table 1. To prepare larger batches, proportional increase in the components was made.

Product yield

Product yield is a measure of the efficiency of a manufacturing process. It is expressed in percentage and calculated from the following equation:

Dried weight of microcapsule

Product yield (%) = --------------------------------------- X100

Total weight of solid in the feed

Fourier transform infra-red (FTIR) spectroscopy

Infra red spectra of pure drug and formulations were obtained using FTIR-8400S spectrophotometer (Shimadzu, Japan) by diffuse reflectance method. The samples were dried under vacuum prior to obtaining any spectra in order to remove the influence of residual moisture. Scans were performed at a resolution of 4 cm^-1 in a frequency range of 4000-600 cm^-1.

Differential scanning calorimetry (DSC)

The DSC thermogram of pure drug, polymers, physical mixtures of the drug with individual polymers (in 1:1 ratio) and formulations were obtained using DSC 50 (Shimadzu, Japan). Samples were sealed in aluminium pans and scanned from 30°C to 300°C in an inert atmosphere (N₂, flow rate: 50 mL/min) at a heating rate of 5°C /min.

Morphology of microcapsules

The surface morphology of the microcapsules was studied using scanning electron microscopy (SEM; JMS-6390, JEOL, Tokyo, Japan) [14]. The samples were coated using platinum to increase the conductivity of the electron beam. The images at required magnifications were captured under an aaccelerating voltage of 10 kV, working distance of 12 mm at spot size of 45.

Particle size

The microcapsules were examined by optical microscopy using eye-piece micrometer which was priory calibrated with a stage micrometer. A random sample of dried microcapsules was placed on glass slide with a drop of liquid paraffin and the size was measured using an optical microscope. The mean particle size also gives an idea about the uniformity of the batches of the microcapsules prepared.

Drug loading (DL) and encapsulation efficiency (EE)

Microcapsules theoretically equivalent to 10 mg of aceclofenac were weighed accurately and dissolved in the 10 ml of methanol. The solution was filtered, diluted suitably and drug content was analysed at 273 nm by UV spectrophotometer (UV 1800, Shimadzu, Japan). All determinations were made in triplicate. DL and EE were calculated for all batches using the following equations.

Estimated drug amount

DL (%) =----------------------------- X100

Weight of microcapsules

Estimated drug amount

EE (%) =----------------------------- X100

Theoretical drug content

Powder flow behaviour

The prepared microcapsules were characterized by their powder characteristics such as bulk density, tapped density, Carr’s compressibility index, Hausner ratio and angle of repose [15-17].

The bulk density is defined as the mass of the powder divided by bulk volume. The bulk density is calculated by dividing the weight of the sample in gram by final volume in cm³.

Mass of microcapsules

Bulk density =-----------------------------------------------

Volume of microcapsules before tapping

To determine tapped density, the cylinder containing known weight of microspheres was tapped for 1 min on a tapped density apparatus (Electrolab, India) until it gives constant volume.

Mass of microcapsules

Tapped density =-----------------------------------------------

Volume of microcapsules after tapping

Carr’s compressibility Index and Hausner’s ratio are important parameters in maintaining uniform weight. It is calculated using following equation:

Tapped density- Bulk density

Compressibility Index (%) =-------------------------------X100

Tapped density

Hausner’s Ratio = -----------------------------

Bulk density

The angle of repose of microcapsules was determined by the funnel method. Accurately weighed sample was taken in the funnel. The height of the funnel was adjusted such that the tip of the funnel is at 1cm from the surface. The powder blend was allowed to flow through the funnel freely on to the surface until the apex of powder cone touch the tip of the funnel. The diameter of the powder cone was measured and angle of repose was calculated using the following equation.

tan θ = h/r

Where ‘h’ and ‘r’ are the height and radius of the powder cone respectively.

In vitro drug release

Drug release from the microcapsules were studied in vitro using USP dissolution apparatus type 1 (rotating basket method; TDT-08L, Electrolab, India) in 0.1N HCl up to 2 h and in phosphate buffer of pH 6.8 from 2 to 8 h. Dissolution medium (900 mL) was maintained at 37±0.5 °C and basket rotation speed was 50 rpm throughout the experiment. The quantity of microcapsules equivalent to 50 mg of drug was taken in basket. Dissolution fluids (5 mL) were withdrawn at regular intervals and were replaced with equal volume of fresh dissolution media. The samples were filtered and analyzed by UV-visible spectrophotometer (UV 1800, Shimadzu, Japan) at 273 nm.

Mechanism of drug release

To study the mechanism of drug release, in vitro drug release profile of the formulations was fitted to kinetic models such as zero order, first order, Higuchi’s model and Korsmeyer - Peppas’ model [18-20].

RESULTS AND DISCUSSION:

Microcapsules were prepared by ‘polymer deposition by solvent evaporation’ method. Aceclofenac was dispersed in liquid paraffin with the aid of tween 80. Polymer (CAP/CAP-EC) was dissolved in acetone and was added slowly into liquid paraffin under constant stirring to form an emulsion. This emulsion was further stirred to evaporate acetone completely. The evaporation of acetone causes the precipitation of polymer, which preferably occur on the surface of dispersed drug particles (solid – liquid interface) as a result of interfacial tension. Thus, the process efficiently produces microcapsules with good yield and entrapment efficiency.

A large number of process variables exist that can affect the characteristics of product. In this study, effects of process variables like drug-polymer ratio and stirring time were considered by trial formulations (data not shown). It was found that when stirring time was 1 h, the product yield was less and mean particle size was larger compared to formulation prepared with 2 h stirring time. This may be attributed to the complete precipitation and size reduction of the polymer within 2 hours of stirring. Drug-polymer ratio of 1:0.5 resulted in a low drug loading and drug polymer ratio of 1:4 resulted in rapid precipitation of polymer to form lumps. From these observations, drug polymer ratio of 1:1, 1:2 and 1:3 at stirring time of 2 h were selected to formulate final batches of microcapsules (Data not shown).

FA batches were prepared with CAP alone and FB batches were prepared from a combination of CAP and EC (CAP-EC). The product yield of different batches was determined by weighing the microcapsules after drying. The percentage yield of different formulation were in the range of 81- 90% (Table 2), which are exceptionally good, indicating the efficiency of encapsulation process with low loss during process. The physicochemical characteristics were evaluated by scanning electron microscopy (SEM), infrared spectroscopy and differential scanning calorimetry. The microcapsules were then evaluated for particle size, drug content, entrapment efficiency, powder and flow characteristics and in vitro drug release.

Fourier transform infra-red (FTIR) spectroscopy

The FTIR spectra of pure aceclofenac and two formulations (FA1 and FB1) are shown in Fig. 1. Aceclofenac showed major peaks at 3306 cm^-1 (N–H stretching of secondary amine), 1718 cm^-1 (C=O stretch of carbonyl group), 1581 cm¹, 1500 cm^-1, 1435 cm^-1 (aromatic C=C stretching), 1149 cm^-1 (C–O stretching of ester), 763 cm^-1 (C–Cl stretching of alkyl halide) and 1253 cm^-1 (C–H stretching of methyl group), which corroborates with previous reports [21]. These peaks were found to be preserved in case of formulation without considerable shift in peak positions. This suggests that there were no drug-excipient interactions or process incompatibilities.

Differential scanning calorimetry

The DSC analysis of pure drug, physical mixture and formulations were carried out to evaluate the possible drug - polymer interactions. The DSC thermogram was obtained at a heating rate of 5°C/min from ambient to 300°C under nitrogen flow rate of 50 ml/min (Fig. 2). The DSC thermogram of pure aceclofenac showed sharp melting endotherm at 153.46°C (Fig. 2a), which was in conformity with the reported value [21]. The thermogram of physical mixture of aceclofenac with EC (Fig. 2d) and CAP (Fig. 2e) also showed the sharp melting endotherm without considerable shift, at 152.37°C and at 152.30°C respectively. The three component system with aceclofenac, EC and CAP (Fig. 2f) showed the melting endotherm at 151.51°C. The observations confirmed that there were no major interaction between drug and polymers.

Table 2: Physicochemical characteristics of microcapsules

	% Yield	PS (µm)	% DL	% EE	BD (g/mL)	TD (g/mL)	HR	CI	θ
FA1	90.0±0.8165	476.04	42.84±0.298	85.68	0.562	0.580	1.03	3.1	21°03'
FA2	88.0±0.9793	485.58	26.68±0.124	80.00	0.624	0.651	1.04	4.15	21°41'
FA3	83.8±1.0270	526.06	19.92±0.222	79.68	0.622	0.648	1.04	4.01	22°14'
FB1	86.1±1.0270	397.32	40.61±0.354	83.23	0.596	0.621	1.04	4.02	23°3'
FB2	82.6±0.4713	410.24	27.37±0.900	82.11	0.610	0.633	1.04	3.63	22°56'
FB3	81.5±1.3380	459.09	19.12±0.586	76.48	0.592	0.616	1.04	3.89	22°27'

PS- particle size, DL- drug loading, EE- entrapment efficiency, BD- bulk density, TD- true density, HR- Hausner Ratio, CI- Carr’s Index, θ- angle of repose

Fig. 5 In vitro drug release pattern of microcapsules

Drug loading and encapsulation efficiency

In micro-encapsulation, EE of the process depends primarily on the solubility of the drug in the continuous phase. High solubility of drug in the continuous phase may result in diffusion of drug into the continuous phase and low degree of encapsulation [23,24]. FA and FB series of microcapsules were prepared by emulsion evaporation technique using light liquid paraffin as continuous phase, in which solubility of aceclofenac is negligible. Hence, loss due to solubilization of drug in the continuous phase and diffusion from the formed particles were least and the entrapment efficiency was high. EE was found good in both FA and FB series, which ranged from 76.48% to 85.68% and was found decreasing with increasing fraction of polymer (Table 2). The reason may be the self assembling of polymer at a high polymer concentration upon evaporation of acetone, rather than depositing on the drug particles. As expected, DL decreased with increase in polymer fraction and ranged from 19.12 to 42.84%. No much variation in DL and EE was observed when a fraction of CAP was substituted by EC.

Powder flow behaviour

The powder properties such as bulk density, tapped density, Carr’s compressibility index, Hausner ratio and angle of repose were determined to understand the flow, packing and compacting behaviour of the microcapsules [15,16,17]. The results are compiled in Table 2. It has been stated that bulk density values less than 1.2 g/mL indicate good flow characteristics of the powder blend. It can be observed in table 2 that the bulk density values were less than 1.2 g/mL, indicating good flow characteristics of the microcapsules. Angle of repose less than or equal to 40° indicates free flowing property of the microcapsules.The angle of repose for all the formulations were found to be between 21°03' and 23°30' indicating good flow property.

Compressibility index and Hausner ratio are measure of cohesiveness and flow property. If the compressibility index is ≤ 10 and Hausner ratio is in the range 1.0 – 1.11, the powder blend can be said to have good flow characteristics. Increase in both the parameters indicates a more compressible powder blend with a less flow. Compressibility index and Hausner ratio of all the formulations were in the above mentioned range, indicating excellent flow characteristics. It is also indicative of uniform and spherical nature of microcapsules.

In vitro drug release

The drug release from the microcapsules was studied in 0.1N HCl up to 2 h and then in phosphate buffer of pH 6.8 upto 8 h. Release rate of aceclofenac from all the polymeric microcapsules was slow and extended. Fig. 5 shows the drug release profile of six batches of microcapsules. The release of aceclofenac in 0.1N HCl was very slow which could be due to the poor permeability of acidic medium through the polymeric coat. After 2 h, when the microcapsules were transferred to phosphate buffer at pH 6.8, there was a burst release of the drug. This could be explained as CAP, being an enteric coating polymer, is impermeable to acidic fluids and when placed in phosphate buffer of pH 6.8, the medium rapidly diffused through the polymer coat and dissolved the drug. Drug release rate was found to be decreasing with the increasing fraction of polymer. This could be due to a decreased amount of drug present close to the surface and formation of a thicker polymer coat. When a fraction of CAP was substituted with EC, the drug release rate reduced further, owing to EC being less permeable to both the media.

Mechanism of drug release

The drug release profiles were subjected to various kinetic models like zero order, first order and Higuchi’s model. The release pattern of formulations were best fitted to first order equation as the correlation coefficient values (ranged from 0.975 to 0.991) were higher than that of zero order and Higuchi’s model, indicating the dependency of release rate on drug concentration in the core. Korsmeyer – Peppas release exponent (n) was determined to elucidate the mechanism of drug release. As a general rule, n = 0.5 indicate Fickian diffusion, 0.5< n <1 indicate anomalous transport, n = 1 indicate case II transport and n > 1 indicate super-case II transport [25,26]. Case II transport occurs when the sorption is entirely controlled by stress induced relaxations taking place at a sharp boundary separating an outer swollen shell from an un-penetrated glassy core. A transport corresponding to coupled drug diffusion in the hydrated matrix and polymer relaxation is termed anomalous diffusion. The ‘n’ values for the present microcapsule systems were greater than 1, indicating a super-case II transport mechanism. This is indicative of a drastic increase in sorption with time [27]. The reason for this observation could be the replacement of dissolution media, 0.1 N HCl, after 2 h, with phosphate buffer and CAP being highly permeable to solutions of neutral or slightly alkaline pH, the penetration into the core increased drastically. This is in accordance with the observation of a burst drug release at 3^rd hour during the dissolution study.

CONCLUSION:

Aceclofenac is an effective non-steroidal anti-inflammatory agent with a short half life and gastric irritant property. The said properties make it a good candidate for delayed and extended release formulation. The present study proved that ‘polymer deposition by solvent evaporation’ is a simple and reproducible method for the preparation of aceclofenac microcapsules. It was found that the prepared microcapsules were spherical, free flowing with high entrapment efficiency and yield. In vitro dissolution studies illustrated a typical slow rate of drug release in acidic media and faster rate of drug release in alkaline media. This, in turn, result in a delayed drug release in the small intestine, in vivo. The drug release was concentration dependant and exhibited super case II transport. Thus the present study illustrates a successful systematic development of extended release microcapsules of aceclofenac using ethyl cellulose and cellulose acetate phthalate, suitable for oral delivery.

ACKNOWLEDGMENTS:

The authors would like to thank Comed Pharmaceuticals Ltd. (Gujrat, India) for providing gift sample of aceclofenac. The authors are thankful to Central Instrumentation Facility, Birla Institute of Technology, India for providing instrumental back up. The authors Rajesh Palva and Praveen R are thankful to UGC India for providing GPAT fellowship and Basic Scientific Research fellowship respectively.

CONFLICT OF INTEREST:

The authors have no conflict of interest. This article does not contain any studies with animal or human subjects.

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Components	FA1	FA2	FA3	FB1	FB2	FB3
Aceclofenac (g)	0.1	0.1	0.1	0.1	0.1	0.1
CAP (g)	0.1	0.2	0.3	0.05	0.1	0.15
EC (g)	-	-	-	0.05	0.1	0.15
Acetone (mL)	10	10	10	10	10	10
Liquid paraffin (mL)	20	20	20	20	20	20
Tween 80 (mL)	0.4	0.4	0.4	0.4	0.4	0.4