INTRODUCTION:

Ion exchange resins are solid insoluble high molecular weight polyelectrolyte that can exchange their mobile ions of equal charge with surrounding medium, reversibly and stoichiometrically [3].

Various ion exchange materials available can be classified as shown in Figure 1.1, on the basis of nature of structural and functional component and ion exchange process. The most important class, organic ion exchangers are widely used in pharmaceutical field. These include ion exchange resins, ion exchange filters and ion selective membranes.

The principle property of these resins is their capacity to exchange bound or insoluble ions with those in solution. Anion exchange resins involve basic functional groups (usually a polyamine) capable of removing anions from acidic solutions and usually contain polystyrene polymers with quaternary ammonium and polyalkylamine groups. Cation exchange resins contain acidic functional groups. Although their exact composition may vary, they usually contain polystyrene polymers with sulfonic, carboxylic or phenolic groups.

The use of ion exchange resins to prolong the effect of drugs is based on the principle that positively or negatively charged drug moieties, combined with appropriate resins yields insoluble polysalt resinates.

R–SO₃ H⁺ + H₂ N^-A « R–SO₃–H₃N^-A

R–NH₃OH^- + HOOC–B « N⁺H₃–OOC–B + H₂O

Where, H₂N^-A and HOOC–B represent basic and acidic drug respectively, and R-SO₃^–H⁺ and R-NH₃⁺OH^- represent cationic and anionic exchange resins respectively. Ion exchange resin complexes are likely to spend 1 to 2 hours in contact with an acidic fluid of pH 1.2, and then move in to intestine where they are in contact for more than six hours with a fluid of slightly alkaline pH [4].

Ion exchange process in stomach

R^-SO₃^-H₃N⁺–A+ HCl « R-SO₃^-H⁺ + AN⁺H₃ Cl^-

R–N⁺H₃ ^-COO–B + HCl « R-N⁺H₃Cl^- + B–COOH

and in intestine

R–SO₃^-H₃N⁺–A + NaCl « R–SO₃^-Na⁺ + A–N⁺H₃Cl^-

R–N⁺ H₃^-OOC–B + NaCl « R–N⁺H₃Cl^- + B-COO Na⁺

In last few years, the fairly new applications for ion exchange resins have been noticed. Avari and Bhalekar reported improved dissolution of sparfloxacin bound to weak cation exchanger. They reported faster dissolution of this poorly soluble drug bound to weak cation exchanger Indion 204, as compared to marketed formulation [25]. Rohm and Hass claims certain new uses of ion exchange resins. These uses include reduction in deliquescence, hygroscopicity and polymorphism [34].

Drugs which form crystalline solids often can exist in more than one crystal form, each of which may have distinct properties in terms of solubility, melting point etc.

Invariably one crystal form may be more active or easier to handle than another although the conditions under which the various crystal forms appears may be so close to be very difficult to control on the large scale. In same case one crystal form can be transformed into another on storage and this can cause the problem with effectiveness of formulation. This effect is known as polymorphism is increasing concern to the pharmaceutical industry by loading drugs on functional polymers; many problems associated with polymorphs can be eliminated to give consistent drug properties [35]. Recent use of ion exchange resins for potential nasal delivery of insulin has been reported [36]. Ion exchangers for ocular delivery of betoxolol increased ocular comfort and increased bioavailability [37].Ion exchange resins have also been used for site specific [38,39] and transdermal drug delivery [40].

MATERIALS AND METHODS:

Salbutamol sulphate was gifted by Zim Labs, Nagpur, Amberlite IRP 69 was gifted by Röhm and Haas, Sodium hydroxide, n-hexane and Hydrochloric acid from Ranbaxy Fine Chemicals Ltd, Monobasic potassium phosphate, Acetone, Span 80 were obtained from S.D. fine chemicals, Sodium chloride and Ethanol were obtained from Samar chemicals, Liquid paraffin was obtained from Rankem.

The drug resin complexes were prepared by batch process [21]. An accurately weighed amount of salbutamol sulfate (500 mg) was dissolved in 100 ml distilled water. The known quantity of ion exchange resin was added to the solution and stirred on magnetic stirrer. The time to reach equilibrium was determined by periodically measuring concentration of drug remaining in solution. It was found that 4 hours is optimum period for attainment of equilibrium loading. Resinate thus formed was washed with an excess amount of distilled water, which was collected and added to previous filtrates. Resinates were dried overnight in a hot air oven at 50 ⁰C and then stored in tightly closed in desiccators. As shown in table 1, formulation batches of polymeric beads were prepared by changing the formulation parameters.

Table 1 Formulation batches of drug resin complex

Batch code	Parameters
Batch code	Core: coat ratio (Resinate: polymer)	Eudragit RS: RL ratio	Solvent composition Acetone or Acetone: ethanol solvent blend (%)
K5	1:1	50:50	Acetone
K6	1:1	50:50	50:50
K7	1:1	70:30	Acetone
K8	1:1	70:30	50:50
K9	1:2	50:50	Acetone
K10	1:2	50:50	50:50
K11	1:2	70:30	Acetone
K12	1:2	70:30	50:50
K13	1:3	50:50	Acetone
K15	1:3	50:50	50:50
K16	1:3	70:30	Acetone
K17	1:3	70:30	50:50

Selection of resin

Resins are selected on the basis of nature of drug and requirements of formulation.

The selected drug salbutamol sulphate contains amine group i.e. cationic center. Therefore strong cation exchange resin (Amberlite IRP 69) was selected, which is specifically recommended for sustained drug delivery.

Moisture content determination of resin [53]

One gram of accurately weighed resin was kept in oven (previously heated to 100 ⁰C) for 24 hours; the moisture content was determined using following formula

Particle size distribution of resin [54]

The sieves were arranged in a nest with coarsest at the top. Ten gram of accurately weighed sample of resin was placed on top sieve, and the sieves were shaken on mechanical sieve shaker for five minutes, the resin retained on each sieve was weighed. Weight retained on each sieve was plotted against mean particle size, which gives frequency distribution curve.

Development of drug resin complex

The drug resin complexes were prepared by batch process [21]. An accurately weighed amount of salbutamol sulfate (500 mg) was dissolved in 100 ml distilled water. The known quantity of ion exchange resin was added to the solution and stirred on magnetic stirrer. The time to reach equilibrium was determined by periodically measuring concentration of drug remaining in solution. It was found that 4 hours is optimum period for attainment of equilibrium loading. Resinate thus formed was washed with an excess amount of distilled water, which was collected and added to previous filtrates. Resinates were dried overnight in a hot air oven at 50 ⁰C and then stored in tightly closed in desiccator. The drug content in the final filtrate and washing was analyzed spectrophotometrically at 276.6 nm.

The amount of drug loaded on the complexes was determined by subtracting the remaining amount of drug in the final filtrate from initial amount.

Effect of pH on drug loading

In 100 ml distilled water 500 mg of salbutamol sulfate was dissolved. To it 500 mg resin was added. The pH of resultant dispersion was adjusted individually to 2, 2.5, 3, 3.5, 4.0, and 4.5, and stirred on magnetic stirrer for 4 hours. The formed resinates were collected by filtration, washed with 100 ml distilled water to remove unbound drug and dried at 50^oC. The drug content was determined by calculation of unbound drug remaining in solution.

Selection of drug resin ratio

The drug resin complexes were prepared in the ratios of 1:1, 1:2, 1:3. The pH of solution was adjusted to 4.0. The solutions were stirred for 4 hours on magnetic stirrer. Resinates obtained were separated by filtration, washed with 3X100 ml deionizer water. The drug content was determined.

Effect of temperature on drug loading

These studies were carried out using drug: resin ratio of 1:1 in pH 4.0 condition. Temperature was kept at 20, 30 and at 40 ^oC. The drug loading was calculated by method described previously.

Evaluation of drug resinate preparation

Physical properties

Different physical parameters of resinates like shape, flow properties, bulk density, tap density and packing ability were studied.

Shape

The shape of the resins and resinates was observed under microscope. The observations are given in table 6.4.

Flow property

The frictional force in the powder can be measured by the angle of repose. Angle of repose was calculated by fixed funnel method. Angle of repose was calculated using equation:

Tan q = h/r

Where, h = height of heap in cm

r = radius of heap in cm.

Bulk density

For determination of bulk density a sample of about 25g was poured into a 100 ml graduated cylinder. The cylinder was dropped at 2 seconds interval onto a hard wooden surface three times from a height of 2.5 cm. The volume was recorded and bulk density was calculated using formula:

Bulk Density = Weight of sample taken /volume occupied

Tapped Density

A sample of 25 g was poured gently into a 100 ml graduated cylinder. The cylinder was dropped at 2 seconds interval on to a hard wooden surface from 2.5 cm height. Tapped density was calculated by measuring final volume after 50 taps on wooden surface from since height and was expressed in g/cm³.

Packing ability

The packing ability of resins and resinates were evaluated from the change in volume due to rearrangement of packing occurring during tapping. It was expressed as Carr’s compressibility index (Cc %) and was calculated as follows;

X–ray diffraction studies

The salbutamol sulfate, resins, resinates and physical mixture (drug and resin) were subjected to X–ray diffraction studies for confirmation of complex formation.

In vitro drug release studies

Effect of pH on in vitro drug release

A claimed advantage of ion exchange delivery system is that release of drug is independent of pH of the dissolution medium. This prospect was investigated by preparing buffer solutions of different pH (1.2 and 6.8) with ionic strength adjusted to m @ 0.1. In vitro release drug resinate was carried out in these pH, using conditions described in previous section.

Effect of ionic strength on in vitro drug release

To study effect of ionic strength on in vitro release of drug resinate, pH 1.2 buffer with ionic strength adjusted to m = 0.1, 0.15, 0.20 was prepared. The release study was carried out at these ionic strengths using method described in previous section.

Microencapsulation

Development of polymeric beads [48]

Microencapsulation was carried out by solvent evaporation method, as described by Georgarakis M. and Amperiadou A. (1994). Solvent evaporation method involves two phases, namely internal phase and external phase. Since salbutamol sulphate is water soluble, both the selected phases were non-solvent for drug. Acetone or acetone + ethanol, and liquid paraffin were selected as three phases in accordance with detailed studies by Sprockel OL. and Prapatrakul W.(1990). Both, acetone and liquid paraffin are virtually immiscible with each other. Span 80 was used as a surfactant. (Georgarakis M. and Amperiadou A.1994)

Steps involved in microencapsulation [43]

The polymer was dissolved in acetone (10 ml) or acetone + ethanol (5+5ml). Resinate was suspended in it. This suspension was sonicated for 10 seconds to ensure the complete dispersion. Span 80 was dissolved in 100 ml low viscosity liquid paraffin. The preformed suspension was gradually added to liquid paraffin. The emulsion was stirred at 500 rpm by mechanical stirrer for 5 hours to evaporate the acetone. Formed polymeric beads were collected on suction pump, washed with 3X25 ml n-hexane

Effect of operational parameters on optimized formulation batches

As shown in table 6, 7, 8 and 9, the optimized formulation batches were selected to study the effect of stirring speed, surfactant concentration and temperature.

Table 6 Effect of operational parameters on beads

Parameters	Levels
Stirring speed	500 rpm	1000 rpm	1500 rpm
Surfactant concentration (%w/v))	1	1.5	2
Temperature ( ^oC)	Room Temp.	20	40

Table 7 Effect of stirring speed on size and shape of beads

Parameters	Levels
Stirring speed	500 rpm	1000 rpm	1500 rpm
Surfactant concentration (%w/v))	1	1.5	2
Temperature ( ^oC)	Room Temp.	20	40

Batch K7 was modified further to study the effect of stirring speed on size and shape of beads keeping other parameters constant.

Table 8 Effect of surfactant concentration on size and shape of beads

Batch code	Core: coat	Eudragit RS: RL	Solvent composition	Surfactant conc.
K7	1:1	70:30	Acetone	500
L7	1:1	70:30	Acetone	1000
E7	1:1	70:30	Acetone	1500

Similarly, batch K7 was modified further to study the effect of surfactant concentration on size and shape of beads keeping other parameters constant.

Table 9 Effect of temperature on encapsulation efficiency

Batch code	Temperature ( ^oC)	Core: coat ratio	Eudragit RS: RL ratio	Acetone : ethanol solvent blend ratio
K12	Room temp.	1:2	70:30	50:50
L12	20	1:2	70:30	50:50
H12	40	1:2	70:30	50:50

Batch K12 was further modified to study the effect of temperature on encapsulation efficiency.

All the parameters of batch were kept constant with only change in operating temperature.

Effect of stirring speed: At each level of stirring speed, other parameters were kept constant. Effect of stirring speed was observed for change in particle size and size distribution as well as further effect of size on drug release rate. Rate of evaporation was observed at respective stirring speeds.

Effect of surfactant: Surfactant concentration was varied to study the quality of polymeric beads produced.

Effect of temperature: Change in temperature affects the physicochemical properties like permeability and coating efficiency of polymers, hence effect of temperature was studied.

Evaluation of polymeric beads

The polymeric beads or microcapsules were evaluated for following characteristics

Yield of polymeric beads [56]

This value gives idea of overall efficiency of the process to form the polymeric beads. It compares total amount of starting materials with formulation prepared and is given by

Entrapment efficiency [56]

Accurately weighed 100 mg of beads were sonicated for 45 minutes in 1.2 pH buffer. The sonicated, filtered solution was analyzed spectrophotometrically. The amount of drug entrapped was calculated from calibration curve.

Physical properties

Size and size distribution

Size and size distribution was performed using optical microscope Motic Image Plus 2.0 MI. Diameter of about 40 to 50 beads from each optimized batch was calculated with correction factor.

Bulk density

Bulk density is defined as mass of powder divided by bulk volume. Weighed quantity of beads was filled in 10 ml measuring cylinder. Initial volume was noted. Cylinder was tapped 3 times on hard wooden surface, from height of 2.5 cm approximately. Final volume after tapping was noted.

Tapped density

A sample was poured gently into a 10 ml graduated cylinder. The cylinder was dropped at 2 seconds interval on to a hard wooden surface from 2.5 cm height. Tap density was calculated by measuring final volume after 50 taps on wooden surface and was expressed in g/cm³.

Angle of repose [57]

The frictional force in the powder can be measured by the angle of repose. Angle of repose is calculated by fixed funnel method. Angle of repose can be calculated using equation

Tan q = h/r

Where, h = height of heap in cm

r = radius of heap in cm.

Surface topography studies by scanning slectron microscopy

Selected samples were gold sputtered and then scanned for surface characterization by scanning electron microscopy (JEOL 6328 Tokyo, Japan).

Effect of core: coat ratio, polymer composition and solvent composition on in vitro drug release

Effects of core: coat ratio, polymer composition and solvent composition on drug release profile were studied on the formulation batches K5 to K12. Each parameter was varied keeping other parameters constant. The batches were designed according table showing batches K5 to K17.

RESULTS AND DISCUSSION:

Evaluation of drug-resin complex

Moisture content

The moisture content of Amberlite IRP 69 resin was found to be 4.5% to 6%.

Particle size distribution

As shown in figure 6.1, the resin particles were found in the range of 40 to50 µm.

Effect of pH on drug loading

The effect of pH on drug resin binding is represented in table 11 and figure 6.4.

pH	Drug loading (%) (Mean±S.D., n=3)
2	49.58 ± 0.45
2.5	52.66 ± 0.58
3	55.85 ±0.48
3.5	63.46 ± 0.95
4	64.78 ± 1.15
4.5	64.81 ± 1.2

Significant effect of pH was not observed on drug loading, as evident from the graph. Still, drug binding was comparatively low at lower pH. This may be due to higher competition between drug and cations for binding with resin. The maximum drug loading was observed in pH 4.0 condition.

Figure 6.4 Effect of pH on drug loading

Selection of drug resin ratio

Table 12 Selection of drug: resin ratio

Drug resin ratio	% Drug content
1:1	63.28
1:2	49.86
1:3	38.89

Figure 6.5 Effect of drug resin ratio on drug loading

Table 12 and figure 6.5 confirmed that optimum drug resin ratio was 1:1, where maximum drug was loaded. When the proportion of resin was increased and proportion of drug was kept constant, the drug loading was increased upto 63.28%. This was due to proportionately increased exchange capacity of resin (grams per unit dry weight).

Effect of temperature on drug loading

These studies were carried out using drug: resin ratio of 1:1 within pH range of 4 to 4.5. Effect of temperature was observed at 20, 30and at 40 ^oC.

Table 13 Effect of temperature on drug loading

Temperature (^oC)	Drug loading (%)
20	63.0
30	63.28
40	63.87

The drug: resin ratio of 1:1 in pH range of 4 has shown no significant difference for drug loading at higher temperature. The drug loading was thus observed to be independent of temperature as evident from table 13.

Evaluation of physical properties of drug resin complex

Table 14 Evaluation of physical properties of drug resin complex

Parameters	Amberlite IRP 60	Drug-resin complex
Shape	Irregular	Irregular
Angle of repose	28.6 ^o	29.2 ^o
Bulk density (g/cm³)	0.788	0.802
Tap density (g/cm³)	0.894	0.838
Carr’s index (%)	12.54	14.12

Shape

The shape of Amberlite IRP 69 resin and resinate was found to be irregular. The shape of resinate was found to affect the flow and packing properties.

Flow properties

The angle of repose of resinate was 29.2 ^o. Angle of repose larger than 45° exists as cohesive powders. The results showed that resin and resinate exhibited good flow properties.

Density

It has been stated that bulk density less than 1.2 gm/cm³ exhibit good packing ability. Both resin and resinate has shown good packing ability.

Packing ability

Carr’s compressibility index indicates the packing ability of powders. When compressibility ranges from 5 to 16 the materials have acceptable flow property and packing ability.

The results have shown that resin as well as resinate had good flow properties and packing ability.

X- ray diffraction studies

Figure 6.6 X-ray diffraction pattern of salbutamol sulphate

Figure 6.7 X-ray diffraction pattern of Amberlite IRP 6

Figure 6.8 X-ray diffraction pattern of salbutamol sulphate and Amberlite IRP 69 complex

X-ray diffraction pattern of drug alone and drug-resin complex was found to be different as evident from figures above. Sharp peaks in X-ray diffraction of salbutamol sulphate show the presence of crystalline drug, while X-ray diffraction of drug-resin complex clearly indicates conversion of crystalline drug to amorphous form. Diffused pattern also confirms drug resin binding has taken place at molecular level as well as stoichiometrically. X-ray diffraction of Amberlite 69 appears more diffused than that of drug-resin complex since complex contains some of the pure drug in unbound form.

All the figures 6.6, 6.7 and 6.8 confirmed that drug was bound to resin at molecular level.

In vitro drug release profile from resinate

Table 15 In vitro drug release from resinate

Time (Hours)	Cumulative % drug released in pH 1.2 buffer
1	39.46 ± 1.25
2	57.86 ± 2.34
3	71.4 ± 1.76
4	86.54 ± 1.59
5	94.12 ± 2.87

Figure 6.9 In vitro release profile of drug from resinate

In vitro drug release from resinate showed initial burst release (about 40% in first hour) as shown in figure 6.9.This might be due to presence of about 25% of resin fraction in size range below 30 µm. This showed that particle size of resin might have played important role in drug release from resinate.

Effect of pH on drug release from resinate

Table 16 Effect of pH on drug release from resinate

Time (Hours)	Cumulative % drug released
Time (Hours)	pH 1.2 buffer (Mean±S.D., n=3)	pH 6.8 phosphate buffer (Mean±S.D., n=3)
1	36.24±0.86	31.24±1.23
2	48.69±1.36	39.19±1.12
3	57.86±1.58	46.89±1.09
4	71.42±1.02	54.56±0.59
5	86.54±1.21	68.33±1.0
6	96.12±1.33	77.27±0.56
7	-	84.96±1.69
8	-	91.58±1.41
9	-	96.68±1.06

Figure 6.10 Effect of pH on drug release from resinate

As shown in table 16 and figure 6.10, the rate of exchange in case of pH 1.2 buffer was more as compared to pH 6.8 phosphate buffer. It was due to higher ionic size of K⁺ ions as compared to H⁺ ions.

Effect of ionic strength on drug release

Table 17 Effect of ionic strength on drug release

Time (Hours)	Cumulative % drug released
Time (Hours)	μ₁= 0.1121	μ₂= 0.2105
1	36.24±0.86	42.32±0.68
2	48.69±1.36	51.5±1.21
3	57.86±1.58	59.86±1.33
4	71.42±1.02	76.58±0.98
5	86.54±1.21	86.95±1.44
6	94.12±1.33	95.51±2.54
7	98.58±1.33	98.88±1.62

Figure 6.11 Effect of ionic strength on drug release

Observations from figure 6.11 and table17 revealed that with increase of ionic strength, the rate of release of drug was increased. As ionic concentration was increased, influx of ions for salbutamol exchange was increased upto certain limit. This led to speeding up of exchange process and hence increased exchange of ion and drug.

Studies on these formulation batches showed effect of core: coat ratio, eudragit composition and solvent composition on yield (%), entrapment efficiency (%), size (µm) and shape. In general, it was observed that increasing core: coat ratio increased entrapment efficiency. As core quantity remained same as compared to coating material, the coating polymer solution became viscous and entrapment efficiency was enhanced. Hence batches K9 to K12 (core: coat ratio1:2) has shown higher encapsulation and yield than batches K5 to K8 (core: coat ratio1:1).

Change in polymer composition did not affect physical parameters, as shown in table18. Polymer composition was found to affect only release rate of the drug.

Variations in yield and entrapment efficiency were thought to be due to variations in solvent compostition. Acetone was found to evaporate at faster rate than ethanol. As organic phase evaporated, polymer gradually formed thin coat around the core. Therefore, it was concluded that thickness of a coat around core was a function of viscosity of coating solution and solvent composition indicating that acetone + ethanol together resulted in viscous solution (as compared to acetone alone) and hence higher yield. The theory “more the viscous solution, polymer in solution gets more time to form a coat around a core resulting in more yield and encapsulation.

Effect of various parameters on optimized batches

The optimized formulation batches K5 to K12 were further selected to study the effect of various parameters. Parameters and their levels are shown in table 19.

Table 19 Effect of various parameters on optimized batches

Parameters	Levels
Stirring speed	500 rpm	1000 rpm	1500 rpm
Surfactant concentration (%))	1	1.5	2
Temperature (^oC)	Room Temp.	20	40

Effect of stirring speed on size and shape of polymeric beads

Table 20 Effect of stirring speed on size and shape of polymeric beads

Batch code	Stirring speed (rpm)	Size (µm) (Mean ± S.D., n=3)	Shape
K7	500	375±30	Spherical
M7	1000	220±30	Irregular
H7	1500	<160	Irregular

\Batch K7 was modified to study effects of stirring speed on size and shape of polymeric beads. It was evident from the table 20 and figure 6.12 that as stirring speed was increased, size range of beads was decreased. It was observed due to rapid evaporation of organic phase. As acetone evaporated rapidly, it became increasingly difficult for polymer in solution to form a uniform coat around core. Hence, it was concluded that very fast evaporation of organic coat causes irregular coat formation with distortion in shape and size of polymeric beads.

Figure 6.12 Effect of stirring speed on bead size and shape of polymeric beads

Effect of surfactant on size and shape of polymeric beads

Table 21 Effect of surfactant on size and shape of polymeric beads

Batch code	Span 80 (%)	Size (µm) (Mean ± S.D., n=3)	Shape
K7	1	375±30	Spherical
L7	1.5	320±30	Semi spherical
E7	2	250±40	Irregular

As shown in table 21, effect of surfactant concentration was studied on modified K7 batches. Basic function of surfactant is to lower the surface tension between two immiscible phases. When other parameters were kept constant, span 80 formed fine emulsion with increasing concentration. As surfactant concentration was raised from 1 to 2% in formulation batches K7 to E7, size of beads was reduced with change in sphericity. Higher surfactant concentration helped in constant reduction in surface tension between two immiscible phases which resulted in finer emulsion. Very fine droplets resulted in beads with much finer size with loss of sphericity, as seen in the case of batch E7 where span 80 at 2%w/v concentration level was employed. These results are shown in table 21 and figure 6.13.

Figure 6.13 Effect of surfactant concentration on size range

Effect of temperature on encapsulation efficiency

Figure 6.14 Effect of temperature on encapsulation efficiency

As shown in table 22 and figure 6.14, as forced evaporation of organic phase was done by increasing temperature, encapsulation efficiency was found to reduce. Much of the core material was not encapsulated due to very quick film formation around core. Since glass transition temperature (Tg) of eudragit RS and RL is 40±2^oC, polymer started to liquefy reducing encapsulation efficiency. This was evident with batch H12 where operating temperature was 40^oC. Room temperature was around 30 ^oC, at which maximum encapsulation was observed. At lower temperature in case of batch L12, slow evaporation of solvent caused repeated coat or layer formation around same core material with insignificant change in encapsulation efficiency.

Yield of polymeric beads

As shown in table 23, increase in core: coat ratio was found to increase yield with increase in entrapment efficiency upto the ratio of 1:2 as found with batch K12. Higher polymer concentration successfully entrapped the core with better film consistency. Increase in stirring speed led to lower yield with more polymer and core material sticking to walls of container. At very high surfactant concentration (>2%) and temperature (>40^oC) polymer formed clumps as a result of change in its chemical properties.

Table 23 Yield of polymeric beads

Batch code	Core: coat ratio	Solvent composition	Yield (%) (Mean±S.D., n=3)
K10	1:2	Acetone	90.54±1.89
K8	1:1	50:50	86.48±1.21
K12	1:2	50:50	91.47±1.54
K13	1:3	50:50	42.58±2.04

Table 22 Effect of temperature on encapsulation efficiency of beads

Batch code	Temperature ( ^oC)	Core: coat ratio	Eudragit RS: RL ratio	Acetone : ethanol solvent blend	Encapsulation efficiency (%) (Mean ± S.D., n=3)
K12	Room temp.	1:2	70:30	50:50	80.12±1.12
L12	20	1:2	70:30	50:50	75.48±1.28
H12	40	1:2	70:30	50:50	42.68±1.44

Results were consistent with the studies which stated that-yield increases with increasing core: coat ratio till certain level, as confirmed from batches K9 to K12. Batches with core: coat ratio of 1:3 resulted in low yield.

Entrapment efficiency of process

The core: coat ratio of 1:2 entrapped more quantity of core material as observed in table 24. Any change in variable which reduced bead size lead to slightly lower entrapment efficiency, as seen with increase in stirring speed (> 500 rpm) as in the case of batch H7 and surfactant concentration (> 2%) as in case of batch E7.

Table 24 Entrapment efficiency of process

Batch code	Core: coat ratio	Solvent composition	Entrapment efficiency (%) (Mean ± S.D., n=3)
K7	1:1	Acetone	73.65±1.28
K8	1:1	50:50	76.86±1.15
K12	1:2	50:50	82.56±1.94
K11	1:2	Acetone	80.20±1.68

Results showed that encapsulation efficiency is not a function of any single factor, but depended on multiple factor. Change in solvent composition as well as core: coat ratio increased entrapment efficiency upto certain level. Batch K12 with core: coat ratio of 1:2 and solvent combination of acetone and ethanol resulted in highest encapsulation efficiency. Still other factors like operating temperature, surfactant level have changed the encapsulation efficiency but with minor values.

Bulk density of polymeric beads

Bulk density is indicative of packing capacity of microcapsules. It changes with change in mass as well as volume.

Table 25 Bulk density of polymeric beads

Batch code	Core: coat ratio	Bulk density (g/cm³) (Mean ± S.D., n=3)
K7	1:1	0.550±0.110
K12	1:2	0.682±0.124
K13	1:3	0.758±0.164

As shown in table 25, as coating level was increased, bead with larger size and thicker coat were formed, with increased bulk density.

Angle of repose

Polymeric beads with maximum sphericity and smooth surface showed better flowability. As shown in table 26, the shape of beads was found to be dependant on stirring speed and hence affected the flow characteristics evidenced in change in angle of repose. Higher stirring speed was found to reduce sphericity of beads, which resulted in poor flow. Optimum speed was found to be 500 rpm.

Table 26 Angle of repose

Batch code	Stirring speed (rpm)	Angle of repose
K7	500	26.17^o
M7	1000	20.14^o
H7	1500	18.42^o

Surface topography studies by scanning electron microscopy (SEM)

Surface studies of beads depicted the effect of solvent composition and other operating parameters. Although all polymeric beads showed rough surfaces, number of pores and their depth varied significantly according to operating conditions. Acetone produced microcapsules with large number of rough surface and deep pores as compared to those with acetone + ethanol (50:50). Surface of beads formulated with combination of solvents showed few wrinkles and occasional pores. Effect of acetone alone could be explained due to fact that it evaporated earlier, resulting in quicker drying and rigidization of film before uniform coat is produced .This may have left more wrinkled surface with deep fissures.

This effect of solvent on surface of beads was further found to influence drug release pattern. Similar surface characteristics were observed when rate of evaporation was enhanced by increasing temperature and stirring speed.

Figure 6.15 Scanning electron micrograph of formulation batch K12

As shown in figure 6.15, scanning electron micrograph of batch K12 showed spherical surface with minor wrinkles on the surface. The presence of wrinkles on the bead surface was supposed to be due to the higher rate of stirring. The sphericity was good indicating optimum operating conditions.

Figure 6.16 Scanning electron micrograph of batch K12.

Figure 6.17 Scanning electron micrograph of batch K12

Figure 6.18 Scanning electron micrograph of batch K6.

As shown in figure 6.18, beads with acetone + ethanol showed smooth surface as compared to acetone alone. Surface characteristic studies thus helped to predict the drug release profile of various batches. It was clear from above figures and dissolution profiles that increase in number of pores enhanced dug release. With large and deep pores, it became easy for ions to cross the polymeric barrier and exchange with ionized drug molecule.

When beads were prepared with forced evaporation of solvent at higher temperature, highly rough surface with deep pores was obtained. The fast evaporation of acetone before film deposition around core has been resulted in type of surface as evident from figures 6.15 and 6.16.

Higher stirring speeds induced greater tangential forces. This higher thrust continually tended to disturb the film being deposited on core material, reducing entrapment efficiency and rough surface.

The beads obtained from batch K6 were with very smooth surface. Batch K6 was prepared with acetone and ethanol in ratio of 50:50, room temperature and stirring speed of 500rpm. These operating conditions were best suitable for obtaining beads with good surface quality.

Figure 6.19 Scanning electron micrograph of batch H12 at 40 ^oC

As evident from fig. 6.19, higher temperature resulted in distorted surface of beads with loss of surface smoothness and sphericity. At higher temperature, film deposited quickly on the core surface and solvent evaporated before film hardening, resulting in rough surface with pores.

FT-IR studies

Fourier transform infrared spectroscopy (FT-IR) was performed on beads and pure drug. Figure 6.20 and 6.21 shows the FT-IR of salbutamol sulphate and formulated beads respectively. No difference in the positions of absorption bands was observed in the spectra of salbutamol sulphate and polymeric beads, indicating no chemical interaction between drug and polymer in solid state.

Figure 6.20 FT-IR spectrum of salbutamol sulphate

Figure 6.21 FT-IR spectrum of beads

Spectra of salbutamol sulphate showed sharp bands at wavelength of about 1000 cm^-1,while spectra of beads alsoshow bands at same wavelength, but somewhat less intense due to drug resin complex. This indicated less probability of chemical interaction of drug with other excipients.

In vitro drug release studies

Figure 6.22 and table 27 show the in vitro drug release profile of optimized batches.

Figure 6.22 Drug release profile of optimized formulation batches

Table 27 Drug release profile of optimized batches

Time (Hour)	Cumulative % drug released (Mean ± S.D., n=3)
Time (Hour)	Batch K7	Batch K8	Batch K12
1	18.45±1.69	14.2±1.777	12.5±2.15
2	25.69±1.25	20.68±2.41	18.65±1.84
3	34.98±1.24	27.59±2.98	22.4±1.44
4	46.54±1.45	35.54±1.57	26.91±1.85
5	53.21±2.87	47.12±2.47	31.85±2.48
6	64.68±2.04	55.45±1.68	42.23±3.47
7	72.57±1.47	67.85±1.48	51.32±1.69
8	81.98±1.11	75.47±2.59	63.29±1.21
9	90.74±0.96	82.69±2.44	76.98±1.27

Batches K7, K8 and K12 were optimized bathes in comparison to other batches. Eudragit RS: RL ratio of 70:30 was found to be optimum ratio for microencapsulation.. Eudragit RS being hydrophobic due to lower percentage of quaternary ammonium groups resulted in retarded drug release. As shown in figure 6.22 and table 27, in batches K8 and K12, acetone and ethanol combination in 50:50 ratio was used which helped in retardation of drug release. Batch K12 was formulated using core: coat ratio of 1:2, giving better control over drug release, while in batches K7 and K8 core: coat ratio was 1:1.

Effect of core: coat ratio on drug release

Three levels of core: coat ratios were selected, 1:1, 1:2 and 1:3.Out of these ratios, 1:1 and 1:2 were found to give satisfactory results.

Figure 6.23 Effect of core: coat ratio on in vitro drug release

Table 28 Effect of core: coat ratio on drug release

Time (Hours)	Cumulative % drug released (Mean ± S.D., n=3)
	K8	K12	K13
	1:1	1:2	1:3
1	14.22±1.25	12.5±1.87	12.02±2.54
2	20.68±1.58	18.65±2.24	16.25±1.24
3	27.59±2.54	22.4±2.18	21.47±1.20
4	35.54±2.58	26.91±1.84	24.85±1.24
5	47.12±1.98	31.85±1.47	30.69±1.36
6	55.45±2.12	42.23±1.09	36.89±1.67
7	67.85±1.04	51.3±22.58	41.14±1.29
8	75.47±1.51	63.29±3.57	49.87±1.54
9	82.69±1.56	72.98±1.89	58.4±2.10

Figure 6.23 and table 28 show that core: coat ratio had significant impact on drug release profile. As ratio was increased, thicker coat formed around the core retarded the drug release. Thick polymer coat acted as barrier for drug release. As the coat thickness was increased, path length for drug diffusion through film was increased, retarding drug release. Hence batch K13 with core: coat ratio of 1:3 showed retarded drug release as compared to other batches.

Effect of solvent composition on drug release

Figure 6.24 Effect of solvent composition on drug release

Table 29 Effect of solvent composition on drug release

Time (Hours)	Cumulative % drug released (Mean ± S.D., n=3)
	Batch K5	Batch K6
	Acetone	Acetone : Ethanol solvent blend (50:50)
1	18.45±1.26	12.5±1.79
2	24.14±2.62	18.65±1.24
3	30.47±2.32	22.4±1.59
4	36.87±1.02	26.91±1.21
5	46.25±1.22	31.85±1.68
6	57.69±1.58	42.23±2.39
7	69.96±2.28	51.32±2.52
8	81.98±2.10	63.29±1.68
9	94.54±1.36	70.15±1.30

The effect of solvent composition was evident as acetone was evaporated quickly as compared with acetone and ethanol solvent blend. This was seen in scanning electron micrograph of beads, as those prepared with acetone and ethanol showed smooth surface and less pores. Thus batch K6 formulated with acetone and ethanol resulted in retarded release as compared with batch K5.The results are shown in figure 6.24 and figure 29.

Acetone was found to leave polymer solution rapidly, forming rough film with large number of pores. These pores were deep enough to enhance the diffusion of drug out of beads. Acetone: ethanol solvent blend in 50:50 ratio has shown beads with desied drug release profile.

Effect of polymer composition on drug release

Figure 6.25 Effect of polymer composition on drug release

Table 30 Effect of polymer composition on drug release

Time (Hours)	Cumulative % drug released (Mean ± S.D., n=3)
	Batch K10	Batch K12
	Eudragit RS:RL ratio (50:50)	Eudragit RS:RL ratio (70:30)
1	18.54±1.23	12.5±1.87
2	23.14±1.58	18.65±2.24
3	30.59±2.54	22.4±2.18
4	37.87±1.69	26.91±1.84
5	47.82±1.91	31.85±1.47
6	55.28±2.1	42.23±1.09
7	67.22±1.36	51.3±22.58
8	78.46±1.89	63.29±3.57
9	86.69±2.58	72.98±1.89

Eudragit is copolymer of polymethyl methacrylate and methacrylic acid ester, whose permeability is independent of pH.

Eudragit RL 100 contains about 8% of quaternary ammonium group, which makes it more permeable. Eudragit RS 100 contains about 4% of quaternary ammonium group, making it less permeable as compared to RL 100. Hence, observations in table 30 and figure 6.25 show that batch K10 containing 50:50 ratio of RS: RL was more permeable than batch K12 which contained RS: RL in 70:30 ratio. The RL 100 was therefore found more suitable for controlling drug release rate.

Mechanism of drug release from beads

Table 31 Drug release mechanism from beads

Batch code→ Parameter↓	Batch K7	Batch K8	Batch K12
r²value	0.991	0.985	0.996
n value	0.584	0.549	0.689
Best fit model	Higuchi	Higuchi	Zero order

The r²value gives information about which kind of drug release mechanism is being mimicked by the formulation. Higuchi equation says that drug release is directly proportional to square root of the time i.e. drug release is chiefly by diffusion method. Batch K12 with core: coat ratio 1:2 and eudragit RS: RL ratio 70:30 was found to be most suitable for retarding the drug release. Zero order drug release from formulation batch K12 was found to follow the drug release kinetics independent of drug remained in the formulation.

Figure 6.26 Basis of ion exchange process

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Received on 02.08.2011 Accepted on 01.10.2011

Asian J. Pharm. Tech. 1(4): Oct. - Dec. 2011; Page 104-118

Parameters→ Batch code↓	Formulation parameters			Characteristics of polymeric beads (Mean ±S.D.,n=3)
Parameters→ Batch code↓	Core: coat ratio	Eudragit RS: RL	Acetone: ethanol ratio	Yield (%)	Entrapment efficiency (%)	Size (µm)	Shape
K5	1:1	50:50	Acetone	82.12 ±1.58	72.14±1.89	360±25	Spherical
K6	1:1	50:50	50:50	85.06±1.86	76.24±1.92	400±28	Spherical
K7	1:1	70:30	Acetone	83.33±1.86	73.65±1.28	375±30	Spherical
K8	1:1	70:30	50:50	86.48±1.21	76.86±1.15	430±38	Spherical
K9	1:2	50:50	Acetone	89.22±1.25	80.41±1.65	550±45	Spherical
K10	1:2	50:50	50:50	90.54±1.89	81.55±2.18	585±38	Spherical
K11	1:2	70:30	Acetone	89.12±	80.20±1.68	550±45	Spherical
K12	1:2	70:30	50:50	91.47±1.54	82.56±1.94	620±55	Spherical