Formulation and pharmacodynamic evaluation of

Journal of Microencapsulation, June 2006; 23(4): 389–404

Formulation and pharmacodynamic evaluation of captopril sustained release microparticles

AMAL H. EL-KAMEL, DOAEA H. AL-SHORA, & YOUSRY M. EL-SAYED Department of Pharmaceutics, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia (Received 14 June 2005; accepted 22 August 2005)

Abstract Cellulose propionate (CP) microparticles containing captopril (CAP) were prepared by solvent evaporation technique. The effects of polymer molecular weight, polymer composition and drug : polymer ratios on the particle size, flow properties, morphology, surface properties and release characteristics of the prepared captopril microparticles were examined. The anti-hypertensive effect of the selected CAP formulation in comparison with aqueous drug solution was also evaluated in vivo using hypertensive rats. The formulation containing drug : polymer blend ratio 1 : 1.5 (1 : 1 low : high molecular weight CP), namely F7, was chosen as the selected formulation with regard to the encapsulation efficiency (75.1%), flow properties ( ¼ 24 , Carr index ¼ 5%, Hausner ratio ¼ 1.1, packing rate ¼ 0.535) and release characteristics. Initial burst effect was observed in the release profile of all examined formulations. DSC and SEM results indicated that the initial burst effect could be attributed to dissolution of CAP crystals present on the surface or embedded in the superficial layer of the matrix. The release kinetics of CAP from most microparticle formulations followed diffusion mechanism. After oral administration of the selected microparticle formulation (F7) to hypertensive rats, systolic blood pressure decreased gradually over 24 h compared to reference drug solution. These results may suggest the potential application of cellulose propionate microparticles as a suitable sustained release drug delivery system for captopril Keywords: Captopril, cellulose propionate, microencapsulation, release mechanism, anti-hypertensive effect

Introduction Captopril (CAP) is an orally active angiotensen converting enzyme inhibitor. It has proven to have excellent clinical effectiveness in the treatment of essential hypertension. However, after single oral dose, the anti-hypertensive action is only effective for 6–8 h. Hence, clinical use requires a daily dose of 37–75 mg to be taken three times in divided doses (Nur and Zhang 2000a), development of a controlled delivery system for captopril would be advantageous especially in long-term therapy to maintain relatively constant blood levels for Correspondence: Amal Hassan El-Kamel, Associate Professor of Pharmaceutics, Department of Pharmaceutics, College of Pharmacy, King Saud University, PO 22452, Riyadh 11495, Saudi Arabia. E-mail: [email protected] ISSN 0265–2048 print/ISSN 1464–5246 online ß 2006 Informa UK Ltd. DOI: 10.1080/02652040500444230

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a long period of time. However, the development of oral controlled release formulation for CAP is somewhat difficult (Nur and Zhang 2000a). This could be due to the fact that the drug suffering in vitro and in vivo instability. Besides that the drug is absorbed passively and actively from the GIT. In addition, the drug being water soluble could suffer from dose dumping and burst phenomenon. On the other hand, its bioavailability decreases in the presence of food. Several attempts have been made to formulate sustained release captopril formulations, for example coated tablets (Guittard et al. 1993), floating tablets and, bioadhesive systems (Nur and Zhang 2000b), sub-lingual tablets (Chetty et al. 2001), beadlets (Joshi et al. 1988), pulsatile delivery system (Wilding et al. 1992), biodegradable (Mandal 1998) and non-biodegradable microcapsules (Singh and Robinson 1988). Concerning in vitro evaluation of these formulations, there were many limitations that affect there suitability as sustained release formulations (Nur and Zhang 2000a). On the other hand, evaluation of most proposed formulations are based on in vitro data only. Indeed, the presence of an in vivo set of experiments along with an in vitro test will definitely facilitate the decision regarding the efficiency of the developed CAP sustained release formulation. Despite of all these performed research work, there are likely to be no well established CAP sustained release dosage forms reported to be in the drug market. The objective of this study was to formulate sustained release captopril-cellulose propionate microparticles. The effects of polymer molecular weights and polymer ratios on the particle size, flow properties, morphology, surface properties and the release characteristics of the prepared captopril microparticles were examined. A comparison of the release properties between the prepared microparticles and marketed conventional tablets was studied. The anti-hypertensive effect of the selected CAP formulation in comparison with aqueous drug solution was also evaluated in vivo using hypertensive rats.

Materials and methods Materials Captopril powder (CAP) and captopril commercial tablets (CapotenÕ , 50 mg) (Squibb, USA). Cellulose propionate (CP) Mw 15 000 and 75 000 (Aldrich chemical Co, Milwaukee, WI, USA). Span 85 (Eingetragene Markeder Atlas Chemical Inc, USA). Liquid paraffin, acetone, L-ascorbic acid and tri-sodium orthophosphate (Win Lab, UK). Methanol, hexane and hydrochloric acid (BDH Poole, UK). Ethanol and phosphoric acid (Rieddehae¨n, Germany). Theophylline anhydrous (Boeringer Ingelheim, Germany). Preparation of microparticles The microparticles were prepared using an emulsion solvent evaporation technique (Khidr et al. 1995). The polymeric solution was prepared by dissolving cellulose propionate in acetone. The drug was dissolved in the polymeric solution forming the internal phase. The prepared drug–polymer solution was added drop-wise by a syringe with a needle gauge 21 to liquid paraffin (external phase) containing 1.5% span 85 and was emulsified by stirring at 800 rpm. The stirring was continued at room temperature until the polymer solvent was evaporated. The produced microparticles were decanted and washed 3 times with 20 ml hexane and dried overnight in a hood. F1, F2, F3 formulations contained 1 : 1, 1 : 1.5 and 1 : 2 CAP : CP low molecular weight, respectively. F4, F5, F6 formulations contained 1 : 1, 1 : 1.5 and 1 : 2 CAP : CP high molecular weight, respectively. On the other hand, F7, F8, F9

Formulation and pharmacodynamic evaluation

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formulations contained 1 : 1.5 CAP : CP blend (1 : 1, 1 : 1.5 and 1 : 2 low : high molecular weight CP, respectively). HPLC assay of captopril Stock solution of CAP was prepared in methanol and a different series of standard diluted solutions were prepared in deionized water to give 3, 4, 6, 8, 10 mg ml1. Theophylline was added as an internal standard to give a final concentration of 20 mg ml1. Aliquots of 20 ml of standard solution was injected onto the HPLC C18 column (Pu–1580 pump, UV–1575 detector, Jasco, Japan). The column was eluted with 0.1% phosphoric acid and methanol (70 : 30 v/v) at flow rate of 1 ml min1. The drug concentration was detected at ¼ 220 nm (Khan et al. 2000). Determination of drug loading, encapsulation efficiency and microparticles yield The average drug content was measured by extracting a sample of 20 mg of microparticles using absolute ethanol. After filtration and appropriate dilution with ethanol, the concentration was determined using the HPLC method (Khan et al. 2000). Percentage drug loading was calculated using the following equation: Loading ð%Þ ¼

Weight of drug 100 Weight of microparticle

The encapsulation efficiency was determined by the following equation: Encapsulation efficiency ð%Þ ¼

Calculated drug content 100 Theoretical drug content

The yield % of the produced microparticles was calculated for each batch by dividing the weight of microparticles (M) by the total expected weight of drug and polymer (M0): Yield ð%Þ ¼ ðM=M0 Þ 100 Each determination was performed in triplicate. Determination of the physicochemical properties of captopril microparticles Differential scanning calorimetry. Thermograms of drug, polymers and microparticles were obtained using Dupont 2100 series thermal analysis system. Heating rate was 5 C min1. Scanning electron microscopy. The microparticles were coated uniformly with gold after fixing the sample in individual stubs. All samples were examined for surface morphology using scanning electron microscope (Jeol, SEM model JSM-25SII, Tokyo, Japan). Particle size analysis Particle size was determined by sieve method. The mean particle size was calculated after sieving as follows (Parrott 1987) X .X n nd dave ¼ where dave is the arithmetic mean diameter of microparticles, n is percentage weight fraction retained on smaller sieve and d is the arithmetic mean size of sieve opening.

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Flow properties Angle of repose of different formulations was measured according to the fixed funnel standing cone method (Banker and Anderson 1987) and was given by: ¼ tan1 h r1 where is the repose angle, r is the radius and h is the height. Bulk density was measured by tapping method (Martin 1993). Kawakita equation (Kawakita 1964) was used to calculate the packing rate (b) according to the following equation: n=c ¼ ð1=abÞ þ ðn=aÞ c ¼ ðv0 vn Þ=v0 where a and b are constants representing the proportion of consolidation at the closest packing attained and packing rate, respectively, n is the number of taps, v0 and vn are the powder bed volumes at initial and tapped state, respectively. Compressibility index (Ci) or carr index (Carr 1965) values of microparticles was computed according to the following equation: ðtapped density fluff densityÞ 100 Carr% ¼ tapped density Hausner ratios of microparticles were determined by comparing the tapped density to the fluff density using the equation (Hausner 1967): Hausner ratio ¼

tapped density fluff density

In vitro release studies Hard gelatin capsules were filled with microparticles (equivalent to 75 mg captopril) and ascorbic acid (1 mg drug : 5 mg ascorbic acid). Dissolution studies were performed at 37 C 0.5 using dissolution apparatus I. The baskets were rotated at 50 rpm. The dissolution medium was 750 ml 0.1 N HCl (pH 1.2) for 2 h then rendered alkaline (pH 6.8) using 0.2 M trisodium phosphate. The concentration was then determined by HPLC method. Samples of 2 ml were withdrawn at the following time intervals: 0, 5, 15, 30, 45, 60, 120, 180, 240, 300, 360 and 420 min. Captopril concentration was determined in each sample using HPLC assay. All experiments were run in triplicate. Kinetic treatment of release data The obtained dissolution data were fitted to zero order (Najib and Suleiman 1985), first order (Desai et al. 1966), Higuchi (Higuchi 1963), Korsmeyer-Peppas (Korsmeyer et al. 1983), Baker-Lonsdale (Baker and Lonsdale 1974), Hixson-Crowell (Hixson and Crowell 1931) and Weibull models (Costa and Lobo 2001) to determine the mechanism of CAP release from the prepared microparticles. Anti-hypertensive effect studies Three different groups (each of six animals) of male hypertensive rats of 12 weeks (mean weight 200 20 g; mean systolic blood pressure 225 25 mmHg) were used. An aliquot of


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120 100

% Yeild

80 60 40 20 0 F1

F2

F3

F4

F5

F6

F7

F8

F9

Formulation

Figure 1.

Percentage yield of different captopril microparticles formulations.

0.5 ml of aqueous solution of CAP equivelant to 6.75 mg kg1 (group I) or carboxymethylcellulose suspension of an exactly weighed amount of CAP-CP microparticles (F7) containing the same dose per kg of drug (group II) were given orally to the rat using an intragastric tube. Drug unloaded microparticles was used as control (group III). Systolic blood pressure (SBP) was recorded by a tail cuff method using a blood pressure recorder (IITC-179, Victory BLVD, Woodland, Hills, CA, USA) (Gerold and Tschirky 1968). SBP was measured immediately before the experiment and 1, 2, 3, 4, 5 and 24 h after the administration. Statistics Analysis of variance (ANOVA) was used to test the differences between the calculated parameters using SPSS Statistical Package (Version 10, SPSS Inc, 1999, USA). Statistical differences yielding p 0.05 were considered to be significant. Duncan multiple comparison was applied when necessary.

Results and discussion Percentage yield, percentage loading and encapsulation efficiency of captopril microparticles The percentage yield of microparticles was shown in Figure 1. The percentage yield for microparticles prepared using high molecular weight polymer was higher than that obtained for low molecular weight polymer. Increasing the molecular weight of the polymer led to subsequent increase in its hydrophobicity. Consequently, it will react better with non-solvent phase (liquid paraffin) leading to more efficient precipitation of the polymer at the droplet interface with subsequent higher yield. Regardless of molecular weight, increasing polymer ratio in the formulation led to increase the product yield. The low percentage yield in some formulations may be also due to microparticles lost during the washing process. Three concentrations of drug in microparticles were evaluated: 33, 40 and 50% w/w. Statistical analysis showed significant increase (p 0.05) in the actual drug loading as the theoretical drug loading increased as shown in Table I. This could be due to fixed solubility

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Percentage loading and encapsulation efficiency of captopril microparticles formulations.

Formulation (drug : polymer) F1 F2 F3 F4 F5 F6 F7 F8 F9

Theoretical drug loading (%)

Actual drug loading (%) SD

Encapsulation efficiency (%) SD

50 40 33 50 40 33 40 40 40

35.4 3.3 27.2 2.2 22.6 1.5 35.9 2.1 33.3 0.9 25.2 0.7 31.3 2.4 33.3 0.7 26.5 2.1

63.9 6.4 66.9 5.3 66.6 4.5 67.3 4.2 81.8 2.4 76.1 2.2 75.1 3.2 84.3 1.7 64.3 5.6

(1:1) (1:1.5) (1:2) (1:1) (1:1.5) (1:2) (1:1.5) (1:1.5) (1:1.5)

Table II.

Micromeritic properties of different captopril microparticles formulations. Bulk density (gm ml1)

Formulation

Angle of repose ( )

Before tapping

After tapping

Carr index (Ci) (%)

Hausner ratio

Packing rate (b)*

F1 F2 F3 F4 F5 F6 F7 F8 F9

19.8 0.5 26.0 0.0 31.6 1.3 24.7 1.0 20.8 1.1 34.1 1.6 24.0 0.8 23.6 0.8 23.5 1.7

0.341 0.351 0.257 0.120 0.091 0.064 0.095 0.159 0.091

0.378 0.473 0.401 0.139 0.117 0.093 0.100 0.190 0.113

10 0.0 26 2.0 36 0.0 14 0.0 22 1.1 32 0.0 5 1.1 16 0.0 20 0.0

1.1 0.0 1.3 0.0 1.5 0.0 1.1 0.0 1.3 0.0 1.4 0.0 1.1 0.0 1.2 0.0 1.3 0.0

0.0879 0.1125 0.2990 0.0949 0.1375 0.2368 0.5352 0.1735 0.1542

*Calculated from Kawakita equiation (Kawakita 1964).

of drug in liquid paraffin, so the amount of drug remained and available for encapsulation increased as the theoretical drug loading increased. Consequently, the actual drug loading increased. The percentage encapsulation efficiency of microparticles prepared from high molecular weight cellulose propionate was found to be higher than that determined for low molecular weight ones at all drug : polymer ratios used. This was in good correlation with the results obtained by Weiland-Berghausen et al. (2002) and Uddin et al. (2001) who also observed an increasing encapsulation efficiency with increasing molecular weight of ethylcellulose. As the molecular weight of the polymer increased its hydrophobocity increased, leading to better precipitation of polymer at the boundary phase of the droplets. Consequently, partitioning of drug to the non-solvent phase will be minimal. Flow properties of microparticles The examined microparticle formulations exhibited angle of repose value between 19.8 and 34.1 (less than 40 ), as shown in Table II, indicating good flowing nature (Banker and Anderson 1987). The Ci values for microparticles prepared ranged from 5–36%. F7 had the lowest Ci index indicating excellent compressibility. The flowability of all formulations was accepted according to Hausner ratio (1.1–1.4) except F3 that had Hausner ratio of 1.5 which indicated poor flowability.


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Table III. Mean particle size of various captopril microparticles formulations.

Formulation

Mean particle size SD (mm)

F1 F2 F3 F4 F5 F6 F7 F8 F9

904.3 8.3 1009.4 268 950.6 14.6 987.1 10.6 1059.4 25.0 900.1 3.2 733.2 27.8 751.3 17.2 922.9 11.2

In general it was found that the bulk density of the formulations decreased as the molecular weight of the polymer increased. On the other hand, at fixed polymer molecular weight, as the ratio of the polymer increased the density of the formulations decreased. These results were in agreement with that reported by Efentakis and Vlachou (2000), who found that the bulk density increased when the molecular weight of the polymer decreased, thus the highest molecular weight polymer exhibited the lowest densities. The packing rate (b) was also calculated from Kawakita equation (Kawakita 1964). The larger the value of parameter, b, the higher the packing rate of the formulation. At fixed molecular weight, as the polymer ratio increased the b parameter also increased, as shown in Table II. Taken together, F7 showed the best flow properties since it had angle of repose value of 24 , lowest Carr (5%) and Husner (1.1) values as well as the highest b (0.535) value. Particle size study Table III shows the mean particle size of microparticles of various drug-to-polymer ratios, keeping the total amount of polymer and drug constant. One of the most important factors that affect the particle size is the drug : polymer ratio (Babu et al. 2001). A change in the polymer content or dispersed phase changed its viscosity and influenced the interfacial tension between the dispersed phase and the dispersion medium with a pronounced effect on the size distribution of microparticles (Babu et al. 2001). An increase in total polymer concentration increased the relative viscosity of the dispersed phase and the sub-division of the dispersed phase into smaller ones was prevented by higher interfacial viscosity. Consequently, increasing the concentration of polymer was expected to increased the particle size. This was in agreement with the obtained results up to a drug : polymer ratio 1 : 1.5 above which (1 : 2) the mean particle size decreased as shown in Table III. Release of captopril from the prepared microparticle formulations Figure 2 shows the effect of different drug : polymer ratios and the molecular weight of the polymer on the release of CAP from the microparticles. While 91% CAP released from the conventional commercial brand in 30 min, the release of drug from the microparticles extended over a period of time depending on the drug : polymer ratio. The release was retarded by increasing the polymer ratio and polymer molecular weight. In contrast,

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A. H. El-Kamel et al. (a) 100

% Drug released

80 60 40 F1 F2 F3 commercial

20 0 0

100

200

300

400

Time (minute)

(b) 100

% Drug released

80 60 40 20

F4 F5 F6 commercial

0 0

100

200

300

400

Time (min)

(c) 100

% Drug released

80 60 40 F7 F8 F9 commercial

20 0 0

100

200

300

400

Time (min)

Figure 2. Captopril release profiles from (a) low, (b) high molecular weight and (c) blend of different ratios of low : high molecular weight polymer formulations in comparison to commercial products.

Formulation and pharmacodynamic evaluation 70 60

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F1 F2 F3

% Burst

50 F4

40

F7 F8 F9

F5

30 F6

20 10 0 Low Mwt

High Mwt

Polymer mix

Polymer composition

Figure 3. Percentage captopril burst from the prepared formulations as a function polymer molecular weight and composition.

Khidr et al. (1998) found that the release of meclofenamic acid was not affected by the molecular weight of CP polymer. The observed in vitro drug release profiles from the microparticles were all biphasic: an initial rapid drug release phase (burst) was followed by slow and prolonged phase. Similar burst effects were reported by Khidr et al. (1995), for metoclopramide hydrochloride microspheres and by Sato et al. (2004), for riboflavin micro-balloons. The burst effect may be beneficial because a high initial release ensures a prompt effect, which can be subsequently maintained for a prolonged period by a slower but continuous release of CAP. The rank order for percentage drug burst regardless of molecular weight of polymer was as follows: 1 : 1 > 1 : 1.5 > 1 : 2 drug : polymer ratio, as shown in Figure 3. At a fixed drug : polymer ratio the rank order of percentage burst depending on the molecular weight of polymer was as follows: low Mw > high Mw. In order to improve the characteristics and modulate the release of CAP from the prepared formulations they were also prepared by mixing various ratios of different molecular weights of cellulose propionate, keeping the drug : polymer blend ratio constant (1 : 1.5). These formulations showed intermediate percentage drug burst, as shown in Figure 3. In order to explain the initial drug burst, further investigations were conducted employing SEM and DSC. SE micrographs of various formulations of CAP microparticles are displayed in Figure 4. These micrographs illustrated how external morphology of microparticles varies with drug loading or polymer content. Increase in polymer content (F1 to F3 and F4 to F6 for low and high molecular weight polymer formulations, respectively) gradually produced smooth and less textured surface in which less CAP crystals were embedded. This could be due to the presence of a high amount of the polymer that can incorporate all the available drug. The drug crystals deposited on the surface are probably due to the rapid formation of emulsion droplets before complete drug entrapment (Khidr et al. 1998). In the case of a high molecular weight polymer, F4 and F5, it can be observed from the microphotographs that the drug particles are not located directly on the surface but embedded in the surface layer of the polymer matrix. Consequently, the percentage drug

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A. H. El-Kamel et al. (a)

F3

F2

F1

(b)

F6

F5

F4

(c)

F9

F8

F7

Figure 4. Scanning electron micrographs of captopril microparticles of (a) low, (b) high molecular weight and (c) blend of different ratios of low : high molecular weight polymer formulations.


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F9

F8 F7 Blank F7 F6 F5 F4 CP high Mwt F3 F2 F1 CP low Mwt CAP

Temperature (C)

Figure 5. DSC thermogram of captopril, low and high molecular weight polymer and various captopril microparticle formulations.

burst, in general, was less than that obtained for low molecular weight particles at all drug : polymer ratios. The rapid initial phase of release was thought to occur mainly by dissolution and diffusion of drug entrapped close to or at the surface of microparticles. The second and slower release phase was thought to involve the diffusion of drug entrapped within the inner part of the polymer matrix by means of aqueous channels of a network of pores. DSC thermograms of low and high molecular weight microparticles are shown in Figure 5. The thermogram of CAP displayed a single sharp endothermic peak at 107.93 C corresponding to the melting point of the drug. Pure low and high molecular weight polymer exhibited endothermic peaks at 180.4 and 190.46 C, respectively. In microparticle thermograms, the peak of the low and high molecular weight polymers disappeared, indicating that the polymers were present in the formulations in an amorphous form. On the other hand, at 1 : 1 and 1 : 1.5 polymer ratios for both low and high molecular weight microparticles formulations, CAP exhibited a small endothermic peak shifted to lower temperature at 99.3 and 104 C, respectively, which is indicative of a certain loss of drug crystallinity. However, at a 1 : 2 drug : polymer ratio the endothermic peak of the drug was completely disappeared. This indicated that at a 1 : 2 drug polymer ratio the drug present in amorphous form totally. On the other hand, it is partially crystalline in microparticles composed of 1 : 1 and 1 : 1 : 5 drug : polymer ratios. These findings were in agreement with the results of examination of SEM photomicrographs in which the drug crystals disappeared from the surface of the particles prepared from a 1 : 2 drug : polymer ratio.

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It has been reported that an initial burst effect in release profile was observed especially when the drug solubility is high, as in the case of CAP, and loading dose in the matrix is large (Velasco et al. 1999). It has been demonstrated also that, in addition to high drug solubility, a burst effect may be due to lack of critical polymer concentration threshold and distribution in the matrix (Mitchell et al. 1993). Additionally, when polymer concentration is low, the hydrated matrix would be highly porous with a low degree of tortuosity leading to low gel strength and rapid diffusion of the drug from the matrix (Khurahashi et al. 1996). For low molecular weight microparticles, at all drug : polymer ratios, > 80% of drug was released in about only 2 h. This was considered as too fast a release rate for controlled release formulations. In addition, these formulations showed a relatively high percentage of drug burst. On the other hand, for 1 : 1.5 (F5) and 1 : 2 (F6) drug : high molecular weight polymer, less than 40% drug released in 7 h with lower percentage drug burst. These results indicated that the prepared microparticles needed some improvement to optimize the release behaviour. Since high encapsulation efficiency is a desirable goal for controlled release studies, F5 that contained 1 : 1.5 drug : polymer ratio which had highest encapsulation efficiency (81.8%) was chosen to be modulated by changing the polymer composition to optimize the drug release behaviour. Mixture of the low and high molecular weight polymers in different ratios were then employed to prepare microparticles with a 1 : 1.5 drug : polymer blend ratio. Statistical analysis (ANOVA) of the percentage captopril released showed a statistically significant effect (p 0.0001) for the molecular weight, ratio and composition of the polymer on the percentage drug released. Post-hoc test (Duncan) showed that the release of captopril from the prepared formulations was in the following order: F1 > F2 ¼ F3 > F4 ¼ F7 > F8 ¼ F9 > F5 > F6. Understanding the nature and composition of the prepared microparticles can help in illustration and explanation of the release order. The melting point of high molecular weight cellulose propionate was 190.4 C, while those of low molecular weight was 171.5 C. Consequently, it could be expected that drug molecules diffuse slower in the polymer that have higher melting point (high Mw CP) because this polymer has less free volume and the mobility of chains is lower (Saltzman 2001). It has also been reported that in vitro release of water-soluble drugs is mainly controlled by diffusion out of the gel layer (Colombo et al. 2000; Bettini et al. 2001). Cellulose propionate is a non-water-soluble polymer; therefore, the release of water soluble drugs is mainly driven by a permeation of drug through the hydrophobic polymer matrix within water filled pores (Tirkonnen and Paronen 1992). The release profile of CAP was strongly influenced by drugto-polymer ratio. It was shown earlier that an increase in polymer concentration causes an increase in viscosity of the gel as well as a decrease in drug release rate (Chukwu et al. 1991; Velasco et al. 1999). Release kinetics To gain more insight in the release behaviour, kinetic analyses were performed on the whole dissolution profile. The dissolution data were fitted to zero order, first order, Higuchi, Weibull, Hixson-Crowell, Baker and Peppas model. Several of the applied models were well fitted to each dissolution profile as indicated by the value of determination coefficient (R2) (Table IV). Criteria for selecting the most appropriate model was based on best goodness of fit (Bamba et al. 1979) indicated by the value of mean standard error (MSE) for linear regression, determination coefficient (R2) nearer to 1 and the F-value calculated from


401

Table IV. Determination coefficients (R2), mean standard errors (MSE) and F-value calculated from ANOVA for release data after fitting of the whole release profile of captopril from various formulations into different mathematical models. Formulations Release models

F1

F2

F3

F4

0.538 0.774 0.811 0.797 R2 F 10.4 30.8 38.6 35.4 MSE 5.26 4.24 3.92 2.157 First order R2 0.478 0.720 0.732 0.764 F 8.23 23.1 24.6 29.21 MSE 0.033 0.032 0.034 0.0218 Higuchi R2 – 0.977 0.898 0.901 F 130 35.1 81.87 MSE 0.817 1.81 1.136 Hixson-Crowell R2 0.695 0.873 0.897 0.955 F 9.1 34.4 35.0 128.3 MSE 0.026 0.015 0.019 0.006 Weibull R2 0.917 0.894 0.938 0.928 F 99.89 75.7 135.9 116.0 MSE 0.039 0.040 0.034 0.016 b 0.483 0.428 0.488 0.222 Td(h) 0.23 0.77 0.95 10.4 Peppas R2 – 0.984 0.965 0.941 F 188.5 111.9 142.3 MSE 0.008 0.013 0.010 n 0.212 0.235 0.159 Baker-Lonsdale R2 0.667 0.789 0.898 0.815 F 17.99 33.6 79.3 39.6 MSE 0.038 0.024 0.016 0.005

Zero order

F5

F6

0.686 19.6 1.500 0.666 17.9 0.0196 0.813 39.0 0.874 0.819 22.6 0.010 0.896 77.4 0.013 0.146 34 0.903 83.5 0.010 0.117 0.658 17.3 0.002

0.834 45.3 1.55 0.754 27.5 0.036 0.941 142.9 0.698 0.906 48.2 0.012 0.973 329.6 0.013 0.297 100 0.966 258.36 0.013 0.270 0.829 43.7 0.0018

F7

F8

F9

0.560 0.669 0.542 11.44 18.17 10.6 3.39 2.95 3.190 0.487 0.549 0.483 8.53 10.9 8.4 0.040 0.044 0.040 0.731 0.831 0.970 24.46 44.38 160.8 1.99 1.58 0.808 0.609 0.752 0.760 7.78 12.1 15.8 0.027 0.025 0.022 0.891 0.943 0.883 73.69 148.9 68.09 0.024 0.019 0.024 0.256 0.295 0.247 7.5 12 18.5 0.874 0.924 0.971 62.3 109.8 168.4 0.019 0.018 0.014 0.192 0.234 0.269 0.661 0.846 0.561 17.5 49.42 11.49 0.0059 0.0034 0.0059

analysis of variance of release data after fitting to each dissolution model. Priority should be given to the model with the highest F-value and lower MSE. The fitting of the whole release profile indicated that the coefficient of determinations were found to be maximum in case of Weibull model for F1, F6, F7, F8 and Peppas model for F2, F3, F5 and F9. Although the Weibull and Peppas model cannot adequately characterize the release kinetic properties of the drug from all the examined formulations, they can describe the dissolution curve in terms of applicable parameters and reported to be excellent models and possessed parameters that were sensitive to the ranges of dissolution profiles (Koester et al. 2004). The Weibull shape parameter, b, characterizes the curve as either exponential (b ¼ 1) (case 1), sigmoid, S-shaped, with upward curvature followed by a turning point (b > 1) (case 2) or parabolic, with higher initial slope and after that consistent with the exponential (b < 1) (case 3). For all examined formulations, after fitting of the whole dissolution curve, the shape factor, b, was less than 1, indicating that the dissolution curves followed case 3. Dissolution time Td that represents the time interval necessary to release 63.2% of drug present in pharmaceutical dosage form was calculated for each formulation and presented in Table IV. The time required for 63.2% drug release was 0.23, 0.77 and 0.95 h for F1, F2, F3 (low molecular weight polymer formulations), respectively, which considered as short time for sustained release preparation. On the other hand, the calculated Td for F4, F5, F6 (high molecular weight polymer formulations) was too long, 10.4, 34, and 100 h, respectively. However, Td values calculated for F7, F8 and F9 (polymer blend formulations) was

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SBP(mmHg)

250 200 150 100 solution F7 F7 placebo

50 0 0

2

4

6

8

10 12 14 16 18 20 22 24 Time (hours)

Figure 6. Mean SBP after administration of F7, aqueous CAP, and drug unloaded F7 to hypertensive rats.

reasonable, 7.5, 12 and 18.5 h, respectively. The results indicated that, as dissolution was slowed, the Weibull parameter Td grew larger, a result was in agreement with the interpretation that this parameter reflects the scale of time for dissolution process. Peppas used n-value to characterize different release mechanisms. When n takes a value of 0.5, the drug diffuses through and is released from the polymer following Fickian diffusion mechanism. For n > 0.5, an anomalous, non-Fickian solute diffusion mechanism is observed. The calculated n-values for all examined formulations were found to be less than 0.5, indicating Fickian diffusion drug release. Taken together, the selected developed captopril formulation (F7) was chosen as a selected formulation by virtue of its good encapsulation efficiency, flow properties, surface characteristics and in vitro release properties. Anti-hypertensive effect studies To verify the capability of cellulose propionate—CAP microparticles to prolong the in vivo duration of drug effect, the anti-hypertensive effect of captopril released from the selected microparticles formulation (F7) was recorded and compared with that of captopril solution containing the same drug amount. Drug unloaded microparticles were used as control. No significant effect (p > 0.05) on systolic blood pressure was noticed after administration of drug unloaded microparticles. A rapid fall in SBP peaked at 2 h after the administration of aqueous solution of CAP, however, at 3 h blood pressure started to increase. On the other hand, a more gradual and sustained SBP reduction was shown up to 24 h after administration of microparticles formulation, as shown in Figure 6. The gradual release showed in vitro profile may justify the slow and gradual SBP lowering effect of the examined F7 microparticle formulation. One way analysis of variance of the percentage decrease in SBP after 24 h of administration of F7, aqueous CAP solution and drug unloaded microparticles showed statistically significant differences (p 0.01) between the three examined formulations in the percentage reduction in SBP. Duncan test showed that the rank order of percentage decrease in SBP was as follows: F7 > aqueous CAP ¼ drug unloaded F7.


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Taken together, the proper selection of formulation composition are very important to control the release of CAP from cellulose propionate microparticles. The in vivo performance of the selected formulation suggests the successful cellulose propionate microparticles as a sustained release formulation for CAP.

Acknowledgements The authors are grateful to the Research Center, King Saud University, Women StudentsMedical Studies & Sciences Sections and to King Abdullaziz City for Science and Technology for the financial support.

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