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JPRDP 2

                     Adegoke et al., J. Pharm. Res. Dev. & Pract., December, 2016, Vol. 1 No. 1, P 12-24 ISSN:2579-0455

 

Formulation of Metronidazole Suppositories with Modified Cocoa Butter and Shea Butter Bases for Enhanced Stability in Tropical Environment

 

                                         A. ADEGOKE, F.A OLADIMEJI*, A.O. OYEDELE,             

 

      Department of Pharmaceutics, Faculty of Pharmacy, Obafemi Awolowo University, Ile-Ife, Nigeria.

____________________________________________________________________________

              *Corresponding Author’s E-mail:  faolad@oauife.edu.ng .  GSM:+2348062691909

______________________________________________________________________________

 

ABSTRACT

The objectionable side effects associated with oral administration of metronidazole can be circumvented by its formulation as suppository for rectal administration. Bases employed are an essential vehicle in the formulation of rectal suppositories. Cocoa butter and shea butter are valuable suppository bases indigenous to tropical countries. However, their physical stability and melting characteristics are vulnerable at normal tropical temperatures. The objective of this study was to modify cocoa butter and shea butter as bases for formulating stable metronidazole suppositories having desirable drug-release properties in tropical environment. The physical properties of cocoa butter and shea butter were modified using different concentrations of beeswax. The modified bases were used in preparing 400 mg metronidazole suppositories with Tween® 20 incorporated at different concentrations as drug release enhancer. The physical and dissolution properties of the suppositories were determined by standard methods and the dissolution data evaluated for best-fitting release kinetic models. Inclusion of beeswax into the bases improved their mechanical strength whilst repressing their drug release propensity. Addition of Tween® 20 to the modified bases resulted in reduction of the mechanical strength of the suppositories but with enhanced release of metronidazole. Physically stable metronidazole suppositories suitable for tropical climate storage, and with optimal drug release properties were obtained when 20 %w/w  beeswax and 4 %w/w  Tween® 20 had been incorporated into cocoa butter and shea butter based suppositories. The release mechanisms of metronidazole from the suppositories were both Fickian and non-Fickian diffusion with release exponents (n) ranging between 0.38 and 0.67.

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Keywords: Metronidazole, suppository formulations, modified cocoa butter, modified shea butter, tropical climate.                                                                 

 

 

Introduction

Cocoa butter (theobroma oil) obtained from fruits of the cocoa tree (Theobroma cacao, family Sterculiaceae)1 has been widely used as suppository base, being compatible with commonly used medicinal chemicals2. Shea butter, obtained from seeds of the tree, Vitellaria paradoxa Gaertner F., family Sapotaceae3, has physical stability and melting characteristicsimilar to those described for cocoa butter4, which make shea butter suitable for general use as a base for suppository formulation. Hence, shea butter has been variously studied for such application5,6,7. However, the high-level environmental temperatures of the tropics could predispose the suppositories formulated with these fatty bases to untimely and undesirable melting and disintegration prior to the points of their needed use, especially where constant refrigeration of the products and consistent electricity supply are not guaranteed8,9. Hence, the need to modify the physical properties of these bases.

Metronidazole suppositories that are official preparations10 have been shown to be effective for prophylaxis and therapy of anaerobic infections in patients undergoing abdominal and gynecological surgery11,12. The suppositories provide systemic administration of the drug by a route other than the oral route, thus overcoming the side effects of the drug on gastrointestinal tract, and also serve as alternative to the intravenous administration when this is inconvenient13,14,15. 

The physical stability and drug release properties of metronidazole suppositories depend greatly on the bases and excipients employed16,17,18. Formulation of natural fat-based metronidazole suppository intended for tropical use, where ambient temperatures may range from 30-40ºC8,9, would require modification of the base. A stiffening agent, such as beeswax, has commonly been used to improve the physical consistency of natural fats employed as suppository base2,7. The presence of a stiffener may, however, cause limitation of the drug-release potential of the resultant suppository product19 which, on the other hand, may be countered by incorporation of surfactant(s) such as polysorbate 20 (Tween® 20) to enhance drug release potential of the resultant suppository formulations20

Therefore, the aims of the present study are to ascertain the suitability and optimal concentration of beeswax employed for modifying the physical consistency of cocoa butter and shea butter in use as bases for metronidazole suppository formulations intended for tropical environments, and to determine appropriateness of using Tween® 20 in addition, as drug-release enhancer.

 

Materials and Methods

Materials

Metronidazole powder (Fidson Healthcare Ltd., Sango-Otta, Nigeria); Cocoa butter (theobroma oil) (Starmark Cocoa Processing Company Ltd., Ondo, Nigeria); Shea butter (procured at market in Shaki, Nigeria); Beeswax (Fluka, Switzerland); Sodium Hydroxide (BDH Laboratory, Poole BH15, England), Potassium dihydrogen orthophospha-te (Surechemproducts Ltd, England), Polysorbate 20 (Tween 20®) (Sigma Chemical Co, St. Louis, USA), UV-visible spectrophotometer (Cecil CE 3041 3000 series), Manesty Tablet Disintegration Apparatus, Digital Tablet Dissolution Test Apparatus Model VDA-8D (PharmaChem Machineries, Mumbai, India), Mettler Toledo PB 153 analytical balance (Switzerland),

 

Methods

Purification of shea butter samples:

The shea butter was purified by hot filtration at 60°C and left to stand for 5 days at the ambient temperature (28±2 °C) prior to use in the study4,21.  

Determination of baseline physicochemical properties of suppository bases:

Physicochemical properties of the cocoa butter and purified shea butter (including iodine, acid, ester, and saponification values), as baseline data, were determined by British Pharmacopoeia10 methods. The melting ranges of the cocoa butter and shea butter were determined using ascending melting point method18.  The base was melted in a water bath at 40oC. One end of a 10 cm capillary tube was dipped into the molten base. The molten mass was allowed to rise to 5 cm mark on the capillary tube and stored for 24 h in the refrigerator. The capillary was then attached to a thermometer calibrated in 1oC increments and lowered into a thermo-regulated water bath so that the temperature rose at the rate of 1oC/2 min. The temperature at which the base began to melt was recorded as the softening/slip point, while the temperature at complete liquefaction of the base was defined as the melting point7. The two temperatures marked the melting range of the base. The results obtained were average of four determinations.

The congealing temperatures of the bases were carried out using the modified method described in the United States Pharmacopoeia22. One gram of the base placed in a test tube was heated in a water bath at a temperature of 5oC above its melting point. A thermometer with an attached wire stirrer (bent into a horizontal loop at its lower end) was suspended in the centre of the liquefied base. The set up was clamped and inserted in a water bath previously heated to 42oC, which was provided with a thermometer. The liquefied base was stirred continuously and the temperature at which the stirrer became immobile was defined as its congealing temperature. The results obtained were average of four determinations.

 

Modification of suppository bases

White beeswax at concentrations of 5 to 50 %w/w was employed in the modification of physical properties of the cocoa butter and shea butter bases. The beeswax was melted in a platinum dish on a water bath regulated at about 5 oC above the melting point of beeswax23. The base was chopped to tiny bits and added to the molten beeswax while congealing and stirred until cool. The modified bases were stored for 48 h after which their melting point range and congealing temperatures were determined as indicated for the pure bases.

Preparation of metronidazole suppositories:

The suppositories were prepared by fusion method using a 2 g metal mould with six cavities24. Suppositories, each containing sifted 400 mg metronidazole were prepared. The quantities of both neat and modified bases of cocoa butter and shea butter required in each formula were determined by the drug’s displacement value25. Batches of suppositories containing varied concentrations of Tween 20® (1 – 10 %w/w)were also prepared. The Tween 20® was mixed with the melted bases prior to the addition of the metronidazole. The suppositories were stored in a refrigerator (4±1oC) and analysis carried out 24 hours after formulation.

Evaluation of physico-chemical properties of metronidazole suppositories

Uniformity of weight test:

Twenty suppositories were randomly selected from each batch of the formulations and weighed individually using a Mettler analytical balance. The mean weight and percentage relative standard deviations (RSD) were determined. The deviations of the individual weight from the theoretical weight of the suppositories were also calculated.

Determination of softening and melting points:

The softening and melting point values of metronidazole suppository bases were determined with a modified method that was used for the neat and modified bases26. The suppository sample was placed in a clean test tube with a thermometer inserted. The tube was clamped vertically, immersed at 8-cm depth in a water bath. Temperature of the water bath was gradually increased (1 °C/ 2 min). The temperature at which the suppository sample began to melt was recorded as the softening point, while the temperature of its complete liquefaction was defined as the melting point. The difference between the two temperature values gave the melting range of the suppository. The results obtained were average of four determinations.

Determination of content uniformity:

The method described by Setnikar and Fontani27 was used. A suppository taken randomly from each batch was weighed and placed in a beaker containing 100 ml of phosphate buffer solution at pH 7.2.  The suppository was melted by heating the beaker gradually on a water bath. The beaker was shaken gently while the melting proceeded. When the suppository had been completely dispersed, the mixture was chilled and the oil layer was removed by filtration through a cotton plug. The aqueous portion was further filtered through Sinter glass number 3. The aqueous filtrate (1 ml) was diluted to 100 ml using phosphate buffer solution.  The absorbance was measured by UV spectrometer at 326 nm. The concentration of the solution was calculated from a standard Beer-Lambert curve and the drug content was determined. The result was an average of six determinations per batch of suppositories.

Determination of mechanical strength:

The hardness of suppository samples was determined using the Monsato Hardness Tester28. Ten suppositories randomly selected from five test batches of 30 suppositories were used. Prior to testing, the suppositories were removed from refrigeration and kept for 24 h at the ambient temperature (25±2 °C). The force under which each suppository sample collapsed was recorded.

Determination of disintegration time:

The disintegration time of the suppositories was evaluated with the Manesty tablet disintegration apparatus by the method for uncoated tablets10. The apparatus consists of six cylindrical glass tubes immersed in water-filled bath maintained at constant temperature (37±1 °C), and held by a basket rack attached to a vertical metal rod that is fixed to a mechanical device capable of raising and lowering the device through a distance of 60 mm at a frequency of 32 cycles per minute. The lower end of each glass tubes is fitted with woven stainless steel wire mesh (2-mm aperture). For disintegration-time testing, one suppository was placed in each glass tube and a metal disc weighing 50 g was added to each tube26. The time required for complete deformation of each suppository sample was noted from which the mean value and standard deviation from the six cylinders were calculated. 

Evaluation of release profile of metronidazole from the suppositories

The United States Pharmacopeia22 basket method was employed for the dissolution studies, using digital tablet dissolution test apparatus. Phosphate buffer solution (900 ml) at pH 7.2 was used as the dissolution medium. Each suppository was embedded in a bed of 12 glass beads (to prevent floating of the suppository) within the dissolution basket and lowered into a flask containing the dissolution medium maintained at constant temperature (37.0±0.5 °C). The basket was rotated at the constant speed of 100 rpm. At appropriate time intervals, 5-ml samples were withdrawn over a period of 360 min. The volume of the dissolution medium was kept constant by replacing the volume of the sample withdrawn with an equal volume of fresh buffer solution maintained at the same temperature. The withdrawn samples were filtered and 2 ml of this was diluted with an equal volume of the buffer solution and the absorbance of the diluted sample was determined by UV spectrometer at 326 nm.  The amount of drug released was calculated from a standard Beer-Lambert calibration curve. The mean of three determinations was used in calculating drug release from each batch of suppositories. The drug release parameters: percentage of drug released at 360 min, 60 min and the time (min) for 50 % (T50%) of the drug to be released were calculated.

Determination of drug release kinetics:

The dissolution data were fitted into four release kinetics models29,30 namely; Zero-Order (Q vs t), First-Order (log (Qo – Qt) vs t), Higuchi model (Q vs t½) and Korsmeyer-Peppas model (logQt vs nlogt),   where Q is the amount of drug released at time t,  Qo is the initial amount of the drug, Qt is the amount remaining at the time t and “n”  the release exponent from Korsmeyer-Peppas model . Dissolution data were evaluated using Microsoft Excel spreadsheet and DDSolver software31,32. The best-fit dissolution model was identified by coefficient of determination R2,  R2adjusted, and Model Selection Criteria (MSC), where model with the highest R2  and R2adjusted (≥ 0.950), MSC (≥ 3.00) values within the set of the models was considered the best fit31.

Statistical analyses:

Statistical differences among the results from the various physicochemical tests and dissolution rate studies were assessed by employing Microsoft Excel and GraphPad Prism 5 software with minimum level of significance established at 5 %.

Results and Discussion

Baseline physicochemical properties of the suppository bases:

Table 1 shows the baseline physicochemical properties of cocoa butter and purified shea butter used in this study, and it also illustrates the difference between the 2 fatty bases. The higher iodine, acid, saponification and ester values of shea butter compared with those of cocoa butter indicate that the cocoa butter is of better quality than the shea butter. The free acid value of the shea butter indicates that the locally purchased shea butter belongs to the “Third Grade” 33.

The purpose of modifying the bases is to make them suitable for tropical climate where the average room temperature may be higher than 30oC. The physical properties of the modified cocoa butter and shea butter, using beeswax are indicated in Table 2. There was an increase in the melting range of the fatty bases with increase in the concentration of beeswax incorporated to them. Previous researchers have reported that a fat-based suppository should have a softening point above 32oC to remain solid at ambient temperature in the tropics2. A softening point not more than 36oC for fast release of active ingredients from fatty bases was also suggested2. The most suitable softening and melting temperatures of the modified bases for tropical climate handling were produced when 20 %w/w beeswax was incorporated in the cocoa butter and shea butter bases (Table 2). Hence, these respective formulations were selected for use in the formulation of the metronidazole suppositories.  

 

 

 

Table 1:  Physicochemical properties of cocoa butter and shea butter.

 

Properties

Cocoa butter*

Shea butter

Iodine value

     32.45±0.35

     43.54±0.05

Acid value

     1.78±0.45

     3.95±0.60

Saponification value

191.31±0.51

196.81±0.26

Ester value

189.53±0.04

192.86±0.05

Congealing temp. (0 C)

     32

     31

Melting range temp. (0 C)

     32– 36

     31 – 34

*Cocoa butter physicochemical properties obtained from Starmark

Cocoa Processing Company Limited, Ondo.

 

Table 2:  Physical properties of modified cocoa butter and shea butter suppository bases

Beeswax Conc. (%w/w)

Cocoa  butter

Shea butter

Melting Range (0C)

Congealing temp. (0C)

Melting Range (°C)

Congealing temp. (°C)

5

33- 35

33.5±0.5

32- 35

33.0±0.5

7.5

33.5 – 35.5

33.5±0.5

32- 35

33.0±0.5

10

34 – 36

34.5±0.0

32- 35.5

33.5±0.3

12.5

35 – 36

35.0±0.3

32.5- 35.5

34.0±0.0

15

35.5-  36.5

35.5±0.6

33- 36

34.0±0.3

20

35.5 – 37

36.0±0.5

34.5- 37

35.5±0.6

30

36- 39

36.5±0.3

35-39

36.5±0.5

40

36- 41

37.5±0.5

36- 40

37.0±0.6

50

39 – 42

40.0±0.8

37 – 42.5

38.0±0.5

 

 

 

 

 

 

 

 

 

 

 

 

 

Physical properties of metronidazole suppositories:

The physical properties of 400 mg metronidazole suppositories prepared with modified cocoa butter and shea butter bases are indicated in Table 3. A comparison of the bland modified bases with those containing 400 mg metronidazole (formula MC0 and MS0) showed that inclusion of the drug in the modified bases increased the melting points of the medicated suppositories with no significant change in the softening points. While it has been reported that some drugs could lower the softening points of some suppository bases34, the inclusion of metronidazole in the modified bases has no such effect. Thus, the modification of the bases with beeswax and incorporation of metronidazole to them synergistically increased the melting range of the suppositories formulated with the cocoa butter and shear butter bases. Incorporation of Tween® 20 into the metronidazole suppositories formulated with the modified bases led to concentration dependent decrease in both softening and melting points of the suppositories. Linear regression equation established a correlation between the Tween® 20 concentration and the softening points of the metronidazole suppositories with coefficient of determination (R2) of 0.978 and 0.958 obtained for suppositories prepared with modified cocoa butter and modified shear butter, respectively.

The weight variations of each formulation were in conformity with BP10 requirement, with relative standard deviation less than 3.5 %. None of the individual suppository weights deviated from the theoretical weight by more than 5 %, which indicated that the calibration of the mould was correct. The percentage coefficient of weight variations decreased with increase in concentrations of Tween® 20 in the formulation. Similarly, the drug content of all the suppositories tested also met the BP10 guideline of remaining within 100±15 % of the expected drug content. The addition of Tween® 20 to the modified bases resulted to concentration dependent improvement in the content uniformity of the suppositories as evidenced from the decrease in their standard deviations (Table 3).

The mechanical strengths (hardness) of the cocoa butter and shea butter suppository bases containing 20 %w/w beeswax (without drug or Tween® 20) were 20.85 N and 13.70 N, respectively. On inclusion of 400 mg of the drug  into the modified bases, the mechanical strength of the resultant medicated suppositories increased to 26.8 N (MC0) and 19.2 N (MS0) (Table 3). Some drugs have been reported to repress the mechanical strength of some suppository bases34, the finding in this study proved otherwise. The above finding may necessitate further study to determine if the effect of the drug on mechanical strength of the suppositories could possibly lead to their hardening on storage with resultant decrease in drug release properties of the suppositories.

All the metronidazole suppositories have disintegration time less than 15 min (Table 3). Incorporation of the drug into the modified cocoa butter and modified shea butter bases without Tween® 20 (MC0, MS0) increased the disintegration times of the medicated suppositories. Inclusion of Tween® 20 in the formulation reduced the disintegration periods, which could be due to de-plasticizing effect of the surfactant on the matrix of the suppositories. The effect of Tween® 20 on the disintegration times of the formulations was highly significant (P < 0.01) and concentration dependent with coefficient of determination (R2) of 0.982 and 0.962 established for formulations prepared with modified cocoa butter and modified shea butter, respectively.

Release properties of metronidazole suppositories:

The modified cocoa butter base without Tween® 20 (MC0) demonstrated higher metronidazole release propensities than their modified shea butter base (MS0) counterparts (Table 1, Figure 1). However, both types of formulation released not more than 55 % of their drug content over 3 h. Beeswax was added to modify the bases because the neat fatty bases are reportedly unsuitable to handle in tropical climate without modification of their physical characteristics7. The use of beeswax and other stiffening agents is however, generally known to cause retardation of drug release from suppositories7,19. Apart from the drug release-retarding effect of the beeswax, presence of the drug, as reported earlier could produce an increase in the melting points of the modified bases which could retard the liquefaction tendency of the suppositories and subsequent diffusion of the drug into the dissolution medium. Different approaches have been used in improving solubility and dissolution rate of poorly water-soluble drugs from suppositories5,35,36. In this study, Tween® 20 was incorporated to the formulations at concentrations 1 – 10 %w/w.

 

 

 

Table 3: Physical and release parameters for Metronidazole Suppository Formulations containing varied Concentration of Tween 20

Modified  Base /

Tween 20 conc.(%w/w)

 

Formulation

code

 

Softening point( 0C)

 

Melting

Point

        (0C)

 

Mean weight

 (g)

 

Drug content

 (%)

 

Mechanical strength  (N)

 

DT (min)

 

%D(360min)

 

T50%(min)

MCB

 

 

 

 

 

 

 

 

 

0

MC0

36.2±

0.8

40.5±

0.6

2.077±

0.054

98.7±

10.8

26.8±

5.4

11.2±

0.2

52.8±

6.2

350.0

1

MC1

36.0±

0.4

39.5±

0.9

2.092±

0.073

100.5±

10.9

24.7±

0.9

9.3±

0.4

68.0±

6.9

130.0

2

MC2

35.5±

0.8

38.0±

0.5

2.093±

0.049

101.1±

6.3

21.9±

2.0

7.8±

0.4

73.7±

2.0

110.0

4

MC4

35.0±

0.5

37.5±

0.4

2.097±

0.019

100.3±

  7.4

20.6±

2.2

7.4±

0.5

83.7±

3.6

90.0

6

MC6

34.6±

0.2

37.3±

0.7

2.111±

0.026

100.0±

  6.2

18.6±

1.4

5.8±

0.4

58.1±

1.5

280.0

8

MC8

33.0±

0.7

37.0±

0.9

2.110±

0.029

101.1±

  5.1

11.7±

1.1

5.9±

0.5

53.4±

2.4

330.0

10

MC10

31.9±

0.5

36.5±

0.5

2.115±

0.024

100.9±

  4.5

10.4±

2.3

5.3±

0.3

47.8±

2.5

>360.0

MSB

 

 

 

 

 

 

 

 

 

0

MS0

35.8±

1.0

39.0±

0.5

2.088±

0.042

97.6±

  11.5

19.2±

0.8

10.2±

0.5

40.3±

1.7

>360.0

1

MS1

35.0±

0.6

38.0±

0.3

2.114±

0.024

98.1±

  10.6

17.6±

1.1

8.1±

1.1

55.7±

1.9

270.0

2

MS2

34.8±

0.6

37.0±

0.2

2.117±

0.036

101.4±

  6.1

14.5±

1.3

7.4±

0.1

66.1±

1.3

240.0

4

   MS4

34.0±

0.8

36.5±

0.3

2.122±

0.014

99.7±

5.1

13.2±

0.9

6.6±

0.3

76.1±

2.0

120.0

6

MS6

33.0±

0.4

36.5±

0.6

2.127±

0.011

98.6±

7.3

10.8±

0.6

5.5±

0.2

62.1±

2.1

250.0

8

MS8

32.8±

0.8

36.0±

0.4

2.133±

0.020

101.0±

  4.9

9.7±

0.7

5.0±

0.8

59.3±

1.1

290.0

10

MS10

  31.1±

    0.5

36.0±

0.8

2.142±

0.013

101.5±

  3.4

9.2±

0.9

3.7±

0.4

53.0±

1.2

340.0

                             

 

MCB and MSB are modified cocoa butter and shea butter respectively; DT = Disintegration time (min); %D(360min) = % drug released in 360 min;

T50%(min) = Time (min) required for 50% of the suppository drug content to dissolve; %D(60min) = % drug released in 60 min.

Mechanical strength of MCB and MSB suppositories without Tween 20 and metronidazole are 20.85±0.92 N and 13.70±1.36 N, respectively

Disintegration time of MCB and MSB suppositories without Tween 20 and metronidazole are 9.63±0.74 min and 9.31±0.21 min, respectively

 

 

 

 

The release of metronidazole from the modified bases increased with increase in concentration of Tween® 20 up to 4 %w/w after which there was a decrease in the release of the drug (Figure 1). Thus, 4 %w/w Tween® 20 gave the optimal concentration for drug release, which was significantly higher (P < 0.05) than that of other concentrations. The dissolution parameters (%D360min and T50%min) indicated in Table 3 shows that only suppositories formulated with 4 %w/w Tween® 20 (MC4 and MS4) released more than 75 % of their drug content at 360 min. The in-vitro metronidazole release promoting effects of Tween® 20 (1-4 %w/w) in the modified bases has been attributed to the surfactant’s ability to decrease the surface and interfacial tension of the molten bases and facilitate the drug’s penetration by the dissolution medium thereby aiding desorption of the embedded drug out of the suppository matrix36,37. Tween® 20 with high Hydrophile-Lipophile Balance (HLB) of 16.7 could enhance the moistening of the modified fatty bases by the dissolution medium, and also increase their hydrophilic property with a reduction in the affinity of the poorly water soluble metronidazole for the base thereby promoting the drug release. The addition of Tween® 20 to the modified bases caused reduction in the softening, mechanical strength, and disintegration of the suppositories, which to a large extent possibly aided the release of the drug from the modified bases. The USP22 requires that suppository formulations intended for immediate release of the drug must release 75 % of their drug content within 60 min. Figure 1 shows that none of the formulations released up to 75 % of their drug content in 60 min. The limited water solubility of metronidazole (BP)10, coupled with decrease in its solubility at high pH of the dissolution medium38, and its affinity for the fatty bases24,29  are other factors that could have appreciably affected the observed dissolution profile of the suppositories.

There was a reduction in the metronidazole released from the suppositories when the concentration of the incorporated Tween® 20 was ≥ 6 %w/w.  Previous studies have ascribed such finding to the formation of micelles within the suppository bases that trapped the drug36,37. The significant reduction in the softening point, melting point and disintegration time of the suppositories as a result of addition of Tween® 20 at concentration ≥  6 %w/w  did not promote the release of the drug from the formulations, suggesting that, micellisation leading to entrapment of the drug was the dominant factor in decreasing the release of metronidazole from the formulations at concentration ≥  6 %w/w  as earlier observed.

 

 

MCB: modified cocoa butter; MSB: modified shea butter

 

Figure 1: Effect of Tween 20 concentration on the percentage of metronidazole released in 60 min from the modified bases

 

Release kinetics of metronidazole suppositories:

The dissolution parameters in Table 3 and Figure 1 indicate increase in percentage of drug released from the suppositories with increase in concentration of Tween® 20 up to 4 %w/w after which a decrease in released was observed. Consequently, suppositories containing 0, 1, 4 and 10 %w/w Tween® 20 were selected for the determination of the release kinetics of metronidazole from the modified bases. The dissolution profiles in Figure 2 were subjected to Higuchi, Zero-Order, First-Order and Korsmeyer-Peppas release kinetics models and the model with the highest R2, R2adjusted  and MSC values was selected as the best fit model. The release profiles of metronidazole from the formulations indicated in Figure 2 were not linear, suggesting that the drug release mechanism was not Zero-Order. This was confirmed by low R2adjusted (0.11 – 0.903) and MSC values (0.11 – 2.20). The model fitting parameters in Table 4 indicate that the release mechanism of metronidazole from formulations MC0, MC1, MC10, MS0, MS1 and MS10 could be classified under Higuchi model indicating that diffusion is one of the primary mechanisms governing metronidazole release from the suppositories. The addition of Tween® 20 to the modified natural fatty bases increased the release rate constants (KH) of the drug from the suppositories (MC0 vs. MC1 and MC4; MS0 vs. MS1 and MS4) (Table 4). However, at higher concentration of Tween® 20 (10 %w/w), there was a decrease in the release rate constants (MC10 and MS10). This has been previously alluded to formation of micelles within the suppository bases that entrapped the drug36,37.

 

Figure 2: Release profile of metronidazole from modified cocoa butter and shea butter bases   incorporated with 0 %, 1 %, 4 % and 10 %w/w Tween 20.

 

 

 

Table 4: Derived release rate constants and model fitting parameters for release kinetics of metronidazole suppository formulations

 

Release Kinetic Model

Release parameter

Formulation code

MC0

MC1

MC4

MC10

MS0

MS1

MS4

MS10

Higuchi

KH (mg/min1/2)

10.080

15.364

19.008

9.148

7.836

11.612

16.552

10.596

R2

0.960

0.947

0.938

0.958

0.983

0.990

0.946

0.991

R2_adj

0.960

0.947

0.938

0.958

0.983

0.990

0.946

0.991

MSC

3.09

2.80

2.66

3.03

3.93

4.47

2.78

4.58

Zero-order

Ko (min-1)

0.644

0.956

1.172

0.584

0.496

0.732

1.024

0.668

R2

0.903

0.419

0.110

0.875

0.803

0.700

0.215

0.680

R2_adj

0.903

0.419

0.110

0.875

0.803

0.700

0.215

0.680

MSC

2.20

0.41

-0.02

1.95

1.49

1.07

0.11

1.01

First-order

K1 (mg/min)

0.008

0.016

0.002

0.008

0.008

0.012

0.002

0.008

R2

0.973

0.821

0.798

0.937

0.882

0.878

0.743

0.848

R2_adj

0.973

0.821

0.798

0.937

0.882

0.878

0.743

0.848

MSC

3.47

1.59

1.47

2.64

2.00

1.97

1.23

1.75

Korsmeyer-Peppas

KKP (mg/minn)

4.132

20.828

35.836

4.252

5.12

10.208

28.756

9.772

n

0.67

0.44

0.38

0.64

0.58

0.52

0.40

0.52

R2

0.997

0.956

0.994

0.986

0.994

0.991

0.984

0.992

R2_adj

0.997

0.953

0.994

0.985

0.993

0.991

0.984

0.991

MSC

5.70

2.86

4.82

4.01

4.81

4.46

3.87

4.51

ko, k1, kH = zero-order, first-order and Higuchi release constants, respectively; kKP, n = release constant and  release exponent in Korsmeyer-Peppas model, respectively; R2 = coefficient of determination; R2_adj = adjusted coefficient of determination; MSC = Model Selection Criterion.

 

 

In Korsmeyer-Peppas model, the value of the release exponent n characterizes the release mechanism of the drug, in which 0.45 ≤  n corresponds to a Fickian diffusion mechanism, 0.45 < < 0.89 to  non-Fickian diffusion transport (anomalous transport drug release mechanism), n = 0.89 to Case II (equivalent of Zero-Order release kinetics) and n > 0.89 to Super Case II (First-Order release kinetics)40,41. Based on n values obtained (0.38-0.67), the release mechanism of metronidazole from the formulations could be classified under Fickian diffusion and non-Fickian transport with release rate constants (Kkp) of the following order: MC4 > MS4 > MC1 > MS1 >MS10 > MS0 > MC10 > MC0. The n values in the Korsmeyer-Peppas model for MC4 and MS4 were associated with Fickian diffusion mechanism. The exponential values n obtained from the Korsmeyer-Peppas model showed that incorporation of Tween® 20 to the modified fatty bases changed the release mechanism of metronidazole from the formulations i. e. MC0 vs.MC4; MS0 vs. MS4 (Table 4). The n values obtained for MC4 and MS4 were the lowest but with highest release rate constants (Table 4).

Conclusions

The most suitable softening and melting characteristics of beeswax-modified cocoa butter and shea butter suppository bases for tropical handling and storage were obtained when 20% w/w beeswax was incorporated. The suppository hardness and disintegration time, however, declined progressively as 1–10 %w/w Tween® 20 was incorporated. The modified cocoa butter based suppositories demonstrated higher metronidazole release propensities than the modified shea butter based counterparts at concentrations of Tween® 20 ≤  4 %w/w. Optimal release of metronidazole from the suppositories was achieved at 4 %w/w Tween® 20, while higher concentrations resulted to a significant (P < 0.01) reduction in the release. The release mechanisms of metronidazole from the modified bases was best classified by the Korsmeyer-Peppas model with release exponents n of 0.38 - 0.67, demonstrating both Fickian and non-Fickian diffusion mechanisms.  The release profile of metronidazole from the suppository formulations indicated that Tween® 20 at the optimal concentration could not enhance the release of 75 % metronidazole content of the suppositories within 60 min as specified by the official pharmacopoeia. This limitation, couple with the reduction in mechanical strength of the suppositories by increasing concentration of Tween® 20 should warrant further studies on use of release enhancers that will not significantly compromise the mechanical strength and softening points of suppositories formulated with modified natural fatty bases.

 Acknowledgements

The authors are grateful to Fidson Healthcare Ltd., Sango-Otta, Nigeria, for providing metronidazole powder for this study. We also gratefully acknowledge processed cocoa butter supplied by Starmark Cocoa Processing Company Ltd., Ondo, Nigeria.

 

 

 

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