Development and Evaluation of Taste Masked Granular Formulation of Satranidazole by Melt Granulation Technique

Drugs from nitroimidazole category are generally bitter in taste. Oral formulation with bitter taste is not palatable. Geriatrics and pediatrics patients usually suffer from swallowing difficulties. Many other patients in some disease conditions avoid swallowing tablets. Satranidazole is a new nitro-imidazole derivative with bitter taste and is available in market as film coated tablet. The purpose of this research was to mask the bitter taste of Satranidazole by coating complexation with low melting point wax and Eudragit EPO. Different types of wax (glyceryl monostearate, stearic acid and cetyl alcohol) were tried for taste masking. The drug to stearic acid ratio 1 : 2 was found to be optimum on the basis of taste evaluation and in vitro release. The formulated granules were found to possess good flow property. FTIR studies confirmed that there was no interaction between drug and excipients. Scanning Electron Microscopy of drug and the optimized batch of granules was performed. The in vitro release of drug from granules was compared with marketed tablet formulation. The taste masked granules of optimized batch showed 87.65% release of drug in 1 hr which is comparable to that of marketed tablet formulation.


Introduction
Masking the bitter taste of drugs is a challenge in development of all oral formulations and a necessity to ensure better patient compliance and product value where the process and formulation should be economic, rapid, and easy, involving least number of equipment, and processing steps and minimum number of excipients without adverse effect on drug bioavailability [1]. Bitter taste of drugs is a major problem in pediatric and geriatric formulations. Numerous methods are employed for effective taste masking, for example, use of flavors and sweeteners, microencapsulation, complexing with ion exchange resin, use of insoluble prodrug, formation of inclusion complexes, gelation, liposome, multiple emulsions, granulation, and so forth.
Taste masking can also be carried out using melt granulation technique [2]. Melt granulation is a process by which pharmaceutical powders are efficiently agglomerated by the use of a binder which can be a molten liquid or a solid that melts during the process. Different types of low melting point wax like glyceryl monostearate, palmitic acid, myristic acid, and so forth can also be used for taste masking as they form dense coating. Eudragit E100 is utilized along with low melting point waxes for dissolving it and spray cooled and granulated in a spray drying apparatus to obtain dense coating whereas if Eudragit E100 when dissolved in organic solvent forms porous coating which is difficult to mask the unpleasant taste [3].
Satranidazole (STZ) is a new nitroimidazole derivative with potent antiamoebic action. It is used in the treatment of intestinal and hepatic amoebiasis, giardiasis, trichomoniasis, and anaerobic infections. Its dose is 300 mg twice daily for 3-5 days in the treatment of amoebiasis and 600 mg as a single dose in the treatment of giardiasis and trichomoniasis. It is reported that Satranidazole exhibits significantly higher plasma concentrations than metronidazole and has a plasma elimination half-life of 1.01 hour which is significantly shorter than the corresponding metronidazole half-life of 3.62 hour 2 Journal of Pharmaceutics [4]. Also, Satranidazole is having better tolerability, absence of neurological, and disulfiram-like reactions and it can be preferred in patients with susceptible neurological symptoms [5].
The present study aims for the development, optimization and evaluation of Satranidazole taste masked granular formulation using different types of low melting point wax with Eudragit EPO as a functional polymer by melt granulation technique.

Materials.
Satranidazole was obtained as a gift sample from Alkem Laboratories, Mumbai. Eudragit EPO was obtained from Evonik Degussa, Mumbai. Satrogyl tablets (strength: 300 mg) were purchased from local market. All the chemicals and reagents used were of analytical grade.

Preformulation
Studies. UV and FTIR spectrum of Satranidazole were taken to confirm the identity of drug. Compatibility study between the drug and the excipients was done using FTIR. IR spectra were recorded in Fourier Transform Infrared spectrophotometer (Shimadzu Corporation) with KBr pellets.

Development and Optimization of Granular Formulation.
In preliminary study, three different trial batches of granules were prepared using glyceryl monostearate, stearic acid, and cetyl alcohol. The purpose of these trials was to select the wax suitable for taste masking of Satranidazole in low concentration since the dose of the drug is high. The required quantity of wax was weighed and melted in porcelain dish. Eudragit EPO was dissolved in molten wax and drug was added followed by mixing. At slightly lower temperature all the other ingredients were added and mixed. Finally the mixture was allowed to cool slightly and the solidified mass was passed through 12 # sieve and then through 16 #. The granules obtained were then lubricated with flavorant, sweetener and lubricant. The composition of granules is shown in Table 1.
Different batches of granules were prepared using wax selected in above trials to optimize drug to ratio for successful taste masking of STZ. The composition of the formulated batches is as shown in Table 2.

Evaluation of Granules.
The prepared batches were evaluated for the following parameters.

Flow Properties.
The granules were evaluated for following flow properties using following parameters [6,7].

Angle of Repose.
Fixed funnel method was used to determine angle of repose and was calculated using following equation: where ℎ is height and is radius.

Bulk Density and Tapped Density.
To measure density, the granules were filled in a 100 mL capacity measuring cylinder up to at least 3/4th the height. Bulk density is the quotient of weight to the volume of the sample. Tapped density is the quotient of weight of the sample to the volume after taping a measuring cylinder 500 times from a height of ∼1.5 in.

Hausner Ratio and Carr's Index.
Hausner's ratio was calculated using the following formula: Hausner's Ratio = Tapped Density Bulk Density .
The percentage compressibility (Carr's index) was calculated as 100 times the ratio of the difference between tapped density and bulk density to the tapped density: Carr's index = (Tapped Density − Bulk Density) Tapped Density × 100. (3)

Granular
Friability. Ten grams of uncoated granules was subjected to friabilator at 25 rpm. After 4 minutes, the granules were sieved on a 200 mesh. The amount of granules passed through 200 mesh was calculated as percentage granular friability.

Particle Size Distribution.
Particle size distribution was performed on optimized batch using nest of standard sieves (20 #, 40 #, 60 #, 100 #, and 120 #). The sieves were agitated mechanically for 10 minutes on a sieve shaker and the weight of granules retained on each smaller sieve was noted.

Scanning Electron Microscopy (SEM).
The surface morphology of uncoated and coated granules was examined using a scanning electron microscope (Zeiss Ultra Plus-FESEM). The samples of granules were previously sputter-coated with gold.

In Vitro Dissolution Studies in 0.1 N Hydrochloric Acid (HCl).
In vitro dissolution studies were carried out using USP II Apparatus (Paddle Method) rotating at 75 rpm in 900 mL of 0.1 N HCl as dissolution media maintained at 37 ± 0.5 ∘ C. Sampling was done at different time intervals of 10, 15, 30, 45, and 60 minutes by withdrawing 5 mL of the dissolution medium and replacing it with the same amount of medium to maintain sink conditions. The withdrawn samples were filtered and the contents of sample were determined spectrophotometrically at 320 nm. The dissolution of marketed tablet formulation (Satrogyl Tablet, 300 mg) was also carried using the similar dissolution conditions. The Percent cumulative release of drug was calculated using standard calibration curve of STZ prepared in 0.1 N HCl.

Determination of Drug Content (Percent Assay) Using
HPLC. The assay of different batches of granules of Satranidazole with stearic acid was carried out using previously developed and validated HPLC method. Isocratic elution at a flow rate of 1.0 mL/min was employed on BDS Hypersil C18 L u b r i c a n t In vitro taste evaluation by UV ( g/mL) 37.81 23.33 33.14 - * All the batches showed less than threshold value in in vitro taste evaluation by UV.

In Vitro Evaluation of Bitter Taste of Granules.
Granules equivalent to 25 mg of STZ were placed in a volumetric flask with 50 mL of phosphate buffer (pH 6.8) and stirred for 5 minutes. The mixture was filtered, and the filtrate was analyzed for STZ concentration at 320 nm by UV-Visible spectrophotometer and that was compared with the threshold value [11].

In Vivo Taste Evaluation.
Two procedures were used for in vivo taste evaluation.
(1) Gustatory sensation taste: informed consent was first obtained from 8 healthy volunteers and taste evaluation study was carried out. The whole dose was added to 100 mL of water for 15 seconds. STZ was used as control. After 15 seconds, 1 mL of dispersion was held in the mouth of each volunteer for 30 seconds and then spat out [8]. The bitterness level was recorded by using the numerical scale shown in Table 3.
(2) Granules equivalent to 50 mg STZ were held in mouth of each volunteer for 30 seconds. After expectoration, bitterness level was recorded by using the numerical scale shown in Table 3.

Stability Studies.
The optimized granular formulation was subjected for stability study for one month according to ICH guidelines. Tests were conducted at room temperature (RT) and accelerated stability conditions. The samples were designated as time 0 and 1 month for RT and 0 and 1 month for accelerated studies. Samples designed for RT storage were kept at 25 ± 2 ∘ C and 60 ± 5% relative humidity (RH). The samples in the accelerated stability study were kept at 40 ± 2 ∘ C and 75 ± 5% RH in humidity chamber. Samples were tested for its appearance, flow properties, taste masking, in vitro release, and drug content using the previously described procedures.

Results and Discussion
The UV spectrum of STZ in methanol is shown in Figure 1.
The max of Satranidazole sample was found to be 320 nm. STZ exhibited characteristic peaks at 1687, 1743, 1066, 1537, and 1215 cm −1 attributed to C=N stretching, C=O stretching, S=O stretching, C-NO 2 stretching, and C-N vibrations. FTIR spectra showed all the important peaks of drug in spectra of drug-excipient mixture and formulation. FTIR of STZ, physical mixture of STZ with Eudragit EPO and stearic  acid, and the optimized formulation is represented in Figures  2, 3, 4, and 5, respectively. All the excipients were selected based on preformulation study results and extensive literature survey. Low melting point wax was used to form dense coating around the drug particle to mask the taste. Eudragit EPO was used as a functional polymer which is insoluble above pH 5 and thus prevents release of STZ in mouth. Preliminary trials were taken for the selection of wax. The ratio of the drug to wax was selected based on literature survey [2,3]. Initially, three different granular formulations were prepared using three different types of wax (glyceryl monostearate, stearic acid, and cetyl alcohol) individually in drug to wax ratio 1 : 2. In preliminary trials, it was found that granules of STZ formed with stearic acid (Trial 2) were better taste masked. The granules formed with glyceryl monostearate and cetyl alcohol in the same ratio with drug were bitter in taste. Hence, the ratio of STZ to stearic acid was varied from 1 : 1 to 1 : 2.5 in further optimization and formed batches were evaluated for taste masking and other parameters such as flow property, drug content (assay), and in vitro drug release.
The results of bulk density, tapped density, Carr's index, Hausner ratio, and angle of repose are summarized in Table 4. The results indicated that the prepared granules possess good flow property [12].
The taste recognition threshold of STZ was determined on the basis of Table 5. The threshold was found to be 28 g/mL.
Under in vitro taste masking evaluation study, the drug release in pH 6.8 phosphate buffer was studied. It was observed that the granules formed with glyceryl monostearate were showing more release than threshold concentration in 6.8 phosphate buffer and were less hard and the granules formed with cetyl alcohol were found to be very 1.14 ± 0.011 1.12 ± 0.009 1.1153 ± 0.01 1.1320 ± 0.01 * All above readings are average ± standard deviation, = 6.   hard as cetyl alcohol used to quickly solidifies on melting and therefore the drug was not completely coated with it and showed more release than threshold concentration in 6.8 phosphate buffer. Results of drug release from granules of batches F1 and F2 were found to be more than the threshold value, that is, 28 g/mL. The drug release from granules of batches F3 and F4 was found to be less than the threshold value which indicated successful taste masking of granules. Gustatory sensation test results obtained from 8 healthy volunteers also indicated that the granules of batches F3 and F4 were better taste masked. Table 6 indicates the in vivo and in vitro taste masking evaluation results of the granular formulations F1 to F4.
Typical HPLC chromatogram of test solution is represented in Figure 6. The STZ peak was eluted at the retention time of about 4.32 minutes. The percentage of drug content for different batches (F1-F4) was found to be in the range of 98-102%.
The results of in vitro dissolution studies and drug content (assay) of different batches (F1-F4) are summarized in Table 7. It was found that as the amount of stearic acid increases in the granules and the % drug release in 0.1 N HCl   Journal of Pharmaceutics    Figure 7. The Granular friability of granules of optimized batch (F3) was found to be 0.15%. The size of the granules was found in the range of 341.93 m-487.5 m. SEM photographs of drug and granules are as shown in Figures 8 and 9, respectively. It was found that the surface of the drug particle was smooth and that of the granules was rough.
Stability study results indicated that there was no change in physical appearance of granules and taste at room temperature as well as accelerated conditions. The results of percent dissolution, assay, and flow properties of the optimized batch F3 at RT and accelerated stability conditions are summarized in Table 8. The optimized formulation was found stable during stability study.

Conclusion
Taste masking of bitter drugs with higher dose is challenging. STZ is having a dose of 300 mg twice daily in amoebiasis  and is available in market in the form of film coated tablet. Difficulty in swallowing tablets is a major problem especially in case of geriatrics and pediatrics as well as patients who are not able to swallow tablets. In the above technique, low melting point wax was used for dissolving Eudragit EPO instead of organic solvent which led to the formation of dense coating around the drug and also step of removal of organic solvent by carrying out drying step was not required by using wax. Granules formed by coating with Eudragit EPO showed similar release of STZ like that of marketed tablet. From the above results, it can be concluded that complete taste masking of bitter STZ is achieved by granulating drug with stearic acid and Eudragit E100 by melt granulation technique. The efficiency of stearic acid was tested for the purpose of taste masking with the possible method of melt granulation. Taste masking by the above technique is achieved by decreasing the surface area of the drug by increasing its particle size and the method was found to be very simple.