INTRODUCTION:

Diabetes has reached epidemic proportions in all over world. Type 2 diabetes is a progressive and chronic condition and requires continued monitoring by a patient and physician, and in addition to diet and exercise, a patient may need to take multiple medications at any time in order to help maintain glucose control. During long therapy it is observed a high secondary failure occurs with monotherapy treatment leading to poor glycemic control. Hence it is helpful to make use of two or more drug therapy. Now-a-days bilayer tablets were tried for dual component delivery for different drugs to modulate the drug release which is efficient in managing drug therapy and patient compliance. In such cases a steady state blood level which is therapeutically effective and non toxic for an extended period of time is preferred¹.

Received on 03.09.2014 Modified on 12.09.2014

Research J. Pharm. and Tech. 8(1): Jan. 2015; Page 06-12

DOI: 10.5958/0974-360X.2015.00002.5

Bilayered tablets are tablets containing subunits that may be either the same (homogeneous) or different (heterogeneous). Bilayer tablets are prepared with one layer of drug for immediate release and second layer designed to release the drug as a second dose or for extended release. These tablets are suitable for sequential release of two drugs in combination, separate two incompatible substances and when the release profiles of the two drugs are different from one another^2,9.

Combination therapy has various advantages over monotherapy such as problem of dose-dependent side effects are minimized. A low dose combination of two different agent reduces the dose related risk, the addition of agent may counteract some deleterious effects of the other, using low dosage of two different agents minimize the clinical and metabolic effects that occur with maximal dose of individual component of the combined tablet.³

The preferred treatment option in the treatment of chronic diseases i.e. hypertension, diabetes and allergic rhinitis is combination therapy in the form of bilayer tablets. The advantages include minimization of side effects, and a reduction of dose related risk. Using low dosage of two different agents minimizes the clinical and metabolic effects that may occur with higher doses of individual components of the combined tablet².

MET is an oral antidiabetic drug in the biguanide class. It is the first line drug of choice for the treatment of type 2 diabetes. The absorption of antidiabetic agent, MET in humans is incomplete and the drug is excreted mainly in urine with a half life of 4 to 6 hrs⁴ .MET is protonated under physiological pH condition. Ionized MET is absorbed into the negatively charged intestinal epithelium. The absorption window is predominantly in the small intestine and colonic absorption in healthy subject is poor .A conventional oral sustained release formulations releases the drug through the small intestine and colon. However, the drug release after the small intestine would be of no therapeutic value and conventional strategy of prolonging the MET release from the dosage form through GI tract will not be effective. The lack of gastroretentive (GRT) tendency of controlled released oral formulation would result in the displacement of the dosage form from the site of absorption and erratic absorption as drug passes on colon. Floating systems have bulk density lower than that of the gastric fluid, and thus remain buoyant in stomach and released slowly at a desired rate. This results in an increase in the GRT and a better control of fluctuations in the plasma drug concentration⁴.Glimepiride is one of the third generation sulfonylureas used for treatment of type 2 diabetes.

MATERIALS AND METHODS:

MET and GLP were gift sample obtained from Flamingo Pharma. (Mumbai, India).

Preparation of Bilayer and Floating Tablet:

Bilayer tablets containing two layers i.e. floating layer of MET and immediate release layer of GLP. MET, HPMC K100 M, HPMCK4M were passed through a sieve (40#) and mixed well in mortar. Granules of the floating layer were prepared using a 10 % (w/v) PVP in isopropyl alcohol solution by mixing all ingredients mentioned in table except lubricants. After lubrication weighed quantities of floating layer were subjected to mild compression. Weighed powder of immediate release layer of GLP was added to the compressed layer and both the layers were then compressed in capsule shape die. (D –Tooling) [Table 1]

Compression of bilayer tablets:

The prepared granules of both the layers were compressed using 18.00× 7.00mm, ‘D’ tooling standard concave, flat faced modified capsule shaped punch. Single punch machine was used to make bi-layer tablets. Hardness was kept between 4-5 kp. The bottom layer (MET) was first compressed with lower pressure, which was then followed by filling of the die cavity by the upper layer GLP blend. The final compression was done only after both the granules occupied the die cavity one on top of the other. Both the layers were identified on the basis of colour since the immediate release layer had pink colour and the sustained release layer had white colour.[ Table 2]

Formulation of bilayer tablet:

Table 1: Formulation of gastroretentive MET layer

S.N.	Name of Ingredient	Qty. per tablet in mg
S.N.	Name of Ingredient	B1	B2	B3	B4	B5	B6	B7	B8	B9
1	MET	500	500	500	500	500	500	500	500	500
2	HPMC K100 M	50	50	50	50	50	50	50	50	50
3	HPMCK4M	50	60	75	80	90	100	125	150	160
4	Carbopol 934 P	30	30	30	30	30	30	30	30	30
5	Micro crystalline cellulose	20	20	20	20	20	20	-	2	10
6	Sodium bicarbonate	100	50	100	75	50	25	75	50	25
7	Citric acid	50	50	50	50	50	50	50	50	50
8	Sodium Alginate	-	-	20	-	-	-	25	-	-
9	PVP K	10	10	10	10	10	10	10	10	10
10	Magnesium stearate	3	3	3	3	3	3	3	3	3
11	Talc	2	2	2	2	2	2	2	2	2
12	Aerosil	3	3	3	3	3	3	5	3	3
13	Lactose	30	72	15	27	52	107	-	-	37
Av. Wt (mg)		850	850	850	850	850	898	850	850	850

Table 2: formulation of GLP layer

Ingredient	Qty. per tablet in mg.
GLP	1
Sodium starch glycolate	7.5
MCC	61
Lactose	58
Colour	1.25
Magnesium Stearate	1.25
Av.Wt.	130

Compatibility Study:

Fourier Transform Infrared Spectroscopy:

The Fourier transform infrared (FT-IR) spectra of samples were obtained using FT-IR spectrophotometer (Shimadzu 84005 Spectrophotometer). About 2–3 mg of samples was mixed with dried potassium bromide of equal weight and compressed to form a KBr disc. The samples were scanned from 400 to 4,000 cm^-1wave number. [Fig.3,Table 4,5]

Differential Scanning Calorimetry:

Differential scanning calorimetry (DSC) experiments were also carried out to characterize the physical state of MET in bilayer floating tablets as well as to find out the presence of any interaction among drug and the excipients. The heating rate was 10°C/min; nitrogen served as purging gas and the system was cooled down by liquid nitrogen. The differential thermal analyzer (Seiko SII DSC 6220) was used for this purpose.

Evaluation of Granules:

Angle of repose:

Fixed funnel method was used to measure the flow properties where the granules were poured from funnel walls to form conical heap in which its lower tip is 2-5 cm away from the hard surface. Static angle of repose was measured by using the formula ,

θ = tan^-1(h/r)

Where, h – height of the heap, r – radius of heap

Bulk and Tapped Density:

Blend was sieved to ensure free from agglomeration and was introduced into a calibrated measuring cylinder. The initial volume was observed and then the cylinder was allowed to tap onto a hard surface from 2.5 cm height at 2” intervals. The tapping was continued to get saturated volume. From the above values, both poured bulk density and tapped density were determined.

Hausner's ratio and Compressibility Index:

Hausner's found that the ratio of tapped volume and poured volume was related to its inter particle friction and can be used as a direct tool for flow property evaluation. Compressibility index was determined by using the formula,

Compressibility index = [(D_F – D_O)/D_F] × 100

Where, D_O – Initial density, D_F – Final Density

Characterization of Tablets:

The prepared floating tablets were evaluated for thickness, diameter, hardness, friability, uniformity of weight and drug content. The thickness and diameter of tablets were measured by vernier caliper. Hardness of tablets was tested using Monsanto hardness tester. Friability of tablets was determined by using Friability test apparatus. The drug content in each formulation was determined by taking 20 tablets from each batch which were weighed and powdered^5,10.[ Table 8]

The drug content of the tablets was determined using distilled water as a solvent, and the samples were analyzed spectrophotometrically (JASCO, V-530, Japan) at 235 nm for MET layer. For GLP layer HPLC analysis was done by using Mobile Phase -Phosphate buffer pH 2.5 –Acetonitrile (40: 60)

Buoyancy Lag-Time Studies:

The buoyancy lag-time of the tablets was studied at 37 ± 0.5°C, in 100 ml 0.1 mol/l HCl (pH 1.2). The time required for the tablet to rise to the surface and float was taken as the buoyancy lag-time.[ Fig.1]

Dissolution Studies:

MET Floating layer dissolution

The release rate of MET from floating tablets was determined using USP dissolution testing apparatus II (Paddle type). The dissolution test was performed using 900 ml distilled water, at 37 ± 0.5°C and 75 rpm. A sample (10 ml) of the solution was withdrawn from the dissolution apparatus hourly for 12 h, and the samples were replaced with fresh dissolution medium. The samples were passed through Whatman filter paper and the absorbance of these solutions was measured at 235 nm. The cumulative percentage drug release was calculated.

Dissolution apparatus: USP type II (Paddle type)

Dissolution test medium: 900 ml of distilled water

Temperature: 37± 0.5°C

Rpm: 75

Time interval (h): 0.5, 1, 2, 4, 6, 8, 12

Aliquot: 10 ml

GLP Dissolution:

The release rate of GLP from floating tablets was determined using USP dissolution testing apparatus II (Paddle type). The dissolution test was performed using 900 ml Phosphate buffer pH 2.5, at 37 ± 0.5°C and 75 rpm. A sample (10 ml) of the solution was withdrawn from the dissolution apparatus at specific intervals of 0.5, 1, 2, 4, 6, 8, 10 min and same volume was replaced with dissolution medium. The samples were filtered through Whatman filter paper .Sample was analysed by HPLC. Percent cumulative drug release was plotted against time in minutes to obtain dissolution profile.

Mobile Phase- Acetonitrile: Phosphate Buffer pH 2.5 (60:40)

Flow rate - 1.2ml /min

Injection volume - 20ml

Detection - UV detector at 226 nm

Column- HiQSilC18HS(4.6 mm µ X 250 mm)

Swelling Characteristics:

The swelling properties of HPMC matrices containing drug were determined by placing the tablet matrices in the dissolution test apparatus, in 900 ml distilled water at 37 ± 0.5°C. The tablets were removed periodically from the dissolution medium and, after removing free water, the weight gain was measured. The swelling characteristics were expressed in terms of the percentage water uptake (WU%) according to the equation [ Fig. 10]

Floating Capability:

Tablets were placed in a 400-ml flask at pH 1.2, and both the time needed to go upward and float on the surface of the fluid and the floating durations were determined.⁷

Optimization of Formulation by Factorial Design:

The tablet formulated was initially optimized for the levels of polymer and gas generating agent. A 3² randomized full factorial design was used in this study. Two factors were evaluated, each at 3 levels, and experimental trials were performed on all 9 possible combinations (Table 2). The amount of HPMC K4M (X1) and sodium bicarbonate (X2) were selected as independent variables. In vitro release data of MET from bilayer tablets of optimized formulations were subjected to the analysis of variance (ANOVA) at three concentrations of polymer and gas generating level.

MET Floating Layer Optimization:

Regression polynomials for the individual dependant variables (% drug release from 12 hrs and floating lag time from the dose) were calculated with the help of Design Expert 9.00 software and applied to approximate the response surface and contour plots.

3²Full Factorial Design Layout:

A 3² randomized full factorial design was utilized in the present study. In this design two factors were evaluated, each at three levels, and experimental trials were carried out at all nine possible combinations. The design layout and coded value of independent factor is shown in Table 3. The factors were selected based on preliminary study. The concentration of HPMC K4M (X1) and concentration of sodium bicarbonate (X2) were selected as independent variables⁸.Overlayand 3D surface plots are displayed in fig.5, 6, 7.

Table 3.Details of selected dependent variables

Translation of coded levels in actual units
Variable level	Low (-1)	Medium (0)	High (+1)
X₁	50	125	175
X₂	50	100	150

(c)

Table 4: Major IR peaks of MET

Band Frequency (cm^-1)	Chemical moiety
3369	N-H primary stretching vibration
3294	N-H secondary stretching
1626 cm^-1and 1567 cm^-1.	C = N stretching

Table 5: Major IR peaks of GLP

Band Frequency (cm^-1)	Chemical moiety
3373.7	N-H vibration
2934.57 and 2855.22	C-H vibration
2789.04 and 2706.85	o-H vibration
1529.59,1462.94,1346.73	N= O vibration
1025.67	C-N
1157.94 and 1123.29	C-O vibration

Table 6: Physical properties of blend ready for compression

Test	Results
Untapped bulk density	0.532+ 0.05
Tapped bulk density	0.634 + 0.05
% Compressibilty	30.00+ 0.5%
Angle of repose	12.34+ 0.5⁰
Flow rate (gm/sec)	1

Time in h.

Fig.4: Dissolution profile for optimized batches

Table 7: Diffusion kinetics model fitting data of bilayer tablet.

Model	Equation	R²
Zero order	Y=6.048 X +12.25	0.866
First order	Y=0.046+1.934	0.936
Korsmeyer -Peppas	Y=0.639+1.315	0.982
Higuchi Square Root	Y=33.38X -15.76	0.983
Hixon crowell cube root	Y=0.166X +5.050	0.973

Observation:

The in vitro drug release profile was subjected to various kinetic models in order to find out the mechanism of drug release. The best fit with the highest coefficient of regression was seen with Higuchi model. The rate constants were calculated from the slope of the respective plots. Drug release followed Higuchi kinetic model which describes the diffusion controlled drugs release.

Fig.5: Overlay plot for floating lag time and % release

Fig. 6: 3D Surface response plot for optimization

Fig.7: 3D Surface response plot for optimization of % release

From figure 6 we can conclude that HPMC K4M 175mg and Sodium bicarbonate upto 150 mg gives floating lag time less than 1 min. We can conclude that HPMC K 4M upto 175 mg and sodium bicarbonate upto 150 mg gives t_80%in the range of 10 to 11 hrs.

Fig. (8a): SEM image of GLP immediate release layer

The DSC profiles of pure components MET and GLP binary systems in melting range of the drug and carriers dehydration are shown in Fig. 2. The thermogram of MET was typical, characterized by a sharp endothermic peak at 235.8^o C which corresponded to its melting. The DSC thermogram of GLP showed a sharp endothermic peak at 211.1^o C. The endothermic peaks of drugs were retained in DSC of drug and excipients. No significant Shifts of reduction in intensity of the FTIR bands of MET & GLP were observed. MET bilayer tablet prepared by direct compression using HPMCK100M and HPMC K15M showed floating time less than 1 min but hardness was less and friability was more than 1% and total floating time was 3 to 4 hrs. Tablets prepared by wet granulation with HPMCK100 M,HPMC K 15M and showed total floating time more than 24 hrs. Floating lag time was 0.75 to 5 min. Optimized tablets showed total floating time more than 24 hrs and floating lag time less than a min.

The image of the tablet taken after dissolution showed a network of channels indicating diffusion of dissolution medium into the tablet, thereby by hinting towards diffusion controlled mechanism of drug release.

Fig.10: The comparative swelling index for the formulations F3, F4, F6, F8, F9

Table 8: Evaluation of optimized batches

Batch Code	Weight Variation Mean± SD	Hardness Kg/ cm²	Friability (%)	Drug content (%)
Batch Code	Weight Variation Mean± SD	Hardness Kg/ cm²	Friability (%)	GLP	MET
F1	982.3+0.23	4+ 0.4	0.443+0.123	99.71	99.74
F2	978.4+0.16	4.5+ 0.3	0.321+ 0.032	101.32	100.34
F3	981.5+ 0 .15	4.4+0.5	0.356+0.143	100.05	100.43
F4	984.4+ 0 .45	4+ 0.4	0.245+ 0.132	98.32	99.34
F5	981.2+ 0.122	4.3+ 0.5	0.325+ 0.124	99.54	101.32
F6	976.2+ 0.37	4.4+0.5	0.315+ 0.045	101.43	101.42
F7	984.5+ 0.46	4.3+ .5	0.345+ 0.042	102.32	99.42
F8	982.3+ 0.23	4+ 0.4	0.243+ 0.074	101.34	99.43
F9	985.6+ 0.57	4.5+ 0.3	0.420+ 0.324	101.23	101.24

CONCLUSION:

This study discusses the preparation and evaluation of gastroretentive bilayer tablets of MET and GLP. The effervescent based floating drug delivery was a promising approach to achieve in vitro buoyancy. The addition of gel forming polymer HPMC K4M and gas generating agent sodium bicarbonate was essential to achieve in vitro buoyancy. Stable and persistent buoyancy was achieved by trapping the gas in the gel formed by the hydration of HPMC K4M.Tablets containing HPMC K4M showed satisfactory buoyancy characteristics and longer floatation time. The drug release from the tablets depends upon the nature of gel matrix. It was observed that polymer swelling play an important role in drug release from the floating tablets. Hence it can be concluded that the effervescent based floating drug delivery is a promising approach to achieve buoyancy.

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