INTRODUCTION:

Formulation of in-lay tablet has been used in the recent years for the development of controlled release formulation to obtain the desired results. The in-lay tablet is the type of multiple compression tablets and is similar to layered tablets. It is formulated by core tablet, which is surrounded by outer layer of drug. During the preparation, the bottom of the die cavity is filled with outer layer material and core is placed upon it. By applying the compression force, the outer layer material forms the strongly compressed layer. ^{[1, 2]} Simple lines diagram for the preparation of in-lay tablets its top view and cross section is represented in Figure 1. In-lay tablet is suitable for predetermined release of two drugs in combination in which one core is formulated as sustained release and second core is designed as immediate release.

The drugs are dispersed uniformly in their respective parts. After ingestion, matrixed tablets undergo hydration in contact with water. The polymeric matrix absorbs the water and swells. Finally the drug diffusion occurs by erosion of outer surface of tablet matrix. Telmisartan is water insoluble antihypertensive drug used in the treatment of hypertension and belongs to a group of angiotensin converting enzyme (ACE) inhibitors. Telmisartan has the longest biological half life (24 h) compared to any other antihypertensive drug and once in a day dose are the two basic principles for making its sustained release dosage form. ^[3] Hydrochlorothiazide is a diuretic drug used worldwide for lowering the blood pressure individually and in the combination of antihypertensive drugs.^[4] The combination provides greater blood pressure reduction and safety as compared to other combination of telmisartan and/or valsartan with hydrochlorothiazide and also monotherapy of individual drugs.^[5]

The literature survey revealed that the design and evaluation of fast release tablet of telmisartan by applying direct compression method was reported for in-vitro drug release up to 8 h.^[6] Development of extended release formulation of telmisartan using materials such as cyclodextrin, aminoclay has also been reported by complexation method. ^[7] Formulation and in-vitro evaluation of fast disintegration tablets of hydrochlorothiazide were reported by various techniques.^[8-10] Formulation and evaluation of bilayer tablets containing telmisartan and hydrochlorothiazide, both as immediate release (IR) form using wet granulation method has also been reported.^[11-14] Formulating telmisartan as IR is not advantageous due to fast disintegration. Therefore to obtain the desired therapeutic effectiveness of telmisartan SR formulation is essential for extended time period. The objective of the present research work is to formulate and evaluate in-lay matrix tablets containing telmisartan as sustained release (SR) outer core and hydrochlorothiazide as immediate release (IR) from inner core by wet granulation method. The developed tablets will provide and maintain the effective concentration of the drugs in the body at the theoretically determined release rate.

Figure 1: Simple lines diagram for the preparation of in-lay tablets, its top view and cross section.

MATERIALS AND METHODS:

Materials

Telmisatan and Hydrochlorothiazide were kindly supplied as gift sample by Systopic Pharmaceuticals Ltd. (New Delhi, India). HPMC K100, HPMC K100M, Carbopol 71G, polyvinyl pyrrolidone (PVP), sodium starch glycollate (SSG), and magnesium stearate was purchased from S.D. Fine Chemicals Ltd., Mumbai, India. HPLC grade water, acetonitrile, methanol, and potassium dihydrogen phosphate were purchased from Fluka analytical, Sigma-Aldrich Corporation, St. Louis, MO, USA). All other chemicals and reagents were of analytical grade.

Methods

Preparation of In-lay Tablets

Wet granulation method was applied for preparation of granules for tablets. Both the inner and outer cores of in-lay tablets were prepared separately by applying the following methods

Granulation for the SR Core

The SR cores were prepared by mixing the ingredients in different compositions as presented in Table 1. HPMC K100M and co-polymer carbopol 71G at different ratios were blended with telmisartan in a planetary mixer for 5 min. The granules were prepared from the mixed powders using sieve of mesh 20 screen, and dried at 30 ºC for 1 h. After drying the granules were again sized by a mesh 20 screen and mixed with magnesium stearate for 2 min. The final weight of each trial formulation was kept at 150 mg.

Table 1: Composition of various trial formulations for SR core containing 80 mg telmisartan

Ingredients (mg)	Formulation Code
Ingredients (mg)	F1	F2	F3	F4
Telmisartan	80	80	80	80
HPMC K 100M	50	40	30	25
Carbopol 71G	-	10	20	25
Polyvinyl Pyrrolidone	10	10	10	10
Magnesium Stearate	10	10	10	10
Total weight	150	150	150	150

Granulation for the IR Core

The IR cores were prepared by mixing the ingredients in different compositions as presented in Table 2. HPMC K100, PVP and SSG at different ratios were blended with hydrochlorothiazide in a planetary mixer for 5 min. The granules were prepared from the mixed powders using sieve of mesh 20 screen, and dried at 30 ºC for 1 h. After drying the granules were again sized by a mesh 20 screen and mixed with magnesium stearate for 2 min. Final weight of each trial formulation was kept at 75 mg.

Table 2: Composition of various trial formulations for IR core containing 25 mg hydrochlorothiazide

Ingredients (mg)	Formulation Code
Ingredients (mg)	F1	F2	F3	F4
Hydrochlorothiazide	25	25	25	25
HPMC K100	25	20	15	10
Sodium Starch Glycollate	5	10	15	20
Polyvinyl Pyrrolidone	10	10	10	10
Magnesium Stearate	10	10	10	10
Total weight	75	75	75	75

Compression of In-lay Tablets

In-lay tablets were prepared by compressing the granules to form inner core tablet and then granules of outer core compressed to form bigger tablet. The granules of hydrochlorothiazide (IR Core) were precompressed and then placed along with the granules of telmisartan (SR Core) for final compression for in-lay tablets using 23-station compression tablet punching machine (Cadmach, Mumbai, India). The total weight of single tablet was 225 mg to an average hardness of 7 Kg/cm². Finally the formulation codes for tablets were named as F1, F2, F3, F4, where composition of F1 is SR1 for the SR core and IR1 for the IR core.

Evaluation of Physical Properties of Tablets

The various quality control tests were carried out for the prepared tablets such as weight variation, friability, Hardness, and thickness test. Evaluation of weight variation of the tablets was conducted on 20 tablets using an electronic balance. Friability of the tablets was determined by Roche friabilator by taking 10 tablets. The hardness of tablets was tested on 10 tablets by using Mosanto hardness tester. The thickness of tablets was measured by Vernier caliper on 10 tablets.

Drug Content Studies

Twenty tablets were weighed accurately and powdered. Powder equivalent to 80 mg of telmisartan and 25 mg of hydrochlorothiazide was taken and transferred to a 50 mL volumetric flask. The powder was dissolved with approximately 25 mL of methanol and ultrasonicated for 10 min. The final volume was made up with methanol. This solution was filtered through a 0.45 mm nylon membrane filter to remove all the excipients. The resultant filtrate was further diluted with methanol: water (50:50, v/v) to obtain the required concentration of both the drugs. The solutions were filtered through a 0.20 mm nylon syringe filter and injected in to the HPLC-UV system for analysis. The amount of telmisartan and hydrochlorothiazide in tablets were determined by calibration equations obtained from the respective calibration curve.

HPLC-UV Conditions

HPLC was performed with a Shimadzu HPLC system (Shimadzu, Japan) equipped with a binary solvent manager and UV detector. The chromatographic separation was achieved on HPLC C₁₈(100.0 × 2.1 mm, 5 µm) column using isocratic mobile phase consisting of acetonitrile-phosphate buffer (50:50, v/v) at a flow rate of 1.0 mL/min. The UV detection was carried out at 295 nm for both the drugs. The 100 µL of sample solution was injected in each run. The total chromatographic run time was 10 min.

Validation of the Method

The method was validated according to ICH validation guidelines.^[15] The parameters addressed were linearity and range, limit of detection and quantitation, precision, and accuracy. Different standard concentrations each of telmisartan and hydrochlorothiazide in the range of 1-1000 µg/mL were prepared separately in methanol: water (50:50, v/v). The solutions were filtered through a 0.20 mm nylon syringe filter and injected in to the HPLC-UV system for analysis. Average peak area at each concentration level was subjected to linear regression analysis with the least squares method. Linearity was described by slope, intercept, and correlation coefficient obtained from regression equations.

Drug Release Studies

Tablets were subjected to in-vitro drug release studies using USP paddle apparatus, Veego VDR-8DR (Veego Instruments, Mumbai, India). The hydrochloric acid of pH 1.2 (0.1N) was used as dissolution medium maintained at 37 ºC. The rotation speed of the paddles was 100 rpm. A single tablet was added to the dissolution medium in each vessel. 5 mL of samples were taken at different time intervals initially every 10 min and continue for every hour up to 20 h. An equivalent amount of fresh medium was added to the dissolution medium. The samples were filtered through 0.45 mm nylon membrane filter to remove all the excipients. The filtrates were further diluted with methanol: water (50:50, v/v) and again filtered through 0.20 mm nylon syringe filter and injected in to the HPLC system for analysis. The release profile of drugs from tablets in terms of percentage was calculated by the amount of drugs dissolved.

Theoretical Release Profile of Telmisartan from Tablets

The Rawlins equation was applied in the calculation of total dose of telmisartan for a once daily SR formulation ^[16] by taking the available pharmacokinetic data on telmisartan. ^{[6, 7]}

D_Total = Dose_IR (1+0.693×t/t_1/2)

Where, D_Total = Total dose of drug, Dose_IR = Dose of IR core, t = Time (h) during which the SR is desired (20 h), and t_1/2= Half life of drug (24 h)

Dose_IR (1+0.693×20/24) = 80; Dose_IR= 50.71 mg

Hence the formulation should release 50.71 mg in 1 h and 1.57 mg per h up to 20 h thereafter.

Drug Release Kinetics

Higuchi’s model of drug diffusion was applied to understand the mechanism of telmisartan release from SR core of tablets, which is given by equation (Q = k_Ht^1/2). Where, Q is the amount of the drug release at time t, and k_H is the Higuchi’s diffusion constant. A graph was plotted between amount of drug released and square root of time. If the release of drug is diffusion controlled, then the obtained graph will be linear. ^[17,
18]

Stability Studies

The prepared tablets were subjected to stability studies as per ICH Q1A (R2) guidelines. ^[19] Tablets were stored in stability chambers for long-term condition at 30 ± 2°C/65 ± 5% RH (Relative humidity) and for accelerated condition at 40 ± 2°C/75 ± 5% RH for 3 months with testing frequency of every months. The tablets were evaluated for assay of drugs and presence of their related degradation products by applying the previously developed validated HPLC-UV method.

RESULTS AND DISCUSSION:

Physical Properties of Tablets

The effects of two polymers, HPMC and carbopol on the physical properties of tablets were studied. It was found that corbopol improve the physical properties such as tablet hardness and friability. The results of evaluation tests for different tablet formulations are given in Table 3. The obtained results were found similar according to the specification limits of United States Pharmacopoeia (USP, 2002). ^[20]

Validation of the Method

The retention time (R_t) was found to be 3.50 min for telmisartan, and 7.50 min for hydrochlorothiazide with the total chromatographic run time of 10 min for each compound, as shown in Figure 2. For telmisartan and hydrochlorothiazide calibration curves were linear over the concentration range of 1-1000 µg/mL with correlation coefficient was more than 0.999. The limit of detection of both the drugs was 1 µg/mL. The obtained results indicated that higher sensitivity of the method. The RSD less than 2% were obtained for both the drugs by evaluation of precision and accuracy. The results of linearity, LOD, and LOQ are presented in Table 4. Validation results showed that method was suitable for drug dissolution and drug content studies.

Table 4: Results obtained from Linearity, LOD, and LOQ

Parameters	Telmisartan	Hydrochlorothiazide
Linear range (µg/mL)	1-1000	1-1000
Slope	11.50	10.55
Intercept	248.45	130.95
Correlation coefficient^a	0.9997	0.9998
LOD (µg/mL)	0.1	0.2
LOQ (µg/mL)	1	1

^aMean of six replicates (n = 6).

Drug Release Studies

Dissolution samples were analyzed by HPLC-UV method described previously in the section drug release studies. Carbopol is a synthetic high molecular weight polymer of acrylic acid that is chemically cross-linked with allyl ethers of pentaerythritol. Each chain is containing carboxylic groups and connected with covalent bond and appeared as network of cross-linked structures. Chemically HPMC is a linear chain of hydroxyl groups. Due to this reason the carbopol is more hydrophilic and swellable than HPMC. The tablet matrix prepared with carbopol and HPMC K100M showed the slower rate of polymer hydration and extended drug effect. The HPMC K100 is comparatively less viscous and rapidly water absorbable HPMC polymer. After mixing with sodium starch glycollate (SSG) it leads to fast disintegration of hydrochlorothiazide and shows its immediate release.^{[21, 22]} From the conducted study it was found that the best sustained release effect of telmisartan can be achieved when it is formulated with equal amounts of HPMC and carbopol 71G rather than formulated individually. Carbopol is highly water absorbable polymer and it increases the water absorption in the HPMC network and produces the synergistic increase in the viscosity. The enhanced viscosity of whole matrix is occurred due to the hydrogen bonding between the carboxylic groups of carbopol and hydroxyl groups of HPMC, resulted in complex formation between two polymers leading to more sustained release of drug. ^[23,
24] The schematic representation of mechanism of drug diffusion through polymeric matrix is shown in Figure 3. Each polymeric particle (HPMC and Carbopol) are inter connected with cross-linking and capable of forming strong gel layer after absorption of water. The gel layer within the matrix system acted as barrier layer for drug diffusion. After complete absorption of water by matrix, the gel layer slowly degraded and at the same time diffusion of drug takes place.

Figure 3: The schematic representation of mechanism of drug diffusion through polymeric matrix.

The formulation F4 was found to release the telmisartan in sustained manner up to 20 h. The telmisartan in-vitro release profile from in-lay matrix tablets in F1, F2, F3, and F4 is shown in Figure 4. The study also shows that when the amount of SSG increases in the IR core, the disintegration time also increases. Hence the tablets from formulation 4 were provided the best immediate release effect of hydrochlorothiazide within 30 min. Hence formulation F4 was selected the optimum formulation and compared with the marketed combination tablets. The optimized tablets showed the better sustained release of telmisartan (99.75%) up to 20 h when compared to the marketed combination tablets (95.95%) for 1 h as shown in Figure 5. The theoretical release profile calculation is important to evaluate the formulation with respect to release rates and to ascertain whether it releases the drug in a predetermined manner. According to the theoretical release pattern, a once daily telmisartan SR tablet should release 50.71 mg in 1 h and 1.57 mg per h up to 20 h thereafter. Theoretically, telmisartan release should be 63.38% and 100% in 1 and 20 h, respectively. Drug release mechanism of SR telmisartan from in-lay tablet, the data was fitted to Higuchi’s equation. The kinetic parameters for telmisartan release from the tablets (F1 to F4) showed high linearity (R²: 0.994-0.999). Formulation F4 was found to release the telmisartan in sustained manner up to 20 h and hydrochlorothiazide as immediate release within 30 min. Hence formulation F4 was considered optimum for stability studies.

Stability Studies

The prepared tablets were subjected to stability study. The tablets were found physically stable and the content of both the drugs in tablets did not change significantly during storage compared to their initial values (p>0.05, ANOVA). Tablets did not show any degradation products during storage of stated conditions but under the stress testing both the drugs were degraded in pure form.

CONCLUSIONS:

In-lay FDC tablets containing telmisartan as sustained release and hydrochlorothiazide as immediate release was prepared by wet granulation method. The telmisartan sustained release was obtained by mixing it with hydrophilic polymer such as HPMC and carbopol 71G in different ratios. The hydrochlorothiazide is highly water soluble and its immediate release is achieved by addition of superdisintegrant such as sodium starch glycollate. It was observed that the use of carbopol along with HPMC improves the tablet properties and produce the additive effect on release of telmisartan. The drug release from prepared tablets was compared with marketed combination tablets. It was found that prepared tablets provided sustained release of telmisartan over a period of 20 h and immediate release of hydrochlorothiazide within 30 min.

ACKNOWLEDGEMENTS:

The authors are grateful to Systopic Pharmaceuticals Ltd., New Delhi, India, for providing gift samples of telmisartan and hydrochlorothiazide.

REFERENCES:

1. Santosh K, Jyoti M. Inlay tablets: A novel approach to drug delivery. Int. J. Adv. Res. Pharm. Biosci., 3(2); 2013:127-130.

2. Rajalakshmi R, Sireesha A, Lakshmi SM. Inlay tablets: A novel approach. Int. J. Adv. Pharm., 1(1); 2011:1-10.

3. Merck Index: An Encyclopedia of Chemicals, Drugs and Biologicals, 14^th edition, Merck Research Laboratories, Merck and Co., Inc., White House Station, NJ, USA, 2006; pp. 1569.

4. British Pharmacopoeia. HMSO: London, 2008; pp. 1036-1037.

5. Ruilope LM, Schumacher H. Telmisartan 80 mg/Hydrochlorothiazide 25 mg single-pill combination in the treatment of hypertension. Expert Opinion on Pharmacotherapy. 13; 2012:2417-2425.

6. Ashutoshkumar S, Arunachalam A, Karthikeyan M, Manidipa1 S, Ravishankar V, Senthilraj R. Design and evaluation of sustained release tablet of telmisartan. Int. J. Pharm. Sci., 8(1); 2010:595-603.

7. Sangwai M, Vavia P. Amorphous ternary cyclodextrin nanocomposites of telmisartan: Improved solubility and reduced pharmacokinetic variability. International Journal of Pharmaceutics. 453(2); 2013:423-432.

8. Rangole US, Kawtikwar PS, Sakarkar DM. Formulation and In-vitro evaluation of rapidly disintegrating tablets using hydrochlorothiazide as a model drug. Res. J. Pharm. Tech., 1; 2008:349-352.

9. Corveleyn S, Remon, JP. Formulation and production of rapidly disintegrating tablets by lyophilization using hydrochlorothiazide as a model drug. International Journal of Pharmaceutics. 152; 1997:215-225.

10. Corveleyn S, Remon JP. Bioavailability of hydrochlorothiazide: Conventional versus freeze dried tablets. International Journal of Pharmaceutics. 173; 1998:149-155.

11. Patel N, Natarajan R, Rajendran N, Rangapriya M. Formulation and evaluation of immediate release bilayer tablets of telmisartan and hydrochlorothiazide. Int. J. Pharm. Sci. Nanotech., 4; 2011:1477-1482.

12. Dandare MS, Sarage RD, Bhaskaran S. Bilayer tablet: A novel approach for immediate release of telmisartan and hydrochlorothiazide in combination. Int. J. Pharm. Technol., 4; 2011:3970-3983.

13. Natarajan R, Patel N, Rajendran NN. Formulation and evaluation of immediate release bilayer tablets of telmisartan and hydrochlorothiazide. Int. J. Ayur. Herb. Med., 1; 2011:1-5.

14. Noor Ahmed VH, Niharika G, Deepak P, Nazan S, Mohammed SA. Formulation design, characterization and In-vitro evaluation of bilayered tablets containing Telmisartan and Hydrochlorothiazide. International Journal of Biopharmaceutics. 4; 2013:1-9.

15. International Conference on Harmonization, ICH Q2 (R1), Validation of Analytical Procedures: Text and methodology, Geneva, 2005.

16. Rawlins EA. Bentley’s Textbook of Pharmaceutics; All India Traveller Book Seller: Delhi, India, 2004; pp. 641-665.

17. Higuchi WI. Analysis of data on the medicament release from ointments. Journal of Pharmaceutical Sciences, 51; 1962:802-804.

18. Subrahmanyam CVS. Textbook of Physical Pharmaceutics; Vallabh Publication: Delhi, India, 2001; pp. 123-125.

19. International Conference on Harmonization, ICH Q1A (R2), Stability Testing of New Drug Substances and Products, Geneva, 2003.

20. United States Pharmacopoeia. United States Pharmacopoeia-national formulary (USP24/NF 19): United States Pharmacopoeial Convention, Rockville, MD, 2002; pp. 16.

21. Hogan JE. Hydroxypropyl methylcellulose sustained release technology. Drug Development and Industrial Pharmacy. 15(6-7); 1989: 975-999.

22. Rajabi-Siahboomi A, Jordan MP. Slow release HPMC matrix systems. Eur. Pharm. Rev., 5(4); 2000: 21-23.

23. Luprizol, Application of Carbopol 71G NF Polymer in controlled release tablets. Pharmaceutical Bulletin. 32; 2008: 2-9.

24. Luprizol, Formulating Controlled Release Tablets and Capsules with Carbopol Polymers. Pharmaceutical Bulletin. 31; 2008:2-22.

Received on 26.06.2015 Modified on 20.07.2015

Res. J. Pharm. Dosage Form. and Tech. 7(3): July-Sept., 2015; Page 193-198

DOI: 10.5958/0975-4377.2015.00029.4

Formulation Code	Weight Variation (%)^b	Thickness (mm)^a	Hardness (Kg/cm²)^a	Friability (% w/w)^a	Drug Contents (%)^b
Formulation Code	Weight Variation (%)^b	Thickness (mm)^a	Hardness (Kg/cm²)^a	Friability (% w/w)^a	Telmisartan	Hydrochlorothiazide
F1	1.44 ± 0.22	7.00 ± 0.01	7.12 ± 0.10	0.72± 0.22	98.25 ± 1.15	99.11 ± 0.75
F2	1.50 ± 0.32	7.10 ± 0.02	7.15 ± 0.10	0.70± 0.25	98.35 ± 1.34	98.72 ± 0.92
F3	1.62 ± 0.15	7.10 ± 0.01	7.22 ± 0.10	0.74± 0.15	98.32 ± 1.20	98.90 ± 0.85
F4	1.15 ± 0.45	7.00 ± 0.02	7.10 ± 0.20	0.71± 0.24	99.35 ± 0.25	99.52 ± 0.22