INTRODUCTION:^[1]

Tuberculosis was for centuries a major killer disease. With the development of the first line drugs, it came to be regarded as an easily curable condition. This is no longer easily curable as the bacteria Mycobacterium tuberculosis which causes it has come back to haunt us. Multidrug-resistant strains are now common and recent evidence suggests that strains with increased virulence have emerged¹. One of the main reasons for the development of this resistance is the mono drug therapy. This may be due to the drug-drug interactions as the therapy includes combination of various fixed dose of drugs leading to poor bioavailability². One such commonly reported interaction is between Rifampicin and Isoniazid, the two extensively used drugs in the treatment of Tuberculosis leading to the poor stability and bioavailability of Rifampicin³.

The present study deals with improving the stability of Rifampicin by modifying the formulation and design of the dosage forms by enteric coating the Isoniazid tablets⁴. Only Isoniazid was enteric coated instead of Rifampicin as Rifampicin has shown more solubility and permeability when it is released in the stomach⁵.

MATERIAL AND METHOD:

Materials:

Rifampicin and Isoniazid were purchased from Yarrow chemicals Ltd. Eudragit L-100 was obtained from Dr. Reddy’s labs Pvt. Ltd. Super Tab 11SD was obtained from DFE Pharma, Germany. Talc and Magnesium stearate were purchased from Himedia chemicals Ltd. Conc. Hydrochloric acid, Acetone, Glycerol and Isopropyl alcohol were purchased from SD-fine chemicals Ltd.

Equipment

UV-visible spectrophotometer- UV 1601 PC, Shimadzu, Japan.

Tablet compression machine- Rimek Mini Press, Karnavati Engineering Ltd., Ahmadabad

Pan coating equipment- Instacoat, Pharma RandD coater, Ideal Cures Pvt. Ltd., Mumbai

Weighing balance- Model FB-200 of EssaeTeraoka Ltd.

Friabilator- Model EF-1W of Electrolab

Disintegration Tester- Model ED-2L of Electrolab

Hardness Tester- Monsanto of Tab machines

Dissolution apparatus- Dissolution tester (USPII), TDT-08L, Electrolab

Mortar and pestle

Objective

The objective of the present study was to formulate once-daily oral fixed-dose combination tablets of Rifampicin and Isoniazid, which facilitates the segregated delivery of these drugs for improved Rifampicin stability.

Methodology

To achieve the above said objective, two different formulations were manufactured. Formulation I includes Rifampicin and Isoniazid as immediate releaseuncoated tablets and Formulation II includes combination of Rifampicin and Isoniazid in enteric coated tablet. The formulation ingredients of Rifampicin and Isoniazid tablets are given in the table1 and table2 respectively.EudragitL-100 was used as enteric coating polymer and the tablets were coated by pan coating technique⁹.

Preparation of the formulations:

The required quantities of the drugs¹⁰and the excipients (as mentioned in the tables 1, 2 and 3) which include the directly compressible vehicle, lubricant and glident were mixed uniformly by triturating them in a mortar and pestle. After adjusting the die cavity in the tablet compression machine with respect to the weight required by the vehicle, the tablets were prepared by direct compression method^{6, 7}. The prepared Rifampicin and Isoniazid tablets were evaluated for the physical parameters like hardness, weight variation, friability, disintegration and dissolution.

Table 1: Rifampicin Formulation

Ingredients	Quantities (mg)
Rifampicin	600
Talc	10
Magnesium stearate	10
SuperTab 11SD	q.s. to 700

Table 2: Isoniazid Formulation

Ingredients	Quantities (mg)
Isoniazid	300
Talc	6
Magnesium stearate	6
SuperTab 11SD	q.s. to 400

Table 3:Combination tablet formulation

Ingredients	Quantities (mg)
Rifampicin	600
Isoniazid	300
Talc	10
Magnesium stearate	10
SuperTab 11SD	q.s. to 1000

Weight Variation:

Rifampicin, Isoniazid and combination tablets were weighed and the % deviation of each and every tablet from the mean weight was calculated. The procedure for weight variation was followed as per Indian Pharmacopoeia⁸.

Hardness:

The uncoated tablets were carefully placed in the hardness tester, the pressure was applied and the pressure at which the tablets break was noted.

Friability:

The prescribed number of uncoated tablets were weighed and placed into the friabilator and were rotated (100 rpm as per Indian Pharmacopoeia⁸) and the final weight of the tablets was noted and the percentage weight loss was computed.

Disintegration Test:

Required numbers of uncoated tablets of Rifampicin and Isoniazid as per Indian Pharmacopoeia⁸ were added into the Disintegration Test apparatus and the disintegration time was noted. For enteric coated combination tablets it was done in 0.1 N Hydrochloric acid for 2hours and then in phosphate buffer of pH 6.8.

Dissolution test:

Required numbers of tablets according to Indian Pharmacopoeia⁸ were added into the Dissolution Test apparatus (USP II) and the release of the Rifampicin was analysed at 336nm wavelength using UV-visible spectrophotometer for both Formulation I and II in 0.1N Hydrochloric acid for 2 hours. After 2 hours the formulation II was shifted to phosphate buffer of pH 6.8 and analysed for Rifampicin release.

Enteric coating of Isoniazid tablets⁹:

Polymer : Eudragit L-100

Plasticizer : Glycerol

Solvent system : Acetone and Isopropyl Alcohol

Technique : Pan Coating Technique

Enteric coating process parameters:

Atomization air pressure : 20psi

Pan RPM : 30

Pump RPM : 1

Inlet air temperature : 40⁰C

Distance between spray gun and tablet bed was maintained at 10cm

Results:

Hardness: The average hardness of Isoniazid tablets was found to be 4.1kg/cm² whereas it was 4.5 and 4.4 kg/cm²for Rifampicin and combination tablets respectively. The results are tabulated in table4 and figure1.

Table 4: Hardness

	Average hardness (Kg/cm2)
Isoniazid	4.0
Rifampicin	4.5
combination	4.4

Figure 1:

Friability:

The uncoated tablets of both the formulations have passed the IP limits for friability. The results are shown in table5 and figure2.

Table 5: Friability

	% Loss in weight
Isoniazid	0.1421
Rifampicin	0.1255
combination	0.1826

Figure 2:

Weight variation:

All the tablets were within the limits as per IP with respect to weight variation test. The results are shown in the figure3.

Figure 3:

Disintegration test:

The disintegration time of uncoated tablets of both the drugs of formulation I confirmed to IP specifications. The combination tablet did not disintegrate in 0.1N Hydrochloric acid for 2 hours but it disintegrated in phosphate buffer of pH 6.8. The results are shown in table6 and figure4.

Table 6: Disintegration test

	Disintegration time (min)
Isoniazid	8.37
Rifampicin	13.12
Combination (in pH 6.8)	14.11

Figure 4:

Enteric coating of Isoniazid tablets:

The weight gain was found to be 10% w/w of the core combination tablet.

Dissolution studies:

Dissolution studies were performed for Formulation I and II separately in 0.1N HCl. Then the cumulative percentage drug release for Rifampicin was calculated from the obtained data. In case of Formulation I, the cumulative percentage drug release for Rifampicin after 2hr in 0.1N HCl was found to be around 80% whereas for Formulation II did not release Rifampicin in 0.1N HCl but it has shown a phenomenal increase in release of Rifampicin up to 88% in phosphate buffer of pH 6.8.The results are given in table7 and figure5 respectively.

Table 7: Dissolution test

Time (min)	Formulation I	Formulation II
0	0	0
5	10.31±1.02	0
10	12.5±1.21	0
20	30.66±1.54	0
30	44.56±1.39	0
45	56.88±1.68	0
60	63.68±1.18	0
90	72.14±1.86	0
120	80.221.79	0
125		11.56±1.22
130		17.1±1.48
140		28.26±1.51
150		48.44±1.42
165		62.55±1.71
180		70.22±1.63
210		79.42±1.58
240		88.11±1.47

Figure 5:

CONCLUSION:

This study proves that Rifampicin interacts with Isoniazid and undergoes degradation to a phenomenal extent in presence of Isoniazid in acidic medium of stomach. This interaction and degradation of Rifampicin can be reduced and the stability can be enhanced by formulating an enteric coated combination tablet of both these drugs so that the physical contact between these two drugs can be prevented in the stomach by releasing these drugs in intestine.

REFERENCES:

1. Rang, H.P, Dale, M.M, Ritter, J.M. Pharmacology. Antibacterial drugs. Fourth edition, page no. 703. Churchill Living Stone Publishers Ltd, 2000.

2. Sankar, R, Nishi Sharda, Saranjit Singh. Behavior of decomposition of rifampicin in the presence of isoniazid in the pH range 1–3. Drug Dev. Ind. Pharm. 2003; 29(7), 733–738.

3. Shishoo, C.J, Shah, S.A, Rathod, I.S, Savale, S.S, Kotecha, J.S, Shah, P.B. Stability of rifampicin in dissolution medium in presence of isoniazid. Int. J. Pharm. 1999; 190, 109–123.

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6. William CG, Joseph LK. Tablets. The Theory and Practice of Industrial Pharmacy. Edited by Leon Lachman, Herbert AL and Joseph LK. New York. 1976: 2^ndedition: pp.321-358.

7. Peter D. Oral solid dosage forms. Pharmaceutical Preformulation and Formulation. Edited by Mark Gibson, UK.2001: 1st Indian edition: pp. 379 – 456.

8. Pharmaceutical Methods, General Chapters, Indian Pharmacopoeia, 2007, 5th edition, Vol-1, pp. 177-183.

9. John RE, Elliot BP, Anton HA. Tablet Coating. The Theory and Practice of Industrial Pharmacy. Edited by Leon Lachman, Herbert AL and Joseph LK. New York. 1976: 2^nd edition: pp.359-388.

10. Standardized treatment regimens. Treatment of Tuberculosis: guidelines for national programmes, World Health Organization, Geneva, 2003, 3^rd edition, pp 27- 39.

Received on 18.09.2013 Modified on 10.10.2013

Research J. Pharm. and Tech. 6(12): Dec. 2013; Page 1468-1471