Formulation and In vitro Dissolution Characteristics of Sustained-Release Matrix Tablets of Tizanidine Hydrochloride

Purpose: To formulate sustained-release (SR) matrix tablets of tizanidine hydrochloride (THC) and to investigate the effect of matrix polymer type on drug release profile of drug.
Methods: Matrix tablets of THC were prepared by direct compression method using a combination of hydroxypropylmethylcellulose (HPMC) and ethylcellulose (EC) in varying ratios. In all the formulations, the amount of THC was 6.87 mg (equivalent to 6 mg base). USP type-I (basket) apparatus was used for the dissolution study. The dissolution study was performed in 0.1M HCl for the first 2 h and in phosphate buffer (pH 6.8) for another 10 h. The dissolution data were subjected to drug release models to ascertain the kinetics of drug release. Additionally, in vitro swelling and buoyancy studies were carried out on the optimized formulation. The optimized formulation was compared with a commercial reference product using similarity factor (ƒ₂) test.
Results: F4 formulation, containing 145 g of HPMC only, (with ƒ₂ value of 67.38) was selected as optimized formulation (compared to the reference commercial product), and it released 97.84 % of the drug in 12 h. The release data showed best fit to first-order kinetics (R² = 0.9963 - 0.9989), though nonsignificantly (p > 0.05) different from the Higuchi model (R² = 0.9813 - 0.9955) except for formulation F6. Based on Koppcha model data, drug release mechanism involved both diffusion and erosion (n = 0.513 - 0.597) with diffusion being dominant. The optimized formulation exhibited 162 % swelling at the end of 11 h, after which no further weight gain occurred.
Conclusion: Suitable sustained-release tablets of tizanidine hydrochloride have been successfully prepared using direct compression Drug release is sustained by increasing the content of the matrix polymers used.

Keywords
Tizanidine; HPMC; EC; Koppcha; Hixson-Crowell; Stokes-Einstein’s equation