Effect of Coating Solvent Ratio on the Drug Release Lag Time of Coated Theophylline Osmotic Tablets

Purpose: The aim of the study was to investigate the effect of hydro-alcohol coating solvent ratio on the surface texture and lag time of porous theophylline osmotic tablet.
Methods: Porous theophylline osmotic pump tablets were formulated by direct compression and coated by spraying with varying ratios of water-alcohol containing hydroxypropyl methylcellulose (HPMC, 5 cps) ® as primary swelling layer and EudragitRSPO and RLPO as porous layer. The viscosity of HPMC coating solution was determined using Brookfield viscometer. Surface morphology was investigated with scanning electron microscopy (SEM). In vitro drug release studies to assess lag time was performed in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF).
Results: No significant change in the viscosity of HPMC coating solution was found when water-alcohol solvent ratio was varied. SEM revealed profound distortions in coating texture with a high proportion of either solvent in the coating solution. Increasing the amount of either water or alcohol in the coating solution increased the roughness of coated surface which contributed to a burst release of drug due to early opening of the tablet as a result of the high osmotic pressure and low mechanical strength of the coated layer. The optimum coating solvent ratio for spray-coating the osmotic tablets was water : alcohol (60:34) and it resulted in a coating with smooth texture and which successfully modulated drug releaselag time of the coated tablets.
Conclusion: The optimum ratio of water/alcohol (60:34) as coating solvent produced a smooth coated tablet surface texture and modulated the drug release lag time of the tablets.

Keywords
Coating solvent, Drug release, Lag time, Osmotic tablet, HPMC, Methacrylic acid co-polymers