OSMOTIC PUMPING RELEASE FROM KCL TABLETS COATED WITH POROUS AND NONPOROUS ETHYLCELLULOSE

A simple model is described for osmotic pumping drug release from membrane-coated formulations. The process requires at least two different areas in parallel in the membrane coat, which exhibit differing reflectivities to solute (drug and/or excipients). The model was used to calculate the reflectivities of membrane coats on potassium chloride tablets. Tablet coats of pure ethylcellulose (EC) had a low reflectivity of less than 0.005, and was not a major release rate regulating parameter. The higher reflectivity of the same membrane was 0.58, significantly lower than for a free relaxed film. The reduction in high reflectivity resulted in a corresponding lower drug release rate. Incorporation of hydroxypropyl methylcellulose (HPMC) in EC reduces the high reflectivity of the tablet coats even further. The reduction is accelerated when the HPMC content exceeds 24%. Since freezing water could be detected in free EC films with HPMC content of 24% and above, the formation of pores in these membrane coats is proposed. The pores are formed by the leaching of the water-soluble polymer HPMC, which is also found in the soaking liquid.