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CA pseudolatices have been extensively investigated in the development of osmotic-and diffusion-controlled drug delivery systems [21-26]. Bindschaedler et al. [27] reported that plasticizer levels from 160% to 320%, by weight of CA, provide films that are comparable to those prepared from organic solutions. King and Wheatly [28] evaluated the effect of plasticizer levels on the glass transition temperature and found that the most effective plasticizers for CA pseudolatex are glyceryl diacetate, glyceryl triacetate, and triethyl citrate (Table 2). Films containing glyceryl diacetate and triethyl citrate exhibited the greatest change in the modulus of elasticity. The water permeability of the CA pseudolatex films is strongly dependent on the nature of the plasticizer

and the processing conditions used. Generally, high boiling plasticizers produce more permeable membranes [26]. Kelbert and Bechard [23] evaluated the release of propranol HC1 from tablets coated with a CA pseudolatex plasticized with triacetin and triethyl citrate, separately, with an without the presence of permeation enhancers. The results showed no release of KC1 for the first 8 hr from tablets that were coated with 150% triacetin and 120% triethyl citrate even though the water had penetrated through them and the tablets swelled. Scanning electron photomicrographs revealed the films/coatings on the tablets to be dense, nonporous, homogeneous, and free of any defects at

Tg CO GTAC

tablet edges. In contrast, tablets coated with CA dispersions containing 40% sucrose provided a steady release of propranolol HC1 over a 12 hr period time with a 2 hr lag time. The addition of 10% PEG 8000 to the pseudolatex, in addition to 40% sucrose, maintained the extended 12 hr release profile and reduced the lag time to 1 hr. It has been postulated that drug transport occurs mainly in the porous CA structure, and the mechanism responsible for it is a combination of molecular diffusion/osmotic pressure via water transport through the porous cellulose acetate membrane.