Release rates, drugs from polymer

Fig. 3 Release of drug from various shapes of pol5mer membrane permeation-controlled drug-delivery systems (A) sphere-type, (B) cylinder-type, and (C) sheet-type. In (D), the drug concentration gradients across the rate-controlling polymeric membrane and hydrodynamic diffusion layer exist in series. Both the polymer membrane, which is either porous or non-porous, and the diffusion layer have a controlled thickness and h, respectively).

In most reservoir and transdermal systems (Figure 9.2), release of drug from the reservoir into the external solution occurs in three steps (1) dissolution of the drug in the polymer, (2) diffusion of drug across the polymer membrane, and (3) dissolution of the drug into the external phase. Assuming that the rate of diffusion across the membrane is much slower than the rate of dissolution/... 

In this chapter we present iri vitro and vivo norethindrone release rate data (from experiments in which we used the basic salt sodium carbonate and the neutral salt sodium sulfate), discuss the effect of thermal treatment on polymer properties, and finally discuss problems that arise when an extremely water-insoluble drug such as levonorgestrel is used. 

The use of water-soluble polymers in ternary CD complexes has also been utilized for controlling the rate of release of drug from cyclodextrin formulation. It seems that ternary complexes consisting of polymers as ternary component with low viscosity (PVP) tend to dissolve rapidly [25], while polymers with matrix-forming property (HPMC) lead to sustained drug release [26] in comparison to corresponding binary systems. 

Release of tetracycUne hydrochloride from PCL fibers was evaluated as a means of controlled administration to periodontal pockets (69). Only small amounts of the drug were released rapidly in vitro or in vivo, and poly(ethylene-co-vinyl acetate) gave superior results. Because Fickian diffusion of an ionic hydrochloride salt in a UpophiUc polymer is unlikely, and because PCL and EVA have essentially identical Fickian permeabilities, we attribute this result to leaching of the charged salt by a mechanism similar to release of proteins from EVA (73). Poly-e-caprolactone pellets have been found unsuitable for the release of methylene blue, another ionic species (74,75). In this case, blending PCL with polyvinyl alcohol (75% hydrolyzed) increased the release rate. 

Chloropromazine (8—34 wt% loading) has been microencapsulated in PCL-cellulose propionate blends by the emulsion solvent evaporation method (61). Phase separation for some ratios of the two polymers was detectable by SEM. The release rate from microcapsules in the size range of 180-250 pm in vitro (Fig. 11) was directly proportional to the PCL content of the blend, the half-life (50% drug release)... 

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