Multiple-dose units have many kinetic and therapeutic advantages over single-dose sustained-release units. We therefore investigated the influence of pellet core properties and different coating parameters on in vitro theophylline release. Pellet size and pellet surface have a distinct effect on the release behaviour, with the surface morphology also playing a major role. One organic solvent-based and two aqueous dispersion-based polymer systems for sustained-release diffusion membranes were compared with respect to the influence of coating process parameters. The product bed temperature, an important process parameter, proved to be noncritical for the organic solvent-based system within a product bed temperature range of 30-45 degrees C, as was demonstrated by well reproducible release values. Since the release values showed a distinctly higher relative standard deviation at the lower spray rate, probably due to spray losses, the spray rate has to be carefully optimised. With one of the aqueous sustained-release systems, both the product bed temperature and the subsequent curing phase which was carried out al a fixed relative humidity and which needed to be sufficiently long had a decisive effect on the release rare. Modern fluid-bed techniques render anti-adhesives, such as talc, largely superfluous for the aforementioned sustained-release systems. When opting for organic solvent-based sustained-release systems, the technicalities of solvent recovery must be taken into consideration due to legal requirements concerning their emission. Compared with organic solvent-based lacquer systems, aqueous latex systems involve higher development and production costs, as has been demonstrated by the influence of the product bed temperature and the subsequent curing. (C) 1997 Elsevier Science B.V.
