Bead coating. I. Change in release kinetics (and mechanism) due to coating levels.

Beads containing 50% acetaminophen (APAP) and 50% microcrystalline cellulose (Avicel PH 101) were prepared and then coated using an aqueous ethylcellulose based dispersion (Aquacoat) to evaluate the effect of the coating level on drug release. The APAP release was shown to be dependent on levels of the coating and a change in mechanism was suggested. Drug release from incompletely coated beads at low levels of coating can be described with the square root of time model, while drug release from beads with a high level of coating appears to be best described by zero-order release. At low coating levels, the drug release rate constant based on the square root relationship seems to be linear with the coating level. At high coating levels, drug release rate in terms of a zero-order model appears to be proportional to the reciprocal of the coating level.