Evaluating critical film coating characteristics of sustained- release coated pellets with different size using terahertz pulsed imaging

In this study different sized sustained-release coated pellets (6, 2.5 and 1 mm in diameter) were investigated using terahertz pulsed imaging (TPI). Ten pellets from each batch of metoprolol succinate layered sugar starter cores coated with a 75:25 (w/w) polymer blend of Kollicoat SR and Kollicoat IR (approximate coating thickness of 60 μm, according to the weight gain) were mapped individually to evaluate the effect of size on coating thickness and morphology (depicted by the terahertz electric field peak strength, TEFPS). The TPI measurements were carried out on a pellet surface area of approximately 33, 2.2 and 0.4 mm for pellets with 6, 2.5 and 1 mm diameters, respectively, and the interface between polymer coating/drug layer and drug layer/sugar core was successfully determined. Results indicated a large variation in the mean coating thickness (CT) between all pellets sizes. Smaller pellets showed a higher mean CT of 70 μm (1 mm) and 81 μm (2.5 mm) compared to 6 mm pellets (50 μm), suggesting a better coating efficiency for smaller pellets. With no differences in surface morphology observed using scanning electron microscopy (SEM), differences in the mean TEFPS values between 6 mm pellets (17.6%) and 2.5/1 mm pellets (2.2 and 2.6%, respectively) were related to signal distortion due to the increase in curvature of smaller pellets. Although the largest pellets showed the thinnest CT, the fastest drug release was obtained from the smallest pellets due to the larger surface area exposed to the dissolution media. TPI proved highly suitable to evaluate film coating characteristics as well as to detect the drug layer/core interface of different sized sustained-release pellets.