Coating tablets involves selecting a specific color and gloss that are used to distinguish a drug product and build brand
identity for that product. In addition to the issue of color and gloss, the coating must have appropriate surface-slip properties
and adequately adhere to the tablet. The coating process and the materials used in the coating system can be optimized using
a quality-by-design (QbD) approach to meet these requirements. The authors describe a QbD study that was performed to optimize
a coating system.
Critical quality attributes (CQAs) for a tablet coating are color, slip, gloss, and adhesion. A quality-by-design (QbD) study
that uses a statistical design and data analysis can identify the critical process parameters (CPPs) needed to obtain the
CPPs include both the material composition (i.e., formula) and parameters in the coating process. In this study, quality function
deployment (QFD) was used to identify the expected effects of material composition on the CQAs. QFD can be used to minimize
the number of experiments, which limits experimental error. Based on the QFD shown in Table I, the amount of titanium dioxide (TiO2) and a red dye were identified as the CQAs for color, and it was determined that a mixture design was not necessary to test
the effect of material composition on color. Instead, a three-factor full factorial with a center point for color effect
was chosen as a practical design of experiment (DOE).
Table I: Quality function deployment for effects of material on coating properties (Spectrablend II Pink, Sensient Pharmaceutical
Coating Systems). (ALL TABLES ARE COURTESY OF THE AUTHORS)
The coating system compositions were studied. The level of red dye (Red 40 Lakes, 12–14% concentrate) and the level of TiO2were varied to set the design space for the color effect. Medium-chain triglyceride (MCT) levels were adjusted to maintain
the mass balance. Table II lists the temperature and spray-rate process variables used to test each of the nine coating-system compositions.
Table II: Experimental coating conditions.
A 1200-g batch of placebo tablets was coated with each of the nine coating systems. After spray coating, 3 and 5% weight-gain
samples were collected and rolled in the coating pan for additional 5 min. The percentage of weight gain was based on theoretical
coating solution consumption and did not account for coating efficiency. A total of 45 coating experiments were conducted,
and 90 samples were collected. Samples were tested, with five replicates, for color, gloss, slip, and adhesion.