Feature Of The Week 9/8/13: Austrian Researchers Investigate OCT for Analysis of Coatings in Pharmaceutical Tablets
Tablet coating is a common pharmaceutical technique to apply a thin continuous layer of solid on the top of a tablet or a granule containing active pharmaceutical ingredients (APIs). The coating itself can serve for different purposes. On the one hand we have the so-called “immediate-release coatings”, which do not have any special effect on the pharmaceutical impact of the tablet, but well on the acceptance among patients and therefore on the commercial success. Examples are increasing the visual attractiveness, taste masking and brand recognition.
On the other hand it is possible to apply coatings which actually influence the overall effect of the pharmaceutical tablet - the so-called “modified release coatings”. Such functional coatings enable formulators to modify the dependency of the initial drug release kinetics on the pH by making it resistant to gastric juice through enteric coating. This is an important task, since for many active pharmaceutical ingredients it has been shown that they work best if released within the small intestine and not in the stomach. Insufficient coating can result in ineffective gastric resistance, whereas the drug release can be seriously delayed when the dosage form passes into the small intestine due to applying too much coating. Therefore, thickness and homogeneity of the coating are critical parameters regarding the drug release rate, and consequently a direct or indirect monitoring strategy of these critical process parameters is essential.
Several measurement and monitoring approaches (measurement of weight and diameter gain, spectroscopic or imaging means) are already commonly standard in the pharmaceutical industry; however all of them have certain limitations in terms of speed or resolution, or simply do not deliver quantitative data.
One technique overcoming all of these drawbacks and limitations is optical coherence tomography (OCT). This purely optical approach offers the possibility of acquiring high resolution depth resolved data in a fast and fully non-destructive way. With the aid of OCT it is not only possible to measure the absolute coating thickness, but also to detect inhomogeneities, defects, inclusions or voids in the coating or substrate material. All of these advantages turn OCT a candidate for the in-line monitoring of the coating process.
In this work the possible application of OCT as in-line method for monitoring pharmaceutical tablet film coating was studied. In a first step off-line investigations of several different commercially available tablets with film coating were performed using two different SD-OCT systems, an in-house developed system employing two different light sources centred at 830 nm (providing different axial resolutions), and a commercially available Thorlabs Telesto system with a centre wavelength of 1330 nm. Based on the excellent results and the achievable axial resolution the spectral window centered at 830 nm was chosen for the further experiments.
In a second step we introduced a relative movement between the tablet bed and the sensor head in order to analyze the in-line capability of OCT. The influence of moving tablets on OCT images was investigated by considering a static tablet bed and moving the sensor head along the tablet bed. The sensor head was moved with the aid of a 3 axis gantry system at velocities ranging from 0.025 m/s to 0.7 m/s. For this whole range of velocities it was possible to perfectly determine the thickness of the coatings. The detection of imperfections in the coating, however, is limited by the broadening of the image size due to the additional relative speed between sensor head and tablet. For this specific OCT configuration the upper limit is approximately at 0.3 m/s, which could be increased by decreasing the exposure time of the CCD.
This study shows the high potential of OCT as in-line monitoring tool in the pharmaceutical industry. However, in order to apply OCT for real-time quality control, an automatic evaluation of the OCT measurements providing information about the coating thickness but also about the coating homogeneity has to be developed.
For more information see recent Article. Courtesy of Michael Leitner from RECENDT.