(T1430-08-46) Using Raman Mapping to Detect and Identify Challenging Degradation Products and Complex Drug-Drug Interactions in a Tablet

Purpose: Raman mapping is a powerful technique for the analysis of pharmaceutical tablets. Thousands of spectra can be collected across the surface of a microtomed tablet, and these spectra can be analyzed to determine how the various components are spatially distributed. In addition, mapping data can also be used to detect and identify very low amounts of a component that would not be detectable if a bulk Raman technique was used. In this current study, we will present the results collected from area maps on two different regions of an expired tablet, where peak height ratio profiles and spectral subtractions were used to locate and identify degradation products.

Methods: An extra-strength Excedrin® tablet (expired July 2009) was prepared for analysis by shaving thin layers off the top until the coating was removed and approximately one fifth of the tablet thickness was removed. Raman mapping was conducted with a HORIBA Scientific XploRA series confocal Raman microscope on an Olympus series BX51TRF optical platform. Area maps consisting of 2601 spectra over a grid of 100 microns by 100 microns were collected near the edge of the tablet and within the interior of the tablet. Reference spectra were obtained of the individual ingredients listed on the label. The spectra were examined for specificity and spectral peaks were identified that could be used to create the mapping profiles. A peak height ratio profile was most often used. Once the profiles were created for each ingredient, spectra were extracted from the map and compared to the reference spectra to confirm their identity. At this point, peaks were identified in the mapping data that did not belong to any of the known components. Profiles were created for this unknown compound which indicated that it was located between two of the active ingredients. Additionally, it was more prevalent towards the edge of the tablet. The unknown was identified, and a hypothesis was formed to explain its appearance. The hypothesis was then applied to a third active ingredient in the tablet and its degradation product was also found. Spectral subtractions were essential for confirming the presence of this last degradation product.

Results: Profiles were created for acetaminophen, acetylsalicylic acid, caffeine, microcrystalline cellulose, and HPMC/HPC; locating and identifying the various excipients and active ingredients were accomplished. Of more interest, however, was the appearance of spectral features that did not correspond to any of the known ingredients. Acetylsalicylic acid (an active ingredient) can decompose to form salicylic acid upon exposure to humidity. In addition, caffeine (an active ingredient) can react with salicylic acid to form caffeine salicylate. Synthesized caffeine salicylate was analyzed by Raman microscopy, compared to the unknown spectrum from the map, and confirmed in the tablet map. To spatially separate the caffeine further from the caffeine salicylate in the mapping images, profiles were created comparing the peak height ratio of each one versus the other. This approach is very powerful at separating closely related compounds or detecting a compound present at a very low level. The resulting profiles indicated that the caffeine salicylate was present in-between regions of caffeine and acetylsalicylic acid. Given the age of the tablet, water may have migrated through the coating, causing the decomposition of acetylsalicylic acid to salicylic acid, which then reacted with caffeine to form caffeine salicylate. If this hypothesis is true, then the third active ingredient (acetaminophen) would degrade upon exposure to moisture to form para-aminophenol. The mapping data were examined to determine if para-aminophenol could be detected. Specificity was poor for this compound, providing an additional challenge to the interpretation. A small shift (2-4 wavenumbers) of a relatively strong peak that did not overlap with other tablet ingredients was found. Mapping profiles were created to interrogate this slight difference, and the resulting profiles revealed small regions where 4-aminophenol was more likely to be present. Spectra for acetaminophen and for 4-aminophenol were extracted from the map and subtracted from one another, finally confirming the identity of the suspected degradation product.

Conclusion: Although the use of Raman mapping to determine the spatial location of various ingredients in a tablet may be straight-forward, identifying an unknown component and determining a likely cause for its appearance requires a more complex approach. Specific profile strategies, knowledge of possible chemical reactions that could be present, and spectral subtractions are necessary to solve such problems. For the expired Excedrin tablet evaluated, evidence of degradation of both acetaminophen and acetylsalicylic acid was found. In addition, caffeine salicylate, a reaction product from a degradation product with caffeine was also found.