Transmission Raman Spectroscopy for Quantitative Analysis of Pharmaceutical Solids

Transmission Raman Spectroscopy for Quantitative Analysis of Pharmaceutical Solids Anders Sparén, Jonas Johansson, Olof Svensson, Staffan Folestad, & Mike Claybourn AstraZeneca R&D At the same time, keep in mind that the product is yours, and your organization will be responsible for communicating with regulatory agencies as the product moves through clinical trials and toward launch. Abstract Raman spectroscopy is today a versatile analytical tool for pharmaceutical analysis. With the development of transmission Raman spectroscopy that offers better sampling for solids than conventional backscatter Raman, the technique is on the edge of a breakthrough to become a more widespread and used analytical tool. In particular, for quantitative analysis of solids it is equally applicable in early development through to manufacturing quality control in the pharmaceutical industry. Introduction Commercial pressure and patient needs are drivers for reducing the development time for pharmaceuticals to market. This leads to increased demands also on pharmaceutical analysis that can no longer afford to rely solely on traditional measurement strategies that may require days for results to be produced. Future analytical techniques must be fast, robust and rich on information, need minimal method development and be applied from early development to production, still having sufficient accuracy and precision. In this context, optical spectroscopy offers a range of alternatives that depending on the sample type and analytical requirements can be used for various applications. Most spectroscopic techniques are non-destructive and solid samples can be analysed in their native state without any sample preparation. Spectroscopic techniques such as Near-Infrared (NIR), Infrared (IR), Raman, Fluorescence, and UV-Vis absorption can give chemical information, such as content of the active pharmaceutical ingredient (API), but also physical information on e.g. polymorph content by Raman, particle size, porosity and density by NIR prediction. 62 APR January February 2009

NIR spectroscopy has been used for a number of applications in the pharmaceutical industry, such as identity, water content, blending, API content, etc [1-4]. The technique underwent a major improvement some ten years ago, when transmission mode NIR spectroscopy was presented and a significant improvement with lowered prediction errors as compared with reflectance mode NIR spectroscopy was reported [31,32]. This improvement was attributed to the increased sampling volume in transmission mode. Raman spectroscopy is an interesting alternative to NIR for pharmaceutical analysis, and provides non-invasive measurements with high selectivity and sensitivity. While IR spectra are generated by a change in the dipole moment of a molecule, Raman scattering occurs as a result of changes in its polarisability. The high selectivity offered by Raman spectroscopy allows the information on the molecular properties of the measured sample to be interpreted in a straightforward way, often directly from the spectral bands. Raman spectroscopy is also relatively insensitive to water, which, depending on the application, can be both an advantage and a drawback. In addition to these qualitative features, Raman also enables accurate quantitative assessment of concentration of compounds in various types of samples. The high selectivity offered by the sharp spectral features in Raman simplifies data modelling, interpretation and communication with authorities, as well as enabling information on solid state characteristics. The usefulness of Raman spectroscopy in pharmaceutical development has been demonstrated for applications such as the monitoring of synthesis [5,6], the determination of polymorphic forms of the active component [7], and blend monitoring [8], while its use for quantitative assessment of API of tablets has only been shown in a limited number of cases. Surprisingly, the use of Raman in the pharmaceutical industry is still sparse compared with the use of IR and NIR. In particular, since Raman is less sensitive to variations in physical parameters than NIR, Raman would be expected to offer advantages such as ease-of-use for quantitative analysis. The reason for the less widespread use of Raman is probably due to more expensive equipment relative to NIR, but also to the inherent weaknesses in the sampling of solids, as well as the possible interference by fluorescence. A major limitation with Raman spectroscopy has until recently been the small sampling volume that may lead to sub-sampling within single samples, such as tablets. This is related to the fact that with the limited light penetration depth in the NIR region, only a small volume fraction of a tablet will be analysed and with the heterogeneity of powder blends and tablets, the method accuracy is most often limited by subsampling [9]. This was first addressed by using sample spinning or/ and translating devices [10] and later by large area sampling probes that made possible a more representative sampling. Recently, the presentation of Spatially Offset Raman Spectroscopy (SORS) [11-14] and transmission Raman spectroscopy [15-18] further improved the sampling of solid samples [13, 19]. In this paper, we discuss the role of Raman spectroscopy for formulation development and manufacturing quality control in the pharmaceutical industry, and exemplify with the use of transmission Raman spectroscopy for quantitative determination of the active pharmaceutical ingredient (API) of whole pharmaceutical tablets and capsules. Raman Spectroscopy in Pharmaceutical Analysis Raman spectroscopy has a great potential to be the first choice analytical tool for rapid assessment of solid samples in the pharmaceutical industry. It has been demonstrated to provide useful information in very diverse applications as discussed above. It is flexible in terms of the different sample types that can be analysed. This is primarily due to the fact that Raman spectra of most active compounds exhibit a significant spectral features in the 1600-1700 cm -1 that are not present in the spectra of the most commonly used excipients. This spectral selectivity together with narrow spectral features lead to highly selective analytical methods and thus open a potential for more robust methodology, since the spectra will be dominated by relevant information of content and solid state form and not by other effects such as variations in physical parameters of samples or sampling conditions. Figure 1 Figure 1. Strategy for Raman measurements in tablet manufacturing [20]. Raman spectroscopy is furthermore flexible in terms of the measurement geometry required for various pharmaceutical development and manufacturing unit operations. A number of different sampling devices have been developed, e.g. immersion probes for liquid environments, non-contact probes for moving samples and automated sample units for high-throughput analysis of finished products. However, still missing is an analytical tool that allows flexible assessment of formulations early in the development chain as well as near the production line without the need for repeated re-calibration of the assessment models. Transmission Raman paired with simple and automated data evaluation show great promise and may very well be the desired tool. Figure 1 proposes a strategy for the use of Raman spectroscopy in tablet manufacturing. 66 APR January February 2009

Principles of Transmission Raman Spectroscopy Figure 2 Figure 2. The principle of Raman transmission spectroscopy: A tablet is illuminated with laser light of a certain wavelength and some of it passes through the tablet, after having been scattered within the tablet. When a solid sample, such as a tablet, is illuminated with laser light as in conventional backscatter mode, the Raman signal obtained mainly comes from the illuminated surface of the sample. In transmission Raman, the light is collected on the opposite side from where it was illuminated. Since the incident light that reaches the detector has to travel through the tablet, the sample volume affected will be much larger. Figure 2 shows a simplified schematic representation of the principle of transmission Raman spectroscopy. Why Transmission Raman Spectroscopy? The difference in the efficiency of sampling the bulk of a tablet in backscatter and transmission modes can be illustrated by measuring a two-layer tablet, consisting of an API (propranolol) on one side and an excipient (mannitol) on the other. Figure 3a shows backscatter Raman spectra of the two sides of the tablet. It can clearly be seen that the spectra of the two sides are quite different, one giving information mainly of the API (e.g. the peak at 1387 cm -1 ) and the other giving information mainly from the excipient side (e.g. the peak at 877 cm -1 ). The spectra obtained on the two sides of the tablet measured in transmission mode (Figure 3b) in contrast, are almost identical, which shows that both the API and the excipient contribute to the Raman signal, no matter from which side the tablet is measured. Similarly, a tablet with accurate dose content, but in which the API is not completely homogeneously distributed within the dose, will be more representatively sampled using Raman in transmission mode compared with backscatter mode. APR January February 2009 67

Figure 3 A critical question is of course what effect the improved sampling obtained in transmission mode has on the accuracy and robustness of quantitative calibrations. This concept was tested [15], using a simple model formulation of tablets consisting of an API (propranolol), a filler (mannitol) and a lubricant (sodium stearyl fumarate). The tablets were directly compressed and had a thickness of about 3.3 mm. The API varied between 16 and 24 %w/w and the tablets were measured in both backscatter and transmission modes, using a laser emitting light at 785 nm. Partial-least squares regression (PLS), with one PLS-component, was used to calculate the calibration models and the predictions for the test set. Figure 5 Figure 3. Raman spectra of both sides of a two-layer tablet, consisting of API (propranolol) on one side and an excipient (mannitol) on the other: (a): backscatter Raman; (b) transmission Raman. Measurement Accuracy and Robustness Figure 4 Figure 4. Prediction of the concentration of propranolol tablets in an independent test set. Comparison of (a) backscatter Raman and (b) transmission Raman. Figure 5. Model robustness: the effect of reducing the number of samples in calibration models on the prediction errors for independent test sets. Filled circles represent transmission mode while empty circles represent backscatter mode. The figure shows mean prediction errors for reduced models built on the original and the exchanged models, and the error bars show max and min prediction errors. The same test set was used for all reduced models originating from the same full model. The results show that the accuracy in transmission mode (Figure 4b) was considerably better compared with the one obtained in backscatter mode (Figure 4a), yielding a mean prediction error for an independent test set that was about 25-30 % lower in transmission mode. Figure 4a also shows that when measured in backscatter mode, the data set contains an outlier, which was not detected in transmission mode. This is likely a result of sub-sampling of the API within the tablet, which could have a more pronounced effect in backscatter mode, compared with transmission mode. In an additional study on a pharmaceutical formulation having five ingredients, the mean prediction error was about 35-40 % lower in transmission mode compared with backscatter mode. The robustness of the measurement system was also tested. Figure 5 shows the robustness towards a decrease in the number of samples in the calibration, and it was estimated with the prediction error for an independent test set. Because of the more representative sampling volume, transmission Raman spectroscopy was relatively insensitive to reducing the number of samples in the calibration to very few. In backscatter mode, the prediction error increased with a decreasing number of samples. 68 APR January February 2009

Sample presentation can be a critical issue when applying spectroscopic techniques to solid samples. This has for example been discussed for NIR spectroscopy [21]. The robustness towards sample presentation was investigated for both transmission and backscatter modes by tilting a tablet in the sample well. Table 1 shows, for transmission mode, how the prediction of the API content in a pharmaceutical tablet was affected by tablet tilting. The results show that a slight or moderate tilt did not have any significant effect on the predicted result, nor did turning the tablet upside down. Not until the tablet was heavily tilted (measurement A_tilt_5), were the predictions severely affected. The results were very similar for backscatter mode, and the sample presentation system can be considered to be robust for both transmission and backscatter mode measurements. Transmission Raman spectroscopy has also been applied to quantitative studies on capsules [15, 17]. These pieces of work show that the content of API in capsules can be well quantified and that the transmission geometry increases the Raman signal from the bulk compared with the Raman signal from the capsule shell. If the capsule spectrum has an interfering fluorescence background, the Raman signal from the bulk will increase compared with the fluorescence background in transmission mode in the same way as described for the Raman signal of the capsule. Both these effects are due to the big difference in sampling volume between the backscatter and transmission geometries. Table 1: The effect of tilting a tablet in the sampling well: The tablet was first measured on side A and tilted more and more and finally measured on side B. Predictions of the concentration of the active component were made on all measurements. Figure 6 Calibration Transfer Calibration transfer has been extensively discussed in the NIR area [22-28], where elaborate means are needed to ensure accurate and precise performance after transferring a qualitative or quantitative calibration from one (master instrument) to another (slave) NIR instrument. The possibility of using calibrations developed on one Raman instrument on one or several other instruments, possibly located at other sites, could therefore be a potential critical issue for the future application of Raman spectroscopy in the pharmaceutical industry, especially in the Process Analytical Technology (PAT) area. In a feasibility study, we transferred a calibration developed on one Raman instrument to another. Figure 6a shows two transmission Raman spectra of the same tablet measured on two different Raman instruments, after x-scale adjustment. It can clearly be seen that there is a difference in offset between the two Raman systems. Predictions from spectra measured on the slave instrument, using a calibration developed on corresponding spectra measured on the master instrument, gave large prediction errors, mainly consisting of bias (data not shown). Applying spectral pretreatment (SNV) [29] and a calibration transfer function (local centring) [30] dramatically reduced the prediction error (Figure 6b). This simple feasibility study indicates that calibration transfer techniques well established for NIR spectroscopy, may be directly applicable to Raman spectroscopy. However, more work is needed in this area. Figure 6. (a) Comparison of Raman spectra of the same tablet measured on two different Raman instruments, after x-scale adjustment. (b) Prediction of the concentration of API in tablets measured on the slave instrument, using a calibration developed on Raman spectra of similar tablets measured on the master instrument. X-scale adjustment, SNV and local centring were applied to the spectra measured on the slave instrument. APR January February 2009 69

Future Prospects Raman spectroscopy, with its unique features, presents very interesting opportunities in pharmaceutical manufacturing. Its main strength is the combination of a fairly high chemical selectivity, with simplicity and flexibility of use. With the transmission geometry applied to Raman spectroscopy much of the remaining obstacles with the technique appear to be solved. The long debated sampling problem for tablets will not be an issue in transmission mode and one can start to benefit from the demonstrated robustness and simplicity that is offered. It will be interesting to follow the future applications of Raman spectroscopy, on the verge of taking the final step to become a widespread tool in pharmaceutical development and manufacturing. References 1. M. Blanco, J. Coello, H. Iturriaga, S. Maspoch, C. de la Pezuela, Near-infrared Spectroscopy in the Pharmaceutical Industry, Analyst 123(8), 135R-150R (1998). 2. B. F. MacDonald, K. A. 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[ ] RAMAN Jonas Johansson started his career at Division of Atomic Physics at Lund University as a PhD student and received his doctor degree in 1993. After the dissertation, he was appointed research scientist and lecturer which he continued for 5 years. Main research areas were medical diagnostics, remote sensing of biomass and analytical separation techniques. In 1997, he joined AstraZeneca as a specialist in spectroscopy at Product Analysis in Mölndal. Since 2003, he has a position as Principal Scientist in Vibrational Spectroscopy and is Adjunct Professor at Lund University. Besides vibrational spectroscopy responsibilities include fiberoptic dissolution testing, process analysis, laser safety, etc. Anders Sparén is an Associate Principal Scientist in process characterisation at AstraZeneca Sweden. He joined the company in 1995 and has mainly been working with analytical method development, vibrational spectroscopy, the design of experiments, multivariate analysis and process analytical technology. He has an MSc in chemical engineering and a PhD in analytical chemistry, both from the Royal Institute of Technology in Stockholm, Sweden. Olof Svensson received both his M.Sc. and Ph.D. degrees in chemistry from Göteborg University, Sweden, in 1989 and 1999, respectively. He spent one year between 1999 and 2000 together with professors John F. MacGregor and Theodora Kourti as a post doctoral fellow at McMaster University in Canada working with the development of multivariate techniques for evaluation of data from chemical processes and vibrational spectroscopy measurements. He has been employed by AstraZeneca since 1989, and holds at present a position as an associate principal scientist in vibrational spectroscopy. Prof. Staffan Folestad, is a Senior Principal Scientist in AstraZeneca with responsibilities in the fields of Manufacturing Sciences and Quality by Design (QbD), including Process Analytical Technology (PAT). He obtained his PhD in Chemistry 1985 from the University of Göteborg. As an Associate Professor, he directed research activities comprising development of Capillary Separation Techniques, Laser-Based Spectroscopy and UltraTrace Analysis. In 1994, he joined AstraZeneca (ex Astra Hässle). As a Senior Principal Scientist he has been leading global R&D activities in PAT and QbD during a decade. As a Leader for the AZ Center of Excellence for PAT, his research comprised development of advanced in-situ Process Sensors and in silico Process Modeling. Between 1999 and 2006, he held a part time position as Professor in Analytical Chemistry at Uppsala University. Among several external appointments he is member of the Royal Swedish Academy of Engineering Sciences, board member of the Center for Chemical Process Engineering at Chalmers University of Technology, Göteborg, and has been member of the EFPIA PAT Topic Group. He also chaired the first pan-european science conferences EuPAT1, EuPAT2, and EuPAT3. Recently he received the New Safe Medicines Faster Award 2007 from the European Federation of Pharmaceutical Sciences (EUFEPS). Dr Mike Claybourn is a Principal Scientist at AstraZeneca based at Charnwood in the UK. He has worked in the area of vibrational spectroscopy for nearly 25 years in both academia and industry. His current focus is new approaches in vibrational spectroscopy to support the design and development of pharmaceutical processes and formulations. To correspond with the author, please email him at: Anders.Sparen@astrazeneca.com To read more on Raman, please visit our website (americanpharmaceuticalreview.com) and type Raman in the advanced search box APR January February 2009 71