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In situ monitoring of crystalline transformation of carbamazepine using probe type low frequency Raman spectroscopy Motoki Inoue, Hiroshi Hisada, Tatsuo Koide, James Carriere, Randy Heyler, and Toshiro Fukami Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00329 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Organic Process Research & Development

In situ monitoring of crystalline transformation of carbamazepine using probe type low frequency Raman spectroscopy Motoki Inoue1*, Hiroshi Hisada1, Tatsuo Koide2, James Carriere3, Randy Heyler3, and Toshiro Fukami1 1

Department of Molecular Pharmaceutics, Meiji Pharmaceutical University, Kiyose, Tokyo 204-

8588 Japan, 2Division of Drugs, National Institute of Health Sciences, Setagaya, 158-8501 Tokyo Japan, 3 Ondax Inc. Duarte Rd Monrovia, 91016 CA USA *Corresponding author: Motoki INOUE PhD Department of Molecular Pharmaceutics, Meiji Pharmaceutical University 2-522-1 Noshio, Kiyose, Tokyo 204-8588 Japan TEL & FAX: +81-42-495-8915 E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Crystallization is one of the most useful processes for the separation and purification of crystalline compounds. In crystallization processes, real-time monitoring is essential to obtain constant quality of crystalline compounds. This paper is the first to report in situ monitoring of crystalline transformations of active pharmaceutical ingredients by probe-type low frequency Raman spectroscopy. In this study, carbamazepine was used as model active pharmaceutical ingredient. We attempted to monitor crystalline transformation of carbamazepine during heattreatment and addition of solvent in a one-pot reaction. When carbamazepine form III was heated to 170 ºC, the indicative spectrum of carbamazepine form I appeared over time. Subsequent addition of ethanol with heat treatment caused the carbamazepine form I spectrum to disappear. After cooling to room temperature, the spectrum of carbamazepine form III reappeared. To optimize the solvent ratio, we monitored carbamazepine form III as it dispersed into a mixture of ethanol/water with different compositions (75/25, 62.5/37.5, 50/50, 37.5/62.5, and 25/75(v/v)). The spectra of carbamazepine dihydrate were observed in all solvent compositions. When the mixture of ethanol/water was 62.5/37.5 (v/v), the conversion time to carbamazepine dihydrate was fastest. Therefore, probe-type low frequency Raman spectroscopy can be used for in situ monitoring of crystalline transformation and may become a useful process analytical technology technique. KEYWORDS: low frequency Raman spectroscopy; Crystallization; Process analytical technology; Carbamazepine


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1. INTRODUCTION Crystallization is often used to isolate the active pharmaceutical ingredient (API) since it is typically an efficient method to purify compounds.1, 2 However, problems in the crystallization process can occur, including 1) the quality of crystallinity may not be stable in every batch, and 2) after scaling-up the process, polymorphs or pseudo-polymorphs of API were sometimes generated in the system. To ensure high quality crystalline API product, it is one option that is frequently used to monitor the process.

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In practice, particle size based measurements using

focused beam reflectance measurement (FBRM) technology,6 and vibrational spectroscopy including near infrared (NIR), infrared (IR) spectroscopy and Raman spectroscopy are commonly used as PAT.

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NIR and IR are listed in US, EU and Japanese pharmacopoeia as

general methods. However, NIR and IR measurements have several issues; 1) in the case of the compounds which have peaks near the water peak, adjustment of optical path length for each measurement is required due to the high molecular extinction coefficient of water, 7 2) The NIR spectral region is the overtone and combination region of the mainly C-H stretching which are generally broad and overlapping.15, 16 On the other hand, Raman spectroscopy avoids these issues and can obtain compound-specific spectra with a non-contact, non-destructive test. Low frequency Raman spectroscopy can be especially useful in differentiating crystalline forms derived from lattice vibrations and has been a ubiquitous technique in the pharmaceutical industry.17-21 This feature can identify crystalline polymorphs and analyze API with different crystalline forms. Therefore, it is expected that probe type low frequency Raman spectroscopy is suitable for in situ monitoring of crystallization reaction and solvent-mediated transformation of API. In situ monitoring of polymorphic transformation has been measured and reported using a low frequency Raman spectrometer equipped with a heating apparatus.22 The report indicated


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that low frequency Raman spectroscopy is a molecular-specific tool for tracking phase transitions. However, the reported tool was limited to monitoring small amount of samples. In order to obtain constant quality of the crystalline form, in situ process monitoring using probetype Raman spectroscopy is useful. In this study, we investigate the properties of probe type low frequency Raman spectroscopy; we demonstrate its ability for monitoring one-pot crystalline transformation and the optimization of solvent conditions for crystalline transformations.

2. EXPERIMENT 2.1. Materials Carbamazepine (CBZ) has been previously used as a model API by groups engaged in the study of crystalline polymorphism and was purchased from Tokyo Chemical Industry (Tokyo, JAPAN).

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Crystalline forms of CBZ were confirmed by comparing the powder X-ray

diffraction (PXRD) results with published data (Supporting Information S1). Ethanol was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, JAPAN). 2.2. Low frequency Raman spectroscopy Scheme 1 shows the experimental setup consisting of a spectrometer and a low frequency Raman probe for in situ monitoring of crystalline transformations. THz-Raman® Probe system (TR-PROBE, excitation laser wave length: 853 nm, notch filter, Ondax Inc., Monrovia, CA, USA) with in BallProbe (1/2 inch in diameter, 10 inch in length by MarqMetrix, Seattle, WA, USA) was attached to the RXN2 Raman spectrometer (Kaiser Optical Systems Inc, Ann Arbor,


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MI, USA). The tip of the probe was immersed into sample heated with oil bath. A dedicated computer system was used to collect the spectra with the HoloGrams software version 4.2 (Kaiser Optical Systems Inc). Measurement region is -900 cm-1 - +900 cm-1 at 853 nm excitation and the spectral resolution is 4 cm-1. The obtained spectra were pre-treated by normalization, and then analyzed with HoloReact software (Kaiser Optical Systems Inc). The analysis region is 10200 cm-1.24 Real-time measurements were obtained with an analysis program called HoloReact (Kaiser Optical Systems Inc). The analysis of crystallization was performed by a multivariate curve resolution (MCR) algorithm installed in the HoloReact program (Supporting Information Figure S2).

3. RESULTS AND DISCUSSION 3.1. In situ monitoring of CBZ crystalline transformation Tian et al. reported the conversion of dihydrate (DH) from the mixture of each polymorphic form of CBZ (I and III) using conventional Raman spectroscopy.

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In the report, they put CBZ

polymorphs into sample component of FT-Raman spectrometer to collect spectra by off-line measurements and quantified each polymorph by partial least squares (PLS). However, these spectra have only slight differences between the polymorphs, and they were measured by off-line measurement. In our study, we monitored the crystalline transformation of carbamazepine under heat-treatment and the addition of solvent in situ and real-time using probe-type low frequency Raman spectroscopy. Figure 1 shows the low-frequency Raman spectrum of CBZ I, III, and DH, spectrum are different. Since clear differences are observed between the polymorphs, it was expected that CBZ crystalline transformations can be monitored using MCR. One gram of CBZ


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form III was heated to 170 ºC for 3 hours using an oil bath.23 Next, CBZ was dissolved in 20 mL of ethanol and heated to 60 ºC then cooled to room temperature (R.T.). Figure 2 shows the changes in the low frequency Raman spectrum with these different temperature and solvent conditions. When CBZ was heated to 170 ºC, indicative peaks at 169 and 183 cm-1 of CBZ form III disappeared, and the peak at 175 cm-1 derived from form I appeared. Consequently, spectra of CBZ form I and III disappeared after the addition of ethanol and heat-treatment, as both polymorphs dissolved. When the vessel was returned to ambient temperature, the specific peaks of 169, 183 cm-1 derived from form III gradually reappeared. This is the first reported example of crystalline transformation of API monitoring by in situ probe-type low frequency Raman spectroscopy. 3.2. Optimization of solvent ratio of ethanol and water Next we investigated the effect of solvent conditions on polymorph transition by suspending CBZ in different ratios of ethanol and water. The transition rate to DH from form III was examined with 25/75, 37.5/62.5, 50/50, 62.5/37.5, and 75/25 ratios of ethanol and water. As the duration of each process was 20 min and 2 times/ min, all the spectra were collected per process. Figure 3 shows the change in the low frequency Raman spectra, when CBZ form III was suspended in the different ratios of ethanol/water. In all compositions, the characteristic peak of form III at 39 cm-1 decreased immediately, and the specific peak of DH at 111 cm-1 increased gradually over time. Therefore, this result indicated that we successfully monitored the conversion of CBZ to DH from CBZ III. MCR is one of the chemometric tools that can be used to extract one spectral component from a multi-component spectrum. When multiple components are generated in the reaction process, the relative concentration and time profile of each component can be obtained using MCR. Venkat et al. reported the characterization of


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hydrogenation reactions using Raman spectroscopy and MCR data analysis.26 We applied MCR to our data, resulting in a pure compound (PC-1) with peaks at 21, 75, 111, 171 cm-1, and another pure compound (PC-2) with peaks at 39, 105, 169, 183 cm-1. It was determined that PC-1 and PC-2 corresponded to CBZ DH and form III, respectively (Supporting Information Figure S2). From this prediction, each factor of PC-1 and PC-2 was calculated and plotted in Figure 4. The 50% conversion time is the fastest at 62.5/37.5 = ethanol/water. It was thought that the solubility of CBZ is relatively high and hydrated DH is easily precipitated. On the other hand at 25/75 = ethanol/water, the solubility of CBZ was decreased with increasing water content, suggesting that the particles of CBZ less homogeneous. Therefore, the 25/75 data was scattered because it becomes difficult to uniformly disperse the CBZ compared with the other solvent ratios. Therefore, the probe-type low frequency Raman spectroscopy can be used not only for crystalline form monitoring, but also optimization of the composition of crystallization solvent ratio. In conclusion, it was clarified that probe-type low frequency Raman spectroscopy can detect in situ crystalline transformation of API without any sampling. Since probe-type low frequency Raman spectroscopy can calculate reaction rates using data obtained by MCR analysis, it can be applied as a process analytical technology (PAT) tool.

ACKNOWLEDGEMENT This work was supported by JSPS KAKENHI Grant Numbers 26460045 (to T.F.) and 16K18867 (to M.I.)


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Scheme 1 Experimental setup for in situ monitoring of crystalline transformation (a) RXN2 Raman spectrometer, (b) THz-Raman® Probe system, (c) holographic transmission grating, (d) optical fiber, (e) ultra narrow-band notch filter, (f) probe, (h) filter, (g) excitation laser (853 nm)


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Figures

Figure 1 Low-frequency Raman spectrum of CBZ I, III, and DH.


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Figure 2 Changes in the low-frequency Raman spectrum of CBZ I, III, and DH with different conditions: heat-treatment to 170 ºC (a), dissolved into ethanol (b), and the return to room temperature (c).


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Figure 3 Changes in the low-frequency Raman spectrum with different ratios of ethanol/water.


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Relative concentration

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0.8

Ethanol/water (v/v) 25/75＝DH：○, III: ● 37.5/62.5＝DH：△, III: ▲ 50/50＝DH：□, III: ■ 62.5/37.5＝DH：▽, III: ▼ 75/25＝DH：◇, III: ◆

0.6 0.4 0.2 0

0

5

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Time (min) Figure 4 Changes of CBZ III and DH composition over time with different ratios of ethanol / water.


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