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Analysis and stability of polymorphs in tablets: The case of Risperidone
Talanta 71 (2007) 1382–1386
Analysis and stability of polymorphs in tablets: The case of Risperidone I. Karabas a,b , M.G. Orkoula a,b , C.G. Kontoyannis a,b,∗ a
b
Department of Pharmacy, University of Patras, Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, P.O. Box 1414, Patras GR-26500, Greece
Received 23 March 2006; received in revised form 13 June 2006; accepted 6 July 2006 Available online 5 September 2006
Abstract Identification of the crystal phase of an active pharmaceutical ingredient (API) in a pharmaceutical tablet is of outmost importance since different polymorphs exhibit different physicochemical properties. Furthermore, some of the crystal phases are protected by patents. Identification of Risperidone polymorph A in film coated commercial tablets was attempted using IR spectroscopy, Raman spectroscopy and X-ray powder diffraction (XRPD). The stability of this polymorph through time and during the manufacturing process was also examined. The inability of IR and Raman techniques to identify the presence of polymorph A in the tablets, despite their lower detection limits for Risperidone, left the XRPD as the only technique that could be used for identifying the presence of Risperidone A against the other crystal phases in the presence of the excipients. Polymorph A was proved to be stable during the manufacturing process and after a storage period of 2 years. © 2006 Elsevier B.V. All rights reserved. Keywords: X-ray powder diffraction; FT-IR; Raman; Risperidone; Polymorphs; Tablets
1. Introduction Many pharmaceutical solids exhibit polymorphism, which is frequently defined as the ability of a substance to exist as two or more crystalline phases that have different arrangements and/or conformations of the molecules in the crystal lattice [1]. Polymorphism may also include solvation or hydration products (also known as pseudopolymorphs) and amorphous forms [2]. Polymorphs and/or solvates of an active pharmaceutical ingredient (API) can have different chemical and physical properties such as melting point, chemical reactivity, apparent solubility, dissolution rate, optical and electrical properties, vapor pressure and density. These properties can have a direct impact on the process-ability of drug substances and the quality/performance of drug products, such as stability, dissolution and bioavailability. One polymorph may convert to another during manufacturing and storage, particularly when a metastable form is used. ∗ Corresponding author at: Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, P.O. Box 1414, Patras GR-26500, Greece. Tel.: +30 2610 997727/969328; fax: +30 2610 997658. E-mail address: [email protected] (C.G. Kontoyannis).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.07.009
Since an amorphous form is thermodynamically less stable than any crystalline form, inadvertent crystallization from an amorphous drug substance may occur. As a consequence of the higher mobility and ability to interact with moisture, amorphous drug substances are also more likely to undergo solid-state reactions. In addition, phase conversions of some drug substances are possible when exposed to a range of manufacturing processes [3]. Milling/micronization operations may result in polymorphic form conversion of a drug substance. In the case of wet granulation processes, where the usual solvents are aqueous, one may encounter a variety of interconversions between anhydrates and hydrates, or between different hydrates. Spray-drying processes have been shown to produce amorphous drug substances. The most stable polymorphic form of a drug substance is often used because it has the lowest potential for conversion from one polymorphic form to another while the metastable form may be used to enhance the bioavailability. Therefore, it is essential to ensure that polymorphic differences are addressed via design and control of formulation and process conditions to physical and chemical stability of the product over the intended shelflife. Based on these, some regulatory agents may refuse to approve an application for a new dug referencing a listed drug if the
I. Karabas et al. / Talanta 71 (2007) 1382–1386
application contains insufficient information to show that the drug substance is the “same” and remains the “same” as that of the reference listed drug during the expected shelf-life. Even in the cases where the regulatory agent accepts the crystal phases as equivalent, an additional complication stems from the patented protection of some metastable polymorphs. Physicochemical measurements and techniques that are commonly used to determine the crystal phases are [4]: melting point (including hot-stage microscopy), solid-state IR, X-ray powder diffraction, thermal analysis procedures (DSC, TGA and DTA), Raman spectroscopy, optical microscopy and solid-state NMR. Microscopy, thermal analysis methodology and solid-state NMR are generally considered as sources of supporting information. The definite criterion for the existence of polymorphism is via demonstration of a non-equivalent crystal structure, usually by comparison of the X-ray diffraction patterns. Although most of these techniques can be easily applied to pure crystal phases it is mostly Raman spectroscopy and X-ray powder diffraction (XRPD) that have been used for rapid screening and identification of new crystal phases in high-throughput crystallization [5]. Despite the large array of available techniques the identification of an active ingredient in a formulation in the presence of numerous excipients is neither easy nor straightforward. In this work, an attempt was made to identify the presence of the crystal phases of the active ingredient Risperidone, a benzisoxazole derivative that is used as an antipsychotic drug, in commercially available film coated tablets, using Raman and IR spectroscopy as well as XRPD. Furthermore, the possibility of the transformation of the API from one crystal phase to another with time or due to the manufacturing process was examined, through these techniques. There are three known polymorphs of Risperidone (A, B and E) for which only the XRPD spectra are available [6]. 2. Experimental 2.1. Materials used Risperidone is commercially available in film-coated tablets containing 0.5, 1, 2, 4, 6 and 8 mg of the active ingredient. Film coated Risperidone A tablets with 8 mg strength as well as placebo tablets and pure Risperidone form A were kindly provided by Specifar Pharmaceuticals SA, Athens, Greece. The tablets contained the following excipients: lactose, maize starch, microcrystalline cellulose, Hypromellose 2910, magnesium stearate, colloidal anhydrous silica, sodium lauryl sulphate and propylene glycol. Each tablet weighted 185 mg. The film coating was carefully removed from the tablets with a scalpel. Attempts were made to prepare Risperidone B and E following the recipes given in the relevant patent [4]. One recipe yielded the form B while several others yielded mixtures of A and B. Polymorph E could not be obtained. 2.2. Characterization of the samples The X-ray powder diffraction analysis was performed (Philips 1830/40) on finely powdered samples using the Cu K␣
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Fig. 1. XRPD spectra of the three known Risperidone polymorphs (A, B and E).
(40 kV and 30 mA) radiation and Ni filter with a scanning speed of 0.005◦ 2θ s−1 . Time constant was set at 2 s. XRPD spectra of the pure A and B Risperidone polymorphs are shown in Fig. 1. The Raman spectra were recorded using a FRA-106/S FT-Raman (Bruker) with the following characteristics: the laser excitation line used, was the 1064 nm of a Nd:YAG laser. A secondary filter was used to remove the Rayleigh line. The scattered light was collected at an angle of 180◦ . The system was equipped with a liquid N2 cooled Ge detector (D 418). The power of the incident laser beam was about 370 mW on the sample’s surface. Typical spectral line width was 0.5 cm−1 while the recorded spectra were the average of 300 scans. Raman spectra from the samples that were in the form of powders, e.g. raw Risperidone, were recorded by placing in aluminum cylindrical cups with 10 mm diameter and 4 mm height having a cavity in the center of 2 mm diameter and 1 mm depth. Homogeneity of the powders was verified by obtaining five Raman spectra for each mixture, focusing the laser beam at randomly selected spots of the surface. Spectra of uncoated and the as-received intact film coated tablets were recorded by placing directly into laser’s beam path. Raman spectra of the pure A and B Risperidone polymorphs are shown in Fig. 3A and B, respectively. All infrared spectra were recorded from 4000–400 cm−1 , using the EQUINOX 55 (Bruker) FT-IR spectrometer. Each spectrum was the average of 200 scans using a spectral resolution of 4 cm−1 . The IR spectra were recorded and stored using a spectroscopic software (OPUS, Version 2.0). FT-IR spectra of the pure substances and tablets were obtained by grounding and mixing 2 mg in an agate mortar with 200 mg of desiccated IR grade potassium bromide (Merck) in a dry box. Pellets were obtained by compression of the powdered mixtures in a stainlesssteel die at 8 MPa for 5 min. IR spectra of the pure A and B Risperidone polymorphs are shown in Fig. 5A and B, respectively.
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3. Results and discussion 3.1. Comparison of spectra of Risperidone polymorphs 3.1.1. X-ray powder diffraction spectra The XRPD spectra of the synthetically prepared A and B crystal phases of Risperidone can be seen in Fig. 1 and are in accordance with the spectra quoted in the relevant patent [4]. In Fig. 1, the XRPD spectrum of polymorph E, from ref. [4], can also be seen. It is apparent that there are significant differences in the spectra which can be used to identify their appearance in the sample. For example, Risperidone A exhibits a major reflection at 21.05◦ 2θ while Risperidone B and E at 21.6◦ 2θ. Furthermore, the peak at 18.83◦ and the double peak at 23.03◦ and 23.43◦ 2θ can differentiate A polymorph against B and E. The presence of the other two crystal phases (B and E) can be verified by existence of peaks 17.4◦ and the double peak at 15.5◦ and 16.4◦ , respectively. Purity of A crystal form can be ascertained by absence of any peak at the area of 17.5◦ and 27◦ 2θ. Though differentiation between the two polymorphs in a mixture of two appears to be feasible, the discrimination should also be checked in the case of a formulation where a number of other substances are present. An XRPD spectrum of the placebo of the commercial tablet is quoted in Fig. 2 as well as the spectrum of a powdered tablet containing 8 mg Risperidone in a total amount of 185 mg of the tablet. It is noted that the API cannot be detected in the spectrum of the tablet which is overwhelmed by the presence of placebo, indicating that XRPD detection limit in the case of Risperidone is higher than 8 mg/tablet. 3.1.2. Raman spectra The Raman spectra of the synthetically prepared A and B crystal phases of Risperidone can be seen in Fig. 3. There are numerous differences between Risperidone A and B such as the
Fig. 2. XRPD spectra of: (A) Risperidone A; (B) Risperidone A recorded from the same powder 2 years after obtaining spectrum A; (P) placebo of Risperidone; (C) powdered tablet of commercial 8 mg Risperidone; (D) powdered tablet with 25% (w/w) Risperidone A; (E) powdered tablet with 25% (w/w) Risperidone A recorded from the same powdered tablet 2 years after obtaining spectrum D.
Fig. 3. FT-Raman spectra of: (A) Risperidone A; (B) Risperidone B; (C) tablet with 25% (w/w) Risperidone; (P) Risperidone placebo; (T) commercial tablet with 8 mg Risperidone (uncoated).
double peak at 3051 and 3069 cm−1 for Risperidone A, having the 3069 cm−1 vibration stronger than the former, while Risperidone B exhibits a single peak at 3055 cm−1 with a shoulder at 3069 cm−1 . An additional strong peak for polymorph B can be seen at 2963 cm−1 . Details of some of the additional differences between Risperidone A and B can be seen in Fig. 4. Polymorph A exhibits a double peak at 432 and 440 cm−1 , a double peak at 1260 and 1271 cm−1 and a single peak at 1397 cm−1 as opposed to polymorph B whereas single peaks appear at 434 cm−1 , 1266 cm−1 and a double peak at 1390 and 1402 cm−1 . The Raman spectra that were excited from 8 mg intact commercial tablets, after the removal of the coating, as well as from tablets of placebo, can also be seen in Fig. 3. It can be seen that though the concentration of the API is only 4.3% (w/w) in the commercial tablet, its presence can be certified easily as the spectra (Fig. 3T and P) differ in three regions, marked with arrows. These differences are probably sufficient to identify the presence of API in the tablet though, as shown earlier, it is not possible to distinguish between A and B crystal forms. At this point it is worth noting that although the detection limit of Raman spectroscopy for Risperidone is lower than the
Fig. 4. FT-Raman spectra of: (A) Risperidone A; (B) Risperidone B; (C) tablet with 25% (w/w) Risperidone; (P) Risperidone placebo.
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Fig. 5. FT-IR spectra of: (A) Risperidone A; (B) Risperidone B; (C) powdered tablet with 25% (w/w) Risperidone; (T) powdered commercial tablet with 8 mg Risperidone (uncoated); (P) Risperidone placebo.
respective for XRPD this advantage is not useful for the task at hand. 3.1.3. IR spectra The IR spectra of the synthetically prepared A and B crystal phases of Risperidone, the placebo and the uncoated powdered commercial tablet with 8 mg API can be seen in Fig. 5. The major bands for both polymorphs appear at 1535 and 1650 cm−1 , but the major difference between them is that the vibration at 1650 cm−1 appears as single peak in polymorph A whereas in polymorph B multiple peaks can be observed. In the spectrum obtained from the powdered tablet (Fig. 5T) the presence of the API could not be detected with the exception of two minor noiselike wide bands, marked with arrows, which appear at around 1535 and 1650 cm−1 . Unfortunately, these bands cannot be used for identification of the API. 3.2. Polymorph A stability tests 3.2.1. Twenty-five percent Risperidone tablets Among the targets of this work was to verify if Risperidone A in tablets transforms with time or through the manufacturing process to a different crystal phase. Since neither the FT-IR nor the Raman was capable of differentiating between the crystal phases, despite their better detection limits than the XRPD, a different approach was followed. A labscale batch of Risperidone tablets with 25% (w/w) Risperidone was manufactured by the pharmaceutical company following the same manufacturing process as with the commercially available concentrations. Working with a 25% (w/w) of Risperidone was chosen after testing mixtures of 10 and 15% (w/w). In the XRPD spectra obtained from the 10 and 15% (w/w) mixtures (not shown) only the main reflection of Risperidone A could be barely observed at 21.0◦ 2θ on the top of a placebo reflection indicating that the XRPD detection limit for Risperidone A was approximately 10% (w/w). The Raman and IR spectra recorded from the same mixtures (not
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shown) were not more helpful than those recorded from the tablet with 8 mg API. Although a 10% (w/w) mixture could have been used for testing the stability of A polymorph based on XRPD data, a 25% (w/w) was thought to be more helpful for the analysis due to the appearance of additional Risperidone reflections, as well as to have Raman and IR spectra that might permit the identification of Risperidone A. The Raman spectrum obtained from the tablet with 25% (w/w) Risperidone A can be seen in Fig. 3C. The increased quantity of the active ingredient allows the positive identification of Risperidone A in these tablets. The wide band at 436 cm−1 (Fig. 4C) stems from the addition of polymorph A double peak at 432 and 440 cm−1 and the placebo vibration at 436 cm−1 (Fig. 4P). The polymorph A vibrations at 1271 and 1397 cm−1 (Fig. 4C) as well as at 3051 and 3069 cm−1 (Fig. 3C) are unaltered due to the absence of the respective placebo vibrations. It should be noted though that the presence of these vibrations does not rule out the possibility of the simultaneous presence of small quantity of B crystal phase. The FT-IR spectrum obtained from the tablet with 25% (w/w) Risperidone A can be seen in Fig. 5C. The presence of a single peak at 1650 cm−1 indicates the presence of Risperidone A but this does not also exclude the possibility of the simultaneous presence of small amounts of B crystal phase since the multiple peaks of B at 1650 cm−1 can be hidden under the apparently single peak at 1650 cm−1 . 3.2.2. Manufacturing process stability test In the sample with 25% (w/w) Risperidone (Fig. 2D) the characteristic reflections of Risperidone polymorph A (Fig. 2A), as well as the diffraction peaks of the compounds consisting the placebo (Fig. 2P), were detected. Furthermore, no additional reflections were detected including the most prominent diffraction peaks at 21.6◦ 2θ of polymorphs B and E. This indicates that during the manufacturing process no transformation of Risperidone A took place. Therefore, in the case of Risperidone, XRPD proved to be the only reliable technique for identifying the presence of A polymorph and simultaneously exclude the possibility of the presence of the other two polymorphs. 3.2.3. Stability tests with time In order to test the stability of Risperidone A with time, the XRPD spectra of raw Risperidone A in powder form were recorded with a time interval of 2 years (Fig. 2A and B). The XRPD spectra of tablets with 25% (w/w) Risperidone A were also recorded shortly after their manufacture (Fig. 2D) and after 2 years (Fig. 2E). In both cases, and in order to simulate reallife scenario, in the interim period the powders were placed in a laboratory cabinet for protection against the light but neither the temperature nor the humidity was controlled. No difference was observed indicating there is no transformation or degradation of the active ingredient after a storage period of 2 years. 4. Conclusions From the application of X-ray powder diffraction, FT-IR and FT-Raman spectroscopy on the above mentioned specimens and
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raw materials as well as on tablets, it is apparent that there is no straightforward process that can be applied in order: (a) to identify the presence of different crystal phases of the active ingredient and/or (b) to test their stability. Product oriented methodologies should be devised using all available techniques depending on a number of factors including the percentage of the active ingredient in the formulation, the problems arising from the presence of the excipients, the detection limits of the different techniques for the active substance in question and the availability of all crystal forms, including the patented. For Risperidone A, the case study of this work, it can be safely concluded that no transformation takes places during the manufacturing process or during a storage period of 2 years. None of the techniques used could positively identify the presence of polymorph A in the commercial tablets with 8 mg API. In tablets with increased API strength IR and Raman proved to be incapable of excluding the simultaneous presence of other crystal phases although positive identification of polymorph A was accomplished. Only the information obtained from XRPD could be used for the identification of form A and the exclusion of forms B and E.
Acknowledgments Authors would like to thank Specifar Pharmaceuticals SA for its help and in particular Ms. Eirini Vasilopoulos from the Department of Product Development. References [1] D.J. Grant, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 1–31 (Chapter 1). [2] International Conference on Harmonization Q6A Guideline: Specifications for New Drug Substances and Products: Chemical Substances, October 1999. [3] H.G. Brittain, E.F. Fiese, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 279–330 (Chapter 8). [4] H.G. Brittain, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 227–271 (Chapter 6). [5] S. Morissette, O. Almarsson, M. Peterson, J. Remenar, M. Read, A. Lemmo, S. Ellis, M. Cima, C. Gardner, Adv. Drug Deliv. Rev. 56 (2004) 275. [6] B. Krochmal, D. Diller, B.-Z. Dolitzky, J. Aronhime, Preparation of Risperidone, U.S. Patent No. 6,750,341 and PCT/WO 02/12200 A1, Feb 14, 2002.
Analysis and stability of polymorphs in tablets: The case of Risperidone I. Karabas a,b , M.G. Orkoula a,b , C.G. Kontoyannis a,b,∗ a
b
Department of Pharmacy, University of Patras, Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, P.O. Box 1414, Patras GR-26500, Greece
Received 23 March 2006; received in revised form 13 June 2006; accepted 6 July 2006 Available online 5 September 2006
Abstract Identification of the crystal phase of an active pharmaceutical ingredient (API) in a pharmaceutical tablet is of outmost importance since different polymorphs exhibit different physicochemical properties. Furthermore, some of the crystal phases are protected by patents. Identification of Risperidone polymorph A in film coated commercial tablets was attempted using IR spectroscopy, Raman spectroscopy and X-ray powder diffraction (XRPD). The stability of this polymorph through time and during the manufacturing process was also examined. The inability of IR and Raman techniques to identify the presence of polymorph A in the tablets, despite their lower detection limits for Risperidone, left the XRPD as the only technique that could be used for identifying the presence of Risperidone A against the other crystal phases in the presence of the excipients. Polymorph A was proved to be stable during the manufacturing process and after a storage period of 2 years. © 2006 Elsevier B.V. All rights reserved. Keywords: X-ray powder diffraction; FT-IR; Raman; Risperidone; Polymorphs; Tablets
1. Introduction Many pharmaceutical solids exhibit polymorphism, which is frequently defined as the ability of a substance to exist as two or more crystalline phases that have different arrangements and/or conformations of the molecules in the crystal lattice [1]. Polymorphism may also include solvation or hydration products (also known as pseudopolymorphs) and amorphous forms [2]. Polymorphs and/or solvates of an active pharmaceutical ingredient (API) can have different chemical and physical properties such as melting point, chemical reactivity, apparent solubility, dissolution rate, optical and electrical properties, vapor pressure and density. These properties can have a direct impact on the process-ability of drug substances and the quality/performance of drug products, such as stability, dissolution and bioavailability. One polymorph may convert to another during manufacturing and storage, particularly when a metastable form is used. ∗ Corresponding author at: Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, P.O. Box 1414, Patras GR-26500, Greece. Tel.: +30 2610 997727/969328; fax: +30 2610 997658. E-mail address: [email protected] (C.G. Kontoyannis).
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.07.009
Since an amorphous form is thermodynamically less stable than any crystalline form, inadvertent crystallization from an amorphous drug substance may occur. As a consequence of the higher mobility and ability to interact with moisture, amorphous drug substances are also more likely to undergo solid-state reactions. In addition, phase conversions of some drug substances are possible when exposed to a range of manufacturing processes [3]. Milling/micronization operations may result in polymorphic form conversion of a drug substance. In the case of wet granulation processes, where the usual solvents are aqueous, one may encounter a variety of interconversions between anhydrates and hydrates, or between different hydrates. Spray-drying processes have been shown to produce amorphous drug substances. The most stable polymorphic form of a drug substance is often used because it has the lowest potential for conversion from one polymorphic form to another while the metastable form may be used to enhance the bioavailability. Therefore, it is essential to ensure that polymorphic differences are addressed via design and control of formulation and process conditions to physical and chemical stability of the product over the intended shelflife. Based on these, some regulatory agents may refuse to approve an application for a new dug referencing a listed drug if the
I. Karabas et al. / Talanta 71 (2007) 1382–1386
application contains insufficient information to show that the drug substance is the “same” and remains the “same” as that of the reference listed drug during the expected shelf-life. Even in the cases where the regulatory agent accepts the crystal phases as equivalent, an additional complication stems from the patented protection of some metastable polymorphs. Physicochemical measurements and techniques that are commonly used to determine the crystal phases are [4]: melting point (including hot-stage microscopy), solid-state IR, X-ray powder diffraction, thermal analysis procedures (DSC, TGA and DTA), Raman spectroscopy, optical microscopy and solid-state NMR. Microscopy, thermal analysis methodology and solid-state NMR are generally considered as sources of supporting information. The definite criterion for the existence of polymorphism is via demonstration of a non-equivalent crystal structure, usually by comparison of the X-ray diffraction patterns. Although most of these techniques can be easily applied to pure crystal phases it is mostly Raman spectroscopy and X-ray powder diffraction (XRPD) that have been used for rapid screening and identification of new crystal phases in high-throughput crystallization [5]. Despite the large array of available techniques the identification of an active ingredient in a formulation in the presence of numerous excipients is neither easy nor straightforward. In this work, an attempt was made to identify the presence of the crystal phases of the active ingredient Risperidone, a benzisoxazole derivative that is used as an antipsychotic drug, in commercially available film coated tablets, using Raman and IR spectroscopy as well as XRPD. Furthermore, the possibility of the transformation of the API from one crystal phase to another with time or due to the manufacturing process was examined, through these techniques. There are three known polymorphs of Risperidone (A, B and E) for which only the XRPD spectra are available [6]. 2. Experimental 2.1. Materials used Risperidone is commercially available in film-coated tablets containing 0.5, 1, 2, 4, 6 and 8 mg of the active ingredient. Film coated Risperidone A tablets with 8 mg strength as well as placebo tablets and pure Risperidone form A were kindly provided by Specifar Pharmaceuticals SA, Athens, Greece. The tablets contained the following excipients: lactose, maize starch, microcrystalline cellulose, Hypromellose 2910, magnesium stearate, colloidal anhydrous silica, sodium lauryl sulphate and propylene glycol. Each tablet weighted 185 mg. The film coating was carefully removed from the tablets with a scalpel. Attempts were made to prepare Risperidone B and E following the recipes given in the relevant patent [4]. One recipe yielded the form B while several others yielded mixtures of A and B. Polymorph E could not be obtained. 2.2. Characterization of the samples The X-ray powder diffraction analysis was performed (Philips 1830/40) on finely powdered samples using the Cu K␣
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Fig. 1. XRPD spectra of the three known Risperidone polymorphs (A, B and E).
(40 kV and 30 mA) radiation and Ni filter with a scanning speed of 0.005◦ 2θ s−1 . Time constant was set at 2 s. XRPD spectra of the pure A and B Risperidone polymorphs are shown in Fig. 1. The Raman spectra were recorded using a FRA-106/S FT-Raman (Bruker) with the following characteristics: the laser excitation line used, was the 1064 nm of a Nd:YAG laser. A secondary filter was used to remove the Rayleigh line. The scattered light was collected at an angle of 180◦ . The system was equipped with a liquid N2 cooled Ge detector (D 418). The power of the incident laser beam was about 370 mW on the sample’s surface. Typical spectral line width was 0.5 cm−1 while the recorded spectra were the average of 300 scans. Raman spectra from the samples that were in the form of powders, e.g. raw Risperidone, were recorded by placing in aluminum cylindrical cups with 10 mm diameter and 4 mm height having a cavity in the center of 2 mm diameter and 1 mm depth. Homogeneity of the powders was verified by obtaining five Raman spectra for each mixture, focusing the laser beam at randomly selected spots of the surface. Spectra of uncoated and the as-received intact film coated tablets were recorded by placing directly into laser’s beam path. Raman spectra of the pure A and B Risperidone polymorphs are shown in Fig. 3A and B, respectively. All infrared spectra were recorded from 4000–400 cm−1 , using the EQUINOX 55 (Bruker) FT-IR spectrometer. Each spectrum was the average of 200 scans using a spectral resolution of 4 cm−1 . The IR spectra were recorded and stored using a spectroscopic software (OPUS, Version 2.0). FT-IR spectra of the pure substances and tablets were obtained by grounding and mixing 2 mg in an agate mortar with 200 mg of desiccated IR grade potassium bromide (Merck) in a dry box. Pellets were obtained by compression of the powdered mixtures in a stainlesssteel die at 8 MPa for 5 min. IR spectra of the pure A and B Risperidone polymorphs are shown in Fig. 5A and B, respectively.
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3. Results and discussion 3.1. Comparison of spectra of Risperidone polymorphs 3.1.1. X-ray powder diffraction spectra The XRPD spectra of the synthetically prepared A and B crystal phases of Risperidone can be seen in Fig. 1 and are in accordance with the spectra quoted in the relevant patent [4]. In Fig. 1, the XRPD spectrum of polymorph E, from ref. [4], can also be seen. It is apparent that there are significant differences in the spectra which can be used to identify their appearance in the sample. For example, Risperidone A exhibits a major reflection at 21.05◦ 2θ while Risperidone B and E at 21.6◦ 2θ. Furthermore, the peak at 18.83◦ and the double peak at 23.03◦ and 23.43◦ 2θ can differentiate A polymorph against B and E. The presence of the other two crystal phases (B and E) can be verified by existence of peaks 17.4◦ and the double peak at 15.5◦ and 16.4◦ , respectively. Purity of A crystal form can be ascertained by absence of any peak at the area of 17.5◦ and 27◦ 2θ. Though differentiation between the two polymorphs in a mixture of two appears to be feasible, the discrimination should also be checked in the case of a formulation where a number of other substances are present. An XRPD spectrum of the placebo of the commercial tablet is quoted in Fig. 2 as well as the spectrum of a powdered tablet containing 8 mg Risperidone in a total amount of 185 mg of the tablet. It is noted that the API cannot be detected in the spectrum of the tablet which is overwhelmed by the presence of placebo, indicating that XRPD detection limit in the case of Risperidone is higher than 8 mg/tablet. 3.1.2. Raman spectra The Raman spectra of the synthetically prepared A and B crystal phases of Risperidone can be seen in Fig. 3. There are numerous differences between Risperidone A and B such as the
Fig. 2. XRPD spectra of: (A) Risperidone A; (B) Risperidone A recorded from the same powder 2 years after obtaining spectrum A; (P) placebo of Risperidone; (C) powdered tablet of commercial 8 mg Risperidone; (D) powdered tablet with 25% (w/w) Risperidone A; (E) powdered tablet with 25% (w/w) Risperidone A recorded from the same powdered tablet 2 years after obtaining spectrum D.
Fig. 3. FT-Raman spectra of: (A) Risperidone A; (B) Risperidone B; (C) tablet with 25% (w/w) Risperidone; (P) Risperidone placebo; (T) commercial tablet with 8 mg Risperidone (uncoated).
double peak at 3051 and 3069 cm−1 for Risperidone A, having the 3069 cm−1 vibration stronger than the former, while Risperidone B exhibits a single peak at 3055 cm−1 with a shoulder at 3069 cm−1 . An additional strong peak for polymorph B can be seen at 2963 cm−1 . Details of some of the additional differences between Risperidone A and B can be seen in Fig. 4. Polymorph A exhibits a double peak at 432 and 440 cm−1 , a double peak at 1260 and 1271 cm−1 and a single peak at 1397 cm−1 as opposed to polymorph B whereas single peaks appear at 434 cm−1 , 1266 cm−1 and a double peak at 1390 and 1402 cm−1 . The Raman spectra that were excited from 8 mg intact commercial tablets, after the removal of the coating, as well as from tablets of placebo, can also be seen in Fig. 3. It can be seen that though the concentration of the API is only 4.3% (w/w) in the commercial tablet, its presence can be certified easily as the spectra (Fig. 3T and P) differ in three regions, marked with arrows. These differences are probably sufficient to identify the presence of API in the tablet though, as shown earlier, it is not possible to distinguish between A and B crystal forms. At this point it is worth noting that although the detection limit of Raman spectroscopy for Risperidone is lower than the
Fig. 4. FT-Raman spectra of: (A) Risperidone A; (B) Risperidone B; (C) tablet with 25% (w/w) Risperidone; (P) Risperidone placebo.
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Fig. 5. FT-IR spectra of: (A) Risperidone A; (B) Risperidone B; (C) powdered tablet with 25% (w/w) Risperidone; (T) powdered commercial tablet with 8 mg Risperidone (uncoated); (P) Risperidone placebo.
respective for XRPD this advantage is not useful for the task at hand. 3.1.3. IR spectra The IR spectra of the synthetically prepared A and B crystal phases of Risperidone, the placebo and the uncoated powdered commercial tablet with 8 mg API can be seen in Fig. 5. The major bands for both polymorphs appear at 1535 and 1650 cm−1 , but the major difference between them is that the vibration at 1650 cm−1 appears as single peak in polymorph A whereas in polymorph B multiple peaks can be observed. In the spectrum obtained from the powdered tablet (Fig. 5T) the presence of the API could not be detected with the exception of two minor noiselike wide bands, marked with arrows, which appear at around 1535 and 1650 cm−1 . Unfortunately, these bands cannot be used for identification of the API. 3.2. Polymorph A stability tests 3.2.1. Twenty-five percent Risperidone tablets Among the targets of this work was to verify if Risperidone A in tablets transforms with time or through the manufacturing process to a different crystal phase. Since neither the FT-IR nor the Raman was capable of differentiating between the crystal phases, despite their better detection limits than the XRPD, a different approach was followed. A labscale batch of Risperidone tablets with 25% (w/w) Risperidone was manufactured by the pharmaceutical company following the same manufacturing process as with the commercially available concentrations. Working with a 25% (w/w) of Risperidone was chosen after testing mixtures of 10 and 15% (w/w). In the XRPD spectra obtained from the 10 and 15% (w/w) mixtures (not shown) only the main reflection of Risperidone A could be barely observed at 21.0◦ 2θ on the top of a placebo reflection indicating that the XRPD detection limit for Risperidone A was approximately 10% (w/w). The Raman and IR spectra recorded from the same mixtures (not
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shown) were not more helpful than those recorded from the tablet with 8 mg API. Although a 10% (w/w) mixture could have been used for testing the stability of A polymorph based on XRPD data, a 25% (w/w) was thought to be more helpful for the analysis due to the appearance of additional Risperidone reflections, as well as to have Raman and IR spectra that might permit the identification of Risperidone A. The Raman spectrum obtained from the tablet with 25% (w/w) Risperidone A can be seen in Fig. 3C. The increased quantity of the active ingredient allows the positive identification of Risperidone A in these tablets. The wide band at 436 cm−1 (Fig. 4C) stems from the addition of polymorph A double peak at 432 and 440 cm−1 and the placebo vibration at 436 cm−1 (Fig. 4P). The polymorph A vibrations at 1271 and 1397 cm−1 (Fig. 4C) as well as at 3051 and 3069 cm−1 (Fig. 3C) are unaltered due to the absence of the respective placebo vibrations. It should be noted though that the presence of these vibrations does not rule out the possibility of the simultaneous presence of small quantity of B crystal phase. The FT-IR spectrum obtained from the tablet with 25% (w/w) Risperidone A can be seen in Fig. 5C. The presence of a single peak at 1650 cm−1 indicates the presence of Risperidone A but this does not also exclude the possibility of the simultaneous presence of small amounts of B crystal phase since the multiple peaks of B at 1650 cm−1 can be hidden under the apparently single peak at 1650 cm−1 . 3.2.2. Manufacturing process stability test In the sample with 25% (w/w) Risperidone (Fig. 2D) the characteristic reflections of Risperidone polymorph A (Fig. 2A), as well as the diffraction peaks of the compounds consisting the placebo (Fig. 2P), were detected. Furthermore, no additional reflections were detected including the most prominent diffraction peaks at 21.6◦ 2θ of polymorphs B and E. This indicates that during the manufacturing process no transformation of Risperidone A took place. Therefore, in the case of Risperidone, XRPD proved to be the only reliable technique for identifying the presence of A polymorph and simultaneously exclude the possibility of the presence of the other two polymorphs. 3.2.3. Stability tests with time In order to test the stability of Risperidone A with time, the XRPD spectra of raw Risperidone A in powder form were recorded with a time interval of 2 years (Fig. 2A and B). The XRPD spectra of tablets with 25% (w/w) Risperidone A were also recorded shortly after their manufacture (Fig. 2D) and after 2 years (Fig. 2E). In both cases, and in order to simulate reallife scenario, in the interim period the powders were placed in a laboratory cabinet for protection against the light but neither the temperature nor the humidity was controlled. No difference was observed indicating there is no transformation or degradation of the active ingredient after a storage period of 2 years. 4. Conclusions From the application of X-ray powder diffraction, FT-IR and FT-Raman spectroscopy on the above mentioned specimens and
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raw materials as well as on tablets, it is apparent that there is no straightforward process that can be applied in order: (a) to identify the presence of different crystal phases of the active ingredient and/or (b) to test their stability. Product oriented methodologies should be devised using all available techniques depending on a number of factors including the percentage of the active ingredient in the formulation, the problems arising from the presence of the excipients, the detection limits of the different techniques for the active substance in question and the availability of all crystal forms, including the patented. For Risperidone A, the case study of this work, it can be safely concluded that no transformation takes places during the manufacturing process or during a storage period of 2 years. None of the techniques used could positively identify the presence of polymorph A in the commercial tablets with 8 mg API. In tablets with increased API strength IR and Raman proved to be incapable of excluding the simultaneous presence of other crystal phases although positive identification of polymorph A was accomplished. Only the information obtained from XRPD could be used for the identification of form A and the exclusion of forms B and E.
Acknowledgments Authors would like to thank Specifar Pharmaceuticals SA for its help and in particular Ms. Eirini Vasilopoulos from the Department of Product Development. References [1] D.J. Grant, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 1–31 (Chapter 1). [2] International Conference on Harmonization Q6A Guideline: Specifications for New Drug Substances and Products: Chemical Substances, October 1999. [3] H.G. Brittain, E.F. Fiese, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 279–330 (Chapter 8). [4] H.G. Brittain, in: H.G. Brittain (Ed.), Polymorphism in Pharmaceutical Solids, Marcel Dekker, Inc., New York, 1999, pp. 227–271 (Chapter 6). [5] S. Morissette, O. Almarsson, M. Peterson, J. Remenar, M. Read, A. Lemmo, S. Ellis, M. Cima, C. Gardner, Adv. Drug Deliv. Rev. 56 (2004) 275. [6] B. Krochmal, D. Diller, B.-Z. Dolitzky, J. Aronhime, Preparation of Risperidone, U.S. Patent No. 6,750,341 and PCT/WO 02/12200 A1, Feb 14, 2002.