STUDY OF POLYMORPHIC TRANSFORMATION OF ORNIDAZOLE DRUG BY DIFFERENTIAL SCANNING CALORIMETRY AND OTHER COMPLEMENTARY TECHNIQUES

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1 Acta Poloniae Pharmaceutica ñ Drug Research, Vol. 65 No. 4 pp. 409ñ413, 2008 ISSN Polish Pharmaceutical Society STUDY OF POLYMORPHIC TRANSFORMATION OF ORNIDAZOLE DRUG BY DIFFERENTIAL SCANNING CALORIMETRY AND OTHER COMPLEMENTARY TECHNIQUES SATISH R. DESAI and SANJIV R. DHARWADKAR* Department of Chemistry, The Institute of Science, 15, Madam Cama Road, Mumbai , India Abstract: Differential scanning calorimetric (DSC) curves recorded for ornidazole drug during heating and cooling showed that the drug which melted around 86.1 O C undercooled to well below ambient room temperature of 27 O C during the cooling cycle. The undercooled melt kept in the freezer at 0 O C for 10 days duration also remained in the viscous liquid form. This liquid on taking out from the freezer after ten days and ageing at ambient room temperature of 27 O C for 12 h transformed into white powder. The DSC pattern recorded for this white powder consisted of two prominent endothermic peaks beginning at 73.2 and 85.9 O C, respectively, suggesting that the powder consisted of a mixture of more than one phase. The Xray diffraction (XRD) pattern recorded for this powder showed it to be a mixture of semicrystalline phase and the original compound. The semicrystalline phase melted at 73.2 O C prior to the melting of original compound at 85.9 O C. This phase on further ageing for 7 days transforms almost completely to its original form. DSC observations were corroborated by XRD and scanning electron microscopy (SEM) techniques. Keywords: ornidazole, polymorph, differential scanning calorimeter, Xray diffraction, scanning electron microscopy Polymorphism in drugs is the most common phenomenon (1) and has great bearing on its efficacy (2). Different polymorphs of a drug may exhibit significantly different biological activities due to their different solubility. The polymorphic transformations of the drugs have been studied extensively in recent years employing differential scanning calorimetry (DSC), supported by other complementary techniques like Xray diffraction, scanning electron microscopy (SEM) and FTIR (3ñ5). The present paper deals with observations on the polymorphic transformation in ornidazole drug studied by DSC, Xray diffraction and SCM techniques. Ornidazole is one of the antibacterial and antiprotozoal tropical drugs recently introduced in the Indian market. There is, however, very limited data on this drug which can be used to asses its long term stability and compatibility with suitable excipients used in drug formulations. DSC in conjunction with other techniques like XRD and SEM have been employed extensively in recent years in the study of polymorphic transformations in the drugs. The present investigation was aimed at finding the existence, if any, of the polymorphic form of ornidazole drug employing differential scanning calorimetry, Xray diffraction technique and scanning electron microscopy. EXPERIMENTAL Ornidazole sample used in this study was procured from M/s. Aarti drugs. It was found to be 99.6 pure using HPLC. The DSC patterns for various samples were recorded employing Shimadzu Differential Scanning Calorimeter (model: DSC60). The experiments were performed on about 8 mg samples in flowing nitrogen at the heating and the cooling rate of 5 O C/min. The builtin software has been provided with the instrument for the determination of phase transformation temperatures from the recorded curves. The instrument was calibrated employing the standard reference materials and procedure recommended by Hohne et al. (6). The Xray diffraction patterns for the drug samples were recorded with Philips Xray diffractometer (Model: PW1710) employing CuK α radiation at the scanning speed of 2 2Θ/min. * Corresponding author: address: srdharwadkar@hotmail.com 409

2 410 SATISH R. DESAI and SANJIV R. DHARWADKAR Scanning electron microscopic (SEM) experiments were carried out by mounting sample on specimen stubs with doublesided adhesive tape. The drug samples investigated in this study were very sensitive to electron beam and developed considerable charge on being bombarded by electrons. The charging of the specimen causes artifacts and also focusing problem in the SEM. To avoid charging, the specimen was coated with a thin layer (250 ñ 300 Å) of conducting material (gold) using sputter coater and examined in Philips (Model: XL30). scanning electron microscope at 12 KV accelerating voltage with tilt angle of 45 O was used with suitable magnification of RESULTS The DSC curve recorded for the as received ornidazole sample (Fig.1a) yielded the sharp melting point at 86.1 O C during the heating cycle. However, no peak was observed due to freezing even on cooling the sample down to the ambient Table 1. Xray diffraction data obtained on original ornidazole sample, chilled viscous mass aged at 27 O C for 12 h and chilled viscous mass aged at 27 O C for 7 days. No Ornidazole (as received) Ornidazole chilled Ornidazole chilled viscous mass at viscous mass aged at 27 O C for 12 h 27 O C for 7days Intensity Intensity Intensity

3 Study of polymorphic transformation of ornidazole drug Figure 1. Differential scanning calorimetric (DSC) traces for (a) original ornidazole sample recorded in nitrogen at the heating and cooling rate of 5 O C/min, first heating and cooling sequence. (b) Chilled viscous mass obtained by freezing ornidazole melt for 10 days. (c) Chilled viscous mass aged at 27 O C for 12 h. (d) Chilled viscous mass aged at 27 O C for 7 days. temperature of 27 O C (Fig.1a). The melt on further cooling to 0 O C in freezer for 10 days remained in the viscous liquid form and on heating in DSC instrument did not yield any peak (Fig.1b). The chilled melt after taking out from the freezer and ageing at the room temperature of 27 O C for 12 h, transformed to white powder. The DSC pattern recorded for this powder (Fig.1c) showed two prominent endothermic peaks beginning around 73.2 and 85.9 O C, respectively. The latter peak corresponded to the melting point of the pure drug (Fig.1a). The additional prominent endothermic peak at 73.2 O C suggests that the white powder formed after ageing of the refrigerated product at 27 O C yielded the polymorphic form of the drug different from original sample. The white powder consisting of the mixture of two phases, on further ageing at room temperature for 7 days, showed nearly complete transformation to pure ornidazole. DSC scan showed a sharp endothermic peak beginning at 86.5 O C (Fig.1d) with heat of transformation 93.0 mj/mg which is in close agreement with that of the heat of fusion 94.4 mj/mg of original drug. The heat of fusion derived from the first peak in Figure 1c is 47.6 mj/mg and that

4 412 SATISH R. DESAI AND SANJIV R. DHARWADKAR Figure 2. Xray diffraction pattern for (a) original ornidazole sample. (b) Chilled viscous mass obtained by freezing ornidazole melt for 10 days. (c) Chilled viscous mass aged at 27 C for 12 h. (d) Chilled viscous mass aged at 27 C for 7 days. derived from the second peak is 54.1mJ/mg, respectively. These heats of fusion per unit mass, however, may not be representative of those of the pure polymorphic forms, since the drug is a mixture of two polymorphic forms and their relative amounts in the sample are not known. The Xray diffraction pattern of the chilled viscous liquid film showed absence of crystalline phase in the melt (Fig. 2b). The Xray diffraction pattern for the powder formed by ageing the chilled mass at the ambient temperature of 27 O C for 12 h (Fig. 2c) showed the material to be poorly crystalline (semicrystalline) with the peaks at 16.3 O (10.6), and 28.2 O (22.3) which were not present in original ornidazole sample. The four additional major peaks Figure 3. SEM photographs for (a) original ornidazole sample. (b) Chilled viscous mass aged at 27 C for 12 h. (c) Chilled viscous mass aged at 27 C for 7 days. at 19.6 O (20.2), 22.2 O (14.9), 38.4 O (62.8) and 44.7 O (100) observed in the semicrystalline material are identical to those in original sample, suggesting that the frozen mass is a mixture of two phases, thereby supporting the observations made by DSC. The chilled mass aged further for 7 days transformed almost completely to original form of the drug yielding Xray diffraction pattern completely in agreement with that of original sample (Fig. 2a and Table 1), albeit with relatively smaller crystallite size, as indicated by the width of the X

5 Study of polymorphic transformation of ornidazole drug ray diffraction peaks. The new lines which were observed in chilled viscous mass aged at 27 O C for 12 h were absent, as expected. The scanning electron microscopic (SEM) patterns recorded for original ornidazole (Fig. 3a) and the samples obtained by ageing the chilled liquid mass for 12 h and 7 days, respectively, are presented in Figures 3b and 3c. It can be seen from them that the sample obtained after ageing the chilled melt at 27 O C for 12 h consisted of two phases, i.e. the smaller needle like crystals, as present in original sample, and larger blocks, which could probably represent the semicrystalline second polymorphic form of the ornidazole drug. The same chilled mass aged at 27 O C for 7 days contained relatively much larger proportion of the original form of the drug which is also corroborated from Xray diffraction pattern (Fig. 2c). DISCUSSION AND CONCLUSION The first peak in DSC recorded for the semicrystalline powder obtained by ageing the previously refrigerated melt at 27 O C for 12 hours (Fig.1c) corresponded to the new polymorphic form of ornidazole which melted at 73.2 O C, without reverting back to its original crystallographic form. This conjecture is further supported by the heat of fusion determined for melting of the original component in the mixture recorded at 85.9 O C.The heat of fusion derived from 2 nd peak in Fig.1c would have been the same as that of the as received drug if this peak represented only polymorphic transformation and not the physical change of state (i.e. melting). If the new polymorph formed during ageing had transformed to the original sample by solid state transformation during heating, then the heat of fusion for melting derived from the peak at 85.9 O C in Fig.1c should have been identical to that obtained for the peak at 86.1 O C in Fig.1a and would correspond to the heat of fusion of pure ornidazole. It can be concluded from DSC observations that compound melting at 73.2 O C is new polymorphic form of the drug coexisting with the original form of the drug melting prior to the as received drug The mixture of two phases on ageing at room temperature showed nearly complete transformation to pure ornidazole suggesting that the compound melting at 73.2 O C is indeed the polymorphic form of the drug. It is indeed interesting to note that the data obtained by DSC is in mutual agreement with the observations made by other complementary techniques, namely, Xray diffraction and SEM. It can be concluded from the above studies that as received ornidazole drug melting at 86.1 C exhibits large changes under cooling and remains in the form of molten viscous liquid mass even up to 0 C for several days. This liquid mass on subjecting to ageing at ambient temperature of 27 C gradually transforms to semicrystalline phase, yielding the mixture of two phases comprising of the original drug and its new semicrystalline polymorphic form. This mixture on further ageing transforms almost completely to the original form of the as received drug. Acknowledgment The authors thank M/s Indoco Remedies Limited, Mumbai and CIRCOT, Mumbai for their help in providing instrumental facilities. REFERENCES 1. Giron D.: Thermochim. Acta 248, 1 (1995). 2. Higuchi T.: J. Am. Pharm. Assoc. 47, 657 (1958). 3. Desai S.R., Shaikh M.M., Dharwadkar S.R.: Thermochim. Acta 399, 81 (2003). 4. Gu X. J., Jiang W.: J. Pharm. Sci. 84, 1438 (1995). 5. Martins P. D., Conflant P., Drache M., Huvenne J. P., GuyotHermann A. M.: J. Thermal Anal. Calorim. 48, 447 (1997). 6 Hˆhne G.W.H., Hemminger W., Flammersheim H.J.: Differential Scanning Calorimetry ñ An Introduction for Practitioners, Springer Verlag, Berlin Received: