- Email: [email protected]
Spectroscopic studies of solid-state forms of donepezil free base and salt forms with various salicylic acids
Journal of Molecular Structure xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic studies of solid-state forms of donepezil free base and salt forms with various salicylic acids Harry G. Brittain ⇑ Center for Pharmaceutical Physics, 10 Charles Road, Milford, NJ 08848, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Non-solvated crystalline salts were
formed by donepezil with salicylic or 4-methylsalicylic acids. Solvated crystalline salts were formed by donepezil salts with 3-methylsalicylic and 5-methylsalicylic acids. Solid-state fluorescence from non-solvated donepezil salts came from the salicylate fluorophore. Desolvation of solvated donepezil salts yielded glassy solids exhibiting strong green fluorescence.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Polymorphism Salt formation Infrared spectroscopy X-ray powder diffraction Differential scanning calorimetry Fluorescence spectroscopy
a b s t r a c t The polymorphic forms of donepezil free base have been studied using X-ray powder diffraction, Fourier transform infrared absorption spectroscopy, and differential scanning calorimetry. None of the free base crystal forms was observed to exhibit detectable fluorescence in the solid state under ambient conditions. Crystalline salt products were obtained by the reaction of donepezil with salicylic and methyl-substituted salicylic acids, with the salicylate and 4-methylsalicylate salts being obtained as non-solvated products, and the 3-methylsalicylate and 5-methylsalicylate salts being obtained as methanol solvated products. The intensity of solid-state fluorescence from donepezil salicylate and donepezil 4-methylsalicylate was found to be reduced relative to the fluorescence intensity of the corresponding free acids, while the solid-state fluorescence intensity of donepezil 3-methylsalicylate methanolate and donepezil 5-methylsalicylate methanolate was greatly increased relative to the fluorescence intensity of the corresponding free acids. Desolvation of the solvated salt products led to formation of glassy solids that exhibited strong green fluorescence. Ó 2014 Published by Elsevier B.V.
1. Introduction The hydrochloride salt of donepezil [(R,S)-2-[(1-benzyl-4piperidyl)methyl]-5,6-dimethoxy-2,3-dihydroinden-1-one]:
⇑ Tel.: +1 908 996 3509. E-mail address: [email protected] http://dx.doi.org/10.1016/j.molstruc.2014.01.044 0022-2860/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: H.G. Brittain, J. Mol. Struct. (2014), http://dx.doi.org/10.1016/j.molstruc.2014.01.044
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H.G. Brittain / Journal of Molecular Structure xxx (2014) xxx–xxx
has been approved for use in the treatment of mild to moderate Alzheimer’s disease [1], and has been shown to inhibit acetylcholinesterase activity in human erythrocytes and increase extracellular acetylcholine levels in the cerebral cortex and hippocampus of the rat [2]. Donepezil free base is characterized by unacceptably low aqueous solubility, but the hydrochloride salt has been reported to have a water solubility of 55 mg/mL at 25 °C [3]. When performed in conjunction with crystallographic investigation, solid-state spectroscopic studies are now generally accepted as being able to provide important information regarding the properties of polymorphic systems [4–6], as long as the spectroscopic properties of the molecule are affected by differences in crystal structures. The synergistic use of solid-state infrared absorption, Raman, and nuclear magnetic resonance spectroscopies has been demonstrated in numerous works, and in several studies solid-state fluorescence spectroscopy has been shown able to yield supporting structural information for substances exhibiting luminescence in the solid state [7–10]. In the present work, solid-state fluorescence spectroscopy has been used as part of an overall characterization study of the polymorphic forms of donepezil free base, and several donepezil salts with various salicylic acids. While donepezil free base was not fluorescent in the solid state, its protonation through salt formation yielded a wide range of interesting photophysical behavior. Differences in solid-state fluorescence were observed between solvated and non-solvated salt products, and drastic differences in photophysics were observed between crystalline and amorphous substances. 2. Materials and methods 2.1. Materials The three polymorphic forms of donepezil free base were generously provided by Dr. Reddy’s Laboratories, while 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid were purchased from Aldrich Chemicals (Milwaukee, WI). Salts containing these acidic coformers were prepared by dissolving equimolar amounts of donepezil and acid in methanol, and allowing the solution to evaporate to dryness. The spontaneously crystallized yields were ground to a fine powder, and characterized without further processing.
aluminum DSC pan, and then covered with an aluminum lid that was inverted and pressed down so as to tightly contain the powder between the top and bottom aluminum faces of the lid and pan. The samples were then heated over the temperature range of 20–200 °C, at a heating rate of 10 °C/min. Measurements of total volatile content were made using an Ohaus model MB45 system. The samples were heated isothermally at a temperature of 100 °C for a period of 10 min, whereupon the tested samples were found to have reached constant weight loss. Fluorescence spectra of the solid samples were obtained on a spectrometer where the appropriate excitation energy of a 250 W xenon arc lamp was isolated by using a combination of glass and solution filters [11]. Approximately 500 mg of sample was packed into a 5-mm glass NMR tube, which was irradiated using front-face excitation. The resulting fluorescence was analyzed by a 0.5 m monochromator (Spex model 1870), having a grating blazed at 500 nm and ruled at 1200 g/mm. The dispersion of this monochromator was 1.2 nm/mm slit width, so with the input and output slits set at 0.5 mm, the resolution of the spectra was 0.8 nm and the reported wavelength values were therefore rounded to the nearest 0.5 nm. The fluorescence was detected by an end-on photomultiplier tube (Thorn EMI type 9558QB having S-20 response). 3. Donepezil free base The XRPD patterns of the three studied polymorphs of donepezil free base (Forms A, B, and C) are shown in Fig. 1, and these patterns were found to be completely equivalent with those disclosed in a United States patent [12]. The existence of the polymorphism disclosed in this patent is therefore confirmed, as all three diffraction patterns are sufficiently distinctive so as to confirm the existence of different crystals merely by inspection. DSC thermograms obtained for the three polymorphic forms of donepezil free base are shown in Fig. 2. The melting endothermic transitions of Forms A, B, and C were found to be similar, an observation also disclosed in a US patent [13]. The temperature maximum observed in the DSC thermogram of Form-A was found to be 97.3 °C (enthalpy of fusion equal to 67 J/g), that of Form-B was 96.6 °C (enthalpy of fusion equal to 75 J/g), and that of Form-C was found to be 94.4 °C (enthalpy of fusion equal to 81 J/ g). The baselines were all very flat, and did not contain any evidence for a phase transition. The trend in fusion enthalpies indicated that the order of stability of these polymorphic forms
X-ray powder diffraction (XRPD) patterns were obtained using a Rigaku MiniFlex powder diffraction system, equipped with a horizontal goniometer in the h/2h mode. The X-ray source was nickelfiltered K-a emission of copper (1.54056 Å). Samples were packed into an aluminum holder using a back-fill procedure, and were scanned over the range of 50–6° 2h, at a scan rate of 0.5° 2h/min. Using a data acquisition rate of 1 point per second, the scanning parameters equate to a step size of 0.0084° 2h. Calibration of the diffractometer system was effected using purified talc as a reference material. Fourier-transform infrared absorption (FTIR) spectra were obtained at a resolution of 4 cm 1 using a Shimadzu model 8400S spectrometer, with each spectrum being obtained as the average of 40 individual spectra. The data were acquired using the attenuated total reflectance (ATR) sampling mode, where the samples were clamped against the ZnSe crystal of a Pike MIRacle™ single reflection horizontal ATR sampling accessory. Measurements of differential scanning calorimetry (DSC) were obtained on a TA Instruments 2910 thermal analysis system. Samples of approximately 1–2 mg were accurately weighed into an
Relative Intensity
2.2. Methods
Form-C
Form-B
Form-A
5
10
15
20
25
30
35
40
Scattering Angle (degrees 2θ) Fig. 1. X-ray powder diffraction patterns of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C.
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crystal structures do not exert a strong influence on the pattern of intramolecular vibrations in the solids. The high-frequency region of the FTIR spectra were devoid of any spectral features that would indicate the existence of either a hydrate or solvated crystal form, confirming that the endothermic features observed in the DSC thermograms corresponded to genuine melting phase transitions. None of the polymorphic forms of donepezil free base were found to exhibit any detectable fluorescence when excited between 275 nm and 400 nm.
Heat Flow (W/g)
Form-C
Form-B
Form-A
4. Donepezil salt forms with salicylic acids
25
40
55
70
85
100
115
130
Temperature (°C) Fig. 2. Differential scanning calorimetry thermograms of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C.
at room temperature was C > B > A, while the trend in melting temperature indicated that the order of stability near the melting point was A > B > C. Using the heat of fusion rule, it was determined that the three polymorphic forms of the free base bore enantiotropic relationships to each other, and that none of the forms were related by monotropy. The FTIR spectra in the fingerprint region of the three polymorphs of donepezil free base are shown in Fig. 3, and were extremely similar to each other. Although similar, the spectra did contain shifts in corresponding bands. For example, the carbonyl stretching band associated Form-A was observed at 1688 cm 1, and was accompanied by a definite shoulder at 1696 cm 1 (which is marked in the inset of Fig. 3). The carbonyl peaks of Form-B (1683 cm 1) and Form-C (1686 cm 1) were observed as single peaks that did not contain a shoulder at higher wavenumbers. This high degree of similarity in the FTIR spectra is somewhat surprising given the large differences between the respective crystal structures (as evidenced by the XRPD patterns), but it is clear that the intermolecular interactions among molecules in the different
Since the pKa value associated with the weakly basic tertiary nitrogen of donepezil has been determined to be 8.90 [3], the compound is capable of forming salts only with strongly acidic substances. The efficiency of salt formation can be predicted using a published procedure [14], and in doing so one calculates that salts would form with better than 99% degree of conversion is the pKa of the acid coformer was 3.5 or less. Salicylic acid and its methyl derivatives are known to be more acidic than most other benzenecarboxylic acids, as shown in Table 1. Since the family of salicylic acids is sufficiently acidic, and since the salicylate group is known to be an effective fluorophore [16], the spectroscopic properties of donepezil salts with this series of salt-formers were investigated. XRPD patterns of the salicylic, 3-methylsalicylic, 4-methylsalicylic, and 5-methylsalicylic acids are shown in Fig. 4, and the non-equivalence of these with the XRPD patterns of the starting materials demonstrates formation of the respective salt forms. DSC thermograms of the donepezil salt products with the studies salicylic acids are shown in Fig. 5, and it is evident from the
Table 1 Ionization constants of benzoic and substituted salicylic acids. Log K [15] (25 °C; ionic strength = 0.1) Benzoic acid Salicylic acid 3-Methylsalicylic acid 4-Methylsalicylic acid 5-Methylsalicylic acid
4.00 2.81 2.82 2.97 2.90
Form-C
Absorbance
Form-C
Form-B Form-B
Form-A Form-A
1750 1725 1700 1675 1650 1625
W avenumber (cm-1)
1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 3. Infrared absorption spectra within the fingerprint region of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C. The left inset shows a scale expansion of the carbonyl absorption bands, and the shoulder on the Form-A band is marked.
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Relative Intensity
5-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Salicylic
5
10
15
20
25
30
35
(enthalpy = 92 J/g), respectively. The thermograms of these salts did not contain any evidence for desolvation phenomena at lower temperatures, indicating that these salts were effectively non-solvated and anhydrous. On the other hand, the DSC analysis of the salts formed by donepezil with 3-methylsalicylic and 5-methylsalicylic acids demonstrated that these were obtained as solvates. Donepezil 3-methylsalicylate exhibited a strong desolvation transition at 84 °C (enthalpy = 55 J/g), while donepezil 5-methylsalicylate exhibited a desolvation endotherm at 88 °C (enthalpy = 95 J/g). In both instances, the desolvation process yielded an amorphous product that did not exhibit a subsequent melting transition. The total volatile content of donepezil 3-methylsalicylate solvate was found to be approximately 5.4%, which would be consistent with the existence of a mono-methanol solvate (total volatile content calculated to be 5.68%). The total volatile content of the donepezil 5-methylsalicylate solvate was found to be approximately 5.8%, which is also consistent with the existence of a mono-methanol solvate. As shown in Fig. 6, the fingerprint region of the FTIR spectra of the various salt forms was greatly complicated due to a substantial overlapping of absorption bands of donepezil with those of the salicylic acid salt coformers. However, the absorption associated with
40
Scattering Angle (degrees 2θ) 5-Me-Salicylic
Fig. 4. X-ray powder diffraction patterns of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
Absorbance
4-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Heat Flow (W/g)
5-Me-Salicylic
Salicylic
3-Me-Salicylic
Salicylic 1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1) Fig. 6. Infrared absorption spectra within the fingerprint region of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
25
45
65
85
105
125
145
165
185
Temperature (°C) Fig. 5. Differential scanning calorimetry thermograms of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
figure that the system was capable of crystallizing into two types of products. The DSC thermograms of the salts formed by donepezil formed with salicylic and 4-methylsalicylic acids only contained endothermic transitions at relative high temperature, characterized by maxima at 173 °C (enthalpy = 88 J/g) and 167 °C
Table 2 Carbonyl stretching band maxima of the substituted salicylic acids and their salt products with donepezil.
Salicylic acid 3-Methylsalicylic acid 4-Methylsalicylic acid 5-Methylsalicylic acid
Carboxylic acid dimeric stretching mode (wavenumbers)
Ionized carboxylate stretching mode (wavenumbers)
1653 1664 1645 1655
1557 1591 1589 1568
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the asymmetric stretching mode of the initial carboxylic acids (which is shifted to lower wavenumbers owing to dimerization of the acids in the solid state) was not observed in the FTIR spectra of the salts, and was replaced by a new absorption band associated with the ionized carboxylate group. Table 2 contains a summary of the peak values for these bands.
Absorbance
5-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Salicylic 4000
3800
3600
3400
3200
3000
2800
-1
Wavenumber (cm ) Fig. 7. Infrared absorption spectra within the high-frequency region of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
(a)
Whereas the high-frequency region of the FTIR spectrum was not highly informative with respect to the polymorphs of donepezil free base, this region (shown in Fig. 7) of the FTIR spectra of donepezil 3-methylsalicylate methanolate and 5-methylsalicylate methanolate solids clearly demonstrated substantial absorption associated with the solvent of crystallization. The non-solvated nature of the donepezil salicylate and donepezil 3-methylsalicylate salts was also clearly evident in the high-frequency FTIR region. Protonation of the donepezil moiety by salt formation was found to drastically alter the photophysical properties of the compound, with differences being observed between solvated and nonsolvated salt products, as well as differences between crystalline and amorphous substances. As shown in Fig. 8, the solid-state fluorescence spectra of donepezil salicylate, donepezil 4-methylsalicylate, and their respective free acids all consisted of a single peak having a maximum around 420–425 nm. The consistence in peak maxima indicates that the fluorescence processes are effectively localized in the salicylate fluorophore. In both cases, the solid-state fluorescence intensity of the salt form was found to be substantially reduced relative to the solid-state fluorescence intensity of the corresponding free acids. As shown in Fig. 9, the solid-state fluorescence intensities of the methanol solvates of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate were significantly increased relative to the solid-state fluorescence spectra of their corresponding free acids. In addition, the fluorescence peak maximum of the salts was significantly red-shifted relative to the peak maxima of the free acids. Perhaps most interesting was the finding that not only did desolvation of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate yield glassy solids, these solids exhibited strong green fluorescence characterized by band maxima at approximately 510–520 nm. The ability of amorphous films of mixed organic solids to exhibit fluorescence (especially when prepared by epitaxial methods) has been the focus of considerable research into the optoelectronic properties of such films [17]. In these films, the observed photophysical properties have been explained by the generation of large permanent dipole moments associated with structural asym-
(b)
salicylic-acid 50
4-Me-salicylic-acid 40
Relative Intensity
Relative Intensity
40
30
20
30
20
10
10
salicylate 4-Me-salicylate 0
400
430
460
490
520
Wavelength (nm)
550
580
0
400
430
460
490
520
550
580
Wavelength (nm)
Fig. 8. (a) Fluorescence spectra obtained for salicylic acid and donepezil salicylate. (b) Fluorescence spectra obtained for 4-methylsalicylic acid and donepezil 4methylsalicylate.
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(a)
25
(b) 3-Me-salicylate
20
Relative Intensity
Relative Intensity
10
5
60
DES-5-Me-salicylate
40
20
3-Me-salicylic-acid 0
5-Me-salicylate
80
DES-3-Me-salicylate
15
100
400
430
460
490
5-Me-salicylic-acid 520
550
580
0
400
Wavelength (nm)
430
460
490
520
550
580
Wavelength (nm)
Fig. 9. (a) Fluorescence spectra obtained for 3-methylsalicylic acid, donepezil 3-methylsalicylate, and desolvated donepezil 3-methylsalicylate. (b) Fluorescence spectra obtained for 5-methylsalicylic acid, donepezil 5-methylsalicylate, and desolvated donepezil 5-methylsalicylate.
metry of the molecules in the film that inhibits the formation of crystallites during growth of the amorphous phase. The interaction of the dipoles leads to the generation of local ordering within the amorphous film, and this process results in a red-shifting of the fluorescence spectrum through exciton delocalization [18]. Other systems are known that are mildly fluorescent in the crystalline state, but whose fluorescence becomes more intense and red-shifted when the crystallinity is destroyed [19,20]. In these systems, the high degree of planarity resulting from p–p stacking in the crystal leads to efficient energy transfer and a significant degree of fluorescence quenching. Reducing the crystallinity serves to induce non-parallel packing through the introduction of alternate modes of hydrogen bonding, and breaks the pathway of non-radiative energy transfer. This latter mechanism not only results in a fluorescence enhancement, but also in a strong red-shifting of the emission spectrum. The DSC thermograms of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate demonstrate that desolvation of the crystalline hydrate structures causes the formation of amorphous solids. When this desolvation was carried out on the bulk level, the desolvated solids were observed to fuse into a contiguous film, and it was this film that exhibited the observed green fluorescence. It is hypothesized that removal of the solvent molecule from the crystalline solids causes a total collapse of the structures, thus facilitating the re-ordering of molecules in the amorphous film so that a non-radiative delocalization of excitation energy can occur. Ultimately, the delocalized energy becomes trapped at defect centers in the film, and the green fluorescence originates from these defect sites. 5. Conclusions While the various polymorphic forms of donepezil free base are non-fluorescent in the solid state, differing types of fluorescence have been obtained from various salicylate salts of donepezil. Crystalline salt products were obtained by the reaction of donepezil with salicylic and methyl-substituted salicylic acids, with the salicylate and 4-methylsalicylate salts being obtained as non-solvated products. For these salts, the solid-state fluorescence was
effectively localized on the salicylate fluorophore. On the other hand, the 3-methylsalicylate and 5-methylsalicylate salts were obtained as solvates, and desolvation of these salts led to formation of glassy solids that exhibited strong green fluorescence. This fluorescence was attributed to an energy transfer process associated with local order in the amorphous solids. References [1] Y.A. Asiri, G.A.E. Mostafa, Donepezil, in: H.G. Brittain (Ed.), Profiles of Drug Substances, Excipients, and Related Methodology, vol. 35, Elsevier, Academic Press, Amsterdam, 2010, pp. 117–150 (Chapter 3). [2] H.M. Bryson, P. Benfield, Drugs Aging 10 (3) (1997) 234–239. [3] Y. Donepezil, in: A.C. Moffat, M.D. Osselton, B. Widdop (Eds.), Clarke’s Analysis of Drugs and Poisons, vol. 2, fourth ed., Pharmaceutical Press, London, 2011, pp. 1296–1297. [4] J. Bernstein, Polymorphism in Molecular Crystals, Clarendon Press, London, 2002. [5] R. Hilfiker, Polymorphism in the Pharmaceutical Industry, Wiley-VCH, Weinheim, 2006. [6] H.G. Brittain, Polymorphism in Pharmaceutical Solids, second ed., Informa Healthcare, New York, 2009. [7] H.G. Brittain, J. Pharm. Sci. 93 (2004) 375–383. [8] H.G. Brittain, B.J. Elder, P.K. Isbester, A.H. Salerno, Pharm. Res. 22 (2005) 999– 1006. [9] H.G. Brittain, J. Pharm. Sci. 96 (2007) 2757–2764. [10] H.G. Brittain, Fluorescence Studies of Various Solvated and Desolvated Solvatomorphs of Erythromycin A, in: C.D. Geddes (Ed.), Reviews in Fluorescence, vol. 4, Springer, New York, 2009, pp. 379–392. [11] C.A. Parker, Photoluminescence of Solutions, Elsevier, Amsterdam, 1968. pp. 186–191. [12] A. Imai, T. Ichinohe, T. Endo, T. Tsurugi, M. Uemura, Donepezil Polycrystals and Process for Producing the Same, United States Patent 6,245,911, 2001 (issued June 12, 2001). [13] A. Imai, H. Watanabe, T. Kajima, Y. Ishihama, A. Ohtsuka, T. Tanaka, Polymorphs of Donepezil Hydrochloride and Process for Production, United States Patent 6,140,321, 2000 (issued October 31, 2000). [14] H.G. Brittain, Am. Pharm. Rev. 12 (7) (2009) 62–65. [15] A.E. Martell, R.M. Smith, Critical Stability Constants, vol. 3, Plenum Press, New York, 1977. pp. 16, 187–188. [16] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, second ed., Academic Press, New York, 1971. p. 166. [17] S.R. Forrest, Chem. Rev. 97 (1997) 1793–1896. [18] M.A. Baldo, Z.G. Soos, S.R. Forrest, Chem. Phys. Lett. 347 (2001) 297–303. [19] M. Mao, S. Xiao, J. Li, Y. Zou, R. Zhang, J. Pan, F. Dan, K. Zou, T. Yi, Tetrahedron 68 (2012) 5037–5041. [20] S. Wang, S. Xiao, X. Chen, R. Zhang, Q. Cao, K. Zou, Dyes Pigm. 99 (2013) 543– 547.
Please cite this article in press as: H.G. Brittain, J. Mol. Struct. (2014), http://dx.doi.org/10.1016/j.molstruc.2014.01.044
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic studies of solid-state forms of donepezil free base and salt forms with various salicylic acids Harry G. Brittain ⇑ Center for Pharmaceutical Physics, 10 Charles Road, Milford, NJ 08848, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Non-solvated crystalline salts were
formed by donepezil with salicylic or 4-methylsalicylic acids. Solvated crystalline salts were formed by donepezil salts with 3-methylsalicylic and 5-methylsalicylic acids. Solid-state fluorescence from non-solvated donepezil salts came from the salicylate fluorophore. Desolvation of solvated donepezil salts yielded glassy solids exhibiting strong green fluorescence.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Polymorphism Salt formation Infrared spectroscopy X-ray powder diffraction Differential scanning calorimetry Fluorescence spectroscopy
a b s t r a c t The polymorphic forms of donepezil free base have been studied using X-ray powder diffraction, Fourier transform infrared absorption spectroscopy, and differential scanning calorimetry. None of the free base crystal forms was observed to exhibit detectable fluorescence in the solid state under ambient conditions. Crystalline salt products were obtained by the reaction of donepezil with salicylic and methyl-substituted salicylic acids, with the salicylate and 4-methylsalicylate salts being obtained as non-solvated products, and the 3-methylsalicylate and 5-methylsalicylate salts being obtained as methanol solvated products. The intensity of solid-state fluorescence from donepezil salicylate and donepezil 4-methylsalicylate was found to be reduced relative to the fluorescence intensity of the corresponding free acids, while the solid-state fluorescence intensity of donepezil 3-methylsalicylate methanolate and donepezil 5-methylsalicylate methanolate was greatly increased relative to the fluorescence intensity of the corresponding free acids. Desolvation of the solvated salt products led to formation of glassy solids that exhibited strong green fluorescence. Ó 2014 Published by Elsevier B.V.
1. Introduction The hydrochloride salt of donepezil [(R,S)-2-[(1-benzyl-4piperidyl)methyl]-5,6-dimethoxy-2,3-dihydroinden-1-one]:
⇑ Tel.: +1 908 996 3509. E-mail address: [email protected] http://dx.doi.org/10.1016/j.molstruc.2014.01.044 0022-2860/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: H.G. Brittain, J. Mol. Struct. (2014), http://dx.doi.org/10.1016/j.molstruc.2014.01.044
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H.G. Brittain / Journal of Molecular Structure xxx (2014) xxx–xxx
has been approved for use in the treatment of mild to moderate Alzheimer’s disease [1], and has been shown to inhibit acetylcholinesterase activity in human erythrocytes and increase extracellular acetylcholine levels in the cerebral cortex and hippocampus of the rat [2]. Donepezil free base is characterized by unacceptably low aqueous solubility, but the hydrochloride salt has been reported to have a water solubility of 55 mg/mL at 25 °C [3]. When performed in conjunction with crystallographic investigation, solid-state spectroscopic studies are now generally accepted as being able to provide important information regarding the properties of polymorphic systems [4–6], as long as the spectroscopic properties of the molecule are affected by differences in crystal structures. The synergistic use of solid-state infrared absorption, Raman, and nuclear magnetic resonance spectroscopies has been demonstrated in numerous works, and in several studies solid-state fluorescence spectroscopy has been shown able to yield supporting structural information for substances exhibiting luminescence in the solid state [7–10]. In the present work, solid-state fluorescence spectroscopy has been used as part of an overall characterization study of the polymorphic forms of donepezil free base, and several donepezil salts with various salicylic acids. While donepezil free base was not fluorescent in the solid state, its protonation through salt formation yielded a wide range of interesting photophysical behavior. Differences in solid-state fluorescence were observed between solvated and non-solvated salt products, and drastic differences in photophysics were observed between crystalline and amorphous substances. 2. Materials and methods 2.1. Materials The three polymorphic forms of donepezil free base were generously provided by Dr. Reddy’s Laboratories, while 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid were purchased from Aldrich Chemicals (Milwaukee, WI). Salts containing these acidic coformers were prepared by dissolving equimolar amounts of donepezil and acid in methanol, and allowing the solution to evaporate to dryness. The spontaneously crystallized yields were ground to a fine powder, and characterized without further processing.
aluminum DSC pan, and then covered with an aluminum lid that was inverted and pressed down so as to tightly contain the powder between the top and bottom aluminum faces of the lid and pan. The samples were then heated over the temperature range of 20–200 °C, at a heating rate of 10 °C/min. Measurements of total volatile content were made using an Ohaus model MB45 system. The samples were heated isothermally at a temperature of 100 °C for a period of 10 min, whereupon the tested samples were found to have reached constant weight loss. Fluorescence spectra of the solid samples were obtained on a spectrometer where the appropriate excitation energy of a 250 W xenon arc lamp was isolated by using a combination of glass and solution filters [11]. Approximately 500 mg of sample was packed into a 5-mm glass NMR tube, which was irradiated using front-face excitation. The resulting fluorescence was analyzed by a 0.5 m monochromator (Spex model 1870), having a grating blazed at 500 nm and ruled at 1200 g/mm. The dispersion of this monochromator was 1.2 nm/mm slit width, so with the input and output slits set at 0.5 mm, the resolution of the spectra was 0.8 nm and the reported wavelength values were therefore rounded to the nearest 0.5 nm. The fluorescence was detected by an end-on photomultiplier tube (Thorn EMI type 9558QB having S-20 response). 3. Donepezil free base The XRPD patterns of the three studied polymorphs of donepezil free base (Forms A, B, and C) are shown in Fig. 1, and these patterns were found to be completely equivalent with those disclosed in a United States patent [12]. The existence of the polymorphism disclosed in this patent is therefore confirmed, as all three diffraction patterns are sufficiently distinctive so as to confirm the existence of different crystals merely by inspection. DSC thermograms obtained for the three polymorphic forms of donepezil free base are shown in Fig. 2. The melting endothermic transitions of Forms A, B, and C were found to be similar, an observation also disclosed in a US patent [13]. The temperature maximum observed in the DSC thermogram of Form-A was found to be 97.3 °C (enthalpy of fusion equal to 67 J/g), that of Form-B was 96.6 °C (enthalpy of fusion equal to 75 J/g), and that of Form-C was found to be 94.4 °C (enthalpy of fusion equal to 81 J/ g). The baselines were all very flat, and did not contain any evidence for a phase transition. The trend in fusion enthalpies indicated that the order of stability of these polymorphic forms
X-ray powder diffraction (XRPD) patterns were obtained using a Rigaku MiniFlex powder diffraction system, equipped with a horizontal goniometer in the h/2h mode. The X-ray source was nickelfiltered K-a emission of copper (1.54056 Å). Samples were packed into an aluminum holder using a back-fill procedure, and were scanned over the range of 50–6° 2h, at a scan rate of 0.5° 2h/min. Using a data acquisition rate of 1 point per second, the scanning parameters equate to a step size of 0.0084° 2h. Calibration of the diffractometer system was effected using purified talc as a reference material. Fourier-transform infrared absorption (FTIR) spectra were obtained at a resolution of 4 cm 1 using a Shimadzu model 8400S spectrometer, with each spectrum being obtained as the average of 40 individual spectra. The data were acquired using the attenuated total reflectance (ATR) sampling mode, where the samples were clamped against the ZnSe crystal of a Pike MIRacle™ single reflection horizontal ATR sampling accessory. Measurements of differential scanning calorimetry (DSC) were obtained on a TA Instruments 2910 thermal analysis system. Samples of approximately 1–2 mg were accurately weighed into an
Relative Intensity
2.2. Methods
Form-C
Form-B
Form-A
5
10
15
20
25
30
35
40
Scattering Angle (degrees 2θ) Fig. 1. X-ray powder diffraction patterns of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C.
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crystal structures do not exert a strong influence on the pattern of intramolecular vibrations in the solids. The high-frequency region of the FTIR spectra were devoid of any spectral features that would indicate the existence of either a hydrate or solvated crystal form, confirming that the endothermic features observed in the DSC thermograms corresponded to genuine melting phase transitions. None of the polymorphic forms of donepezil free base were found to exhibit any detectable fluorescence when excited between 275 nm and 400 nm.
Heat Flow (W/g)
Form-C
Form-B
Form-A
4. Donepezil salt forms with salicylic acids
25
40
55
70
85
100
115
130
Temperature (°C) Fig. 2. Differential scanning calorimetry thermograms of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C.
at room temperature was C > B > A, while the trend in melting temperature indicated that the order of stability near the melting point was A > B > C. Using the heat of fusion rule, it was determined that the three polymorphic forms of the free base bore enantiotropic relationships to each other, and that none of the forms were related by monotropy. The FTIR spectra in the fingerprint region of the three polymorphs of donepezil free base are shown in Fig. 3, and were extremely similar to each other. Although similar, the spectra did contain shifts in corresponding bands. For example, the carbonyl stretching band associated Form-A was observed at 1688 cm 1, and was accompanied by a definite shoulder at 1696 cm 1 (which is marked in the inset of Fig. 3). The carbonyl peaks of Form-B (1683 cm 1) and Form-C (1686 cm 1) were observed as single peaks that did not contain a shoulder at higher wavenumbers. This high degree of similarity in the FTIR spectra is somewhat surprising given the large differences between the respective crystal structures (as evidenced by the XRPD patterns), but it is clear that the intermolecular interactions among molecules in the different
Since the pKa value associated with the weakly basic tertiary nitrogen of donepezil has been determined to be 8.90 [3], the compound is capable of forming salts only with strongly acidic substances. The efficiency of salt formation can be predicted using a published procedure [14], and in doing so one calculates that salts would form with better than 99% degree of conversion is the pKa of the acid coformer was 3.5 or less. Salicylic acid and its methyl derivatives are known to be more acidic than most other benzenecarboxylic acids, as shown in Table 1. Since the family of salicylic acids is sufficiently acidic, and since the salicylate group is known to be an effective fluorophore [16], the spectroscopic properties of donepezil salts with this series of salt-formers were investigated. XRPD patterns of the salicylic, 3-methylsalicylic, 4-methylsalicylic, and 5-methylsalicylic acids are shown in Fig. 4, and the non-equivalence of these with the XRPD patterns of the starting materials demonstrates formation of the respective salt forms. DSC thermograms of the donepezil salt products with the studies salicylic acids are shown in Fig. 5, and it is evident from the
Table 1 Ionization constants of benzoic and substituted salicylic acids. Log K [15] (25 °C; ionic strength = 0.1) Benzoic acid Salicylic acid 3-Methylsalicylic acid 4-Methylsalicylic acid 5-Methylsalicylic acid
4.00 2.81 2.82 2.97 2.90
Form-C
Absorbance
Form-C
Form-B Form-B
Form-A Form-A
1750 1725 1700 1675 1650 1625
W avenumber (cm-1)
1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 3. Infrared absorption spectra within the fingerprint region of the polymorphic forms of donepezil free base, Form-A, Form-B, and Form-C. The left inset shows a scale expansion of the carbonyl absorption bands, and the shoulder on the Form-A band is marked.
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Relative Intensity
5-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Salicylic
5
10
15
20
25
30
35
(enthalpy = 92 J/g), respectively. The thermograms of these salts did not contain any evidence for desolvation phenomena at lower temperatures, indicating that these salts were effectively non-solvated and anhydrous. On the other hand, the DSC analysis of the salts formed by donepezil with 3-methylsalicylic and 5-methylsalicylic acids demonstrated that these were obtained as solvates. Donepezil 3-methylsalicylate exhibited a strong desolvation transition at 84 °C (enthalpy = 55 J/g), while donepezil 5-methylsalicylate exhibited a desolvation endotherm at 88 °C (enthalpy = 95 J/g). In both instances, the desolvation process yielded an amorphous product that did not exhibit a subsequent melting transition. The total volatile content of donepezil 3-methylsalicylate solvate was found to be approximately 5.4%, which would be consistent with the existence of a mono-methanol solvate (total volatile content calculated to be 5.68%). The total volatile content of the donepezil 5-methylsalicylate solvate was found to be approximately 5.8%, which is also consistent with the existence of a mono-methanol solvate. As shown in Fig. 6, the fingerprint region of the FTIR spectra of the various salt forms was greatly complicated due to a substantial overlapping of absorption bands of donepezil with those of the salicylic acid salt coformers. However, the absorption associated with
40
Scattering Angle (degrees 2θ) 5-Me-Salicylic
Fig. 4. X-ray powder diffraction patterns of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
Absorbance
4-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Heat Flow (W/g)
5-Me-Salicylic
Salicylic
3-Me-Salicylic
Salicylic 1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1) Fig. 6. Infrared absorption spectra within the fingerprint region of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
25
45
65
85
105
125
145
165
185
Temperature (°C) Fig. 5. Differential scanning calorimetry thermograms of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
figure that the system was capable of crystallizing into two types of products. The DSC thermograms of the salts formed by donepezil formed with salicylic and 4-methylsalicylic acids only contained endothermic transitions at relative high temperature, characterized by maxima at 173 °C (enthalpy = 88 J/g) and 167 °C
Table 2 Carbonyl stretching band maxima of the substituted salicylic acids and their salt products with donepezil.
Salicylic acid 3-Methylsalicylic acid 4-Methylsalicylic acid 5-Methylsalicylic acid
Carboxylic acid dimeric stretching mode (wavenumbers)
Ionized carboxylate stretching mode (wavenumbers)
1653 1664 1645 1655
1557 1591 1589 1568
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the asymmetric stretching mode of the initial carboxylic acids (which is shifted to lower wavenumbers owing to dimerization of the acids in the solid state) was not observed in the FTIR spectra of the salts, and was replaced by a new absorption band associated with the ionized carboxylate group. Table 2 contains a summary of the peak values for these bands.
Absorbance
5-Me-Salicylic
4-Me-Salicylic
3-Me-Salicylic
Salicylic 4000
3800
3600
3400
3200
3000
2800
-1
Wavenumber (cm ) Fig. 7. Infrared absorption spectra within the high-frequency region of the salt products formed by donepezil with 2-hydroxybenzoic acid (salicylic acid), 3-methylsalicylic acid, 4-methylsalicylic acid, and 5-methylsalicylic acid.
(a)
Whereas the high-frequency region of the FTIR spectrum was not highly informative with respect to the polymorphs of donepezil free base, this region (shown in Fig. 7) of the FTIR spectra of donepezil 3-methylsalicylate methanolate and 5-methylsalicylate methanolate solids clearly demonstrated substantial absorption associated with the solvent of crystallization. The non-solvated nature of the donepezil salicylate and donepezil 3-methylsalicylate salts was also clearly evident in the high-frequency FTIR region. Protonation of the donepezil moiety by salt formation was found to drastically alter the photophysical properties of the compound, with differences being observed between solvated and nonsolvated salt products, as well as differences between crystalline and amorphous substances. As shown in Fig. 8, the solid-state fluorescence spectra of donepezil salicylate, donepezil 4-methylsalicylate, and their respective free acids all consisted of a single peak having a maximum around 420–425 nm. The consistence in peak maxima indicates that the fluorescence processes are effectively localized in the salicylate fluorophore. In both cases, the solid-state fluorescence intensity of the salt form was found to be substantially reduced relative to the solid-state fluorescence intensity of the corresponding free acids. As shown in Fig. 9, the solid-state fluorescence intensities of the methanol solvates of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate were significantly increased relative to the solid-state fluorescence spectra of their corresponding free acids. In addition, the fluorescence peak maximum of the salts was significantly red-shifted relative to the peak maxima of the free acids. Perhaps most interesting was the finding that not only did desolvation of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate yield glassy solids, these solids exhibited strong green fluorescence characterized by band maxima at approximately 510–520 nm. The ability of amorphous films of mixed organic solids to exhibit fluorescence (especially when prepared by epitaxial methods) has been the focus of considerable research into the optoelectronic properties of such films [17]. In these films, the observed photophysical properties have been explained by the generation of large permanent dipole moments associated with structural asym-
(b)
salicylic-acid 50
4-Me-salicylic-acid 40
Relative Intensity
Relative Intensity
40
30
20
30
20
10
10
salicylate 4-Me-salicylate 0
400
430
460
490
520
Wavelength (nm)
550
580
0
400
430
460
490
520
550
580
Wavelength (nm)
Fig. 8. (a) Fluorescence spectra obtained for salicylic acid and donepezil salicylate. (b) Fluorescence spectra obtained for 4-methylsalicylic acid and donepezil 4methylsalicylate.
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(a)
25
(b) 3-Me-salicylate
20
Relative Intensity
Relative Intensity
10
5
60
DES-5-Me-salicylate
40
20
3-Me-salicylic-acid 0
5-Me-salicylate
80
DES-3-Me-salicylate
15
100
400
430
460
490
5-Me-salicylic-acid 520
550
580
0
400
Wavelength (nm)
430
460
490
520
550
580
Wavelength (nm)
Fig. 9. (a) Fluorescence spectra obtained for 3-methylsalicylic acid, donepezil 3-methylsalicylate, and desolvated donepezil 3-methylsalicylate. (b) Fluorescence spectra obtained for 5-methylsalicylic acid, donepezil 5-methylsalicylate, and desolvated donepezil 5-methylsalicylate.
metry of the molecules in the film that inhibits the formation of crystallites during growth of the amorphous phase. The interaction of the dipoles leads to the generation of local ordering within the amorphous film, and this process results in a red-shifting of the fluorescence spectrum through exciton delocalization [18]. Other systems are known that are mildly fluorescent in the crystalline state, but whose fluorescence becomes more intense and red-shifted when the crystallinity is destroyed [19,20]. In these systems, the high degree of planarity resulting from p–p stacking in the crystal leads to efficient energy transfer and a significant degree of fluorescence quenching. Reducing the crystallinity serves to induce non-parallel packing through the introduction of alternate modes of hydrogen bonding, and breaks the pathway of non-radiative energy transfer. This latter mechanism not only results in a fluorescence enhancement, but also in a strong red-shifting of the emission spectrum. The DSC thermograms of donepezil 3-methylsalicylate and donepezil 5-methylsalicylate demonstrate that desolvation of the crystalline hydrate structures causes the formation of amorphous solids. When this desolvation was carried out on the bulk level, the desolvated solids were observed to fuse into a contiguous film, and it was this film that exhibited the observed green fluorescence. It is hypothesized that removal of the solvent molecule from the crystalline solids causes a total collapse of the structures, thus facilitating the re-ordering of molecules in the amorphous film so that a non-radiative delocalization of excitation energy can occur. Ultimately, the delocalized energy becomes trapped at defect centers in the film, and the green fluorescence originates from these defect sites. 5. Conclusions While the various polymorphic forms of donepezil free base are non-fluorescent in the solid state, differing types of fluorescence have been obtained from various salicylate salts of donepezil. Crystalline salt products were obtained by the reaction of donepezil with salicylic and methyl-substituted salicylic acids, with the salicylate and 4-methylsalicylate salts being obtained as non-solvated products. For these salts, the solid-state fluorescence was
effectively localized on the salicylate fluorophore. On the other hand, the 3-methylsalicylate and 5-methylsalicylate salts were obtained as solvates, and desolvation of these salts led to formation of glassy solids that exhibited strong green fluorescence. This fluorescence was attributed to an energy transfer process associated with local order in the amorphous solids. References [1] Y.A. Asiri, G.A.E. Mostafa, Donepezil, in: H.G. Brittain (Ed.), Profiles of Drug Substances, Excipients, and Related Methodology, vol. 35, Elsevier, Academic Press, Amsterdam, 2010, pp. 117–150 (Chapter 3). [2] H.M. Bryson, P. Benfield, Drugs Aging 10 (3) (1997) 234–239. [3] Y. Donepezil, in: A.C. Moffat, M.D. Osselton, B. Widdop (Eds.), Clarke’s Analysis of Drugs and Poisons, vol. 2, fourth ed., Pharmaceutical Press, London, 2011, pp. 1296–1297. [4] J. Bernstein, Polymorphism in Molecular Crystals, Clarendon Press, London, 2002. [5] R. Hilfiker, Polymorphism in the Pharmaceutical Industry, Wiley-VCH, Weinheim, 2006. [6] H.G. Brittain, Polymorphism in Pharmaceutical Solids, second ed., Informa Healthcare, New York, 2009. [7] H.G. Brittain, J. Pharm. Sci. 93 (2004) 375–383. [8] H.G. Brittain, B.J. Elder, P.K. Isbester, A.H. Salerno, Pharm. Res. 22 (2005) 999– 1006. [9] H.G. Brittain, J. Pharm. Sci. 96 (2007) 2757–2764. [10] H.G. Brittain, Fluorescence Studies of Various Solvated and Desolvated Solvatomorphs of Erythromycin A, in: C.D. Geddes (Ed.), Reviews in Fluorescence, vol. 4, Springer, New York, 2009, pp. 379–392. [11] C.A. Parker, Photoluminescence of Solutions, Elsevier, Amsterdam, 1968. pp. 186–191. [12] A. Imai, T. Ichinohe, T. Endo, T. Tsurugi, M. Uemura, Donepezil Polycrystals and Process for Producing the Same, United States Patent 6,245,911, 2001 (issued June 12, 2001). [13] A. Imai, H. Watanabe, T. Kajima, Y. Ishihama, A. Ohtsuka, T. Tanaka, Polymorphs of Donepezil Hydrochloride and Process for Production, United States Patent 6,140,321, 2000 (issued October 31, 2000). [14] H.G. Brittain, Am. Pharm. Rev. 12 (7) (2009) 62–65. [15] A.E. Martell, R.M. Smith, Critical Stability Constants, vol. 3, Plenum Press, New York, 1977. pp. 16, 187–188. [16] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, second ed., Academic Press, New York, 1971. p. 166. [17] S.R. Forrest, Chem. Rev. 97 (1997) 1793–1896. [18] M.A. Baldo, Z.G. Soos, S.R. Forrest, Chem. Phys. Lett. 347 (2001) 297–303. [19] M. Mao, S. Xiao, J. Li, Y. Zou, R. Zhang, J. Pan, F. Dan, K. Zou, T. Yi, Tetrahedron 68 (2012) 5037–5041. [20] S. Wang, S. Xiao, X. Chen, R. Zhang, Q. Cao, K. Zou, Dyes Pigm. 99 (2013) 543– 547.
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