Toward a Better Understanding of the Physical Stability of Amorphous

Subscriber access provided by - Access paid by the | UCSB Libraries

Article

Towards a better understanding of the physical stability of amorphous anti-inflammatory agents: the role of molecular mobility and molecular interaction patterns M. Rams-Baron, Z. Wojnarowska, K. Grzybowska, M. Dulski, J. Knapik, K. Jurkiewicz, W. Smolka, W. Sawicki, A. Ratuszna, and M. Paluch Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.5b00351 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on September 6, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Molecular Pharmaceutics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Molecular Pharmaceutics

Towards a Better Understanding of the Physical Stability of Amorphous AntiInflammatory Agents: The Role of Molecular Mobility and Molecular Interaction Patterns M. Rams-Baron 1,2*, Z. Wojnarowska 1,2, K. Grzybowska 1,2, M. Dulski 3, J. Knapik 1,2, K. Jurkiewicz 1,2, W. Smolka 4, W. Sawicki 5, A. Ratuszna 1,2, M. Paluch 1,2 1

Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland

2

Silesian Center for Education and Interdisciplinary Research, 75 Pułku Piechoty 1a, 41-500

Chórzow, Poland 3

Institute of Material Sciences, University of Silesia, 75 Pułku Piechoty 1a, 41-500 Chorzów,

Poland 4

Silesian Medical University, Department of Otolaryngology, Francuska 20/27, 40-027

Katowice, Poland 5

Department of Physical Chemistry, Medical University of Gdansk, Hallera 107, 84-416 Gdansk,

Poland

*corresponding author: [email protected]

Abstract The aim of this article is to examine the crystallization tendencies of three chemically related amorphous anti-inflammatory agents, etoricoxib, celecoxib and rofecoxib. Since the molecular mobility is considered as one of the factors affecting the crystallization behavior of a given material broadband dielectric spectroscopy (BDS) was used to gain insight into the molecular dynamics of the selected APIs. Interestingly, our experiments did not reveal any significant

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

differences in their relaxation behavior, both in the supercooled liquid as well as in the glassy state. Hence, as a possible explanation for the enhanced physical stability of etoricoxib, its ability to undergo a tautomerization reaction was recognized. The occurrence of intramolecular proton transfer in the disordered etoricoxib was proven experimentally by time-dependent dielectric and infrared measurements. Additionally, IR spectroscopy combined with DFT calculations pointed out that in the etoricoxib drug, being in fact a binary mixture of tautomers, the individual isomers may interact with each other through a hydrogen bonding network. A possible explanation of this issue was achieved by performing dielectric experiments at elevated pressure. Since compression results in etoricoxib recrystallization, the possible influence of pressure on the observed stabilization effect is also carefully discussed.

Key words amorphous drug, molecular dynamics, physical stability, hydrogen bonding, molecular interactions, tautomerisation

Introduction In recent years much experimental and intellectual effort has been dedicated to advancing the understanding of the fundamental properties of amorphous materials in order to take the benefits from their undeniable advantages for pharmaceutical applications. It is well recognized that preparation of an amorphous material with high free energy may be an exciting approach leading to the improved solubility and dissolution rate of poorly water-soluble active pharmaceutical ingredients (API).1 From a commercial point of view a major limitation of amorphous systems is their thermodynamic instability accompanied by the intensified molecular mobility, both leading to undesirable drug re-crystallization.2,3 Thus, detailed understanding of the critical factors that affect the physical stability of amorphous drugs is crucial both from a scientific standpoint as well as practical applications. It is well known that the propensity of amorphous drugs to convert into more stable crystalline forms depends on several factors.1 However, as indicated by many literature reports, among them the molecular mobility is pointed out to be crucial.4,5 Pharmaceuticals with disordered molecular structures may exist in supercooled liquid (T > Tg) or glassy (T < Tg) states which are characterized by the different time scale of molecular rearrangements. The distinct dynamic


Page 2 of 38

Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


properties of both states are also reflected in various temperature dependences of their characteristic relaxation times. The molecular mobility in the supercooled liquid state is attributed to the highly cooperative movement of molecules (rotational and translational) which are described as α-relaxation. These motions which are seen to slow down with decreasing temperature are responsible for the glass transition. However, they can be also altered by the addition of a plasticizer, defined as, an additive that decreases the Tg of given material. The most commonly known example of such components are molecules of water. Thus, in the case of highly hygroscopic pharmaceuticals the effect of water (reported Tg equal to 135 K)6 on the molecular mobility should not be neglected. Below Tg, in the glassy state, the material is “kinetically frozen”. Then, the local motions of molecules or intramolecular reorientations, well known as β-relaxations, are observed. Some of them, occurring as whole molecular motions, are coupled to α-relaxations and usually called Johari-Goldstein (JG) relaxations.7 However, there is no established theory that relates the type of relaxation phenomena with the stability of amorphous phases. It was found that both, the structural relaxation as well as the local mobility can be correlated with the crystallization kinetics of different glass forming liquids.4,8 Nevertheless, the general opinion is that storage of amorphous materials at temperatures much below Tg, where the molecular mobility is predominantly limited, should reduce the risk of drug recrystallization. Such optimal storage conditions can be specified by Kauzmann temperature that is usually approximated by Tg – 50 K rule. 9 However, some exceptions from this principle have also been reported.10 The slowing down of molecular mobility can be achieved by appropriate excipients (e.g. polymer, sugar or amino acid) added to the amorphous system. This well-established strategy, based on binary co-amorphous mixtures, leads to the significant improvement of physical stability of such formulations. Among an increasing effect of an excipient on the Tg value, the specific interactions between amorphous drug and an additive seems to be crucial in the stabilization effect.11 Extremely interesting is the concept of formation of amorphous mixtures with dual functionality e.g. naproxen and indomethacin12 or the combination of indomethacin and ranitidine hydrochloride13. This is because in addition to the stability enhancement it may also offer some medical benefits. In the discussion concerning amorphous pharmaceuticals the case of drugs in which tautomerization reaction takes place cannot be neglected. According to literature reports, the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

liquid-glass conversion can be accompanied by reversible internal proton transfer reactions. As a consequence the sample can exist in dynamic equilibrium between two or even more isomeric forms. Herein one can recall the case of glibenclamide14 or indapamid15. While the effect of tautomerization process on the biological activity as well as therapeutic efficiency of drugs seems to be obvious, the investigations clarifying the possible role of tautomers in drug stability are still limited. Since the amorphous materials in which proton transfer reaction takes place become in fact a binary mixture of tautomers, one can only expect that their re-crystallization tendency should be highly reduced. In the present study we have chosen three structurally related compounds: etoricoxib, rofecoxib and celecoxib. These anti-inflammatory agents belong to the class of selective cyclooxygenase-2 (COX-2) inhibitors. In recent years there has been debate regarding the benefits and risks associated with the application of coxibs. This is due to the high number of cardiovascular events being found among patients taking rofecoxib and valdecoxib. For this reason some time ago these two drugs were removed from the market.16 At the same time, much effort has been made to overcome the problem of APIs poor water solubility, indicating their conversion into amorphous form as a promising approach.17 Consequently many reports addressed various aspects of coxibs stability enhancement.18-23 The molecular structures of selected APIs are presented in Figure 1. Despite some similarities in the three-ring structural profile, the different substituents, in particular the lack of NH groups in the case of etoricoxib and rofecoxib cause different character of their intermolecular associations. Furthermore, in the case of etoricoxib the tautomerization ability was reported.24 The main idea of our work was to characterize the molecular mobility of selected anti-inflammatory agents in the glassy and supercooled liquid states by means of broadband dielectric spectroscopy (BDS). To check if the molecular mobility is responsible for the physical instability of the studied APIs we investigated structural and secondary relaxations over a wide temperature and frequency range. Additionally, it is of particular interest to verify, whether or not the differences in the ability of hydrogen bond formation among the tested compounds can directly affect their crystallization tendency from the amorphous state. Finally, the possible influence of observed intramolecular proton transfer reaction in etoricoxib molecule on its physical stability will be also discussed.


Page 4 of 38

Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Chemical structure of three anti-inflammatory drugs selected for study: rofecoxib (a), etoricoxib (b) and celecoxib (c).

Experimental section Materials The samples under tests represent a group of selective COX-2 non-steroidal anti-inflammatory drugs (NSAIDs). Crystalline etoricoxib and rofecoxib were supplied from Sigma-Aldrich, while celecoxib was purchased from Polpharma. All tested materials were characterized with purity greater than 98%. The amorphous samples were prepared by rapid cooling (quenching) of the melt in a glovebox at relative humidity in the range of 10 - 20%. The crystalline powder was kept on a heating block until a complete melting was achieved. After that the sample was quickly transferred to a very cold metal plate. Samples prepared in such way were completely amorphous, confirmed by XRD and DSC experiments. At the same time high-performance liquid chromatography confirmed that no degradation occurred during preparation of the amorphous forms. The water content of each analyzed sample determined by means of Karl-Fisher titration was lower than 0.1%.

Methods Differential Scanning Calorimetry (DSC) Thermograms of amorphous and crystalline forms of the investigated drugs were recorded using Mettler-Toledo DSC apparatus equipped with liquid nitrogen cooling accessory and HSS8 ceramic sensor (heat flux sensor with 120 thermocouples). Temperature and enthalpy calibrations



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

were performed using high-purity standards of indium and zinc. During the measurements the aluminum crucibles of volume equal to 40µL were filled with crystalline samples (1.64 mg, 1.88 mg and 1.55 mg for etoricoxib, celecoxib and rofecoxib, respectively). After that the crucibles were closed using the crucible sealing press. The samples amorphization was performed in DSC apparatus. In the first scan the examined materials were heated to the melting temperature (heating rate 10 K/min). Next, the melted samples were cooled down with the rate equal to 20 K/min until the temperature 298 K was reached. Within the subsequent heating scans the rates equal to 10 K/min and 0.5 K/min were applied. Crystallization temperature and melting point were determined as an onset of the peak, while the glass transition temperature was assessed as the midpoint of the heat capacity increment. DSC measurements were done in triplicate with high reproducibility.

Broadband dielectric spectroscopy (BDS) Isobaric dielectric measurements at atmospheric pressure were performed using a Novocontrol GMBH Alfa analyzer with frequency values limited to 106 Hz. The dielectric spectra were measured over a wide temperature range from T = 153 K to T = 393 K, with various temperature intervals (from ∆T = 10 K at low temperatures to ∆T = 2K in the vicinity of Tg). The temperature was precisely controlled by Quatro temperature controller using a nitrogen gas cryostat (accuracy better than 0.1 K). During the measurements the tested samples were placed between steel electrodes of the capacitor (15 mm diameter) with fixed distance between electrodes (0.1 mm) provided by fused silica spacer fibers supplied by Novocontrol. Since the silica spacers do not give any contribution to the overall dielectric response the subtraction procedure was not required. The same experimental setup was applied to monitor the progress of tautomerization at temperature equals to 346 K. In order to obtain greater signal-to-noise ratio and consequently data of better quality we repeated the rofecoxib measurements using a capacitor of 40 mm diameter. To verify the influence of pressure on the overall behavior of etoricoxib we conducted dielectric measurements using the automatic high-pressure system developed by Unipress. As previously, the investigated sample was placed between the steel electrodes of capacitor (diameter15 mm; gap 0.1 mm; Teflon spacer). In the next step, the capacitor was placed inside Teflon ring and sealed. Tested sample, out of contact with compression medium, was fixed to pressure chamber


Page 6 of 38

Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


filled with silicon oil. The time-dependent measurements were performed under strictly controlled temperature (T = 353 K) and pressure condition (p = 25 MPa). Essentially identically treated reference sample was measured under ambient pressure conditions. Additionally, we performed isothermal dielectric measurements at T = 353 K in the pressure range between p = 10 MPa and p = 65 MPa with pressure intervals equal to 5 MPa.

Fourier-transform Infrared Spectroscopy Infrared measurements were carried out using an Agilent Cary 660 FTIR spectrometer equipped with a standard source and a DTGS Peltier-cooled detector. The spectra have been collected using GladiATR diamond accessory (Pike Technologies) in the 4000 - 400 cm-1 range. All spectra were accumulated with a spectral resolution of 4 cm-1 and recorded by accumulating of 16 scans. Finally, time-dependent infrared measurements were carried out at 345 K to follow the tautomerisation process.

Theoretical calculations Theoretical studies of etoricoxib were performed by density functional theory (DFT) method25-27 using Gaussian 09 software package28. The geometry of etoricoxib tautomers, as well as its heterodimers were optimized using Becke’s three-parameter hybrid functional with the LeeYang-Parr correlation functional (B3LYP)29,30 and 6-31G(d,p) basis set31. The obtained results were visualized using GaussView 5.0.8 software. All the optimized structures showed positive harmonic vibrations indicating a true energy minimum.28 Finally, dipole moment of each tautomeric form was determined based on the previously optimized geometry.

X-ray diffraction (XRD) The X-ray diffraction experiment was performed using a Rigaku-Denki D/MAX RAPID II-R diffractometer equipped with a rotating anode Ag tube (λKα = 0.5608 Å), an incident beam (002) graphite monochromator, and an image plate in the Debye-Scherrer geometry as a detector. The X-ray beam width at the sample was 0.3 mm. Crystalline etoricoxib sample and sample which recrystallize during experiment were placed inside glass capillaries with a diameter of 1.5 mm and wall thickness of 0.01 mm. The measurements were carried out for the capillaries filled with samples and empty. The diffraction intensity for the empty capillary was then subtracted. The



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 38

obtained two-dimensional diffraction patterns were converted into one-dimensional functions of intensity versus the scattering angle using suitable Rigaku/XRD software which is a comprehensive package controlling the video image of sample, data collection, and X-ray image processing.

Results and discussion Thermal behavior of tested drugs. Thermal methods, such as differential scanning calorimetry (DSC), are usually employed both for analyzing the ability of a drug to be transformed into the glassy state as well as for understanding the amorphous product stability over the shelf life.2 The DSC heating curves obtained during the heating of the crystalline samples (rate 10 K/min) are presented in Figure 2a. Well resolved endotherms corresponding to melting of crystalline forms are noticed. The lower panel (b) shows DSC heating curves of amorphous forms prepared in situ in DSC by quench cooling of the melt (with cooling rate 20 K/min). From the nonisothermal measurements performed by means of DSC technique we found that amorphous celecoxib reveals the highest tendency towards recrystallization during heating above Tg. The cold crystallization for this API does not occur but only at a relatively high heating rate of 25 K/min. At the same time, to observe the rofecoxib devitrification a very slow heating rate equal to 0.5 K/min has to be applied. In contrast, no signs of crystallization were visible in the case of etoricoxib. As a result, the following order of tendency to recrystallization was experimentally established: celecoxib>rofecoxib>etoricoxib. The thermodynamic quantities obtained from DSC scans are collected in Table 1. As one can easily see, the tested compounds are characterized by similar Tg values (variations in the order of 4K) but significantly different values of Tm. To compare their re-crystallization tendencies we used the so-called reduced crystallization temperature (Tred) proposed by Zhou et al. and defined as follows:32

Tred = (Tc − Tg ) /(Tm − Tg )

(1)

where Tg, Tm and Tc denote the glass transition temperature, the onset of melting point and the onset of crystallization temperature obtained after reheating of amorphous samples, respectively. The concept of reduced crystallization temperature, by scaling between 0 and 1, allows for normalization of results in such a way that the lower Tred values tend to indicate a higher


Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tendency towards glass devitrification. In the case of etoricoxib under the experimental time scale recrystallization did not occur thus we assumed that Tred = 1 (which implies Tc = Tm). Other tested compounds possess Tred values lower than etoricoxib. They are equal to 0.74 and 0.76 for celecoxib for rofecoxib, respectively. Various re-crystallization tendencies of the tested compounds, together with the properties arising from their molecular structures, make them perfect materials to study the impact of chemical structure on the observed variations in the physical stability. To put more light on this issue, the potential contribution of various thermodynamic and molecular factors in the crystallization event must be considered. Several years ago the thermodynamic properties of etoricoxib, celecoxib and rofecoxib were compared by Kaushal et al.33 Using the conventional and modulated DSC technique the authors reported amorphous rofecoxib as the one that holds the highest overall thermodynamic force to re-crystallization.

Table 1. Thermodynamic quantities obtained from DSC heating curves of tested compounds in the crystalline and amorphous state (10 K/min). The onset and temperature range of crystallization were determined from DSC curves recorded with heating rate of 0.5 K/min.

crystallization range [K] -

Tred

444

443.9 - 444.0

0.76

406

406.4 - 407.5

0.74

Tg [K]*

Tm [K]**

Tc [K]**

etoricoxib

330

407

-

rofecoxib

325

482

celecoxib

330

434

* determined as midpoint, ** determined as onset.


1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 38

Figure 2. DSC heating curves tested compounds in the crystalline (a) and amorphous (b) state.

Molecular mobility in supercooled liquid and glassy state. To examine the molecular dynamics of investigated APIs we performed dielectric measurements over a wide temperature and frequency range. Dielectric spectra collected above Tg (see Figure SI in supplementary information) of etoricoxib, celecoxib and rofecoxib reveal well-pronounced α-relaxation peak corresponding to cooperative movement of drug molecules. One can easily see that the maximum of α-peak moves towards lower frequencies with cooling indicating an increase in structural relaxation time (τα). To compare the properties of the investigated drugs in the supercooled liquid state we analyzed the temperature dependencies of their structural relaxation times defined as τα=1/2πfmax. As shown in Figure 3 α-relaxation times determined for all studied herein systems plotted against 1000/T reveal non-Arrhenius behavior that can be satisfactorily parameterized by means of Vogel-Fulcher-Tammann equation (see supplementary information). From the VFT fits we found the temperature at which the structural relaxation time reaches 100 s, assigned to the liquid-glass transition of the studied systems (see Table 2). The determined Tg values, equal to


Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


328 K for etoricoxib, 327 K for celecoxib and 324 K for rofecoxib, are roughly similar and are in good agreement with DSC results. The variation in dynamic behavior of the investigated APIs were also analyzed in terms of their fragilities. In general, fragile materials show a significant deviation from Arrhenius-like behavior. This is as a result of the notable changes in molecular dynamics in the vicinity of Tg.34, 35 Since the concept of fragility is clearly related to the mobility of glassy systems, its potential role as a stability indicator is under debate. Liquids classified as fragile are often considered as physically less stable in comparison to the strong ones.36 The calculated m values of the tested compounds are very similar (m = 98 ± 2 for etoricoxib, 104 ± 2 for rofecoxib and 97 ± 3 for celecoxib) and indicate that the studied APIs can be classified as fragile glass formers. At the same time, one can note that there is no relation between m values and resistance against crystallization of examined systems. Interestingly, we found that the obtained values of steepness index differs from those predicted by Kaushal et al. on the basis of calorimetric data.33 The authors of recalled paper made the following assumptions to evaluate the VFT parameters, T0, D and τ∞ and consequently calculate m parameter: (i) the VFT divergence temperature T0 is equal to Kauzmann temperature TK, (ii) the preexponential factor τ ∞= 10-14 s, and (iii) the structural relaxation time τα = 100 s at Tg. However, as can be seen in Table 2, the values of τ∞ determined directly from logτα(T) dependences of examined systems are much higher than 10-14 s. What is more even if we fix the parameter τ∞ = 10-14 the obtained m value37 is still different than those reported by Kaushal et al. The next step of our studies was dedicated to analysis of relaxation time distribution manifested by the value of βKWW parameter. According to Shamblin et al. the improved stability of amorphous pharmaceutical systems should correlate with an increasing value of βKWW.38 To determine the value of βKWW for the examined herein anti-inflammatory agents the dielectric spectra of the investigated APIs were shifted horizontally to overlap each other and analyzed in terms of the Kohlrausch–Williams–Watts (KWW) function (see Figure SIII in supporting information). The obtained values of stretching parameters are equal to βKWW = 0.63 for etoricoxib, 0.64 for rofecoxib and 0.67 for celecoxib. Again, we observed that small variations in βKWW values does not accurately reflect the real tendencies to recrystallization of the drugs investigated. Considering the potential impact of molecular mobility on the physical stability of amorphous drugs the secondary relaxation processes cannot be neglected. This issue is of particular interest



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 38

because amorphous drugs are usually stored in temperatures much below their Tg. Interestingly, the strong tendency toward crystallization found for celecoxib, was recently related to its intense molecular mobility manifested in many secondary modes.37 Since all tested APIs possess similar three ring based chemical structure we expected that the degree of local mobility, generally related to intramolecular motions, should also be similar. Indeed, as illustrated in Figure 3 some analogies between the secondary relaxation dynamics of investigated APIs were found. Namely, all samples are characterized by the same number of processes with similar values of activation energies. The only striking difference is the presence of β-process in celecoxib assigned previously as Johari-Goldstein relaxation (JG) involving fast reorientations of entire drug molecules as well as the presence of δ-relaxation in etoricoxib that is slower than δ-modes observed in the others studied APIs. To accurately determine which part of molecule is responsible for this process the quantum mechanical calculations should be performed. However, through the analogy to celecoxib in which the δ-relaxation was assigned to the rotation of the phenyl-CH3 one can suppose that the origin of δ-process in etoricoxib is the rotation of pyridineCH3 part.39 Moreover, the characteristics of such movements will highly depend on the molecular interactions within the system due to the presence of NH group (as an effect of tautomerization) capable to hydrogen bond formation. Consequently, such motions may be slowed down what actually is observed when we compare the temperature dependencies of δ-relaxation times obtained for investigated drugs. Thus, one can state that the highest degree of celecoxib molecular mobility corresponds well with its large devitrification tendency. In contrast, the experimental data of rofecoxib and etoricoxib, having similar secondary relaxation patterns and completely different crystallization behavior, indicate that there is no correlation between glassy mobility and physical stability of these systems. In the context of experimental data presented above the question regarding a plausible explanation for the better stability of amorphous etoricoxib still remains unresolved. Therefore, in the next part of our paper the physical stability of amorphous anti-inflammatory agents will be discussed in terms of inter- and intramolecular interactions.


Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. The relaxation map of investigated APIs. VFT equation was applied to describe structural relaxation times, while the temperature dependences of secondary relaxations were fitted to the Arrhenius equation.

Table 2 The most important parameters obtained from dielectric measurements: VFT fitting parameters, glass transition temperatures, fragilities and activation energies of the secondary relaxations. Tg [K]

Fragility

Activation energy [kJ/mol]

(for τα = 100 s)

m

β-relaxation γ-relaxation δ-relaxation

266.2

328

98±2

-

53

21

267.3

324

104±2

-

50

14

celecoxib* -19.0 13.3 256.5

327

97±3

80**

51

21

D

T0[K]

etoricoxib

-16.6 9.8

rofecoxib

-16.3 8.8

material

log τ∞

* in the previous paper37 we fixed the value of the parameter log τ∞ = -14.0 as it is usually done to predict the dynamic fragility from calorimetric data; accordingly other parameters were: D = 6.2; T0 = 280.7 K; Tg = 328 K; m = 109. **from37 Inter- and intramolecular interactions. In order to find an explanation for the high stability of etoricoxib drug, in comparison to other investigated coxib-type systems, we focused on the chemical structure of this compound. The observed difference in the crystallization behavior may arise from the ability of etoricoxib to undergo a tautomerization reaction. In general, tautomerization is described as a reversible proton migration within one molecule, resulting in changes in its chemical structure as well as physicochemical properties.40 Such phenomenon can



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

have positive as well as negative consequences. On the one hand, in the case of drugs or some biologically important molecules, such mismatching process may have serious medical side effects. On the other hand, the isomerization reaction may positively affect certain properties of amorphous drugs, such as physical stability. To date, the correlation between the higher stability affected by the tautomerization ability of amorphous API has been found for two pharmaceutically important molecules, i.e. indapamid15 and glibenclamid41. One can suspect that in the case of drug being a mixture of tautomers the stabilization effect can be similar to those observed for amorphous drug-drug formulations. In such systems the molecular interactions between drugs play crucial role in preventing crystallization due to the heterodimer formation.1,12 The possibility of tautomeric conversion in the etoricoxib molecule was mentioned previously by Kaushal et al.24 The proton transfer was indicated between methyl group linked directly to the pyridine ring and pyridine nitrogen atom, as shown in Figure 4. This means that the disordered state of etoricoxib may exist in dynamic equilibrium between two different isomeric forms, described as E1 (methyl group attached to the pyridine ring) and E2 (methylene group in the pyridine ring). To verify this statement we performed time-dependent FT-IR experiments. Due to the substantial sensitivity to structural changes, infrared spectroscopy is a perfect method to study the changes in the molecular architecture associated with the tautomerization phenomenon. In our study the FT-IR spectra were collected during the isothermal annealing of vitrified etoricoxib at 345 K, which is 18 K above its Tg temperature (see Figure 4). Basically, the tautomeric equilibrium is very sensitive to the changes of local environment conditions but sometimes the total equilibration time can be highly elongated. At a temperature equal to T = 345 K several hours were required to achieve the tautomeric equilibrium. The changes in the infrared absorption, determined during the time-dependent experiment, indicate that band intensities are sensitive to the structural changes of etoricoxib. In the analysis, the FT-IR spectrum of crystalline etoricoxib was used as a reference. The most significant changes were observed in the 3200 and 2800 cm-1 regions where the bands are generally due to the CH stretching vibrations, such as νCH3 asym/sym, νCH2 asym/sym and νCH (see Figure 4). In the crystalline sample such vibrations were assigned to the sharp maxima at 3059, 3025, 2988 cm-1 (νCH3 asym/sym) and 2914 cm-1 (νCH). While for supercooled material, the corresponding bands were observed at 3036 cm-1 (νCH3 asym), 2988 cm-1 (νCH3 sym) and 2923 cm-1 (νCH). Additionally, the bands originating from methylene stretching vibrations were detected at 3009 cm-1 (νCH2 asym) and


Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2964 cm-1 (νCH2 sym). The occurrence of methylene stretching modes, typical for the tautomer form, indicate the presence of E2 fraction in the isomer mixture. The appearance of tautomeric methylene isomer in the supercooled etoricoxib was also observed in the region between 3700 and 3200 cm-1, where NH and OH stretching vibrations occur. The broad FT- IR band located at 3519 cm-1 was attributed to the stretching vibration of primary amine (νNH) in E2, while band at 3617 cm-1 was due to vibration of free hydroxyl groups or very weak H-bond. Our results confirmed the co-existence of various isomers in the examined sample. Interestingly, the time evolution of FT-IR spectra pointed out a gradual increase of NH band population accompanied by a well-defined decrease of CH3 band population, suggesting that the equilibrium is favored mainly to E2 form (Figure 4).

Figure 4. The analysis of tautomerization progress in etoricoxib drug at T = 345 K performed by means of FT-IR spectroscopy. The comparison of infrared spectra (400 – 4000 cm-1) of crystalline and amorphous etoricoxib is presented; the marked area (2450 – 3800 cm-1) corresponding to methyl and hydroxyl region was analyzed in details. The analysis of exemplary IR spectra of etoricoxib measured at T = 345 K in a time-dependent manner is depicted. The



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

presented data corresponds to the initial spectrum measured immediately after sample vitrification (t = 0 min) and the final spectrum registered after t = 350 min. Additionally, the time-evolution of CH3 and NH bands in comparison to the crystal is shown. Furthermore, chemical structures of E1 and E2 isomers are presented, together with calculated values of dipole moments.

Another method applied to monitor the kinetics of tautomerization reaction was BDS spectroscopy. If the tautomeric conversion leads to changes in viscosity or average dipole moment of the sample, then the tautomerization progress can be easily tracked by measuring the changes in the dielectric response of investigated systems. The spectral changes observed over time can include both an increase and decrease of dielectric strength (Δε). According to Eq.2 the Δε value, being the actual difference between the static dielectric constant (εs) and its high frequency analogue (ε∞), depends on the number of mobile dipoles (N) and the average electric dipole moment of the sample (µ):

∆ε = ε s − ε ∞ ≈ Nµ 2

(2)

Therefore, variations of Δε will reflect the changes in isomers ratio, if only the particular isomers possess different magnitudes of dipole moments.42 Figure 5 shows the time-evolution of dielectric spectra registered at T = 346 K. One can see that the Δε value increases with time, indicating that the average dipole moment of the studied system is also going up. It means that during the annealing process the population of isomers with higher dipole moment dominates in the sample. The dipole moment of each tautomeric form was obtained from DFT method using 631G(d,p) basis set and B3LYP hybrid functional. According to our calculation the dipole moments values of E1 and E2 isomer were equal to 3.92 D and 5.24 D, respectively. It means that in the dielectric experiment the tautomers equilibrium is greatly shifted toward E2 form that is consistent with the result of infrared spectra analysis. Herein, it should be stressed that the preferences of E2 tautomer may arise for different reasons. It has been found that multiple factors, such as polarity, aromaticity, intra- and/or intermolecular interactions, substituent and solvent effects, can markedly influence the tautomeric equilibria.43


Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Representative dielectric spectra of vitrified etoricoxib measured in a time-dependent manner at temperature equals to 346 K, illustrating the progress of tautomeric conversion. Selected curves represent the first spectrum recorded after temperature stabilization (green circles) and the last one registered after sample re-equilibration (blue circles). Additionally, chemical structures of E1 and E2 isomers are presented, together with calculated values of dipole moments.

The nature of molecular interactions existing in the studied samples are governed by their chemical structures. Although the tested systems are structurally related to each other there are certain differences in their functional groups. As a result the variations in their H-bonding abilities take place. The analysis of FTIR spectra of all drugs in amorphous and crystalline state, performed by Kaushal et al., has shed more light on their hydrogen-bonding patterns. The primary structural difference concerns the presence of strong hydrogen bond donors. While celecoxib possesses two NH- groups, indicating its high H-bonding potential, in the other structures only the H-bond acceptors are found. Despite this for rofecoxib no evidence of associations between the drug molecules were found.24 On the other hand, in the case of etoricoxib relatively weak directional interactions between atoms in E1 form were reported. What is more, our FT-IR results and theoretical data have shown that there are some interactions between both isomers, E1 and E2 (see Figure 6). The applied computational approach enabled us to distinguish the most stable structures created by the hydrogen bonds between possible etoricoxib monomers. The infrared bands located between 2450 and 2800 cm-1 might be assigned



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

to such hydrogen-bond interactions occurring between hydrogen in NH group of E2 and the pyridine nitrogen of E1. Theoretical calculations confirmed the strong red shift of infrared bands due to the presence of moderate hydrogen bond with donor-acceptor distance dXH…A of 1.92 Ǻ.44,45 Moreover, DFT data revealed that absorption maxima observed at 3155, 3435 and 3586 cm-1 might be assigned to the hydrogen-bonded structures with donor-acceptor distance dXH…A between 2.35-2.56 Ǻ. An example of such structure involving hydrogen and nitrogen atoms in the central pyridine ring of two molecules (dXH…A ≈ 2.56Ǻ) is depicted in Figure 6. It is well known that crystallization tendency of a drug strongly depends on how easily the molecules can be orderly packed. Therefore, the molecular interactions are frequently considered as a crucial factor controlling the physical stability of various APIs.5 In the literature one can find a lot of reports showing that the nucleation rate in the binary drug-drug systems is suppressed due to the heterodimers formation.12,13,46,47 This is because to initiate the crystallization process the disruption of heterodimer must occur as well as the rearrangement of molecules into homodimers which can form crystal nuclei. Since we have proved that etoricoxib exists in dynamic equilibrium between interacting tautomers its reorganization into a crystal lattice should be also hindered. Another aspect discussed in the literature in the context of crystallization performance is the differences in the hydrogen-bond strength between amorphous and crystalline states of a material. Marsac et al. shows that calcium channel blocker felodipine with stronger hydrogen bonds in the amorphous state reveal lower tendency to crystallization in comparison to nifedipine which is characterized by stronger hydrogen bonding network in the crystalline state.48,49 This is because the bonds existing in the amorphous state have to be disrupted to promote structural rearrangement leading to the crystalline state formation. In the case of etoricoxib molecules the differences in hydrogen bond patterns in amorphous and crystalline are incontestable. The presence of H-donating NH group attached to the pyridine ring of tautomeric E2 form will impact significantly the hydrogen-bonding ability of amorphous etoricoxib (E1 form can only weakly interact with other molecules).


Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Theoretical model illustrating H-bond formation between etoricoxib isomers.

The simple way to verify whether or not the nature of molecular interactions has a real impact on the physical stability of supercooled etoricoxib, is the application of hydrostatic pressure. It is well known that compression favors the dense packing of molecules. Since the hydrogen-bonds strength depends on the distance between donor and acceptor atoms, the applied mechanical stress should reinforce the nature of molecular interactions effect.50 To verify the possible effect of high pressure on the physical stability of supercooled etoricoxib we applied dielectric spectroscopy once again. We performed isothermal measurements at T = 353 K under elevated pressure. Because the α-relaxation exhibited notably strong sensitivity to pressure, the narrow pressure range (10 - 65 MPa) was sufficient to shift the maximum of α-peak outside the experimentally accessible frequency range. The recorded dielectric loss spectra together with the pressure dependence of α-relaxation times are depicted in Figure 7. The determined Pg value,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

corresponding to τα= 100 s, together with the Tg value received from ambient-pressure isobaric measurements were used to estimate the pressure sensitivity of etoricoxib reflected in the value of dTg/dPg coefficient. The estimated value of 0.246 K/MPa is comparable to those reported for glibenclamide (0.249 K/MPa)14 or ibuprofen51 (0.195 K/MPa) and indicates the remarkable sensitivity of molecular dynamics to pressure-induced changes in the local density.

Figure 7. Upper panel shows dielectric loss spectra of supercooled etoricoxib drug at T = 353 K under various pressures, increasing from p = 10 MPa to p = 65 MPa. Lower panel presents pressure dependence of determined structural relaxation times described by means of pressure counterpart of VFT fit function, defined elsewhere.52 Inset shows the relation between Tg values (isobaric measurements at ambient pressure) and Pg values (isothermal measurements at T = 363 K) used to estimate dTg/dPg value. Tg and Pg values were indicated for relaxation time τ α = 100 s.

To find out the possible impact of applied pressure on the crystallization behavior of etoricoxib we have performed time-dependent dielectric measurements at T = 353 K and p = 20 MPa. As a


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reference we conducted a similar experiment at the same structural relaxation time but at p = 0.1 MPa. Just like in the case of our previous observations, at ambient pressure conditions, no crystallization signs were detected. On the other hand, the compression of supercooled liquid rapidly induces the crystallization process of etoricoxib. It is manifested by fast decrease of both static epsilon εs as well as amplitude of structural relaxation peak. Interestingly, at the same time new relaxation mode associated with some intramolecular movements of crystalline material becomes visible in the experimental window. The dielectric spectra showing the progress of crystallization of etoricoxib drug under pressure are presented in Figure 8. Herein, it should be noted that by tracking the time-evolution of dielectric strength one can easily monitor the kinetics of crystallization process. At the beginning we have normalized the real part of dielectric permittivity function ε’(f) by means of the following equation:

ε ' (0) − ε ' (t ) ε = ' ε (0) − ε ' (∞) ' N

(3)

' ' where ε (0) and ε (∞) denotes the values of normalized real permittivity at the beginning and at

the end of the crystallization process, respectively. Additionally, we presented the crystallization data as the dependence of εN’ and its first derivative against ln t i.e. in the form of the so-called Avramov approach.53 The analysis of crystallization kinetics of etoricoxib showed that after 26 hours the drug was completely in the crystalline state, as confirmed by DSC and XRD results. Moreover, the detailed analysis of XRD diffraction patterns, presented in the inset of Figure 8, showed that the crystalline structure obtained under pressure was identified as triclinic lattice with the P1 space group (tabulated in CCDC no. 855683). In contrast, the commercially available etoricoxib forms the pure orthorhombic crystalline structure with the Pca21 space group (CCDC no. 855682). Such behavior was previously found also for other glass-forming systems.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

Figure 8. Left panel shows the real (ε’) and imaginary (ε”, in inset) part of etoricoxib dielectric spectra recorded during monitoring the progress of its crystallization at T = 353 K under p = 20 MPa. The spectra depicting initial and final stages of crystallization process are depicted as green circles and blue circles, respectively. Right panel presents the relationship of normalized dielectric constant ε N' (blue circles) and its first derivative (grey circles) against logarithm of time. Inset shows X-ray powder diffraction patterns of original crystalline etoricoxib and sample converted to crystalline form during the high pressure experiment.

The induction of API recrystallization under pressure is well documented for several amorphous pharmaceuticals (e.g. indomethacin54,55, celecoxib56, ibuprofen57). However, the effect of compression on changes in tautomers ratio is poorly explored so far. The main limitation is due to experimental difficulties related to high pressure measurements. Nevertheless, the recent example of benzo[1,3]oxazine being investigated in terms of pressure-modulated molecular switching ability clearly proved that tautomerization is strictly pressure dependent.58 Thus, one can expect that compression is somehow influencing the tautomers ratio in etoricoxib drug. In the literature there are examples of drug-drug mixtures, being stabilized by molecular interactions between components, which shown that mixtures with 1:1 drug ratio are more stable if compared to 2:1 and 1:2 mixtures.12,

47

Thus one can imagine that shifting the equilibrium towards any of the

etoricoxib isomer can facilitate crystallization process. It seems to be highly reasonable that such


Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


variations in tautomers concentration and/or reinforcing of interactions between isomers under pressure may be responsible for etoricoxib recrystallization. However, to explore this issue in more detail, high pressure infrared spectroscopy should be applied in the future. Concluding this section it should be noted that high pressure investigations are of special practical importance because mechanical stress applied during the formulation process can lead to unexpected events, such as those found for etoricoxib, stability changes or new polymorph formation, both influencing the effectiveness and safety of the final amorphous product.

Conclusions and Perspectives This article highlights a highly important issue associated with the identification of factors governing the physical instability of amorphous pharmaceuticals. This research direction is important both from a technological as well as scientific point of view. This is because only the fully described physical background of observed processes may result in significant progress in the amorphous drugs production. As the focus of our studies we have chosen a group of clinically and chemically related compounds that reveal opposite crystallization behaviors. Additionally, interesting differences in their molecular interaction patterns were observed. In particular, the ability of etoricoxib to undergo a tautomerization reaction is discussed in detail and has been proven experimentally. Since intense mobility is one factor limiting the stability of amorphous phases, initially we focused on the use of broadband dielectric spectroscopy to study the molecular dynamics of the tested drugs. Interestingly, the obtained results showed many similarities in the structural relaxation parameters and glassy dynamics. Searching for an explanation for the greater stability of amorphous etoricoxib, we focused our research on the tautomerisation effect, the phenomenon that distinguishes this drug from others. The infrared and dielectric measurements indicated clearly that supercooled etoricoxib may exist as a dynamic mixture of two different tautomers. Moreover, based on theoretical and experimental data, we pointed out that these isomers may interact with each other due to the hydrogen bonding network. In our opinion, the existence of such interactions between isomers, similar to those existing in binary amorphous drug-drug mixtures, may be responsible for the observed stabilization enhancement. What is more, our further experiments showed that the drug stability can change dramatically after compression. At this moment we cannot clearly explain whether or not the pressure-induced instability was

due to the reinforcing of molecular interactions and/or is



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

associated with the shifting of the tautomer equilibrium towards the one isomer. This issue still remains open and becomes a challenge for further studies.

Acknowledgment The authors M.R.B., Z.W., W.S. and M.P. are deeply grateful for the financial support by the National Science Centre within the framework of the Opus3 project (Grant No. DEC2012/05/B/NZ7/03233). Z.W. acknowledges the financial assistance from FNP START (2014). This research was supported in part by PL-Grid Infrastructure.

Supporting Information Details of analysis of dielectric spectra recorded above and below Tg. Dielectric loss spectra of tested compounds above and below glass transition temperature. Temperature dependencies of structural relaxation times of rofecoxib, celecoxib and etoricoxib. Analysis of dielectric loss curves of etoricoxib, rofecoxib and celecoxib in terms of KWW function. Typical sample arrangements used during dielectric measurements at ambient and elevated pressures. This information is available free of charge via the Internet at http://pubs.acs.org/.

Literature 1. Laitinen, R.; Löbmann, K.; Strachan, C. J.; Grohganz, H.; Rades, T. Emerging trends in the stabilization of amorphous drugs. Int J Pharm. 2013, 453(1), 65-79. 2. Baird, J. A.; Taylor, L. S. Evaluation of amorphous solid dispersion properties using thermal analysis techniques. Adv Drug Deliv Rev. 2012, 64(5), 396-421. 3. Mahlin, D.; Bergström, C. A. Early drug development predictions of glass-forming ability and physical stability of drugs. Eur J Pharm Sci. 2013, 49(2), 323-32. 4. Bhattacharya, S.; Suryanarayanan, R. Local mobility in amorphous pharmaceuticalscharacterization and implications on stability. J Pharm Sci. 2009, 98(9), 2935-53. 5. Bhugra, C.; Pikal, M. J. Role of thermodynamic, molecular, and kinetic factors in crystallization from the amorphous state. J Pharm Sci. 2008, 97(4), 1329-49.


Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Hancock, B. C.; Zografi, G. The relationship between the glass transition temperature and the water content of amorphous pharmaceutical solids. Pharm Res. 1994, 11(4), 471-7. 7. Ngai, K. L.; Paluch M. Classification of secondary relaxation in glass-formers based on dynamic properties. J Chem Phys. 2004, 120(2), 857-73. 8. Kothari, K.; Ragoonanan, V.; Suryanarayanan, R. Influence of molecular mobility on the physical stability of amorphous pharmaceuticals in the supercooled and glassy states. Mol Pharm. 2014, 11(9), 3048-55 . 9. Hancock, B.C; Shamblin, S. L.; Zografi, G. Molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures. Pharm Res. 1995, 12(6), 799-806. 10. Vyazovkin, S.; Dranca, I. Physical stability and relaxation of amorphous indomethacin. J Phys Chem B. 2005, 109(39), 18637-44. 11. Vo, C. L.; Park, C.; Lee, B. J. Current trends and future perspectives of solid dispersions containing poorly water-soluble drugs. Eur J Pharm Biopharm. 2013, 85, 799-813. 12. Löbmann, K.; Laitinen, R.; Grohganz, H.; Gordon, K. C.; Strachan, C.; Rades, T. Coamorphous drug systems: enhanced physical stability and dissolution rate of Indomethacin and Naproxen. Mol Pharm. 2011, 8(5), 1919-1928. 13. Chieng, N.; Aaltonen, J.; Saville, D.; Rades, T. Physical characterization and stability of amorphous indomethacin and ranitidine hydrochloride binary systems prepared by mechanical activation. Eur J Pharm Biopharm. 2009, 71(1), 47-54. 14. Wojnarowska, Z.; Grzybowska, K.; Adrjanowicz, K.; Kaminski, K.; Paluch, M.; Hawelek, L.; Wrzalik, R.; Dulski, M.; Sawicki, W.; Mizgalska, J.; Tukalska, A.; Bieg, T. Study of the amorphous glibenclamide drug: analysis of the molecular dynamics of guenched and cryomilled material. Mol Pharm. 2010, 7(5), 1692-1707. 15. Wojnarowska, Z.; Grzybowska, K.; Hawelek, L.; Dulski, M.; Wrzalik, R.; Gruszka, I.; Paluch, M.; Pienkowska, K.; Sawicki, W.; Bujak, P.; Paluch, K. J.; Tajber, L.;Markowski, J. Molecular dynamics, physical stability and solubility advantage from amorphous indapamide drug. Mol Pharm. 2013, 10(10), 3612-27.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

16. Shi, S.; Klotz, U. Clinical use and pharmacological properties of selective COX-2 inhibitors. Eur J Clin Pharmacol. 2008, 64(3), 233-52. 17. Gupta, P.; Chawla, G.; Bansal, A. K. Physical stability and solubility advantage from amorphous celecoxib: the role of thermodynamic quantities and molecular mobility. Mol Pharm. 2004, 1(6), 406-13. 18. Gupta, P.; Bansal, A. K. Molecular interactions in celecoxib-PVP-meglumine amorphous system. J Pharm Pharmacol. 2005, 57(3), 303-10. 19. Chawla, G.; Gupta, P.; Thilagavathi, R.; Chakraborti, A. K.; Bansal, A. K. Characterization of solid-state forms of celecoxib. Eur. J. Pharm. Sci. 2003, 20, 305-317. 20. Gupta, P.; Thilagavathi, R.; Chakraborti, A. K.; Bansal, A. K. Role of molecular interaction in stability of celecoxib-PVP amorphous systems. Mol Pharm. 2005, 2(5), 384-91. 21. Dantuluri, A. K. R.; Amin, A., Puri, V.; Bansal A. K. Role of r-Relaxation on Crystallization of Amorphous Celecoxib above Tg Probed by Dielectric Spectroscopy, Mol. Pharmaceutics, 2011, 8(3), 814-822 22. Ambike, A. A.;. Mahadik, K. R.; Paradkar, A.; Stability study of amorphous valdecoxib. Int. J. Pharm. 2004, 282, 151-162. 23. Skiba, M.; Skiba, M.; Milon, N.; Bounoure, F.; Fessi, H. Preparation and characterization of amorphous solid dispersions of nimesulide in cyclodextrin copolymers. J Nanosci Nanotechnol. 2014, 14, 2772-9. 24. Kaushal, A. M.; Chakraborti, A. K.; Bansal, A. K. FTIR studies on differential intermolecular association in crystalline and amorphous states of structurally related nonsteroidal anti-inflammatory drugs. Mol Pharm. 2008, 5(6), 937-45. 25. Hehre, W. J.; Radom, L.; Schleyer R., Pople, J. Ab Initio molecular orbital theory. Wiley: New York, 1986; 20-29, 65-88. 26. Parr, R. G.; Yang, W. Density functional theory of atoms and molecules. Oxford University Press: New York, 1989;142-197.


Page 26 of 38

Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Dobson, J. F.; Vignale, G.; Das, M. P. Electronic density functional theory: recent progress and new directions. Springer US, 1998. 28. Frisch, M. J. et al. Gaussian, Inc. Gaussian 09, Revision A.1. Wallingford CT, 2009. 29. Becke, A. D. A new mixing of Hartree-Fock and local density-functional theories. J. Chem. Phys., 1993, 98, 5648-5652. 30. Becke A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys Rev A. 1988, 38, 3098-3100. 31. Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation energy formula into a functional of the electron density. Phys. Rev. B. Condens Matter, 1988, 37, 785-789 . 32. Zhou, D.; Zhang, G. G.; Law, D.; Grant, D. J.; Schmitt, E. A. Physical stability of amorphous pharmaceuticals: importance of configurational thermodynamic quantities and molecular mobility. J Pharm Sci. 2002, 91(8), 1863-72. 33. Kaushal, A. M. K.; Bansal, A. Thermodynamic behavior of glassy state of structurally related compounds. Eur J Pharm Biopharm. 2008, 69(3), 1067-76. 34. Crowley, K. J.; Zografi, G. The use of thermal methods for predicting glass-former fragility. Thermochim. Acta. 2001, 2, 79-93. 35. Green, J. L.; Ito, K.; Xu, K.; Angell, C. A.; Fragility in liquids and polymers: new, simple quantifications and interpretations. J. Phys. Chem. B. 1999, 103 (20), 3991-3996. 36. Shintani, H.; Tanaka, H.; Frustration on the way to crystallization in glass. Nature Physics. 2006, 2, 200-206. 37. Grzybowska, K.; Paluch, M.; Grzybowski, A.; Wojnarowska, Z.; Hawelek, L.; Kolodziejczyk, K.; Ngai, K. L. Molecular Dynamics and Physical Stability of Amorphous Anti-Inflammatory Drug: Celecoxib. J. Phys. Chem. B. 2010, 114 (40), 12792-12801. 38. Shamblin, S. L.; Hancock, B. C.; Dupuis, Y.; Pikal, M. J. Interpretation of relaxation time constants for amorphous pharmaceutical systems. J. Pharm. Sci. 2000, 89, 417-427.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

39. Grzybowska, K.; Paluch, M.; Wlodarczyk, P.; Grzybowski, A.; Kaminski, K.; Hawelek, L. Enhancement of amorphous celecoxib stability by mixing it with octaacetylmaltose: the molecular dynamics study. Mol. Pharmaceutics, 2012, 9 (4), 894-904. 40. Antonov, L. Tautomerism. Methods and theories. John Wiley & Sons, 2013. 41. Wojnarowska, Z.; Wlodarczyk, P.; Kaminski, K.; Grzybowska, K.; Hawelek, L.; Paluch, M. On the kinetics of tautomerizm in drugs: new application of broadband dielectric spectroscopy. J Chem Phys. 2010, 133(9), 094507. 42. Wlodarczyk, P.; Kaminski, K.; Haracz, S.; Dulski, M.; Paluch, M. Kinetic processes in supercooled monosaccharides upon melting: application of dielectric spectroscopy in the mutarotationstudies of D-ribose. J. Chem. Phys. 2010, 132, 195104. 43. Raczyńska, E. D.; Kamińska B. Variations of the tautomeric preferences and π-electron delocalization for the neutral and redox forms of purine when proceeding from the gas phase (DFT) to water (PCM). J Mol Model. 2013, 19, 3947-3960. 44. Jeffrey, G. A. An introduction to hydrogen bonding. Oxford University Press, 1997. 45. Steiner, T. The Hydrogen Bond in the Solid State,. Angew. Chem. Int. Ed. Engl. 2002, 41, 49-76 46. Löbmann, K.; Laitinen, R.; Grohganz, H.; Strachan, C.; Rades, T.; Gordon, K. C. A theoretical and spectroscopic study of co-amorphous naproxen and indomethacin. Int. J. Pharm. 2013, 453, 80-87. 47. Allesø, M.; Chieng, N., Rehder, S.; Rantanen, J.; Rades, T.; Aaltonen, J. Enhanced dissolution rate and synchronized release of drugs in binary systems through formulation: Amorphous naproxen-cimetidine mixtures prepared by mechanical activation, J Control Release, 2009, 136, 45-53. 48. Tang, X.C.; Pikal, M. J.; Taylor, L. S. A spectroscopic investigation of hydrogen bond patterns in crystalline and amorphous phases in dihydropyridine calcium channel blockers. Pharm Res. 2002, 19(4), 477-83. 49. Marsac, P. J.; Konno, H.; Taylor, L. S. A comparison of the physical stability of amorphous felodipine and nifedipine systems. Pharm Res. 2006, 23(10), 2306-16.


Page 28 of 38

Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50. Stieger, N.; Liebenberg, W.; Recrystallization of Active Pharmaceutical Ingredients, INTECH Open Access Publisher, 2012, 183-205. 51. Adrjanowicz, K.; Kaminski, K.; Wojnarowska, Z.; Dulski, M.; Hawelek, L.; Pawlus, S.; Paluch, M.; Sawicki, W. Dielectric relaxation and crystallization kinetics of ibuprofen at ambient and elevated pressure. J Phys Chem B. 2010, 114(19), 6579-93. 52. Paluch, M.; Rzoska, S. J.; Habdas, P.; Ziolo, J. Isothermal and high-pressure studies of dielectric relaxation in supercooled glycerol. J. Phys.: Condens. Matter. 1996, 8, 10885. 53. Avramov, I.; Avramova, K.; Rüssel, C.; New method to analyze data on overall crystallization kinetics. Cryst Growth. 2005, 285 (3), 394-399. 54. Wojnarowska, Z.; Adrjanowicz, K.; Wlodarczyk, P.; Kaminska, E.; Kaminski, K.; Grzybowska, K.; Wrzalik, R.; Paluch, M.; Ngai, K.Broadband dielectric relaxation study at ambient and elevated pressure of molecular dynamics of pharmaceutical: indomethacin. J. Phys. Chem. B 2009, 113 (37), 12536-12545. 55. Adrjanowicz,K.; Grzybowski, A.; Grzybowska, K.; Pionteck, J.; Paluch, M. Toward better understanding crystallization of supercooled liquids under compression: isochronal crystallization kinetics approach, Cryst. Growth Des. 2013, 13, 4648-4654. 56. Joshi, A. B.; Patel, S.; Kaushal, A. M.; Bansal, A. K. Compaction studies of alternate solid forms of celecoxib. Adv. Powder Technol. 2010, 21 (4), 452-460. 57. Adrjanowicz, K.; Kaminski, K.; Wojnarowska, Z.; Dulski, M.; Hawelek, M., Pawlus, S.; Paluch, M.; Sawicki, W. Dielectric relaxation and crystallization kinetics of Ibuprofen at ambient and elevated pressure. J. Phys. Chem. B, 2010, 114 (19), 6579-6593. 58. Wang, Y.; Tan, X.; Zhang, Y.-M.; Zhu, S.; Zhang, I.; Yu, B.; Wang, K.; Yang, B.; Li, M.; Zou, B.; Zhang, S. X.-A. Dynamic Behavior of Molecular Switches in Crystal under Pressure and Its Reflection on Tactile Sensing. J. Am. Chem. Soc., 2015, 137, 931-939.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

47x26mm (600 x 600 DPI)


Page 30 of 38

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


57x19mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

109x146mm (300 x 300 DPI)


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


57x40mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

338x190mm (96 x 96 DPI)


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


65x52mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

127x204mm (300 x 300 DPI)


Page 36 of 38

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


82x98mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

91x46mm (300 x 300 DPI)


Page 38 of 38

Toward a Better Understanding of the Physical Stability of Amorphous

Recommend Documents