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Polymorphs I and II of pridopidine hydrochloride are enantiotropically related, with form I being stable at ambient conditions and form II being stabl...
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Polymorphs of Pridopidine Hydrochloride Anne Zimmermann,† Brian Frøstrup,† and Andrew D. Bond*,‡ †

NeuroSearch A/S, Pederstrupvej 93, 2750 Ballerup, Denmark Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark

‡

S Supporting Information *

ABSTRACT: Pridopidine hydrochloride (Huntexil, NeuroSearch A/S, Ballerup, Denmark) is a dopaminergic stabilizer, currently in development for the treatment of motor symptoms associated with Huntington’s disease. In this study, two polymorphic forms are characterized, forms I and II. The crystal structures of both polymorphs contain N+− H···Cl−···N+−H··· interactions, and the polymorphism can be viewed as alternative orientations (parallel or antiparallel) of comparable molecular columns while retaining the ···N+− H···Cl−···N+−H··· motif between columns. Forms I and II have melting points of 199 and 210 °C, respectively. Following melting of form I, a kinetically controlled crystallization to form II is observed. The polymorphs are enantiotropically related, form I being stable at ambient conditions and form II being stable above 127 °C. At around 180 °C, the differential scanning calorimetry thermogram of a mixture of forms I and II shows a solid-state transition from form I to form II, which is not observed for pure form I samples. Transformation of form II to form I below the transition temperature is solution-mediated. Because of the thermodynamic relation and the propensity for transformations, the safety, quality, and efficacy of pridopidine hydrochloride is not affected by the existence of form II.

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INTRODUCTION Different solid forms of a molecular compound (e.g., polymorphs, salts, hydrates) can exhibit different physicochemical properties, such as solubility and physical stability,1,2 or mechanical properties such as tabletability.3−5 When dealing with active pharmaceutical ingredients (APIs), these differences may ultimately affect the bioavailability and shelf life of a marketed product, and in this way impact drug product safety, quality, and efficacy.1,6,7 As a result, regulatory guidelines have emerged that encourage pharmaceutical companies to search for polymorphic forms alternative to any known form(s) and to characterize these forms in the solid state. If the presence of different polymorphic forms in a drug product has a potential impact on safety, quality, and efficacy, the polymorphic content should be controlled, or performance tests for controlling the quality attributes should be included in the specification.8−10 Thermodynamics dictate that only one polymorphic form can be stable at a given temperature and pressure.1,11 It is important to elucidate the thermodynamic stability relationship between polymorphs as a function of temperature for several reasons. In order to control the polymorphic outcome of chemical synthesis of an API, the stability relation at the manufacturing temperature should be known. From the point of view of shelf life, it is valuable to know which form is stable under ambient conditions and at the various conditions of temperature and humidity to which a drug product may be exposed. Furthermore, since the API is exposed to various conditions during processing, the risk of processing-induced transformations between polymorphs should also be taken into consideration.12 Thus, a sound understanding of a polymorphic © 2012 American Chemical Society

system is a prerequisite for building quality into the product during the drug development phase. This approach can be part of an overall “quality by design” strategy for product development.13−15 In this paper, we describe the solid-state properties of the API salt pridopidine hydrochloride (Scheme 1). Pridopidine Scheme 1. Pridopidine Hydrochloride

(Huntexil, NeuroSearch A/S, Ballerup, Denmark) is a dopidine, a new class of compounds that act as dopaminergic stabilizers. It is currently in phase III clinical development for treatment of motor symptoms associated with Huntington's disease (HD). HD is an autosomal dominant, progressive neurodegenerative disease that produces a range of cognitive, behavioral, and motor deficits that impact substantially patients and their families. Two recent Cochrane reviews failed to recommend strongly any intervention for symptomatic or disease-modifying effects for symptoms of HD.16,17 However, phase IIb and III studies in patients with HD show that pridopidine (90 mg/day) improves overall motor function and is well tolerated.18,19 The Received: February 7, 2012 Revised: April 13, 2012 Published: May 8, 2012 2961

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aluminum pan (20 μL) and heated from 30 to 300 °C at a heating rate of 10 °C/min. Other heating rates were applied in special cases to study the kinetics of the observed events, as described in the Results and Discussion section. The DSC was continuously purged with dry N2 gas at a flow rate of 20 mL/min. TGA was conducted on a MettlerToledo TGA/SDTA 851e instrument (Mettler-Toledo, Glostrup, Denmark), using Mettler-Toledo StarE software (ver. 9.2). Approximately 10 mg of sample was placed in an open aluminum crucible (100 μL) and heated from 30 to 300 °C at a heating rate of 10 °C/ min. The TGA was continuously purged with dry N2 gas at a flow rate of 20 mL/min. Polarized-Light Microscopy (PLM) and Hot-Stage Microscopy (HSM). PLM and HSM were performed using an Olympus BX50 microscope fitted with cross-polarizing filters. The sample was dispersed in a drop of silicone oil to enhance the refractive index. The HSM analyses were conducted using a Mettler-Toledo FP90 temperature controller and Mettler-Toledo FP82HT hot stage. The temperature was ramped at 10 °C/min in the appropriate temperature ranges. Time-accelerated live sequences were recorded digitally and combined automatically with the temperature data using in-house software. Dynamic Vapor Sorption (DVS). DVS measurements were conducted on a TA Instruments Q5000 SA instrument, using TA Instrument Advantage software (ver. 5.2.6). Samples were analyzed in aluminum pans (100 μL) placed on platinum hangers. The humidity was brought down to 0% relative humidity (RH) and the sample dried until the weight had stabilized within a given limit. The temperature was held constant at 25 °C and the RH was subsequently ramped in steps of 10% (5% for the interval 90−95% RH). Physical Stability of Form II to Humidity. Approximately 50 mg of pridopidine hydrochloride form II was placed in open containers in desiccators of varying relative humidity (11, 30, 53, 62, 75, 80 and 88%), obtained by adding saturated aqueous solutions of appropriate inorganic salts to the desiccators.21 After 7 and 30 days of storage at room temperature the samples were analyzed by PXRD. Slurrying Experiments. Slurrying experiments were performed under ambient conditions in ethanol, 2-propanol, and N,Ndimethylacetamide. To saturated solutions of pridopidine hydrochloride, a mixture of solid forms I and II was added and the suspensions were stirred for 24 h. The solid phase was isolated following the 24 h slurry time and immediately examined by PXRD. Slurrying at elevated temperature was carried out in 1,2-dichlorobenzene (bp 180 °C). 1,2-Dichlorobenzene (1 mL) was heated to the relevant temperature (100−170 °C) in an oil bath, and then form I was added to the vial to form a saturated solution. Excess amounts of both forms I and II were then added and the suspension was left in the oil bath to stir for 4 h. Following the 4 h slurry time, PXRD was performed immediately on the suspension. For a few of the samples the experiment was repeated with a slurry time of 24 h. Solid-State Milling. Solid samples of form I and form II were milled using a Retsch MM400 ball mill, using 25 mL stainless steel containers with one stainless steel ball (Ø 15 mm), shaken at 17.5 Hz for 20 and 60 min. The milled solids were analyzed immediately by PXRD. Scanning Electron Microscopy (SEM). SEM micrographs were recorded on a JSM 5200 scanning electron microscope (Jeol Ltd., Japan). The particles were sputtered with gold (E5200 auto sputter coater, Bio Rad, Watford, England) for 120 s before microscopy. Bulk Density and Tapped Density. Measurements of bulk density were performed according to Ph Eur 2.9.3422 method 1 with the following exceptions: the material was not sieved prior to analysis and 50 g of material was used instead of 100 g. The accurately weighed sample was poured gently into a measuring cylinder and the volume was read. Tapped density was measured according to Ph Eur 2.9.34 method 1.22 50 g of powder was poured into a measuring cylinder, which was then placed in a settling apparatus. The sample was tapped 2500 times at 100 taps/min.

primary mode of action of pridopidine is via functional antagonism of dopamine type 2 receptors with rapid dissociation.20 The compound is a tertiary amine base with molecular weight 281.42 g/mol, pKa 9.6, and log P 2.3. Following a salt screen involving various counterions, the hydrochloride was chosen for development (MW 317.88 g/ mol, water solubility >200 mg/mL). Only one solid form of pridopidine hydrochloride had been encountered during chemical synthesis, and this form is designated form I. During the course of the development work, a second polymorph, designated form II, was discovered. This paper reports the structures of forms I and II of pridopidine hydrochloride, their thermodynamic stability relationship, and the propensity for transformation between the forms when exposed to various conditions.

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EXPERIMENTAL SECTION

Materials. Pridopidine hydrochloride (4-[3-(methylsulfonyl)phenyl]-1-propylpiperidine hydrochloride) is a NeuroSearch A/S (Ballerup, Denmark) development compound. Numerous batches of form I were applied in the study, all of which were of GMP quality. Crystals of form I suitable for single-crystal X-ray diffraction were obtained by evaporative crystallization from N,N-dimethylacetamide (DMA). Bulk samples of form II were prepared by dissolving form I under heating to reflux at approximately 180 °C. The applied solids concentration was at or below 50 mg/mL. At this level of saturation, spontaneous precipitation of form I was avoided while the temperature was lowered to 165 °C. Then, seed crystals of form II were added and stirring was initiated as soon as the seed crystals began to grow. Over 30 min, the temperature was lowered to 150 °C. After another 30 min, the suspension was filtered at 150 °C, washed with heptane, and dried under vacuum to provide a bulk sample of form II. The form II seed crystals were prepared in the differential scanning calorimetry (DSC) instrument by heating a sample of form I to 180 °C at 10 °C/min, then to 200 °C at 0.5 °C/min, then to 203 °C at 0.1 °C/min, followed by cooling to room temperature. The identity of the material was verified by powder X-ray diffraction (PXRD). Single crystals of form II suitable for X-ray diffraction were picked from the bulk sample prepared as described above. Powder X-ray Diffraction PXRD. (PXRD) patterns were measured on a PANalytical X’Pert Pro diffractometer equipped with a PIXcel detector or a Bruker D8 Advance diffractometer equipped with a scintillation detector. Samples were placed on a zerobackground silicon holder for measurements at ambient conditions and a continuous 2θ scan was performed in the range 5° to 30° using CuKα radiation (λ = 1.5418 Å). Sample spinning was employed during measurements to minimize preferred-orientation effects. Single-Crystal X-ray Diffraction (SXRD). SXRD data were collected using graphite-monochromated MoKα radiation (λ = 0.7107 Å) on a Bruker-Nonius X8-APEXII CCD diffractometer. Data were collected at room temperature (25 °C) to provide structures that could be matched directly against room-temperature PXRD data. Data collection and processing were performed using the programs in the Bruker-Nonius suite Apex2 (ver. 1.0-22). Data reduction and cell refinement were performed using SAINT (ver. 7.06a) and a multiscan correction was applied using SADABS (ver. 2.10). The structures were solved using direct methods and refined by full-matrix least-squares on all F2 data using Bruker SHELXTL (ver. 6.10). The H atom associated with the NH group of the pridopidine cation was visible in the difference Fourier map in both cases, but placed in an idealized position and refined as riding for the final refinements. For the non-centrosymmetric form I, the crystal was refined explicitly as an inversion twin, with a refined batch scale factor of 0.31(5). Thermal Analysis. Differential scanning calorimetry (DSC) was conducted on a Mettler-Toledo 821e DSC instrument (MettlerToledo, Glostrup, Denmark), using Mettler-Toledo StarE software (ver. 9.2). Approximately 3 mg of sample was placed in a pinholed 2962

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RESULTS AND DISCUSSION Discovery and Crystallization of Form II. A standard polymorph screen using a variety of solvents and crystallization methods (Supporting Information) did not yield any evidence of polymorphism. The existence of form II was discovered by examination of the DSC thermograms of some form I batches. Figure 1 shows a DSC trace of a representative form I batch,

Table 1. Selected Crystallographic Data for the Polymorphs of Pridopidine Hydrochloride (25 °C) empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β (°) volume (Å3) Z calculated density (mg/m3) absorption coeff (mm−1) F(000) crystal size (mm) data collected unique data Rint observed data [I > 2σ(I)] R1 [I > 2σ(I)] wR2 (all data) goodness-of-fit (on F2) diff density (e·Å−3) CCDC

Figure 1. DSC thermogram of form I, showing melting of form I at ca. 199 °C, followed by crystallization of form II and subsequent melting of form II at ca. 210 °C.

where the first endotherm with an onset of 199.3 °C corresponds to the melting of form I. This endotherm is immediately followed by an exotherm, then a second sharp endotherm starting around 210 °C, indicating crystallization of a different crystalline phase, followed by its melting. In order to isolate this crystalline phase, a method was designed where a sample of form I was heated in a controlled manner within the DSC instrument and then left to cool, as described in the Experimental Section. PXRD of the resulting material showed a diffractogram different from that of form I, and a melting point of 210 °C was observed on reheating the material within the DSC. The form II sample produced within the DSC instrument was subsequently used as seed crystals to obtain the bulk samples of form II. Crystal Structures of Forms I and II. Crystallographic data for forms I and II are summarized in Table 1. Form I crystallizes in the non-centrosymmetric space group Pna21, frequently with some degree of inversion twinning. Form II crystallizes in the centrosymmetric space group P21/c. The molecular conformation is closely comparable in the two forms: the central piperidine group adopts a chair conformation, with both the methylsulfonylphenyl group and propyl chain in equatorial positions. Overlay of the pridopidine molecule in the two polymorphs illustrates that the propyl chain is oriented in opposite directions with respect to the terminal CH2CH3 group in the two crystal structures (Figure 2). The packing of the molecules shows ···N+−H···Cl−···N+−H··· chains, whereby the Cl− anion accepts a hydrogen bond from the N+−H group, and also approaches the N atom of a neighboring pridopidine molecule from the backside of the N+−H bond (Figure 3). Identical interaction motifs are present in forms I and II, running along the c and b axes, respectively. The metric parameters of the N+−H···Cl− hydrogen bonds are identical in the two structures (N+(−H)···Cl− = 3.065(2) Å for form I vs 3.062(2) Å for form II), but the Cl−···N+ contacts are longer for

Form I

Form II

C15H24ClNO2S 317.85 orthorhombic Pna21 10.4668(4) 23.0660(9) 6.9667(2) 90 1681.95(10) 4 1.255 0.352 680 0.35 × 0.20 × 0.08 11689 2830 0.036 2420 0.030 0.067 1.04 −0.15, 0.17 864449

C15H24ClNO2S 317.85 monoclinic P21/c 12.2441(4) 13.4701(4) 10.2207(3) 91.117(1) 1685.37(9) 4 1.253 0.352 680 0.30 × 0.20 × 0.08 26810 3198 0.031 2461 0.038 0.106 1.07 −0.39, 0.47 864450

Figure 2. Overlay of the pridopidine molecules in the crystal structures of form I (red) and form II (blue). H atoms omitted.

form II (3.948(2) Å) compared to form I (3.909(3) Å). In form I (Figure 3a), adjacent pridopidine molecules along the chains are related by translation (an isotactic arrangement), while in form II (Figure 3b), the chains run along 21 screw axes so that every second pridopidine molecule in a given chain is related by translation (a syndiotactic arrangement). The form I and II structures display similarity in one dimension, with the molecules lying in directly comparable columns, along the a axis in form I and c axis in form II. Thus, the molecules are packed identically in one direction perpendicular to propagation of the ···N+−H···Cl−···N+−H··· chain, and the polymorphism corresponds to alternative orientations (parallel or antiparallel) of these entire columns while retaining the consistent ···N+−H···Cl−···N+−H··· motif. Powder X-ray Diffraction. The PXRD patterns measured for the bulk samples of forms I and II (Figure 4) are in good agreement with those calculated from the single-crystal X-ray structures, and the two forms are readily distinguishable. Measured patterns for form I frequently exhibit preferred orientation serving to diminish the intensity of the (011) and (031) reflections at 2θ ≈ 13.2 and 17.2°, respectively. This is consistent with the observed crystal morphology (see Figure 9), which shows a needle-like shape with the {011} faces capping each end. The needle axis corresponds to the direction of propagation of the ···N+−H···Cl−···N+−H··· motifs. PXRD patterns of form II frequently display preferred orientation 2963

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17 different batches, the melting onset of form I occurs at 199.1 ± 0.5 °C. For the batch shown in Figure 1, melting of form I is followed by crystallization and melting of form II. In general, however, the extent to which crystallization of form II is observed is variable between form I batches, and some do not display any evidence of form II. In order to verify whether this could be attributed to the crystallization kinetics of form II, replicate DSC analyses were run for a single form I batch (Figure 5). It is evident that the rate of crystallization of form II

Figure 5. DSC thermograms (identical heating rates) for replicate analyses from a single form I batch, showing variable extent of form II crystallization. The uppermost trace does not show any crystallization of form II.

Figure 3. View of the ···N+−H···Cl−···N+−H··· motifs in the crystal structures of form I (a) and form II (b). The molecular packing in the direction of the projection (along the b axis in form I and c axis in form II) is directly comparable (see text).

can be variable, even for material sampled from the same batch. In cases where form II crystallizes slowly, the experimental temperature can increase above the form II melting point before form II crystallizes, thereby giving a DSC thermogram showing only melting of form I. For bulk samples of form II, the DSC thermogram exhibits only one endotherm with an onset at ca. 210 °C, corresponding to melting. In some batches, a small endotherm could also be discerned at ca. 180 °C, suggesting a transition of a minority form I component to form II. This was confirmed by preparation of mixtures of forms I and II, for which DSC clearly shows an endothermic transition at ca. 180 °C followed by a melting endotherm only for form II. DSC thermograms for a representative form II batch, and a mixture of form I and II containing 15% form II, are shown in Figure 6. Similar behavior has been described previously for tulobuterol, where a transition is seen in DSC only for mixtures of polymorphs and not for the pure form.23 Cases have also been reported in which the transition occurs from a pure phase.24,25 Thermogravimetric analysis (TGA) of forms I and II (Supporting Information) shows closely comparable behavior, with essentially no weight loss observed between 25 and 200 °C, with rapid weight loss observed first at ca. 220 °C. Since this is well above the melting temperature of form I (and since form II is established to crystallize from the form I melt), there is no molecular decomposition associated with melting of form I. Thermodynamic Relationship Between Forms I and II. The melting points and heats of fusion established for forms I and II are listed in Table 2. The values represent triple determinations of one form I and one form II batch. According to the “heat of fusion rule” of Burger and Ramberger,11 the

Figure 4. Experimental and calculated PXRD data for forms I and II.

serving to enhance the intensity of the (200) reflection at 2θ ≈ 14.4°. This is also consistent with the morphology for form II, which shows a plate-like shape with {100} as the principal faces (see Figure 9). Thermal Analysis. A DSC thermogram for one form I batch has been illustrated in Figure 1. For that batch, the endotherm observed with onset at around 199 °C was verified by HSM to represent the melting of form I. From an average of 2964

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(Cp ,L − Cp ,II) =

(ΔHt − ΔHm, I + ΔHm, II ) (Tm,II − Tm,I)

(4)

For pridopidine hydrochloride, a linear correlation is observed between the amount of form II in mixtures of form I and II and ΔHt observed in DSC (see Supporting Information). This linear relationship was extrapolated to provide a value of ΔHt for 100% form I, giving (Cp,L − Cp,II) = 0.13 kJ mol−1 K−1. Using eqs 1−3, ΔH0 = 4.69 kJ mol−1, ΔS0 = 0.01172 kJ mol−1 K−1, and Tt = 400 K, corresponding to 127 °C. This finding was examined experimentally by slurrying experiments using a mixture of the two forms at elevated temperatures in 1,2dichlorobenzene. Up to 125 °C, the product was form I after 4 h of slurrying. At 130 °C, a mixture remained after 4 h of slurrying, but conversion to form II was found after 24 h. Thus, the enantiotropic transition temperature is confirmed to lie close to 127 °C. Figure 7 illustrates a schematic summary of the thermodynamic relationship between forms I and II.

Figure 6. DSC thermograms of form II and of a form I/form II (85:15%) mixture, showing an endothermic transition from I to II starting at ca. 180 °C, followed by melting of form II at ca. 210 °C.

Table 2. Melting Points and Heats of Fusion Established for Forms I and II Tm (K) ΔHfus (kJ/mol)

Form I

Form II

472.0 ± 0.16 39.34 ± 1.04

483.5 ± 0.05 36.16 ± 1.16

polymorphs are enantiotropically related because form II has a higher melting point and lower heat of fusion than form I. The “heat of transition rule”11 is also consistent with an enantiotropic relationship, since an endothermic transition of form I to form II is observed below the melting point of the two forms for a mixed sample (as exemplified in Figure 6). If such a transition is observed, a transition temperature (Tt) is located below this temperature. Since the transition occurs in the direction form I → form II as the mixed sample is heated, form I is thermodynamically stable below Tt while form II is the more stable form above Tt. A thermodynamic model for the calculation of Tt from thermal data has been proposed by Yu.26 Assuming a linear dependence of ΔG on T and ΔG(Tt) = 0, Tt can be calculated from eq 1:

Tt =

ΔH0 ΔS0

Figure 7. Schematic temperature-free energy diagram for pridopidine hydrochloride polymorphs.

A common experimental method to determine Tt for enantiotropic polymorphs is to measure the solubility (or intrinsic dissolution rate) of both forms and construct a Van’t Hoff plot.27 For pridopidine hydrochloride, however, the transition from form II to form I is so rapid when in contact with any solvent that a reliable solubility value for form II could not be obtained. Transitions Between Forms I and II. As described above, a solid-state transition from form I to form II is observed above Tt during DSC of mixed samples but not in samples of pure form I. This suggests that the presence of form II can in some way initiate or catalyze the transition. Like any crystallization process, a solid−solid transformation involves nucleation of the new phase followed by growth. Typically, the nucleation and growth processes take place at imperfections present in the crystals.28,29 Since pure form I batches do not show any solidstate transition to form II during the DSC analysis, the rate of form II nucleation in the solid form I sample appears to be slow (compared to the duration of the DSC analysis). If form II crystals are present in close contact with the form I crystals, however, they may be able to act as seeds from which transformation can proceed, and in this way bypass the step involving formation of form II nuclei. DSC of pure form I even

(1)

where ΔH0 = ΔHm,I − ΔHm,II + (Cp ,L − Cp ,II)(Tm,II − Tm,I) (2)

and

ΔS0 =

ΔHm,I Tm,I

−

ΔHm,II Tm,II

⎛ Tm,II ⎞ ⎟⎟ + (Cp ,L − Cp ,II) ln⎜⎜ ⎝ Tm,I ⎠

(3)

The term (Cp,L − Cp,II) represents the difference between the heat capacity of the stable solid (in this case form II) and the unstable, supercooled liquid at a temperature between Tm,I and Tm,II. Because the range of integration is normally small, this value is assumed to be constant. It can be calculated as described by eq 4, provided that the heat of transition (ΔHt) can be determined: 2965

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Figure 8. HSM images showing the transition of form I crystals to form II, followed by melting of the form II crystals. The transition takes place without any visible change in the crystal morphology.

at low heating rate (0.5 °C/min) did not provide enough time for the transition to happen in the absence of form II seeds. Thus, the presence of form II greatly accelerates the process. Similar observations have been made for chloramphenicol palmitate: adding seeds of the stable form A to samples of unstable form B or C during grinding caused a significant increase in the rate of transformation to the stable form A.30 HSM of the mixed sample showed that the shape of the form I crystals remained unchanged during the transformation, and that the transition seemed to propagate through the crystals in several directions (Figure 8). Similar behavior has also been described for tolbutamide, mefenamic acid, and phenylbutazone.24,31 Since the crystal shape does not change during the transition, the rearrangement of molecules occurs without passing through a disordered phase. As evidenced from the crystal structures, half of the pridopidine molecules must rotate by 180° in order to transform from form I to form II. As described by Castro et al., this may impose considerable kinetic constraints on the transition.25 For pridopidine hydrochloride, this is evidenced by the fact that the solid-state transition is observed well above Tt. Furthermore, DSC of the mixed sample performed at varying heating rates showed that the onset temperature for the transition moved from 180.8 at 10 °C/min, to 177.3 at 5 °C/min and 172.9 at 2 °C/min (Supporting Information). The transition between polymorphs of Rcinacalcet hydrochloride as observed in DSC and XRPD has also been found to be dependent on heating rate.32 Since form I is the stable form under ambient conditions, form II should eventually transform to form I at temperatures below Tt, either by a solid-state or a solution-mediated path. It is evident from the solid-form screen that crystallization of form I is favored when employing a variety of crystallization methods and solvents at temperatures below Tt, even though, according to Ostwald’s rule,33 the metastable form II should, at least in some cases, crystallize out first. One reason why form II was not observed in the solid-form screen could be that transition from II to I is solution-mediated and occurs so rapidly that form II is no longer present when the crystals are harvested. Slurry experiments where excess amounts of form II were slurried in ethanol, iso-propanol, and N,N-dimethylacetamide showed that conversion to form I had occurred within 24 h, supporting a solution-mediated mechanism of conversion.

To investigate the effect of moisture, samples of form II were placed in desiccators of increasing RH. After 30 days, the samples were withdrawn and examined by PXRD. Samples placed in conditions at or above 62% RH had transformed to form I, whereas samples stored at or below 53% RH were still form II. This indicates that a critical amount of solvent is necessary for the transformation to occur. According to Cardew and Davey,34 solution-mediated transformations can be either dissolution- or growth-controlled, depending on whether dissolution of molecules from one form, or self-association and growth of the other form is rate determining. Once the pridopidine form II crystals have dissolved, nucleation and growth of form I crystals seem to occur rapidly, indicating that the transformation is dissolution-controlled. Milling of Forms I and II. Dry milling of form I in a ball mill for 20 min showed no change for the sample. Milling of form II for 20 min produced only a minor form I component in the PXRD pattern, and continued milling for a further 60 min increased the relative intensity of the form I diffraction peaks only minimally (see Supporting Information). Thus, form II converts only very slowly to form I under dry milling conditions. Addition of a single drop of water (solvent-drop grinding), however, resulted in complete conversion to form I within only a few minutes, consistent with the suggestion that the form II → form I conversion is solution-mediated. Hygroscopicity. From the point of view of stability, minor water content in a bulk API is usually acceptable, whereas a higher amount may cause chemical instability, especially if the substance is degraded by hydrolysis. Apparent potency may also be affected if a powder contains significant amounts of water. Furthermore, moisture may cause polymorphic transitions and, therefore, it is important to investigate the tendency of a substance to take up water. The dynamic vapor sorption (DVS) profiles of forms I and II at 25 °C (Supporting Information) are very similar. During the first adsorption cycle, essentially no moisture is adsorbed below the 80% RH step. Then, between 80% RH and 95% RH, a dramatic weight gain is observed as the solids take up approximately 70% moisture. A moisture uptake this extensive suggests that both polymorphs are deliquescent, which means that adsorbed moisture condenses and ultimately dissolves the highly water-soluble salt (the aqueous solubility of pridopidine hydrochloride is >200 mg/mL).35 Upon visual 2966

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the API salt at or below 60 °C, and there is no prospect for polymorphic transition under ambient conditions. In the unlikely event that form II should be present within bulk samples of form I, it converts rapidly to form I on contact with any solvent, so that dissolution and absorption of the API salt is not affected.

inspection following the sorption cycles, the material appeared like an amorphous gel. During desorption, the moisture is not lost as abruptly as it was adsorbed, suggesting that the material does not recrystallize. In the second cycle, both adsorption and desorption follow the desorption curve of the first cycle. Particulate Properties and Crystal Morphology. The calculated crystal densities of forms I and II are essentially identical (Table 1). The bulk densities, however, are quite different, with form II having a much higher bulk density than form I (Table 3). For the tapped density, where the bulk

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* Supporting Information Details of the applied polymorph screen; FTIR spectra; TGA thermograms; DVS isotherms; correlation between amount of form II in form I/form II mixtures and ΔHt observed in DSC; DSC performed at varying heating rates on a mixed form I/ form II sample, PXRD patterns of milled samples. This information is available free of charge via the Internet at http:// pubs.acs.org/. CCDC-864449 and CCDC-864450 contain the supplementary crystallographic data, including structure factors. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk.

Table 3. Measured Bulk Density and Tapped Density for Forms I and II bulk density (g/cm3) tapped density (g/cm3)

Form I

Form II

0.212 ± 2.2% 0.264 ± 1.1%

0.382 ± 0.3% 0.486 ± 1.1%

ASSOCIATED CONTENT

S

samples have been stamped within the measuring cylinder, the difference is even more pronounced. These measurements describe the efficiency with which the crystals fill space within a bulk sample. The denser packing of bulk samples of form II compared to form I can be explained by the morphological differences between the forms. Figure 9 shows the observed

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+45) 6550 2545. E-mail. [email protected]. Notes

The authors declare the following competing financial interest(s):Pridopidine hydrochloride is a development compound owned by NeuroSearch A/S, Ballerup, Denmark (registered trade name Huntexil). Both AZ and BF were NeuroSearch employees at the time that the work was performed, but are no longer employees of the company.

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ACKNOWLEDGMENTS The work of Helle Gluver, Linda Setina Ilm, and Laila Flint Knudsen, NeuroSearch A/S, Department of Preformulation, regarding characterization of pridopidine hydrochloride is greatly appreciated. Tove Thomsen, NeuroSearch A/S, Medicinal Chemistry, is acknowledged for her efforts in synthesizing form II, and Tommy Munk, University of Copenhagen, Department of Pharmaceutics and Analytical Chemistry, is thanked for providing SEM images and performing measurements of bulk and tapped density.

Figure 9. SEM micrographs of form I (left, × 2000), and form II (right, × 50).

morphology: form I crystallizes as needles, whereas form II exhibits a plate-like morphology. The form I crystals are generally much smaller than the form II crystals. The form II plates may be stacked on top of each other to form a dense bulk sample, especially following stamping, whereas the form I needles may be oriented in a more random fashion. This is also consistent with the tendencies observed for preferred orientation in the measured PXRD patterns (Figure 4). The bulk density of a sample can be important, for example, if a capsule formulation is desired.

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REFERENCES

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CONCLUSIONS The two polymorphs of pridopidine hydrochloride are enantiotropically related with a transition temperature of 127 °C. Form I is the stable form at room temperature and form II is stable above the transition temperature. Transformation of form I to form II at elevated temperature can occur either as a solid-state transition, which is accelerated in the presence of form II seeds, or from the melt of form I. The latter process exhibits variable kinetics, and may not be observed in DSC analyses, especially at higher heating rates. Under ambient conditions, form II transforms to form I via a dissolutioncontrolled solvent-mediated path. Although pridopidine hydrochloride is polymorphic, the transition temperature of 127 °C is sufficiently high to ensure that form II will not occur during production crystallization of 2967

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