Dependence of Heterogeneous Nucleation on Hydrogen Bonding

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Dependence of Heterogeneous Nucleation on Hydrogen Bonding Lifetime and Complementarity Vivek Verma, Jacek Zeglinski, Sarah Hudson, Peter Davern, and Benjamin K. Hodnett Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01302 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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Crystal Growth & Design

Dependence of Heterogeneous Nucleation on Hydrogen Bonding Lifetime and Complementarity Vivek Verma, Jacek Zeglinski, Sarah Hudson, Peter Davern, Benjamin K. Hodnett* Synthesis and Solid State Pharmaceutical Centre, Department of Chemical Sciences, and Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland

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Crystal Growth & Design 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

Abstract

The crystallization of seven active pharmaceutical ingredients (APIs) (acetaminophen (AAP), carbamazepine (CBMZ), caffeine (CAF), phenylbutazone (PBZ), risperidone (RIS), clozapine base (CPB) and fenofibrate (FF)) was studied in the absence and presence of microcrystalline cellulose (MCC) which acted as a heterosurface. Two of the APIs, namely AAP and CBMZ, possess hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) functionalities whereas the other five possess HBA functionality only. Density Functional Theory (DFT) and Molecular dynamics calculations complemented the experimental study. The smallest nucleation rate enhancement was observed for CBMZ at 1.4 times and the largest was observed for FF at 16 times. For all the APIs studied, the interfacial energy was similar for crystallizations performed in the presence and absence of the heterosurface. By contrast, the pre-exponential factor was larger by a factor of ca. 2 and more for crystallizations carried out in the presence of the heterosurface. Arising from this study, a model of heterogeneous crystallization was developed wherein two influencing factors were identified. The first involves the issue of hydrogen bond complementarity between heterosurface and API. Hence, a HBD-rich heterosurface will provide a hydrogen-bond mediated option for API cluster formation that would otherwise not be specifically available in solution to APIs possessing HBAs only. The second factor identified is that the lifetime of the hydrogen bond made by an individual API molecule or small API cluster with the heterosurface is up to 1,000 times longer than (i) the lifetime of API-API interactions in a solution phase, or (ii) the time required for an API molecule to add to a growing crystal. This lifetime effect arises from the greater stability of an adsorbed species, and this extended lifetime increases the probability that other molecules or small clusters of the API in solution will add to the already adsorbed or attached species thus encouraging the heterogeneous route to crystallization. 2|Page ACS Paragon Plus Environment

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1. Introduction Crystallization is universal in nature and is of particular significance to the chemical and pharmaceutical industry.1 Notably, crystallisation is an attractive isolation step for manufacturing as this process not only combines particle formation and purification but also controls other properties such as size distribution, morphology and polymorphism, to ensure product quality and therefore performance.2-3 Control and scale up of the crystallization process remains a challenging task. However, it can be controlled by targeting nucleation, a critical step in the process. In recent years, heterogeneous nucleation has emerged as a valuable method to not only control nucleation and polymorphism but also to facilitate a reduction in induction time.3-9 Unlike homogeneous nucleation which occurs in an idealised supersaturated liquid, where the internal fluctuations of the liquid trigger the crossing of the kinetic barrier to nucleation, heterogeneous nucleation occurs in “impure” liquids where surface walls or nucleating agents can engender heterogeneity. Heterogeneous nucleation is the process in which molecules in solution aggregate on a heterosurface to form crystal nuclei at a solid-liquid interface. This assembly of the molecules on the heterosurface occurs through selective and specific interactions such as functional group matching10 and lattice matching3; or through nonspecific adsorption of the molecules on the heterosurface11. In the past two decades, materials such as self-assembled monolayers,12-14 silanised glass substrates,15-17 pharmaceutical excipients,10, 18-20 biocompatible polymers,6-7 synthetic polymers21-26 and porous substrates27 have been used as heterosurfaces to induce nucleation. Despite a significant amount of research work published in this area, the exact mechanism of the heterogeneous nucleation process remains unclear. The present work focusses on the homogeneous and heterogeneous nucleation of seven active pharmaceutical ingredients (APIs) with different molecular functionalities in the absence and 3|Page ACS Paragon Plus Environment


presence of a commonly used excipient, microcrystalline cellulose (MCC). The aim is to investigate the extent to which hydrogen bonding between the API and the excipient may play a role in the mechanism for the heterogeneous nucleation process. It is well-known that molecular functionality at the heterosurface / API crystal interface is responsible for facilitating nucleation but the exact mechanism is still unclear.22, 28-29 Recently, researchers have discussed the influence that API-excipient hydrogen bonding may have on polymorphic control during the heterogeneous nucleation process. For example, Lopez-Mejias et al.29 contended that the heterogeneous nucleation of orthorhombic acetaminophen (AAP) in the presence of poly (methyl methacrylate) (PMMA) was a result of the hydrogen bonding interaction between PMMA’s carbonyl groups (C=O) and AAP’s phenolic hydroxyl group; by contrast, monoclinic AAP nucleated in the presence of poly (n-butyl methacrylate) (PBMA) since the latter’s surface is dominated by terminal butyl groups which impart a steric effect that alters the orientation of the surface C=O groups, thus hindering hydrogen bond formation between PBMA and AAP. Furthermore, Chadwick et al.10 proposed that the ca. 5fold reduction in AAP’s induction time upon crystallisation in the presence of D-mannitol was due to the favourable functional group matching between AAP and D-mannitol. In this light, therefore, this study examined the crystallization of the following seven APIs in the presence and absence of MCC with a view to further probing the underlying mechanism of heterogeneous nucleation: acetaminophen (AAP), a common analgesic and antipyretic drug; carbamazepine (CBMZ), an anticonvulsant drug applied to treat epilepsy and trigeminal neuralgia; fenofibrate (FF), an oral medicine used in the treatment of high cholesterol; clozapine base (CPB), an atypical antipsychotic agent; risperidone (RIS), an antipsychotic medicine mainly used to treat schizophrenia, bipolar disorder, and irritability in people with autism; phenylbutazone (PBZ), a nonsteroidal anti-inflammatory drug (NSAID) for the short-


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term treatment of pain and fever in animals, and caffeine (CAF), a central nervous system stimulant. These seven APIs were selected on the basis of their particular molecular functionalities. As such, AAP, CPB and CBMZ each possess hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) groups, whereas RIS, PBZ, CAF and FF possess only HBA groups. MCC was selected as the heterosurface based on its widespread use as an excipient in pharmaceutical industry, and on its previously reported19 very low solubility in MeOH under the prevailing crystallizations conditions of this study. In the context of its molecular functionality, MCC possesses both HBA and HBD groups. It was also used because of its positive and discriminating behaviour in heterogeneous nucleation19-20 and its processability during the later stages of pharmaceutical production.20 This study is based on the hypotheses that (a) the heterogeneous nucleation parameters for API crystallisation in the presence of excipients could be optimised;19 (b) the nucleation rate of APIs would be enhanced upon crystallisation in the presence of excipient; and (c) the functional group complementarity between API and excipient favours heterogeneous nucleation. Heterogeneous nucleation studies reported previously from the group18-20 showed that excipients such as MCC, α/β-Lactose and D-mannitol have positive and discriminating effects on the heterogeneous nucleation of CBMZ,20 FF18 and AAP.19 Functional group complementarity encouraged hydrogen bond formation between API and excipients, and was a strong driving force for the nucleation of CBMZ, FF and AAP in the presence of all the excipients studied. The optimisation of such process parameters as API supersaturation, APIexcipient contact time, and maximum attainable API loading was successfully achieved, thus allowing for good control over the final API crystal size distribution to be exercised. On the contrary, these studies have left unanswered questions on (a) how the rate of desupersaturation of a range of APIs would vary in the absence and presence of MCC, and 5|Page ACS Paragon Plus Environment


(b) the influence of API structures and functionalities on heterogeneous nucleation. The goal of this work was to crystallise CBMZ, AAP, CAF, PBZ, RIS, CPB and FF in the absence and presence of MCC at different supersaturations to optimise heterogeneous nucleation parameters and to use classical nucleation theory to deduce the kinetic parameters of nucleation for each API. A second objective was to derive a plausible mechanism for heterogeneous nucleation of the APIs in the presence of MCC. By elucidation of the mechanism for heterogeneous nucleation for a range of API molecules, appropriate crystallisation conditions and heterosurfaces can be selected in silico for individual APIs to then generate crystals of desired size and morphology in controlled heterogeneous crystallisation processes. 2. Experimental Section 2.1. Materials Methanol (MeOH, >99.9 %), phenylbutazone (PBZ, >98% pure, CCDC BPYZDO21), and acetaminophen (AAP, >98% pure, CCDC HXACAN01) were supplied by Sigma-Aldrich and used as-received. Clozapine base (CPB, >98% pure, CCDC NDNHCL01) and carbamazepine (CBMZ, >98% pure, CCDC CBMZPN01) were supplied by Novartis and used as-received. Caffeine (CAF, >98% pure, CCDC NIWFEE04) was supplied by Alfa Aesar and was used as received, while fenofibrate (FF, >98% pure, CCDC TADLIU01) and risperidone (RIS, >98% pure, CCDC WASTEP) were supplied by Kemprotec and used as received. Microcrystalline cellulose (MCC) (MW: 36000 g/mol) was supplied by FMC International and was used as-received. The molecular structures of the excipient, the solvent and all seven APIs are presented in Figure 1.


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Figure 1: The chemical structures of the excipient, the solvent and the seven APIs used in this study with the number of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) groups in each structure shown.

2.2. Determination of the metastable zone widths for selected API-MeOH solutions in the absence of excipient Previously reported metastable zone width (MSZW) data for CBMZ20, AAP19 and FF18 were used to define appropriate processing conditions for their respective crystallizations from saturated MeOH solutions in the presence of MCC. As such, saturation temperatures (Tsat), crystallization temperatures (Tcry) and cooling rates (°C/min) were defined to ensure that each API’s crystallization occurred inside or within 0.5 ̊C of its MSZW. This was done with the aim of reducing the likelihood of homogeneous API nucleation (and subsequent crystal growth) from the supersaturated MeOH solutions at Tcry and to promote the selective



proliferation of API crystal particles via heterogeneous nucleation onto the suspended MCC particles. MSZWs for the remaining four APIs (CAF, PBZ, CPB and RIS) were determined at a 600 mL scale in a 1 L LabMax Automated Reactor (Mettler Toledo). Saturated API-MeOH solutions, at their respective saturation temperatures, were cooled to 0 ˚C at a cooling rate of 0.1 ˚C/min for CAF, CPB and RIS, and 0.3 ˚C/min for PBZ while the onset of crystallization was monitored in situ via attenuated total reflectance (ATR) - Fourier transform infrared (FTIR) spectrometry (iC15 (Mettler Toledo)) in conjunction with iControl software (Mettler Toledo). Cooling crystallizations were performed in triplicate and the average temperature for the onset of crystallization was used to determine the MSZW 2.3. Crystallization of API from MeOH in the presence and absence of Excipient The crystallization of each API in the absence and presence of excipient from its corresponding supersaturated MeOH solution was performed at a 600 mL scale in the 1 L LabMax Automated Reactor (Mettler Toledo). Nucleation and %-desupersaturation were monitored in situ via ATR-FTIR (iC15 (Mettler Toledo)) in conjunction with iControl software (Mettler Toledo). All crystallizations proceeded under isothermal conditions following cooling of the respective API-MeOH solution from its saturation temperature (Tsat) to a crystallization temperature (Tcry) which generated the desired level of supersaturation (S) to effect nucleation; (S = c/c*, where c = the API solubility at Tsat in g API / kg MeOH, and c* = the API solubility concentration at Tcry in g API / kg MeOH). Tsat and Tcry used to generate the desired S for each API-MeOH solution are presented in Table 1. For those API crystallizations performed in the presence of excipient, the amount of excipient added to achieve the required maximum attainable API loading (% w/w) at complete desupersaturation


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of the API-MeOH metastable solutions was calculated as defined in Equation 120. The maximum attainable API loading (% w/w) in each case was 25% w/w.

(% /) =

( ) × 100 ( ) + ( )

(1)

2.4. Solid State characterization of the API-excipient composites 2.4.1. Powder X-ray Diffraction (PXRD) Powder X-ray diffractograms (PXRDs) were recorded on a PANalytical Empyrean diffractometer using a Cu radiation source (λ=1.541 Å) at 40 mA and 40 kV. Scans were performed between 5 and 35° 2θ at a scan rate of 0.013° 2θ/min. 2.4.2. Scanning Electron Microscopy The habit of the isolated API-excipient composite solid particles was examined by scanning electron microscopy (SEM) (JCM-5700 (JEOL)). Samples were gold coated (S150B, Edward) and the surface appearances of the isolated API-excipient composite solids were compared. 2.5. Computational Methods 2.5.1. Density Functional Theory Calculations Density Functional Theory (DFT) calculations were applied to investigate the 1:1 API-API and API-excipient pair interactions. For this purpose, an API molecule and a dimer of the same API were extracted from the crystal lattice of its stable form crystal structure and optimized with a B97-D3 Grimme’s functional,30 and a Gaussian-type 6-31G(d,p) basis set.31



(Note: for CAF, AAP, and PBZ, the dimers were constructed from the available structural information using the Material Studio 7.0 software.) The molecular geometries changed only slightly upon gas-phase optimization, preserving the original crystal-like conformation. The binding energy of each API dimer was calculated for the lowest energy configuration, after probing molecular interactions. Single point energies were calculated using a double hybrid B2PLYP-D332 functional combined with an exact Hartree Fock exchange with an MP2-like correlation and long-range dispersion corrections with a basis set of quadruple-ζ valence quality (def2-QZVPP).33 This methodology has previously been successfully applied for small and medium-sized API molecules.34-38 The binding energy was calculated as follows: ∆Ebind = EA-B – (EA + EB) where, EA-B is the single point energy of the given dimer, while EA and EB are the respective single point energies of the isolated monomers, A and B, in the gas phase. A similar approach was followed to probe the API-excipient pair interactions. To simplify the calculations, the monomer of MCC, glucose, was used to represent the excipient. The possible interactions between the hydroxyl groups (-OH) of glucose and the carbonyl group (C=O) on APIs were focused on, except for CPB which only contain amine nitrogens as the most electronegative centres. 2.5.2

Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were employed to quantify the interaction energy of a single API molecule in each of the following solid phases: (i) amorphous cellulose, (ii) API cluster, and (iii) crystalline API. MD simulations were carried out with two APIs, namely AAP and FF. In order to more closely emulate the structure of MCC, an oligomer comprising

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5 glucose units was constructed. In periodic simulations, a distance of 20 Å between a molecule and its mirror images is considered sufficient to prevent self-interactions. In order to ensure that the solute molecules are sufficiently separated from their mirror images, the simulation cells constructed were ≥ 45 Å x 45 Å x 45 Å. Simulation cells were constructed with the following molecular compositions: (a) 100 molecules of cellulose + 1 molecule of FF; (b) 200 molecules of FF (cluster); (c) 288 molecules of FF (crystal, CCDC TADLIU01); (d) 100 molecules of cellulose + 1 molecule of AAP; (e) 450 molecules of AAP (cluster); (f): 560 molecules of AAP (crystal, CCDC HXACAN01). All simulation cells were equilibrated within periodic boundary conditions for 350 – 500 ps (solutions) and 100 ps (crystals) at 298 K and 1 atm, using NPT ensemble, Nose thermostat and Berendsen barostat. Ewald summation was used to calculate the electrostatic interactions. At these conditions, uniform molecular densities and constant interaction energies were obtained; this gives confidence that the simulation times were sufficiently long. The interaction energies were calculated according to the procedure described by Zeglinski et

al.39 with the following modification: the final energies were calculated for the geometry optimised structures taken from six time intervals of the final (fully equilibrated) part of the MD trajectory. Thus the computed interaction energies represent mean values (n=6 ± SD). The simulations were performed using the COMPASS II force field40 along with the force field assigned charges as implemented in the Forcite module of the Materials Studio 7.0 software package. 2.6. Hydrogen Bond Propensity Calculations In order to find the probable HBAs and HBDs for all the APIs, hydrogen bond propensities were calculated41 using the program Mercury 3.10.1 as follows. The training dataset for the statistical models was composed of molecules extracted from the Cambridge Structural



Database (CSD) that contain all of the functional groups present in the target APIs. A logistic regression was then applied to the training dataset that allowed the predictions in the form of H-bond propensities upon consideration of the environmental variables for the functional groups (e.g. aromaticity, steric density) of the target API. The H-bond propensity for a donor–acceptor pair can take a value between 0 and 1, where 0 indicates no likelihood of Hbond formation and 1 indicates that a hydrogen bond will always be found.

3. Results and Discussion 3.1. Determination of MSZW Table 1 summarizes the key processing parameters used during the various crystallizations of the seven APIs from their respective saturated MeOH solutions in the presence of the excipient heterosurface, MCC. The MSZWs determined during this study for CAF, PBZ,RIS and CPB are included in Table 1 along with the previously reported MSZWs for CBMZ20, AAP19 and FF18. The MSZWs of CAF (Tsat = 30 ̊C), PBZ (Tsat = 25 ̊C), CPB (Tsat = 40 ̊C) and RIS (Tsat = 25 ̊C) were determined to be 5 ̊C (0.1 C ̊ /min), 9 ̊C (0.3 ̊C/min), 19 ̊C (0.1 ̊C/min) and 21 C ̊ (0.1 ̊C/min) respectively. Table 1 also summarizes the saturation and crystallisation temperatures for all the APIs along with their respective solubilities (in g/g MeOH) at these temperatures. Table 1 further indicates that all crystallization temperatures used to generate the desired supersaturations reside inside or within 0.5 C ̊ of the MSZW based on the data presented, thereby favoring heterogeneous nucleation for all the APIs in the presence of MCC at the applied supersaturations.


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Table 1: The applied supersaturations relevant to the Tsat and Tcry temperatures used; the solubilities in MeOH at the indicated Tsat and Tcry temperatures; and the MSZW data for the seven APIs used in this study.

API

Applied Supersaturation (S)

Saturation temperature (T , ̊C), (Solubility

Crystallisation temperature (T , ̊C), (Solubility

at T

at T

sat

sat

(g/g MeOH))

1.15 Carbamazepine (CBMZ)

Acetaminophen (AAP)

Caffeine (CAF)

Phenylbutazone (PBZ)

Risperidone (RIS)

Clozapine (CPB)

Fenofibrate (FF)

1.23

cry

cry

(g/g MeOH))

25 (0.097)

18 (0.079) 15 (0.072)

1.12

20 (0.297) 25 (0.332)

15 (0.265)

1.33

12 (0.25)

1.15

27.5 (0.0143)

1.21

30 (0.0164)

26.5 (0.0135)

1.32

25 (0.0124)

1.15

22.5 (0.073)

1.22

25 (0.084)

21.5 (0.069)

1.31

20 (0.064)

1.23

20 (0.044)

1.35

25 (0.054)

18 (0.04)

1.51

15 (0.036)

1.09

20 (0.0745)

1.18

25 (0.0809)

15 (0.0686)

1.28

10 (0.063)

1.28

21 (0.06)

1.37 1.50

MSZW (̊C)

20 (0.084)

1.35

1.25

Solubility data Reference

25 (0.077)

20 (0.0564)

42

9.520

43

19

19

44

5this work

45

9 this work

46

21 this work

47

19 this work

18

18

12.5

18.5 (0.051)

3.2. Influence of MCC on the induction time of the various APIs from supersaturated MeOH solutions Figure 2 presents the change in induction times and nucleation rates for the crystallization of the seven APIs from supersaturated MeOH solutions in the absence and presence of dispersed MCC in terms of variations in API solution concentrations (as measured via in situ FTIR). As summarised in Table 2, the induction times for all seven APIs decreased when crystallized in



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the presence of MCC relative to in its absence. The extent of this decrease varied considerably among the APIs despite all crystallizations being designed to generate a maximum attainable API loading of 25% w/w on the dispersed excipient heterosurface. As such, MCC displays a positive yet discriminating influence over the heterogeneous nucleation of the seven APIs. Table 2 also summarises API nucleation rates in the absence and presence of MCC (J* and J, respectively, and calculated according to Equation 2 previously reported by Mealey et al.34) along with the corresponding nucleation rate ratios (J/J*). When compared for the seven APIs, these nucleation rate ratios provide a measure of MCC’s discriminating influence over heterogeneous API nucleation. In this regard, MCC’s positive influence is most pronounced in the case of FF where its nucleation rate ratio is 16 (S=1.37), least pronounced for CBMZ with its ratio of just 1.4 (S=1.35). The nucleation rate ratios of the remaining five APIs reside between these two extremes. Being a ‘hydroxyl group rich’ hemiacetal, MCC is readily capable of forming hydrogen bonds with APIs possessing HBA and HBD groups alike. This capability may facilitate the formation of hydrogen bond mediated API clusters on its insoluble heterosurface. In terms of enhancing the likelihood of API cluster formation, one might reasonably expect APIs possessing only HBA groups to benefit disproportionately more from this facility than those possessing HBA and HBD groups, since molecules of the latter already have an inherent ability to hydrogen bond with each other en route to cluster formation during nucleation.

# # ∗ =

1 %&' (

(2)

where, J or J* is the nucleation rate (nuclei m-3 s-1), tind is the induction time (s), and V is the volume of the crystallization solution (m3).


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Figure 2: %-Desupersaturation for each of the seven API-MeOH solutions in the absence (red curve) and presence (blue curve) of MCC at the supersaturations and temperatures indicated; volume = 600 mL.



3.3. Solid state characterisation of the isolated solids With the exception of CPB, the thermodynamically stable form of all the APIs was nucleated in the presence of MCC, as confirmed using PXRD and presented in Figure 3. In the specific case of CPB, the CPB-MeOH solvate was nucleated in both the absence and presence of MCC, as previously reported.47 It is important to note that the applied supersaturations used in this work were all calculated with respect to the thermodynamically stable form. Although not a primary focus of this work, it is unlikely that sufficiently high supersaturations were applied to be supersaturated with respect to metastable polymorphs.

Figure 3: PXRD patterns of (i) the composite solids isolated after complete desupersaturation of each of the seven API-MeOH solutions in the presence of MCC at the indicated supersaturations and temperatures (red patterns), (ii) MCC (green patterns), and (iii) CPBMeOH solvate (calculated) and the other six APIs (blue patterns).


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Figure 4 presents the SEM micrographs for all the APIs crystallised in the presence of MCC along with an SEM micrograph of MCC as a reference. In each case, the irregular shape of the API crystal at the API-MCC interface provides evidence to support the heterogeneous nucleation and subsequent growth of the API crystals on the surface of the MCC because the independent (i.e. homogeneous) crystallisation of an API would typically yield API crystals with well-defined edges throughout. Further evidence to support this observed mode of API crystal attachment to the MCC surface was obtained using SEM-Raman and was reported previously.20

Figure 4: SEM micrographs of the composite solids obtained following the crystallization of each of the seven APIs from MeOH solutions in the presence of MCC at the indicated supersaturations, along with an SEM micrograph of MCC; API crystals are marked with a red star.

3.4. Influence of supersaturation on API crystallization in the presence of MCC The free energy barrier to nucleation (∆G*c) plays a significant role in determining the number and nature of crystals produced, and its magnitude depends on temperature, supersaturation and the interfacial energy at the crystal-solution interface (Equation 3). At low supersaturations ∆G*c is relatively large, thus critical nuclei are rare. As the supersaturation increases, ∆G*c diminishes thus reducing the barrier to the generation of 17 | P a g e ACS Paragon Plus Environment


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nuclei. Crystals can grow either by following the classical model of exchanging monomers with other crystals in a structured way, or via the two-step process where monomers firstly combine in an unstructured way, resulting in the formation of poorly structured particles or clusters that subsequently crystallise in a second step.48

∆*+∗

2 16-. / 01 34 = 36 2 7 2 2 8

(3)

where, γ is the interfacial energy, vm is the molecular volume, Na is Avogadro’s constant, k is the Boltzmann constant, T is temperature (in Kelvin), and S is the supersaturation.


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Table 2: The influence of supersaturation on the induction time, the nucleation rate and the nucleation rate ratio during the crystallization of each of the seven APIs from MeOH solutions in the absence and presence of MCC MCC absent API

Carbamazepine (CBMZ)

Acetaminophen (AAP)

Caffeine (CAF)


Risperidone (RIS)

ClozapineMeOH solvate (CPB-MeOH)

Fenofibrate (FF)

MCC present

Supersaturation (S)

Induction time (min)

Nucleation rate (J*, nuclei m-3 s-1)

Induction time (min)

Nucleation rate (J, nuclei m-3 s-1)

Nucleation rate ratio (J/J*)

1.16

2160

0.01

960

0.03

2.25

1.22

670 ± 40

0.041 ± 0.002

600 ± 30

0.046 ± 0.002

1.12 ± 0.01

1.34

110 ± 30

0.25 ± 0.07

80 ± 20

0.35 ± 0.09

1.4 ± 0.01

1.12

140 ± 30

0.20 ± 0.04

100 ± 20

0.28 ± 0.06

1.4 ± 0.01

1.25

100 ± 30

0.28 ± 0.08

60 ± 10

0.46 ± 0.08

1.7 ± 0.01

1.32

80 ± 10

0.35 ± 0.04

70 ± 8

0.4 ± 0.05

1.14 ± 0.01

1.15

1260

0.02

600

0.05

2.1

1.21

850 ± 60

0.03 ± 0.002

360 ± 25

0.08 ± 0.006

2.4 ± 0.03

1.32

700 ± 20

0.04 ± 0.001

240 ± 10

0.12 ± 0.005

2.9 ± 0.04

1.15

1200

0.02

360

0.07

3.3

1.21

350 ± 30

0.08 ± 0.007

100 ± 10

0.28 ± 0.03

3.5 ± 0.04

1.31

60 ± 10

0.46 ± 0.08

30 ± 5

0.92 ± 0.15

2 ± 0.02

1.23

2280

0.01

870

0.03

2.6

1.34

1370 ± 30

0.02 ± 0.001

370 ± 60

0.08 ± 0.013

4 ± 0.3

1.51

450 ± 30

0.06 ± 0.004

280 ± 30

0.1 ± 0.011

1.7 ± 0.03

1.08

1020 ± 45

0.03 ± 0.001

70 ± 10

0.4 ± 0.057

15 ± 0.43

1.18

510 ± 30

0.05 ± 0.003

36 ± 5

0.77 ± 0.1

15 ± 0.36

1.28

256 ± 20

0.11 ± 0.008

12 ± 3

2.31 ± 0.58

21 ± 0.70

1.28

460 ± 45

0.06 ± 0.006

38 ± 5

0.73 ± 0.01

12 ± 0.16

1.36

240 ± 20

0.12 ± 0.01

15 ± 3

1.85 ± 0.37

16 ± 0.37

1.50

33 ± 10

0.84 ± 0.26

8±2

3.47 ± 0.87

4 ± 0.03

Table 2 presents the induction times, the nucleation rates and the nucleation rate ratios (J/J*) determined for the crystallization of each of the seven APIs from MeOH solutions in the absence and presence of MCC at three different supersaturations. The supersaturations were 19 | P a g e ACS Paragon Plus Environment


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devised such that crystallisation occurred inside or within 0.5 ̊C of the MSZW, thereby reducing the likelihood of homogeneous nucleation in the presence of MCC. The data illustrate the extent to which nucleation behaviour can vary among a set of APIs, many possessing broadly similar chemical functionalities, i.e. heterocyclic, aromatic, and/or carbonyl-rich. For example, the nucleation rate of RIS increased by ca. 4-fold (S = 1.35) in the presence of MCC whereas the corresponding increase for FF was far more pronounced at ca. 16-fold (S = 1.37). Table 2 also highlights the general, though clearly not absolute, trend towards decreasing nucleation rate ratios (J/J*) as S increases. This may be reflective of how the driving force for heterogeneous nucleation can be influenced by the particular interplay between a given supersaturation and whether a heterosurface is present or not. Indeed, this interplay may be further complicated by the potential of the heterosurface to reduce the width of the metastable zone. As such, the MSZW is known to be affected by hydrodynamic conditions (agitation rate, solution volume, reactor and impeller characteristic dimensions), operating conditions (cooling rate, saturation temperature) and other factors such as the presence of particulates.49 Hence, to better observe the specific effect of MCC on the nucleation of the different APIs, lower supersaturations in the range of 1.25 – 1.35 would likely be more apt. The nucleation data presented in Table 2 was also used to calculate, the interfacial energy (γ* and γ) and the pre-exponential factor (A* and A) in the absence and presence of MCC, respectively for each API, Table 3. Kinetic parameters were calculated using the nucleation rate equation (Equation 4) published by Kaschiev et al.50 and also used in a previous publication from our group.47

#(8) = 8 exp (−=/ 2 8) where,


(4)

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J = Nucleation rate (nuclei m-3 s-1); J(S) implies the nucleation rate at a given supersaturation, S A = Pre-exponential factor (nuclei m-3 s-1) S = Supersaturation / = = 16-0>2 .?@ /3(67)/

vo = Molecular volume (m3) (for CBMZ: 3.09 × 10-28 m3, AAP: 2 × 10-28 m3, CAF: 2.21 × 10-28 m3, PBZ: 4.36 × 10-28 m3, RIS: 4.92 × 10-28 m3, CPB: 4.1 × 10-28 m3 and FF: 5.08 × 10-28 m3)51

γef = Interfacial energy of the cluster/solution interface, for heterogeneous nucleation (J/m2) k = Boltzmann constant (J/K) (1.38 ×10-23 J/K) T = Crystallization temperature (K) Equation 3 can be re-written as follows:

(#⁄8) = − (=/ 2 8)


(5)


Figure 5: A plot of ‘ln(J/S) or ln(J*/S)’ against (1/(T3ln2S)) × 106 that illustrates the dependence of nucleation rate on supersaturation for the crystallization of the seven APIs from MeOH in the absence (blue diamond) and presence (red square) of MCC at different supersaturations; the lines shown are the best linear fits and include the respective line equations and R2 values. 22 | P a g e ACS Paragon Plus Environment

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Best fit linear equations, as presented in Figure 5, were used to calculate the γ* and γ, and A* and A for each of the seven crystallization systems in the absence and presence of MCC respectively. Previously published literature states that an “interface promotes nucleation by lowering interfacial energy of aggregates”29, 52, however the data presented in Table 3 suggest that the presence or absence of MCC does not appreciably influence the interfacial energy for any of the seven crystallization systems studied here. Yet the interesting finding from this study and our previous work47 is that the pre-exponential factor is a major contributing parameter to the crystallization process as observed from Table 3. Based on dimensional analysis of Equation 4, A must have the same units as J, namely nuclei (or cluster) m-3 s-1. Hence we interpret A as the number of clusters of a size less than the critical radius which form per cubic metre of supersaturated solution each second. As such, Table 3 indicates that the presence of MCC coincides with an increase in the pre-exponential (A*) factor of ca. 2fold or more for the crystallization of all the APIs except CBMZ. This signifies that the rate of sub-critical size cluster formation is at least 2-fold greater in the presence of MCC for most of the APIs, thereby promoting heterogeneous nucleation. Table 3: Interfacial energies and pre-exponential factors for the crystallization of the seven APIs from MeOH solutions in the absence and presence of MCC, and the related nucleation rate ratios. API

Crystallization in the absence of MCC γ* (mJm-2) A* (nuclei m-3s-1)

Crystallization in the presence of MCC γ (mJm-2) A (nuclei m-3s-1)

Nucleation rate ratio (J/J*), (S)


1.51

0.42

1.4

0.41

1.4 (1.35)

Acetaminophen (AAP)

0.81

0.27

0.92

0.45

1.7 (1.25)

Caffeine (CAF)

1.02

0.04

1.21

0.11

2.9 (1.32)


1.1

0.78

1.17

1.53

3.5 (1.22)

0.97

0.05

1.07

0.1

4 (1.35)

0.54

0.08

0.58

1.42

15 (1.18)

1.54

1.82

1.29

5.66

16 (1.37)

Risperidone (RIS) ClozapineMeOH solvate (CPB-MeOH) Fenofibrate (FF)



3.5. Analysis of Intermolecular Interactions Many APIs possess hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) which facilitate the formation of strong or weak intermolecular or intramolecular hydrogen bond interactions.53 It is well known in the literature that strong and specific hydrogen bond interactions can play a significant role in crystallisation kinetics54 and the thermodynamic stability of solids.41 In this context, the logit hydrogen-bonding propensity (LHP) model is a method used to predict which donors and acceptors form hydrogen bonds in a crystal structure, based on the statistical analysis of hydrogen bonds in the Cambridge Structural Database (CSD).41 The model defines hydrogen bond propensity, π, as a probability measure for the formation of an intermolecular hydrogen bond between specified donor and acceptor atoms on two adjacent molecules within a crystal structure, with an assumption that the hydrogen bond interaction responsible for the formation of a given crystal will be between the strongest possible donor-acceptor pairs.41 The extent of intermolecular hydrogen bond interactions is influenced by the following factors: (a) the presence of HBDs and HBAs groups on the molecules, (b) the degree of steric hindrance related to the HBDs and HBAs, and (c) the chemical and electronic influence of these HBDs and HBAs.55 The π values between API-API molecules relevant to this work are presented in Table 4. Therefore, based on the logit LHP model, CBMZ and AAP are capable of bonding inter-molecularly with molecules of their own kind, since they both possess HBDs and HBAs. Hence CBMZ and AAP exhibit hydrogen bond propensities of 0.79 and 0.59 respectively, as shown in Table 4. By contrast, CAF, PBZ, RIS and FF all lack HBDs and only contain HBAs; they are therefore not capable of hydrogen bonding with molecules of their own kind, hence their hydrogen bond propensity values of zero in Table 4 for them. CPB forms CPB-MeOH solvate upon crystallisation from methanol in the absence and presence of MCC. In CPBMeOH crystal structure, methanol blocks the only HBD site of CPB, leaving HBAs to 24 | P a g e ACS Paragon Plus Environment

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hydrogen bond with MCC, as reported in the work published previously from the group.47 This changes the π value to zero for the CPB-MeOH solvate from a value of 0.22 for CPB base. Comparing the hydrogen bond propensity of each API with its nucleation rate ratio shows that APIs with large values of π, presented lower nucleation rate ratios whereas the APIs with π value of zero exhibited higher nucleation rate ratios. This signifies that MCC has a weaker influence on the nucleation rate of APIs capable of forming a hydrogen bond with themselves, such as CBMZ and AAP. All APIs can hydrogen bond with MCC, thereby providing support to immobilise molecules of these APIs on the MCC surface and hence increase the nucleation rate ratios. As discussed before, MCC has several hydrogen bond donors, hydroxyl groups (-OH), on its surface which can hydrogen bond with the HBA polar atoms of APIs, i.e. nitrogen and oxygen. This acceptor-donor interaction between MCC and APIs increases the nucleation rate of APIs in the presence of MCC in order of FF followed by CPB-MeOH solvate, RIS, PBZ, CAF, AAP and CBMZ. This also explains the small change in A* for CBMZ (Table 3) because of its high hydrogen bond propensity to another molecule of CBMZ, thereby favouring homogeneous and heterogeneous nucleation concomitantly, as reported previously.20



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Table 4: Correlation of number of hydrogen bond donor and acceptor group of each API with their hydrogen bond propensity, binding energy between API-API and API-Glucose, and nucleation rate ratio Binding energy (kJ/mol) API-API API-Glucose

API

Number of HBDs

Number of HBAs


1

3

-33.8

Acetaminophen (AAP)

2

2

Caffeine (CAF)

0


Hydrogen bond propensity, π

Nucleation rate ratio (J/J*), (S)

-17.6

0.79

1.4 (1.35)

-60.0

-20.9

0.59

1.7 (1.25)

6

-24.7

-23.4

0

2.9 (1.32)

0

4

-18

-22.8

0

3.5 (1.22)

Risperidone (RIS)

0

6

-12

-34

0

4 (1.35)

Clozapine-MeOH solvate (CPB-MeOH)

0

4

-11.9*

-22.5*

0

15 (1.18)

Fenofibrate (FF)

0

3

-7.9

-34.6

0

16 (1.37)

*CPB-MCC

binding energy was calculated instead of CPB-MeOH solvate and MCC, to avoid the complexity of methanol in the system

3.6. Interaction energies from DFT calculations Table 4 also presents the binding energies of the various API-API dimers and API-glucose dimers. These data support the argument that hydrogen bonding between the HBDs of MCC and the HBAs of the APIs is the driving force for heterogeneous nucleation of the APIs during their crystallization in the presence of MCC. Though the HBAs of each API can form hydrogen bonds with the HBDs of glucose, the extent of these interactions will likely vary depending on each API’s specific molecular structure. As such, the API-API interaction is stronger in the cases of AAP and CBMZ than the corresponding API-glucose interaction, an observation that correlates with the rather modest increase in their respective nucleation rate ratios. By contrast, the binding energies of the FF-FF dimer and the FF-glucose dimer are considerably different, at -7.9 kJ/mol and -34.6 kJ/mol respectively. Indeed the API-API binding energies are less than the API-glucose binding energy for all the APIs possessing only HBA’s. This correlates to the significant increase in the nucleation rate ratio for those particular APIs. Also, the calculated differences in binding energies between the various API26 | P a g e ACS Paragon Plus Environment

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API and API-glucose dimer pairs are consistent with the related API hydrogen bond propensities, and both these parameters in turn are good indicators of the trend in nucleation rate ratios. In essence, APIs with only HBAs will only hydrogen bond with the HBDs of the heterosurface, but this interaction ultimately facilitates the sequestering of an API cluster on the heterosurface thus increasing the nucleation rate ratio of the API. Figure 6 presents the optimized geometry of the API-API and API-Glucose dimers for each of the seven APIs along with the respective binding energies in kJ mol-1. Each of the API-glucose structures in Figure 6 features H-bonding interactions. (Note: CPB-MCC binding energy was calculated instead of CPB-MeOH solvate and MCC, to avoid the complexity of methanol in the system)



.

Figure 6: Optimized geometries of the various API-API and API-Glucose dimers (B97D3/6-31G(d,p) level). Binding energy in kJ mol-1, calculated at B2PLYP-D3/def2-QZVPP level; Hydrogen – white, carbon – grey, oxygen – red, nitrogen – blue, fluorine – pale green, chlorine – green. 28 | P a g e ACS Paragon Plus Environment

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3.7. Interaction energies from Molecular Dynamics Simulations The DFT calculations presented in Table 4 offer a useful insight into the specific intermolecular interactions between the molecules in dimers. However, using the simple 1:1 models, it is not possible to fully capture and quantify the whole interaction space around an API molecule. To overcome this limitation, molecular dynamics simulation was employed; a simulation cell filled with hundreds of molecules was used. Here an oligomer comprising five glucose units was also used in order to more closely approximate the bulk structure of MCC. MD simulations were considered for two APIs, one had induction times influenced only slightly (AAP) and one significantly (FF) by the presence of the MCC additive. A comparison of the interaction of a single molecule of each API in three different scenarios (namely embedded in bulk cellulose, as part of an amorphous API cluster, and as part of the API crystalline phase) is shown in Figure 7.

Figure 7: Graphical representations and interaction energies for a single molecule of AAP and FF embedded within: a matrix of cellulose, (a) and (d); an amorphous API cluster, (b) 29 | P a g e ACS Paragon Plus Environment


and (e); an API crystalline structure, (c) and (f). The API molecule in (a) and (d) is highlighted in yellow. For AAP, the weakest interaction was observed for the single API molecule interacting with cellulose (-216.3 kJ mol-1), followed by a significantly stronger interaction when in the amorphous cluster phase (-252.1 kJ mol-1), while the strongest (i.e. most stable) interaction was for the AAP molecule in its crystalline phase (-276.8 kJ mol-1). In the case of FF, the interaction strength was the highest (i.e. most stable) for the FF molecule interacting with bulk cellulose (-330.9 kJ mol-1), followed closely by the interaction energy in the FF crystal (-327.7 kJ mol-1), while the interaction strength in its amorphous cluster phase was significantly lower (-292.5 kJ mol-1). When comparing the two different systems, it appears that an AAP molecule is energetically stabilised by 36 kJ mol-1 in the amorphous phase relative to its interaction with cellulose. Further stabilisation takes place when this amorphous cluster transforms to a crystalline phase. In contrast, FF’s most preferential state is its interaction with cellulose, which is comparable with the API-API interaction within FF crystals. The lowest interaction energy (i.e. the least stable interaction) is recorded for the FF molecule embedded in its amorphous phase.


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Discussion The lifetimes of hydrogen bond interactions play an important role in governing the heterogeneous nucleation process.18,

47

Table 5 collates typical literature values for the

average lifetimes of single molecules attached to a surface by hydrogen bonds and the time required to form critical sized nuclei (at ca. 25 C ̊ ) attached to solid surfaces via hydrogenbonding. These lifetimes vary in the range from < 10 ns to < 70 ns for the first interaction of single molecules and 1-200 ns for critical nuclei depending on the desorption energy of the hydrogen bond involved.56-59 In general, the lifetime of a specific hydrogen bond interaction in solution is typically 100 -1000 time less at 0.001 to 0.07 ns.60-61 Table 6 presents the time required (tm) to add a single molecule to growing crystals of the seven APIs investigated in this work. These values were calculated from the growth phases of Figure 2 for each of the APIs using Equation 6;62 tm values were calculated in the range 0.001-0.3 ns molecule-1.

1 =

B C DEFG (H 3E

(6)

where, tg is time required for a single crystal to grow to a certain size (s) (obtained from the growth phase of APIs in Figure 2), Mw is the molecular mass of the API (g/mol), ρAPI is density of the API (g/m3) (for APP: 1.26 × 106 g/m3, CBMZ: 1.30 × 106 g/m3, PBZ: 1.20 × 106 g/m3, CAF: 1.23 × 106 g/m3, CPB: 1.3 × 106 g/m3, RIS: 1.4 × 106 g/m3, FF: 1.18 × 106 g/m3),51 Vp is the volume of the API particle (m3) calculated using the particle diameter from SEM micrographs (particle diameter for AAP: 20µm, CBMZ: 20µm, PBZ: 100µm, CAF: 50µm, CPB-MeOH solvate: 100µm, RIS: 70µm and FF: 76µm), and NA is Avogadro’s constant (6.023 × 1023 mol-1).



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Table 5: Literature values for the interaction lifetimes for various heterogeneous nucleating systems. System

First interaction lifetime (ns)

Critical nuclei/crystal formation lifetime (ns)

CO2 hydrate on SiO2

< 70

1 – 160

57

Ice nucleation on AgI surface

< 11

36

56

< 10

15

58

< 15

15 – 200

59

Ice nucleation on Graphite Secondary nucleation on Bicalutamide surface

Reference

In general, large values of tm were calculated when the particle size was small or when the growth time was long. The lifetime of a single API molecule attached to a solid surface by hydrogen bonding as well as the average lifetime for nucleus formation (Table 5) is several orders of magnitude longer than the time required to add a single molecule to a growing API crystal (Table 6). Alternatively, the adsorbed API can exist attached to a surface for a time scale which will allow the attachment of multiple API molecules from the solution phase and facilitate the formation of nuclei and eventually stable crystals. In the solution phase API-API interactions are much shorter-lived increasing the difficulty of nucleus formation. Table 6: Time required for each API to add one molecule to an API cluster/crystal API

Time required to add one molecule, tm (ns molecule-1)


0.3

Acetaminophen (AAP)

0.23

Caffeine (CAF)

0.046


0.002

Risperidone (RIS)

0.052

Clozapine-MeOH solvate (CPB-MeOH)

0.001

Fenofibrate (FF)

0.003


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The lifetime effect also helps to explain the modest enhancement in nucleation rates for CBMZ and AAP. All of our previous studies with AAP and CBMZ indicate that mature crystals of these APIs still attach to the MCC surface apparently favouring the heterogeneous route to crystallization. Here we argue that those AAP or CBMZ molecules which adhere to MCC stay on the surface for longer times than the duration of the corresponding API-API solution phase interactions.

This increases the probability that other AAP or CBMZ

molecules will attach as individual molecules or as small clusters to the MCC-bound molecules, thus facilitating the eventual development of mature crystals attached to the MCC surface. Hence, the heterogeneous nucleation observed for AAP and CBMZ is due to the “lifetime effect” whereby the MCC attached AAP or CBMZ molecule exists for long enough to allow other AAP or CBMZ molecules or small clusters to attached via API-API interactions. For FF and other APIs without HBD functionality the lifetime effect operates in conjunction with the complementarity of functional groups to give much stronger accelerations of the crystallization of these molecules in the presence of MCC. Our experimental and modelling results allow us to propose a mechanism for the heterogeneous nucleation of APIs in the presence of the MCC. A schematic representation of the two distinct modes of the API aggregation in solution in the presence MCC is shown in Figure 8 for AAP which possesses HBD and HBA functionality and for FF which possesses only HBA functionality.



Figure 8: Schematic representation of two distinct modes of progression of solute aggregation in the presence of MCC: left (A-C) – formation of H-bond clusters of acetaminophen (AAP) in solution and on MCC surface; right (D-E) – adsorption and stabilisation of the fenofibrate (FF) cluster at polar surface of MCC. H-bonds between MCC and API molecules shown as dotted lines. Hence for AAP, we can envisage AAP-AAP interactions in the solution phase which would result in a relatively quick formation of small molecular aggregates of solute molecules, which, being reinforced by H-bonds may be sufficiently stable to further grow into crystal nuclei in the absence of a heterosurface. The introduction of a polar heterosurface (provided by MCC) promotes a modest reduction in AAP’s induction time relative to the reductions


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observed for the other APIs studied. The AAP crystals which form all seem to be associated with the MCC surface despite the binding energies for AAP-Glucose and CBMZ-Glucose both being less than the AAP-AAP and CBMZ-CBMZ binding energies. This effect is explained here in terms of the lifetime effect discussed above. Essentially, the mechanism outlined in Figure 8 envisages the formation of small clusters of AAP in a solution in contact with MCC. These may attach as small clusters or as individual molecules to the MCC. The longer lifetimes of these attached structures over the corresponding structures in solution favour the heterogeneous crystallization route. APIs possessing only HBAs, such as FF, are incapable of forming H-bond mediated aggregates/clusters of solute molecules. Thus aggregates rely instead on Van der Waals interaction to mediate cluster formation in solution, and that these interactions are much weaker than H-bond interactions. This results in relatively longer nucleation times in the absence of the heterosurface. It appears from the experimental results that by introducing the polar MCC heterosurface, the nucleation process of such APIs becomes significantly shortened. Both the DFT and MD modelling results show consistently that the polar hydroxyl groups present on MCC are efficient H-bond donors towards API molecules possessing only HBAs,

such

as

FF.

Thus

the

polar

heterosurface

facilitates

efficient

immobilisation/stabilisation of the FF molecules at the MCC particles. It could be envisaged that initially individual molecules or small clusters of FF would be adsorbed, followed by formation of surface adsorbed semi-structured aggregates of API, which upon reaching sufficient stability and size, could subsequently convert into crystalline phases. In effect, APIs without HBD functionality are encouraged to nucleate in the presence of MCC by virtue of the complementary HBD capacity of MCC and through the “lifetime effect” whereby individual API molecules can exist on the MCC surface for 10-1000 folds longer than the time required to add a single molecule to a developing nucleus. 35 | P a g e ACS Paragon Plus Environment


Conclusions The heterogeneous nucleation of seven APIs, namely CBMZ, AAP, CAF, PBZ, RIS, CPB and FF, was performed from methanol solutions in the presence of MCC. MCC displayed a positive yet discriminating influence over each API’s heterogeneous nucleation in terms of the enhancement in API nucleation rate it conferred. As such, though all seven APIs nucleated at a faster rate in the presence of MCC than in its absence, those APIs possessing both HBD and HBA groups (and therefore able to hydrogen bond with other molecules of themselves) experienced a smaller enhancement in their nucleation rate in the presence of MCC relative to the APIs possessing HBA groups only. Despite the observed increases in nucleation rates, the thermodynamically stable form of each API was nucleated in the presence of MCC, as confirmed using PXRD and SEM; in the specific case of CPB, the thermodynamically stable form is the methanol solvate. DFT and molecular dynamics simulations were consistent with the experimental results, whereby APIs possessing only HBA groups readily formed hydrogen bonds with the HBD groups of MCC; and this interaction facilitated the sequestering of the API molecules on to the MCC surface, thus promoting heterogeneous nucleation of the API. The experimental data support the view that MCC’s presence did not influence the heterogeneous nucleation by lowering the interfacial energy required for nucleation but instead by increasing the pre-exponential factor (A*) by 2fold and more. In conclusion, when crystallized in the presence of HBD-rich heterosurfaces, APIs possessing only HBA functionality will exhibit a more pronounced reduction in induction times than APIs possessing HBAs and HBDs. This phenomenon will also be influenced by the lifetime of the interactions between the API and the heterosurface whereby adsorbed APIs are characterised by a much longer lifetime than the lifetime of API-API interactions in solution or the time required to add new API molecules to a growing API crystal. 36 | P a g e ACS Paragon Plus Environment

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Conflicts of Interest There are no conflicts of interest to declare. Acknowledgements This work was funded by Science Foundation Ireland under Grant 12/RC/2275. This work was conducted under the framework of the Irish Government's Programme for Research in Third Level Institutions Cycle 5, with the assistance of the European Regional Development Fund.



References 1. Mullin, J. W., Crystallisation. Butterworth Heinemann: Oxford, UK: 2001. 2. Lee, A. Y.; Erdemir, D.; Myerson, A. S., Crystal Polymorphism in Chemical Process Development. Annu Rev Chem Biomol 2011, 2, 259-280. 3. López-Mejías, V.; Myerson, A. S.; Trout, B. L., Geometric Design of Heterogeneous Nucleation Sites on Biocompatible Surfaces. Cryst. Growth Des. 2013, 13, 3835-3841. 4. Yazdanpanah, N.; Testa, C. J.; Perala, S. R. K.; Jensen, K. D.; Braatz, R. D.; Myerson, A. S.; Trout, B. L., Continuous Heterogeneous Crystallization on Excipient Surfaces. Cryst. Growth Des. 2017, 17, 3321-3330. 5. Wijethunga, T. K.; Baftizadeh, F.; Stojaković, J.; Myerson, A. S.; Trout, B. L., Experimental and Mechanistic Study of the Heterogeneous Nucleation and Epitaxy of Acetaminophen with Biocompatible Crystalline Substrates. Cryst. Growth Des. 2017, 17, 3783-3795. 6. Eral, H. B.; López-Mejías, V.; O’Mahony, M.; Trout, B. L.; Myerson, A. S.; Doyle, P. S., Biocompatible Alginate Microgel Particles as Heteronucleants and Encapsulating Vehicles for Hydrophilic and Hydrophobic Drugs. Cryst. Growth Des. 2014, 14, 2073-2082. 7. Eral, H. B.; O’Mahony, M.; Shaw, R.; Trout, B. L.; Myerson, A. S.; Doyle, P. S., Composite Hydrogels Laden with Crystalline Active Pharmaceutical Ingredients of Controlled Size and Loading. Chem. Mater. 2014, 26, 6213-6220. 8. Quon, J. L.; Chadwick, K.; Wood, G. P.; Sheu, I.; Brettmann, B. K.; Myerson, A. S.; Trout, B. L., Templated nucleation of acetaminophen on spherical excipient agglomerates. Langmuir 2013, 29, 3292-300. 9. Chadwick, K.; Myerson, A.; Trout, B., Polymorphic control by heterogeneous nucleation - A new method for selecting crystalline substrates. CrystEngComm 2011, 13, 6625-6627. 10. Chadwick, K.; Chen, J.; Myerson, A. S.; Trout, B. L., Toward the Rational Design of Crystalline Surfaces for Heteroepitaxy: Role of Molecular Functionality. Cryst. Growth Des. 2012, 12, 1159-1166. 11. Weissbuch, I.; Addadi, L.; Leiserowitz, L., Molecular Recognition at Crystal Interfaces. Science 1991, 253, 637-645. 12. Aizenberg, J.; Black, A. J.; Whitesides, G. M., Oriented Growth of Calcite Controlled by SelfAssembled Monolayers of Functionalized Alkanethiols Supported on Gold and Silver. J. Am. Chem. Soc. 1999, 121, 4500-4509. 13. Aizenberg, J.; Black, A. J.; Whitesides, G. M., Control of crystal nucleation by patterned selfassembled monolayers. Nature 1999, 398, 495-498. 14. Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L., Ceramic Thin-Film Formation on Functionalized Interfaces Through Biomimetic Processing. Science 1994, 264, 48-55. 15. Yang, H.; Song, C. L.; Lim, Y. X. S.; Chen, W.; Heng, J. Y. Y., Selective crystallisation of carbamazepine polymorphs on surfaces with differing properties. CrystEngComm 2017, 19, 65736578. 16. Parambil, J. V.; Poornachary, S. K.; Hinder, S. J.; Tan, R. B. H.; Heng, J. Y. Y., Establishing template-induced polymorphic domains for API crystallisation: the case of carbamazepine. CrystEngComm 2015, 17, 6384-6392. 17. Parambil, J. V.; Poornachary, S. K.; Tan, R. B. H.; Heng, J. Y. Y., Template-induced polymorphic selectivity: the effects of surface chemistry and solute concentration on carbamazepine crystallisation. CrystEngComm 2014, 16, 4927-4930. 18. Arribas Bueno, R.; Crowley, C. M.; Davern, P.; Hodnett, B. K.; Hudson, S., Heterogeneous Crystallization of Fenofibrate onto Pharmaceutical Excipients. Cryst. Growth Des. 2018, 18, 21512164. 19. Arribas Bueno, R.; Crowley, C. M.; Hodnett, B. K.; Hudson, S.; Davern, P., Influence of Process Parameters on the Heterogeneous Nucleation of Active Pharmaceutical Ingredients onto Excipients. Org. Process Res. Dev. 2017, 21, 559-570. 38 | P a g e ACS Paragon Plus Environment

Page 38 of 42

Page 39 of 42 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


20. Verma, V.; Peddapatla, R. V. G.; Crowley, C. M.; Crean, A. M.; Davern, P.; Hudson, S.; Hodnett, B. K., Experimental Study on the Influence of Excipients on the Heterogeneous Crystallization and Dissolution Properties of an Active Pharmaceutical Ingredient. Cryst. Growth Des. 2018, 18, 338-350. 21. Pfund, L. Y.; Price, C. P.; Frick, J. J.; Matzger, A. J., Controlling pharmaceutical crystallization with designed polymeric heteronuclei. J. Am. Chem. Soc. 2015, 137, 871-875. 22. McClelland, A. A.; López-Mejías, V.; Matzger, A. J.; Chen, Z., Peering at a Buried Polymer−Crystal Interface: Probing Heterogeneous NucleaKon by Sum Frequency GeneraKon Vibrational Spectroscopy. Langmuir : the ACS journal of surfaces and colloids 2011, 27, 2162-2165. 23. López-Mejías, V.; Kampf, J. W.; Matzger, A. J., Polymer-Induced Heteronucleation of Tolfenamic Acid: Structural Investigation of a Pentamorph. J. Am. Chem. Soc. 2009, 131, 4554-4555. 24. Grzesiak, A. L.; Uribe, F. J.; Ockwig, N. W.; Yaghi, O. M.; Matzger, A. J., Polymer-induced heteronucleation for the discovery of new extended solids. Angewandte Chemie 2006, 45, 25532556. 25. Price, C. P.; Grzesiak, A. L.; Matzger, A. J., Crystalline Polymorph Selection and Discovery with Polymer Heteronuclei. J. Am. Chem. Soc. 2005, 127, 5512-5517. 26. Meidong, L.; Grzesiak, A. L.; Matzger, A. J., The Use of Polymer Heteronuclei for Crystalline Polymorph Selection. J. Am. Chem. Soc. 2002, 124, 14834-14835. 27. Ling, J.; Chadwick, K., Heterogeneous Crystallization Inside Microporous Polymer Particles as a Process Intensification Technology for the Manufacture of Drug Formulations. Org. Process Res. Dev. 2017, 21, 827-834. 28. Diao, Y.; Myerson, A. S.; Hatton, T. A.; Trout, B. L., Surface design for controlled crystallization: the role of surface chemistry and nanoscale pores in heterogeneous nucleation. Langmuir 2011, 27, 5324-5334. 29. López-Mejías, V.; Knight, J. L.; Brooks, C. L.; Matzger, A. J., On the Mechanism of Crystalline Polymorph Selection by Polymer Heteronuclei. Langmuir 2011, 27, 7575-7579. 30. Grimme, S.; Ehrlich, S.; Goerigk, L., Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456-1465. 31. Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A., 6-31G* basis set for third-row atoms. J. Comput. Chem. 2001, 22, 976-984. 32. Goerigk, L.; Grimme, S., Efficient and Accurate Double-Hybrid-Meta-GGA Density Functionals—Evaluation with the Extended GMTKN30 Database for General Main Group Thermochemistry, Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2011, 7, 291309. 33. Weigend, F.; Ahlrichs, R., Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305. 34. Mealey, D.; Zeglinski, J.; Khamar, D.; Rasmuson, A. C., Influence of solvent on crystal nucleation of risperidone. Faraday Discuss. 2015, 179, 309-328. 35. Khamar, D.; Zeglinski, J.; Mealey, D.; Rasmuson, Å. C., Investigating the Role of Solvent– Solute Interaction in Crystal Nucleation of Salicylic Acid from Organic Solvents. J. Am. Chem. Soc. 2014, 136, 11664-11673. 36. Zhang, S.; Rasmuson, Å. C., Thermodynamics and Crystallization of the Theophylline–Glutaric Acid Cocrystal. Cryst. Growth Des. 2013, 13, 1153-1161. 37. Bannigan, P.; Zeglinski, J.; Lusi, M.; O’Brien, J.; Hudson, S. P., Investigation into the Solid and Solution Properties of Known and Novel Polymorphs of the Antimicrobial Molecule Clofazimine. Cryst. Growth Des. 2016, 16, 7240-7250. 38. Padrela, L.; Zeglinski, J.; Ryan, K. M., Insight into the Role of Additives in Controlling Polymorphic Outcome: A CO2-Antisolvent Crystallization Process of Carbamazepine. Cryst. Growth Des. 2017, 17, 4544-4553.



39. Zeglinski, J.; Kuhs, M.; Khamar, D.; Hegarty Avril, C.; Devi Renuka, K.; Rasmuson Åke, C., Crystal Nucleation of Tolbutamide in Solution: Relationship to Solvent, Solute Conformation, and Solution Structure. Chem.-Eur. J. 2018, 24, 4916-4926. 40. Sun, H., COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds. J. Phys. Chem. B 1998, 102, 7338-7364. 41. Galek, P. T. A.; Fabian, L.; Motherwell, W. D. S.; Allen, F. H.; Feeder, N., Knowledge-based model of hydrogen-bonding propensity in organic crystals. Acta Crystallogr., Sect. B: Struct. Sci. 2007, 63, 768-782. 42. O’Mahony, M. A.; Croker, D. M.; Rasmuson, Å. C.; Veesler, S.; Hodnett, B. K., Measuring the Solubility of a Quickly Transforming Metastable Polymorph of Carbamazepine. Org. Process Res. Dev. 2013, 17, 512-518. 43. Granberg, R. A.; Rasmuson, Å. C., Solubility of Paracetamol in Pure Solvents. J. Chem. Eng. Data 1999, 44, 1391-1395. 44. Shalmashi, A.; Golmohammad, F., Solubility of caffeine in water, ethyl acetate, ethanol, carbon tetrachloride, methanol, chloroform, dichloromethane, and acetone between 298 and 323 K. Lat. Am. Appl. Res. 2010, 40, 283-285. 45. Liang, A.; Wang, S.; Qu, Y., Determination and Correlation of Solubility of Phenylbutazone in Monosolvents and Binary Solvent Mixtures. J. Chem. Eng. Data 2017, 62, 864-871. 46. Mealey, D.; Svärd, M.; Rasmuson, Å. C., Thermodynamics of risperidone and solubility in pure organic solvents. Fluid Phase Equilib. 2014, 375, 73-79. 47. Davern, P.; Verma, V.; Bannigan, P.; Lusi, M.; Crowley, C.; Hudson, S.; Hodnett, B. K., The Heterogeneous Crystallization of a Novel Solvate of Clozapine Base in the presence of Excipients. CrystEngComm 2018, 20, 4347-4516. 48. De Yoreo, J. J.; Gilbert, P. U. P. A.; Sommerdijk, N. A. J. M.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Cölfen, H.; Dove, P. M., Crystallization by particle attachment in synthetic, biogenic, and geologic environments. Science 2015, 349, aaa6760. 49. Nordstrom, F. L.; Svard, M.; Rasmuson, A. C., Primary nucleation of salicylamide: the influence of process conditions and solvent on the metastable zone width. CrystEngComm 2013, 15, 7285-7297. 50. Kashchiev, D.; van Rosmalen, G. M., Review: Nucleation in solutions revisited. Cryst. Res. Technol. 2003, 38, 555-574. 51. http://www.chemspider.com/, 2018. 52. Liu, Y.-X.; Wang, X.-J.; Lu, J.; Ching, C.-B., Influence of the Roughness, Topography, and Physicochemical Properties of Chemically Modified Surfaces on the Heterogeneous Nucleation of Protein Crystals. J. Phys. Chem. B 2007, 111, 13971-13978. 53. Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J., Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv Drug Deliver Rev 2012, 64, 4-17. 54. Cui, P.; Zhang, X.; Yin, Q.; Gong, J., Evidence of Hydrogen-Bond Formation during Crystallization of Cefodizime Sodium from Induction-Time Measurements and In Situ Raman Spectroscopy. Ind. Eng. Chem. Res. 2012, 51, 13663-13669. 55. Desiraju, G. R., Crystal engineering: from molecule to crystal. J. Am. Chem. Soc. 2013, 135, 9952-9967. 56. Zielke, S. A.; Bertram, A. K.; Patey, G. N., A Molecular Mechanism of Ice Nucleation on Model AgI Surfaces. J. Phys. Chem. B 2015, 119, 9049-9055. 57. Bai, D.; Chen, G.; Zhang, X.; Wang, W., Microsecond Molecular Dynamics Simulations of the Kinetic Pathways of Gas Hydrate Formation from Solid Surfaces. Langmuir : the ACS journal of surfaces and colloids 2011, 27, 5961-5967.


Page 40 of 42

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58. Lupi, L.; Hudait, A.; Molinero, V., Heterogeneous Nucleation of Ice on Carbon Surfaces. J. Am. Chem. Soc. 2014, 136, 3156-3164. 59. Anwar, J.; Khan, S.; Lindfors, L., Secondary Crystal Nucleation: Nuclei Breeding Factory Uncovered. Angewandte Chemie 2015, 54, 14681-14684. 60. Zhang, X.; Zhang, Q.; Zhao, D.-X., Act Phys. Chim. Sin. 2011, 27, 2547-2552. 61. Yani, Y.; Chow, P. S.; Tan, R. B. H., Glycine Open Dimers in Solution: New Insights into αGlycine Nucleation and Growth. Cryst. Growth Des. 2012, 12, 4771-4778. 62. Bueno, R. A. Crystallization of active pharmaceutical ingredients in the presence of heterosurfaces. Crystallization of active pharmaceutical ingredients in the presence of heterosurfaces, University of Limerick, 2018.



“For Table of Contents use only”

Dependence of Heterogeneous Nucleation on Hydrogen Bonding Lifetime and Complementarity Vivek Verma, Jacek Zeglinski, Sarah Hudson, Peter Davern, Benjamin K. Hodnett*

Influence of hydrogen bonding complementarity and lifetime in heterogeneous nucleation of active pharmaceutical ingredients.


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Dependence of Heterogeneous Nucleation on Hydrogen Bonding

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