Solubility-Limited Impurity Purge in Crystallization - Crystal Growth

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Solubility-Limited Impurity Purge in Crystallization Fredrik L. Nordstrom,*,# Brian Linehan,# Rattavut Teerakapibal,§ and Huayu Li# #

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API Engineering, Material & Analytical Sciences, Boehringer-Ingelheim, 900 Ridgebury Road, Ridgefield, Connecticut 06877, United States § School of Pharmacy, University of Wisconsin-Madison, 777 Highland Avenue, Madison, Wisconsin 53705, United States ABSTRACT: Crystallization from solution is a key unit operation utilized across the synthetic scheme to remove impurities. However, little is still known of the underlying impurity purge mechanisms that are responsible for controlling the final purity of the product. Reported herein is the solubility-limited impurity purge mechanism in which the impurity exists as a separate solid phase with its own solubility. A mathematical framework is presented that describes the separation of the impurity in the solid and liquid phases based on the relative solubilities of the product and impurity, and initial impurity level. Three theoretical solubility-limited impurity purge mechanisms are derived that are confirmed experimentally using salicylic acid, ibuprofen, and acetaminophen as model compounds. A practical experimental test is introduced that is used to identify if the impurity is rejected by solubility-limitation and its corresponding type. Finally, development strategies are presented to remove impurities that are purged based on their solubilities.

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INTRODUCTION The importance of purity control in the chemical industry in general, and in the pharmaceutical industry in particular, cannot be overstated.1,2 Impurities are present and dealt with at all stages of development from starting materials, intermediates, drug substance, and even in the drug product.3,4 Many different types of impurities can form across the synthetic scheme. Examples include inorganic and organic impurities, reaction side-products or unreacted reagents, chiral impurities, degradants from exposure to light, oxidation or pH changes, and perhaps the most critical being genotoxic impurities.5−9 In the pharmaceutical industry, a combination of scientific fields are involved in controlling the purity of the active pharmaceutical ingredient or API. In analytical chemistry impurities are identified and analyzed, and their chemical structures are resolved using a variety of analytical tools, methodologies, and workflows.10,11 In synthetic chemistry, the formation and rejection of impurities are monitored, evaluated, and assessed across the synthetic route and over different unit operations. In toxicology, impurities are evaluated for mutagenicity and toxicity, and impurity limits are established based on dose ranges.12 Comprehensive purity controls are put in place throughout development and ultimately formalized at the time of filing. Stringent regulatory controls and oversight are also in place to ensure the safety and efficacy of the developed medicines for the patients. Purity control thus permeates through the Chemistry, Manufacturing, Controls (CMC) organizations from start to finish of development and onward to production for market supply. The unit operation of crystallization is routinely carried out across all synthetic steps. Besides enabling the product to be isolated as an easy-to-handle solid in sufficient yield, crystallization also controls the crystal form, powder properties, © XXXX American Chemical Society

and purity profile of the product. While the crystal form and particle size are often critical for the API, impurity purges in crystallizations are relied upon throughout the synthetic scheme to control the purity of the API. Most of the engineering research done on the unit operation crystallization is focused on crystal forms and particle size, while less attention is paid to how impurities are purged during crystallization. Typically, impurity purge in crystallization is evaluated more or less empirically, focusing primarily on the change in purity profile after the isolation.13 In early development, the impurity rejection ability in a crystallization process is typically established based on the lot and batch history. Design space experiments are carried out later in development as part of developing process control justifications. A common type of experiment is to spike in excess of the impurities in question to evaluate whether or not they are purged during the crystallization.14 However, a mechanistic understanding of why certain impurities are rejected and others are not is seldom established. On the basis of the available literature on impurity purge in crystallization, the following mechanisms have been proposed, which are both thermodynamic and kinetic in nature, namely, solubility-limited impurity purge, solid solution (substitutional or interstitial), surface adsorption, kinetic incorporation, and inclusion.15 Since the two former mechanisms are thermodynamic in nature, changes in the crystallization procedure may not alter the purge notably. On the contrary, appreciable improvements in the impurity levels are possible for the latter Received: November 20, 2018 Revised: December 20, 2018 Published: January 2, 2019 A

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impurity and product before crystallization so that the desired impurity levels were obtained postcrystallization. SLIP Test. SLIP tests were carried out using 4 and 20 mL vials with caps. Crystallized lots from MeOH/H2O that were enriched in the selected impurity were added to the vials together with either pure water or 10 w% MeOH in H2O. Components were mixed in different proportions to reach different process volumes (Vp). It is useful to exploit the full range of process volumes up to the critical process volume of the product (Vp,crit p ) to uncover the entire trend in the solid and liquid phases. Suspensions were left to equilibrate at room temperature for at least 1 day, commonly 3−4 days using a rotating rack. All the vials were wrapped with parafilm to minimize solvent loss through evaporation. After equilibrium had been reached, a sample of the suspension was filtered using a syringe and disposable 0.2 μm filter. The liquid sample was diluted with ACN before being analyzed by HPLC. The solid phase was separated from the liquid phase by either centrifuge filtration when at a smaller scale, or through a disposable filter unit (Chemglass, article no. OP-6602-14) when at a larger scale. Solids were not washed as this may preferably dissolve or crystallize one of the components. The solids were completely dissolved in the diluent solvent ACN prior to HPLC analysis. Theory. The mathematical basis for calculating impurity rejection in crystallizations was given in an earlier contribution.13 Within this framework a product and an impurity in a crude material are separated into a solid and liquid phase through, e.g., crystallization or slurrying. The designation crude refers herein to any type of solid material containing some level of an impurity that is undergoing changes in purity after contact with a liquid phase. The mass balance describing this relationship is given as Product (p)

two mechanisms, as these are kinetically related, e.g., by establishing control of supersaturation in the crystallization. Thus, solely by differentiating between thermodynamic and kinetic mechanisms, an understanding of the purge ability can be acquired, which helps to direct future development work. As of today, no simple or practical tests are available that can quickly identify the underlying impurity purge mechanism. Presented in this contribution is a comprehensive analysis of one of the most common purge mechanisms, namely, solubility-limited impurity purge, henceforth abbreviated as SLIP. The objectives are 3-fold. First, establish a mathematical framework to quantify the purity of the solid and liquid phases at different process conditions for a SLIP mechanism. Second, provide a fast and practical experimental approach to identify a SLIP mechanism using crystallized material. Third, provide developmental strategies to enable rejection of impurities based on the type of solubility-limitation.

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EXPERIMENTAL SECTION

Materials. Salicylic acid, SA (Sigma-Aldrich, Allentown, PA, USA), acetaminophen, APAP (Sigma-Aldrich, Allentown, PA, USA), and ibuprofen, IBF (Bishop, TX, USA), were used as is. All solid reagents were of ACS or pharmaceutical grade. Acetonitrile, ACN, and methanol, MeOH, were both received from Sigma-Aldrich (ACS or HPLC grade). The in-house water was distilled and deionized prior to use. Procedures. High Performance Liquid Chromatography (HPLC) Analysis. HPLC-UV was used to determine concentration for all samples in the study. Standard curves and response factors were generated for each component, and samples were diluted to remain within the linearity range for their respective components. APAP/SA samples were separated on an Agilent 1100 HPLC with DAD using an Agilent Zorbax SB-Aq column (4.6 × 150 mm, 3.5 μm), at a column temperature of 40 °C, a flow rate of 1.2 mL/min, an injection volume of 2 μL, a run time of 10 min, and a detection wavelength of 240 nm. The mobile phase was a gradient of ACN/ 0.1% H3PO4 in H2O. Response factors for SA and APAP were 3.351 × 106 and 6.450 × 106 respectively. The linear ranges for quantitation were below 1800 mAU (R2 = 1.0000) for SA and below 2700 mAU (R2 = 0.9997) for APAP. SA/IBF samples were separated on an Agilent 1100 HPLC with DAD using a Halo Phenyl Hexyl column (4.6 × 150 mm, 2.7 μm), at a column temperature of 20 °C, a flow rate of 1.0 mL/min, an injection volume of 2 μL, a run time of 13 min, and a detection wavelength of 220 nm. The mobile phase was a gradient of ACN/ 0.1% H3PO4 in H2O. Response factors for IBF and SA were 1.662 × 104 and 1.410 × 104 respectively. The linear ranges for quantitation were below 8000 mAU (R2 = 0.9990) for SA and below 6000 mAU (R2 = 0.9940) for IBF. Crystallization in MeOH/H2O. To simulate process-related impurities in product lots the product was recrystallized from solutions comprising product and impurity in MeOH/H2O. Approximately 4 g of the product (SA or APAP) was added to a reactor. Different amounts of the simulated impurity (IBF or SA) were added to the same reactor depending on the desired impurity level. Approximately 8 g of MeOH was added, and the resulting suspension was mixed until all solids were dissolved. Occasionally heating was applied to facilitate the dissolution. Water was then added to the clear solution until a final solvent composition of 10 w% MeOH in H2O was reached, typically 72 g of H2O. The crystallizations were fairly fast, and the final slurries were filtered after a few hours. The filtration was normally completed within 1 min, not leaving much mother liquor left in the filter cake. The filtered solids were not washed and were dried in vacuo at 50 °C. The final impurity levels in the crystallized solids were measured by HPLC. On the basis of the known solubility profiles in MeOH/H2O of the pure product and impurity, it was possible to adjust the relative amounts of

mPC = mPS + mPL

(1)

Impurity (i)

miC = miS + miL

(2)

where the superscripts C, S, and L denote the crude, solid, and liquid, respectively. The impurity rejection, Ri, and product yield, Yp, are quantified as Ri =

YP =

miL miC

(3)

mPS mPC

(4)

The purity level in the crude, solid, or liquid phase (x = C, S or L) can be expressed as weight fraction, ω: ωix =

ωpx =

mix mix + mpx

(5)

mpx m ix

+ mpx

(6)

when only one impurity is considered. For this case it follows that ωpx + ωix = 1

(7)

For the liquid phase the product and impurity level can be expressed in terms of concentrations, C (e.g., in g of substance/g of solvent). Cp =

Ci =

mpL msolvent

(8)

miL msolvent

(9)

where msolvent is the amount of solvent. Process volume, Vp, is defined as L of solvent/kg of product. Hence, B

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msolvent δsolventmpC

Table 1. Types of SLIP Mechanisms (10)

solubility-limitation

where δsolvent is the density of the solvent. Product yield can be expressed as a function of process volumes by combining eqs 1, 4, 8, and 10 Yp = 1 − δsolventVpC p

SLIP SLIP SLIP SLIP SLIP SLIP SLIP

(11)

The impurity rejection can also be made a function of process volume through eqs 3, 5, 6, 9, and 10 ij ωpC yz R i = VpCiδsolventjjjj C zzzz jω z k i {

< Vp,crit SLIP 1 Mechanism (Vi,crit p p ). A SLIP 1A mechanism is in place when the process volume is lower than the critical process volume of both the impurity and product. In this scenario a physical mixture of product and impurity particles are suspended in a solution comprised of partially dissolved impurity and product with a concentration equal to their respective solubilities (left side in Figure 1). The impurity level in the liquid phase is governed by eq 13. Inserting eqs 11 and 12 into eq 13 yields after rearrangements:

(12)

The impurity level in the liquid phase can be derived as a function of impurity rejection and product yield via eqs 1, 3−5, and 7 ij R i yz jj (1 − Y ) zz p { k ωi = ij R i yz ijj ωpC yzz jj (1 − Y ) zz + jj C zz p { k k ωi { L

ωiL = ωiL(eq) =

(13)

Cieq

Cieq + C peq

Similarly, the impurity level in the solid phase can be obtained through eqs 2−6 ij 1 − R i yz jj Y zz k p { ωiS = ij 1 − R i yz ijj ωpC yzz jj Y zz + jj C zz k p { k ωi {

1 δsolventC peq

Figure 1. Impurity level in the liquid and solid phases as a function of process volumes for a SLIP 1 mechanism. The transition from SLIP 1A (left) to SLIP 1B (right) takes place at the critical process volume for the impurity, Vi,crit p .

(15)

Thus, the impurity level in the liquid, ωLi , will be constant with a value of ωLi (eq) from process volumes ranging from 0 to Vi,crit p . This constant is solely governed by the solubility of the impurity and product. The purity profile of the solid phase, as described by eq 14, can similarly be rearranged through eqs 11 and 12:

where Ceq p corresponds to the solubility of the product. The second boundary condition relates to the solubility of the impurity. Similar to above, with increasing dilution all the impurity will become dissolved so that mCi = mLi and the concentration of the impurity will equal its solubility. This critical process volume is denoted Vi,crit p . Through eqs 5, 9, and 10 it follows that

Vpi,crit

ij ωiC yz jjj C zzz ωp = eqk { Ci δsolvent

(17)

(14)

Equations 13 and 14 form the basis to describe the purity profile in the liquid and solid phases for a crude contacted with a liquid phase (e.g., crystallization or slurry) when the impurity and product do not react (mass balance holds).13 It can be seen that the impurity level in both the liquid and solid are only functions of the crude impurity level, impurity rejection, and product yield. For all these terms, the numeric values range from 0 to 1 (or 0% to 100%). The product yield and impurity rejection terms are further described by eqs 11 and 12, which both include process volume and concentration as variables. Boundary Conditions. For a SLIP mechanism two boundary conditions can be inferred. The first boundary condition relates to the solubility of the product. At a certain process volume all the product will be dissolved so that mCp = mLp , and the concentration of the product equals its solubility. This critical process volume thus constitutes the first boundary condition and is herein represented as Vp,crit p . By eqs 8 and 10 we can write Vpp,crit =

criteria Vi,crit < Vp,crit p p Vp < Vi,crit < Vp,crit p p p,crit Vi,crit ≤ V p p ≤ Vp p,crit i,crit Vp < Vp Vp < Vp,crit < Vi,crit p p i,crit Vp,crit ≤ V p p ≤ Vp p,crit Vi,crit = V p p

1 1A 1B 2 2A 2B 3

ωiS =

ωiC − ωpCVpδsolventCieq 1 − ωpC Vpδsolvent(Cieq + C peq)

(18)

Equation 18 thus applies for the solid phase from process volumes S C S i,crit from 0 up to Vi,crit p , where ωi = ωi at Vp = 0, and ωi = 0 at Vp = Vp . The impurity dependence with process volumes in the solid and liquid can thus be established graphically (left side in Figure 1). A SLIP 1B mechanism exists when the process volume is between the critical process volume of the impurity and the critical process volume of the product. In this case the concentration of the impurity is below its solubility, and the impurity is therefore completely dissolved in the liquid phase. No impurity exists in the solid phase, and pure product crystals are surrounded by a solution comprising both impurity and product (right side in Figure 1). When the

(16)

Different impurity rejection scenarios follow depending on which of the critical process volumes is larger: the product or impurity. For simplicity, the nomenclature in Table 1 is used to differentiate between the various SLIP scenarios. In the subsequent theoretical discussion on SLIP mechanisms, only equilibrium conditions are considered. No kinetic scenarios are discussed such as desaturation or crystal growth. In addition, it assumes no chemical reaction. C

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boundary condition Vp = Vi,crit is met, the impurity rejection, Ri, equals p 100%. This is the upper limit for impurity rejection. Above this process volume it follows from eq 14 that ωSi = 0 when Ri = 100%. The impurity level in the liquid phase can thus be determined by setting Ri to unity in eq 13 and using eq 11.

This equation simply describes a dilution effect of the impurity, which results in an almost linear increase with process volumes starting from ωLi (eq) and ending with complete dissolution of the impurity so that the impurity level of the liquid is reverted back to that of the crude (right side in Figure 2). The extent of deviation from linearity increases with higher impurity levels in the crude. At relatively low levels (approximately ωCi < 5%), the curve can be approximated by eq 21 with a line having the slope Ceq i δsolvent.

1

ωiL = 1+

δsolventVpC peq jijj ωiC zzyz jjj C zzz ω k p{

ωiL ≈ VpCieqδsolvent

(19) Vi,crit p

(21) (Vi,crit p

This equation thus applies from process volumes ranging from up to Vp,crit p . At the former limit the purity of the liquid phase is also determined by eq 17. At the latter limit ωLi = ωCi as both the impurity and product are now fully dissolved in the liquid phase and no solid phase exists. On the basis of these limits and the functional form of eq 19 the graphical representation of the SLIP 1B impurity level as a function of process volumes is shown on the right in Figure 1. < Vi,crit SLIP 2 Mechanism (Vp,crit p p . In a SLIP 2A scenario a physical mixture of product and impurity particles are suspended in a solution comprised of partially dissolved impurity and product with a concentration equal to their respective solubilities. The state is similar to SLIP 1A as discussed above, and the impurity levels in the liquid and solid phases are also described by the same equations as for SLIP 1A, i.e., eqs 17 and 18. However, in this case, all of the product will dissolve before the impurity upon increasing process volumes. Accordingly, the SLIP 2A scenario is in place for process volumes ranging from 0 to Vp,crit p . The impurity profiles with process volumes are also inverted as compared to Type 1A in which the impurity level is higher in the solids than the liquid. While the impurity level in the liquid is constant and equal to ωLi (eq), the impurity level in the solid phase increases with increasing process volumes until only the impurity particles remain (ωSi = 100%). On the basis of eqs 17 and 18 < Vi,crit the impurity levels in the liquids and solids can be when Vp,crit p p shown graphically (left side in Figure 2).

Vp,crit p ).

SLIP 3 Mechanism = For this unlikely case to occur the crude impurity level and solubility of impurity and product must be perfectly balanced so that the critical process volumes are exactly equal. By eq 15 and 16 we can write the condition for this to happen: ωiC(SLIP3) =

Cieq

Cieq + C Peq

(22)

This critical value thus shows the exact impurity level in the crude to achieve a SLIP 3 mechanism. If ωCi is below this value in a lot then a SLIP 1 is obtained and vice versa if ωCi is above then a SLIP 2 is obtained. Combining eqs 22, 17, and 18 results in the following equation for the purity of the solid and liquid phases: ωiL = ωiS = ωiC

(23)

The trend in impurity level in the liquid and solid phases is or Vi,crit therefore constant for process volumes from 0 up to Vp,crit p p (left side in Figure 3). Above this critical volume (right side in Figure 3), all the solids are completely dissolved, and hence ωLi = ωCi .

Figure 3. Impurity level in the liquid and solid phases as a function of process volumes for a SLIP 3 mechanism. Separation Efficiency, β. In crystallizations involving purification, impurity rejection alone is not sufficient to compare between different solvent systems. Product yield must also be taken into account when making a fair assessment on purification power. We therefore use the term β (introduced in ref 13), which represents a separation efficiency over the crystallization. The variable β is expressed as

Figure 2. Impurity level in the liquid and solid phases as a function of process volumes for a SLIP 2 mechanism. The transition from SLIP 2A (left) to SLIP 2B (right) takes place at the critical process volume for the product. In a SLIP 2B case, all of the product solids have dissolved, and only solids belonging to the impurity exist at equilibrium with a solution of dissolved product and partially dissolved impurity. This is illustrated on the right in Figure 2. The impurity level in the solid phase is thus 100%, and the product yield is 0%. Inserting Yp equaling 0 in eq 13 and using eq 12 yields

ωiL =

β=

(24)

and represents a normalized value on the extent of impurity rejection per loss of product in the liquid phase. A value of β above unity indicates that the isolated solids are purified in the crystallization, whereas a value below unity implies that the impurity is enriched in the solids. When β is unity then no separation of product from impurity is obtained. Through eqs 11 and 12, eq 24 can be further expressed as

VpCieqδsolvent VpCieqδsolvent + 1

Ri 1 − Yp

(20) D

DOI: 10.1021/acs.cgd.8b01734 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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impurity levels in order to properly demonstrate the various SLIP mechanisms. The three model compounds were recrystallized in MeOH/ H2O at 23 °C through a simple antisolvent crystallization where one of the components was spiked in to simulate an impurity. All the components were fully dissolved in pure MeOH prior to crystallization through the addition of water. The experimental solubilities of the pure APAP, SA, and IBF in MeOH/H2O have been reported in the literature.16−18 Each isolated material contained different amounts of impurity levels. These materials were then subjected to the so-called SLIP test, explained above. The three different SLIP mechanisms could be verified experimentally in the following case studies and are summarized in Table 2.

(25)

When the concentrations of the product and impurity are equal to their solubilities (SLIP 1A, 2A, and 3), we can simplify eq 25 through eqs 15 and 16 β=

Vpp,crit Vpi,crit

(26)

Thus, for a solubility-limited impurity purge at equilibrium the separation efficiency is directly governed by the critical process volumes of the product and impurity. The change in separation efficiency as well as impurity rejection with process volumes are graphically depicted in Figure 4 for all SLIP mechanisms.

Table 2. Overview of Three Experimental Case Studies Representing SLIP 1, 2, and 3 Mechanisms case study SLIP mechanism product impurity level solvent system

1 1 APAP 5.2 wt % SA H2O

2 2 SA 9.7 wt % IBF 10 wt% MeOH in H2O

3 3 APAP 13 wt % SA H2O

Case Study 1. APAP (product) was recrystallized in MeOH/H2O from a solution comprising 9.1 wt % SA (impurity). After isolation the SA content was reduced to 5.2 wt %. This material was then subjected to the SLIP test performed in pure water at RT at process volumes ranging from 8 to 57 L/kg. The experimental trends in impurity level with process volumes for both the solid and liquid phases are presented in Figure 5. Inserted in the top figure are also the calculated trends (as full lines) for a SLIP 1 mechanism, based on eq 17−19 and the measured solubility. The impurity levels in the liquid are much higher than those in the solids. The fit to the experimental data clearly confirms that SA impurity was purged in the APAP crystallization per a SLIP 1 mechanism. The experimental data show that the impurity level in the solid phase drops drastically until reaching approximately 20 L/kg. Above that process volume the solids are essentially free from the impurity. The transition at ∼20 L/ kg would then correspond to the critical process volume of the impurity (Vi,crit p ). The solution concentrations in the liquid samples of both the product (APAP) and impurity (SA) are presented in Figure 5. It can be seen that the product concentrations measured from the liquid samples correspond to the solubility of APAP. The concentrations of the impurity correspond to its solubility below 20 L/kg. Above this point is a reduction in concentration taking place as the impurity is being diluted. The measured solubility of APAP in this case study was 15.0 mg/g (std dev 0.26 mg/g, n = 10), which is consistent with literature data for pure APAP.16 The measured solubility of SA was 2.75 mg/g (std dev 0.06 mg/g, n = 2), which is also close to the literature values for pure SA.17 Hence, the impurity effect on solubility of both APAP and SA is small. This can be explained by no changes taking place in the solid phase (physical mixture of crystalline APAP and SA) and the concentration in the liquid phase is relatively low (98% is still water). Case Study 2. In this case study SA is used as the product and IBF is spiked in to simulate a poorly soluble impurity. SA was recrystallized in MeOH/H2O with 9.0 wt % IBF. After

Figure 4. (a−c) Change in impurity rejection, Ri (red), product recovery, Yp (blue), and separation efficiency, β (green), as a function of process volumes for SLIP mechanisms.

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RESULTS Acetaminophen (APAP), salicylic acid (SA), and ibuprofen (IBP) were used as model compounds in this study as these are common small molecule pharmaceuticals having different properties and solubility profiles. It was important to use different chemical structures to avoid having other impurity purge mechanisms appearing during the crystallization, and to reduce the possibility of cocrystal formation. It also became necessary to select certain solvent compositions and crude E

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Figure 5. Case study 1. (Top) Change in impurity level in liquid and solid phases with increasing process volumes. The squares and diamonds correspond to experimental data in the liquid (red) and solid (blue), respectively. The lines are based on theoretical predictions for a SLIP 1 mechanism. (Bottom) Change in product (APAP) and impurity (SA) concentration of liquid phase with process volumes.

recrystallization the impurity level increased to 9.7 wt %; i.e., IBF was enriched in the product. The SLIP test was carried out in 10 wt % MeOH in H2O at ambient conditions at process volumes ranging from 49 to 881 L/kg. There is an excellent fit between the experimental data and the calculated lines based on a SLIP 2 mechanism (eqs 17, 18, and 20), which clearly shows that the IBF impurity is purged in the crystallization via a SLIP 2 mechanism (compare Figure 2 and Figure 6). The experimental impurity levels in the liquid phase are constant up to 350−400 L/kg at which point the impurity level increases. This corresponds to the critical process volume of the product. Some deviation to the theoretical predictions can be seen above 700 L/kg. This was found to be due to surface adsorption effects in which the impurity, IBF in this case, preferentially adsorbed to the vial walls. At these process volumes, the slurries are so thin that no solid phase could be filtered out and analyzed. A trend shift can also be seen in the experimental HPLC data for the solid phase. The impurity level increases dramatically until only the impurity remains as solid particles, i.e., the critical process volume of the product for a SLIP 2 mechanism. The concentration of the impurity (IBF) remains constant at 0.062 mg/g (standard deviation of 0.04 mg/g, n = 13) across the entire Vp range and is equal to its solubility. The reported solubility of pure IBF in 10 wt % MeOH in H2O at 25 °C is 0.096 mg/g according to Jimenez et al., 2016.18 The concentration of salicylic acid is equal to its solubility up to 350−400 L/kg, in this study at 2.95 mg/g (standard deviation

Figure 6. Case study 2. Change in impurity level in liquid (top) and solid (middle) phases with increasing process volumes. The squares and diamonds correspond to experimental data in the liquid and solid, respectively. The lines are based on theoretical predictions for a SLIP 2 mechanism. (Bottom) Change in product (SA) and impurity (IBF) concentration of liquid phase with process volumes.

of 0.067 mg/g, n = 7). This is within the range for pure salicylic acid reported by Matsuda et al.17 Case Study 3. In case study 3 several attempts were made to identify the exact right conditions to reach the theoretical SLIP 3 scenario. This requires that the solubilities of the product and impurity are perfectly balanced relative the impurity level in the crude, as given by eq 22. APAP was used as the product and SA as the impurity and a level of 13.0 wt % SA was obtained in the isolated solids. The SLIP test was carried out of this material at 5, 10, and 15 L/kg in pure water and the results are provided in Figure 7. No separation of product from impurity was achieved at any process volume. The experimental impurity levels in the solid and liquid phases are the same as the starting impurity level; F

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dissolved completely. When the dissolved product is made supersaturated through, e.g., antisolvent addition or cooling, the impurity may become supersaturated, especially if the impurity exhibits similar solubility profile as the product. For primary nucleation, there is a lag time, or induction time, before crystallization initiates from the supersaturated solution. As a result, the supersaturated level of impurity may not necessarily lead to nucleation before filtration is performed. In fact, a reoccurring issue in chemical processing is that several pilot plant batches of a product have been made over a certain period of time without prior impurity issues. Then one particular impurity is suddenly elevated in one batch, seemingly for no obvious reason. In such cases it is very tempting to start investigating what went wrong with this particular process by identifying process parameters that may have deviated to some degree from previous processing. In reality, however, this impurity may have been supersaturated in all the previous batches but did not nucleate and crystallize out prior to filtration. This type of example is a telltale sign of an impurity undergoing SLIP mechanism in the crystallization. Primary nucleation as the underlying phenomenon for this behavior is known to be highly sensitive to a number of parameters such as mixing, temperature, purity, particulates, etc.19 It is also known to be inherently stochastic,19,20 meaning that the onset of nucleation varies considerably even for the same material. As such, it is very difficult to control. A better approach to ensure robust impurity rejection in crystallizations with SLIP mechanisms is to operate at undersaturated conditions so that the impurity can never form a solid phase. This requires that the specific SLIP mechanism for each particular process is identified. Identification of SLIP Mechanism. Impurities being rejected in the crystallization through the SLIP mechanism often exhibit irregular behaviors depending on if or to what extent the impurity crystallized out from solution. Reviewing batch history or lab experiments that were carried out without major process alterations may provide clues through the impurity levels in the isolated solids, especially when comparing to other impurities. Another indication of SLIP mechanism is content uniformity issues in the isolated solids. A result of the SLIP mechanism in crystallization is that the impurity forms its own individual particles, i.e., a physical mixture with product and impurity particles. These particles may exhibit a different particle size and morphology as compared to the product crystals, which may lead to segregation during the crystallization and in subsequent solids handling. Analyses from different parts of the batch could then give different impurity levels. A direct confirmation of the SLIP mechanism can be achieved by applying the SLIP test to the crystallized materials, explained in the Experimental Procedure Section. The crystallized material is simply resuspended in the solvent it was crystallized from at different process volumes, e.g., at the final solvent composition. The impurity level trends with process volumes of both the solids and liquid phase will not just enable identification of the SLIP mechanism, but also the type of SLIP mechanism. However, even without a very detailed trending with process volumes, as in case studies 1−3, the SLIP mechanism can still be identified. A key feature is that the impurity level must be constant in one of the phases over a certain range in process volumes. This is the case for SLIP 1A, 2A, and 3 in the liquid and SLIP 1B (at 0%), 2B (at 100%),

Figure 7. Change in impurity level of product and impurity with increasing process volumes in case study 3. The squares and diamonds correspond to experimental data in the liquid and solid, respectively. The dashed line is based on theoretical predictions for a SLIP 3 mechanism.

i.e., the criteria for a SLIP 3 mechanism has been achieved (compare Figure 3 and Figure 7).

■

DISCUSSION All the data from the case studies are in complete agreement with the theoretical derivations for SLIP mechanisms. The presented case studies show that all three SLIP mechanisms can be obtained in crystallizations executed under typical process conditions used in the pharmaceutical industry. In fact, the SLIP mechanism is often observed in crystallization of pharmaceutical compounds. Determination of the relevant SLIP mechanism for a specific process, however, does not only depend on the relative solubility of the impurity and product, but also on the impurity level in the crude. It is possible to switch between SLIP 1, 2, and 3 in the same crystallization process just by having different impurity levels in the crude. This was experimentally demonstrated in case studies 1 and 3 where the impurity level after crystallization was 5.2 wt % SA in case study 1 but 13.0 wt % SA in case study 3. If the crude impurity level was further increased, then a SLIP 2 mechanism would be obtained, which would have caused the impurity to be enriched rather than rejected in the solids after crystallization. The sensitivity in moving from different SLIP mechanisms during crystallization depends on where the current crude impurity level is located relative to the theoretical crude impurity level for a SLIP 3 case given by eq 27. For example, if a chemical reaction historically has provided crude impurity levels ranging from 0.5 to 1.0 w% and the SLIP 3 crude impurity level is 5 w%, then there is an appreciable margin for ensuring that a SLIP 1 mechanism will always be obtained, resulting in adequate rejection of the impurity in the crystallization. However, if the SLIP 3 crude impurity level is 0.9 wt %, then it is possible that a few reactions may cause the impurity purge mechanism in the crystallization to switch to SLIP 2, resulting in enrichment of the impurity in the solids. At this point, repeating the crystallization would only reduce the purity further as the impurity will now be even more enriched in the solids. All of these considerations are based on that thermodynamic equilibrium has been achieved. Obviously, kinetics can also play a role. When a product is crystallized or recrystallized, the product is frequently completely dissolved in a suitable cosolvent. That normally means that the impurity is also G

DOI: 10.1021/acs.cgd.8b01734 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 3. Parameter Trends with Increasing Process Volumes for Different SLIP Mechanisms (SLIP Test) parameter

SLIP 1A

SLIP 1B

SLIP 2A

SLIP 2B

SLIP 3

impurity level in solid phase (ωSi ) impurity level in liquid phase (ωLi ) ωSi /ωLi separation efficiency, β

decreasing constant (= ωLi )(eq)) decreasing (1)

0% decreasing 0 decreasing (≥1)

increasing constant (= ωLi )(eq)) increasing (>1) constant (1) increasing (≤1)

ωCi constant (= ωCi ) 1 1

suspending the dirty product lot in the same solvent system used for crystallization. It is also possible to just compare the impurity solubility with the impurity concentration in the liquid phase after equilibration of the dirty product lot. However, this requires that the concentration is in equilibrium with the solid impurity phase; i.e., the impurity cannot be completely dissolved. It is therefore important to analyze the impurity level in both the liquid and solid phase. Analyses of both phases also allow for an additional verification of the SLIP mechanism, as well as control of the mass balance13 should the impurity react or degrade over time. II. Impurity Is Known but Is Not Available. In some cases the impurity has been identified to a great certainty (e.g., by LC-MS or NMR), but it has never been synthesized or isolated. Since solubility has not been determined of the pure impurity, it is difficult to verify the SLIP mechanism using the approach above. However, by knowing the chemical structure of the impurity some assumptions can be made regarding the response factor for the HPLC chromatograms. For example, a common impurity is a dimer of the product, which can be presumed to exhibit a very similar response factor to the product, i.e., the relative response factor (F) is close to unity in eq 27. In other cases MS fragmentation data can be used to estimate the relative response factor. With this type of incomplete information, the SLIP mechanism can be identified using the SLIP test. A crystallized lot that is enriched in the impurity, i.e., the dirty product lot, is suspended at different process volumes, typically at the end condition in the crystallization solvents from where the dirty lot was isolated from. The reason for using the same solvent system is that suspending the dirty product lot in other solvents may lead to form conversion to different solid forms, which would erase the “history” of the crystallized solids. Form conversions of the impurity itself may also take place, which would be difficult to detect. Furthermore, the impurity purge mechanism may change altogether when suspended in a new solvent system. After equilibrium is reached the suspension is separated, and the solid and liquid phases are analyzed by, e.g., HPLC. If the same trends with process volumes appear, as presented in the theory section and experimentally shown in the case studies, then the relevant SLIP mechanism can be inferred. In addition, if the peak integrations in the HPLC chromatograms can be converted into wt % using eqs 27 and 28 through the estimation of the relative response factor, then the solubility of the impurity can be estimated without actually isolating it. For SLIP 1A and 2A, eqs 17 and 27 can easily be rearranged to express the solubility of the impurity as a function of product solubility and impurity level in the liquid phase from the SLIP test:

and 3 in the solid. A guide for change in common purification parameters is provided in Table 3. The extent of confirmation of impurity rejection mechanism also depends on how much is known of the impurity in question. Three development scenarios can be outlined, i.e., (i) the impurity is known, has been isolated, and is readily available, (ii) the impurity is known but is not available, (iii) the impurity is unknown and has not been isolated. I. Impurity Has Been Isolated and Is Available. In the event that the impurity has been identified and isolated, the methodology to establish a SLIP mechanism is fairly straightforward. The solubility of the (pure) impurity is determined in the same solvent system it was isolated from over a relevant range of process conditions, e.g., solvent composition or temperature. The solubility of the product is normally already available as part of the crystallization process design. The next step is to use a product lot that was crystallized from the same system and enriched in the impurity, referred to as the dirty product lot. Accurate measurements of the impurity level in both the solid and liquid phases are helpful if the impurity level is elevated. The impurity level in the dirty product lot is measured as peak area % from HPLC, but can be converted into wt % impurity level (ωi) by knowing the relative response factor, F, between the product and impurity. ωi =

1

(

F

1 Ωi

)

−1 +1

(27)

where Ωi is the peak area % from the HPLC chromatogram given as Ωi =

Γi Γi + Γp

(28)

where Γ represents the integrated peak area (normally in mAUs units) for the product or impurity. On the basis of the solubility profiles of the product and impurity, as well as the impurity level in the dirty product lot, the corresponding SLIP mechanism and impurity level in the solid and liquid phases can be calculated. For example, at the end of the crystallization, the solubility of the product is 2 mg/ mL, and the solubility of the impurity is 0.2 mg/mL. The dirty product lot contains 2 wt % of the impurity in question. Hence at the end of the crystallization the critical process volume of the product is 500 L/kg (eq 15), and the critical process volume of the impurity is 102 L/kg (eq 16). These values imply a possible SLIP 1 scenario. The dirty product lot containing the 2 wt % impurity is then suspended in, e.g., 50 L/kg so that if a SLIP mechanism is in place the impurity will be present in both the solid and liquid phase. If the experimental impurity levels in the solids and liquids fit the calculated impurity levels per eqs 17 and 18 then a SLIP 1 mechanism can be inferred. With higher impurity levels in the product, solubilizing effects relative to pure product may follow. This effect can be measured and accounted for when

ij ωiL(eq) yz zz = Cieq = Cpeq jjj j 1 − ω L(eq) zz i k { F

Cpeq

(

H

1 Ω iL(eq)

)

−1

(29)

DOI: 10.1021/acs.cgd.8b01734 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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level in the crude on the rejection ability at equilibrium. In certain cases an increase in the crude impurity level can cause a switch in the type of solubility-limited impurity rejection, e.g., from SLIP 1 to 2, which would be highly detrimental to the purity of the product. The exact crude impurity level for this to occur was given by eq 22. When a SLIP 1 is in effect, the first step is to determine Vi,crit p , and hence locate the boundary between SLIP 1A and 1B. This is especially important at the end of the crystallization before filtration as it most likely constitutes a critical point. The crystallization process should then preferably operate above (in the SLIP 1B region) to avoid the impurity to become Vi,crit p supersaturated and potentially crystallize out. At these conditions 100% impurity rejection is possible in theory. If the crystallization is carried out within the SLIP 1A domain, it is important to determine the theoretical impurity rejection at equilibrium, and then evaluate whether this will be acceptable. Frequently there will be a trade-off between yield of the product and purity since the solubility of the product and impurity often trends similarly with the process conditions, e.g., solvent composition. In these cases, it is worthwhile to determine the optimal condition so that maximum yield is achieved while also meeting the purity target. A critical unit operation for SLIP 1 is filtration. In the initial filtration, a portion of the mother liquor will remain in the filter cake. Up to process volumes of 1−2 L/kg of the crystallization solvent can easily be left in the filter cake. If the final process volume is low (