Cocrystal solubility advantage diagrams as a means to control

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Cocrystal solubility advantage diagrams as a means to control dissolution, drug supersaturation and precipitation Yaohui Huang, Gislaine Kuminek, Lilly Roy, Katie L. Cavanagh, Qiuxiang Yin, and Naír Rodríguez-Hornedo Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.9b00501 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Molecular Pharmaceutics

Cocrystal solubility advantage diagrams as a means to control dissolution, supersaturation and precipitation

Yaohui Huang1, Gislaine Kuminek2, Lilly Roy2,3, Katie L. Cavanagh2, Qiuxiang Yin1, Naír Rodríguez-Hornedo2* 1

School of Chemical Engineering and Technology, Key Laboratory for Modern Drug

Delivery and High Efficiency, Tianjin University, Tianjin 300072, People’s Republic of China 2

Department of Pharmaceutical Sciences, College of Pharmacy, University of Michigan,

Ann Arbor, Michigan 48109-1065, United States 3

Present address: Vertex Pharmaceuticals Inc., Boston, MA 02205

* Corresponding author.

ACS Paragon Plus Environment

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Molecular Pharmaceutics 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

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ABSTRACT Cocrystals are often more soluble than needed and pose unnecessary risks for precipitation of less soluble forms of the drug during processing and dissolution. Such conversions lead to erratic cocrystal behavior and nullify the cocrystal solubility advantage over parent drug (SA = Scocrystal/Sdrug). This work demonstrates a quantitative method for additive selection to control cocrystal disproportionation based on cocrystal SA diagrams.

The tunability of cocrystal SA is dependent on the selective drug

solubilizing power of surfactants (SPdrug = (ST/Saq)drug). This cocrystal property is used to generate SA - SP diagrams that facilitate surfactant selection and provide a framework for evaluating how SA influences drug concentration-time profiles associated with cocrystal dissolution, drug supersaturation and precipitation (DSP). Experimental results with indomethacin-saccharin cocrystal (IND-SAC) and surfactants (sodium lauryl sulfate, Brij, and Myrj) demonstrate the log-linear relationship characteristic of SA - SP diagrams and the dependence of σmax and dissolution AUC on SA with characteristic maxima at a threshold supersaturation where drug nucleation occurs. This approach is expected to streamline cocrystal formulation as it facilitates additive selection by considering the interplay between thermodynamic (SA) and kinetic (DSP) processes. Keywords: Dissolution, supersaturation, precipitation, cocrystals, solubility enhancement, solubilizing agent, surfactant


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INTRODUCTION Cocrystals have gained much interest in a range of scientific fields, including chemistry, crystal engineering, materials engineering, and pharmaceutical sciences due to their ability to improve physicochemical and biopharmaceutical properties of drugs, especially solubility.1-4 Cocrystals are composed of two or more different molecular components in a crystal lattice with a well-defined stoichiometry. They are distinguished from solvates in that the cocrystal components are solids at room temperature.5-7 Cocrystal formation usually involves hydrogen bonding between a neutral drug molecule and a benign molecule or coformer.8, 9 In terms of drug molecules, which cannot be ionized or form polymorphs or solvates, cocrystal formation provides an alternative method to improve pharmaceutical properties.10-12 Solubility is one of the most important properties for oral drug absorption. Drug dissolution rate and bioavailability can be increased by enhancing solubility when the drug absorption process is limited by dissolution.13,

14

Cocrystal formation has been

demonstrated to increase drug solubility by orders of magnitude.15,

16

However, this

cocrystal solubility advantage over drug (SA=Scocrystal/Sdrug) presents a risk for precipitation to less soluble forms of the drug. Determining the precise additive(s) and concentration(s) that will slow drug precipitation and maintain desired drug supersaturation levels is difficult and subtle. Additive selection has generally relied on a case-by-case basis by using surfactants and polymers to control cocrystal dissolution, drug supersaturation and precipitation (DSP).1721

We have shown that cocrystal SA is dependent on the preferential drug solubilization

by additives,22-24 and hypothesize that this cocrystal property enables the rational selection of additives to control DSP, since SA represents the driving force for drug precipitation during cocrystal dissolution. In this work, we propose a method for additive selection to control cocrystal disproportionation based on cocrystal SA diagrams. The mathematical basis for the SA and additive solubilizing power (SP) diagrams has been demonstrated.24 Based on this knowledge, SA can be fine-tuned to a desired value by rationally selecting drug solubilizing agents and concentrations.25, 26


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Our experimental results with indomethacin-saccharin cocrystal (IND-SAC) demonstrate how SA-SP diagrams facilitate additive selection and provide a framework for evaluating drug concentration versus time profiles associated with cocrystal DSP behavior. IND-SAC has an SA value of 25 that results in fast drug precipitation in pH 2.1 buffer.27 To determine the influence of drug solubilizing agents on SA and the utility of SA in anticipating cocrystal to drug conversions during dissolution, we studied three surfactants used in pharmaceutical applications: sodium lauryl sulfate (SLS), Myrj52, and Brij99. MATERIALS AND METHODS Theoretical basis The molar solubility of a 1:1 cocrystal as a function of ionization and solubilization of its components is given by an equation of the general form 𝑆!"!#$%&'(,! =

𝐾!" 𝛿!"#$,! 𝛿!"#"$%&$,!

(1)

where Ksp is the product of the free drug and coformer molar concentrations at equilibrium, [drug][coformer]. δT represents the total (T) contributions of ionization (I) and solubilization (S) on cocrystal solubility for both drug and coformer, where δdrug,T = δdrug,I + δdrug,S and δcoformer,T = δcoformer,I + δcoformer,S.28, 29 Table 1 shows the expressions for δ in terms of the respective equilibrium constants (ionization constant Ka, and solubilization constant Ks) pH, and solubilizing agent concentration [M])26,

30

for a

cocrystal of monoprotic acidic components such as IND and SAC. For surfactants, [M] corresponds to the micellar concentration obtained from the difference between the surfactant total concentration and the critical micellar concentration (CMC) given by [M] = [surfactant]T - CMC. The molar drug solubility can be calculated from the general equation 𝑆!"#$,! = 𝑆!,!"#$ 𝛿!"#$,!

(2)


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Table 1. Ionization (δI) and solubilization (δS) terms for 1:1 cocrystal with monoprotic acidic components used to calculate cocrystal solubility, drug solubility, and solubility advantage according to Equations 1-6. δI

δS

δT = δI + δS

1 + 10!"!!"!,!"#$

𝐾!,!"#$ 𝑀

1 + 10!"!!"!,!"#$ + 𝐾!,!"#$ 𝑀

1 + 10!"!!"!,!"#"$%&$

𝐾!,!"#"$%&$ 𝑀

1 + 10!"!!"!,!"#"$%&$ + 𝐾!,!"#"$%&$ 𝑀

Monoprotic Acidic Drug Monoprotic Acidic Coformer

SA is defined as the molar ratio of cocrystal to drug solubilities in the same experimental conditions, such as solvent, pH, additive concentration, and temperature. SA is given by the ratio of Equations 1 to 2 as

𝑆𝐴 =

!!"!#$%&'(,! !!"#$,!

=

!!" !!"#$,! !!"#"$%&$,! !!,!"#$ !!"!",!

(3)

The above equation can be expressed in terms of SAaq and drug solubilizing power of an additive (SPdrug) by substitution of

𝐾!" = 𝑆!"!#$%&'(,!"

!

(4)

and !

!"#$,! (5) 𝑆𝑃!"#$ = ! !"#$,!"

to give

𝑆𝐴 =

!"!"

!!"#"$%&$,!

!"!"#$

!!"#"$%&$,!

(6)


5


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SAaq describes the SA in aqueous media without additive solubilization.

SAaq is

dependent on pH and temperature. Sdrug,T is the sum of the concentrations of all species dissolved (Sdrug,T = Sdrug,aq + Sdrug,M). Sdrug,aq is the drug aqueous solubility at a given pH in the absence of solubilizing agent and is the sum of the un-ionized and ionized contributions to the aqueous solubility (Sdrug,aq = Sun,aq + Sion,aq). Sdrug,m represents the drug solubilized by micelles (Sdrug,M = Sun,M + Sion,M) and contributions from the unionized and ionized species as appropriate. When coformer solubilization by additives is negligible, Equation 6 becomes

𝑆𝐴 =

!"!" !"!"#$

(7)

In aqueous media or when there is no drug solubilization by additive, SPdrug = 1 and SA = SAaq. Equation 7 is applicable to cocrystals with hydrophobic drugs and hydrophilic coformers, as with those in this study. Experimental measurements of IND and SAC in our laboratory confirm the validity of this assumption.31

When coformer solubilization

is not negligible then equation 6 should be used. The influence of coformer solubilization on SA is the subject of a subsequent manuscript. Equation 7 clearly suggests that cocrystal SA is fine-tunable by varying SPdrug via drug solubilizing agents and their concentrations. Additives are not limited to surfactants but include polymers, complexing agents, lipids, and other additives that preferentially solubilize drug over coformer. Generation of cocrystal SA - SP diagrams (log(SA) vs log(SPdrug)) is based on the log-linear plot of log(SA) vs log(SPdrug) according to the logarithmic form of equation 7:

𝑙𝑜𝑔 𝑆𝐴 = 𝑙𝑜𝑔 𝑆𝐴!" −

! !

𝑙𝑜𝑔 𝑆𝑃!"#$

(8)

Knowledge of any cocrystal SA value and a log-linear slope of -1/2 allows for the generation of SA-SP diagrams for a 1:1 cocrystal.


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Materials. Indomethacin γ-form (IND) and SAC were purchased from Sigma Chemical Company (St. Louis, MO) and used as received. IND polymorph γ was confirmed by differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). Polyoxyethylene (20) oleyl ether (Brij99®), sodium lauryl sulfate (SLS) and polyoxyethylene (40) stearate (Myrj52®) were also purchased from Sigma Chemical Company (St. Louis, MO). High performance liquid chromatography (HPLC) grade acetonitrile and monosodium phosphate were purchased from Fisher Scientific (Fair Lawn, NJ). Phosphoric acid was purchased from Acros Organics (NJ). Water used in this study was filtered through a double deionized purification system (Milli Q Plus Water System) from Millipore Co. (Bedford, MA). Media Preparation. Phosphate buffer at pH 2.1 ± 0.1 was prepared by dissolving 17.2 g monosodium phosphate (NaH2PO4·H2O) in 7.3 mL of 85 % phosphoric acid (H3PO4) and deionized water to prepare 1 L of 0.2 M buffer. Brij 99, SLS, or Myrj 52 was dissolved in the buffer to make different concentrations of media.

All dissolution media were

adjusted to target pH with 1 M NaOH and 85 % phosphoric acid. Cocrystal Synthesis. IND-SAC cocrystal was prepared by reaction crystallization32 at room temperature. 1:1 molar ratio of IND (1.1985 g) and SAC (0.6181 g) was added to 0.05 M SAC solution in ethyl acetate (10 mL). Solid phases were characterized by PXRD and DSC, and the stoichiometries were verified by HPLC. Drug solubility and surfactant solubilizing power, SPdrug. IND γ solubility was measured by adding excess solid to pH 2.1 phosphate buffer with a pre-dissolved concentration of surfactants. Suspensions were magnetically stirred and kept at 25.0 ± 0.2 °C using a water bath for 72−96 h. Solution concentrations were measured using HPLC, and solid phases were analyzed by DSC and PXRD.

SPdrug values were

calculated from equation 5 using experimentally measured drug solubilities in the presence of surfactant (Sdrug,T) and previously reported aqueous drug solubility at pH 2.1 (Sdrug,aq).27


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Eutectic concentrations and cocrystal solubility advantage (SA). Cocrystal SA was experimentally determined from measurement of eutectic drug and coformer concentration, [drug]eu and [coformer]eu, in solutions that are doubly saturated with cocrystal and drug according to

𝐾!" =

!"#"$%&$ !" !"#$ !"

=

!!"!#$%&'( !!"#$

!

= 𝑆𝐴

!

(9)

where Keu is the eutectic constant29 and the terms in brackets refer to molar concentrations. Concentrations and solubilities are the analytical concentrations and total solubilities under the solution conditions studied (pH, solubilizing agents, solvent, temperature). We have demonstrated the utility of this relationship for several cocrystals as a function of pH27, 29, 30 and drug solubilization.25, 31 The eutectic point between cocrystal and drug solid phases in equilibrium with solution was reached by suspending 50−100 mg of cocrystal and 25−50 mg of drug in 3 mL of aqueous buffer pH 2.1 and in the presence of drug solubilizing agents (Brij 99 concentrations of 0.3, 2, 3, 10, 20 and 36 mM, 0.4 mM Myrj 52 and 8 mM SLS). Suspensions were stirred until equilibrium was reached (72−96 h) at 25 ± 0.2 °C. Equilibrium solution concentrations of drug and coformer at eutectic point ([drug]eu and [coformer]eu) were analyzed by HPLC. Solid phases at equilibrium were confirmed by PXRD. Cocrystal SA values were determined from the ratio of [drug]eu and [coformer]eu according to the Keu equation (Equation 9). Cocrystal and Drug Powder Dissolution.

9 mg IND-SAC or 6 mg IND were

suspended in 30 mL of dissolution media. Both drug and cocrystal powders were sieved through mesh screens, and particle sizes between 106 and 125 µm were used. The resulting slurry was stirred by an overhead stirrer with a glass propeller at 150 rpm over 3 h at 25.0 ± 0.2 °C. Aliquots of 0.5 mL were withdrawn at appropriate time points for up to 180 min and filtered through a 0.45 µm PVDF syringe filter. The solution concentrations of drug and coformer were analyzed with HPLC.


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High Performance Liquid Chromatography (HPLC). IND and SAC concentrations were analyzed by a Waters HPLC equipped with a UV spectrometer detector. A C18 Thermo Electron Corporation (Quebec, Canada) column (5 µm, 250 × 4.6 mm) was used for separation at ambient temperature. Waters’ operation software Empower 2 was used to collect and process data. The mobile phase composed of 70 % acetonitrile and 30 % water with 0.1 % trifluoroacetic acid. The flow was set at 1 mL/min, and the sample injection volume was 20 µL. Absorbance of IND and SAC was monitored at 265 nm and the retention times were 7.2 and 3.5 minutes for IND and SAC. Powder X-ray Diffraction (PXRD). X-ray powder diffractograms of solid phases were recorded on a benchtop Rigaku Miniflex X-ray diffractometer (Danvers, MA) using Cu Kα radiation (λ = 1.54 Å), a tube voltage of 30 kV, and a tube current of 15 mA. Data was collected between 5° and 40° in 2θ at a continuous scan rate of 2.5°/min. Thermal Analysis.

Solid phases were analyzed by DSC using a TA Instruments

(Newark, DE) 2910MDSC system equipped with a refrigerated cooling unit. Thermal measurements were performed by heating the samples at a rate of 10 °C/min under a dry nitrogen atmosphere. A high purity indium standard was used for temperature and enthalpy calibration. Standard aluminum sample pans were used for all measurements. Inverted Light Microscopy. IND-SAC cocrystal dissolution and drug precipitation behavior was studied by bright field inverted microscopy using a Leica DMi8 microscope. 200 µL aliquots of reaction media were transferred to 96-well plates and examined for cocrystal dissolution and drug nucleation mechanisms (surface and bulk nucleation). RESULTS AND DISCUSSION Quantitative predictive model of cocrystal SA dependence on surfactant solubilizing power (SP). Figure 1 shows the SA-SP diagram for IND-SAC cocrystal predicted according to equation 8


9


𝑙𝑜𝑔 𝑆𝐴 = 𝑙𝑜𝑔 𝑆𝐴!" −

! !

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𝑙𝑜𝑔 𝑆𝑃!"#$ .

Experimental SA values were obtained from Keu measurements according to Equation 9. SP values were determined from Equation 5. Both Keu and SP were determined in aqueous solutions of surfactants (Brij 99, Myrj 52, and SLS). The predicted log-linear dependence was calculated from the experimentally determined Keu of 642, which corresponds to a cocrystal aqueous solubility advantage SAaq of 25 at pH 2.1 in the absence of drug solubilization, and the theoretical slope of -1/2 for 1:1 cocrystals. Scocrystal (0.072 mM) and Sdrug (0.00285 mM) were previously reported by our group.27

Figure 1. Cocrystal SA - SP diagram showing the log-linear dependence of solubility advantage (SA) on drug solubilization power (SPdrug) predicted from equation 8. Symbols represent SA and SPdrug values determined from experimental measurements of (1) drug and coformer eutectic concentrations, and (2) drug solubility in buffer with and without surfactant, as follows: buffer (black x), Brij 99 (green triangles), Myrj 52 (pink circle), and SLS (purple square). Numbers by the symbols correspond to surfactant mM concentrations. Dashed arrows indicate SA, SP, and surfactant concentrations used in the dissolution studies. Error bars (within the points) represent standard deviations of SA and SPdrug. The results show that IND-SAC cocrystal SA decreased from 25 to values less than 1 by increasing the drug solubilization power of surfactants.


Furthermore, the

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dependence of SA on SPdrug appears to be independent of the surfactant, as shown for non-ionic (Brij99 and Myrj 52) and ionic (SLS) surfactants in this work. Studies with other drug solubilizing agents confirm this behavior and are the subject of another publication (in preparation). The purpose of the present study was to determine the relationship between cocrystal SA and DSP behavior.

We used the SA-SP diagram in

Figure 1 to fine-tune SA by surfactant selection and assess DSP behavior. Cocrystal SA values were determined from experimental measurements of drug and coformer concentrations at the cocrystal/drug eutectic points in three surfactants as a function of drug solubilization.

The ratio of coformer to drug molar concentrations at

the eutectic point is referred to the eutectic constant, Keu, which has been found to be a key cocrystal stability indicator.25, 27, 29 A cocrystal is at a transition point when its Keu value is equal to that of its stoichiometry (coformer and drug ratio in the cocrystal). We have derived the mathematical relationship between cocrystal SA and Keu demonstrated its utility for cocrystals of different stoichiometries.27,

29

29

and

For a 1:1

cocrystal, the relationship is given by Equation 9 in the methods section as

𝐾!"

𝑆!"!#$%&'( 𝑐𝑜𝑓𝑜𝑟𝑚𝑒𝑟 !" = = 𝑑𝑟𝑢𝑔 !" 𝑆!"#$

!

= 𝑆𝐴

!

Figure 2 shows the experimental values and predicted behavior for IND-SAC cocrystal according to the log-linear representation of equation 9 𝑙𝑜𝑔 𝐾!" = 2 𝑙𝑜𝑔 𝑆𝐴

(10)

These results demonstrate that Keu and SA decrease as surfactant concentration increases, in agreement with the predicted behavior, and become equal to 1 at the transition point when Scocrystal = Sdrug.


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Figure 2. Predicted and experimental Keu and SA values for IND-SAC. Keu and SA decrease with increasing surfactant concentration as drug solubilization increases. Line was generated according to log(Keu) = 2 log(SA). Symbols represent experimentally determined Keu and SA values. Numbers by the symbols correspond to surfactant mM concentrations. Figure 3 presents a flowchart of the steps required to generate cocrystal SA-SP diagrams for the intelligent selection of additives so that SA provides a basis to achieve optimal DSP behavior. Based on Keu, SA, and SP calculations (Equations 5, 7 and 9) and their relationships (Figures 1 and 2), additives can be rationally selected to control drug kinetic supersaturation during cocrystal dissolution.


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Figure 3. Flowchart to generate cocrystal SA-SP diagrams for the rational selection of drug solubilizing agents to control DSP behavior. The solid phases at the eutectic point are (1:1) cocrystal and drug. Since permeability and absorption of single component drug phases from the gastrointestinal tract are reduced by drug solubilizing agents,33-35 it is important to consider their influence on cocrystals. Drug solubilizing agents increase the total solubility by increasing the bound drug aqueous concentrations, which is not available for absorption. Cocrystal superior solubility over drug is due to increasing the free drug concentration, which is available for absorption.

The presence of drug solubilizing

agents reduces this free drug concentration, but the cocrystal will maintain a higher free drug concentration compared to drug as long as SA is greater than 1. In other words, drug solubilization softens an unnecessarily high cocrystal SA while still providing superior drug exposure and drug dose dissolution. Cocrystal SA and DSP behavior. Figure 4 demonstrates that there is a parabolic-like dependence of cocrystal DSP parameters on SA. The cocrystal DSP kinetic parameters include maximum or highest supersaturation (σmax) and relative areas under the dissolution curves (RAUC =


13


AUCcocrystal / AUCdrug).

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This parabolic behavior indicates that there is an optimal SA

value associated with the highest σmax and RAUC values. At the highest SA of 25, in buffer without drug solubilization, σmax exhibited the greatest departure from SA and the lowest RAUC due to fast drug precipitation. Photomicrographs presented in the next section demonstrate the nucleation pathways. SA decreased from 25 to 2 by addition of drug solubilizing agents, and DSP parameters reached maximum values at an SA of 10. This SA value with two different surfactants, SLS (1.7 mM) and Brij 99 (0.3 mM), resulted in similar σmax values of 7 and higher RAUC value for SLS than for Brij, suggesting that SLS is a more effective drug precipitation inhibitor at this SA.

Figure 4. Influence of cocrystal SA on DSP behavior, σmax and RAUC= AUCcocrystal/AUCdrug. There is a critical SA value at which σmax and RAUC achieve their highest levels. SA ≤ 4 sustained the indicated drug supersaturations without a maximum in the dissolution curve as shown in Figure 9. The observed DSP dependence on SA follows the nucleation rate dependence on supersaturation,36, 37 where spontaneous nucleation occurs above a threshold or critical supersaturation. Below this supersaturation level there is no instantaneous nucleation and supersaturation levels are sustained. Above the critical supersaturation, drug nucleation


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limits the achievable kinetic supersaturation levels as observed by decreases in σmax and RAUC as SA increases. The critical supersaturation for IND cocrystals and amorphous systems has been reported to be 4 to 12,17, 38-44 which is in agreement to the values observed in our studies. How SA relates to kinetically achievable supersaturation levels and the utility of SA diagrams has to our knowledge not been exploited for kinetic control of cocrystal DSP behavior. Results in Figs. 1, 2 and 4 validate the power of these relationships to predict the thermodynamic cocrystal SA in the presence of drug solubilizing agents and to assess their influence on kinetic DSP behavior. Additives that modulate nucleation have dual functionality: (1) thermodynamic and (2) kinetic.45-47 The thermodynamic functionality is determined from the SA-SP relationship, while the kinetic functionality is determined from DSP-SA plots and supersaturation-time profiles presented in Figure 9. This thermodynamic functionality criterion in relation to SA and SP is a unique property of cocrystals that does not apply to other non-stoichiometric solubility-enhancing solid forms such as amorphous solid dispersions. Nucleation mechanisms and SA. Figure 5 shows the influence of SA on cocrystal dissolution, which is represented by final percentage cocrystal dissolved, and precipitation, which is represented by the final ratio between SAC and IND molar concentrations. Percentage cocrystal dissolved was calculated from the moles of cocrystal dissolved relative to that initially added. Mass of dissolved cocrystal was obtained from the measured dissolved coformer concentrations since solutions are undersaturated with respect to coformer in these studies. With decreasing SA, percent cocrystal dissolved was found to increase, whereas [SAC]/[IND] decreased. This ratio represents the extent of IND precipitation, since only IND not SAC can precipitate. There was no detectable drug precipitation at SA ≤ 4, since [SAC]/[IND] was equal to 1. The extent of drug precipitation was observed to increase, [SAC]/[IND] of 4 to 29, with SA values from 10 to 25. These findings provide additional insight to the DSP results in Figure 4, as it appears that relative exposure at SA 2 and 4 is limited by low cocrystal SA where there is no drug precipitation, whereas relative exposure at SA ≥ 10 was limited by drug precipitation.


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Cocrystal dissolution and drug nucleation and growth were investigated by microscopy at three SA values: 25 (pH 2.1 buffer), 10 (0.30 mM Brij99) and 2 (7.78 mM Brij99).

Figure 6 shows photomicrographs of samples withdrawn from the reaction

media at various times. At SA of 25, drug precipitation and growth mainly occurs on the cocrystal surface (final [SAC]/[IND] = 29). These clusters are not well organized and appear to coat the cocrystal, hindering its dissolution. The percent cocrystal dissolved at 3 hours was the lowest (30 %) shown in Figure 5, which supports the impediment of dissolution by drug surface nucleation. At SA of 10 where final [SAC]/[IND] = 4, drug precipitation was observed both on the particle surface and in the bulk medium.

Bulk

nucleation was higher than that observed at SA 25. This may be a result of higher drug supersaturation in solution where indomethacin nucleation can occur. At SA of 2 no drug precipitation was visually observed at the 3h dissolution final time point ([SAC]/[IND] = 1) and cocrystal dissolved was 84% .

Figure 5. Influence of SA on the ratio of dissolved [SAC]/[IND] molar concentrations, and the percent cocrystal dissolved at the end of 3 hour dissolution studies with and without Brij 99. The [SAC]/[IND] ratio represents the extent of IND precipitation and is equal to 1 when there is no drug precipitation at SA values less than or equal to 4. IND precipitation increases with SA. The percent of cocrystal dissolved decreases with SA reaching a steady value of 30% at SA>10. The % cocrystal dissolved was calculated !"#$% !"# !"##$%&'! from !"!#!$% !"#$% !" !"!#$%&'( !""#" 100. Lines were drawn between data points for clarity.


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Figure 6. Microscope images of cocrystal DSP behavior in pH 2.1 buffer and buffer with 0.30 mM Brij99 (SA=10) and 7.78 mM Brij99 (SA=2). Y axis represents cocrystal SA and DSP media. X axis represents the time after cocrystal was suspended in DSP media. Influence of SA on drug concentration- and supersaturation-time profiles during cocrystal dissolution. Cocrystals induce faster and higher dissolved drug concentrations compared to drug crystal, as shown in Figures 7 and 8. SA appears to be an important parameter in determining cocrystal dissolution and precipitation behavior, as indicated by the dissolved drug concentration-time profiles and the shape of the curves. At low SA (≤ 4) values, cocrystal dissolution curves are characterized by reaching and sustaining the highest dissolved drug concentrations (there is no maximum in the curve). At higher SA values (≥ 10), the dissolution curves exhibit a maximum that is sustained for some time before drug nucleation decreases the dissolved drug levels. This behavior suggests that as SA increases, cocrystal dissolution and drug supersaturation are dampened by drug


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nucleation. The observed behavior is complemented with the findings from microscopy studies that relate the drug nucleation mechanisms to cocrystal SA. The supersaturation profiles were quantified in terms of the RAUC as discussed earlier and presented in Figure 4. (a)

(b)

Figure 7. Influence of SA on the drug concentration-time profiles during dissolution of (a) cocrystal IND-SAC and (b) drug IND. SA was modulated from 25 to 2 by increasing the surfactant concentration. (b)

(a)

Figure 8. Influence of SA on the concentration-time profiles during dissolution of (a) cocrystal IND-SAC and (b) drug IND. Different surfactants and concentrations were selected to achieve SA values in a range of 10 to 12. Figure 9 shows the drug supersaturation-time profiles for the purpose of examining how the cocrystal dissolution and drug nucleation interplay generates the observed drug supersaturations.

The driving force for drug nucleation is the

supersaturation; thus, one can determine the threshold supersaturation, the extent that it is


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sustained, or if it decreases and at what rate.

The highest (σmax = 7.4) and most

sustainable supersaturations were observed at SA of 12. Supersaturations of about 7 were reached by 40 minutes and sustained at this level until 60 minutes before decreasing to a supersaturation of 3 and sustaining this value for the length of the study, 3 hours. SA less than or equal to 4 did not show a maximum in the supersaturation-time curves, as this value is below the threshold supersaturation for IND. While under these conditions the supersaturations are lower, the dissolved drug concentration levels are higher. Since the drug solubility increases with decreasing SA, there is a lower driving force for drug nucleation and cocrystal conversion is less favorable. (a)

(b)

Figure 9. Influence of SA on the drug supersaturation-time profiles during cocrystal dissolution. SA=25 in the absence of surfactant. (a) Brij 99 at various concentrations that correspond to SA values between 2 and 15 and (b) in different surfactants with SA range of 10 to 12. The dashed line represents a drug saturated solution, SA=1. The selection of a target SA for a given cocrystal should be based on the drug dose and its solubility. The dose number (D0) is well recognized as a unitless parameter to assess the extent to which the dose dissolves. 48 The dose will fully dissolve when D0 is ≤ 1. Dose number is defined as 𝐷! =

!! /!!

(11)

!!

where M0 is the dose (mmol), V0 is the volume (L) taken with dose or luminal volume (0.25 L), and ST is the drug or cocrystal solubility (mM). The dose of IND is 50 mg (0.14 mmoles), ST is 0.00285 mM in pH 2.1, and D0 is calculated to be 196. This means that IND solubility would have to be increased by a factor of 196 to fully dissolve.


The

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Page 20 of 25

cocrystal solubility is higher than drug solubility, but there is a compromise between a very high SA and the kinetically achievable supersaturation. Therefore, based on the results presented here, an SA of 10 for the cocrystal is able to reach and sustain supersaturation. Cocrystal formulations can then be designed to enhance bioavailability of poorly soluble drugs by considering the approaches presented here, with or without addition of precipitation inhibitors in addition to drug solubilizing agents. Conclusion In this work, we propose a quantitative method that enables cocrystal SA tunability and control of cocrystal DSP based on the solubilizing power of additives. Our experimental results with IND-SAC cocrystal demonstrate how SA-SP diagrams facilitate additive selection and provide a framework for evaluating drug concentration versus time profiles associated with cocrystal DSP behavior. Notes The authors declare no competing financial interest. Acknowledgment Research reported in this publication was partially supported by the Upjohn Award from the College of Pharmacy, University of Michigan, Tianjin Municipal Natural Science Foundation (No.11JCZDJC20700), and the National Institute of General Medical Sciences of the National Institutes of Health under award number R01GM107146. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health, and other funding sources.

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For Table of Contents Use Only


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Cocrystal solubility advantage diagrams as a means to control

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