Micronization of Copper Indomethacin Using Gas Antisolvent Processes

Ind. Eng. Chem. Res. 2002, 41, 1993-2004

1993

Micronization of Copper Indomethacin Using Gas Antisolvent Processes Barry Warwick, Fariba Dehghani, and Neil R. Foster* School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia

John R. Biffin† and Hubertus L. Regtop† Biochemical Veterinary Research Pty. Ltd., Braemar, New South Wales 2575, Australia

The nonsteroidal antiinflammatory drug Cu2(indomethacin)4L2 [Cu-Indo; L ) dimethylformamide (DMF)] was successfully micronized by the gas antisolvent technique (GAS) and aerosol solvent extraction systems (ASES) using DMF as the solvent and CO2 as the antisolvent. The effect of changing process variables on the size and morphology of the particles produced was investigated at temperatures between 25 and 40 °C, pressures between 6.6 and 14.5 MPa, and solute concentrations between 5 and 200 mg‚g-1. The most dominant variable was found to be solute concentration. As the Cu-Indo concentration was increased from 5 to 200 mg‚g-1, the morphology of the particles produced from the GAS process was changed from rhombic to bipyramidal. The particles obtained from the ASES process were changed from bipyramidal to spherical as the concentration was increased from 5 to 100 mg‚g-1. A further increase in the solute concentration to 200 mg‚g-1 resulted in large porous spheres when processing Cu-Indo by ASES. The nature of the solvent has a significant impact on the morphology of Cu-Indo in both GAS and ASES processes. The particle size of Cu-Indo produced by ASES was one-fifth of those produced by GAS. Spherical particles of Cu-Indo with diameters of less than 8 µm were formed at 24 °C and 6.89 MPa by processing solutions concentrated more than 20 mg‚g-1 using ASES. The immediate benefit of micronizing Cu-Indo was demonstrated with an eightfold increase in the dissolution rate in water compared with the unprocessed drug. Introduction The product quality of materials including explosives, catalysts, pigments, and pharmaceuticals can be significantly influenced by physical properties such as particle size distribution and morphology. The particle size is especially significant in the synthesis of pharmaceuticals because this characteristic can have a significant effect on the applicability of a particular delivery system and/or the pharmaceutical action of the drug. If the product obtained from the actual synthesis does not meet the stipulated requirements, postsynthesis processing such as ball milling or spray drying may be necessary for size reduction. Particles produced by ball milling commonly possess a broad particle size distribution. High temperatures associated with spray drying also make this method unattractive for the micronization of many pharmaceutical compounds. The gas antisolvent (GAS) and aerosol solvent extraction system (ASES) processes have been demonstrated to be effective techniques for the micronization of various materials.1 In these processes, dense gases, i.e., those that are near or above their critical point, generally with a reduced temperature and pressure between 0.9 and 1.2 are used as the antisolvent. In both methods, the organic solvent must be miscible with the dense gas at operating conditions, while the solute is insoluble (or * To whom correspondence should be addressed. Tel: (61) (2) 9385 4341. Fax: (61) (2) 9385 5966. E-mail: N.Foster@ unsw.edu.au. † Tel: (61) (2) 48713155. Fax: (61) (2) 48713161. E-mail: [email protected].

minimally soluble) in the dense gas. Carbon dioxide has been used commonly as the dense gas because it is inexpensive, less toxic than conventional solvents, and easy to remove, resulting in the production of microparticles with negligible or zero levels of residual solvent. GAS involves the gradual addition of a dense gas to expand a static solution until supersaturation occurs and the solute precipitates. Once the solute has precipitated, additional dense gas is added to remove the residual solvent and produce a dry precipitate upon depressurization. The second technique (ASES) involves spraying an organic solution via a nozzle into a flowing or static dense gas. As the solution is sprayed into the dense gas, high degrees of supersaturation result in the precipitation of fine particles. In general, precipitation using the dense GAS processes is rapid, requires mild operating pressures and temperatures, and does not require an additional solvent removal stage. The primary aim of this work was to determine the applicability of the GAS and ASES processes for the micronization of Cu2(indomethacin)4L2 [Cu-Indo; L ) dimethylformamide (DMF)], a novel nonsteroidal antiinflammatory drug. Micronized Cu-Indo offers a number of advantages over the particles produced from the conventional synthesis process. Cu-Indo is relatively insoluble in water, and micronization is expected to increase the dissolution rate of the drug in water. The enhancement in the dissolution rate increases the bioavailability of Cu-Indo and hence the possibility of decreasing the dose required for therapeutic response. A lower dose reduces the possibility of toxic side effects

10.1021/ie010760y CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002

1994

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002

and offers a commercial advantage of increased units for a quantity of drug. It has been demonstrated previously that the GAS process is an effective technique for the synthesis of CuIndo.2 The possibility of micronizing Cu-Indo during the synthesis and therefore eliminating the need for postsynthesis processing was of particular interest in this study. The GAS and ASES process variables that affect Cu-Indo particle formation were investigated. Conflicting results have been reported in the literature regarding the effect of process parameters on drug particles formed when using dense gases as antisolvents.1 These conflicts have arisen primarily when micronizing by ASES, which is indicative of the complexity of the particle formation mechanisms in operation during the process. Experimental Section Materials. Cu-Indo was provided by Biochemical Veterinary Research Co. (g90% purity). Copper(II) acetate and ethyl acetate (98% purity) were purchased from Sigma-Aldrich, isopropyl alcohol (99.9% purity) from Aldrich Chemical Co., and DMF (99.9% purity) from Burdick and Jackson. N-Methylpyrrolidone (NMP; 99.5% purity) and dimethyl sulfoxide (DMSO; 99.8% purity) were supplied from ISP Technologies Inc. and Ajax Chemicals, respectively. Carbon dioxide (BOC Gases Industrial Grade, 99.95% purity) was used as the antisolvent. All chemicals and reagents were used without further purification. Procedure. DMF is a poor solvent for Cu-Indo. The solutions used for the GAS and ASES experiments were prepared by different methods. The low-concentration solutions were prepared by dissolving Cu-Indo in pure DMF. The high-concentration solutions were made by mixing copper acetate and indomethacin (reactants) in a 1:2 molar ratio in DMF to synthesize Cu-Indo.2 CuIndo was formed in the presence of acetic acid during the synthesis. The presence of acetic acid enhances the solubility of Cu-Indo, therefore enabling dissolution of up to 200 mg‚g-1 of Cu-Indo in DMF. (a) GAS. The schematic diagram of the GAS process is shown in Figure 1a. Micronization by the GAS technique was conducted by charging the vessel (Jerguson sight gauge series no. 32) with 5-10 mL of solution. The vessel was then brought to the desired pressure by passing CO2 from the pump (Isco LC-500 syringe pump) through the filter (0.5 µm) from the bottom. Pressure monitoring was made possible by the use of a Druck pressure transducer (model DPI 260 ( 0.007 MPa). The temperature was controlled to within 0.1 °C by submerging the vessel in a constant-temperature water bath heated with a recirculation heater (Thermoline Unistat 130 water heater). The rate of pressurization was controlled by means of a needle valve (Whitey SS-41XS2). Two pressurization rates were conducted by increasing the pressure from atmospheric to the desired maximum over 40 and 1 min. A magnetic stirrer (∼50 rpm) was designed for the high-pressure vessel to ensure the solution was well mixed before and during sampling. Once the precipitation was completed, the liquid was removed at constant pressure by passing CO2 from the top of the vessel and purging the liquid through the filter. Cu-Indo that precipitated was washed and dried by removing the solvent at a constant pressure between 5.7 and 5.9 MPa and a CO2 flow rate of 2-4 mL‚min-1.

Figure 1. Schematic diagrams of (a) GAS and (b) ASES processes.

In each batch 200-400 mL of CO2 was used to minimize the residual solvent. The system was depressurized and a sample taken for analysis. (b) ASES. The schematic diagram of the ASES apparatus is shown in Figure 1b. Micronization by the ASES process was conducted by first pressurizing the vessel to the required pressure and then allowing the CO2 to flow through the vessel from the top. The CO2 flow rate was controlled by the needle valve located before the vent, until the pump was flowing at 4-5 mL‚min-1. After the system was at steady state, the solution was pumped into the precipitation vessel through the nozzle (10 cm long stainless steel tubes with internal diameters of 229 and 1020 µm) using the highperformance liquid chromatography pump (Waters 500). Once sufficient precipitate had been collected for analysis, the flow of solution was stopped, and the precipitate was washed with at least 200 mL of CO2 at a specified pressure. The volume of CO2 used for washing the precipitate was measured using the CO2 pump. After washing was complete, a small amount of CO2 was purged through the nozzle to remove any remaining solution. The reaction vessel was then depressurized, and samples of precipitate were taken for analysis. Particle morphology of the solid powders was determined using a scanning electron microscope (SEM; Hitachi S4500). The samples were mounted on metal plates and gold-coated using a sputter coater under vacuum. Particle size distributions of the powders were determined using laser diffraction (Mastersizer, Malvern, U.K.). The samples were suspended in isopropyl alcohol and sonicated for 1 min prior to analysis. X-ray diffraction (XRD) was used to compare the level of crystallinity between the powders formed. Measurements were made on a Siemans D-500 diffractometer using Cu KR radiation (λ ) 1.540 56 Å). Samples were

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1995 Table 1. Results and Experimental Conditions for Micronization of Cu-Indo from a DMF Solution by the GAS Process T/°C

C/mg‚g-1

expansionb

morphology

particle size/µm

25 25 25 25 40 40 25 25 25 25 25

5 5 5 5 5 5 200c 200c 100c 50c 20c

slowa rapida slow rapid slow rapid slow rapid slow slow slow

Rh Rh-BP Rh Rh Rh Rh BP BP BP BP BP

20-50 20-100 10-50 10-50 50 50 2-20 2-20 2-20 2-20 2-20

Rh: rhombic. BP: bipyramidal. a No stirring. b Slow expansion: 0-5.8 MPa over 40 min. Rapid expansion: 0-5.8 MPa over 1 min. c The solution was formed during the synthesis of Cu-Indo and contained acetic acid.

placed in aluminum sample holders and were scanned from 3 to 23° at a scanning rate of 2° min-1. (c) Dissolution Studies. In vitro dissolution studies were performed using the USP paddle method. A phosphate buffer solution (pH 6.3, 1 L) was used as the dissolution medium. The determinations were conducted at a rotational speed of 50 rpm and at a constant temperature of 37 °C. Samples containing the same amount of Cu-Indo were added into the dissolution medium simultaneously. Aliquots (≈4 mL) were withdrawn over certain time intervals and passed through a 0.45 µm filter. The amounts of Cu-Indo in the withdrawn samples were measured using UV spectrophotometry (Hewlett-Packard 8453). Results and Discussion Micronization of Cu-Indo by the GAS Process. The GAS process is essentially one of nucleation and growth of particles from a supersaturated solution. Any process parameter that changes the degree of supersaturation in the solution can influence the nucleation and growth rates and hence the particle characteristics. The process parameters that have been examined when micronizing Cu-Indo by the GAS process are expansion rate, stirring, temperature, solvent type, and solute concentration. The experimental conditions and results are listed in Table 1. (a) Effect of Expansion Rate and Stirring. The size of particles produced by micronization with GAS was dominated by the rate of expansion of the solution.3-6 The size of particles may then be reduced at high expansion rates because of the high levels of supersaturation generated, which results in rapid rates of nucleation and consequently little particle growth. The morphology of particles formed from the GAS process has also been reported to be affected by the rate of expansion of the solution.3,7-9 The relationship between the particle morphology and the rate of expansion of solution arises from the stability of the environment surrounding a growing particle. The uniformity of crystal morphology is dependent on the stability of the growth environment during expansion. A slow expansion rate provides a stable environment for crystal growth resulting in a uniformly crystalline solid. Conversely, a fast expansion rate results in a turbulent environment that results in the formation of particles with less crystalline nature. The effect of expansion rate was studied for the precipitation of Cu-Indo from a 5 mg‚g-1 DMF solution

Figure 2. SEM images of Cu-Indo particles produced by the GAS process at 25 °C from a 5 mg‚g-1 solution of Cu-Indo in DMF: (a) slow expansion; (b) rapid expansion.

by CO2 at 25 °C and 5.8 MPa. At a slow expansion rate, the solution was homogeneously expanded and rhombic crystals of Cu-Indo precipitated. As shown in Figure 2a, the precipitate consisted of uniform particles, and the sizes varied between 20 and 50 µm. The particle morphology was not affected when a stirrer was added to the precipitator, indicating that sufficient mixing was provided by sparging CO2 into the solution. The Cu-Indo particles generated from the rapid expansion were not uniform. The large rhombic crystals, approximately 100 and 20 µm bipyramidal crystals that were precipitated at a high expansion rate, are shown in Figure 2b. Inefficient mixing of the solution resulted in the temporary formation of a lower DMF/Cu-Indorich liquid phase and an upper CO2-rich liquid phase. At low pressures a precipitate was formed in the interface of two liquid phases because of the higher concentration of CO2 in this region than the bulk DMF solution and the high degree of supersaturation. The two liquid phases merged upon an increase in the pressure of the system. Further precipitation then occurred from the single expanded liquid phase. The Cu-Indo particles with a bipyramidal morphology are thought to occur at the interface between the two phases and the larger rhombic crystals from the system once the two phases merged. Similar observations have been reported in the literature when a liquid-liquid-phase separation occurs during the GAS process.10,11 The level of supersaturation

1996

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002

Figure 3. SEM images of Cu-Indo particles produced by the GAS process at 40 °C from a stirred 5 mg‚g-1 solution of Cu-Indo in DMF.

is thought to be one of the main factors that affect crystal habit.12,13 The change in habit with varying supersaturation levels arises from the change in the relative growth rates of the various crystal faces. At higher levels of supersaturation, the nucleation and growth of crystals will be more rapid and it is possible for a kinetically favored crystal form to develop. The significant difference between the particles produced with and without stirring was the degree of aggregation and the particle size distribution. Stirring prevented the formation of a second liquid phase. Upon addition of the stirrer to the system at high expansion rate, only rhombic particles with sizes between 10 and 50 µm were formed. Two distinct crystal types were formed in the agitated system. The particles produced were either single rhombic crystals of approximately 50 µm in size or clusters of rhombic crystals less than 10 µm in size. It is known that stirring reduces the level of supersaturation achievable in a liquid solution.14 The reduction in the level of supersaturation with agitation may explain the similarity in the crystals formed at slow and rapid expansion rates. The low level of supersaturation achievable with stirring is also evidenced by the fact that the thermodynamically stable rhombic crystal form was produced for both the slow and rapid expansion experiments. These results also confirm that the bipyramidal crystals were formed at the interface of the two liquid phases. (b) Effect of Temperature. An increase in the temperature is expected to have various effects on the crystallization of a compound. At a specific pressure the density of CO2 decreases as the temperature increases, which diminishes the CO2 solubility in DMF and hence lowers the degree of supersaturation in the solution. The solubility of Cu-Indo in DMF and the rate of crystal growth are, however, increased as the temperature increases.14 These effects imply that the degree of supersaturation decreases at higher temperatures, thus resulting in larger particle size formation. The effect of temperature on the precipitation of CuIndo from a 5 mg‚g-1 DMF solution with CO2 as the antisolvent at 7.5 MPa was examined. Examples of the particles collected are shown in Figure 3. Single rhombic crystals of approximately 50 µm were obtained from both slow and fast expansion rates at 40 °C. The particles produced were only slightly larger than those

Figure 4. SEM images of Cu-Indo particles produced by the GAS process at 25 °C: (a) 200 mg‚g-1 and (b) 50 mg‚g-1 solutions of Cu-Indo in DMF.

produced at 25 °C. The negligible effect of temperature on particle size has been observed in other GAS systems.6,13,15 The Cu-Indo particles produced at 40 °C suggest that the rate of nucleation is rapid enough to offset any increase in the particle growth rate. (c) Effect of Concentration. In terms of solvent usage and processing steps, it would be more economical to use higher concentration solutions. To determine the effect of increasing the concentration of Cu-Indo and the presence of acetic acid on the Cu-Indo particles produced from the GAS process, DMF solutions of CuIndo at concentrations of 20, 50, 100, and 200 mg‚g-1 were expanded with CO2 at 25 °C. Variation of the concentration of the solution had a significant impact on particle characteristics. At low concentration (5 mg‚g-1), rhombic particles were formed, while at higher concentration (20-200 mg‚g-1), bipyramidal crystals with particle sizes between 2 and 20 µm were formed. Examples of the Cu-Indo particles produced from the higher concentration of DMF solutions are shown in Figure 4. The particles produced from a 200 mg‚g-1 solution (Figure 4a) were smaller and more uniform than those precipitated from less concentrated solutions (Figure 4b). The change in crystal morphology from rhombic to bipyramidal, upon an increase in the concentration of Cu-Indo in DMF, can be explained in terms of the thermodynamic behavior of the system. Upon pressurization with CO2, the solutions of higher concentra-

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1997

tion formed a second liquid phase at approximately 4.8 MPa, which was stable even when the solution was stirred. The precipitation occurred in the upper liquid phase but not in the lower solute-rich phase. As the pressure was increased above 4.8 MPa, rapid nucleation occurred in the lower solute-rich liquid phase. After precipitation the two liquid phases merged. An analogous observation has been made for a number of other systems containing solute in high concentration.16,17 The formation of a second liquid phase is thought to be the result of intermolecular forces between the solute and solvent. The strong interactions decrease the free solvent available to interact with CO2, resulting in a second liquid phase forming at higher pressures. The hypothesis that the bipyramidal crystals are the kinetically favored morphology is supported by these highconcentration experiments. The formation of the second liquid phase results in CO2 being unable to dissolve in the solute-rich phase until the pressure reaches values where the Cu-Indo solutions of lower concentration underwent exponential expansion. As the pressure increases, a point is reached where the CO2-DMF interactions are favored over the DMF-Cu-Indo interactions. At this point DMF is extracted from the solute-rich phase, thus inducing a high level of supersaturation and the formation of a kinetically favored crystal form. In the case of Cu-Indo, this is the bipyramidal morphology. (d) Effect of Solvent. The effect of solvent on the size and morphology of Cu-Indo particles produced from GAS was examined. The solvents included NMP and DMSO. Carbon dioxide was added to the Cu-Indo solution until the pressure approached 5.8 MPa. The particles produced from both NMP (Figure 5a) and DMSO (Figure 5b) consisted of clusters of needlelike crystals ranging in size from 50 to 100 µm. Solvent type has been reported to have a dramatic effect on the morphology of particles produced by the GAS process.7,8,10 The differences in morphology can be attributed to the solvent-solute interactions. The solvent forms an integral part of the Cu-Indo molecular structure in that the solvent molecule occupies a coordination site on each copper atom. It would therefore be expected that the morphologies of the crystals would differ upon a change in the solvent. Micronization of Cu-Indo by the ASES Process. It has been proposed that the precipitation mechanism in an ASES system is a function of operating conditions.18 At subcritical conditions, the precipitation mechanism is based on the hydrodynamics of the process, atomization, and droplet formation. At supercritical conditions, when antisolvent and solvent are miscible, droplets are not formed and the rapid decrease in the surface tension around the jet results in the solvent spreading out in a manner similar to that of a gaseous jet. Nucleation and growth of particles occurs within this gaseous plume. Both mechanisms are supported by results obtained in the literature, and the mechanism in operation is system-dependent. Process parameters that determine the particle size depend on the particle formation mechanism. Adjusting process variables of the ASES technique and their effect on the particles produced can aid in determination of the dominant particle formation mechanism. The process parameters that have been examined for the micronization of Cu-Indo using the ASES technique

Figure 5. SEM images of Cu-Indo particles produced by the GAS process at 25 °C from a stirred 5 mg‚g-1 solution of Cu-Indo in (a) NMP and (b) DMSO.

include the solution flow rate, temperature, nozzle diameter, solvent type, and antisolvent density. (a) Effect of Concentration. As mentioned previously, two types of solutions were examined: pure CuIndo solutions and synthesis solutions. The results obtained from the ASES process for both types of solutions are listed in Table 2. In general, the Cu-Indo particles produced from the ASES process from DMF solutions were a quarter of the size of those produced from the GAS process. Increasing the concentration of Cu-Indo in DMF from 5 to 200 mg‚g-1 results in a dramatic change in the particle size and morphology. The Cu-Indo particles produced from solutions at a concentration of 5 mg‚g-1 (Figure 6a) had a bipyramidal morphology and were less than 5 µm in size. The Cu-Indo particles produced from solutions at a concentration of 20 mg‚g-1 (Figure 6b) had a predominantly spherical morphology with diameters of less than 5 µm. Some particles with a bipyramidal morphology and similar particle size were also evident among the particles with a spherical structure. The Cu-Indo particles produced from solutions with a concentration of 100 mg‚g-1 (Figure 6c) were spherical with diameters of less than 5 µm. At a Cu-Indo concentration of 200 mg‚g-1 (Figure 6d), the particles produced were spherical with diameters ranging from 20 to 50 µm. Further examples of the particles produced from 200 mg‚g-1 solutions are shown in Figure 7. In some cases, the surfaces of the spheres were made up of smaller

1998

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002

Table 2. Results and Experimental Conditions for Micronization of Cu-Indo Using the ASES Techniquea C/mg‚g-1

T/°C

P/MPa

morphology

size/µm

5 5 5 5 5 20b 20b 20b 20b 100b 100b 100b,c 100b,d 100b 100b 100b 200b 200b 200b

25 25 25 40 40 25 25 25 40 25 25 25 25 25 25 40 25 25 25

6.89 6.89 13.79 14.48 14.48 6.89 10.34 6.89 14.48 6.89 13.79 6.89 6.89 6.89 6.55 14.48 6.89 10.34 6.89

BP BP BP BP BP S + BP S + BP S + BP BP S S S BP S S IS S + BP S + BP S + BP