- Email: [email protected]
Solubility and Precipitation of Nicotinic Acid in Supercritical Carbon Dioxide
Solubility and Precipitation of Nicotinic Acid in Supercritical Carbon Dioxide M. REHMAN,1 B. Y. SHEKUNOV,1 P. YORK,1,2 P. COLTHORPE3 1
Drug Delivery Group, School of Pharmacy, University of Bradford, Bradford, BD7 1DP, United Kingdom
2
Bradford Particle Design Ltd., 69 Listerhills Science Park, Campus Road, Bradford, BD7 1HR, United Kingdom
3
AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, Leics, LE11 5RH, United Kingdom
Received 21 November 2000; revised 5 April 2001; accepted 17 April 2001
ABSTRACT: Solubilities of a model compound (nicotinic acid) in pure supercritical carbon dioxide (SC-CO2) and SC-CO2 modi®ed with methanol have been measured in the pressure range of 80±200 bar and between temperatures of 35 and 908C. On-line ultraviolet detection enabled a simple and relatively fast measurement of very low levels of solubility (10ÿ7 mol fraction) with good accuracy in pure and modi®ed SC-CO2. The solute solubility in both pure SC-CO2 and SC-CO2 modi®ed with methanol increased with pressure at all investigated temperatures. A retrograde solubility behavior was observed in that, at pressures below 120 bar, a solubility decrease on temperature increase occurred. Solubility data were used to calculate supersaturation values and to de®ne optimum operating conditions to obtain crystalline particles 1±5 mm in diameter using the solution-enhanced dispersion by supercritical ¯uids (SEDSTM) process, thereby demonstrating the feasibility of a one-step production process for particulate pharmaceuticals suitable for respiratory drug delivery. ß 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:1570±1582, 2001
Keywords: antisolvent precipitation; nicotinic acid; supercritical CO2; solubility; particle formation; SEDSTM
INTRODUCTION Solubility of a drug in the chosen supercritical ¯uid is a critical parameter for the design and optimization of all particle formation supercritical ¯uid (SCF) processes. Because solubility behavior plays a key role in de®ning the rate, yield, and economic feasibility of any SCF process, it is important to gain knowledge of solubility at an early stage in any process development. Whereas a number of large-scale commercial SCF extraction processes are being used, SCF particle technology is relatively in its infancy. One of the major obstacles of the wider commercializaCorrespondence to: B.Y. Shekunov (Telephone: 44-1274235592; Fax: 44-1274-305570; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 90, 1570±1582 (2001) ß 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association
1570
tion of SCF technology is the lack of suf®cient design data required for the reliable scale up of laboratory-based processes. To successfully design a scale-up process as well as develop applications in wider areas of SCF technology, it is necessary to have knowledge of phase behavior and solubility of the solute species in the SCF medium. In SCF extraction applications, the solubility of the solute to be extracted is required to be relatively high and a substantial database exists in this area. The solute-SCF system most frequently investigated is that of naphthalene in supercritical carbon dioxide (SC-CO2), with solubility data ®rst reported by Tsekhanskaya et al.1 Many researches have studied the same system as a standard for calibrating solubility measuring equipment. Schmitt and Reid2 examined the solubility of a wide range of lipophilic solutes in SC-CO2 and
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
other SCF solvents and showed that solute solubility is dependent on the choice of the SCF with typical solubility levels in the order of 10ÿ4 to 10ÿ3 mol fraction. Such a relatively high solubility measurement can be quanti®ed by using gravimetric techniques, whereas solutes with low solubility in SC-CO2 require more sensitive chemical analysis and analytical procedures. Despite the extensive studies, solubility calculations based on equations of state are still not capable of predicting accurate solubility values in even the simplest binary systems.3 In addition, the models need some physicochemical properties of the solute that are not readily available. In the past, it has been demonstrated that correlation of solubility measurements with calculated data using the equations of state, even with high solubility compounds, is unreliable. Hutchenson and Foster3 observed discrepancies of up to 50% between experimental and calculated solubilities using the Peng-Robinson equation of state for the naphthalene-SC-CO2 system near the critical point, although both components in the system were available in high purity and easily analyzed. Because SCFs are non-ideal systems, a priori predictions of singlecomponent solubility in even the most commonly used SCFs such as CO2 are not yet possible. Therefore, accurate and reliable experimental solubility data are essential. Additionally, SCF technologies frequently incorporate entrainers (cosolvents) to form binary solvent-SCF mixtures, and accurate solubility data for such systems are also limited and remain a challenge. The limited experimental solubility data available for pure and mixed solutes in SCF binary mixtures have been obtained under sub-critical conditions. Ting et al.4 have demonstrated that cosolvents can enhance the solubility of solids in SCFs by orders of magnitude. This phenomenon plays a key role in the design of SCF processes. Over recent years, the potential of SCF technologies for the production of micron-sized pharmaceutical particulates has attracted considerable attention. At present two main approaches dominate, each depending on solvating power of the SCFs, that is, high solute solubility in SCF (precipitation by rapid expansion of the SCFÐthe technique is known as RESS) and low solute solubility (precipitation by SCF antisolvent). In both techniques, material is precipitated by rapid increase in the supersaturation, that is, reduction in solubility of the solute in the SCF. Because most pharmaceutical materials are inso-
1571
luble or sparingly soluble in SCFs, the antisolvent method is of greater importance. Liquid antisolvent techniques, based on the use of two miscible liquid solvents such as water and alcohol, are widely used in the pharmaceutical industry. The solute is soluble in the ®rst solvent, but insoluble, or poorly soluble, in the second solvent (antisolvent). Therefore, an addition of antisolvent supersaturates and precipitates the solute while a solution of the two liquids is formed.5 When a SCF is used, an important bene®t is derived because it can be easily separated from the solid product and liquid cosolvent, and recycled. Carbon dioxide is the most widely used SCF for pharmaceutical applications because of its relatively low critical temperature (31.38C) and pressure (73.8 bar), low toxicity, and low cost. SC-CO2 is miscible (or partly miscible below the critical mixture point) with most organic solvents, readily forming binary and ternary organic solvent-CO2 systems. Such mixtures dramatically enhance the solvation power and selectivity of SC-CO2, making it an extremely versatile solvent. Several SCF antisolvent processes have been presented, and they are distinguished by the method of introduction of the antisolvent into the process. Methods include supercritical anti solvent (SAS),6 gas anti solvent (GAS),7,8 aerosol solvent extraction system (ASES),9 and precipitation by compressed antisolvent (PCA).10±13 Gallaghar and co-workers14 found that rapid introduction of SC-CO2 to cyclohexanone solutions of cyclotrimethylenetrinitramine (RDX), an explosive, resulted in monodisperse micron-sized RDX particles. Chang and Randolph15 crystallized b-carotenes from mixtures of b-carotenes and carotene oxides in cyclohexanone or toluene/ butanol using SC-CO2 as a gas antisolvent. Furthermore, manipulating process variables such as temperature, ¯ow rate, and pressure can control the morphology of the particles. York and Hanna16 have developed the solution enhanced dispersion by supercritical ¯uids (SEDSTM) technique to overcome constraints of RESS and GAS processes to produce controlled particulate materials with de®ned morphology using SCF technology. This process features a highly turbulent ¯ow of solvent and CO2, leading to a very fast mixing or dispersion. Thus, mass transfer is not limited by molecular diffusion or convective phenomena. By using this technique, it is possible to control the size, shape and morphology of the material of interest.17 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1572
REHMAN ET AL.
In all antisolvent techniques, nucleation and growth of particles are controlled by the magnitude and uniformity of supersaturation, created by mixing or dispersion of solvent in SC antisolvent or vice versa. Shekunov and co-workers18 have demonstrated the effect of high dependencies of nucleation rate on equilibrium solubility (C0) in particle formation kinetics. The aim of the present study was to determine the solubility of a model compound for pulmonary delivery in pure and modi®ed SC-CO2 and relate this behavior to the precipitation process. Nicotinic acid (NA) was selected based on its low solubility in pure CO2 (in the order of 10ÿ7 mol fraction), and relatively simple organic molecular structure. The solubility of NA was studied in pure SC-CO2 and SC-CO2 modi®ed with methanol in the pressure range 80±200 bar, between temperatures of 35 and 908C, using a dynamic ¯ow through solubility system. The dynamic system was previously validated with o-hydroxybenzoic (salicylic) acid and m-hydroxybenzoic acid in pure SC-CO2 and methanol-modi®ed SCCO2, respectively.19 In the present work, solubility was related to the supersaturation value. Experimental solubility data generated were also used to de®ne optimum operating conditions for the SEDSTM process to obtain crystalline 1±5-mm particles, thereby demonstrating the feasibility of a one-step production method for pharmaceutical particulates suitable for respiratory drug delivery. Such a one-step process eliminates the need for micronization of drug substance after crystallization.
EXPERIMENTAL METHODS Dynamic Flow Through Solubility System A schematic diagram of the dynamic ¯ow through solubility system is shown in Figure 1. Initially, the lowest pressure for the solubility study was established by using pure SC-CO2 or SC-CO2 modi®ed with methanol pumped through the system at de®ned ¯ow rates using high-pressure liquid chromatography (HPLC) pumps (Jasco PU980/6, Tokyo, Japan). An HPLC ultraviolet (UV) detector (Jasco UV-1575 equipped with a high-pressure UV ¯ow cell (Jasco UV-975) was placed between the oven and back pressure regulator (Jasco 880±881). Both six-way Rheodyne values were switched to blank mode to direct the SC-CO2 ¯ow along a bypass line and through JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 1. A schematic diagram of the setup for dynamic solubility measurement.
the high-pressure ¯ow cell. UV data were captured and processed by using a computer with Borwin version 1.21 software (JMBS Developments, Lefontanil, France). In the blank mode, the detector response was set to zero to establish a stable baseline and then the valves were switched to sample mode, diverting the SCF through the extraction vessel. As NA extracted into the SCF, a characteristic UV response plateau was observed. The pressure was then raised to the next selected level at the back pressure regulator. This procedure was repeated at a range of pressures and temperatures to establish a comprehensive data set at de®ned conditions. A range of solvent modi®cations were made by altering ¯ow rate of the pumps. An on-line UV detector calibration curve was prepared using a series of dilutions of NA solution (0.1% w/v) in methanol. A wide range of calibration standards were examined at 263 nm, to adequately bracket solubility values. The separate calibration curves were prepared to cover low solubility of NA in pure SC-CO2 and moderate solubility in modi®ed SC-CO2. Sample response plateau data were interpolated from the calibration curve to determine the concentration of NA present in the SCF. Corrections were made to account for density changes of the SCF at different operating conditions. Supersaturation (s) is de®ned as the difference between measured solute concentration, C, and equilibrium solute concentration (solubility), C0, at a given pressure
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1573
and temperature. It can be expressed by the following equation: ln C=C0 ln CNA fNA =C0 fNA fCO2
1
where fNA and fCO2 are ¯ow rates (mol/s) of the methanol solution and CO2, respectively. CNA is the given concentration of NA in methanol. Sample Preparation Acid-washed glass beads (220±300 mm diameter; Sigma Chemicals, Poole, UK) were mixed with NA in a 4:1 ratio by wegith of glass beads: NA using a Turbula mixer (T2C, Nr. 860961, Glen Creston, Stanmore, England) for 30 min to ensure that a homogeneous mix was achieved. The presence of the glass beads enhances surface area for solute/ solvent interaction, prevents caking of the drug, and eliminates channeling of the SCF through the bed. A 10-ml stainless-steel pressure vessel was carefully packed with the glass beads/NA mixture and placed into a fan-assisted oven (Applied Separations, Allentown, USA) to maintain isothermal conditions. The glass beads/NA mixture was sandwiched between circular paper ®lters (Schleicher & Schuell, Dassel, Germany) to minimize movement of the packing material and avoid any entrainment of material. All samples were investigated in the pressure range of 80±200 bar and between temperatures of 35±908C. NA (>99% purity) was purchased from Sigma Chemicals, and solvents were of HPLC grade from Fisher Scienti®c (Loughborough, Leicestershire, UK). Liquid CO2 (99.99% purity) was supplied by BOC Limited (Guilford, Surrey, UK). Solution Enhanced Dispersion by Supercritical Fluids (SEDS) Process A schematic diagram of the SEDS apparatus is shown in Figure 2. A stainless-steel particle formation vessel (50-ml volume) with a specially designed two-¯ow coaxial nozzle, capable of withstanding a pressure of 500 bar was placed in an air-assisted heated oven. Pressure in the system was maintained within 1 bar by an automated back pressure regulator (Jasco 880-881). NA solution was delivered by a reciprocating HPLC pump (Jasco PU-980) and varied between 0.1±0.8 ml/min. Liquid CO2 (ÿ108C) was pumped by a water-cooled reciprocating HPLC pump (Jasco PU-986) and ¯ow was varied between
Figure 2. A schematic diagram of the SEDS apparatus for particle formation.
4.5±20 ml/min. The CO2 passed through a heat exchanger to ensure that it was supercritical before entering the nozzle, which consisted of two concentric tubes and a small premixing chamber. The diameter of the nozzle ori®ce was maintained at 0.2 mm throughout the study. The high velocity of the SC-CO2 stream thoroughly mixes and disperses the methanol stream and extracts methanol, leaving dry NA powder in the vessel. The powder (typically about 200 mg batch) was collected from the vessel and analyzed. A range of temperatures (35±1208C) and pressures (80± 200 bar) were applied to produce NA powder using the SEDS process. Powder Analysis SEDS products were analyzed using the following analytical techniques: 1.
Scanning electron microscopy (SEM): Powder samples were manually dispersed on an aluminum stub with a thin self-adhered carbon ®lm. The samples were coated with a thin layer of gold using an ion sputter (Emitech K550, Houston, TX) under 0.5 mbar argon atmosphere. Particle morphology was examined using SEM (Hitachi S-520, Tokyo, Japan). 2. X-ray powder diffraction (XRPD): Powder samples were gently ¯attened by a glass slide on a brass sample holder and examined using an X-ray powder diffractometer (D5000, Siemens, Karlsruhe, Germany) in the 2y angle range between 1.58 and 458. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1574
REHMAN ET AL.
3.
Differential scanning calorimetry (DSC): Samples (1±10 mg) were accurately weighed into pierced crimped aluminum pans and analyzed by heating at 108C minÿ1, over a heating range of 25±2508C, under a nitrogen purge, using a PerkinElmer differential scanning calorimeter (PEDSC 7 series, Beacons®eld, UK). 4. Particle sizing: A small amount of powder was analyzed using a laser diffraction Rodos/VIBRI dispersing system (Helos/ Rodos; Sympatec GmbH, Clausthal-Zellerfeld, Germany) operating between 2 and 3 bar air pressure. The instrument consisted of laser sensor HELOS and dry-powder airdispersion system RODOS. Different measuring ranges of the laser sensor were provided by interchangeable objectives R1 (0.1±35 mm) and R2 (0.25±87.5 mm). The rate of powder dispersion was controlled by means of adjusting pressure of the compressed air ¯ow. A pressure of 2 bar was suf®cient to achieve deagglomeration of primary particles without the attrition effect. The particle density in aerosol is measured using optical concentration, which is de®ned as a fraction of light intensity scattered by the particles relative to the reference light intensity. The optimum optical concentration for NA particles was between 20 and 30%. The graphic presentation of particle size distribution (PSD) is made in terms of volume cumulative distribution, Q3, and volume density distribution, q3, which is calculated from the cumulative data. These parameters are related by the following equation: q3 ln x x
dQ xq3 ln 10 d log x
5% within 95% con®dence level.20 The reproducibility of measurements was within 3% (including deviation caused by sampling and dispersion process). The reproducibility of the instrument itself, measured using a certi®ed SiC powder, was within 0.5%. This method was also validated for extension into submicron measurements.21 Measurement Accuracy The PSD measurements were also repeated using Aerosizer (TSI, Minneapolis, USA) time-of-¯ight particle-size analysis. The data obtained with this instrument was accurate within 10% compared with the laser diffraction method. In addition, the data are in good agreement with particle-size analysis by SEM. Thus, in the case of NA powders, these measurement techniques showed relatively high accuracy and were consistent in terms of particle-size changes observed for powders obtained at different processing conditions.
RESULTS Solubility of NA Solubility data obtained at 35, 55, and 908C and pressures of 80±200 bar for NA in pure SC-CO2 and methanol modi®ed SC-CO2 using the ¯ow through dynamic system with on-line (UV detector) analysis have shown both pressure and temperature dependence. The addition of methanol enhanced NA solubility by 10-fold (Figs. 3
2
The volume-moment mean particle diameter, d(4,3), is de®ned as: Zxmax xq3 x dx
d 4; 3
3
xmin
where xmin and xmax are minimum and maximum particle diameter. The typical amount of sample used was about 20 mg per measurement, repeated in triplicate. Because the coarsest particle-size class was typically below 30 mm, the sampling error was JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 3. Online solubility pro®le of NA in pure SCCO2.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1575
Figure 6. Effects of varying methanol mole fraction on solubility of NA at 358C/200 bar and 758C/120 bar.
Figure 4. Online solubility pro®le of NA in MeOHmodi®ed (2 mole %) SC-CO2 at variable temperatures and pressures.
and 4). The results were found to be reproducible within an experimental error of 15%. Solubility in pure SC-CO2 and methanol-modi®ed SC-CO2 increased signi®cantly at higher temperatures with increasing pressure (Fig. 3 and 4). At 908C and between 80 and 200 bar, the solubility of NA was found to increase by an order of magnitude. The region of the lowest solubility was between 65±758C and pressure below 120 bar (Figs. 4 and 5). The relationship between solubility of NA and mol fraction of methanol is shown in Figure 6. Points 358C/200 bar and 758C/120 bar were
selected because they correspond to the operating conditions of extreme CO2 densities which are still in the miscibility region and where the different solubility behavior could be expected. This exponential relationship results in a signi®cant increase in NA concentration at higher methanol concentrations. At concentrations above approximately 4% mole fraction of methanol, the modi®ed SC-CO2 could be effectively considered as a solvent rather than an antisolvent. This imposes a limitation on the maximum solution ¯ow rate during any antisolvent precipitation process. An increase in ¯ow rate of modi®ed SC-CO2 had no signi®cant effect on solubility of NA within the experimental error of 15% (Fig. 7). This indicates that the dynamic ¯ow through technique measured equilibrium solubility. Thus, an online measurement can be considered as a reliable and rapid technique to measure low concentrations of NA.19 Effect of Solubility on the SCF Precipitation Process
Figure 5. Effects of temperature of solubility pro®le of NA in MeOH-modi®ed (2 mole %) SC-CO2 at 100 and 200 bar.
A series of experiments were performed using different mole fractions of methanol in SC-CO2 while maintaining a constant concentration of NA solution at 10 mg/ml. The particle size of the material produced was measured in each case. At 908C/90 bar, the particle size of NA increased ®vefold with increase in the mole fraction of MeOH (0.25±1.0 mol % methanol), as shown in Figure 8. Under different temperature and pressure conditions, the morphology changed from JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1576
REHMAN ET AL.
Figure 7. Effects of varying CO2 ¯ow rates on extraction of NA using 2 mole % MeOH-modi®ed SCCO2 at 358C and 200 bar.
small round particles to large smooth tabular particles (Fig. 9). The optimum conditions were 908C and 90 bar pressure because at these conditions, smooth micron-sized (2±4 mm) particles with good crystallinity were obtained [Fig. 9(c)]. Different CO2 ¯ow rates with a ®xed mole fraction (2 mole %) of methanol were investigated under the optimum conditions (908C/90 bar), as shown in Figure 10. The particle size did not
Figure 8. Effects of MeOH mole fraction of particle size of NA determined at 908C/90 bar using CO2 ¯ow of 18 mL/min. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
signi®cantly change at low ¯ow rates of CO2 and then increased at CO2 ¯ow rates above approximately 13 ml/min. However, previous work on the ¯uid dynamics of precipitation in SCFs has shown that large ¯ow velocities can increase particle size because of reduced residence time in the nozzle and increased variability of solvent concentration in the ¯ow.18,22 A wide range of different concentrations of NA solution was investigated. An increase in NA concentration ®rst gave a reduction in particle size, then an increase in particle size at NA concentrations above 1% w/v (Fig. 11). A relationship between particle size and temperature at constant pressure was found, as illustrated in Figure 12. The results indicate that a rise in temperature reduces particle size because of a decrease in solubility. The in¯uence of pressure on particle size and morphology was investigated at a range of temperature (Fig. 13). The results show that particle size increased at higher pressures irrespective of temperature, as a result of increased solubility of NA, with more uniform prismatic morphology obtained at high temperature and a pressure of 90 bar. Many NA samples were photographed under a scanning electron microscope at different conditions and assessed in terms of particle size and morphology in comparison with unprocessed NA material. The results con®rm that particle size decreased with increase in temperature. Micron-sized, smooth-faced particles were formed at 908C and 90 bar (Fig. 9). Thus, these are the optimum conditions for formation of particles of NA with typical d(4,3) & 4.43 mm, d50 and d90 3.35 and 8.20 mm, respectively (see Table 2). A possible explanation is that, under optimum conditions (908C/90 bar), a maximum s was accomplished because of high temperature and low CO2 density (see Table 1). XRPD pro®les indicated that SEDS-processed material has increased crystallinity compared with the starting material (see Figs. 14 and 15). SEDS-processed NA exhibits a sharp pro®le without the amorphous humps present in unprocessed NA between 10±328 2y. This increased crystallinity was con®rmed by the speci®c enthalpy of melting (DH) from DSC measurements (101.0 J/g for unprocessed NA and 200.0 J/g for SEDS processed). Higher DH values indicate lower levels of defects and impurities present in the crystal structure and a higher degree of crystallinity. The particle-size distribution for SEDS-processed NA material is narrow in comparison with unprocessed material, as shown in Figure 16.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1577
Figure 9. SEM photomicrographs of unprocessed and SEDS-processed NA under the following conditions: CO2 ¯ow 18 ml/min and 2.2% MeOH.
DISCUSSION The experimental data suggest a strong dependence of solubility on pressure at constant
temperature. Such behavior is well known and can be explained by the increase of density and the solvating power of SC-CO2 at high pressure. Figures 3 and 4 indicate an increase by order of JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1578
REHMAN ET AL.
Figure 10. Effects of total ¯ow rate of SC-CO2 composition on particle size determined at 908C/90 bar.
magnitude (i.e., from 10ÿ6 to 10ÿ5 mole fraction) in solubility of NA because of the addition of methanol (polar cosolvent). The retrograde behavior of the solubility was observed at a pressure of 120 bar. Above the crossover point, solubility of NA rises with increase of both pressure and temperature, whereas below this point, solubility of NA again rises with the increase of pressure but, in contrast, decreases as the temperature is raised. The retrograde behavior is a manifestation of the complex relationship between the solubility and the solvent temperature and density.23 Generally, at a ®xed pressure, the solubility decreases with increasing temperature close to the critical point because of a decrease in the density of the solvent/cosolvent, but the solubility rises with
Figure 11. Effects of NA concentration on particle size determined at 908C/90 bar using CO2 ¯ow of 18 ml/ min and MeOH 2.2%. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 12. Particle size of NA as a function of temperature at constant pressure (80 bar) obtained under the following conditions: CO2 ¯ow 18 ml/min and MeOH 2.2%.
increasing temperature distal to the critical point because of the increased vapor pressure of the solute. Solubility is a strong function of the density of the vapor phase for the CO2-NA binary system. However, the effect of density on the solubility becomes weaker with an increase in methanol concentration. For a polar cosolvent such as methanol, with polar solutes such as NA, a large increase in solubility with solvent mole fraction would be expected to result from speci®c chemical
Figure 13. Particle size of NA as a function of pressure at different temperatures using CO2 ¯ow of 18 ml/min and MeOH 2.2%.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1579
Table 1. Effects of Temperature on CO2 Density and Supersaturation Value Temperature (8C) 35 45 75 90 90
Pressure (bar)
CO2 Density (g/mL)
Supersaturation (s)
100 100 100 100 160
0.714 0.496 0.233 0.203 0.408
1.7 2.0 2.3 2.7 0.88
interactions such as hydrogen bonding or charge transfer complex formation in the solution state. In this case, one explanation of the solubility trends is that the interaction between the NA and methanol is relatively strong, through hydrogen bonding, and becomes the dominant effect on solubility as the concentration of methanol increases. With this hypothesis, the density effect can be neglected. Some authors have also pointed out that cosolvents interact non-ideally24 (forming clusters of solvent-solute molecules) for systems containing polar solutes and polar cosolvents,25,26 as is the case for the methanol/NA system. NA has displayed an increased solubility, typical for many pharmaceuticals, in the methanol-modi®ed SC-CO2 at high pressure, and therefore precipitation/crystallization behavior is strongly in¯uenced by its solubility as a function of pressure and temperature. For example, at 80 bar and 358C, higher solubility (10ÿ5, mol fraction) results in low s and low crystal growth producing a small number of large ¯aky particles. Under the identi®ed optimum conditions of 90 bar and 908C, low solubility (10ÿ6 mole fraction) results in high s, and fast nucleation rate, producing a large number of small prismatic particles, as shown in SEM photomicrographs (Fig. 9 and Table 1). Clearly, solubility also affects Table 2. Comparison Between Aerosizer Sympatec Particle-Size Measurements of NA
Sample As-supplied NA Sample A (358C/80 bar) Sample B (558C/80 bar) Sample C (758C/80 bar) Sample D (908C/80 bar) Sample E (908C/90 bar)
and
Sympatec API Aerosizer GmbH d(4,3) (mm) d(4,3) (mm) 22.98 10.28 8.36 6.73 12.99 4.28
35.03 14.6 8.31 5.67 8.01 4.43
the product yield, that is, the higher the solubility, the lower the yield, and vice versa. In a methanol-rich phase, equilibrium concentration of NA increased signi®cantly and, as a result, low s was generated. Therefore, with slow nucleation rates, large particles were produced (Fig. 8). The mechanism of achieving s in the droplets is more complicated in comparison with completely miscible solvents because both the droplet expansion and evaporation processes must be considered. In general, high methanol concentration leads to larger droplets and longer evaporation time. Shekunov et al.18 have stated that the thermodynamic behavior of cosolvent (methanol, ethanol, etc.) droplets in CO2 above and below the critical mixture point has a pronounced in¯uence on precipitation kinetics. Therefore, when the processing pressure is lower than that of the critical mixture pressure, the mol fraction of solvent in the solvent-rich and CO2rich phases are signi®cantly different. This is attributed to non-zero interfacial tension of droplets produced in the two-phase region. Therefore, the solvent-rich phase is unable to be condensed below a certain droplet size, which is stabilized by the surface tension as in the spraying process, and thus the nucleation and post-nucleating growth are con®ned within the solvent-rich phase. Fast droplet evaporation and fast crystallization kinetics at high temperature, combined with low NA solubility in the gaseous phase lead to production of small and uniform NA crystals. High turbulence of SC-CO2 should generally result in more ef®cient mixing and lead to higher s and production of many small particles.22 However, the large-scale inhomogeneity at high methanol ¯ow rates has an opposite effect (reduction of the local supersaturation) and consequent increase of particle size (Fig. 10). A minimum PSD was noted in Figure 11 at intermediate concentrations. This can be explained by a combined effect of two factors: JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1580
REHMAN ET AL.
Figure 14.
XRPD pro®le of unprocessed NA.
®rst, a decrease of primary particle size with an increase of the solute concentration (supersaturation), and second, particle agglomeration at high NA concentration. A large reduction in particle size occurred with increased temperature at constant pressure as indicated in Figure 12. This is because of high s, and low CO2 density at increased temperature (see Table 1). In contrast, a signi®cant increase in particle size was observed with an increase in pressure irrespective of temperature (Fig. 13). This is because of low s and high CO2 density. For example, at 908C, the solubility of NA increased with pressure by two
Figure 15.
orders of magnitude with a corresponding decrease in s from 2.7 at 100 bar to 0.88 at 160 bar. NA processed by SEDSTM at optimum conditions has shown increased crystallinity, and a narrow and uniform particle-size distribution with a mean particle size of between 2±4 mm. This material would therefore be suitable if required for respiratory formulations. The SEDSTM-processed samples were uncharged, easy ¯owing, and gentle aeration readily generated primary particles. Particles also exhibited uniform morphology and habit.
XRPD diffraction pro®le of SEDS-processed NA.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
Figure 16. Particle-size analysis of unprocessed and SEDS-processed NA.
CONCLUSIONS The dynamic method with on-line UV detection enabled a simple and relatively fast measurement of low levels of solubility (10ÿ7 mole fraction) of the model compound (NA), with good accuracy in pure and modi®ed SC-CO2. The solubility of NA clearly demonstrated a dependence on pressure, temperature, and methanol ¯ow rate. The solubility data were successfully applied in optimizing SEDSTM conditions to obtain crystalline particles suitable for respiratory drug delivery directly from solution in a one-step process. In addition, solubility data were also used to calculate supersaturation at different experimental conditions.
ACKNOWLEDGMENTS The authors gratefully acknowledge ®nancial support of this work from the EPSRC and AstraZeneca R&D Charnwood, Loughborough, UK.
REFERENCES 1. Tsekhanskaya YV, Iomtev MB, Mushinka EV. 1964. Russ J Phys Chem 38:1173±1176. 2. Schmitt WJ, Reid RC. 1986. Solubility of monofunctional organic solids in chemically diverse supercritical ¯uids. J Chem Eng Data 31:204±212. 3. Hutchenson KW, Foster NR. 1994. Innovations in supercritical ¯uids science and technology. ACS Symp Ser 608:1±28.
1581
4. Ting SST, Macnaughton SJ, Tomasko DL, Foster NR. 1993. Solubility of naproxen in supercritical carbon dioxide with and without cosolvents. Ind Eng Chem Res 32:1471±1481. 5. SoÈhnel O, Garside J. 1992. Precipitation: Basic principles and industrial applications. Oxford UK: Butterworth-Heinmann Ltd. 6. Debenedetti PG, Tom JW, Yeo S-D, Lim G-B. 1993. Application of supercritical ¯uids for production of sustained delivery devices. J Controlled Release 24:27±44. Ã ber neue 7. Zsigismondy R, Bachmann W. 1918. U Filter Z Anorg. Allg Chem 103:119. 8. Gallagher PM, Coffey MP, Krukonis VJ, Klasutis N. 1989. Gas antisolvent recrystallization: New process to recrystallize compounds insoluble in supercritical ¯uids. ACS Symp Ser 406:334±354. 9. Bleich J, MuÈller BW, Waûmus W. 1993. Aerosol solvent extraction system: A new microparticle production technique. Int J Pharm 97:111±117. 10. Dixon DJ, Johnston KP, Bodmeier RP. 1993. Polymeric materials formed by precipitation with a compressed ¯uid antisolvent. AIChE J 39:127±139. 11. Randolph TW, Randolph AJ, Mebes M, Yeung S. 1993. Sub-micrometer-sized biodegradable particles of poly(L-lactic acid) via the gas antisolvent precipitation process. Biotechnol Prog 9:429±435. 12. Subramaniam B. 1998. Micronization and coating applications with supercritical carbon dioxide. Proc. Of the AAPS annual meeting. Pharm Sci 1:S±615. 13. Reverchon E. 1999. Supercritical antisolvent precipitation of micro and nano particles. J Supercrit Fluids 15:1±21. 14. Gallaghar PM, Coffey MP, Krukonis VJ. 1992. Gas anti-solvent recrystallization of RDX: formation of ultra ®ne particles of a dif®cult-to-comminute explosive. J Supercrit Fluids 5:130±142. 15. Chang CJ, Randolph AD. 1991. Separation of bcarotene mixtures precipitated from liquid solvents with high pressure CO2. Biotechnol Prog 7:275± 278. 16. York P, Hanna M. 1996. Particle engineering by supercritical technologies for powder inhalation drug delivery. Proc. of Respiratory Drug Delivery V, Pheonix: AZ. p 231±239. 17. York P. 1999. Strategies for particle design using supercritical ¯uid technologies. Pharm Sci Technol Today 2:430±440. 18. Shekunov BYu, Baldyga J, York P. 2001. Particle formation by mixing with supercritical antisolvent at high Reynolds numbers. Chem Eng Sci 56:1±13. 19. Bristow S, Shekunov BYu, York P. 2001. Solubility analysis of drug compounds in supercritical carbon dioxide using static and dynamic extraction systems. Ind Eng Chem Res 40:1732±1739. 20. Allen T. 1997. Particle size measurements, 5th ed. London: Chapman & Hall, Vol. 1, p 38. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1582
REHMAN ET AL.
21. RoÈthele S, Naumann H, Heuer M. 1998. The application of Fraunhoffer diffraction below 1 mm to particle size analysis from 0.1 mm to 2000 mm. Proc. of the 7th European Symposium on Particle Characterisation, NuÈrnberg, Germany. 22. Shekunov BYu, Hanna M, York P. 1999. Crystallization process in turbulent supercritical ¯ows. J Cryst Growth 198/199:1345±1351. 23. Johnston KP, Barry SE, Read NK, Holcomb TR. 1987. Separation of isomers using retrograde crystallisation from supercritical ¯uids. Ind Eng Chem Res 26:2372±2377.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
24. Debenedetti PG. 1987. Clustering in dilute, binary supercritical ¯uid mixtures based on molecular distribution functions. Chem Eng Sci 42:2203± 2212. 25. Dobbs MJ, Wong JM, Lahiere RJ, Johnston KP. 1987. Modi®cation of supercritical ¯uid phase behaviour using polar cosolvents. Ind Eng Chem Res 26:56±65. 26. Gurdial GS, Macnaughton SJ, Tomasko DL, Foster NR. 1993. In¯uence of chemical modi®ers on the solubility of o- and m-hydroxybenezoic acid in supercritical CO2. Ind Eng Chem Res 32:1488±1497.
Drug Delivery Group, School of Pharmacy, University of Bradford, Bradford, BD7 1DP, United Kingdom
2
Bradford Particle Design Ltd., 69 Listerhills Science Park, Campus Road, Bradford, BD7 1HR, United Kingdom
3
AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, Leics, LE11 5RH, United Kingdom
Received 21 November 2000; revised 5 April 2001; accepted 17 April 2001
ABSTRACT: Solubilities of a model compound (nicotinic acid) in pure supercritical carbon dioxide (SC-CO2) and SC-CO2 modi®ed with methanol have been measured in the pressure range of 80±200 bar and between temperatures of 35 and 908C. On-line ultraviolet detection enabled a simple and relatively fast measurement of very low levels of solubility (10ÿ7 mol fraction) with good accuracy in pure and modi®ed SC-CO2. The solute solubility in both pure SC-CO2 and SC-CO2 modi®ed with methanol increased with pressure at all investigated temperatures. A retrograde solubility behavior was observed in that, at pressures below 120 bar, a solubility decrease on temperature increase occurred. Solubility data were used to calculate supersaturation values and to de®ne optimum operating conditions to obtain crystalline particles 1±5 mm in diameter using the solution-enhanced dispersion by supercritical ¯uids (SEDSTM) process, thereby demonstrating the feasibility of a one-step production process for particulate pharmaceuticals suitable for respiratory drug delivery. ß 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:1570±1582, 2001
Keywords: antisolvent precipitation; nicotinic acid; supercritical CO2; solubility; particle formation; SEDSTM
INTRODUCTION Solubility of a drug in the chosen supercritical ¯uid is a critical parameter for the design and optimization of all particle formation supercritical ¯uid (SCF) processes. Because solubility behavior plays a key role in de®ning the rate, yield, and economic feasibility of any SCF process, it is important to gain knowledge of solubility at an early stage in any process development. Whereas a number of large-scale commercial SCF extraction processes are being used, SCF particle technology is relatively in its infancy. One of the major obstacles of the wider commercializaCorrespondence to: B.Y. Shekunov (Telephone: 44-1274235592; Fax: 44-1274-305570; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 90, 1570±1582 (2001) ß 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association
1570
tion of SCF technology is the lack of suf®cient design data required for the reliable scale up of laboratory-based processes. To successfully design a scale-up process as well as develop applications in wider areas of SCF technology, it is necessary to have knowledge of phase behavior and solubility of the solute species in the SCF medium. In SCF extraction applications, the solubility of the solute to be extracted is required to be relatively high and a substantial database exists in this area. The solute-SCF system most frequently investigated is that of naphthalene in supercritical carbon dioxide (SC-CO2), with solubility data ®rst reported by Tsekhanskaya et al.1 Many researches have studied the same system as a standard for calibrating solubility measuring equipment. Schmitt and Reid2 examined the solubility of a wide range of lipophilic solutes in SC-CO2 and
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
other SCF solvents and showed that solute solubility is dependent on the choice of the SCF with typical solubility levels in the order of 10ÿ4 to 10ÿ3 mol fraction. Such a relatively high solubility measurement can be quanti®ed by using gravimetric techniques, whereas solutes with low solubility in SC-CO2 require more sensitive chemical analysis and analytical procedures. Despite the extensive studies, solubility calculations based on equations of state are still not capable of predicting accurate solubility values in even the simplest binary systems.3 In addition, the models need some physicochemical properties of the solute that are not readily available. In the past, it has been demonstrated that correlation of solubility measurements with calculated data using the equations of state, even with high solubility compounds, is unreliable. Hutchenson and Foster3 observed discrepancies of up to 50% between experimental and calculated solubilities using the Peng-Robinson equation of state for the naphthalene-SC-CO2 system near the critical point, although both components in the system were available in high purity and easily analyzed. Because SCFs are non-ideal systems, a priori predictions of singlecomponent solubility in even the most commonly used SCFs such as CO2 are not yet possible. Therefore, accurate and reliable experimental solubility data are essential. Additionally, SCF technologies frequently incorporate entrainers (cosolvents) to form binary solvent-SCF mixtures, and accurate solubility data for such systems are also limited and remain a challenge. The limited experimental solubility data available for pure and mixed solutes in SCF binary mixtures have been obtained under sub-critical conditions. Ting et al.4 have demonstrated that cosolvents can enhance the solubility of solids in SCFs by orders of magnitude. This phenomenon plays a key role in the design of SCF processes. Over recent years, the potential of SCF technologies for the production of micron-sized pharmaceutical particulates has attracted considerable attention. At present two main approaches dominate, each depending on solvating power of the SCFs, that is, high solute solubility in SCF (precipitation by rapid expansion of the SCFÐthe technique is known as RESS) and low solute solubility (precipitation by SCF antisolvent). In both techniques, material is precipitated by rapid increase in the supersaturation, that is, reduction in solubility of the solute in the SCF. Because most pharmaceutical materials are inso-
1571
luble or sparingly soluble in SCFs, the antisolvent method is of greater importance. Liquid antisolvent techniques, based on the use of two miscible liquid solvents such as water and alcohol, are widely used in the pharmaceutical industry. The solute is soluble in the ®rst solvent, but insoluble, or poorly soluble, in the second solvent (antisolvent). Therefore, an addition of antisolvent supersaturates and precipitates the solute while a solution of the two liquids is formed.5 When a SCF is used, an important bene®t is derived because it can be easily separated from the solid product and liquid cosolvent, and recycled. Carbon dioxide is the most widely used SCF for pharmaceutical applications because of its relatively low critical temperature (31.38C) and pressure (73.8 bar), low toxicity, and low cost. SC-CO2 is miscible (or partly miscible below the critical mixture point) with most organic solvents, readily forming binary and ternary organic solvent-CO2 systems. Such mixtures dramatically enhance the solvation power and selectivity of SC-CO2, making it an extremely versatile solvent. Several SCF antisolvent processes have been presented, and they are distinguished by the method of introduction of the antisolvent into the process. Methods include supercritical anti solvent (SAS),6 gas anti solvent (GAS),7,8 aerosol solvent extraction system (ASES),9 and precipitation by compressed antisolvent (PCA).10±13 Gallaghar and co-workers14 found that rapid introduction of SC-CO2 to cyclohexanone solutions of cyclotrimethylenetrinitramine (RDX), an explosive, resulted in monodisperse micron-sized RDX particles. Chang and Randolph15 crystallized b-carotenes from mixtures of b-carotenes and carotene oxides in cyclohexanone or toluene/ butanol using SC-CO2 as a gas antisolvent. Furthermore, manipulating process variables such as temperature, ¯ow rate, and pressure can control the morphology of the particles. York and Hanna16 have developed the solution enhanced dispersion by supercritical ¯uids (SEDSTM) technique to overcome constraints of RESS and GAS processes to produce controlled particulate materials with de®ned morphology using SCF technology. This process features a highly turbulent ¯ow of solvent and CO2, leading to a very fast mixing or dispersion. Thus, mass transfer is not limited by molecular diffusion or convective phenomena. By using this technique, it is possible to control the size, shape and morphology of the material of interest.17 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1572
REHMAN ET AL.
In all antisolvent techniques, nucleation and growth of particles are controlled by the magnitude and uniformity of supersaturation, created by mixing or dispersion of solvent in SC antisolvent or vice versa. Shekunov and co-workers18 have demonstrated the effect of high dependencies of nucleation rate on equilibrium solubility (C0) in particle formation kinetics. The aim of the present study was to determine the solubility of a model compound for pulmonary delivery in pure and modi®ed SC-CO2 and relate this behavior to the precipitation process. Nicotinic acid (NA) was selected based on its low solubility in pure CO2 (in the order of 10ÿ7 mol fraction), and relatively simple organic molecular structure. The solubility of NA was studied in pure SC-CO2 and SC-CO2 modi®ed with methanol in the pressure range 80±200 bar, between temperatures of 35 and 908C, using a dynamic ¯ow through solubility system. The dynamic system was previously validated with o-hydroxybenzoic (salicylic) acid and m-hydroxybenzoic acid in pure SC-CO2 and methanol-modi®ed SCCO2, respectively.19 In the present work, solubility was related to the supersaturation value. Experimental solubility data generated were also used to de®ne optimum operating conditions for the SEDSTM process to obtain crystalline 1±5-mm particles, thereby demonstrating the feasibility of a one-step production method for pharmaceutical particulates suitable for respiratory drug delivery. Such a one-step process eliminates the need for micronization of drug substance after crystallization.
EXPERIMENTAL METHODS Dynamic Flow Through Solubility System A schematic diagram of the dynamic ¯ow through solubility system is shown in Figure 1. Initially, the lowest pressure for the solubility study was established by using pure SC-CO2 or SC-CO2 modi®ed with methanol pumped through the system at de®ned ¯ow rates using high-pressure liquid chromatography (HPLC) pumps (Jasco PU980/6, Tokyo, Japan). An HPLC ultraviolet (UV) detector (Jasco UV-1575 equipped with a high-pressure UV ¯ow cell (Jasco UV-975) was placed between the oven and back pressure regulator (Jasco 880±881). Both six-way Rheodyne values were switched to blank mode to direct the SC-CO2 ¯ow along a bypass line and through JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 1. A schematic diagram of the setup for dynamic solubility measurement.
the high-pressure ¯ow cell. UV data were captured and processed by using a computer with Borwin version 1.21 software (JMBS Developments, Lefontanil, France). In the blank mode, the detector response was set to zero to establish a stable baseline and then the valves were switched to sample mode, diverting the SCF through the extraction vessel. As NA extracted into the SCF, a characteristic UV response plateau was observed. The pressure was then raised to the next selected level at the back pressure regulator. This procedure was repeated at a range of pressures and temperatures to establish a comprehensive data set at de®ned conditions. A range of solvent modi®cations were made by altering ¯ow rate of the pumps. An on-line UV detector calibration curve was prepared using a series of dilutions of NA solution (0.1% w/v) in methanol. A wide range of calibration standards were examined at 263 nm, to adequately bracket solubility values. The separate calibration curves were prepared to cover low solubility of NA in pure SC-CO2 and moderate solubility in modi®ed SC-CO2. Sample response plateau data were interpolated from the calibration curve to determine the concentration of NA present in the SCF. Corrections were made to account for density changes of the SCF at different operating conditions. Supersaturation (s) is de®ned as the difference between measured solute concentration, C, and equilibrium solute concentration (solubility), C0, at a given pressure
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1573
and temperature. It can be expressed by the following equation: ln C=C0 ln CNA fNA =C0 fNA fCO2
1
where fNA and fCO2 are ¯ow rates (mol/s) of the methanol solution and CO2, respectively. CNA is the given concentration of NA in methanol. Sample Preparation Acid-washed glass beads (220±300 mm diameter; Sigma Chemicals, Poole, UK) were mixed with NA in a 4:1 ratio by wegith of glass beads: NA using a Turbula mixer (T2C, Nr. 860961, Glen Creston, Stanmore, England) for 30 min to ensure that a homogeneous mix was achieved. The presence of the glass beads enhances surface area for solute/ solvent interaction, prevents caking of the drug, and eliminates channeling of the SCF through the bed. A 10-ml stainless-steel pressure vessel was carefully packed with the glass beads/NA mixture and placed into a fan-assisted oven (Applied Separations, Allentown, USA) to maintain isothermal conditions. The glass beads/NA mixture was sandwiched between circular paper ®lters (Schleicher & Schuell, Dassel, Germany) to minimize movement of the packing material and avoid any entrainment of material. All samples were investigated in the pressure range of 80±200 bar and between temperatures of 35±908C. NA (>99% purity) was purchased from Sigma Chemicals, and solvents were of HPLC grade from Fisher Scienti®c (Loughborough, Leicestershire, UK). Liquid CO2 (99.99% purity) was supplied by BOC Limited (Guilford, Surrey, UK). Solution Enhanced Dispersion by Supercritical Fluids (SEDS) Process A schematic diagram of the SEDS apparatus is shown in Figure 2. A stainless-steel particle formation vessel (50-ml volume) with a specially designed two-¯ow coaxial nozzle, capable of withstanding a pressure of 500 bar was placed in an air-assisted heated oven. Pressure in the system was maintained within 1 bar by an automated back pressure regulator (Jasco 880-881). NA solution was delivered by a reciprocating HPLC pump (Jasco PU-980) and varied between 0.1±0.8 ml/min. Liquid CO2 (ÿ108C) was pumped by a water-cooled reciprocating HPLC pump (Jasco PU-986) and ¯ow was varied between
Figure 2. A schematic diagram of the SEDS apparatus for particle formation.
4.5±20 ml/min. The CO2 passed through a heat exchanger to ensure that it was supercritical before entering the nozzle, which consisted of two concentric tubes and a small premixing chamber. The diameter of the nozzle ori®ce was maintained at 0.2 mm throughout the study. The high velocity of the SC-CO2 stream thoroughly mixes and disperses the methanol stream and extracts methanol, leaving dry NA powder in the vessel. The powder (typically about 200 mg batch) was collected from the vessel and analyzed. A range of temperatures (35±1208C) and pressures (80± 200 bar) were applied to produce NA powder using the SEDS process. Powder Analysis SEDS products were analyzed using the following analytical techniques: 1.
Scanning electron microscopy (SEM): Powder samples were manually dispersed on an aluminum stub with a thin self-adhered carbon ®lm. The samples were coated with a thin layer of gold using an ion sputter (Emitech K550, Houston, TX) under 0.5 mbar argon atmosphere. Particle morphology was examined using SEM (Hitachi S-520, Tokyo, Japan). 2. X-ray powder diffraction (XRPD): Powder samples were gently ¯attened by a glass slide on a brass sample holder and examined using an X-ray powder diffractometer (D5000, Siemens, Karlsruhe, Germany) in the 2y angle range between 1.58 and 458. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1574
REHMAN ET AL.
3.
Differential scanning calorimetry (DSC): Samples (1±10 mg) were accurately weighed into pierced crimped aluminum pans and analyzed by heating at 108C minÿ1, over a heating range of 25±2508C, under a nitrogen purge, using a PerkinElmer differential scanning calorimeter (PEDSC 7 series, Beacons®eld, UK). 4. Particle sizing: A small amount of powder was analyzed using a laser diffraction Rodos/VIBRI dispersing system (Helos/ Rodos; Sympatec GmbH, Clausthal-Zellerfeld, Germany) operating between 2 and 3 bar air pressure. The instrument consisted of laser sensor HELOS and dry-powder airdispersion system RODOS. Different measuring ranges of the laser sensor were provided by interchangeable objectives R1 (0.1±35 mm) and R2 (0.25±87.5 mm). The rate of powder dispersion was controlled by means of adjusting pressure of the compressed air ¯ow. A pressure of 2 bar was suf®cient to achieve deagglomeration of primary particles without the attrition effect. The particle density in aerosol is measured using optical concentration, which is de®ned as a fraction of light intensity scattered by the particles relative to the reference light intensity. The optimum optical concentration for NA particles was between 20 and 30%. The graphic presentation of particle size distribution (PSD) is made in terms of volume cumulative distribution, Q3, and volume density distribution, q3, which is calculated from the cumulative data. These parameters are related by the following equation: q3 ln x x
dQ xq3 ln 10 d log x
5% within 95% con®dence level.20 The reproducibility of measurements was within 3% (including deviation caused by sampling and dispersion process). The reproducibility of the instrument itself, measured using a certi®ed SiC powder, was within 0.5%. This method was also validated for extension into submicron measurements.21 Measurement Accuracy The PSD measurements were also repeated using Aerosizer (TSI, Minneapolis, USA) time-of-¯ight particle-size analysis. The data obtained with this instrument was accurate within 10% compared with the laser diffraction method. In addition, the data are in good agreement with particle-size analysis by SEM. Thus, in the case of NA powders, these measurement techniques showed relatively high accuracy and were consistent in terms of particle-size changes observed for powders obtained at different processing conditions.
RESULTS Solubility of NA Solubility data obtained at 35, 55, and 908C and pressures of 80±200 bar for NA in pure SC-CO2 and methanol modi®ed SC-CO2 using the ¯ow through dynamic system with on-line (UV detector) analysis have shown both pressure and temperature dependence. The addition of methanol enhanced NA solubility by 10-fold (Figs. 3
2
The volume-moment mean particle diameter, d(4,3), is de®ned as: Zxmax xq3 x dx
d 4; 3
3
xmin
where xmin and xmax are minimum and maximum particle diameter. The typical amount of sample used was about 20 mg per measurement, repeated in triplicate. Because the coarsest particle-size class was typically below 30 mm, the sampling error was JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 3. Online solubility pro®le of NA in pure SCCO2.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1575
Figure 6. Effects of varying methanol mole fraction on solubility of NA at 358C/200 bar and 758C/120 bar.
Figure 4. Online solubility pro®le of NA in MeOHmodi®ed (2 mole %) SC-CO2 at variable temperatures and pressures.
and 4). The results were found to be reproducible within an experimental error of 15%. Solubility in pure SC-CO2 and methanol-modi®ed SC-CO2 increased signi®cantly at higher temperatures with increasing pressure (Fig. 3 and 4). At 908C and between 80 and 200 bar, the solubility of NA was found to increase by an order of magnitude. The region of the lowest solubility was between 65±758C and pressure below 120 bar (Figs. 4 and 5). The relationship between solubility of NA and mol fraction of methanol is shown in Figure 6. Points 358C/200 bar and 758C/120 bar were
selected because they correspond to the operating conditions of extreme CO2 densities which are still in the miscibility region and where the different solubility behavior could be expected. This exponential relationship results in a signi®cant increase in NA concentration at higher methanol concentrations. At concentrations above approximately 4% mole fraction of methanol, the modi®ed SC-CO2 could be effectively considered as a solvent rather than an antisolvent. This imposes a limitation on the maximum solution ¯ow rate during any antisolvent precipitation process. An increase in ¯ow rate of modi®ed SC-CO2 had no signi®cant effect on solubility of NA within the experimental error of 15% (Fig. 7). This indicates that the dynamic ¯ow through technique measured equilibrium solubility. Thus, an online measurement can be considered as a reliable and rapid technique to measure low concentrations of NA.19 Effect of Solubility on the SCF Precipitation Process
Figure 5. Effects of temperature of solubility pro®le of NA in MeOH-modi®ed (2 mole %) SC-CO2 at 100 and 200 bar.
A series of experiments were performed using different mole fractions of methanol in SC-CO2 while maintaining a constant concentration of NA solution at 10 mg/ml. The particle size of the material produced was measured in each case. At 908C/90 bar, the particle size of NA increased ®vefold with increase in the mole fraction of MeOH (0.25±1.0 mol % methanol), as shown in Figure 8. Under different temperature and pressure conditions, the morphology changed from JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1576
REHMAN ET AL.
Figure 7. Effects of varying CO2 ¯ow rates on extraction of NA using 2 mole % MeOH-modi®ed SCCO2 at 358C and 200 bar.
small round particles to large smooth tabular particles (Fig. 9). The optimum conditions were 908C and 90 bar pressure because at these conditions, smooth micron-sized (2±4 mm) particles with good crystallinity were obtained [Fig. 9(c)]. Different CO2 ¯ow rates with a ®xed mole fraction (2 mole %) of methanol were investigated under the optimum conditions (908C/90 bar), as shown in Figure 10. The particle size did not
Figure 8. Effects of MeOH mole fraction of particle size of NA determined at 908C/90 bar using CO2 ¯ow of 18 mL/min. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
signi®cantly change at low ¯ow rates of CO2 and then increased at CO2 ¯ow rates above approximately 13 ml/min. However, previous work on the ¯uid dynamics of precipitation in SCFs has shown that large ¯ow velocities can increase particle size because of reduced residence time in the nozzle and increased variability of solvent concentration in the ¯ow.18,22 A wide range of different concentrations of NA solution was investigated. An increase in NA concentration ®rst gave a reduction in particle size, then an increase in particle size at NA concentrations above 1% w/v (Fig. 11). A relationship between particle size and temperature at constant pressure was found, as illustrated in Figure 12. The results indicate that a rise in temperature reduces particle size because of a decrease in solubility. The in¯uence of pressure on particle size and morphology was investigated at a range of temperature (Fig. 13). The results show that particle size increased at higher pressures irrespective of temperature, as a result of increased solubility of NA, with more uniform prismatic morphology obtained at high temperature and a pressure of 90 bar. Many NA samples were photographed under a scanning electron microscope at different conditions and assessed in terms of particle size and morphology in comparison with unprocessed NA material. The results con®rm that particle size decreased with increase in temperature. Micron-sized, smooth-faced particles were formed at 908C and 90 bar (Fig. 9). Thus, these are the optimum conditions for formation of particles of NA with typical d(4,3) & 4.43 mm, d50 and d90 3.35 and 8.20 mm, respectively (see Table 2). A possible explanation is that, under optimum conditions (908C/90 bar), a maximum s was accomplished because of high temperature and low CO2 density (see Table 1). XRPD pro®les indicated that SEDS-processed material has increased crystallinity compared with the starting material (see Figs. 14 and 15). SEDS-processed NA exhibits a sharp pro®le without the amorphous humps present in unprocessed NA between 10±328 2y. This increased crystallinity was con®rmed by the speci®c enthalpy of melting (DH) from DSC measurements (101.0 J/g for unprocessed NA and 200.0 J/g for SEDS processed). Higher DH values indicate lower levels of defects and impurities present in the crystal structure and a higher degree of crystallinity. The particle-size distribution for SEDS-processed NA material is narrow in comparison with unprocessed material, as shown in Figure 16.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1577
Figure 9. SEM photomicrographs of unprocessed and SEDS-processed NA under the following conditions: CO2 ¯ow 18 ml/min and 2.2% MeOH.
DISCUSSION The experimental data suggest a strong dependence of solubility on pressure at constant
temperature. Such behavior is well known and can be explained by the increase of density and the solvating power of SC-CO2 at high pressure. Figures 3 and 4 indicate an increase by order of JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1578
REHMAN ET AL.
Figure 10. Effects of total ¯ow rate of SC-CO2 composition on particle size determined at 908C/90 bar.
magnitude (i.e., from 10ÿ6 to 10ÿ5 mole fraction) in solubility of NA because of the addition of methanol (polar cosolvent). The retrograde behavior of the solubility was observed at a pressure of 120 bar. Above the crossover point, solubility of NA rises with increase of both pressure and temperature, whereas below this point, solubility of NA again rises with the increase of pressure but, in contrast, decreases as the temperature is raised. The retrograde behavior is a manifestation of the complex relationship between the solubility and the solvent temperature and density.23 Generally, at a ®xed pressure, the solubility decreases with increasing temperature close to the critical point because of a decrease in the density of the solvent/cosolvent, but the solubility rises with
Figure 11. Effects of NA concentration on particle size determined at 908C/90 bar using CO2 ¯ow of 18 ml/ min and MeOH 2.2%. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
Figure 12. Particle size of NA as a function of temperature at constant pressure (80 bar) obtained under the following conditions: CO2 ¯ow 18 ml/min and MeOH 2.2%.
increasing temperature distal to the critical point because of the increased vapor pressure of the solute. Solubility is a strong function of the density of the vapor phase for the CO2-NA binary system. However, the effect of density on the solubility becomes weaker with an increase in methanol concentration. For a polar cosolvent such as methanol, with polar solutes such as NA, a large increase in solubility with solvent mole fraction would be expected to result from speci®c chemical
Figure 13. Particle size of NA as a function of pressure at different temperatures using CO2 ¯ow of 18 ml/min and MeOH 2.2%.
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
1579
Table 1. Effects of Temperature on CO2 Density and Supersaturation Value Temperature (8C) 35 45 75 90 90
Pressure (bar)
CO2 Density (g/mL)
Supersaturation (s)
100 100 100 100 160
0.714 0.496 0.233 0.203 0.408
1.7 2.0 2.3 2.7 0.88
interactions such as hydrogen bonding or charge transfer complex formation in the solution state. In this case, one explanation of the solubility trends is that the interaction between the NA and methanol is relatively strong, through hydrogen bonding, and becomes the dominant effect on solubility as the concentration of methanol increases. With this hypothesis, the density effect can be neglected. Some authors have also pointed out that cosolvents interact non-ideally24 (forming clusters of solvent-solute molecules) for systems containing polar solutes and polar cosolvents,25,26 as is the case for the methanol/NA system. NA has displayed an increased solubility, typical for many pharmaceuticals, in the methanol-modi®ed SC-CO2 at high pressure, and therefore precipitation/crystallization behavior is strongly in¯uenced by its solubility as a function of pressure and temperature. For example, at 80 bar and 358C, higher solubility (10ÿ5, mol fraction) results in low s and low crystal growth producing a small number of large ¯aky particles. Under the identi®ed optimum conditions of 90 bar and 908C, low solubility (10ÿ6 mole fraction) results in high s, and fast nucleation rate, producing a large number of small prismatic particles, as shown in SEM photomicrographs (Fig. 9 and Table 1). Clearly, solubility also affects Table 2. Comparison Between Aerosizer Sympatec Particle-Size Measurements of NA
Sample As-supplied NA Sample A (358C/80 bar) Sample B (558C/80 bar) Sample C (758C/80 bar) Sample D (908C/80 bar) Sample E (908C/90 bar)
and
Sympatec API Aerosizer GmbH d(4,3) (mm) d(4,3) (mm) 22.98 10.28 8.36 6.73 12.99 4.28
35.03 14.6 8.31 5.67 8.01 4.43
the product yield, that is, the higher the solubility, the lower the yield, and vice versa. In a methanol-rich phase, equilibrium concentration of NA increased signi®cantly and, as a result, low s was generated. Therefore, with slow nucleation rates, large particles were produced (Fig. 8). The mechanism of achieving s in the droplets is more complicated in comparison with completely miscible solvents because both the droplet expansion and evaporation processes must be considered. In general, high methanol concentration leads to larger droplets and longer evaporation time. Shekunov et al.18 have stated that the thermodynamic behavior of cosolvent (methanol, ethanol, etc.) droplets in CO2 above and below the critical mixture point has a pronounced in¯uence on precipitation kinetics. Therefore, when the processing pressure is lower than that of the critical mixture pressure, the mol fraction of solvent in the solvent-rich and CO2rich phases are signi®cantly different. This is attributed to non-zero interfacial tension of droplets produced in the two-phase region. Therefore, the solvent-rich phase is unable to be condensed below a certain droplet size, which is stabilized by the surface tension as in the spraying process, and thus the nucleation and post-nucleating growth are con®ned within the solvent-rich phase. Fast droplet evaporation and fast crystallization kinetics at high temperature, combined with low NA solubility in the gaseous phase lead to production of small and uniform NA crystals. High turbulence of SC-CO2 should generally result in more ef®cient mixing and lead to higher s and production of many small particles.22 However, the large-scale inhomogeneity at high methanol ¯ow rates has an opposite effect (reduction of the local supersaturation) and consequent increase of particle size (Fig. 10). A minimum PSD was noted in Figure 11 at intermediate concentrations. This can be explained by a combined effect of two factors: JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1580
REHMAN ET AL.
Figure 14.
XRPD pro®le of unprocessed NA.
®rst, a decrease of primary particle size with an increase of the solute concentration (supersaturation), and second, particle agglomeration at high NA concentration. A large reduction in particle size occurred with increased temperature at constant pressure as indicated in Figure 12. This is because of high s, and low CO2 density at increased temperature (see Table 1). In contrast, a signi®cant increase in particle size was observed with an increase in pressure irrespective of temperature (Fig. 13). This is because of low s and high CO2 density. For example, at 908C, the solubility of NA increased with pressure by two
Figure 15.
orders of magnitude with a corresponding decrease in s from 2.7 at 100 bar to 0.88 at 160 bar. NA processed by SEDSTM at optimum conditions has shown increased crystallinity, and a narrow and uniform particle-size distribution with a mean particle size of between 2±4 mm. This material would therefore be suitable if required for respiratory formulations. The SEDSTM-processed samples were uncharged, easy ¯owing, and gentle aeration readily generated primary particles. Particles also exhibited uniform morphology and habit.
XRPD diffraction pro®le of SEDS-processed NA.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
SOLUBILITY OF NICOTINIC ACID IN SC-CO2
Figure 16. Particle-size analysis of unprocessed and SEDS-processed NA.
CONCLUSIONS The dynamic method with on-line UV detection enabled a simple and relatively fast measurement of low levels of solubility (10ÿ7 mole fraction) of the model compound (NA), with good accuracy in pure and modi®ed SC-CO2. The solubility of NA clearly demonstrated a dependence on pressure, temperature, and methanol ¯ow rate. The solubility data were successfully applied in optimizing SEDSTM conditions to obtain crystalline particles suitable for respiratory drug delivery directly from solution in a one-step process. In addition, solubility data were also used to calculate supersaturation at different experimental conditions.
ACKNOWLEDGMENTS The authors gratefully acknowledge ®nancial support of this work from the EPSRC and AstraZeneca R&D Charnwood, Loughborough, UK.
REFERENCES 1. Tsekhanskaya YV, Iomtev MB, Mushinka EV. 1964. Russ J Phys Chem 38:1173±1176. 2. Schmitt WJ, Reid RC. 1986. Solubility of monofunctional organic solids in chemically diverse supercritical ¯uids. J Chem Eng Data 31:204±212. 3. Hutchenson KW, Foster NR. 1994. Innovations in supercritical ¯uids science and technology. ACS Symp Ser 608:1±28.
1581
4. Ting SST, Macnaughton SJ, Tomasko DL, Foster NR. 1993. Solubility of naproxen in supercritical carbon dioxide with and without cosolvents. Ind Eng Chem Res 32:1471±1481. 5. SoÈhnel O, Garside J. 1992. Precipitation: Basic principles and industrial applications. Oxford UK: Butterworth-Heinmann Ltd. 6. Debenedetti PG, Tom JW, Yeo S-D, Lim G-B. 1993. Application of supercritical ¯uids for production of sustained delivery devices. J Controlled Release 24:27±44. Ã ber neue 7. Zsigismondy R, Bachmann W. 1918. U Filter Z Anorg. Allg Chem 103:119. 8. Gallagher PM, Coffey MP, Krukonis VJ, Klasutis N. 1989. Gas antisolvent recrystallization: New process to recrystallize compounds insoluble in supercritical ¯uids. ACS Symp Ser 406:334±354. 9. Bleich J, MuÈller BW, Waûmus W. 1993. Aerosol solvent extraction system: A new microparticle production technique. Int J Pharm 97:111±117. 10. Dixon DJ, Johnston KP, Bodmeier RP. 1993. Polymeric materials formed by precipitation with a compressed ¯uid antisolvent. AIChE J 39:127±139. 11. Randolph TW, Randolph AJ, Mebes M, Yeung S. 1993. Sub-micrometer-sized biodegradable particles of poly(L-lactic acid) via the gas antisolvent precipitation process. Biotechnol Prog 9:429±435. 12. Subramaniam B. 1998. Micronization and coating applications with supercritical carbon dioxide. Proc. Of the AAPS annual meeting. Pharm Sci 1:S±615. 13. Reverchon E. 1999. Supercritical antisolvent precipitation of micro and nano particles. J Supercrit Fluids 15:1±21. 14. Gallaghar PM, Coffey MP, Krukonis VJ. 1992. Gas anti-solvent recrystallization of RDX: formation of ultra ®ne particles of a dif®cult-to-comminute explosive. J Supercrit Fluids 5:130±142. 15. Chang CJ, Randolph AD. 1991. Separation of bcarotene mixtures precipitated from liquid solvents with high pressure CO2. Biotechnol Prog 7:275± 278. 16. York P, Hanna M. 1996. Particle engineering by supercritical technologies for powder inhalation drug delivery. Proc. of Respiratory Drug Delivery V, Pheonix: AZ. p 231±239. 17. York P. 1999. Strategies for particle design using supercritical ¯uid technologies. Pharm Sci Technol Today 2:430±440. 18. Shekunov BYu, Baldyga J, York P. 2001. Particle formation by mixing with supercritical antisolvent at high Reynolds numbers. Chem Eng Sci 56:1±13. 19. Bristow S, Shekunov BYu, York P. 2001. Solubility analysis of drug compounds in supercritical carbon dioxide using static and dynamic extraction systems. Ind Eng Chem Res 40:1732±1739. 20. Allen T. 1997. Particle size measurements, 5th ed. London: Chapman & Hall, Vol. 1, p 38. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
1582
REHMAN ET AL.
21. RoÈthele S, Naumann H, Heuer M. 1998. The application of Fraunhoffer diffraction below 1 mm to particle size analysis from 0.1 mm to 2000 mm. Proc. of the 7th European Symposium on Particle Characterisation, NuÈrnberg, Germany. 22. Shekunov BYu, Hanna M, York P. 1999. Crystallization process in turbulent supercritical ¯ows. J Cryst Growth 198/199:1345±1351. 23. Johnston KP, Barry SE, Read NK, Holcomb TR. 1987. Separation of isomers using retrograde crystallisation from supercritical ¯uids. Ind Eng Chem Res 26:2372±2377.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 90, NO. 10, OCTOBER 2001
24. Debenedetti PG. 1987. Clustering in dilute, binary supercritical ¯uid mixtures based on molecular distribution functions. Chem Eng Sci 42:2203± 2212. 25. Dobbs MJ, Wong JM, Lahiere RJ, Johnston KP. 1987. Modi®cation of supercritical ¯uid phase behaviour using polar cosolvents. Ind Eng Chem Res 26:56±65. 26. Gurdial GS, Macnaughton SJ, Tomasko DL, Foster NR. 1993. In¯uence of chemical modi®ers on the solubility of o- and m-hydroxybenezoic acid in supercritical CO2. Ind Eng Chem Res 32:1488±1497.