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Phase Behavior of Ketoprofen-Poly(lactic acid) Drug Particles Formed by Rapid Expansion of Supercritical Solutions Muhammad Imran ul-haq, Egor Chasovskikh, and Ruth Signorell* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada Received June 29, 2010. Revised Manuscript Received August 12, 2010 The present contribution investigates whether it is possible to form stable amorphous particles of ketoprofenpoly(lactic acid), naproxen-poly(lactic acid), and indomethacin-poly(lactic acid). Amorphization and micronization of these poorly water-soluble drugs offer a combined way to improve the solubility and enhance the dissolution rate. The particles were formed by pulsed rapid expansion of supercritical CO2 solutions and characterized in the aerosol phase with rapid-scan infrared spectroscopy and after collection with scanning electron microscopy and X-ray diffraction. None of the three drug-poly(lactic acid) mixtures showed long-term stability on the order of weeks against the reversion from the amorphous to the crystalline state. Ketoprofen was the only drug that formed mixed amorphous particles with at least short-term stability. The long-term products turned out to be submicrometer- to micrometer-sized particles with a crystalline drug core and an amorphous poly(lactic acid) shell. Moreover, we found that the poly(lactic acid) coating stabilizes the particles against agglomeration.
1. Introduction Ketoprofen (KET), naproxen (NAP), and indomethacin (IND) (Figure 1) belong to the class of nonsteroidal anti-inflammatory drugs. Ketoprofen, for example, acts by inhibiting isoforms of cyclo-oxygenase 1 and 2. It has an activity to treat inflammatory rheumatoid diseases and relieve acute pain and is effective against period pains, pain after surgery, and fever. All three drugs contain carboxylic acid groups and are poorly soluble in water. There are several strategies to improve the solubility and enhance the dissolution rate of poorly water-soluble drugs. In the present contribution we study the combination of two of these strategies, namely amorphization of the drugs by mixing with another biocompatible compound1-3 and micronization of the drugs into small particles.4-11 To convert the drugs into an amorphous state, we use poly(lactic acid) (PLA) as a second compound. With its carbonyl groups it has the potential to form hydrogen bonds with the OH acid groups of KET, NAP, and IND, thereby breaking up the crystalline structure of the pure drugs. PLA has been approved for both in vivo applications and drug delivery formulations and is widely used for medical and pharmaceutical applications, as are its copolymers. Because of their biocompatibility and biodegradability, these polymers are ideal base materials to produce parenteral drug delivery *Corresponding author: Fax þ1 604 822 2847; e-mail signorell@ chem.ubc.ca.
(1) Gupta, M. K.; Vanwert, A.; Bogner, R. H. J. Pharm. Sci. 2003, 92, 563–551. (2) Manna, L.; Banchero, M.; Sola, D.; Ferri, A.; Ronchetti, S.; Sicardi, S. J. Supercrit. Fluids 2007, 42, 378–384. (3) Blasi, P.; Schoubben, A.; Giovagnoli, S.; Perioli, L.; Ricci, M.; Rossi, C. AAPS Pharm. Sci. Technol. 2007, 8, Article 37. (4) Tom, J. W.; Debenedetti, P. G. J. Aerosol Sci. 1991, 22, 555–584. (5) Tom, W.; Debenedetti, P. G. Biotechnol. Prog. 1991, 7, 403–411. (6) Kim, J.-H.; Paxton, T. E.; Tomasko, D. L. Biotechnol. Prog. 1996, 12, 650– 661. (7) T€urk, M.; Hils, P.; Helfgen, B.; Lietzow; Schaber, R. K. Part. Part. Syst. Charact. 2002, 19, 327–335. (8) T€urk, M.; Upper, G.; Hils, P. J. Supercrit. Fluids 2006, 39, 253–263. (9) T€urk, M. J. Supercrit. Fluids 2009, 47, 537–545. (10) Gadermann, M.; Kular, S.; Al-Marzougi, A.; Signorell, R. Phys. Chem. Chem. Phys. 2009, 39, 7861–7868. (11) Hermsdorf, D.; Jauer, S.; Signorell, R. Mol. Phys. 2007, 8, 951–959.
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Figure 1. Molecular structures of (a) PLA, (b) ketoprofen, (c) naproxen, and (d) indomethacin.
systems, such as microparticles, nanoparticles, slabs, pellets, and in situ formed implants. For micronization we use rapid expansion of supercritical CO2 solutions (RESS), which is an attractive method to produce submicrometer- to micrometer-sized drug particles.4-11 The process conditions are comparatively mild, and the particles are generated free from solvent residues. Micronization results in a huge increase in the surface to volume ratio compared with bulk material, which strongly enhances the bioavailability of the drug. Several previous investigations were devoted to the micronization of mixed drug/PLA systems by RESS.4,6,8,10,11 The major focus of
Published on Web 08/26/2010
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Figure 2. Scheme of the RESS setup: (a) mixing unit; (b) expansion unit. See ref 12 for details.
these studies was to clarify whether PLA is able to suppress the strong agglomeration and coagulation of primary particles observed after micronization of pure drugs. Agglomeration and coagulation reduce the effective surface and thus the effects gained by micronization. To the best of our knowledge, the possibility of amorphization of drugs by PLA and its combination with micronization has not been studied before. This led us to explore whether stable amorphous drug/PLA particles can be formed by RESS. In a first step, we have investigated which of the three drugs (KET, NAP, IND) is able to form amorphous films with PLA (section 3). These bulk measurements give important hints to which of the drugs have the potential to form amorphous particles with PLA. The results for particles and their stability over time are reported in section 4.
2. Experimental Section Materials. Ketoprofen (2-(3-benzoylphenyl)propionic acid; C16H14O3), naproxen ((S)-(þ)-2-(6-methoxy-2-naphthyl)propionic acid; C14H14O3), indomethacin (1-(4-chlorobenzoyl)-5-methoxy-2-methyl-3-indoleacetic acid; C19H16ClNO4), DL-poly(lactic acid), and dichloromethane for film preparation were purchased from Sigma-Aldrich. CO2 gas (Technical grade, 4.0, Praxair) was used without further purification to prepare the supercritical solution. CO2 was chosen as supercritical solvent because it is nonflammable, inexpensive, and nontoxic and has low critical data (Tcrit = 304 K, pcrit = 7.38 MPa) which allows processing of sensitive drugs at moderate temperatures and pressures. Preparation of Films and Physical Mixtures. The conversion of the crystalline drugs to the amorphous state by mixing with PLA was tested for films prior to particle formation. For that purpose, desired amounts of PLA and drug (KET, NAP, or IND) were dissolved in 10 mL of dichloromethane by stirring at room temperature. The solutions thus obtained were cast on a Petri dish and allowed to evaporate overnight at room temperature and at 14952 DOI: 10.1021/la1026224
60 °C under vacuum for an additional 12 h to remove any trace of the residual volatile solvent. Dried films with drug mass fractions of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7 were prepared and analyzed with infrared (IR) spectroscopy, X-ray diffraction, and differential scanning calorimetry (DSC). Physical mixtures of polymer and drug were prepared by mixing and grinding PLA with drugs and were afterward characterized with the same methods as the films. While film preparation as described above is expected to promote the formation of homogeneous mixtures of polymer and drug, it is unlikely that any mixing on a molecular level takes place in the physical mixtures. Particle Formation. Pure drug particles, pure PLA particles, and mixed drug-PLA particles were generated by rapid expansion of supercritical CO2 solutions.4-11 A scheme of our RESS setup is shown in Figure 2 and described in more detail in ref 12. It consists of two units: the mixing unit where the supercritical solutions are prepared and the expansion unit where particle formation takes place. Drug and/or PLA were filled into the extractor together with glass beads and mixed with CO2 up to a pressure of p = 38 MPa and heated to a temperature of T = 333 K. This pressure and temperature achieve the maximal solubility for all three drugs over the range of conditions accessible with our setup. For the present experiments, the extractor and reservoir were connected and kept at the same pressure p and temperature T. The supercritical solution was then expanded through a small nozzle (diameter of 400 μm and length of 250 μm; T = 333 K) into the expansion chamber (0.08 m3). Rapid expansion leads to supersaturation and thus to particle formation. The nozzle was operated in a pulsed mode (pulse repetition rates of 1 min-1 and nozzle opening time of 500 ms). For each experiment, eight pulses were accumulated before recording any data. In-situ particle characterization with infrared spectroscopy as well as particle collection for off-line characterization with scanning electron (12) Bonnamy, A.; Hermsdorf, D.; Ueberschaer, R.; Signorell, R. Rev. Sci. Instrum. 2005, 76, 53904–53908.
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microscopy (SEM) and X-ray diffraction was performed in the expansion chamber. Characterization Methods. Films, physical mixtures, and particles were characterized by mid-infrared spectroscopy with a Bruker IFS 66v/S Fourier transform infrared spectrometer with a spectral resolution of 2 cm-1. The spectrometer was equipped with a mid-infrared Globar light source, a KBr beam splitter, and an MCT detector. Time sequences of infrared (IR) spectra of the particles were recorded in situ in the aerosol phase directly after particle formation (see Figure.2) with a time resolution of up to 30 ms.12 In-situ measurements in the aerosol phase allow us to monitor time-dependent structural changes of the particles. Infrared spectroscopy was also exploited to determine the mass fraction of drug in the films, physical mixtures, and particles analogous to the procedure described in ref 10. X-ray powder diffraction patterns of films, physical mixtures, and particles were measured with a Bruker D8 Advance diffractometer. The differential scanning calorimetry (DSC) analysis of films and physical mixtures was performed on a TA Instruments DSC Q100 (New Castle, DE) equipped with a liquid nitrogen cooling system. Accurately weighed samples (∼2-5 mg) were hermetically sealed in an aluminum pan and heated from 273 to 373 K at a rate of 10 K/min under nitrogen flow. Particle shape and size were determined with scanning electron microscopy (SEM, Hitachi S4700). For this purpose, the aerosol particles were collected onto silica plates located in the expansion chamber (Figure 2) where they were allowed to settle directly from the aerosol phase. The silica plates with polymers particles were coated with gold (5 nm) in a sputter coater.
3. Results: Films and Physical Mixtures Appearance of KET-PLA Films. Preparation of films by solvent casting as described above is a simple way to check whether stable homogeneous mixtures of drug and PLA form at all. The physical appearance of these films already provides information on the type of film. For KET-PLA we have found pronounced changes of the film appearance as a function of the ratio of KET and PLA (Figure S1). Similar to pure PLA films (Figure S1a), KET-PLA films are transparent up to a KET mass fraction of 0.3 with smooth surface and no visible crystals (Figure S1c). These are the first indications that homogeneous mixtures of KET and PLA are formed. Furthermore, we found that these films were stable for several weeks. By contrast, films with a KET content above 0.4 were no longer transparent and had rough surfaces with visible crystals similar to the pure KET films (Figures S1d and S1b, respectively). These findings indicate that the “miscibility window” of KET and PLA lies between a KET mass fraction of ∼0.1 and 0.3. IR Spectra of KET-PLA Mixtures. Figure 3 illustrates that transparent PLA-KET films (up to KET mass fraction of 0.3) show characteristic shifts in the KET bands that are observed neither for the nontransparent films (KET mass fraction above 0.4) nor for the physical mixtures. The IR spectra of pure amorphous PLA and pure crystalline KET in the carbonyl stretching region (1840-1600 cm-1) are shown in Figures 3a and 3b, respectively. PLA shows a broad carbonyl band around ∼1752 cm-1. The position of this broad band is not sensitive to mixing with KET. Pure crystalline KET consists of dimeric subunits (Figure S2a).13 It has two characteristic sharp symmetric carbonyl bands at 1694 and 1655 cm-1, which arise from the dimeric carboxylic and ketonic stretching vibrations, respectively.13 The position of these two bands is sensitive to structural changes, which for example are caused by homogeneous mixing with other substances. No shift of (13) Seong-Ho, C.; Soo-Yeon, K.; Jae Jeong, R.; Ji Yeon, P.; Kwang-Pill, L. Anal. Sci. 2001, 17, 785–788.
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Figure 3. Infrared spectra of pure and mixed PLA and KET films and physical mixtures in the region between 1840 and 1600 cm-1: (a) pure amorphous PLA bulk; (b) pure crystalline KET bulk; (c) physical mixture with a KET mass fraction of 0.5; (d) film with a KET mass fraction of 0.5; (e) physical mixture with a KET mass fraction of 0.2; (f) film with a KET mass fraction of 0.2; (g) pure amorphous KET.
these bands is observed for any of the physical mixtures (Figures 3c,e) or for the films with a KET mass fraction larger than 0.4 (Figure 3d). This is a clear indication that PLA and KET are not mixed on a molecular level. KET still has a crystalline structure, which is consistent with the observation that these samples have a crystalline appearance (Figure S1). Characteristic shifts are only observed for the transparent films with KET mass fractions below 0.3 (Figure 3f). The KET band originally observed at 1694 cm-1 is shifted to 1708 cm-1 for these films. This is ascribed to the breaking of the intermolecular hydrogen bond of the KET dimer (Figure S2a), which allows the formation of new intermolecular bonds of KET with the carbonyl moiety of the polymer chains (Figures S2b and S2c). Moreover, the band of the ketonic group at 1655 cm-1 in the crystal shifts to 1660 cm-1 for the transparent films as a consequence of the loss of the crystalline structure. KET dimers, which are essential for the formation of the crystal lattice, do not form in the transparent films, which leads to the desired amorphization of the drug. Figure 3g illustrates that similar band positions (1708 and 1660 cm-1) as for the transparent films are indeed found for amorphous KET samples (formed by RESS, see next section). Note that the band at 1708 cm-1 arises from the free acid carbonyl group, which is present in the amorphous but not in the crystalline state. Table 1 summarizes band positions for films and physical mixtures with various amounts of KET. X-ray Diffraction and DSC of KET-PLA Mixtures. The X-ray diffraction patterns in Figure 4 confirm the results derived from infrared spectra. Pure PLA is an amorphous polymer and as such does not show any sharp features (Figure 4a). Ketoprofen, by contrast, is crystalline, which is confirmed by the characteristic DOI: 10.1021/la1026224
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Table 1. Infrared Band Positions of the Ketonic Carbonyl Band and the Acidic Carbonyl Band of Ketoprofen in Films, Physical Mixtures, and Particles sample
mass fraction of KET
ketonic (CdO)
acidic (CdO)
pure KET bulk 1 1655 1694 pure KET film 1 1655 1694 physical mixtures 0.1-0.7 1655 1694 mixed films 0.4- 0.7 1655 1694 mixed films 0.1-0.3 1660 1708 pure KET particles in the aerosol phase 1 1655-1660 1694-1708 pure KET particles collected after RESS 1 1655 1694 1660 1708 mixed particles in the aerosol phase ∼0.3a a mixed KET particles collected after RESS ∼0.3 1655 1694 a Note that the mass fraction of KET in the mixed particles after micronization was ∼0.3 almost independently of the the extractor.
Figure 4. X-ray powder diffraction patterns of (a) pure amorphous PLA bulk, (b) pure crystalline KET bulk, (c) physical mixture with a KET mass fraction of 0.5, (d) film with a KET mass fraction of 0.5, (e) physical mixture with a KET mass fraction of 0.2, and (f) film with a KET mass fraction of 0.2.
sharp peaks in the X-ray diffraction spectrum (Figure 4b). All physical mixtures of KET with PLA (Figures 4c,e) and all films with a KET content above 0.4 (Figure 4d) show the same peaks as pure crystalline KET. This confirms that KET is present in the crystalline state in these mixtures. The only mixtures that do not show contributions from crystalline KET are the films with KET fractions below 0.3 (Figure 4f). The absence of sharp features is another strong indication that the transparent films are amorphous homogeneous KET-PLA mixtures. Consistent results were obtained from the DSC measurements. All physical mixtures and all films with KET fractions above 0.4 have KET melting temperatures Tm around ∼92 °C, close to the melting of pure KET (95 °C), and glass transition temperatures Tg around 50 °C, close to the glass transition temperature of pure PLA (50 °C). This indicates that in all these mixtures PLA and KET do not interact on a molecular level. By contrast, the 14954 DOI: 10.1021/la1026224
phase crystalline crystalline crystalline crystalline amorphous amorphous to crystalline crystalline amorphous crystalline ratio of KET and PLA in
presence of molecular mixtures in all films with KET fractions below 0.3 is evident from the disappearance of the melting peak of KET and the decrease of the glass temperature of PLA. The disappearance of the crystalline KET melting peak is due to the interaction of KET and PLA and indicates complete amorphization of KET. As KET molecules interact with PLA chains, they disrupt the interactions between the polymer chains, removing barriers to bond rotation and chain mobility, which decreases Tg. NAP-PLA and IND-PLA Mixtures. NAP-PLA and IND-PLA mixtures both behave differently from KET-PLA mixtures. None of the NAP-PLA films (NAP mass fractions between 0.1 and 1) were transparent. All films showed crystals similar to the pure NAP films, which indicates that NAP does not form stable homogeneous mixtures with PLA. Further evidence comes from the IR spectra of these films: The position of the carbonyl band of pure crystalline NAP at 1730 cm-1 does not change its value for any of the mixtures (see ref 10 for the IR spectrum of NAP). In line with this observation all NAP-PLA mixtures show the characteristic X-ray diffraction patterns of crystalline NAP. The crystalline nature of NAP in all these samples clearly demonstrates the failure of its amorphization by PLA, in marked contrast to the case of KET. IND-PLA films showed a behavior intermediate between KET-PLA and NAP-PLA films. Films with an IND mass fraction above ∼0.5 and all physical mixtures had a crystalline appearance. Films with mass fractions below this value showed transparent regions similar to the transparent KET-PLA films, but in contrast to the latter transparent regions in IND-PLA films were always accompanied by crystalline regions (Figure S3). The fraction of these crystalline regions increased with increasing IND fraction. Infrared spectra of the transparent regions hint that IND and PLA are homogeneously mixed in these regions, causing the characteristic shifts observed for the carbonyl bands (1695 and 1677 cm-1; note that contributions of pure IND are also visible in this spectrum) compared with pure crystalline IND (1713 and 1689 cm-1) (Figures S4c and S4b, respectively).1 However, we observed that the transparent regions were unstable over time and converted to the crystalline form within hours to days.
4. Results: Drug Particles Pure PLA and Pure KET Particles. Pure PLA, pure KET, and mixed KET-PLA particles were formed by RESS under the same conditions as described in section 2. SEM images of pure PLA particles are depicted in Figure 5. They provide information on primary particle size, agglomeration, and particle shape. PLA particles formed by RESS do not show any agglomeration and have an almost perfectly spherical shape.14 For the PLA used here (14) Imran ul-haq, M.; Acosta-Ramı´ rez, A.; Mehrkhodavandi, P.; Signorell, R. J. Supercrit. Fluids 2010, 51, 376–383.
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Figure 6. Time-dependent (top to bottom) infrared spectra of pure KET aerosol particles in the region between 1840 and 1600 cm-1. The spectra were recorded directly after the expansion (t = 0 min) in the aerosol phase.
Figure 5. SEM images of pure PLA particles. Note the different scale in panels a and b.
the mean particle diameter lies around ∼700 nm. The spherical shape of the particles hints to an amorphous structure analogous to bulk PLA, which is confirmed by infrared spectroscopy and powder diffraction. Infrared spectra of PLA particles recorded in situ in the aerosol phase show the same broad bands as the amorphous bulk with the strong carbonyl band around ∼1752 cm-1 also apparent in Figure 3a. The amorphous structure does reflect not only in broad infrared bands but also in the same structureless powder diffraction pattern (Figure S5a) as shown for amorphous bulk PLA in Figure 4a. Since pure KET bulk is crystalline, we expected that pure KET particles remain crystalline after micronization. However, timedependent infrared spectra of pure KET particles recorded directly after particle formation in the aerosol phase reveal a more complicated behavior. The band positions in these IR spectra shift over time, and the bands become narrower as illustrated by the sequence of IR spectra in Figure 6 (from top to bottom). Only after about an hour (bottom trace in Figure 6), the band positions stabilize at values that are characteristic for crystalline KET (1694 and 1655 cm-1 for the carbonyl region; see Figure 3b and Table 1). Initially (top spectrum in Figure 6), the particles are formed in an amorphous state, exhibiting three broad carbonyl bands at 1740, 1708, and 1660 cm-1. The first band is assigned to free acid carbonyl groups (Figure S2b), which therefore disappears in the crystalline state (bottom trace). The shift of the second and third band compared with crystalline KET is a consequence of the breaking of KET dimer as described above. Figure 6 shows the crystallization of initially amorphous aerosol particles to crystalline particles (see Table 1). Since the crystallization Langmuir 2010, 26(18), 14951–14957
Figure 7. SEM images of pure KET particles. Note the different scale in panels a and b. DOI: 10.1021/la1026224
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Figure 8. Time-dependent (top to bottom) infrared spectra of KET-PLA aerosol particles in the region between 1840 and 1600 cm-1. The spectra were recorded directly after the expansion (t = 0 min) in the aerosol phase. The KET mass fraction is 0.3.
is complete after about an hour, it is not surprising that the KET particles in the SEM images in Figure 7 have a crystalline appearance. The primary particles are small rodlike crystals with average lengths of about 4 μm and average widths of about 800 nm. These primary particles aggregate strongly, forming networklike structures with sizes well above several micrometers (Figure 7a). The crystalline state of these particles is also confirmed by X-ray powder diffraction (Figure S5b). Mixed KET-PLA Particles. We have demonstrated in section 3 that mixed KET-PLA films with KET mass fractions below 0.3 form amorphous transparent films that are stable over weeks. Amorphization and micronization both improve the dissolution behavior. The combination of both would thus be desirable. The time-dependent aerosol infrared spectra in Figure 8 illustrate that it is indeed possible to form amorphous PLA-KET particles by micronization. The spectra show the characteristic bands of amorphous KET at 1708 and 1660 cm-1 (Table 1). In contrast to pure KET particles, no changes in the band positions and band widths were observed within the first 3 h after particle formation. The mixtures stay amorphous over this time. The SEM images recorded a few days after collection of the particles in the expansion chamber, however, are not in agreement with amorphous particles (Figure 9). The rodlike particles with average lengths of about 7 μm and average widths of about 1 μm clearly have a crystalline appearance. The crystalline state of KET in these deposited particles is confirmed by powder diffraction (Figure S5c) and IR spectra (carbonyl bands at 1694 and 1655 cm-1; Table 1). The finite size of these particles obviously promotes crystallization compared with the transparent mixed films discussed above, which were found to be stable over weeks. It is likely that demixing of KET and PLA is more efficient in 14956 DOI: 10.1021/la1026224
Figure 9. SEM images of KET-PLA particles. Note the different scale in panels a and b.
these small particles than in films because of the large surface-tovolume ratio. A conceivable mechanism would be the migration of one substance to the nearby surface region, eventually leading to a core-shell structure with one substance in the core and the other one in the shell. The finite size of a particle would clearly facilitate such a process compared with the film. Once regions of pure KET are formed, the crystallization sets in as found above for the pure KET particles (Figure 6). The appearance of the particles in Figure 9 strongly hints at the formation of core-shell particles with crystalline KET in the core surrounded by a thin amorphous PLA shell. The pronounced crystalline shape of the particles most likely arises from the crystalline KET in the core since it is hard to imagine that such regular rods could form with spherical amorphous PLA cores. The same argument speaks against the formation of separate domains of pure KET and pure PLA in the same particle. Free energy arguments would also favor the formation of a core-shell structures over multiple domain particles since the former minimizes the interfacial region. A comparison of the SEM images for pure KET particles in Figure 7 with mixed KET-PLA particles in Figure 9 reveals two major differences. The mixed PLA-KET particles form nearperfect crystals with very regular shapes. We believe that the formation of such well-defined crystals is the consequence of slower crystallization of the mixed particles compared with pure KET particles. Slow crystallization promotes the formation of regular crystals. The second difference is the degree of agglomeration of primary particles. Agglomeration is significantly reduced for the mixed particles (Figure 9a) compared with pure KET particles (Figure 7a). It is known from previous studies that Langmuir 2010, 26(18), 14951–14957
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coating of particles with PLA leads to stabilization against particle agglomeration.6,8,10,11 The fact that we observe the same behavior here further supports our above conclusion that PLA forms a shell around the KET cores. Mixed NAP-PLA and Mixed IND-PLA Particles. As mentioned in section 3, NAP and PLA do not form amorphous films. It is thus not surprising that the same holds for mixed NAP-PLA particles. NAP in these mixed particles is already crystalline immediately after the expansion. As we have demonstrated in a previous publication,10 PLA coats these NAP particles and stabilizes them against agglomeration. The major problem with the formation of pure IND and mixed IND-PLA particles is the solubility of IND in supercritical CO2 which seems to be much lower that of KET and NAP.15,16 We could not generate enough pure IND particles with our lowthroughput setup to properly analyze the properties of these particles. The addition of PLA to IND in the extractor increased the solubility of IND so that we were at least able to record infrared spectra of the mixed IND-PLA particles in the aerosol phase after particle formation. The positions of the carbonyl bands of these particles (1713 and 1686 cm-1; Figure S4d) are closer to crystalline IND (1713 and 1689 cm-1; Figure S4b) and to amorphous IND (1695 and 1677 cm-1; Figure S4c). This observation shows that these particles do not form stable amorphous mixtures. Because of the low particle yield, our sensitivity was too low to observe time-dependent infrared spectra to search for any phase transitions. Neither could we collect enough particles from the expansion chamber to perform any further analysis.
5. Conclusion Previous studies have demonstrated that amorphization of crystalline drugs can be achieved by mixing with other compounds, in particular with certain biocompatible polymers.1-3 It was also shown that some of these amorphous mixtures were stable over weeks.1 For the three drugs KET, IND, and NAP studied here, we found that KET is the only drug that forms amorphous films with PLA that are stable over weeks. Even though amorphous IND-PLA films were formed in the first place, these films were only stable for hours to days. In the case of NAP it was not possible to obtain any amorphous films at all by mixing with PLA. The major goal of the present study was to explore the possibility of forming stable amorphous micrometer-sized drugPLA particles. Amorphization and miconization both increase the solubility of the drug. Such a combined effect would thus be desirable to enhance the bioavailability. Rapid expansion of supercritical KET-PLA-CO2 solutions indeed generates (15) Coimbra, P.; Fernandes, D.; Gil, M. H.; de Sousa, H. C. J. Chem. Eng. Data 2008, 53, 1990–1995. (16) Gupta, R. B. Solubility in sc-dioxide: CRC Press: Boca Raton, FL, 2007; pp 457-458.
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submicrometer- to micrometer-sized amorphous KET-PLA particles. The stability of these particles, however, is limited to only a few days, in contrast to the corresponding films, which were observed to be stable for weeks. After a few days, the particles show complete demixing of the two substances and crystallization of KET. The originally amorphous particles have converted to particles with a crystalline KET core coated with a thin amorphous PLA shell. Consistent with the findings for the films, expansion of NAP-PLA-CO2 solutions produces crystalline NAP particles that are coated with PLA. Amorphous particles were not observed for this drug at all, not even as transient species.10 IND-PLA showed an intermediate behavior between that of KET-PLA and NAP-PLA particles. Mixed particles with at least some amorphous contributions were observed. As the films, these amorphous particles crystallize quickly. The low particle yield, however, prevented more detailed studies of these mixtures. All three substances KET, NAP, and IND are carboxylic acidcontaining drugs, which in the crystalline state consist of hydrogen-bonded dimer units (Figure S2a). In the amorphous mixed state, hydrogen bonds are formed between the acid OH groups of the drug and the carbonyl groups of PLA. Our study shows that the amorphous mixtures are thermodynamically less stable than separate pure drug and pure PLA. The kinetics of converting the amorphous mixed forms into pure components, however, are surprisingly different for the three different drugs. The two steps involved in this process, demixing of drug and PLA and crystallization of pure drug domains, are influenced by steric effects, diffusion processes, and the kinetics of bond breaking and making. Even though permanent amorphization beyond several weeks could not be achieved with PLA for any of the three drugs, we have shown that the drugs can be micronized and be further stabilized against agglomeration by coating with PLA. Micronization to particles with submicrometer to micrometer size is an important way to increase the solubility of poorly water-soluble compounds. The wide range of different time scales we have observed for the reversion of drug particles from the amorphous to the crystalline state suggests that long-term amorphization might still be achievable by a suitable choice of polymer, possibly a suitably designed copolymer with a slightly higher affinity to the drug in question. Acknowledgment. This project was supported financially by the Natural Sciences and Engineering Research Council of Canada and by the Canada Foundation for Innovation. Supporting Information Available: Images of KET films, molecular structures of hydrogen-bonded clusters, images of IND films, infrared spectra of PLA-IND films and aerosol particles, and powder diffraction patterns of PLA, KET, and mixed PLA-KET particles. This material is available free of charge via the Internet at http://pubs.acs.org.
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