Amphiphilic Block Copolymer as a Crystal Habit Modifier Kuldipkumar,†
Tan,‡
Anuj Yvonne T. F. Mark Geoff G. Z. Zhang,*,# and Glen S. Kwon*,†
Goldstein,#
Yukio
Nagasaki,⊥
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1781-1785
Department of Pharmaceutical Sciences, School of Pharmacy, University of Wisconsin-Madison, 777 Highland Avenue, Madison, Wisconsin 53705, School of Pharmaceutical Sciences, Universiti Sains Malaysia, Minden 11800, Penang, Malaysia, Pharmaceutics and New Technology Center, Global Pharmaceutical R & D, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, Illinois 60064-6120, Department of Materials Science, Science University of Tokyo, Chiba 278-8510, Japan Received February 4, 2005;
Revised Manuscript Received May 24, 2005
ABSTRACT: The purpose of this study is to evaluate amphiphilic block copolymers, a new class of additives, for the habit modification of pharmaceuticals. The additive chosen is poly(ethylene glycol)-block-poly(lactic acid) (PEGb-PLA). The model compound chosen is tolazamide, an oral hypoglycemic pharmaceutical agent. Crystallization was carried out in aqueous media by pH adjustment, with or without additives. PEG-b-PLA was found to be very effective in modifying the crystal habit of tolazamide. The habit changed from a needle shape to a plate shape at a concentration of 12.5 µg/mL. This change in habit required both blocks and could not be achieved in the presence of either block alone. A small amount of copolymer (0.55% weight) was found on/in tolazamide particles crystallized in the presence of 30 µg/mL of copolymer. Washing the crystals with toluene effectively removed the copolymer. It is thus concluded that the block copolymer is mainly adsorbed on the crystal surface(s) rather than being incorporated into the lattice of tolazamide crystals. The recrystallized tolazamide is of the same crystal form as that of the original solid, regardless of the concentration of the copolymer used. Crystallization is most commonly employed as the final step in the preparation of an active pharmaceutical ingredient (API). During this process, impurities are removed from the API, a desirable crystal form of the API is obtained, and the particles of the API attain certain characteristics such as particle size and morphology. Crystal morphology, or habit, can influence many downstream pharmaceutically relevant properties, such as filterability, flowability, syringeability, compactability, and dissolution profile.1 Therefore, proper crystal habit is essential for manufacturing processes such as powder mixing, capsule filling, and tabletting. However, very often, APIs and excipients crystallize in an acicular or needle shape, thus rendering them with undesirable flow characteristics, low bulk density, propensity to cake, and difficulties in packaging and handling of the material.2 To improve mixing efficiency and content uniformity, milling is often employed to remove the distinct shape of the particles and to reduce particle size. However, it is well documented that milling may generate amorphous content.3,4 These amorphous phases, although a small fraction, could have a significant impact on the quality of the final product. For instance, residual moisture content may be higher; degradation is therefore accelerated because of the increased mobility of the molecules.4,5 Crystal* To whom correspondence should be addressed: (G.S.K.) Telephone: (608) 265-5183. Fax: (608) 262-5345. E-mail: GSKwon@ pharmacy.wisc.edu. (G.G.Z.Z.) Pharmaceutics and New Technology Center, Global Pharmaceutical R & D, Department R4P3, Bldg. AP9, 100 Abbott Park Road, Abbott Park, IL 60064-6120. Telephone: (847) 937-4702. Fax: (847) 938-4434. E-mail:
[email protected]. † University of Wisconsin-Madison. ‡ Universiti Sains Malaysia. # Abbott Laboratories. ⊥ Science University of Tokyo.
lization of the amorphous regions on storage may lead to tablet hardening, delayed disintegration, and/or dissolution, which may subsequently lead to lower absorption and bioavailability. Therefore, it is desirable to generate a suitable crystal habit through solution crystallization. One major factor that affects the crystal habit is the internal structure, i.e., molecular arrangement in the crystal lattice. As a result, different polymorphs and solvates may have different crystal habits. On the other hand, the external environment from which the crystals form is equally important, if not more. To improve the processing characteristics of the powder of a particular compound without changing the crystal form, crystal engineering/habit modification is often employed. The use of different solvents for crystallization and variation of the crystallization conditions have been previously shown to impact the habit.6,7 The choice of solvent as crystallization medium is, however, limited by the toxicity and cost of the solvent, crystallization efficiency, and the purity of the final product. Hence, it is not always possible to find a suitable solvent that yields a desired habit. To overcome this, researchers have used different “impurities” in the crystallization medium such as tailor-made additives (i.e., structurally related compounds),8-15 surfactants,15-18 and homopolymers19-23 to control/modify the habit. Because of the structural similarities, the use of tailor-made additives often results in irreversible incorporation of these additives into the lattice of the crystallizing solute (host), forming solid solutions.24-30 This leads to contamination of the API that is not desirable. Surfactants may also be incorporated into the lattice of the host owing to their smaller size. Some polymers (natural
10.1021/cg050049m CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
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polymers or synthetic homopolymers) are effective in modifying the habit of certain compounds when present in sufficient quantities. However, this may lead to an increase in the viscosity of the crystallization medium and subsequent difficulty in the filtration step. Because of these limitations, crystal engineering using additives is seldom practiced in the industrial crystallization of APIs. Therefore, there exists a strong need in the field of crystal engineering to identify new classes of additives. Two characteristics are considered necessary for these new classes of additives. First, they should be very effective in habit modification; thus, only small quantities would be required in the crystallization medium. Second, and more importantly, they should have an insignificant extent of incorporation into the host lattice during crystallization. Additives that possess both these characteristics may become attractive for habit modification in an industrial setting. Amiphiphilic block copolymers (ABCs) are polymers that have a linear arrangement of two blocks of different composition. The techniques to synthesize these molecules having a welldefined composition and molecular weight have made tremendous progress in the past decade.31 The continued interest in ABCs arises due to their unique solution properties, including their ability to self-associate in selective solvents to form micelles. Consequently, ABCs have been extensively studied as vehicles for the delivery of poorly soluble drugs.32 The presence of two dissimilar functionalities within the same molecule while being separated from one another allows for greater flexibility in modulating the properties of ABCs. Previously, amphiphilic triblock copolymers, poloxamers, have been shown to reduce the particle size and change the habit of ethyl p-hydroxybenzoate.33,34 However, even at a high concentration of 0.15% w/w, a significant fraction of the crystals still exhibit the original morphology. More recently, double hydrophilic block copolymers (DHBCs) have been shown to be effective in engineering the crystal habit of some inorganic materials.35,36 Because of the surface activity of ABCs, we hypothesize that ABCs could preferentially adsorb onto the different growing crystal faces and thus be effective in the habit modification of some pharmaceuticals during crystallization from aqueous solutions. Furthermore, we hypothesize that these ABCs will mainly be adsorbed on the surfaces of the grown host crystals rather than be incorporated into the crystal lattice owing to the distinct differences in the molecular sizes between the ABC and the host. In this study, we choose poly(ethylene glycol)-block-poly(lactic acid) (PEG-b-PLA) as a representative ABC. The model pharmaceutical compound used is tolazamide, an oral hypoglycemic agent with no reported polymorphs. As expected from its molecular structure, the solubility of tolazamide remains essentially unchanged at pHs below its pKa and increases log linearly as the pH increases beyond its pKa (Figure 1). The experimental solubility values were fitted to the theoretical equation, giving an intrinsic solubility of 69.99 ( 1.75 µg/mL and a pKa value of 5.90 ( 0.02. From the pH-solubility profile, it is clear that we could perform crystallization studies by adjusting the pH of the solution from high
Kuldipkumar et al.
Figure 1. pH-Solubility profile of tolazamide at 25 °C.
to low. Therefore, an aqueous buffer solution with pH 5.9 was chosen as the crystallization medium. Tolazamide was dissolved in 0.1 N NaOH at a concentration of 1% w/v, and this solution was added to the pH 5.9 buffer. To maintain the pH of the crystallization medium, an equal amount of 0.1 N HCl solution was added to the crystallization medium simultaneously. The photomicrographs of tolazamide crystals obtained in the absence of PEG-b-PLA have a needle or acicular habit (Figure 2a). It is well documented that the habit of a crystal is determined by the relative growth rates of its different faces.6 These face growth rates in turn are determined by numerous internal/structural factors such as crystal packing and defects as well as by external parameters such as solvent, supersaturation, and the presence of impurities.37 The large differences in the growth rates of the individual crystal faces result in crystals with extreme habits (e.g., acicular). This seems to be the case for tolazamide crystals obtained from aqueous solution in the absence of any additive. In the presence of 12.5 µg/mL of PEG-b-PLA, tolazamide crystals with two different habits are obtained: needle and plate shaped (Figure 2b). Upon further increase in the concentration of PEG-b-PLA to 50 µg/ mL, only plate-shaped crystals of tolazamide are obtained (Figure 2c). This indicates that the PEG-b-PLA affects the growth rates of the tolazamide individual crystal faces differently. It is well documented that the presence of an additive that is surface active can have a profound influence on the growth rates of the individual crystal faces.38 In our case, we speculate that the hydrophobic PLA block is selectively adsorbed onto the growing crystal faces. This causes the hydrophilic PEG block to protrude from these faces into the solution, thus forming an interfacial layer that sterically hinders the approach of incoming tolazamide molecules from solution to these faces. Moreover, the adsorption of these block copolymer molecules on the surface effectively reduces the number of incorporation sites on the crystal surfaces that are critical for crystal growth. As a result of this selective adsorption of the ABC and the resulting decrease in the growth rates, a change in the habit of tolazamide crystals occurs. Both blocks of the copolymer are necessary for the additive to be effective in habit modification. This was demonstrated by the control experiments performed using the individual homopolymers of the ABC. There
Amphiphilic Block Copolymer as a Crystal Habit Modifier
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Figure 2. Photomicrographs of tolazamide crystals obtained by recrystallization (a) in the absence of additive, (b) in the presence of 12.5 µg/mL of PEG-b-PLA, (c) in the presence of 50 µg/mL of PEG-b-PLA, (d) in the presence of 1000 µg/mL of PEG 4600, and (e) in the presence of saturated solution of PLA 6000-16000. Magnification for (a-c) is 400× and for (d) and (e) is 200×.
is no change in the habit of tolazamide when it is crystallized from aqueous solutions containing either PEG 4600 (Figure 2d) or PLA 6000-16000 (Figure 2e) alone. Various additives, including tailor-made additives, low molecular weight surfactants, natural polymers, synthetic homopolymers, and polymeric surfactants, have previously been studied for their ability to modify the habit of organic crystals. The effectiveness of these
additives varies widely. The minimum concentration at which the additives are effective varies from 0.0002% (w/v) to as high as 10% (w/w).8,10-16,18-26,28-29,33-34 Compared to these additives, PEG-b-PLA is one of the most effective ones in habit modification. Previously, the amount and the location of contamination of the crystallizing solute from additives in the crystallization medium were studied by continuous24,29 or stepwise25,26,28 washing, followed by HPLC analysis
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Figure 3. Residual PEG-b-PLA in/on the tolazamide crystals as a function of washing.
of the washing solution. In this study, we developed a 1H NMR-based analytical technique and performed experiments on the washed crystals to quantify the level of PEG-b-PLA associated with tolazamide crystals obtained via crystallization in the presence of the ABC. The detection and quantitation limits in the analytical solution are 2.62 × 10-3 and 8.74 × 10-3 µg/g of DMSO, respectively. Using these values together with the typical sample preparation procedure of tolazamide crystals, the detection and quantitation limits of the ABC associated with the sample crystals are 6.24 × 10-3 and 2.08 × 10-2% (weight), respectively. These low values suggest that we will be able to detect and quantify very low levels of PEG-b-PLA associated with the recrystallized tolazamide crystals. When tolazamide is crystallized in the presence of 30 µg/mL of PEG-b-PLA, the level of PEG-b-PLA associated with the crystals is 0.55% (weight). This is a small but significant contamination, in the context of pharmaceutical applications. Washing the tolazamide crystals with water does not seem to remove the ABC effectively as shown in Figure 3. However, washing the crystals with toluene gradually removes the ABC associated with the crystals (Figure 3). After only two washes, the amount of PEG-b-PLA associated with the crystals decreases to approximately 0.03% (weight), which is insignificant and is very close to the quantitation limit. The solubility of tolazamide in toluene was determined to be 0.8 mg/mL. Therefore, the maximum amount of tolazamide that could be dissolved in the washing solvent after two washes is 16 mg, out of the total solid of 196 mg. Assuming isotropic dissolution from the tolazamide crystals and that the dissolution is homogeneous throughout the population of the crystals, this weight loss corresponds to only a 2.8% decrease in each dimension, on average. The above calculation is based on the solubility value of tolazamide in toluene, and therefore it represents the worst case. However, in reality, the samples were washed with toluene for only
Kuldipkumar et al.
Figure 4. Melting point of the recrystallized tolazamide crystals as a function of washing. The upper/lower limits represent average ( standard deviation of three replicates.
10 min, and the actual concentration of tolazamide in the washing solvent is not likely to reach solubility. Hence, the actual size decrease upon washing with toluene is expected to be below 2.8%. Therefore, it is safe to conclude that PEG-b-PLA is mainly adsorbed onto the crystal surface(s), rather than being incorporated into the tolazamide crystal lattice. The crystal form of the recrystallized tolazamide was also evaluated to ensure that the presence of additive in the crystallization medium did not change the crystal form, although there were no reported polymorphs for tolazamide in the literature. Meanwhile, the crystallinity of the recrystallized tolazamide was evaluated to ensure that it was not adversely affected by the additive. In the powder X-ray diffraction (PXRD) patterns (see Supporting Information) of the tolazamide crystals obtained in the absence and the presence of the PEGb-PLA, strong diffraction peaks are observed with no amorphous halo, indicating high crystallinity for all samples. Many of the peaks overlap, some vary in intensity, and some appear and disappear. This variation in PXRD patterns is likely due to the preferred orientation of the crystals, not the change in crystal form, because the habit is changed from needle- to plateshaped crystals.39 The differential scanning calorimetry (DSC) thermograms (see Supporting Information), show a single sharp endotherm with similar melting temperatures. The slight depression of melting point for the tolazamide samples crystallized in the presence of the ABC is of concern. It could be an indication of decreased crystallinity. However, we have shown above that a small amount of ABC contamination was observed, which could also contribute to the slight decrease in the melting point. Therefore, another DSC study was performed using the washed tolazamide crystals obtained in the presence of 30 µg/mL PEG-b-PLA. The melting points of the tolazamide crystal samples are plotted in Figure 4. They correlate very well with the removal of
Amphiphilic Block Copolymer as a Crystal Habit Modifier
the adsorbed ABC. Beyond two washings, when the residual ABC is very low, the melting point of tolazamide crystals is equal to or higher than that of the original solid. These results strongly suggest that the slight melting point depression is caused by the surfaceadsorbed polymer instead of the decrease in crystallinity. On the basis of the above results, we conclude that the presence of ABC, at a concentration up to 50 µg/mL in the crystallization medium, does not change the crystal form or decrease the crystallinity of the recrystallized tolazamide crystals. In conclusion, we have demonstrated, as hypothesized, that an ABC is very effective in modifying the habit of tolazamide crystals during crystallization from aqueous media, in a concentration-dependent manner. It was also shown that both blocks of the additive are necessary, and this change in habit cannot be achieved by the presence of either of the blocks alone in the solution. The crystal form and crystallinity of tolazamide were not affected by the presence of the ABC during crystallization. Washing the tolazamide crystals obtained with an appropriate solvent effectively removed the residual ABC, indicating that the ABC is mainly adsorbed on the crystal surface rather than being incorporated into the lattice of the crystals. This change in habit from needle to plate shape will help improve the flow characteristics of the powder, which in turn may improve the quality of the pharmaceutical end products and broaden the pharmaceutical formulation and processing options (such as direct compression). The mechanism of this habit modification by PEG-bPLA is currently under investigation. Especially important are the mechanism of tolazamide crystallization from aqueous solution and the specific interactions between PEG-b-PLA and the crystal surface(s). Acknowledgment. The authors thank Dr. Devalina Law (Abbott Laboratories) for helpful discussions. This work was supported in part by NIH Grant A104334607. Supporting Information Available: Materials and methods; molecular structure of PEG-b-PLA and tolazamide (Figure 1); solution 1H NMR spectra of tolazamide crystallized in the presence of PEG-b-PLA. (Figure 2); standard curve for the determination of PEG-b-PLA (Figure 3); PXRD patterns and DSC thermograms of tolazamide crystallized in the presence of ABC (Figure 4). This material is available free of charge via the Internet at http://pubs.acs.org.
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CG050049M
