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The Ester Group: How Hydrofluoroalkane-philic Is It? Robson P. S. Peguin, Libo Wu, and Sandro R. P. da Rocha* Department of Chemical Engineering and Materials Science, Wayne State UniVersity, 5050 Anthony Wayne DriVe, Detroit, Michigan 48202 ReceiVed April 5, 2007. In Final Form: May 18, 2007 Pressurized metered-dose inhalers (pMDIs) have been recognized as potential devices for the delivery of systemically acting drugs, including biomolecules, to and through the lungs. Therefore, the development of novel excipients capable of imparting stability to suspension formulations in hydrofluoroalkane (HFA) propellants is of great relevance because many of the drugs of interest are poorly soluble in HFAs. In this work, we use ab initio calculations and chemical force microscopy (CFM) to determine the HFA-philicity of the biodegradable and biocompatible ester moiety quantitatively. The complementary information obtained from the binding energy calculations and adhesion force measurements are used to gain microscopic insight into the relationship between the chemistry of the moiety of interest and its solvation in HFA. A lactide (LA)-based copolymer surfactant was synthesized and characterized, and its ability to stabilize a dispersion of micronized budesonide in HFA227 was demonstrated. These results corroborate the ab initio calculations and CFM and show that the LA-based moiety is a suitable candidate for enhancing the stability of dispersions in HFA-based pMDIs.
Introduction Pressurized metered-dose inhalers (pMDIs) are currently used to deliver small drug compounds to the lungs for the treatment of asthma and chronic pulmonary diseases.1 These formulations are either solution- (the drug is soluble in the propellant) or suspension-based (the drug is insoluble in the propellant).2 pMDIs are also potential candidates for the delivery of systemically acting drugs to and through the lungs.3-6 They are the least expensive and most widely used oral inhalation therapy devices.7 With the replacement of CFCs, as required by the Montreal Protocol, the non-ozone-depleting and biocompatible 1,1,1,2tetrafluoroethane (HFA134a) and 1,1,1,2,3,3,3-heptafluoropropane (HFA227) have become the propellants of choice for pMDIs.8 However, there have been many challenges associated with the transition to HFA-based pMDIs, with the low solubility of FDA-approved surfactants9 in HFA propellants being one of the most notable.10,11 Such a problem arises because of the mismatch between the hydrogenated tails of the amphiphiles and the semifluorinated (and somewhat polar) nature of the HFA propellants.12 Amphiphiles are generally required excipients in pMDIs and are used to improve dosage reproducibility and for valve lubrication.13,14 Whereas cosolvents can be used to enhance * To whom correspondence should be addressed. E-mail: sdr@ eng.wayne.edu. Tel: +1-313-577-4669. Fax: +1-313-577-3810. (1) Courrier, H. M.; Butz, N.; Vandamme, T. F. Crit. ReV. Ther. Drug 2002, 19, 425-498. (2) Johnson, K. A. In Inhalation Aerosols: Physical and Biological Basis for Therapy, 2nd ed.; Hickey, A. J., Ed.; Informa Healthcare: New York, 2007; pp 347-371. (3) Keller, M. Int. J. Pharm. 1999, 186, 81-90. (4) Laube, B. L. Resp. Care 2005, 50, 1161-1176. (5) Owens, D. R.; Zinman, B.; Bolli, G. Diabetic Med. 2003, 20, 886-898. (6) Patton, J. S. AdV. Drug DeliVery ReV. 1996, 19, 3-36. (7) Terzano, C. Pulm. Pharmacol. Ther. 2001, 14, 351-366. (8) FDA; Use of Ozone-Depleting Substances; RemoVal of Essential-Use Designations; Final Rule; National Archives and Records Administration: Washington, DC, 2005; Vol. 2003P-0029, pp 17168-17192. (9) In practice, what is approved by the FDA is the formulation and not the surfactant. The surfactant is one of the excipients present in the formulation. In this letter, we use the term “FDA-approved surfactant” loosely, indicating those surfactants present in FDA-approved oral inhalation formulations. (10) Blondino, F. E.; Byron, P. R. Drug DeV. Ind. Pharm. 1998, 24, 935-945. (11) Dickinson, P. A.; Seville, P. C.; McHale, H.; Perkins, N. C.; Taylor, G. J. Aerosol Med. 2000, 13, 179-186. (12) Vervaet, C.; Byron, P. R. Int. J. Pharm. 1999, 186, 13-30.
surfactant solubility in HFAs, they may decrease the overall chemical and physical stability of the formulation.12,15 Because many of the drugs of interest (including biomolecules) are not soluble in the HFA propellants, the development of novel excipients capable of imparting stability to suspension formulations in HFAs is of great relevance to the field.16-18 Steric stabilization is one possible mechanism for stabilizing colloidal dispersions in low dielectric media such as the HFAs.19,20 Effective surface-active agents will have an anchor moiety that interacts with the dispersed phase (active drug) and a stabilizing group that is well solvated by the propellant medium (HFA).20 The objective of this work is to quantify the HFA-philicity (affinity of a solute to HFA) of a biodegradable and biocompatible ester moiety (lactide - LA),21-23 which is a potential group (HFAphile) to be used as a general stabilizing moiety as, for example, in surfactants or polymeric coatings of particles. The evaluation of the HFA-philicity of a moiety starts with the calculation of the nonbonded pair interaction (binding) energy (Eb) between the solvent (HFA227) and a representative fragment of the candidate stabilizing group (LA, Figure 1a). The adhesion force (Fad) between a chemically modified AFM tip and substrate (with the same chemistry, in our case LA) is also employed to quantify the degree of solvation of the moiety of interest. We compare and contrast our results with the alkyl-based tail (CH3), (13) Hickey, A. J. In Inhalation Aerosols: Physical and Biological Basis for Therapy; Lenfant, C., Ed.; Marcel Dekker: New York, 1996; Vol. 94, pp 417436. (14) Rogueda, P. Expert Opin. Drug DeliVery 2005, 2, 625-638. (15) Tzou, T.-Z.; Pachuta, R. R.; Coy, R. B.; Schultz, R. K. J. Pharm. Sci. 1997, 86, 1352-1357. (16) Patton, J. S. Ther. Proteins 1993, 329-347. (17) Byron, P. R. AdV. Drug DeliVery ReV. 1990, 5, 107-132. (18) Wu, L.; Peguin, R. P. S.; Selvam, P.; Chokshi, U.; da Rocha, S. P. R. Molecular Scale Behavior in Alternative Propellant-Based Inhaler Formulations. In Inhalation Aerosols: Physical and Biological Basis for Therapy, 2nd ed.; Hickey, A. J., Ed.; Informa Healthcare U.S.A.: New York, 2007; pp 373-397. (19) Tadros, T. F. In Colloid Stability: The Role of Surface Forces; Tadros, T. F., Ed.; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2007; Vol. 1, Part I, pp 1-22. (20) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: Orlando, FL, 1983; p 428. (21) James, S. S. Drug DeliVery Tech. 2002, 2, 62, 64-69. (22) James, S. S.; Duan, D. C.; Myrdal, P. B. Respir. Drug DeliVery VII 2000, 83-90. (23) Tokiwa, Y.; Calabia, B. P. Appl. Microbiol. Biotechnol. 2006, 72, 244251.
10.1021/la700996x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/28/2007
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Figure 1. Optimized structures (MP2/6-31g+(d,p)) of the (a) HFA227-LA and (b) HFA227-CH3 complexes. The interaction points between the pairs are indicated by dashed lines. Distances are in angstroms. Cartesian coordinates of the optimized structures are available in Supporting Information.
which is the baseline group (i.e., tails of the amphiphiles in FDA-approved formulations that have low solubility in HFAs). To validate the ab initio (Eb) and chemical force microscopy (CFM)24 (Fad) results, we synthesize a copolymer amphiphile containing the LA fragment and test its ability to stabilize a dispersion of budesonide in HFA227 at ambient temperature and saturation pressure of the propellant. Ab Initio Calculations. To assess the enthalpic interactions between the HFA propellant (here we selected HFA227) and candidate tail moieties, the binding energy (Eb) between the propellant and selected tail fragments25,26 was computed using the supermolecule approach27
Eb ) Est - Es - Et
(1)
where Est is the total energy of the complex HFA227 + tail fragment and Es and Et are the energies of the isolated HFA227 and fragment, respectively. With this approach, more negative Eb’s represent more energetically favorable pairs. Here we investigate an ester-based fragment (lactide, LA; Figure 1a) and its hydrogenated analog (isohexane, CH3; Figure 1b). CH3 represents the tails of surfactants in FDA-approved formulations and serves as a baseline in our studies. The LA group was selected as a promising moiety because of the wide applicability of esterbased copolymers in the pharmaceutical industry and as implantable materials.28 Both raw and corrected energies (corrected for basis set superposition error, BSSE) are reported in Supporting Information. The BSSE was determined using the counterpoise (CP) method of Boys and Bernadi.29 Calculations were carried out in Gaussian 03.30 Complete geometry optimization calculations were made for each fragment and the complexes in the second-order Møller-Plesset perturbation method (MP2) and the 6-31+g(d,p) basis set. Single-point MP2 energy calculations have been carried out with the aug-cc-pVDZ basis set.31 The results of these optimizations correspond to energy minima because no imaginary frequencies were observed.32 We discuss the results in terms of the average between the raw and CP-corrected interaction energies. The average value has been shown to be a good approximation to the complete basis set limit, thus avoiding computationally expensive calculations at (24) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. ReV. Mater. Sci. 1997, 27, 381-421. (25) Selvam, P.; Peguin, R. P. S.; Chokshi, U.; da Rocha, S. R. P. Langmuir 2006, 22, 8675-8683. (26) Wu, L.; Peguin, R. P. S.; da Rocha, S. P. R. J. Phys. Chem. B, accepted for publication, 2007. (27) Morokuma, K.; Kitaura, K. In Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J., Eds.; Wiley: New York, 1980; Vol. 1, pp 21-87. (28) Agrawal, C. M.; Ray, R. B. J. Biomed. Mater. Res. 2001, 55, 141-150. (29) Gutowski, M.; Van Duijneveldt-Van de Rijdt, J. G. C. M.; Van Lenthe, J. H.; Van Duijneveldt, F. B. J. Chem. Phys. 1993, 98, 4728-4737.
higher levels of theory and larger basis sets.33-35 Also, the overestimation of the energy at the MP2 level of theory is minimized when using the average.34,36 Chemical Force Microscopy (CFM). Perhaps the closest microscopic experimental analog of the ab initio calculations discussed above is the CFM. A schematic of the experimental procedure is shown in Figure 2a. Details of materials and surface modification procedures used for chemical functionalization of the AFM tip and substrate necessary for the CFM experiments are given in Supporting Information. The adhesion force (Fad) between the chemically modified tip and substrate was measured at 298 K by vapor deposition.37-39 The force-distance curves were measured in isooctane and in 2H,3H-perfluoropentane (HPFP), which is a solvent that mimics HFAs,25,40-45 in order to evaluate their solvation effects on the chemistries of interest.24,46,47 Tips and substrates were modified with both octyltrichlorosilane (CH3) and 2-acetoxyethyltrichlorosilane (LA). These silanes are representative of the fragments that were investigated using the ab initio calculations discussed above and are given the same name. All of the tips and substrates were rinsed with chloroform and dried under N2 just prior to mounting them into the AFM fluid cell. Fad measurements were performed at seven contact points that were randomly distributed on the sample surfaces. At each contact point, 25 force-distance curves were measured. A histogram of Fad was then constructed. The average and deviation of the Fad’s were determined by fitting a Gaussian distribution to the histograms. The substrates were moved toward and away from the tip in the range of 2000 nm, with a speed of 4 s‚cycle-1 and set force of 1.8 nN. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision A.01; Gaussian, Inc.: Wallingford, CT, 2004. (31) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007-1023. (32) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods: A Guide to Using Gaussian, 2nd ed.; Gaussian: Pittsburgh, PA, 1998; pp 70-72. (33) Baradie, B.; Shoichet, M. S.; Shen, Z.; McHugh, M. A.; Hong, L.; Wang, Y.; Johnson, J. K.; Beckman, E. J.; Enick, R. M. Macromolecules 2004, 37, 7799-7807. (34) Kilic, S.; Michalik, S.; Wang, Y.; Johnson, J. K.; Enick, R. M.; Beckman, E. J. Ind. Eng. Chem. Res. 2003, 42, 6415-6424. (35) Feller, D.; Jordan, K. D. J. Phys. Chem. A 2000, 104, 9971-9975. (36) Hyla-Kryspin, I.; Haufe, G.; Grimme, S. Chem.sEur. J. 2004, 10, 34113422. (37) Duchet, J.; Chabert, B.; Chapel, J. P.; Gerard, J. F.; Chovelon, J. M.; Jaffrezic-Renault, N. Langmuir 1997, 13, 2271-2278. (38) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274. (39) Jung, G.-Y.; Li, Z.; Wu, W.; Chen, Y.; Olynick, D. L.; Wang, S.-Y.; Tong, W. M.; Williams, R. S. Langmuir 2005, 21, 1158-1161. (40) Ashayer, R.; Luckham, P. F.; Manimaaran, S.; Rogueda, P. Eur. J. Pharm. Sci. 2004, 21, 533-543. (41) Peguin, R. P. S.; Selvam, P.; da Rocha, S. R. P. Langmuir 2006, 22, 8826-8830. (42) Rogueda, P. G. A. Drug DeV. Ind. Pharm. 2003, 29, 39. (43) Traini, D.; Rogueda, P.; Young, P. M.; Price, R. Pharm. Res. 2005, 22, 816-825. (44) Traini, D.; Young, P. M.; Rogueda, P.; Price, R. Int. J. Pharm. 2006, 320, 58-63. (45) Young, P. M.; Price, R.; Lewis, D.; Edge, S.; Traini, D. J. Colloid Interface Sci. 2003, 262, 298-302. (46) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943-7951. (47) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. ReV. 1999, 99, 2845-2890.
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Figure 2. (a) Schematic diagram of the CFM experiment showing the chemically modified AFM tip and substrate immersed in solvent (HPFP). (b) Adhesion force (Fad) normalized by the radius of curvature of the AFM tip (R ) 33 nm) for hydrogenated (CH3) and ester-based (LA) tip-substrate combinations in HPFP or isooctane (ISO) solvent.
Results and Discussion Ab initio calculations and CFM were used to relate the chemistry of fragments of interest to their HFA-philicity quantitatively. The Eb calculations revealed that HFA227 has strong enthalpic interactions with LA, as indicated by an Eb of -8.00 Kcal‚mol-1. The interaction points of the HFA227-LA pair consist of a strong H‚‚‚O bond and several weaker F‚‚‚H contacts, as shown in Figure 1a (bond distances in angstroms). This is in sharp contrast to the weak interaction observed with CH3, the hydrogenated analog to LA, of -4.44 Kcal‚mol-1 that arises because of the presence of weak F‚‚‚H interactions only (Figure 1b). These results are in agreement with the experimentally observed low solubility and inability of methyl-based surfactants to stabilize drug dispersions in HFAs.15 The closest microscopic experimental analog to the Eb calculations described above is the adhesion force (Fad) obtained by CFM.24 In this work, both an AFM tip and a substrate (silicone wafer) were chemically modified as schematically illustrated in Figure 2a. 2-Acetoxyethyltrichlorosilane and octyltrichlorosilane moieties were used as representative of the LA and CH3 fragments, respectively. The Fad was determined in HPFP, which is a liquid HFA under ambient conditions.42 To put the results into perspective, force-distance curves between the modified tip and substrate were also determined in isooctane (ISO). The average Fad curves normalized by the AFM tip radius of curvature are shown in Figure 2b. The much lower Fad in HPFP between the ) 4.8 ( 1.8 mN‚m-1, LA-modified tip and substrate, FLA-LA ad indicates that the ester moiety is significantly better solvated by 3-CH3 HPFP than the hydrogenated tail, which produced an FCH ad -1 of 68.2 ( 17.6 mN‚m . At the same time, one can argue that there is still room for improvement in the selection of the HFAphile when 4.8 mN‚m-1 is contrasted with the zero adhesion force observed for the methyl-modified tip and substrate in ISO, 3-CH3 FCH ) 0, which is the situation closest to “ideal” solvation. ad The low FLA-LA in HPFP is in direct agreement with the more ad negative Eb’s determined with the ab initio calculations. However, CFM results provide an important additional piece of information, which is the degree of solvation of the candidate moiety relative to ideal solvation. Combined, the ab initio and CFM results indicate that HFAs have strong interactions with LA, indicating that this is a promising HFA-phile. To confirm the above results, we investigated the physical stability of a 2 mg‚mL-1 budesonide suspension in (a) pure HFA227 and (b) an LA-based surfactant solution in HFA227. We synthesized a copolymer of lactide and -caprolactone (LA340-
Figure 3. Physical stability of the budesonide suspension (2 mg‚mL-1) in (a) pure HFA227 and (b) 0.4 mg‚mL-1 LA340CL209 in HFA227.
CL209). The synthesis and characterization of the copolymer and the preparation of the HFA227 suspension are described in Supporting Information. Snapshots of the budesonide formulations in HFA227 as a function of the elapsed time after dispersion are shown in Figure 3. Whereas low-molecular-weight oligolactic acid is highly soluble in HFA134a,21,22 the solubility of the copolymer containing the stabilizing PCL moiety was found to be fairly limited. Therefore, comparative stability studies were not performed in HFA134a. The budesonide particles start to cream out of the pure HFA227 in just a few minutes, and the dispersion is almost completely phase separated after 12 h. A significant improvement can be accomplished by the addition of LA340CL209 to the formulation. No signs of flocculation or creaming for over 12 h are observed when in the presence of the LA-based amphiphile. The physical stability of the drug suspension in HPFP was observed to be similar to that in HFA227. The more hydrophobic CL segments are expected to serve as anchors, interacting with the hydrophobic budesonide particles. The LA tails are expected to interact with the HFA propellant, thus protecting the particles from flocculating. These results are in direct agreement with the ab initio and Fad results discussed above. LA-based amphiphiles are expected to be capable of stabilizing other drugs in HFA propellants if a suitable anchor moietysone that interacts well with the drugsis found.
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Conclusions
based pMDI formulations. These results may be helpful in extending the applicability of these simple and inexpensive devices (pMDIs) to the systemic delivery of biomolecules to and through the lungs.
We have quantitatively determined the HFA-philicity of the LA fragment using a combined computational and experimental microscopic approach. The enthalpic interaction between HFA227 and LA was determined using binding energy (Eb) calculations and chemical force microscopy (adhesion force, Fad). The results show that HFA interacts much more favorably with the LA fragment than with its hydrogenated counterpart (CH3), as indicated by a very negative Eb for the HFA227-LA complex of -8.00 Kcal‚mol-1 and a low Fad of 4.8 ( 1.8 mN‚m-1 for the LA-modified tip and substrate in HPFP. We have synthesized a biodegradable and biocompatible copolymer containing the LA-based moiety (LA340CL209), which dramatically improved the stability of budesonide crystals dispersed in HFA227. These bulk physical stability studies corroborate the microscopic information obtained from ab initio calculations and chemical force microscopy. Combined, the results show that LA is a viable HFA-phile that can be used to generate stable suspensions of nonsoluble drugs in HFAs. Understanding the solvation in HFAs is of great relevance for the development of novel dispersion-
Acknowledgment. We thank Wayne State University for start-up funds, a Ph.D. assistantship for L.W., and a GRA (IMR at WSU) for R.P. S.P. We also acknowledge Dr. Schlegel’s group at Wayne State University for useful discussions related to the ab initio calculations, GRID/WSU for computer time, Solvay Fluor and Derivate GmbH & Co., Hannover, Germany, for the HFA227 samples, and NSF-CBET 0553537 for financial support. Supporting Information Available: Details of materials and surface modification procedures used for chemical functionalization of the AFM tip and substrate. Synthesis procedure and characterization of the LA-based copolymer. Cartesian coordinates of the optimized structures. Raw, CP-corrected, and average Eb’s. This material is available free of charge via the Internet at http://pubs.acs.org. LA700996X
