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Oil-Filled Silica Nanocapsules for Lipophilic Drug Uptake: Implications for Drug Detoxification Therapy Royale S. Underhill,† Aleksa V. Jovanovic,† Stephen R. Carino,† Manoj Varshney,‡ Dinesh O. Shah,§,| Donn M. Dennis,⊥,# Timothy E. Morey,⊥ and Randolph S. Duran*,† Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, Department of Chemistry, Hamdard University, New Delhi, India, and Center for Surface Science & Engineering, and Departments of Chemical Engineering, Anesthesiology, and Pharmacology & Therapeutics, University of Florida, Gainesville, Florida 32611 Received March 4, 2002. Revised Manuscript Received September 16, 2002
Oil-filled nanocapsules were synthesized using the oil droplets of an O/W microemulsion as templates. A polysiloxane/silicate shell was formed at the surface of the oil droplet by cross-linking n-octadecyltrimethoxysilane and tetramethoxysiloxane. The shell imparted stability to the oil droplets against coalescence. The nanocapsules can be used in a number of applications (i.e., biomedical or environmental) where the free concentration of lipophilic compounds must be reduced. As a proof, the nanocapsules (1.4% w/v oil content in saline) were shown to sequester quinoline (8 µM) from saline in 97% of 5, the 0.9% w/v oil content removed 45 ( 5% of 5, the 0.3% w/v oil content removed 35 ( 5% of 5, and the 0.1% w/v oil content removed 32 ( 5% of 5 (Figure 4 inset). In all cases, a monoexponential decay equation with plateau gave the best fit. This is consistent with the kinetics observed by Ding and Liu52 for the loading of Rhodamine B in water-soluble hollow nanospheres. Specifically, a biexponential decay equation with plateau did not give a statistically improved fit over a monoexponential decay equation with plateau for the 0.1% w/v oil content nanocapsule solution (P ) 0.88), 0.3% w/v oil content nanocapsule solution (P ) 0.36), 0.8% oil content nanocapsule solution (P ) 0.93), and 1.4% w/v oil content nanocapsule solution (P ) 0.10) data. To verify that the loss of micelles and microemulsion droplets had occurred and was responsible for the drop in removal efficiency, particle size analysis was performed after the addition of 5 (Figure 5B). The peak at 8 ( 1 nm is due to mixed micelles, a new peak appeared at 15(1 nm, and the microemulsion droplet and nanocapsule peaks at 33 ( 5 and 113 ( 33 nm are retained. The peak at 15 ( 1 nm is due to the mixed micelles being swollen with 5. This confirms that some of the removal efficiency is due to the micelles. This solution was then diluted 2-fold with saline to yield 0.8% w/v oil content nanocapsule concentration and allowed to equilibrate with stirring for 24 h. After 24 h, particle size analysis resulted in Figure 5C. The peaks at 8 ( 1, 15 ( 1, and 33 ( 5 nm were reduced. A 15-fold dilution of the original stock solution containing 5, to produce 0.1% w/v oil content, resulted in the loss of the 33 ( 5 nm peak altogether. The micellar peaks at 8 ( 1 and 15 ( 1 nm were replaced by a broad peak centered at 15 ( 2 (52) Ding, J.; Liu, G. J. Phys. Chem. B 1998, 102, 6107.
Silica Nanocapsules for Lipophilic Drug Uptake
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Conclusions
Figure 6. Removal of varying concentrations of bupivacaine as determined by HPLC. The data at low concentrations shows a linear fit with a correlation of 0.997; the dotted line shows the intersection indicating the capacity of the nanocapsules.
nm. The dilution reduced the presence of micelles and microemulsion droplets, thus proving that, at high dilution, the nanocapsules were responsible for the uptake of the quinoline. Thus, the 0.1% w/v oil content data in Figure 4 inset show that the nanocapsules sequester 32 ( 5% of 8 µM molecule 5. The objective for creating these nanocapsules was the removal of drugs from blood so a preliminary uptake study was performed with bupivacaine (6), a local anesthetic. Molecule 6 does not have intense bands in the UV-visible region of convenience to measure, so its removal from saline was monitored using HPLC. Current emulsion-based treatments such as Intralipid, used to deliver the antifungal, propofol, are used at concentrations of ≈1% w/v total oil content in the blood. The upper concentration limit for the commercially available products is 1.5% w/v oil content in blood. With this in mind, the stock nanocapsule solution was diluted to yield a solution that was 0.1% w/v oil content. To the 0.1% w/v oil content nanocapsule solution, 6 was added to yield final drug concentrations of 7, 27, 49, 96, 181, 363, 536, 1030, 3400, and 10380 µM. These solutions were allowed to equilibrate at room temperature for >30 min, after which they were centrifuged in Centrifree filter devices to remove the nanocapsules. The filtrate was analyzed by HPLC to determine the amount of free 6 present. As Figure 6 illustrates, for concentrations 99% of 6 when the initial concentration is ≈200 µM. The amount sequestered becomes constant at 1919 µM while the initial concentration is increased. The leveling off of the amount of drug sequestered is reasonable since the nanospheres have a finite capacity and once this is reached they cannot uptake more no matter how much is added. Figure 6 indicates the capacity of the nanocapsules at 0.1% w/v oil is 1900 µM. Uptake studies in serum are in progress.
We have shown that oil-in-water microemulsions can be made using 2 with 1 as the cosurfactant. The microemulsions can template the formation of nanocapsules for use in varying conditions by the addition of 3, which polymerizes a polysiloxane/silicate shell around the droplets. The spherical shell fortifies the microemulsion droplets against coagulation or rupture. The resulting dispersions have a trimodal size distribution of micelles, microemulsion droplets, and nanocapsules. These nanocapsules were tested for their ability to sequester hydrophobic compounds from saline. Results show that the nanocapsules sequester the hydrophobic drug mimics quinoline and 1-dodecene and the drug bupivacaine over a relatively short period. This is promising for clinical applications in treating patients exhibiting toxic levels of lipophilic drugs, where time is critical. The removal of bupivacaine at concentrations from 10 to 10 000 µM is demonstrated. The resulting nanocapsules may be effective in other biomedical applications such as drug delivery.1,3,53 Other areas which may benefit from the stability imparted by the hard shell include environmental applications54 such as decontamination of industrial wastes55 and agricultural sprays56 where instability of conventional micelles and micremulsion due to changes in environment could be a major problem. Acknowledgment. The authors thank Dr. R. Baney and Dr. L. Gower for valuable discussions about silica and particle chemistry. Preliminary NMR studies were performed by Dr. J. Rathke, Dr. R. E. Gerald, and Dr. R. J. Klingler at the Chemical Technologies Division of the Argonne National Laboratories. The authors wish to thank the Electron Microscopy Core Laboratory of the Interdisciplinary Center for Biotechnology Research at the University of Florida for the use of their EM facilities. Financial support was provided by the Engineering Research Center (ERC) for Particle Science and Technology at the University of Florida, The National Science Foundation (NSF) (Grants EEC-94-02989 and NSF-CPE 8005851) and DOE-BES (Grant DE-FG0296ER45589). CM0202299 (53) Mathiowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedi, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410. (54) Kim, J.-Y.; Cohen, C.; Shuler, M. L.; Lion, L. W. Environ. Sci. Technol. 2000, 34, 4133. (55) Jeong, B. C.; Hawes, C.; Bonthrone, K. M.; Macaskie, L. E.; Microbiology 1997, 143, 2497. (56) Chappat, M. Colloids Surf. A 1994, 91, 57.
