- Email: [email protected]
Effect of ultrasound on the release of micro-encapsulated drugs
Ultrasonics
36 (1998) 709-712
Effect of ultrasound on the release of micro-encapsulated Peter J.A. Frinking a Thorurcenter,
drugs
a,*, Ayache Bouakaz a, Nice de Jong a-c, Folkert J. Ten Cate b, Siobhan Keating d
Erasmus University. P. 0. Box 1738, 3000 DR Rotterdam. The Nethrrlunds b Dijkzigt Hospital, Rotterdam, The Netherlunds ’ Interuniversity Cardiology Institute, Utrecht, The Netherlands ’ Andaris Ltd., I Mere Way, Nottinghum, UK
Abstract Although
ultrasound
for external controlled
is used extensively in medical therapies and diagnostics, it has been recognized only recently as a method delivery of drugs. In this paper, firstly, a literature review on drug delivery and the combination with
ultrasound irradiation,
is given. Then an experiment is described on measuring from a polymer carrier. 0 1998 Elsevier Science B.V.
Keywords:
Ultrasound;
Microspheres;
the release
of a model
drug (hexabrix)
under
ultrasound
Drug delivery
1. Introduction
2. Controlling the release rate
For many years, the major focus of drug research has been on the synthesis or discovery of new drugs. While this continues to be important, in the last few years attention has been paid to research aimed at creating new drug delivery systems. In the early days, sustained delivery was attempted by combining drugs with substances that decreased their solubility, coating them with materials that did not dissolve in stomach acid, compressing them in dense tablets or putting them into suspensions or emulsions. Although the drugs were effective for a longer period of time, the release kinetics were still strongly influenced by patient variations. To improve the delivery systems, new approaches have been developed [ 1,2] e.g.: ( 1) drug modification by chemical means. A drug may be chemically modified to alter properties like biodistribution, pharmacokinetics, solubility or antigenicity; (2) drug entrapment within pumps or polymer materials that are placed into the desired bodily compartments (in these delivery systems the release rate is almost exclusively controlled by the design of the polymer system or pumps); (3) drug entrapment in small vesicles that are injected into the bloodstream.
Controlled drug release systems provide several advantages over conventional drug therapies [ 1,2], i.e. (i) controlling drug release patterns, continuous versus pulsatile (these significantly affect the therapeutic response); (ii) localized delivery, so lowering the drug level; (iii) preservation of medication that are destroyed rapidly by the body; (iv) reduced need for follow-up care; (v) increased patient comfort and (vi) improved patient compliance. Langer [ 1,2] mentions several options how to embed drugs into polymer materials, each depending on the different release mechanism such as diffusion, chemical reaction or solvent activation. Diffusion may occur through a reservoir, in which a drug core is surrounded by a polymer film, or in a matrix, where the drug is uniformly distributed through the polymeric system. Chemical control is accomplished either by polymer degradation or chemical cleavage of the drug from the polymer. Solvent activation involves either swelling of the polymer or osmotic effects.
3. External drug control * Corresponding author. Tel: (31) IO 408 8035; fax: (31) 10436 5191; e-mail [email protected] 0041-624X/98/$19.00 6 1998 Elsevier Science B.V. All rights reserved. PI1 s0041-624x(97)00122-4
The polymer drug release systems can provide a sustained release of drugs over a long period of time.
710
P.J. A. Frinking
et al. I Ultrasonics
However, the release rate of the drugs is constant or decays over time. There is no possibility of changing the drug release on demand once it has started. A solution is to control the release by external means. Magnetic fields, electric pulses and ultrasound irradiation have been used [ 1,3,4]. The remainder of this article focuses on the approach using ultrasound to externally control the release of embedded drugs.
4. Ultrasound Ultrasound has been used extensively for medical diagnostics and to a certain extent in medical therapy ultrasonic surgery, hyperthermia). (physiotherapy, Nevertheless, it has only recently become popular as a technique to enhance drug release from drug delivery systems. A number of studies suggest the use of ultrasound as an external mean of delivering drugs at increased rates and at desired times. Some of them will be mentioned below. Ultrasound has been used to enhance transcutaneous drug delivery [5-91. Since the skin represents the major barrier to deliver drugs, low frequency ultrasound (20 kHz-1 MHz), compared to diagnostic ultrasound, is thought to increase the permeability of the skin. Although the exact mechanism which is responsible for the enhanced delivery using ultrasound is not clearly understood, the effect is explained by cavitation and heating. Ultrasound combined with drug release from polymer systems have been used in the field of cancer chemotherapy [lo]. 5-fluorouracil (S-FU) was embedded in an ethylene-vinyl alcohol copolymer using both the reservoir and matrix system. Ultrasound at 1 MHz was used and showed in both cases an increase in release rate which turned back to baseline after the ultrasound irradiation was stopped. Increasing temperature in the delivery system was speculated to be the cause of increased diffusion of the 5-FU from the polymer. Kost et al. [ll] investigated the effect of ultrasound on the degradation of polymers and the release rate of drugs incorporated in these polymers. Both an increase of the release of the drugs and polymer degradation was observed. Several mechanisms were found to be responsible for the effect of ultrasound. The release rate increased as a function of the applied intensity. Temperature and mixing were relatively unimportant in effecting enhanced polymer degradation, whereas cavitation appeared to play a significant role. Increased release rates were also observed when ultrasound was applied to biodegradable polymers implanted in rats.
5. Novel drug delivery systems Novel drug delivery systems have been described microspheres containing a therapeutic compound
using [ 121.
36 (19%)
709-712
These systems can be administered with special drug delivery catheters [ 13,141 and offer numerous advantages over conventional drug therapy. Especially gaseous containing microspheres, used as ultrasound contrast agents in the medical diagnostic field. They offer the possibility to be visualized during their transport in the human body using standard ultrasonic imaging systems. Therefore, the release can be controlled externally once the microspheres reach a diseased region [ 15,161. Ishihara [ 171 explains a method for using ‘micromachines’ (gas-filled microspheres) as drug carriers to selectively administer the drug to a localized region, and utilizing the fact that when the drug carrier is irradiated by ultrasound from a external source at the resonance frequency, ultrasonic energy is absorbed most effectively. This way, drugs embedded within the microspheres can be released due to rupture of the microspheres. Unger [ 181 also describes the use of gas-filled microspheres as delivery devices. The microspheres contain a therapeutic compound and a temperature activated gaseous precursor which becomes a gas (fluid-gas phase shift) upon activation at a selected temperature. Several ways to incorporate the therapeutic compound within the microspheres are mentioned. After administering the microspheres into the human body, the fluid-gas phase shift makes the microspheres detectable for diagnostic ultrasound. They can be monitored in real time until they are detected in a region of interest. Therapeutic ultrasound, using a lower frequency compared to diagnostic ultrasound, is then applied to the region in order to rupture the microspheres and release the therapeutic compound.
6. Experiment To get more experience in the field of ultrasonic induced drug delivery an experiment was set up at Andaris Ltd. Nottingham. Small polymeric microspheres were produced by spray-drying a 50150 Poly (DL Lactide-co-Glycolide) polymer, inherent viscosity Hexabrix, a standard X-ray contrast of 0.55 dl g-l. agent which is highly soluble in water, was used as a model drug. As a result of the spray-drying the microspheres were gas-filled and the Hexabrix was embedded within the shell (5% loading). The product obtained was a dry powder with a mean size of 2-3 urn (Fig. 1). To see if the microspheres could be detected by ultrasound, images were made with a standard ultrasound machine used for medical diagnosis (HP Sonos 1500, 3.5 MHz). First a baseline image was obtained (Fig. 2(A)) of 250 ml Isoton in a beaker which was put down on a mechanical stirrer. Then 27 mg of the product was suspended in 5 ml Isoton, gently inversed and added into the beaker. Again an image was obtained
01 0
30
60
90
I?0
I50
~imelmli
Fig. I. Microscopic view of the polymeric of 400. grid 5ize is 2 pm per div.
Fig. 2. Upper
(Fig. 2(B)) and a significant
microspheres;
panel shows baseline
magnification
Fig. 3. Percentage ultrasound alter
release
image: lower panel shows echo response
increase in echo genicity can be appreciated. The main purpose of this experiment was to see if the release of drugs embedded within a polymeric microsphere carrier can be controlled by ultrasound. To measure the release, 5.2 mg of the product was suspended in 0.01 M phosphate buffered saline (pH 7.4). At t = 0 min, a 1 ml sample was filtered through a 0.2 pm Sartorius RC15 filter and analyzed by a Beckman UV/VIS spectrophotometer, which was calibrated for Hexabrix, at 243 nm using quartz cuvettes. At this wavelength the absorbens of UV light is maximal for Hexabrix and does not overlap the maximum in absorbens of the polymer. Every 5-10 min a new sample was analyzed up to t = 55 min and from t= 120 min to t = 140 min. In between sampling the product was left on a rollermixer and gently mixed. The release was calculated by taking the theoretical loading of 5% into account. This experiment was carried out in threefold and the mean value and SD are shown by the drawn line in Fig. 3. After 140 min only 35% of the Hexabrix is released from the polymer. At t = 0 there is an offset in the release due to Hexabrix which is weakly bounded to the outer surface of the microspheres. Experiments
overtime.
sust;lineJ
release;
- - -.
t = I20 min.
when the product
is added
to determine the actual loading subscribed this explanation [ 191 (actual loading of 90- 95%). A fourth measurement was conducted in the same way as the previous one, only this time the product was put in an ultrasonic bath after t = I30 min and irradiated for 1.5 min at 35 kHz at maximum intensity. As appears by the dotted line Fig. 3. there is a significant increase in release up to 100%. This experiment only shows the release of a compound embedded within a microsphere with and without ultrasound. The efl’ect of a wide variety of parameters still have to be investigated. i.e. acoustic pressure amplitude, frequency and duty cycle. The degradation of the polymer was not measured in this experiment.
7. Discussion The release rate of drugs from a carrier can be controlled depending on three predominant parameters: ( 1 ) carrier composition, (2) drug incorporation and (3 ) ultrasonic parameters. The physical characteristics of the drug carrier determines the release rate depending
712
P.J. A. Fritzking et ~11./ Ultrusonics 36 (1998) 709-712
on its degradation, rupture rate or temperature rise. The drug incorporation to a microsphere carrier can be done in different ways. The drug can be encapsulated in the microsphere, so within the gas containing lumen. A disadvantage is that by putting a therapeutic compound in this place the acoustic behavior (echogenicity) of the microsphere will change. It is also difficult to make this kind of delivery system. An easier way is to embed the drug in the shell of the microsphere. In this way the microsphere will contain its acoustic properties and can be located in a specific region within the human body. The last method is to attach or chemically link the drug to the outer surface of the shell. In case of chemical linking it is important to know how the drug is linked (strong or weak) because this will have consequences on the effect that ultrasound will have on the release rate. Attaching an antibody to the outer surface of the shell is also possible. In this way the drugs can be delivered to very specific parts i.e. targeted delivery. According to the drug position, the release will be gradual or instantaneous, depending on the desired application. Although ultrasound is used extensively in medical therapy and diagnostics, it has been recognized only recently as a method for external controlled delivery of drugs. Several studies have shown that the release rate is directly related to the ultrasonic intensity, and that ultrasound has an effect on polymer degradation. However, the exact underlying mechanism is as yet unknown. Other ultrasonic parameters, such as frequency, duty cycle and duration may have an effect on the release rate and the local absorption of drugs. From the field of ultrasound contrast agents, a lot of experience is available on the interaction of ultrasound
and gas-filled microspheres. It is the conviction of the authors that this expertise is of great value for developing ultrasound directed drug delivery systems.
References [ I] R. Langer. Science 249 ( 1990) 1527. [2] R. Langer, Chem. Brit. (March 1990) 233. [3] D.S.T. Hsieh, R. Langer, Proc. Nat]. Acad. Sci. USA 78 (1981) 1863. [4] M.R. Prausnitz, V.G. Bose, R. Langer, Proc. Natl. Acad. Sci. USA 90 (1993) 1054. [5] J. Kost, Clin. Mat. 13 (1993) 155. [6] J. Kost, J. Contr. Release 24 (1993) 247. [7] N.N. Byl, Phys. Therapy 75 (1995) 539. [S] S. Mitragotri, D.A. Edwards, B. Blankschtein, R. Langer, J. Pharm. Science 84 ( 1995) 697. [9] S. Mitragotri, B. Blankschtein, R. Langer, Science 269 ( 1995) 850. [IO] S. Miyazaki, W.M. Hou, M. Takada, Chem. Pharm. Bull. 33 (1985) 428. [I 11 J. Kost, K. Leong, R. Langer, Proc. Natl. Acad. Sci. USA 86 ( 1989) 7663. [ 121 R.L. Wilenskey, K.L. March, Seminars in Interventional Cardiology I (1996) 48. [ 131 R.L. Wilenskey. K.L. March, D.R. Hathaway, Am. Heart Journ. 122(4)(1991) 1136. [ 141 L.A. Guzman, V. Labhasetwar, C. Song, Y. Jang, A.M. Lincoff. R. Levy, E.J. Topol, Circulation 94 (6) (1996) 1441. [15] T. Porter, S. Li, K. Kilzer, J. Desjardins, P. Iversen, 2nd Thoraxcenter European Symposium on Ultrasound Contrast Imaging, 1997, p. 25 (abstract). [ 161 E.C. Unger, 2nd Thoraxcenter European Symposium on Ultrasound Contrast Imaging, 1997, p. 54 (abstract). [ 171 K. Ishihara, Proc. Sec. Intern. Micromachine Symp., 1996, p. 69. [ 181 E.C. Unger, T. Fritz, T. Matsunaga, V. Ramaswami, D. Yellowhair. G. Wu. US patent Nr. 5 542 935 (Aug. 1996). [ 191 S. Keating, Andaris Ltd., Nottingham, Personal Communication.
36 (1998) 709-712
Effect of ultrasound on the release of micro-encapsulated Peter J.A. Frinking a Thorurcenter,
drugs
a,*, Ayache Bouakaz a, Nice de Jong a-c, Folkert J. Ten Cate b, Siobhan Keating d
Erasmus University. P. 0. Box 1738, 3000 DR Rotterdam. The Nethrrlunds b Dijkzigt Hospital, Rotterdam, The Netherlunds ’ Interuniversity Cardiology Institute, Utrecht, The Netherlands ’ Andaris Ltd., I Mere Way, Nottinghum, UK
Abstract Although
ultrasound
for external controlled
is used extensively in medical therapies and diagnostics, it has been recognized only recently as a method delivery of drugs. In this paper, firstly, a literature review on drug delivery and the combination with
ultrasound irradiation,
is given. Then an experiment is described on measuring from a polymer carrier. 0 1998 Elsevier Science B.V.
Keywords:
Ultrasound;
Microspheres;
the release
of a model
drug (hexabrix)
under
ultrasound
Drug delivery
1. Introduction
2. Controlling the release rate
For many years, the major focus of drug research has been on the synthesis or discovery of new drugs. While this continues to be important, in the last few years attention has been paid to research aimed at creating new drug delivery systems. In the early days, sustained delivery was attempted by combining drugs with substances that decreased their solubility, coating them with materials that did not dissolve in stomach acid, compressing them in dense tablets or putting them into suspensions or emulsions. Although the drugs were effective for a longer period of time, the release kinetics were still strongly influenced by patient variations. To improve the delivery systems, new approaches have been developed [ 1,2] e.g.: ( 1) drug modification by chemical means. A drug may be chemically modified to alter properties like biodistribution, pharmacokinetics, solubility or antigenicity; (2) drug entrapment within pumps or polymer materials that are placed into the desired bodily compartments (in these delivery systems the release rate is almost exclusively controlled by the design of the polymer system or pumps); (3) drug entrapment in small vesicles that are injected into the bloodstream.
Controlled drug release systems provide several advantages over conventional drug therapies [ 1,2], i.e. (i) controlling drug release patterns, continuous versus pulsatile (these significantly affect the therapeutic response); (ii) localized delivery, so lowering the drug level; (iii) preservation of medication that are destroyed rapidly by the body; (iv) reduced need for follow-up care; (v) increased patient comfort and (vi) improved patient compliance. Langer [ 1,2] mentions several options how to embed drugs into polymer materials, each depending on the different release mechanism such as diffusion, chemical reaction or solvent activation. Diffusion may occur through a reservoir, in which a drug core is surrounded by a polymer film, or in a matrix, where the drug is uniformly distributed through the polymeric system. Chemical control is accomplished either by polymer degradation or chemical cleavage of the drug from the polymer. Solvent activation involves either swelling of the polymer or osmotic effects.
3. External drug control * Corresponding author. Tel: (31) IO 408 8035; fax: (31) 10436 5191; e-mail [email protected] 0041-624X/98/$19.00 6 1998 Elsevier Science B.V. All rights reserved. PI1 s0041-624x(97)00122-4
The polymer drug release systems can provide a sustained release of drugs over a long period of time.
710
P.J. A. Frinking
et al. I Ultrasonics
However, the release rate of the drugs is constant or decays over time. There is no possibility of changing the drug release on demand once it has started. A solution is to control the release by external means. Magnetic fields, electric pulses and ultrasound irradiation have been used [ 1,3,4]. The remainder of this article focuses on the approach using ultrasound to externally control the release of embedded drugs.
4. Ultrasound Ultrasound has been used extensively for medical diagnostics and to a certain extent in medical therapy ultrasonic surgery, hyperthermia). (physiotherapy, Nevertheless, it has only recently become popular as a technique to enhance drug release from drug delivery systems. A number of studies suggest the use of ultrasound as an external mean of delivering drugs at increased rates and at desired times. Some of them will be mentioned below. Ultrasound has been used to enhance transcutaneous drug delivery [5-91. Since the skin represents the major barrier to deliver drugs, low frequency ultrasound (20 kHz-1 MHz), compared to diagnostic ultrasound, is thought to increase the permeability of the skin. Although the exact mechanism which is responsible for the enhanced delivery using ultrasound is not clearly understood, the effect is explained by cavitation and heating. Ultrasound combined with drug release from polymer systems have been used in the field of cancer chemotherapy [lo]. 5-fluorouracil (S-FU) was embedded in an ethylene-vinyl alcohol copolymer using both the reservoir and matrix system. Ultrasound at 1 MHz was used and showed in both cases an increase in release rate which turned back to baseline after the ultrasound irradiation was stopped. Increasing temperature in the delivery system was speculated to be the cause of increased diffusion of the 5-FU from the polymer. Kost et al. [ll] investigated the effect of ultrasound on the degradation of polymers and the release rate of drugs incorporated in these polymers. Both an increase of the release of the drugs and polymer degradation was observed. Several mechanisms were found to be responsible for the effect of ultrasound. The release rate increased as a function of the applied intensity. Temperature and mixing were relatively unimportant in effecting enhanced polymer degradation, whereas cavitation appeared to play a significant role. Increased release rates were also observed when ultrasound was applied to biodegradable polymers implanted in rats.
5. Novel drug delivery systems Novel drug delivery systems have been described microspheres containing a therapeutic compound
using [ 121.
36 (19%)
709-712
These systems can be administered with special drug delivery catheters [ 13,141 and offer numerous advantages over conventional drug therapy. Especially gaseous containing microspheres, used as ultrasound contrast agents in the medical diagnostic field. They offer the possibility to be visualized during their transport in the human body using standard ultrasonic imaging systems. Therefore, the release can be controlled externally once the microspheres reach a diseased region [ 15,161. Ishihara [ 171 explains a method for using ‘micromachines’ (gas-filled microspheres) as drug carriers to selectively administer the drug to a localized region, and utilizing the fact that when the drug carrier is irradiated by ultrasound from a external source at the resonance frequency, ultrasonic energy is absorbed most effectively. This way, drugs embedded within the microspheres can be released due to rupture of the microspheres. Unger [ 181 also describes the use of gas-filled microspheres as delivery devices. The microspheres contain a therapeutic compound and a temperature activated gaseous precursor which becomes a gas (fluid-gas phase shift) upon activation at a selected temperature. Several ways to incorporate the therapeutic compound within the microspheres are mentioned. After administering the microspheres into the human body, the fluid-gas phase shift makes the microspheres detectable for diagnostic ultrasound. They can be monitored in real time until they are detected in a region of interest. Therapeutic ultrasound, using a lower frequency compared to diagnostic ultrasound, is then applied to the region in order to rupture the microspheres and release the therapeutic compound.
6. Experiment To get more experience in the field of ultrasonic induced drug delivery an experiment was set up at Andaris Ltd. Nottingham. Small polymeric microspheres were produced by spray-drying a 50150 Poly (DL Lactide-co-Glycolide) polymer, inherent viscosity Hexabrix, a standard X-ray contrast of 0.55 dl g-l. agent which is highly soluble in water, was used as a model drug. As a result of the spray-drying the microspheres were gas-filled and the Hexabrix was embedded within the shell (5% loading). The product obtained was a dry powder with a mean size of 2-3 urn (Fig. 1). To see if the microspheres could be detected by ultrasound, images were made with a standard ultrasound machine used for medical diagnosis (HP Sonos 1500, 3.5 MHz). First a baseline image was obtained (Fig. 2(A)) of 250 ml Isoton in a beaker which was put down on a mechanical stirrer. Then 27 mg of the product was suspended in 5 ml Isoton, gently inversed and added into the beaker. Again an image was obtained
01 0
30
60
90
I?0
I50
~imelmli
Fig. I. Microscopic view of the polymeric of 400. grid 5ize is 2 pm per div.
Fig. 2. Upper
(Fig. 2(B)) and a significant
microspheres;
panel shows baseline
magnification
Fig. 3. Percentage ultrasound alter
release
image: lower panel shows echo response
increase in echo genicity can be appreciated. The main purpose of this experiment was to see if the release of drugs embedded within a polymeric microsphere carrier can be controlled by ultrasound. To measure the release, 5.2 mg of the product was suspended in 0.01 M phosphate buffered saline (pH 7.4). At t = 0 min, a 1 ml sample was filtered through a 0.2 pm Sartorius RC15 filter and analyzed by a Beckman UV/VIS spectrophotometer, which was calibrated for Hexabrix, at 243 nm using quartz cuvettes. At this wavelength the absorbens of UV light is maximal for Hexabrix and does not overlap the maximum in absorbens of the polymer. Every 5-10 min a new sample was analyzed up to t = 55 min and from t= 120 min to t = 140 min. In between sampling the product was left on a rollermixer and gently mixed. The release was calculated by taking the theoretical loading of 5% into account. This experiment was carried out in threefold and the mean value and SD are shown by the drawn line in Fig. 3. After 140 min only 35% of the Hexabrix is released from the polymer. At t = 0 there is an offset in the release due to Hexabrix which is weakly bounded to the outer surface of the microspheres. Experiments
overtime.
sust;lineJ
release;
- - -.
t = I20 min.
when the product
is added
to determine the actual loading subscribed this explanation [ 191 (actual loading of 90- 95%). A fourth measurement was conducted in the same way as the previous one, only this time the product was put in an ultrasonic bath after t = I30 min and irradiated for 1.5 min at 35 kHz at maximum intensity. As appears by the dotted line Fig. 3. there is a significant increase in release up to 100%. This experiment only shows the release of a compound embedded within a microsphere with and without ultrasound. The efl’ect of a wide variety of parameters still have to be investigated. i.e. acoustic pressure amplitude, frequency and duty cycle. The degradation of the polymer was not measured in this experiment.
7. Discussion The release rate of drugs from a carrier can be controlled depending on three predominant parameters: ( 1 ) carrier composition, (2) drug incorporation and (3 ) ultrasonic parameters. The physical characteristics of the drug carrier determines the release rate depending
712
P.J. A. Fritzking et ~11./ Ultrusonics 36 (1998) 709-712
on its degradation, rupture rate or temperature rise. The drug incorporation to a microsphere carrier can be done in different ways. The drug can be encapsulated in the microsphere, so within the gas containing lumen. A disadvantage is that by putting a therapeutic compound in this place the acoustic behavior (echogenicity) of the microsphere will change. It is also difficult to make this kind of delivery system. An easier way is to embed the drug in the shell of the microsphere. In this way the microsphere will contain its acoustic properties and can be located in a specific region within the human body. The last method is to attach or chemically link the drug to the outer surface of the shell. In case of chemical linking it is important to know how the drug is linked (strong or weak) because this will have consequences on the effect that ultrasound will have on the release rate. Attaching an antibody to the outer surface of the shell is also possible. In this way the drugs can be delivered to very specific parts i.e. targeted delivery. According to the drug position, the release will be gradual or instantaneous, depending on the desired application. Although ultrasound is used extensively in medical therapy and diagnostics, it has been recognized only recently as a method for external controlled delivery of drugs. Several studies have shown that the release rate is directly related to the ultrasonic intensity, and that ultrasound has an effect on polymer degradation. However, the exact underlying mechanism is as yet unknown. Other ultrasonic parameters, such as frequency, duty cycle and duration may have an effect on the release rate and the local absorption of drugs. From the field of ultrasound contrast agents, a lot of experience is available on the interaction of ultrasound
and gas-filled microspheres. It is the conviction of the authors that this expertise is of great value for developing ultrasound directed drug delivery systems.
References [ I] R. Langer. Science 249 ( 1990) 1527. [2] R. Langer, Chem. Brit. (March 1990) 233. [3] D.S.T. Hsieh, R. Langer, Proc. Nat]. Acad. Sci. USA 78 (1981) 1863. [4] M.R. Prausnitz, V.G. Bose, R. Langer, Proc. Natl. Acad. Sci. USA 90 (1993) 1054. [5] J. Kost, Clin. Mat. 13 (1993) 155. [6] J. Kost, J. Contr. Release 24 (1993) 247. [7] N.N. Byl, Phys. Therapy 75 (1995) 539. [S] S. Mitragotri, D.A. Edwards, B. Blankschtein, R. Langer, J. Pharm. Science 84 ( 1995) 697. [9] S. Mitragotri, B. Blankschtein, R. Langer, Science 269 ( 1995) 850. [IO] S. Miyazaki, W.M. Hou, M. Takada, Chem. Pharm. Bull. 33 (1985) 428. [I 11 J. Kost, K. Leong, R. Langer, Proc. Natl. Acad. Sci. USA 86 ( 1989) 7663. [ 121 R.L. Wilenskey, K.L. March, Seminars in Interventional Cardiology I (1996) 48. [ 131 R.L. Wilenskey. K.L. March, D.R. Hathaway, Am. Heart Journ. 122(4)(1991) 1136. [ 141 L.A. Guzman, V. Labhasetwar, C. Song, Y. Jang, A.M. Lincoff. R. Levy, E.J. Topol, Circulation 94 (6) (1996) 1441. [15] T. Porter, S. Li, K. Kilzer, J. Desjardins, P. Iversen, 2nd Thoraxcenter European Symposium on Ultrasound Contrast Imaging, 1997, p. 25 (abstract). [ 161 E.C. Unger, 2nd Thoraxcenter European Symposium on Ultrasound Contrast Imaging, 1997, p. 54 (abstract). [ 171 K. Ishihara, Proc. Sec. Intern. Micromachine Symp., 1996, p. 69. [ 181 E.C. Unger, T. Fritz, T. Matsunaga, V. Ramaswami, D. Yellowhair. G. Wu. US patent Nr. 5 542 935 (Aug. 1996). [ 191 S. Keating, Andaris Ltd., Nottingham, Personal Communication.