DRUG DELIVERY THROUGH LIPOSOME BY ULTRASONIC CAVITATION

Transcription

1 BIMEDICAL ENGINEERING- APPLICATINS, BASIS & CMMUNICATINS 47 DRUG DELIVERY THRUGH LIPSME BY ULTRASNIC CAVITATIN P-LIN CHIU*, CHUNG-YU WU*, JENH TSA*, AND FU-HSIUNG CHANG** *Department of Electrical Engineering, National Taiwan University, **Institute of Biochemistry and Molecular Biology, National Taiwan University Biomed. Eng. Appl. Basis Commun : Downloaded from ABSTRACT Ultrasonic.las<nic cavitation has has found potential usages in in medical applications, and and we we are are developing a new drug delivery control system based onliposome and cavitation technology. In In order order to reduce to reduce the side effects of f drugs and increase the the efficacy of of drugs, we we combine the the advantage of the of the usage usage of of liposome lipasome and ultrasonic cavitation to to attain the the system. In In this this system, imaging unit unit and and power unit unit are essential elements, elements. There are are many parameters which need need to to be be controlled: controlled. By this By this drug drug delivery delivery system, stems, we are to verify its safety and availability. In this paper, we we describe how this this drug delivery system %tex b by,' itttrasvnie ultrasonic cavitation to to be be designed and and evaluated. Biomed Eng Appl Basis Comm, 21 (February); 13: INTRDUCTIN It is important for the physicists and pharmacists to understand how the drug is distributed, delivered, and precisely controlled. Many investigators have indicated the problems of various drug delivery systems, which have been studied in pharmacokinetic field intensively, like control release. The most important concern of drug delivery is the efficacy of the drug and safety. Its efficacy depends strongly on its stability, distribution, and absorptive efficiency of the drug. How to increase the efficacy and decrease its side effects are always the important issues in research and clinical field. In order to decrease the side effects, we may localize our drug release nearby target organ, and it could reduce the probability of the unwanted effects occurred on the other organs. We may choose a long half-life drug carrier to sustain its stability in circulatory system, and it might improve the drug tolerance. Received: Jan. 29, 21; accepted: Feb.2, 21. Address for correspondence: Jenho Tsao, Dept of Electrical Engineering, National Taiwan University, Taipei. Taiwan. The dosage is also a major factor to affect the efficacy. The field of medical ultrasound has gone through greatly change since its inception in early 195s. The advance of ultrasound in diagnostic and therapeutic usage is accompanied with rapid development of electronic technique and biomedical science. Today ultrasonic system is the most common and equipped instrument in hospital. It has been widely used in imaging, lithotripsy, measuring blood flow, scaling, thermal therapy and physical therapy, etc. This article will evaluate a new kind of drug delivery system using ultrasonic imaging system as a detector and acoustic cavitation as a destructor for the drug carrier to make drug release. And this control system may calculate the dose used in the organ. And also in this article, we attempt to show a new therapeutic strategy in drug delivery that is controlled by a medical ultrasonic system. Because much of the effectiveness of ultra-sound-induced drug delivery appears to be associated with bubbles and the occurrence of cavitation, we introduce some background in acoustic cavitation, then describe its medical applications and therapeutic strategies in detail. 2. DRUG DELIVERY BY ULTRASNIC CAVITATIN -47-

2 Biomed. Eng. Appl. Basis Commun : Downloaded from 48 TECHNIQUE We introduce several medical applications of acoustic cavitation here in order to obtain more sights into application of cavitation in medicine, especially the drug delivery system. What is cavitation? "Cavitation is the behavior of the formation of bubbles in liquid." The word `formation' refers both to the creation of a new cavity and to the expansion of a pre-existing bubble to a critical size. Cavitation may occur in the normal human circulatory system because of elastic property of the blood vessel. There are turbulence and large pressure changes in blood vessels resulting from heart beating. Some researchers suggested the possibility of bubbles in the cardiac chambers could be detected by the application of the Doppler principle, as it flows through the heart of large vessels. At the time, cavitation may cause damage to heart valves, and at the junction of arteries may result in arteriosclerosis. It is sufficient to note that all of the effects of cavitation could potentially be used and seen in vivo if ultrasound is applied at proper combination of frequency and intensity. In pharmaceutical field, there has been considerable interest over recent years in the targeted delivery of pharmaceutical agents. Drug delivery system aims at making the availability of the drug to a reasonably good level. The availability of the drug is strongly determined by the stability and safety of the drug itself. There are many factors, like the drug concentration, the ph value in the stomach and intestine, and the ways of the administration we choose, etc., that determine the available level of the drug. As we choose the suitable pathway of administration and dosage form, we will design the suitable pharmacokinetic model for the drug. The designed objective of the drug delivery system is to change the half-life time of the drug, to reduce its side effects, and to increase its efficacy. There are several major fields in drug delivery system by ultrasound: transdermal drug delivery (TDD), hyperthermia, and drug release by direct cavitation technique (K Ishihara, and T. Furukawa, 1991). In these active fields, the role of ultrasound is to make the drug release in the local area in order to reduce the unwanted effects of the other organs, or to increase the permeability of the drug to the skin barrier, etc. For example, in the transdermal drug delivery system, the ultrasound acts both by surface erosion of the polymer matrix of the drug and by accelerating the natural hydrolysis reaction. Much work has been done on the transdermal drug delivery by ultrasound. Acoustic cavitation generated by ultrasound introduces defects that are devoid of lipid molecules in stratum corneum, and air pockets may have been trapped in the pores. Because of the existing defects, ultrasound can enhance the passage of Vol. 13 No. 1 February 21 the drug molecules. Transdermal drug delivery can avoid of the destruction of the gastric acid, so that it can be used in the patients that cannot administer the drugs by oral or injection. The rate of permeation can be increased by the use of chemical enhancers such as water, ethanol, N-methyl pyrollidone (NMP), polyethylene glycols or dimethyl sylphoxide (DMS) (Walters et al., 1993). In the hyperthermia therapy, the drug made by a special kind of polymer is injected into the circulatory system, and then we use the ultrasonic wave to heat it around the local area. Because of the heat sensitivity of polymer, it can be dissolved in the bloodstream. Also, hyperthermia has been shown to change the circulatory dynamics and blood vessel permeability (Dewhist, 1994). For example, hyperthermia enhances uptake and effect of liposomeal chemotherapy (Gaber et al., 1996; Huang et al., 1994; Kakinuma et al., 1996). The drug release depends on local temperature. This strategy could effectively make the drug release in the target region, and it improves the efficacy of the drug. But the human body has a thermoregulatory center-hypothalamus. As the temperature rises, hypothalamus can sensitize it and send the signals out, like hormone and nervous signal, in order to balance the temperature. It makes the control of drug delivery more difficult. n the other hand, it is hard to quantify the dosage in vivo, because the thermal therapy cannot secure a position where the drug is. The other strategy is achieved by cavitation technique. Before applying this technique, we have to make sure what kind of drug carrier to be used. We locate the drug by ultrasonic imaging system, and tune the frequency and intensity to induce cavitation on the drug carrier. The advantage of this strategy is its ability to control the dose release, and by correct detection, we could focus on the place where we want it to be released. Because of complicated compositions and properties of bloodstream, it is hard to trace the elements that flow in the vessel precisely. Many researchers have reported different techniques for bubble detection. By Doppler ultrasound, we could estimate the average velocity of the local flow, and predict the location of the bubble. Ken Ishihara and Toshiyuki Furukawa (1991) have shown a 2D-MTI method, and its principle is serial image accumulation of substracted images of echoic particles in blood. This cavitation technique can be made local to the desired site of action so that large concentrations of the active agent are not present elsewhere in the body. The drug efficacy is strongly dependent on its half-life. Since the safety of cavitation in vivo is not clear, many researchers have reported that it is safe in some kinds of therapy. The new method that this article will evaluate is based on this idea. -48-

3 BIMEDICAL ENGINEERING- APPLICATINS, BASIS & CMMUNICATINS 49 Biomed. Eng. Appl. Basis Commun : Downloaded from (a) Dissolving bubble (b) Steady growth (c) Low amplitude pulsation (d) Decaying pulsation (e) Transient cavitation Fig. 1. The illustrations of pictures of the range of bubble dynamics. 3. BUBBLE CAVITATIN In this section, we will briefly discuss the physics of acoustic cavitation. By the preceding definition of cavitation, there are many kinds of cavitation and for example, acoustic cavitation is produced by sound waves in liquid due to pressure variation. And what are the possible bubble behaviors in the liquid? When we monitor the bubbles in the liquid without acoustic effects, the bubbles may expect to be dissoluted slowly (Figure 1(a)), or to be expanded under decompression or heating (Figure 1(b)) (Leighton and Walton, 1987). Given low frequency acoustic field, the bubble has a natural frequency to oscillate as a mechanical oscillator possessing stiffness and inertia (Figure 1(c)). Its frequency might matches the coupling of frequency and intensity of acoustic wave. Despite damping, the bubble pulsations are shown not to decrease in amplitude (Figure 1(d)), and this suggests a continuous-wave insonation. Sudden expansion and then rapid collapse of the bubble may occur under specific conditions (Figure 1(e)) (Apfel, 1981), and they are the characteristics of inertial cavitation. The occurrence of inertial cavitation is a threshold phenomenon. Whether it occurs or not, for a free-floating spherical bubble, depends primarily on the acoustic pressure amplitude, the acoustic frequency, and the size of the bubbles. Resonance is an important property of the bubble, & quency (MHz) 5 6 Fig.2. The frequency response of bubbles in the acoustic field. especially its nonlinear property. We might roughly say that the resonance frequency of the bubble is inversely proportional to the bubble radius under some assumed conditions. As the bubble pulsation amplitude increases near resonance, the nonlinear characteristic of the oscillation becomes more pronounced. There is also an important property -scattering cross-section. It is defined as the amount of energy scattered by an incident wave divided by the intensity of incident wave. The scattering cross-section can be computed base on Born approximation. It can be shown that if applied frequency is resonant frequency, the scattering cross-section will increase greatly. Figure 2 shows the relationship between scattering crosssection, bubble radius, and frequency (Nico de Jong, 1996). Thus, given a particle of a specific radius, the scattering cross-section is strongly influenced by the frequency. As far, knowing cavitation occurrence and bubble dynamics may help us to use and control the acoustic cavitation precisely later. 4. THE THERAPEUTIC STRATEGY. We propose a new concept for a drug delivery in which air-filled drug carriers are monitored with echography and are controlled with resonant ultrasound. We choose a suitable drug carrier, which can encapsulate the gas and carry drugs, and monitor the drug carrier by ultrasonic imaging system. As it flows into and out of the interest region, we can cavitate the carrier to make it release the drug. In our research, we choose liposome as the drug carrier, and tune the parameters of the transmitting transducer in order to find out the best model of cavitation. These parameters include the frequency, the intensity, and PRF, etc. Liposome is a vesicle in which an aqueous volume is entirely enclosed by a membrane composed of -49-

4 5 Vol. 13 No. 1 February 21 Biomed. Eng. Appl. Basis Commun : Downloaded from phospholipids. The size of liposome may range from 1nm to 1,u m in diameter. In preparation, we can add different types of lipid in order to control its stability and specificity, and we may conjugate some kinds of antibodies on the surface of liposomes to localize the desired region. The composition of the liposomes also determines the distribution in vivo. The pharmacokinetics of liposomes in animal body is the most concern issue as using it a drug carrier, and there are many reports for the distribution of liposomes, especially in liver (D. Liu, 1997). Some researchers have conjugated some kinds of antibodies to control its specificity. In order to enhance its echogenity, there are several papers reported that various factors affects the echogenity of liposomes in ultrasonic field (Hayat et al., 1996). Lipid compositions, particle size, encapsulated gas, and loaded drugs etc., all affect the liposome echogenity in ultrasonic field to a degree. Some researchers have reported the behavior of cavitation of liposomes in animal model. n the part of the bubble detection, we greatly use the nonlinear characteristics of the bubble to obtain the information of bubble dynamics when they are inside the circulatory system. As we have prepared the liposomes, we have to measure its size in order to predict the resonance frequency of the liposomes in the liquid. Because of the great different acoustic impedence of the gas and liquid, the magnitude of the resonance frequency of the liposomes is larger than other static tissue in the frequency spectrum. And we could identify the resonance that the bubble occurs. We develop a Drug Administration (Pharmacokinetic model) new method -the spectrum difference, which is to be published latter-to detect the bubble. It could help us to identify the resonance frequency and the location of the bubble. So we obtain the bubble size by measuring the resonant frequency of the bubble, so that we can tune the transmitting frequency and intensity to cavitate the bubbles. By precise detection, we could quantify the total dose of the drug in the sample volume. By tuning to right frequency and locating right position of the liposomes with imaging, we can transmit the acoustic wave to cavitate the liposome to make the drug release in the local region. The strategy, which we just introduced, is a powerful technique for tumor therapy. There are a lot of side effects by using conventional chemotherapy, if we could localize the distribution of liposomes in specific region and release the quantitative dose nearby the tumor region, we might obtain a high drug efficacy to kill the tumor cell and prevent it from damaging the normal cell. For example, 5-fluorouracil has been used in clinical trial. 5-fluorouracil is embedded in an ethylene-vinyl alcohol copolymer, and as the temperature is getting raised, the release of 5-fluorouracil from the copolymer will increase (Miyazaki et al., 1985). There are several papers reported that cavitation might cause side effects of specific conditions in the animal model, like thrombolysis (Everbach et al., 2). Many reports have proved that lower intensity ultrasound accelerate enzymatic fibrinolysis in vitro and in vivo (Laur et al., 1992; Tachibana, 1992). These side effects are still under research, and their mechanisms are not Fig.3. The diagram of system units. -5-

5 Biomed. Eng. Appl. Basis Commun : Downloaded from BIMEDICAL ENGINEERING- APPLICATINS, BASIS & CMMUNICATINS clear. The therapeutic strategy we established might have some advantages : (a) it is easy to control for the physicians, (b) we could quantify the dose precisely, and (c) it could reduce side effects of the drug very effectively 5. SYSTEM REQUIREMENTS In order to realize the operations in the idea that we discussed above, it requires some essential elements and system controls. As drug is injected into human body, it might be targeted to the desired organ by specific recognition between the drug carrier and the organ. Then we may use ultrasonic imaging to trace the drug in the circulatory system and calculate the dose passing the vessel at the same time. The physicians could choose the region of interest on the image. After the parameters of the drug are measured in the local area, we may focus the ultrasound beam on the target of drug carriers and make them cavitate and release. The system must have the functions of the detecting unit and power unit. With the same transducer, we might know the information of the position and the dose of the drug by imaging, the size of the drug carrier by specific signal processing, and the power to cavitate the drug carrier. Illustration of the ideal system is shown in Figure 3. In our ultrasonic therapeutic system, there are two essential elements : imaging unit and power unit. We define imaging unit as a detecting tool, which could trace the behaviors of bubbles. The imaging unit is similar as current ultrasonic imaging system, and we might change the implementation by some other ways to be able to transmit specific pulse, as defined by the power unit. The control unit is able to process the signals from the imaging unit, for tracing, focusing and monitoring. It is also tuned to right frequency, PRF, and intensity to transmit pulses and then cavitate the drug carrier through the power unit. Drug administration is also an important unit in this therapeutic system, and it includes all the relevant knowledge from pharmacokinetic and pharmacodynamic fields. The control unit might monitor it. After the therapy, we might evaluate the efficiency of the system. Until now, we have known that the release of the drug is dependent on drug carrier composition and ultrasonic beam pattern in this system, and have developed techniques to observe bubble size in acoustic field, and find out several suitable parameters that control the cavitation of the drug carriers. 6. CNCLUSIN We predict that this new therapeutic strategy may improve the efficacy of the drug enormously, but we have to verify its safety through clinical trials. In order to use it in hospital effectively, we will design a control system for our drug delivery system in the future. It will help physicians to monitor the drug concentration in the patient, and to make drug delivery more efficient. The research for bubble detection and quantified bubble of cavitation is our subjects in near future. There is a need for further work on this idea, which is ripe for collaboration between the medical physicists, the pharmacists and the biomedical engineers. We hope that there will be an automatic control system to handle all the system elements described above in the future. REFERENCE M. W. Miller. Gene Transfection and Drug Delivery, Ultrasound in Med. & Biol. 26 (2), S59-S C. J. Diederich and K. Hynynen. Ultrasound Technology for Hyperthermia. Ultrasound in Med. & Bio., 25(1999), K A. Walters and J. Hadgraft. Pharmaceutical Skin Penetration Enhancement, New York: Dekker, M. W. Dewhist. Future Directions in Hyperthermia Biology. Int. J'Hyperthermia, 1(1994), M. H. Gaber, N. Z. Wu, K Hong, S. K. Huang, M. W. Dewhirst, and D. Papahadjopoulos. Thermosensitive Liposomes : Extravasation and Release of Contents in Tumor Microvascular Networks. Int. J Radiat. ncol. Biol. Phys., 36(1996), Peter J.A. Frinking, Ayache Bouakaz, Nico de Jong, Folkert J. Tencate, and Siobhan Keating. Effect of Ultrasound on the Release of Micro-Encapsulated Drugs, Ultrasonics, 36(1998), J. Wu, J. Chappelow, J. Yang and L. Weimann. Defects Generated in Human Stratum Corneum Specimens by Ultrasound, Ultrasound in Med. & Biol., 24(1998), A. J. Walton and G. T. Reynolds. Sonoluminescence, Adv. in Phys. 33(1984), F. R. Young. Cavitation, London: McGraw-Hill, T. G. Leighton and A. J. Walton. An Experimental Study of the Sound Emitted from Gas Bubbles in a Liquid, Euro. J. Phys., 8(1987), R. E. Apfel. Acoustic Cavitation, Methods in Exp. Phys., New York: Academic Press, 19(1981), Nico de Jong. Improvements in Ultrasound Contrast Agents, IEEE Engineering in Medicine and Biology, (1996), R. E. Apfel. Acoustic Cavitation: A Possible Con- -51-

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