Continuous sonication method for preparing protein encapsulated microbubbles.

FIELD OF INVENTION

0001This invention relates to ultrasonic imaging of the human body for diagnostic purposes; and, more particularly, to the preparation of ultrasonic imaging agents by sonication of protein solutions.

BACKGROUND OF INVENTION

0002It has been known since 1968-70 that con­trast echocardiography can be used to delineate intra­cardiac structures, assess valvular competence, demon­strate intracardiac shunts, and identify pericardial effusion. (Gramiak and Shah, 1968; and Feigenbaum et al., 1970). Ultrasonic imaging of the heart potential­ly has important advantages of convenience, safety, and reduced cost over present diagnostic procedures, such as angiography, which requires the use of radio-opaque dyes for X-ray imaging, or the use of radionuclide imaging agents for radioimaging. However, progress in practical applications of ultrasonic imaging has been delayed by the lack of effective clinically-usable imaging agents.

0003Ultrasonic imaging ultilizes an ultrasonic scanner to generate and receive sound waves. The scan­ner is placed on a body surface overlying the area to be imaged, and sound waves are directed toward that area. The scanner detects reflected sound waves and translates that data into images. When ultrasonic energy is transmitted through a substance, the acoustic properties of the substance depend upon the velocity of the transmission and the density of the substance. Changes in the substance's acoustic properties (e.g., variation in acoustic impedance) are most prominent at the interfaces of different substances, such as a liquid-solid or liquid-gas interface. Consequently, when ultrasonic energy is directed through media, changes in acoustic properties will result in more intense sound reflection signals for detection by the ultrasonic scanner.

0004Ultrasonic imaging agents can consist of small solid or gaseous particles which, when injected in the circulatory system, provide improved sound re­flection and image clarity. Microbubble-type imaging agents consist of minute bubbles of a gas (usually air) which are dispersed in a carrier liquid for parenteral injection. The "microbubbles" are carried by the cir­culatory system to the organ being imaged.

0005It has been proposed to form a dispersion of air microbubbles in a warm aqueous gelatin solution, and cooling the solution to a solidification tempera­ture to trap the microbubbles For administration, the gelled dispersion is to be warmed until it liquefies, and parenterally administered with the microbubbles dispersed in the liquefied gelatin. (Tickner, et al. U.S. Patent 4,276,885; and Tickner, et al., National Technical Information Service Report HR-69217-1A, April, 1977). Gelatin-trapped microbubbles on intro­duction into the bloodstream have a short lifetime. They rapidly dissipate. Another disadvantage is that the microbubbles are too large to pass through capil­lary beds, and are therefore not suitable for heart imaging by peripheral intravenous administration.

0006The discovery by Dr. Steven B. Feinstein of sonication-produced microbubble imaging agents repre­sented an important advance in this art. Using viscous aqueous solutions, such as 70% sorbitol or dextrose, Dr. Feinstein produced a dispersion of microbubbles by high energy sonication of the solutions. The resulting microbubbles had sizes less than 10 microns, and were capable of passing through capillary beds. The persistence of the microbubbles, although of the order of a few minutes, permitted the imaging agent to be prepared and administered intravenously for heart imaging. (Feinstein, et al., 1984; and Feinstein U.S. Patent 4,572,203.)

0007Subsequently Dr. Feinstein sought to improve the persistence of the microbubbles. He found that by sonication of a heat-sensitive protein, such as albu­min, microbubbles of improved stability were obtained (Feinstein U.S. Patents 4,572,203 and 4,718,433). Concentrations of microbubbles of 10 to 14 x 10⁶ microbubbles per milliliter were obtained with bubble sizes from 2 to 9 microns (Keller, Feinstein, and Wat­son, 1987). The microbubbles persisted for 24 to 28 hours. However, the sonication-produced albumin micro­bubble imaging of Feinstein was not sufficiently stable for commercial manufacture.

0008Stabilities of the order of weeks or months (rather than hours or days) are required to permit an imaging agent to be manufactured at a central location and distributed to hospitals in the United States and other countries. For commercially feasible manufac­ture, shipment and hospital storage prior to use, a stability time of at least four weeks is needed and preferably at least eight weeks, or longer. Also, for the most effective imaging, it is desirable to have the highest obtainable concentration of microbubbles in the imaging agent. An imaging agent of very high microbub­ble concentration is inherently better, and a safety factor is provided. These advances in the sonication generation of albumin encapsulated microbubbles have been partially achieved by Molecular Biosystems, Inc., San Diego, CA. The experimental "ALBUNEX" product of this company comprises microspheres having microbubble centers with insolubilized albumin walls.

0009Prior to the present invention, however, the "ALBUNEX" microspheres have only been prepared in small quantities on a batch-by-batch basis. It was not known whether large scale commercial production was feasible. No one knew how microbubbles encapsulated with albumin could be produced on a continuous basis while obtaining size control, high microbubble concentration, and long-­term stability.

SUMMARY OF INVENTION

0010This invention provides a sonication method for continuously producing microspheres comprising pro­tein encapsulated microbubbles in controlled small sizes, effective concentration ranges, and commercially and clinically practical stabilities. As in prior practice, a dilute aqueous solution of a heat-denatur­able, water-soluble, biocompatible protein is prepared. For example, as in prior practice, a sterile 5% aqueous solution of human serum albumin can be used. For con­tinuous production, it has been found to be critical to rapidly heat increments of the protein solution to a temperature of incipient denaturation for the protein. An indirect flow-through heat exchanger can be used for this purpose, but it is important the the temperature of the heat exchange liquid medium be carefully con­trolled. Essentially the heat exchange medium should be at the desired incipient denaturation temperature. At the conclusion of the rapid heating step, the pro­tein solution is on the verge of denaturation but should not contain denatured protein. The heating to the temperature of incipient denaturation is carried out without appreciate protein insolubilization.

0011Another critical feature of the process is to introduce a biocompatible gas into the protein solu­tion. Sterile air is preferably employed for this purpose. The introduction of air provides an excess of air for formation of the microspheres. Air addition can be carried out before, during, or after the heat­ing. In a preferred procedure, however, the gas is introduced immediately following the heating, concur­rently with the introduction of the heated solution into the sonication chamber.

0012The gas-containing heated increments of protein solution are continuously passed through a chamber enclosing an operating sonicator, which pro­vides a sonicator horn in contact with the solution The sonication produces gas microbubbles in the solu­tion while the protein therein is quickly heated to insolubilize around the microbubble. Since the solu­tion has already been heated to a temperature of in­cipient denaturation, only a few degrees of additional heating is required to produce the insolubilized pro­tein, which forms the walls of the microspheres.

0013The formation of a large population of minute microspheres (less than 10 microns in diameter) takes place with extreme rapidity due to the excess air present tin the solution. The insolubilized albumin is formed with equal rapidity because of the preheated condition of the solution. these factors result in ex­tremely rapid formation of the protein-encapsulated microbubbles, referred to herein as "microspheres". The residence time of a solution increment in the sonicator chamber can be very brief.

0014By the method of this invention it is pos­sible to form the microsphere imaging agent on a con­tinous, high-production basis. The solution as discharged from the sonication chamber already contains the stable, small size, protein-encapsulated microbub­bles. Only a small amount of oversize or undersize microspheres are present. This eliminates the need for any time-consuming fractionations. Intense foaming of the solution occurs during the sonication, and the solution as discharged from the sonication chamber will have a foamy character. But the foam can be easily dissipated.

THE DRAWINGS

0015The sonication method of this invention for continuous production of albumin encapsulated microbub­bles can be carried out in a sequence of operations. These operations are illustrated diagrammatically by the accompanying drawings, in which-- <ul id="ul0001" list-style="none"><li>FIG. 1 is a flow sheet illustrating the preparation of the albumin solution;</li><li>FIG. 2 is a diagrammatic flow sheet of the sonication operation, including the preheating, air introduction, and continous sonication;</li><li>FIG. 3 is an enlarged cross-sectional view of a sonicator which may be used instead of the soni­cation vessel of FIG. 2;</li><li>FIG. 4 is a flow sheet illustrating the further processing of the suspension of albumin microspheres in a separator/concentrator; and</li><li>FIG. 5 is an alternate separator/concentrat­or which may be used instead of the apparatus illustra­ted in FIG. 4.</li></ul>

DETAILED DESCRIPTION

0016The starting material for practicing this invention is an aqueous solution of a heat-denaturable, water-soluble biocompatible protein. The encapsulating protein should be heat-sensitive so that it can be partially insolubilized by heating during sonication. More specifically, coincident with the sonication, a small portion of the dissolved protein material is insolubilized. This results in a small volume of solid phase materials, which forms the encapsulating layers around the microspheres. Heat-sensitive proteins may be used such as albumin, hemoglobin, collagen, etc. For administration to humans, human protein is pre­ferred. Human serum albumin (HSA) is especially suitable. HSA is available commercially as a sterile 5% aqueous solution, which can be used directly as the starting material for preparing the microspheres. However, other concentrations of albumin or other heat-denaturable proteins can be used. HSA concentra­tion can be varied, for example, within the range from 1 to 25% by weight. In the continuous process of the present invention, it is desirable to utilize the pro­tein in the form of a dilute aqueous solution. For al­bumin, it is preferred that the solution contain from 0.5 to 7.5% by weight of the albumin. Because of the extremely favorable conditions established for micro­bubble generation, protein insolubilization, and re­sulting encapsulation, concentrations within 5% albumin may be used, such as from 0.5 to 3%.

0017Commercially-available equipment may be used in practicing this invention. The feed preparation operation utilizes stainless steel tanks and process filters which can be obtained from Walker Stainless Equipment Co., New Lisbon, WI, and Milliporr, Bedford, MA, respectively, as well as other companies. This operation insures that all feed media to be sonicated will be consistent with FDA requirements and regula­tions.

0018The sonication operation utilizes both a heat exchanger and a flow through sonicating vessel, in series. Heat exchanger equipment of this type can be obtained from ITT Standard, Buffalo, New York, and other companies. The heat exchanger maintains operat­ing temperature for the sonication process. Its tem­perature control of the sonication media ranges from 65°C to 80°C, depending on the media's makeup.

0019Sonication equipment's vibration frequencies can vary over a considerable range, such as from 5 to 40 kilohertz (kHz), but most commercially-available sonicators operate at 20 kHz or 10 kHz. The 20 kHz sonicators perform well for purpose of this invention. Such sonicating equipment can be obtained from Sonics &amp; Materials, Inc., Danbury, CT, and other companies. Sonics &amp; Materials Vibra-Cell or similar model can be used with a flat tip sonicator horn. The power applied to the sonicator horn can be varied over power settings scaled from 1 to 10 by the manufacturer, as with Sonics &amp; Materials Vibra-Cell Model VL1500. An intermediate power setting can be used (viz. from 5 to 9). The vibrational frequency and the power supplied must be sufficient to produce cavitation in the liquid being sonicated. Feed flow rates range from 50 ml/min to 1000 ml/min. Residence times in the sonication vessel can range from 1 sec to 4 mins. Gaseous fluid addition rates range from 10cc/min to 100cc/min or 5% to 25% of the feed flow rate.

0020Sonication is deliberately carried out in such manner as to produce intense foaming of the solu­tion, contrary to conventional sonications, where it is desirable to avoid foaming. For the purpose of the present invention, foaming and aerosolating are impor­tant for obtaining the imaging agent of enhanced con­centration and stability. To promote foaming, the power input to the sonicator horn may be increased, as well as operating the process under slight pressure (i.e., 1-5 psi). The foaming produced from the sonica­tion is immediately detectable by the cloudy appearance of the solution, and by the foam produced.

0021By means of the continuous sonication pro­cess, comprising the cavitation phase followed by a foaming phase, the concentration of the encapsulated microbubbles, referred to herein as "microspheres", can be greatly increased. Concentrations in excess of 40 x 10⁶ microspheres per milliliter are easily obtainable, such as from 40 to 200 x 10⁶ concentrations. Moreover, the resulting microspheres will be predominately of di­ameters less than 10 microns. For example, 80% or more of the microspheres can hve diameters in the range from 1 to 9 microns with a mean diameter of 4 to 6 microns.

0022When the sonication is carried out in contact with air as the gaseous fluid, the microspheres will have air centers. Air is believed to be the most con­venient gaseous fluid, but if desired sonication could be carried out under other gaseous fluids (viz., nitro­gen, oxygen, carbon dioxide, etc.).

0023The continous sonication process allows for a continuous or at least a semi-continuous separation/­concentration operation. Again a stainless steel tank/­vessel can be obtained from Walker Stainless Equipment Co., New Lisbon, WI, or other companies. The separation/­concentration operation allows for total control of the product output in terms of microsphere concentrations and overall means sphere size.

0024Since microspheres are buoyant they tend to rise to the surface of the dispersion. By holding the dispersion without agitation for a number of hours (viz. for 1 to 8 hours), most of the microspheres will rise to the surface and concentrate in an upper layer above the clarified solution. By this separation/concentration operation or "float-separation" of the microspheres microspheres into an upper layer, portions of the clar­ified solution can be removed from below the micro­spheres, thereby obtaining a dispersion of greater microsphere concentration. For exmple, from 50 to 75% of the solution volume may be removed in this concen­tration process. This clarified solution may be re­cycled back to the feed preparation operation.

0025If needed, either before or after the above-­described concentration, float-separation of oversized microspheres can be obtained. Large size microspheres such as one having diameters greater than 10 microns have relatively greater buoyancy. They will therefore rise more rapidly to the surface of the solution. By utilizing a short holding time, such as from 15 to 45 minutes, the largest size microspheres can be selec­tively collected in a small upper layer above a disper­sion which will still contain substantially all of the microspheres of small size. By removing this micro­sphere dispersion from beneath the layer of oversize microspheres, a fractionation may be achieved in which the larger microspheres will remain in the vessel in which the fractionation is carried out. However, the inherent size control obtained by the continuous soni­cation of this invention makes it unnecessary to carry out lengthy seperation steps for removal of over or under size microspheres.

0026The imaging agent produced by this combina­tion of continuous sonication and the separation/­concentration can have a homogeneously-dispersed con­centration of greater than 22 x 10⁶, such as from 100 to 1200 x 10⁶ (1 to 12 x 10⁸) microspheres per mil­liliter. High concentrations can be maintained for long periods of holding at ambient room temperatures (20-25°C). Concentrations above 200 and typically above 44 x 10⁶ microspheres per milliliter can be maintained for periods of at least four and usually eight weeks or longer.

ILLUSTRATIVE EMBODIMENTS

0027Figures 1, 2 and 4 illustrate the three operations of a manufacturing plant for producing the microsphere imaging agent. The feed medium, comprising the albumin solution, is first subject to a feed preparation operation. The medium is then transported to the sonication operation. After the medium is heated and gaseous fluid is added, the medium is sonicated. It is then transported to a separation operation, where the microspheres are concentrated. The clarified medium can be removed from the micro­sphere suspension, and recycled back to the feed prepa­ration operation.

0028Figure 1 details the feed preparation. The feed medium is pumped through a series of filters to validate the feed medium under FDA specifications. The filtered medium is then placed in one or more feed tanks for the sonication operation.

0029Figure 2 details the sonication operation. The medium is passed through a flow control valve and a heat exchanger before entering the sonication vessel. A gaseous fluid, preferably air, is introduced into the feed medium either at or before the sonication vessel at a controlled rate. For example, air may be supplied from a pressurized source, such as bolted compressed air, or supplied by an air pump. The air should be in sterilized condition before it is added to the solu­tion.

0030Figure 4 details the separation and concen­tration operation Here the microspheres are concen­trated by float-separation at or near the top of the vessel, which may contain a static defoamer as shown. This defoamer can also serve to disrupt large size microspheres which are floating on top of the solution. The microspheres concentrate is selectively removed. The clarified medium in the bottom of the separator can be transported back to the feed preparation operation and recycled. In this recycling, all solid material and particles are removed, leaving only the protein in solution.

0031Figure 2 illustrates the operation which is the heart of the process. As indicated an insulation jacket surrounds the heat exchanger and sonicator ves­sel. The prepared albumin solution is pumped through a flow meter controller into the feed end of the tube and shell heat exchanger. In the heat exchanger, the al­bumin solution passes through multiple tubes which ex­tend to the discharge end of the exchanger. These tubes may be double jacketed to assure separation from the heat transfer medium, which enters the space sur­rounding the tubes near the discharge end, and flows countercurrently to the albumin solution before dis­charge near the feed end of the exchanger. Suitable liquid heat transfer media are water and mineral oil.

0032The temperature of the heat transfer medi­um is carefully controlled to a temperature correspond­ing to the target incipient denaturation temperature of the albumin. For human serum albumin, the incipient denaturation range is between 70°C and 75°C. Tempera­tures above 75° can result in substantial insolubiliza­tion of the albumin. A desirable operating range for the heat transfer medium is from 72 to 74°C. The al­bumin solution is heated rapidly with a single pass through the heat exchanger from an inlet temperature of 20 to 30°C to an outlet temperature of 72 to 73°C. In practice, the residence time in the heat exchanger can be less than one minute, such as from about 45 to 55 seconds.

0033As shown in Figure 2, after rapid heating to an incipient denaturation temperature for the albumin, the solution is passed to the inlet of a jacketed soni­cation vessel. This vessel may be of small volume, such as from 25 to 400 liters. In the vessel, there is mounted a sonicator horn for direct contact with the solution as it flows through the vessel. Into the bot­tom of the vessel there is introduced a continous flow of sterile, filtered air or the sonication proceeds. The air is dispersed in the solution, and rapidly formed into microspheres. The temperature of the solution in the sonicator is raised a few degrees above its entry temperature, the increase being sufficient to insolu­bilize a portion of the albumin. For example, from 0.5 to 3% of the albumin introduced into the vessel may be denatured and thrown out of solution to provide the en­capsulating protein. Under steady flow conditions, the control temperatures of the solution in the sonication vessel can be 74°C ± 0.2. The suspension of micro­spheres as discharged from the sonication vessel will have this temperature, which may be sensed by a tempera­ture controller, using a feedback to a regulator for the flow rate of the heat transfer medium.

0034The removed foamy suspension of the micro­spheres can be passed through a static mixer, but this is optional. If used, the static mixer can begin to break up the foam, and provide a homogeneous dispersion of the microspheres. As shown in Fig. 4, the foamy sus­pension is passed to a separator and concentrator unit which includes a static defoamer. The microspheres are removed with the solution flowing over the top of the weir. The defoamed solution is removed and passed through a static mixer. The mixer assures that the mi­crospheres are homogeneously dispersed in the solution.

0035Preparation of the imaging agent is complete at this point. Albumin microspheres of the desired size are within the range from 1 to 10 microns For example, they may be predominately sized from 3 to 8 microns. The microsphere concentrate preferably has a concentra­tion in the range from 400 to 800 10⁶ microspheres per milliliter. The suspension of the microspheres may be packaged in suitable vials under sterile conditions for subsequent administration as an ultrasonic imaging agent.

0036Figure 3 illustrates an alternate sonication apparatus. Similar sonicator units are obtainable from Sonics &amp; Materials, Danbury, CT. A small size chamber can be provided, which may have an internal volume of about 50 to 150 cc. The sonicator provides a horn which extends into the sonication chamber, and the chamber has a cavity-providing wall portion, which as shown is in the form of a cup. This cup is arranged in closely-­spaced opposed relation to the sonicator horn. The son­icator horn may extend into the cavity of the sonicator cup. Through a passage extending into the bottom of the sonicator cup, a premixed air/albumin solution is intro­duced. The albumin solution has been passed through the heat exchanger, as previously described, and the air is introduced from a source of pressurized air under ster­ile conditions. The residence time of the solution in the sonicator can be very short, such as of the order of 1 to 20 seconds. The foamy suspension of albumin micro­spheres as discharged from the sonication chamber is processed as previously described with reference to Figs. 2 and 4.

0037In Fig. 5, there is shown an alternate sepa­rator/concentrator in the form of a funnel-shaped ves­sel equipped at its bottom with a valve-controlled out­let. A series of such separatory funnels may be used, permitting increments of the foamy microsphere suspen­ sion to be held for defoaming and size separation. The foam collects on top of the solution together with over-­size microspheres. Undersize microspheres concentrate toward the bottom of the funnel, and can be removed to­gether with the albumin solution to be recycled as the first drainage fraction from the funnel The next frac­tion will comprise the albumin microspheres which are passed to a static mixer, as previously described, and then to packaging. The final fraction removed from the separatory funnel will comprise the residual foam and oversize microspheres, and is discharged to waste.

OPERATING PROCEDURES

Feed Preparation

:

0038Fill the 100 or 200 liter feed tanks with aqueous albumin, 5%. Pump the albumin through the fil­ters at rates between 50ml/min to 1L min. The filter albumin is placed in the sonication operation's feed tanks.

Sonication

:

0039A controlled flow of albumin, between 50 ml/min and 1 liter/min, is transported through a heat exchanger, equipped with a feedback temperature control loop designed to control the albumin temperature during the sonication process. The temperatures of the heat transfer medium and the heated solution are as previous­ly described. A filtered gas (i.e., air at 25cc/min to 200 cc/min) is added to the sonication process. This air greatly enhances cavitation and formation of micro­bubbles during the sonication process. Using a sonica­tion vessel as shown in Fig. 2, the total residence time in the sonication vessel is between 1.0 and 4 minutes. The sonication power (or energy) setting can be set at a range of 6 to 10 (on a scale of 1 to 10). The static mixer after sonication defoams the less dense foam. Op­tionally, the sonication product may be placed in a holding tank and mixed before proceeding to the separa­tion operation.

Separation/Concentration

:

0040The sonication operations product settles out between 1-8 hours without agitation in a separator/con­centrator vessel. When substantially all the micro­spheres have formed a layer on the top, drain approxi­mately two-thirds of the volume from the bottom. The top layer is microsphere product. The bottom layer is returned to the feed preparation operation to be recy­cled.

Optional Fractionation

:

0041Resuspend the microspheres and fill a 60 ml syringe with them. Let sit 30 minutes, then drain all but about the last 3-4 ml into a collection vessel. The oversize microspheres are left. Count a sample and cal­culate the concentration, mean diameter, and percentage less than 10 microns. If less than 90% are less than 10 microns, re-fractionate. If required for redispersion, the concentration may be adjusted with 5% human serum albumin.

REFERENCES

0042Feigenbaum, <u style="single">et al</u>. (1970), <u style="single">Circulation</u> 41:515-521. Feinstein, U.S. Patent 4,572,203. Feinstein, U.S. Patent 4,718,433. Feinstein, <u style="single">et al</u>. (1984), <u style="single">J. Am. Coll. Cardiol.</u> 3:14-20. Gramiak and Shah (1968), <u style="single">Invest. Radiol.</u> 3:356-358. Keller, Feinstein and Watson (1987), <u style="single">Amer. Heart. J.</u>, 114:570-575. Tickner, <u style="single">et al</u>. National Technical Information Service Report HR 62917-1A, April, 1977, pages 34-40.