Vascular magnetic imaging method

CROSS REFERENCE TO RELATED APPLICATIONS

This application corresponds to a continuation-in-part of applicants' prior co-pending U.S. Application Serial No. 067,586, filed June 26, 1987, which is, in turn, a continuation-in-part of applicants' prior co-pending U.S. Application Serial No. 882,044, filed July 3, 1986.

1.

INTRODUCTION

This invention relates to the use of a biodegradable superparamagnetic metal oxide particle, without resort to a receptor blocking agent, in the manufacture of vascular contrast agents for vascular MR imaging of animal or human subjects, and to the use of said metal oxide particle in the manufacture of brightening contrast agents for MR imaging. These vascular agents, in turn, allow one to image organ or tissue perfusion, as well as blood flow. The use of these contrast agents, preferably administered as a superparamagnetic fluid, offers significant advantages over existing methodologies including <u>inter</u><u>alia</u> high proton relaxivities, rapid biodegradability, control of blood lifetimes, versatility in choice of pulse sequence weighting schemes and other MR experimental parameters, low dosage requirements, low osmolality, and little or no toxicity.

2.

BACKGROUND OF THE INVENTION

Magnetic resonance (MR) imaging is widely regarded as a powerful technique for probing, discovering, and diagnosing the presence and progress of a pathological condition or disease. The MR method of imaging is also regarded as the least invasive of the imaging techniques presently available and does not expose the patient or subject to potentially harmful high-energy radiation, such as X-rays, or radioactive isotopes, such as technetium-99m. Technological developments in both instrumentation and computer software continue to improve the availability and quality of the images produced. Researchers discovered quickly, however, that the relative differences between the chemical and magnetic environments of water molecules, whose proton nuclei provide by far the largest source of measurable signal intensity within the body, are often quite small, regardless of whether these molecules are located in organs, tissues, tumors, or in the vascular compartment. Consequently, the resulting images are poorly resolved. Fortunately, this inherent limitation can be overcome by the use of proton relaxation agents, also known as contrast agents, which are absorbed selectively by different types of tissues and/or sets of organs, and thus create a temporary condition in which the magnetic environments of neighboring water molecules are measurably dissimilar.

According to their magnetic properties, there are three general types of MR contrast agents: paramagnetic, ferromagnetic, and superparamagnetic. The weak magnetism of paramagnetic substances arises from the individual unpaired electrons while the stronger magnetism of ferromagnetic and superparamagnetic materials results from the coupling of unpaired electrons made possible by their presence in crystalline lattices. Ferromagnetic materials retain their magnetism in the absence of an applied magnetic field, while superparamagnetic materials lose their magnetism when the applied magnetic field is removed. With respect to effects on proton relaxation, paramagnetic agents have been termed T₁ type agents because of their ability to enhance spin-lattice or longitudinal relaxation of proton nuclei. Ferromagnetic agents have been termed T₂ type agents because of their specific effects on T₂, which is sometimes called the spin-spin or transverse relaxation time.

There are three major disadvantages to the use of paramagnetic chelates as vascular MR contrast agents. The first is that many low molecular weight, ionic materials which are commonly used as paramagnetic chelates are hypertonic. The use of hypertonic solutions often results in adverse reactions upon injection. The second disadvantage is that paramagnetic chelates have short blood lifetimes whereas a vascular MR contrast agent should remain confined to the vascular compartment for long periods of time. Third, removal of paramagnetic chelates from the vascular compartment can result in release of the paramagnetic ion from the chelate. Paramagnetic ions of iron, manganese, and gadolinium, for example, are toxic in their free ionic form. Vascular MR contrast agents, however, should have a benign metabolic fate after removal from the vascular compartment. These three disadvantages are explained further below.

Paramagnetic materials that have been used as T₁ contrast agents generally include organic free radicals as well as transition metal salts and chelates. These compounds can be quite soluble and, in the case of most transition metal complexes, are highly charged, ionic species. Due to their relatively low relaxivities (their ability to increase the relaxation rates of protons as a function of dose), high concentrations of transition metal chelates are needed to effect useful alterations in the relaxation times of blood. In addition, the ionic nature of many transition metal ion salts and chelates contributes to the high osmotic pressure of the injected diagnostic solutions. The end result is that solutions of paramagnetic materials, whether they are used as MR agents or not, tend to be hyperosmotic relative to blood. The administration of hyperosmotic solutions into the subject is widely believed to be a major cause of adverse reactions to radiographic and MR contrast media (<u>See</u>, McClennan, <u>B.L. Diagnostic Imaging Supplement</u><b>1987</b>, 16-18 (December); "Contrast Media: Biological Effects and Clinical Application," Vol. I, ch. 5, Parvez, Z., Moncada, R., and Sovak, M. (Eds.), CRC Press, Boca Raton, Florida (1987)).

The usual approach to the development of MR contrast agents confined to the vascular compartment for long periods of time is to increase the molecular weight of paramagnetic chelates by attaching the chelates to high molecular weight polymers. After injection, high molecular weight forms of the chelates cannot be excreted by glomerular filtration. Consequently, they have longer residence times within the vascular compartment. High molecular weight forms of chelates, such as gadolinium can be made by covalently attaching chelators to macromolecules such as human serum albumin (Schmiedl et al. <u>Radiology</u><b>1987</b>, <u>162</u>, 205-210)). With this approach to the design of vascular MR contrast agents, the fate of the gadolinium after degradation of the agent presents serious problems. The long term retention of gadolinium not eliminated by glomerular filtration, and the potential for delayed toxicity from that element, pose major obstacles to the administration of high molecular weight gadolinium chelates to humans.

A major disadvantage of present ferromagnetic contrast agents is that such materials are relatively large and, frankly particulate in character. Frankly particulate materials, those generally having overall dimensions between about 0.5-10 µm, are quickly removed from the blood by the phagocytic action of the cells of the reticuloendothelial system, limiting the duration of their effects on the spin-spin and spin-lattice relaxation times of blood. Hence their usefulness as vascular MR contrast agents is limited. All particulate agents suffer a similar limitation.

3.

DEFINITIONS

Unless otherwise noted, the term "sterilized" describes a sample or a preparation which has been subjected to any method known in the art which completely destroys all bacteria and other infectious agents which may be present in the sample or preparation. Nonlimiting examples of such methods include autoclaving, ultraviolet or gamma irradiation, cold membrane filtration, or chemical treatment. The resulting preparation is then suitable for <u>in</u><u>vivo</u> and/or clinical use with or without further treatment. The term "biodegradable" describes a property of a compound or complex which allows the compound or complex to be broken down into smaller innocuous components and be excreted from or utilized within the body. The term "polyfunctional" applies to a molecule which contains a plurality of identical functional groups. A "macromolecular species" may include any molecule, natural or synthetic, which has a molecular weight in excess of 1 kilodalton. Typically, the macromolecular species will have a molecular weight of about 1 to about 250 kilodaltons.

Abbreviations used in the text are defined below. <dl id="dl0001" compact="compact"><dt>T₁ =</dt><dd>spin-lattice relaxation time</dd><dt>T₂ =</dt><dd>spin-spin relaxation time</dd><dt>EM =</dt><dd>transmission electron microscopy</dd><dt>MR =</dt><dd>magnetic resonance or nuclear magnetic resonance (NMR)</dd><dt>RES =</dt><dd>reticuloendothelial system</dd><dt>MW =</dt><dd>molecular weight</dd><dt>kD =</dt><dd>kilodalton</dd></dl>

4.

SUMMARY OF THE INVENTION

This invention relates to the use of a biodegradable superparamagnetic metal oxide particle, in the manufacture of vascular and brightening contrast agents for MR imaging an animal or human subject. The enhanced MR image is realized by administering to such an animal or human subject an effective amount of a contrast agent prepared from aggregates of individual biodegradable superparamagnetic metal oxide crystals. The aggregates, whose metal oxide crystals may be associated with a macromolecular species, are dispersed in a physiologically acceptable medium, preferably an aqueous citrate buffer, which stabilises the aggregates of the resulting fluid against further aggregation even under the sometimes extreme conditions of sterilization. Depending on the manner in which the biodegradable superparamagnetic aggregates are prepared, their properties and characteristics, such as overall sizes and blood circulation lifetimes, for example, may be varied.

One of the objects of the present invention is to provide a pharmaceutical preparation of a biodegradable superparamagnetic contrast agent suitable for human clinical use in magnetic resonance experiments. In particular, preferred embodiments of the invention have great utility in obtaining enhanced images of the vascular system, of the blood pool, or of organ or tissue perfusion. By practicing the methods of the invention, blood vessels, arteries, veins, capillaries, as well as lymph vessels may be imaged successfully. In particular, capillary beds of various organs or tissues such as the brain, the kidney, the lung, and the heart can be visualized. Such methods can thus provide valuable information regarding the condition of these vessels, organs, or tissues, including the presence of microocclusions, embolisms, aneurysm, restricted blood flow, and the onset or recession of arterial disease.

It is also an object of this invention to provide a contrast agent which can serve as a brightening or a darkening agent, or both, and which is effective at low doses. This invention also seeks to provide a contrast agent which is rapidly biodegraded, being either excreted from or utilized within the body of the subject, as evidenced by a return of the proton relaxation rates of affected tissue to pre-administration levels within about one week of the initial administration. Yet another object of the present invention is to provide sterile solutions of a contrast agent which have a low osmotic pressure, low enough, in fact, that added salt may be included in the formulation, if so desired. Thus, sterile solutions of contrast media may be prepared which are isotonic and hypotonic, if so desired. It is also an object of the present invention to provide a contrast agent whose blood lifetime may be adjusted to accommodate the variable needs of the MR imaging experiment.

It is thus an object of the instant invention to provide an effective contrast agent with all the advantages enumerated above, which, however is substantially free of the limitations and objectionable aspects associated with paramagnetic and ferromagnetic materials.

After a thorough consideration of the foregoing disclosures and those that follow, other objects of the present invention will become readily apparent to those readers skilled in the art.

5.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a chromatogram obtained from a Sephrosse B gel exclusion fractionation of a biodegradable superparamagnetic fluid.

Fig. 2 is a plot of the blood spin-lattice relaxation rate versus time for unfractionated and fractionated superparamagnetic material.

Fig. 3 is an electron micrograph of a sterilized superparamagnetic fluid prepared according to the standard methods of the invention. The particles numbered 1-4 in Fig. 3 have the following overall mean diameters: 9 nm, 122 nm, 68 nm, and 32 nm respectively.

Fig. 4 is a plot of the "brightening" effect of a number of contrast agents as a function of their solution concentration.

6.

DETAILED DESCRIPTION OF THE INVENTION

Superparamagnetic metal oxides are a unique class of magnetic materials because of their large effects on both the T₁ and T₂ relaxation times of magnetically active nuclei. We have described in detail the utility of biodegradable superparamagnetic materials as MR contrast agents in related U.S. Patents Nos. 4,827,945 and 4,770,183. The disclosures in both Patents are incorporated herein by reference. Others have described the use of superparamagnetic contrast agents since (<u>See</u>, for example, U.S. Patent No. 4,675,173 issued to Widder; Hemmingson et al. <u>Acta. Radiologics</u><b>1987</b>, <u>28(6)</u> 703; Saini et al. <u>Radiology</u><b>1987</b>, <u>162</u>, 211). When present as a component of a superparamagnetic lattice, transition metals, such as iron, are more potent enhancers of proton relaxation than the corresponding central metal ion of a simple paramagnetic chelate. This increased "potency" of the superparamagnetic iron is reflected in, for example, the spin lattice relaxivity value measured for a biodegradable superparamagnetic fluid, comprising the biodegradable superparamagnetic iron oxide and a citrate buffer, which value is 50 times greater than that found for a chelated form of iron. Indeed, as the sizes of the aggregates of iron oxide crystals decrease, their effects on T₁ increase relative to their effects on T₂. As disclosed in the present invention, superparamagnetic fluids comprised of very small metal oxide aggregates are excellent intravascular MR contrast agents and can be used in a variety of modes, including T₁-weighted (brightening agents), T₂-weighted (darkening agents), or combinations of these and other pulse techniques.

An important prerequisite for the successful use of biodegradable superparamagnetic materials as intravascular agents in humans is the ability to control the blood lifetimes of the materials. In other words, an effective intravascular agent must have a useful lifetime in the circulatory system so that the MR experiment may be completed before a significant proportion of the administered contrast agent is removed from circulation by organs or tissues of, for example, the reticuloendothelial system (RES). The injection of higher concentrations of any contrast agent will, of course, increase its longevity in the vascular compartment and prolong its effect on the nuclear relaxation times of the blood. Such a strategy, while useful in demonstrating the efficacy of an MR contrast agent in animal models of human pathology, is not ideal for the formulation of a pharmaceutical, clinical form of the contrast agent. Safety considerations preclude the use of overly large doses of a pharmaceutical or diagnostic agent. Imaging techniques, such as those described herein, which are able to enhance the blood pool while maintaining the minimum effective dose, are highly desirable. Moreover, the ability of the practitioner to manipulate or control the blood lifetime of the biodegradable superparamagnetic contrast agents of the invention permits an unsurpassed flexibility previously unavailable in the design and operation of the MR experiment. A variety of pharmacokinetic patterns might be used, for example. Hence, for an experiment designed to delineate tissue blood pool volume, an agent with a long blood lifetime would be used having all the advantages outlined above for biodegradable superparamagnetic materials. In other applications, an agent which is rapidly removed from the vascular compartment may be used to "wash in" to damaged tissue and to take an image very soon thereafter. Areas where the blood brain barrier have broken down due to the presence of tumors may be successfully examined using these methods. Moreover, because discrete size ranges of the contrast agent can be obtained, for example, by size exclusion chromatography, the practitioner can tailor and control the amount of time (e.g., blood halflives) that these agents spend circulating within the vascular compartment. Generally, the blood lifetime is inversely proportional to the overall size of the contrast agent. Some agents of the present invention, because of their extremely small dimensions, are able to circulate within the vascular compartment for relatively long periods of time. Furthermore, it is the crystalline nature of these agents which provides solutions of low osmolality compared with paramagnetic agents. This feature, in combination with the above-mentioned properties and characteristics, make these biodegradable superparamagnetic metal oxides uniquely suitable as intravascular contrast agents.

The biodegradable superparamagnetic metal oxides can be prepared as a superparamagnetic dispersion or as a superparamagnetic fluid by general methods described previously in applicants' related U.S. Patents identified above. These procedures include precipitating the metal oxide crystals, in the presence or absence of a macromolecular or polymeric species, from a solution of the trivalent and divalent ions of a metal by the addition of a sufficient quantity of base. Preferred metal ions include those of chromium, cobalt, copper, iron, manganese, molybdenum, nickel and tungsten. A solution of hydrated ferric and ferrous salts is particularly preferred. The alkaline reaction may be carried out in the presence of a macromolecular species which aids in dispersing the resultant superparamagnetic metal oxide crystals. In general, the aggregates that form initially, in the absence of the polymeric substance, are larger. Some suitable macromolecular species include polymeric organosilanes, proteins, polypeptides, carbohydrates, mono-, oligo-, and polysaccharides. Preferred macromolecules include N-2-aminoethyl-3-aminopropyltrimethoxy silane, human and bovine serum albumin, polyglutamate, polylysine, heparin, hydroxyethyl starch, gelatin, and dextran, with the lattermost being particularly preferred. The dark slurry which forms after the addition of base is then sonicated to further reduce the size of the aggregates. The sonication also serves to oxygenate the mixture and assures the full oxidation of the divalent metal ion to the trivalent oxidation state. Larger particulate aggregates that may form during the procedure may be removed by centrifugation. The resulting supernatant contains only nonparticulate-sized aggregates.

The next stage of the preparation involves the dilution of the supernatant with distilled water and subsequent ultrafiltration through a hollow fiber dialysis apparatus. The diluted supernatant is dialyzed against an aqueous solution or buffer of a polyfunctional organic molecule. Molecules containing positively charged groups may be utilized. These molecules may include but are not limited to polylysine, polyornithine, and polyarginine. However, the functional groups of the organic molecule are preferably ionizable to give negatively charged groups such as phosphates, phosphinates, phosphonates, sulfinates, sulfonates, carboxylates, and the like. Polycarboxylate compounds are particularly effective, with salts of citrate ion being most preferred. Besides being commonly used and widely accepted as safe in clinical preparations, the aqueous citrate buffer serves to stabilize the aggregates of the resulting superparamagnetic fluid to further clustering or aggregation. The inventors have found, quite unexpectedly, that the resulting fluid is stable even under the extreme conditions of prolonged heating in an autoclave. In addition, the dialysis step also removes most of the macromolecular species, e.g. dextran. Also, dialysis against citrate scavengers isolated free metal ions probably through coordination with the anionic groups of the polyfunctional molecule. If so desired, greater than ninety percent of the macromolecular species used initially can be removed in this manner.

The resulting dialysed superparamagnetic fluid is then diluted further and sterilized by any means well known in the art. Sterilization may be accomplished by autoclaving, irradiation, sterile filtration, chemical treatment, or other means. Sealing the samples in appropriate containers and heating them in an autoclave is most convenient. It is understood that, depending upon the particular application at hand, substantially all the added macromolecular species may be removed or another introduced by any appropriate means well known in the art, including exhaustive dialysis.

The size distribution and architecture of the metal oxide crystal aggregates can be examined by light scattering methods and by electron microscopy (EM). The EM studies are particularly revealing and show that the aggregates are indeed comprised of individual metal oxide crystals which are interconnected to form irregularly shaped three-dimensional structures (See, for example, Fig. 3). The dimensions of the aggregates depend on the number of component crystals present and, for one specific example, fall in the range of 10-125 nm. The majority of the aggregates have dimensions smaller than about 50 nm. The results of light scattering methods give similar, but slightly larger, values. This slight difference is probably due to the fact that larger particles will tend to scatter more light and, thus, skew the value slightly toward the higher end. Light scattering methods give a value of about 75 nm for the overall mean diameter of the sample described above.

In addition, light scattering measurements reflect that the aggregates generally fall within the range of about 1 to about 400 nm. In this context, aggregates having an overall mean diameter of 300 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less can be detected.

In a specially modified procedure, extremely. small superparamagnetic materials particles are obtained at the outset. In this modification, a solution of, for example, ferric and ferrous salts is adjusted first to a pH of about 2.3 before the addition of dextran. The resulting mixture is stirred and heated to about 60-70°C for several minutes and then allowed to cool to a temperature of about 40-45°C. To this solution is added a sufficient amount of ammonium hydroxide to bring the pH to about 10. The resulting suspension is then heated to about 1OO°C until a black suspension forms. After an ultrafiltration step to remove most of the unbound dextran, light scattering experiments showed that dextranized particles of this preparation have an overall mean diameter of about 40 nm.

The superparamagnetic fluid prepared according to the general methods can be separated into fractions of decreasing aggregate diameter by size exclusion gel chromatography, with a suitable buffer as eluent. The collected fractions can be analyzed by UV-vis spectroscopy for metal oxide content and by light scattering methods for an estimate of their crystal aggregate dimensions. Predictably, the larger aggregates elute first, followed by successively smaller aggregates. The fractions can be divided into four groups, for the sake of convenience, and tested separately. A standard solution comprised of dextran blue (2,000 kD), ferritin (443 kD), and bovine serum albumin (65 kD) can be fractionated under the same conditions as the superparamagnetic fluid. The fraction number or elution volume at which the known proteins emerge serve as points of reference from which the average "molecular weights" of the crystal aggregates of the contrast agent fractions can be related. The results are summarized in Section 7.5.

Proton relaxation experiments are carried out <u>in</u><u>vivo</u> by intravascularly injecting mice with solutions of fractionated as well as unfractionated contrast agents. The effect on proton relaxation, T₁ for example, may be monitored as a function of time for each of the samples, and the results will show that, the smaller the aggregate size, the longer the blood serum lifetime of the contrast agent will be. The unfractionated superparamagnetic contrast agent shows a moderate serum blood halflife, Blood t_1/2, of about 13.5 minutes, while smaller aggregates having a Blood t_1/2 value of 49.6 minutes can be isolated.

These superparamagnetic metal oxide preparations serve as excellent contrast agents for MR imaging. They may be successfully used as brightening agents, for example, as can be demonstrated by a relative comparison of the Brightness/Intensity effects of the materials of the invention versus samples of pure water and aqueous solutions of typical paramagnetic T₁ relaxation agents, such as manganese(II) chloride or ferric chloride. All the superparamagnetic metal oxide preparations described above are able to brighten water signals relative to pure water and, in one case, the brightness effects even exceeds those of manganese(II) chloride, heretofore recognized as a very effective T₁ contrast agent (See Fig. 4).

7.

EXAMPLES

7.1.

PREPARATION OF STERILIZED SUPERPARAMAGNETIC FLUIDS

To a vigorously stirred aqueous 16% ammonium hydroxide solution (5 liters) of dextran (2.500 kg, 10-15 kilodalton) is added gradually, and over a 5 minute period, an aqueous solution (5 liters) of ferric chloride hexahydrate (FeCl₃.6H₂O, 0.755 kg) and ferrous chloride tetrahydrate (FeCl₂.4H₂O, 0.320 kg). The black magnetic slurry which forms is then pumped at a rate of about 0.4 liters per minute through a continuous flow sonicating apparatus. It is sometimes advantageous, although not necessary, to heat the mixture through a sonicating apparatus which comprises a sonicator connected in series to a 100°C heating coil unit followed by a cooling coil unit. The resulting dispersed mixture is next centrifuged at 2,000 x g for 20 minutes to separate the larger aggregates which are discarded.

The supernatant is diluted with deionized, sterile water to a volume of about 20 liters. Ultrafiltration of the diluted supernatant against water and citrate buffer is then carried out in a noncontinuous fashion using a large hollow fiber dialysis/concentrator, Model DC 10 (Amicon Corp., Danvers, Massachusetts), equipped with a 100 kilodalton molecular weight cutoff dialysis cartridge. In this manner, a substantial amount of the dextran used initially is removed along with any free metal ions.

After ultrafiltration, a solution of 0.20 M Fe and 0.025 M citrate is obtained by the addition of 1 M citrate and adjusting to a pH of about 8 with 1 N NaOH. The solution is then autoclaved for 30 minutes (121°C). Autoclaving is the preferred technique for sterilization since the bottle or container need not be sterile prior to filling. An alternative to sterilization is filtration, but superparamagnetic fluids at high concentrations filter poorly. Dilute superparamagnetic fluids can be filter sterilized, but the added water must then be removed under sterile conditions.

7.2.

FURTHER REMOVAL OF DEXTRAN FROM STERILIZED SUPERPARAMAGNETIC FLUIDS

The dextran remaining after ultrafiltration is largely dissociated from the superparamagnetic iron oxide-dextran by autoclaving.

To determine what part of the dextran is attached to superparamagnetic iron oxide after autoclaving, dialysis is used to separate dextran from superparamagnetic iron oxide-dextran complexes. The autoclaved 0.20 M Fe solution from above is diluted into 25 mM citrate buffer (pH 8) to an iron concentration of about 2 mM and the solution is incubated overnight at 37°C to release any dextran which is weakly adsorbed to the superparamagnetic iron oxide. The solution is then applied to a centrifuge micropartition system equipped with a cellulose acetate membrane (Amicon, Danvers, MA), where the filtrate is forced through the membrane by centrifugation. The dextran employed initially has a molecular weight of 10-15 kD and passes through the membrane while superparamagnetic iron oxide does not.

Prior to autoclaving (but after ultrafiltration) there were 8.45 mg/mℓ of dextran present in a solution containing 11.2 mg/mℓ of Fe (0.2 M Fe = 11.2 mg Fe/mℓ). The concentration of dextran passing through the micropartition system membrane was 7.49 mg/mℓ, indicating that most of the dextran had detached from the superparamagnetic iron oxide by the autoclaving step. Iron is measured by atomic absorption spectrophotometry after dissolving the iron oxide in 0.01 N HCl. Dextran is measured by a phenol-sulfuric acid method for total carbohydrates (<u>See</u>, C.E. Meloan and Y. Pomeranz, "Food Analysis Laboratory Experiments," The Avi Publishing Co., Westport Conn., pp. 85-86 (1973)).

The small aggregates of superparamagnetic iron oxide crystals seen in EM studies are largely uncoated after autoclaving and are stabilized through their interactions with polyvalent anions such as citrate. The majority of that dextran is present as a dialyzable species free in solution.

7.3

MODIFIED PROCEDURE FOR THE PREPARATION OF EXTREMELY SMALL BIODEGRADABLE SUPERPARAMAGNETIC AGGREGATES

To an aqueous solution (250 mℓ) of FeCl₃.6H₂0 (35 g) and FeCl₂.4H₂0 (16 g) is added a sufficient amount of aqueous 10% sodium carbonate to bring the pH of the solution to a value of about 2.3. Solid dextran (150 g) is then added. The solution is stirred and heated to about 60-70°C for about 15 min and then allowed to cool to 40-45°C. To the reddish solution is added aqueous 7.5% NH₄OH to a final pH between 9.5 and 10.0. A greenish suspension is produced which is subsequently heated to 95-100°C for 15 min. The resulting black suspension is then subjected to an ultrafiltration step using an Amicon RA 2000 hollow fiber dialysis unit equipped with a cartridge having a nominal cutoff of 100 kilodaltons. Light scattering measurements reveal that the dextranized particles have an overall mean diameter of about 40 nm.

7.4

GEL EXCLUSION CHROMATOGRAPHY OF SUPERPARAMAGNETIC MATERIALS

The superparamagnetic fluid described in Section 7.1 can be applied to the top of a chromatography column (2.5 cm x 100 cm) packed with Sepharose 4B. The sample is eluted with 20 mM sodium citrate buffer (pH = 8). Fractions are collected and analyzed for their optical density at 340 nm. A separate sample containing a mixture of known proteins can be treated and fractionated under the same chromatography conditions. A plot of the optical density (0D) versus the fraction number and elution volume yields a chromatogram which is illustrated in Fig. 1. The overall mean diameter in nanometers may be determined for a given set of superparamagnetic fluid fractions by light scattering methods. These results are listed across the top of Fig. 1. As is readily evident from the chromatogram and plot, the overall size decreases with increasing fraction number and elution time. Likewise, the higher molecular weight protein standards elute faster than the smaller ones. The eluted fractions of superparamagnetic materials can be combined as follows: Sample A, fractions 29-31; Sample B, fractions 34-37; Sample C, fractions 43-49; and Sample D, fractions 55-62.

7.5

DETERMINATION OF BLOOD LIFETIMES OF SUPERPARAMAGNETIC MATERIALS AS A FUNCTION OF SIZE

The samples B, C, and D, described above, are intravenously injected separately into a rat at the same dose of 2 mg of iron per kg of rat. The spin-lattice relaxation rate, 1/T₁, can then be measured as a function of time and reflects the blood concentration. Fig. 2 is a graphical representation of the results for samples B, C, D, and unfractionated superparamagnetic fluid. An examination of the plot in Fig. 2 shows that the blood lifetimes of the different fractions increase with decreasing overall size. Table I summarizes the results of the above experiments. The blood half-life (Blood t_1/2) is determined from a nonlinear least squares fit to a single exponential decay process. When a set of standard proteins are also run, the molecular weight (MW) values of samples A, B, C, and D can be estimated from extrapolated values derived from the standard proteins. <tables id="tabl0001" num="0001"><table frame="all"><title>TABLE I</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col5" align="center">PROPERTIES OF NATIVE AND FRACTIONATED SUPERPARAMAGNETIC FLUID</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Sample</entry><entry namest="col2" nameend="col2" align="center">Fraction#</entry><entry namest="col3" nameend="col3" align="left">OMD^a</entry><entry namest="col4" nameend="col4" align="center">Blood t_1/2</entry><entry namest="col5" nameend="col5" align="left">Dose</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Native^b</entry><entry namest="col2" nameend="col2" align="left">All</entry><entry namest="col3" nameend="col3" align="left">75 nm</entry><entry namest="col4" nameend="col4" align="left">13.5 min</entry><entry namest="col5" nameend="col5" align="left">1 mg/kg</entry></row><row><entry namest="col1" nameend="col1" align="left">A</entry><entry namest="col2" nameend="col2" align="left">29-31</entry><entry namest="col3" nameend="col3" align="left">97</entry><entry namest="col4" nameend="col4" align="left">-</entry><entry namest="col5" nameend="col5" align="left">2</entry></row><row><entry namest="col1" nameend="col1" align="left">B</entry><entry namest="col2" nameend="col2" align="left">34-37</entry><entry namest="col3" nameend="col3" align="left">46</entry><entry namest="col4" nameend="col4" align="left">4.05</entry><entry namest="col5" nameend="col5" align="left">2</entry></row><row><entry namest="col1" nameend="col1" align="left">C</entry><entry namest="col2" nameend="col2" align="left">43-49</entry><entry namest="col3" nameend="col3" align="left">18</entry><entry namest="col4" nameend="col4" align="left">16.6</entry><entry namest="col5" nameend="col5" align="left">2</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">D</entry><entry namest="col2" nameend="col2" align="left">55-62</entry><entry namest="col3" nameend="col3" align="left">&lt;15</entry><entry namest="col4" nameend="col4" align="left">49.6</entry><entry namest="col5" nameend="col5" align="left">2</entry></row></tbody></tgroup><tgroup cols="5" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><tbody valign="top"><row><entry namest="col1" nameend="col5" align="justify">^aOverall mean diameter as measured by light scattering methods.</entry></row><row><entry namest="col1" nameend="col5" align="justify">^bUnfractionated fluid prepared by the general methods.</entry></row></tbody></tgroup></table></tables>

7.6.

TRANSMISSION ELECTRON MICROSCOPY STUDIES

An electron micrograph of the sterilized superparamagnetic fluid, prepared according to 7.1 is shown in Figure 3. The EM study shows the individual particle to be aggregates of iron oxide, with individual iron oxide crystals having a diameter of about 5-7 nm. The crystals of each particle are in direct contact with each other and are not separated by, or embedded in, a matrix or polymer of any kind. EM studies (beyond those shown in Fig. 3) have indicated the presence of aggregates from as small as 10 nm to as large as 300 nm. Both EM and gel chromatography (see Section 7.4) indicate that the superparamagnetic fluid is a heterogeneous mixture of different-sized particles. EM studies reveal the presence of a large number of small aggregates which comprise a very small percentage of the volume of iron mass. Consequently, the average volume of an aggregate by EM studies is only about 20 nm. (Average volume = the total volume of all aggregates divided by the total number of aggregates.)

For EM studies it is important to prevent changes in size distribution of particles during sample preparation. The preparation of the sample for EM studies is as follows. A solution of 5% molten agar is added to the sterile superparamagnetic fluid to give a final agar concentration of 1% and the agar allowed to harden. The agar is minced into pieces of about a millimeter and dehydrated by addition of ethanol/water mixtures of decreasing water content until water is replaced by 100% ethanol. The ethanol is then replaced by propylene oxide in an analogous fashion. The resulting propylene resin, yielding a 100% epoxy resin, is hardened by curing overnight at 60°C. The hardened resin is then sliced with an ultramicrotone (0.5 micron thick sections) and placed on a 400 mesh copper grid.

A Phillips model 410 LS transmission electron microscope is used to take micrographs at 100 kV.

7.7.

SUPERPARAMAGNETIC FLUIDS AS BRIGHTENING AGENTS

Superparamagnetic materials are used as MR contrast agents due to their ability to promote the relaxation of magnetically active nuclei. As the particle size of the metal oxide cluster in superparamagnetic fluids becomes smaller and smaller, the clusters become more powerful as brightening agents in T₁-weighted MR pulse sequences. Fig. 4 shows the effects of different concentrations of four MR contrast agents on the signal intensity (S) of an MR image using a T₁-weighted pulse sequence of TE = 200 msec and TR = 15 msec. The imager was a GE CSI 2 Tesla imager. The signal intensity of distilled water is set at 1.0.

Various concentrations of agents were added to distilled water and the molar concentration of metal is given on the x-axis. Agents that brighten increase the value of S, while those that darken decrease the value of S. A standard T₁ type MR brightening agent is MnCl₂, which has a strong image brightening effect over a wide range of concentrations. A standard, more purely T₂ type MR agent is provided by a large silanized cluster of superparamagnetic materials made according to the teachings of U.S. Patent No. 4,695,392, which is incorporated herein by reference. On a molar basis, the smallest fraction obtained from the chromatogram of Fig. 1 (D of Table I) is a more potent brightening agent than MnCl₂, while the larger parent material (Native of Table I) is considerably less potent as a brightening agent. As shown in Fig. 4, the large silanized cluster is the least potent T₁ type agent. The strong effects of sterile superparamagnetic fluids on MR image brightening, particularly as particle size decreases, allows such materials to be used as vascular MR imaging with T₁- or T₂- weighted pulse sequences. When T₁-weighted sequences are used, such materials perform as contrast agents in a manner analogous to paramagnetic brightening agents such as Gd/DTPA or MnCl₂.