Drug – bio-affecting and body treating compositions – In vivo diagnosis or in vivo testing – Magnetic imaging agent
Reexamination Certificate
2001-05-10
2003-06-10
Padmanabhan, Sreeni (Department: 1617)
Drug, bio-affecting and body treating compositions
In vivo diagnosis or in vivo testing
Magnetic imaging agent
C424S009321, C424S009360, C424S009510, C424S009520, C600S420000
Reexamination Certificate
active
06576220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of magnetic resonance imaging, more specifically to the use of stabilized gas filled vesicles as contrast media for magnetic resonance imaging (MRI) directed ultrasound surgery.
There are a variety of imaging techniques that have been used to diagnose disease in humans. One of the first imaging techniques employed was X-rays. In X-rays, the images produced of the patients' body reflect the different densities of body structures. To improve the diagnostic utility of this imaging technique, contrast agents are employed to increase the density of tissues of interest as compared to surrounding tissues to make the tissues of interest more visible on X-ray. Barium and iodinated contrast media, for example, are used extensively for X-ray gastrointestinal studies to visualize the esophagus, stomach, intestines and rectum. Likewise, these contrast agents are used for X-ray computed tomographic studies (that is, computer assisted tomography or CAT) to improve visualization of the gastrointestinal tract and to provide, for example, a contrast between the tract and the structures adjacent to it, such as the vessels or lymph nodes. Such contrast agents permit one to increase the density inside the esophagus, stomach, intestines and rectum, and allow differentiation of the gastrointestinal system from surrounding structures.
Magnetic resonance imaging (MRI) is a relatively new imaging technique which, unlike X-rays, does not utilize ionizing radiation. Like computer assisted tomography (CAT), MRI can make cross-sectional images of the body, however MRI has the additional advantage of being able to make images in any scan plane (i.e., axial, coronal, sagittal or orthogonal). Unfortunately, the full utility of MRI as a diagnostic modality for the body is hampered by the need for new or better contrast agents. Without suitable agents, it is often difficult using MRI to differentiate the target tissue from adjacent tissues. If better contrast agents were available, the overall usefulness of MRI as an imaging tool would improve, and the diagnostic accuracy of this modality would be greatly enhanced.
MRI employs a magnetic field, radio frequency energy and magnetic field gradients to make images of the body. The contrast or signal intensity differences between tissues mainly reflect the T1 (longitudinal) and T2 (transverse) relaxation values and the proton density (effectively, the free water content) of the tissues. In changing the signal intensity in a region of a patient by the use of a contrast medium, several possible approaches are available. For example, a contrast medium could be designed to change either the T1, the T2 or the proton density.
2. Brief Description of the Prior Art
In the past, attention has mainly been focused on paramagnetic contrast media for MRI. Paramagnetic contrast agents contain unpaired electrons which act as small local magnets within the main magnetic field to increase the rate of longitudinal (T1) and transverse (T2) relaxation. Most paramagnetic contrast agents are metal ions which in most cases are toxic. In order to decrease toxicity, these metal ions are generally chelated using ligands. The resultant paramagnetic metal ion complexes have decreased toxicity. Metal oxides, most notably iron oxides, have also been tested as MRI contrast agents. While small particles of iron oxide, e.g., under 20 nm diameter, may have paramagnetic relaxation properties, their predominant effect is through bulk susceptibility. Therefore magnetic particles have their predominant effect on T2 relaxation. Nitroxides are another class of MRI contrast agent which are also paramagnetic. These have relatively low relaxivity and are generally less effective than paramagnetic ions as MRI contrast agents. All of these contrast agents can suffer from some toxic effects in certain use contexts and none of them are ideal for use as perfusion contrast agents by themselves.
The existing MRI contrast agents suffer from a number of limitations. For example, positive contrast agents are known to exhibit increased image noise arising from intrinsic peristaltic motions and motions from respiration or cardiovascular action. Positive contrast agents such as Gd-DTPA are subject to the further complication that the signal intensity depends upon the concentration of the agent as well as the pulse sequence used. Absorption of contrast agent from the gastrointestinal tract, for example, complicates interpretation of the images, particularly in the distal portion of the small intestine, unless sufficiently high concentrations of the paramagnetic species are used (Kornmesser et al.,
Magn. Reson. Imaging
6:124 (1988)). Negative contrast agents, by comparison, are less sensitive to variation in pulse sequence and provide more consistent contrast. However at high concentrations, particulates such as ferrites can cause magnetic susceptibility artifacts which are particularly evident, for example, in the colon where the absorption of intestinal fluid occurs and the superparamagnetic material may be concentrated. Negative contrast agents typically exhibit superior contrast to fat, however on T1-weighted images, positive contrast agents exhibit superior contrast versus normal tissue. Since most pathological tissues exhibit longer T1 and T2 than normal tissue, they will appear dark on T1-weighted and bright on T2-weighted images. This would indicate that an ideal contrast agent should appear bright on T1-weighted images and dark on T2-weighted images. Many of the currently available MRI contrast media fail to meet these dual criteria.
Toxicity is another problem with the existing contrast agents. With any drug there is some toxicity, the toxicity generally being dose related. With the ferrites there are often symptoms of nausea after oral administration, as well as flatulence and a transient rise in serum iron. The paramagnetic contrast agent Gd-DTPA is an organometallic complex of gadolinium coupled with the complexing agent diethylene triamine pentaacetic acid. Without coupling, the free gadolinium ion is highly toxic. Furthermore, the peculiarities of the gastrointestinal tract, for example, wherein the stomach secretes acids and the intestines release alkalines, raise the possibility of decoupling and separation of the free gadolinium or other paramagnetic agent from the complex as a result of these changes in pH during gastrointestinal use. Certainly, minimizing the dose of paramagnetic agents is important for minimizing any potential toxic effects.
New and/or better contrast agents useful in magnetic resonance imaging as well as improved imaging techniques are needed. The present invention is directed, inter alia, to these important ends.
In the work on MRI contrast agents described above for application Ser. No. 07/507,125, filed Apr. 10, 1990, it has been disclosed how gas can be used in combination with polymer compositions and paramagnetic or superparamagnetic agents as MRI contrast agents. Therein it has been shown how the gas stabilized by said polymers would finction as an effective susceptibility contrast agent to decrease signal intensity on T2 weighted images; and that such systems are particularly effective for use as gastrointestinal MRI contrast media.
Widder et al. published application EP-A-0 324 938 discloses stabilized microbubble-type ultrasonic imaging agents produced from heat-denaturable biocompatible protein, e.g., albumin, hemoglobin, and collagen.
There is also mentioned a presentation believed to have been made by Moseley et al., at a 1991 Napa, Calif. meeting of the Society for Magnetic Resonance in Medicine, which is summarized in an abstract entitled “Microbubbles: A Novel MR Susceptibility Contrast Agent.” The microbubbles which are utilized comprise air coated with a shell of human albumin. The stabilized gas filled vesicles of the present invention are not suggested.
For intravascular use, however, the inventors have found that for optimal results, it is advantageous that any gas be
IMARX Therapeutics, Inc.
Padmanabhan Sreeni
Wells Lauren Q.
Woodcock & Washburn LLP
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