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Title:  Methods for functional kidney imaging using small dendrimer contrast agents

United States Patent:  6,852,842

Issued:  February 8, 2005

Inventors:  Brechbiel; Martin W. (Annandale, VA); Star; Robert A. (Bethesda, MD); Kobayashi; Hisataka (Nishinomiya, JP)

Assignee:  The United States of America as represented by the Secretary of the (Washington, DC)

Appl. No.:  229316

Filed:  August 26, 2002

Abstract

Small dendrimer-based MRI contrast agents are disclosed to accumulate in renal tubules. The accumulation enables visualization of renal structure and function, permitting assessment of structural and functional damage to the kidneys. In a disclosed embodiment, six, small dendrimer-based MRI contrast agents were synthesized, and their pharmacokinetics, whole body retention, and renal MRI images were evaluated in mice. Surprisingly, despite having unequal renal clearance properties, all of the dendrimer agents clearly visualized the renal anatomy and proximal straight tubules of the mice better than Gd-[DTPA]-dimeglumine. Dendrimer conjugate contrast agents prepared from PAMAM-G2D, DAB-G3D, and DAB-G2D dendrimers were excreted rapidly and may be acceptable for use in clinical applications.

Description of the Invention

FIELD OF THE INVENTION

Methods of imaging kidney tissue are disclosed. More specifically, the disclosure relates to MRI methods that provide images that may be used to assess renal structure and function, and to detect renal injury.

BACKGROUND OF THE INVENTION

MRI is a technique that allows whole body in vivo imaging in three dimensions at high resolution. In MRI, a static magnetic field is applied to the object of interest while simultaneously or subsequently applying pulses of radio frequency (RF) to change the distribution of the magnetic moments of protons in the object. The change in distribution of the magnetic moments of protons in the object from their equilibrium (normal) distribution to a non-equilibrium distribution and back to the normal distribution (via relaxation processes) constitute the MRI signal.

The longitudinal relaxation time, T1, is defined as the time constant of the exponential recovery of proton spins to their equilibrium distribution along an applied magnetic field after a disturbance (e.g. a RF pulse). The transverse relaxation time, T2, is the time constant that describes the exponential loss of magnetization in a plane transverse to the direction of the applied magnetic field, following a RF pulse that rotates the aligned magnetization into the transverse plane. Magnetic resonance (MR) contrast agents assist this return to a normal distribution by shortening T1 and/or T2 relaxation times.

Signal intensity in biological MRI depends largely on the local value of the longitudinal relaxation rate (1/T1), and the transverse relaxation rate (1/T2) of water protons. Contrast agents will increase 1/T1 and/or 1/T2, depending on the nature of the agent and the strength of the applied field. MRI pulse sequences that emphasize changes in 1/T1 are referred to as T1 -weighted and those that emphasize changes in 1/T2 are referred to as T2 -weighted. MR contrast agents that include gadolinium (III) ions increase both 1/T1 and 1/T2, and are primarily used with T1 -weighted imaging sequences, since the relative change in 1/T1 in tissue is typically much greater than the change in 1/T2. Iron particles, by contrast, provide larger relative changes in 1/T2, and are best visualized in a T2 -weighted image.

Advances in MRI have tended to favor T1 agents such as gadolinium based contrast agents. Faster scans with higher resolution require more rapid RF pulsing, and can lead to loss of the MRI signal through saturation effects. T1 agents relieve this saturation and restore signal intensity by stimulating relaxation of nuclear spins between RF pulses.

Because many paramagnetic metal ions, including gadolinium (III), are toxic, they are often administered in a sequestered form, for example, as metal chelates. However, metal chelates, because of their small size and relative hydrophillicity, tend to be cleared rapidly from blood, giving rise to limited cell penetration (but good tolerability). Conjugation of metal chelates to macromolecules to form macromolecular imaging agents is one approach to altering the pharmacological properties (e.g., blood retention, tissue perfusion, and excretion) and biophysical properties (e.g., relaxivity, which is defined as the increase in longitudinal or transverse relaxation rate per millimolar concentration of a contrast agent) of metal chelates. For example, high molecular weight macromolecular imaging agents tend to be retained in the vascular space by virtue of their size, and are useful for blood pool imaging in a technique called magnetic resonance angiography (MRA). Tissue specific accumulation and/or image enhancement are features that may be exhibited by macromolecular contrast agents due to their pharmacokinetic properties, but the mechanisms that lead to such by which this occurs are poorly understood outside of immunologically active contrast agents, such as antibodies conjugated to metal chelates.

Dendrimers are a class of highly branched, often spherical, macromolecular polymers that exhibit greater monodispersity (i.e. a smaller range of molecular weights, sizes, and shapes) than linear polymers of similar size. These three-dimensional oligomeric structures are prepared by reiterative reaction sequences starting from a core molecule that has multiple reactive groups. When monomer units, also having multiple reactive groups, are reacted with the core, the number of reactive groups comprising the outer bounds of the dendrimer increases. Successive layers of monomer molecules may be added to the surface of the dendrimer, with the number of branches and reactive groups on the surface increasing geometrically each time a layer is added. The number of layers of monomer molecules in a dendrimer may be referred to as the "generation" of the dendrimer. The total number of reactive functional groups on a dendrimer's outer surface ultimately depends on the number of reactive groups possessed by the core, the number of reactive groups possessed by the monomers that are used to grow the dendrimer, and the generation of the dendrimer.

The reactive functional groups that form the outer surface of a dendrimer may be conjugated to metal chelates, such as gadolinium (III) chelates, to provide macromolecular MRI contrast agents. Conjugation of multiple metal chelates to a dendrimer core to provide a dendrimer conjugate may provide a contrast agent exhibiting high relaxivities and altered pharmacokinetics relative to the metal chelates themselves. Unfortunately, selection of a dendrimer-based contrast agent that is suitable for imaging a particular tissue or tissue function (by virtue of a combination of distribution to the tissue and image enhancement of particular features of the tissue) is complicated by the current understanding of how molecules are processed and ultimately excreted (e.g. through the kidney and liver) from the body.

In particular, selection of dendrimeric agents for imaging of renal tissue is difficult in view of the complex way in which molecules are processed by renal tissue. For example, blood clearance and renal excretion of low-molecular weight molecules depends on glomerular filtration, which in turn relies on molecular shape and size (See, for example, Chang et al., "Permselectivity of the glomerular capillary wall to macromolecules," Biophys. J, 15: 887-906 (1975)). Molecular charge also affects renal filtration (See, for example, Guasch et al., "Charge selectivity of the glomerular filtration barrier in healthy and nephrotic humans," J. Clin. Invest., 92: 2274-2282 (1993)). Once filtered, low-molecular weight molecules generally undergo endocytosis, another process that depends at least in part upon charge (See, for example, Kobayashi et al., "The pharmacokinetic characteristics of glycolated humanized anti-Tac Fabs are determined by their isoelectric points," Cancer Res., 59: 422-430 (1999)). Furthermore, the relative hydrophobicity or hydrophillicity of a molecule has been shown to affect accumulation in the kidney. For example, a more hydrophobic DAB-Am-64-(1B4M)64 dendrimer contrast agent has been shown to accumulate significantly more in the liver and less in the kidney than a relatively hydrophillic PAMAM-G4D-(1B4M)64 contrast agent (Kobayashi et al., "Novel Liver Macromolecular MR Contrast Agent with a Polypropylenimine Diaminobutyl Dendrimer Core: Comparison to the Vascular MR Contrast Agent with the Polyamidoamine Dendrimer Core," Magn. Res. Med., 46: 795-802 (2001)). The complicated dependence of renal processing on size, shape, charge and hydrophobicity (hydrophillicity) of a macromolecule makes it difficult to predict which agents will provide contrast for renal structure and function.

Further complicating the discovery of clinically suitable macromolecular MRI contrast agents is the danger that such agents will be retained for extended periods of time in the body, leading to increased potential toxicity from unstable Gd (III) chelates. For example, only 20 percent of the injected dose (% ID) of the PAMAM-G4D based contrast agent was excreted from mice during the first two days following administration (See, Sato et al., "Pharmacokinetics and enhancement patterns of macromolecular MR contrast agents with various sizes of polyamidoamine dendrimer cores," Magn. Reson. Med. 46: 1169-1173 (2001)). Efforts to increase the excretion rate of dendrimeric contrast agents can lead to altered image enhancement patterns. (See, for example, Kobayashi et. al., "Novel Intravascular Macromolecular MRI Contrast Agent With Generation-4 Polyamidoamine Dendrimer Core: Accelerated Renal Excretion with Coinjection of Lysine," Magn. Res. Med., 46:457-464 (2001)).

SUMMARY OF THE INVENTION

Contrast agents and methods useful for imaging the internal structure and function of the mammalian kidney are disclosed. Contrast agents prepared from small DAB and PAMAM dendrimers are disclosed to provide visualization of the proximal straight tubules of the kidney. In one disclosed method, an image-enhancing amount of contrast agent prepared from a DAB-G2D, DAB-G3D or PAMAM-G2D dendrimer is administered to a subject. Following administration, accumulation of the dendrimeric contrast agents in the proximal straight tubules may be monitored to assess kidney function. Abnormal function of the kidney may be inferred from differences in the way that renal tissue processes the contrast agent in comparison to normal renal tissue.

DETAILED DESCRIPTION OF THE INVENTION

In order to facilitate review of the various embodiments of the invention, the following explanations of specific abbreviations and terms are provided:

I. Abbreviations

MR--magnetic resonance

MRI--magnetic resonance imaging

DAB--diaminobutane

DTPA--diethylenetriaminepentaacetic acid

PAMAM--polyamidoamine

1B4M--2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid

DAB-G2--DAB-Am16-(Gd-1B4M)16

DAB-G3--DAB-Am-32-(Gd-1B4M)32

DAB-G4--DAB-Am-64-(Gd-1B4M)64

PAMAM-G2--PAMAM-G2D-(Gd-1B4M)16

PAMAM-G3--PAMAM-G3D-(Gd-1B4M)32

PAMAM-G4--PAMAM-G4D-(Gd-1B4M)64

II. Terms

Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Definitions of common terms in magnetic resonance imaging may be found, for example, in Bushong, "Magnetic resonance imaging: physical and biological principles," Mosby, St. Louis Mo., 1996.

The singular terms "a," "an," and "the" include plural referents unless context clearly indicates otherwise. Similarly, the word "or" is intended to include "and" unless the context clearly indicates otherwise. The term "comprises" means "includes."

In one aspect, dendrimer conjugates useful for imaging kidney structure and function are disclosed. The term "dendrimer conjugate" refers to a dendrimer attached to one or more metal chelates. The term "metal chelate" refers to a complex of a metal ion and a group of atoms that serves to bind the metal ion. In some embodiments, a dendrimer conjugate may have fewer bound metal ions than it has groups of atoms capable of binding metal ions.

The term "bifunctional chelating agent" refers to a molecule that has at least two functional groups, one of which is a reactive group which can form a bond, such as a covalent bond, with another molecule, and one of which is a metal binding group. Bifunctional chelating agents may be reacted with dendrimers to provide dendrimer conjugates. Conjugation between a dendrimer and a metal chelate typically refers to formation of a covalent bond between the dendrimer and the metal chelate(s). However, in some instances ion-ion bonds, ion-dipole bonds, dipole-dipole bonds and hydrophobic interactions may be used to conjugate a dendrimer and a metal chelate.

As used herein, the term "DAB dendrimer" refers to a dendrimer having a diaminobutane core and polyalkylenimine branches. In general, DAB dendrimers may have polyalkylenimine branches, such as polyethyleneimine, polypropylenimine and polybutyleneimine branches. The term "DAB-Am dendrimer" refers to a DAB dendrimer having polypropylenimine branches and one or more surface amino groups. The term "DAB-Am-X" refers to a DAB-Am dendrimer having X number of surface amino groups. For example, DAB-Am-4 denotes a diaminobutane-core dendrimer having polypropylenimine branches and 4 amino groups at its surface. Additional examples of DAB-Am-X dendrimers include DAB-Am-8, DAB-Am-16, DAB-Am-32 and DAB-Am-64. DAB-Am-X dendrimers having 4, 8, 16, 32 and 64 surface amine groups are also known, respectively, as N,N,N',N'-tetrakis(3-aminopropyl)-1,4-butanediaminepolypropylenimine tetraamine, 4,17-bis(3-aminopropyl)-8,13-bis[3-[bis(3-aminopropyl)-amino]propyl]-4,8,1 3,17-tetraazaeicosane-1,20-diamine, polypropylenimine hexadecaamine dendrimer [--CH2 CH2 N(CH2)3 N[(CH2)3 NH2 ]2 ]2 ]2 ]2 ], polypropylenimine dotriacontaamine dendrimer [--CH2 CH2 N(CH2)3 N[(CH2)3 NH2 ]2 ]2 ]2 ]2 ]2 ], and polypropylene tetrahexacontaamine dendrimer [--CH2 CH2 N(CH2)3 N[(CH2)3 NH2 ]2 ]2 ]2 ]2 ]2 ]2 ]. As used herein, DAB-Am-X dendrimers having 4, 8, 16, 32 and 64 surface amine groups are also referred to, respectively, as DAB-G1D, DAB-G1D, DAB-G2D, DAB-G3D and DAB-G4D dendrimers, where the numbers refer to the generation of the dendrimer. A variety of DAB-Am-X dendrimers, including the specific examples listed immediately above, are available from Aldrich (Milwaukee, Wis.). Note, however, that Aldrich refers to these specific DAB-Am-X dendrimers as generations 1 through 5. In order to permit a comparison between generations of DAB-Am dendrimers and PAMAM dendrimers (discussed below) having equal numbers of amine groups on their surfaces, the generation numbers used by Aldrich may be reduced by one. The two types of dendrimers are compared in FIG. 1, which shows a schematic representation of the structures and generation numbers of some examples of both DAB-Am and PAMAM dendrimers.

DAB-Am dendrimers also may be synthesized according to the methods disclosed in Womer, and Mulhaupt, Angew Chem., Int. Ed. Engl, 32: 1306-1308, 1993. De Brabander-van den Berg and Meijer (Angew. Chem., Int. Ed. Engl., 32:1308, 1993) also describe similar methods. Polypropylenimine dendrimers having other initiator cores, such as ammonia, ethylenediamine, propylenediamine, and other polyamines such as tris-aminoethylamine, cyclene, hexaazacyclooctadecane, 1,5 diaminopentane, ethylenetriamine, triethylenetetramine, 1,4,8,11-tetraazaundecane, 1,5,8,12-tetraazaundodecane, and 1,5,9,13-tetraazatridecane. Typically, the surface of the polypropylenimine dendrimer will have one or more amino groups. However, some or all of the surface amino groups may be modified, for example, to provide other reactive groups or charged, hydrophilic, and/or hydrophobic groups such as carboxylate, hydroxyl and alkyl groups on the surface. Similar schemes may be used to synthesize polybutylenimine and higher polyalkylenimine dendrimers.

The term "PAMAM dendrimer" refers to a dendrimer having polyamidoamine branches. Like the DAB dendrimers discussed above, a variety of PAMAM dendrimers are commercially available from Aldrich (Milwaukee, Wis.). In particular, generation 1.0, 2.0, 3.0 and 4.0 PAMAM dendrimers, having ethylenediamine cores and, respectively, 8, 16, 32 and 64 surface amine groups, are commercially available. As used herein, these particular dendrimers are referred to, respectively, as PAMAM-G1D, PAMAM-G2D, PAMAM-G3D and PAMAM-G4D dendrimers. PAMAM dendrimers, also may be synthesized from a variety of core molecules (e.g., those described above for DAB dendrimers) according to the methods disclosed in U.S. Pat. No. 5,338,532. Dendrimers having other surface groups, such as carboxylate and hydroxyl, also are available commercially (Aldrich, Milwaukee, Wis.) or may be provided by the methods disclosed in U.S. Pat. No. 5,338,532.

The metal chelate in a dendrimer conjugate is a complex of a metal ion and a metal binding group (a group of atoms that serves to bind or chelate the metal ion). Examples of metal binding groups include natural and synthetic amines, porphyrins, aminocarboxylic acids, iminocarboxylic acids, ethers, thiols, phenols, glycols and alcohols, polyamines, polyaminocarboxylic acids, polyiminocarboxylic acids, aminopolycarboxylic acids, iminopolycarboxylic acids, nitrilocarboxylic acids, dinitrilopolycarboxlic acids, polynitrilopolycarboxylic acids, ethylenediaminetetracetates, diethylenetriaminepenta or tetraacetates, polyethers, polythiols, cryptands, polyetherphenolates, polyetherthiols, ethers of thioglycols or alcohols, polyaminephenols, all either acyclic, macrocyclic, cyclic, macrobicyclic or polycyclic, or other similar ligands which produce stable metal chelates or cryprates (including sepulchrates, sacrophagines, and crown ethers).

Specific examples of metal chelating groups include diethylenetriaminepentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecanetetraacetic acid (DOTA), 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A), 1-oxa-4,7,10-triazacyclododecane-triacetic acid (DOXA), 1,4,7-triazacyclononanetriacetic acid (NOTA), 1,4,8,11-tetraazacyclotetradecanetetraacetic acid (TETA), DOTA-N(2-aminoethyl)amide and DOTA-N-(2-aminophenethyl)amide, BOPTA, HP-DO3A, DO3MA, and various derivatives and combinations thereof. Other examples are provided in Caravan et al., Chem. Rev., 99: 2293-2352, 1999. Since it is advantageous for in vivo imaging to select a metal chelating group capable of tightly binding a metal ion, a high stability constant for the metal chelate is desired.

Metals ions of the metal chelates may be paramagnetic ions if the imaging agent is to be used as a MRI contrast agent. Suitable ions include ions of metals having atomic numbers of 22-29 (inclusive), 42, 44 and 58-70 (inclusive) and combinations thereof. In particular embodiments, the metal ions have an oxidation state of 2 or 3. Examples of such metal ions are chromium (III), manganese (II), iron (II), iron (III), cobalt (II), nickel (II), copper (II), praseodymium (III), neodymium (III), samarium (III), gadolinium (III), terbium (III), dysprosium (III), holmium (III), erbium (III) and ytterbium (III), and combinations thereof. Particular examples of useful ions for MRI include the paramagnetic ions of gadolinium, dysprosium, cobalt, manganese, and iron. In a particular disclosed embodiment, the metal ion is a Gd (III) ion.

If the macromolecular imaging agent is to be used as an X-ray contrast agent, the metal ion may be selected from the ions of W, Bi, Hg, Os, Pb, Zr, lanthanides, and combinations thereof. If a combined MRI/X-ray contrast agent is desired, the metal ion may be selected from the paramagnetic lanthanide ions. If a radiographic imaging agent is desired, the metal may be radioactive, such as the radioactive isotopes of In, Tc, Y, Re, Pb, Cu, Ga, Sm, Fe, or Co.

The unique localization properties of the disclosed dendrimer conjugates also make it possible to use them for delivery of therapeutic radiation to particular tissues and tissue structures. Examples of metal ions useful for therapy include ions of the radioactive isotopes of Pb, Bi and Y.

Bifunctional chelating agents may be used to form a dendrimer conjugate. A bifunctional chelating agent is a molecule capable of forming a bond with another molecule, such as a dendrimer, and also capable of forming a metal chelate by binding a metal ion. Appropriate bifunctional chelating agents therefore include a reactive group and a metal chelating group, such as the metal chelating groups described above.

The reactive group of a bifunctional chelating agent is a group of atoms that that will undergo a reaction with a surface group of a dendrimer to form a bond, such as a covalent bond. Examples of reactive groups include carboxylic acid groups, diazotiazable amine groups, N-hydroxysuccinimidyl, esters, aldehydes, ketones, anhydrides, mixed anhydrides, acyl halides, maleimides, hydrazines, benzimidates, nitrenes, isothiocyanates, azides, sulfonamides, bromoacetamides, iodocetamides, carbodiimides, sulfonylchlorides, hydroxides, thioglycols, or any reactive group known in the art as useful for forming conjugates. If the dendrimer is a DAB-Am dendrimer, the reactive group may be a functional group capable of undergoing reaction an amino group of the DAB-Am dendrimer.

Specific examples of bifunctional chelating agents include bifunctional diethylenetriaminepentaacetic acid (DTPA) derivatives such as those disclosed in U.S. Pat. No. 5,434,287 to Gansow et al. Other examples include polysubstituted diethylenetriaminepentaacetic acid chelates such as those described by Gansow et al. in U.S. Pat. No. 5,246,692. Bifunctional chelating agents comprising 1,4,7,10-Tetraazacyclododecane-N,N',N",N'"-tetraacetic acid (DOTA) and its derivatives are also useful. Examples of bifunctional DOTA derivatives are provided in U.S. Pat. No. 5,428,154 to Gansow et al. and references therein. A particular example of a bifunctional imaging agent is 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid (1B4M).

Macromolecular imaging agents may be prepared by reacting a surface group of a dendrimer with the reactive group of a bifunctional chelating agent and then reacting the metal chelating group of the bifunctional chelating agent with a metal ion. Alternatively, a metal ion is reacted with the metal chelating group of the bifunctional chelating agent prior to reacting the reactive group of the bifunctional chelating agent with a surface groups of the dendrimer. Metal chelation is typically carried out in solution, and desirably avoids the use of strong acids or bases. In particular embodiments, a dendrimer selected from the group consisting of DAB-G2D, DAB-G3D, DAB-G4D, PAMAM-G2D, PAMAM-G3D and PAMAM G4D is reacted with 1B4M and gadolinium ions (in either order discussed above) to provide dendrimer conjugates suitable for kidney imaging.

Thus, in one aspect, dendrimer conjugates suitable for kidney imaging are provided. Particular examples include DAB-G2 [DAB-Am16-(Gd-1B4M)16 ], DAB-G3 [DAB-Am-32-(Gd-1B4M)32 ], DAB-G4 [DAB-Am-64-(Gd-1B4M)64 ], PAMAM-G2 [PAMAM-G2D-(Gd-1B4M)16 ], PAMAM-G3 [PAMAM-G3D-(Gd-1B4M)32 ] and PAMAM-G4 [PAMAM-G4D-(Gd-1B4M)64 ]. Table 1 compares some properties of these conjugates and the simple gadolinium chelate, Gd-DTPA-dimeglumine.

                             TABLE 1
              Comparison of Example Contrast Agents.
                                                Commercial name
    Name used                          Core MW  (of dendrimer core
    herein.      MW (kD)   Gd atoms     (kD)    where applicable)
    PAMAM-G4       59         64         14.2   PAMAM G4
    PAMAM-G3       29         32          6.9   PAMAM G3
    PAMAM-G2       14         16          3.5   PAMAM G2
    DAB-G4         51         64          7.1   DAB-Am-64
    DAB-G3         25         32          3.5   DAB-Am-32
    DAB-G2         12         16          1.7   DAB-Am-16
    Gd-DTPA-        0.8        1         N/A    Magnevist
    dimeglumine

In particular embodiments, dendrimer conjugates are disclosed where the conjugates comprise a dendrimer selected from the group consisting of DAB-G2D, DAB-G3D and PAMAM-G2D and a gadolinium (III) chelate of 1B4M. In more particular embodiments, the dendrimer is selected from the group consisting of DAB-G2D and DAB-G3D.

In another aspect, methods are disclosed for imaging kidney tissue. These methods include administering to a subject an image-enhancing amount of a dendrimer conjugate, the dendrimer conjugate comprising a dendrimer selected from the group consisting of DAB-G2D, DAB-G3D, DAB-G4D, PAMAM-G2D, PAMAM-G3D, PAMAM-G4D and combinations thereof, and a metal chelate. Once administered, a difference in a magnetic resonance signal intensity of at least a portion of the kidney is detected. Differences in intensity may be used to detect structural and/or functional features of the kidney. For example, a portion of the kidney comprising the proximal straight tubules or the pelvis of the kidney may be imaged, and the image(s) used to detect whether the kidney is functioning to process the dendrimer conjugate in a manner characteristic of normal kidney tissue. Images characteristic of normal kidney function may be obtained from a subject that has no clinical indications of renal dysfunction (e.g. elevated BUN levels or creatine levels), and such images may be compared with images obtained from subjects that do not exhibit clinical indications of renal dysfunction. In yet other embodiments, cortical thinning associated with chronic renal failure may be detected by measuring the thickness of the cortex and/or outer medulla in an image obtained according to the disclosed methods. Such images may be useful for differentiating acute from chronic renal failure, and may also be used to detect renal cysts associated with chronic renal failure.

In particular embodiments of the disclosed methods, detecting a difference in a magnetic resonance signal intensity of at least a portion of the kidney comprises detecting a difference in a T1-weigthed signal at a position corresponding to the proximal straight tubules of the kidney, for example, the outer stripe of the outer medulla of the kidney. Alternatively, detecting a difference in a magnetic resonance signal intensity of at least a portion of the kidney comprises detecting a signal associated with the appearance of urine in the pelvis of the kidney as the dendrimer conjugate is excreted. For example, an increase in the signal intensity in the region of the kidney pelvis may be followed over time and used to assess kidney function.

In other embodiments, the methods also include comparing the difference in magnetic resonance signal intensity detected for the subject to a difference in signal intensity detected in a second subject to determine whether the kidneys of the subject and the second subject are similar in structure or function. For example, a second subject exhibiting acute renal failure may be used for comparison to detect whether the subject also exhibits a MRI signal indicative of acute renal failure.

In some embodiments, the methods include administering a dendrimer conjugate to a subject where the metal chelate of the dendrimer conjugate is selected from the group consisting of metal chelates of diethylenetriaminepentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecanetetraacetic acid (DOTA), 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A), 1-oxa-4,7,10-triazacyclododecane-triacetic acid (DOXA), 1,4,7-triazacyclononanetriacetic acid (NOTA), 1,4,8,11-tetraazacyclotetradecanetetraacetic acid (TETA), DOTA-N(2-aminoethyl)amide and DOTA-N-(2-aminophenethyl)amide, BOPTA, HP-DO3A, DO3MA, and derivatives and combinations thereof. The metal chelate may comprise an ion of a metal selected from the group consisting of the metals having atomic numbers of 22-29, 42, 44 and 58-70 and combinations thereof. In particular embodiments, the ion is selected from the group consisting of chromium (III), manganese (II), iron (II), iron (III), cobalt (II), nickel (II), copper (II), praseodymium (III), neodymium (III), samarium (III), gadolinium (III), terbium (III), dysprosium (III), holmium (III), erbium (III), ytterbium (III) and combinations thereof. In particular embodiments, the dendrimer conjugate is a 1B4M conjugate and is selected from the group consisting of DAB-G2, DAB-G3, DAB-G4, PAMAM-G2, PAMAM-G3, PAMAM-G4 and combinations thereof.

When a dendrimer conjugate is used as an imaging agent for imaging the kidney of a subject (i.e. a mammal such as a mouse or a human), the conjugate is administered in an image enhancing amount [i.e. an amount sufficient to produce detectable (e.g. visually detectable or electronically detectable) differences in the image of the kidney at some time following administration]. For MRI, such differences may be detected in either a T1 - or T2 -weighted image taken at some time after the imaging agent is administered. The differences may be due to either an increase or a decrease in the intensity of the kidney or a portion thereof, relative to surrounding tissue in comparison to an image obtained before administration of the agent. In one embodiment, dendrimer conjugates for MRI are administered in dosages that are 1/4 to 1/3 the dosages required for simple chelates such as Gd-DOTA and Gd-DPTA. In particular embodiments, a detectable difference in kidney MRI image intensity may be provided by administering between about 0.001 mmol Gd/kg and about 0.10 mmol Gd/kg, for example, administering between about 0.003 mmol Gd/kg and about 0.03 mmol Gd/kg intravenously or parenterally. Imaging may begin immediately or anywhere from about 1 min to about 2 hrs after administration, such as between about 3 minutes and 60 minutes after administration.

Claim 1 of 49 Claims

We claim:

1. A dendrimer conjugate, comprising:

a dendrimer selected from the group consisting of DAB-G2D and DAB-G3D; and

a gadolinium (III) chelate of 1B4M.


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