Gadolinium expressed lipid nanoparticles for magnetic resonance imaging

Abstract

Lipid nanoparticles expressing metal ions and methods for using the compositions for magnetic resonance imaging.

Claims

1. A nanoparticle, comprising: (a) a phospholipid selected from distearoyl-phosphocholine (DSPC) or dipalmitoyl sphingomyelin (DPSP); (b) |N-(carbonyl-methoxypolyethyleneglycol)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (mPEG-DSPE); and (c) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-diethylenetriaminepenta-acetyl (DSPE-DTPA) with a chelated Gd.sup.3+ ion, wherein the nanoparticle is not a liposome, and wherein the nanoparticle has a T1 relaxivity constant (r.sub.1) of at least about 133 sec.sup.−1 mM.sup.−1 as determined by a magnetic resonance (MR) scanner; wherein the mPEG-DSPE is present in the amount from about 5 to about 20 mole percent of the combined phospholipid, the mPEG-DSPE, and DSPE-DTPA, and wherein the phospholipid:mPEG-DSPE:DSPE-DTPA ratio is from 8:1:0.9 to 8:2:1.

2. The nanoparticle of claim 1, wherein the N-(carbony]l-methoxypolyethyleneglycol)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (mPEG-DSPE) is N-(carbonyl-methoxypolyethyleneglycol-2000)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (mPEG-2000-DSPE).

3. The nanoparticle of claim 1, further comprising a targeting moiety.

4. The nanoparticle of claim 3, wherein the targeting moiety is selected from the group consisting of a protein, a polypeptide, a peptide, an antibody or fragment thereof, a small molecule, a sugar or polysaccharide or derivative thereof, and a nucleic acid.

5. A composition, comprising a carrier and a plurality of the nanoparticles of claim 1.

6. The nanoparticle of claim 1, wherein the nanoparticle comprises 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and N-(carbonyl-methoxypolyethyleneglycol-2000)-1,2-distearoyl-sn-glycero-3 phosphoethanolamine (mPEG-2000-DSPE).

7. The nanoparticle of claim 1, wherein the nanoparticle does not have a solid core structure.

Description

DESCRIPTION OF THE DRAWINGS

(1) The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings.

(2) FIG. 1 is a graph illustrating reduction of calcein fluorescence as a function of Gd.sup.3+:DTPA-PE (m/m) ratio for a representative formulation of the invention. The formulation was incubated with calcein and its fluorescence measured. Free Gd.sup.3+ binds to calcein and reduces its fluorescence.

(3) FIGS. 2A and 2B compare relaxivity as a function of Gd.sup.3+ concentration.

(4) FIG. 2A compares Gd.sup.3+ dose effect on T1 relaxation rate; R.sub.1 (1/T1) values of Gd.sup.3+ concentrations (μmol/mL) measured in a 1.5T magnetic resonance (MR) scanner using a standard spin-echo sequence for a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC) compared to other formulations. FIG. 2B compares Gd.sup.3+ dose effect on T2 relaxation rate; R.sub.2 (1/T2) values of Gd.sup.3+ concentrations (μmol/mL) measured in a 1.5T magnetic resonance (MR) scanner using a standard spin-echo sequence for a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC) compared to other formulations.

(5) FIGS. 3A-3E illustrate time-course coronal images of ventral cavity of M. Nemestrina up to 24 hours after injection of a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC). FIGS. 3A-3D are time sequence images of the liver before (FIG. 3A) and after 20 min (FIG. 3B), 6 hr (FIG. 3C), and 24 hr (FIG. 3D) administration. FIG. 3E illustrates time sequence images of lymph nodes. The lymph nodes and liver show high contrast compared to background tissues.

(6) FIG. 4A is a graph comparing MR dynamic contrast enhanced image intensity of the lymph nodes (LN 1, LN 2, and LN3) and liver (Liver) in M. Nemestrina up to 24 hr after subcutaneous injection of 24.4 μmol/kg of a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC). Results are for images shown in FIGS. 3A-3D. FIG. 4B is a graph comparing the time course of tissue specific MR signal (lymph node) compared to adjacent control tissue.

(7) FIGS. 5A-5D are time course MR images of a rat after intravenous injection of a representative formulation of the invention (0.01 mmol/kg Gd-DPTA-PE:mPEG-PE:DSPC): pre-dose (FIG. 5A); 5 min (FIG. 5B); 14 min (FIG. 5C); and 24 hr (FIG. 5D). Contrast enhancement is localized mainly in vasculature and vascularized tissues in lymph nodes at 5 min and 14 min. The arrow in FIG. 5C indicates the beginning of gadolinium elimination into the gastrointestinal tract through the biliary route. By 24 hr (FIG. 5D) most of the contrast agent is eliminated and the residual fraction appears in the intestine.

(8) FIGS. 6A-6C are time course MR images of a rat after intravenous injection of a commercially available gadolinium-based contrast agent (0.05 mmol/kg Gd-DPTA, OMNISCAN): pre-dose (FIG. 6A); 5 min (FIG. 6B); and 15 min (FIG. 6C). Contrast enhancement is diffused throughout and does not localize in the vasculature, well-perfused tissues, or the lymph nodes at either 5 min or 15 min. Distribution of contrast media to periphery and extremities is apparent.

(9) FIGS. 7A-7D compare time course MR images of a rat after intravenous injection of a commercially available gadolinium-based contrast agent (0.05 mmol/kg Gd-DPTA, OMNISCAN, pre-dose (FIG. 7A), 5 min (FIG. 7B), and 15 min (FIG. 7C)) and an MR image of a rat 15 min after administration of a representative formulation of the invention (0.01 mmol/kg Gd-DPTA-PE:mPEG-PE:DSPC (FIG. 7D)).

(10) FIGS. 8A-8F compare dose response MR images of a rat after intravenous injection of a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC): 0.0 mmol/kg (FIG. 8A); 0.00125 mmol/kg (FIG. 8B); 0.0025 mmol/kg (FIG. 8C); 0.005 mmol/kg (FIG. 8D); and 0.010 mmol/kg (FIG. 8E and FIG. 8F). Lymph nodes are clearly apparent in FIG. 8F.

(11) FIGS. 9A-9C compare MR images of rats after intravenous injection of two commercially available gadolinium-based contrast agents (0.05 mmol/kg MS-325, FIG. 9A; 0.05 mmol/kg MAGNEVIST Gd-DPTA, FIG. 9B) and a representative formulation of the invention (0.01 mmol/kg Gd-DPTA-PE:mPEG-PE:DSPC, FIG. 9C). Images were acquired near peak enhancement, about 1 to 2 min after administration.

(12) FIG. 10 is a schematic illustration of the preparation of a representative lipid-containing gadolinium chelate: DSPE-DOTA-Gd.

(13) FIG. 11 illustrates three metal chelators useful for preparing representative lipid-containing chelates: p-SCN-Bn-DOTA; CHX-A″-DTPA; and p-SCN-Bn-DTPA.

(14) FIG. 12 is a schematic illustration of the preparation of representative lipid-containing gadolinium chelates.

DETAILED DESCRIPTION OF THE INVENTION

(15) The present invention provides compositions expressing metal ions and methods for using the compositions. In one embodiment, the compositions are lipid nanoparticles that include paramagnetic metal ions and are useful for magnetic resonance imaging. In another embodiment, the compositions are lipid nanoparticles that include ions of radio-isotopes and are useful for delivery of radio-cancer therapeutic agents.

(16) In one aspect, the invention provides compositions and methods for magnetic resonance imaging. The compositions and methods enhance gadolinium distribution and accumulation in lymphatics. The invention provides a gadolinium composition (referred to herein as “Gd-DTPA-lipid nanoparticle”) that is suitable for both intravenous and subcutaneous administration. Subcutaneous administration allows direct access to lymphatic system. The composition enhances T1 weighted MR signal in the lymph nodes as well as increases the residence time of the contrast agent in the lymphatics. Upon intravenous administration, the composition exhibits at least 100-fold enhancement over soluble Gd-DTPA as a vascular imaging agent and eliminates predominantly through biliary, rather than renal clearance. The composition was shown to significantly increase signal-to-noise ratio by more than 300-fold for MR visualization of lymph nodes in macaques.

(17) The composition of the invention includes a lipid, a polyalkylene-containing lipid, and a lipid-containing metal chelator. In one embodiment, the composition further includes a chelated metal ion.

(18) In one embodiment, the composition of the invention is a chelator- (or metal chelate-) expressing particle. As used herein, the term “expressing” refers to the particle presenting or having available the chelator or chelated metal for activity. As noted above, in one embodiment, the composition of the invention is a lipid nanoparticle having chelated gadolinium ion (Gd.sup.+3) (e.g., Gd-DTPA-lipid nanoparticle). In the lipid nanoparticle, chelated gadolinium ion is expressed.

(19) The lipid nanoparticles of the invention are biocompatible and are readily administered. The nanoparticles have a diameter of from about 5 nm to about 2 μm. In one embodiment, the nanoparticles have a diameter of from about 10 nm to about 100 μm. In one embodiment, the nanoparticles have a diameter of about 70 nm.

(20) As noted above, the composition of the invention (e.g., lipid nanoparticles) includes a lipid, a polyalkylene-containing lipid, and a lipid-containing metal chelator.

(21) Lipids.

(22) The lipid component of the nanoparticles of the invention comprise the nanoparticle core.

(23) Representative lipids useful in the compositions include phospholipids, sphingolipids, cholesterol and steroid derivatives, bile acids and derivatives, cardilipin, acyl-glycerides and derivatives, glycolipids, acyl-peptides, fatty acids, carbohydrate-based polymers (e.g., cellulose polymers), suitably functionalized silica, lipophilic polymers (e.g., polyanhydrides, polylactate-glycolate), and lipophilic bioploymers (e.g., proteins, sugar polymers).

(24) In one embodiment, the lipid is disteroylamidomethylamine.

(25) In one embodiment, the lipid is a phospholipid. Representative phospholipids include 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC); dipalmitoyl phosphatidylcholine; dimysristoyl phosphatidyl choline; dioleoyl phosphatidyl choline; trans-esterified phospholipids derived from eggs, soybean, flaxseed, and the like; and phosphatidylcholine substituted with phosphatidyl ethanolamine, phosphatidylglycerol, phosphatidyl serine, and phosphatidic acids. In one embodiment, the phospholipid is 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC).

(26) Polyalkalene-Containing Lipids.

(27) The polyalkylene-containing lipid component of the nanoparticles of the invention serve as surface hydrating agents.

(28) Representative polyalkylene-containing lipids include polyoxyethylene-containing lipids and polyoxypropylene-containing lipids. In one embodiment, the polyalkylene-containing lipid is a phospholipid functionalized with polyethylene glycol (e.g., PEGylated phospholipid). Suitable PEGylated phospholipids include a polyethylene glycol having a number average molecular weight of from about 500 to about 20,000. In one embodiment, the PEGylated phospholipid is N-(carbonyl-methoxypolyethyleneglycol-2000)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (mPEG-2000-DSPE) (also referred to herein as “mPEG-DSPE” and “mPEG-PE”).

(29) In addition to polyalkylene-containing lipid, in other embodiments, the surface hydrating agent is hydrophilic biomaterials such as a carbohydrate polymer, a polyamine, a polyvinyl pyrrolidone, a poly(aspartate), or a poly(L-amino acid).

(30) Other useful surface hydrating agents include covalent conjugates of polyethoxyl, polymethylene glycol, or propylene glycol and a lipid or other hydrophobic moiety (e.g., long chain hydrocarbon).

(31) The surface hydrating agent is preferably present from about 5 to about 50 mole percent of the composition (i.e., lipid, polyalkylene-containing lipid (surface hydrating agent), and lipid-containing metal chelator).

(32) Lipid-Containing Metal Chelator.

(33) The lipid-containing metal chelator component of the nanoparticles of the invention are expressed on the surface of the nanoparticle and serve to chelate metal ions. Suitable lipid-containing metal chelators include two moieties: (1) a lipid moiety and (2) a metal chelator moiety.

(34) Representative lipid-containing metal chelators include 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-diethylenetriaminepentaacetyl (DSPE-DTPA), tetraazacyclododecane, tetraacety(gadodiamide or OMNISCAN)-PE, and lipid-functionalized-[N,N-bis[2-[bis(carboxymethyl)amino]ethyl]-glycinato-(5″)](MAGNEVIST).

(35) Representative metal chelators include BOPTA, DO3A, and DOTA chelators.

(36) In one embodiment, the metal chelator includes a PEGylated lipid moiety. Representative PEGylated metal chelators include DSPE-BOPTA, a DSPE-DO3A, and a DSPE-DOTA. In one embodiment, the metal chelator is a PEGylated DTPA (DPTA-PE).

(37) The metal chelator is preferably present from about 5 to about 50 mole percent of the lipid, polyalkylene-containing lipid (surface hydrating agent), and metal chelator.

(38) Chelated Metal Ion.

(39) The compositions of the invention are effective carriers of metal ions. In these embodiments, the composition (e.g., lipid nanoparticles) further includes a chelated metal ion.

(40) For MR applications, useful metal ions include paramagnetic metal ions. Representative paramagnetic metal ions include Gd.sup.3+, Cu.sup.2+, Fe.sup.3+, Fe.sup.2+, and Mn.sup.2+ ions.

(41) For other applications such as imaging and therapeutic ion delivery, useful metal ions include ions of radio-isotopes. Representative radio-isotopes include ions of .sup.68Ga, .sup.55Co, .sup.86Y, .sup.90Y, .sup.177Lu, and .sup.111In.

(42) For embodiments that include chelated metal ions, the ratio of metal ion:metal chelator is 0.1-1.0:1.0 (less than or equal to 1:1).

(43) Targeting Agents.

(44) The compositions of the invention can be used to target specific tissues. In these embodiments, the composition (e.g., lipid nanoparticles) further includes a targeting moiety. Representative targeting moieties include proteins, polypeptide, and peptides; antibodies and derivatives (fragments); small molecules; sugars, polysaccharides, and derivatives; and nucleic acids, such as nucleotide polymers (e.g., aptamers), DNAs; and RNAs. Representative targeting moiety targets include cancer cells and virus infected cells.

(45) Lipid Nanoparticle Formulations.

(46) The lipid nanoparticles of the invention can be formulated into compositions for administration. Suitable compositions for administration include a carrier and a plurality of the lipid nanoparticles. Representative carriers include pharmaceutically acceptable carriers, such as saline for injection or dextrose for injection.

(47) The lipid nanoparticle of the invention is not a liposome and does not form liposomes when formulated.

(48) Methods for Tissue Imaging.

(49) In another aspect, the invention provides methods for imaging tissues (e.g., occluded tissues). In one embodiment, the method includes administering to a subject to be imaged a diagnostically effective amount of a composition of the invention. The composition can be administered by a variety of techniques including subcutaneously and intravenously. The method is effective for imaging tissues such as lymphoid, cardiovascular, liver, kidney, brain, heart, muscle, and gastrointestinal tract tissues, and other tissues accessible by the lymphatic or vascular (blood) systems. The method is effective for imaging the tissues above to determine whether the tissues are occluded. For magnetic resonance imaging methods, the composition includes a paramagnetic metal ion (e.g., Gd.sup.3+).

(50) In general, the effective amount is from about 0.001 to about 5 mmol metal/kg subject. In one embodiment, the effective amount is from about 0.005 to about 0.050 mmol metal/kg subject. In one embodiment, the effective amount is about 0.010 mmol metal/kg subject.

(51) Methods for Radio-Cancer Therapeutic Agent Delivery.

(52) In another aspect, the invention provides methods for delivering a radio-cancer therapeutic agent to a cancer cell. In one embodiment, the method includes administering to a subject in need thereof a therapeutically effective amount of a composition of the invention in which the chelated metal ion is a radio-isotope (e.g., .sup.68Ga, .sup.55Co, .sup.86Y, .sup.90Y, .sup.177Lu, and .sup.111In). The composition can be administered by a variety of techniques including subcutaneously and intravenously. The method is effective for delivery to tissues such as lymphoid, cardiovascular, liver, kidney, brain, heart, muscle, and gastrointestinal tract tissues, and other tissues accessible by the lymphatic or vascular (blood) systems.

(53) The following is a description of the preparation, characterization, and imaging results for representative lipid nanoparticles of the invention.

(54) Lipid nanoparticles were prepared composed of 10 mole percent of surface-bound DTPA. These lipid nanoparticles contained distearoyl-phosphatidylcholine and PEGylated lipid, mPEG-2000-DSPE. They were allowed to complex with Gd.sup.3+ (presented as Gd.sup.3+. CE) at varying Gd.sup.3+-to-DTPA-PE mole ratios. The presence of free Gd.sup.3+ in the admixture was determined by the ability of free Gd.sup.3+ to quench the fluorescence of calcein. With up to a Gd.sup.3+-to-DTPA-PE mole ratio of 4, no free Gd.sup.3+ could be detected by the calcein quenching assay. At a 6 or higher Gd.sup.3+-to-DTPA-PE mole ratio, free or unbound Gd.sup.3+ was detected (see FIG. 1). To determine the effects of Gd.sup.3+ on DTPA-expressing lipid nanoparticles, we determined the particle diameter by photon correlation spectroscopy. The diameter of DTPA-expressing lipid nanoparticles was Gd.sup.3+ concentration dependent. At or below a Gd.sup.3+-to-DTPA mole ratio of 1, the presence of Gd.sup.3+ did not influence the diameter of lipid nanoparticles. At a Gd.sup.3+-to-DTPA mole ratio of 2, the apparent diameter of Gd.sup.3+-DTPA lipid nanoparticle increases by about 2- to 3-fold, while there is no apparent decrease in the degree of Gd associated with DTPA-lipid nanoparticles (see Table 1). These lipid nanoparticles appeared to be stable as no significant change in diameter was detected over 24 hr at room temperature. Collectively, these data suggest that the ratio of Gd.sup.3+-to-DTPA influence the degree of Gd.sup.3+ incorporation into Gd-DTPA lipid nanoparticles and their apparent diameters. At or below Gd.sup.3+-to-DTPA mole ratio of 2, substantially all Gd.sup.3+ was associated with lipid nanoparticles. In one embodiment, the Gd.sup.3+-to-DTPA mole ratio in the composition of the invention is about 1.

(55) The contrast properties of the Gd.sup.3+-expressed lipid nanoparticles was determined by comparing the effects of the various Gd.sup.3+ formulations on the R1 (1/T1) relaxivity of Gd.sup.3+. Lipid nanoparticles composed of distearoyl-phosphatidylcholine (DSPC) with or without PEGylated lipid (mPEG-2000-DSPE, referred to herein as “mPEG-DSPE” or “mPEG-PE”) and fixed Gd.sup.3+-to-DTPA mole ratio at 1. The T1 and T2 measurements were collected with a 1.5 T MR scanner. A clinically-used Gd-DTPA preparation (OMNISCAN, commercially available from GE Healthcare, Princeton, N.J.) was included as a comparison. As shown in FIG. 2A, the representative composition of the invention, Gd-DTPA lipid nanoparticle containing mPEG-PE (Gd-DTPA-PE:mPEG-PE:DSPC), exhibited significantly higher R1 value than other preparations, including OMNISCAN and lipid nanoparticles that did not contain mPEG. The effects of mPEG on Gd-DTPA lipid nanoparticles are less for T2 measurement (see FIG. 2B). However, the R1 and R2 values for both Gd-DTPA lipid nanoparticle compositions (Gd-DTPA-PE:mPEG-PE:DSPC and Gd-DTPA-PE:DSPC) were significantly higher than that of the soluble Gd-DTPA commercial preparation (OMNISCAN) or free Gd.sup.3+ in solution (Gd w/o DTPA). To our knowledge the R1 values for Gd-DTPA nanoparticles of the invention are the highest value observed to date including those reported with Gd.sup.3+-expressed liposome containing PEG. In fact these results were more than 10-fold higher than the data collected with Gd-DTPA liposomes containing egg lectin and cholesterol that express PEG (MW=5000) linked to egg transesterified PE, instead of mPEG linked to DSPE.

(56) Both of the nanoparticle formulations containing Gd.sup.3+ had higher R1 values than soluble Gd-DTPA. As expected, the DSPC and DSPC plus mPEG-2000-PE control formulations without Gd.sup.3+ showed no significant effect on relaxivity. The data indicates that the PEG-containing Gd-DTPA-lipid nanoparticles provide a much greater increase in R1 compared to the other formulations. Up to a 100-fold increase in R1 relaxivity was achieved when compared to the commercially available OMNISCAN. The Gd-DTPA nanoparticle formulation without surface PEG (Gd-DTPA-PE:DSPC) also showed higher R1 values than OMNISCAN, but much less than the PEG-containing Gd-DTPA-lipid nanoparticle formulation (Gd-DTPA-PE:mPEG-PE:DSPC) (see FIG. 2A).

(57) Because positive contrast generated by Gd.sup.3+ in an MR image is dependant on a low R2/R1 ratio, changes of R2 values were also determined. FIG. 2B demonstrates the effects of the various Gd.sup.3+ formulations on the R2 (1/T2) relaxivity measurements. Once again the PEGylated Gd-DTPA-lipid nanoparticles (Gd-DTPA-PE:mPEG-PE:DSPC and Gd-DTPA-PE:DSPC) showed a greater increase in R2 compared to the other formulas, however the magnitude of the change was less. The mPEGylated Gd-DTPA-lipid nanoparticles (Gd-DTPA-PE:mPEG-PE:DSPC) showed about eight-fold increase in R2, compared to OMNISCAN. The Gd-DTPA in DSPC (without mPEG) nanoparticle formulation (Gd-DTPA-PE:DSPC) showed a greater increase in R2 than R1, which may limit its effectiveness as a positive contrast agent formulation. The DSPC and DSPC plus mPEG-2000-PE control formulations without Gd.sup.3+ (DTPA-PE:mPEG-PE:DSPC and DTPA-PE:DSPC) again showed no effect on relaxivity. The relaxivity experiments show that the mPEGylated Gd-DTPA-lipid nanoparticle formulation greatly increases the R1 relaxivity compared to other formulations. However, at high concentrations (greater than 3 μmol/ml), the T1 spillover into T2 becomes significant, leading to an apparent reduction of T1 signal (data not shown). Optimal T1 tissue contrast could be obtained with minimum Gd.sup.3+ concentration. These data clearly demonstrate much higher contrast “potency” than that provided by the commercial product, OMNISCAN.

(58) The use of the Gd-DTPA lipid nanoparticles containing mPEG-2000-PE (Gd-DTPA-PE:mPEG-PE:DSPC or “Gd-DTPA lipid nanoparticles”) for MR imaging studies in primates is described below.

(59) FIGS. 3A-3D show several coronal MR images of ventral cavity of M. Nemestrina both pre-contrast and up to 24 hours following subcutaneous injection of Gd.sup.3+-DTPA-lipid nanoparticles. The auxiliary lymph nodes are clearly visible with a high degree of intensity enhancement compared to the surrounding tissue (FIG. 3E). The enhancement of the lymph nodes can be seen as early as twenty minutes while lasting up to twenty four hours. FIGS. 4A and 4B show the time course of dynamic contrast enhanced MRI, showing relative intensities of various organs vs. time. This data shows that the Gd.sup.3+-DTPA-lipid nanoparticles quickly reach the lymph nodes tissue within twenty minutes after injection, and maintain the contrast enhancement for at least twenty four hours. Compared to a low signal that dissipates within 20 min in lymphatic system of animals treated intravenously with soluble Gd-DTPA (OMNISCAN preparations), a single subcutaneous Gd-DTPA lipid nanoparticles dose provided greatly enhanced signal for extended time.

(60) The MR image enhancing property of the Gd-DTPA-lipid nanoparticles can be used to minimize the IV dose need to produce vascular image enhancement and also reduce renal burden. Administration of 0.01 mmole/kg Gd-DTPA nanoparticles (about ⅕ of current dose for human) in rats produced a high quality MR image with clearly discernable central and peripheral vasculature of rat within 5 min (FIG. 5B). It begins to clear through the bile and gut within 15 min (FIG. 5C), and by the clearance process appeared to complete by 24 hr (FIG. 5D). In contrast, 0.05 mmole soluble Gd-DTPA (OMNISCAN) produce diffuse contrast localization with no vascular definition; it also appeared to distribute to periphery an extremities (FIG. 6A). In a limited dose-response study, only about to 10-20-fold lower dose of the Gd-DTPA-lipid nanoparticles, compared to current Gd-DTPA formulation, is needed in rat to produce equivalent or better definition of contrast enhanced MR images in rats. The 0.0025 mmole/kg Gd in DTPA-lipid nanoparticles produce equivalent image quality of 0.05 mmole/kg Gd-DTPA (OMNISCAN) preparation. Therefore, the Gd-DTPA-lipid nanoparticles may overcome challenges in the clinical use with currently approve Gd contrast agents due to renal insufficiency and neurotoxicity.

(61) The PEGylated lipid nanoparticles of the invention having surface-bound gadolinium ion exhibited a great improvement over other preparations in contrast enhanced MR lymphography and vascular imaging. These lipid nanoparticles showed high degree of accumulation in the lymph nodes after subcutaneous injection. The contrast enhancement in lymphoid tissue begins within 20 minutes of injection and is maintained for 24 hours. When given intravenously this agent produced high quality images of vasculature in much higher sensitivity than the current agents. Intravenously administered lipid nanoparticles are cleared almost exclusively through biliary route and appeared to complete within 24 hr. Surface modification by adding mPEG in lipid nanoparticles increased the MR signal of Gd.sup.3+ through coordination of water molecules. This leads to a much higher R1 relaxivity and lymph node image enhancement. The lipid nanoparticle formulation may allow using a low dose to achieve a high signal-to-noise MR contrast ratio for increasing the metastatic nodal discrimination and allowing for a much wider time frame for imaging. The potentially lower dose and more favorable elimination route of Gd.sup.3+ needed for MR contrast could provide higher safety margin.

(62) The formulations of the invention provide effective contrast at relatively low dose compared to currently available and approved contrast agents. FIGS. 8A-8F compare dose response MR images of a rat after intravenous injection of a representative formulation of the invention (Gd-DPTA-PE:mPEG-PE:DSPC): 0.0 mmol/kg (FIG. 8A); 0.00125 mmol/kg (FIG. 8B); 0.0025 mmol/kg (FIG. 8C); 0.005 mmol/kg (FIG. 8D); and 0.010 mmol/kg (FIG. 8E and FIG. 8F). Lymph nodes are clearly apparent in FIG. 8F.

(63) As noted above, the formulations of the invention offer advantages over currently available and approved contrast agents. FIGS. 9A-9C compare MR images of rats after intravenous injection of two commercially available gadolinium-based contrast agents (0.05 mmol/kg MS-325, EPIX Pharmaceuticals, Inc., Cambridge, Mass., FIG. 9A; and 0.05 mmol/kg MAGNEVIST Gd-DPTA, Bayer HealthCare Pharmaceuticals, FIG. 9B) and a representative formulation of the invention (0.01 mmol/kg Gd-DPTA-PE:mPEG-PE:DSPC, FIG. 9C). Images were acquired near peak enhancement, about 1 to 2 min after administration. As can be seen from the images, the representative formulation of the invention demonstrates significantly greater contrast than the currently available agents.

(64) FIG. 10 is a synthetic scheme for preparing a representative Gd complexes useful in the invention (DSPE-DOTA-Gd) by reacting DSPE with p-SCN-Bn-DOTA followed by Gd metallation. FIG. 11 illustrates three representative chelating agents (isothiocyanates, —N═C═S or —NCS) (p-SCN-Bn-DOTA, CHX-A″-DTPA, and p-SCN-Bn-DTPA) that are reactive toward phospholipids and useful in the invention.

(65) FIG. 12 illustrates four representative lipophilic compounds (DSA, Diether PE, DSPE-PEG(2000) Amine, and DSPE) and a synthetic scheme for preparing a representative Gd complex useful in the invention (DSPE-DOTA-Gd) by reacting DSPE with p-SCN-Bn-DOTA followed by Gd metallation. Lipid-containing metal chelators useful in the invention are readily prepared as shown in FIGS. 10 and 12. Suitably reactive metal chelators (e.g., isothiocyanate-functionalized metal chelators, see FIG. 11) are reacted with lipid compounds containing suitably reactive groups (e.g., amino groups in DSPE, DSPE-PEG(2000) Amine, Diether PE, DSA, see FIGS. 10 and 12) to provide lipid-containing metal chelators in which the lipid moiety is covalently coupled to the metal chelator. For isothiocyanate-functionalized metal chelators and reactive amine-containing lipids, the product lipid-containing metal chelator includes a thiourea (—NH—C(═S)—NH—) linkage coupling the lipid to the chelator. It will be appreciated that other suitably reactive metal chelators (e.g., isocyanate) and lipid compounds containing suitably reactive groups (e.g., alcohol) can provide lipid-containing metal chelators useful in the invention. Metallation of the chelators provides the metal ion-containing compounds.

(66) TABLE-US-00001 TABLE 1 Effects of the Gd.sup.3+ to DTPA mole ratio on particle size and stability of Gd.sup.3+-lipid nanoparticles.sup.a. Gd.sup.3+:DTPA Particle diameter (nm).sup.c (m/m) ratio.sup.b After preparation At 24 hr Free or unbound Gd.sup.3+d 0 52 ± 0.05 65 ± 0.3 — 0.5 46 ± 0.09  49 ± 0.07 1.4 ± 2.7% 1 41 ± 0.1  48 ± 0.1 1.6 ± 4.8% 2 137 ± 0.15  145 ± 0.2  0.8 ± 2.0% .sup.aDTPA is expressed on lipid nanoparticles composed of DSPC, mPEG-PE and DTPA-PE (8:1:0.9 mole ratio, as described in Example 1. .sup.bThe nanoparticles were exposed to GdCl.sub.3 at indicated DTPA to Gd.sup.3+ mole ratio. .sup.cThe diameter of DTPA expressed nanoparticles were measured by photon correlation spectroscopy and data expressed were mean ± SD of quadruplicate samples at indicated time points. .sup.dThe presence of unbound or free Gd.sup.3+ was estimated with calcein fluorescence quenching assay.

(67) The following examples are provided for the purpose of illustration, not limiting, the invention.

EXAMPLES

Example 1

Materials and Methods

(68) Lipid Nanoparticle Preparation.

(69) 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC, Avanti Polar Lipids, AL), N-(carbonyl-methoxypolethyleneglycol-2000)-1,2-distearoyl-sn-glycero-3-phospoethanolamine (mPEG-2000-DSPE, Genzyme, MA), and 1,2-di stearoyl-sn-glycero-3-phophoethanolamine-N-DTPA (DSPE-DTPA, Avanti Polar Lipids, AL) were combined in chloroform (DSPC:mPEG-DSPE:DSPE-DTPA) in a ratio of 8:2:1 and dried into a thin film under nitrogen and then under high vacuum overnight. The mPEG-DSPE containing PEG polymers of various molecular weights (or chain lengths) are also purchased from Genzyme, MA. At this point phosphate buffered saline (PBS, pH 7.4) was added to the film and sonicated in a bath-type sonicator (Laboratory Supplies Company, New York). The vesicle diameter, as measured by dynamic light scattering using a Malvern Zetasizer 5000 photon correlation spectroscopy (Malvern Instruments, PA), was 50 nm. The nanoparticles in suspension were mixed with gadolinium (III) chloride hexahydrate (Aldrich, St Louis, Mo.) for 20 minutes at indicated mole ratio to form Gd-DTPA-lipid nanoparticles. To determine the unbound Gd.sup.3+, the nanoparticles were incubated with calcein (0.5 M) (Sigma, St Louis, Mo.) in PBS, pH 7.4, and the fluorescence was measured at 485/535 nm using a Victor3V 1420 multilabel counter (PerkinElmer, Waltham, Mass.). Free ionic Gd.sup.3+ quenches calcein fluorescence in [Gd.sup.3+] dependent manner. To determine the final Gd concentration, elemental Gd mass was determined using the inductively coupled plasma atomic emission spectrometry. The particles along with control particles without Gd.sup.3+ were used in the studies described herein.

(70) Relaxivity Studies.

(71) Dilutions of Gd-DTPA-lipid nanoparticles were prepared with Gd.sup.3+ concentrations between 0-5 μmol/ml. For comparison several samples were prepared from commercial agents such as OMNISCAN (Gd-DTPA-BMA) with Gd.sup.3+ concentrations from 0-5 μmol/ml. The relaxation time T1 was measured using the standard spin-echo sequence on a 1.5T MR scanner with a volume head coil as RF receiver. For T1 measurements, TE was fixed to 9 ms and seven TR were 133, 200, 300, 500, 750, 1000 and 2000 ms, respectively. For T2 measurements, TR was fixed to 2000 ms and four TE were 15, 30, 45, and 60 ms, respectively. The imaging intensities were fitted to obtain the corresponding T1 and T2 values, which were plotted versus Gd.sup.3+ concentration.

(72) Primate Lymphatic MRI Study.

(73) In vivo imaging of the lymph nodes using Gd-DTPA-lipid nanoparticles for dynamic contrast enhanced (DCE) MRI was performed in a 1.5T MR scanner. The pigtailed macaque (M. Nemestrina) was anesthetized with inhaled isofluorane (1-2%) and closely monitored during the experiments. A pre-contrast image of the primate was recorded to determine proper lymph node location and fine-tune the imaging parameters. The animal was removed from the MR scanner and injected subcutaneously at four sites. Each injection site received 2, 5, 5, and 8.5 mL, respectively of 6.1 μmol/ml Gd-DTPA-lipid nanoparticles to allow probing of dose effects and contrast diffusion from injection sites. The total dose of Gd is estimated to be 24.4 μmol/kg for the primate studies. The images were recorded on a Signa 1.5T Scanner using a surface coil 12×12 inch.sup.2. A standard spin-echo imaging sequence was used with TR=500 ms, TE=15 ms, slice thickness of 3 mm, 21 slices, slice gap=0.5 mm, FOV (field of view)=320×320 mm.sup.2, matrix size=512×512, which gives an in-plane resolution of 0.63×0.63 mm.sup.2 and a temporal resolution is 3.1 min.

(74) Rat Vascular MRI Study.

(75) In vivo imaging of the rat using Gd-DTPA-lipid nanoparticles for dynamic contrast enhanced (DCE) MRI was performed in a 3.0T MR scanner. The rats (SD) was anesthetized with inhaled isofluorane (1-2%) and closely monitored during the experiments. A pre-contrast image of the rat was recorded to determine proper location, orientation and fine-tune the imaging parameters. The animal was removed from the MR scanner and injected with 400 L of indicated Gd contrast media through femoral vein. The images were recorded on a Signa 1.5T Scanner using a surface coil 12×12 inch. A standard spin-echo imaging sequence was used with TR=500 ms, TE=15 ms, slice thickness of 3 mm, 21 slices, slice gap=0.5 mm, FOV (field of view)=320×320 mm.sup.2, matrix size=512×512, which gives an in-plane resolution of 0.63×0.63 mm.sup.2 and a temporal resolution is 3.1 min.

(76) While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.