Marga Spiller

NC100150 injection, a preparation of optimized iron oxide nanoparticles for positive-contrast MR angiography

A preparation of monocrystalline iron oxide nanoparticles with an oxidized starch coating, currently in clinical trials (NC100150 Injection; CLARISCAN™), was characterized by magnetization measurements, relaxometry, and photon correlation spectroscopy. By combining the results with a measure of iron content, one can obtain the size and magnetic attributes of the iron cores, including the relevant correlation times for outer sphere relaxation (τSO and τD), and information about the interaction of the organic coating with both core and solvent. The results are 6.43 nm for the iron oxide core diameter, a magnetic moment of 4.38 × 10−17 erg/G, and a water‐penetrable coating region of oxidized oligomeric starch fragments and entrained water molecules. The latter extends the hydrodynamic diameter to 11.9 nm and lowers the average diffusivity of solvent about 64% (which increases τD accordingly). The nanoparticles show little size‐polydispersity, evidenced by the lowest value of r2/r1 at 20 MHz reported to date, an asset for magnetic resonance angiography. J. Magn. Reson. Imaging 2000;11:488–494.

Species Dependence on Plasma Protein Binding and Relaxivity of the Gadolinium-based Mri Contrast Agent Ms-325

Rationale and Objectives:We sought to determine whether there is a species dependence on plasma protein and serum album binding and/or relaxivity of the MR contrast agent MS-325. Methods:Equilibrium binding of MS-325 to plasma proteins or purified serum albumin was determined as a function of chelate concentration. T1 and T2 values were determined at 0.47 and 1.41 T, and NMRD profiles were measured to determine the changes in relaxivity over varying field strengths from 0.002 to 1.2 T. Results:The binding of MS-325 to either animal plasma or serum albumin plateaus at chelate concentrations less than 0.1 mM with human, pig, and rabbit plasmas showing maximum binding. Human and pig plasmas show the greatest observed relaxivity enhancement in the presence of MS-325. Conclusions:MS-325 exhibits increased relaxivity in blood plasma as the result of plasma protein binding. Binding ranged from 64% to 91% and was species dependent: human > pig ∼ rabbit > dog ∼ rat ∼ mouse.

‘NC100150’, a preparation of iron oxide nanoparticles ideal for positive-contrast MR angiography

A laboratory-scale synthesis of NC100150 (iron oxide particles with an oxidized starch coating) was characterized by magnetization measurements (vibrating sample magnetometry, VSM), relaxometry (1/T1 NMRD profiles and 1/T2 at 10 and 20 MHz), and dynamic light scattering (photon correlation spectroscopy, PCS). The results were related to give a self-consistent physical description of the particles: a water-impenetrable part making up 12% of the total particle volume, 82% of this volume consisting of an iron oxide core and the remaining 18% consisting of an oxidized starch rind; and, a water-penetrable part making up 88% of the total particle volume, consisting of oxidized starch polymers and entrained water molecules. Relating the magnetization to the relaxometry results required that the oxidized starch coating slows the diffusivity of solvent water molecules in the vicinity of the iron oxide cores. The effect of the organic coating on water diffusivity, not previously considered in the application of relaxation theory to iron oxide nanoparticles, is supported by the much greater (factor of about 2) diameter obtained from the dynamic light scattering measurements in comparison to that obtained from the magnetization measurements. The present work shows that three physical techniques—VSM, relaxometry, and PCS—are needed for properly assessing iron oxide nanoparticles for use as contrast agents for magnetic resonance angiography (MRA). It is also shown that NC100150 has a narrow range of diameters and the smallest value ofr2/r1 reported to date, an asset for MRA.

Magnetic field dependence (NMRD profile) of 1/T1 of rabbit kidney medulla and urine after intravenous injection of Gd(DTPA).

The measurement of NMRD profiles of water protons of excised tissues containing paramagnetic metal ions is one of the few ways of determining the biochemical and biophysical state of these ions in vivo. It is of critical importance, for example, to verify that Gd, injected as Gd(DTPA) to enhance contrast in MRI, remains chelated, since free Gd ions are highly toxic. We have investigated this in the renal medulla of rabbits. Fitting the magnetization data at each field of the dispersion to a single exponential shows that Gd accumulates predominantly in the renal medulla, from which it is cleared within 18 hours, and that Gd(DTPA) introduced intravenously into rabbits is excreted as Gd(DTPA) in the urine as rotationally mobile as in neat water. Taking a larger data set at each field and fitting it to the sum of two exponentials, since the errors of the single exponential analysis were larger than for other tissues, shows that the relaxation behavior of the renal medulla, free of contrast agent, can be well-described by a single relaxation rate at 37 degrees C. For increasing concentrations of Gd in the medulla, as determined by ICP analysis, two relaxation rates are required to account for the data, due to compartmentalization of tissue water and inhomogeneous distribution of Gd. These results, and similar data after mild mechanical disruption of renal structures, show unequivocally that the Gd in the renal medulla remains the chelate complex Gd(DTPA) and rotationally mobile, for dosages up to 300 mumoles/kg injected.

Evidence for weak protein binding of commercial extracellular gadolinium contrast agents

It is widely assumed that commercial extracellular gadolinium‐based contrast agents do not bind to proteins. Here, nuclear magnetic relaxation dispersion was used to characterize the interaction between the contrast agents gadodiamide and gadopentetate dimeglumine and the proteins human serum albumin, chicken egg white lysozyme, egg white proteins, or milk proteins. In all cases, contrast agent relaxivity was increased at all field strengths measured (0.0002 to 1.4 T) when protein was added. A distinct peak in relaxivity was observed between 0.5 and 0.7 T that is consistent with fractional protein binding and that could not be attributed to changes in solution viscosity. This peak was observed for gadodiamide with all four protein solutions and for gadopentetate dimeglumine with lysozyme, human serum albumin, and milk proteins. Protein binding was both contrast agent and protein dependent. For gadodiamide, the highest affinity was to egg white and milk proteins, while gadopentetate dimeglumine interacted most strongly with lysozyme. Protein binding was estimated at 30–40% for a 0.7 mmol/kg solution of gadodiamide in egg white or milk proteins. These results have implications for the accurate determination of contrast agent concentration in vivo. Weak protein binding may be an additional discriminating factor in understanding differences in the toxicokinetics of contrast agents. Magn Reson Med, 2010.

Calcification can shorten T2, but not T1, at magnetic resonance imaging fields. Results of a relaxometry study of calcified human meningiomas.

RATIONALE AND OBJECTIVES.Water content and waterproton relaxation rates are reported for fresh, histologically characterized, surgical specimens of calcified human intracranial meningiomas and compared with results for noncalcified meningiomas from an earlier study and with calcium hydroxyapatite (CaHA) suspensions to elucidate the influence of calcification on magnetic resonance imaging (MRI) signal intensity of calcified meningiomas. METHODS.The magnetic field dependence of 1/T1 of water protons (nuclear magnetic relaxation dispersion profile) and dry weights are reported for 38 calcified nonhemorrhagic and 3 hemorrhagic specimens of known histologic subtype, a subset of the 67 specimens measured earlier. Calcification was considered mild or heavy when the dry weight was within or above the range for noncalcified meningiomas. Preliminary 1/T1 profiles for pure Call A and a single high-field 1 /T2 value also are reported. RESULTS.The ranges of dry weights and of low-field 1/T1 values were twice as large for calcified as for noncalcified meningiomas. No correlation was found between low-field 1/T1 and either histologic subtype or dry weight. Mild calcification produced the highest low-field 1/T1 values; the most heavily calcified tumor had slightly increased low-field 1/T1. Calcium hydroxyapatite increases low-field 1/T1 significantly but not high-field 1/T1; high-field 1/T2 is large. For calcified hemorrhagic meningiomas, increases in both low-field and high-field 1/T1 were seen. CONCLUSION.For mild calcification, MRI signal voids result from an increased high-field 1/T2; for heavier calcification, reduced proton density (from excluded water) becomes of increasing importance. Cellular CaHA appears to brighten the signal in T1-weighted MRI in the presence of hemorrhage.

NMRD ASSESSMENT OF GD-DTPA-BIS(METHOXYETHYLAMIDE) (GD-DTPA-BMEA), A NONIONIC MRI AGENT

RATIONALE AND OBJECTIVES Gd-DTPA-BMEA, a nonionic bis(methoxyethylamide) derivative of Gd-DTPA, is the active ingredient of OptiMARK, now awaiting FDA approval. In this study, we compare the relaxivities of Gd-DTPA-BMEA (OptiMARK) with those of the commercially available DTPA-based agents Gd-DTPA2- (Magnevist) and Gd-DTPA-BMA (Omniscan) at different field strengths (1/T1 nuclear magnetic relaxation dispersion (NMRD) profiles). In addition, we study how changes in structural attributes of small paramagnetic chelate complexes of Gd3+ ions influence 1/T1 NMRD profiles. METHODS 1/T1 NMRD profiles of Gd-DTPA-BMEA (OptiMARK) were measured at 5 degrees and 35 degrees C and a set of values for the parameters that describe relaxation by Gd(3+)-proton magnetic dipolar interactions was obtained. The rotational (tau R) and the diffusional (tau D) correlation times for Gd-DTPA-BMA were adjusted for the 15% greater molecular weight of Gd-DTPA-BMEA. tau M (the resident lifetime of Gd(3+)-bound water) was obtained from available 17O NMR relaxation data. For tau S0 and tau V (the low-field relaxation time of the Gd3+ moment and its correlation time), Gd-DTPA-BMA values were taken as initial values and tau S0 refined as needed. RESULTS Although, at 35 degrees C, tau M is comparable for the two neutral agents and an order of magnitude longer than that for Gd-DTPA2-, the 1/T1 NMRD profiles of Gd-DTPA-BMEA are indistinguishable from those of Gd-DTPA2- and Gd-DTPA-BMA. A 40% increase in the value of tau S0 from Gd-DTPA2- is required for agreement of data and theory for Gd-DTPA-BMEA. CONCLUSIONS Based on their 1/T1 NMRD profiles, the efficacy of the three agents should be identical in typical clinical MRI applications. The data can be fit reliably to theory, and differences in the fit parameters (and structure) have no effect on the three profiles at 35 degrees C. The relatively long values of tau M for the two neutral agents would only be of importance at low temperatures.

Important considerations in the design of iron oxide nanoparticles as contrast agents for Tl-weighted MRI and MRA

Organically coated magnetic monocrystalline iron oxide nanoparticles are being considered as contrast agents for T1-weighted magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) (1–4). For such Tl-weighted applications, imaging efficacy is strongly dependent on the physical characteristics of the individual nanoparticles (5). In particular, three stringent physical requirements must be satisfied. First, the magnetic cores must be of an optimal size with essentially a monodisperse size distribution. If the cores are too small, rl (the Tl relaxivity) will be too small for practical applications as T1 agents; if the cores are too large, r2 (the T2 relaxivity) may be so large relative to rl that the T1 efficacy of the particles will be diminished. Second, the high-field magnetization of the cores, typically close to its saturation value, must be sufficient to relax water protons effectively, also implying that the organic coating must not compromise access of solvent to the core. This high-field limit is related to both the total iron content of the cores and their geometry. Third, the nanoparticles cannot become agglomerated in vivo. Given that r2 is very sensitive to agglomeration and rl is not, agglomeration preferentially increases r2, thereby diminishing the efficacy of the nanoparticles as Tl agents. In the current work, the relative physical characteristics of two distinct nanoparticle preparations are compared with respect to their efficacy for Tl-weighted MRI and MRA: MION-46L and NC100150 Injection. 1/Tl nuclear magnetic relaxation dispersion (NMRD) profiles suffice for such a comparison, and magnetization data (as discussed by Koenig et al [6] in this supplement) are not needed.

Three Types of Physical Measurements Needed to Characterize Iron Oxide Nanoparticles for MRI and MRA: Magnetization, Relaxometry, and Light Scattering

Iron oxide nanoparticles, with monocrystalline cores (generally magnetite or maghemite) and coated with organic polymer to increase chemical stability and solubility, have generated widespread interest as negative contrast (susceptibility) agents for magnetic resonance imaging (MRI) (1,2). For negative agents, the requisite T2 shortening depends on the geometry of the physiologic structures that take up the agent (2) but is relatively insensitive to the size and structure of the nanoparticles themselves. Nanoparticles also have a potential as positive agents for magnetic resonance angiography (MRA) (3–6), an application that depends on T1 shortening in the vasculature by individual nanoparticles, provided that T2 is not too short (7). To optimize their suitability for MRA, the agents must be synthesized and characterized reproducibly, the size of the magnetic core must be carefully controlled, and the interaction of the organic coating with nearby solvent molecules must be understood (8). Here we describe a set of physical techniques for measuring the parameters that determine MRA efficacy: the magnetic moment and diameter d of the iron oxide cores, and the interaction of organic coating with solvent. We study magnetization (8) as a function of magnetic field B0; l/Tl nuclear magnetic relaxation dispersion (NMRD) profiles (9) for B0 from 0.24 mT to 1.2 T (0.01–50 MHz) and 1/T2 at 0.47 T (20 MHz); and solute translational diffusivity by photon correlation spectroscopy (PCS) (10). As demonstrated earlier (8), magnetization data, being thermodynamic, yield and the saturation magnetization of the particles, Msat. Combined with total iron content and the known density of the core, one obtains d. When these results are combined with NMRD data—which depend on and d and yield certain correlation times—one can detect any water-opaque organic “rind” and measure the diffusivity D of outer sphere solvent, a measure of viscous interactions between solvent and polymer. These interactions can be quantitated by PCS, which yields the effective radius of gyration of the polymers (11), including contributions from both polymer and viscously entrained water molecules. This trio of measurements lets one characterize the physical properties of solute nanoparticles self-consistently, deduce their behavior in MRA applications, and guide the development of improved agents. We report on two preparations of coated iron oxide nanoparticles: (a) CLARISCAN, a Nycomed Amersham product in clinical trials (8); and (b) MION-46L, a laboratory preparation (11) of MION-46 obtained from Massachusetts General Hospital.

Water-proton relaxation by a noncovalent albumin-binding gadolinium chelate: an NMRD study of a potential blood pool agent.

Magnetic dipolar interactions between solvent water protons and paramagnetic solutes, such as chelated Gd3 ions, provide an efficient means for increasing both the longitudinal (1/T1) and transverse (1/T2) relaxation rates of solvent protons. For small paramagnetic chelate complexes, interactions by direct water coordination to the metal ion (inner sphere) and diffusion in the outer sphere environment of the metal complexes are the two main contributors to relaxation enhancement. At magnetic resonance imaging (MRI) fields and physiologic conditions, these contributions are additive and generally comparable in magnitude, whereas for macromolecular complexes, inner sphere contributions tend to dominate relaxation. The inner sphere contribution to 1/T1 is given by: 1/T1p q[Gd3 ]/55.5(T1M M). Here, q is the number of coordinated inner sphere water molecules, T1M is the longitudinal relaxation time of the coordinated water protons, and M is their lifetime on the metal ion. For coordinated Gd3 ions with q 1, both T1M and M are of the order of microseconds. 1/T1 is a complicated function of the strength of the interaction, the magnitude of the applied MRI field B0, and an overall correlation time, C. C is given by C 1 R 1 S 1 M 1, the inverse of the sums of the reciprocals of the orientational relaxation time of the complex R, the electronic relaxation time of the paramagnetic metal ion S, and M, respectively. For Gd3 ions, S can also be a function of B0, often characterizable by another correlation time V. In any event, the enhancement is maximized for kinetically labile complexes, when exchange of the inner sphere water is rapid (but not too rapid), such that C M T1M (1–3). Attachment of Gd3 chelates to macromolecules generally leads to a peak in 1/T1 in the MRI field range (2,3). This phenomenon is well understood and is associated with the long R of macromolecular complexes and related to the fact that S becomes very long at MRI fields. Hence, formation of a rigid macromolecular paramagnetic complex is a design goal for obtaining high-relaxivity contrast agents. To date, several covalently bonded Gd3 macromolecular complexes have been explored, but the peak relaxivity has been limited either by lack of rigidity or by a long value of M (4–8). Relatively rigid paramagnetic macromolecular complexes have been generated in vivo, in the blood pool, through reversible noncovalent hydrophobic interactions of small lipophilic Gd3 chelates with serum (9,10). Depending on the specifics of the docking interaction at the known hydrophobic patches on albumin, highly efficient intravascular contrast agents can be produced. In addition, the reversibility of the noncovalent interactions ensures increased circulation lifetime of the agent and ease of elimination by the kidneys as small molecules, thereby reducing toxicity problems. A monomeric nonaromatic MR agent (code-named MP-2269) that binds to serum albumin has been synthesized. MP-2269 is the Gd3 complex of 4-pentylbicyclo[2.2.2]octane-1-carboxyl-di-L-aspartyl-lysine-derived-DTPA (see Fig 1 for MP-2269 structure). The lysine-DTPA derivative is -(N,N-bis-[2-[N ,N -bis(carboxy-methyl-amino)]]ethyl)-L-lysine (11). The hydrophobic pentylbicyclo[2.2.2]octane side chains associate with albumin, and negAcad Radiol 2002; 9(suppl 1):S11–S16