Blood-pool contrast agents for MRI: a critical evaluation

D-Alpha-Tocopheryl Poly(ethylene Glycol 1000) Succinate-Coated Manganese-Zinc Ferrite Nanomaterials for a Dual-Mode Magnetic Resonance Imaging Contrast Agent and Hyperthermia Treatments

Manganese-zinc ferrite (MZF) is known as high-performance magnetic material and has been used in many fields and development. In the biomedical applications, the biocompatible MZF formulation attracted much attention. In this study, water-soluble amphiphilic vitamin E (TPGS, d-alpha-tocopheryl poly(ethylene glycol 1000) succinate) formulated MZF nanoparticles were synthesized to serve as both a magnetic resonance imaging (MRI) contrast agent and a vehicle for creating magnetically induced hyperthermia against cancer. The MZF nanoparticles were synthesized from a metallic acetylacetonate in an organic phase and further modified with TPGS using an emulsion and solvent-evaporation method. The resulting TPGS-modified MZF nanoparticles exhibited a dual-contrast ability, with a longitudinal relaxivity (35.22 s-1 mM Fe-1) and transverse relaxivity (237.94 s-1 mM Fe-1) that were both higher than Resovist®. Furthermore, the TPGS-assisted MZF formulation can be used for hyperthermia treatment to successfully suppress cell viability and tumor growth after applying an alternating current (AC) electromagnetic field at lower amplitude. Thus, the TPGS-assisted MZF theranostics can not only be applied as a potential contrast agent for MRI but also has potential for use in hyperthermia treatments.

MRI study of subconjunctival and intravitreal injections

Previous magnetic resonance imaging (MRI) studies to investigate the routes of penetration and barriers in ocular delivery have provided insights into the mechanisms of transscleral and intraocular drug delivery. The objective of the present study was to investigate ocular penetration and clearance after subconjunctival and intravitreal injections using a contrast agent at concentrations higher than those in the previous studies. This high concentration approach was hypothesized to allow the visualization of the contrast agent in the eye that could not be achieved previously. Subconjunctival and intravitreal injections of contrast agent Magnevist, a model hydrophililc probe, were performed in rabbits, and the distribution and clearance of the probe after the injections were examined by MRI. After subconjunctival injection in vivo, significant contrast agent penetration into the anterior chamber was observed but not into the vitreous. A clearance pathway of the hydrophilic probe from the subconjunctival depot to the regions near the periocular fat behind the eye was found. After intravitreal injection in vivo, the contrast agent was observed in the anterior chamber, optic nerve, and tissues surrounding the eye during clearance. MRI continues to provide insights into the transport barriers and clearance pathways of hydrophilic molecules in ocular delivery.

Coating thickness of magnetic iron oxide nanoparticles affects R2 relaxivity

PURPOSE

To evaluate the effect of coating thickness on the relaxivity of iron oxide nanoparticles.

MATERIALS AND METHODS

Monocrystalline superparamagnetic iron oxide nanoparticles (MIONs), coated with a polyethylene glycol (PEG)-modified, phospholipid micelle coating, with different PEG molecular weights, were prepared. The particle diameters were measured with dynamic light scattering (DLS) and electron microscopy (EM). The R1 and R2 of MIONs were measured using a bench-top nuclear magnetic resonance (NMR) relaxometer. pH was varied for some measurements. Monte Carlo simulations of proton movement in a field with nanometer-sized magnetic inhomogeneities were performed.

RESULTS

Increasing the molecular weight of the PEG portion of the micelle coating increased overall particle diameter. As coating thickness increases, the R2 decreases and the R1 increases. Changing pH has no effect on relaxivity. The Monte Carlo simulations suggest that the effect of coating size on R2 relaxivity is determined by two competing factors: the physical exclusion of protons from the magnetic field and the residence time for protons within the coating zone.

CONCLUSION

Coating thickness can significantly impact the R2, and the R2/R1 ratio, of a MION contrast agent. An understanding of the relationship between coating properties and changes in relaxivity is critical for designing magnetic nanoparticle probes for molecular imaging applications using MRI.

Relaxation of protons by radicals in rotationally immobilized proteins

Proton spin-lattice relaxation by paramagnetic centers may be dramatically enhanced if the paramagnetic center is rotationally immobilized in the magnetic field. The details of the relaxation mechanism are different from those appropriate to solutions of paramagnetic relaxation agents. We report here large enhancements in the proton spin-lattice relaxation rate constants associated with organic radicals when the radical system is rigidly connected with a rotationally immobilized macromolecular matrix such as a dry protein or a cross-linked protein gel. The paramagnetic contribution to the protein-proton population is direct and distributed internally among the protein protons by efficient spin diffusion. In the case of a cross-linked-protein gel, the paramagnetic effects are carried to the water spins indirectly by chemical exchange mechanisms involving water molecule exchange with rare long-lived water molecule binding sites on the immobilized protein and proton exchange. The dramatic increase in the efficiency of spin relaxation by organic radicals compared with metal systems at low magnetic field strengths results because the electron relaxation time of the radical is orders of magnitude larger than that for metal systems. This gain in relaxation efficiency provides completely new opportunities for the design of spin-lattice relaxation based contrast agents in magnetic imaging and also provides new ways to examine intramolecular protein dynamics.

DTPA-bisamide-based MR sensor agents for peroxidase imaging

[reaction: see text] The synthesis and some properties of two novel DTPA-bisamides are reported. These derivatives were designed as enzyme-activated contrast agents (CA) for magnetic resonance imaging. Both derivatives bear tyramido or 5-hydroxytryptamido groups that could be oligomerized in situ in the presence of peroxidase/H(2)O(2) pair resulting in a net increase in longitudinal (R1) relaxivity.

Assessment of T1 and T2* effects in vivo and ex vivo using iron oxide nanoparticles in steady state--dependence on blood volume and water exchange

Accurate knowledge of the relationship between contrast agent concentration and tissue relaxation is a critical requirement for quantitative assessment of tissue perfusion using contrast-enhanced MRI. In the present study, using a pig model, the relationship between steady-state blood concentration levels of an iron oxide nanoparticle with a hydrated diameter of 12 nm (NC100150 Injection) and changes in the transverse and longitudinal relaxation rates (1/T2* and 1/T1, respectively) in blood, muscle, and renal cortex was investigated at 1.5 T. Ex vivo measurements of 1/T2* and 1/T1 were additionally performed in whole pig blood spiked with different concentrations of the iron oxide nanoparticle. In renal cortex and muscle, 1/T2* increased linearly with contrast agent concentration with slopes of 101 +/-22 s(-1)mM(-1) and 6.5 +/-0.9 s(-1)mM(-1) (mean +/- SD), respectively. In blood, 1/T2* increased as a quadratic function of contrast agent concentration, with different quadratic terms in the ex vivo vs. the in vivo experiments. In vivo, 1/T1 in blood increased linearly with contrast agent concentration, with a slope (T1-relaxivity) of 13.9 +/- 0.9 s(-1)mM(-1). The achievable increase in 1/T1 in renal cortex and muscle was limited by the rate of water exchange between the intra- and extravascular compartments and the 1/T1-curves were well described by a two-compartment water exchange limited relaxation model.

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 (tau(SO) and tau(D)), 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 4.38x10(-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 tau(D) accordingly). The nanoparticles show little size-polydispersity, evidenced by the lowest value of r(2)/r(1) at 20 MHz reported to date, an asset for magnetic resonance angiography.

Contrast agents for MRA: future directions

The purpose of this review is to outline recent developments in contrast agents for magnetic resonance angiography (MRA) and to give some idea of what the future might hold. Up to now, non-binding gadolinium (Gd) chelates have been the agents used for MRA. Modified paramagnetic Gd-based agents with varying degrees of protein interaction have been developed, and these, together with new superparamagnetic compounds, are currently under clinical evaluation. It is likely that two different types of contrast agent will soon be available: extracellular agents for first-pass MRA and intravascular agents mainly for steady-state MRA. Several agents also exhibit certain tissue specific properties in addition to conventional extracellular properties. This will lead to more comprehensive imaging approaches.

Magnetic field dependence of solvent proton relaxation by solute dysprosium (III) complexes

RATIONALE AND OBJECTIVES

Many magnetic resonance imaging (MRI) agents are Gd(III)-based; its half-filled f-shell has an S-ground state and hence a long electronic relaxation time, leading to comparably large effects on 1/T1 and 1/T2 of water protons with no shift in the water-proton resonance frequency. 1/T1 and 1/T2 nuclear magnetic relaxation dispersion (NMRD) profiles of the Dy(III) aquo ion and its chelates have been reported recently. Dy(III) ions differ magnetically from Gd(III); the large spin-orbit interaction of its non-S-ground state reduces the electronic relaxation time 100-fold, and can have a large effect on proton 1/T2 and resonance frequency. Relaxation theory is well-developed and applicable to both ions but, for Dy(III), the phenomena are more wide-ranging. Recent interpretations have suggested that the data are anomolous, requiring a new mechanism for their explanation. The authors explain published Dy(III) data in terms of known theory, guided by experience with Gd(III) agents.

METHODS

For fields below 1 T, the authors incorporate the shortened electronic relaxation time into the usual low-field theory for magnetic dipolar interactions between water protons and Dy(III) magnetic moments. Both inner- and outer-sphere relaxations are included. At higher fields (and unusual for small single-ion agents) one must include dipolar interactions of protons with the magnetization of the Dy(III) moments. This "Curie magnetization" causes a quadratic dependence of 1/T1 on field, and--through dipolar-induced shifts--an even greater quadratic dependence of 1/T2.

RESULTS

All published data can be explained by magnetic dipolar interactions. For Dy(III), the Curie term has a longer correlation time than the low-field term, namely, the rotation of solute for 1/T1 and the even longer water exchange lifetime tau M for 1/T2. This exchange modulates the shift, producing phenomena not seen with Gd(III).

CONCLUSIONS

Relaxation by Dy(III) chelates can be explained by the same well-established theory of dipolar interactions used for their Gd(III) analogs. Interestingly, for MRI applications, tau M should be long for Dy(III)-based agents and short for Gd(III)-based agents.