Modification of Gd-DTPA cystine copolymers with PEG-1000 optimizes pharmacokinetics and tissue retention for magnetic resonance angiography

Evaluation of methemoglobin as an intravascular contrast agent: T1 relaxation time effect in a rabbit model

OBJECTIVE

Alternative contrast agents for MRI are needed for individuals who may respond adversely to gadolinium, and need an intravascular agent for specific indications. One potential contrast agent is intracellular methemoglobin, a paramagnetic molecule that is normally present in small amounts in red blood cells. An animal model was used to determine whether methemoglobin modulation with intravenous sodium nitrite transiently changes the T1 relaxation of blood.

METHODS

Four adult New Zealand white rabbits were treated with 30 mg intravenous sodium nitrite. 3D TOF and 3D MPRAGE images were acquired before (baseline) and after methemoglobin modulation. T1 of blood was measured with 2D ss EPl acquisitions with inversion recovery preparation performed at two-minute intervals up to 30 min. T1 maps were calculated by fitting the signal recovery curve within major blood vessels.

RESULTS

Baseline T1 was 1758 ± 53 ms in carotid arteries and 1716 ± 41 ms in jugular veins. Sodium nitrite significantly changed intravascular T1 relaxation. The mean minimum value of T1 was 1126 ± 28 ms in carotid arteries 8 to 10 min after the injection of sodium nitrite. The mean minimum value of T1 was 1171 ± 52 ms in jugular veins 10 to 14 min after the injection of sodium nitrite. Arterial and venous T1 recovered to baseline after a period of 30 min.

CONCLUSION

Methemoglobin modulation produces intravascular contrast on T1-weighted MRI in vivo. Additional studies are needed to safely optimize methemoglobin modulation and sequence parameters for maximal tissue contrast.

Surface impact on nanoparticle-based magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) is one of the most widely used diagnostic tools in the clinic. To improve imaging quality, MRI contrast agents, which can modulate local T1 and T2 relaxation times, are often injected prior to or during MRI scans. However, clinically used contrast agents, including Gd3+-based chelates and iron oxide nanoparticles (IONPs), afford mediocre contrast abilities. To address this issue, there has been extensive research on developing alternative MRI contrast agents with superior r1 and r2 relaxivities. These efforts are facilitated by the fast progress in nanotechnology, which allows for preparation of magnetic nanoparticles (NPs) with varied size, shape, crystallinity, and composition. Studies suggest that surface coatings can also largely affect T1 and T2 relaxations and can be tailored in favor of a high r1 or r2. However, the surface impact of NPs has been less emphasized. Herein, we review recent progress on developing NP-based T1 and T2 contrast agents, with a focus on the surface impact.

A neutral polydisulfide containing Gd(III) DOTA monoamide as a redox-sensitive biodegradable macromolecular MRI contrast agent

This work aims to develop safe and effective gadolinium (III)-based biodegradable macromolecular MRI contrast agents for blood pool and cancer imaging. A neutral polydisulfide containing macrocyclic Gd-DOTA monoamide (GOLS) was synthesized and characterized. In addition to studying the in vitro degradation of GOLS, its kinetic stability was also investigated in an in vivo model. The efficacy of GOLS for contrast-enhanced MRI was examined with female BALB/c mice bearing 4T1 breast cancer xenografts. The pharmacokinetics, biodistribution, and metabolism of GOLS were also determined in mice. GOLS has an apparent molecular weight of 23.0 kDa with T1 relaxivities of 7.20 mM(-1) s(-1) per Gd at 1.5 T, and 6.62 mM(-1) s(-1) at 7.0 T. GOLS had high kinetic inertness against transmetallation with Zn(2+) ions, and its polymer backbone was readily cleaved by L-cysteine. The agent showed improved efficacy for blood pool and tumor MR imaging. The structural effect on biodistribution and in vivo chelation stability was assessed by comparing GOLS with Gd(HP-DO3A), a negatively charged polydisulfide containing Gd-DOTA monoamide GODC, and a polydisulfide containing Gd-DTPA-bisamide (GDCC). GOLS showed high in vivo chelation stability and minimal tissue deposition of gadolinium. The biodegradable macromolecular contrast agent GOLS is a promising polymeric contrast agent for clinical MR cardiovascular imaging and cancer imaging.

Gd-based macromolecules and nanoparticles as magnetic resonance contrast agents for molecular imaging

As we move towards an era of personalized medicine, molecular imaging contrast agents are likely to see an increasing presence in routine clinical practice. Magnetic resonance (MR) imaging has garnered particular interest as a platform for molecular imaging applications due its ability to monitor anatomical changes concomitant with physiologic and molecular changes. One promising new direction in the development of MR contrast agents involves the labeling and/or loading of nanoparticles with gadolinium (Gd). These nanoplatforms are capable of carrying large payloads of Gd, thus providing the requisite sensitivity to detect molecular signatures within disease pathologies. In this review, we discuss some of the progress that has recently been made in the development of Gd-based macromolecules and nanoparticles and outline some of the physical and chemical properties that will be important to incorporate into the next generation of contrast agents, including high Gd chelate stability, high "relaxivity per particle" and "relaxivity density", and biodegradability.

An extracellular MRI polymeric contrast agent that degrades at physiological pH

Macromolecular contrast agents have the potential to assist magnetic resonance imaging (MRI) due to their high relaxivity, but are not clinically useful because of toxicity due to poor clearance. We have prepared a biodegradable ketal-based polymer contrast agent which is designed to degrade rapidly at physiological pH by hydrolysis, facilitating renal clearance. In vitro, the agent degraded more rapidly at lower pH, with complete fragmentation after 24 h at pH 7.4. In vitro relaxivity measurements showed a direct correlation between molecular weight and relaxivity. We compared our polymer contrast agent with commercially available Magnevist in vivo by MRI imaging, as well as measuring the Gd concentration in blood. Our results show that our polymer contrast agent gives a higher contrast and intensity in the same organs and areas as Magnevist and is cleared from the blood at a similar rate. We aim to improve our polymer contrast agent design to develop it for use as a MRI contrast agent, and explore its use as a platform for other imaging modalities.

Development of intravascular contrast agents for MRI using gadolinium chelates

Two MRI contrast agents (CAs) composed of Gd-DO3A conjugated to amino acid building blocks derived from glutamic acid (CA1) and lysine (CA2) have been synthesized by using novel alkyne and propionate linkers, and subsequently characterized. In vitro cell viability assays showed insignificant cytotoxicity of both CAs at low concentrations up to 0.2 mM. The longitudinal relaxivities (r(1) ) of CA1 and CA2 measured at 9.4 T are 6.4 and 5.4 mM(-1)  s(-1) in H(2) O at 25 °C, respectively. Both r(1) values are higher than those of CAs in clinical use: Gd-DTPA (Magnevist, Bayer Schering, Germany) and Gd-DOTA (Dotarem, Guerbet, France). In vivo imaging in Wistar rats demonstrated considerable signal enhancement (∼50 %) in the brain artery by CA2, but lower signal enhancement (∼30 %) by CA1. In contrast to Dotarem, which showed a similar signal enhancement as CA2, the enhancement by CA2 remained high (∼30 %), even at 52 min post-injection. This demonstrates that CA2 has a much longer blood half-life (68.1 min), which could be advantageous for angiography and tissue targeting.

Multivalent protein polymer MRI contrast agents: controlling relaxivity via modulation of amino acid sequence

Magnetic resonance imaging is a noninvasive imaging modality with high spatial and temporal resolution. Contrast agents (CAs) are frequently used to increase the contrast between tissues of interest. To increase the effectiveness of MR agents, small molecule CAs have been attached to macromolecules. We have created a family of biodegradable, macromolecular CAs based on protein polymers, allowing control over the CA properties. The protein polymers are monodisperse, random coil, and contain evenly spaced lysines that serve as reactive sites for Gd(III) chelates. The exact sequence and length of the protein can be specified, enabling controlled variation in lysine spacing and molecular weight. Relaxivity could be modulated by changing protein polymer length and lysine spacing. Relaxivities of up to approximately 14 mM(-1) s(-1) per Gd(III) and approximately 461 mM(-1) s(-1) per conjugate were observed. These CAs are biodegradable by incubation with plasmin, such that they can be easily excreted after use. They do not reduce cell viability, a prerequisite for future in vivo studies. The protein polymer CAs can be customized for different clinical diagnostic applications, including biomaterial tracking, as a balanced agent with high relaxivity and appropriate molar mass.

Polydisulfide Based Biodegradable Macromolecular Magnetic Resonance Imaging Contrast Agents

Macromolecular Gd(III) complexes are advantageous over small molecular Gd(III) complexes in contrast enhanced magnetic resonance imaging (MRI) because of their prolonged blood circulation and preferential tumor accumulation. However, macromolecular contrast agents have not been approved for clinical applications because of the safety concerns related to their slow body excretion. Polydisulfide Gd(III) complexes have been designed and developed as biodegradable macromolecular MRI contrast agents to alleviate the concerns by facilitating the clearance of Gd(III) complexes from the body. These agents initially behave as macromolecular agents and result in superior contrast enhancement in the vasculature and tumor tissues. They can then be readily degraded in vivo into small molecular chelates that can rapidly excrete from the body via renal filtration after the MRI examinations. Various polydisulfide Gd(III) complexes have been prepared as biodegradable macromolecular MRI contrast agents. These agents have resulted in strong contrast enhancement in the vasculature and tumor tissue in animal models with minimal long-term tissue accumulation comparable to small molecular contrast agents. Polydisulfide Gd(III) complexes are promising for further clinical development as safe and effective biodegradable macromolecular MRI contrast agents for cardiovascular and cancer imaging. The review summarizes the chemistry and properties of polydisulfide Gd(III) complexes.

Noninvasive evaluation of antiangiogenic effect in a mouse tumor model by DCE-MRI with Gd-DTPA cystamine copolymers

The efficacy of polydisulfide-based biodegradable macromolecular Gd(III) complexes, Gd-DTPA cystamine copolymers (GDCC), for assessing tumor microvascular characteristics and monitoring antiangiogenesis therapy was investigated in a mouse model using dynamic contrast-enhanced MRI (DCE-MRI). The mice bearing human colon tumor xenografts were intraperitoneally injected with an antiangiogenesis agent Avastin three times in a week at a dose of 200 mug/mouse. DCE-MRI with GDCC of 40 kDa (GDCC-40) was performed before and at 36 h after the first treatment with Avastin and at the end of treatment (7 days). Gd(DTPA-BMA) was used as a low molecular weight control. The tumor vascular parameters, endothelial transfer coefficient K(trans) and factional plasma volume f(PV), were calculated from the DCE-MRI data with a two-compartment model. The K(trans) and f(PV) in tumor periphery estimated by DCE-MRI with GDCC-40 before and after the antiangiogenesis treatment correlated well to tumor growth before and after the treatment in the tumor model. In contrast, the parameters estimated by Gd(DTPA-BMA) did not show significant correlation to the therapeutic efficacy. This study demonstrates that DCE-MRI with the biodegradable macromolecular MRI contrast agent can provide effective assessment of the antiangiogenic efficacy of Avastin in the animal tumor model based on measured vascular parameters in tumor periphery.

Structural effect on degradability and in vivo contrast enhancement of polydisulfide Gd(III) complexes as biodegradable macromolecular MRI contrast agents

The structural effect of biodegradable macromolecular magnetic resonance imaging (MRI) contrast agents, polydisulfide gadolinium (Gd)(III) chelates, on their in vitro degradability, and cardiovascular and tumor imaging were evaluated in mice. Polydisulfide Gd(III) chelates, Gd-DTPA cystamine copolymers (GDCC), Gd-DTPA l-cystine copolymers (GDCP), Gd-DTPA d-cystine copolymers (dGDCP) and Gd-DTPA glutathione (oxidized) copolymers (GDGP), with different sizes and narrow molecular weight distribution were prepared and evaluated both in vitro and in vivo in mice bearing MDA-MB-231 tumor xenografts. GDGP with large steric hindrance around the disulfide bonds had greater T(1) and T(2) relaxivities than GDCC, GDCP and dGDCP. The degradability of the polydisulfide by the endogenous thiols decreased with increasing steric effects around the disulfide bonds in the order of GDCC>GDCP, dGDCP>GDGP. The size and degradability of the contrast agents had a significant impact on vascular contrast enhancement kinetics. The agents with a large size and low degradability resulted in more prolonged vascular enhancement than the agents with a small size and high degradability. It seems that the size and degradability of the agents did not significantly affect tumor enhancement. All agents resulted in significant contrast enhancement in tumor tissue. This study has demonstrated that the vascular enhancement kinetics of the polydisulfide MRI contrast agents can be controlled by their sizes and structures. The polydisulfide Gd(III) chelates are promising biodegradable macromolecular MRI contrast agents for magnetic resonance angiography and cancer imaging.