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

Functional nanoparticles for magnetic resonance imaging

Nanoparticle-based magnetic resonance imaging (MRI) contrast agents have received much attention over the past decade. By virtue of a high payload of magnetic moieties, enhanced accumulation at disease sites, and a large surface area for additional modification with targeting ligands, nanoparticle-based contrast agents offer promising new platforms to further enhance the high resolution and sensitivity of MRI for various biomedical applications. T ₂ * superparamagnetic iron oxide nanoparticles (SPIONs) first demonstrated superior improvement on MRI sensitivity. The prevailing SPION attracted growing interest in the development of refined nanoscale versions of MRI contrast agents. Afterwards, T ₁ -based contrast agents were developed, and became the most studied subject in MRI due to the positive contrast they provide that avoids the susceptibility associated with MRI signal reduction. Recently, chemical exchange saturation transfer (CEST) contrast agents have emerged and rapidly gained popularity. The unique aspect of CEST contrast agents is that their contrast can be selectively turned 'on' and 'off' by radiofrequency saturation. Their performance can be further enhanced by incorporating a large number of exchangeable protons into well-defined nanostructures. Besides activatable CEST contrast agents, there is growing interest in developing nanoparticle-based activatable MRI contrast agents responsive to stimuli (pH, enzyme, etc.), which improves sensitivity and specificity. In this review, we summarize the recent development of various types of nanoparticle-based MRI contrast agents, and have focused our discussions on the key advantages of introducing nanoparticles in MRI. WIREs Nanomed Nanobiotechnol 2016, 8:814-841. doi: 10.1002/wnan.1400 For further resources related to this article, please visit the WIREs website.

Oligoethylenimine grafted PEGylated poly(aspartic acid) as a macromolecular contrast agent: properties and in vivo studies

A macromolecular contrast agent based on PEGylated poly(aspartic acid) was prepared and well characterized, which may provide helpful insights for the further development of sensitive and biocompatible MRI probes.

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.

A facile synthetic approach to a biodegradable polydisulfide MRI contrast agent

A biodegradable novel polydisulfide MRI contrast agent forming self-assembly in aqueous solution with a low cytotoxicity and a higherr₁is promising for producing better MRI imaging with fewer side effects.

Synthesis and evaluation of a polydisulfide with Gd-DOTA monoamide side chains as a biodegradable macromolecular contrast agent for MR blood pool imaging

Macromolecular Gd(III)-based contrast agents are effective for contrast-enhanced blood pool and cancer MRI in preclinical studies. However, their clinical applications are impeded by potential safety concerns associated with slow excretion and prolonged retention of these agents in the body. To minimize the safety concerns of macromolecular Gd contrast agents, we have developed biodegradable macromolecular Gd contrast agents based on polydisulfide Gd(III) complexes. In this study, we designed and synthesized a new generation of the polydisulfide Gd(III) complexes containing a macrocyclic Gd(III) chelate, Gd-DOTA monoamide, to improve the in vivo kinetic inertness of the Gd(III) chelates. (N6-Lysyl)lysine-(Gd-DOTA) monoamide and 3-(2-carboxyethyldisulfanyl)propanoic acid copolymers (GODC) were synthesized by copolymerization of (N6-lysyl)lysine DOTA monoamide and dithiobis(succinimidylpropionate), followed by complexation with Gd(OAc)3. The GODC had an apparent molecular weight of 26.4 kDa and T1 relaxivity of 8.25 mM(-1) s(-1) per Gd at 1.5 T. The polymer chains of GODC were readily cleaved by L-cysteine and the chelates had high kinetic stability against transmetallation in the presence of an endogenous metal ion Zn(2+). In vivo MRI study showed that GODC produced strong and prolonged contrast enhancement in the vasculature and tumor periphery of mice with breast tumor xenografts. GODC is a promising biodegradable macromolecular MRI contrast agent with high kinetic stability for MR blood pool imaging.

Biodegradable polysilsesquioxane nanoparticles as efficient contrast agents for magnetic resonance imaging

Polysilsesquioxane (PSQ) nanoparticles are crosslinked homopolymers formed by condensation of functionalized trialkoxysilanes, and provide an interesting platform for developing biologically and biomedically relevant nanomaterials. In this work, the design and synthesis of biodegradable PSQ particles with extremely high payloads of paramagnetic Gd(III) centers is explored, for use as efficient contrast agents for magnetic resonance imaging (MRI). Two new bis(trialkoxysilyl) derivatives of Gd(III) diethylenetriamine pentaacetate (Gd-DTPA) containing disulfide linkages are synthesized and used to form biodegradable Gd-PSQ particles by base-catalyzed condensation reactions in reverse microemulsions. The Gd-PSQ particles, PSQ-1 and PSQ-2, carry 53.8 wt% and 49.3 wt% of Gd-DTPA derivatives, respectively. In addition, the surface carboxy groups on the PSQ-2 particles can be modified with polyethylene glycol (PEG) and the anisamide (AA) ligand to enhance biocompatibility and cell uptake, respectively. The Gd-PSQ particles are readily degradable to release the constituent Gd(III) chelates in the presence of endogenous reducing agents such as cysteine and glutathione. The MR relaxivities of the Gd-PSQ particles are determined using a 3T MR scanner, with r1 values ranging from 5.9 to 17.8 mMs(-1) on a per-Gd basis. Finally, the high sensitivity of the Gd-PSQ particles as T1 -weighted MR contrast agents is demonstrated with in vitro MR imaging of human lung and pancreatic cancer cells. The enhanced efficiency of the anisamide-functionalized PSQ-2 particles as a contrast agent is corroborated by both confocal laser scanning microscopy imaging and ICP-MS analysis of Gd content in vitro.

Targeted biodegradable dendritic MRI contrast agent for enhanced tumor imaging

Highly sensitive and safe contrast agents (CAs) are essential for magnetic resonance imaging (MRI) to achieve accurate tumor detection and imaging. Dendrimer-based macromolecular MRI contrast agents are advantageous owing to their tumor-targeting ability, enhanced imaging contrast and enlarged imaging window. However, most of them have drawbacks of non-degradability and thereby long-term retention in body and toxicity. Herein, a tumor-targeting biodegradable dendritic CA (DCA) (FA-PEG-G2-DTPA-Gd) was prepared from a polyester dendrimer conjugated with gadolinium (Gd) chelates and PEG chains with distal folic acid. The DCA had a high longitudinal relaxivity up to 17.1mM(-1)s(-1), 4 times higher than the clinically used CA Magnevist. The MRI contrasted by FA-PEG-G2-DTPA-Gd outlined the inoculated tumor more clearly, and had much higher contrast enhancement for a much longer time than Magnevist. More importantly, the biodegradable FA-PEG-G2-DTPA-Gd gave much less Gd retentions in all the organs or tissues than non-degradable DCAs. Thus, the high efficiency in MRI contrast enhancement and low Gd retention merit it a promising CA for contrast enhanced tumor MRI.

Biodegradable polydisulfide dendrimer nanoclusters as MRI contrast agents

Gadolinium-conjugated dendrimer nanoclusters (DNCs) are a promising platform for the early detection of disease; however, their clinical utility is potentially limited due to safety concerns related to nephrogenic systemic fibrosis (NSF). In this paper, biodegradable DNCs were prepared with polydisulfide linkages between the individual dendrimers to facilitate excretion. Further, DNCs were labeled with premetalated Gd chelates to eliminate the risk of free Gd becoming entrapped in dendrimer cavities. The biodegradable polydisulfide DNCs possessed a circulation half-life of >1.6 h in mice and produced significant contrast enhancement in the abdominal aorta and kidneys for as long as 4 h. The DNCs were reduced in circulation as a result of thiol-disulfide exchange, and the degradation products were rapidly excreted via renal filtration. These agents demonstrated effective and prolonged in vivo contrast enhancement and yet minimized Gd tissue retention. Biodegradable polydisulfide DNCs represent a promising biodegradable macromolecular MRI contrast agent for magnetic resonance angiography and can potentially be further developed into target-specific MRI contrast agents.

Gadolinium-based contrast agents for magnetic resonance cancer imaging

Magnetic resonance imaging (MRI) is a clinical imaging modality effective for anatomical and functional imaging of diseased soft tissues, including solid tumors. MRI contrast agents (CA) have been routinely used for detecting tumor at an early stage. Gadolinium-based CA are the most commonly used CA in clinical MRI. There have been significant efforts to design and develop novel Gd(III) CA with high relaxivity, low toxicity, and specific tumor binding. The relaxivity of the Gd(III) CA can be increased by proper chemical modification. The toxicity of Gd(III) CA can be reduced by increasing the agents' thermodynamic and kinetic stability, as well as optimizing their pharmacokinetic properties. The increasing knowledge in the field of cancer genomics and biology provides an opportunity for designing tumor-specific CA. Various new Gd(III) chelates have been designed and evaluated in animal models for more effective cancer MRI. This review outlines the design and development, physicochemical properties, and in vivo properties of several classes of Gd(III)-based MR CA tumor imaging.

Molecular imaging probe development: a chemistry perspective

Molecular imaging is an attractive modality that has been widely employed in many aspects of biomedical research; especially those aimed at the early detection of diseases such as cancer, inflammation and neurodegenerative disorders. The field emerged in response to a new research paradigm in healthcare that seeks to integrate detection capabilities for the prediction and prevention of diseases. This approach made a distinct impact in biomedical research as it enabled researchers to leverage the capabilities of molecular imaging probes to visualize a targeted molecular event non-invasively, repeatedly and continuously in a living system. In addition, since such probes are inherently compact, robust, and amenable to high-throughput production, these probes could potentially facilitate screening of preclinical drug discovery, therapeutic assessment and validation of disease biomarkers. They could also be useful in drug discovery and safety evaluations. In this review, major trends in the chemical synthesis and development of positron emission tomography (PET), optical and magnetic resonance imaging (MRI) probes are discussed.

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.

Polydisulfide manganese(II) complexes as non-gadolinium biodegradable macromolecular MRI contrast agents

PURPOSE

To develop safe and effective manganese(II) -based biodegradable macromolecular MRI contrast agents.

MATERIALS AND METHODS

In this study, we synthesized and characterized two polydisulfide manganese(II) complexes, Mn-DTPA cystamine copolymers and Mn-EDTA cystamine copolymers, as new biodegradable macromolecular MRI contrast agents. The contrast enhancement of the two manganese-based contrast agents were evaluated in mice bearing MDA-MB-231 human breast carcinoma xenografts, in comparison with MnCl(2) .

RESULTS

The T(1) and T(2) relaxivities were 4.74 and 10.38 mM(-1) s(-1) per manganese at 3T for Mn-DTPA cystamine copolymers (M(n) = 30.50 kDa) and 6.41 and 9.72 mM(-1) s(-1) for Mn-EDTA cystamine copolymers (M(n) = 61.80 kDa). Both polydisulfide Mn(II) complexes showed significant liver, myocardium and tumor enhancement.

CONCLUSION

The manganese-based polydisulfide contrast agents have a potential to be developed as alternative non-gadolinium contrast agents for MR cancer and myocardium imaging.

Molecular magnetic resonance contrast agents for the detection of cancer: past and present

Magnetic resonance imaging (MRI) is a powerful diagnostic tool with unsurpassed spatial resolution that is capable of providing detailed information about the structure and composition of tumors. The use of exogenously administered contrast agents allows compartment-specific enhancement of tumors, enabling imaging of functional blood and interstitial volumes. Current efforts are directed at enhancing the capabilities of MRI in oncology by adding contrast agents with molecular specificities to the growing armamentarium of diagnostic probes that produce signal by changing local proton relaxation times as a consequence of specific contrast agent binding to cell surface receptors or extracellular matrix components. We review herein the most notable examples, illustrating major trends in the development of specific probes for high-resolution imaging in molecular oncology.

Nutritional and medicinal aspects of D-amino acids

This paper reviews and interprets a method for determining the nutritional value of D-amino acids, D-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as L-lysine (L-Lys), L-methionine (L-Met), L-phenylalanine (L-Phe), and L-tryptophan (L-Trp) as well as the semi-essential amino acids L-cysteine (L-Cys) and L-tyrosine (L-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding L-amino acid. Because the organism is forced to use the D-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and pharmacological aspects of 27 individual D-amino acids, D-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results provide a valuable record and resource for further progress on the multifaceted aspects of D-amino acids in food and biological samples.

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.

Synthesis and evaluation of nanoglobule-cystamine-(Gd-DO3A), a biodegradable nanosized magnetic resonance contrast agent for dynamic contrast-enhanced magnetic resonance urography

Dynamic contrast-enhanced magnetic resonance imaging has been recently shown to be effective for diagnostic urography. High-resolution urographic images can be acquired with T1 contrast agents for the kidney and urinary tract with minimal noise in the abdomen. Currently, clinical contrast agents are low molecular weight agents and can rapidly extravasate from blood circulation, leading to slow contrast agent elimination through kidney and consequently providing limited contrast enhancement in urinary tract. In this study, a new biodegradable macromolecular contrast agent, nanoglobule-G4-cystamine-(Gd-DO3A), was prepared by conjugating Gd-DO3A chelates on the surface of a generation 4 nanoglobule, poly-l-lysine octa(3-aminopropyl)silsesquioxane dendrimer, via a disulfide spacer, where the carrier had a precisely defined nanosize that is far smaller than the renal filtration threshold. The in vivo contrast enhancement and dynamic imaging of the urinary tract of the agent was evaluated in nude mice using a low molecular weight agent Gd(DTPA-BMA) as a control. The agent eliminated rapidly from blood circulation and accumulated more abundantly in urinary tract than Gd(DTPA-BMA). The fast elimination kinetics is ideal for functional evaluation of the kidneys. The morphology of the kidneys and urinary tract was better visualized by the biodegradable nanoglobular contrast agent than Gd(DTPA-BMA). The agent also resulted in low liver contrast enhancement, indicating low nonspecific tissue deposition. These features render the G4 nanoglobule-cystamine-(Gd-DO3A) conjugate a promising contrast agent for magnetic resonance urography.

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.