Pharmacokinetics, biodistribution and contrast enhanced MR blood pool imaging of Gd-DTPA cystine copolymers and Gd-DTPA cystine diethyl ester copolymers in a rat model

Fe3+@PDOPA‑b‑PSar Nanoparticles for Magnetic Resonance Imaging and Cancer Chemotherapy

Purpose

Chemotherapy treatments for cancer are always accompanied by a low concentration of drug delivered in the tumor area and severe side effects including systemic toxicity. Improving the concentration, biocompatibility, and biodegradability of regional chemotherapy drugs is a pressing challenge in the field of materials.

Methods

N-Phenyloxycarbonyl-amino acids (NPCs) which exhibit significant tolerance to nucleophiles, such as water and hydroxyl-containing compounds, are promising monomers for the synthesis of polypeptides and polypeptoids. Cell line and mouse models were used to comprehensively explore how to enhance the tumor MRI signal and evaluate the therapeutic effect of Fe@POS-DOX nanoparticles.

Results

In this study, poly(3,4-dihydroxy-_L-phenylalanine)-b-polysarcosine (PDOPA-b-PSar, simplified as POS) was synthesized by the block copolymerization of DOPA-NPC with Sar-NPC. Fe@POS-DOX nanoparticles were prepared in order to utilize the strong chelation of catechol ligands to iron (III) cations and the hydrophobic interaction between DOX and DOPA block to deliver chemotherapeutics to tumor tissue. The Fe@POS-DOX nanoparticles exhibit high longitudinal relaxivity (r ₁ = 7.06 mM^-1·s^-1) and act as T ₁-weighted magnetic resonance (MR) imaging contrast agents. Further, the main focus was improving tumor site-specific bioavailability and achieving therapeutic effects through the biocompatibility and biodegradability of Fe@POS-DOX NPs. The Fe@POS-DOX treatment exhibited excellent antitumor effects.

Conclusion

Upon intravenous injection, Fe@POS-DOX delivers DOX specifically to the tumor tissues, as revealed by MR, and leads to the inhibition of tumor growth without overt toxicity to normal tissues, thus displaying considerable potential for use in clinical applications.

EDTMP ligand-enhanced water interactions endowing iron oxide nanoparticles with dual-modal MRI contrast ability

Single-modal magnetic resonance imaging (MRI) contrast agents sometimes cause signal confusion in clinical diagnosis. Utilizing ligands to endow iron oxide nanoparticles (IO NPs) with excellent dual-modal MRI contrast efficiency might be an effective strategy to improve diagnostic accuracy. This work presents the development of a special ligand-assisted one-pot approach for the preparation of super-hydrophilic magnetic NPs with excellent water dispersion, biocompatibility and T₁-T₂ dual-modal contrast enhancement properties. In addition, the strong binding capacity between the ethylenediamine tetramethylenephosphonic acid (EDTMP) ligand and water molecules induced by the presence of abundant hydrogen bonds significantly improves spin-lattice (T₁) and spin-spin (T₂) imaging of the IO core. After being modified with the EDTMP ligand, the T₂ relaxation rate of the IO core is dramatically increased from 71.78 mM^-1 s^-1 to 452.38 mM^-1 s^-1, and a moderate T₁ relaxation rate (11.61 mM^-1 s^-1) is observed simultaneously, implying that the NPs with an average size of 9.7 nm may be potential candidates as high-efficiency T₁-T₂ MRI contrast agents. This fundamental technique of using super-hydrophilicity ligands to endow IO NPs with dual-modal contrast properties without size change and damage in the T₂ contrast effect may provide a useful strategy to facilitate the application of magnetic NPs in the field of medical diagnosis.

Construction of arsenic-metal complexes loaded nanodrugs for solid tumor therapy: A mini review

Arsenic trioxide (As₂O₃), a front-line therapeutic agent against acute promyelocytic leukemia, has a broad spectrum against malignancies. Unfortunately, the clinical application of As₂O₃ in treating hematological cancers has not been transformed to solid tumors, for its dose-limited toxicity and undesirable pharmacokinetics. The ordinary As₂O₃ loaded nanodrugs (such as liposomes, polymer micelles, albumin-based nanodrugs, and silica-based nanodrugs, etc.) still could not fuel up pharmaceuticals and eradicate toxicity for low delivery efficiency caused by the instability and severe drug leakage of formulations during circulation. Recently, the approach of forming and delivering arsenic-metal complexes which will dissociate in the tumoral environment caught our mind. This is the most effective strategy to reduce drug leakage in circulation and accumulate arsenite ions in tumor sites, therefore promote the anti-tumor effect and lighten the toxicity of the drug. This review aims to explain the formation mechanism of arsenic-metal nanocomposites and summarize the constructing strategies of the arsenic-metal nanocomplexes (arsenic-nickel, arsenic-manganese, arsenic-platinum, arsenic-gadolinium, arsenic-zinc, and arsenic-iron nanobins) loaded nanodrugs for solid tumor therapy. Furthermore, the expectations and challenges of arsenic-metal complexes containing nanodrugs for cancer therapy in the future were discussed.

A novel manganese chelated macromolecular MRI contrast agent based on O-carboxymethyl chitosan derivatives

Currently used Gd-based and Mn-based small molecular MRI contrast agents fail to meet the requirements for the long-term monitoring, and the potential safety risk under high administration dose or repeat dosing needs to be considered. In the present study, a biocompatible macromolecular magnetic resonance imaging (MRI) contrast agents based on O-carboxymethyl chitosan (CMCS), CMCS-(Mn-DTPA)_n was designed and synthesized. The relaxivity of CMCS-(Mn-DTPA)_n is approximately 3.5 and 5.5 times higher than that of Gd-DTPA and Mn-DPDP in aqueous solution, respectively. The MRI signal intensity in the kidney and liver of Sprague Dawley (SD) rats is significantly increased at a dose of 0.03 mM Mn/kg b.w. CMCS-(Mn-DTPA)_n accompanied by a long effective imaging window. According to in vitro studies, CMCS-(Mn-DTPA)_n exhibits good cellular and blood biocompatibility at the dose necessary for MRI imaging. Based on the results from in vivo studies, manganese (Mn) is completely excreted from SD rats within ten days after administration and does not exert a pathological effect on the liver. CMCS-(Mn-DTPA)_n represents a potentially novel MRI contrast agent due to its excellent relaxivity, long effective imaging window and good biocompatibility.

Gastric Parietal Cell and Intestinal Goblet Cell Secretion: a Novel Cell-Mediated In Vivo Metal Nanoparticle Metabolic Pathway Enhanced with Diarrhea Via Chinese Herbs

Up to date, the way in which metal nanoparticles are cleared in vivo has yet to be elucidated well. Herein, we report a novel intestinal goblet cell-mediated in vivo clearance pathway to remove metal nanoparticles. Typical metal nanoparticles such as triangular silver nanoplates, magnetic nanoparticles, gold nanorods, and gold nanoclusters were selected as representative examples. These metal nanoparticles were prepared, characterized, and injected via tail vein into a mice model with common bile duct (CBD) ligation. The feces and urines were collected for 7 days to be followed by the sacrifice of the mice and collection of the intestinal and gastric tissues for further analysis. The results showed that all four selected metal nanoparticles were located inside the goblet cells (GCs) of the whole intestinal tissue and were excreted into the gut lumen through the secretion of intestinal GC. Moreover, triangular silver nanoplates and gold nanorods were located inside the gastric parietal cells (PCs). Importantly, nanoparticles did not cause obvious pathological changes in intestinal tissues. In this study, we confirmed that the blood corpuscles are involved in the GCs secretion pathway. Furthermore, we found that the secretion of nanoparticles from intestinal GCs and PCs is accelerated by diarrhea induced via Chinese herbs. In conclusion, metal nanoparticles such as triangular silver nanoplates, magnetic nanoparticles, gold nanorods, and gold nanoclusters can be cleaned away by intestinal GCs and PCs. This novel pathway of in vivo clearance of metal nanoparticles has a great potential for future applications such as new drug design and development, nanoparticle-based labeling and in vivo tracking, and biosafety evaluation of in vivo nanoparticles.

Gadolinium accumulation in organs of Sprague-Dawley® rats after implantation of a biodegradable magnesium-gadolinium alloy

Biodegradable magnesium implants are under investigation because of their promising properties as medical devices. For enhancing the mechanical properties and the degradation resistance, rare earth elements are often used as alloying elements. In this study Mg10Gd pins were implanted into Sprague-Dawley® rats. The pin volume loss and a possible accumulation of magnesium and gadolinium in the rats' organs and blood were investigated in a long-term study over 36weeks. The results showed that Mg10Gd is a fast disintegrating material. Already 12weeks after implantation the alloy is fragmented to smaller particles, which can be found within the intramedullary cavity and the cortical bones. They disturbed the bone remodeling until the end of the study. The results concerning the elements' distribution in the animals' bodies were even more striking, since an accumulation of gadolinium could be observed in the investigated organs over the whole time span. The most affected tissue was the spleen, with up to 3240μgGd/kg wet mass, followed by the lung, liver and kidney (up to 1040, 685 and 207μgGd/kg). In the brain, muscle and heart, the gadolinium concentrations were much smaller (less than 20μg/kg), but an accumulation could still be detected. Interestingly, blood serum samples showed no accumulation of magnesium and gadolinium. This is the first time that an accumulation of gadolinium in animal organs was observed after the application of a gadolinium-containing degradable magnesium implant. These findings demonstrate the importance of future investigations concerning the distribution of the constituents of new biodegradable materials in the body, to ensure the patients' safety.

STATEMENT OF SIGNIFICANCE

In the last years, biodegradable Mg alloys are under investigation due to their promising properties as orthopaedic devices used for bone fracture stabilization. Gadolinium as Rare Earth Element enhances the mechanical properties of Mg-Gd alloys but its toxicity in humans is still questionable. Up to now, there is no study investigating the elements' metabolism of a REE-containing Magnesium alloy in an animal model. In this study, we examined the gadolinium distribution and accumulation in rat organs during the degradation of Mg10Gd. Our findings showed that Gd is accumulating in the animal organs, especially in spleen, liver and kidney. This study is of crucial benefit regarding a safe application of REE-containing Magnesium alloys in humans.

Au Nanocage Functionalized with Ultra-small Fe3O4 Nanoparticles for Targeting T1-T2Dual MRI and CT Imaging of Tumor

Diagnostic approaches based on multimodal imaging of clinical noninvasive imaging (eg. MRI/CT scanner) are highly developed in recent years for accurate selection of the therapeutic regimens in critical diseases. Therefore, it is highly demanded in the development of appropriate all-in-one multimodal contrast agents (MCAs) for the MRI/CT multimodal imaging. Here a novel ideal MCAs (F-AuNC@Fe₃O₄) were engineered by assemble Au nanocages (Au NC) and ultra-small iron oxide nanoparticles (Fe₃O₄) for simultaneous T₁–T₂dual MRI and CT contrast imaging. In this system, the Au nanocages offer facile thiol modification and strong X-ray attenuation property for CT imaging. The ultra-small Fe₃O₄ nanoparticles, as excellent contrast agent, is able to provide great enhanced signal of T₁- and T₂-weighted MRI (r₁ = 6.263 mM⁻¹ s⁻¹, r₂ = 28.117 mM⁻¹ s⁻¹) due to their ultra-refined size. After functionalization, the present MCAs nanoparticles exhibited small average size, low aggregation and excellent biocompatible. In vitro and In vivo studies revealed that the MCAs show long-term circulation time, renal clearance properties and outstanding capability of selective accumulation in tumor tissues for simultaneous CT imaging and T₁- and T₂-weighted MRI. Taken together, these results show that as-prepared MCAs are excellent candidates as MRI/CT multimodal imaging contrast agents.

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.

Real-time monitoring of arsenic trioxide release and delivery by activatable T(1) imaging

Delivery of arsenic trioxide (ATO), a clinical anticancer drug, has drawn much attention to improve its pharmacokinetics and bioavailability for efficient cancer therapy. Real-time and in situ monitoring of ATO behaviors in vivo is highly desirable for efficient tumor treatment. Herein, we report an ATO-based multifunctional drug delivery system that efficiently delivers ATO to treat tumors and allows real-time monitoring of ATO release by activatable T1 imaging. We loaded water-insoluble manganese arsenite complexes, the ATO prodrug, into hollow silica nanoparticles to form a pH-sensitive multifunctional drug delivery system. Acidic stimuli triggered the simultaneous release of manganese ions and ATO, which dramatically increased the T1 signal (bright signal) and enabled real-time visualization and monitoring of ATO release and delivery. Moreover, this smart multifunctional drug delivery system significantly improved ATO efficacy and strongly inhibited the growth of solid tumors without adverse side effects. This strategy has great potential for real-time monitoring of theranostic drug delivery in cancer diagnosis and therapy.

One-step synthesis of water-dispersible ultra-small Fe3O4 nanoparticles as contrast agents for T1 and T2 magnetic resonance imaging

Uniform, highly water-dispersible and ultra-small Fe3O4 nanoparticles were synthesized via a modified one-step coprecipitation approach. The prepared Fe3O4 nanoparticles not only show good magnetic properties, long-term stability in a biological environment, but also exhibit good biocompatibility in cell viability and hemolysis assay. Due to the ultra-small sized and highly water-dispersibility, they exhibit excellent relaxivity properties, the 1.7 nm sized Fe3O4 nanoparticles reveal a low r2/r1 ratio of 2.03 (r1 = 8.20 mM(-1) s(-1), r2 = 16.67 mM(-1) s(-1)); and the 2.2 nm sized Fe3O4 nanoparticles also appear to have a low r2/r1 ratio of 4.65 (r1 = 6.15 mM(-1) s(-1), r2 = 28.62 mM(-1) s(-1)). This demonstrates that the proposed ultra-small Fe3O4 nanoparticles have great potential as a new type of T1 magnetic resonance imaging contrast agents. Especially, the 2.2 nm sized Fe3O4 nanoparticles, have a competitive r1 value and r2 value compared to commercial contrasting agents such as Gd-DTPA (r1 = 4.8 mM(-1) s (-1)), and SHU-555C (r2 = 69 mM(-1) s(-1)). In vitro and in vivo imaging experiments, show that the 2.2 nm sized Fe3O4 nanoparticles exhibit great contrast enhancement, long-term circulation, and low toxicity, which enable these ultra-small sized Fe3O4 nanoparticles to be promising as T1 and T2 dual contrast agents in clinical settings.

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.

Secretion of intestinal goblet cells: a novel excretion pathway of nanoparticles

Understanding the excretion pathway is one of the most important prerequisites for the safe use of nanoparticles in biomedicine. However, the excretion of nanoparticles in animals remains largely unknown, except for some particles very small in size. Here we report a novel natural pathway for nanoparticle excretion, the intestinal goblet cell (GC) secretion pathway (IGCSP). Direct live observation of the behavior of 30-200nm activated carbon nanoparticles (ACNP) demonstrated that ACNP microinjected into the yolk sac of zebrafish can be excreted directly through intestinal tract without involving the hepato-biliary (hap-bile) system. Histopathological examination in mice after ligation of the common bile duct (CBD) demonstrated that the intravenously-injected ACNP were excreted into the gut lumen through the secretion of intestinal GCs. ACNP in various secretion phases were revealed by histopathological examination and transmission electron microscopy (TEM). IGCSP, in combination with renal and hap-bile pathways, constitutes a complete nanoparticle excretion mechanism.

FROM THE CLINICAL EDITOR

Nanoparticle elimination pathways are in the forefront of interest in an effort to optimize and enable nanomedicine applications. This team of authors reports a novel natural pathway for nanoparticle excretion, the intestinal goblet cell (GC) secretion pathway (IGCSP). Direct live observation of the behavior of activated carbon nanoparticles has shown excretion directly through the intestinal tract without involving the hepato-biliary (hap-bile) system in a zebrafish model.

Engineered iron-oxide-based nanoparticles as enhanced T1 contrast agents for efficient tumor imaging

We report the design and synthesis of small-sized zwitterion-coated gadolinium-embedded iron oxide (GdIO) nanoparticles, which exhibit a strong T1 contrast effect for tumor imaging through enhanced permeation and retention effect and the ability to clear out of the body in living subjects. The combination of spin-canting effects and the collection of gadolinium species within small-sized GdIO nanoparticles led to a significantly enhanced T1 contrast effect. For example, GdIO nanoparticles with a diameter of ∼4.8 nm exhibited a high r1 relaxivity of 7.85 mM(-1)·S(-1) and a low r2/r1 ratio of 5.24. After being coated with zwitterionic dopamine sulfonate molecules, the 4.8 nm GdIO nanoparticles showed a steady hydrodynamic diameter (∼5.2 nm) in both PBS buffer and fetal bovine serum solution, indicating a low nonspecific protein absorption. This study provides a valuable strategy for the design of highly sensitive iron-oxide-based T1 contrast agents with relatively long circulation half-lives (∼50 min), efficient tumor passive targeting (SKOV3, human ovarian cancer xenograft tumor as a model), and the possibility of rapid renal clearance after tumor imaging.

Macromolecular Imaging Agents Containing Lanthanides: Can Conceptual Promise Lead to Clinical Potential?

Macromolecular magnetic resonance imaging (MRI) contrast agents are increasingly being used to improve the resolution of this noninvasive diagnostic technique. All clinically-approved T₁ contrast agents are small molecule chelates of gadolinium [Gd(III)] that affect bound water proton relaxivity. Both the small size and monomeric nature of these agents ultimately limits the image resolution enhancement that can be achieved for both contrast enhancement and pharmacokinetic/biodistribution reasons. The multimeric nature of macromolecules, such as polymers, dendrimers, and noncovalent complexes of small molecule agents with proteins, have been shown to significantly increase the image contrast and resolution due to their large size and ability to incorporate multiple Gd(III) chlelation sites. Also, macromolecular agents are advantageous as they have the ability to be designed to be nontoxic, hydrophilic, easily purified, aggregation-resistant, and have controllable three-dimensional macromolecular structure housing the multiple lanthanide chelation sites. For these reasons, large molecule diagnostics have the ability to significantly increase the relaxivity of water protons within the targeted tissues and thus the image resolution for many diagnostic applications. The FDA approval of a contrast agent that consists of a reversible, non-covalent coupling of a small Gd(III) chelate with serum albumin for blood pool imaging (marketed under the trade names of Vasovist and Ablivar) proved to be one of the first diagnostic agent to capitalize on these benefits from macromolecular association in humans. However, much research and development is necessary to optimize the safety of these unique agents for in vivo use and potential clinical development. To this end, recent work in the field of polymer, dendrimer, and noncovalent complex-based imaging agents are reviewed herein and the future outlook of this field is 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.

Multifunctional mesoporous silica nanospheres with cleavable Gd(III) chelates as MRI contrast agents: synthesis, characterization, target-specificity, and renal clearance

Mesoporous silica nanospheres (MSNs) are a promising material for magnetic resonance imaging (MRI) contrast agents. In this paper multifunctional MSNs with cleavable Gd(III) chelates are synthesized and characterized, and their applicability as MRI contrast agents is demonstrated both in vitro and in vivo. The MSNs contain Gd(III) chelates that are covalently linked via a redox-responsive disulfide moiety. The MSNs are further functionalized with polyethylene glycol (PEG) and an anisamide ligand to improve their biocompatibility and target specificity. The effectiveness of MSNs as an MRI imaging contrast agent and their targeting ability are successfully demonstrated in vitro using human colon adenocarcinoma and pancreatic cancer cells. Finally, the capability of this platform as an in vivo MRI contrast agent is tested using a 3T scanner. The Gd(III) chelate was quickly cleaved by the blood pool thiols and eliminated through the renal excretion pathway. Further tuning of the Gd(III) chelate release kinetics is needed before the MSN system can be used as target-specific MRI contrast agents in vivo.

Strategies for the preparation of bifunctional gadolinium(III) chelators

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.

Stability and biodistribution of a biodegradable macromolecular MRI contrast agent Gd-DTPA cystamine copolymers (GDCC) in rats

PURPOSE

The aim of this study was to evaluate stability and Gd tissue distribution of a biodegradable macromolecular MRI contrast agent, GDCC.

METHODS

Kinetic stability of GDCC was evaluated based on transmetallation with endogenous metal ions Zn2+ and Cu2+ in rat plasma in comparison with Omniscan, MultiHance and ProHance. In vivo transmetallation of GDCC was evaluated by determining metal content in the urine samples of Spague-Dawley rats. The biodistribution of the agents was determined in rats at 48 h post-injection.

RESULTS

A new method of using ultrafiltration was developed for study of kinetic stability against transmetallation of Gd(III)-based MRI contrast agents. Both in vitro and in vivo stability of the contrast agents towards transmetallation with Zn2+ were in the order of ProHance > MultiHance approximately GDCC > Omniscan. No significant transmetallation with Cu2+ was observed for the contrast agents. GDCC had comparable retention to the control agents in most organs and tissues with slightly high retention in the liver and kidneys at 48 h post-injection.

CONCLUSION

Ultrafiltration is efficient and accurate for characterizing the kinetic stability of Gd(III)-based MRI contrast agents. The novel biodegradable macromolecular contrast agent GDCC is promising for further development for contrast enhanced MRI.

Macromolecular and dendrimer-based magnetic resonance contrast agents

Magnetic resonance imaging (MRI) is a powerful imaging modality that can provide an assessment of function or molecular expression in tandem with anatomic detail. Over the last 20-25 years, a number of gadolinium-based MR contrast agents have been developed to enhance signal by altering proton relaxation properties. This review explores a range of these agents from small molecule chelates, such as Gd-DTPA and Gd-DOTA, to macromolecular structures composed of albumin, polylysine, polysaccharides (dextran, inulin, starch), poly(ethylene glycol), copolymers of cystamine and cystine with GD-DTPA, and various dendritic structures based on polyamidoamine and polylysine (Gadomers). The synthesis, structure, biodistribution, and targeting of dendrimer-based MR contrast agents are also discussed.

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.

Biodistribution of gadolinium-based contrast agents, including gadolinium deposition

The biodistribution of approved gadolinium (Gd)-based contrast agents (GBCAs) is reviewed. After intravenous injection GBCAs distribute in the blood and the extracellular space and transiently through the excretory organs. Preclinical animal studies and the available clinical literature indicate that all these compounds are excreted intact. Elimination tends to be rapid and, for the most part, complete. In renally insufficient patients the plasma elimination half-life increases substantially from hours to days depending on renal function. In patients with impaired renal function and nephrogenic systemic fibrosis (NSF), the agents gadodiamide, gadoversetamide, and gadopentetate dimeglumine have been shown to result in Gd deposition in the skin and internal organs. In these cases, it is likely that the Gd is no longer present as the GBCA, but this has still not been definitively shown. In preclinical models very small amounts of Gd are retained in the bone and liver, and the amount retained correlates with the kinetic and thermodynamic stability of the GBCA with respect to Gd release in vitro. The pattern of residual Gd deposition in NSF subjects may be different than that observed in preclinical rodent models. GBCAs are designed to be used via intravenous administration. Altering the route of administration and/or the formulation of the GBCA can dramatically alter the biodistribution of the GBCA and can increase the likelihood of Gd deposition. J. Magn. Reson. Imaging 2009;30:1259-1267. (c) 2009 Wiley-Liss, Inc.

Application of a biodegradable macromolecular contrast agent in dynamic contrast-enhanced MRI for assessing the efficacy of indocyanine green-enhanced photothermal cancer therapy

PURPOSE

To investigate the effectiveness of a polydisulfide-based biodegradable macromolecular contrast agent, (Gd-DTPA)-cystamine copolymers (GDCC), in assessing the efficacy of indocyanine green-enhanced photothermal cancer therapy using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

MATERIALS AND METHODS

Breast cancer xenografts in mice were injected with indocyanine green and irradiated with a laser. The efficacy was assessed using DCE-MRI with GDCC of 40 kDa (GDCC-40) at 4 hours and 7 days after the treatment. The uptake of GDCC-40 by the tumors was fit to a two-compartment model to obtain tumor vascular parameters, including fractional plasma volume (f(PV)), endothelium transfer coefficient (K(PS)), and permeability surface area product (PS).

RESULTS

GDCC-40 resulted in similar tumor vascular parameters at three doses, with larger standard deviations at lower doses. The values of f(PV), K(PS), and PS of the treated tumors were smaller (P < 0.05) than those of untreated tumors at 4 hours after the treatment and recovered to pretreatment values (P > 0.05) at 7 days after the treatment.

CONCLUSION

DCE-MRI with GDCC-40 is effective for assessing tumor early response to dye-enhanced photothermal therapy and detecting tumor relapse after the treatment. GDCC-40 has a potential to noninvasively monitor anticancer therapies with DCE-MRI.

Tumor characterization with dynamic contrast enhanced magnetic resonance imaging and biodegradable macromolecular contrast agents in mice

PURPOSE

To investigate the efficacy of polydisulfide-based biodegradable macromolecular contrast agents of different degradability and molecular weight for tumor characterization based on angiogenesis using dynamic contrast enhanced MRI (DCE-MRI).

METHODS

Biodegradable macromolecular MRI contrast agents, Gd-DTPA cystamine copolymers (GDCC) and Gd-DTPA cystine copolymers (GDCP), with molecular weight of 20 and 70 KDa were evaluated for tumor characterization. Gd(DTPA-BMA) and a prototype of macromolecular contrast agent, albumin-(Gd-DTPA), were used as controls. The DCE-MRI studies were performed in nude mice bearing MDA PCa 2b and PC-3 human prostate tumor xenografts. Tumor angiogenic kinetic parameters including endothelium transfer coefficient (K(trans)) and fractional tumor plasma volume (f(PV)) were calculated from the DCE-MRI data using a two-compartment model and compared between the two different tumor models for each contrast agent.

RESULTS

There was no significant difference in the f(PV) values between two tumor models estimated with the same agent except for GDCC-70. The K(trans) values in both tumor models decreased with the increase of molecular weight of contrast agents. With the same high molecular weight (70 KDa), GDCC-70 showed a higher K(trans) values than GDCP-70 due to high degradability of the former in both tumor models (p < 0.05). The K(trans) values of MDA PCa 2b tumors were significantly higher than those of PC-3 tumors estimated by Gd(DTPA-BMA), GDCC-20, GDCC-70, GDCP-70, and albumin-(Gd-DTPA) (p < 0.05).

CONCLUSIONS

The polydisulfide-based biodegradable macromolecular MRI contrast agents are promising in tumor characterization and differentiation with dynamic contrast enhanced MRI.

Characterization of tumor angiogenesis with dynamic contrast-enhanced MRI and biodegradable macromolecular contrast agents in mice

The efficacy of polydisulfide-based biodegradable macromolecular contrast agents for characterizing tumor angiogenesis was investigated in a mouse model using dynamic contrast-enhanced MRI (DCE-MRI). Biodegradable macromolecular MRI contrast agents, gadopentetate dimeglumine (Gd-DTPA) cystamine copolymers (GDCC), and Gd-DTPA cystine copolymers (GDCP), with molecular weights of 20 and 70 kDa were used in the study. Gadodiamide (Gd [DTPA-BMA]) and albumin labeled with Gd-DTPA [albumin-(Gd-DTPA)] were used as the controls. The DCE-MRI studies were performed in nude mice bearing prostate tumor xenografts from the MDA-PCa-2b cell line. Tumor angiogenic kinetic parameters, including endothelial transfer coefficient (K(PS)), fractional tumor plasma volume (f(PV)), and permeability surface area product (PS), were estimated from the DCE-MRI data using a two-compartment model. The K(PS) and f(PV) values estimated by the biodegradable macromolecular contrast agents were between those estimated by Gd(DTPA-BMA) and albumin-(Gd-DTPA). The parameters estimated by the agent with a slow degradation rate and high molecular weight, GDCP-70 (K(PS) = 2.09 +/- 0.50 ml/min/100 cc and f(PV) = 0.075 +/- 0.021), were closer to those by albumin-(Gd-DTPA) (K(PS) = 1.43 +/- 0.64 ml/min/100 cc and f(PV) = 0.044 +/- 0.007) than by other agents with relatively low molecular weight or rapid degradation rate. The polydisulfide-based biodegradable macromolecular contrast agents are promising for characterizing tumor vascularity and angiogenesis with DCE-MRI.

Clearance properties of nano-sized particles and molecules as imaging agents: considerations and caveats

Nanoparticles possess enormous potential as diagnostic imaging agents and hold promise for the development of multimodality agents with both imaging and therapeutic capabilities. Yet, some of the most promising nanoparticles demonstrate prolonged tissue retention and contain heavy metals. This presents serious concerns for toxicity. The creation of nanoparticles with optimal clearance characteristics will minimize toxicity risks by reducing the duration of exposure to these agents. Given that many nanoparticles possess easily modifiable surface and interior chemistry, if nanoparticle characteristics associated with optimal clearance from the body were well established, it would be feasible to design and create agents with more favorable clearance properties. This article presents a thorough discussion of the physiologic aspects of nanoparticle clearance, focusing on renal mechanisms, and provides an overview of current research investigating clearance of specific types of nanoparticles and nano-sized macromolecules, including dendrimers, quantum dots, liposomes and carbon, gold and silica-based nanoparticles.

Synthesis and evaluation of globular Gd-DOTA-monoamide conjugates with precisely controlled nanosizes for magnetic resonance angiography

The purpose of this study was to design and prepare macromolecular contrast agents (CAs) with a precisely defined globular structure for MR angiography and tumor angiogenesis imaging. Generations 1 through 3 (Gd-DOTA-monoamide)-poly-L-lysine octasilsesquioxane dendrimers were prepared as nanoglobular MRI CAs. The nanoglobular Gd(III) chelates had a well-defined compact globular structure and high loading of Gd-DOTA-monoamide at their surface. The size of the G1, G2, and G3 nanoglobular MRI CAs was approximately 2.0, 2.4, and 3.2 nm, respectively. The T1 relaxivity of G1, G2, and G3 nanoglobular MRI CAs was approximately 6.4, 7.2, and 10.0 mM(-1) sec(-1) at 3T, respectively. The nanoglobular MRI CAs showed size-dependent contrast enhancement within the mouse vasculature, which gradually decayed to baseline after a 60 min session. The G3 nanoglobular CA resulted in more significant and prolonged vascular enhancement than the smaller nanoglobular agents at 0.03 mmol Gd/kg. The G3 agent also provided significant and prolonged contrast enhancement in the heart and vasculature at a dose as low as 0.01 mmol Gd/kg, 1/10th of the regular clinical dose. Significant enhancement was observed in tumor for all CAs. The nanoglobular CAs cleared via renal filtration and accumulated in the urinary bladder as shown in the dynamic MR images. The nanoglobular Gd(III) chelates are effective intravascular MRI CAs at substantially reduced doses. The nanoglobular MRI CAs are promising for further preclinical development for MR angiography and MR imaging of tumor angiogenesis.

Contrast-enhanced MRI-guided photodynamic cancer therapy with a pegylated bifunctional polymer conjugate

PURPOSE

To study contrast-enhanced MRI guided photodynamic therapy with a pegylated bifunctional polymer conjugate containing an MRI contrast agent and a photosensitizer for minimally invasive image-guided cancer treatment.

METHODS

Pegylated and non-pegylated poly-(L-glutamic acid) conjugates containing mesochlorin e6, a photosensitizer, and Gd(III)-DO3A, an MRI contrast agent, were synthesized. The effect of pegylation on the biodistribution and tumor targeting was non-invasively visualized in mice bearing MDA-MB-231 tumor xenografts with MRI. MRI-guided photodynamic therapy was carried out in the tumor bearing mice. Tumor response to photodynamic therapy was evaluated by dynamic contrast enhanced MRI and histological analysis.

RESULTS

The pegylated conjugate had longer blood circulation, lower liver uptake and higher tumor accumulation than the non-pegylated conjugate as shown by MRI. Site-directed laser irradiation of tumors resulted in higher therapeutic efficacy for the pegylated conjugate than the non-pegylated conjugate. Moreover, animals treated with photodynamic therapy showed reduced vascular permeability on DCE-MRI and decreased microvessel density in histological analysis.

CONCLUSIONS

Pegylation of the polymer bifunctional conjugates reduced non-specific liver uptake and increased tumor uptake, resulting in significant tumor contrast enhancement and high therapeutic efficacy. The pegylated poly(L-glutamic acid) bifunctional conjugate is promising for contrast enhanced MRI guided photodynamic therapy in cancer treatment.

In Vivo evaluation of a PAMAM-cystamine-(Gd-DO3A) conjugate as a biodegradable macromolecular MRI contrast agent

Macromolecular Gd(III) chelates are superior magnetic resonance imaging (MRI) contrast agents for blood pool and tumor imaging. However, their clinical development is limited by the safety concerns related to the slow excretion and long-term gadolinium tissue accumulation. A generation 6 PAMAM Gd(III) chelate conjugate with a cleavable disulfide spacer, PAMAM-G6-cystamine-(Gd-DO3A), was prepared as a biodegradable macromolecular MRI contrast agent with rapid excretion from the body. T(1) and T(2) relaxivities of the contrast agent were 11.6 and 13.3 mM(-1)sec(-1) at 3T, respectively. Blood pool and tumor contrast enhancement of the agent were evaluated in female nude mice bearing MDA-MB-231 human breast carcinoma xenografts with a nondegradable conjugate PAMAM-G6-(Gd-DO3A) as a control. PAMAM-G6-cystamine-(Gd-DO3A) resulted in significant contrast enhancement in the blood for about 5 mins, and Gd-DO3A was released from the conjugate and rapidly excreted via renal filtration after the disulfide spacer was cleaved. The nondegradable control had much longer blood circulation and excreted more slowly from the body. PAMAM-G6-cystamine-(Gd-DO3A) also resulted in more prominent tumor contrast enhancement than the control. However, PAMAM-G6-cystamine-(Gd-DO3A) demonstrated high toxicity due to the intrinsic toxicity of PAMAM dendrimers. In conclusion, although PAMAM-G6-cystamine-(Gd-DO3A) showed some advantages compared with the nondegradable control, PAMAM dendrimers are not suitable carriers for biodegradable macromolecular MRI contrast agents, due to their high toxicity.

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

The purpose of this study was to investigate the effect of PEGylation of novel biodegradable macromolecular polydisulfide Gd(III) complexes, gadolinium diethylenetriaminepentaacetate (GdDTPA) cystine copolymers (GDCP), on their pharmacokinetics and long-term Gd(III) tissue retention, and to demonstrate the potential application of PEGylated GDCP (PEG-GDCP) for MR angiography (MRA). The pharmacokinetics, biodistribution, and metabolic excretion of PEG(1000)-GDCP (42.1-52.1 kDa; PEG: MW = 1000 Da) with three different PEG grafting degrees and GDCP (43.3 kDa) were investigated in Sprague-Dawley rats. Pharmacokinetic data were analyzed by means of an open two-compartment model. Initially all three PEG(1000)-GDCP contrast agents (CAs) had a higher plasma concentration than GDCP, but after 30 min the Gd(III) concentration from the PEGylated agents rapidly decreased, resulting in significantly lower elimination half-life values. All of the biodegradable macromolecular CAs demonstrated low long-term Gd(III) tissue accumulation, while PEG(1000)-GDCP had significantly lower accumulation in the liver than GDCP. In the rats, all CAs showed excellent vascular contrast enhancement in an MRA protocol with a long image acquisition time. Because PEG(1000)-GDCP remained intravascular for an acceptable period for effective contrast-enhanced (CE)-MRA, and then excreted rapidly from the vasculature with minimal tissue retention, PEG(1000)-GDCP shows a great promise as a blood-pool CA for MRA.

Gadolinium(III)-based blood-pool contrast agents for magnetic resonance imaging: status and clinical potential

Blood-pool MRI contrast agents have enormous potential to aid sensitive magnetic resonance detection and yield definitive diagnostic data of cancer and diseases of the cardiovascular system. Many attempts have been initiated to design macromolecular gadolinium (Gd[III]) complexes as magnetic resonance imaging blood-pool contrast agents, as macromolecules do not readily diffuse across healthy vascular endothelium, and remain intravascular. Although extremely efficacious in detecting and characterizing pathologic tissue, clinical development of these agents has been limited by potential toxicity concerns from incomplete Gd(III) clearance. Recent innovative technologies, such as reversible protein-binding contrast agents and biodegradable macromolecular contrast agents, may be valuable alternatives that combine the effective imaging characteristics of an intravascular contrast agent and the safety of clinically approved low-molecular-weight Gd(III) chelates.

Noninvasive Visualization of Pharmacokinetics, Biodistribution and Tumor Targeting of Poly[N-(2-hydroxypropyl)methacrylamide] in Mice Using Contrast Enhanced MRI

PURPOSE

To study a non-invasive method of using contrast enhanced magnetic resonance imaging (MRI) to visualize the real-time pharmacokinetics, biodistribution and tumor accumulation of paramagnetically labeled poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA) copolymer conjugates with different molecular weights and spacers in tumor-bearing mice.

MATERIALS AND METHODS

Paramagnetically labeled HPMA copolymer conjugates were synthesized by free radical copolymerization of HPMA with monomers containing a chelating ligand, followed by complexation with Gd(OAc)(3). A stable paramagnetic chelate, Gd-DO3A, was conjugated to the copolymers via a degradable spacer GlyPheLeuGly and a non-degradable spacer GlyGly, respectively. The conjugates with molecular weights of 28, 60 and 121 kDa and narrow molecular weight distributions were prepared by fractionation with size exclusion chromatography. The conjugates were injected into athymic nude mice bearing MDA-MB-231 human breast carcinoma xenografts via a tail vein. MR images were acquired before and at various time points after the injection with a 3D FLASH sequence and a 2D spin-echo sequence at 3T. Pharmacokinetics, biodistribution and tumor accumulation of the conjugates were visualized based on the contrast enhancement in the blood, major organs and tumor tissue at various time points. The size effect of the conjugates was analyzed among the conjugates.

RESULTS

Contrast enhanced MRI resulted in a real-time, three-dimensional visualization of blood circulation, pharmacokinetics, biodistribution and tumor accumulation of the conjugates, and the size effect on these pharmaceutical properties. HPMA copolymer conjugates with high molecular weight had a prolonged blood circulation time and high passive tumor targeting efficiency. Non-biodegradable HPMA copolymers with molecular weights higher than the threshold of renal filtration demonstrated higher efficiency for tumor drug delivery than biodegradable poly(L-glutamic acid).

CONCLUSIONS

Contrast enhanced MRI is an effective method for non-invasive visualization of in vivo properties of the paramagnetically labeled polymer conjugates in preclinical studies.