In vivo micro-computed tomography evaluation of radiopaque, polymeric device degradation in normal and inflammatory environments

Polymeric biomedical implants are an important clinical tool, but degradation remains difficult to determine post-implantation. Computed tomography (CT) could be a powerful tool for device monitoring, but polymers require incorporation of radiopaque contrast agents to be distinguishable from tissue. In addition, immune response to radiopaque devices must be characterized as it modulates device function. Radiopaque devices and films were produced by incorporating 0-20 wt% TaOx nanoparticles into polymers: polycaprolactone (PCL) and poly(lactide-co-glycolide) (PLGA). In vitro inflammatory responses of mouse bone marrow-derived macrophages to polymer matrix incorporating TaOx nanoparticles was determined by monitoring cytokine secretion. Nanoparticle addition stimulated a slight inflammatory reaction, increasing TNFα secretion, mediated by changes in polymer matrix properties. Subsequently, devices (PLGA 50:50 + 20 wt% TaOx) were implanted subcutaneously in a mouse model of chronic inflammation, that featured a sustained increase in inflammatory response local to the implant site over 12 weeks. No changes to device degradation rates or foreign body response were noted between a normal and chronically stimulated inflammatory environment. Serial CT device monitoring post-implantation provided a detailed timeline of device collapse, with no rapid, spontaneous release of nanoparticles that occluded matrix visualization. Importantly, repeat CT sessions did not ablate the immune system or alter degradation kinetics. Thus, polymer devices incorporating radiopaque nanoparticles can be used for in situ monitoring and be readily combined with other medical imaging techniques, for a dynamic view biomaterial and tissue interactions. STATEMENT OF SIGNIFICANCE: A growing number of implantable devices are in use in the clinic, exposing patients to inherent risks of implant movement, collapse, and infection. The ability to monitor implanted devices would enable faster diagnosis of failure and open the door for personalized rehabilitation therapies - both of which could vastly improve patient outcomes. Unfortunately, polymeric materials which make up most biomedical devices are not radiologically distinguishable from tissue post-implantation. The introduction of radiopaque nanoparticles into polymers allows for serial monitoring via computed tomography, without affecting device degradation. Here we demonstrate for the first time that nanoparticles do not undergo burst release from devices post-implantation and that inflammatory responses - a key determinant of device function in vivo - are also unaffected by nanoparticle addition.

Quantitative imaging approaches to understanding biological processing of metal ions

Faster, more sensitive, and higher resolution quantitative instrumentation are aiding a deeper understanding of how inorganic chemistry regulates key biological processes. Researchers can now image and quantify metals with subcellular resolution, leading to a vast array of new discoveries in organismal development, pathology, and disease. Metals have recently been implicated in several diseases such as Parkinson's, Alzheimers, ischemic stroke, and colorectal cancer that would not be possible without these advancements. In this review, instead of focusing on instrumentation we focus on recent applications of label-free elemental imaging and quantification and how these tools can lead to a broader understanding of metals role in systems biology and human pathology.

Rapid isolation and profiling of a diverse panel of human monoclonal antibodies targeting the SARS-CoV-2 spike protein

Antibodies are a principal determinant of immunity for most RNA viruses and have promise to reduce infection or disease during major epidemics. The novel coronavirus SARS-CoV-2 has caused a global pandemic with millions of infections and hundreds of thousands of deaths to date1,2. In response, we used a rapid antibody discovery platform to isolate hundreds of human monoclonal antibodies (mAbs) against the SARS-CoV-2 spike (S) protein. We stratify these mAbs into five major classes on the basis of their reactivity to subdomains of S protein as well as their cross-reactivity to SARS-CoV. Many of these mAbs inhibit infection of authentic SARS-CoV-2 virus, with most neutralizing mAbs recognizing the receptor-binding domain (RBD) of S. This work defines sites of vulnerability on SARS-CoV-2 S and demonstrates the speed and robustness of advanced antibody discovery platforms.

Rapid isolation and profiling of a diverse panel of human monoclonal antibodies targeting the SARS-CoV-2 spike protein

Antibodies are a principal determinant of immunity for most RNA viruses and have promise to reduce infection or disease during major epidemics. The novel coronavirus SARS-CoV-2 has caused a global pandemic with millions of infections and hundreds of thousands of deaths to date ^1,2 . In response, we used a rapid antibody discovery platform to isolate hundreds of human monoclonal antibodies (mAbs) against the SARS-CoV-2 spike (S) protein. We stratify these mAbs into five major classes based on their reactivity to subdomains of S protein as well as their cross-reactivity to SARS-CoV. Many of these mAbs inhibit infection of authentic SARS-CoV-2 virus, with most neutralizing mAbs recognizing the receptor-binding domain (RBD) of S. This work defines sites of vulnerability on SARS-CoV-2 S and demonstrates the speed and robustness of new antibody discovery methodologies.

Water-Soluble Nanoconjugate for Enhanced Cellular Delivery of Receptor-Targeted Magnetic Resonance Contrast Agents

ProGlo is an efficient steroid receptor-targeted magnetic resonance (MR) imaging contrast agent (CA). It has been shown to bind to the progesterone receptor (PR) and produce enhanced image contrast in PR-positive cells and tissues in vitro and in vivo. However, the hydrophobicity of the steroid targeting domain of ProGlo (logP = 1.4) limits its formulation and delivery at clinically relevant doses. In this work, a hydrophobic moiety was utilized to drive efficient adsorption onto nanodiamond (ND) clusters to form a water-soluble nanoconstruct (logP = -2.4) with 80% release in 8 h under biological conditions. In cell culture, the ND-ProGlo construct delivered increased concentrations of ProGlo to target cells compared to ProGlo alone. Importantly, these results were accomplished without the use of solvents such as DMSO, providing a significant advance toward formulating ProGlo for translational applications. Biodistribution studies confirm the delivery of ProGlo to PR(+) tissues with enhanced efficacy over untargeted controls. These results demonstrate the potential for a noncovalent ND-CA construct as a general strategy for solubilizing and delivering hydrophobic targeted MR CAs.

A Markedly Improved Synthetic Approach for the Preparation of Multifunctional Au-DNA Nanoparticle Conjugates Modified with Optical and MR Imaging Probes

We describe a new, and vastly superior approach for labeling spherical nucleic acid conjugates (SNAs) with diagnostic probes. SNAs have been shown to provide the unique ability to traverse the cell membrane and deliver surface conjugated DNA into cells while preserving the DNA from nuclease degradation. Our previous work on preparing diagnostically labeled SNAs was labor intensive, relatively low yielding, and costly. Here, we describe a straightforward and facile preparation for labeling SNAs with optical and MR imaging probes with significantly improved physical properties. The synthesis of Gd(III) labeled DNA Au nanoparticle conjugates is achieved by sequential conjugation of 3'-thiol-modified oligonucleotides and cofunctionalization of the particle surface with the subsequent addition of 1,2 diothiolate modified chelates of Gd(III) (abbreviated: DNA-Gd^III@AuNP). This new generation of SNA conjugates has a 2-fold increase of DNA labeling and a 1.4-fold increase in Gd(III) loading compared to published constructs. Furthermore, the relaxivity ( r₁) is observed to increase 4.5-fold compared to the molecular dithiolane-Gd(III) complex, and 1.4-fold increase relative to previous particle constructs where the Gd(III) complexes were conjugated to the oligonucleotides rather than directly to the Au particle. Importantly, this simplified approach (2 steps) exploits the advantages of previous Gd(III) labeled SNA platforms; however, this new approach is scalable and eliminates modification of DNA for attaching the contrast agent, and the particles exhibit improved cell labeling.

Synthesis and evaluation of MR probes for targeted-reporter imaging

Visualizing disease heterogeneity remains a challenging task since most imaging agents are targeted to a single receptor. We describe the development of an MR platform able to report on multiple molecular events. Enzyme activation and enhanced cellular uptake of this modular probe make it suitable for subsequent targeted-reporter imaging applications.

Exploring the Association Between Demographics, SLC30A8 Genotype, and Human Islet Content of Zinc, Cadmium, Copper, Iron, Manganese and Nickel

A widely prevalent single nucleotide polymorphism, rs13266634 in the SLC30A8 gene encoding the zinc transporter ZnT8, is associated with an increased risk for T2DM. ZnT8 is mostly expressed in pancreatic insulin-producing islets of Langerhans. The effect of this variant on the divalent metal profile in human islets is unknown. Additionally, essential and non-essential divalent metal content of human islets under normal environmental exposure conditions has not been described. We therefore examined the correlation of zinc and other divalent metals in human islets with rs13266634 genotype and demographic characteristics. We found that the diabetes risk genotype C/C at rs13266634 is associated with higher islet Zn concentration (C/C genotype: 16792 ± 1607, n = 22, C/T genotype: 11221 ± 1245, n = 18 T/T genotype: 11543 ± 6054, n = 3, all values expressed as mean nmol/g protein ± standard error of the mean, p = 0.040 by ANOVA). A positive correlation between islet cadmium content and both age (p = 0.048, R2 = 0.09) and female gender (women: 36.88 ± 4.11 vs men: 21.22 ± 3.65 nmol/g protein, p = 0.007) was observed. Our results suggest that the T2DM risk allele C is associated with higher islet zinc levels and support prior evidence of cadmium's higher bioavailability in women and its long tissue half-life.

Nanodiamond-Gadolinium(III) Aggregates for Tracking Cancer Growth In Vivo at High Field

The ability to track labeled cancer cells in vivo would allow researchers to study their distribution, growth, and metastatic potential within the intact organism. Magnetic resonance (MR) imaging is invaluable for tracking cancer cells in vivo as it benefits from high spatial resolution and the absence of ionizing radiation. However, many MR contrast agents (CAs) required to label cells either do not significantly accumulate in cells or are not biologically compatible for translational studies. We have developed carbon-based nanodiamond-gadolinium(III) aggregates (NDG) for MR imaging that demonstrated remarkable properties for cell tracking in vivo. First, NDG had high relaxivity independent of field strength, a finding unprecedented for gadolinium(III) [Gd(III)]-nanoparticle conjugates. Second, NDG demonstrated a 300-fold increase in the cellular delivery of Gd(III) compared to that of clinical Gd(III) chelates without sacrificing biocompatibility. Further, we were able to monitor the tumor growth of NDG-labeled flank tumors by T1- and T2-weighted MR imaging for 26 days in vivo, longer than was reported for other MR CAs or nuclear agents. Finally, by utilizing quantitative maps of relaxation times, we were able to describe tumor morphology and heterogeneity (corroborated by histological analysis), which would not be possible with competing molecular imaging modalities.

Gd(III)-Gold Nanoconjugates Provide Remarkable Cell Labeling for High Field Magnetic Resonance Imaging

In vivo cell tracking is vital for understanding migrating cell populations, particularly cancer and immune cells. Magnetic resonance (MR) imaging for long-term tracking of transplanted cells in live organisms requires cells to effectively internalize Gd(III) contrast agents (CAs). Clinical Gd(III)-based CAs require high dosing concentrations and extended incubation times for cellular internalization. To combat this, we have devised a series of Gd(III)-gold nanoconjugates (Gd@AuNPs) with varied chelate structure and nanoparticle-chelate linker length, with the goal of labeling and imaging breast cancer cells. These new Gd@AuNPs demonstrate significantly enhanced labeling compared to previous Gd(III)-gold-DNA nanoconstructs. Variations in Gd(III) loading, surface packing, and cell uptake were observed among four different Gd@AuNP formulations suggesting that linker length and surface charge play an important role in cell labeling. The best performing Gd@AuNPs afforded 23.6 ± 3.6 fmol of Gd(III) per cell at an incubation concentration of 27.5 μM-this efficiency of Gd(III) payload delivery (Gd(III)/cell normalized to dose) exceeds that of previous Gd(III)-Au conjugates and most other Gd(III)-nanoparticle formulations. Further, Gd@AuNPs were well-tolerated in vivo in terms of biodistribution and clearance, and supports future cell tracking applications in whole-animal models.

Gd(III)-Dithiolane Gold Nanoparticles for T1-Weighted Magnetic Resonance Imaging of the Pancreas

Pancreatic adenocarcinoma has a 5 year survival of approximately 3% and median survival of 6 months and is among the most dismal of prognoses in all of medicine. This poor prognosis is largely due to delayed diagnosis where patients remain asymptomatic until advanced disease is present. Therefore, techniques to allow early detection of pancreatic adenocarcinoma are desperately needed. Imaging of pancreatic tissue is notoriously difficult, and the development of new imaging techniques would impact our understanding of organ physiology and pathology with applications in disease diagnosis, staging, and longitudinal response to therapy in vivo. Magnetic resonance imaging (MRI) provides numerous advantages for these types of investigations; however, it is unable to delineate the pancreas due to low inherent contrast within this tissue type. To overcome this limitation, we have prepared a new Gd(III) contrast agent that accumulates in the pancreas and provides significant contrast enhancement by MR imaging. We describe the synthesis and characterization of a new dithiolane-Gd(III) complex and a straightforward and scalable approach for conjugation to a gold nanoparticle. We present data that show the nanoconjugates exhibit very high per particle values of r1 relaxivity at both low and high magnetic field strengths due to the high Gd(III) payload. We provide evidence of pancreatic tissue labeling that includes MR images, post-mortem biodistribution analysis, and pancreatic tissue evaluation of particle localization. Significant contrast enhancement was observed allowing clear identification of the pancreas with contrast-to-noise ratios exceeding 35:1.

Cell-Permeable Esterase-Activated Ca(II)-Sensitive MRI Contrast Agent

Calcium [Ca(II)] is a fundamental transducer of electrical activity in the central nervous system (CNS). Influx of Ca(II) into the cytosol is responsible for action potential initiation and propagation, and initiates interneuronal communication via release of neurotransmitters and activation of gene expression. Despite the importance of Ca(II) in physiology, it remains a challenge to visualize Ca(II) flux in the central nervous system (CNS) in vivo. To address these challenges, we have developed a new generation, Ca(II)-activated MRI contrast agent that utilizes ethyl esters to increase cell labeling and prevent extracellular divalent Ca(II) binding. Following labeling, the ethyl esters can be cleaved, thus allowing the agent to bind Ca(II), increasing relaxivity and resulting in enhanced positive MR image contrast. The ability of this probe to discriminate between extra- and intracellular Ca(II) may allow for spatiotemporal in vivo imaging of Ca(II) flux during seizures or ischemia where large Ca(II) fluxes (1-10 μM) can result in cell death.

DNA-gadolinium-gold nanoparticles for in vivo T1 MR imaging of transplanted human neural stem cells

The unambiguous imaging of transplanted cells remains a major challenge to understand their biological function and therapeutic efficacy. In vivo imaging of implanted cells is reliant on tagging these to differentiate them from host tissue, such as the brain. We here characterize a gold nanoparticle conjugate that is functionalized with modified deoxythymidine oligonucleotides bearing Gd(III) chelates and a red fluorescent Cy3 moiety to visualize in vivo transplanted human neural stem cells. This DNA-Gd@Au nanoparticle (DNA-Gd@AuNP) exhibits an improved T1 relaxivity and excellent cell uptake. No significant effects of cell uptake have been found on essential cell functions. Although T1 relaxivity is attenuated within cells, it is sufficiently preserved to afford the in vivo detection of transplanted cells using an optimized voxel size. In vivo MR images were corroborated by a post-mortem histological verification of DNA-Gd@AuNPs in transplanted cells. With 70% of cells being correctly identified using the DNA-Gd-AuNPs indicates an overall reliable detection. Less than 1% of cells were false positive for DNA-Gd@AuNPs, but a significant number of 30% false negatives reveals a dramatic underestimation of transplanted cells using this approach. DNA-Gd@AuNPs therefore offer new opportunities to visualize transplanted cells unequivocally using T1 contrast and use cellular MRI as a tool to derive biologically relevant information that allows us to understand how the survival and location of implanted cells determines therapeutic efficacy.

Water-soluble lipophilic MR contrast agents for cell membrane labeling

Long-term cell tracking using MR imaging necessitates the development of contrast agents that both label and are retained by cells. One promising strategy for long-term cell labeling is the development of lipophilic Gd(III)-based contrast agents that anchor into the cell membrane. We have previously reported the efficacy of monomeric and multimeric lipophilic agents and showed that the monomeric agents have improved labeling and contrast enhancement of cell populations. Here, we report on the synthesis, characterization, and in vitro testing of a series of monomeric lipophilic contrast agents with varied alkyl chain compositions. We show that these agents disperse in water, localize to the cell membrane, and label HeLa and MCF7 cells effectively. Additionally, these agents have up to tenfold improved retention in cells compared to clinically available ProHance(®).

Multimeric Near IR-MR Contrast Agent for Multimodal In Vivo Imaging

Multiple imaging modalities are often required for in vivo imaging applications that require both high probe sensitivity and excellent spatial and temporal resolution. In particular, MR and optical imaging are an attractive combination that can be used to determine both molecular and anatomical information. Herein, we describe the synthesis and in vivo testing of two multimeric NIR-MR contrast agents that contain three Gd(III) chelates and an IR-783 dye moiety. One agent contains a PEG linker and the other a short alkyl linker. These agents label cells with extraordinary efficacy and can be detected in vivo using both imaging modalities. Biodistribution of the PEGylated agent shows observable fluorescence in xenograft MCF7 tumors and renal clearance by MR imaging.

Nanodiscs as a Modular Platform for Multimodal MR-Optical Imaging

Nanodiscs are monodisperse, self-assembled discoidal particles that consist of a lipid bilayer encircled by membrane scaffold proteins (MSP). Nanodiscs have been used to solubilize membrane proteins for structural and functional studies and deliver therapeutic phospholipids. Herein, we report on tetramethylrhodamine (TMR) tagged nanodiscs that solubilize lipophilic MR contrast agents for generation of multimodal nanoparticles for cellular imaging. We incorporate both multimeric and monomeric Gd(III)-based contrast agents into nanodiscs and show that particles containing the monomeric agent (ND2) label cells with high efficiency and generate significant image contrast at 7 T compared to nanodiscs containing the multimeric agent (ND1) and Prohance, a clinically approved contrast agent.

High relaxivity Gd(III)-DNA gold nanostars: investigation of shape effects on proton relaxation

Gadolinium(III) nanoconjugate contrast agents (CAs) have distinct advantages over their small-molecule counterparts in magnetic resonance imaging. In addition to increased Gd(III) payload, a significant improvement in proton relaxation efficiency, or relaxivity (r1), is often observed. In this work, we describe the synthesis and characterization of a nanoconjugate CA created by covalent attachment of Gd(III) to thiolated DNA (Gd(III)-DNA), followed by surface conjugation onto gold nanostars (DNA-Gd@stars). These conjugates exhibit remarkable r1 with values up to 98 mM(-1) s(-1). Additionally, DNA-Gd@stars show efficient Gd(III) delivery and biocompatibility in vitro and generate significant contrast enhancement when imaged at 7 T. Using nuclear magnetic relaxation dispersion analysis, we attribute the high performance of the DNA-Gd@stars to an increased contribution of second-sphere relaxivity compared to that of spherical CA equivalents (DNA-Gd@spheres). Importantly, the surface of the gold nanostar contains Gd(III)-DNA in regions of positive, negative, and neutral curvature. We hypothesize that the proton relaxation enhancement observed results from the presence of a unique hydrophilic environment produced by Gd(III)-DNA in these regions, which allows second-sphere water molecules to remain adjacent to Gd(III) ions for up to 10 times longer than diffusion. These results establish that particle shape and second-sphere relaxivity are important considerations in the design of Gd(III) nanoconjugate CAs.

Graphene oxide enhances cellular delivery of hydrophilic small molecules by co-incubation

The delivery of bioactive molecules into cells has broad applications in biology and medicine. Polymer-modified graphene oxide (GO) has recently emerged as a de facto noncovalent vehicle for hydrophobic drugs. Here, we investigate a different approach using native GO to deliver hydrophilic molecules by co-incubation in culture. GO adsorption and delivery were systematically studied with a library of 15 molecules synthesized with Gd(III) labels to enable quantitation. Amines were revealed to be a key chemical group for adsorption, while delivery was shown to be quantitatively predictable by molecular adsorption, GO sedimentation, and GO size. GO co-incubation was shown to enhance delivery by up to 13-fold and allowed for a 100-fold increase in molecular incubation concentration compared to the alternative of nanoconjugation. When tested in the application of Gd(III) cellular MRI, these advantages led to a nearly 10-fold improvement in sensitivity over the state-of-the-art. GO co-incubation is an effective method of cellular delivery that is easily adoptable by researchers across all fields.

Progesterone-targeted magnetic resonance imaging probes

Determination of progesterone receptor (PR) status in hormone-dependent diseases is essential in ascertaining disease prognosis and monitoring treatment response. The development of a noninvasive means of monitoring these processes would have significant impact on early detection, cost, repeated measurements, and personalized treatment options. Magnetic resonance imaging (MRI) is widely recognized as a technique that can produce longitudinal studies, and PR-targeted MR probes may address a clinical problem by providing contrast enhancement that reports on PR status without biopsy. Commercially available MR contrast agents are typically delivered via intravenous injection, whereas steroids are administered subcutaneously. Whether the route of delivery is important for tissue accumulation of steroid-modified MRI contrast agents to PR-rich tissues is not known. To address this question, modification of the chemistry linking progesterone with the gadolinium chelate led to MR probes with increased water solubility and lower cellular toxicity and enabled administration through the blood. This attribute came at a cost through lower affinity for PR and decreased ability to cross the cell membrane, and ultimately it did not improve delivery of the PR-targeted MR probe to PR-rich tissues or tumors in vivo. Overall, these studies are important, as they demonstrate that targeted contrast agents require optimization of delivery and receptor binding of the steroid and the gadolinium chelate for optimal translation in vivo.

Gd(III)-labeled peptide nanofibers for reporting on biomaterial localization in vivo

Bioactive supramolecular nanostructures are of great importance in regenerative medicine and the development of novel targeted therapies. In order to use supramolecular chemistry to design such nanostructures, it is extremely important to track their fate in vivo through the use of molecular imaging strategies. Peptide amphiphiles (PAs) are known to generate a wide array of supramolecular nanostructures, and there is extensive literature on their use in areas such as tissue regeneration and therapies for disease. We report here on a series of PA molecules based on the well-established β-sheet amino acid sequence V3A3 conjugated to macrocyclic Gd(III) labels for magnetic resonance imaging (MRI). These conjugates were shown to form cylindrical supramolecular assemblies using cryogenic transmission electron microscopy and small-angle X-ray scattering. Using nuclear magnetic relaxation dispersion analysis, we observed that thermal annealing of the nanostructures led to a decrease in water exchange lifetime (τm) of hundreds of nanoseconds only for molecules that self-assemble into nanofibers of high aspect ratio. We interpret this decrease to indicate more solvent exposure to the paramagnetic moiety on annealing, resulting in faster water exchange within angstroms of the macrocycle. We hypothesize that faster water exchange in the nanofiber-forming PAs arises from the dehydration and increase in packing density on annealing. Two of the self-assembling conjugates were selected for imaging PAs after intramuscular injections of the PA C16V3A3E3-NH2 in the tibialis anterior muscle of a murine model. Needle tracts were clearly discernible with MRI at 4 days postinjection. This work establishes Gd(III) macrocycle-conjugated peptide amphiphiles as effective tracking agents for peptide amphiphile materials in vivo over the timescale of days.

Cell labeling via membrane-anchored lipophilic MR contrast agents

Cell tracking in vivo with MR imaging requires the development of contrast agents with increased sensitivity that effectively label and are retained by cells. Most clinically approved Gd(III)-based contrast agents require high incubation concentrations and prolonged incubation times for cellular internalization. Strategies to increase contrast agent permeability have included conjugating Gd(III) complexes to cell penetrating peptides, nanoparticles, and small molecules which have greatly improved cell labeling but have not resulted in improved cellular retention. To overcome these challenges, we have synthesized a series of lipophilic Gd(III)-based MR contrast agents that label cell membranes in vitro. Two of the agents were synthesized with a multiplexing strategy to contain three Gd(III) chelates (1 and 2) while the third contains a single Gd(III) chelate (3). These new agents exhibit significantly enhanced labeling and retention in HeLa and MDA-MB-231-mcherry cells compared to agents that are internalized by cells (4 and Prohance).

Structural optimization of Zn(II)-activated magnetic resonance imaging probes

We report the structural optimization and mechanistic investigation of a series of bioactivated magnetic resonance imaging contrast agents that transform from low relaxivity to high relaxivity in the presence of Zn(II). The change in relaxivity results from a structural transformation of the complex that alters the coordination environment about the Gd(III) center. Here, we have performed a series of systematic modifications to determine the structure that provides the optimal change in relaxivity in response to the presence of Zn(II). Relaxivity measurements in the presence and absence of Zn(II) were used in conjunction with measurements regarding water access (namely, number of water molecules bound) to the Gd(III) center and temperature-dependent (13)C NMR spectroscopy to determine how the coordination environment about the Gd(III) center is affected by the distance between the Zn(II)-binding domain and the Gd(III) chelate, the number of functional groups on the Zn(II)-binding domain, and the presence of Zn(II). The results of this study provide valuable insight into the design principles for future bioactivated magnetic resonance probes.

pH-Responsive Theranostic Polymer-Caged Nanobins (PCNs): Enhanced Cytotoxicity and T1 MRI Contrast by Her2-Targeting

A PCN theranostic platform comprises a doxorubicin (DXR)-loaded liposomal core and an acid-sensitive polymer shell that is functionalized with Herceptin and Gd^III-based MRI contrast agents. In vitro testing reveals a 14-fold increase in DXR-based cytotoxicity versus a non-targeted analogue and an 120-fold improvement in cellular Gd^III-uptake in comparison with clinically approved DOTA-Gd^III, leading to significant T₁ MRI contrast enhancement.

Mechanisms of Gadographene-Mediated Proton Spin Relaxation

Gd(III) associated with carbon nanomaterials relaxes water proton spins at an effectiveness that approaches or exceeds the theoretical limit for a single bound water molecule. These Gd(III)-labeled materials represent a potential breakthrough in sensitivity for Gd(III)-based contrast agents used for magnetic resonance imaging (MRI). However, their mechanism of action remains unclear. A gadographene library encompassing GdCl3, two different Gd(III)-complexes, graphene oxide (GO), and graphene suspended by two different surfactants and subjected to varying degrees of sonication was prepared and characterized for their relaxometric properties. Gadographene was found to perform comparably to other Gd(III)-carbon nanomaterials; its longitudinal (r1) and transverse (r2) relaxivity is modulated between 12-85 mM-1s-1 and 24-115 mM-1s-1, respectively, depending on the Gd(III)-carbon backbone combination. The unusually large relaxivity and its variance can be understood under the modified Florence model incorporating the Lipari-Szabo approach. Changes in hydration number (q), water residence time (τM), molecular tumbling rate (τR), and local motion (τfast) sufficiently explain most of the measured relaxivities. Furthermore, results implicated the coupling between graphene and Gd(III) as a minor contributor to proton spin relaxation.

Accumulation of cadmium in insulin-producing β cells

Evidence suggests that chronic low level cadmium exposure impairs the function of insulin-producing β cells and may be associated with type-2 diabetes mellitus. Herein, we describe the cadmium content in primary human islets and define the uptake kinetics and effects of environmentally relevant cadmium concentrations in cultured β cells. The average cadmium content in islets from 10 non-diabetic human subjects was 29 ± 7 nmol/g protein (range 7 to 72 nmol/g protein). Exposure of the β-cell line MIN6 to CdCl 2 concentrations between 0.1 and 1.0 µmol/L resulted in a dose- and time-dependent uptake of cadmium over 72 h. This uptake resulted in an induction of metallthionein expression, likely enhancing cellular cadmium accumulation. Furthermore, cadmium accumulation resulted in an inhibition of glucose stimulated insulin secretion in MIN6 cells and primary mouse islets. Our results indicate that this impairment in β-cell function is not due to an increase in cell death or due to an increase in oxidative stress. We conclude that mouse β cells accumulate cadmium in a dose- and time-dependent manner over a prolonged time course at environmentally relevant concentrations. This uptake leads to a functional impairment of β-cell function without significant alterations in cell viability, expression of genes important for β-cell function or increase in oxidative stress.

Synthesis and biological evaluation of water-soluble progesterone-conjugated probes for magnetic resonance imaging of hormone related cancers

Progesterone receptor (PR) is strongly associated with disease prognosis and therapeutic efficacy in hormone-related diseases such as endometriosis and breast, ovarian, and uterine cancers. Receptor status is currently determined by immunohistochemistry assays. However, noninvasive PR imaging agents could improve disease detection and help elucidate pathological molecular pathways, leading to new therapies and animal disease models. A series of water-soluble PR-targeted magnetic resonance imaging (MRI) probes were synthesized using Cu(I)-catalyzed click chemistry and evaluated in vitro and in vivo. These agents demonstrated activation of PR in vitro and preferential accumulation in PR(+) compared to PR(-) human breast cancer cells with low toxicity. In xenograft tumor models, the agents demonstrated enhanced signal intensity in PR(+) tumors compared to PR(-) tumors. The results suggest that these agents may be promising MRI probes for PR(+) diseases.

A steroid-conjugated magnetic resonance probe enhances contrast in progesterone receptor expressing organs and tumors in vivo

Progesterone receptor (PR) is a significant biomarker in diseases such as endometriosis and breast, ovarian, and uterine cancers that is associated with disease prognosis and therapeutic efficacy. While receptor status is currently determined by immunohistochemistry assays, the development of noninvasive PR imaging agents could improve molecular characterization, treatment decisions, and disease monitoring. ProGlo, a progesterone-conjugated magnetic resonance imaging (MRI) contrast agent, was evaluated in vivo to determine whether it targets and enhances signal intensity in organs and tumors that express high PR levels. A tissue distribution study indicated that ProGlo accumulates in the PR-rich uterus, which was confirmed by in vivo imaging studies. Ex vivo images of these organs revealed that ProGlo was distributed in the substructures that express high PR levels. In xenograft tumor models, ProGlo was taken up to a greater extent than the nonfunctionalized Gd-DO3A in tumors, particularly in PR(+) tumors. The ability to accumulate and enhance signal intensity in PR(+) organs and tumors suggests that ProGlo may be a promising MRI probe for PR(+) diseases.

Probing the Chemical Stability of Mixed Ferrites: Implications for MR Contrast Agent Design

Nanomaterials with mixed composition, in particular magnetic spinel ferrites, are emerging as efficient contrast agents for magnetic resonance imaging (MRI). Many factors, including size, composition, atomic structure, and surface properties are crucial in the design of such nanoparticle-based probes due to their influence on the magnetic properties. Silica-coated iron oxide (IO-SiO(2)) and cobalt ferrite (CoIO-SiO(2)) nanoparticles were synthesized using standard high temperature thermal decomposition and base-catalyzed water-in-oil microemulsion techniques. Under neutral aqueous conditions, it was found that 50-75% of the cobalt content in the CoIO-SiO(2) nanoparticles leached out of the core structure. Leaching caused a 7.2-fold increase in longitudinal relaxivity and an increase in the saturation magnetization from ~48 emu/g core to ~65 emu/g core. X-ray absorption fine structure studies confirmed that the atomic structure of the ferrite core was altered following leaching, while TEM and DLS confirmed that the morphology and size of the nanoparticle remained unchanged. The CoIO-SiO(2) nanoparticles converted from a partially inverted spinel cation arrangement (unleached state) to an inverse spinel arrangement (leached state). The control IO-SiO(2) nanoparticles remained stable with no change in structure and negligible changes in magnetic behavior. This detailed analysis highlights how important understanding the properties of nanomaterials is in the development of reliable agents for diagnostic and therapeutic applications.

Ultrasmall, Water-Soluble Magnetite Nanoparticles with High Relaxivity for Magnetic Resonance Imaging

Ultrasmall (3, 4, 5, and 6 nm), water-soluble Fe3O4 magnetic nanoparticles were synthesized in diethylene glycol (DEG) via a facile one-pot reaction. Hydrodynamic size and relaxation time measurements did not show particle aggregation when Fe3O4 nanoparticles were dispersed in phosphate buffered saline, fetal bovine serum, or calf bovine serum for 1 week. Furthermore, the new Fe3O4 nanoparticles tolerated high salt concentrations (≤1 M NaCl) and a wide pH range from 5 to 11. Surface modification of the nanoparticles with poly(ethylene glycol) bis(carboxymethyl) ether (HOOC-PEG-COOH, 600 g/mol) was accomplished through a ligand-exchange reaction. The effects of PEG modification on magnetization and relaxivity of the Fe3O4 nanoparticles were investigated, and the results indicate that the increase in transverse relaxivity after PEG modification may be due to the increased volume of slowly diffusing water surrounding each nanoparticle. In vitro experiments showed that the DEG- and PEG-coated Fe3O4 nanoparticles have little effect on NIH/3T3 cell viability.

Cell-permeable MR contrast agents with increased intracellular retention

Magnetic resonance imaging (MRI) is a technique used in both clinical and experimental settings to produce high-resolution images of opaque organisms without ionizing radiation. Currently, MR imaging is augmented by contrast agents, and the vast majority these small molecule Gd(III) chelates are confined to the extracellular regions. As a result, contrast agents are confined to vascular regions reducing their ability to provide information about cell physiology or molecular pathology. We have shown that polypeptides of arginine have the capacity to transport Gd(III) contrast agents across cell membranes. However, this transport is not unidirectional, and once inside the cell, the arginine-modified contrast agents efflux rapidly, decreasing the intracellular Gd(III) concentration and corresponding MR image intensity. By exploiting the inherent disulfide reducing environment of cells, thiol compounds, Gd(III)-DOTA-SS-Arg 8 and Gd(III)-DTPA-SS-Arg 8, are cleaved from their cell-penetrating peptide transduction domains upon cell internalization. This reaction prolongs the cell-associated lifetime of the chelated Gd(III) by cleaving it from the cell transduction domain.

Rational Design, Synthesis, and Biological Evaluation of Progesterone-Modified MRI Contrast Agents

A series of contrast agents for magnetic resonance imaging (MRI) aimed at noninvasively determining the hormone receptor status of cancer in vitro was developed. These MRI contrast agents were prepared by conjugating progesterone to clinically used Gd(III) chelates. These agents exhibited higher progesterone receptor binding affinities in the nanomolar range and intracellular accumulation. High logP values of the modified compounds suggested that the lipophilicity of the steroid conjugates may have contributed to membrane permeability. Synchrotron radiation X-ray fluorescence microscopy and magnetic resonance images revealed that the synthesized conjugates showed the greatest cellular accumulation and significant increase in relaxivity in vitro compared to the previously developed steroid-modified agent. Transcriptional assays using the progesterone response element linked to luciferase indicated that the contrast agents entered the cell, interacted with the biological target, and drove specific progesterone-mediated transcription.

Cellular Delivery of MRI Contrast Agents

Magnetic resonance imaging (MRI) is a powerful tool for acquiring images of opaque living animals with the benefit of tracking events over extended periods of time on the same specimen. Contrast agents are used to enhance regions, tissues, and cells that are magnetically similar but histologically distinct. A principal barrier to the development of MRI contrast agents for investigating biological questions is the delivery of agents across cellular membranes. Here, we describe the synthesis and in vitro testing of Gd(III)-based MRI contrast agents containing varying length polyarginine oligomers capable of permeating cell membranes. We examine the effect of the length of oligomer on T(1) enhancement and cellular uptake. Furthermore, the effect of incubation time, concentration, and cell type on uptake is explored. Toxicity and washout studies are performed in addition to MRI phantom studies.