Functional imaging using fluorine ((19)F) MR methods: basic concepts

Kidney-associated pathologies would greatly benefit from noninvasive and robust methods that can objectively quantify changes in renal function. In the past years there has been a growing incentive to develop new applications for fluorine ((19)F) MRI in biomedical research to study functional changes during disease states. (19)F MRI represents an instrumental tool for the quantification of exogenous (19)F substances in vivo. One of the major benefits of (19)F MRI is that fluorine in its organic form is absent in eukaryotic cells. Therefore, the introduction of exogenous (19)F signals in vivo will yield background-free images, thus providing highly selective detection with absolute specificity in vivo. Here we introduce the concept of (19)F MRI, describe existing challenges, especially those pertaining to signal sensitivity, and give an overview of preclinical applications to illustrate the utility and applicability of this technique for measuring renal function in animal models. This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis

Gd(III)-Dithiolane Gold Nanoparticles for <i>T</i>₁‑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 <i>r</i>₁ 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

¹⁹F Magnetic Resonance Imaging Signals from Peptide Amphiphile Nanostructures Are Strongly Affected by Their Shape

Magnetic resonance imaging (MRI) is a noninvasive imaging modality that provides excellent spatial and temporal resolution. The most commonly used MR probes face significant challenges originating from the endogenous ¹H background signal of water. In contrast, fluorine MRI (¹⁹F MRI) allows quantitative probe imaging with zero background signal. Probes with high fluorine content are required for high sensitivity, suggesting nanoscale supramolecular assemblies containing ¹⁹F probes offer a potentially useful strategy for optimum imaging as a result of improved payload. We report here on supramolecular nanostructures formed by fluorinated peptide amphiphiles containing either glutamic acid or lysine residues in their sequence. We identified molecules that form aggregates in water which transition from cylindrical to ribbon-like shape as pH increased from 4.5 to 8.0. Interestingly, we found that ribbon-like nanostructures had reduced magnetic resonance signal, whereas their cylindrical counterparts exhibited strong signals. We attribute this drastic difference to the greater mobility of fluorinated tails in the hydrophobic compartment of cylindrical nanostructures compared to lower mobility in ribbon-like assemblies. This discovery identifies a strategy to design supramolecular, self-assembling contrast agents for ¹⁹F MRI that can spatially map physiologically relevant changes in pH using changes in morphology

Calcium-Induced Morphological Transitions in Peptide Amphiphiles Detected by ¹⁹F‑Magnetic Resonance Imaging

Misregulation of extracellular Ca²⁺ can indicate bone-related pathologies. New, noninvasive tools are required to image Ca²⁺ fluxes and fluorine magnetic resonance imaging (¹⁹F-MRI) is uniquely suited to this challenge. Here, we present three, highly fluorinated peptide amphiphiles that self-assemble into nanoribbons in buffered saline and demonstrate these nanostructures can be programmed to change ¹⁹F-NMR signal intensity as a function of Ca²⁺ concentration. We determined these nanostructures show significant reduction in ¹⁹F-NMR signal as nanoribbon width increases in response to Ca²⁺, corresponding to ¹⁹F-MR image intensity reduction. Thus, these peptide amphiphiles can be used to quantitatively image biologically relevant Ca²⁺ concentrations

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