Bimodal MR-PET agent for quantitative pH imaging

Improved pH measurements with a single PARACEST MRI contrast agent

The measurement of extracellular pH has potential utility for assessing the therapeutic effects of pH-dependent and pH-altering therapies. A PARAmagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agent, Yb-DO3A-oAA, has two CEST effects that are dependent on pH. A ratio derived from these CEST effects was linearly correlated with pH throughout the physiological pH range. The pH can be measured with a precision of 0.21 pH units and an accuracy of 0.09 pH units. The pH measurement is independent of concentration and T₁ relaxation times, but is dependent on temperature. Although MR coalescence affects the CEST measurements, especially at high pH, the ratiometric analysis of the CEST effects can account for incomplete saturation of the agent's amide and amine that results from MR coalescence. Provided that an empirical calibration is determined with saturation conditions, magnetic field strength and temperature that can be used for subsequent studies, these results demonstrate that this single PARACEST MRI contrast agent can accurately measure pH.

Cell-permeable Ln(III) chelate-functionalized InP quantum dots as multimodal imaging agents

Quantum dots (QDs) are ideal scaffolds for the development of multimodal imaging agents, but their application in clinical diagnostics is limited by the toxicity of classical CdSe QDs. A new bimodal MRI/optical nanosized contrast agent with high gadolinium payload has been prepared through direct covalent attachment of up to 80 Gd(III) chelates on fluorescent nontoxic InP/ZnS QDs. It shows a high relaxivity of 900 mM(-1) s(-1) (13 mM(-1 )s(-1) per Gd ion) at 35 MHz (0.81 T) and 298 K, while the bright luminescence of the QDs is preserved. Eu(III) and Tb(III) chelates were also successfully grafted to the InP/ZnS QDs. The absence of energy transfer between the QD and lanthanide emitting centers results in a multicolor system. Using this convenient direct grafting strategy additional targeting ligands can be included on the QD. Here a cell-penetrating peptide has been co-grafted in a one-pot reaction to afford a cell-permeable multimodal multimeric MRI contrast agent that reports cellular localization by fluorescence and provides high relaxivity and increased tissue retention with respect to commercial contrast agents.

Synthesis and high performance of magnetofluorescent polyelectrolyte nanocomposites as MR/near-infrared multimodal cellular imaging nanoprobes

Here, we describe an easy but robust chemical strategy to synthesize high-performance magnetic resonance (MR)/near-infrared (NIR) multimodal imaging nanoprobes. Poly(γ-glutamic acid) was used for the convenient phase transfer of MnFe(2)O(4) nanoparticles dispersed in organic solvents into aqueous solutions and facilitated further ionic gelation with poly(l-lysine). During the gelation process, MnFe(2)O(4) nanoparticulate satellites were encapsulated in the ionic nanocomplex, which induced synergistic magnetism and resulted in huge T(2) relaxivity (r(2)). The positively charged outer surfaces were assembled with other negatively charged NIR emitting fluorescent nanocrystals and enabled the highly efficient delivery of the magnetofluorescent polyelectrolyte nanocomposites (MagFL-PEN) into cancer cells. The enhancement of negative contrast of MagFL-PEN at 2 μg/mL concentration was similar to that of Resovist at 20 μg/mL concentration. The NIR fluorescence microscopy images of the MagFL-PEN-labeled cells even at 12.5 pM were able to be clearly observed. The labeling efficiency of MagFL-PEN was approximately 65-fold higher compared to that of the commercialized fluorescent nanocrystals, only after 3 h incubation period, even at the test concentration (100 pM). Due to the high-performance capabilities both in materials properties and cell labeling efficiency, the MagFL-PEN is expected to be used as a highly efficient MR/NIR dual-modality imaging nanoprobe in the detection of cancer cells and monitoring of therapeutic cells 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.

Next-generation imaging development for nanoparticle biodistribution measurements

As nanotechnologies move closer to use in humans, quantitative imaging methods will play a vital role in answering questions of biodistribution. Accurate knowledge of the location and quantity of in vivo nanoconstructs and carriers is a challenging task, and new methods of quantitative imaging at appropriate resolutions are being developed and tested. Sustaining simultaneous advancement in both imaging development and nanotechnology research requires multidisciplinary research teams conducting experiments with interconnected goals. On an even greater scale, networks of multidisciplinary teams focused on similar issues of imaging and probe development offer opportunities for leveraging resources, as well as providing a forum for sharing ideas and creating consensus on solutions to common challenges. The Network for Translational Research (NTR): Optical Imaging in Multimodal Platforms from the National Cancer Institute is just such a network. Four multidisciplinary centers are accepting the challenges of developing and optimizing multimodal imaging hardware and software along with imaging probe development. These efforts are similar to the efforts that will be required for future studies of in vivo nanoparticle biodistribution. In addition to technology development and optimization, the network is organized to confront the challenges of validation of the imaging hardware and associated imaging agents, similar to the methods needed for validating nanomedicine.

MRI & MRS assessment of the role of the tumour microenvironment in response to therapy

MRI and MRS techniques are being applied to the characterisation of various aspects of the tumour microenvironment and to the assessment of tumour response to therapy. For example, kinetic parameters describing tumour blood vessel flow and permeability can be derived from dynamic contrast-enhanced MRI data and have been correlated with a positive tumour response to antivascular therapies. The ongoing development and validation of noninvasive, high-resolution anatomical/molecular MR techniques will equip us with the means to detect specific tumour biomarkers early on, and then to monitor the efficacy of cancer treatments efficiently and reliably, all within a clinically relevant time frame. Reliable tumour microenvironment imaging biomarkers will provide obvious advantages by enabling tumour-specific treatment tailoring and potentially improving patient outcome. However, for routine clinical application across many disease types, such imaging biomarkers must be quantitative, robust, reproducible, sufficiently sensitive and cost-effective. These characteristics are all difficult to achieve in practice, but image biomarker development and validation have been greatly facilitated by an increasing number of pertinent preclinical in vivo cancer models. Emphasis must now be placed on discovering whether the preclinical results translate into an improvement in patient care and, therefore, overall survival.

Eu(III) complexes as anion-responsive luminescent sensors and paramagnetic chemical exchange saturation transfer agents

The Eu(III) complex of (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (S-THP) is studied as a sensor for biologically relevant anions. Anion interactions produce changes in the luminescence emission spectrum of the Eu(III) complex, in the (1)H NMR spectrum, and correspondingly, in the PARACEST spectrum of the complex (PARACEST = paramagnetic chemical exchange saturation transfer). Direct excitation spectroscopy and luminescence lifetime studies of Eu(S-THP) give information about the speciation and nature of anion interactions including carbonate, acetate, lactate, citrate, phosphate, and methylphosphate at pH 7.2. Data is consistent with the formation of both innersphere and outersphere complexes of Eu(S-THP) with acetate, lactate, and carbonate. These anions have weak dissociation constants that range from 19 to 38 mM. Citrate binding to Eu(S-THP) is predominantly innersphere with a dissociation constant of 17 μM. Luminescence emission peak changes upon addition of anion to Eu(S-THP) show that there are two distinct binding events for phosphate and methylphosphate with dissociation constants of 0.3 mM and 3.0 mM for phosphate and 0.6 mM and 9.8 mM for methyl phosphate. Eu(THPC) contains an appended carbostyril derivative as an antenna to sensitize Eu(III) luminescence. Eu(THPC) binds phosphate and citrate with dissociation constants that are 10-fold less than that of the Eu(S-THP) parent, suggesting that functionalization through a pendent group disrupts the anion binding site. Eu(S-THP) functions as an anion responsive PARACEST agent through exchange of the alcohol protons with bulk water. The alcohol proton resonances of Eu(S-THP) shift downfield in the presence of acetate, lactate, citrate, and methylphosphate, giving rise to distinct PARACEST peaks. In contrast, phosphate binds to Eu(S-THP) to suppress the PARACEST alcohol OH peak and carbonate does not markedly change the alcohol peak at 5 mM Eu(S-THP), 15 mM carbonate at pH 6.5 or 7.2. This work shows that the Eu(S-THP) complex has unique selectivity toward binding of biologically relevant anions and that anion binding results in changes in both the luminescence and the PARACEST spectra of the complex.

A smart magnetic resonance imaging contrast agent responsive to adenosine based on a DNA aptamer-conjugated gadolinium complex

We report a general strategy for developing a smart MRI contrast agent for the sensing of small molecules such as adenosine based on a DNA aptamer that is conjugated to a Gd compound and a protein streptavidin. The binding of adenosine to its aptamer results in the dissociation of the Gd compound from the large protein, leading to decreases in the rotational correlation time and thus change of MRI contrast.

Activatable T₁ and T₂ magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) has become one of the most important diagnosis tools available in medicine. Typically MRI is not capable of sensing biochemical activities. However, recently emerged activatable MRI contrast agents (CAs), whose relaxivity is variable in response to a specific parameter change in the surrounding physiological microenvironment, potentially allow for MRI to indicate biological processes. Among the various factors influencing the relaxivity of a CA, the number of inner-sphere water molecules (q) directly coordinated to the metal center, the residence time of the coordinated water molecule (τ (m)), and the rotational correlation time representing the molecular tumbling time of a complex (τ (R)) contribute strongly to the relaxivity of an activatable CA. Tuning the ligand structure and properties has been the subject of intensive research for activatable MR CA designs. This review summarizes a variety of activatable MRI CAs sensitive to common variables in microenvironment in vivo, i.e., pH, luminescence, metal ions, redox, and enzymes, etc., with emphasis on the influence of ligand design on parameters q, τ (m), and τ (R).

Bimodal thrombus imaging: simultaneous PET/MR imaging with a fibrin-targeted dual PET/MR probe--feasibility study in rat model

PURPOSE

To image thrombus by using magnetic resonance (MR) imaging and positron emission tomography (PET) simultaneously in a rat arterial thrombus model with a dual PET/MR probe.

MATERIALS AND METHODS

Animal studies were approved by the institutional animal use committee. A dual PET/MR probe was synthesized by means of partial exchange of gadolinium for copper 64 ((64)Cu) in the fibrin-targeted MR probe EP-2104R. A preformed 25-mm thrombus was injected into the right internal carotid artery of a rat. Imaging was performed with a clinical 3.0-T MR imager with an MR-compatible human PET imager. Rats (n = 5) were imaged prior to and after systemic administration of the dual probe by using simultaneous PET/MR. The organ distribution of (64)Cu and gadolinium was determined ex vivo (n = 8), 2 hours after injection by using well counting and inductively coupled plasma mass spectrometry, respectively. Signal intensity ratios (SIRs) between the thrombus-containing and contralateral vessel were computed from PET images and MR data before and after probe administration.

RESULTS

The dual probe was synthesized with greater than 98% radiochemical purity. Thrombus enhancement was observed in all five animals at both MR (SIR([postprobe])/SIR([preprobe]) = 1.71 ± 0.35, P = .0053) and PET (SIR = 1.85 ± 0.48, P = .0087) after injection of the dual PET/MR probe. Ex vivo analysis at 2 hours after injection showed the highest (64)Cu and gadolinium concentrations, after the excretory organs (kidney and liver), to be in the thrombus.

CONCLUSION

A fibrin-targeted dual PET/MR probe enables simultaneous, direct MR and PET imaging of thrombus.

Quantitative cardiovascular magnetic resonance for molecular imaging

Cardiovascular magnetic resonance (CMR) molecular imaging aims to identify and map the expression of important biomarkers on a cellular scale utilizing contrast agents that are specifically targeted to the biochemical signatures of disease and are capable of generating sufficient image contrast. In some cases, the contrast agents may be designed to carry a drug payload or to be sensitive to important physiological factors, such as pH, temperature or oxygenation. In this review, examples will be presented that utilize a number of different molecular imaging quantification techniques, including measuring signal changes, calculating the area of contrast enhancement, mapping relaxation time changes or direct detection of contrast agents through multi-nuclear imaging or spectroscopy. The clinical application of CMR molecular imaging could offer far reaching benefits to patient populations, including early detection of therapeutic response, localizing ruptured atherosclerotic plaques, stratifying patients based on biochemical disease markers, tissue-specific drug delivery, confirmation and quantification of end-organ drug uptake, and noninvasive monitoring of disease recurrence. Eventually, such agents may play a leading role in reducing the human burden of cardiovascular disease, by providing early diagnosis, noninvasive monitoring and effective therapy with reduced side effects.