Gd-loaded-RBCs for the assessment of tumor vascular volume by contrast-enhanced-MRI

Mn(iii), Fe(iii) and Zn(ii)-serum albumin as innovative multicolour contrast agents for photoacoustic imaging

Here we propose innovative photoacoustic imaging (PAI) contrast agents, based on the loading of Mn(iii)-, Fe(iii)- or Zn(ii)-protoporphyrin IX in serum albumin. These systems show different absorption wavelengths, opening the way to multicolor PA imaging. They were characterized in vitro for assessing stability, biocompatibility, and their optical and contrastographic properties. Finally, a proof of concept in vivo study was carried out in breast cancer bearing mice, to evaluate its effectiveness for cancer imaging.

Compartmentalized agents: A powerful strategy for enhancing the detection sensitivity of chemical exchange saturation transfer contrast

Since the very beginnings of the chemical exchange saturation transfer (CEST) technique, poor overall sensitivity has appeared to be one of its strongest limitations for future applications. Research has therefore focused on designing systems, such as supramolecular and nanosized agents, that contain a high number of magnetically equivalent mobile spins. However, the number of mobile spins offered by these systems is still limited by their composition and surface/volume ratio. The design of compartmentalized agents, that is, systems where an aqueous inner core is separated from the MRI-detected bulk pool via a semipermeable barrier/membrane, is very much a step forward for the technique. These vesicular systems can (i) act as biocompatible and versatile carriers for dia-, para-, and hetero-nuclear CEST probes, thus offering new application options; and (ii) act as CEST probes themselves via the encapsulation of a suitable agent (e.g., a paramagnetic shift reagent) that can change the resonance frequency of the spin pool in the inner compartment only. LipoCEST agents were the pioneers in the latter category, as they are able to grant picomolar sensitivity (in terms of nanoparticle concentration), and paved the way for new applications for CEST agents, especially in the theranostic research area. The use of larger, natural vesicular systems, such as yeasts and cells, in which the huge number of intravesicular spins lowers the detection threshold to a femtomolar limit, is a further step forward in the development of compartmentalized CEST agents. Finally, interesting combinations of nanovesicular and cellular compartmentalized systems have been proposed, thus highlighting how the approach has the potential to drive CEST agents towards completing their journey to mature clinical translation.

A Multifunctional Magnetic Red Blood Cell-Mimetic Micromotor for Drug Delivery and Image-Guided Therapy

Inspired by nature, innovative devices have been made to imitate the morphology and functions of natural red blood cells (RBCs). Here, we report a red blood cell-mimetic micromotor (RBCM), which was fabricated based on a layer-by-layer assembly method and precisely controlled by an external rotating uniform magnetic field. The main framework of the RBCM was constructed by the natural protein zein and finally camouflaged with the RBC membrane. Functional cargos such as Fe₃O₄ nanoparticles and the chemotherapeutic agent doxorubicin were loaded within the wall part of the RBCM for tumor therapy. Due to the massive loading of Fe₃O₄ nanoparticles, the RBCM can be precisely navigated by an external rotating uniform magnetic field and be used as a magnetic resonance imaging contrast agent for tumor imaging. The RBCM has been proven to be biocompatible, biodegradable, magnetically manipulated, and imageable, which are key requisites to take micromotors from the chalkboard to clinics. We expect the RBC-inspired biohybrid device to achieve wide potential applications.

Studies of the hydrophobic interaction between a pyrene - containing dye and a tetra-aza macrocyclic gadolinium complex

An in vivo and in vitro investigation of the hydrophobic interaction between HPTS and gadolinium(III)-complex of tetra-aza macrocyclic ligand HP-DO3A‡ (Gd(HP-DO3A)) is reported. UV-spectra at variable pH showed that the...

Re-engineered imaging agent using biomimetic approaches

Recent progress in biomedical technology, the clinical bioimaging, has a greater impact on the diagnosis, treatment, and prevention of disease, especially by early intervention and precise therapy. Varieties of organic and inorganic materials either in the form of small molecules or nano-sized materials have been engineered as a contrast agent (CA) to enhance image resolution among different tissues for the detection of abnormalities such as cancer and vascular occlusion. Among different innovative imaging agents, contrast agents coupled with biologically derived endogenous platform shows the promising application in the biomedical field, including drug delivery and bioimaging. Strategy using biocomponents such as cells or products of cells as a delivery system predominantly reduces the toxic behavior of its cargo, as these systems reduce non-specific distribution by navigating its cargo toward the targeted location. The hypothesis is that depending on the original biological role of the naïve cell, the contrast agents carried by such a system can provide corresponding natural designated behavior. Therefore, by combining properties of conventional synthetic molecules and nanomaterials with endogenous cell body, new solutions in the field of bioimaging to overcome biological barriers have been offered as innovative bioengineering. In this review, we will discuss the engineering of cell and cell-derived components as a delivery system for various contrast agents to achieve clinically relevant contrast for diagnosis and study underlining mechanism of disease progression. This article is categorized under: Nanotechnology Approaches to Biology > Cells at the Nanoscale Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Investigating plasma volume expanders as novel macromolecular MRI-CEST contrast agents for tumor contrast-enhanced imaging

PURPOSE

The aim of this study was to investigate two clinically approved plasma volume expanders (dextran 70 and voluven) as macromolecular MRI-chemical exchange saturation transfer (CEST) contrast agents to assess tumor vascular properties.

METHODS

CEST contrast efficiency of both molecules (6% w/v) was measured in vitro at various irradiation saturation powers (1-6 μT for 5 s) and pH values (range, 5.5-7.9) and the exchange rate of hydroxyl protons was calculated. In vivo studies in a murine adenocarcinoma model (n = 4 mice for each contrast agent) upon i.v. injection provided CEST-derived perfusion tumor properties that were compared with those obtained with a gadolinium-based blood-pool agent (Gd-AAZTA-Madec).

RESULTS

In vitro measurements showed a marked CEST contrast dependency to pH, with higher CEST contrast at lower pH values for both molecules. The measured prototropic exchange rates confirmed a base-catalyzed exchange rate that was faster for dextran 70 in comparison to voluven. Both molecules showed a similar CEST contrast increase (ΔST% > 3%) in the tumor tissue up to 30 min postinjection, with heterogeneous accumulation. In tumors receiving both CEST and T₁ -weighted agents, a voxel-by-voxel analysis indicated moderate spatial correlation of perfusion properties between voluven/dextran 70 and Gd-AAZTA-Madec, suggesting different distribution patterns according to their molecular size.

CONCLUSIONS

The obtained results showed that both voluven and dextran 70 can be exploited as MRI-CEST contrast agents for evaluating tumor enhancement properties. Their increased accumulation in tumors and prolonged contrast enhancement promote their use as blood-pool MRI-CEST agents to examine tumor vascularization.

Monitoring tissue implants by field-cycling 1H-MRI via the detection of changes in the 14N-quadrupolar-peak from imidazole moieties incorporated in a "smart" scaffold material

This study is focused on the development of innovative sensors to non-invasively monitor the tissue implant status by Fast-Field-Cycling Magnetic Resonance Imaging (FFC-MRI). These sensors are based on oligo-histidine moieties that are conjugated to PLGA polymers representing the structural matrix for cells hosting scaffolds. The presence of 14N atoms of histidine causes a quadrupolar relaxation enhancement (also called Quadrupolar Peak, QP) at 1.39 MHz. This QP falls at a frequency well distinct from the QPs generated by endogenous semisolid proteins. The relaxation enhancement is pH dependent in the range 6.5-7.5, thus it acts as a reporter of the scaffold integrity as it progressively degrades upon lowering the microenvironmental pH. The ability of this new sensors to generate contrast in an image obtained at 1.39 MHz on a FFC-MRI scanner is assessed. A good biocompatibility of the histidine-containing scaffolds is observed after its surgical implantation in healthy mice. Over time the scaffold is colonized by endogenous fibroblasts and this process is accompanied by a progressive decrease of the intensity of the relaxation peak. In respect to the clinically used contrast agents this material has the advantage of generating contrast without the use of potentially toxic paramagnetic metal ions.

Supramolecular adducts between macrocyclic Gd(iii) complexes and polyaromatic systems: a route to enhance the relaxivity through the formation of hydrophobic interactions

The set-up of reversible binding interactions between the hydrophobic region of macrocyclic GBCAs (Gadolinium Based Contrast Agents) and SO₃ ^-/OH containing pyrene derivatives provides new insights for pursuing relaxivity enhancements of this class of MRI contrast agents. The strong binding affinity allows attaining relaxation enhancements up to 50% at pyrene/GBCA ratios of 3 : 1. High resolution NMR spectra of the Yb-HPDO3A/pyrene system fully support the formation of a supramolecular adduct based on the set-up of hydrophobic interactions. The relaxation enhancement may be accounted for in terms of the increase of the molecular reorientation time (τ _R) and the number of second sphere water molecules. This effect is maintained in blood serum and in vivo, as shown by the enhancement of contrast in T _1w-MR images obtained by simultaneous injection of GBCA and pyrene derivatives in mice.

Co(II) Macrocyclic Complexes Appended with Fluorophores as paraCEST and cellCEST Agents

Four high-spin macrocyclic Co(II) complexes with hydroxypropyl or amide pendants and appended coumarin or carbostyril fluorophores were prepared as CEST (chemical exchange saturation transfer) MRI probes. The complexes were studied in solution as paramagnetic CEST (paraCEST) agents and after loading into Saccharomyces cerevisiae yeast cells as cell-based CEST (cellCEST) agents. The fluorophores attached to the complexes through an amide linkage imparted an unusual pH dependence to the paraCEST properties of all four complexes through of ionization of a group that was attributed to the amide NH linker. The furthest shifted CEST peak for the hydroxypropyl-based complexes changed by ∼90 ppm upon increasing the pH from 5 to 7.5. At acidic pH, the Co(II) complexes exhibited three to four CEST peaks with the most highly shifted CEST peak at 200 ppm. The complexes demonstrated substantial paramagnetic water proton shifts which is a requirement for the development of cellCEST agents. The large shift in the proton resonance was attributed to an inner-sphere water at neutral pH, as shown by variable temperature ¹⁷O NMR spectroscopy studies. Labeling of yeast with one of these paraCEST agents was optimized with fluorescence microscopy and validated by using ICP mass spectrometry quantitation of cobalt. A weak asymmetry in the Z-spectra was observed in the yeast labeled with a Co(II) complex, toward a cellCEST effect, although the Co(II) complexes were toxic to the cells at the concentrations necessary for observation of cellCEST.

Saccharomyces cerevisiae and Candida albicans Yeast Cells Labeled with Fe(III) Complexes as MRI Probes

The development of MRI probes is of interest for labeling antibiotic-resistant fungal infections based on yeast. Our work showed that yeast cells can be labeled with high-spin Fe(III) complexes to produce enhanced T2 water proton relaxation. These Fe(III)-based macrocyclic complexes contained a 1,4,7-triazacyclononane framework, two pendant alcohol groups, and either a non-coordinating ancillary group and a bound water molecule or a third coordinating pendant. The Fe(III) complexes that had an open coordination site associated strongly with Saccharomyces cerevisiae upon incubation, as shown by screening using Z-spectra analysis. The incubation of one Fe(III) complex with either Saccharomyces cerevisiae or Candida albicans yeast led to an interaction with the β-glucan-based cell wall, as shown by the ready retrieval of the complex by the bidentate chelator called maltol. Other conditions, such as a heat shock treatment of the complexes, produced Fe(III) complex uptake that could not be reversed by the addition of maltol. Appending a fluorescence dye to Fe(TOB) led to uptake through secretory pathways, as shown by confocal fluorescence microscopy and by the incomplete retrieval of the Fe(III) complex by the maltol treatment. Yeast cells that were labeled with these Fe(III) complexes displayed enhanced water proton T2 relaxation, both for S. cerevisiae and for yeast and hyphal forms of C. albicans.

Relaxometric studies of erythrocyte suspensions infected by Plasmodium falciparum: a tool for staging infection and testing anti-malarial drugs

PURPOSE

Malaria is a global health problem with the most malignant form caused by Plasmodium falciparum (P. falciparum). Parasite maturation in red blood cells (RBCs) is accompanied by changes including the formation of paramagnetic hemozoin (HZ) nanocrystals, and increased metabolism and variation in membrane lipid composition. Herein, MR relaxometry (MRR) was applied to investigate water exchange across RBCs' membrane and HZ formation in parasitized RBCs.

METHODS

Transverse water protons relaxation rate constants (R₂ = 1/T₂ ) were measured for assessing HZ formation in P. falciparum-parasitized human RBCs. Moreover, water exchange lifetimes across the RBC membrane (τ_i ) were assessed by measuring longitudinal relaxation rate constants (R₁ = 1/T₁ ) at 21.5 MHz in the presence of a gadolinium complex dissolved in the suspension medium.

RESULTS

τ_i increased after invasion of parasites (ring stage, mean τ_i / τ i 0 = 1.234 ± 0.022) and decreased during maturation to late trophozoite (mean τ_i / τ i 0 = 0.960 ± 0.075) and schizont stages (mean τ_i / τ i 0 = 1.019 ± 0.065). The HZ accumulation in advanced stages was revealed by T₂ -shortening. The curves reporting R₂ (1/T₂ ) vs. magnetic field showed different slopes for non-parasitized RBCs (npRBCs) and parasitized RBCs (pRBCs), namely 0.003 ± 0.001 for npRBCs, 0.009 ± 0.002, 0.028 ± 0.004 and 0.055 ± 0.002 for pRBCs at ring-, early trophozoite-, and late trophozoite stage, respectively. Antimalarial molecules dihydroartemisinin and chloroquine elicited measurable changes in parasitized RBCs, namely dihydroartemisinin modified τ_i , whereas the interference of chloroquine with HZ formation was detectable by a significant T₂ increase.

CONCLUSIONS

MRR can be considered a useful tool for reporting on P. falciparum blood stages and for screening potential antimalarial molecules.

Photoacoustic ratiometric assessment of mitoxantrone release from theranostic ICG-conjugated mesoporous silica nanoparticles

A theranostic nanosystem based on indocyanine green (ICG) covalently conjugated to mesoporous silica nanoparticles (MSNs) loaded with the anticancer drug mitoxantrone (MTX) is proposed as an innovative photoacoustic probe. Taking advantage of the characteristic PA signal displayed by both ICG and MTX, a PA-ratiometric approach was applied to assess the drug release profile from the MSNs. After complete in vitro characterization of the nanoprobe, a proof-of-concept study has been carried out in tumour-bearing mice to evaluate in vivo its effectiveness for cancer imaging and chemotherapeutic agent delivery.

Fast RBC loading by fluorescent antibodies and nuclei staining dye and their potential bioanalytical applications

This study was designed to load different antibodies (Abs) and a fluorescent dye onto the red blood cell (RBC) surface. We have used fluorescein isothiocyanate (FITC)-conjugate anti-human Ab, CD22-PE (B-cell marker-phycoerythrin Ab), and 4',6-diamidino-2-phenylindole (DAPI) for insertion over the RBC surface. In a first step, conjugation experiments were performed: in dimethyl sulfoxide (DMSO), RBCs were conserved and modified by succinic anhydride to create an additional -COOH group, and then activated with 3-(3-dimethylaminopropyl)carbodiimide-N-hydroxysuccinimide (EDC-NHS) in 2-(N-morpholino) ethanesulfonic acid hydrate buffer for insertion of labeled Abs or DAPI. In a second step, fluorescence signals were evaluated by microscopy and the mean fluorescence intensities of cell lysates were measured by spectrofluorometry. The results showed clear evidence for adsorption of FITC- and PE-labeled Abs to activated conserved RBCs. DAPI was adsorbed well also to DMSO-conserved RBCs without the need for an activation step. The DMSO conservation step was enough to create reactive RBCs for insertion of specific Abs and fluorescent dyes. The additional modification by succinic anhydride and activation with EDC-NHS resulted in two- to seven-fold increase in fluorescence signals, indicating a much higher RBC loading capacity. These Ab- and fluorescent dye-functionalized RBCs have potentially high application in developing new biomedical diagnostic and in vitro assay techniques.

Biomimetic surface modification of discoidal polymeric particles

The rationale for the design of drug delivery nanoparticles is traditionally based on co-solvent self-assembly following bottom-up approaches or in combination with top-down approaches leading to tailored physiochemical properties to regulate biological responses. However, the optimal design and control of material properties to achieve specific biological responses remain the central challenge in drug delivery research. Considering this goal, we herein designed discoidal polymeric particles (DPPs) whose surfaces are re-engineered with isolated red blood cell (RBC) membranes to tailor their pharmacokinetics. The RBC membrane-coated DPPs (RBC-DPPs) were found to be biocompatible in cell-based in vitro experiments and exhibited extended blood circulation half-life. They also demonstrated unique kinetics at later time points in a mouse model compared to that of bare DPPs. Our results suggested that the incorporation of biomimicry would enable the biomimetic particles to cooperate with systems in the body such as cells and biomolecules to achieve specific biomedical goals.

CEST-MRI for glioma pH quantification in mouse model: Validation by immunohistochemistry

In glioma, the acidification of the extracellular tumor microenvironment drives proliferation, angiogenesis, immunosuppression, invasion and chemoresistance. Therefore, quantification of glioma extracellular pH (pHe) is of crucial importance. This study is focused on the application of the YbHPDO3A (ytterbium 1,4,7-triscarboxymethyl-1,4,7,10-tetraazacyclododecane) probe for in vivo glioma pHe quantification using chemical exchange saturation transfer (CEST)-MRI and its correlation with tumor metabolism assessed by immunohistochemistry. The U87 glioma mouse model was used (n = 18) and MRI performed at 4.7 T. CEST-MRI of YbHPDO3A solutions at different pH values showed two resolved CEST spectra at 71 ppm and 99 ppm, both sensitive to pH variations, allowing therefore calculation of the ratiometric curve for in vivo pH quantification. In vivo MRI sequences consisted of T_2w for tumor localization, T_2w * to assess YbHPDO3A biodistribution by exploiting its magnetic susceptibility effect and CEST for glioma pHe mapping. T_2w * images show that YbHPDO3A extravasates in tumor in regions with damaged blood-brain barrier. The pHe is calculated only in these regions. Hematoxylin/eosin histology and Ki-67, CA-IX (carbonic anhydrase 9) and NHE-1 immunohistochemical staining were performed; their expression rates were compared with the in vivo pHe values. On the basis of the cell proliferation marker Ki-67, two groups were defined: one group with a lower mitotic index (MI% < 20% = mean value) and a mean pHe value of 7.00 (low-proliferation/high-pH group) and the other with MI% > 20% and an acidic pHe of 6.6 (high-proliferation/low-pH group). CA-IX and NHE-1 were over-expressed in the high-proliferation/low-pH group (CA-IX, 92 ± 7% versus 30 ± 13%; NHE-1, 84 ± 8% versus 35 ± 11%), indicating an acidic/hypoxic microenvironment. These immunohistochemical results are consistent with our pHe mapping (Pearson correlation coefficient > 0.70) and provide evidence for the feasibility of the CEST-MRI method with the YbHPDO3A probe for glioma pHe quantification at 4.7 T. Importantly, the YbHPDO3A probe has similar chemical and biological properties to the clinically approved MRI contrast agent GdHPDO3A. This makes the method promising for a clinical translation.

Intracellular Delivery by Membrane Disruption: Mechanisms, Strategies, and Concepts

Intracellular delivery is a key step in biological research and has enabled decades of biomedical discoveries. It is also becoming increasingly important in industrial and medical applications ranging from biomanufacture to cell-based therapies. Here, we review techniques for membrane disruption-based intracellular delivery from 1911 until the present. These methods achieve rapid, direct, and universal delivery of almost any cargo molecule or material that can be dispersed in solution. We start by covering the motivations for intracellular delivery and the challenges associated with the different cargo types-small molecules, proteins/peptides, nucleic acids, synthetic nanomaterials, and large cargo. The review then presents a broad comparison of delivery strategies followed by an analysis of membrane disruption mechanisms and the biology of the cell response. We cover mechanical, electrical, thermal, optical, and chemical strategies of membrane disruption with a particular emphasis on their applications and challenges to implementation. Throughout, we highlight specific mechanisms of membrane disruption and suggest areas in need of further experimentation. We hope the concepts discussed in our review inspire scientists and engineers with further ideas to improve intracellular delivery.

Generation of multiparametric MRI maps by using Gd-labelled- RBCs reveals phenotypes and stages of murine prostate cancer

Prostate Cancer (PCa) is the second most common and fifth cause of cancer-related mortality in males in Western Countries. The development of innovative tools for an early, more precise and noninvasive diagnosis is a medical need. Vascular volume (Vv) and hypoxia are two of the most important tumor hallmarks. Herein, they have been assessed in TRAMP mice by using MRI. Their quantification has been carried out by injecting autologous Red Blood Cells (RBCs), ex vivo labelled with Gd-HPDO3A or Gd-DOTP complexes, respectively. Gd-labelled-RBCs are stably confined in the intravascular space, also in presence of a very leaky tumor endothelium, thus representing efficient probes for vascular space analysis. Vv enhancement and hypoxia onset have been demonstrated to be present at early stages of PCa and their expression largely increases with tumor development. Moreover, also Diffusion weighted MRI and Amide Proton Transfer MRI have been herein applied to characterize PCa. The herein applied multiparametric MRI (mpMRI) analysis allows a detailed in vivo characterization of PCa, in which each histotype and cancer stage displays a specific MRI pattern. This provides an unprecedented opportunity to feature prostate tumor, making possible a non-invasive, precise and early diagnosis, which could direct treatments towards a more personalized medicine.

Ferucarbotran-loaded red blood cells as long circulating MRI contrast agents: first in vivo results in mice

AIM

The encapsulation of superparamagnetic iron oxide contrast agents in red blood cells (RBCs) could overcome their rapid removal by reticulo-endothelial system improving their stability in blood circulation.

MATERIALS & METHODS

Murine ferucarbotran-loaded RBCs were tested in vivo as new contrasting agents in MRI application.

RESULTS

A superior visualization of organs and cerebral vessels was evidenced in ferucarbotran-loaded RBCs-treated mice compared with the controls. The signal enhancement lasted for days, while the contrast from bulk ferucarbotran disappeared after few minutes.

CONCLUSION

Ferucarbotran-loaded RBCs showed to improve diagnostic imaging and their use may extend the time frame for MRI and magnetic resonance angiography since to date the time frame for data acquisition has been limited to the first pass.

Nano-sized and other improved reporters for magnetic resonance imaging of angiogenesis

Magnetic Resonance Imaging (MRI) enables to provide anatomical, functional and molecular information of pathological angiogenesis when used with properly tailored imaging probes. Functional studies have been the domain of Dynamic Contrast Enhancement (DCE) -MRI protocols from which it is possible to extract quantitative estimations on key parameters such as the volumes of vascular and extracellular compartments and the rates of the bidirectional exchange of the imaging reporters across the endothelial barrier. Whereas paramagnetic Gd-complexes able to reversibly bind to serum albumin act better than the clinically used small-sized, hydrophilic species, new findings suggest that an accurate assessment of the vascular volume is possible by analyzing images acquired upon the i.v. administration of Gd-labelled Red Blood Cells (RBCs). As far as it concerns molecular MRI, among the many available biomarkers, α_vβ₃ integrins are the most investigated ones. The low expression of these targets makes mandatory the use of nano-sized systems endowed with the proper signal enhancing capabilities. A number of targeted nano-particles have been investigated including micelles, liposomes, iron oxides and perfluorocarbon containing systems. Finally, a growing attention is devoted to the design and testing of "theranostic" agents based on the exploitation of MRI to monitor drug delivery processes and therapeutic outcome.

External Magnetic Field-Induced Targeted Delivery of Highly Sensitive Iron Oxide Nanocubes for MRI of Myocardial Infarction

Magnetic field responsive nanocubes (MFRFs) are synthesized as nanoplatforms for external magnetic field-induced selective targeting of macrophages in the infarcted tissue and magnetic resonance imaging (MRI) monitoring. MFRFs have uniform size, favorable colloidal stability, and high magnetic properties. Under the influence of external magnetic field, MFRFs perform qualitative and quantitative MRI of myocardial infarction.

LipoCEST and cellCEST imaging agents: opportunities and challenges

From the early days of CEST agents' disclosure, it was evident that their potential for in vivo applications was strongly hampered by the intrinsic low sensitivity. Therefore, much work has been devoted to seek out suitable routes to achieve strong CEST contrast enhancement. The use of nanosized systems turned out to be a strategic choice, because a very large amount of CEST agents can be delivered at the site of interest. However, the breakthrough innovation in term of increase of sensitivity was found by designing the lipoCEST agents. The naturally inspired, liposomes vesicles, when loaded with paramagnetic lanthanide-based shift reagents, can be transformed into CEST probes. The large number of water molecules entrapped inside the inner cavity of the nanovesicles represents an enormous pool of exchanging protons for the generation of CEST contrast, whereas the presence of the shift reagent increases the separation in chemical shift of their nuclear magnetic resonance signal from that of the bulk water, thus allowing for a proper exchange regime for the activation of CEST contrast. From lipoCEST, it has been rather straightforward to evolve to cellCEST in order to exploit the cytoplasmatic water molecules as source of the CEST effect, once cells have been loaded with the proper shift reagent. The red blood cells were found to be particularly suitable for the development of the cellCEST concept. Finally, an understanding of the main determinants of the CEST effects in nanosized and cellular-sized agents has allowed the design of innovative lipoCEST/RBC aggregates for potential theranostic applications. WIREs Nanomed Nanobiotechnol 2016, 8:602-618. doi: 10.1002/wnan.1385 For further resources related to this article, please visit the WIREs website.

An MRI Method To Map Tumor Hypoxia Using Red Blood Cells Loaded with a pO2-Responsive Gd-Agent

Hypoxia is a typical hallmark of many solid tumors and often leads to therapy resistance and the development of a more aggressive cancer phenotype. Oxygen content in tissues has been evaluated using numerous different methods for several imaging modalities, but none has yet reached the required standard of spatial and temporal resolution. Magnetic Resonance Imaging (MRI) appears to be the technique of choice and several pO2-responsive probes have been designed for it over the years. In vivo translation is often hampered in Gd-relaxation agents as it is not possible to separate effects that arise from changes in local concentration from those associated with responsive properties. A novel procedure for the MRI based assessment of hypoxia is reported herein. The method relies on the combined use of Gd-DOTP- and Gd-HPDO3A-labeled red blood cells (RBCs) where the first probe acts as a vascular oxygenation-responsive agent, while the second reports the local labeled RBC concentration in a transplanted breast tumor mouse model. The MRI assessment of oxygenation state has been validated by photoacoustic imaging and ex vivo immunofluorescence. The method refines tumor staging in preclinical models and makes possible an accurate monitoring of the relationship between oxygenation and tumor growth.