A new gadolinium-based contrast agent for magnetic resonance imaging of brain tumors: kinetic study on a C6 rat glioma model

Review of the Efficacy and Safety of Gadopiclenol: A Newly Emerging Gadolinium-Based Contrast Agent

Gadolinium-based contrast agents (GBCAs) are one of the most commonly used agents in magnetic resonance imaging. Gadopiclenol is a new GBCA aimed at providing improved diagnostic efficacy with a favorable safety profile. The proposed advantages are due to its specific pharmacological properties, one of which is high relaxivity values. The aim of this review is to assess the efficacy, diagnostic accuracy, and safety of gadopiclenol in comparison to other currently used gadolinium-based contrast agents. PubMed and other database systems were used to identify relevant studies. The Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines were followed, resulting in 10 articles that were included in the review. The outcomes were reviewed according to several factors regarding efficacy and accuracy in terms of qualitative and quantitative descriptors relative to properties of enhancement provided by the contrast agent. In terms of safety profile, a number of outcomes were assessed such as the occurrence of serious adverse effects, severe kidney injury, and organ-based contrast retention. Gadopiclenol was found to provide outcomes comparable to other commonly used GBCAs at lower doses with further favorable results at higher doses while maintaining an acceptable safety profile. However, it was found to have high rates of retention within the liver and can cause nonsignificant QT prolongation in healthy individuals, which arguably creates the need for further research regarding more long-term implications of these possible adverse effects.

Pulsed high magnetic field-induced reversible blood-brain barrier permeability to enhance brain-targeted drug delivery

The present study aimed to select an effective Pulsed High Magnetic Field (PHMF) stimulation protocol that would induce the Blood-Brain Barrier's (BBB) reversible permeability to enhance brain-targeted drug delivery. PHMF was applied to the skull over the right hemisphere of 60 Wistar rats. The sham group contained other 10 rats that did not receive PHMF stimulation. The investigated parameters were repetition frequencies (0.25, 1, and 4 Hz as well as the effective low frequency combined with 10 Hz) and numbers of pulses in each train. Evans Blue Dye (EBD) uptake within the brain parenchyma was measured to select an effective PHMF stimulation protocol. BBB reversibility was evaluated by measuring EBD uptake and Gadobutrol retention, through MRI signal intensity enhancement, within brain parenchyma after exposure to the effective PHMF stimulation protocol at different time points including 0.5, 1, and 24 hours. The obtained results showed that the PHMF stimulation increased the BBB's reversible permeability; this increase was more significant for 28 pulses with 1 Hz frequency (P < .0001). Changes in EBD uptake and MRI signal intensity in the exposed side (right hemisphere) peaked within 0.5-1 hour and returned to normal levels 24 hours after exposure to the effective protocol of PHMF stimulation (28 pulses with 1 Hz frequency). The Contrast-Enhanced MRI (CE-MRI) signal intensity confirmed the changes in EBD concentration. PHMF stimulation can be used as an effective protocol for enhancing the permeability reversibly of BBB, hence considered a potential clinical approach to brain-targeted drug delivery.

Metabolic NMR mapping with microgram tissue biopsy

This study explores the potential of profiling a microgram-scale soft tissue biopsy by NMR spectroscopy. The important elements of high resolution and high sensitivity for the spectral data are achieved through a unique probe, HR-μMAS, which allowed comprehensive profiling to be performed on microgram tissue for the first time under MAS conditions. Thorough spatially resolved metabolic maps were acquired across a coronal brain slice of rat C6 gliomas, which rendered the delineation of the tumor lesion. The results present a unique ex vivo NMR possibility to analyze tissue pathology that cannot be fully explored by the conventional approach, HR-MAS and in vivo MRS. Aside from the capability of analyzing a small localized region to track its specific metabolism, it could also offer the possibility to carry out longitudinal investigations on live animals due to the feasibility of minimally invasive tissue excision.

Contrast-to-Dose Relationship of Gadopiclenol, an MRI Macrocyclic Gadolinium-based Contrast Agent, Compared with Gadoterate, Gadobenate, and Gadobutrol in a Rat Brain Tumor Model

Background Detection of cerebral lesions at MRI may benefit from a chemically stable and more sensitively detected gadolinium-based contrast agent (GBCA). Gadopiclenol, a macrocyclic GBCA with at least twofold higher relaxivity, is currently undergoing clinical trials in humans. Purpose To determine the relationship between MRI contrast enhancement and the injected dose of gadopiclenol in a glioma rat model compared with those of conventional GBCA at label dose. Materials and Methods Between April and July 2012, 32 rats implanted with C6 glioma received two intravenous injections at a 24-hour interval. The injections were randomly selected among five doses of gadopiclenol (0.025, 0.05, 0.075, 0.1, and 0.2 mmol/kg) and three reference GBCAs (gadoterate meglumine, gadobutrol, and gadobenate dimeglumine) at 0.1 mmol/kg. MRI tumor enhancement was assessed on T1-weighted images before and up to 30 minutes after injection. Two blinded radiologists visually and qualitatively scored contrast enhancement, border delineation, and visualization of tumor morphology. Quantitatively, variations in contrast-to-noise ratio (ΔCNR) between tumor and contralateral parenchyma were calculated at each time point and were compared for each treatment at 5 minutes by using a mixed model after normality test. Results A total of 24 rats underwent the complete protocol (n = 5-7 per group). A linear dose-dependent ΔCNR relationship was observed between 0.025 and 0.1 mmol/kg for gadopiclenol (R ² = 0.99). No difference in ΔCNR was observed between the three reference GBCAs (P ≥ .55). Gadopiclenol resulted in twofold higher ΔCNR at 0.1 mmol/kg (P < .001 vs gadobutrol and gadoterate, P = .002 vs gadobenate) and similar ΔCNR at 0.05 mmol/kg (P = .56, P > .99, and P = .44 compared with gadobutrol, gadobenate, and gadoterate, respectively). For both readers, 0.05 mmol/kg of gadopiclenol improved contrast enhancement, border delineation, and visualization of tumor morphology (scores > 3 compared with scores between 2 and 3 for the marketed GBCA). Conclusion Gadopiclenol at 0.05 mmol/kg yielded comparable change in contrast-to-noise ratio and morphologic characterization of brain tumors compared with gadobenate, gadoterate, or gadobutrol at 0.1 mmol/kg. Published under a CC BY 4.0 license. Online supplemental material is available for this article. See also the editorial by Tweedle in this issue.

Technical Note: Clinical translation of the Rapid-Steady-State-T1 MRI method for direct cerebral blood volume quantification

PURPOSE

In preclinical studies, the Rapid-Steady-State-T1 (RSST1) MRI method has advantages over conventional MRI methods for blood volume fraction (BVf) mapping, since after contrast agent administration, the BVf is directly quantifiable from the signal amplitude corresponding to the vascular equilibrium magnetization. This study focuses on its clinical implementation and feasibility.

METHODS

Following sequence implementation on clinical Philips Achieva scanners, the RSST1-method is assessed at 1.5 and 3 T in the follow-up examination of neurooncological patients receiving 0.1-0.2 mmol/kg Gd-DOTA to determine the threshold dose needed for cerebral BVf quantification. Confounding effects on BVf quantification such as transendothelial water exchange, transverse relaxation, and contrast agent extravasation are evaluated.

RESULTS

For a dose≥0.13 mmol/kg at 1.5 T and ≥0.16 mmol/kg at 3 T, the RSST1-signal time course in macrovessels and brain tissue with Gd-DOTA impermeable vasculature reaches a steady state at maximum amplitude for about 8 s. In macrovessels, a BVf of 100% was obtained validating cerebral microvascular BVf quantification (3.5%-4.5% in gray matter and 1.5%-2.0% in white matter). In tumor tissue, a continuously increasing signal is detected, necessitating signal modeling for tumor BVf calculation.

CONCLUSIONS

Using approved doses of Gd-DOTA, the steady state RSST1-signal in brain tissue is reached during the first pass and corresponds to the BVf. The first-pass duration is sufficient to allow accurate BVf quantification. The RSST1-method is appropriate for serial clinical studies since it allows fast and straightforward BVf quantification without arterial input function determination. This quantitative MRI method is particularly useful to assess the efficacy of antiangiogenic agents.

Diffusion properties of conventional and calcium-sensitive MRI contrast agents in the rat cerebral cortex

Calcium-sensitive MRI contrast agents can only yield quantitative results if the agent concentration in the tissue is known. The agent concentration could be determined by diffusion modeling, if relevant parameters were available. We have established an MRI-based method capable of determining diffusion properties of conventional and calcium-sensitive agents. Simulations and experiments demonstrate that the method is applicable both for conventional contrast agents with a fixed relaxivity value and for calcium-sensitive contrast agents. The full pharmacokinetic time-course of gadolinium concentration estimates was observed by MRI before, during and after intracerebral administration of the agent, and the effective diffusion coefficient D* was determined by voxel-wise fitting of the solution to the diffusion equation. The method yielded whole brain coverage with a high spatial and temporal sampling. The use of two types of MRI sequences for sampling of the diffusion time courses was investigated: Look-Locker-based quantitative T(1) mapping, and T(1) -weighted MRI. The observation times of the proposed MRI method is long (up to 20 h) and consequently the diffusion distances covered are also long (2-4 mm). Despite this difference, the D* values in vivo were in agreement with previous findings using optical measurement techniques, based on observation times of a few minutes. The effective diffusion coefficient determined for the calcium-sensitive contrast agents may be used to determine local tissue concentrations and to design infusion protocols that maintain the agent concentration at a steady state, thereby enabling quantitative sensing of the local calcium concentration.

Characterization of tumor angiogenesis in rat brain using iron-based vessel size index MRI in combination with gadolinium-based dynamic contrast-enhanced MRI

This study aimed at combining an iron-based, steady-state, vessel size index magnetic resonance imaging (VSI MRI) approach, and a gadolinium (Gd)-based, dynamic contrast-enhanced MRI approach (DCE MRI) to characterize tumoral microvasculature. Rats bearing an orthotopic glioma (C6, n=14 and RG2, n=6) underwent DCE MRI and combined VSI and DCE MRI 4 h later, at 2.35 T. Gd-DOTA (200 mumol of Gd per kg) and ultrasmall superparamagnetic iron oxide (USPIO) (200 micromol of iron per kg) were used for DCE and VSI MRI, respectively. C6 and RG2 gliomas were equally permeable to Gd-DOTA but presented different blood volume fractions and VSI, in good agreement with histologic data. The presence of USPIO yielded reduced K(trans) values. The K(trans) values obtained with Gd-DOTA in the absence and in the presence of USPIO were well correlated for the C6 glioma but not for the RG2 glioma. It was also observed that, within the time frame of DCE MRI, USPIO remained intravascular in the C6 glioma whereas it extravasated in the RG2 glioma. In conclusion, VSI and DCE MRI can be combined provided that USPIO does not extravasate with the time frame of the DCE MRI experiment. The mechanisms at the origin of USPIO extravasation remain to be elucidated.

Cerebral blood volume quantification in a C6 tumor model using gadolinium per (3,6-anhydro) alpha-cyclodextrin as a new magnetic resonance imaging preclinical contrast agent

In magnetic resonance imaging (MRI), cerebral blood volume (CBV) quantification is dependent on the MRI sequence and on the properties of the contrast agents (CAs). By using the rapid steady-state T(1) method, we show the potential of gadolinium per (3,6-anhydro) alpha-cyclodextrin (Gd-ACX), a new MRI paramagnetic CA (inclusion complex of Gd(3+) with per (3,6-anhydro)-alpha-cyclodextrin), for the CBV quantification in the presence of blood-brain barrier lesions. After biocompatibility and relaxivity experiments, in vivo experiments on rats were performed on a C6 tumor model with 0.05 mmol Gd-ACX/kg (<1/10 of the median lethal dose) injected at a 25 mmol/L concentration, inducing neither nephrotoxicity nor hemolysis. On T(1)-weighted images, a signal enhancement of 170% appeared in vessels after injection, but not in the tumor (during the 1 h of observation), in contrast to the 90% signal enhancement obtained with Gd-DOTA (a clinical MRI CA) injected at a T(1) isoefficient dose. This result shows the absence of Gd-ACX extravasation into the tumor tissue and its confinement to the vascular space. Fractional CBV values were found similar to Gd-ACX and Gd-DOTA in healthy brain tissue and in the contralateral hemisphere of tumor-bearing rats, whereas only Gd-ACX was appropriate for CBV quantification in tumor regions.

Serial In vivo Spectroscopic Nuclear Magnetic Resonance Imaging of Lactate and Extracellular pH in Rat Gliomas Shows Redistribution of Protons Away from Sites of Glycolysis

The acidity of the tumor microenvironment aids tumor growth, and mechanisms causing it are targets for potential therapies. We have imaged extracellular pH (pHe) in C6 cell gliomas in rat brain using 1H magnetic resonance spectroscopy in vivo. We used a new probe molecule, ISUCA [(±)2-(imidazol-1-yl)succinic acid], and fast imaging techniques, with spiral acquisition in k-space. We obtained a map of metabolites [136 ms echo time (TE)] and then infused ISUCA in a femoral vein (25 mmol/kg body weight over 110 min) and obtained two consecutive images of pHe within the tumor (40 ms TE, each acquisition taking 25 min). pHe (where ISUCA was present) ranged from 6.5 to 7.5 in voxels of 0.75 μL and did not change detectably when [ISUCA] increased. Infusion of glucose (0.2 mmol/kg·min) decreased tumor pHe by, on average, 0.150 (SE, 0.007; P &lt; 0.0001, 524 voxels in four rats) and increased the mean area of measurable lactate peaks by 54.4 ± 3.4% (P &lt; 0.0001, 287 voxels). However, voxel-by-voxel analysis showed that, both before and during glucose infusion, the distributions of lactate and extracellular acidity were very different. In tumor voxels where both could be measured, the glucose-induced increase in lactate showed no spatial correlation with the decrease in pHe. We suggest that, although glycolysis is the main source of protons, distributed sites of proton influx and efflux cause pHe to be acidic at sites remote from lactate production. [Cancer Res 2007;67(16):7638–45]

A new Magnetic Resonance Imaging method for mapping the cerebral blood volume fraction: the rapid steady-state T1 method

This paper describes a new rapid steady-state T(1) (RSST(1)) method for mapping the cerebral blood volume fraction (CBVf) by magnetic resonance imaging (MRI). The principle is based on a two-compartment model of the brain (intra- and extravascular), and the effects of paramagnetic contrast agents on the intravascular longitudinal relaxation time T(1). Using appropriate parameters, an Inversion-Recovery-Fast-Low-Angle-Shot sequence acts like a low pass T(1) filter, suppressing signals from tissues with T(1)>>TR (TR=repetition time). It was shown in vivo that, exceeding a particular contrast agent dose, the signal reaches its maximum (corresponding to the intravascular equilibrium magnetization), and is maintained for a duration related to the dose. Acquisitions during this steady state divided by an additional measure of the overall (intra- and extravascular) magnetization at thermal equilibrium provides the CBVf. Experiments were performed on healthy rats at 2.35 T using P760 (Gd(3+)-compound from Guerbet Laboratories) and Gd-DOTA. Because of its high longitudinal relaxivity, P760 is more convenient, and was used to show the feasibility of the method. The CBVf in different structures of the rat brain was compared. The average CBVf for the whole brain slice is 3.29%+/-0.69% (n=15). The influence of transendothelial water exchange was quantified and transversal relaxation effects were found negligible in microvasculature. Finally, the sensitivity of the method to CBVf increases under hypercapnia was evaluated (1%/mm Hg PaCO(2)), demonstrating its potential for longitudinal studies and functional MRI. Clinical applications are feasible since equivalent results were obtained with Gd-DOTA.

In vivo assessment of tumoral angiogenesis

Vessel size imaging (VSI) for brain tumor characterization was evaluated and the vessel size index measured by MRI (VSIMRI) was correlated with VSI obtained by histology (VSIhisto). Blood volume (BV) and VSI maps were obtained on 12 rats by simultaneous measurements of R2* and R2, before and after the injection of a macromolecular contrast agent, AMI-227. Immunostaining of collagen IV in vessels was performed. An expression was derived for evaluating VSI from stereologic measurements on histology data (VSIhisto). On BV and VSI images obtained from large-size tumors (n = 9), three regions could be distinguished and correlated well with histological sections: a high BV region surrounding the tumor, a necrotic area where BV is very low, and a viable tumor tissue region showing lower BV but higher VSI than the normal rat cortex, with the presence of larger vessels. The quantitative analysis showed a good correlation (Spearman rank's rho = 0.74) between VSIhisto and VSIMRI with a linear regression coefficient of 1.17. The good correlation coefficient supports VSI imaging as a quantitative method for tumor vasculature characterization.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.

Comparison between GdDTPA and two gadolinium polyoxometalates as potential MRI contrast agents

Two gadolinium polyoxometalates, Gd(2)P(2)W(18)O(62) and K(15)[(GdO)(3)(PW(9)O(34))(2)], have been evaluated by in vivo as well as in vitro experiments as the candidates of tissue-specific magnetic resonance imaging (MRI) contrast agents. T(1)-relaxivities of 28.4 mM(-1).s(-1) for Gd(2)P(2)W(18)O(62) and 11.2 mM(-1).s(-1) for K(15)[(GdO)(3)(PW(9)O(34))(2)] (400 MHz, 25 degrees C) were higher than that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in bovine serum albumin and human serum transferrin were also reported. The favorable liver-specific contrast enhancement and renal excretion capability in in vivo MRI with Sprague-Dawley rats after i.v. administration of K(15)[(GdO)(3)(PW(9)O(34))(2)] was demonstrated. In vivo and in vitro assay showed that K(15)[(GdO)(3)(PW(9)O(34))(2)] is a promising liver-specific MRI contrast agent. However, Gd(2)P(2)W(18)O(62) did not show the favorable quality in vivo as expected from its high relaxivity in vitro, which was attributed to low bioavailability, indicating that it is of limited value as tissue-specific MRI contrast agent.

Small animal imaging. current technology and perspectives for oncological imaging

Advances in the biomedical sciences have been accelerated by the introduction of many new imaging technologies in recent years. With animal models widely used in the basic and pre-clinical sciences, finding ways to conduct animal experiments more accurately and efficiently becomes a key factor in the success and timeliness of research. Non-invasive imaging technologies prove to be extremely valuable tools in performing such studies and have created the recent surge in small animal imaging. This review is focused on three modalities, PET, MR and optical imaging which are available to the scientist for oncological investigations in animals.

MR microscopy and high resolution small animal MRI: applications in neuroscience research

The application of magnetic resonance (MR) imaging in the study of human disease using small animals has steadily evolved over the past two decades and strongly established the fields of "small animal MR imaging" and "MR microscopy." An increasing number of neuroscience related investigations now implement MR microscopy in their experiments. Research areas of growth pertaining to MR microscopy studies are focused on (1). phenotyping of genetically engineered mice models of human neurological diseases and (2). rodent brain atlases. MR microscopy can be performed in vitro on tissue specimens, ex vivo on brain slice preparations and in vivo (typically on rodents). Like most new imaging technologies, MR microscopy is technologically demanding and requires broad expertise. Uniform guidelines or "standards" of a given MR microscopy experiment are non-existent. The main focus therefore of this review will be on biological applications of MR microscopy and the experimental requirements. We also take a critical look at the biological information that small animal (rodent) MR imaging has provided in neuroscience research.

An evaluation of gadolinium polyoxometalates as possible MRI contrast agent

Two gadolinium polyoxometalates, K(9)GdW(10)O(36) and K(11)[Gd(PW(11)O(39))(2)], have been evaluated both in vivo and in vitro as candidates for tissue-specific MRI contrast agents. T(1)-relaxivities of 6.89 mM(-1). s(-1) for K(9)GdW(10)O(36) and 5.27 mM(-1). s(-1) for K(11)[Gd(PW(11)O(39))(2)] are slightly higher than that of the commercial MRI contrast agent (Gd-DTPA). Both compounds bind with bovine serum albumin and human serum transferrin and favorable liver-specific contrast enhancement in in vivo MRI with Sprague-Dawley rats after i.v. administration has been demonstrated. Imaging studies demonstrate that the two agents have a long residence time, showing MR signal enhancement in the liver for more than 40 min, longer than commercially available contrast agents. In vivo and in vitro assays showed that GdW(10) and Gd(PW(11))(2) are promising liver-specific MRI contrast agents and GdW(10) may be used in the diagnosis of the pathological state. However, with the higher acute toxicity, the two gadolinium polyoxometalates need to be modified and studied further before clinical use.