Albumin-binding MR blood pool agents as MRI contrast agents in an intracranial mouse glioma model

The Distribution Volume of 18F-Albumin as a Potential Biomarker of Antiangiogenic Treatment Efficacy

Objective: 18F-albumin, a vascular imaging agent, may have potential to assess tumor responses to anti-angiogenic therapies. In these studies tumor distribution volume of 18F-albumin were first determined in various human tumor xenografts from biodistribtuion measurments and then one of the tumor type was used to evaluate changes in 18F-albumin uptake in anti-angiognic tumor model. Method: 18F-albumin was synthesized via conjugation of 6-[18F]fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [18F]F-Py-TFP, with rat albumin. From the biodistribution of 18F-albumin in various human tumor xenografts tumor distribution volumes (DVs; tumor%ID/g:blood%ID/g) were first determined at various time points. Then, the ability of 18F-albumin to detect tumor angiogenic inhibition in one of these tumor types (U87MG) following treatment with sunitinib was evaluated by position emission tomography (PET) imaging at 0, 7, 14, and 21 days post treatment. Caliper measurements of tumor dimensions were also made at these same times. At Day 21, following imaging, biodistributions, autoradiography of tumor tissues and tumor blood vessel counts (CD31 IHC) were performed. Results: 18F-albumin retention in various tumors steadily increased over time with U87MG tumor exhibiting the highest uptake (DV) at all times. Significant decreases in 18F-albumin DVs were observed one week post-treatement (-39%) vs. controls whereas tumor caliper volumes were not significantly decreased until days 14 and 21. At day 21 the significant decrease in DVs in the treatment group (-44%) paralleled biodistribution DV measurements and was consistent with autoradiography and CD31 IHC findings. Conclusion: These data suggest that 18F-albumin DVs obtained by imaging may serve as an early biomarker of the effectiveness of anti-angiogenic therapy and thus aid in patient management and treatment planning.

Perspectives: MRI of angiogenesis

Angiogenesis, the expansion of the vascular bed, is an important component in remodeling of tissues and organs. Such remodeling is essential for coping with substantial and sustained increase in the demands for supply of oxygen and nutrients and the timely removal of waste products. The vasculature, and its effectiveness in systemic delivery to all parts of the body, regulates the distribution of immune cells and the delivery of therapeutics as well as the dissemination of disease. Therefore, the vascular bed is possibly one of the key organs involved in homeostasis, in health and disease. The critical role of the vasculature in health, and the accessibility to non invasive probing by MRI, renders MRI as a modality of choice for monitoring the vasculature and its adaption to challenges.

Contrast Agents for MR Imaging

Contrast media are essential to the practice of MR imaging. An increasing variety of agents have been approved for clinical use, specific contrast agents can often be tailored to a specific clinical question. Compared with CT contrast media, MR imaging contrast is well tolerated with an excellent safety record and a low incidence of adverse events. In this article, we review the pharmacology, indications, and the common adverse events of the intravenous and oral MR contrast agents most commonly used in contemporary imaging practice, including gadolinium-based contrast, manganese and iron-based agents and the most common oral contrast agents.

Measurement of rat brain tumor kinetics using an intravascular MR contrast agent and DCE-MRI nested model selection

PURPOSE

Using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in a rat glioma model, and nested model selection (NMS), to compare estimates of the pharmacokinetic parameters vp , K(trans) , and ve for two different contrast agents (CAs)-gadofosveset, which reversibly binds to human serum albumin, and gadopentetate dimeglumine, which does not.

MATERIALS AND METHODS

DCE-MRI studies were performed on nine Fisher 344 rats inoculated intracerebrally with 9L gliosarcoma cells using both gadofosveset and gadopentetate. The parameters vp , K(trans) , and ve were estimated using NMS.

RESULTS

K(trans) estimates using gadofosveset, compared to gadopentetate, differed in their means (gadofosveset 0.025 ± 0.008 min(-1) vs. gadopentetate 0.046 ± 0.011 min(-1) ; P = 0.0039). This difference notwithstanding, the intraclass correlation coefficient (ICC) for the two estimates of K(trans) showed nearly perfect linear dependence (ICC = 0.8479 by Pearson's r). Other estimates, ve (gadofosveset 22.7 ± 4.7% vs. gadopentetate 23.6 ± 5.6%; P = 0.4258) and vp (gadofosveset 1.5 ± 0.5% vs. gadopentetate 1.6 ± 0.4%; P = 0.25), were not different in their means between the two CAs, and there was almost perfect agreement for ve (ICC = 0.8798) and substantial agreement for vp (ICC = 0.7981) between the two CAs.

CONCLUSION

Estimates of K(trans) were statistically different using gadofosveset and gadopentetate, whereas ve and vp were similar with two CAs. NMS produced robust estimates of pharmacokinetic parameters using DCE-MRI that show promise as important measures of tumor physiology and microenvironment.

MR imaging features of high-grade gliomas in murine models: how they compare with human disease, reflect tumor biology, and play a role in preclinical trials

Murine models are the most commonly used and best investigated among the animal models of HGG. They constitute an important weapon in the development and testing of new anticancer drugs and have long been used in preclinical trials. Neuroimaging methods, particularly MR imaging, offer important advantages for the evaluation of treatment response: shorter and more reliable treatment end points and insight on tumor biology and physiology through the use of functional imaging DWI, PWI, BOLD, and MR spectroscopy. This functional information has been progressively consolidated as a surrogate marker of tumor biology and genetics and may play a pivotal role in the assessment of specifically targeted drugs, both in clinical and preclinical trials. The purpose of this Research Perspectives was to compile, summarize, and critically assess the available information on the neuroimaging features of different murine models of HGGs, and explain how these correlate with human disease and reflect tumor biology.

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.

In vivo detection of inducible nitric oxide synthase in rodent gliomas

Increased iNOS expression is often found in brain tumors, such as gliomas. The goal of this study was to develop and assess a novel molecular MRI (mMRI) probe for in vivo detection of iNOS in rodent models for gliomas (intracerebral implantation of rat C6 or RG2 cells or ethyl nitrosourea-induced glioma). The probe we used incorporated a Gd-DTPA (gadolinium(III) complex of diethylenetriamine-N,N,N',N'',N''-pentaacetate) backbone with albumin and biotin moieties and covalent binding of an anti-iNOS antibody (Ab) to albumin (anti-iNOS probe). We used mMRI with the anti-iNOS probe to detect in vivo iNOS levels in gliomas. Nonimmune normal rat IgG coupled to albumin-Gd-DTPA-biotin was used as a control nonspecific contrast agent. By targeting the biotin component of the anti-iNOS probe with streptavidin Cy3, fluorescence imaging confirmed the specificity of the probe for iNOS in glioma tissue. iNOS levels in glioma tumors were also confirmed via Western blots and immunohistochemistry. The presence of plasma membrane-associated iNOS in glioma cells was established by transmission electron microscopy and gold-labeled anti-iNOS Ab. The more aggressive RG2 glioma was not found to have higher levels of iNOS compared to C6. Differences in glioma vascularization and blood-brain barrier permeability between the C6 and the RG2 gliomas are discussed. In vivo assessment of iNOS levels associated with tumor development is quite feasible in heterogeneous tissues with mMRI.

Measuring Brain Tumor Growth: Combined Bioluminescence Imaging–Magnetic Resonance Imaging Strategy

Small-animal tumor models are essential for developing translational therapeutic strategies in oncology research, with imaging having an increasingly important role. Magnetic resonance imaging (MRI) offers tumor localization, volumetric measurement, and the potential for advanced physiologic imaging but is less well suited to high-throughput studies and has limited capacity to assess early tumor growth. Bioluminescence imaging (BLI) identifies tumors early, monitors tumor growth, and efficiently measures response to therapeutic intervention. Generally, BLI signals have been found to correlate well with magnetic resonance measurements of tumor volume. However, in our studies of small-animal models of malignant brain tumors, we have observed specific instances in which BLI data do not correlate with corresponding MRIs. These observations led us to hypothesize that use of BLI and MRI together, rather than in isolation, would allow more effective and efficient measures of tumor growth in preclinical studies. Herein we describe combining BLI and MRI studies to characterize tumor growth in a mouse model of glioblastoma. The results led us to suggest a cost-effective, multimodality strategy for selecting cohorts of animals with similar tumor growth patterns that improves the accuracy of longitudinal in vivo measurements of tumor growth and treatment response in preclinical therapeutic studies.

DCE-MRI using small-molecular and albumin-binding contrast agents in experimental carcinomas with different stromal content

OBJECTIVES

To compare DCE-MRI experiments performed using a standard small-molecular (Gd-DTPA) and an albumin-binding (MS-325) contrast agent in two carcinoma models with different stromal content.

MATERIALS AND METHODS

DU-145 or BXPC-3 cancer cells were subcutaneously injected into nude mice. DCE-MRI was performed by a bolus injection of Gd-DTPA or MS-325 about 2 weeks after inoculation. For quantitative analysis a volume of interest was manually drawn over each tumor. To address the heterogeneous enhancement, each tumor volume was then divided into the 20% most-enhancing and the remaining 80% least-enhancing fractions. Mean tumor enhancement was calculated over these selected tumor volumes and compared between tumor groups and contrast agents. Maps of differential enhancement, peak enhancement and time-to-peak were used for visual evaluation. CD31 and VEGF immunohistochemistry were performed in excised tumors.

RESULTS

In the 80% least-enhancing volume, at late time points of the dynamic scan, the mean enhancement elicited by MS-325 was higher in BXPC-3 than in DU-145 tumors. In the 20% most-enhancing volume, using either contrast agents, significant difference between the two tumors types were observed only early, while at later time points of the dynamic scan the difference were obscured by the faster washout observed in the BXPC-3 tumors. Enhancement maps confirmed that BXPC-3 tumors were characterized by marked washout rate using either contrast agent, particularly in the higher enhancing peripheral rim. With MS-325 this washout pattern appeared to be specific to the BXPC-3 carcinomas, since it was not observed in the DU-145 tumors. Finally, in both tumor types, MS-325 produced significantly higher enhancement than Gd-DTPA in the late phase of the dynamic scan. Ex vivo analysis confirmed the marked presence of aberrant infiltrative stroma in BXPC-3 tumors, in which tumor vessels were embedded. In all tumors the central portion was less viable and less infiltrated by stromal tissue then the peripheral areas.

CONCLUSIONS

Contrast distribution proved to be related to stromal content, which presumably produced the higher enhancement and faster washout observed in the BXPC-3 tumors. In particular, 'early' contrast-enhanced MRI, appeared as the most sensitive technique to detect the tumor portions characterized by a high stromal content, i.e. the peripheral rim of the BXPC-3 tumors. Since the same tumor models were recently investigated using FDG-PET imaging, showing inverse relationship between FDG uptake and stromal content, contrast-enhanced MRI and FDG-PET could provide complementary and comprehensive sensitivity in the assessment of carcinomas.

In vivo myocardial tissue kinetics of Gd(ABE-DTTA), a tissue-persistent contrast agent

The phenomenological tissue kinetics of Gd(ABE-DTTA) was investigated in myocardial infarction (MI). Reperfused infarction was generated by balloon catheter in closed-chest canines (N=11). Forty-eight hours thereafter, inversion-recovery (IR)-prepared fast gradient-echo control images were acquired with varying inversion times (TIs). Precontrast R(1) maps were calculated from the TI dependence of signal intensity (SI) using nonlinear curve fitting. Then 0.05 mmol/kg Gd(ABE-DTTA) was administered I.V. In 11 dogs postcontrast R(1) maps were generated at 24 hr and 48 hr postcontrast. In five dogs measurements were also repeated at 108 hr and 12 days. In one dog early measurement was carried out at 4 hr. Delta R(1) values for blood and viable and infarcted myocardium were calculated at each time point by subtracting the precontrast R(1) from the postcontrast R(1). Gd(ABE-DTTA) showed significant, progressive accumulation into infarcts during the first 2 days (k(in)=0.39 hr(-1)) and a delayed clearance (k(out) = 0.005 hr(-1)). Among the time points sampled, the maximum infarct Delta R(1) was detected at 48 hr (1.72 s(-1)). Contrast agent (CA) in infarcted tissue was detectable for 12 days. Clearance from blood and viable myocardium occurred in parallel and was completed by 108 hr. Gd(ABE-DTTA) displays slow, tissue-persistent kinetics and partly intravascular, partly extravascular characteristics. It demonstrates high affinity for infarcted myocardium and induces highlighting of infarcts between 4 hr and 12 days following administration.

MRI of mouse models of neurological disorders

MRI has contributed to significant advances in the understanding of neurological diseases in humans. It has also been used to evaluate the spectrum of mouse models spanning from developmental abnormalities during embryogenesis, evaluation of transgenic and knockout models, through various neurological diseases such as stroke, tumors, degenerative and inflammatory diseases. The MRI techniques used clinically are technically more challenging in the mouse because of the size of the brain; however, mouse imaging provides researchers with the ability to explore cellular and molecular imaging that one day may translate into clinical practice. This article presents an overview of the use of MRI in mouse models of a variety of neurological disorders and a brief review of cellular imaging using magnetically tagged cells in the mouse central nervous system.

In vivo imaging in a murine model of glioblastoma

OBJECTIVE

To use in vivo imaging methods in mice to quantify intracranial glioma growth, to correlate images and histopathological findings, to explore tumor marker specificity, to assess effects on cortical function, and to monitor effects of chemotherapy.

METHODS

Mice with DBT glioma cell tumors implanted intracranially were imaged serially with a 4.7-T small-animal magnetic resonance imaging (MRI) scanner. MRI tumor volumes were measured and correlated with postmortem histological findings. Different nonspecific and specific positron emission tomography radiopharmaceuticals, [18F]2-fluoro-2-deoxy-d-glucose, [18F]3'-deoxy-3'-fluorothymidine, or [11C]RHM-I, a sigma2-receptor ligand, were visualized with microPET (CTI-Concorde MicroSystems LLC, Knoxville, TN). Intrinsic optical signals were imaged serially during contralateral whisker stimulation to study the impact of tumor growth on cortical function. Other groups of mice were imaged serially with MRI after one or two doses of the antimitotic N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU).

RESULTS

MRI and histological tumor volumes were highly correlated (r2 = 0.85). Significant binding of [11C]RHM-I was observed in growing tumors. Over time, tumors reduced and displaced (P # 0.001) whisker-activated intrinsic optical signals but did not change intrinsic optical signals in the contralateral hemisphere. Tumor growth was delayed 7 days after a single dose of BCNU and 18 days after two doses of BCNU. Mean tumor volume 15 days after DBT implantation was significantly smaller for treated mice (1- and 2-dose BCNU) compared with controls (P = 0.0026).

CONCLUSION

Mouse MRI, positron emission tomography, and optical imaging provide quantitative and qualitative in vivo assessments of intracranial tumors that correlate directly with tumor histological findings. The combined imaging approach provides powerful multimodality assessments of tumor progression, effects on brain function, and responses to therapy.

Albumin marks pseudopodia of astrocytoma cells responding to hepatocyte growth factor or serum

It is well accepted that dysfunction in the blood brain barrier (BBB) allows permeation of albumin from the bloodstream into astrocytic brain tumors, especially glioblastomas, the most aggressive astrocytomas. In vitro, bovine serum albumin (BSA) aids functional cell assays by maintaining cytokines and growth factors in solution and delivering its cargo of fatty acids. Earlier, we showed that BSA was prominent in lysates prepared from pseudopodia formed by U87 astrocytoma cells. The present studies investigated the association of albumin with pseudopodia formed by U87 and LN229 astrocytoma cells. With hepatocyte growth factor (HGF) stimulation, cell migration was enhanced and BSA, especially its dimerized form, was prominent in pseudopodia compared to unmigrated cells on one-dimensional gels and immunoblots. When lysates were equalized for levels of glyceraldehyde-3-phosphate dehydrogenase, the rise for BSA levels in pseudopodia vs migrated cells was comparable or greater than levels noted for established pseudopodial proteins, beta-actin and ezrin. The increase for dimerized BSA in pseudopodia compared to unmigrated cells was greater than the rise in levels of beta-actin, ezrin, HGF, and phosphorylated Met when pseudopodia were harvested from filters with 1 mum pores using either cell line. Fluorescein (F)-labeled BSA co-localized with HGF on actin-rich cellular protrusions and with CM-DiI labeled pseudopodial plasma membranes. The F-BSA highlighted small, individual pseudopodial profiles more so than complex pseudopodial networks (reticulopodia) or unmigrated cells. Labeled human serum albumin also decorated pseudopodia preferentially. Albumin's association with pseudopodia may help to explain its selective accumulation in astrocytomas in vivo. The leaky BBB permits serum albumin to enter the microenvironment of astrocytomas thus allowing their invasive cells contact with serum albumin as a source of fatty acids that would be useful for remodeling cell membranes in pseudopodia. Thus, albumin potentially aids and marks invasion as it accumulates in these tumors.

Species dependence on plasma protein binding and relaxivity of the gadolinium-based MRI contrast agent MS-325

RATIONALE AND OBJECTIVES

We sought to determine whether there is a species dependence on plasma protein and serum album binding and/or relaxivity of the MR contrast agent MS-325.

METHODS

Equilibrium binding of MS-325 to plasma proteins or purified serum albumin was determined as a function of chelate concentration. T1 and T2 values were determined at 0.47 and 1.41 T, and NMRD profiles were measured to determine the changes in relaxivity over varying field strengths from 0.002 to 1.2 T.

RESULTS

The binding of MS-325 to either animal plasma or serum albumin plateaus at chelate concentrations less than 0.1 mM with human, pig, and rabbit plasmas showing maximum binding. Human and pig plasmas show the greatest observed relaxivity enhancement in the presence of MS-325.

CONCLUSIONS

MS-325 exhibits increased relaxivity in blood plasma as the result of plasma protein binding. Binding ranged from 64% to 91% and was species dependent: human > pig approximately rabbit > dog approximately rat approximately mouse.

Application of arsenazo III in the preparation and characterization of an albumin-linked, gadolinium-based macromolecular magnetic resonance contrast agent

A macromolecular magnetic resonance contrast agent (MMCA) was prepared by linking bovine serum albumin (BSA) to gadolinium (Gd) via a chelating agent, diethylenetriaminepentaacetic acid (DTPA). Colorimetric testing with 2,7-bis(o-arsenophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid (arsenazo III) was performed to check for the appearance of free gadolinium during preparation and to quantify the Gd content in the final product. The complex was purified by dialysis, concentrated by lyophilyzation and characterized by magnetic resonance (MR) proton relaxation times. The resultant product had a molecular weight of about 90 kDa, Gd:BSA ratio of 14:1, and T1 and T2 relaxation times of 128.3 and 48.9 ms, respectively, at a field strength of 7Tesla (T) and at 20% concentration. Contrast enhancement of Gadomer-17 (a dendritic MMCA) and Gd-linked to BSA (Gd-BSA) was sequentially evaluated in a rat brain gliosarcoma model (n = 5) by MR imaging (MRI). Following intravenous injection, the blood concentration of Gadomer-17 fell rapidly, whereas that of Gd-BSA was almost constant for the duration of imaging. The areas of enhancement of both MMCAs were comparable. The spatial distribution of Gd-BSA showed good agreement with Evans blue-tagged albumin. Treatment with dexamethasone decreased Gd-BSA enhancement in the tumor. These results suggest that the arsenazo III method is applicable in preparing Gd-BSA to image brain tumors and their response to treatment. This simple method may also be useful for preparing other gadolinium-linked MMCAs.

Magnetic Resonance Macromolecular Agents for Monitoring Tumor Microvessels and Angiogenesis Inhibition

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using macromolecular contrast media enables assessments of the tumor vasculature based on the differential distribution of the contrast agent within normal and pathologic tissues. Quantitative assays of both morphologic and functional properties can provide useful diagnostic insight into tissue angiogenesis. The use of MRI enhanced with macromolecular agents for the characterization of tumor microvessels has been experimentally demonstrated in a range of malignant tumor types. Kinetic analysis of DCE-MRI data can be used to estimate microvascular permeability and tumor blood volume. By measuring these functional tumor properties, an accurate, noninvasive, and quantitative description of the microcirculation of individual tumors can be acquired, improving the specificity of imaging examinations for cancer diagnosis and for treatment and follow up. The noninvasive MRI assessment of tumor angiogenesis can be applied in the diagnostic differentiation between benign and malignant tumors and can also provide means for in vivo monitoring of antitumor therapy. In this review, the potential clinical applications and limitations of various macromolecular contrast agents applied for evaluations of tumor angiogenesis, with and without drug interventions, are discussed.

Detection and Localization of Proteinuria by Dynamic Contrast-Enhanced Magnetic Resonance Imaging Using MS325

After renal transplantation, persistent glomerular disease affecting the native kidneys typically causes albuminuria, at least for a period of time, making it difficult to determine in a noninvasive fashion whether proteinuria originates in the native kidneys or the renal allograft. To address this problem, dynamic contrast-enhanced magnetic resonance imaging (MRI) using gadolinium (Gd)-based albumin-bound blood pool contrast agent (MS325) to localize proteinuria was investigated. Glomerular proteinuria was induced in Sprague-Dawley rats by intravenous injection of puromycin aminonucleoside (PAN), whereas control rats received physiologic saline vehicle. Both groups of animals underwent a 40-min dynamic contrast-enhanced MRI using radio frequency spoiled gradient echo imaging sequence after injection of Gd-labeled MS325. Contrast uptake and clearance curves for cortex and medulla were determined from acquired MR images. Compared with controls, proteinuric rats exhibited significantly lower elimination rate constants. The use of gadopentetate dimeglumine (Gd-DTPA) as a contrast agent showed smaller and less specific differences between proteinuric and control groups. In rats with one proteinuric kidney (PAN-treated) and one normal kidney (transplanted from a normal rat), MRI using MS325 was able to differentiate between the two kidneys. The results suggest that MRI with an albumin-bound blood pool contrast agent may be a useful noninvasive way to localize proteinuria. If this technique can be successfully applied in human patients, it may allow for the localization of proteinuria after kidney transplant and thereby provide a noninvasive way to detect disease affecting the renal allograft.

High-resolution longitudinal screening with magnetic resonance imaging in a murine brain cancer model

One of the main limitations of intracranial models of diseases is our present inability to monitor and evaluate the intracranial compartment noninvasively over time. Therefore, there is a growing need for imaging modalities that provide thorough neuropathological evaluations of xenograft and transgenic models of intracranial pathology. In this study, we have established protocols for multiple-mouse magnetic resonance imaging (MRI) to follow the growth and behavior of intracranial xenografts of gliomas longitudinally. We successfully obtained weekly images on 16 mice for a total of 5 weeks on a 7-T multiple-mouse MRI. T2- and T1-weighted imaging with gadolinium enhancement of vascularity was used to detect tumor margins, tumor size, and growth. These experiments, using 3D whole brain images obtained in four mice at once, demonstrate the feasibility of obtaining repeat radiological images in intracranial tumor models and suggest that MRI should be incorporated as a research modality for the investigation of intracranial pathobiology.