Robert C. Brasch

Dynamic contrast-enhanced magnetic resonance imaging as a surrogate marker of tumor response to anti-angiogenic therapy in a xenograft model of glioblastoma multiforme

To evaluate the effects of a neutralizing anti‐vascular endothelial growth factor (anti‐VEGF) antibody on tumor microvascular permeability, a proposed indicator of angiogenesis, and tumor growth in a rodent malignant glioma model.

MRI characterization of tumors and grading angiogenesis using macromolecular contrast media: status report

Magnetic resonance imaging (MRI) enhanced with a macromolecular contrast medium (MMCM) has been applied successfully to assay tumor microvascular characteristics. These MRI-assayed characteristics correlate closely with histologic microvascular density, an established surrogate of tumor angiogenesis, and with pathologic tumor grade. The utility of MMCM-enhanced MRI for tumor characterizations has been established experimentally in a range of cancer types including breast, ovary, fibrosarcoma, and prostate. The MMCM-enhanced MRI technique can also be applied to monitor changes in tumor vessels that result from administration of an angiogenesis inhibitor, antibody against vascular endothelial growth factor (VEGF). Suppression of microvascular permeability (up to 98%) induced by this inhibitor of angiogenesis was detected and quantified as soon as 24 h after initiation of therapy. Thus, MRI assays of tumor microvascular characteristics, particularly macromolecular permeability, provide a means to non-invasively characterize tumors for prognostication, for individualization and optimization of treatment, and for monitoring therapeutic response. Pending successful completion of drug trials, now in progress, the availability of MMCM should permit the immediate application of these powerful techniques in clinical practice.

In vivo monitoring of tumor angiogenesis with MR imaging

Magnetic resonance (MR) imaging is a widely employed diagnostic method for the evaluation of patients with tumors. This method is noted for its remarkable soft-tissue definition, absence of ionizing radiation, high spatial and temporal resolution, and ability to generate images in any plane of the entire body. Equipment costs and, thus, examination costs are relatively high, however. MR imaging has been proposed and tested, both experimentally and clinically, as a method to characterize tumors regarding their state of angiogenesis. Multiple approaches to the challenge of MR imaging assays of angiogenesis have been proposed, some of which are potentially additive; all are intended to provide information regarding tumor microvessels. The quantitative end points that are sought include tissue plasma/blood volume, transendothelial permeability to water or solutes, perfusion/flow, and relative concentration of angiogenesis-specific molecules. The available approaches can be divided into intrinsic (non-contrast material enhanced) and contrast material-enhanced methods. The latter methods can be further divided by the type of contrast medium employed: small molecular agents that distribute rapidly in the extracellular space (so-called nonspecific or extracellular-fluid-space [ECF] agents), large molecular agents designed for prolonged intravascular retention (socalled macromolecular contrast media [MMCM] or bloodpool agents), and targeted agents intended to accumulate at the sites of concentrated angiogenesis mediator. Today, ECF contrast agents are commercially available and being used in clinical evaluations of antiangiogenesis drug treatments. Macromolecular contrast media are currently in clinical trials, but they are not now approved for use in humans. Molecular-targeted contrast media are in preclinical development. This section summarizes some of the many reports dealing with MR imaging assays of angiogenesis. For clarity, the discussion is divided by the specific MR imaging approach used. The rationale for that approach, limited information regarding the technique itself, accumulated experience, and limitations are provided, as well.

Tumor microvascular changes in antiangiogenic treatment: Assessment by magnetic resonance contrast media of different molecular weights

To test magnetic resonance (MR) contrast media of different molecular weights (MWs) for their potential to characterize noninvasively microvascular changes in an experimental tumor treatment model.

Factors affecting nitroxide reduction in ascorbate solution and tissue homogenates

Because of their paramagnetic properties, nitroxides are potentially useful as contrast agents in magnetic resonance imaging (MRI). They are reduced in vivo to their corresponding hydroxylamines which are nonparamagnetic and have no contrast enhancing property. Nitroxides with high resistance to reduction would be advantageous as pharmaceutical contrast enhancing agents. We show that in the presence of ascorbic acid and in tissue homogenates, the reduction is faster for piperidine than for pyrrolidine nitroxides and for positively-charged than for negatively-charged derivatives. The data also suggest that nitroxide reduction in tissue homogenates is mainly due to sulfhydryl groups on proteins and that endogenous ascorbic acid plays a relatively minor role.

Gadolinium-Ethoxybenzyl-DTPA, a New Liver-Specific Magnetic Resonance Contrast Agent: Kinetic and Enhancement Patterns in Normal and Cholestatic Rats

OBJECTIVES Gadolinium-ethoxybenzyl-DTPA (Gd-EOB-DTPA) is a new hepatobiliary magnetic resonance imaging (MRI) contrast agent with a dual elimination: 70% via the liver and bile and 30% via the kidney in normal rats. The abdominal enhancement patterns of this new compound and the uptake mechanism by the liver were studied in rats using tissue relaxometry and MRI. METHODS Twelve normal rats, 33 rats treated with agents designed to inhibit biliary excretion of the agent, and 6 rats with surgically ligated common bile ducts received Gd-EOB-DTPA intravenously. Distribution and excretion were measured by MR relaxometry. MR signal intensity was measured over time for liver, kidney, and bowel. RESULTS In normal animals, 0.1 mmol/kg Gd-EOB-DTPA induced a significantly greater (200%) and more prolonged liver signal enhancement (100% at 30 minutes) than Gd-DTPA at the same dose. Either hyperbilirubinemia, induced by common bile duct ligation, or bromosulfophtalein (BSP) infusion inhibited liver uptake of Gd-EOB-DTPA, resulting in a preferential elimination via the kidney. Taurocholate (TC), an inhibitor of the bile acid transporter, was unable to block the liver uptake of Gd-EOB-DTPA. Blood half-lives of Gd-EOB-DTPA in rats were 2.4 minutes for the first component and 8.2 minutes for the second. CONCLUSIONS Data indicate that transport of Gd-EOB-DTPA through the liver into bile is driven by the organic anion transporter. The relation between enhancement of liver and kidney may be diagnostically useful to indirectly evaluate liver excretory function. Yet, persistent enhancement of liver, even in the presence of severe hyperbilirubinemia, should be sufficient to identify focal mass lesions.

Correlation of Microvascular Permeability Derived from Dynamic Contrast-Enhanced MR Imaging with Histologic Grade and Tumor Labeling Index

RATIONALE AND OBJECTIVES Dynamic contrast material-enhanced magnetic resonance (MR) imaging may be used to quantify fractional blood volume (fBV) and microvascular permeability in human brain tumors. Hypothesis is that these measurements correlate with tumor histologic grade and immunohistologically assessed mitotic activity. MATERIALS AND METHODS Thirty-eight patients with newly diagnosed gliomas underwent MR imaging consisting of dynamic three-dimensional spoiled gradient-recalled acquisition in the steady state image sets following bolus injections of a single dose of gadodiamide. Signal intensity changes in blood and tissue were kinetically analyzed, yielding estimates of fBV and microvascular permeability (k). Tumor specimens were graded with the World Health Organization-II four-point grading score. MIB-1 immunohistochemical labeling (anti-Ki-67 monoclonal antibody) was performed in 22 patients to evaluate mitotic activity. RESULTS Histologic study revealed nine grade 2, 14 grade 3, and 15 grade 4 tumors. fBV ranged from 0.4% to 24%, k from -0.4 to 31.4 mL/100 cm3 x min, and MIB-1 labeling indexes from 1.7% to 42.8%. Correlation to the tumor grade was highest for permeability (r = 0.73), followed by the MIB-1 index (r = 0.63), and fBV (r = 0.48). Correlation between k and MIB-1 index was strong (r = 0.84). There was no statistically significant difference between the fBV of any of the groups. Despite some overlap between the permeability values of specific tumors from different grades, differences were statistically significant. The MIB-1 index was significantly different between grades 3 and 4 but not between grades 2 and 3. CONCLUSION Dynamic contrast-enhanced MR imaging allows noninvasive determination of tumor fBV and microvascular permeability k. k is more reliable than the MIB-1 labeling index for differentiating grade 2 from grade 3 tumors.

Assessment of myocardial salvage after ischemia and reperfusion using magnetic resonance imaging and spectroscopy.

To test the hypothesis that contrast-enhanced magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) can differentiate reversible from irreversible myocardial injury, these modalities were used to study ischemia and reperfusion in a rat model. The presence of ischemia and reperfusion were confirmed with radiolabeled microspheres (n = 6). Groups of animals were subjected to either 16 (n = 17), 30 (n = 14), 60 (n = 11), or 90 (n = 14) minutes of left coronary artery (LCA) occlusion and 60 minutes reperfusion. After albumin-gadolinium (Gd)-DTPA injection, contrast-enhanced, T1-weighted, spin-echo proton images were acquired at baseline and every 16 minutes during LCA occlusion and reperfusion. In separate experiments, 31phosphorus (31P) spectra were acquired at similar time points during ischemia and reperfusion. After 16 minutes occlusion, normally perfused myocardium enhanced significantly compared with ischemic myocardium on MRI (104 +/- 7.9% vs. 61 +/- 11.0%, p less than 0.05, n = 5, mean +/- SEM, % of baseline value). MRS showed reduced phosphocreatine (PCr) and adenosine triphosphate (ATP) (58.8 +/- 2.4%, p less than or equal to 0.01; 81.4 +/- 2.4, p less than or equal to 0.01, n = 12). After 16 or 30 minutes ischemia, reflow resulted in uniform MRI signal intensity of the ischemic zone compared with normal myocardium (93.5 +/- 11.3 vs. 80.9 +/- 7.0, p = NS, n = 11, % of baseline value at 30 minutes reperfusion) and PCr recovery on MRS (94.3 +/- 4.0%, p = NS, n = 20, % baseline value at 30 minutes reflow). After 60 and 90 minutes ischemia, reflow resulted in marked enhancement of reperfused compared with normal myocardium on MRI (254.0 +/- 30.0 vs. 78.3 +/- 9.2, p less than or equal to 0.01, n = 10) and no recovery of PCr on MRS (64.1 +/- 3.0, p = NS, n = 14). Triphenyltetrazolium chloride (TTC) staining revealed transmural myocardial infarction (MI) in all hearts subjected to 60 or 90 minutes ischemia and reflow, and small nontransmural MIs in only 2/11 hearts subjected to 16 or 30 minutes ischemia and reperfusion. Thus, 1) MRI with albumin-Gd-DTPA is useful for identifying myocardial ischemia by enhancing the contrast between normally perfused and ischemic myocardia; 2) MRI with albumin-Gd-DTPA is useful for identifying reperfusion after myocardial ischemia; and 3) after reperfusion, reversible can be distinguished from irreversible myocardial injury by characteristic findings on MRI and MRS.

Influence of Severity of Myocardial Injury on Distribution of Macromolecules: Extravascular Versus Intravascular Gadolinium-Based Magnetic Resonance Contrast Agents☆

OBJECTIVES This study sought to 1) compare the distribution of extravascular (573 Da) and intravascular (92 kDa) magnetic resonance (MR) contrast agents in reperfused infarcted myocardium, and 2) investigate the effect of injury severity on these distribution patterns. BACKGROUND Myocardial distribution of low and high molecular weight contrast agents depends on vascular permeability, diffusive/convective transport within the interstitium and accessibility of the intracellular compartment (cellular integrity). METHODS To vary the severity of myocardial injury, 72 rats were subjected to 20, 30, 45 or 75 min (n = 18, respectively) of coronary artery occlusion. After 2 h of reflow, the animals received either 0.05 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (Gd-DTPA-BMA) (n = 24), (Gd-DTPA)30-albumin (n = 24) or saline (control group, n = 24). Three minutes after injection, the hearts were excised and imaged (spin-echo imaging parameters: repetition time 300 ms, echo time 8 ms, 2-tesla system), followed by triphenyltetrazolium chloride staining for infarct detection and sizing. RESULTS Histomorphometric and MR infarct size (expressed as percent of slice surface) correlated well: r = 0.96 for Gd-DTPA-BMA; r = 0.95 for (Gd-DTPA)30-albumin. On Gd-DTPA-BMA-enhanced images, reperfused myocardial infarctions were homogeneously enhanced. The ratio of signal intensity of infarcted/ normal myocardium increased with increasing duration of ischemia (overall p < 0.0001, analysis of variance [ANOVA]), indicating an increase in the distribution volume of Gd-DTPA-BMA in postischemic myocardium. On (Gd-DTPA)30-albumin-enhanced images, reperfused infarctions consisted of a bright border zone and a less enhanced central core. The extent of the core increased with increasing duration of ischemia (overall p value < 0.0001, ANOVA). CONCLUSIONS At 2 h of reperfusion, the distribution of MR contrast agents in postischemic myocardium is 1) specific for extravascular and intravascular agents, and 2) modulated by the duration of ischemia.

Correlation of dynamic contrast-enhanced magnetic resonance imaging with histologic tumor grade: comparison of macromolecular and small-molecular contrast media

Background. The endothelial integrity of microvessels is disrupted in malignant tumors. Quantitative assays of tumor microvascular characteristics based on dynamic magnetic resonance imaging (MRI) were correlated with histopathologic grade in mammary soft tissue tumors. Materials and methods. A spectrum of tumors, benign through highly malignant, was induced in 33 female rats by administration of N -ethyl-N -nitrosourea (ENU), a potent carcinogen. Dynamic contrast-enhanced MRI was performed using a small-molecular contrast medium [gadopentetate, MW = 0.5 kDa] and a macromolecular contrast medium [albumin-(Gd-DTPA)30, MW = 92 kDa] at an interval of 1–2 days. Permeability surface area product (PS), as estimated by the corresponding endothelial transfer coefficient (KPS), and fractional plasma volume (fPV) were calculated for each tumor and each contrast agent using a two-compartment bi-directional kinetic model. MRI microvascular characteristics were correlated with histopathologic tumor grade. Results. Tumor permeability to macromolecular contrast medium, characterized by KPS, showed a highly positive correlation with tumor grade (r2 = 0.76, P < 10− 10). KPS values were zero for all benign and some low-grade carcinomas, greater than zero in all other carcinomas, and increased in magnitude with higher tumor grade. A considerably smaller but significantly positive correlation was found between fPV and tumor grade using macromolecular contrast medium (r2 = 0.25, P < 0.003). No correlation between KPS or fPV values and tumor grade was found using gadopentetate (r2 = 0.01, P > 0.95 and r2 = 0.03, P > 0.15, respectively). Conclusion. Quantitative tumor microvascular permeability assays generated with macromolecular MRI contrast medium correlate closely with histologic tumor grade. No significant correlation is found using small-molecular gadopentetate.