Macromolecular contrast media-enhanced MRI estimates of microvascular permeability correlate with histopathologic tumor grade

In vivo monitoring of sorafenib therapy effects on experimental prostate carcinomas using dynamic contrast-enhanced MRI and macromolecular contrast media

Purpose: To investigate dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with macromolecular contrast media (MMCM) to monitor the effects of the multikinase inhibitor sorafenib on subcutaneous prostate carcinomas in rats with immunohistochemical validation. Materials and methods: Copenhagen rats, implanted with prostate carcinoma allografts, were randomized to the treatment group (n = 8) or the control group (n = 8). DCE-MRI with albumin-(Gd-DTPA)₃₅ was performed at baseline and after 1 week using a clinical 3-Tesla system. The treatment group received sorafenib, 10 mg/kg body weight daily. Kinetic analysis yielded quantitative parameters of tumor endothelial permeability–surface area product (PS; ml/100 ml/min) and fractional blood volume (V_b, %). Tumors were harvested on day 7 for immunohistochemical analysis. Results: In sorafenib-treated tumors, PS (0.62 ± 0.20 vs 0.08 ± 0.09 ml/100 ml/min; P < 0.01) and V_b (5.1 ± 1.0 vs 0.56 ± 0.48%; P < 0.01) decreased significantly from day 0 to day 7. PS showed a highly significant inverse correlation with tumor cell apoptosis (TUNEL; r = −0.85, P < 0.001). Good, significant correlations of PS were also observed with tumor cell proliferation (Ki-67; r = 0.67, P < 0.01) and tumor vascularity (RECA-1; r = 0.72, P < 0.01). MRI-assayed fractional blood volume V_b showed a highly significant correlation with tumor vascularity (RECA-1; r = 0.87, P < 0.001) and tumor cell proliferation (Ki-67; r = 0.82, P < 0.01). Conclusion: Results of DCE-MRI with MMCM demonstrated good, significant correlations with the immunohistochemically assessed antiangiogenic, antiproliferative, and proapoptotic effects of a 1-week, daily treatment course of sorafenib on experimental prostate carcinoma allografts.

Imaging in renal cell carcinoma

Imaging plays a central role in the detection, diagnosis, staging, and follow-up of renal cell carcinoma (RCC). Most renal masses are incidentally detected with modern, high-resolution imaging techniques and a variety of management options exist for the renal masses encountered today. This article discusses the role of multiple imaging modalities in the diagnosis of RCC and the imaging features of specific pathologic subtypes and staging techniques. Future directions in RCC imaging are presented, including dynamic contrast-enhanced and unenhanced techniques, as well as the development of novel tracers for positron emission tomography.

Breast cancers: MR imaging of folate-receptor expression with the folate-specific nanoparticle P1133

PURPOSE

To assess the capability of the folate receptor (FR)-targeted ultrasmall superparamagnetic iron oxide (USPIO) P1133 to provide FR-specific enhancement of breast cancers on magnetic resonance (MR) images.

MATERIALS AND METHODS

This study was approved by the institutional Animal Care and Use Committee. The FR-targeted contrast agent P1133 was incubated with various FR-positive human breast cancer cell lines, with and without free folic acid (FFA) as a competitor. Labeling efficiencies were evaluated with MR imaging and inductively coupled plasma mass spectrometry. Subsequently, six athymic rats with implanted FR-positive MDA-MB-231 breast cancers underwent MR imaging at 3 T before and up to 1 hour and 24 hours after injection of P1133. Six athymic rats with implanted FR-positive MDA-MB-231 cancers injected with the non-FR-targeted USPIO P904 and nine athymic rats with implanted FR-negative A549 lung cancers injected with P1133 (n = 6) or P904 (n = 3) served as controls. Data of the in vitro studies were compared for significant differences with the Wilcoxon test for two independent samples. Tumor signal-to-noise-ratios (SNRs) were compared between different experimental groups by using the Kruskal-Wallis test and were correlated with histopathologic findings. Differences with P < .05 were considered significant.

RESULTS

FR-positive breast cancer cells showed a significant P1133 uptake which was inhibited by FFA. MDA-MB-231 cells showed the highest level of P1133 uptake and the strongest T2 effect on MR images. In vivo, all tumors showed an initial perfusion effect. At 24 hours after injection, only MDA-MB-231 tumors injected with P1133 showed significantly decreased SNR data compared with baseline data (P < .05). MR findings were confirmed by using histopathologic findings.

CONCLUSION

The FR-targeted USPIO P1133 demonstrates a specific retention in FR-positive breast cancers. Because FR expression correlates with tumor aggressiveness and prognosis, persistent P1133 tumor enhancement may be used as a noninvasive indicator for tumors with poor outcome.

In vivo monitoring of angiogenesis inhibitory treatment effects by dynamic contrast-enhanced computed tomography in a xenograft tumor model

RATIONALE AND OBJECTIVES

To evaluate the potential of dynamic CT enhanced by iohexol or a novel macromolecular contrast agent, PEG12000-Gen4-triiodo, to monitor microvascular changes in tumors treated with the angiogenesis inhibitor bevacizumab.

MATERIALS AND METHODS

Ten female nude rats with MDA-MB 435 xenograft tumors were treated with 1 mg intraperitoneal bevacizumab when tumors reached 1 cm diameter and, for 4 rats, treated again 7 days later. Just before and 24 hours after the first injection of anti-VEGF antibody, the tumors were imaged by dynamic CT scans enhanced with PEG12000-Gen4-triiodo (n = 3 rats) or iohexol (n = 3 rats). The other 4 rats underwent dynamic CT scans enhanced with PEG12000-Gen4-triiodo just before and 24 hours after the second injection of anti-VEGF antibody. Microvascular leakiness (K(PS)) was calculated for the tumors using a 2-compartment tissue model.

RESULTS

PEG12000-Gen4-triiodo-enhanced CT scans showed progressive reductions in K(PS) from day 1 to 2 to 9 (from 2.55 to 1.27 to 0.69 microL min(-1) cm(-3), respectively, P < 0.005 for each comparison of day 1-2 and day 2-9). No significant difference was seen in the K(PS) estimates derived from iohexol-enhanced CT scans obtained before or after treatment (276 vs. 223.8 microL min(-1) cm(-3), respectively, P = 0.54). The microvascular leak (K(PS)) was significantly larger for iohexol than for PEG12000-Gen4-triiodo-enhanced CT, P < 0.05.

CONCLUSION

Dynamic macromolecular contrast-enhanced CT can be used to monitor serial decreases in tumor microvessel leakiness induced by repeated doses of an angiogenesis inhibitor drug.

Colloidal gold nanoparticles as a blood-pool contrast agent for X-ray computed tomography in mice

OBJECTIVES

To present the pharmacokinetics and computed tomographic imaging efficacy of colloidal gold nanoparticles (AuNPs) as a blood-pool agent for x-ray computed tomography (CT).

METHODS AND MATERIALS

To prepare the colloidal AuNPs, gold nanocrystals were modified using sulfhydrated polyethylene glycol (PEG). Cytotoxicity and histopathologic tests were carried out for toxicity evaluation. Six adult Balb/c mice underwent microcomputed tomography scans after injection of colloidal AuNPs (2.5 micromol Au/g body weight). Four mice with HT-1080 tumors were imaged for visualization of the tumor vasculature.

RESULTS

The PEG coated colloidal AuNPs appeared as spherical nanoparticles with 38-nm diameters. The AuNPs-PEG showed a biocompatibility without toxicity in the mice. We identified a stable imaging window for visualizing the vasculature system, immediately to 24 hours after injection. Microcomputed tomography imaging using AuNPs-PEG clearly visualized the tumor vascular structures.

CONCLUSION

Colloidal AuNPs show potential as a blood-pool agent for x-ray CT imaging.

Gadolinium(III)-based blood-pool contrast agents for magnetic resonance imaging: status and clinical potential

Blood-pool MRI contrast agents have enormous potential to aid sensitive magnetic resonance detection and yield definitive diagnostic data of cancer and diseases of the cardiovascular system. Many attempts have been initiated to design macromolecular gadolinium (Gd[III]) complexes as magnetic resonance imaging blood-pool contrast agents, as macromolecules do not readily diffuse across healthy vascular endothelium, and remain intravascular. Although extremely efficacious in detecting and characterizing pathologic tissue, clinical development of these agents has been limited by potential toxicity concerns from incomplete Gd(III) clearance. Recent innovative technologies, such as reversible protein-binding contrast agents and biodegradable macromolecular contrast agents, may be valuable alternatives that combine the effective imaging characteristics of an intravascular contrast agent and the safety of clinically approved low-molecular-weight Gd(III) chelates.

Multi-slice DCE-MRI data using P760 distinguishes between metastatic and non-metastatic rodent prostate tumors

An intermediate molecular weight contrast agent P760 was used to investigate the ability of multi-slice dynamic contrast-enhanced MRI (DCE-MRI) to distinguish metastatic from non-metastatic rodent prostate tumors. Non-metastatic AT2.1 and metastatic AT3.1 prostate tumors originally derived from the Dunning prostate cancer model were implanted on the hind leg of Copenhagen rats. Multi-sliced DCE-MRI data were acquired on a SIGNA 1.5 T scanner and analyzed using a recently developed empirical mathematical model. The P760 multi-slice DCE-MRI data in combination with the empirical mathematical model successfully distinguished between metastatic and non-metastatic rodent prostate tumors. Specifically, fitting the data with the empirical model showed that metastatic tumors had significantly faster contrast media uptake (p<0.001) and slower washout rates (p<0.01) than non-metastatic tumors.

MR and optical approaches to molecular imaging

With an increasing understanding of the molecular basis of disease, various new imaging targets have recently been defined that potentially allow for an early, sensitive, and specific diagnosis of disease or monitoring of treatment response. Different approaches to depict these molecular structures in vivo are currently being explored by the molecular imaging community. We briefly review methodologies for molecular imaging by magnetic resonance imaging and optical methods. Special emphasis is put on different contrast agent designs (e.g., targeted and smart probes). New technical developments in optical imaging are briefly discussed. In addition, current research results are put into a clinical perspective to elucidate the potential merits one might expect from this new research field.

Functional tumor imaging with dynamic contrast-enhanced magnetic resonance imaging

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is the acquisition of serial MRI images before, during, and after the administration of an MR contrast agent. Unlike conventional enhanced MRI, which simply provides a snapshot of enhancement at one point in time, DCE-MRI permits a fuller depiction of the wash-in and wash-out contrast kinetics within tumors, and thus provides insight into the nature of the bulk tissue properties. Such data is readily amenable to two-compartment pharmacokinetic modeling from which parameters based on the rates of exchange between the compartments can be generated. These parameters can be used to generate color-encoded images that aid in the visual assessment of tumors. DCE-MRI is used currently to characterize masses, stage tumors, and noninvasively monitor therapy. While DCE-MRI is in clinical use, there are also a number of limitations, including overlap between malignant and benign inflammatory tissue, failure to resolve microscopic disease, and the inconsistent predictive value of enhancement pattern with regard to clinical outcome. Current research focuses on improving understanding of the meaning of DCE-MRI at a molecular level, evaluating macromolecular and targeted contrast agents, and combining DCE-MRI with other physiologic imaging techniques such as positron emission tomography. Efforts to standardize DCE-MRI acquisition, analysis, and reporting methods will allow wider dissemination of this useful functional imaging technique.

A molecular CT blood pool contrast agent

RATIONALE AND OBJECTIVES

A molecular-based computed tomographic (CT) contrast agent with prolonged vascular residence time is needed for vascular and tumor imaging. No particulate agents have reached clinical practice due to nonspecific macrophage activation. The authors' objective was to synthesize a water-soluble macromolecular agent.

MATERIALS AND METHODS

Dysprosium-DTPA-dextran was synthesized through activation of the hydroxyl units of dextran PM40 with allylbromine and subsequent reaction with amino ethanethiol to produce amino-terminated leashes. These leashes were then coupled to DTPA by means of the mixed anhydride method. Complexation of dysprosium by DTPA-dextran was achieved in an acidic solution of 0.2 M dysprosium chloride. One rabbit with a VX2 tumor was imaged with [Dy]DTPA-dextran (0.5 mL, 3.1 g, 1.15 mmol of dysprosium per kilogram). Transaxial scans were acquired through the liver and tumor for 45 minutes. A second healthy rabbit was imaged with Optiray-320 (6.0 mL, 5.0 mmol of iodine per kilogram) at 1-minute intervals for 10 minutes and again at 20 minutes.

RESULTS

Each dextran PM40 molecule (diameter, 8.8 nm) contained 95 [Dy]DTPA groups, increasing its average molecular weight from 40,500 to 101,537 g/mol. The baseline-corrected inferior vena cava (IVC) enhancement for [Dy]DTPA-dextran decreased, with an 8-minute half-time during the first 15 minutes followed by a nearly zero slope for the rest of the observation period. The IVC remained brighter than liver throughout the observation period. The solid portion of the tumor was enhanced by 5-10 CT numbers, rendering areas of necrosis more apparent. The baseline-corrected IVC curve for Optiray-320 also demonstrated two phases, with half-times of 2.5 and 45 minutes. The IVC became less dense than liver within 5-8 minutes.

CONCLUSION

[Dy]DTPA-dextran is water soluble and can be synthesized without intermolecular cross-linking to carry a high load of dysprosium. It provides blood pool enhancement characteristics with a long intravascular dwell time.

Dynamic contrast-enhanced MRI of Implanted VX2 tumors in rabbit muscle: comparison of Gd-DTPA and NMS60

We studied the dynamics of injected contrast enhancement in implanted VX2 tumors in rabbit thigh muscle. We compared two contrast agents Gd-DTPA and NMS60, a novel gadolinium containing trimer of molecular weight 2.1 kd. T1-weighted spin echo images were acquired preinjection and at 5-60 min after i.v. injection of 0.1 mmol/kg of agent. Dynamic T1-weighted SPGR images (1.9 s/image) were acquired during the bolus injection. Male NZW rabbits (n = 13) were implanted with approximately 2 x 10(6) VX2 tumor cells and grew tumors of 28+/-27 mL over 12 to 21 days. NMS60 showed significantly greater peak enhancement in muscle, tumor rim, and core compared to DTPA in both T1-weighted and SPGR images. NMS60 also showed delayed peak enhancement in the dynamic scans (compared to Gd-DTPA) and significantly reduced leakage rate constant into the extravascular space for tumor rim (K21 = 5.1 min(-1) vs. 11.5 min(-1) based on a 2 compartment kinetic model). The intermediate weight contrast agent NMS60 offers greater tumor enhancement than Gd-DTPA and may offer improved regional differentiation on the basis of vascular permeability in tumors.

MRI enhancement and microvascular density in gliomas. Correlation with tumor cell proliferation

RATIONALE AND OBJECTIVES

Angiogenesis and proliferation activity are important indicators of tumor behavior in human gliomas. The authors studied how tumor enhancement in MR imaging and intratumoral vascular density were correlated with cell proliferation in cerebral gliomas.

METHODS

The authors studied retrospectively 62 cerebral gliomas. Patients were examined before surgery with contrast-enhanced MR imaging. Microvessel density and the cell proliferation rate of tumor specimens were measured immunohistochemically using factor VIII and MIB-1 antibodies. Contrast enhancement of the tumors was evaluated by two radiologists.

RESULTS

Contrast enhancement was observed in 45 tumors and was correlated with histologic cell proliferation (P = 0.0007) and microvessel density (P = 0.01). There was also a correlation between tumor vascular density and the cell proliferation rate (r = 0.51, P < 0.0001). Histologic tumor grade was associated with vascular density (P = 0.001).

CONCLUSIONS

Lesion enhancement on preoperative contrast-enhanced MR imaging correlates with vascularity and proliferation activity of gliomas. The additional correlation between tumor vascularity and proliferation suggests that intratumoral microvessel density could be useful in estimating tumor proliferation.