Applications of magnetic resonance in model systems: tumor biology and physiology

Blood-pool MRI assessment of myocardial microvascular reactivity

Purpose

The ability to non-invasively image myocardial microvascular dilation and constriction is essential to assessing intact function and dysfunction. Yet, conventional measurements based on blood oxygenation are not specific to changes in blood volume. The purpose of this study was to extend to the heart a blood-pool MRI approach for assessing vasomodulation in the presence of blood gas changes and investigate if sex-related differences exist.

Methods

Animals [five male and five female healthy Sprague Dawley rats (200-500 g)] were intubated, ventilated, and cycled through room air (normoxia) and hypercapnia (10% CO2) in 10-minute cycles after i.v. injection of blood-pool agent Ablavar (0.3 mmol/kg). Pre-contrast T1 maps and T1-weighted 3D CINE were acquired on a 3 Tesla preclinical MRI scanner, followed by repeated 3D CINE every 5 min until the end of the gas regime. Invasive laser Doppler flowmetry of myocardial perfusion was performed to corroborate MRI results.

Results

Myocardial microvascular dilation to hypercapnia and constriction to normoxia were readily visualized on T1 maps. Over 10 min of hypercapnia, female myocardial T1 reduced by 20% (vasodilation), while no significant change was observed in the male myocardium. After return to normoxia, myocardial T1 increased (vasoconstriction) in both sexes (18% in females and 16% in males). Laser Doppler perfusion measurements confirmed vasomodulatory responses observed on MRI.

Conclusion

Blood-pool MRI is sensitive and specific to vasomodulation in the myocardial microcirculation. Sex-related differences exist in the healthy myocardium in response to mild hypercapnic stimuli.

Stereotactic Magnetic Resonance-Guided Adaptive and Non-Adaptive Radiotherapy on Combination MR-Linear Accelerators: Current Practice and Future Directions

Stereotactic body radiotherapy (SBRT) is an effective radiation therapy technique that has allowed for shorter treatment courses, as compared to conventionally dosed radiation therapy. As its name implies, SBRT relies on daily image guidance to ensure that each fraction targets a tumor, instead of healthy tissue. Magnetic resonance imaging (MRI) offers improved soft-tissue visualization, allowing for better tumor and normal tissue delineation. MR-guided RT (MRgRT) has traditionally been defined by the use of offline MRI to aid in defining the RT volumes during the initial planning stages in order to ensure accurate tumor targeting while sparing critical normal tissues. However, the ViewRay MRIdian and Elekta Unity have improved upon and revolutionized the MRgRT by creating a combined MRI and linear accelerator (MRL), allowing MRgRT to incorporate online MRI in RT. MRL-based MR-guided SBRT (MRgSBRT) represents a novel solution to deliver higher doses to larger volumes of gross disease, regardless of the proximity of at-risk organs due to the (1) superior soft-tissue visualization for patient positioning, (2) real-time continuous intrafraction assessment of internal structures, and (3) daily online adaptive replanning. Stereotactic MR-guided adaptive radiation therapy (SMART) has enabled the safe delivery of ablative doses to tumors adjacent to radiosensitive tissues throughout the body. Although it is still a relatively new RT technique, SMART has demonstrated significant opportunities to improve disease control and reduce toxicity. In this review, we included the current clinical applications and the active prospective trials related to SMART. We highlighted the most impactful clinical studies at various tumor sites. In addition, we explored how MRL-based multiparametric MRI could potentially synergize with SMART to significantly change the current treatment paradigm and to improve personalized cancer care.

Non-contrast enhanced molecular characterization of C6 rat glioma tumor at 7 T

PURPOSE

The aim of this study was to investigate, how to assess the relevant magnetization changes in the rat brain tissue due to the present of glioma tumor and its growth at a 7 T animal magnetic resonance imaging (MRI) system.

MATERIAL AND METHODS

For this study, a custom-built two dimensional (2D) chemical exchange saturation transfer (CEST) pulse sequence was optimized for different tissue properties using fresh and cooked quail eggs. C6 tumor cells were investigated by in-vivo and post-mortem measurements in six Wistar rats using the optimized CEST sequence up to 5 weeks. Magnetization transfer ratio (MTR)- and asymmetric MTR (MTR_asym)-maps of rat brains were created at different frequency offsets. In-vivo results were verified by ¹H spectroscopic, histological and also in-vitro C6 cell culture examinations.

RESULTS

The CEST module for the optimal visualization of magnetization effects consists of five RF-pulses, each with a duration of 20 ms and a flip angle of 180°. In-vivo and post-mortem z-spectra of the cerebrospinal fluid (CSF), cortex, myelinated/demyelinated, healthy and tumorous tissue and tumor rim were obtained. The magnetization level and shape of the z-spectra on the upfield and downfield from the water peak were not the same. The magnetization dips on the upfield and downfield from the water peak of the z-spectra disappeared due to the thermal denaturation in cooked quail eggs and due to formaldehyde-induced fixation in post-mortem rat brains. The z-spectra of the rat brain in a range of ±2 to ±4 ppm displayed valuable information about the differentiation of various brain regions from the tumor tissue. Histological examinations confirmed our results. The C6 cell culture examinations showed that the observed magnetization changes in the rat brain occurred only due to the interaction between glioma cells and their environment in the rat brain and not from the C6 tumor cells.

CONCLUSIONS

Based on our in-vivo and post-mortem results, it is to be recommended to create the MTR-maps at a special offset frequency depending on the aim of research project instead of MTR_asym-maps. Otherwise, the desired effect attenuates or vanishes.

Volumetric assessment of apparent diffusion coefficient predicts outcome following chemoradiation for cervical cancer

OBJECTIVE

To determine the utility of volumetric diffusion weighted imaging (DWI) compared to other clinical factors for predicting recurrence and survival in cervical cancer patients treated with definitive chemoradiation.

METHODS AND MATERIALS

We retrospectively studied cervical cancer patients treated with definitive chemoradiation between 2009-2013 at a single institution with a baseline MRI with DWI and 18F-FDG positron emission tomography/computed tomography (FDG-PET) scan. To identify clinical and imaging metrics correlated with survival and recurrence endpoints, variable importance values were calculated from random forest models. To provide clinically relevant threshold values, recursive partitioning analysis dichotomized patients into potential risk groups based on selected metrics. Cox's proportional hazard models assessed the effect of clinical and imaging factors on survival endpoints.

RESULTS

Ninety-three patients were included in the analysis (median age 50 years). At a median follow-up of 35.6 months, 32 patients (34%) had disease recurrence. In the best multivariate model including clinical and imaging parameters, 90th percentile ADC < 1.917 was the only significantly associated factor with worse progression free survival (PFS). Overall survival, PFS, and distant metastasis free survival (DMFS) were significantly different between patient groups divided on 90th percentile ADC with threshold of 1.917 × 10^-3 mm²/s and MRI volume with threshold of 18.9 cc (P = 0.037, P = 0.0002, P = 0.001). High MRI volume and low ADC were associated with worse clinical outcomes.

CONCLUSIONS

Volumetric 90th percentile ADC value of the primary tumor on pretreatment MRI was a significant predictor of PFS and DMFS in cervical cancer patients, independent of established clinical factors and SUV on FDG-PET.

Bridging the translational gap: Implementation of multimodal small animal imaging strategies for tumor burden assessment in a co-clinical trial

In designing co-clinical cancer studies, preclinical imaging brings unique challenges that emphasize the gap between man and mouse. Our group is developing quantitative imaging methods for the preclinical arm of a co-clinical trial studying immunotherapy and radiotherapy in a soft tissue sarcoma model. In line with treatment for patients enrolled in the clinical trial SU2C-SARC032, primary mouse sarcomas are imaged with multi-contrast micro-MRI (T1 weighted, T2 weighted, and T1 with contrast) before and after immune checkpoint inhibition and pre-operative radiation therapy. Similar to the patients, after surgery the mice will be screened for lung metastases with micro-CT using respiratory gating. A systems evaluation was undertaken to establish a quantitative baseline for both the MR and micro-CT systems against which others systems might be compared. We have constructed imaging protocols which provide clinically-relevant resolution and contrast in a genetically engineered mouse model of sarcoma. We have employed tools in 3D Slicer for semi-automated segmentation of both MR and micro-CT images to measure tumor volumes efficiently and reliably in a large number of animals. Assessment of tumor burden in the resulting images was precise, repeatable, and reproducible. Furthermore, we have implemented a publicly accessible platform for sharing imaging data collected during the study, as well as protocols, supporting information, and data analyses. In doing so, we aim to improve the clinical relevance of small animal imaging and begin establishing standards for preclinical imaging of tumors from the perspective of a co-clinical trial.

Assessment of pharmacokinetics for microvessel proliferation by DCE-MRI for early detection of physeal bone bridge formation in an animal model

OBJECTIVES

Bone bridge formation occurs after physeal lesions and can lead to growth arrest if not reversed. Previous investigations on the underlying mechanisms of this formation used histological methods. Therefore, this study aimed to apply a minimally invasive method using dynamic contrast-enhanced MRI (DCE-MRI).

MATERIALS AND METHODS

Changes in functional parameters related to the microvessel system were assessed in a longitudinal study of a cohort of an animal model applying a reference region model. The development of morphology of the injured physis was investigated with 3D high-resolution MRI. To acquire complementary information for MRI-related findings qRT-PCR and immunohistochemical data were acquired for a second cohort of the animal model.

RESULTS

The evaluation of the pharmacokinetic parameters showed a first rise of the transfer coefficient 7 days post-lesion and a maximum 42 days after operation. The analysis of the complementary data showed a connection of the first rise to microvessel proliferation while the maximum value was linked to bone remodeling.

CONCLUSION

The pharmacokinetic analysis of DCE-MRI provides information on a proliferation of microvessels during the healing process as a sign for bone bridge formation. Thereby, DCE-MRI could identify details, which up to now required analyses of highly invasive methods.

Vascular Reactivity Maps in Patients with Gliomas Using Breath-Holding BOLD fMRI

BACKGROUND AND PURPOSE

To evaluate whether breath-holding (BH) blood oxygenation level-dependent (BOLD) fMRI can quantify differences in vascular reactivity (VR), as there is a need for improved contrast mechanisms in gliomas.

METHODS

16 patients (gliomas, grade II = 5, III = 2, IV = 9) were evaluated using the BH paradigm: 4-second single deep breath followed by 16 seconds of BH and 40 seconds of regular breathing for five cycles. VR was defined as the difference in BOLD signal between the minimal signal seen at the end of the deep breath and maximal signal seen at the end of BH (peak-to-trough). VR was measured for every voxel and compared for gray versus white matter and tumor versus normal contralateral brain. VR maps were compared to the areas of enhancement and FLAIR/T2 abnormality.

RESULTS

VR was significantly lower in normal white matter than gray matter (P < .05) and in tumors compared to the normal, contralateral brain (P < 0.002). The area of abnormal VR (1103 ± 659 mm²) was significantly greater (P = .019) than the enhancement (543 ± 530 mm²), but significantly smaller (P = .0011) than the FLAIR abnormality (2363 ± 1232 mm²). However, the variability in the areas of gadolinium contrast enhancement versus VR abnormality indicates that the contrast mechanism elicited by BH (caused by abnormal arteriolar smooth muscles) appears to be fundamentally different from the contrast mechanism of gadolinium enhancement (caused by the presence of "leaky" gap junctions).

CONCLUSIONS

BH maps based on peak-to-trough can be used to characterize VR in brain tumors. VR maps in brain tumor patients appear to be caused by a different mechanism than gadolinium enhancement.

The Tumor Microenvironment Modulates Choline and Lipid Metabolism

An increase of cellular phosphocholine (PC) and total choline (tCho)-containing compounds as well as alterations in lipids have been consistently observed in cancer cells and tissue. These metabolic changes are closely related to malignant transformation, invasion, and metastasis. The study of cancer cells in culture plays an important role in understanding mechanisms leading to altered choline (Cho) and lipid metabolism in cancer, as it provides a carefully controlled environment. However, a solid tumor is a complex system with a unique tumor microenvironment frequently containing hypoxic and acidic regions and areas of nutrient deprivation and necrosis. Cancer cell–stromal cell interactions and the extracellular matrix may also alter Cho and lipid metabolism. Human tumor xenograft models in mice are useful to mimic the growth of human cancers and provide insights into the influence of in vivo conditions on metabolism. Here, we have compared metabolites, obtained with high resolution ¹H MRS of extracts from human breast and prostate cancer cells in a 2-dimensional (2D) monolayer culture and from solid tumor xenografts derived from these cells, as well as the protein expression of enzymes that regulate Cho and lipid metabolism. Our data demonstrate significant differences in Cho and lipid metabolism and protein expression patterns between human breast and prostate cancer cells in culture and in tumors derived from these cells. These data highlight the influence of the tumor microenvironment on Cho and lipid metabolism.

Diffusion-Weighted Magnetic Resonance Imaging as a Predictor of Outcome in Cervical Cancer After Chemoradiation

PURPOSE

To determine whether apparent diffusion coefficient (ADC) value is predictive of survival after definitive chemoradiation for cervical cancer independent of established imaging and clinical prognostic factors.

METHODS AND MATERIALS

Between 2011 and 2013, the pretreatment MRI scans for 69 patients treated with definitive chemoradiation for newly diagnosed cervical cancer were retrieved. Scans were acquired with a 1.5-T magnetic resonance scanner, including diffusion-weighted imaging sequences. Mean ADC value was measured within a region of interest in the primary cervical cancer on the baseline MRI scan. Baseline tumor maximum standardized uptake value on positron emission tomography/computed tomography was determined by the reading radiologist. Treatment included external beam radiation therapy to the pelvis followed by brachytherapy in 97%, and with concurrent weekly cisplatin in 99% of patients. Univariate and multivariate analyses were done to investigate the association of clinical and imaging variables with disease control and survival endpoints using a Cox proportional hazard test.

RESULTS

Median follow-up was 16.7 months (range, 3.1-44.2 months). The 1-year overall survival, locoregional recurrence-free survival, and disease-free survival rates were 91%, 86%, and 74%, respectively. The median ADC value was 0.941 × 10^-3 mm²/s (range, 0.256-1.508 × 10^-3 mm²/s). The median standardized uptake value in the primary tumor was 15 (range, 6.2-43.4). In multivariate analysis, higher ADC value (hazard ratio [HR] 0.36, 95% confidence interval [CI] 0.15-0.85, P=.02), higher stage (HR 2.4, 95% CI 1.1-5.5, P=.033), and nonsquamous histology (HR 0.23, 95% CI 0.07-0.82, P=.024) were independent predictors of disease-free survival.

CONCLUSIONS

The mean ADC value of the primary tumor on pretreatment MRI was the only imaging feature that was an independent predictor of disease-free survival in cervical cancer patients treated with chemoradiation. Further validation will be needed to determine whether ADC values may prove useful in identifying cervical patients at high risk of recurrence.

Quantitative correlational study of microbubble-enhanced ultrasound imaging and magnetic resonance imaging of glioma and early response to radiotherapy in a rat model

PURPOSE

Radiotherapy remains a major treatment method for malignant tumors. Magnetic resonance imaging (MRI) is the standard modality for assessing glioma treatment response in the clinic. Compared to MRI, ultrasound imaging is low-cost and portable and can be used during intraoperative procedures. The purpose of this study was to quantitatively compare contrast-enhanced ultrasound (CEUS) imaging and MRI of irradiated gliomas in rats and to determine which quantitative ultrasound imaging parameters can be used for the assessment of early response to radiation in glioma.

METHODS

Thirteen nude rats with U87 glioma were used. A small thinned skull window preparation was performed to facilitate ultrasound imaging and mimic intraoperative procedures. Both CEUS and MRI with structural, functional, and molecular imaging parameters were performed at preradiation and at 1 day and 4 days postradiation. Statistical analysis was performed to determine the correlations between MRI and CEUS parameters and the changes between pre- and postradiation imaging.

RESULTS

Area under the curve (AUC) in CEUS showed significant difference between preradiation and 4 days postradiation, along with four MRI parameters, T2, apparent diffusion coefficient, cerebral blood flow, and amide proton transfer-weighted (APTw) (all p < 0.05). The APTw signal was correlated with three CEUS parameters, rise time (r = - 0.527, p < 0.05), time to peak (r = - 0.501, p < 0.05), and perfusion index (r = 458, p < 0.05). Cerebral blood flow was correlated with rise time (r = - 0.589, p < 0.01) and time to peak (r = - 0.543, p < 0.05).

CONCLUSIONS

MRI can be used for the assessment of radiotherapy treatment response and CEUS with AUC as a new technique and can also be one of the assessment methods for early response to radiation in glioma.

Quantitative multiparametric MRI assessment of glioma response to radiotherapy in a rat model

BACKGROUND

The inability of structural MRI to accurately measure tumor response to therapy complicates care management for patients with gliomas. The purpose of this study was to assess the potential of several noninvasive functional and molecular MRI biomarkers for the assessment of glioma response to radiotherapy.

METHODS

Fourteen U87 tumor-bearing rats were irradiated using a small-animal radiation research platform (40 or 20 Gy), and 6 rats were used as controls. MRI was performed on a 4.7 T animal scanner, preradiation treatment, as well as at 3, 6, 9, and 14 days postradiation. Image features of the tumors, as well as tumor volumes and animal survival, were quantitatively compared.

RESULTS

Structural MRI showed that all irradiated tumors still grew in size during the initial days postradiation. The apparent diffusion coefficient (ADC) values of tumors increased significantly postradiation (40 and 20 Gy), except at day 3 postradiation, compared with preradiation. The tumor blood flow decreased significantly postradiation (40 and 20 Gy), but the relative blood flow (tumor vs contralateral) did not show a significant change at most time points postradiation. The amide proton transfer weighted (APTw) signals of the tumor decreased significantly at all time points postradiation (40 Gy), and also at day 9 postradiation (20 Gy). The blood flow and APTw maps demonstrated tumor features that were similar to those seen on gadolinium-enhanced T1-weighted images.

CONCLUSIONS

Tumor ADC, blood flow, and APTw were all useful imaging biomarkers by which to predict glioma response to radiotherapy. The APTw signal was most promising for early response assessment in this model.

Detection of (19)F-labeled biopharmaceuticals in cell cultures with magnetic resonance

Magnetic resonance (MR) studies of the therapeutic efficacy of fluorinated drugs have recently become possible due to improvements in detection including the application of very strong magnetic fields up to 9.4Tesla (T). These advances allow tracking, identification, and quantification of (19)F-labeled biopharmaceuticals using (19)F MR imaging ((19)F MRI) and spectroscopy ((19)F MRS). Both techniques are noninvasive, are nondestructive, and enable serial measurements. They also allow for controlled and systematic studies of cellular metabolism in cancerous tissue in vivo (small animals and humans) and in vitro (body fluids, cells culture, tissue extracts and isolated tissues). Here we provide an overview of the (19)F MRI and (19)F MRS techniques used for tracking (19)F labeled anticancer chemotherapeutics and antibodies which allow quantification of drug uptake in cancer cells in vitro.

In vivo maps of extracellular pH in murine melanoma by CEST-MRI

PURPOSE

A novel method based on the use of Yb-HPDO3A as MRI Para-CEST agent for in vivo pH mapping of the tumor region in a melanoma murine model is reported. This method does not require the knowledge of the concentration of the imaging agent.

METHODS

C57BL/6-mice were inoculated with B16-F10 cells. CEST-MR images of tumor and bladder were acquired upon the i.v. administration of Yb-HPDO3A (1.2 mmol/Kg). pH was assessed by the use of a ratiometric method.

RESULTS

Yb-HPDO3A distributes well in the extracellular space of the tumor allowing the detection of good levels of saturation transfer (ST). It is excreted throughout kidneys and accumulated in the bladder thus yielding a strong CEST signal from urine. By comparing the ST% obtained upon selective irradiation of the two OH resonances belonging to the two isomeric forms of Yb-HPDO3A, it has been possible to measure the extracellular pH for each voxel (0.22 mm(3) ). The obtained pH-maps of tumors show a great heterogeneity. Marked differences are associated to tumor staging.

CONCLUSION

The application of Yb-HPDO3A to measure extracellular tumor pH provides a good spatio-temporal resolution and it does not require the prior knowledge of the contrast agent concentration. The herein reported data support the potential clinical translation of Yb-HPDO3A.

Effects of HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) on NEU/HER2 overexpressing mammary tumours in MMTV-NEU-NT mice monitored by Magnetic Resonance Spectroscopy

BACKGROUND

The importance of ERBB2/NEU/HER2 in the response of breast tumours to the heat shock protein 90 (HSP90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG; tanespimycin) has been demonstrated in the clinic. ERBB2 is an oncoprotein client that is highly dependent on HSP90. This and other oncogenic client proteins (e.g. B-RAF, C-RAF, ALK and CDK4) are depleted by 17-AAG in both animal tumours and patients. Here we investigate by Magnetic Resonance Spectroscopy (MRS) the metabolic response of 17-AAG in spontaneous, NEU/HER2 driven mammary tumours in transgenic MMTV-NEU-NT mice and in cells isolated and cultured from these tumours.

METHODS

Mammary tumours were monitored by 31P MRS in vivo and in tumour extracts, comparing control and 17-AAG treated mice. A cell line derived from NEU/HER2 mammary tumours was also cultured and the effect of 17-AAG was measured by 31P MRS in cell extracts. Molecular biomarkers were assessed by immunoblotting in extracts from cells and tumours. For comparison of tumour volume, metabolite concentrations and Western blot band intensities, two-tailed unpaired t-tests were used.

RESULTS

The NEU/HER2 mammary tumours were very sensitive to 17-AAG and responded in a dose-dependent manner to 3 daily doses of 20, 40 and 80mg/kg of 17-AAG, all of which caused significant regression. At the higher doses, 31P MRS of tumour extracts showed significant decreases in phosphocholine (PC) and phosphoethanolamine (PE) whereas no significant changes were seen at the 20mg/kg dose. Extracts of isolated cells cultured from the mammary carcinomas showed a significant decrease in viable cell number and total PME after 17-AAG treatment. Western blots confirmed the expected action of 17-AAG in inducing HSP72 and significantly depleting HSP90 client proteins, including NEU/HER2 both in tumours and in isolated cells.

CONCLUSIONS

The data demonstrate the high degree of sensitivity of this clinically relevant NEU/HER2-driven tumour model to HSP90 inhibition by 17-AAG, consistent with the clinical data, and suggest that the metabolic signature of choline phospholipids obtained by MRS could be useful both as a preclinical and clinical tool for investigating surrogate markers of response to treatment.

Application of ex vivo micro-computed tomography for assessment of in vivo fluorescence and plain radiographic imaging for monitoring bone metastases and osteolytic lesions

The intracardiac injection model is a commonly used in vivo model to test therapeutic response in bone metastases. However, few studies have critically compared the performance of different imaging methods in terms of sensitivity and quantitative assessment of osteolytic lesions. We performed in vivo optical and plain radiographic imaging of bone metastases followed by high-sensitivity ex vivo micro-computed tomography (micro-CT) imaging. This approach allowed for quantitative assessment of in vivo imaging techniques using fluorescence and plain radiography. Comparison of lesions detected in vivo by fluorescent optical imaging with ex vivo micro-CT revealed that the limited spatial resolution of fluorescent optical imaging may underestimate the number of bone metastases. Radiography was compared with micro-CT for the detection of osteolytic lesions. When using dichotomous yes/no grading, there was a 64% agreement in detection of osteolytic lesions. When subjective semiquantitative grading methods were used to assess the extent of osteolytic lesions, a positive association between the micro-CT grades and the square root of the radiography-based grades was observed (p < 0.05). Micro-CT also showed a significant association with fluorescent optical values; however, no such association was observed between lesion scores based on radiographs and those based on fluorescent imaging. The findings reveal an approximate two-fold sensitivity for micro-CT compared to plain radiography in the detection of osteolytic lesions. Significant associations between micro-CT-based osteolytic lesion grade and tumor growth characterized by increased fluorescent area document the value of these two techniques for the assessment of osteolytic bone metastases.

Compressive sensing could accelerate 1H MR metabolic imaging in the clinic

PURPOSE

To retrospectively evaluate the fidelity of magnetic resonance (MR) spectroscopic imaging data preservation at a range of accelerations by using compressed sensing.

MATERIALS AND METHODS

The protocols were approved by the institutional review board of the university, and written informed consent to acquire and analyze MR spectroscopic imaging data was obtained from the subjects prior to the acquisitions. This study was HIPAA compliant. Retrospective application of compressed sensing was performed on 10 clinical MR spectroscopic imaging data sets, yielding 600 voxels from six normal brain data sets, 163 voxels from two brain tumor data sets, and 36 voxels from two prostate cancer data sets for analysis. The reconstructions were performed at acceleration factors of two, three, four, five, and 10 and were evaluated by using the root mean square error (RMSE) metric, metabolite maps (choline, creatine, N-acetylaspartate [NAA], and/or citrate), and statistical analysis involving a voxelwise paired t test and one-way analysis of variance for metabolite maps and ratios for comparison of the accelerated reconstruction with the original case.

RESULTS

The reconstructions showed high fidelity for accelerations up to 10 as determined by the low RMSE (< 0.05). Similar means of the metabolite intensities and hot-spot localization on metabolite maps were observed up to a factor of five, with lack of statistically significant differences compared with the original data. The metabolite ratios of choline to NAA and choline plus creatine to citrate did not show significant differences from the original data for up to an acceleration factor of five in all cases and up to that of 10 for some cases.

CONCLUSION

A reduction of acquisition time by up to 80%, with negligible loss of information as evaluated with clinically relevant metrics, has been successfully demonstrated for hydrogen 1 MR spectroscopic imaging.

Therapeutic response in musculoskeletal soft tissue sarcomas: evaluation by MRI

This article provides a literature review of the use of MRI in monitoring the treatment response of soft tissue sarcomas. The basic classification and physiology of soft tissue tumors are introduced. Then, the major treatment options for soft tissue sarcomas are summarized with brief coverage of possible responses and grading systems. Four major branches of MRI techniques are covered, including conventional T(1) - and T(2) -weighted imaging, contrast-enhanced MRI, MR diffusion and perfusion imaging, and MRS, with a focus on the tumor microenvironment. Although this literature survey focuses on recent clinical developments using these MRI techniques, research venues in preclinical studies, as well as in potential applications other than soft tissue sarcomas, are also included when comparable and/or mutually supporting. Examples from other less-discussed MRI modalities are also briefly covered, not only to complement, but also to expand, the scope and depth of information for various kinds of lesions.

MR evaluation of response to targeted treatment in cancer cells

The development of molecular technologies, together with progressive sophistication of molecular imaging methods, has allowed the further elucidation of the multiple mutations and dysregulatory effects of pathways leading to oncogenesis. Acting against these pathways by specifically targeted agents represents a major challenge for current research efforts in oncology. As conventional anatomically based pharmacological endpoints may be inadequate to monitor the tumor response to these targeted treatments, the identification and use of more appropriate, noninvasive pharmacodynamic biomarkers appear to be crucial to optimize the design, dosage and schedule of these novel therapeutic approaches. An aberrant choline phospholipid metabolism and enhanced flux of glucose derivatives through glycolysis, which sustain the redirection of mitochondrial ATP to glucose phosphorylation, are two major hallmarks of cancer cells. This review focuses on the changes detected in these pathways by MRS in response to targeted treatments. The progress and limitations of our present understanding of the mechanisms underlying MRS-detected phosphocholine accumulation in cancer cells are discussed in the light of gene and protein expression and the activation of different enzymes involved in phosphatidylcholine biosynthesis and catabolism. Examples of alterations induced in the MRS choline profile of cells exposed to different agents or to tumor environmental factors are presented. Current studies aimed at the identification in cancer cells of MRS-detected pharmacodynamic markers of therapies targeted against specific conditional or constitutive cell receptor stimulation are then reviewed. Finally, the perspectives of present efforts addressed to identify enzymes of the phosphatidylcholine cycle as possible novel targets for anticancer therapy are summarized.

Magnetic resonance spectroscopy of living tissues

The comprehensive work of both clinical and basic science colleagues has demonstrated a clear proof of concept for "in vitro discovered- in vivo validated" biomarkers in translational metabolic profiling research using magnetic resonance techniques. Major tissue metabolites (initially discovered by high-resolution in vitro techniques on cancer specimens) can be translated into in vivo protocols based on noninvasive magnetic resonance spectroscopy (MRS). Using (1)H- and (31)P-MRS on living animals or patients, a decrease in citrate and polyamines in prostate cancer, an increase of cholines in breast cancer, as well as a decreased NAA and an increased lactate in gliomas during cancer progression can be assessed noninvasively. MRS can be used to follow up conventional cytotoxic as well as targeted anticancer therapies, which has been extensively done in animal models of cancer. This review focuses on applications and protocol development for in vivo (1)H- and (31)P-MRS on small animal models as well as on larger animals in cancer research, diagnosis, and treatment.

Monitoring of gliomas in vivo by diffusion MRI and (1)H MRS during gene therapy-induced apoptosis: interrelationships between water diffusion and mobile lipids

The measurement of water diffusion by diffusion-weighted MRI (DWI) in vivo offers a non-invasive method for assessing tissue responses to anti-cancer therapies. The pathway of cell death after anti-cancer treatment is often apoptosis, which leads to accumulation of mobile lipids detectable by (1)H MRS in vivo. However, it is not known how these discrete MR markers of cell death relate to each other. In a rodent tumour model [i.e. ganciclovir-treated herpes simplex thymidine kinase (HSV-tk) gene-transfected BT4C gliomas], we studied the interrelationships between water diffusion (Trace{D}) and mobile lipids during apoptosis. Water diffusion and water-referenced concentrations of mobile lipids showed clearly increasing and interconnected trends during treatment. Of the accumulating (1)H MRS-visible lipids, the fatty acid --CH==CH-- groups and cholesterol compounds showed the strongest associations with water diffusion (r(2) = 0.30; P < 0.05 and r(2) = 0.48; P < 0.01, respectively). These results indicate that the tumour histopathology and apoptotic processes during tumour shrinkage can be interrelated in vivo by DWI of tissue water and (1)H MRS of mobile lipids, respectively. However, there is considerable individual variation in the associations, particularly at the end of the treatment period, and in the relative compositions of the accumulating NMR-visible lipids. The findings suggest that the assessment of individual treatment response in vivo may benefit from combining DWI and (1)H MRS. Absolute and relative changes in mobile lipids may indicate initiation of tumour shrinkage even when changes in tissue water diffusion are still small. Conversely, greatly increased water diffusion probably indicates that substantial cell decomposition has taken place in the tumour tissue when the (1)H MRS resonances of mobile lipids alone can no longer give a reliable estimate of tissue conditions.

Time-dependent effects of imatinib in human leukaemia cells: a kinetic NMR-profiling study

The goal of this study was to evaluate the time course of metabolic changes in leukaemia cells treated with the Bcr-Abl tyrosine kinase inhibitor imatinib. Human Bcr-Abl⁺ K562 cells were incubated with imatinib in a dose-escalating manner (starting at 0.1 μM with a weekly increase of 0.1 μM imatinib) for up to 5 weeks. Nuclear magnetic resonance spectroscopy and liquid-chromatography mass spectrometry were performed to assess a global metabolic profile, including glucose metabolism, energy state, lipid metabolism and drug uptake, after incubation with imatinib. Initially, imatinib treatment completely inhibited the activity of Bcr-Abl tyrosine kinase, followed by the inhibition of cell glycolytic activity and glucose uptake. This was accompanied by the increased mitochondrial activity and energy production. With escalating imatinib doses, the process of cell death rapidly progressed. Phosphocreatine and NAD⁺ concentrations began to decrease, and mitochondrial activity, as well as the glycolysis rate, was further reduced. Subsequently, the synthesis of lipids as necessary membrane precursors for apoptotic bodies was accelerated. The concentrations of the Kennedy pathway intermediates, phosphocholine and phosphatidylcholine, were reduced. After 4 weeks of exposure to imatinib, the secondary necrosis associated with decrease in the mitochondrial and glycolytic activity occurred and was followed by a shutdown of energy production and cell death. In conclusion, monitoring of metabolic changes in cells exposed to novel signal transduction modulators supplements molecular findings and provides further mechanistic insights into longitudinal changes of the mitochondrial and glycolytic pathways of oncogenesis.

Enhancement of histological volumes through averaging and their use for the analysis of magnetic resonance images

Magnetic resonance imaging (MRI) of small animals is routinely performed in research centers. But despite its many advantages, MR still suffers from limited spatial resolution which makes the interpretation and quantitative analysis of the images difficult, particularly for small structures of interest within areas of significant heterogeneity. One possibility to address this issue is to complement the MR images with histological data, which requires reconstructing 3D volumes from a series of 2D images. A number of methods have been proposed recently in the literature to address this issue, but deformation or tearing during the slicing process often produces reconstructed volumes with visible artifacts and imperfections. In this paper, we show that a possible solution to this problem is to work with several histological volumes, reconstruct each of these separately and then compute an average. The resulting histological atlas shows structures and substructures more clearly than any individual volume. We also propose an original approach to normalize intensity values across slices, a required preprocessing step when reconstructing histological volumes. We show that the histological atlas we have created can be used to localize structures and substructures, which cannot be seen easily in MR images. We also create an MR atlas that is associated with the histological atlas. We show that using the histological volumes to create the MR atlas is better than using the MR volumes only. Finally, we validate our approach quantitatively on MR image volumes by comparing volumetric measurements obtained manually and obtained automatically with our atlases.

The metabolites citrate, myo-inositol, and spermine are potential age-independent markers of prostate cancer in human expressed prostatic secretions

OBJECTIVES

Due to specific physiological functions, prostatic tissues and fluids have unique metabolic profiles. In this study, proton nuclear magnetic resonance spectroscopy ((1)H-NMRS) is used to assess potential metabolic markers of prostate cancer (PCa) in human expressed prostatic secretions (EPS).

METHODS

Metabolic profiles of EPS from 52 men with PCa and from 26 healthy controls were analyzed using quantitative (1)H-NMRS. The metabolites quantified included citrate, spermine, myo-inositol, lactate, alanine, phosphocholine, glutamine, acetate, and hydroxybutyrate. Logistic regression (LR) was used to model the risk of PCa based on metabolite concentrations while adjusting for age.

RESULTS

The average age of the EPS donors with PCa was 58.0+/-7.0 years and 52.2+/-12.1 for the healthy donors. The median Gleason score for the men with PCa was 7 (range 5-9). The LR models indicated that the absolute concentrations of citrate, myo-inositol, and spermine were highly predictive of PCa and inversely related to the risk of PCa. The areas under the receiver operating characteristic curves (AUROC) for citrate, myo-inositol and spermine were 0.89, 0.87, and 0.79, respectively. At 90% sensitivity, these metabolites had specificities of 74%, 51%, and 34%, respectively. The LR analysis indicated that absolute levels of these three metabolites were independent of age.

CONCLUSIONS

The results indicate that citrate, myo-inositol and spermine are potentially important markers of PCa in human EPS. Further, the absolute concentrations of these metabolites in EPS appear to be independent of age, increasing the potential utility of these markers due to elimination of age as a confounding variable.

The effects of tumor-derived platelet-derived growth factor on vascular morphology and function in vivo revealed by susceptibility MRI

Platelet-derived growth factors (PDGF) play a major role in pericyte recruitment in tumor capillaries. Pericytes are required for proper vessel development, and contribute to tumor angiogenesis by promoting stabilization and maturation of newly formed vessels. To investigate the effects of pericyte coverage on tumor vessel morphology and function in vivo, tumors derived from B16 melanoma cells transfected with either control plasmid (B16/ctr) or plasmid encoding full-length PDGF-BB (B16/PDGF), the latter previously shown to have enhanced blood vessel pericyte coverage and an increased tumor growth rate, were assessed using histopathological methods, Hoechst 33342-based perfusion analyses, and two noninvasive susceptibility magnetic resonance imaging (MRI) methods. Susceptibility-contrast MRI, incorporating the use of ultrasmall superparamagnetic iron oxide particles, revealed a significant (p < 0.05) reduction in vessel size index (R(v)) of B16/PDGF tumors, and which was validated histologically by the presence of significantly smaller (p < 0.001), more punctate blood vessels identified by fluorescence microscopy of the perfusion marker Hoechst 33342. Intrinsic-susceptibility MRI was used to measure the transverse MRI relaxation rate R(2)*, sensitive to changes in endogenous paramagnetic [deoxyhaemoglobin], and used to probe for vascular maturation and function. Hypercapnia (5% CO(2)/95% air) induced a negligible Delta R(2)* response in the B16/ctr and B16/PDGF tumors. In contrast, hyperoxia (5% CO(2)/95% O(2)) induced a significantly greater R(2)* reduction in the B16/PDGF tumors (p < 0.02). Together the susceptibility MRI-derived biomarkers reveal novel pericyte-dependent changes in the morphology and function of the perfused tumor vasculature in vivo.

A comparative evaluation of EPR and OxyLite oximetry using a random sampling of pO(2) in a murine tumor

Methods currently available for the measurement of oxygen concentrations (oximetry) in viable tissues differ widely from each other in their methodological basis and applicability. The goal of this study was to compare two novel methods, particulate-based electron paramagnetic resonance (EPR) and OxyLite oximetry, in an experimental tumor model. EPR oximetry uses implantable paramagnetic particulates, whereas OxyLite uses fluorescent probes affixed on a fiber-optic cable. C3H mice were transplanted with radiation-induced fibrosarcoma (RIF-1) tumors in their hind limbs. Lithium phthalocyanine (LiPc) microcrystals were used as EPR probes. The pO(2) measurements were taken from random locations at a depth of approximately 3 mm within the tumor either immediately or 48 h after implantation of LiPc. Both methods revealed significant hypoxia in the tumor. However, there were striking differences between the EPR and OxyLite readings. The differences were attributed to the volume of tissue under examination and the effect of needle invasion at the site of measurement. This study recognizes the unique benefits of EPR oximetry in terms of robustness, repeatability and minimal invasiveness.

MRI-measured water mobility increases in response to chemotherapy via multiple cell-death mechanisms

Numerous pre-clinical and clinical reports have demonstrated that the MRI-measured apparent diffusion coefficient of water (ADC) increases early in the response to a wide variety of anti-cancer therapies. It has been proposed that this increase in ADC generally results from an increase in the tumor extracellular volume fraction leading to a greater degree of unrestricted water motion. Furthermore, an increase in extracellular volume has been ascribed to the cell shrinkage that occurs early in the process of programmed cell death. However, other modes of death can be initiated soon after beginning therapy. These other modes of death include mitotic catastrophe and necrosis, and may also involve changes in the fraction of water with unrestricted motion. This work examines whether MRI-measured ADC is altered in response to therapies that induce cell death via non-apoptotic mechanisms and correlates ADC changes with cell death modalities regionally within the tumor. Apoptotic responses were limited to the tumor periphery in apoptosis-proficient tumors. Apoptosis was not observed in deficient tumors. Mitotic catastrophe was observed after treatment at the periphery and deeper into the tumor. Necrosis was the predominant response in the center of the tumor. ADC changes were moderate in the periphery and larger in the center. The results indicate that early and significant changes in ADC can occur in concert with mitotic catastrophe and lytic necrosis in the absence of apoptosis. Hence, changes in ADC may be a generalized measure of cytotoxic response to chemotherapy.

Measuring oxygenation in vivo with MRS/MRI--from gas exchange to the cell

Oxygen plays a major role as a substrate in metabolic processes in numerous signaling pathways, in redox metabolism, and in free radical metabolism. To study the role of oxygen in normal and pathophysiological states, methods that can be used noninvasively are required. This review examines the potential of nuclear magnetic resonance techniques to study tissue oxygenation. It is written from a systems perspective, looking at detection methods with respect to the path that oxygen takes in the mammalian system-from the lungs, through the vascular system, into the interstitial space, and finally into the cell. Methods discussed range from those that are quantifiable, such as the assessment of spin lattice relaxation time in fluorocarbon solutions, to those that are more correlative, such as assessment of lactate and high energy phosphates. Since the methods vary in their site of application, sensitivity, and specificity to the quantification of oxygen, this review provides examples of how each method has been applied. This may facilitate the reader's understanding of how to optimally apply different methods to study specific biomedical problems.

MR Imaging: Brief Overview and Emerging Applications

Magnetic resonance (MR) imaging has become established as a diagnostic and research tool in many areas of medicine because of its ability to provide excellent soft-tissue delineation in different areas of interest. In addition to T1- and T2-weighted imaging, many specialized MR techniques have been designed to extract metabolic or biophysical information. Diffusion-weighted imaging gives insight into the movement of water molecules in tissue, and diffusion-tensor imaging can reveal fiber orientation in the white matter tracts. Metabolic information about the object of interest can be obtained with spectroscopy of protons, in addition to imaging of other nuclei, such as sodium. Dynamic contrast material-enhanced imaging and recently proton spectroscopy play an important role in oncologic imaging. When these techniques are combined, they can assist the physician in making a diagnosis or monitoring a treatment regimen. One of the major advantages of the different types of MR imaging is the ability of the operator to manipulate image contrast with a variety of selectable parameters that affect the kind and quality of the information provided. The elements used to obtain MR images and the factors that affect formation of an MR image include MR instrumentation, localization of the MR signal, gradients, k-space, and pulse sequences.

Early changes in apparent diffusion coefficient predict the quantitative antitumoral activity of capecitabine, oxaliplatin, and irradiation in HT29 xenografts in athymic nude mice

PURPOSE

The purpose of this study was to evaluate the possible use of changes in apparent diffusion coefficient (ADC) measured by magnetic resonance imaging for pretreatment prediction and early detection of tumor response in a mouse model during fractionated chemoradiotherapy.

MATERIALS AND METHODS

Athymic mice with bilateral HT29 xenografts on rear flanks were allocated into three groups: control, capecitabine, and capecitabine and oxaliplatin. The left flanks of the mice received daily irradiation. T2 and diffusion images were acquired before therapy and weekly for the following 9 weeks. Pretreatment and changes in ADC were calculated and compared with tumor doubling growth delay.

RESULTS

No correlations between pretreatment ADC and changes in tumor volumes after therapy were seen. All treated tumors, except those receiving capecitabine (P = .06), showed increased mean tumor ADC values 11 days after initialization of therapy (P < .05) before returning to pretreatment values within 5 days posttherapy (day 18 after onset of therapy). This increase in mean tumor ADC showed a strong positive correlation (r = 0.92, P < .01) with mean tumor doubling growth delay.

CONCLUSIONS

Pretreatment ADC values did not predict the effectiveness of therapy, whereas early changes in mean ADC quantitatively correlated with treatment outcome.

An objective method for combining multi-parametric MRI datasets to characterize malignant tumors

Medical imaging has made significant contributions to the characterization of malignant tumors. In many cases, however, maps from multiple modalities may be required for more complete tumor mapping. In this manuscript we propose an objective method for combining multiple imaging datasets with the goal of characterizing malignant tumors. We refer to the proposed technique as the percent overlap method (POM). To demonstrate the power and flexibility of the POM analysis, we present four patients with recurrent glioblastoma multiforme. Each patient had multiple magnetic resonance imaging procedures resulting in seven different parameter maps. Chemical shift imaging was used to provide three metabolite ratio maps (Cho:NAA, Cho:Cre, Lac:Cre). A perfusion scan provided regional cerebral blood volume and permeability maps. Diffusion and carbogen-based hypoxia mapping data were also acquired. Composite maps were formed for each patient using POM, then were compared to results from the ISODATA clustering technique. The POM maps of likely recurrent tumor regions were found to be consistent with the ISODATA clustering method. This manuscript presents an objective method for combining parameters from multiple physiologic imaging techniques into a single composite map. The accuracy of the map depends strongly on the sensitivity of the chosen imaging parameters to the disease process at the time of image acquisition. Further validation of this method may be achieved by correlation with histological data.

Dynamic Contrast Enhanced Magnetic Resonance Imaging in Oncology: Theory, Data Acquisition, Analysis, and Examples

Dynamic contrast enhanced MRI (DCE-MRI) enables the quantitative assessment of tumor status and has found application in both pre-clinical tumor models as well as clinical oncology. DCE-MRI requires the serial acquisition of images before and after the injection of a paramagnetic contrast agent so that the variation of MR signal intensity with time can be recorded for each image voxel. As the agent enters into a tissue, it changes the MR signal intensity from the tissue to a degree that depends on the local concentration. After the agent is transported out of the tissue, the MR signal intensity returns to its' baseline value. By analyzing the associated signal intensity time course using an appropriate mathematical model, physiological parameters related to blood flow, vessel permeability, and tissue volume fractions can be extracted for each voxel or region of interest.In this review we first discuss the basic physics of this methodology, and then present technical aspects of how DCE-MRI data are acquired and analyzed. We also discuss appropriate models of contrast agent kinetics and how these can be used to elucidate tissue characteristics of importance in cancer biology. We conclude by briefly summarizing some future goals and demands of DCE-MRI.

Tumour metabolomics in animal models of human cancer

Multinuclear Nuclear Magnetic Resonance Spectroscopy (MRS) and mass spectrometry (MS) are the key analytical techniques used in an increasing manner to explore tumor metabolite profiles. Recent work has revealed that metabolite profiles in various tumor preparations (i.e., cultured cells, tissue specimens, and tumors in vivo) show strong correlations with tumor type, proliferation, metabolic activity, and cell death. These data are regarded as highly promising for tumor diagnosis as well as assessment of prognosis and treatment response in a clinical setting. In this pursuit, animal models of human cancer have played a central role. In this short account, we review the potentials of MRS and MS techniques for animal tumor metabolomic work, as well as highlight some interesting applications of these techniques for various animal tumor types.

Tumor-specific antivascular effect of TZT-1027 (Soblidotin) elucidated by magnetic resonance imaging and confocal laser scanning microscopy

TZT-1027 (soblidotin), an antimicrotubule agent, has previously been evaluated in terms of its antivascular effects. In this study, Evans blue perfusion, magnetic resonance imaging (MRI), and confocal laser scanning microscopy (CLSM) were utilized to further elucidate the antivascular effect of TZT-1027 in female nude mice and rats bearing human breast tumor MX-1, as well as in female Sprague-Dawley rats that developed breast tumors induced by dimethylbenz(a)anthracene (DMBA). Therapeutic doses of TZT-1027 caused nearly complete regression of implanted MX-1 tumors in nude mice and rats as well as DMBA-induced tumors in rats. The perfusion in MX-1 tumor implanted in nude mice was drastically reduced within 30 min after TZT-1027 administration and was completely inhibited after 6 h or more, although not reduced in normal tissue of kidney. The study using MRI demonstrated that rich blood flow within tumors was remarkably reduced 1-3 h after TZT-1027 administration both in nude rats bearing MX-1 tumors and in rats with DMBA-induced tumors. Furthermore, the study with CLSM in nude mice bearing MX-1 tumors revealed a disruption of tumor microvessels at 1 h and a destruction of tumor microvessel network at 3 h after TZT-1027 administration. In contrast, these types of vascular disorders were not observed in heart and kidney. These results suggest that TZT-1027 specifically damages tumor vasculatures, leading to extensive tumor necrosis within tolerable dose range, and confirms earlier observations that TZT-1027 exerts a considerable antivascular effect in addition to an excellent cytotoxic effect.

Promise and Progress for Functional and Molecular Imaging of Response to Targeted Therapies

Biomarkers to predict or monitor therapy response are becoming essential components of drug developer's armamentaria. Molecular and functional imaging has particular promise as a biomarker for anticancer therapies because it is non-invasive, can be used longitudinally and provides information on the whole patient or tumor. Despite this promise, molecular or functional imaging endpoints are not routinely incorporated into clinical trial design. As the costs of clinical trials and drug development become prohibitively more expensive, the need for improved biomarkers has become imperative and thus, the relatively high cost of imaging is justified. Imaging endpoints, such as Diffusion-Weighted MRI, DCE-MRI and FDG-PET have the potential to make drug development more efficient at all phases, from discovery screening with in vivo pharmacodynamics in animal models through the phase III enrichment of the patient population for potential responders. This review focuses on the progress of imaging responses to new classes of anti-cancer therapies targeted against PI3 kinase/AKT, HIF-1alpha and VEGF. The ultimate promise of molecular and functional imaging is to theragnostically predict response prior to commencement of targeted therapy.

Therapeutic targets and biomarkers identified in cancer choline phospholipid metabolism

Choline phospholipid metabolism is altered in a wide variety of cancers. The choline metabolite profile of tumors and cancer cells is characterized by an elevation of phosphocholine and total choline-containing compounds. Noninvasive magnetic resonance spectroscopy can be used to detect this elevation as an endogenous biomarker of cancer, or as a predictive biomarker for monitoring tumor response to novel targeted therapies. The enzymes directly causing this elevation, such as choline kinase, phospholipase C and phospholipase D may provide molecular targets for anticancer therapies. Signal transduction pathways that are activated in cancers, such as those mediated by the receptor tyrosine kinases breakpoint cluster region-abelson (Bcr-Abl), c-KIT or epidermal growth factor receptor (EGFR), correlate with the alterations in choline phospholipid metabolism of cancers, and also offer molecular targets for specific anticancer therapies. This review summarizes recently discovered molecular targets in choline phospholipid metabolism and signal transduction pathways, which may lead to novel anticancer therapies potentially being monitored by magnetic resonance spectroscopy techniques.

Assessing responses to cancer therapy using molecular imaging

Tumor responses to therapy in the clinic are still evaluated primarily from non-invasive imaging measurements of reductions in tumor size. This approach, however, lacks sensitivity and can only give a delayed indication of a positive response to treatment. Major advances in our understanding of the molecular mechanisms responsible for cancer, combined with new targeted clinical imaging technologies designed to detect the molecular correlates of disease progression and response to treatment, are set to revolutionize our approach to the detection and treatment of the disease. We describe here the imaging technologies available to image tumor cell proliferation and migration, metabolism, receptor and gene expression, apoptosis and tumor angiogenesis and vascular function, and show how measurements of these parameters can be used to give early indications of positive responses to treatment or to detect drug resistance and/or disease recurrence. Special emphasis has been placed on those applications that are already used in the clinic and those that are likely to translate into clinical application in the near future or whose use in preclinical studies is likely to facilitate translation of new treatments into the clinic.

Evaluation of antiangiogenic treatment effects on tumors' microcirculation by Bayesian physiological pharmacokinetic modeling and magnetic resonance imaging

A physiological pharmacokinetic (PBPK) model was used to estimate tumor microcirculation in nude mice with a grafted tumor. The kinetics of a rapid clearance blood pool agent, Vistarem, were investigated by dynamic MRI after bolus administration. Signal enhancements were recorded in arterial blood and in tumor tissue. To analyze these data, we developed a whole-body mathematical model of the agent's biodistribution using physiological parameters. The model included six compartments: arterial and venous plasma, tumor (split into capillaries and interstitium), and the rest of the body (also split into capillaries and interstitium). As an application, changes in tumor microcirculation parameters were evaluated in mice receiving either an antiangiogenic treatment (ZD4190) or a placebo. The analysis was performed in a Bayesian framework, and the model was fitted to experimental data using Markov Chain Monte Carlo techniques. Results showed a significant difference in tumor microcirculation between the two groups of mice when the microcirculation parameters are considered together. This whole-body physiological model enables to analyze jointly data in tumor tissue and in arterial blood. This leads to accurate estimates of microcirculation parameters and the evaluation of their uncertainty.

Measurement of kinetic parameters in skeletal muscle by magnetic resonance imaging with an intravascular agent

The purpose of this work was to investigate the use of an intravascular contrast agent to determine perfusion kinetics in skeletal muscle. A two-compartment kinetic model was used to represent the flux of contrast agent between the intravascular space and extravascular extracellular space (EES). The relationship between the image signal-to-noise ratio (SNR) and errors in estimating permeability surface area product (Ktrans), interstitial volume (ve), and plasma volume (vp) for linear and nonlinear curve-fitting methods was estimated from Monte Carlo simulations. Similar results were obtained for both methods. For an image SNR of 60, the estimated errors in these parameters were 10%, 22%, and 17%, respectively. In vivo experiments were conducted in rabbits to examine physiological differences between these parameters in the soleus (SOL) and tibialis anterior (TA) muscles in the hind limb. Values for Ktrans were significantly higher in the SOL (3.2+/-0.9 vs. 2.0+/-0.5x10(-3) min-1), as were values for vp (3.4+/-0.8 vs. 2.1+/-0.7%). Differences in ve for the two muscles (8.7+/-2.2 vs. 8.5+/-1.6%) were not found to be significant. These results demonstrate that relevant physiological metrics can be calculated in skeletal muscle using MRI with an intravascular contrast agent.

Amide proton transfer imaging of human brain tumors at 3T

Amide proton transfer (APT) imaging is a technique in which the nuclear magnetization of water-exchangeable amide protons of endogenous mobile proteins and peptides in tissue is saturated, resulting in a signal intensity decrease of the free water. In this work, the first human APT data were acquired from 10 patients with brain tumors on a 3T whole-body clinical scanner and compared with T1- (T1w) and T2-weighted (T2w), fluid-attenuated inversion recovery (FLAIR), and diffusion images (fractional anisotropy (FA) and apparent diffusion coefficient (ADC)). The APT-weighted images provided good contrast between tumor and edema. The effect of APT was enhanced by an approximate 4% change in the water signal intensity in tumor regions compared to edema and normal-appearing white matter (NAWM). These preliminary data from patients with brain tumors show that the APT is a unique contrast that can provide complementary information to standard clinical MRI measures.

Practices and Pitfalls of Mouse Cancer Models in Drug Discovery

Mouse models of cancer are critical tools for elucidating mechanisms of cancer development, as well as for assessment of putative cancer therapies. However, there are ongoing concerns about the value of mouse cancer models for predicting therapeutic efficacy in humans. This chapter reviews the most commonly used transplanted tumor models, including subcutaneous and orthotopic tumors in mice. It also reviews commonly utilized in vivo study endpoints. Even small improvements in predictive value achieved through careful selection of models and endpoints have the potential to have large impacts on productivity and overall drug development costs.

Detection of resistance to imatinib by metabolic profiling: clinical and drug development implications

Acquired resistance to imatinib mesylate is an increasing and continued challenge in the treatment of BCR-ABL tyrosine kinase positive leukemias as well as gastrointestinal stromal tumors. Stable isotope-based dynamic metabolic profiling (SIDMAP) studies conducted in parallel with the development and clinical testing of imatinib revealed that this targeted drug is most effective in controlling glucose transport, direct glucose oxidation for RNA ribose synthesis in the pentose cycle, as well as de novo long-chain fatty acid synthesis. Thus imatinib deprives transformed cells of the key substrate of macromolecule synthesis, malignant cell proliferation, and growth. Tracer-based magnetic resonance spectroscopy studies revealed a restitution of mitochondrial glucose metabolism and an increased energy state by reversing the Warburg effect, consistent with a subsequent decrease in anaerobic glycolysis. Recent in vitro SIDMAP studies that involved myeloid cells isolated from patients who developed resistance against imatinib indicated that non-oxidative ribose synthesis from glucose and decreased mitochondrial glucose oxidation are reliable metabolic signatures of drug resistance and disease progression. There is also evidence that imatinib-resistant cells utilize alternate substrates for macromolecule synthesis to overcome limited glucose transport controlled by imatinib. The main clinical implications involve early detection of imatinib resistance and the identification of new metabolic enzyme targets with the potential of overcoming drug resistance downstream of the various genetic and BCR-ABL-expression derived mechanisms. Metabolic profiling is an essential tool used to predict, clinically detect, and treat targeted drug resistance. This need arises from the fact that targeted drugs are narrowly conceived against genes and proteins but the metabolic network is inherently complex and flexible to activate alternative macromolecule synthesis pathways that targeted drugs fail to control.

In vivo magnetic resonance spectroscopy in cancer

Magnetic resonance spectroscopy (MRS) has been used for more than two decades to interrogate metabolite distributions in living cells and tissues. Techniques have been developed that allow multiple spectra to be obtained simultaneously with individual volume elements as small as 1 uL of tissue (i.e., 1 x 1 x 1 mm(3)). The most common modern applications of in vivo MRS use endogenous signals from (1)H, (31)P, or (23)Na. Important contributions have also been made using exogenous compounds containing (19)F, (13)C, or (17)O. MRS has been used to investigate cardiac and skeletal muscle energetics, neurobiology, and cancer. This review focuses on the latter applications, with specific reference to the measurement of tissue choline, which has proven to be a tumor biomarker that is significantly affected by anticancer therapies.

Continuous low-dose (metronomic) chemotherapy on rat prostate tumors evaluated using MRI in vivo and comparison with histology

Continuous low-dose (metronomic) therapy, based on cyclophosphamide (CTX) combined with thalidomide (Tha), was evaluated on Dunning prostate R3327-AT1 rat tumors. Significantly delayed tumor growth (P < .001) was observed with oral CTX alone at a low dose (metronomic cyclophosphamide or M-CTX; 30 mg/kg per day) or combined with Tha. To investigate dynamic changes in tumor physiology during early stages of treatment, magnetic resonance imaging (MRI) was applied before and during the M-CTX or M-CTX + Tha therapy. Dynamic contrast-enhanced MRI revealed significant changes in the tumor center by day 3 (P < .01); by day 7, only a thin peripheral tumor region showed high signal enhancement. There was a significant correlation between poorly enhancing fraction on day 7 and ultimate tumor growth delay (P < .02). The apparent transverse relaxation rate (R2*) showed similar baseline tumor heterogeneity, but no obvious changes with growth or therapy. Histology confirmed substantial necrosis in the tumor center, leaving a thin live peripheral rim. Immunohistochemistry showed a significant increase in vascular endothelial growth factor, and apoptotic tumor and vascular endothelial cells. These results show the efficacy of the metronomic CTX +/- Tha for delaying tumor growth and indicate that MRI provides insights into the mode of action and early indication of efficacy.