Multimodality imaging for vertebral metastases in a rat osteolytic model

Metastatic human breast cancer to the spine produces mechanical hyperalgesia and gait deficits in rodents

BACKGROUND CONTEXT

Metastases to the spine are a common source of severe pain in cancer patients. The secondary effects of spinal metastases include pain, bone fractures, hypercalcemia, and neurological deficits. As the disease progresses, pain severity can increase until it becomes refractory to medical treatments and leads to a decreased quality of life for patients. A key obstacle in the study of pain-induced spinal cancer is the lack of reliable and reproducible spine cancer animal models. In the present study, we developed a reproducible and reliable rat model of spinal cancer using human-derived tumor tissue to evaluate neurological decline using imaging and behavioral techniques.

PURPOSE

The present study outlines the development and characterization of an orthotopic model of human breast cancer to the spine in immunocompromised rats.

STUDY DESIGN/SETTING

This is a basic science study.

METHODS

Female immunocompromised rats were randomized into three groups: tumor (n=8), RBC3 mammary adenocarcinoma tissue engrafted in the L5 vertebra body; sham (n=6), surgery performed but not tumor engrafted; and control (n=6), naive rats, no surgery performed. To evaluate the neurological impairment due to tumor invasion, functional assessment was done in all rodents at day 40 after tumor engraftment using locomotion gait analysis and pain response to a mechanical stimulus (Randall-Selitto test). Bioluminescence (BLI) was used to evaluate tumor growth in vivo and cone beam computed tomography (CBCT) was performed to evaluate bone changes due to tumor invasion. The animals were euthanized at day 45 and their spines were harvested and processed for hematoxylin and eosin (H&E) staining.

RESULTS

Tumor growth in the spine was confirmed by BLI imaging and corroborated by histological analysis. Cone beam computed tomography images were characterized by a decrease in the bone intensity in the lumbar spine consistent with tumor location on BLI. On H&E staining of tumor-engrafted animals, there was a near-complete ablation of the ventral and posterior elements of the L5 vertebra with severe tumor invasion in the bony components displacing the spinal cord. Locomotion gait analysis of tumor-engrafted rats showed a disruption in the normal gait pattern with asignificant reduction in length (p=.02), duration (p=.002), and velocity (p=.002) of right leg strides and only in duration (p=.0006) and velocity (p=.001) of left leg strides, as compared with control and sham rats. Tumor-engrafted animals were hypersensitive to pain stimulus shown as a significantly reduced response in time (p=.02) and pressure (p=.01) applied when compared with control groups.

CONCLUSIONS

We developed a system for the quantitative analysis of pain and locomotion in an animal model of metastatic human breast cancer of the spine. Tumor-engrafted animals showed locomotor and sensory deficits that are in accordance with clinical manifestation in patients with spine metastasis. Pain response and locomotion gait analysis were performed during follow-up. The Randall-Selitto test was a sensitive method to evaluate pain in the rat's spine. We present a model for the study of bone-associated cancer pain secondary to cancer metastasis to the spine, as well as for the study of new therapies and treatments to lessen pain from metastatic cancer to the neuroaxis.

μFE models can represent microdamaged regions of healthy and metastatically involved whole vertebrae identified through histology and contrast enhanced μCT imaging

Micro-damage formation within the skeleton is an important stimulant for bone remodeling, however abnormal build-up of micro-damage can lead to skeletal fragility. In this study, µCT imaging based micro finite element (μFE) models were used to evaluate tissue level damage criteria in whole healthy and metastatically-involved vertebrae. T13-L2 spinal segments were excised from osteolytic (n=3) and healthy (n=3) female athymic rnu/rnu rats. Osteolytic metastasis was generated by intercardiac injection of HeLa cancer cells. Micro-mechanical axial loading was applied to the spinal motion segments under μCT imaging. Vertebral samples underwent BaSO4 staining and sequential calcein/fuchsin staining to identify load induced micro-damage. μCT imaging was used generate specimen specific μFE models of the healthy and osteolytic whole rat vertebrae. Model boundary conditions were generated through deformable image registration of loaded and unloaded scans. Elevated stresses and strains were detected in regions of micro-damage identified through histological and BaSO4 staining within healthy and osteolytic vertebral models, as compared to undamaged regions. Additionally, damaged regions of metastatic vertebrae experienced significantly higher local stresses and strains than those in the damaged regions of healthy specimens. Areas identified by BaSO4 staining, however, yielded lower levels of stress and strain in damaged and undamaged regions of healthy and metastatic vertebrae as compared to fuschin staining. The multimodal (experimental, image-based and computational) techniques used in this study demonstrated the ability of local stresses and strains computed through µFE analysis to identify trabecular micro-damage, that can be applied to biomechanical analyses of healthy and diseased whole bones.

A rat model of metastatic spinal cord compression using human prostate adenocarcinoma: histopathological and functional analysis

BACKGROUND CONTEXT

Cancer is a major global public health problem responsible for one in every four deaths in the United States. Prostate cancer alone accounts for 29% of all cancers in men and is the sixth leading cause of death in men. It is estimated that up to 30% of patients with cancer will develop metastatic disease, the spine being one of the most frequently affected sites in patients with prostate cancer.

PURPOSE

To study this condition in a preclinical setting, we have created a novel animal model of human metastatic prostate cancer to the spine and have characterized it histologically, functionally, and via bioluminescence imaging.

STUDY DESIGN

Translational science investigation of animal model of human prostate cancer in the spine.

METHODS

Luciferase-positive human prostate tumor cells PC3 (PC3-Luc) were injected in the flank of athymic male rats. PC3-Luc tumor samples were then implanted into the L5 vertebral body of male athymic rats (5 weeks old). Thirty-two rats were randomized into three surgical groups: experimental, control, and sham. Tumor growth was assessed qualitatively and noninvasively via bioluminescence emission, upon luciferin injection. To determine the functional impact of tumor growth in the spine, rats were evaluated for gait abnormalities during gait locomotion using video-assisted gait analysis. Rats were euthanized 22 days after tumor implantation, and spines were subjected to histopathological analyses.

RESULTS

Twenty days after tumor implantation, the tumor-implanted rats showed distinct signs of gait disturbances: dragging tail, right- or left-hind limb uncoordination, and absence of toe clearance during forward limb movement. At 20 days, all rats experienced tumor growth, evidenced by bioluminescent signal. Locomotion parameters negatively affected in tumor-implanted rats included stride length, velocity, and duration. At necropsy, all spines showed evidence of tumor growth, and the histological analysis found spinal cord compression and peritumoral osteoblastic reaction characteristic of bony prostate tumors. None of the rats in the sham or control groups demonstrated any evidence of bioluminescence signal or signs of gait disturbances.

CONCLUSIONS

In this project, we have developed a novel animal model of metastatic spine cancer using human prostate cancer cells. Tumor growth, evaluated via bioluminescence and corroborated by histopathological analyses, affected hind limb locomotion in ways that mimic motor deficits present in humans afflicted with metastatic spine disease. Our model represents a reliable method to evaluate the experimental therapeutic approaches of human tumors of the spine in animals. Gait locomotion and bioluminescence analyses can be used as surrogate noninvasive methods to evaluate tumor growth in this model.

An in vivo mouse model of intraosseous spinal cancer causing evolving paraplegia

The spine is the commonest site of skeletal metastatic disease and uncontrolled growth of cancer in the spine will inevitably cause pain and neurologic compromise. Improved understanding of the pathobiology behind this devastating condition is urgently needed. For this reason, the aim of this study was to establish a clinically relevant, animal model of spinal cancer. A percutaneous orthotopic injection of human breast (MDA-MB-231) or human prostate (PC-3) cancer cells was administered into the upper lumbar spine of nude mice (n = 6). Animals were monitored twice daily for general welfare, gait asymmetry or disturbance, and hindlimb weakness. After sacrifice, plain radiographs, micro-CT imaging and histological analysis of the spines were performed on each mouse. All mice recovered fully from the inoculation procedure and displayed normal gait and behaviour patterns for at least 3 weeks post-inoculation. Subsequently, between 3 and 5 weeks post-inoculation, each mouse developed evolving paralysis in their hindlimbs over 48-72 h. All followed the same pattern of decline following onset of neurological dysfunction; from gait asymmetry and unilateral hindlimb weakness, to complete unilateral hindlimb paralysis and finally to complete bilateral hindlimb paralysis. Plain radiographs, micro-CT scanning and histological analysis confirmed local tumour growth and destruction of the spine in all six mice. An in vivo mouse model of human intraosseous spinal cancer has been established forming cancers that grow within the spine and cause epidural spinal cord compression, resulting in a reproducible, evolving neurological deficit and paralysis that closely resembles the human condition.

A novel animal model of human breast cancer metastasis to the spine: a pilot study using intracardiac injection and luciferase-expressing cells

OBJECT

Metastatic spine disease is prevalent in cancer victims; 10%-30% of the 1.2 million new patients diagnosed with cancer in the US exhibit spinal metastases. Unfortunately, treatments are limited for these patients, as disseminated disease is often refractory to chemotherapy and is difficult to treat with surgical intervention alone. New animal models that accurately recapitulate the human disease process are needed to study the behavior of metastases in real time.

METHODS

In this study the authors report on a cell line that reliably generates bony metastases following intracardiac injection and can be tracked in real time using optical bioluminescence imaging. This line, RBC3, was derived from a metastatic breast adenocarcinoma lesion arising in the osseous spine of a rat following intracardiac injection of MDA-231 human breast cancer cells.

RESULTS

Upon culture and reinjection of RBC3, a statistically significantly increased systemic burden of metastatic tumor was noted. The resultant spine lesions were osteolytic, as demonstrated by small animal CT scanning.

CONCLUSIONS

This cell line generates spinal metastases that can be tracked in real time and may serve as a useful tool in the study of metastatic disease in the spine.

Chapter five--The development of transcription-regulated adenoviral vectors with high cancer-selective imaging capabilities

A clear benefit of molecular imaging is to enable noninvasive, repetitive monitoring of intrinsic signals within tumor cells as a means to identify the lesions as malignant or to assess the ability of treatment to perturb key pathways within the tumor cells. Due to the promising utility of molecular imaging in oncology, preclinical research to refine molecular imaging techniques in small animals is a blossoming field. We will first discuss the several imaging modalities such as fluorescent imaging, bioluminescence imaging, and positron emission tomography that are now commonly used in small animal settings. The indirect imaging approach, which can be adapted to a wide range of imaging reporter genes, is a useful platform to develop molecular imaging. In particular, reporter gene-based imaging is well suited for transcriptional-targeted imaging that can be delivered by recombinant adenoviral vectors. In this review, we will summarize transcription-regulated strategies used in adenoviral-mediated molecular imaging to visualize metastasis and monitor oncolytic therapy in preclinical models.

In vivo animal models of spinal metastasis

The vertebral column is the commonest site for skeletal metastases, with breast, prostate and lung cancers being the most common primary sources. The spine has structural and neural-protective properties thus involvement by metastatic cancer often causes bony instability and fracture, intractable pain and neurological deficit. In vivo animal models which resemble the human condition are essential in order to improve understanding of the pathophysiology behind the spread of metastatic cancer to the spine and its subsequent local growth and invasion, to enable in-depth analysis of the interaction between host and tumour cells and the molecular processes behind local cancer invasion and barriers to invasion as well as to allow assessment of novel treatment modalities for spinal metastases. This review summarizes the current status of the animal models specifically used for the study of spinal metastasis, their relevance, advantages and limitations, and important considerations for the development of future in vivo animal models.

Can micro-imaging based analysis methods quantify structural integrity of rat vertebrae with and without metastatic involvement?

This study compares the ability of μCT image-based registration, 2D structural rigidity analyses and multimodal continuum-level finite element (FE) modeling in evaluating the mechanical stability of healthy, osteolytic, and mixed osteolytic/osteoblastic metastatically involved rat vertebrae. μMR and μCT images (loaded and unloaded) were acquired of lumbar spinal motion segments from 15rnu/rnu rats (five per group). Strains were calculated based on image registration of the loaded and unloaded μCT images and via analysis of FE models created from the μCT and μMR data. Predicted yield load was also calculated through 2D structural rigidity analysis of the axial unloaded μCT slices. Measures from the three techniques were compared to experimental yield loads. The ability of these methods to predict experimental yield loads were evaluated and image registration and FE calculated strains were directly compared. Quantitatively for all samples, only limited weak correlations were found between the image-based measures and experimental yield load. In comparison to the experimental yield load, we observed a trend toward a weak negative correlation with median strain calculated using the image-based strain measurement algorithm (r=-0.405, p=0.067), weak significant correlations (p<0.05) with FE based median and 10th percentile strain values (r=-0.454, -0.637, respectively), and a trend toward a weak significant correlation with FE based mean strain (r=-0.366, p=0.09). Individual group analyses, however, yielded more and stronger correlations with experimental results. Considering the image-based strain measurement algorithm we observed moderate significant correlations with experimental yield load (p<0.05) in the osteolytic group for mean and median strain values (r=-0.840, -0.832, respectively), and in the healthy group for median strain values (r=-0.809). Considering the rigidity-based predicted yield load, we observed a strong significant correlation with the experimental yield load in the mixed osteolytic/osteoblastic group (r=0.946) and trend toward a moderate correlation with the experimental yield load in the osteolytic group (r=0.788). Qualitatively, strain patterns in the vertebral bodies generated using image registration and FEA were well matched, yet quantitatively a significant correlation was found only between mean strains in the healthy group (r=0.934). Large structural differences in metastatic vertebrae and the complexity of motion segment loading may have led to varied modes of failure. Improvements in load characterization, material properties assignments and resolution are necessary to yield a more generalized ability for image-based registration, structural rigidity and FE methods to accurately represent stability in healthy and pathologic scenarios.

Multimodal μCT/μMR based semiautomated segmentation of rat vertebrae affected by mixed osteolytic/osteoblastic metastases

PURPOSE

Multimodal microimaging in preclinical models is used to examine the effect of spinal metastases on bony structure; however, the evaluation of tumor burden and its effect on microstructure has thus far been mainly qualitative or semiquantitative. Quantitative analysis of multimodality imaging is a time consuming task, motivating automated methods. As such, this study aimed to develop a low complexity semiautomated multimodal μCT/μMR based approach to segment rat vertebral structure affected by mixed osteolytic/osteoblastic destruction.

METHODS

Mixed vertebral metastases were developed via intracardiac injection of Ace-1 canine prostate cancer cells in three 4-week-old rnu/rnu rats. μCT imaging (for high resolution bone visualization), T1-weighted μMR imaging (for bone registration), and T2-weighted μMR imaging (for osteolytic tumor visualization) were conducted on one L1, three L2, and one L3 vertebrae (excised). One sample (L1-L3) was processed for undecalcified histology and stained with Goldner's trichome. The μCT and μMR images were registered using a 3D rigid registration algorithm with a mutual information metric. The vertebral microarchitecture was segmented from the μCT images using atlas-based demons deformable registration, levelset curvature evolution, and intensity-based thresholding techniques. The μCT based segmentation contours of the whole vertebrae were used to mask the T2-weighted μMR images, from which the osteolytic tumor tissue was segmented (intensity-based thresholding).

RESULTS

Accurate registration of μCT and μMRI modalities yielded precise segmentation of whole vertebrae, trabecular centrums, individual trabeculae, and osteolytic tumor tissue. While the algorithm identified the osteoblastic tumor attached to the vertebral pereosteal surfaces, it was limited in segmenting osteoblastic tissue located within the trabecular centrums.

CONCLUSIONS

This semiautomated segmentation method yielded accurate registration of μCT and μMRI modalities with application to the development of mathematical models analyzing the mechanical stability of metastatically involved vertebrae and in preclinical applications evaluating new and existing treatment effects on tumor burden and skeletal microstructure.

Mesh morphing and response surface analysis: quantifying sensitivity of vertebral mechanical behavior

Vertebrae provide essential biomechanical stability to the skeleton. In this work novel morphing techniques were used to parameterize three aspects of the geometry of a specimen-specific finite element (FE) model of a rat caudal vertebra (process size, neck size, and end-plate offset). Material properties and loading were also parameterized using standard techniques. These parameterizations were then integrated within an RSM framework and used to produce a family of FE models. The mechanical behavior of each model was characterized by predictions of stress and strain. A metamodel was fit to each of the responses to yield the relative influences of the factors and their interactions. The direction of loading, offset, and neck size had the largest influences on the levels of vertebral stress and strain. Material type was influential on the strains, but not the stress. Process size was substantially less influential. A strong interaction was identified between dorsal-ventral offset and dorsal-ventral off-axis loading. The demonstrated approach has several advantages for spinal biomechanical analysis by enabling the examination of the sensitivity of a specimen to multiple variations in shape, and of the interactions between shape, material properties, and loading.

Rat model of metastatic breast cancer monitored by MRI at 3 tesla and bioluminescence imaging with histological correlation

Background

Establishing a large rodent model of brain metastasis that can be monitored using clinically relevant magnetic resonance imaging (MRI) techniques is challenging. Non-invasive imaging of brain metastasis in mice usually requires high field strength MR units and long imaging acquisition times. Using the brain seeking MDA-MB-231BR transfected with luciferase gene, a metastatic breast cancer brain tumor model was investigated in the nude rat. Serial MRI and bioluminescence imaging (BLI) was performed and findings were correlated with histology. Results demonstrated the utility of multimodality imaging in identifying unexpected sights of metastasis and monitoring the progression of disease in the nude rat.

Methods

Brain seeking breast cancer cells MDA-MB-231BR transfected with firefly luciferase (231BRL) were labeled with ferumoxides-protamine sulfate (FEPro) and 1-3 × 10⁶ cells were intracardiac (IC) injected. MRI and BLI were performed up to 4 weeks to monitor the early breast cancer cell infiltration into the brain and formation of metastases. Rats were euthanized at different time points and the imaging findings were correlated with histological analysis to validate the presence of metastases in tissues.

Results

Early metastasis of the FEPro labeled 231BRL were demonstrated onT2*-weighted MRI and BLI within 1 week post IC injection of cells. Micro-metastatic tumors were detected in the brain on T2-weighted MRI as early as 2 weeks post-injection in greater than 85% of rats. Unexpected skeletal metastases from the 231BRL cells were demonstrated and validated by multimodal imaging. Brain metastases were clearly visible on T2 weighted MRI by 3-4 weeks post infusion of 231BRL cells, however BLI did not demonstrate photon flux activity originating from the brain in all animals due to scattering of the photons from tumors.

Conclusion

A model of metastatic breast cancer in the nude rat was successfully developed and evaluated using multimodal imaging including MRI and BLI providing the ability to study the temporal and spatial distribution of metastases in the brain and skeleton.

Treatment of canine osseous tumors with photodynamic therapy: a pilot study

Photodynamic therapy uses nonthermal coherent light delivered via fiber optic cable to locally activate a photosensitive chemotherapeutic agent that ablates tumor tissue. Owing to the limitations of light penetration, it is unknown whether photodynamic therapy can treat large osseous tumors. We determined whether photodynamic therapy can induce necrosis in large osseous tumors, and if so, to quantify the volume of treated tissue. In a pilot study we treated seven dogs with spontaneous osteosarcomas of the distal radius. Tumors were imaged with MRI before and 48 hours after treatment, and the volumes of hypointense regions were compared. The treated limbs were amputated immediately after imaging at 48 hours and sectioned corresponding to the MR axial images. We identified tumor necrosis histologically; the regions of necrosis corresponded anatomically to hypointense tissue on MRI. The mean volume of necrotic tissue seen on MRI after photodynamic therapy was 21,305 mm(3) compared with a pretreatment volume of 6108 mm(3). These pilot data suggest photodynamic therapy penetrates relatively large canine osseous tumors and may be a useful adjunct for treatment of bone tumors.

The effect of versican G3 domain on local breast cancer invasiveness and bony metastasis

Introduction

Increased versican expression has been associated with local breast cancer invasiveness and a more aggressive tumor phenotype. The cellular mechanisms are not fully understood and this study evaluated versican G3 domain with its EGF-like motifs in influencing tumor invasion and metastasis.

Methods

One recombinant construct was synthesized (a signal peptide for product secretion and the versican G3 domain). The construct was stably transfected into human breast carcinoma MT-1 cells. Cell viability in vitro was evaluated in low serum and serum starvation conditions. In vivo study of tumor growth was evaluated in a nude mouse model. G3 effects on rodent vascular endothelial cells were evaluated in vitro on cell survival, apoptosis, migration, and vascular formation. The effects of VEGF, fibronectin, and G3 on vascular formation were examined. An intracardiac injection model of metastatic human breast carcinoma tested the effect of G3 on distant bony and soft tissue metastasis. Analysis of metastatic burden included histology, radiographs, and micro-CT quantification of osteolysis.

Results

A greater viability of cancer cells was observed in low serum and serum-free conditions in the presence of versican G3. Larger subcutaneous tumors were obtained in the G3 group following tumor cell injection into CD1 mice. G3 induced a greater degree of rodent vascular endothelial cell proliferation and migration in vitro. Simultaneous presence of fibronectin, VEGF, and G3 promoted endothelial cell migration in wound-healing assays as compared to the treatments containing none, one or two of these molecules. Systemic tumor burden to distant bony and soft tissue metastatic sites was greater in the G3 group using the intracardiac injection metastatic model

Conclusion

Versican G3 domain appears to be important in local and systemic tumor invasiveness of human breast cancer. Effects include enhancing cell viability, proliferation, migration and enhancing local tumor growth. Potential effects on angiogenesis include enhancing vascular endothelial proliferation, migration, and vessel formation. The interactions between tumor cells, surrounding stromal components and neo-vascularization in breast cancer may include interactions with VEGF and fibronectin. The propensity of versican G3 to influence tumor invasion to bone and the mechanisms of G3 mediated osteolysis warrants ongoing studies.

In vivo fluorescence imaging of the reticuloendothelial system using quantum dots in combination with bioluminescent tumour monitoring

PURPOSE

We characterised in vivo fluorescence imaging (FLI) of the reticuloendothelial system using quantum dots (QD) and investigated its use in combination with in vivo bioluminescence imaging (BLI).

MATERIALS AND METHODS

In vivo FLI was performed in five mice repeatedly after the intravenous administration of QD without conjugation to targeting ligands. Ex vivo FLI of the excised organs was performed 24 h after QD injection in three mice. Seven days after intravenous inoculation of luciferase-expressing model cells of a haematological malignancy, mice were injected with the QD or saline (n = 5 each), and combined BLI/FLI was performed repeatedly. Additional five mice inoculated with the tumour cells were examined by in vivo BLI/FLI, and the structures harbouring bioluminescent foci were determined by ex vivo BLI. The utility of combining FLI with bioluminescent tumour monitoring was evaluated.

RESULTS

In vivo FLI after QD injection allowed long-term, repeated observation of the reticuloendothelial system in individual mice, although fluorescence intensity and image contrast gradually decreased over time. Ex vivo FLI verified selective accumulation in reticuloendothelial structures. The administration of QD did not affect whole-body bioluminescent signal intensities during longitudinal tumour monitoring. In vivo BLI/FLI, accompanied by fusion of both images, improved the accuracy and confidence level of the localisation of the bioluminescent foci.

CONCLUSIONS

In vivo FLI using QD provides an overview of the reticuloendothelial system in living mice. In combination with bioluminescent tumour monitoring, fluorescent reticuloendothelial imaging is expected to provide valuable information for lesion localisation.