Review of methods used to study oxygen transport at the microcirculatory level

Applications of Intravital Imaging in Cancer Immunotherapy

Currently, immunotherapy is one of the most effective treatment strategies for cancer. However, the efficacy of any specific anti-tumor immunotherapy can vary based on the dynamic characteristics of immune cells, such as their rate of migration and cell-to-cell interactions. Therefore, understanding the dynamics among cells involved in the immune response can inform the optimization and improvement of existing immunotherapy strategies. In vivo imaging technologies use optical microscopy techniques to visualize the movement and behavior of cells in vivo, including cells involved in the immune response, thereby showing great potential for application in the field of cancer immunotherapy. In this review, we briefly introduce the technical aspects required for in vivo imaging, such as fluorescent protein labeling, the construction of transgenic mice, and various window chamber models. Then, we discuss the elucidation of new phenomena and mechanisms relating to tumor immunotherapy that has been made possible by the application of in vivo imaging technology. Specifically, in vivo imaging has supported the characterization of the movement of T cells during immune checkpoint inhibitor therapy and the kinetic analysis of dendritic cell migration in tumor vaccine therapy. Finally, we provide a perspective on the challenges and future research directions for the use of in vivo imaging technology in cancer immunotherapy.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Nano versus Molecular: Optical Imaging Approaches to Detect and Monitor Tumor Hypoxia

Hypoxia is a ubiquitous feature of solid tumors, which plays a key role in tumor angiogenesis and resistance development. Conventional hypoxia detection methods lack continuous functional detection and are generally less suitable for dynamic hypoxia measurement. Optical sensors hereby provide a unique opportunity to noninvasively image hypoxia with high spatiotemporal resolution and enable real-time detection. Therefore, these approaches can provide a valuable tool for personalized treatment planning against this hallmark of aggressive cancers. Many small optical molecular probes can enable analyte triggered response and their photophysical properties can also be fine-tuned through structural modification. On the other hand, optical nanoprobes can acquire unique intrinsic optical properties through nanoconfinement as well as enable simultaneous multimodal imaging and drug delivery. Furthermore, nanoprobes provide biological advantages such as improving bioavailability and systemic delivery of the sensor to enhance bioavailability. This review provides a comprehensive overview of the physical, chemical, and biological analytes for cancer hypoxia detection and focuses on discussing the latest nano- and molecular developments in various optical imaging approaches (fluorescence, phosphorescence, and photoacoustic) in vivo. Finally, this review concludes with a perspective toward the potentials of these optical imaging approaches in hypoxia detection and the challenges with molecular and nanotechnology design strategies.

Monitoring with In Vivo Electrochemical Sensors: Navigating the Complexities of Blood and Tissue Reactivity

The disruptive action of an acute or critical illness is frequently manifest through rapid biochemical changes that may require continuous monitoring. Within these changes, resides trend information of predictive value, including responsiveness to therapy. In contrast to physical variables, biochemical parameters monitored on a continuous basis are a largely untapped resource because of the lack of clinically usable monitoring systems. This is despite the huge testing repertoire opening up in recent years in relation to discrete biochemical measurements. Electrochemical sensors offer one of the few routes to obtaining continuous readout and, moreover, as implantable devices information referable to specific tissue locations. This review focuses on new biological insights that have been secured through in vivo electrochemical sensors. In addition, the challenges of operating in a reactive, biological, sample matrix are highlighted. Specific attention is given to the choreographed host rejection response, as evidenced in blood and tissue, and how this limits both sensor life time and reliability of operation. Examples will be based around ion, O₂, glucose, and lactate sensors, because of the fundamental importance of this group to acute health care.

Combined endogenous MR biomarkers to assess changes in tumor oxygenation induced by an allosteric effector of hemoglobin

Hypoxia is a crucial factor in cancer therapy, determining prognosis and the effectiveness of treatment. Although efforts are being made to develop methods for assessing tumor hypoxia, no markers of hypoxia are currently used in routine clinical practice. Recently, we showed that the combined endogenous MR biomarkers, R₁ and R₂ *, which are sensitive to [dissolved O₂ ] and [dHb], respectively, were able to detect changes in tumor oxygenation induced by a hyperoxic breathing challenge. In this study, we further validated the ability of the combined MR biomarkers to assess the change in tumor oxygenation induced by an allosteric effector of hemoglobin, myo-inositol trispyrophosphate (ITPP), on rat tumor models. ITPP induced an increase in tumor pO₂ , as observed using L-band electron paramagnetic resonance oximetry, as well as an increase in both R₁ and R₂ * MR parameters. The increase in R₁ indicated an increase in [O₂ ], whereas the increase in R₂ * resulted from an increase in O₂ release from blood, inducing an increase in [dHb]. The impact of ITPP was then evaluated on factors that can influence tumor oxygenation, including tumor perfusion, saturation rate of hemoglobin, blood pH and oxygen consumption rate (OCR). ITPP decreased blood [HbO₂ ] and significantly increased blood acidity, which is also a factor that right-shifts the oxygen dissociation curve. No change in tumor perfusion was observed after ITPP treatment. Interestingly, ITPP decreased OCR in both tumor cell lines. In conclusion, ITPP increased tumor pO₂ via a combined mechanism involving a decrease in OCR and an allosteric effect on hemoglobin that was further enhanced by a decrease in blood pH. MR biomarkers could assess the change in tumor oxygenation induced by ITPP. At the intra-tumoral level, a majority of tumor voxels were responsive to ITPP treatment in both of the models studied.

Automated noninvasive evaluation of blood flow and oxygenation in rats integrated with systemic physiological monitoring

BACKGROUND

Many studies evaluating blood flow and oxygen partial pressure (PO2) do not directly measure both parameters, are confined to few locations/microvessels, and depend on investigator's selection of measuring sites. Moreover, clinically/physiologically relevant systemic parameters are not simultaneously recorded. We implemented an automated system for prolonged blood flow/PO2 acquisition in large areas while collecting relevant systemic information.

METHODS

In anesthetized animals, cardiorespiratory parameters were continuously recorded. Other data were collected at baseline and hourly after 4 hours of hemorrhagic shock. A cremaster muscle was spread over a pedestal fixed to a motorized stage. One 2-dimensional tissue scan allowed 16 noninvasive PO2 measurements using oxygen-dependent phosphorescence quenching and fiber optics. Blood flow was estimated using laser speckle contrast imaging in the same areas used for PO2 measurements. At each timepoint, blood was sampled for extensive biochemistry/coagulation profile.

RESULTS

The system was used successfully by different operators. A set of flow/PO2 measurements was completed in less than 90 seconds. Muscle flow and PO2 correlated with some but not several systemic parameters, emphasizing the importance of performing both local and systemic evaluations.

CONCLUSION

System advantages include integration between local and over 40 systemic parameters, unbiased data collection/analysis, improved performance/sampled area, easy expansion, implementation and maintenance, no customized programming, and simplified training. Combining this system with trauma/prolonged HS models will enhance our ability to investigate tissue stability and select better resuscitation strategies to improve outcomes and survival.

LEVEL OF EVIDENCE

Diagnostic test, level V.

Synchronous delivery of oxygen and photosensitizer for alleviation of hypoxia tumor microenvironment and dramatically enhanced photodynamic therapy

Photosensitizer, proper laser irradiation, and oxygen are essential components for effective photodynamic therapy (PDT) in clinical cancer therapy. However, native hypoxic tumoral microenvironment is a major barrier hindering photodynamic reactions in vivo. Thus, we have prepared biocompatible liposomes by loading complexes of oxygen-carrier (hemoglobin, Hb) and photosensitizer (indocyanine green, ICG) for enhanced PDT against hypoxic tumor. Ideal oxygen donor Hb, which is an oxygen-carried protein in red blood cells, makes such liposome which provide stable oxygen supply. ICG, as a photosensitizer, could transfer energy from lasers to oxygen to generate cytotoxic reactive oxygen species (ROS) for treatment. The liposomes loading ICG and Hb (LIH) exhibited efficient tumor homing upon intravenous injection. As revealed by T₂-weighted magnetic resonance imaging and immunohistochemical analysis, the intratumoral hypoxia was greatly alleviated, and the level of hypoxia inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) in tumor was obviously down-regulated. A weak PDT efficiency was found in cells incubated in simulated hypoxia condition in vitro, while PDT effect was dramatically enhanced in LIH treated hypoxia cells under near-infrared (NIR) laser, which was mainly attributed to massive generation of ROS with sufficient oxygen supply. ROS trigger oxidative damage of tumors and induce complete suppression of tumor growth and 100% survival rate of mice, which were also in good health condition. Our work highlights a liposome-based nanomedicine that could effectively deliver oxygen to tumor and alleviate tumor hypoxia state, inducing greatly improved efficacy compared to conventional cancer PDT and demonstrates the promise of modulating unfavorable tumor microenvironment with nanotechnology to overcome limitations of cancer therapies.

Combined endogenous MR biomarkers to predict basal tumor oxygenation and response to hyperoxic challenge

Hypoxia is a common feature of solid tumors, which translates into increased angiogenesis, malignant phenotype cell selection, change in gene expression and greater resistance to radiotherapy and chemotherapy. Therefore, there is a need for markers of hypoxia to stratify patients, in order to personalize treatment to improve therapeutic outcome. However, no modality has yet been validated for the screening of hypoxia in routine clinical practice. Magnetic resonance imaging (MRI) R₁ and R₂ * relaxation parameters are sensitive to tissue oxygenation: R₁ is sensitive to dissolved oxygen and R₂ * is sensitive to intravascular deoxyhemoglobin content. Two rat tumor models with distinct levels of hypoxia, 9L-glioma and rhabdomyosarcoma, were imaged for R₁ and R₂ * under air and carbogen (95% O₂ and 5% CO₂ ) breathing conditions. It was observed that the basal tumor oxygenation level had an impact on the amplitude of response to carbogen in the vascular compartment (R₂ *), but not in the tissue compartment (R₁ ). In addition, the change in tissue oxygenation estimated by ΔR₁ correlated with the change in vascular oxygenation estimated by ΔR₂ *, which is consistent with an increase in oxygen supply generating an elevated tumor pO₂ . At the intra-tumoral level, we identified four types of voxel to which a hypoxic feature was attributed (mild hypoxia, severe hypoxia, normoxia and vascular steal), depending on the carbogen-induced change in R₁ and R₂ * values for each voxel. The results showed that 9L-gliomas present more normoxic fractions, whereas rhabdomyosarcomas present more hypoxic fractions, which is in accordance with a previous study using ¹⁸ F-fluoroazomycin arabinoside (¹⁸ F-FAZA) and electron paramagnetic resonance (EPR) oximetry. The response of the combined endogenous MRI contrasts to carbogen challenge could be a useful tool to predict different tumor hypoxic fractions.

In vivo tumor detection with combined MR–Photoacoustic-Thermoacoustic imaging

Here, we report a new method using combined magnetic resonance (MR)–Photoacoustic (PA)–Thermoacoustic (TA) imaging techniques, and demonstrate its unique ability for in vivo cancer detection using tumor-bearing mice. Circular scanning TA and PA imaging systems were used to recover the dielectric and optical property distributions of three colon carcinoma bearing mice While a 7.0-T magnetic resonance imaging (MRI) unit with a mouse body volume coil was utilized for high resolution structural imaging of the same mice. Three plastic tubes filled with soybean sauce were used as fiducial markers for the co-registration of MR, PA and TA images. The resulting fused images provided both enhanced tumor margin and contrast relative to the surrounding normal tissues. In particular, some finger-like protrusions extending into the surrounding tissues were revealed in the MR/TA infused images. These results show that the tissue functional optical and dielectric properties provided by PA and TA images along with the anatomical structure by MRI in one picture make accurate tumor identification easier. This combined MR–PA–TA-imaging strategy has the potential to offer a clinically useful triple-modality tool for accurate cancer detection and for intraoperative surgical navigation.

Hematocrit distribution and tissue oxygenation in large microcirculatory networks

OBJECTIVE

Oxygen tension in the brain is controlled by the microcirculatory supply of RBC, but the effect of non-Newtonian blood flow rheology on tissue oxygenation is not well characterized. This study assesses different biphasic blood flow models for predicting tissue oxygen tension as a function of microcirculatory hemodynamics.

METHODS

Two existing plasma-skimming laws are compared against measured RBC distributions in rat and hamster microcirculatory networks. A novel biphasic blood flow model is introduced. The computational models predict tissue oxygenation in the mesentery, cremaster muscle, and the human secondary cortex.

RESULTS

This investigation shows deficiencies in prior models, including inconsistent plasma-skimming trends and insufficient oxygen perfusion due to the high prevalence (33%) of RBC-free microvessels. Our novel method yields physiologically sound RBC distributions and tissue oxygen tensions within one standard deviation of experimental measurements.

CONCLUSIONS

A simple, novel biphasic blood flow model is introduced with equal or better predictive power when applied to historic raw data sets. It can overcome limitations of prior models pertaining to trifurcations, anastomoses, and loops. This new plasma-skimming law eases the computations of bulk blood flow and hematocrit fields in large microcirculatory networks and converges faster than prior procedures.

Real-Time Image Reconstruction for Pulse EPR Oxygen Imaging Using a GPU and Lookup Table Parameter Fitting

The importance of tissue oxygenation has led to a great interest in methods for imaging pO₂ in vivo. Electron paramagnetic resonance imaging (EPRI) provides noninvasive, near absolute 1 mm-resolved 3D images of pO₂ in the tissues and tumors of living animals. Current EPRI image reconstruction methods tend to be time consuming and preclude real-time visualization of information. Methods are presented to significantly accelerate the reconstruction process in order to enable real-time reconstruction of EPRI pO₂ images. These methods are image reconstruction using graphics processing unit (GPU)-based 3D filtered back-projection and lookup table parameter fitting. The combination of these methods leads to acceleration factors of over 650 compared to current methods and allows for real-time reconstruction of EPRI images of pO₂ in vivo.

Maximally spaced projection sequencing in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) provides 3D images of absolute oxygen concentration (pO₂) in vivo with excellent spatial and pO₂ resolution. When investigating such physiologic parameters in living animals, the situation is inherently dynamic. Improvements in temporal resolution and experimental versatility are necessary to properly study such a system. Uniformly distributed projections result in efficient use of data for image reconstruction. This has dictated current methods such as equal-solid-angle (ESA) spacing of projections. However, acquisition sequencing must still be optimized to achieve uniformity throughout imaging. An object-independent method for uniform acquisition of projections, using the ESA uniform distribution for the final set of projections, is presented. Each successive projection maximizes the distance in the gradient space between itself and prior projections. This maximally spaced projection sequencing (MSPS) method improves image quality for intermediate images reconstructed from incomplete projection sets, enabling useful real-time reconstruction. This method also provides improved experimental versatility, reduced artifacts, and the ability to adjust temporal resolution post factum to best fit the data and its application. The MSPS method in EPRI provides the improvements necessary to more appropriately study a dynamic system.

In vivo microscopy of microvessel oxygenation and network connections

Abnormal or compromised microvascular function is a key component of various diseases. In vivo microscopy of microvessel function in preclinical models can be useful for the study of a disease state and effects of new treatments. Wide-field imaging of microvascular oxygenation via hemoglobin (Hb) saturation measurements has been applied in various applications alone and in combination with other measures of microvessel function, such as blood flow. However, most current combined imaging methods of microvessel function do not provide direct information on microvessel network connectivity or changes in connections and blood flow pathways. First-pass fluorescence (FPF) imaging of a systemically administered fluorescent contrast agent can be used to directly image blood flow pathways and connections relative to a local supplying arteriole in a quantitative manner through measurement of blood supply time (BST). Here, we demonstrate the utility of information produced by the combination of Hb saturation measurements via spectral imaging with BST measurements via FPF imaging for correlation of microvessel oxygenation with blood flow pathways and connections throughout a local network. Specifically, we show network pathway effects on oxygen transport in normal microvessels, dynamic changes associated with wound healing, and pathological effects of abnormal angiogenesis in tumor growth and development.

Effect of small-molecule modification on single-cell pharmacokinetics of PARP inhibitors

The heterogeneous delivery of drugs in tumors is an established process contributing to variability in treatment outcome. Despite the general acceptance of variable delivery, the study of the underlying causes is challenging, given the complex tumor microenvironment including intra- and intertumor heterogeneity. The difficulty in studying this distribution is even more significant for small-molecule drugs where radiolabeled compounds or mass spectrometry detection lack the spatial and temporal resolution required to quantify the kinetics of drug distribution in vivo. In this work, we take advantage of the synthesis of fluorescent drug conjugates that retain their target binding but are designed with different physiochemical and thus pharmacokinetic properties. Using these probes, we followed the drug distribution in cell culture and tumor xenografts with temporal resolution of seconds and subcellular spatial resolution. These measurements, including in vivo permeability of small-molecule drugs, can be used directly in predictive pharmacokinetic models for the design of therapeutics and companion imaging agents as demonstrated by a finite element model.

What we learn from in vivo EPR oxygen images

Distributions of oxygen concentration (pO₂) are a critical determinant of normal tissue health as well as tumor aggressiveness and response to therapy. A number of studies show the value of normal tissue and tumor tissue oxygenation images and some of these will be discussed here. A strong correlation between tumor hypoxic fraction as measured with electron paramagnetic resonance oxygen imaging and radiation treatment success or failure has been found in two separate cancer types. Oxygen images of the torso of wild type mice show initial reduction of lung, liver, visceral, and muscle pO₂ with cyclic halving of fraction of inspired oxygen (FiO₂), but variation is blunted over an hour. Spontaneous breast cancers in Mouse Mammary Tumor Viral (MMTV) promoted-polyoma middle T antigen (PyMT) mice with BNIP3, a major factor in promotion of mitochondrial autophagy, knocked out will be compared with wild type animals. Preliminary studies for the BNIP3 knock out animals show extremely low pO₂. The wide variety of studies, in which oxygen images can play an integral role, serve to demonstrate the importance of oxygen images.

Noninvasive evaluation of the vascular response to transplantation of alginate encapsulated islets using the dorsal skin-fold model

Alginate encapsulation reduces the risk of transplant rejection by evading immune-mediated cell injury and rejection; however, poor vascular perfusion results in graft failure. Since existing imaging models are incapable of quantifying the vascular response to biomaterial implants after transplantation, in this study, we demonstrate the use of in vivo laser speckle imaging (LSI) and wide-field functional imaging (WiFI) to monitor the microvascular environment surrounding biomaterial implants. The vascular response to two islet-containing biomaterial encapsulation devices, alginate microcapsules and a high-guluronate alginate sheet, was studied and compared after implantation into the mouse dorsal window chamber (N = 4 per implant group). Images obtained over a 14-day period using LSI and WiFI were analyzed using algorithms to quantify blood flow, hemoglobin oxygen saturation and vascular density. Using our method, we were able to monitor the changes in the peri-implant microvasculature noninvasively without the use of fluorescent dyes. Significant changes in blood flow, hemoglobin oxygen saturation and vascular density were noted as early as the first week post-transplant. The dorsal window chamber model enables comparison of host responses to transplanted biomaterials. Future experiments will study the effect of changes in alginate composition on the vascular and immune responses.

Evaluation of hypoxia in a feline model of head and neck cancer using ⁶⁴Cu-ATSM positron emission tomography/computed tomography

Background

Human and feline head and neck squamous cell carcinoma (HNSCC) share histology, certain molecular features, as well as locally aggressive and highly recurrent clinical behavior. In human HNSCC, the presence of significant hypoxia within these tumors is considered an important factor in the development of a more aggressive phenotype and poor response to therapy. We hypothesized that feline head and neck tumors, particularly HNSCC, would exhibit hypoxia and that ⁶⁴Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) positron emission tomography/computed tomography (PET/CT) would permit detection of intratumoral hypoxia.

Methods

12 cats with measureable head and neck tumors were given ⁶⁴Cu-ATSM and iodinated contrast for PET/CT scan. The presence or absence of hypoxia was also assessed using an intratumoral fluorescent life-time probe to quantitate pO₂ and pimonidazole immunohistochemical staining in biopsy specimens. In two cats, intratumoral O₂ and ⁶⁴Cu-ATSM uptake was measured before and after treatment with anti-angiogenic agents to determine the effect of these agents on hypoxia.

Results

Eleven of twelve feline tumors demonstrated significant ⁶⁴Cu-ATSM uptake, regardless of malignant or benign etiology. The presence (and absence) of hypoxia was confirmed using the fluorescent O₂ detection probe in nine tumors, and using pimonidazole staining in three tumors. Squamous cell carcinomas (HNSCC) demonstrated the highest degree of hypoxia, with T_max/M ratios ranging from 4.3 to 21.8. Additional non-neoplastic tissues exhibited ⁶⁴Cu-ATSM uptake suggestive of hypoxia including reactive draining lymph nodes, non-malignant thyroid pathology, a tooth root abscess, and otitis media. In two cats with HNSCC that received anti-vascular agents, the pattern of ⁶⁴Cu-ATSM uptake was altered after treatment, demonstrating the potential of the feline model to study the modulation of tumor oxygenation.

Conclusion

Feline HNSCC serves as a clinically relevant model for the investigation of intratumoral hypoxia including its measurement, modulation and targeting.

Principal component analysis enhances SNR for dynamic electron paramagnetic resonance oxygen imaging of cycling hypoxia in vivo

PURPOSE

Low oxygen concentration (hypoxia) in tumors strongly affects their malignant state and resistance to therapy. These effects may be more deleterious in regions undergoing cycling hypoxia. Electron paramagnetic resonance imaging (EPRI) has provided a noninvasive, quantitative imaging modality to investigate static pO2 in vivo. However, to image changing hypoxia, EPRI images with better temporal resolution may be required. The tradeoff between temporal resolution and signal-to-noise ratio (SNR) results in lower SNR for EPRI images with imaging time short enough to resolve cycling hypoxia.

METHODS

Principal component analysis allows for accelerated image acquisition with acceptable SNR by filtering noise in projection data, from which pO2 images are reconstructed. Principal component analysis is used as a denoising technique by including only low-order components to approximate the EPRI projection data.

RESULTS

Simulated and experimental studies show that principal component analysis filtering increases SNR, particularly for small numbers of sub-volumes with changing pO2 , enabling an order of magnitude increase in temporal resolution with minimal deterioration in spatial resolution or image quality.

CONCLUSION

The SNR necessary for dynamic EPRI studies with temporal resolution required to investigate cycling hypoxia and its physiological implications is enabled by principal component analysis filtering.

Imaging hypoxia in gliomas

Hypoxia plays a central role in tumour development, angiogenesis, growth and resistance to treatment. Owing to constant developments in medical imaging technology, significant advances have been made towards in vitro and in vivo imaging of hypoxia in a variety of tumours, including gliomas of the central nervous system. The aim of this article is to review the literature on imaging approaches currently available for measuring hypoxia in human gliomas and provide an insight into recent advances and future directions in this field. After a brief overview of hypoxia and its importance in gliomas, several methods of measuring hypoxia will be presented. These range from invasive monitoring by Eppendorf polarographic O(2) microelectrodes, positron electron tomography (PET) tracers based on 2-nitroimidazole compounds [(18)F-labelled fluoro-misonidazole ((18)F-MISO) or 1-(2-[((18))F]fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole (FRP-170)], (64)Cu-ATSM Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) or (99m)Tc- and (68)Ga-labelled metronidazole (MN) agents to advanced MRI methods, such as blood oxygenation level dependent (BOLD) MRI, oxygen-enhanced MRI, diffusion-weighted MRI (DWI-MRI), dynamic contrast-enhanced MRI (DCE-MRI) and (1)H-magnetic resonance spectroscopy.

A micro-delivery approach for studying microvascular responses to localized oxygen delivery

BACKGROUND

In vivo video microscopy has been used to study blood flow regulation as a function of varying oxygen concentration in microcirculatory networks. However, previous studies have measured the collective response of stimulating large areas of the microvascular network at the tissue surface.

OBJECTIVE

We aimed to limit the area being stimulated by controlling oxygen availability to highly localized regions of the microvascular bed within intact muscle.

DESIGN AND METHOD

Gas of varying O(2) levels was delivered to specific locations on the surface of the Extensor Digitorum Longus muscle of rat through a set of micro-outlets (100 μm diameter) patterned in ultrathin glass using state-of-the-art microfabrication techniques. O(2) levels were oscillated and digitized video sequences were processed for changes in capillary hemodynamics and erythrocyte O(2) saturation.

RESULTS AND CONCLUSIONS

Oxygen saturations in capillaries positioned directly above the micro-outlets were closely associated with the controlled local O(2) oscillations. Radial diffusion from the micro-outlet is limited to ~75 μm from the center as predicted by computational modeling and as measured in vivo. These results delineate a key step in the design of a novel micro-delivery device for controlled oxygen delivery to the microvasculature to understand the fundamental mechanisms of microvascular regulation of O(2) supply.

A systems approach for tumor pharmacokinetics

Recent advances in genome inspired target discovery, small molecule screens, development of biological and nanotechnology have led to the introduction of a myriad of new differently sized agents into the clinic. The differences in small and large molecule delivery are becoming increasingly important in combination therapies as well as the use of drugs that modify the physiology of tumors such as anti-angiogenic treatment. The complexity of targeting has led to the development of mathematical models to facilitate understanding, but unfortunately, these studies are often only applicable to a particular molecule, making pharmacokinetic comparisons difficult. Here we develop and describe a framework for categorizing primary pharmacokinetics of drugs in tumors. For modeling purposes, we define drugs not by their mechanism of action but rather their rate-limiting step of delivery. Our simulations account for variations in perfusion, vascularization, interstitial transport, and non-linear local binding and metabolism. Based on a comparison of the fundamental rates determining uptake, drugs were classified into four categories depending on whether uptake is limited by blood flow, extravasation, interstitial diffusion, or local binding and metabolism. Simulations comparing small molecule versus macromolecular drugs show a sharp difference in distribution, which has implications for multi-drug therapies. The tissue-level distribution differs widely in tumors for small molecules versus macromolecular biologic drugs, and this should be considered in the design of agents and treatments. An example using antibodies in mouse xenografts illustrates the different in vivo behavior. This type of transport analysis can be used to aid in model development, experimental data analysis, and imaging and therapeutic agent design.

Wide-field functional imaging of blood flow and hemoglobin oxygen saturation in the rodent dorsal window chamber

The rodent dorsal window chamber is a widely used in vivo model of the microvasculature. The model consists of a 1cm region of exposed microvasculature in the rodent dorsal skin that is immobilized by surgically implanted titanium frames, allowing the skin microvasculature to be visualized. We describe a detailed protocol for surgical implantation of the dorsal window chamber which enables researchers to perform the window chamber implantation surgery. We further describe subsequent wide-field functional imaging of the chamber to obtain hemodynamic information in the form of blood oxygenation and blood flow on a cm size region of interest. Optical imaging techniques, such as intravital microscopy, have been applied extensively to the dorsal window chamber to study microvascular-related disease and conditions. Due to the limited field of view of intravital microscopy, detailed hemodynamic information typically is acquired from small regions of interest, typically on the order of hundreds of μm. The wide-field imaging techniques described herein complement intravital microscopy, allowing researchers to obtain hemodynamic information at both microscopic and macroscopic spatial scales. Compared with intravital microscopy, wide-field functional imaging requires simple instrumentation, is inexpensive, and can give detailed metabolic information over a wide field of view.

Analysis of tumor environmental response and oncogenic pathway activation identifies distinct basal and luminal features in HER2-related breast tumor subtypes

Introduction

Breast cancer heterogeneity occurs as a consequence of the dysregulation of numerous oncogenic pathways as well as many non-genetic factors, including tumor microenvironmental stresses such as hypoxia, lactic acidosis, and glucose deprivation. Although the importance of these non-genetic factors is well recognized, it is not clear how to integrate these factors within the genetic framework of cancer as the next logical step in understanding tumor heterogeneity.

Methods

We report here the development of a series of gene expression signatures to measure the influences of microenvironmental stresses. The pathway activities of hypoxia, lactic acidosis, acidosis and glucose deprivation were investigated in a collection of 1,143 breast tumors, which have been separated into 17 breast tumor subgroups defined by their distinct patterns of oncogenic pathways. A validation dataset comprised of 547 breast tumors was also used to confirm the major findings, and representative breast cancer cell lines were utilized to validate in silico results and mechanistic studies.

Results

Through the integrative pathway analysis of microenvironmental stresses and oncogenic events in breast tumors, we identified many known and novel correlations between these two sources of tumor heterogeneity. Focusing on differences between two human epidermal growth factor receptor 2 (HER2)-related subgroups, previously identified based on patterns of oncogenic pathway activity, we determined that these subgroups differ with regards to tumor microenvironmental signatures, including hypoxia. We further demonstrate that each of these subgroups have features consistent with basal and luminal breast tumors including patterns of oncogenic signaling pathways, expression of subtype specific genes, and cellular mechanisms that regulate the hypoxia response. Importantly, we also demonstrate that the correlated pattern of hypoxia-related gene expression and basal-associated gene expression are consistent across HER2-related tumors whether we analyze the tumors as a function of our pathway-based classification scheme, using the intrinsic gene list (ERBB2+), or based on HER2 IHC status. Our results demonstrate a cell lineage-specific phenomenon in which basal-like tumors, HER2-related tumors with high hypoxia, as well as normal basal epithelial cells express increased mRNA levels of HIF-1α compared to luminal types and silencing of HIF-1α results in decreased expression of hypoxia-induced genes.

Conclusions

This study demonstrates differences in microenvironmental conditions in HER2-related subgroups defined by distinct oncogenic pathway activities, and provides a mechanistic explanation for differences in the observed hypoxia response between these subgroups. Collectively, these data demonstrate the potential of a pathway-based classification strategy as a framework to integrate genetic and non-genetic factors to investigate the basis of tumor heterogeneity.

The emerging role of hypoxia, HIF-1 and HIF-2 in multiple myeloma

Hypoxia is an imbalance between oxygen supply and demand, which deprives cells or tissues of sufficient oxygen. It is well-established that hypoxia triggers adaptive responses, which contribute to short- and long-term pathologies such as inflammation, cardiovascular disease and cancer. Induced by both microenvironmental hypoxia and genetic mutations, the elevated expression of the hypoxia-inducible transcription factor-1 (HIF-1) and HIF-2 is a key feature of many human cancers and has been shown to promote cellular processes, which facilitate tumor progression. In this review, we discuss the emerging role of hypoxia and the HIFs in the pathogenesis of multiple myeloma (MM), an incurable hematological malignancy of BM PCs, which reside within the hypoxic BM microenvironment. The need for current and future therapeutic interventions to target HIF-1 and HIF-2 in myeloma will also be discussed.

Optical imaging of tumor hypoxia dynamics

The influence of the tumor microenvironment and hypoxia plays a significant role in determining cancer progression, treatment response, and treatment resistance. That the tumor microenvironment is highly heterogeneous with significant intratumor and intertumor variability presents a significant challenge in developing effective cancer therapies. Critical to understanding the role of the tumor microenvironment is the ability to dynamically quantify oxygen levels in the vasculature and tissue in order to elucidate the roles of oxygen supply and consumption, spatially and temporally. To this end, we describe the use of hyperspectral imaging to characterize hemoglobin absorption to quantify hemoglobin content and oxygen saturation, as well as dual emissive fluorescent∕phosphorescent boron nanoparticles, which serve as ratiometric indicators of tissue oxygen tension. Applying these techniques to a window-chamber tumor model illustrates the role of fluctuations in hemoglobin saturation in driving changes in tissue oxygenation, the two being significantly correlated (r = 0.77). Finally, a green-fluorescence-protein reporter for hypoxia inducible factor-1 (HIF-1) provides an endpoint for hypoxic stress in the tumor, which is used to demonstrate a significant association between tumor hypoxia dynamics and HIF-1 activity in an in vivo demonstration of the technique.

Interleukin 6 mediated recruitment of mesenchymal stem cells to the hypoxic tumor milieu

Mesenchymal stem cells (MSCs) are a heterogeneous population of non-hematopoietic precursor cells predominantly found in the bone marrow. They have been recently reported to home towards the hypoxic tumor microenvironment in vivo. Interleukin-6 is a multifunctional cytokine normally involved in the regulation of the immune and inflammatory response. In addition to its normal function, IL-6 signaling has been implicated in tumorigenesis. Solid tumors develop hypoxia as a result of inadequate O(2) supply. Interestingly, tumor types with increased levels of hypoxia are known to have increased resistance to chemotherapy as well as increased metastatic potential. Here, we present evidence that under hypoxic conditions (1.5% O(2)) breast cancer cells secrete high levels of IL-6, which serve to activate and attract MSCs. We now report that secreted IL-6 acts in a paracrine fashion on MSCs stimulating the activation of both Stat3 and MAPK signaling pathways to enhance migratory potential and cell survival. Inhibition of IL-6 signaling utilizing neutralizing antibodies leads to attenuation of MSC migration. Specifically, increased migration is dependent on IL-6 signaling through the IL-6 receptor. Collectively, our data demonstrate that hypoxic tumor cells specifically recruit MSCs, which through activation of signaling and survival pathways facilitate tumor progression.

Imaging of tumor vasculature using Twente photoacoustic systems

Photoacoustic imaging is a hybrid imaging modality based on the detection of acoustic waves generated by the absorption of short laser pulses in biological tissue. It combines the advantages of excellent contrast achieved in optical techniques with the high resolution of ultrasound imaging. In this article we present a review of the work done at the University of Twente to image tumor angiogenesis in vivo using this technique. We start with a description and the technical details of the different photoacoustic systems developed in our laboratory, with their validation on phantoms. We then discuss small-animal studies with results of serial imaging of angiogenesis over a 10-day period at the site of tumor induction in a rat. Further, we present clinical results using a photoacoustic mammoscope of breast cancer imaging based on angiogenesis-driven optical absorption contrast.

A dual-emissive-materials design concept enables tumour hypoxia imaging

Luminescent materials are widely used for imaging and sensing owing to their high sensitivity, rapid response and facile detection by many optical technologies. Typically materials must be chemically tailored to achieve intense, photostable fluorescence, oxygen-sensitive phosphorescence or dual emission for ratiometric sensing, often by blending two dyes in a matrix. Dual-emissive materials combining all of these features in one easily tunable molecular platform are desirable, but when fluorescence and phosphorescence originate from the same dye, it can be challenging to vary relative fluorescence/phosphorescence intensities for practical sensing applications. Heavy-atom substitution alone increases phosphorescence by a given, not variable amount. Here, we report a strategy for modulating fluorescence/phosphorescence for a single-component, dual-emissive, iodide-substituted difluoroboron dibenzoylmethane-poly(lactic acid) (BF(2)dbm(I)PLA) solid-state sensor material. This is accomplished through systematic variation of the PLA chain length in controlled solvent-free lactide polymerization combined with heavy-atom substitution. We demonstrate the versatility of this approach by showing that films made from low-molecular-weight BF(2)dbm(I)PLA with weak fluorescence and strong phosphorescence are promising as 'turn on' sensors for aerodynamics applications, and that nanoparticles fabricated from a higher-molecular-weight polymer with balanced fluorescence and phosphorescence intensities serve as ratiometric tumour hypoxia imaging agents.

Quantitative optical spectroscopy: a robust tool for direct measurement of breast cancer vascular oxygenation and total hemoglobin content in vivo

We propose the use of a robust, biopsy needle-based, fiber-optic tool for routine clinical quantification of tumor oxygenation at the time of diagnostic biopsy for breast cancer. The purpose of this study was to show diffuse reflectance spectroscopy as a quantitative tool to measure oxygenation levels in the vascular compartment of breast cancers in vivo via an optical biopsy technique. Thirty-five patients undergoing surgical treatment for breast cancer were recruited for the study at Duke University Medical Center. Diffuse reflectance spectroscopy was performed on the tumors in situ before surgical resection, followed by needle-core biopsy of the optically measured tissue. Hemoglobin saturation and total hemoglobin content were quantified from 76 optical spectra-tissue biopsy pairs, consisting of 20 malignant, 23 benign, and 33 adipose tissues. Hemoglobin saturation in malignant tissues was significantly lower than nonmalignant tissues (P<0.002) and was negatively correlated with tumor size and pathologic tumor category (P<0.05). Hemoglobin saturation was positively correlated with total hemoglobin content in malignant tissues (P<0.02). HER2/neu-amplified tumors exhibited significantly higher total hemoglobin content (P<0.05) and significantly higher hemoglobin saturation (P<0.02), which is consistent with a model of increased angiogenesis and tumor perfusion promoted by HER2/neu amplification. Diffuse reflectance spectroscopy could aid in prognosis and prediction in breast cancer via quantitative assessment of tumor physiology at the time of diagnostic biopsy.

Microvascular blood flow and oxygenation during hemorrhagic hypotension

Understanding microvascular oxygen transport requires the knowledge of microvessel topology and geometry, blood flow and oxygen levels. Microvascular hemodynamic responses to hemorrhagic hypotension (HH) such as size-dependent vasoconstriction and blood flow reduction could lead to increased longitudinal oxygen partial pressure (PO(2)) gradients. However, the mesenteric microvascular PO(2) has never been evaluated during HH. Therefore, we studied hemodynamic variables and PO(2) distribution in 165 mesenteric microvessels from 39 anesthetized rats to investigate whether HH-induced vasoconstriction and blood flow reduction were associated with changes in longitudinal PO(2) gradients. Vessels were analyzed according to their position in the network, as well as a few interstitial PO(2) areas. We found that during baseline a small PO(2) gradient exists, but HH is accompanied by more pronounced microvascular longitudinal PO(2) gradients. Decreased blood flow did not seem to completely explain these findings, since blood flow was uniformly diminished in arterioles and venules, independent of diameter and position in the network. During HH, some microvessels presented higher PO(2) than during baseline despite blood flow reduction, possibly due to a combination of systemic hyperoxia and low oxygen consumption of mesentery. The data suggest that blood flow measurements may be a poor indicator of the oxygenation status in some regions of the mesentery. The enhanced mesenteric longitudinal PO(2) gradient may lead to regions with different levels of other physiologically active compounds.

Intravital imaging and cell invasion

The main cause of cancer treatment failure is the invasion of normal tissues by cancer cells that have migrated from a primary tumor. An important obstacle to understanding cancer invasion has been the inability to acquire detailed, direct observations of the process over time in a living system. Intravital imaging, and the rodent dorsal skinfold window chamber in particular, were developed several decades ago to address this need. However, it is just recently, with the advent of sophisticated new imaging systems such as confocal and multiphoton microscopy together with the development of a wide range of fluorescent cellular and intracellular markers, that intravital methods and the window chamber have acquired powerful new potential for the study of cancer cell invasion. Moreover, the interaction of various cell signaling pathways with the integrin class of cell surface receptors has increasingly been shown to play a key role in cancer invasion. The window chamber in combination with integrin-knockout rodent models, integrin-deficient tumor cell lines, and integrin antagonists, allows real-time observation of integrin-mediated cancer invasion and angiogenesis. The present review outlines the history, uses, and recent methods of the rodent dorsal skinfold window chamber. The introduction of labeled tumor cells into the chamber is described, and imaging of tumors and angiogenic vessels within chambers using standard brightfield, confocal, and multiphoton microscopy is discussed in detail, along with the presentation of sample images.

Methods for noninvasive imaging of tissue hypoxia

The purpose of this review is to provide an overview of the methods available for imaging tissue oxygenation. The following imaging methods are reviewed: phosphorescence, near-infrared (NIR), positron emission tomography (PET), magnetic resonance imaging ((19)F MRI and BOLD MRI), and electron paramagnetic resonance (EPR). The methods are based on different principles and differ in their ability to accurately quantify tissue oxygenation, either the absolute value of a particular measure of oxygenation (partial pressure of oxygen, concentration), or a parameter related to it (oxygen saturation). Methods that can provide images of relative changes in oxygenation or visualization of hypoxia in a specific tissue of interest are also considered valuable tools for biomedical research and clinical applications.

Drug Resistance and the Solid Tumor Microenvironment

Resistance of human tumors to anticancer drugs is most often ascribed to gene mutations, gene amplification, or epigenetic changes that influence the uptake, metabolism, or export of drugs from single cells. Another important yet little-appreciated cause of anticancer drug resistance is the limited ability of drugs to penetrate tumor tissue and to reach all of the tumor cells in a potentially lethal concentration. To reach all viable cells in the tumor, anticancer drugs must be delivered efficiently through the tumor vasculature, cross the vessel wall, and traverse the tumor tissue. In addition, heterogeneity within the tumor microenvironment leads to marked gradients in the rate of cell proliferation and to regions of hypoxia and acidity, all of which can influence the sensitivity of the tumor cells to drug treatment. In this review, we describe how the tumor microenvironment may be involved in the resistance of solid tumors to chemotherapy and discuss potential strategies to improve the effectiveness of drug treatment by modifying factors relating to the tumor microenvironment.

A novel method for mapping the heterogeneity in blood supply to normal and malignant tissues in the mouse dorsal window chamber

A novel first-pass imaging method for studying blood flow in tumors and normal tissues in mouse dorsal window chamber preparations is reported in this study. A fluorescence-labeled macromolecule is administered i.v. as a bolus dose, and the signal generated by the bolus is detected as the bolus passes through the vascular network. The sensitivity of the method is sufficiently high that the bolus can be detected in any tumor or normal tissue microvessel. The method is particularly useful for measuring the velocity and direction of blood flow in individual vessel segments of normal and malignant tissues and the blood supply time (BST) of individual tumor vessel segments; i.e., the time required for arterial blood to flow from a supplying tumor artery to a downstream microvessel segment. Since all vessels within a window chamber preparation can be studied simultaneously in a single imaging sequence, a significant benefit of the method is that it can be used to produce maps of velocity of blood flow and BST of the entire microvascular network of xenografted tumors.

Developments in quantitative oxygen-saturation imaging of breast tissue in vivo using multispectral near-infrared tomography

Imaging of oxygen saturation provides a spatial map of the tissue metabolic activity and has potential in diagnosis and treatment monitoring of breast cancer. Oxygen-saturation imaging is possible through near-infrared (NIR) tomography, but has low signal-to-noise ratio (SNR). This can be augmented by using NIR tomography as an add-on to MRI. Presented are results from a free-standing NIR system and a hybrid MR-guided system for breast imaging. In results from imaging 60 healthy volunteers in the initial NIR system, oxygen saturation was a significant discriminator between the BIRADS classifications of adipose tissue, heterogeneously dense, and extremely dense tissue. By using the MR-guided NIR system, more accurate tissue-specific data were obtained on adipose and fibroglandular volumes, with 11 healthy volunteers. In these data, oxygen saturation in the adipose tissue correlated with percentage of adipose tissue. In two case studies of infiltrating ductal carcinomas, oxygen saturation was reduced at the site of the tumor, as compared with the surrounding healthy tissue, agreeing with conventional thought that hypoxia exists in larger solid tumors. The MRI-guided NIR images of oxygen saturation provide higher resolution and superior SNR and will likely be used in the future to study and characterize specific tissue volumes.

Functional neoangiogenesis imaging of genetically engineered mouse prostate cancer using three-dimensional power Doppler ultrasound

We report the first application of high-frequency three-dimensional power Doppler ultrasound imaging in a genetically engineered mouse (GEM) prostate cancer model. We show that the technology sensitively and specifically depicts functional neoangiogenic blood flow because little or no flow is measurable in normal prostate tissue or tumors smaller than 2-3 mm diameter, the neoangiogenesis "switch-on" size. Vascular structures depicted by power Doppler were verified using Microfil-enhanced micro-computed tomography (micro-CT) and by correlation with microvessel distributions measured by immunohistochemistry and enhanced vascularity visualized by confocal microscopy in two GEM models [transgenic adenocarcinoma of the mouse prostate (TRAMP) and PSP94 gene-directed transgenic mouse adenocarcinoma of the prostate (PSP-TGMAP)]. Four distinct phases of neoangiogenesis in cancer development were observed, specifically, (a) an early latent phase; (b) establishment of a peripheral capsular vascular structure as a neoangiogenesis initiation site; (c) a peak in tumor vascularity that occurs before aggressive tumor growth; and (d) rapid tumor growth accompanied by decreasing vascularity. Microsurgical interventions mimicking local delivery of antiangiogenesis drugs were done by ligating arteries upstream from feeder vessels branching to the prostate. Microsurgery produced an immediate reduction of tumor blood flow, and flow remained low from 1 h to 2 weeks or longer after treatment. Power Doppler, in conjunction with micro-CT, showed that the tumors recruit secondary blood supplies from nearby vessels, which likely accounts for the continued growth of the tumors after surgery. The microsurgical model represents an advanced angiogenic prostate cancer stage in GEM mice corresponding to clinically defined hormone-refractory prostate cancer. Three-dimensional power Doppler imaging is completely noninvasive and will facilitate basic and preclinical research on neoangiogenesis in live animal models.

Expression of the hypoxia marker carbonic anhydrase 9 is associated with anaplastic phenotypes in meningiomas

PURPOSE

Hypoxia in the tumor microenvironment triggers a variety of genetic and adaptive responses that regulate tumor growth. Tumor hypoxia is often associated with more malignant phenotypes, resistance to therapy, and poor survival. The purpose of this study was to evaluate the prevalence of hypoxia in meningiomas using the endogenous hypoxia marker carbonic anhydrase 9 (CA9) and to relate the expression of CA9 to tumor vascularity, histopathologic grade, and clinical variables, such as recurrent tumor status.

EXPERIMENTAL DESIGN

Expression of CA9 and CD34, an endothelial cell marker, was examined in serial paraffin-embedded sections by immunohistochemistry in 25 grade 1, 17 grade 2, and 20 grade 3 meningiomas. Areas of immunoreactivity were semiquantitatively scored and correlated to clinical variables using Statistical Analysis System statistical software.

RESULTS

Approximately 50% (29 of 62) of all meningiomas contained regions of hypoxia as judged by expression of CA9, and this expression was significantly associated with higher-grade histology (P = 0.001). In contrast, vascularity, as assessed by the percentage of vascular hotspots, was inversely associated with tumor grade (P = 0.023) and was not associated with CA9 expression. Among lower-grade meningiomas, CA9 expression tended to be more common in recurrent tumors.

CONCLUSIONS

Tumor hypoxia is an endogenous feature of meningiomas, and therapeutic regimens should include strategies to target the subpopulation of hypoxic as well as the normoxic cells within the tumor. Hypoxia in meningiomas is associated with an aggressive phenotype. Further studies to define the contribution of hypoxia to meningioma pathophysiology are warranted.

Methodology for quantifying interactions between perfusion evaluated by DCE-US and hypoxia throughout tumor growth

The objective was to validate a combination of two new technologies to depict tumor physiology both temporally and spatially with dynamic contrast-enhanced sonography and an oximeter. Human cancer prostate tumors xenografted onto mice were followed for three weeks using dynamic contrast-enhanced ultrasonography (DCE-US) to detect tumor perfusion. Time intensity curves in linear data were quantified on four regions-of-interest (ROI, main tumor section and its anterior, central and posterior intra-tumoral areas) to extract three indices of perfusion. An oxygen sensor was guided by sonography to obtain accurate pO(2) measurements in the three predefined areas of tumors during their development. No impact on tumor growth of subsequent pO(2) probe insertion was detected. Among the four ROIs studied, the local central tumor showed significant perfusion and oxygenation variations throughout the experiment. A correlation was observed between local central tumor perfusion and pO(2), both of them decreasing through time (p = 0.0068; r = 0.66). The methodology which we developed demonstrated the potential of combining DCE-US with direct tissue pO(2) measurements, improving the description of complex intratumoral dynamic behavior.

Fluctuating and Diffusion-Limited Hypoxia in Hypoxia-Induced Metastasis

PURPOSE

Most tumors develop regions with hypoxic cells during growth, owing to permanent limitations in oxygen diffusion (chronic or diffusion-limited hypoxia) and/or transient limitations in blood perfusion (acute or fluctuating hypoxia). The aim of this study was to investigate the relative significance of chronic and acute hypoxia in the development of metastatic disease.

EXPERIMENTAL DESIGN

D-12 and R-18 human melanoma xenografts were used as models of human cancer. D-12 tumors metastasize to the lungs, whereas R-18 tumors develop lymph node metastases. Fraction of radiobiologically hypoxic cells (HF(Rad)) was measured in individual primary tumors by using a radiobiological assay based on the paired survival curve method. Fraction of immunohistochemically hypoxic cells (HF(Imm)) was assessed in the same tumors by using a pimonidazole-based immunohistochemical assay optimized with respect to achieving selective staining of chronically hypoxic cells. HF(Imm) and the difference between HF(Rad) and HF(Imm), HF(Rad) - HF(Imm), were verified to be adequate variables for fraction of chronically hypoxic cells and fraction of acutely hypoxic cells, respectively.

RESULTS

Chronic as well as acute hypoxia were found to promote spontaneous metastasis of D-12 and R-18 tumors. Acute hypoxia influenced metastasis to a greater extent than chronic hypoxia, partly because the fraction of acutely hypoxic cells was larger than the fraction of chronically hypoxic cells in most tumors and partly because acutely hypoxic cells showed a higher metastatic potential than chronically hypoxic cells.

CONCLUSIONS

It may be beneficial to focus on fluctuating hypoxia rather than diffusion-limited hypoxia when searching for hypoxia-related prognostic variables and predictive assays.

Noninvasive monitoring of radiotherapy-induced microvascular changes using dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) in a colorectal tumor model

PURPOSE

To examine dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with a macromolecular contrast agent (P792) to visualize effects of radiotherapy (RT) on microvascular leakage in a colorectal cancer model.

METHODS AND MATERIALS

CC531 tumors were induced in WAG/Rij rats. DCE-MRI was performed before and 5 days after 5 x 5 Gy of RT and parametric maps generated of the endothelial transfer constant (K(trans)) and the fractional interstitial space (Ve) according to the Tofts model. Tissue pO2 mapping was performed in each tumor core and rim before and after RT. Microvessel density (MVD), vascular endothelial growth factor (VEGF) expression, and pimonidazole hypoxia staining were compared with a control group of tumor-bearing rats.

RESULTS

Mean K(trans) and v(e) were significantly reduced after RT in all tumor regions. Mean pO2 was 6.8 mm Hg before RT vs. 7.7 mm Hg after RT (p < 0.001) in the tumor rim and 3.5 mm Hg before RT vs. 4.4 mm Hg after RT (p < 0.001) in the tumor core. Mean MVD in the tumor rim was 10.4 in the RT treated group vs. 16.9 in the control group (p = 0.061). VEGF expression was significantly higher in RT-treated rats. After RT, no correlation was found between DCE-MRI parameters and histologic parameters. A correlation was seen after RT between pO2 and K(trans) (r = -0.57, p = 0.08) and between pO2 and v(e) (r = -0.65, p = 0.04).

CONCLUSIONS

Dynamic contrast-enhanced-MRI with P792 allows quantification of microvascular changes in this colorectal model. RT significantly reduces neovascular leakage and enhances tissue oxygenation and VEGF expression. After RT, DCE-MRI parameters are related to tumor pO2, but not to MVD or VEGF expression.

Assessment of fraction of radiobiologically hypoxic cells in human melanoma xenografts by dynamic contrast-enhanced MRI

A noninvasive method for assessment of the extent of hypoxia in experimental and human tumors is highly needed. In this study, the potential usefulness of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was investigated, using gadopentetate dimeglumine (Gd-DTPA) as contrast agent and A-07 human melanoma xenografts as tumor model. DCE-MRI was performed at a voxel size of 0.3 x 0.6 x 2.0 mm3 with spoiled gradient-recalled sequences. Images of E . F (E is the initial extraction fraction of Gd-DTPA and F is perfusion) and lambda (the partition coefficient of Gd-DTPA, which is proportional to extracellular volume fraction) were obtained by Kety analysis of DCE-MRI data. The study was based on the hypothesis that hypoxic tissue would have low E . F (i.e., poor oxygen supply) and/or low lambda (i.e., high cell density and, hence, high oxygen consumption rate). Twenty-two tumors were first subjected to DCE-MRI and then to measurement of fraction of hypoxic cells, using a radiobiological assay. E . F was found to be strongly correlated to fraction of hypoxic cells (P < 0.000001), whereas significant correlation between lambda and fraction of hypoxic cells could not be detected. It is thus possible that E . F may be a useful parameter for the extent of hypoxia in experimental and human tumors with physiologic properties similar to those of A-07 tumors. This possibility warrants further studies involving experimental tumors of several lines, as well as human tumors.

Mapping of the functional microcirculation in vital organs using contrast-enhanced in vivo video microscopy

A functional microcirculation is vital to the survival of mammalian tissues. In vivo video microscopy is often used in animal models to assess microvascular function, providing real-time observation of blood flow in normal and diseased tissues. To extend the capabilities of in vivo video microscopy, we have developed a contrast-enhanced system with postprocessing video analysis tools that permit quantitative assessment of microvascular geometry and function in vital organs and tissues. FITC-labeled dextran (250 kDa) was injected intravenously into anesthetized mice to provide intravascular fluorescence contrast with darker red blood cell (RBC) motion. Digitized video images of microcirculation in a variety of internal organs (e.g., lung, liver, ovary, and kidney) were processed using computer-based motion correction to remove background respiratory and cardiac movement. Stabilized videos were analyzed to generate a series of functional images revealing microhemodynamic parameters, such as plasma perfusion, RBC perfusion, and RBC supply rate. Fluorescence contrast revealed characteristic microvascular arrangements within different organs, and images generated from video sequences of liver metastases showed a marked reduction in the proportion of tumor vessels that were functional. Analysis of processed video sequences showed large reductions in vessel volume, length, and branch-point density, with a near doubling in vessel segment length. This study demonstrates that postprocessing of fluorescence contrast video sequences of the microcirculation can provide quantitative images useful for studies in a wide range of model systems.