Accumulation of Tc-99m HL91 in tumor hypoxia: in vitro cell culture and in vivo tumor model

In vivo noninvasive preclinical tumor hypoxia imaging methods: a review

Tumors exhibit areas of decreased oxygenation due to malformed blood vessels. This low oxygen concentration decreases the effectiveness of radiation therapy, and the resulting poor perfusion can prevent drugs from reaching areas of the tumor. Tumor hypoxia is associated with poorer prognosis and disease progression, and is therefore of interest to preclinical researchers. Although there are multiple different ways to measure tumor hypoxia and related factors, there is no standard for quantifying spatial and temporal tumor hypoxia distributions in preclinical research or in the clinic. This review compares imaging methods utilized for the purpose of assessing spatio-temporal patterns of hypoxia in the preclinical setting. Imaging methods provide varying levels of spatial and temporal resolution regarding different aspects of hypoxia, and with varying advantages and disadvantages. The choice of modality requires consideration of the specific experimental model, the nature of the required characterization and the availability of complementary modalities as well as immunohistochemistry.

Technical Feasibility and Physiological Relevance of Hypoxic Cell Culture Models

Hypoxia is characterized as insufficient oxygen delivery to tissues and cells in the body and is prevalent in many human physiology processes and diseases. Thus, it is an attractive state to experimentally study to understand its inner mechanisms as well as to develop and test therapies against pathological conditions related to hypoxia. Animal models in vivo fail to recapitulate some of the key hallmarks of human physiology, which leads to human cell cultures; however, they are prone to bias, namely when pericellular oxygen concentration (partial pressure) does not respect oxygen dynamics in vivo. A search of the current literature on the topic revealed this was the case for many original studies pertaining to experimental models of hypoxia in vitro. Therefore, in this review, we present evidence mandating for the close control of oxygen levels in cell culture models of hypoxia. First, we discuss the basic physical laws required for understanding the oxygen dynamics in vitro, most notably the limited diffusion through a liquid medium that hampers the oxygenation of cells in conventional cultures. We then summarize up-to-date knowledge of techniques that help standardize the culture environment in a replicable fashion by increasing oxygen delivery to the cells and measuring pericellular levels. We also discuss how these tools may be applied to model both constant and intermittent hypoxia in a physiologically relevant manner, considering known values of partial pressure of tissue normoxia and hypoxia in vivo, compared to conventional cultures incubated at rigid oxygen pressure. Attention is given to the potential influence of three-dimensional tissue cultures and hypercapnia management on these models. Finally, we discuss the implications of these concepts for cell cultures, which try to emulate tissue normoxia, and conclude that the maintenance of precise oxygen levels is important in any cell culture setting.

How to Modulate Tumor Hypoxia for Preclinical In Vivo Imaging Research

Tumor hypoxia is related with tumor aggressiveness, chemo- and radiotherapy resistance, and thus a poor clinical outcome. Therefore, over the past decades, every effort has been made to develop strategies to battle the negative prognostic influence of tumor hypoxia. For appropriate patient selection and follow-up, noninvasive imaging biomarkers such as positron emission tomography (PET) radiolabeled ligands are unprecedentedly needed. Importantly, before being able to implement these new therapies and potential biomarkers into the clinical setting, preclinical in vivo validation in adequate animal models is indispensable. In this review, we provide an overview of the different attempts that have been made to create differential hypoxic in vivo cancer models with a particular focus on their applicability in PET imaging studies.

Tc-99m-HL91 imaging in the early detection of neuronal injury in a neonatal rat model of hypoxic ischemia

OBJECTIVE

Hypoxic-ischemic insult in newborns results in progressive neuronal loss. For neuroprotective therapy to be effective, it is important to identify high-risk neonates soon after birth. 99mTc-labeled imaging agent, Tc-99m-HL91, developed as a putative hypoxic reagent, has been reported to demonstrate increased uptake in ischemic myocardium. We hypothesized that Tc-99m-HL91 is sensitive for the early identification of hypoxic-ischemic injury in neonatal rat brains.

DESIGN

Laboratory investigation.

SETTING

University research laboratory.

SUBJECTS

Sprague-Dawley rat pups.

INTERVENTIONS

Postnatal day-7 pups were divided into four groups: hypoxic-ischemia, hypoxia-only, ischemia-only, and controls. In the early (2 hrs), intermediate (20 hrs), and late (44 hrs) reoxygenation phases, Tc-99m-HL91 in vivo and ex vivo imaging and quantitative autoradiography were performed. Regions of interest were drawn to calculate the contrast ratio of Tc-99m-HL91 uptake between the ipsilateral and contralateral hemispheres. Pathology, cerebral blood flow, and blood-brain barrier damage were determined.

MEASUREMENTS AND MAIN RESULTS

After hypoxic-ischemia, there were very few pyknotic neurons in the early phase, many pyknotic neurons in the intermediate phase, and extensive neuronal loss in the late phase postreoxygenation. Blood-brain barrier damage occurred in the early phase, progressed in the intermediate phase, and became extensive in the late phase. The hypoxia-only and ischemia-only pups showed no neuronal or blood-brain barrier damage and had higher cerebral blood flow postreoxygenation compared with the hypoxia-ischemia pups. Regions of interest analysis of in vivo and ex vivo images and autoradiography revealed significantly higher Tc-99m-HL91 contrast ratio at early and intermediate phases, not late phase of hypoxic-ischemic group. Hypoxic-ischemia group had significantly higher contrast ratio values in the early and intermediate phases than the hypoxia-only and ischemia-only groups. A contrast ratio value of 0.15 in the early phase on postnatal day 7 had a sensitivity of 0.95 and specificity of 0.89 in detecting significant hypoxic-ischemic lesions on postnatal day 21.

CONCLUSION

Tc-99m-HL91 uptake is sensitive for the early detection of hypoxic-ischemic injury in neonatal brains.

Hypoxia imaging predicts success of hypoxia-induced cytosine deaminase/5-fluorocytosine gene therapy in a murine lung tumor model

Tc-99m-HL91 is a hypoxia imaging biomarker. The aim of this study was to investigate the value of Tc-99m-HL91 imaging for hypoxia-induced cytosine deaminase (CD)/5-fluorocytosine (5-FC) gene therapy in a murine lung tumor model. C57BL/6 mice were implanted with Lewis lung carcinoma cells transduced with the hypoxia-inducible promoter-driven CD gene (LL2/CD) or luciferase gene (LL2/Luc) serving as the control. When tumor volumes reached 100 mm(3), pretreatment images were acquired after injection of Tc-99m-HL91. The mice were divided into low and high hypoxic groups based on the tumor-to-non-tumor ratio of Tc-99m-HL91. They were injected daily with 5-FC (500 mg kg(-1)) or the vehicle for 1 week. When tumor volumes reached 1000 mm(3), autoradiography and histological examinations were performed. Treatment with 5-FC delayed tumor growth and enhanced the survival of mice bearing high hypoxic LL2/CD tumors. The therapeutic effect of hypoxia-induced CD/5-FC gene therapy was more pronounced in high hypoxic tumors than in low hypoxic tumors. This study provides the first evidence that Tc-99m-HL91 can serve as an imaging biomarker for predicting the treatment responses of hypoxia-regulated CD/5-FC gene therapy in animal tumor models. Our results suggest that hypoxia imaging using Tc-99m-HL91 has the predictive value for the success of hypoxia-directed treatment regimens.

Molecular imaging of hypoxia with radiolabelled agents

Tissue hypoxia results from an inadequate supply of oxygen (O₂) that compromises biological functions. Structural and functional abnormalities of the tumour vasculature together with altered diffusion conditions inside the tumour seem to be the main causes of tumour hypoxia. Evidence from experimental and clinical studies points to a role for tumour hypoxia in tumour propagation, resistance to therapy and malignant progression. This has led to the development of assays for the detection of hypoxia in patients in order to predict outcome and identify patients with a worse prognosis and/or patients that would benefit from appropriate treatments. A variety of invasive and non-invasive approaches have been developed to measure tumour oxygenation including oxygen-sensitive electrodes and hypoxia marker techniques using various labels that can be detected by different methods such as positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), autoradiography and immunohistochemistry. This review aims to give a detailed overview of non-invasive molecular imaging modalities with radiolabelled PET and SPECT tracers that are available to measure tumour hypoxia.