Determination of tumour hypoxia with the PET tracer [18F]EF3: improvement of the tumour-to-background ratio in a mouse tumour model

Recent Advances of 68Ga-Labeled PET Radiotracers with Nitroimidazole in the Diagnosis of Hypoxia Tumors

Positron emission tomography (PET) is a noninvasive molecular imaging method extensively applied in the detection and treatment of various diseases. Hypoxia is a common phenomenon found in most solid tumors. Nitroimidazole is a group of bioreducible pharmacophores that selectively accumulate in hypoxic regions of the body. Over the past few decades, many scientists have reported the use of radiopharmaceuticals containing nitroimidazole for the detection of hypoxic tumors. Gallium-68, a positron-emitting radioisotope, has a favorable half-life time of 68 min and can be conveniently produced by ⁶⁸Ge/⁶⁸Ga generators. Recently, there has been significant progress in the preparation of novel ⁶⁸Ga-labeled complexes bearing nitroimidazole moieties for the diagnosis of hypoxia. This review provides a comprehensive overview of the current status of developing ⁶⁸Ga-labeled radiopharmaceuticals with nitroimidazole moieties, their pharmacokinetics, and in vitro and in vivo studies, as well as PET imaging studies for hypoxic tumors.

The Effect of Low-Dose Gamma Irradiation on the Uptake of Boronophenylalanine to Enhance the Efficacy of Boron Neutron Capture Therapy in an Orthotopic Oral Cancer Model

The success of boron neutron capture therapy (BNCT) mainly depends on the boron concentration in the tumor and a high tumor/normal tissue (T/N) boron ratio or a high tumor/blood (T/B) boron ratio. Therefore, the effective enhancement of boron ratios is the first priority. Our study investigated whether a low-dose of γ-radiation (LDR) could improve boron ratios and enhance the therapeutic effects of BNCT in an orthotopic human oral squamous cell carcinoma-bearing animal model. SAS/luc cells were used to establish the orthotopic tumor-bearing model. The pharmacokinetics of boronophenylalanine (BPA) administration with 400 mg/kg of body weight both alone and in combination with LDR (0.1 Gy) was evaluated, and BNCT was performed at the Tsing Hua Open-pool Reactor (THOR). The radiation doses were evaluated using a treatment planning system. Moreover, tumor growth and metastasis were monitored via bioluminescence imaging (BLI). The therapeutic effects after BNCT were evaluated using BLI, histopathological findings and the overall survival rate. LDR increased the BPA accumulation in tumors by 52.2%. T/N and T/B ratios were enhanced from 3.77 to 5.31 and from 3.47 to 4.46, respectively. Radiation dose was increased by 44.3%. Notably, tumor recurrence and cervical lymph node metastasis were observed in the BNCT group, which had a survival rate of 50%. Complete responses were found in the combined-treatment group, which had a survival rate of 100%. No toxicity was found according to the histopathological findings. Conclusively, LDR increased BPA accumulation in the tumor and the T/N and T/B ratios, resulting in BNCT efficacy improvement and the overall survival rate extension.

Understanding the hypoxic niche of multiple myeloma: therapeutic implications and contributions of mouse models

Multiple myeloma (MM) is the second most common hematological malignancy and is characterized by the clonal expansion of plasma cells in the bone marrow. Recently, hypoxia has received increased interest in the context of MM, in both basic and translational research. In this review, we describe the discovery of the hypoxic niche in MM and how it can be targeted therapeutically. We also discuss mouse models that closely mimic human MM, highlighting those that allow preclinical research into new therapies that exploit the hypoxic niche in MM.

Resolution in PET hypoxia imaging: voxel size matters

INTRODUCTION

Tumor hypoxia adversely affects treatment outcome, especially in squamous cell carcinomas (SCCs). Image guided radiotherapy (IGRT) based on PET-generated tumor hypoxia maps allows dose boosting to hypoxic sub-volumes and has received considerable interest. However, the combination of slow oxygenation-dependent tracer retention, slow clearance of unbound tracer from non-hypoxic tissue and the necessity to average signal over large non-homogenous tissue areas due to the low PET resolution remains problematic.

MATERIALS AND METHODS

To assess pitfalls inherent to low-resolution imaging we have analyzed the fine-scale distribution of a PET hypoxia tracer (autoradiograms) and tissue architecture (immunofluorescence microscopy) in sectioned experimental SCCs, and compared the results to those obtained when applying macroscopic averaging mimicking the resolution in clinical PET scanners.

RESULTS AND DISCUSSION

We show that tumor areas that would be classified as non-hypoxic based on simple PET threshold identification, often contains foci of hypoxic cells, in particular in tumors where necrosis and severely hypoxic cells are intermixed. In contrast, in a non-necrotic tumor model we found that the risk of missing hypoxic cells was greatly reduced, however, its patchy hypoxic pattern made a clear delineation of a target to boost unfeasible. We discuss the implications of these and other complicating factors in PET hypoxia-imaging and outline future strategies to overcome or circumvent them.

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.

[18F]EF3 is not superior to [18F]FMISO for PET-based hypoxia evaluation as measured in a rat rhabdomyosarcoma tumour model

PURPOSE

The aim of this investigation was to quantitatively compare the novel positron emission tomography (PET) hypoxia marker 2-(2-nitroimidazol-1-yl)-N-(3[(18)F],3,3-trifluoropropyl)acetamide ([(18)F]EF3) with the reference hypoxia tracer [(18)F]fluoromisonidazole ([(18)F]FMISO).

METHODS

[(18)F]EF3 or [(18)F]FMISO was injected every 2 days into two separate groups of rats bearing syngeneic rhabdomyosarcoma tumours. In vivo PET analysis was done by drawing regions of interest on the images of selected tissues. The resulting activity data were quantified by the percentage of injected radioactivity per gram tissue (%ID/g) and tumour to blood (T/B) ratio. The spatial distribution of radioactivity was defined by autoradiography on frozen tumour sections.

RESULTS

The blood clearance of [(18)F]EF3 was faster than that of [(18)F]FMISO. The clearance of both tracers was slower in tumour tissue compared with other tissues. This results in increasing T/B ratios as a function of time post tracer injection (p.i.). The maximal [(18)F]EF3 tumour uptake, compared to the maximum [(18)F]FMISO uptake, was significantly lower at 2 h p.i. but reached similar levels at 4 h p.i. The tumour uptake for both tracers was independent of the tumour volume for all investigated time points. Both tracers showed heterogeneous intra-tumoural distribution.

CONCLUSIONS

[(18)F]EF3 tumour uptake reached similar levels at 4 h p.i. compared with tumour retention observed after injection of [(18)F]FMISO at 2 h p.i. Although [(18)F]EF3 is a promising non-invasive tracer, it is not superior over [(18)F]FMISO for the visualisation of tumour hypoxia. No significant differences between [(18)F]EF3 and [(18)F]FMISO were observed with regard to the intra-tumoural distribution and the extra-tumoural tissue retention.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

Determination of tumour hypoxia with [18F]EF3 in patients with head and neck tumours: a phase I study to assess the tracer pharmacokinetics, biodistribution and metabolism

PURPOSE

The aim of this study was to assess the pharmacokinetics, biodistribution and metabolism of [(18)F]EF3, a labelled 2-nitroimidazole hypoxia marker, in ten patients with head and neck cancer.

METHODS

[(18)F]EF3 was administered intravenously (group 1, n=5, mean dose+/-SD: 324+/-108 MBq; group 2, n=5, mean dose+/-SD: 1,134+/-138 MBq) to patients (nine male, one female). Blood and urine samples and whole-body PET scans were obtained from 20 s to 4-6 h. Radioactivity was determined in several regions of interest.

RESULTS

No serious adverse event was reported. [(18)F]EF3 concentration in blood exhibited a bi-exponential decline. [(18)F]EF3 was mainly eliminated in the urine. By 7 h 40 min after injection, 53+/-14% of the injected dose was collected in the urine. There was no significant difference between the low- and high-dose groups. A progressive accumulation occurred also in the colon, indicating a hepatobiliary excretion. Except in organs involved in the elimination of [(18)F]EF3, the tumour-to-organ ratio remained close to or below unity in muscle, lungs, heart and brain at various times after injection. In one patient, tumour hypoxia was observed with a tumour-to-blood ratio ranging from 1.4 to 1.9. Last, [(18)F]EF3 remained very stable after injection, with percentage of native tracer above 87% in the serum and 84% in the urine.

CONCLUSION

Administration of [(18)F]EF3 in head and neck cancer patients is feasible and safe. Uptake and retention in tumour was observed, indicating the presence of hypoxia.