In vivo evaluation of [18F]fluoroetanidazole as a new marker for imaging tumour hypoxia with positron emission tomography

Initial evaluation of dynamic human imaging using 18F-FRP170 as a new PET tracer for imaging hypoxia

18F-FRP170, 1-(2-fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole, is a new hypoxia imaging agent for positron emission tomography. This compound was synthesized by 18F-labeling of RP170, which was developed as a new hydrophilic 2-nitroimidazole analog. In the present study, we analyzed dynamic whole-body imaging in healthy volunteers and dynamic tumor imaging in three patients with lung cancer.

METHODS

Four healthy male volunteers and three lung cancer patients were enrolled in this study. Volunteers underwent dynamic whole-body scans just after injection of 18F-FRP170 for about 90 min, while the lung cancer patients underwent dynamic tumor imaging for about 60 or 120 min. Data are expressed as standardized uptake values (SUV). Regions of interest were placed over images of each organ or tumor to generate time-SUV curves.

RESULTS

The series of dynamic whole-body scans showed rapid elimination of 18F-FRP170 from the kidneys following elimination from the liver. Very low physiological uptake was observed above the diaphragm. 18F-FRP170 uptake in the lung cancer lesion could be visualized clearly from early after injection. The changes of tumor SUV, tumor/blood ratio, or tumor/muscle ratio about 30 min after injection or later were small.

CONCLUSIONS

Dynamic imaging using 18F-FRP170 demonstrated rapid elimination from the kidney, suggesting the high hydrophilicity of this imaging agent. The background activity above the diaphragm was very low, and patients with lung cancer showed clear tumor uptake of 18F-FRP170 early after injection.

Metabolic functional imaging for tumor radiosensitivity monitoring

Assessing tumor radiosensitivity before and during radiation therapy can be a crucial element in decision-making with regard to treatment. However, no known non-invasive test is available at present, which allows for a reliable evaluation of the radiosensitivity of a tissue subjected to radiotherapy. Among tests being evaluated, positron emission tomography (PET) is considered to be a promising method. The purpose of this review is to identify the tests and research paths that have recently been explored for the evaluation of tumor response to treatment after isotopic labeling revealed by nuclear imaging. The majority of the explored methodologies are based on the indirect evaluation of the radiosensitivity by cell proliferation or apoptosis, tissue oxygenation or hypoxia, intrinsic radiosensitivity of clonogenic cells, tumor metabolism and angiogenesis. The development of such methods would permit the adoption of a therapeutic regimen with respect to a given radiosensitivity of a tissue. Therefore, a given therapeutic strategy could be readjusted (by associating, for instance, a radiosensitizer of hypoxic cells) or even modified if it proved to be inadequate or when it presents an unfavorable cost-effectiveness ratio. We present here a critical review of the radiotracers revealed by nuclear imaging that are developed for radiosensitivity monitoring.

Advances in methods for assessing tumor hypoxia in vivo: implications for treatment planning

Tumor hypoxia and its downstream effects have remained of considerable interest for decades due to its negative impact on response to various cancer therapies and promotion of metastasis. Diagnosing hypoxia non-invasively can provide a significant advancement in cancer treatment and is the dire necessity for implementing specific targeted therapies now emerging to treat different aspects of cancer. A variety of techniques are being proposed to do so. However, none of them has yet been established in the clinical arena. This review summarizes the methods currently available to assess tumor hypoxia in vivo and their respective advantages and shortcomings. It also points out the impedances that need to be overcome to establish any particular method in the clinic, along with a broad overview of requirements for further advancement in this sphere of cancer research.

Tumour angiogenesis: the relevance to surgeons

INTRODUCTION

Angiogenesis is the process of new blood vessel formation from pre-existing vessels, and is a key feature of malignant tumours. Surgeons involved in the management of patients with malignant disease need to be aware of angiogenic mechanisms and their surgical implications.

PATIENTS AND METHODS

A literature search was used to review recent developments in our understanding of the factors and processes involved in tumour angiogenesis, and how these will impact on the care of patients with malignant disease encountered by surgeons.

RESULTS

Angiogenesis is fundamental to all stages of the malignant process, and involves a complex interaction between mediators secreted by tumour cells and host cells. Intense investigation continues into therapies targeting components of the angiogenic cascade. Imaging modalities capable of measuring the angiogenic activity of a tumour are also being studied in order to predict prognosis and select suitable patients for anti-angiogenic therapy.

CONCLUSIONS

As the use of these anti-angiogenic therapies becomes more wide-spread, they may have implications on the healing rates of cutaneous wounds and intracorporeal anastomoses.

Hypoxia: importance in tumor biology, noninvasive measurement by imaging, and value of its measurement in the management of cancer therapy

PURPOSE

The Cancer Imaging Program of the National Cancer Institute convened a workshop to assess the current status of hypoxia imaging, to assess what is known about the biology of hypoxia as it relates to cancer and cancer therapy, and to define clinical scenarios in which in vivo hypoxia imaging could prove valuable.

RESULTS

Hypoxia, or low oxygenation, has emerged as an important factor in tumor biology and response to cancer treatment. It has been correlated with angiogenesis, tumor aggressiveness, local recurrence, and metastasis, and it appears to be a prognostic factor for several cancers, including those of the cervix, head and neck, prostate, pancreas, and brain. The relationship between tumor oxygenation and response to radiation therapy has been well established, but hypoxia also affects and is affected by some chemotherapeutic agents. Although hypoxia is an important aspect of tumor physiology and response to treatment, the lack of simple and efficient methods to measure and image oxygenation hampers further understanding and limits their prognostic usefulness. There is no gold standard for measuring hypoxia; Eppendorf measurement of pO(2) has been used, but this method is invasive. Recent studies have focused on molecular markers of hypoxia, such as hypoxia inducible factor 1 (HIF-1) and carbonic anhydrase isozyme IX (CA-IX), and on developing noninvasive imaging techniques.

CONCLUSIONS

This workshop yielded recommendations on using hypoxia measurement to identify patients who would respond best to radiation therapy, which would improve treatment planning. This represents a narrow focus, as hypoxia measurement might also prove useful in drug development and in increasing our understanding of tumor biology.

Hypoxia in head and neck cancer

A high level of hypoxia in solid tumours is an adverse prognostic factor for the poor outcome of cancer patients following treatment. This review describes the status of research into finding a practical method for measuring hypoxia and treating hypoxic tumours. The application of such methodology would enable the selection of head and neck cancer treatment based on an individual's tumour oxygenation status. This individualization would include the selection not only of surgery or radiotherapy, but also of novel hypoxia-modification strategies.

Minimally invasive pharmacokinetic and pharmacodynamic technologies in hypothesis-testing clinical trials of innovative therapies

Clinical trials of new cancer drugs should ideally include measurements of parameters such as molecular target expression, pharmacokinetic (PK) behavior, and pharmacodynamic (PD) endpoints that can be linked to measures of clinical effect. Appropriate PK/PD biomarkers facilitate proof-of-concept demonstrations for target modulation; enhance the rational selection of an optimal drug dose and schedule; aid decision-making, such as whether to continue or close a drug development project; and may explain or predict clinical outcomes. In addition, measurement of PK/PD biomarkers can minimize uncertainty associated with predicting drug safety and efficacy, reduce the high levels of drug attrition during development, accelerate drug approval, and decrease the overall costs of drug development. However, there are many challenges in the development and implementation of biomarkers that probably explain their disappointingly low implementation in phase I trials. The Pharmacodynamic/Pharmacokinetic Technologies Advisory committee of Cancer Research UK has found that submissions for phase I trials of new cancer drugs in the United Kingdom often lack detailed information about PK and/or PD endpoints, which leads to suboptimal information being obtained in those trials or to delays in starting the trials while PK/PD methods are developed and validated. Minimally invasive PK/PD technologies have logistic and ethical advantages over more invasive technologies. Here we review these technologies, emphasizing magnetic resonance spectroscopy and positron emission tomography, which provide detailed functional and metabolic information. Assays that measure effects of drugs on important biologic pathways and processes are likely to be more cost-effective than those that measure specific molecular targets. Development, validation, and implementation of minimally invasive PK/PD methods are encouraged.

Évaluation de la radiosensibilité tumorale par l'imagerie fonctionnelle et métabolique : de la recherche à l'application clinique. Revue de la littérature

During the last half of century considerable research on radiosensitivity biomarkers has been published. However, to date there is no non-invasive marker of cellular radiosensitivity identified for clinical routinely use. In this review, the main functional and metabolic imaging isotopic techniques for tumor radiosensitivity that have been explored over the last years are being described. This indirect evaluation fall into 3 topics associated with tumor proliferation rate or apoptosis, tumor hypoxic fraction, neoangiogenesis and the intrinsic radiosensitivity of clonogenic tumor cells. The final objective of the radiosensitivity monitoring during radiotherapy would be to adapt treatment strategy for overcoming the identified radioresistance mechanism such as hypoxia by the addition of radiosensitisers for example. This would allow better tumor control rather than continue inefficient and costly treatment delivery, which in addition could compromise outcome.

The progress and promise of molecular imaging probes in oncologic drug development

As addressed by the recent Food and Drug Administration Critical Path Initiative, tools are urgently needed to increase the speed, efficiency, and cost-effectiveness of drug development for cancer and other diseases. Molecular imaging probes developed based on recent scientific advances have great potential as oncologic drug development tools. Basic science studies using molecular imaging probes can help to identify and characterize disease-specific targets for oncologic drug therapy. Imaging end points, based on these disease-specific biomarkers, hold great promise to better define, stratify, and enrich study groups and to provide direct biological measures of response. Imaging-based biomarkers also have promise for speeding drug evaluation by supplementing or replacing preclinical and clinical pharmacokinetic and pharmacodynamic evaluations, including target interaction and modulation. Such analyses may be particularly valuable in early comparative studies among candidates designed to interact with the same molecular target. Finally, as response biomarkers, imaging end points that characterize tumor vitality, growth, or apoptosis can also serve as early surrogates of therapy success. This article outlines the scientific basis of oncology imaging probes and presents examples of probes that could facilitate progress. The current regulatory opportunities for new and existing probe development and testing are also reviewed, with a focus on recent Food and Drug Administration guidance to facilitate early clinical development of promising probes.

Distribution of sigma receptors in EMT-6 cells: preliminary biological evaluation of PB167 and potential for in-vivo PET

Sigma(1) and sigma(2) receptors have been detected in many tissues and are highly expressed in several tumour cell lines from various tissues. The high level of expression observed for sigma receptors and their involvement in cell proliferation and apoptosis has led to the development of several sigma ligands in order to obtain a molecular probe for in-vivo diagnostic imaging techniques such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT). The EMT-6 cells implanted in mice were a good model for evaluating the proliferation of solid tumours by in-vivo PET. Moreover, we developed the sigma ligand PB167, a cyclohexylpiperazine derivative, previously evaluated for sigma(2) receptor affinity and activity in standard protocols. The related results encouraged us to verify if this compound could be developed as a radiotracer for in-vivo PET in order to visualize sigma(2) receptors expressed in EMT-6 cells when implanted in mice. This perspective was thought to be favourable because PB167 bears a methoxy substituent on the tetraline nucleus, an easy point for (11)C labelling. The aims of this preliminary study were both to assess the relative distribution of sigma(1) and sigma(2) receptors in EMT-6 cells and to verify if PB167 could be developed as a sigma(2) radiotracer for in-vivo PET. The results showed that both sigma(1) and sigma(2) receptors were overexpressed in EMT-6 cells and that the ligand PB167 can be positively considered for radiosynthesis preparation in order to suitably visualize sigma(2) receptors by the in-vivo PET technique and correlate their presence to tumour proliferation.

NanoPET imaging of [(18)F]fluoromisonidazole uptake in experimental mouse tumours

PURPOSE

The purpose of this study was to assess the potential and utility of ultra-high-resolution hypoxia imaging in various murine tumour models using the established hypoxia PET tracer [(18)F]fluoromisonidazole ([(18)F]FMISO).

METHODS

[(18)F]FMISO PET imaging was performed with the dedicated small-animal PET scanner NanoPET (Oxford Positron Systems) and ten different human tumour xenografts in nude mice as well as B16 melanoma tumours in syngeneic Balb/c mice. For comparison, [(18)F]fluorodeoxyglucose ([(18)F]FDG) PET scans were also performed in the mice bearing human tumour xenografts.

RESULTS

In 10 out of 11 experimental tumour models, [(18)F]FMISO PET imaging allowed clear-cut visualisation of the tumours. Inter- and intratumoural heterogeneity of tracer uptake was evident. In addition to average TMRR (tumour-to-muscle retention ratio including all voxels in a volume of interest (VOI)), the parameters TMRR(75%) and TMRR(5) (tumour-to-muscle retention ratio including voxels of 75% or more of the maximum radioactivity in a VOI and the five hottest pixels, respectively) also served as measures for quantifying the heterogeneous [(18)F]FMISO uptake in the tumours. The variability observed in [(18)F]FMISO uptake was related neither to tumour size nor to the injected mass of the radiotracer. The pattern of normoxic and hypoxic regions within the human tumour xenografts, however, correlated with glucose metabolism as revealed by comparison of [(18)F]FDG and [(18)F]FMISO images.

CONCLUSION

This study demonstrates the feasibility and utility of [(18)F]FMISO for imaging murine tumour models using NanoPET.

Monitoring multiple angiogenesis-related molecules in the blood of cancer patients shows a correlation between VEGF-A and MMP-9 levels before treatment and divergent changes after surgical vs. conservative therapy

Anti-angiogenic therapies are currently in cancer clinical trials, but to date there are no established tests for evaluating the angiogenic status of a patient. We measured 11 circulating angiogenesis-associated molecules in cancer patients before and after local treatment. The purpose of our study was to screen for possible relationships among the different molecules and between individual molecules and tumor burden. We measured VEGF-A, PlGF, SCF, MMP-9, EDB+ -fibronectin, sVEGFR-2, sVEGFR-1, salphaVbeta3, sTie-2, IL-8 and CRP in the blood of 22 healthy volunteers, 17 early breast, 17 early colorectal, and 8 advanced sarcoma/melanoma cancer patients. Breast cancer patients had elevated levels of VEGF-A and sTie-2, colorectal cancer patients of VEGF-A, MMP-9, sTie-2, IL-8 and CRP, and melanoma/sarcoma patients of sVEGFR-1. salphaVbeta3 was decreased in colorectal cancer patients. A correlation between VEGF-A and MMP-9 was found. After tumor removal, MMP-9 and salphaVbeta3 significantly decreased in breast and CRP in colorectal cancer, whereas sVEGFR-1 increased in colorectal cancer patients. In sarcoma/melanoma patients treated regionally with TNF and chemotherapy we observed a rise in VEGF-A, SCF, VEGFR-2, MMP-9, Tie-2 and CRP, a correlation between CRP and IL-8, and a decreased in sVEGFR-1 levels. In conclusion, among all factors measured, only VEGF-A and MMP-9 consistently correlated to each other, elevated CRP levels were associated with tumor burden, whereas sVEGF-R1 increased after tumor removal in colorectal cancer. Treatment with chemotherapy and TNF induced changes consistent with an angiogenic switch. These results warrant a prospective study to compare the effect of surgical tumor removal vs. chemotherapy on some of these markers and to evaluate their prognostic/predictive value.

Current imaging paradigms in radiation oncology

The technological revolution in imaging during recent decades has transformed the way image-guided radiation therapy is performed. Anatomical imaging (plain radiography, computed tomography, magnetic resonance imaging) greatly improved the accuracy of delineating target structures and has formed the foundation of 3D-based radiation treatment. However, the treatment planning paradigm in radiation oncology is beginning to shift toward a more biological and molecular approach as advances in biochemistry, molecular biology, and technology have made functional imaging (positron emission tomography, nuclear magnetic resonance spectroscopy, optical imaging) of physiological processes in tumors more feasible and practical. This review provides an overview of the role of current imaging strategies in radiation oncology, with a focus on functional imaging modalities, as it relates to staging and molecular profiling (cellular proliferation, apoptosis, angiogenesis, hypoxia, receptor status) of tumors, defining radiation target volumes, and assessing therapeutic response. In addition, obstacles such as imaging-pathological validation, optimal timing of post-therapy scans, spatial and temporal evolution of tumors, and lack of clinical outcome studies are discussed that must be overcome before a new era of functional imaging-guided therapy becomes a clinical reality.

Progrès en imagerie des tumeurs de la sphère cervico-maxillofaciale : la tomographie par émission de positons (TEP-scan)

Positron emission tomography (PET-scan) is a well-established imaging modality in oncology. Using FDG, PET has also a wide range of applications in head and neck tumors for diagnosis, staging, monitoring of response to therapy, and detection of relapse. After a short technical introduction, the current indications of PET-FDG in head and neck tumors are reviewed. Present and future developments of PET are twofold: the use of new tracers for protein synthesis, cellular proliferation or detection of hypoxia etc., and the introduction of metabolic imaging as a adjunct to CT and MRI to determine target-volumes in radiation treatment planning. However, it has to be emphasized that a thorough clinical validation of the methods used is mandatory before their implementation in routine practice.

Hypoxia in head and neck cancer: How much, how important?

BACKGROUND

Hypoxia develops in tumors because of a less ordered, often chaotic, and leaky vascular supply compared with that in normal tissues. In preclinical models, hypoxia has been shown to be associated with treatment resistance and increased malignant potential. In the clinic, several reports show the presence and extent of tumor hypoxia as a negative prognostic indicator. This article reviews the biology and importance of hypoxia in head and neck cancer.

METHODS

A review of literature was carried out and combined with our own experience on hypoxia measurements using exogenous and endogenous markers.

RESULTS

Hypoxia can increase resistance to radiation and cytotoxic drugs and lead to malignant progression, affecting all treatment modalities, including surgery. Hypoxia measurements using electrodes, exogenous bioreductive markers, or endogenous markers show the presence of hypoxia in most head and neck cancers, and correlations with outcome, although limited, consistently indicate hypoxia as an important negative factor. Each hypoxia measurement method has disadvantages, and no "gold standard" yet exists. Distinctions among chronic, acute, and intermediate hypoxia need to be made, because their biology and relevance to treatment resistance differ. Reliable methods for measuring these different forms in the clinic are still lacking. Several methods to overcome hypoxia have been tested clinically, with radiosensitizers (nimorazole), hypoxic cytotoxins (tirapazamine), and carbogen showing some success. New treatments such as hypoxia-mediated gene therapy await proper clinical testing.

CONCLUSIONS

The hypoxia problem in head and neck cancer needs to be addressed if improvements in current treatments are to be made. Increased knowledge of the molecular biology of intermediate, severe, and intermittent hypoxia is needed to assess their relevance and indicate strategies for overcoming their negative influence.

Radiobiological rationale and patient selection for high-LET radiation in cancer therapy

The rationale for introducing ion beams in cancer therapy is the high level of physical selectivity that can be achieved with ions, equal or even better than with proton beams or modern photon techniques, as well as the potential advantage of high-LET radiations for some tumour types and sites. The radiobiological arguments for high-LET radiation in cancer therapy are reviewed: reduction of OER in the case of hypoxic and poorly-reoxygenating tumours, and the lesser importance of repair phenomena which are a problem in controlling repair-proficient photon-resistant tumours. Fast neutrons were the first type of high-LET radiation used clinically, and were often applied under suboptimal technical conditions. Nevertheless, useful clinical information was derived from the neutron experience. A greater benefit from neutrons than from conventional radiotherapy was found for several tumour sites. The present discussion is limited to the results for salivary gland tumours and prostatic adenocarcinoma. Based on the fast neutron experience, radiobiological arguments, and the added benefit of excellent physical selectivity of ion beams, the potential clinical indications for high-LET ions are discussed: hypoxic, slowly growing and well-differentiated photon-resistant tumours. One of the main remaining issues is the selection of individual patients for high- or low-LET radiation. Since the physical selectivity of ions now matches that obtained with other techniques, the selection of patients will be based only on the radiobiological characteristics of the tumour.

Update on PET radiopharmaceuticals: life beyond fluorodeoxyglucose

Twenty-eight years after its inception, 2-[18F]FDG- is still the most widely used radiopharmaceutical for PET studies, but numerous more specific radiotracers have been developed and applied in neuroscience and oncology. The advances in radiotracer chemistry, especially the nucleophilic substitution reaction, have played the pivotal role in synthesizing various no-carrier-added 18F-labeled radiotracers for PET studies of various receptor systems. This article lists some of the radiotracers that are available for PET studies in neuroscience and oncology. The prospects for developing other new radiotracers for imaging other organ diseases also seem to be promising.

The uptake of 3?-deoxy-3?-[18F]fluorothymidine into L5178Y tumours in vivo is dependent on thymidine kinase 1 protein levels

PURPOSE

The aim of this study was to investigate the role of thymidine kinase 1 (TK1) protein in 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) positron emission tomography (PET) studies.

METHODS

We investigated the in vivo kinetics of [18F]FLT in TK1+/- and TK1-/- L5178Y mouse lymphoma tumours that express different levels of TK1 protein.

RESULTS

[18F]FLT-derived radioactivity, measured by a dedicated small animal PET scanner, increased within the tumours over 60 min. The area under the normalised tumour time-activity curve were significantly higher for the TK1+/- compared with the -/- variant (0.89+/-0.02 vs 0.79+/-0.03 MBq ml(-1) min, P=0.043; n=5 for each tumour type). Ex vivo gamma counting of tissues excised at 60 min p.i. (n=8) also revealed significantly higher tumour [18F]FLT uptake for the TK1+/- variant (6.2+/-0.6 vs 4.6+/-0.4%ID g(-1), P=0.018). The observed differences between the cell lines with respect to [18F]FLT uptake were in keeping with a 48% higher TK1 protein in the TK1+/- tumours versus the -/- variant (P=0.043). On average, there were no differences in ATP levels between the two tumour variants (P=1.00). A positive correlation between [18F]FLT accumulation and TK1 protein levels (r=0.68, P=0.046) was seen. Normalisation of the data for ATP content further improved the correlation (r=0.86, P=0.003).

CONCLUSION

This study shows that in vivo [18F]FLT kinetics depend on TK1 protein expression. ATP may be important in realising this effect. Thus, [18F]FLT-PET has the potential to yield specific information on tumour proliferation in diagnostic imaging and therapy monitoring.