Comparison of the biodistribution of two hypoxia markers [18F]FETNIM and [18F]FMISO in an experimental mammary carcinoma

Recent Update on PET/CT Radiotracers for Imaging Cerebral Glioma

Positron emission tomography/computed tomography (PET/CT) has dramatically altered the landscape of noninvasive glioma evaluation, offering complementary insights to those gained through magnetic resonance imaging (MRI). PET/CT scans enable a multifaceted analysis of glioma biology, supporting clinical applications from grading and differential diagnosis to mapping the full extent of tumors and planning subsequent treatments and evaluations. With a broad array of specialized radiotracers, researchers and clinicians can now probe various biological characteristics of gliomas, such as glucose utilization, cellular proliferation, oxygen deficiency, amino acid trafficking, and reactive astrogliosis. This review aims to provide a recent update on the application of versatile PET/CT radiotracers in glioma research and clinical practice.

Advances in PET and MRI imaging of tumor hypoxia

Tumor hypoxia is a complex and evolving phenomenon both in time and space. Molecular imaging allows to approach these variations, but the tracers used have their own limitations. PET imaging has the disadvantage of low resolution and must take into account molecular biodistribution, but has the advantage of high targeting accuracy. The relationship between the signal in MRI imaging and oxygen is complex but hopefully it would lead to the detection of truly oxygen-depleted tissue. Different ways of imaging hypoxia are discussed in this review, with nuclear medicine tracers such as [¹⁸F]-FMISO, [¹⁸F]-FAZA, or [⁶⁴Cu]-ATSM but also with MRI techniques such as perfusion imaging, diffusion MRI or oxygen-enhanced MRI. Hypoxia is a pejorative factor regarding aggressiveness, tumor dissemination and resistance to treatments. Therefore, having accurate tools is particularly important.

A Comparison of PET Tracers in Recurrent High-Grade Gliomas: A Systematic Review

Humans with high-grade gliomas have a poor prognosis, with a mean survival time of just 12-18 months for patients who undergo standard-of-care tumor resection and adjuvant therapy. Currently, surgery and chemoradiotherapy serve as standard treatments for this condition, yet these can be complicated by the tumor location, growth rate and recurrence. Currently, gadolinium-based, contrast-enhanced magnetic resonance imaging (CE-MRI) serves as the predominant imaging modality for recurrent high-grade gliomas, but it faces several drawbacks, including its inability to distinguish tumor recurrence from treatment-related changes and its failure to reveal the entirety of tumor burden (de novo or recurrent) due to limitations inherent to gadolinium contrast. As such, alternative imaging modalities that can address these limitations, including positron emission tomography (PET), are worth pursuing. To this end, the identification of PET-based markers for use in imaging of recurrent high-grade gliomas is paramount. This review will highlight several PET radiotracers that have been implemented in clinical practice and provide a comparison between them to assess the efficacy of these tracers.

Hypoxia Imaging As a Guide for Hypoxia-Modulated and Hypoxia-Activated Therapy

Significance: Oxygen imaging techniques, which can probe the spatiotemporal heterogeneity of tumor oxygenation, could be of significant clinical utility in radiation treatment planning and in evaluating the effectiveness of hypoxia-activated prodrugs. To fulfill these goals, oxygen imaging techniques should be noninvasive, quantitative, and capable of serial imaging, as well as having sufficient temporal resolution to detect the dynamics of tumor oxygenation to distinguish regions of chronic and acute hypoxia. Recent Advances: No current technique meets all these requirements, although all have strengths in certain areas. The current status of positron emission tomography (PET)-based hypoxia imaging, oxygen-enhanced magnetic resonance imaging (MRI), ¹⁹F MRI, and electron paramagnetic resonance (EPR) oximetry are reviewed along with their strengths and weaknesses for planning hypoxia-guided, intensity-modulated radiation therapy and detecting treatment response for hypoxia-targeted prodrugs. Critical Issues: Spatial and temporal resolution emerges as a major concern for these areas along with specificity and quantitative response. Although multiple oxygen imaging techniques have reached the investigative stage, clinical trials to test the therapeutic effectiveness of hypoxia imaging have been limited. Future Directions: Imaging elements of the redox environment besides oxygen by EPR and hyperpolarized MRI may have a significant impact on our understanding of the basic biology of the reactive oxygen species response and may extend treatment possibilities.

Correlation of hypoxia as measured by fluorine-18 fluoroerythronitroimidazole (18F-FETNIM) PET/CT and overall survival in glioma patients

PURPOSE

Hypoxia is important in the biology of glioma in humans. Positron emission tomography/computed tomography (PET/CT) with a hypoxia tracer offers a noninvasive method to differentiate individual tumor biology and potentially modify treatment for patients with malignancies. The purpose of this study was to determine whether hypoxia, as measured by fluorine-18 fluoroerythronitroimidazole (¹⁸F-FETNIM) PET/CT, was associated with tumor grade, overall survival (OS), and immunohistochemical features related to hypoxia, proliferation, angiogenesis, and the invasion of gliomas.

PROCEDURES

Twenty-five patients with gliomas in whom gross maximal resection could be safely attempted were analyzed. All patients underwent ¹⁸F-FETNIM PET/CT studies before surgery. The maximum standardized uptake value (SUVmax) was obtained from the PET images of tumor tissues. Tumor specimens were stereotactically obtained for the immunohistochemical staining of hypoxia-inducible factor-1 alpha (HIF-1α), Ki-67, vascular endothelial growth factor (VEGF), and matrix metalloproteinase 9 (MMP-9).

RESULTS

A correlation between the SUVmax and glioma grade was found (r = 0.881, P < 0.001). The SUVmax was significantly correlated with the expression of HIF-1α, Ki-67, VEGF, and MMP-9 (r = 0.820, 0.747, 0.606, and 0.727; all P < 0.001). Patients with a high SUVmax had significantly worse 3-year OS than those with a low SUVmax (24.4% vs. 82.1%, P = 0.003).

CONCLUSIONS

¹⁸F-FETNIM PET/CT provides an excellent noninvasive assessment of hypoxia in glioma. It can be used to understand the mechanisms by which hypoxia affects the OS of glioma patients.

Hypoxia with 18F-fluoroerythronitroimidazole integrated positron emission tomography and computed tomography (18F-FETNIM PET/CT) in locoregionally advanced head and neck cancer: Hypoxia changes during chemoradiotherapy and impact on clinical outcome

Hypoxia is a well-recognized biological characteristic to therapy resistance and negative prognostic factor in patients with head and neck squamous cell carcinoma (HNSCC). This study aims to investigate the changes of hypoxia measured by F-fluoroerythronitroimidazole (FETNIM) uptake on integrated positron emission tomography and computed tomography (PET/CT) during chemoradiotherapy and its prognostic value of clinical outcome in locoregionally advanced HNSCC.Thirty-two patients with locoregionally advanced HNSCC who received definitive treatment with concurrent chemoradiotherapy underwent FETNIM PET/CT scans before and after 5 weeks of treatment. The intensity of hypoxia using the maximum standardized uptake value (SUVmax) was evaluated both on primary lesion and metastatic lymph node (MLN). The pre-SUVmax and mid-SUVmax were defined as SUVmax on pre- and mid-FETNIM PET/CT. The local control (LC), regional control (RC), distant metastatic-free survival (DMFS), and overall survival (OS) were collected in patient follow-ups.Mid-SUVmax decreased significantly both in the primary tumor (t = 8.083, P < .001) and MLN (t = 6.808, P < .001) compared to pre-SUVmax. With a median follow-up of 54 months, the 5-year LC, RC, DMFS, and OS rates were 55%, 66.7%, 64.7%, and 55%, respectively, for all of the patients. On univariate analysis, patients with high pre-SUVmax in primary tumor had significantly worse LC (56.3% vs 87.5%, P = .046) and OS (43.8% vs 87.5%, P = .023) than other patients. Patients with high mid-SUVmax had significantly worse DMFS (50% vs 84.6%, P = .049) and OS (33.3% vs 73.1%, P = .028) than other patients. The tumor grade and mid-SUVmax were the significant predictors of OS on multivariate analysis.In this study, hypoxia in tumor significantly decreased during chemoradiotherapy. The persistent hypoxia predicted poor OS. The data provided evidence that FETNIM PET/CT could be used dynamically for selecting appropriate patients and optimal timing of hypoxia-adapted therapeutic regimens.

Insights into Hypoxia: Non-invasive Assessment through Imaging Modalities and Its Application in Breast Cancer

Oxygen is crucial to maintain the homeostasis in aerobic cells. Hypoxia is a condition in which cells are deprived of the oxygen supply necessary for their optimum performance. Whereas oxygen deprivation may occur in normal physiological processes, hypoxia is frequently associated with pathological conditions. It has been identified as a stressor in the tumor microenvironment, acting as a key mediator of cancer development. Numerous pathways are activated in hypoxic cells that affect cell signaling and gene regulation to promote the survival of these cells by stimulating angiogenesis, switching cellular metabolism, slowing their growth rate, and preventing apoptosis. The induction of dysregulated metabolism in cancer cells by hypoxia results in aggressive tumor phenotypes that are characterized by rapid progression, treatment resistance, and poor prognosis. A non-invasive assessment of hypoxia-induced metabolic and architectural changes in tumors is advisable to fully improve breast cancer (BC) patient management, by potentially reducing the need for invasive biopsy procedures and evaluating tumor response to treatment. This review provides a comprehensive overview of the molecular changes in breast tumors secondary to hypoxia and the non-invasive imaging alternatives to evaluate oxygen deprivation, with an emphasis on their application in BC and the advantages and limitations of the currently available techniques.

To Explore a Representative Hypoxic Parameter to Predict the Treatment Response and Prognosis Obtained by [18F]FMISO-PET in Patients with Non-small Cell Lung Cancer

PURPOSE

To explore a representative hypoxic parameter to predict the treatment response and prognosis for [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO) positron emission tomography (PET)/X-ray computed tomography (CT) in patients with non-small cell lung cancer (NSCLC).

PROCEDURES

Twenty-nine patients with NSCLC underwent FMISO-PET scans before chemoradiotherapy (CRT). The maximum standard uptake values (SUVmax) in the tumor, normal lung, aortic arch, and vertical ridge muscle were measured, and the tumor-to-lung (T/L) ratios, tumor-to-blood (T/B) ratios, ands tumor-to-muscle (T/M) ratios were calculated and analyzed. Fractional hypoxic volume (FHV) was expressed as percentage of hypoxic volume.

RESULTS

SUVmax, T/L ratio, T/B ratio, and FHV were all significantly different between the responders and the non-responders (SUVmax, 2.07 ± 0.53 vs. 2.61 ± 0.69, P = 0.026; T/L ratio, 3.16 ± 0.85 vs. 4.09 ± 1.46, P = 0.047; T/B ratio, 1.27 ± 0.20 vs. 1.48 ± 0.32, P = 0.042; 38.92 ± 18.47 vs. 52.91 ± 11.29 %, P = 0.020). However, the T/M ratio was not significantly different between the two populations (1.46 ± 0.31 vs. 1.67 ± 0.33, P = 0.098). The correlation ratio between hypoxic parameters and treatment responses ranged from high to low as FHV (r = 0.412); SUVmax (r = 0.400); T/L ratio (r = 0.379), P < 0.05; and T/B ratio (r = 0.355), P = 0.059. According to the area under curve (AUC) to predict response, the hypoxic parameters were arranged as FHV (AUC = 0.748), SUVmax (AUC = 0.731), T/L ratio (AUC = 0.719), and T/B ratio (AUC = 0.705). Binary logistic regression analyses showed that FHV was the only independent predictor for treatment response with the P value of 0.038. In the progression-free survival (PFS) prediction, both FHV and SUVmax reached statistical significance by Kaplan-Meier plots (FHV, 46.99 %, P = 0.010; SUVmax, 1.99, P = 0.046) while only FHV was the independent prognostic factor in multivariate analysis by Cox proportional hazard model (P = 0.037).

CONCLUSION

FHV may be a representative hypoxic parameter to predict the CRT response and PFS in patients with NSCLC.

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.

Molecular mechanisms of hypoxia in cancer

PURPOSE

Hypoxia is a condition of insufficient oxygen to support metabolism which occurs when the vascular supply is interrupted, or when a tumour outgrows its vascular supply. It is a negative prognostic factor due to its association with an aggressive tumour phenotype and therapeutic resistance. This review provides an overview of hypoxia imaging with Positron emission tomography (PET), with an emphasis on the biological relevance, mechanism of action, highlighting advantages, and limitations of the currently available hypoxia radiotracers.

METHODS

A comprehensive PubMed literature search was performed, identifying articles relating to biological significance and measurement of hypoxia, MRI methods, and PET imaging of hypoxia in preclinical and clinical settings, up to December 2016.

RESULTS

A variety of approaches have been explored over the years for detecting and monitoring changes in tumour hypoxia, including regional measurements with oxygen electrodes placed under CT guidance, MRI methods that measure either oxygenation or lactate production consequent to hypoxia, different nuclear medicine approaches that utilise imaging agents the accumulation of which is inversely related to oxygen tension, and optical methods. The advantages and disadvantages of these approaches are reviewed, along with individual strategies for validating different imaging methods. PET is the preferred method for imaging tumour hypoxia due to its high specificity and sensitivity to probe physiological processes in vivo, as well as the ability to provide information about intracellular oxygenation levels.

CONCLUSION

Even though hypoxia could have significant prognostic and predictive value in the clinic, the best method for hypoxia assessment has in our opinion not been realised.

Assessing Tumor Oxygenation for Predicting Outcome in Radiation Oncology: A Review of Studies Correlating Tumor Hypoxic Status and Outcome in the Preclinical and Clinical Settings

Tumor hypoxia is recognized as a limiting factor for the efficacy of radiotherapy, because it enhances tumor radioresistance. It is strongly suggested that assessing tumor oxygenation could help to predict the outcome of cancer patients undergoing radiation therapy. Strategies have also been developed to alleviate tumor hypoxia in order to radiosensitize tumors. In addition, oxygen mapping is critically needed for intensity modulated radiation therapy (IMRT), in which the most hypoxic regions require higher radiation doses and the most oxygenated regions require lower radiation doses. However, the assessment of tumor oxygenation is not yet included in day-to-day clinical practice. This is due to the lack of a method for the quantitative and non-invasive mapping of tumor oxygenation. To fully integrate tumor hypoxia parameters into effective improvements of the individually tailored radiation therapy protocols in cancer patients, methods allowing non-invasively repeated, safe, and robust mapping of changes in tissue oxygenation are required. In this review, non-invasive methods dedicated to assessing tumor oxygenation with the ultimate goal of predicting outcome in radiation oncology are presented, including positron emission tomography used with nitroimidazole tracers, magnetic resonance methods using endogenous contrasts (R₁ and R2*-based methods), and electron paramagnetic resonance oximetry; the goal is to highlight results of studies establishing correlations between tumor hypoxic status and patients’ outcome in the preclinical and clinical settings.

Impact of PET/CT on radiation treatment in patients with esophageal cancer: A systematic review

PURPOSE

With the advances in radiotracers, positron emission tomography/computed tomography (PET/CT) is recognized as a useful adjunct to anatomic imaging with CT, MRI and endoscopic ultrasonography (EUS). The objective of this review was to comprehensively analyze the roles of PET/CT for the radiotherapy of esophageal cancer.

METHODS

In this review, we focused on issues concerning the application of PET/CT in TNM staging, target volume delineation and response to therapy, both for the primary tumor and regional lymph nodes. Furthermore, the following questions were addressed: how does PET/CT guide appropriate treatment protocols, how does it allow accurate tumor delineation and how does it guide prognosis and future treatment decisions.

RESULTS AND CONCLUSION

For the staging of esophageal cancer, PET/CT played a crucial role in exploring distant malignant lymph nodes and metastasis with high sensitivity, specificity and accuracy. PET/CT using different radiotracer provided a serial of thresholding methods based on standardized uptake value (SUV) to assist in auto-contouring the gross tumor volume (GTV). The change in SUV may offer a potential paradigm of personalized treatment to definitive chemoradiotherapy (CRT). In total, PET/CT has sought to further optimize radiotherapy treatment planning for patients with esophageal cancer.

A Comparative Study of Noninvasive Hypoxia Imaging with 18F-Fluoroerythronitroimidazole and 18F-Fluoromisonidazole PET/CT in Patients with Lung Cancer

PURPOSE

This is a clinical study to compare noninvasive hypoxia imaging using 18F-fluoroerythronitroimidazole (18F-FETNIM) and 18F-fluoromisonidazole (18F-FMISO) positron emission tomography/computed tomography (PET/CT) in patients with inoperable stages III-IV lung cancer.

METHODS

A total of forty-two patients with inoperable stages III-IV lung cancer underwent 18F-FETNIM PET/CT (n = 18) and 18F-FMISO PET/CT (n = 24) before chemo/radiation therapy. The standard uptake values (SUVs) of malignant and normal tissues depict 18F-FETNIM PET/CT and 18F-FMISO PET/CT uptake. Tumor-to-blood ratios (T/B) were used to quantify hypoxia.

RESULTS

All patients with lung cancer underwent 18F-FETNIM PET/CT and 18F-FMISO PET/CT successfully. Compared to 18F-FMISO, 18F-FETNIM showed similar uptake in muscle, thyroid, spleen, pancreas, heart, lung and different uptake in blood, liver, and kidney. Significantly higher SUV and T/B ratio with 18F-FMISO (2.56±0.77, 1.98±0.54), as compared to 18F-FETNIM (2.12±0.56, 1.42±0.33) were seen in tumor, P = 0.022, <0.001. For the patients with different histopathological subtypes, no significant difference of SUV (or T/B ratio) was observed both in 18F-FMISO and 18F-FETNIM in tumor. A significantly different SUV (or T/B ratio) was detected between < = 2cm, 2~5cm, and >5cm groups in 18F-FMISO PET/CT, P = 0.015 (or P = 0.029), whereas no difference was detected in 18F-FMISO PET/CT, P = 0.446 (or P = 0.707). Both 18F-FETNIM and 18F-FMISO showed significantly higher SUVs (or T/B ratios) in stage IV than stage III, P = 0.021, 0.013 (or P = 0.032, 0.02).

CONCLUSION

18F-FMISO showed significantly higher uptake than 18F-FETNIM in tumor/non-tumor ratio and might be a better hypoxia tracer in lung cancer.

Molecular imaging of hypoxia in non-small-cell lung cancer

Non-small-cell lung cancer (NSCLC) is the commonest cancer worldwide but survival remains poor with a high risk of relapse, particularly after nonsurgical treatment. Hypoxia is present in a variety of solid tumours, including NSCLC. It is associated with treatment resistance and a poor prognosis, although when recognised may be amenable to different treatment strategies. Thus, noninvasive assessment of intratumoral hypoxia could be used to stratify patients for modification of subsequent treatment to improve tumour control. Molecular imaging approaches targeting hypoxic cells have shown some early success in the clinical setting. This review evaluates the evidence for hypoxia imaging using PET in NSCLC and explores its potential clinical utility.

Positron emission tomography to assess hypoxia and perfusion in lung cancer

In lung cancer, tumor hypoxia is a characteristic feature, which is associated with a poor prognosis and resistance to both radiation therapy and chemotherapy. As the development of tumor hypoxia is associated with decreased perfusion, perfusion measurements provide more insight into the relation between hypoxia and perfusion in malignant tumors. Positron emission tomography (PET) is a highly sensitive nuclear imaging technique that is suited for non-invasive in vivo monitoring of dynamic processes including hypoxia and its associated parameter perfusion. The PET technique enables quantitative assessment of hypoxia and perfusion in tumors. To this end, consecutive PET scans can be performed in one scan session. Using different hypoxia tracers, PET imaging may provide insight into the prognostic significance of hypoxia and perfusion in lung cancer. In addition, PET studies may play an important role in various stages of personalized medicine, as these may help to select patients for specific treatments including radiation therapy, hypoxia modifying therapies, and antiangiogenic strategies. In addition, specific PET tracers can be applied for monitoring therapy. The present review provides an overview of the clinical applications of PET to measure hypoxia and perfusion in lung cancer. Available PET tracers and their characteristics as well as the applications of combined hypoxia and perfusion PET imaging are discussed.

Hypoxia, blood flow and metabolism in squamous-cell carcinoma of the head and neck: correlations between multiple immunohistochemical parameters and PET

Background

The relationship between the uptake of [¹⁸F]fluoroerythronitroimidazole ([¹⁸F]FETNIM), blood flow ([¹⁵O]H₂O) and 2-[¹⁸F]fluoro-2-deoxyglucose ([¹⁸F]FDG) and immunohistochemically determined biomarkers was evaluated in squamous-cell carcinomas of the head and neck (HNSCC).

Methods

[¹⁸F]FETNIM and [¹⁸F]FDG PET were performed on separate days on 15 untreated patients with HNSCC. Hypoxia imaging with [¹⁸F]FETNIM was coupled with measurement of tumor blood flow using [¹⁵O]H₂O. Uptake of [¹⁸F]FETNIM was measured as tumor-to-plasma ratio (T/P) and fractional hypoxic volume (FHV), and that of [¹⁸F]FDG as standardized uptake value (SUV) and the metabolically active tumor volume (TV). Tumor biopsies were cut and stained for GLUT-1, Ki-67, p53, CD68, HIF-1α, VEGF_sc-152, CD31 and apoptosis. The expression of biomarkers was correlated to PET findings and patient outcome.

Results

None of the PET parameters depicting hypoxia and metabolism correlated with the expression of the biomarkers on a continuous scale. When PET parameters were divided into two groups according to median values, a significant association was detected between [¹⁸F]FDG SUV and p53 expression (p =0.029) using median SUV as the cut-off. There was a significant association between tumor volume and the amount of apoptotic cells (p =0.029). The intensity of VEGF stained cells was associated with [¹⁸F]FDG SUV (p =0.036). Patient outcome was associated with tumor macrophage content (p =0.050), but not with the other biomarkers. HIF-1α correlated with GLUT-1 (r_s =0.553, p =0.040) and Ki-67 with HIF-1α (r_s =506, p =0.065). p53 correlated inversely with GLUT-1 (r_s = −618, p =0.019) and apoptosis with Ki-67 (r_s = −638, p =0.014).

Conclusions

A high uptake of [¹⁸F]FDG expressed as SUV is linked to an aggressive HNSCC phenotype: the rate of apoptosis is low and the expressions of p53 and VEGF are high. None of the studied biomarkers correlated with perfusion and hypoxia as evaluated with [¹⁵O]H₂O-PET and [¹⁸F]FETNIM-PET. Increased tumor metabolism evaluated with PET may thus signify an aggressive phenotype, which should be taken into account in the management of HNSCC.

Positron emission tomography radiotracers for imaging hypoxia

Localized hypoxia, the physiological hallmark of many clinical pathologies, is the consequence of acute or chronic ischemia in the affected region or tissue. The versatility, sensitivity, quantitative nature, and increasing availability of positron emission tomography (PET) make it the preclinical and clinical method of choice for functional imaging of tissue hypoxia at the molecular level. The progress and current status of radiotracers for hypoxia-specific PET imaging are reviewed in this article including references mainly focused on radiochemistry and also relevant to molecular imaging of hypoxia in preclinical and clinical studies.

The meaning, measurement and modification of hypoxia in the laboratory and the clinic

Hypoxia was identified as a microenvironmental component of solid tumours over 60 years ago and was immediately recognised as a potential barrier to therapy through the reliance of radiotherapy on oxygen to elicit maximal cytotoxicity. Over the last two decades both clinical and experimental studies have markedly enhanced our understanding of how hypoxia influences cellular behaviour and therapy response. Furthermore, they have confirmed early assumptions that low oxygenation status in tumours is an exploitable target in cancer therapy. Generally such approaches will be more beneficial to patients with hypoxic tumours, necessitating the use of biomarkers that reflect oxygenation status. Tissue biomarkers have shown utility in many studies. Further significant advances have been made in the non-invasive measurement of tumour hypoxia with positron emission tomography, magnetic resonance imaging and other imaging modalities. Here, we describe the complexities of defining and measuring tumour hypoxia and highlight the therapeutic approaches to combat it.

Hypoxia imaging with 18F-fluoroerythronitroimidazole integrated PET/CT and immunohistochemical studies in non-small cell lung cancer

PURPOSE

(18)F-fluoroerythronitroimidazole ((18)F-FETNIM) PET/CT allows a noninvasive assessment of tumor hypoxia. The purpose of this study was to evaluate a noninvasive and simplicity parameter for quantization of (18)F-FETNIM uptake with expectations to predict survival in non-small cell lung cancer surgical patients and investigate the relationship between (18)F-FETNIM uptake and molecular markers related to hypoxia, glucose metabolism, and angiogenesis.

PATIENTS AND METHODS

Thirty-two patients with biopsy-proven non-small cell lung cancer for surgical treatment were enrolled from March 2007 to February 2011. All patients had PET/CT studies with (18)F-FETNIM and subsequently underwent surgery. Twenty-five patients had stage II disease of surgical staging only for statistical analysis. The tumor-to-mediastinum (T/Me) ratio was calculated and correlated with survival and immunohistochemical staining of hypoxia inducible factor 1α (HIF-1α), glucose transporter 1 (GLUT-1), and vascular endothelial growth factor (VEGF).

RESULTS

The actuarial survival was worse for patients showing a high T/Me ratio, the best discriminative cutoff value being 1.9. A statistically significant worse survival was noted in patients having a tumor with a T/Me ratio of 1.9 or greater, compared with patients showing a tumor with a T/Me ratio of less than 1.9, a 3-year survival of 43.8% and 88.9%, respectively (P = 0.034). There was a positive correlation between T/Me ratio and HIF-1α (P = 0.023), GLUT-1 (P = 0.035), and VEGF (P = 0.042).

CONCLUSIONS

T/Me ratio provides a noninvasive parameter for quantization of (18)F-FETNIM uptake on PET/CT. T/Me ratio is correlated with a worse outcome and with the expression of HIF-1α, GLUT-1, and VEGF, all up-regulated under hypoxic conditions.

Hypoxia imaging agents labeled with positron emitters

Imaging hypoxia using positron emission tomography (PET) is of great importance for therapy of cancer. [(18)F]Fluoromisonidazole (FMISO) was the first PET agent for hypoxia imaging, and various radiolabeled nitroimidazole derivatives such as [(18)F]fluoroerythronitroimidazole (FETNIM), [(18)F]1-α-D: -(2-deoxy-2-fluoroarabinofuranosyl)-2-nitroimidazole (FAZA), [(18)F]2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide (EF-5), and [(18)F]fluoroetanidazole (FETA) have been developed successively. To overcome the high cost of cyclotron installation, (68)Ga-labeled nitroimidazole derivatives also have been developed. Another important hypoxia imaging agent is (64)Cu-diacetyl-bis(N (4)-methylthiosemicarbazone) ((64)Cu-ATSM), which can distribute in cancer tissue rapidly due to high lipophilicity. However, its application is limited due to high cost of radionuclide production. Although various hypoxia imaging agents have been reported and tested, hypoxia PET images still have to be improved, because of the low blood flow in hypoxic tissues and resulting low uptake of the agents.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

Development of (99m)Tc-N4-NIM for molecular imaging of tumor hypoxia

The nitro group of 2-nitroimidazole (NIM) enters the tumor cells and is bioreductively activated and fixed in the hypoxia cells. 1,4,8,11-tetraazacyclotetradecane (N4) has shown to be a stable chelator for ^99mTc. The present study was aimed to develop ^99mTc-cyclam-2-nitroimidazole (^99mTc-N4-NIM) for tumor hypoxia imaging. N4-NIM precursor was synthesized by reacting N4-oxalate and 1,3-dibromopropane-NIM, yielded 14% (total synthesis). Cell uptake of ^99mTc-N4-NIM and ^99mTc-N4 was obtained in 13762 rat mammary tumor cells and mesothelioma cells in 6-well plates. Tissue distribution of ^99mTc-N4-NIM was evaluated in breast-tumor-bearing rats at 0.5–4 hrs. Tumor oxygen tension was measured using an oxygen probe. Planar imaging was performed in the tumor-bearing rat and rabbit models. Radiochemical purity of ^99mTc-N4-NIM was >96% by HPLC. Cell uptake of ^99mTc-N4-NIM was higher than ^99mTc-N4 in both cell lines. Biodistribution of ^99mTc-N4-NIM showed increased tumor-to-blood and tumor-to-muscle count density ratios as a function of time. Oxygen tension in tumor tissue was 6–10 mmHg compared to 40–50 mmHg in normal muscle tissue. Planar imaging studies confirmed that the tumors could be visualized clearly with ^99mTc-N4-NIM in animal models. Efficient synthesis of N4-NIM was achieved. ^99mTc-N4-NIM is a novel hypoxic probe and may be useful in evaluating cancer therapy.

Measuring tumor hypoxia with ¹⁸F-FETNIM PET in esophageal squamous cell carcinoma: a pilot clinical study

The purpose of this study was to evaluate hypoxia in esophageal squamous cell carcinoma (SCC) with (18)F-fluoroerythronitroimidazole positron emission tomography/computed tomography ((18)F-FETNIM PET/CT). We determined an imaging threshold for hypoxia, quantified the spatiotemporal variability of hypoxia in untreated tumor, and evaluated the ability of (18)F-FETNIM PET to predict clinical response following concurrent chemoradiotherapy (CCRT). Twenty-eight consecutive patients with inoperable SCC of the esophagus were consecutively accrued between April 2007 and June 2010. The first 10 patients received two pretreatment (18)F-FETNIM PET/CT scans on separate days. The remaining 18 patients only underwent (18)F-FETNIM PET/CT once before CCRT. The ratio of the maximum standardized uptake value (SUV(max) ) of 336 normal tissue regions (i.e. heart, lung, brain, or muscle) to the mean standardized uptake value (SUV(mean)) of the respective patient's spleen was calculated, and the imaging threshold for hypoxia defined as the level of uptake demonstrated by less than 5% of tissue regions. Among the patients with two pretreatment scans, each pair of scans was compared with respect to location and intensity of uptake to assess for baseline spatiotemporal variability. Logistic regression analysis was used to determine whether pretreatment imaging characteristics are predictive of clinical response. The mean and median ratios of the SUV(max) of tissue : SUV(mean) of spleen were nearly identical, and 95% of the ratios fell below 1.3. The mean Dice similarity coefficient for the hypoxic volumes on pretreatment PET scans acquired in the same patient on different days was 0.12 (range, 0.05-0.21). Individuals' tumor SUV(max) and SUV(mean) did not vary significantly, but on average, the geometric centers of hypoxic regions shifted 15 mm (range, 8-20 mm) from the first pretreatment scan to the second. SUV(max) was the imaging characteristic most predictive of treatment response (P= 0.041), with high SUVmax associated with poor clinical response. (18)F-FETNIM PET/CT can depict hypoxia in esophageal SCC. Prior to CCRT, tumor hypoxia demonstrates spatial variability on different days, although overall (18)F-FETNIM uptake remains similar. Baseline SUV(max) may be predictive of treatment response.

Strategies To Assess Hypoxic/HIF-1-Active Cancer Cells for the Development of Innovative Radiation Therapy

Local tumor recurrence and distant tumor metastasis frequently occur after radiation therapy and result in the death of cancer patients. These problems are caused, at least in part, by a tumor-specific oxygen-poor microenvironment, hypoxia. Oxygen-deprivation is known to inhibit the chemical ionization of both intracellular macro-molecules and water, etc., and thus reduce the cytotoxic effects of radiation. Moreover, DNA damage produced by free radicals is known to be more repairable under hypoxia than normoxia. Hypoxia is also known to induce biological tumor radioresistance through the activation of a transcription factor, hypoxia-inducible factor 1 (HIF-1). Several potential strategies have been devised in radiation therapy to overcome these problems; however, they have not yet achieved a complete remission. It is essential to reveal the intratumoral localization and dynamics of hypoxic/HIF-1-active tumor cells during tumor growth and after radiation therapy, then exploit the information to develop innovative therapeutic strategies, and finally damage radioresistant cells. In this review, we overview problems caused by hypoxia/HIF-1-active cells in radiation therapy for cancer and introduce strategies to assess intratumoral hypoxia/HIF-1 activity.

Comparison of 18F-Fluoroerythronitroimidazole and 18F-fluorodeoxyglucose positron emission tomography and prognostic value in locally advanced non-small-cell lung cancer

INTRODUCTION

The aim of this study was to compare glucose metabolism and hypoxia using 18F- fluorodeoxyglucose (18F-FDG) and 18F-fluoroerythronitroimidazole (18F-FETNIM) positron emission tomography (PET) and investigate their prognostic role on survival in patients with locally advanced non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Twenty-six patients with NSCLC were imaged with 18F-FETNIM PET/computed tomography (CT), and 11 cases also with 18F-FDG PET/CT imaging among those with significant 18F-FETNIM uptake, a few days before any chemo/adiation therapy. The maximum standardized uptake value (SUVmax) was used to depict 18F-FDG uptake, and hypoxic volume (HV) and tumor:blood ratio (T/Bmax) were used to quantify hypoxia. Overall survival (OS) after treatment was selected as the endpoint of the study.

RESULTS

Twenty-two patients (84.6%) had significant 18F-FETNIM uptake in the primary tumor. The correlations between the overall tumor SUVmax of 18F-FDG and HV, T/Bmax ratio of 18F-FENTIM in 11 patients were small and without significant difference. In univariate analyses, log-rank tests were used to compare Kaplan-Meier survival curves. 18F-FETNIM T/Bmax ratio and HV were strong predictors for OS, and 18F-FDG uptake of the primary lesions did not have a significant relationship with survival. In multivariate survival analysis, only 18F-FETNIM T/Bmax ratio was found to be an independent prognostic factor.

CONCLUSION

Imaging using both 18F-FETNIM and 18F-FDG appears to be beneficial in the evaluation of solid tumors. 18F-FETNIM imaging provides us with a valuable method to detect tumor hypoxia and predict OS. These preliminary results warrant validation in larger trials.

Synthesis and evaluation of two novel 2-nitroimidazole derivatives as potential PET radioligands for tumor imaging

INTRODUCTION

Nitroimidazole (azomycin) derivatives labeled with radioisotopes have been developed as cancer imaging and radiotherapeutic agents based on the oncological hypoxic mechanism. By attaching nitroimidazole core with different functional groups, we synthesized new nitroimidazole derivatives and evaluated their potentiality as tumor imaging agents.

METHODS

Starting with commercially available 2-nitroimidazole, 2-fluoro-N-(2-(2-nitro-1H-imidazol-1-yl)ethyl)acetamide (NEFA, [(19)F]7) and 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl 2-fluoroacetate (NEFT, [(19)F]8), as well as radiolabeling precursors, the bromo-substituted analogs were quickly synthesized through a three-step synthetic pathway. The precursors were radiolabeled with [(18)F]F(-)/18-crown-6/KHCO(3) in dimethyl sulfoxide at 90°C for 10 min followed by purification with an Oasis HLB cartridge. Biodistribution studies were carried out in EMT-6 tumor-bearing mice. The uptake (%ID/g) in tumors and normal tissues were measured at 30 min postinjection. Liquid chromatography-electrospray ionization mass spectrometry (LC/MS) was used to distinguish metabolites from parent drugs in urine and plasma of rat injected with "cold" NEFA ([(19)F]7) and NEFT ([(19)F]8).

RESULTS

Two radiotracers, [(18)F]NEFA ([(18)F]7) and [(18)F]NEFT ([(18)F]8), were prepared with average yields of 6%-7% and 9%-10% (not decay corrected). Radiochemical purity for both tracers was >95% as determined by HPLC. Biodistribution studies in EMT-6 tumor-bearing mice indicated that the tumor to blood and tumor to liver ratios of both [(18)F]7 (0.96, 0.61) and [(18)F]8 (0.98, 1.10) at 30 min were higher than those observed for [(18)F]FMISO (1) (0.91, 0.59), a well-investigated azomycin-type hypoxia radiotracer. Liquid chromatography-electrospray ionization mass spectrometry analysis demonstrated that fluoroacetate was the main in vivo metabolite for both NEFA ([(19)F]7) and NEFT ([(19)F]8).

CONCLUSIONS

In this research, two new fluorine-18 labeled 2-nitroimidazole derivatives, [(18)F]7 and [(18)F]8, both of which containing in vivo hydrolyzable group, were successfully prepared. Further biological evaluations are warranted to investigate their potential as PET radioligands for imaging tumor.

Review of positron emission tomography tracers for imaging of tumor hypoxia

Hypoxia plays a critical role in tumor development and aggressiveness and is an important prognostic factor for resistance to antineoplastic treatments; therefore, it is required to measure the hypoxic level of tumor for a favorable outcome. The pretherapy information on the oxygenation status of a tumor microenvironment should also have implications for treatment selection. A diffuse distribution of hypoxia in a tumor might suggest a benefit from a systemic approach, such as a hypoxic cell cytotoxin, tirapazamine, or antigrowth factor drugs to combat the limitations of hypoxia. Alternatively, a more focal hypoxia might benefit from a local/regional approach, such as intensity-modulated radiation therapy-based radiation dose escalation to the hypoxic subvolume. This review anticipates that (18)F-FMISO ((18)F-fluoromisonodazole) and (64)Cu-ATSM-positron emission tomography (PET) will prove useful for selecting individual patients for the most appropriate treatment. The advent of new radiotracers has allowed noninvasive assessment of hypoxia, with the most extensively investigated and validated PET radiotracer for hypoxia to date being (18)F-FMISO. This article discusses the relevance and biology of hypoxia in cells and organ systems and reviews the laboratory and clinical applications of (18)F-FMISO and other agents in oncology.

Fast growth associated with aberrant vasculature and hypoxia in fibroblast growth factor 8b (FGF8b) over-expressing PC-3 prostate tumour xenografts

BACKGROUND

Prostate tumours are commonly poorly oxygenated which is associated with tumour progression and development of resistance to chemotherapeutic drugs and radiotherapy. Fibroblast growth factor 8b (FGF8b) is a mitogenic and angiogenic factor, which is expressed at an increased level in human prostate tumours and is associated with a poor prognosis. We studied the effect of FGF8b on tumour oxygenation and growth parameters in xenografts in comparison with vascular endothelial growth factor (VEGF)-expressing xenografts, representing another fast growing and angiogenic tumour model.

METHODS

Subcutaneous tumours of PC-3 cells transfected with FGF8b, VEGF or empty (mock) vectors were produced and studied for vascularity, cell proliferation, glucose metabolism and oxygenation. Tumours were evaluated by immunohistochemistry (IHC), flow cytometry, use of radiolabelled markers of energy metabolism ([18F]FDG) and hypoxia ([18F]EF5), and intratumoral polarographic measurements of pO2.

RESULTS

Both FGF8b and VEGF tumours grew rapidly in nude mice and showed highly vascularised morphology. Perfusion studies, pO2 measurements, [18F]EF5 and [18F]FDG uptake as well as IHC staining for glucose transport protein (GLUT1) and hypoxia inducible factor (HIF) 1 showed that VEGF xenografts were well-perfused and oxygenised, as expected, whereas FGF8b tumours were as hypoxic as mock tumours. These results suggest that FGF8b-induced tumour capillaries are defective. Nevertheless, the growth rate of hypoxic FGF8b tumours was highly increased, as that of well-oxygenised VEGF tumours, when compared with hypoxic mock tumour controls.

CONCLUSION

FGF8b is able to induce fast growth in strongly hypoxic tumour microenvironment whereas VEGF-stimulated growth advantage is associated with improved perfusion and oxygenation of prostate tumour xenografts.

The radiation response of cells from 9L gliosarcoma tumours is correlated with [F18]-EF5 uptake

PURPOSE

Tumour hypoxia affects cancer biology and therapy-resistance in both animals and humans. The purpose of this study was to determine whether EF5 ([2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide]) binding and/or radioactive drug uptake correlated with single-dose radiation response in 9L gliosarcoma tumours.

MATERIALS AND METHODS

Twenty-two 9L tumours were grown in male Fischer rats. Rats were administered low specific activity (18)F-EF5 and their tumours irradiated and assessed for cell survival and hypoxia. Hypoxia assays included EF5 binding measured by antibodies against bound-drug adducts and gamma counts of (18)F-EF5 tumour uptake compared with uptake by normal muscle and blood. These assays were compared with cellular radiation response (in vivo to in vitro assay). In six cases, uptake of tumour versus muscle was also assayed using images from a PET (positron emission tomography) camera (PENN G-PET).

RESULTS

The intertumoural variation in radiation response of 9L tumour-cells was significantly correlated with uptake of (18)F-labelled EF5 (i.e., including both bound and non-bound drug) using either tumour to muscle or tumour to blood gamma count ratios. In the tumours where imaging was performed, there was a significant correlation between the image analysis and gamma count analysis. Intertumoural variation in cellular radiation response of the same 22 tumours was also correlated with mean flow cytometry signal due to EF5 binding.

CONCLUSION

To our knowledge, this is the first animal model/drug combination demonstrating a correlation of radioresponse for tumour-cells from individual tumours with drug metabolism using either immunohistochemical or non-invasive techniques.

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.

Imaging and cancer: a review

Multiple biomedical imaging techniques are used in all phases of cancer management. Imaging forms an essential part of cancer clinical protocols and is able to furnish morphological, structural, metabolic and functional information. Integration with other diagnostic tools such as in vitro tissue and fluids analysis assists in clinical decision-making. Hybrid imaging techniques are able to supply complementary information for improved staging and therapy planning. Image guided and targeted minimally invasive therapy has the promise to improve outcome and reduce collateral effects. Early detection of cancer through screening based on imaging is probably the major contributor to a reduction in mortality for certain cancers. Targeted imaging of receptors, gene therapy expression and cancer stem cells are research activities that will translate into clinical use in the next decade. Technological developments will increase imaging speed to match that of physiological processes. Targeted imaging and therapeutic agents will be developed in tandem through close collaboration between academia and biotechnology, information technology and pharmaceutical industries.