Tumor receptor imaging

Tumor receptors play an important role in carcinogenesis and tumor growth and have been some of the earliest targets for tumor-specific therapy, for example, the estrogen receptor in breast cancer. Knowledge of receptor expression is key for therapy directed at tumor receptors and traditionally has been obtained by assay of biopsy material. Tumor receptor imaging offers complementary information that includes evaluation of the entire tumor burden and characterization of the heterogeneity of tumor receptor expression. The nature of the ligand-receptor interaction poses a challenge for imaging--notably, the requirement for a low molecular concentration of the imaging probe to avoid saturating the receptor and increasing the background because of nonspecific uptake. For this reason, much of the work to date in tumor receptor imaging has been done with radionuclide probes. In this overview of tumor receptor imaging, aspects of receptor biochemistry and biology that underlie tumor receptor imaging are reviewed, with the estrogen-estrogen receptor system in breast cancer as an illustrative example. Examples of progress in radionuclide receptor imaging for 3 receptor systems--steroid receptors, somatostatin receptors, and growth factor receptors-are highlighted, and recent investigations of receptor imaging with other molecular imaging modalities are reviewed.

Site-specific labeling of annexin V with F-18 for apoptosis imaging

Annexin V is useful in detecting apoptotic cells by binding to phosphatidylserine (PS) that is exposed on the outer surface of the cell membrane during apoptosis. In this study, we examined the labeling of annexin V-128, a mutated form of annexin V that has a single cysteine residue at the NH₂ terminus, with the thiol-selective reagent ¹⁸F-labeling agent N-[4-[(4-[¹⁸F]fluorobenzylidene)aminooxy]butyl]maleimide ([¹⁸F]FBABM). We also examined the cell binding affinity of the ¹⁸F-labeled annexin V-128 ([¹⁸F]FAN-128). [¹⁸F]FBABM was synthesized in two-step, one-pot method modified from literature procedure. (Toyokuni et al., Bioconjugate Chem. 2003, 14, 1253−1259). The average yield of [¹⁸F]FBABM was 23 ± 4% (n = 4, decay-corrected) and the specific activity was ∼6000 Ci/mmol. The total synthesis time was ∼92 min. The critical improvement of this study was identifying and then developing a purification method to remove an impurity N-[4-[(4-dimethylaminobenzylidene)aminooxy]butyl]maleimide 4, whose presence dramatically decreased the yield of protein labeling. Conjugation of [¹⁸F]FBABM with the thiol-containing annexin V-128 gave [¹⁸F]FAN-128 in 37 ± 9% yield (n = 4, decay corrected). Erythrocyte binding assay of [¹⁸F]FAN-128 showed that this modification of annexin V-128 did not compromise its membrane binding affinity. Thus, an in vivo investigation of [¹⁸F]FAN-128 as an apoptosis imaging agent is warranted.

Novel imaging approaches to head and neck cancer

An inadequate supply of oxygen, hypoxia, is an important factor contributing to resistance to treatment in a number of tumor types, including head and neck cancer. Novel imaging methods have been applied to studies of this important prognostic factor. Mammalian cells need oxygen to live but O2 also participates in the cytotoxic effects of ionizing radiation. Hypoxia is often the result of abnormal blood vessels supplying the tumor, increased diffusion distances to tumor cells, and reduced O2 transport capacity of the blood. Its consequences are mediated by a series of hypoxia-initiated genomic changes activating angiogenesis, glycolysis, and other processes that enable tumor cells to survive or escape the O2-deficient environment. Hypoxia has been shown to be important in overall diminished therapeutic response, malignant progression, increased probability of recurrence, locoregional spread, and distant metastases. Strategies are being developed to surmount the cure-limiting consequences of hypoxia, but methods are needed to select patients most likely to benefit from these new treatments. Even though hypoxia is a common tumor phenotype, it is by no means universal and is often heterogeneous within an individual patient. This review considers the biology of hypoxia, its consequences with respect to treatment, methods for measuring oxygenation in tissues, modern techniques for imaging of regional hypoxia, and how information about the oxygenation status of tumors might impact treatment.

NCI-sponsored trial for the evaluation of safety and preliminary efficacy of FLT as a marker of proliferation in patients with recurrent gliomas: safety studies

PURPOSE

3'-[F-18]Fluoro-3'-deoxythymidine (FLT) is an analog of thymidine that is being developed for imaging cellular proliferation. The goal of this study was to prove that the dose of FLT used for positron emission tomography imaging produces no significant toxicity.

PROCEDURES

Twelve patients with gliomas with either recurrence or suspected radionecrosis were imaged with FLT. Before and at several time points after imaging, subjects underwent general physical and neurological examinations with review of systems and tests of hematologic, hepatic, renal, and several other metabolic parameters. Vital signs and electrocardiograms were monitored during and after the imaging session.

RESULTS

There were no significant adverse effects from FLT injected at a dose of 0.07 mCi/kg (maximum of 5 mCi) at specific activities of 1.25 Ci/micromol or higher. The FLT mass administered for imaging was 0.0001% to 0.0009% of the least toxic cumulative dose administered in clinical trials of FLT as an antiretroviral agent.

CONCLUSIONS

FLT is a safe radiotracer for quantifying proliferation in the human cancer setting.

Quantitative imaging of estrogen receptor expression in breast cancer with PET and 18F-fluoroestradiol

The PET compound (18)F-fluoroestradiol ((18)F-FES) has been developed and tested as an agent for the imaging of estrogen receptor (ER) expression in vivo. (18)F-FES uptake has been shown to correlate with ER expression assayed in vitro by radioligand binding; however, immunohistochemistry (IHC) rather than radioligand binding is used most often to measure ER expression in clinical practice. We therefore compared (18)F-FES uptake with ER expression assayed in vitro by IHC with both qualitative and semiquantitative measures.

METHODS

Seventeen patients with primary or metastatic breast cancer were studied with dynamic (18)F-FES PET; cancer tissue samples, collected close to the time of imaging, were assayed for ER expression by IHC. For each tumor, partial-volume-corrected measures of (18)F-FES uptake were compared with ER expression measured by 3 different ER scoring methods: qualitative scoring (0-3+), the Allred score (0-10), and a computerized IHC index.

RESULTS

There was excellent agreement (r = 0.99) between observers using IHC as well as the different methods of measuring ER content (P < 0.001). ER-negative tumors had (18)F-FES partial-volume-corrected standardized uptake values of less than 1.0, whereas ER-positive tumors had values above 1.1. Correlation coefficients for the different measures of ER content and the different measures of (18)F-FES uptake ranged from 0.57 to 0.73, with the best correlation being between the computerized IHC index and (18)F-FES partial-volume-corrected standardized uptake values.

CONCLUSION

Our results showed good agreement between (18)F-FES PET and ER expression measured by IHC. (18)F-FES imaging may be a useful tool for aiding in the assessment of ER status, especially in patients with multiple tumors or for tumors that are difficult to biopsy.

Challenges in clinical studies with multiple imaging probes

This article addresses two related issues: (a) When a new imaging agent is proposed, how does the imager integrate it with other biomarkers, either sampled or imaged? (b) When we have multiple imaging agents, is the information additive or duplicative and how is this objectively determined? Molecular biology is leading to new treatment options with reduced normal tissue toxicity, and imaging should have a role in objectively evaluating new treatments. There are two roles for molecular characterization of disease. Molecular imaging measurements before therapy help predict the aggressiveness of disease and identify therapeutic targets and, therefore, help choose the optimal therapy for an individual. Measurements of specific biochemical processes made during or after therapy should be sensitive measures of tumor response. The rules of evidence are not fully developed for the prognostic role of imaging biomarkers, but the potential of molecular imaging provides compelling motivation to push forward with convincing validation studies. New imaging procedures need to be characterized for their effectiveness under realistic clinical conditions to improve the management of patients and achieve a better outcome. The purpose of this article is to promote a critical discussion within the molecular imaging community because our future value to the overall biomedical community will be in supporting better treatment outcomes rather than in detection.

18F-Fluoroestradiol

Estrogen receptor (ER) expression is an important determinant of breast cancer behavior and is critical for response to endocrine therapies such as tamoxifen and aromatase inhibitors. In current practice, ER expression is determined by assay of biopsy material. In more advanced disease, tissue assay may present practical difficulties and be associated with significant sampling error. This and other considerations motivated the development of ER imaging agents for positron emission tomography (PET), of which the most successful has been (18)F-16alpha-17beta-fluoroestradiol (FES). In this review, we highlight aspects of ER biology and the importance of the ER in breast cancer therapy; review the structure and synthesis of FES; describe its kinetics and safety/dosimetry data; and highlight validation studies. Also discussed are early results in patients using FES-PET to localize ER-expressing tumors and associated data pointing toward its accuracy as a predictive assay for breast cancer endocrine therapy. Finally, early data for tumors and sites other than breast cancer are mentioned. Preliminary data strongly point toward potential clinical utility for FES-PET, motivating further validation and future clinical trials with prospective endpoints tested under appropriate regulatory oversight.

Positron emission tomography-guided conformal fast neutron therapy for glioblastoma multiforme

Glioblastoma multiforme (GBM) continues to be a difficult therapeutic challenge. Our study was conducted to determine whether improved survival and tumor control could be achieved with modern delivery of fast neutron radiation using three-dimensional treatment planning. Ten patients were enrolled. Eligibility criteria included pathologic diagnosis of GBM, age >or=18 years, and KPS >or=60. Patients underwent MRI and (18)F-fluorodeoxyglucose PET (FDG PET) as part of initial three-dimensional treatment planning. Sequential targets were treated with noncoplanar fields to a total dose of 18 Gy in 16 fractions over 4 weeks. Median and 1-year overall survival were 55 weeks and 60%, respectively. One patient remains alive at last follow-up 255 weeks after diagnosis. Median progression-free survival was 16 weeks, and all patients had tumor progression by 39 weeks. Treatment was clinically well tolerated, but evidence of mild to moderate gliosis and microvascular sclerosis consistent with radiation injury was observed at autopsy in specimens taken from regions of contralateral brain that received approximately 6-10 Gy. Fast neutron radiation using modern imaging, treatment planning, and beam delivery was feasible to a total dose of 18 Gy, but tumor control probability was poor in comparison to that predicted from a dose-response model based on older studies. Steep dose-response curves for both tumor control and neurotoxicity continue to present a challenge to establishing a therapeutic window for fast neutron radiation in GBM, even with modern techniques.

Tumor-specific positron emission tomography imaging in patients: [18F] fluorodeoxyglucose and beyond

Biochemical and molecular imaging of cancer using positron emission tomography (PET) plays an increasing role in the care of cancer patients. Most clinical work to date uses the glucose analogue [(18)F]fluorodeoxyglucose (FDG) to detect accelerated and aberrant glycolysis present in most tumors. Although clinical FDG PET has been used largely to detect and localize cancer, more detailed studies have yielded biological insights and showed the utility of FDG as a prognostic marker and as a tool for therapeutic response evaluation. As cancer therapy becomes more targeted and individualized, it is likely that PET radiopharmaceuticals other than FDG, aimed at more specific aspects of cancer biology, will also play a role in guiding cancer therapy. Clinical trials designed to test and validate new PET agents will need to incorporate rigorous quantitative image analysis and adapt to the evolving use of imaging as a biomarker and will need to incorporate cancer outcomes, such as survival into study design.

Molecular imaging research in the outcomes era: measuring outcomes for individualized cancer therapy

Advances in molecular imaging, combined with the goal of personalized cancer therapy, call for new approaches to clinical study design for trials testing imaging to guide therapy. The role of cancer imaging must expand and move beyond tumor detection and localization to incorporate quantitative evaluation of regional tumor phenotype. Imaging study design and outcome analysis must move beyond metrics designed to measure the performance for detection to include measures of prognosis, prediction of therapeutic success, and early therapy response. This implies changes in how studies are carried and out, and importantly in the regulatory oversight of cancer imaging. Demonstration that a biochemical or molecular imaging method correctly and accurately measures a specific biologic feature should be sufficient for approval for clinical trials. It may be possible that a combination of imaging procedures known to accurately depict tumor phenotype may be prognostic, even if the individual study cannot be directly validated against patient outcomes. Therefore, it will be important to be able to apply a range of possible imaging studies to different targeted cancer therapy trials. Academia and industry must work together with regulatory agencies and payers to facilitate well designed clinical studies, with appropriate outcome measures, to test the effectiveness of imaging in helping to direct cancer therapy. These will assure the appropriate use of imaging to direct treatment and make an important step towards individualized cancer therapy.

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.

Tumor hypoxia imaging with [F-18] fluoromisonidazole positron emission tomography in head and neck cancer

PURPOSE

Advanced head and neck cancer shows hypoxia that results in biological changes to make the tumor cells more aggressive and less responsive to treatment resulting in poor survival. [F-18] fluoromisonidazole (FMISO) positron emission tomography (PET) has the ability to noninvasively quantify regional hypoxia. We investigated the prognostic effect of pretherapy FMISO-PET on survival in head and neck cancer.

EXPERIMENTAL DESIGN

Seventy-three patients with head and neck cancer had pretherapy FMISO-PET and 53 also had fluorodeoxyglucose (FDG) PET under a research protocol from April 1994 to April 2004.

RESULTS

Significant hypoxia was identified in 58 patients (79%). The mean FMISO tumor/bloodmax (T/Bmax) was 1.6 and the mean hypoxic volume (HV) was 40.2 mL. There were 28 deaths in the follow-up period. Mean FDG standard uptake value (SUV)max was 10.8. The median time for follow-up was 72 weeks. In a univariate analysis, T/Bmax (P=0.002), HV (P=0.04), and the presence of nodes (P=0.01) were strong independent predictors. In a multivariate analysis, including FDG SUVmax, no variable was predictive at P<0.05. When FDG SUVmax was removed from the model (resulting in n=73 with 28 events), nodal status and T/Bmax (or HV) were both highly predictive (P=0.02, 0.006 for node and T/Bmax, respectively; P=0.02 and 0.001 for node and HV, respectively).

CONCLUSIONS

Pretherapy FMISO uptake shows a strong trend to be an independent prognostic measure in head and neck cancer.

Imaging oxygenation of human tumours

Tumour hypoxia represents a significant challenge to the curability of human tumours leading to treatment resistance and enhanced tumour progression. Tumour hypoxia can be detected by non-invasive and invasive techniques but the inter-relationships between these remains largely undefined. ¹⁸F-MISO and Cu-ATSM-PET, and BOLD-MRI are the lead contenders for human application based on their non-invasive nature, ease of use and robustness, measurement of hypoxia status, validity, ability to demonstrate heterogeneity and general availability, these techniques are the primary focus of this review. We discuss where developments are required for hypoxia imaging to become clinically useful and explore potential new uses for hypoxia imaging techniques including biological conformal radiotherapy.

Kinetic analysis of 3'-deoxy-3'-18F-fluorothymidine in patients with gliomas

3'-Deoxy-3'-fluorothymidine (FLT), a thymidine analog, is under investigation for monitoring cellular proliferation in gliomas, a potential measure of disease progression and response to therapy. Uptake may result from retention in the biosynthetic pathway or leakage via the disrupted blood-tumor barrier. Visual analysis or static measures of 18F-FLT uptake are problematic as transport and retention cannot be distinguished.

METHODS

Twelve patients with primary brain tumors were imaged for 90 min of dynamic 18F-FLT PET with arterial blood sampling. Total blood activity was corrected for labeled metabolites to provide an FLT input function. A 2-tissue compartment, 4-rate-constant model was used to determine blood-to-tissue transport (K1) and metabolic flux (K(FLT)). Modeling results were compared with MR images of blood-brain barrier (BBB) breakdown revealed by gadolinium (Gd) contrast enhancement. Parametric image maps of K1 and K(FLT) were produced by a mixture analysis approach.

RESULTS

Similar to prior work with 11C-thymidine, identifiability analysis showed that K1 (transport) and K(FLT) (flux) could be estimated independently for sufficiently high K1 values. However, estimation of K(FLT) was less robust at low K1 values, particularly those close to normal brain. K1 was higher for MRI contrast-enhancing (CE) tumors (0.053 +/- 0.029 mL/g/min) than noncontrast-enhancing (NCE) tumors (0.005 +/- 0.002 mL/g/min; P < 0.02), and K(FLT) was higher for high-grade tumors (0.018 +/- 0.008 mL/g/min, n = 9) than low-grade tumors (0.003 +/- 0.003 mL/g/min, n = 3; P < 0.01). The flux in NCE tumors was indistinguishable from contralateral normal brain (0.002 +/- 0.001 mL/g/min). For CE tumors, K1 was higher than K(FLT). Parametric images matched region-of-interest estimates of transport and flux. However, no patient has 18F-FLT uptake outside of the volume of increased permeability defined by MRI T1+Gd enhancement.

CONCLUSION

Modeling analysis of 18F-FLT PET data yielded robust estimates of K1 and K(FLT) for enhancing tumors with sufficiently high K1 and provides a clearer understanding of the relationship between transport and retention of 18F-FLT in gliomas. In tumors that show breakdown of the BBB, transport dominates 18F-FLT uptake. Transport across the BBB and modest rates of 18F-FLT phosphorylation appear to limit the assessment of cellular proliferation using 18F-FLT to highly proliferative tumors with significant BBB breakdown.

Quantitative Fluoroestradiol Positron Emission Tomography Imaging Predicts Response to Endocrine Treatment in Breast Cancer

Purpose

In breast cancer, [18F]fluoroestradiol (FES) positron emission tomography (PET) correlates with estrogen receptors (ER) expression and predicts response to tamoxifen. We tested the ability of FES-PET imaging to predict response to salvage hormonal treatment in heavily pretreated metastatic breast cancer patients, predominantly treated with aromatase inhibitors.

Patients and Methods

Initial FES uptake measurements in 47 patients with ER-positive tumors were correlated with subsequent tumor response to 6 months of hormonal treatment. Most patients had bone dominant disease and prior tamoxifen exposure. Response was compared to initial FES-PET uptake, measured qualitatively and quantitatively using standardized uptake value (SUV) and estradiol-binding flux.

Results

Eleven of 47 patients (23%) had an objective response. While no patients with absent FES uptake had a response to treatment, the association between qualitative FES-PET results and response was not significant (P = .14). However, quantitative FES uptake and response were significantly associated; zero of 15 patients with initial SUV less than 1.5 responded to hormonal therapy, compared with 11 of 32 patients (34%) with SUV higher than 1.5 (P < .01). In the subset of patients whose tumors did not overexpress HER2/neu, 11 of 24 patients (46%) with SUV higher than 1.5 responded.

Conclusion

Quantitative FES-PET can predict response to hormonal therapy and may help guide treatment selection. Treatment selection using quantitative FES-PET in our patient series would have increased the rate of response from 23% to 34% overall, and from 29% to 46% in the subset of patients lacking HER2/neu overexpression. A multi-institutional collaborative trial would permit definitive assessment of the value of FES-PET for therapeutic decision making.

Hypoxia imaging-directed radiation treatment planning

Increasing evidence supports the role of the tumor microenvironment in modulating cancer behavior. Tissue hypoxia, an important and common condition affecting the tumor microenvironment, is well established as a resistance factor in radiotherapy. Increasing evidence points to the ability of hypoxia to induce the expression of gene products, which confer aggressive tumor behavior and promote broad resistance to therapy. These factors suggest that determining the presence or absence of tumor hypoxia is important in planning cancer therapy. Recent advances in PET hypoxia imaging, conformal radiotherapy, and imaging-directed radiotherapy treatment planning now make it possible to perform hypoxia-directed radiotherapy. We review the biological aspects of tumor hypoxia and PET imaging approaches for measuring tumor hypoxia, along with methods for conformal radiotherapy and image-guided treatment, all of which provide the underpinnings for hypoxia-directed therapy. As a case example, we review emerging data on PET imaging of hypoxia to direct radiotherapy.

Are there lessons to be learned from drug development that will accelerate the use of molecular imaging probes in the clinic?

This special issue of the journal contains contributions from participants of the third La Jolla meeting (The Magic Bullet: A Century Later). The goal of this meeting was twofold: to review approaches to validating molecular imaging agents and to review the progress in advancing the use of molecular imaging from the bench to the bedside, with a special emphasis on how molecular imaging improves patient care and management. Drug development has changed its focus over the years. The original approach depended on direct measurements in patients, whereby, in many cases, the drug was advanced to an NDA based on physiological results (e.g., lowering blood pressure) without identifying a target. Over the past decade, the focus has been on validating a target and choosing the lead compound using combinatorial chemistry and high throughput screening, often at the expense of a focus on the biology of diseases. On the other hand, molecular imaging has been target based since its beginning because of the requirements dictated by external imaging (i.e., a target-to-nontarget ratio). This article explores the possible analogies between current targeted drug development and molecular imaging-targeted probe development with the goal of better defining the path to new molecular imaging probes for the clinic.

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.

[11C]Metahydroxyephedrine and [18F]Fluorodeoxyglucose Positron Emission Tomography Improve Clinical Decision Making in Suspected Pheochromocytoma

BACKGROUND

Pheochromocytomas are rare tumors of chromaffin cells for which the optimal management is surgical resection. Precise diagnosis and localization may be elusive. We evaluated whether positron emission tomography (PET) scanning with the combination of [18F]fluorodeoxyglucose (FDG) and the norepinephrine analogue [11C]metahydroxyephedrine (mHED) would allow more exact diagnosis and localization.

METHODS

Fourteen patients with suspected pheochromocytoma were evaluated by anatomical imaging (computed tomography or magnetic resonance imaging) and [131I]metaiodobenzylguanidine (MIBG) planar imaging. PET imaging was performed by using mHED with dynamic adrenal imaging, followed by a torso survey and FDG with a torso survey. Images were evaluated qualitatively by an experienced observer.

RESULTS

Eight patients had pathology-confirmed pheochromocytoma. Of the other six, two patients had normal adrenal tissue at adrenalectomy, and the other four had subsequent clinical courses inconsistent with a diagnosis of pheochromocytoma. In four of eight patients with pheochromocytoma, MIBG failed to detect one or more sites of pathology-confirmed disease. The mHED-PET detected all sites of confirmed disease, whereas FDG-PET detected all sites of adrenal and abdominal disease, but not bone metastases, in one patient. MIBG and FDG-PET results were all negative in the six patients without pheochromocytoma. One patient with adrenal medullary hyperplasia had a positive mHED-PET scan. PET scanning aided the decision not to operate in three of six patients. The resolution of PET functional imaging was superior to that of MIBG.

CONCLUSIONS

PET scanning for pheochromocytoma offers improved quality and resolution over current diagnostic approaches. PET may significantly influence the clinical management of patients with a suspicion of these tumors and warrants further investigation.

[F-18]-fluorodeoxyglucose positron emission tomography for targeting radiation dose escalation for patients with glioblastoma multiforme: clinical outcomes and patterns of failure

PURPOSE

[F-18]-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging for brain tumors has been shown to identify areas of active disease. Radiation dose escalation in the treatment of glioblastoma multiforme may lead to improved disease control. Based on these premises, we initiated a prospective study of FDG-PET for the treatment planning of radiation dose escalation for the treatment of glioblastoma multiforme.

METHODS AND MATERIALS

Forty patients were enrolled. Patients were treated with standard conformal fractionated radiotherapy with volumes defined by MRI imaging. When patients reached a dose of 45-50.4 Gy, they underwent FDG-PET imaging for boost target delineation, for an additional 20 Gy (2 Gy per fraction) to a total dose of 79.4 Gy (n = 30).

RESULTS

The estimated 1-year and 2-year overall survival (OS) for the entire group was 70% and 17%, respectively, with a median overall survival of 70 weeks. The estimated 1-year and 2-year progression-free survival (PFS) was 18% and 3%, respectively, with a median of 24 weeks. No significant improvements in OS or PFS were observed for the study group in comparison to institutional historical controls.

CONCLUSIONS

Radiation dose escalation to 79.4 Gy based on FDG-PET imaging demonstrated no improvement in OS or PFS. This study establishes the feasibility of integrating PET metabolic imaging into radiotherapy treatment planning.

True tracers: comparing FDG with glucose and FLT with thymidine

As PET metabolic imaging becomes routine in clinical practice, there is a tendency to make imaging and data analysis fast and simple, but interpretation of these pictures by visual inspection does not do justice to the power of PET technology. Tissue data and blood data can be analyzed mathematically to provide parametric images of the PET tracer's biochemistry in terms of a transport parameter and a metabolic flux. The methods for parametric imaging with (11)C tracers of glucose and thymidine have been validated, but the short half-life of this radionuclide and the rapid metabolism of these labeled substrates to [(11)C]CO(2) have led investigators to develop (18)F analogs. While (18)F substitution at critical positions in the natural substrate can block metabolism, it has other effects on the transport and metabolism of the analog tracer. The fidelity with which analog tracers mimic tracers of the authentic substrate is critically evaluated for [(18)F]-2-fluoro-2-deoxyglucose and [(18)F]-3'-fluoro-3'-deoxythymidine.

Evaluation of 18F-annexin V as a PET imaging agent in an animal model of apoptosis

Annexin V is a 36-kDa protein that binds with high affinity to phosphatidylserine lipids in the cell membrane. Because one of the earliest measurable events in apoptosis is the eversion of phosphatidylserine from the inner membrane leaflet to the outer cell surface, annexin V has proven useful for detecting the earliest stages of apoptosis.

METHODS

Annexin V was radiolabeled with 18F using N-succinimidyl-4-18F-fluorobenzoic acid chemistry, to a specific activity of 555-925 kBq/mug of protein. 18F-Annexin V (14.8-51.8 MBq) was administered intravenously to rats after pretreatment with cycloheximide (5 mg/kg) to induce liver apoptosis, and the injected rats were imaged by PET over 2 h. After imaging, rats were dissected and individual organs were weighed and counted.

RESULTS

Pretreatment of rats with cycloheximide resulted in a 3- to 9-fold increase in uptake of 18F-annexin V in the liver of treated animals at 2 h, compared with controls. By morphologic analysis, treated livers showed a 3- to 6-fold higher level of apoptosis than controls, with higher levels also seen with longer exposure to cycloheximide. Terminal deoxynucleotide end-labeling (TUNEL) assays performed on liver slices showed that cycloheximide induced a 5- to 8-fold increase in the number of TUNEL-positive nuclei. These TUNEL results correlated with the uptake of 18F-annexin V in dissected liver tissue, with an r2 value of 0.89. Biodistribution analysis of normal rats showed highest uptake of 18F-annexin V in the kidneys and urinary bladder, indicating rapid renal clearance of 18F-annexin V metabolites.

CONCLUSION

The PET data, the organ-specific uptake data from dissection, and the morphologic and TUNEL measures of apoptosis together indicate that 18F-annexin V binds specifically to apoptotic tissues in this model of chemically induced apoptosis in rat liver. The short physical half-life of 18F-annexin V and the rapid clearance of its metabolites to the urinary system suggest that 18F-annexin V will be useful in early assessment of the clinical response to cancer therapy in individual patients.